Manual browser: c++(1)
GCC(1) | GNU | GCC(1) |
NAME
gcc - GNU project C and C++ compilerSYNOPSIS
gcc [ -c|-S|-E] [-std=standard][-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...
Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same options as gcc.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The "overall options" allow you to stop this process at an intermediate stage. For example, the -c option says not to run the linker. Then the output consists of object files output by the assembler.Other options are passed on to one stage of processing. Some options control the preprocessor and others the compiler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them.
Most of the command-line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly. If the description for a particular option does not mention a source language, you can use that option with all supported languages.
The gcc program accepts options and file names as operands. Many options have multi-letter names; therefore multiple single-letter options may not be grouped: -dv is very different from -d -v.
You can mix options and other arguments. For the most part, the order you use doesn't matter. Order does matter when you use several options of the same kind; for example, if you specify -L more than once, the directories are searched in the order specified. Also, the placement of the -l option is significant.
Many options have long names starting with -f or with -W---for example, -fmove-loop-invariants, -Wformat and so on. Most of these have both positive and negative forms; the negative form of -ffoo is -fno-foo. This manual documents only one of these two forms, whichever one is not the default.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations are in the following sections.- Overall Options
- -c -S -E -o file -no-canonical-prefixes -pipe -pass-exit-codes -x language -v -### --help[=class[,...]] --target-help --version -wrapper @file -fplugin=file -fplugin-arg-name=arg -fdump-ada-spec[-slim] -fada-spec-parent=unit -fdump-go-spec=file
- C Language Options
- -ansi -std=standard -fgnu89-inline -aux-info filename -fallow-parameterless-variadic-functions -fno-asm -fno-builtin -fno-builtin-function -fhosted -ffreestanding -fopenmp -fms-extensions -fplan9-extensions -trigraphs -traditional -traditional-cpp -fallow-single-precision -fcond-mismatch -flax-vector-conversions -fsigned-bitfields -fsigned-char -funsigned-bitfields -funsigned-char
- C++ Language Options
- -fabi-version=n -fno-access-control -fcheck-new -fconstexpr-depth=n -ffriend-injection -fno-elide-constructors -fno-enforce-eh-specs -ffor-scope -fno-for-scope -fno-gnu-keywords -fno-implicit-templates -fno-implicit-inline-templates -fno-implement-inlines -fms-extensions -fno-nonansi-builtins -fnothrow-opt -fno-operator-names -fno-optional-diags -fpermissive -fno-pretty-templates -frepo -fno-rtti -fstats -ftemplate-backtrace-limit=n -ftemplate-depth=n -fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ -fno-default-inline -fvisibility-inlines-hidden -fvisibility-ms-compat -fext-numeric-literals -Wabi -Wconversion-null -Wctor-dtor-privacy -Wdelete-non-virtual-dtor -Wliteral-suffix -Wnarrowing -Wnoexcept -Wnon-virtual-dtor -Wreorder -Weffc++ -Wstrict-null-sentinel -Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions -Wsign-promo
- Objective-C and Objective-C++ Language Options
- -fconstant-string-class=class-name -fgnu-runtime -fnext-runtime -fno-nil-receivers -fobjc-abi-version=n -fobjc-call-cxx-cdtors -fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc -fobjc-nilcheck -fobjc-std=objc1 -freplace-objc-classes -fzero-link -gen-decls -Wassign-intercept -Wno-protocol -Wselector -Wstrict-selector-match -Wundeclared-selector
- Language Independent Options
- -fmessage-length=n -fdiagnostics-show-location=[once|every-line] -fno-diagnostics-show-option -fno-diagnostics-show-caret
- Warning Options
- -fsyntax-only -fmax-errors=n -Wpedantic -pedantic-errors -w -Wextra -Wall -Waddress -Waggregate-return -Waggressive-loop-optimizations -Warray-bounds -Wno-attributes -Wno-builtin-macro-redefined -Wc++-compat -Wc++11-compat -Wcast-align -Wcast-qual -Wchar-subscripts -Wclobbered -Wcomment -Wconversion -Wcoverage-mismatch -Wno-cpp -Wno-deprecated -Wno-deprecated-declarations -Wdisabled-optimization -Wno-div-by-zero -Wdouble-promotion -Wempty-body -Wenum-compare -Wno-endif-labels -Werror -Werror=* -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 -Wno-format-contains-nul -Wno-format-extra-args -Wformat-nonliteral -Wformat-security -Wformat-y2k -Wframe-larger-than=len -Wno-free-nonheap-object -Wjump-misses-init -Wignored-qualifiers -Wimplicit -Wimplicit-function-declaration -Wimplicit-int -Winit-self -Winline -Wmaybe-uninitialized -Wno-int-to-pointer-cast -Wno-invalid-offsetof -Winvalid-pch -Wlarger-than=len -Wunsafe-loop-optimizations -Wlogical-op -Wlong-long -Wmain -Wmaybe-uninitialized -Wmissing-braces -Wmissing-field-initializers -Wmissing-include-dirs -Wno-mudflap -Wno-multichar -Wnonnull -Wno-overflow -Woverlength-strings -Wpacked -Wpacked-bitfield-compat -Wpadded -Wparentheses -Wpedantic-ms-format -Wno-pedantic-ms-format -Wpointer-arith -Wno-pointer-to-int-cast -Wredundant-decls -Wno-return-local-addr -Wreturn-type -Wsequence-point -Wshadow -Wsign-compare -Wsign-conversion -Wsizeof-pointer-memaccess -Wstack-protector -Wstack-usage=len -Wstrict-aliasing -Wstrict-aliasing=n -Wstrict-overflow -Wstrict-overflow=n -Wsuggest-attribute=[pure|const|noreturn|format] -Wmissing-format-attribute -Wswitch -Wswitch-default -Wswitch-enum -Wsync-nand -Wsystem-headers -Wtrampolines -Wtrigraphs -Wtype-limits -Wundef -Wuninitialized -Wunknown-pragmas -Wno-pragmas -Wunsuffixed-float-constants -Wunused -Wunused-function -Wunused-label -Wunused-local-typedefs -Wunused-parameter -Wno-unused-result -Wunused-value -Wunused-variable -Wunused-but-set-parameter -Wunused-but-set-variable -Wuseless-cast -Wvariadic-macros -Wvector-operation-performance -Wvla -Wvolatile-register-var -Wwrite-strings -Wzero-as-null-pointer-constant
- C and Objective-C-only Warning Options
- -Wbad-function-cast -Wmissing-declarations -Wmissing-parameter-type -Wmissing-prototypes -Wnested-externs -Wold-style-declaration -Wold-style-definition -Wstrict-prototypes -Wtraditional -Wtraditional-conversion -Wdeclaration-after-statement -Wpointer-sign
- Debugging Options
- -dletters -dumpspecs -dumpmachine -dumpversion -fsanitize=style -fdbg-cnt-list -fdbg-cnt=counter-value-list -fdisable-ipa-pass_name -fdisable-rtl-pass_name -fdisable-rtl-pass-name=range-list -fdisable-tree-pass_name -fdisable-tree-pass-name=range-list -fdump-noaddr -fdump-unnumbered -fdump-unnumbered-links -fdump-translation-unit[-n] -fdump-class-hierarchy[-n] -fdump-ipa-all -fdump-ipa-cgraph -fdump-ipa-inline -fdump-passes -fdump-statistics -fdump-tree-all -fdump-tree-original[-n] -fdump-tree-optimized[-n] -fdump-tree-cfg -fdump-tree-alias -fdump-tree-ch -fdump-tree-ssa[-n] -fdump-tree-pre[-n] -fdump-tree-ccp[-n] -fdump-tree-dce[-n] -fdump-tree-gimple[-raw] -fdump-tree-mudflap[-n] -fdump-tree-dom[-n] -fdump-tree-dse[-n] -fdump-tree-phiprop[-n] -fdump-tree-phiopt[-n] -fdump-tree-forwprop[-n] -fdump-tree-copyrename[-n] -fdump-tree-nrv -fdump-tree-vect -fdump-tree-sink -fdump-tree-sra[-n] -fdump-tree-forwprop[-n] -fdump-tree-fre[-n] -fdump-tree-vrp[-n] -ftree-vectorizer-verbose=n -fdump-tree-storeccp[-n] -fdump-final-insns=file -fcompare-debug[=opts] -fcompare-debug-second -feliminate-dwarf2-dups -fno-eliminate-unused-debug-types -feliminate-unused-debug-symbols -femit-class-debug-always -fenable-kind-pass -fenable-kind-pass=range-list -fdebug-types-section -fmem-report-wpa -fmem-report -fpre-ipa-mem-report -fpost-ipa-mem-report -fprofile-arcs -fopt-info -fopt-info-options[=file] -frandom-seed=string -fsched-verbose=n -fsel-sched-verbose -fsel-sched-dump-cfg -fsel-sched-pipelining-verbose -fstack-usage -ftest-coverage -ftime-report -fvar-tracking -fvar-tracking-assignments -fvar-tracking-assignments-toggle -g -glevel -gtoggle -gcoff -gdwarf-version -ggdb -grecord-gcc-switches -gno-record-gcc-switches -gstabs -gstabs+ -gstrict-dwarf -gno-strict-dwarf -gvms -gxcoff -gxcoff+ -fno-merge-debug-strings -fno-dwarf2-cfi-asm -fdebug-prefix-map=old=new -femit-struct-debug-baseonly -femit-struct-debug-reduced -femit-struct-debug-detailed[=spec-list] -p -pg -print-file-name=library -print-libgcc-file-name -print-multi-directory -print-multi-lib -print-multi-os-directory -print-prog-name=program -print-search-dirs -Q -print-sysroot -print-sysroot-headers-suffix -save-temps -save-temps=cwd -save-temps=obj -time[=file]
- Optimization Options
- -faggressive-loop-optimizations -falign-functions[=n] -falign-jumps[=n] -falign-labels[=n] -falign-loops[=n] -fassociative-math -fauto-inc-dec -fbranch-probabilities -fbranch-target-load-optimize -fbranch-target-load-optimize2 -fbtr-bb-exclusive -fcaller-saves -fcheck-data-deps -fcombine-stack-adjustments -fconserve-stack -fcompare-elim -fcprop-registers -fcrossjumping -fcse-follow-jumps -fcse-skip-blocks -fcx-fortran-rules -fcx-limited-range -fdata-sections -fdce -fdelayed-branch -fdelete-null-pointer-checks -fdevirtualize -fdse -fearly-inlining -fipa-sra -fexpensive-optimizations -ffat-lto-objects -ffast-math -ffinite-math-only -ffloat-store -fexcess-precision=style -fforward-propagate -ffp-contract=style -ffunction-sections -fgcse -fgcse-after-reload -fgcse-las -fgcse-lm -fgraphite-identity -fgcse-sm -fhoist-adjacent-loads -fif-conversion -fif-conversion2 -findirect-inlining -finline-functions -finline-functions-called-once -finline-limit=n -finline-small-functions -fipa-cp -fipa-cp-clone -fipa-pta -fipa-profile -fipa-pure-const -fipa-reference -fira-algorithm=algorithm -fira-region=region -fira-hoist-pressure -fira-loop-pressure -fno-ira-share-save-slots -fno-ira-share-spill-slots -fira-verbose=n -fivopts -fkeep-inline-functions -fkeep-static-consts -floop-block -floop-interchange -floop-strip-mine -floop-nest-optimize -floop-parallelize-all -flto -flto-compression-level -flto-partition=alg -flto-report -fmerge-all-constants -fmerge-constants -fmodulo-sched -fmodulo-sched-allow-regmoves -fmove-loop-invariants fmudflap -fmudflapir -fmudflapth -fno-branch-count-reg -fno-default-inline -fno-defer-pop -fno-function-cse -fno-guess-branch-probability -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 -fno-sched-interblock -fno-sched-spec -fno-signed-zeros -fno-toplevel-reorder -fno-trapping-math -fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-register-move -foptimize-sibling-calls -fpartial-inlining -fpeel-loops -fpredictive-commoning -fprefetch-loop-arrays -fprofile-report -fprofile-correction -fprofile-dir=path -fprofile-generate -fprofile-generate=path -fprofile-use -fprofile-use=path -fprofile-values -freciprocal-math -free -fregmove -frename-registers -freorder-blocks -freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop -freschedule-modulo-scheduled-loops -frounding-math -fsched2-use-superblocks -fsched-pressure -fsched-spec-load -fsched-spec-load-dangerous -fsched-stalled-insns-dep[=n] -fsched-stalled-insns[=n] -fsched-group-heuristic -fsched-critical-path-heuristic -fsched-spec-insn-heuristic -fsched-rank-heuristic -fsched-last-insn-heuristic -fsched-dep-count-heuristic -fschedule-insns -fschedule-insns2 -fsection-anchors -fselective-scheduling -fselective-scheduling2 -fsel-sched-pipelining -fsel-sched-pipelining-outer-loops -fshrink-wrap -fsignaling-nans -fsingle-precision-constant -fsplit-ivs-in-unroller -fsplit-wide-types -fstack-protector -fstack-protector-all -fstrict-aliasing -fstrict-overflow -fthread-jumps -ftracer -ftree-bit-ccp -ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-coalesce-inline-vars -ftree-coalesce-vars -ftree-copy-prop -ftree-copyrename -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -ftree-loop-if-convert -ftree-loop-if-convert-stores -ftree-loop-im -ftree-phiprop -ftree-loop-distribution -ftree-loop-distribute-patterns -ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize -ftree-parallelize-loops=n -ftree-pre -ftree-partial-pre -ftree-pta -ftree-reassoc -ftree-sink -ftree-slsr -ftree-sra -ftree-switch-conversion -ftree-tail-merge -ftree-ter -ftree-vect-loop-version -ftree-vectorize -ftree-vrp -funit-at-a-time -funroll-all-loops -funroll-loops -funsafe-loop-optimizations -funsafe-math-optimizations -funswitch-loops -fvariable-expansion-in-unroller -fvect-cost-model -fvpt -fweb -fwhole-program -fwpa -fuse-ld=linker -fuse-linker-plugin --param name=value -O -O0 -O1 -O2 -O3 -Os -Ofast -Og
- Preprocessor Options
- -Aquestion=answer -A-question[=answer] -C -dD -dI -dM -dN -Dmacro[=defn] -E -H -idirafter dir -include file -imacros file -iprefix file -iwithprefix dir -iwithprefixbefore dir -isystem dir -cxx-isystem dir -imultilib dir -isysroot dir -M -MM -MF -MG -MP -MQ -MT -nostdinc -P -fdebug-cpp -ftrack-macro-expansion -fworking-directory -remap -trigraphs -undef -Umacro -Wp,option -Xpreprocessor option -no-integrated-cpp
- Assembler Option
- -Wa,option -Xassembler option
- Linker Options
- object-file-name -llibrary -nostartfiles -nodefaultlibs -nostdlib -pie -rdynamic -s -static -static-libgcc -static-libstdc++ -static-libasan -static-libtsan -shared -shared-libgcc -symbolic -T script -Wl,option -Xlinker option -u symbol
- Directory Options
- -Bprefix -Idir -iquotedir -iremapsrc:dst -Ldir -specs=file -I- --sysroot=dir
- Target Options
- -V version -b machine -Bprefix -Idir -iplugindir=dir -iquotedir -Ldir -specs=file -I- --sysroot=dir --no-sysroot-suffix
- Machine Dependent Options
-
AArch64 Options -mbig-endian -mlittle-endian -mgeneral-regs-only -mcmodel=tiny -mcmodel=small -mcmodel=large -mstrict-align -momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer -mtls-dialect=desc -mtls-dialect=traditional -march=name -mcpu=name -mtune=name
- Code Generation Options
- -fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions -fnon-call-exceptions -fdelete-dead-exceptions -funwind-tables -fasynchronous-unwind-tables -finhibit-size-directive -finstrument-functions -finstrument-functions-exclude-function-list=sym,sym,... -finstrument-functions-exclude-file-list=file,file,... -fno-common -fno-ident -fpcc-struct-return -fpic -fPIC -fpie -fPIE -fno-jump-tables -frecord-gcc-switches -freg-struct-return -fshort-enums -fshort-double -fshort-wchar -fverbose-asm -fpack-struct[=n] -fstack-check -fstack-limit-register=reg -fstack-limit-symbol=sym -fno-stack-limit -fsplit-stack -fleading-underscore -ftls-model=model -fstack-reuse=reuse_level -ftrapv -fwrapv -fbounds-check -fvisibility -fstrict-volatile-bitfields -fsync-libcalls
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in that order. GCC is capable of preprocessing and compiling several files either into several assembler input files, or into one assembler input file; then each assembler input file produces an object file, and linking combines all the object files (those newly compiled, and those specified as input) into an executable file.For any given input file, the file name suffix determines what kind of compilation is done:
- file.c
- C source code that must be preprocessed.
- file.i
- C source code that should not be preprocessed.
- file.ii
- C++ source code that should not be preprocessed.
- file.m
- Objective-C source code. Note that you must link with the libobjc library to make an Objective-C program work.
- file.mi
- Objective-C source code that should not be preprocessed.
- file.mm
- file.M
- Objective-C++ source code. Note that you must link with the libobjc library to make an Objective-C++ program work. Note that .M refers to a literal capital M.
- file.mii
- Objective-C++ source code that should not be preprocessed.
- file.h
- C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (default), or C, C++ header file to be turned into an Ada spec (via the -fdump-ada-spec switch).
- file.cc
- file.cp
- file.cxx
- file.cpp
- file.CPP
- file.c++
- file.C
- C++ source code that must be preprocessed. Note that in .cxx, the last two letters must both be literally x. Likewise, .C refers to a literal capital C.
- file.mm
- file.M
- Objective-C++ source code that must be preprocessed.
- file.mii
- Objective-C++ source code that should not be preprocessed.
- file.hh
- file.H
- file.hp
- file.hxx
- file.hpp
- file.HPP
- file.h++
- file.tcc
- C++ header file to be turned into a precompiled header or Ada spec.
- file.f
- file.for
- file.ftn
- Fixed form Fortran source code that should not be preprocessed.
- file.F
- file.FOR
- file.fpp
- file.FPP
- file.FTN
- Fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).
- file.f90
- file.f95
- file.f03
- file.f08
- Free form Fortran source code that should not be preprocessed.
- file.F90
- file.F95
- file.F03
- file.F08
- Free form Fortran source code that must be preprocessed (with the traditional preprocessor).
- file.go
- Go source code.
- file.ads
- Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). Such files are also called specs.
- file.adb
- Ada source code file containing a library unit body (a subprogram or package body). Such files are also called bodies.
- file.s
- Assembler code.
- file.S
- file.sx
- Assembler code that must be preprocessed.
- other
- An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.
You can specify the input language explicitly with the -x option:
- -x language
-
Specify explicitly the language for the following input files (rather than letting the compiler choose a default based on the file name suffix). This option applies to all following input files until the next -x option. Possible values for language are:
c c-header cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input f95 f95-cpp-input
go
java
- -x none
- Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as they are if -x has not been used at all).
- -pass-exit-codes
- Normally the gcc program exits with the code of 1 if any phase of the compiler returns a non-success return code. If you specify -pass-exit-codes, the gcc program instead returns with the numerically highest error produced by any phase returning an error indication. The C, C++, and Fortran front ends return 4 if an internal compiler error is encountered.
If you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc where to start, and one of the options -c, -S, or -E to say where gcc is to stop. Note that some combinations (for example, -x cpp-output -E) instruct gcc to do nothing at all.
- -c
-
Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in the form of an object file for each source file.
- -S
-
Stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler code file for each non-assembler input file specified.
- -E
-
Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output.
- -o file
-
Place output in file file. This applies to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code.
- -v
- Print (on standard error output) the commands executed to run the stages of compilation. Also print the version number of the compiler driver program and of the preprocessor and the compiler proper.
- -###
- Like -v except the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or "./-_". This is useful for shell scripts to capture the driver-generated command lines.
- -pipe
- Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble.
- --help
- Print (on the standard output) a description of the command-line options understood by gcc. If the -v option is also specified then --help is also passed on to the various processes invoked by gcc, so that they can display the command-line options they accept. If the -Wextra option has also been specified (prior to the --help option), then command-line options that have no documentation associated with them are also displayed.
- --target-help
- Print (on the standard output) a description of target-specific command-line options for each tool. For some targets extra target-specific information may also be printed.
- --help={class|[^]qualifier}[,...]
- Print (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers. These are the supported classes:
- optimizers
- Display all of the optimization options supported by the compiler.
- warnings
- Display all of the options controlling warning messages produced by the compiler.
- target
- Display target-specific options. Unlike the --target-help option however, target-specific options of the linker and assembler are not displayed. This is because those tools do not currently support the extended --help= syntax.
- params
- Display the values recognized by the --param option.
- language
- Display the options supported for language, where language is the name of one of the languages supported in this version of GCC.
- common
- Display the options that are common to all languages.
- undocumented
- Display only those options that are undocumented.
- joined
- Display options taking an argument that appears after an equal sign in the same continuous piece of text, such as: --help=target.
- separate
- Display options taking an argument that appears as a separate word following the original option, such as: -o output-file.
--help=target,undocumented
--help=warnings,^joined,^undocumented
--help=target,optimizers
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]
-Q -O2 --help=optimizers
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
- -no-canonical-prefixes
- Do not expand any symbolic links, resolve references to /../ or /./, or make the path absolute when generating a relative prefix.
- --version
- Display the version number and copyrights of the invoked GCC.
- -wrapper
-
Invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma separated list.
gcc -c t.c -wrapper gdb,--args
- -fplugin=name.so
- Load the plugin code in file name.so, assumed to be a shared object to be dlopen'd by the compiler. The base name of the shared object file is used to identify the plugin for the purposes of argument parsing (See -fplugin-arg-name-key=value below). Each plugin should define the callback functions specified in the Plugins API.
- -fplugin-arg-name-key=value
- Define an argument called key with a value of value for the plugin called name.
- -fdump-ada-spec[-slim]
- For C and C++ source and include files, generate corresponding Ada specs.
- -fada-spec-parent=unit
- In conjunction with -fdump-ada-spec[-slim] above, generate Ada specs as child units of parent unit.
- -fdump-go-spec=file
- For input files in any language, generate corresponding Go declarations in file. This generates Go "const", "type", "var", and "func" declarations which may be a useful way to start writing a Go interface to code written in some other language.
- @file
-
Read command-line options from file. The options read are inserted in place of the original @ file option. If file does not exist, or cannot be read, then the option will be treated literally, and not removed.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc; and preprocessed C++ files use the suffix .ii. GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name gcc).However, the use of gcc does not add the C++ library. g++ is a program that calls GCC and automatically specifies linking against the C++ library. It treats .c, .h and .i files as C++ source files instead of C source files unless -x is used. This program is also useful when precompiling a C header file with a .h extension for use in C++ compilations. On many systems, g++ is also installed with the name c++.
When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that the compiler accepts:- -ansi
-
In C mode, this is equivalent to -std=c90. In C++ mode, it is equivalent to -std=c++98.
- -std=
-
Determine the language standard. This option is currently only supported when compiling C or C++.
- c90
- c89
- iso9899:1990
- Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code.
- iso9899:199409
- ISO C90 as modified in amendment 1.
- c99
- c9x
- iso9899:1999
- iso9899:199x
- ISO C99. Note that this standard is not yet fully supported; see < http://gcc.gnu.org/c99status.html> for more information. The names c9x and iso9899:199x are deprecated.
- c11
- c1x
- iso9899:2011
- ISO C11, the 2011 revision of the ISO C standard. Support is incomplete and experimental. The name c1x is deprecated.
- gnu90
- gnu89
- GNU dialect of ISO C90 (including some C99 features). This is the default for C code.
- gnu99
- gnu9x
- GNU dialect of ISO C99. When ISO C99 is fully implemented in GCC, this will become the default. The name gnu9x is deprecated.
- gnu11
- gnu1x
- GNU dialect of ISO C11. Support is incomplete and experimental. The name gnu1x is deprecated.
- c++98
- c++03
- The 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as -ansi for C++ code.
- gnu++98
- gnu++03
- GNU dialect of -std=c++98. This is the default for C++ code.
- c++11
- c++0x
- The 2011 ISO C++ standard plus amendments. Support for C++11 is still experimental, and may change in incompatible ways in future releases. The name c++0x is deprecated.
- gnu++11
- gnu++0x
- GNU dialect of -std=c++11. Support for C++11 is still experimental, and may change in incompatible ways in future releases. The name gnu++0x is deprecated.
- c++1y
- The next revision of the ISO C++ standard, tentatively planned for 2017. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.
- gnu++1y
- GNU dialect of -std=c++1y. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.
- -fgnu89-inline
-
The option -fgnu89-inline tells GCC to use the traditional GNU semantics for "inline" functions when in C99 mode.
This option is accepted and ignored by GCC versions 4.1.3 up to but not including 4.3. In GCC versions 4.3 and later it changes the behavior of GCC in C99 mode. Using this option is roughly equivalent to adding the "gnu_inline" function attribute to all inline functions.
- -aux-info filename
-
Output to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files. This option is silently ignored in any language other than C.
- -fallow-parameterless-variadic-functions
-
Accept variadic functions without named parameters.
- -fno-asm
-
Do not recognize "asm", "inline" or "typeof" as a keyword, so that code can use these words as identifiers. You can use the keywords "__asm__", "__inline__" and "__typeof__" instead. -ansi implies -fno-asm.
- -fno-builtin
- -fno-builtin-function
-
Don't recognize built-in functions that do not begin with __builtin_ as prefix.
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
- -fhosted
- Assert that compilation targets a hosted environment. This implies -fbuiltin. A hosted environment is one in which the entire standard library is available, and in which "main" has a return type of "int". Examples are nearly everything except a kernel. This is equivalent to -fno-freestanding.
- -ffreestanding
- Assert that compilation targets a freestanding environment. This implies -fno-builtin. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at "main". The most obvious example is an OS kernel. This is equivalent to -fno-hosted.
- -fopenmp
- Enable handling of OpenMP directives "#pragma omp" in C/C++ and "!$omp" in Fortran. When -fopenmp is specified, the compiler generates parallel code according to the OpenMP Application Program Interface v3.0 < http://www.openmp.org/>. This option implies -pthread, and thus is only supported on targets that have support for -pthread.
- -fgnu-tm
-
When the option -fgnu-tm is specified, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature.
- -fms-extensions
-
Accept some non-standard constructs used in Microsoft header files.
typedef int UOW;
struct ABC {
UOW UOW;
};
- -fplan9-extensions
-
Accept some non-standard constructs used in Plan 9 code.
- -trigraphs
- Support ISO C trigraphs. The -ansi option (and -std options for strict ISO C conformance) implies -trigraphs.
- -traditional
- -traditional-cpp
- Formerly, these options caused GCC to attempt to emulate a pre-standard C compiler. They are now only supported with the -E switch. The preprocessor continues to support a pre-standard mode. See the GNU CPP manual for details.
- -fcond-mismatch
- Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. This option is not supported for C++.
- -flax-vector-conversions
- Allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types. This option should not be used for new code.
- -funsigned-char
-
Let the type "char" be unsigned, like "unsigned char".
- -fsigned-char
-
Let the type "char" be signed, like "signed char".
- -fsigned-bitfields
- -funsigned-bitfields
- -fno-signed-bitfields
- -fno-unsigned-bitfields
- These options control whether a bit-field is signed or unsigned, when the declaration does not use either "signed" or "unsigned". By default, such a bit-field is signed, because this is consistent: the basic integer types such as "int" are signed types.
Options Controlling C++ Dialect
This section describes the command-line options that are only meaningful for C++ programs. You can also use most of the GNU compiler options regardless of what language your program is in. For example, you might compile a file "firstClass.C" like this:
g++ -g -frepo -O -c firstClass.C
In this example, only -frepo is an option meant only for C++ programs; you can use the other options with any language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
- -fabi-version=n
-
Use version n of the C++ ABI. The default is version 2.
- -fno-access-control
- Turn off all access checking. This switch is mainly useful for working around bugs in the access control code.
- -fcheck-new
- Check that the pointer returned by "operator new" is non-null before attempting to modify the storage allocated. This check is normally unnecessary because the C++ standard specifies that "operator new" only returns 0 if it is declared throw(), in which case the compiler always checks the return value even without this option. In all other cases, when "operator new" has a non-empty exception specification, memory exhaustion is signalled by throwing "std::bad_alloc". See also new (nothrow).
- -fconstexpr-depth=n
- Set the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed to detect endless recursion during constant expression evaluation. The minimum specified by the standard is 512.
- -fdeduce-init-list
-
Enable deduction of a template type parameter as "std::initializer_list" from a brace-enclosed initializer list, i.e.
template <class T> auto forward(T t) -> decltype (realfn (t))
{
return realfn (t);
}
void f()
{
forward({1,2}); // call forward<std::initializer_list<int>>
}
- -ffriend-injection
-
Inject friend functions into the enclosing namespace, so that they are visible outside the scope of the class in which they are declared. Friend functions were documented to work this way in the old Annotated C++ Reference Manual, and versions of G++ before 4.1 always worked that way. However, in ISO C++ a friend function that is not declared in an enclosing scope can only be found using argument dependent lookup. This option causes friends to be injected as they were in earlier releases.
- -fno-elide-constructors
- The C++ standard allows an implementation to omit creating a temporary that is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases.
- -fno-enforce-eh-specs
- Don't generate code to check for violation of exception specifications at run time. This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining NDEBUG. This does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time.
- -fextern-tls-init
- -fno-extern-tls-init
-
The C++11 and OpenMP standards allow thread_local and threadprivate variables to have dynamic (runtime) initialization. To support this, any use of such a variable goes through a wrapper function that performs any necessary initialization. When the use and definition of the variable are in the same translation unit, this overhead can be optimized away, but when the use is in a different translation unit there is significant overhead even if the variable doesn't actually need dynamic initialization. If the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or a use of the variable in the defining TU will be executed before any uses in another TU), they can avoid this overhead with the -fno-extern-tls-init option.
- -ffor-scope
- -fno-for-scope
-
If -ffor-scope is specified, the scope of variables declared in a for-init-statement is limited to the for loop itself, as specified by the C++ standard. If -fno-for-scope is specified, the scope of variables declared in a for-init-statement extends to the end of the enclosing scope, as was the case in old versions of G++, and other (traditional) implementations of C++.
- -fno-gnu-keywords
- Do not recognize "typeof" as a keyword, so that code can use this word as an identifier. You can use the keyword "__typeof__" instead. -ansi implies -fno-gnu-keywords.
- -fno-implicit-templates
- Never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for explicit instantiations.
- -fno-implicit-inline-templates
- Don't emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles with and without optimization need the same set of explicit instantiations.
- -fno-implement-inlines
- To save space, do not emit out-of-line copies of inline functions controlled by #pragma implementation. This causes linker errors if these functions are not inlined everywhere they are called.
- -fms-extensions
- Disable Wpedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax.
- -fno-nonansi-builtins
- Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include "ffs", "alloca", "_exit", "index", "bzero", "conjf", and other related functions.
- -fnothrow-opt
- Treat a "throw()" exception specification as if it were a "noexcept" specification to reduce or eliminate the text size overhead relative to a function with no exception specification. If the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller because the EH cleanups for those variables can be optimized away. The semantic effect is that an exception thrown out of a function with such an exception specification results in a call to "terminate" rather than "unexpected".
- -fno-operator-names
- Do not treat the operator name keywords "and", "bitand", "bitor", "compl", "not", "or" and "xor" as synonyms as keywords.
- -fno-optional-diags
- Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class.
- -fpermissive
- Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using -fpermissive allows some nonconforming code to compile.
- -fno-pretty-templates
- When an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g. "void f(T) [with T = int]" rather than "void f(int)") so that it's clear which template is involved. When an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. If either of these behaviors make it harder to understand the error message rather than easier, you can use -fno-pretty-templates to disable them.
- -frepo
- Enable automatic template instantiation at link time. This option also implies -fno-implicit-templates.
- -fno-rtti
- Disable generation of information about every class with virtual functions for use by the C++ run-time type identification features ( dynamic_cast and typeid). If you don't use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but G++ generates it as needed. The dynamic_cast operator can still be used for casts that do not require run-time type information, i.e. casts to "void *" or to unambiguous base classes.
- -fstats
- Emit statistics about front-end processing at the end of the compilation. This information is generally only useful to the G++ development team.
- -fstrict-enums
- Allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type.
- -ftemplate-backtrace-limit=n
- Set the maximum number of template instantiation notes for a single warning or error to n. The default value is 10.
- -ftemplate-depth=n
- Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11). The default value is 900, as the compiler can run out of stack space before hitting 1024 in some situations.
- -fno-threadsafe-statics
- Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn't need to be thread-safe.
- -fuse-cxa-atexit
- Register destructors for objects with static storage duration with the "__cxa_atexit" function rather than the "atexit" function. This option is required for fully standards-compliant handling of static destructors, but only works if your C library supports "__cxa_atexit".
- -fno-use-cxa-get-exception-ptr
- Don't use the "__cxa_get_exception_ptr" runtime routine. This causes "std::uncaught_exception" to be incorrect, but is necessary if the runtime routine is not available.
- -fvisibility-inlines-hidden
-
This switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.
- -fvisibility-ms-compat
-
This flag attempts to use visibility settings to make GCC's C++ linkage model compatible with that of Microsoft Visual Studio.
- 1.
- It sets the default visibility to "hidden", like -fvisibility=hidden.
- 2.
- Types, but not their members, are not hidden by default.
- 3.
- The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used.
- -fno-weak
- Do not use weak symbol support, even if it is provided by the linker. By default, G++ uses weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++.
- -nostdinc++
- Do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library.)
In addition, these optimization, warning, and code generation options have meanings only for C++ programs:
- -fno-default-inline
-
Do not assume inline for functions defined inside a class scope.
Note that these functions have linkage like inline functions; they just aren't inlined by default.
- -Wabi (C, Objective-C, C++ and Objective-C++ only)
-
Warn when G++ generates code that is probably not compatible with the vendor-neutral C++ ABI. Although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is generating incompatible code. There may also be cases where warnings are emitted even though the code that is generated is compatible.
- •
-
A template with a non-type template parameter of reference type is mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}
- •
-
SIMD vector types declared using "__attribute ((vector_size))" are mangled in a non-standard way that does not allow for overloading of functions taking vectors of different sizes.
- •
-
Incorrect handling of tail-padding for bit-fields. G++ may attempt to pack data into the same byte as a base class. For example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
- •
-
Incorrect handling of tail-padding for virtual bases. G++ does not use tail padding when laying out virtual bases. For example:
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
- •
-
Incorrect handling of bit-fields with declared widths greater than that of their underlying types, when the bit-fields appear in a union. For example:
union U { int i : 4096; };
- •
-
Empty classes can be placed at incorrect offsets. For example:
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
- •
-
Names of template functions whose types involve "typename" or template template parameters can be mangled incorrectly.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
- •
-
For SysV/x86-64, unions with "long double" members are passed in memory as specified in psABI. For example:
union U {
long double ld;
int i;
};
- -Wctor-dtor-privacy (C++ and Objective-C++ only)
- Warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions. Also warn if there are no non-private methods, and there's at least one private member function that isn't a constructor or destructor.
- -Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
- Warn when delete is used to destroy an instance of a class that has virtual functions and non-virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor. This warning is enabled by -Wall.
- -Wliteral-suffix (C++ and Objective-C++ only)
-
Warn when a string or character literal is followed by a ud-suffix which does not begin with an underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens in order to maintain backwards compatibility with code that uses formatting macros from "<inttypes.h>". For example:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>
int main() {
int64_t i64 = 123;
printf("My int64: %"PRId64"\n", i64);
}
- -Wnarrowing (C++ and Objective-C++ only)
-
Warn when a narrowing conversion prohibited by C++11 occurs within { }, e.g.
int i = { 2.2 }; // error: narrowing from double to int
- -Wnoexcept (C++ and Objective-C++ only)
- Warn when a noexcept-expression evaluates to false because of a call to a function that does not have a non-throwing exception specification (i.e. throw() or noexcept) but is known by the compiler to never throw an exception.
- -Wnon-virtual-dtor (C++ and Objective-C++ only)
- Warn when a class has virtual functions and an accessible non-virtual destructor, in which case it is possible but unsafe to delete an instance of a derived class through a pointer to the base class. This warning is also enabled if -Weffc++ is specified.
- -Wreorder (C++ and Objective-C++ only)
-
Warn when the order of member initializers given in the code does not match the order in which they must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
- -fext-numeric-literals (C++ and Objective-C++ only)
- Accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off these suffixes are treated as C++11 user-defined literal numeric suffixes. This is on by default for all pre-C++11 dialects and all GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++1y. This option is off by default for ISO C++11 onwards ( -std=c++11, ...).
The following -W... options are not affected by -Wall.
- -Weffc++ (C++ and Objective-C++ only)
- Warn about violations of the following style guidelines from Scott Meyers' Effective C++, Second Edition book:
- •
- Item 11: Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
- •
- Item 12: Prefer initialization to assignment in constructors.
- •
- Item 14: Make destructors virtual in base classes.
- •
- Item 15: Have "operator=" return a reference to *this.
- •
- Item 23: Don't try to return a reference when you must return an object.
- •
- Item 6: Distinguish between prefix and postfix forms of increment and decrement operators.
- •
- Item 7: Never overload "&&", "||", or ",".
- -Wstrict-null-sentinel (C++ and Objective-C++ only)
- Warn about the use of an uncasted "NULL" as sentinel. When compiling only with GCC this is a valid sentinel, as "NULL" is defined to "__null". Although it is a null pointer constant rather than a null pointer, it is guaranteed to be of the same size as a pointer. But this use is not portable across different compilers.
- -Wno-non-template-friend (C++ and Objective-C++ only)
- Disable warnings when non-templatized friend functions are declared within a template. Since the advent of explicit template specification support in G++, if the name of the friend is an unqualified-id (i.e., friend foo(int)), the C++ language specification demands that the friend declare or define an ordinary, nontemplate function. (Section 14.5.3). Before G++ implemented explicit specification, unqualified-ids could be interpreted as a particular specialization of a templatized function. Because this non-conforming behavior is no longer the default behavior for G++, -Wnon-template-friend allows the compiler to check existing code for potential trouble spots and is on by default. This new compiler behavior can be turned off with -Wno-non-template-friend, which keeps the conformant compiler code but disables the helpful warning.
- -Wold-style-cast (C++ and Objective-C++ only)
- Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts ( dynamic_cast, static_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended effects and much easier to search for.
- -Woverloaded-virtual (C++ and Objective-C++ only)
-
Warn when a function declaration hides virtual functions from a base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
B* b;
b->f();
- -Wno-pmf-conversions (C++ and Objective-C++ only)
- Disable the diagnostic for converting a bound pointer to member function to a plain pointer.
- -Wsign-promo (C++ and Objective-C++ only)
- Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. Previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior.
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves.This section describes the command-line options that are only meaningful for Objective-C and Objective-C++ programs. You can also use most of the language-independent GNU compiler options. For example, you might compile a file "some_class.m" like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; you can use the other options with any language supported by GCC.
Note that since Objective-C is an extension of the C language, Objective-C compilations may also use options specific to the C front-end (e.g., -Wtraditional). Similarly, Objective-C++ compilations may use C++-specific options (e.g., -Wabi).
Here is a list of options that are only for compiling Objective-C and Objective-C++ programs:
- -fconstant-string-class=class-name
- Use class-name as the name of the class to instantiate for each literal string specified with the syntax "@"..."". The default class name is "NXConstantString" if the GNU runtime is being used, and "NSConstantString" if the NeXT runtime is being used (see below). The -fconstant-cfstrings option, if also present, overrides the -fconstant-string-class setting and cause "@"..."" literals to be laid out as constant CoreFoundation strings.
- -fgnu-runtime
- Generate object code compatible with the standard GNU Objective-C runtime. This is the default for most types of systems.
- -fnext-runtime
- Generate output compatible with the NeXT runtime. This is the default for NeXT-based systems, including Darwin and Mac OS X. The macro "__NEXT_RUNTIME__" is predefined if (and only if) this option is used.
- -fno-nil-receivers
- Assume that all Objective-C message dispatches ("[receiver message:arg]") in this translation unit ensure that the receiver is not "nil". This allows for more efficient entry points in the runtime to be used. This option is only available in conjunction with the NeXT runtime and ABI version 0 or 1.
- -fobjc-abi-version=n
- Use version n of the Objective-C ABI for the selected runtime. This option is currently supported only for the NeXT runtime. In that case, Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C 2.0 additions. Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI. If nothing is specified, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit target machines.
- -fobjc-call-cxx-cdtors
-
For each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial default constructor. If so, synthesize a special "- (id) .cxx_construct" instance method which runs non-trivial default constructors on any such instance variables, in order, and then return "self". Similarly, check if any instance variable is a C++ object with a non-trivial destructor, and if so, synthesize a special "- (void) .cxx_destruct" method which runs all such default destructors, in reverse order.
- -fobjc-direct-dispatch
- Allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page.
- -fobjc-exceptions
- Enable syntactic support for structured exception handling in Objective-C, similar to what is offered by C++ and Java. This option is required to use the Objective-C keywords @try, @throw, @catch, @finally and @synchronized. This option is available with both the GNU runtime and the NeXT runtime (but not available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier).
- -fobjc-gc
- Enable garbage collection (GC) in Objective-C and Objective-C++ programs. This option is only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags.
- -fobjc-nilcheck
- For the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual method call. This is the default and can be disabled using -fno-objc-nilcheck. Class methods and super calls are never checked for nil in this way no matter what this flag is set to. Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used.
- -fobjc-std=objc1
- Conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards, which is controlled by the separate C/C++ dialect option flags. When this option is used with the Objective-C or Objective-C++ compiler, any Objective-C syntax that is not recognized by GCC 4.0 is rejected. This is useful if you need to make sure that your Objective-C code can be compiled with older versions of GCC.
- -freplace-objc-classes
- Emit a special marker instructing ld(1) not to statically link in the resulting object file, and allow dyld(1) to load it in at run time instead. This is used in conjunction with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded in the course of program execution, without the need to restart the program itself. Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
- -fzero-link
- When compiling for the NeXT runtime, the compiler ordinarily replaces calls to "objc_getClass("...")" (when the name of the class is known at compile time) with static class references that get initialized at load time, which improves run-time performance. Specifying the -fzero-link flag suppresses this behavior and causes calls to "objc_getClass("...")" to be retained. This is useful in Zero-Link debugging mode, since it allows for individual class implementations to be modified during program execution. The GNU runtime currently always retains calls to "objc_get_class("...")" regardless of command-line options.
- -gen-decls
- Dump interface declarations for all classes seen in the source file to a file named sourcename.decl.
- -Wassign-intercept (Objective-C and Objective-C++ only)
- Warn whenever an Objective-C assignment is being intercepted by the garbage collector.
- -Wno-protocol (Objective-C and Objective-C++ only)
- If a class is declared to implement a protocol, a warning is issued for every method in the protocol that is not implemented by the class. The default behavior is to issue a warning for every method not explicitly implemented in the class, even if a method implementation is inherited from the superclass. If you use the -Wno-protocol option, then methods inherited from the superclass are considered to be implemented, and no warning is issued for them.
- -Wselector (Objective-C and Objective-C++ only)
- Warn if multiple methods of different types for the same selector are found during compilation. The check is performed on the list of methods in the final stage of compilation. Additionally, a check is performed for each selector appearing in a "@selector(...)" expression, and a corresponding method for that selector has been found during compilation. Because these checks scan the method table only at the end of compilation, these warnings are not produced if the final stage of compilation is not reached, for example because an error is found during compilation, or because the -fsyntax-only option is being used.
- -Wstrict-selector-match (Objective-C and Objective-C++ only)
- Warn if multiple methods with differing argument and/or return types are found for a given selector when attempting to send a message using this selector to a receiver of type "id" or "Class". When this flag is off (which is the default behavior), the compiler omits such warnings if any differences found are confined to types that share the same size and alignment.
- -Wundeclared-selector (Objective-C and Objective-C++ only)
- Warn if a "@selector(...)" expression referring to an undeclared selector is found. A selector is considered undeclared if no method with that name has been declared before the "@selector(...)" expression, either explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section. This option always performs its checks as soon as a "@selector(...)" expression is found, while -Wselector only performs its checks in the final stage of compilation. This also enforces the coding style convention that methods and selectors must be declared before being used.
- -print-objc-runtime-info
- Generate C header describing the largest structure that is passed by value, if any.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...). You can use the options described below to control the formatting algorithm for diagnostic messages, e.g. how many characters per line, how often source location information should be reported. Note that some language front ends may not honor these options.- -fmessage-length=n
- Try to format error messages so that they fit on lines of about n characters. The default is 72 characters for g++ and 0 for the rest of the front ends supported by GCC. If n is zero, then no line-wrapping is done; each error message appears on a single line.
- -fdiagnostics-show-location=once
- Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit source location information once; that is, in case the message is too long to fit on a single physical line and has to be wrapped, the source location won't be emitted (as prefix) again, over and over, in subsequent continuation lines. This is the default behavior.
- -fdiagnostics-show-location=every-line
- Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the same source location information (as prefix) for physical lines that result from the process of breaking a message which is too long to fit on a single line.
- -fno-diagnostics-show-option
- By default, each diagnostic emitted includes text indicating the command-line option that directly controls the diagnostic (if such an option is known to the diagnostic machinery). Specifying the -fno-diagnostics-show-option flag suppresses that behavior.
- -fno-diagnostics-show-caret
- By default, each diagnostic emitted includes the original source line and a caret '^' indicating the column. This option suppresses this information.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error.The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by GCC.
- -fsyntax-only
- Check the code for syntax errors, but don't do anything beyond that.
- -fmax-errors=n
- Limits the maximum number of error messages to n, at which point GCC bails out rather than attempting to continue processing the source code. If n is 0 (the default), there is no limit on the number of error messages produced. If -Wfatal-errors is also specified, then -Wfatal-errors takes precedence over this option.
- -w
- Inhibit all warning messages.
- -Werror
- Make all warnings into errors.
- -Werror=
-
Make the specified warning into an error. The specifier for a warning is appended; for example -Werror=switch turns the warnings controlled by -Wswitch into errors. This switch takes a negative form, to be used to negate -Werror for specific warnings; for example -Wno-error=switch makes -Wswitch warnings not be errors, even when -Werror is in effect.
- -Wfatal-errors
- This option causes the compiler to abort compilation on the first error occurred rather than trying to keep going and printing further error messages.
You can request many specific warnings with options beginning with -W, for example -Wimplicit to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning -Wno- to turn off warnings; for example, -Wno-implicit. This manual lists only one of the two forms, whichever is not the default. For further language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.
When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC emits a diagnostic stating that the option is not recognized. However, if the -Wno- form is used, the behavior is slightly different: no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being produced. This allows the use of new -Wno- options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present.
- -Wpedantic
- -pedantic
-
Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++. For ISO C, follows the version of the ISO C standard specified by any -std option used.
- -pedantic-errors
- Like -Wpedantic, except that errors are produced rather than warnings.
- -Wall
-
This enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with macros. This also enables some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.
- -Wextra
-
This enables some extra warning flags that are not enabled by -Wall. (This option used to be called -W. The older name is still supported, but the newer name is more descriptive.)
- •
- A pointer is compared against integer zero with <, <=, >, or >=.
- •
- (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.
- •
- (C++ only) Ambiguous virtual bases.
- •
- (C++ only) Subscripting an array that has been declared register.
- •
- (C++ only) Taking the address of a variable that has been declared register.
- •
- (C++ only) A base class is not initialized in a derived class's copy constructor.
- -Wchar-subscripts
- Warn if an array subscript has type "char". This is a common cause of error, as programmers often forget that this type is signed on some machines. This warning is enabled by -Wall.
- -Wcomment
- Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a Backslash-Newline appears in a // comment. This warning is enabled by -Wall.
- -Wno-coverage-mismatch
- Warn if feedback profiles do not match when using the -fprofile-use option. If a source file is changed between compiling with -fprofile-gen and with -fprofile-use, the files with the profile feedback can fail to match the source file and GCC cannot use the profile feedback information. By default, this warning is enabled and is treated as an error. -Wno-coverage-mismatch can be used to disable the warning or -Wno-error=coverage-mismatch can be used to disable the error. Disabling the error for this warning can result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to an existing code-base. Completely disabling the warning is not recommended.
- -Wno-cpp
-
(C, Objective-C, C++, Objective-C++ and Fortran only)
- -Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)
-
Give a warning when a value of type "float" is implicitly promoted to "double". CPUs with a 32-bit "single-precision" floating-point unit implement "float" in hardware, but emulate "double" in software. On such a machine, doing computations using "double" values is much more expensive because of the overhead required for software emulation.
float area(float radius)
{
return 3.14159 * radius * radius;
}
- -Wformat
- -Wformat=n
-
Check calls to "printf" and "scanf", etc., to make sure that the arguments supplied have types appropriate to the format string specified, and that the conversions specified in the format string make sense. This includes standard functions, and others specified by format attributes, in the "printf", "scanf", "strftime" and "strfmon" (an X/Open extension, not in the C standard) families (or other target-specific families). Which functions are checked without format attributes having been specified depends on the standard version selected, and such checks of functions without the attribute specified are disabled by -ffreestanding or -fno-builtin.
- -Wformat=1
- -Wformat
- Option -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0. Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull. Some aspects of this level of format checking can be disabled by the options: -Wno-format-contains-nul, -Wno-format-extra-args, and -Wno-format-zero-length. -Wformat is enabled by -Wall.
- -Wno-format-contains-nul
- If -Wformat is specified, do not warn about format strings that contain NUL bytes.
- -Wno-format-extra-args
-
If -Wformat is specified, do not warn about excess arguments to a "printf" or "scanf" format function. The C standard specifies that such arguments are ignored.
- -Wno-format-zero-length
- If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed.
- -Wformat=2
- Enable -Wformat plus additional format checks. Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k.
- -Wformat-nonliteral
- If -Wformat is specified, also warn if the format string is not a string literal and so cannot be checked, unless the format function takes its format arguments as a "va_list".
- -Wformat-security
- If -Wformat is specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls to "printf" and "scanf" functions where the format string is not a string literal and there are no format arguments, as in "printf (foo);". This may be a security hole if the format string came from untrusted input and contains %n. (This is currently a subset of what -Wformat-nonliteral warns about, but in future warnings may be added to -Wformat-security that are not included in -Wformat-nonliteral.)
- -Wformat-y2k
- If -Wformat is specified, also warn about "strftime" formats that may yield only a two-digit year.
- -Wnonnull
-
Warn about passing a null pointer for arguments marked as requiring a non-null value by the "nonnull" function attribute.
- -Winit-self (C, C++, Objective-C and Objective-C++ only)
-
Warn about uninitialized variables that are initialized with themselves. Note this option can only be used with the -Wuninitialized option.
int f()
{
int i = i;
return i;
}
- -Wimplicit-int (C and Objective-C only)
- Warn when a declaration does not specify a type. This warning is enabled by -Wall.
- -Wimplicit-function-declaration (C and Objective-C only)
- Give a warning whenever a function is used before being declared. In C99 mode ( -std=c99 or -std=gnu99), this warning is enabled by default and it is made into an error by -pedantic-errors. This warning is also enabled by -Wall.
- -Wimplicit (C and Objective-C only)
- Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall.
- -Wignored-qualifiers (C and C++ only)
-
Warn if the return type of a function has a type qualifier such as "const". For ISO C such a type qualifier has no effect, since the value returned by a function is not an lvalue. For C++, the warning is only emitted for scalar types or "void". ISO C prohibits qualified "void" return types on function definitions, so such return types always receive a warning even without this option.
- -Wmain
- Warn if the type of main is suspicious. main should be a function with external linkage, returning int, taking either zero arguments, two, or three arguments of appropriate types. This warning is enabled by default in C++ and is enabled by either -Wall or -Wpedantic.
- -Wmissing-braces
-
Warn if an aggregate or union initializer is not fully bracketed. In the following example, the initializer for a is not fully bracketed, but that for b is fully bracketed. This warning is enabled by -Wall in C.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
- -Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
- Warn if a user-supplied include directory does not exist.
- -Wparentheses
-
Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about.
{
if (a)
if (b)
foo ();
else
bar ();
}
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
- -Wsequence-point
-
Warn about code that may have undefined semantics because of violations of sequence point rules in the C and C++ standards.
- -Wno-return-local-addr
- Do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the function returns.
- -Wreturn-type
-
Warn whenever a function is defined with a return type that defaults to "int". Also warn about any "return" statement with no return value in a function whose return type is not "void" (falling off the end of the function body is considered returning without a value), and about a "return" statement with an expression in a function whose return type is "void".
- -Wswitch
- Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of the named codes of that enumeration. (The presence of a "default" label prevents this warning.) "case" labels outside the enumeration range also provoke warnings when this option is used (even if there is a "default" label). This warning is enabled by -Wall.
- -Wswitch-default
- Warn whenever a "switch" statement does not have a "default" case.
- -Wswitch-enum
- Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of the named codes of that enumeration. "case" labels outside the enumeration range also provoke warnings when this option is used. The only difference between -Wswitch and this option is that this option gives a warning about an omitted enumeration code even if there is a "default" label.
- -Wsync-nand (C and C++ only)
- Warn when "__sync_fetch_and_nand" and "__sync_nand_and_fetch" built-in functions are used. These functions changed semantics in GCC 4.4.
- -Wtrigraphs
- Warn if any trigraphs are encountered that might change the meaning of the program (trigraphs within comments are not warned about). This warning is enabled by -Wall.
- -Wunused-but-set-parameter
-
Warn whenever a function parameter is assigned to, but otherwise unused (aside from its declaration).
- -Wunused-but-set-variable
-
Warn whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This warning is enabled by -Wall.
- -Wunused-function
- Warn whenever a static function is declared but not defined or a non-inline static function is unused. This warning is enabled by -Wall.
- -Wunused-label
-
Warn whenever a label is declared but not used. This warning is enabled by -Wall.
- -Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only)
- Warn when a typedef locally defined in a function is not used. This warning is enabled by -Wall.
- -Wunused-parameter
-
Warn whenever a function parameter is unused aside from its declaration.
- -Wno-unused-result
- Do not warn if a caller of a function marked with attribute "warn_unused_result" does not use its return value. The default is -Wunused-result.
- -Wunused-variable
-
Warn whenever a local variable or non-constant static variable is unused aside from its declaration. This warning is enabled by -Wall.
- -Wunused-value
-
Warn whenever a statement computes a result that is explicitly not used. To suppress this warning cast the unused expression to void. This includes an expression-statement or the left-hand side of a comma expression that contains no side effects. For example, an expression such as x[i,j] causes a warning, while x[(void)i,j] does not.
- -Wunused
-
All the above -Wunused options combined.
- -Wuninitialized
-
Warn if an automatic variable is used without first being initialized or if a variable may be clobbered by a "setjmp" call. In C++, warn if a non-static reference or non-static const member appears in a class without constructors.
- -Wmaybe-uninitialized
-
For an automatic variable, if there exists a path from the function entry to a use of the variable that is initialized, but there exist some other paths for which the variable is not initialized, the compiler emits a warning if it cannot prove the uninitialized paths are not executed at run time. These warnings are made optional because GCC is not smart enough to see all the reasons why the code might be correct in spite of appearing to have an error. Here is one example of how this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
- -Wunknown-pragmas
- Warn when a "#pragma" directive is encountered that is not understood by GCC. If this command-line option is used, warnings are even issued for unknown pragmas in system header files. This is not the case if the warnings are only enabled by the -Wall command-line option.
- -Wno-pragmas
- Do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas. See also -Wunknown-pragmas.
- -Wstrict-aliasing
- This option is only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. The warning does not catch all cases, but does attempt to catch the more common pitfalls. It is included in -Wall. It is equivalent to -Wstrict-aliasing=3
- -Wstrict-aliasing=n
-
This option is only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. Higher levels correspond to higher accuracy (fewer false positives). Higher levels also correspond to more effort, similar to the way -O works. -Wstrict-aliasing is equivalent to -Wstrict-aliasing=3.
- -Wstrict-overflow
- -Wstrict-overflow=n
-
This option is only active when -fstrict-overflow is active. It warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur. Note that it does not warn about all cases where the code might overflow: it only warns about cases where the compiler implements some optimization. Thus this warning depends on the optimization level.
- -Wstrict-overflow=1
- Warn about cases that are both questionable and easy to avoid. For example, with -fstrict-overflow, the compiler simplifies "x + 1 > x" to 1. This level of -Wstrict-overflow is enabled by -Wall; higher levels are not, and must be explicitly requested.
- -Wstrict-overflow=2
- Also warn about other cases where a comparison is simplified to a constant. For example: "abs (x) >= 0". This can only be simplified when -fstrict-overflow is in effect, because "abs (INT_MIN)" overflows to "INT_MIN", which is less than zero. -Wstrict-overflow (with no level) is the same as -Wstrict-overflow=2.
- -Wstrict-overflow=3
- Also warn about other cases where a comparison is simplified. For example: "x + 1 > 1" is simplified to "x > 0".
- -Wstrict-overflow=4
- Also warn about other simplifications not covered by the above cases. For example: "(x * 10) / 5" is simplified to "x * 2".
- -Wstrict-overflow=5
- Also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example: "x + 2 > y" is simplified to "x + 1 >= y". This is reported only at the highest warning level because this simplification applies to many comparisons, so this warning level gives a very large number of false positives.
- -Wsuggest-attribute=[pure|const|noreturn|format]
- Warn for cases where adding an attribute may be beneficial. The attributes currently supported are listed below.
- -Wsuggest-attribute=pure
- -Wsuggest-attribute=const
- -Wsuggest-attribute=noreturn
- Warn about functions that might be candidates for attributes "pure", "const" or "noreturn". The compiler only warns for functions visible in other compilation units or (in the case of "pure" and "const") if it cannot prove that the function returns normally. A function returns normally if it doesn't contain an infinite loop or return abnormally by throwing, calling "abort()" or trapping. This analysis requires option -fipa-pure-const, which is enabled by default at -O and higher. Higher optimization levels improve the accuracy of the analysis.
- -Wsuggest-attribute=format
- -Wmissing-format-attribute
-
Warn about function pointers that might be candidates for "format" attributes. Note these are only possible candidates, not absolute ones. GCC guesses that function pointers with "format" attributes that are used in assignment, initialization, parameter passing or return statements should have a corresponding "format" attribute in the resulting type. I.e. the left-hand side of the assignment or initialization, the type of the parameter variable, or the return type of the containing function respectively should also have a "format" attribute to avoid the warning.
- -Warray-bounds
- This option is only active when -ftree-vrp is active (default for -O2 and above). It warns about subscripts to arrays that are always out of bounds. This warning is enabled by -Wall.
- -Wno-div-by-zero
- Do not warn about compile-time integer division by zero. Floating-point division by zero is not warned about, as it can be a legitimate way of obtaining infinities and NaNs.
- -Wsystem-headers
- Print warning messages for constructs found in system header files. Warnings from system headers are normally suppressed, on the assumption that they usually do not indicate real problems and would only make the compiler output harder to read. Using this command-line option tells GCC to emit warnings from system headers as if they occurred in user code. However, note that using -Wall in conjunction with this option does not warn about unknown pragmas in system headers---for that, -Wunknown-pragmas must also be used.
- -Wtrampolines
-
Warn about trampolines generated for pointers to nested functions.
A trampoline is a small piece of data or code that is created at run
time on the stack when the address of a nested function is taken, and
is used to call the nested function indirectly. For some targets, it
is made up of data only and thus requires no special treatment. But,
for most targets, it is made up of code and thus requires the stack
to be made executable in order for the program to work properly.
- -Wfloat-equal
-
Warn if floating-point values are used in equality comparisons.
- -Wtraditional (C and Objective-C only)
- Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided.
- •
- Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes place within string literals, but in ISO C it does not.
- •
- In traditional C, some preprocessor directives did not exist. Traditional preprocessors only considered a line to be a directive if the # appeared in column 1 on the line. Therefore -Wtraditional warns about directives that traditional C understands but ignores because the # does not appear as the first character on the line. It also suggests you hide directives like #pragma not understood by traditional C by indenting them. Some traditional implementations do not recognize #elif, so this option suggests avoiding it altogether.
- •
- A function-like macro that appears without arguments.
- •
- The unary plus operator.
- •
- The U integer constant suffix, or the F or L floating-point constant suffixes. (Traditional C does support the L suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in "<limits.h>". Use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases.
- •
- A function declared external in one block and then used after the end of the block.
- •
- A "switch" statement has an operand of type "long".
- •
- A non-"static" function declaration follows a "static" one. This construct is not accepted by some traditional C compilers.
- •
- The ISO type of an integer constant has a different width or signedness from its traditional type. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.
- •
- Usage of ISO string concatenation is detected.
- •
- Initialization of automatic aggregates.
- •
- Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
- •
- Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the assumption that the zero initializer in user code appears conditioned on e.g. "__STDC__" to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case.
- •
- Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these prototypes when compiling with traditional C causes serious problems. This is a subset of the possible conversion warnings; for the full set use -Wtraditional-conversion.
- •
- Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features appear in your code when using libiberty's traditional C compatibility macros, "PARAMS" and "VPARAMS". This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility.
- -Wtraditional-conversion (C and Objective-C only)
- Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed-point argument except when the same as the default promotion.
- -Wdeclaration-after-statement (C and Objective-C only)
- Warn when a declaration is found after a statement in a block. This construct, known from C++, was introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90 and was not supported by GCC versions before GCC 3.0.
- -Wundef
- Warn if an undefined identifier is evaluated in an #if directive.
- -Wno-endif-labels
- Do not warn whenever an #else or an #endif are followed by text.
- -Wshadow
- Warn whenever a local variable or type declaration shadows another variable, parameter, type, or class member (in C++), or whenever a built-in function is shadowed. Note that in C++, the compiler warns if a local variable shadows an explicit typedef, but not if it shadows a struct/class/enum.
- -Wlarger-than=len
- Warn whenever an object of larger than len bytes is defined.
- -Wframe-larger-than=len
- Warn if the size of a function frame is larger than len bytes. The computation done to determine the stack frame size is approximate and not conservative. The actual requirements may be somewhat greater than len even if you do not get a warning. In addition, any space allocated via "alloca", variable-length arrays, or related constructs is not included by the compiler when determining whether or not to issue a warning.
- -Wno-free-nonheap-object
- Do not warn when attempting to free an object that was not allocated on the heap.
- -Wstack-usage=len
-
Warn if the stack usage of a function might be larger than len bytes. The computation done to determine the stack usage is conservative. Any space allocated via "alloca", variable-length arrays, or related constructs is included by the compiler when determining whether or not to issue a warning.
- •
-
If the stack usage is fully static but exceeds the specified amount, it's:
warning: stack usage is 1120 bytes
- •
-
If the stack usage is (partly) dynamic but bounded, it's:
warning: stack usage might be 1648 bytes
- •
-
If the stack usage is (partly) dynamic and not bounded, it's:
warning: stack usage might be unbounded
- -Wunsafe-loop-optimizations
- Warn if the loop cannot be optimized because the compiler cannot assume anything on the bounds of the loop indices. With -funsafe-loop-optimizations warn if the compiler makes such assumptions.
- -Wno-pedantic-ms-format (MinGW targets only)
- When used in combination with -Wformat and -pedantic without GNU extensions, this option disables the warnings about non-ISO "printf" / "scanf" format width specifiers "I32", "I64", and "I" used on Windows targets, which depend on the MS runtime.
- -Wpointer-arith
- Warn about anything that depends on the "size of" a function type or of "void". GNU C assigns these types a size of 1, for convenience in calculations with "void *" pointers and pointers to functions. In C++, warn also when an arithmetic operation involves "NULL". This warning is also enabled by -Wpedantic.
- -Wtype-limits
- Warn if a comparison is always true or always false due to the limited range of the data type, but do not warn for constant expressions. For example, warn if an unsigned variable is compared against zero with < or >=. This warning is also enabled by -Wextra.
- -Wbad-function-cast (C and Objective-C only)
- Warn whenever a function call is cast to a non-matching type. For example, warn if "int malloc()" is cast to "anything *".
- -Wc++-compat (C and Objective-C only)
- Warn about ISO C constructs that are outside of the common subset of ISO C and ISO C++, e.g. request for implicit conversion from "void *" to a pointer to non-"void" type.
- -Wc++11-compat (C++ and Objective-C++ only)
- Warn about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is enabled by -Wall.
- -Wcast-qual
-
Warn whenever a pointer is cast so as to remove a type qualifier from the target type. For example, warn if a "const char *" is cast to an ordinary "char *".
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';
- -Wcast-align
- Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a "char *" is cast to an "int *" on machines where integers can only be accessed at two- or four-byte boundaries.
- -Wwrite-strings
-
When compiling C, give string constants the type "const char[ length]" so that copying the address of one into a non-"const" "char *" pointer produces a warning. These warnings help you find at compile time code that can try to write into a string constant, but only if you have been very careful about using "const" in declarations and prototypes. Otherwise, it is just a nuisance. This is why we did not make -Wall request these warnings.
- -Wclobbered
- Warn for variables that might be changed by longjmp or vfork. This warning is also enabled by -Wextra.
- -Wconversion
-
Warn for implicit conversions that may alter a value. This includes conversions between real and integer, like "abs (x)" when "x" is "double"; conversions between signed and unsigned, like "unsigned ui = -1"; and conversions to smaller types, like "sqrtf (M_PI)". Do not warn for explicit casts like "abs ((int) x)" and "ui = (unsigned) -1", or if the value is not changed by the conversion like in "abs (2.0)". Warnings about conversions between signed and unsigned integers can be disabled by using -Wno-sign-conversion.
- -Wno-conversion-null (C++ and Objective-C++ only)
- Do not warn for conversions between "NULL" and non-pointer types. -Wconversion-null is enabled by default.
- -Wzero-as-null-pointer-constant (C++ and Objective-C++ only)
- Warn when a literal '0' is used as null pointer constant. This can be useful to facilitate the conversion to "nullptr" in C++11.
- -Wuseless-cast (C++ and Objective-C++ only)
- Warn when an expression is casted to its own type.
- -Wempty-body
- Warn if an empty body occurs in an if, else or do while statement. This warning is also enabled by -Wextra.
- -Wenum-compare
- Warn about a comparison between values of different enumerated types. In C++ enumeral mismatches in conditional expressions are also diagnosed and the warning is enabled by default. In C this warning is enabled by -Wall.
- -Wjump-misses-init (C, Objective-C only)
-
Warn if a "goto" statement or a "switch" statement jumps forward across the initialization of a variable, or jumps backward to a label after the variable has been initialized. This only warns about variables that are initialized when they are declared. This warning is only supported for C and Objective-C; in C++ this sort of branch is an error in any case.
- -Wsign-compare
- Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. This warning is also enabled by -Wextra; to get the other warnings of -Wextra without this warning, use -Wextra -Wno-sign-compare.
- -Wsign-conversion
- Warn for implicit conversions that may change the sign of an integer value, like assigning a signed integer expression to an unsigned integer variable. An explicit cast silences the warning. In C, this option is enabled also by -Wconversion.
- -Wsizeof-pointer-memaccess
- Warn for suspicious length parameters to certain string and memory built-in functions if the argument uses "sizeof". This warning warns e.g. about "memset (ptr, 0, sizeof (ptr));" if "ptr" is not an array, but a pointer, and suggests a possible fix, or about "memcpy (&foo, ptr, sizeof (&foo));". This warning is enabled by -Wall.
- -Waddress
- Warn about suspicious uses of memory addresses. These include using the address of a function in a conditional expression, such as "void func(void); if (func)", and comparisons against the memory address of a string literal, such as "if (x == "abc")". Such uses typically indicate a programmer error: the address of a function always evaluates to true, so their use in a conditional usually indicate that the programmer forgot the parentheses in a function call; and comparisons against string literals result in unspecified behavior and are not portable in C, so they usually indicate that the programmer intended to use "strcmp". This warning is enabled by -Wall.
- -Wlogical-op
- Warn about suspicious uses of logical operators in expressions. This includes using logical operators in contexts where a bit-wise operator is likely to be expected.
- -Waggregate-return
- Warn if any functions that return structures or unions are defined or called. (In languages where you can return an array, this also elicits a warning.)
- -Wno-aggressive-loop-optimizations
- Warn if in a loop with constant number of iterations the compiler detects undefined behavior in some statement during one or more of the iterations.
- -Wno-attributes
- Do not warn if an unexpected "__attribute__" is used, such as unrecognized attributes, function attributes applied to variables, etc. This does not stop errors for incorrect use of supported attributes.
- -Wno-builtin-macro-redefined
- Do not warn if certain built-in macros are redefined. This suppresses warnings for redefinition of "__TIMESTAMP__", "__TIME__", "__DATE__", "__FILE__", and "__BASE_FILE__".
- -Wstrict-prototypes (C and Objective-C only)
- Warn if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if preceded by a declaration that specifies the argument types.)
- -Wold-style-declaration (C and Objective-C only)
- Warn for obsolescent usages, according to the C Standard, in a declaration. For example, warn if storage-class specifiers like "static" are not the first things in a declaration. This warning is also enabled by -Wextra.
- -Wold-style-definition (C and Objective-C only)
- Warn if an old-style function definition is used. A warning is given even if there is a previous prototype.
- -Wmissing-parameter-type (C and Objective-C only)
-
A function parameter is declared without a type specifier in K&R-style functions:
void foo(bar) { }
- -Wmissing-prototypes (C and Objective-C only)
- Warn if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. Use this option to detect global functions that do not have a matching prototype declaration in a header file. This option is not valid for C++ because all function declarations provide prototypes and a non-matching declaration will declare an overload rather than conflict with an earlier declaration. Use -Wmissing-declarations to detect missing declarations in C++.
- -Wmissing-declarations
- Warn if a global function is defined without a previous declaration. Do so even if the definition itself provides a prototype. Use this option to detect global functions that are not declared in header files. In C, no warnings are issued for functions with previous non-prototype declarations; use -Wmissing-prototype to detect missing prototypes. In C++, no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces.
- -Wmissing-field-initializers
-
Warn if a structure's initializer has some fields missing. For example, the following code causes such a warning, because "x.h" is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
- -Wno-multichar
- Do not warn if a multicharacter constant ('FOOF') is used. Usually they indicate a typo in the user's code, as they have implementation-defined values, and should not be used in portable code.
- -Wnormalized=<none|id|nfc|nfkc>
-
In ISO C and ISO C++, two identifiers are different if they are different sequences of characters. However, sometimes when characters outside the basic ASCII character set are used, you can have two different character sequences that look the same. To avoid confusion, the ISO 10646 standard sets out some normalization rules which when applied ensure that two sequences that look the same are turned into the same sequence. GCC can warn you if you are using identifiers that have not been normalized; this option controls that warning.
- -Wno-deprecated
- Do not warn about usage of deprecated features.
- -Wno-deprecated-declarations
- Do not warn about uses of functions, variables, and types marked as deprecated by using the "deprecated" attribute.
- -Wno-overflow
- Do not warn about compile-time overflow in constant expressions.
- -Woverride-init (C and Objective-C only)
-
Warn if an initialized field without side effects is overridden when using designated initializers.
- -Wpacked
-
Warn if a structure is given the packed attribute, but the packed attribute has no effect on the layout or size of the structure. Such structures may be mis-aligned for little benefit. For instance, in this code, the variable "f.x" in "struct bar" is misaligned even though "struct bar" does not itself have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
- -Wpacked-bitfield-compat
-
The 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute on bit-fields of type "char". This has been fixed in GCC 4.4 but the change can lead to differences in the structure layout. GCC informs you when the offset of such a field has changed in GCC 4.4. For example there is no longer a 4-bit padding between field "a" and "b" in this structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));
- -Wpadded
- Warn if padding is included in a structure, either to align an element of the structure or to align the whole structure. Sometimes when this happens it is possible to rearrange the fields of the structure to reduce the padding and so make the structure smaller.
- -Wredundant-decls
- Warn if anything is declared more than once in the same scope, even in cases where multiple declaration is valid and changes nothing.
- -Wnested-externs (C and Objective-C only)
- Warn if an "extern" declaration is encountered within a function.
- -Wno-inherited-variadic-ctor
- Suppress warnings about use of C++11 inheriting constructors when the base class inherited from has a C variadic constructor; the warning is on by default because the ellipsis is not inherited.
- -Winline
-
Warn if a function that is declared as inline cannot be inlined. Even with this option, the compiler does not warn about failures to inline functions declared in system headers.
- -Wno-invalid-offsetof (C++ and Objective-C++ only)
-
Suppress warnings from applying the offsetof macro to a non-POD type. According to the 1998 ISO C++ standard, applying offsetof to a non-POD type is undefined. In existing C++ implementations, however, offsetof typically gives meaningful results even when applied to certain kinds of non-POD types (such as a simple struct that fails to be a POD type only by virtue of having a constructor). This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it.
- -Wno-int-to-pointer-cast
- Suppress warnings from casts to pointer type of an integer of a different size. In C++, casting to a pointer type of smaller size is an error. Wint-to-pointer-cast is enabled by default.
- -Wno-pointer-to-int-cast (C and Objective-C only)
- Suppress warnings from casts from a pointer to an integer type of a different size.
- -Winvalid-pch
- Warn if a precompiled header is found in the search path but can't be used.
- -Wlong-long
- Warn if long long type is used. This is enabled by either -Wpedantic or -Wtraditional in ISO C90 and C++98 modes. To inhibit the warning messages, use -Wno-long-long.
- -Wvariadic-macros
- Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU alternate syntax when in pedantic ISO C99 mode. This is default. To inhibit the warning messages, use -Wno-variadic-macros.
- -Wvarargs
- Warn upon questionable usage of the macros used to handle variable arguments like va_start. This is default. To inhibit the warning messages, use -Wno-varargs.
- -Wvector-operation-performance
- Warn if vector operation is not implemented via SIMD capabilities of the architecture. Mainly useful for the performance tuning. Vector operation can be implemented "piecewise", which means that the scalar operation is performed on every vector element; "in parallel", which means that the vector operation is implemented using scalars of wider type, which normally is more performance efficient; and "as a single scalar", which means that vector fits into a scalar type.
- -Wno-virtual-move-assign
- Suppress warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. This is dangerous because if the virtual base is reachable along more than one path, it will be moved multiple times, which can mean both objects end up in the moved-from state. If the move assignment operator is written to avoid moving from a moved-from object, this warning can be disabled.
- -Wvla
- Warn if variable length array is used in the code. -Wno-vla prevents the -Wpedantic warning of the variable length array.
- -Wvolatile-register-var
- Warn if a register variable is declared volatile. The volatile modifier does not inhibit all optimizations that may eliminate reads and/or writes to register variables. This warning is enabled by -Wall.
- -Wdisabled-optimization
- Warn if a requested optimization pass is disabled. This warning does not generally indicate that there is anything wrong with your code; it merely indicates that GCC's optimizers are unable to handle the code effectively. Often, the problem is that your code is too big or too complex; GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time.
- -Wpointer-sign (C and Objective-C only)
- Warn for pointer argument passing or assignment with different signedness. This option is only supported for C and Objective-C. It is implied by -Wall and by -Wpedantic, which can be disabled with -Wno-pointer-sign.
- -Wstack-protector
- This option is only active when -fstack-protector is active. It warns about functions that are not protected against stack smashing.
- -Wno-mudflap
- Suppress warnings about constructs that cannot be instrumented by -fmudflap.
- -Woverlength-strings
-
Warn about string constants that are longer than the "minimum maximum" length specified in the C standard. Modern compilers generally allow string constants that are much longer than the standard's minimum limit, but very portable programs should avoid using longer strings.
- -Wunsuffixed-float-constants (C and Objective-C only)
- Issue a warning for any floating constant that does not have a suffix. When used together with -Wsystem-headers it warns about such constants in system header files. This can be useful when preparing code to use with the "FLOAT_CONST_DECIMAL64" pragma from the decimal floating-point extension to C99.
Options for Debugging Your Program or GCC
GCC has various special options that are used for debugging either your program or GCC:- -g
-
Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF 2). GDB can work with this debugging information.
- -gsplit-dwarf
- Separate as much dwarf debugging information as possible into a separate output file with the extension .dwo. This option allows the build system to avoid linking files with debug information. To be useful, this option requires a debugger capable of reading .dwo files.
- -ggdb
- Produce debugging information for use by GDB. This means to use the most expressive format available (DWARF 2, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible.
- -gpubnames
- Generate dwarf .debug_pubnames and .debug_pubtypes sections.
- -gstabs
- Produce debugging information in stabs format (if that is supported), without GDB extensions. This is the format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output that is not understood by DBX or SDB. On System V Release 4 systems this option requires the GNU assembler.
- -feliminate-unused-debug-symbols
- Produce debugging information in stabs format (if that is supported), for only symbols that are actually used.
- -femit-class-debug-always
- Instead of emitting debugging information for a C++ class in only one object file, emit it in all object files using the class. This option should be used only with debuggers that are unable to handle the way GCC normally emits debugging information for classes because using this option increases the size of debugging information by as much as a factor of two.
- -fdebug-types-section
- When using DWARF Version 4 or higher, type DIEs can be put into their own ".debug_types" section instead of making them part of the ".debug_info" section. It is more efficient to put them in a separate comdat sections since the linker can then remove duplicates. But not all DWARF consumers support ".debug_types" sections yet and on some objects ".debug_types" produces larger instead of smaller debugging information.
- -gstabs+
- Produce debugging information in stabs format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program.
- -gcoff
- Produce debugging information in COFF format (if that is supported). This is the format used by SDB on most System V systems prior to System V Release 4.
- -gxcoff
- Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems.
- -gxcoff+
- Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail with an error.
- -gdwarf-version
-
Produce debugging information in DWARF format (if that is supported). The value of version may be either 2, 3 or 4; the default version for most targets is 4.
- -grecord-gcc-switches
- This switch causes the command-line options used to invoke the compiler that may affect code generation to be appended to the DW_AT_producer attribute in DWARF debugging information. The options are concatenated with spaces separating them from each other and from the compiler version. See also -frecord-gcc-switches for another way of storing compiler options into the object file. This is the default.
- -gno-record-gcc-switches
- Disallow appending command-line options to the DW_AT_producer attribute in DWARF debugging information.
- -gstrict-dwarf
- Disallow using extensions of later DWARF standard version than selected with -gdwarf-version. On most targets using non-conflicting DWARF extensions from later standard versions is allowed.
- -gno-strict-dwarf
- Allow using extensions of later DWARF standard version than selected with -gdwarf-version.
- -gvms
- Produce debugging information in Alpha/VMS debug format (if that is supported). This is the format used by DEBUG on Alpha/VMS systems.
- -glevel
- -ggdblevel
- -gstabslevel
- -gcofflevel
- -gxcofflevel
- -gvmslevel
-
Request debugging information and also use level to specify how much information. The default level is 2.
- -gtoggle
- Turn off generation of debug info, if leaving out this option generates it, or turn it on at level 2 otherwise. The position of this argument in the command line does not matter; it takes effect after all other options are processed, and it does so only once, no matter how many times it is given. This is mainly intended to be used with -fcompare-debug.
- -fsanitize=address
- Enable AddressSanitizer, a fast memory error detector. Memory access instructions will be instrumented to detect out-of-bounds and use-after-free bugs. See < http://code.google.com/p/address-sanitizer/> for more details.
- -fsanitize=thread
- Enable ThreadSanitizer, a fast data race detector. Memory access instructions will be instrumented to detect data race bugs. See < http://code.google.com/p/data-race-test/wiki/ThreadSanitizer> for more details.
- -fdump-final-insns[=file]
- Dump the final internal representation (RTL) to file. If the optional argument is omitted (or if file is "."), the name of the dump file is determined by appending ".gkd" to the compilation output file name.
- -fcompare-debug[=opts]
-
If no error occurs during compilation, run the compiler a second time, adding opts and -fcompare-debug-second to the arguments passed to the second compilation. Dump the final internal representation in both compilations, and print an error if they differ.
- -fcompare-debug-second
-
This option is implicitly passed to the compiler for the second compilation requested by -fcompare-debug, along with options to silence warnings, and omitting other options that would cause side-effect compiler outputs to files or to the standard output. Dump files and preserved temporary files are renamed so as to contain the ".gk" additional extension during the second compilation, to avoid overwriting those generated by the first.
- -feliminate-dwarf2-dups
- Compress DWARF 2 debugging information by eliminating duplicated information about each symbol. This option only makes sense when generating DWARF 2 debugging information with -gdwarf-2.
- -femit-struct-debug-baseonly
-
Emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the struct is defined.
- -femit-struct-debug-reduced
-
Emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the type is defined, unless the struct is a template or defined in a system header.
- -femit-struct-debug-detailed[=spec-list]
-
Specify the struct-like types for which the compiler generates debug information. The intent is to reduce duplicate struct debug information between different object files within the same program.
- -fno-merge-debug-strings
- Direct the linker to not merge together strings in the debugging information that are identical in different object files. Merging is not supported by all assemblers or linkers. Merging decreases the size of the debug information in the output file at the cost of increasing link processing time. Merging is enabled by default.
- -fdebug-prefix-map=old=new
- When compiling files in directory old, record debugging information describing them as in new instead.
- -fno-dwarf2-cfi-asm
- Emit DWARF 2 unwind info as compiler generated ".eh_frame" section instead of using GAS ".cfi_*" directives.
- -p
- Generate extra code to write profile information suitable for the analysis program prof. You must use this option when compiling the source files you want data about, and you must also use it when linking.
- -pg
- Generate extra code to write profile information suitable for the analysis program gprof. You must use this option when compiling the source files you want data about, and you must also use it when linking.
- -Q
- Makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes.
- -ftime-report
- Makes the compiler print some statistics about the time consumed by each pass when it finishes.
- -fmem-report
- Makes the compiler print some statistics about permanent memory allocation when it finishes.
- -fmem-report-wpa
- Makes the compiler print some statistics about permanent memory allocation for the WPA phase only.
- -fpre-ipa-mem-report
- -fpost-ipa-mem-report
- Makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization.
- -fprofile-report
- Makes the compiler print some statistics about consistency of the (estimated) profile and effect of individual passes.
- -fstack-usage
- Makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending .su to the auxname. auxname is generated from the name of the output file, if explicitly specified and it is not an executable, otherwise it is the basename of the source file. An entry is made up of three fields:
- •
- The name of the function.
- •
- A number of bytes.
- •
- One or more qualifiers: "static", "dynamic", "bounded".
- -fprofile-arcs
- Add code so that program flow arcs are instrumented. During execution the program records how many times each branch and call is executed and how many times it is taken or returns. When the compiled program exits it saves this data to a file called auxname.gcda for each source file. The data may be used for profile-directed optimizations ( -fbranch-probabilities), or for test coverage analysis ( -ftest-coverage). Each object file's auxname is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o).
- --coverage
- This option is used to compile and link code instrumented for coverage analysis. The option is a synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the documentation for those options for more details.
- •
- Compile the source files with -fprofile-arcs plus optimization and code generation options. For test coverage analysis, use the additional -ftest-coverage option. You do not need to profile every source file in a program.
- •
- Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
- •
- Run the program on a representative workload to generate the arc profile information. This may be repeated any number of times. You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated. Also "fork" calls are detected and correctly handled (double counting will not happen).
- •
- For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus -fbranch-probabilities.
- •
- For test coverage analysis, use gcov to produce human readable information from the .gcno and .gcda files. Refer to the gcov documentation for further information.
- -ftest-coverage
- Produce a notes file that the gcov code-coverage utility can use to show program coverage. Each source file's note file is called auxname.gcno. Refer to the -fprofile-arcs option above for a description of auxname and instructions on how to generate test coverage data. Coverage data matches the source files more closely if you do not optimize.
- -fdbg-cnt-list
- Print the name and the counter upper bound for all debug counters.
- -fdbg-cnt=counter-value-list
- Set the internal debug counter upper bound. counter-value-list is a comma-separated list of name:value pairs which sets the upper bound of each debug counter name to value. All debug counters have the initial upper bound of "UINT_MAX"; thus "dbg_cnt()" returns true always unless the upper bound is set by this option. For example, with -fdbg-cnt=dce:10,tail_call:0, "dbg_cnt(dce)" returns true only for first 10 invocations.
- -fenable-kind-pass
- -fdisable-kind-pass=range-list
- This is a set of options that are used to explicitly disable/enable optimization passes. These options are intended for use for debugging GCC. Compiler users should use regular options for enabling/disabling passes instead.
- -fdisable-ipa-pass
- Disable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
- -fdisable-rtl-pass
- -fdisable-rtl-pass=range-list
- Disable RTL pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1. range-list is a comma-separated list of function ranges or assembler names. Each range is a number pair separated by a colon. The range is inclusive in both ends. If the range is trivial, the number pair can be simplified as a single number. If the function's call graph node's uid falls within one of the specified ranges, the pass is disabled for that function. The uid is shown in the function header of a dump file, and the pass names can be dumped by using option -fdump-passes.
- -fdisable-tree-pass
- -fdisable-tree-pass=range-list
- Disable tree pass pass. See -fdisable-rtl for the description of option arguments.
- -fenable-ipa-pass
- Enable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
- -fenable-rtl-pass
- -fenable-rtl-pass=range-list
- Enable RTL pass pass. See -fdisable-rtl for option argument description and examples.
- -fenable-tree-pass
- -fenable-tree-pass=range-list
- Enable tree pass pass. See -fdisable-rtl for the description of option arguments.
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll
- -dletters
- -fdump-rtl-pass
- -fdump-rtl-pass=filename
-
Says to make debugging dumps during compilation at times specified by letters. This is used for debugging the RTL-based passes of the compiler. The file names for most of the dumps are made by appending a pass number and a word to the dumpname, and the files are created in the directory of the output file. In case of =filename option, the dump is output on the given file instead of the pass numbered dump files. Note that the pass number is computed statically as passes get registered into the pass manager. Thus the numbering is not related to the dynamic order of execution of passes. In particular, a pass installed by a plugin could have a number over 200 even if it executed quite early. dumpname is generated from the name of the output file, if explicitly specified and it is not an executable, otherwise it is the basename of the source file. These switches may have different effects when -E is used for preprocessing.
- -fdump-rtl-alignments
- Dump after branch alignments have been computed.
- -fdump-rtl-asmcons
- Dump after fixing rtl statements that have unsatisfied in/out constraints.
- -fdump-rtl-auto_inc_dec
- Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions.
- -fdump-rtl-barriers
- Dump after cleaning up the barrier instructions.
- -fdump-rtl-bbpart
- Dump after partitioning hot and cold basic blocks.
- -fdump-rtl-bbro
- Dump after block reordering.
- -fdump-rtl-btl1
- -fdump-rtl-btl2
- -fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two branch target load optimization passes.
- -fdump-rtl-bypass
- Dump after jump bypassing and control flow optimizations.
- -fdump-rtl-combine
- Dump after the RTL instruction combination pass.
- -fdump-rtl-compgotos
- Dump after duplicating the computed gotos.
- -fdump-rtl-ce1
- -fdump-rtl-ce2
- -fdump-rtl-ce3
- -fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if conversion passes.
- -fdump-rtl-cprop_hardreg
- Dump after hard register copy propagation.
- -fdump-rtl-csa
- Dump after combining stack adjustments.
- -fdump-rtl-cse1
- -fdump-rtl-cse2
- -fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common subexpression elimination passes.
- -fdump-rtl-dce
- Dump after the standalone dead code elimination passes.
- -fdump-rtl-dbr
- Dump after delayed branch scheduling.
- -fdump-rtl-dce1
- -fdump-rtl-dce2
- -fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store elimination passes.
- -fdump-rtl-eh
- Dump after finalization of EH handling code.
- -fdump-rtl-eh_ranges
- Dump after conversion of EH handling range regions.
- -fdump-rtl-expand
- Dump after RTL generation.
- -fdump-rtl-fwprop1
- -fdump-rtl-fwprop2
- -fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward propagation passes.
- -fdump-rtl-gcse1
- -fdump-rtl-gcse2
- -fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common subexpression elimination.
- -fdump-rtl-init-regs
- Dump after the initialization of the registers.
- -fdump-rtl-initvals
- Dump after the computation of the initial value sets.
- -fdump-rtl-into_cfglayout
- Dump after converting to cfglayout mode.
- -fdump-rtl-ira
- Dump after iterated register allocation.
- -fdump-rtl-jump
- Dump after the second jump optimization.
- -fdump-rtl-loop2
- -fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.
- -fdump-rtl-mach
- Dump after performing the machine dependent reorganization pass, if that pass exists.
- -fdump-rtl-mode_sw
- Dump after removing redundant mode switches.
- -fdump-rtl-rnreg
- Dump after register renumbering.
- -fdump-rtl-outof_cfglayout
- Dump after converting from cfglayout mode.
- -fdump-rtl-peephole2
- Dump after the peephole pass.
- -fdump-rtl-postreload
- Dump after post-reload optimizations.
- -fdump-rtl-pro_and_epilogue
- Dump after generating the function prologues and epilogues.
- -fdump-rtl-regmove
- Dump after the register move pass.
- -fdump-rtl-sched1
- -fdump-rtl-sched2
- -fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic block scheduling passes.
- -fdump-rtl-see
- Dump after sign extension elimination.
- -fdump-rtl-seqabstr
- Dump after common sequence discovery.
- -fdump-rtl-shorten
- Dump after shortening branches.
- -fdump-rtl-sibling
- Dump after sibling call optimizations.
- -fdump-rtl-split1
- -fdump-rtl-split2
- -fdump-rtl-split3
- -fdump-rtl-split4
- -fdump-rtl-split5
- -fdump-rtl-split1, -fdump-rtl-split2, -fdump-rtl-split3, -fdump-rtl-split4 and -fdump-rtl-split5 enable dumping after five rounds of instruction splitting.
- -fdump-rtl-sms
- Dump after modulo scheduling. This pass is only run on some architectures.
- -fdump-rtl-stack
- Dump after conversion from GCC's "flat register file" registers to the x87's stack-like registers. This pass is only run on x86 variants.
- -fdump-rtl-subreg1
- -fdump-rtl-subreg2
- -fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after the two subreg expansion passes.
- -fdump-rtl-unshare
- Dump after all rtl has been unshared.
- -fdump-rtl-vartrack
- Dump after variable tracking.
- -fdump-rtl-vregs
- Dump after converting virtual registers to hard registers.
- -fdump-rtl-web
- Dump after live range splitting.
- -fdump-rtl-regclass
- -fdump-rtl-subregs_of_mode_init
- -fdump-rtl-subregs_of_mode_finish
- -fdump-rtl-dfinit
- -fdump-rtl-dfinish
- These dumps are defined but always produce empty files.
- -da
- -fdump-rtl-all
- Produce all the dumps listed above.
- -dA
- Annotate the assembler output with miscellaneous debugging information.
- -dD
- Dump all macro definitions, at the end of preprocessing, in addition to normal output.
- -dH
- Produce a core dump whenever an error occurs.
- -dp
- Annotate the assembler output with a comment indicating which pattern and alternative is used. The length of each instruction is also printed.
- -dP
- Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp annotation.
- -dx
- Just generate RTL for a function instead of compiling it. Usually used with -fdump-rtl-expand.
- -fdump-noaddr
- When doing debugging dumps, suppress address output. This makes it more feasible to use diff on debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations.
- -fdump-unnumbered
- When doing debugging dumps, suppress instruction numbers and address output. This makes it more feasible to use diff on debugging dumps for compiler invocations with different options, in particular with and without -g.
- -fdump-unnumbered-links
- When doing debugging dumps (see -d option above), suppress instruction numbers for the links to the previous and next instructions in a sequence.
- -fdump-translation-unit (C++ only)
- -fdump-translation-unit-options (C++ only)
- Dump a representation of the tree structure for the entire translation unit to a file. The file name is made by appending .tu to the source file name, and the file is created in the same directory as the output file. If the -options form is used, options controls the details of the dump as described for the -fdump-tree options.
- -fdump-class-hierarchy (C++ only)
- -fdump-class-hierarchy-options (C++ only)
- Dump a representation of each class's hierarchy and virtual function table layout to a file. The file name is made by appending .class to the source file name, and the file is created in the same directory as the output file. If the -options form is used, options controls the details of the dump as described for the -fdump-tree options.
- -fdump-ipa-switch
- Control the dumping at various stages of inter-procedural analysis language tree to a file. The file name is generated by appending a switch specific suffix to the source file name, and the file is created in the same directory as the output file. The following dumps are possible:
- all
- Enables all inter-procedural analysis dumps.
- cgraph
- Dumps information about call-graph optimization, unused function removal, and inlining decisions.
- inline
- Dump after function inlining.
- -fdump-passes
- Dump the list of optimization passes that are turned on and off by the current command-line options.
- -fdump-statistics-option
- Enable and control dumping of pass statistics in a separate file. The file name is generated by appending a suffix ending in .statistics to the source file name, and the file is created in the same directory as the output file. If the -option form is used, -stats causes counters to be summed over the whole compilation unit while -details dumps every event as the passes generate them. The default with no option is to sum counters for each function compiled.
- -fdump-tree-switch
- -fdump-tree-switch-options
- -fdump-tree-switch-options=filename
- Control the dumping at various stages of processing the intermediate language tree to a file. The file name is generated by appending a switch-specific suffix to the source file name, and the file is created in the same directory as the output file. In case of =filename option, the dump is output on the given file instead of the auto named dump files. If the -options form is used, options is a list of - separated options which control the details of the dump. Not all options are applicable to all dumps; those that are not meaningful are ignored. The following options are available
- address
- Print the address of each node. Usually this is not meaningful as it changes according to the environment and source file. Its primary use is for tying up a dump file with a debug environment.
- asmname
- If "DECL_ASSEMBLER_NAME" has been set for a given decl, use that in the dump instead of "DECL_NAME". Its primary use is ease of use working backward from mangled names in the assembly file.
- slim
-
When dumping front-end intermediate representations, inhibit dumping of members of a scope or body of a function merely because that scope has been reached. Only dump such items when they are directly reachable by some other path.
- raw
- Print a raw representation of the tree. By default, trees are pretty-printed into a C-like representation.
- details
- Enable more detailed dumps (not honored by every dump option). Also include information from the optimization passes.
- stats
- Enable dumping various statistics about the pass (not honored by every dump option).
- blocks
- Enable showing basic block boundaries (disabled in raw dumps).
- graph
-
For each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with GraphViz to file.passid.pass.dot. Each function in the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot.
- vops
- Enable showing virtual operands for every statement.
- lineno
- Enable showing line numbers for statements.
- uid
- Enable showing the unique ID ("DECL_UID") for each variable.
- verbose
- Enable showing the tree dump for each statement.
- eh
- Enable showing the EH region number holding each statement.
- scev
- Enable showing scalar evolution analysis details.
- optimized
- Enable showing optimization information (only available in certain passes).
- missed
- Enable showing missed optimization information (only available in certain passes).
- notes
- Enable other detailed optimization information (only available in certain passes).
- =filename
-
Instead of an auto named dump file, output into the given file name. The file names stdout and stderr are treated specially and are considered already open standard streams. For example,
gcc -O2 -ftree-vectorize -fdump-tree-vect-blocks=foo.dump
-fdump-tree-pre=stderr file.c
- all
- Turn on all options, except raw, slim, verbose and lineno.
- optall
- Turn on all optimization options, i.e., optimized, missed, and note.
- original
- Dump before any tree based optimization, to file.original.
- optimized
- Dump after all tree based optimization, to file.optimized.
- gimple
- Dump each function before and after the gimplification pass to a file. The file name is made by appending .gimple to the source file name.
- cfg
- Dump the control flow graph of each function to a file. The file name is made by appending .cfg to the source file name.
- ch
- Dump each function after copying loop headers. The file name is made by appending .ch to the source file name.
- ssa
- Dump SSA related information to a file. The file name is made by appending .ssa to the source file name.
- alias
- Dump aliasing information for each function. The file name is made by appending .alias to the source file name.
- ccp
- Dump each function after CCP. The file name is made by appending .ccp to the source file name.
- storeccp
- Dump each function after STORE-CCP. The file name is made by appending .storeccp to the source file name.
- pre
- Dump trees after partial redundancy elimination. The file name is made by appending .pre to the source file name.
- fre
- Dump trees after full redundancy elimination. The file name is made by appending .fre to the source file name.
- copyprop
- Dump trees after copy propagation. The file name is made by appending .copyprop to the source file name.
- store_copyprop
- Dump trees after store copy-propagation. The file name is made by appending .store_copyprop to the source file name.
- dce
- Dump each function after dead code elimination. The file name is made by appending .dce to the source file name.
- mudflap
- Dump each function after adding mudflap instrumentation. The file name is made by appending .mudflap to the source file name.
- sra
- Dump each function after performing scalar replacement of aggregates. The file name is made by appending .sra to the source file name.
- sink
- Dump each function after performing code sinking. The file name is made by appending .sink to the source file name.
- dom
- Dump each function after applying dominator tree optimizations. The file name is made by appending .dom to the source file name.
- dse
- Dump each function after applying dead store elimination. The file name is made by appending .dse to the source file name.
- phiopt
- Dump each function after optimizing PHI nodes into straightline code. The file name is made by appending .phiopt to the source file name.
- forwprop
- Dump each function after forward propagating single use variables. The file name is made by appending .forwprop to the source file name.
- copyrename
- Dump each function after applying the copy rename optimization. The file name is made by appending .copyrename to the source file name.
- nrv
- Dump each function after applying the named return value optimization on generic trees. The file name is made by appending .nrv to the source file name.
- vect
- Dump each function after applying vectorization of loops. The file name is made by appending .vect to the source file name.
- slp
- Dump each function after applying vectorization of basic blocks. The file name is made by appending .slp to the source file name.
- vrp
- Dump each function after Value Range Propagation (VRP). The file name is made by appending .vrp to the source file name.
- all
- Enable all the available tree dumps with the flags provided in this option.
- -fopt-info
- -fopt-info-options
- -fopt-info-options=filename
-
Controls optimization dumps from various optimization passes. If the -options form is used, options is a list of - separated options to select the dump details and optimizations. If options is not specified, it defaults to all for details and optall for optimization groups. If the filename is not specified, it defaults to stderr. Note that the output filename will be overwritten in case of multiple translation units. If a combined output from multiple translation units is desired, stderr should be used instead.
- optimized
- Print information when an optimization is successfully applied. It is up to a pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops which got successfully vectorized.
- missed
-
Print information about missed optimizations. Individual passes control which information to include in the output. For example,
gcc -O2 -ftree-vectorize -fopt-info-vec-missed
- note
- Print verbose information about optimizations, such as certain transformations, more detailed messages about decisions etc.
- all
- Print detailed optimization information. This includes optimized, missed, and note.
- ipa
- Enable dumps from all interprocedural optimizations.
- loop
- Enable dumps from all loop optimizations.
- inline
- Enable dumps from all inlining optimizations.
- vec
- Enable dumps from all vectorization optimizations.
gcc -O3 -fopt-info-missed=missed.all
gcc -O3 -fopt-info-inline-optimized-missed=inline.txt
gcc -O3 -fopt-info
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt
- -ftree-vectorizer-verbose=n
-
This option is deprecated and is implemented in terms of -fopt-info. Please use -fopt-info-kind form instead, where kind is one of the valid opt-info options. It prints additional optimization information. For n=0 no diagnostic information is reported. If n=1 the vectorizer reports each loop that got vectorized, and the total number of loops that got vectorized. If n=2 the vectorizer reports locations which could not be vectorized and the reasons for those. For any higher verbosity levels all the analysis and transformation information from the vectorizer is reported.
- -frandom-seed=string
-
This option provides a seed that GCC uses in place of random numbers in generating certain symbol names that have to be different in every compiled file. It is also used to place unique stamps in coverage data files and the object files that produce them. You can use the -frandom-seed option to produce reproducibly identical object files.
- -fsched-verbose=n
-
On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints. This information is written to standard error, unless -fdump-rtl-sched1 or -fdump-rtl-sched2 is specified, in which case it is output to the usual dump listing file, .sched1 or .sched2 respectively. However for n greater than nine, the output is always printed to standard error.
- -save-temps
- -save-temps=cwd
-
Store the usual "temporary" intermediate files permanently; place them in the current directory and name them based on the source file. Thus, compiling foo.c with -c -save-temps produces files foo.i and foo.s, as well as foo.o. This creates a preprocessed foo.i output file even though the compiler now normally uses an integrated preprocessor.
gcc -save-temps -o outdir1/foo.o indir1/foo.c&
gcc -save-temps -o outdir2/foo.o indir2/foo.c&
- -save-temps=obj
-
Store the usual "temporary" intermediate files permanently. If the -o option is used, the temporary files are based on the object file. If the -o option is not used, the -save-temps=obj switch behaves like -save-temps.
gcc -save-temps=obj -c foo.c
gcc -save-temps=obj -c bar.c -o dir/xbar.o
gcc -save-temps=obj foobar.c -o dir2/yfoobar
- -time[=file]
-
Report the CPU time taken by each subprocess in the compilation sequence. For C source files, this is the compiler proper and assembler (plus the linker if linking is done).
# cc1 0.12 0.01
# as 0.00 0.01
0.12 0.01 cc1 <options>
0.00 0.01 as <options>
- -fvar-tracking
-
Run variable tracking pass. It computes where variables are stored at each position in code. Better debugging information is then generated (if the debugging information format supports this information).
- -fvar-tracking-assignments
-
Annotate assignments to user variables early in the compilation and attempt to carry the annotations over throughout the compilation all the way to the end, in an attempt to improve debug information while optimizing. Use of -gdwarf-4 is recommended along with it.
- -fvar-tracking-assignments-toggle
- Toggle -fvar-tracking-assignments, in the same way that -gtoggle toggles -g.
- -print-file-name=library
- Print the full absolute name of the library file library that would be used when linking---and don't do anything else. With this option, GCC does not compile or link anything; it just prints the file name.
- -print-multi-directory
- Print the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist in GCC_EXEC_PREFIX.
- -print-multi-lib
- Print the mapping from multilib directory names to compiler switches that enable them. The directory name is separated from the switches by ;, and each switch starts with an @ instead of the -, without spaces between multiple switches. This is supposed to ease shell processing.
- -print-multi-os-directory
- Print the path to OS libraries for the selected multilib, relative to some lib subdirectory. If OS libraries are present in the lib subdirectory and no multilibs are used, this is usually just ., if OS libraries are present in libsuffix sibling directories this prints e.g. ../lib64, ../lib or ../lib32, or if OS libraries are present in lib/subdir subdirectories it prints e.g. amd64, sparcv9 or ev6.
- -print-multiarch
- Print the path to OS libraries for the selected multiarch, relative to some lib subdirectory.
- -print-prog-name=program
- Like -print-file-name, but searches for a program such as cpp.
- -print-libgcc-file-name
-
Same as -print-file-name=libgcc.a.
gcc -nostdlib <files>... ̀gcc -print-libgcc-file-namè
- -print-search-dirs
-
Print the name of the configured installation directory and a list of program and library directories gcc searches---and don't do anything else.
- -print-sysroot
- Print the target sysroot directory that is used during compilation. This is the target sysroot specified either at configure time or using the --sysroot option, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing.
- -print-sysroot-headers-suffix
- Print the suffix added to the target sysroot when searching for headers, or give an error if the compiler is not configured with such a suffix---and don't do anything else.
- -dumpmachine
- Print the compiler's target machine (for example, i686-pc-linux-gnu)---and don't do anything else.
- -dumpversion
- Print the compiler version (for example, 3.0)---and don't do anything else.
- -dumpspecs
- Print the compiler's built-in specs---and don't do anything else. (This is used when GCC itself is being built.)
- -fno-eliminate-unused-debug-types
- Normally, when producing DWARF 2 output, GCC avoids producing debug symbol output for types that are nowhere used in the source file being compiled. Sometimes it is useful to have GCC emit debugging information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit, for example if, in the debugger, you want to cast a value to a type that is not actually used in your program (but is declared). More often, however, this results in a significant amount of wasted space.
Options That Control Optimization
These options control various sorts of optimizations.Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you expect from the source code.
Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program.
The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them.
Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section.
Most optimizations are only enabled if an -O level is set on the command line. Otherwise they are disabled, even if individual optimization flags are specified.
Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each -O level than those listed here. You can invoke GCC with -Q --help=optimizers to find out the exact set of optimizations that are enabled at each level.
- -O
- -O1
-
Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.
- -O2
-
Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. As compared to -O, this option increases both compilation time and the performance of the generated code.
- -O3
- Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the -finline-functions, -funswitch-loops, -fpredictive-commoning, -fgcse-after-reload, -ftree-vectorize, -fvect-cost-model, -ftree-partial-pre and -fipa-cp-clone options.
- -O0
- Reduce compilation time and make debugging produce the expected results. This is the default.
- -Os
-
Optimize for size. -Os enables all -O2 optimizations that do not typically increase code size. It also performs further optimizations designed to reduce code size.
- -Ofast
- Disregard strict standards compliance. -Ofast enables all -O3 optimizations. It also enables optimizations that are not valid for all standard-compliant programs. It turns on -ffast-math and the Fortran-specific -fno-protect-parens and -fstack-arrays.
- -Og
-
Optimize debugging experience. -Og enables optimizations that do not interfere with debugging. It should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience.
Options of the form -fflag specify machine-independent flags. Most flags have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed---the one you typically use. You can figure out the other form by either removing no- or adding it.
The following options control specific optimizations. They are either activated by -O options or are related to ones that are. You can use the following flags in the rare cases when "fine-tuning" of optimizations to be performed is desired.
- -fno-default-inline
- Do not make member functions inline by default merely because they are defined inside the class scope (C++ only). Otherwise, when you specify -O, member functions defined inside class scope are compiled inline by default; i.e., you don't need to add inline in front of the member function name.
- -fno-defer-pop
-
Always pop the arguments to each function call as soon as that function returns. For machines that must pop arguments after a function call, the compiler normally lets arguments accumulate on the stack for several function calls and pops them all at once.
- -fforward-propagate
-
Perform a forward propagation pass on RTL. The pass tries to combine two instructions and checks if the result can be simplified. If loop unrolling is active, two passes are performed and the second is scheduled after loop unrolling.
- -ffp-contract=style
-
-ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables floating-point expression contraction such as forming of fused multiply-add operations if the target has native support for them. -ffp-contract=on enables floating-point expression contraction if allowed by the language standard. This is currently not implemented and treated equal to -ffp-contract=off.
- -fomit-frame-pointer
-
Don't keep the frame pointer in a register for functions that don't need one. This avoids the instructions to save, set up and restore frame pointers; it also makes an extra register available in many functions. It also makes debugging impossible on some machines.
- -foptimize-sibling-calls
-
Optimize sibling and tail recursive calls.
- -fno-inline
-
Do not expand any functions inline apart from those marked with the "always_inline" attribute. This is the default when not optimizing.
- -finline-small-functions
-
Integrate functions into their callers when their body is smaller than expected function call code (so overall size of program gets smaller). The compiler heuristically decides which functions are simple enough to be worth integrating in this way. This inlining applies to all functions, even those not declared inline.
- -findirect-inlining
-
Inline also indirect calls that are discovered to be known at compile time thanks to previous inlining. This option has any effect only when inlining itself is turned on by the -finline-functions or -finline-small-functions options.
- -finline-functions
-
Consider all functions for inlining, even if they are not declared inline. The compiler heuristically decides which functions are worth integrating in this way.
- -finline-functions-called-once
-
Consider all "static" functions called once for inlining into their caller even if they are not marked "inline". If a call to a given function is integrated, then the function is not output as assembler code in its own right.
- -fearly-inlining
-
Inline functions marked by "always_inline" and functions whose body seems smaller than the function call overhead early before doing -fprofile-generate instrumentation and real inlining pass. Doing so makes profiling significantly cheaper and usually inlining faster on programs having large chains of nested wrapper functions.
- -fipa-sra
-
Perform interprocedural scalar replacement of aggregates, removal of unused parameters and replacement of parameters passed by reference by parameters passed by value.
- -finline-limit=n
-
By default, GCC limits the size of functions that can be inlined. This flag allows coarse control of this limit. n is the size of functions that can be inlined in number of pseudo instructions.
- max-inline-insns-single
- is set to n/2.
- max-inline-insns-auto
- is set to n/2.
- -fno-keep-inline-dllexport
- This is a more fine-grained version of -fkeep-inline-functions, which applies only to functions that are declared using the "dllexport" attribute or declspec
- -fkeep-inline-functions
- In C, emit "static" functions that are declared "inline" into the object file, even if the function has been inlined into all of its callers. This switch does not affect functions using the "extern inline" extension in GNU C90. In C++, emit any and all inline functions into the object file.
- -fkeep-static-consts
-
Emit variables declared "static const" when optimization isn't turned on, even if the variables aren't referenced.
- -fmerge-constants
-
Attempt to merge identical constants (string constants and floating-point constants) across compilation units.
- -fmerge-all-constants
-
Attempt to merge identical constants and identical variables.
- -fmodulo-sched
- Perform swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost loops and reorders their instructions by overlapping different iterations.
- -fmodulo-sched-allow-regmoves
- Perform more aggressive SMS-based modulo scheduling with register moves allowed. By setting this flag certain anti-dependences edges are deleted, which triggers the generation of reg-moves based on the life-range analysis. This option is effective only with -fmodulo-sched enabled.
- -fno-branch-count-reg
-
Do not use "decrement and branch" instructions on a count register, but instead generate a sequence of instructions that decrement a register, compare it against zero, then branch based upon the result. This option is only meaningful on architectures that support such instructions, which include x86, PowerPC, IA-64 and S/390.
- -fno-function-cse
-
Do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly.
- -fno-zero-initialized-in-bss
-
If the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS. This can save space in the resulting code.
- -fmudflap -fmudflapth -fmudflapir
-
For front-ends that support it (C and C++), instrument all risky pointer/array dereferencing operations, some standard library string/heap functions, and some other associated constructs with range/validity tests. Modules so instrumented should be immune to buffer overflows, invalid heap use, and some other classes of C/C++ programming errors. The instrumentation relies on a separate runtime library ( libmudflap), which is linked into a program if -fmudflap is given at link time. Run-time behavior of the instrumented program is controlled by the MUDFLAP_OPTIONS environment variable. See "env MUDFLAP_OPTIONS=-help a.out" for its options.
- -fthread-jumps
-
Perform optimizations that check to see if a jump branches to a location where another comparison subsumed by the first is found. If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false.
- -fsplit-wide-types
-
When using a type that occupies multiple registers, such as "long long" on a 32-bit system, split the registers apart and allocate them independently. This normally generates better code for those types, but may make debugging more difficult.
- -fcse-follow-jumps
-
In common subexpression elimination (CSE), scan through jump instructions when the target of the jump is not reached by any other path. For example, when CSE encounters an "if" statement with an "else" clause, CSE follows the jump when the condition tested is false.
- -fcse-skip-blocks
-
This is similar to -fcse-follow-jumps, but causes CSE to follow jumps that conditionally skip over blocks. When CSE encounters a simple "if" statement with no else clause, -fcse-skip-blocks causes CSE to follow the jump around the body of the "if".
- -frerun-cse-after-loop
-
Re-run common subexpression elimination after loop optimizations are performed.
- -fgcse
-
Perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation.
- -fgcse-lm
-
When -fgcse-lm is enabled, global common subexpression elimination attempts to move loads that are only killed by stores into themselves. This allows a loop containing a load/store sequence to be changed to a load outside the loop, and a copy/store within the loop.
- -fgcse-sm
-
When -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination. This pass attempts to move stores out of loops. When used in conjunction with -fgcse-lm, loops containing a load/store sequence can be changed to a load before the loop and a store after the loop.
- -fgcse-las
-
When -fgcse-las is enabled, the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location (both partial and full redundancies).
- -fgcse-after-reload
- When -fgcse-after-reload is enabled, a redundant load elimination pass is performed after reload. The purpose of this pass is to clean up redundant spilling.
- -faggressive-loop-optimizations
- This option tells the loop optimizer to use language constraints to derive bounds for the number of iterations of a loop. This assumes that loop code does not invoke undefined behavior by for example causing signed integer overflows or out-of-bound array accesses. The bounds for the number of iterations of a loop are used to guide loop unrolling and peeling and loop exit test optimizations. This option is enabled by default.
- -funsafe-loop-optimizations
- This option tells the loop optimizer to assume that loop indices do not overflow, and that loops with nontrivial exit condition are not infinite. This enables a wider range of loop optimizations even if the loop optimizer itself cannot prove that these assumptions are valid. If you use -Wunsafe-loop-optimizations, the compiler warns you if it finds this kind of loop.
- -fcrossjumping
-
Perform cross-jumping transformation. This transformation unifies equivalent code and saves code size. The resulting code may or may not perform better than without cross-jumping.
- -fauto-inc-dec
- Combine increments or decrements of addresses with memory accesses. This pass is always skipped on architectures that do not have instructions to support this. Enabled by default at -O and higher on architectures that support this.
- -fdce
- Perform dead code elimination (DCE) on RTL. Enabled by default at -O and higher.
- -fdse
- Perform dead store elimination (DSE) on RTL. Enabled by default at -O and higher.
- -fif-conversion
-
Attempt to transform conditional jumps into branch-less equivalents. This includes use of conditional moves, min, max, set flags and abs instructions, and some tricks doable by standard arithmetics. The use of conditional execution on chips where it is available is controlled by "if-conversion2".
- -fif-conversion2
-
Use conditional execution (where available) to transform conditional jumps into branch-less equivalents.
- -fdelete-null-pointer-checks
-
Assume that programs cannot safely dereference null pointers, and that no code or data element resides there. This enables simple constant folding optimizations at all optimization levels. In addition, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers; these assume that if a pointer is checked after it has already been dereferenced, it cannot be null.
- -fdevirtualize
- Attempt to convert calls to virtual functions to direct calls. This is done both within a procedure and interprocedurally as part of indirect inlining ("-findirect-inlining") and interprocedural constant propagation ( -fipa-cp). Enabled at levels -O2, -O3, -Os.
- -fexpensive-optimizations
-
Perform a number of minor optimizations that are relatively expensive.
- -free
-
Attempt to remove redundant extension instructions. This is especially helpful for the x86-64 architecture, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half.
- -foptimize-register-move
- -fregmove
-
Attempt to reassign register numbers in move instructions and as operands of other simple instructions in order to maximize the amount of register tying. This is especially helpful on machines with two-operand instructions.
- -fira-algorithm=algorithm
- Use the specified coloring algorithm for the integrated register allocator. The algorithm argument can be priority, which specifies Chow's priority coloring, or CB, which specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring is not implemented for all architectures, but for those targets that do support it, it is the default because it generates better code.
- -fira-region=region
- Use specified regions for the integrated register allocator. The region argument should be one of the following:
- all
- Use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set.
- mixed
- Use all loops except for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed ( -O, -O2, ...).
- one
- Use all functions as a single region. This typically results in the smallest code size, and is enabled by default for -Os or -O0.
- -fira-hoist-pressure
-
Use IRA to evaluate register pressure in the code hoisting pass for decisions to hoist expressions. This option usually results in smaller code, but it can slow the compiler down.
- -fira-loop-pressure
-
Use IRA to evaluate register pressure in loops for decisions to move loop invariants. This option usually results in generation of faster and smaller code on machines with large register files (>= 32 registers), but it can slow the compiler down.
- -fno-ira-share-save-slots
- Disable sharing of stack slots used for saving call-used hard registers living through a call. Each hard register gets a separate stack slot, and as a result function stack frames are larger.
- -fno-ira-share-spill-slots
- Disable sharing of stack slots allocated for pseudo-registers. Each pseudo-register that does not get a hard register gets a separate stack slot, and as a result function stack frames are larger.
- -fira-verbose=n
- Control the verbosity of the dump file for the integrated register allocator. The default value is 5. If the value n is greater or equal to 10, the dump output is sent to stderr using the same format as n minus 10.
- -fdelayed-branch
-
If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions.
- -fschedule-insns
-
If supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to required data being unavailable. This helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating-point instruction is required.
- -fschedule-insns2
-
Similar to -fschedule-insns, but requests an additional pass of instruction scheduling after register allocation has been done. This is especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle.
- -fno-sched-interblock
- Don't schedule instructions across basic blocks. This is normally enabled by default when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
- -fno-sched-spec
- Don't allow speculative motion of non-load instructions. This is normally enabled by default when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
- -fsched-pressure
- Enable register pressure sensitive insn scheduling before register allocation. This only makes sense when scheduling before register allocation is enabled, i.e. with -fschedule-insns or at -O2 or higher. Usage of this option can improve the generated code and decrease its size by preventing register pressure increase above the number of available hard registers and subsequent spills in register allocation.
- -fsched-spec-load
- Allow speculative motion of some load instructions. This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
- -fsched-spec-load-dangerous
- Allow speculative motion of more load instructions. This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
- -fsched-stalled-insns
- -fsched-stalled-insns=n
- Define how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready list during the second scheduling pass. -fno-sched-stalled-insns means that no insns are moved prematurely, -fsched-stalled-insns=0 means there is no limit on how many queued insns can be moved prematurely. -fsched-stalled-insns without a value is equivalent to -fsched-stalled-insns=1.
- -fsched-stalled-insns-dep
- -fsched-stalled-insns-dep=n
- Define how many insn groups (cycles) are examined for a dependency on a stalled insn that is a candidate for premature removal from the queue of stalled insns. This has an effect only during the second scheduling pass, and only if -fsched-stalled-insns is used. -fno-sched-stalled-insns-dep is equivalent to -fsched-stalled-insns-dep=0. -fsched-stalled-insns-dep without a value is equivalent to -fsched-stalled-insns-dep=1.
- -fsched2-use-superblocks
-
When scheduling after register allocation, use superblock scheduling. This allows motion across basic block boundaries, resulting in faster schedules. This option is experimental, as not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm.
- -fsched-group-heuristic
- Enable the group heuristic in the scheduler. This heuristic favors the instruction that belongs to a schedule group. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
- -fsched-critical-path-heuristic
- Enable the critical-path heuristic in the scheduler. This heuristic favors instructions on the critical path. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
- -fsched-spec-insn-heuristic
- Enable the speculative instruction heuristic in the scheduler. This heuristic favors speculative instructions with greater dependency weakness. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
- -fsched-rank-heuristic
- Enable the rank heuristic in the scheduler. This heuristic favors the instruction belonging to a basic block with greater size or frequency. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
- -fsched-last-insn-heuristic
- Enable the last-instruction heuristic in the scheduler. This heuristic favors the instruction that is less dependent on the last instruction scheduled. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
- -fsched-dep-count-heuristic
- Enable the dependent-count heuristic in the scheduler. This heuristic favors the instruction that has more instructions depending on it. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
- -freschedule-modulo-scheduled-loops
- Modulo scheduling is performed before traditional scheduling. If a loop is modulo scheduled, later scheduling passes may change its schedule. Use this option to control that behavior.
- -fselective-scheduling
- Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass.
- -fselective-scheduling2
- Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the second scheduler pass.
- -fsel-sched-pipelining
- Enable software pipelining of innermost loops during selective scheduling. This option has no effect unless one of -fselective-scheduling or -fselective-scheduling2 is turned on.
- -fsel-sched-pipelining-outer-loops
- When pipelining loops during selective scheduling, also pipeline outer loops. This option has no effect unless -fsel-sched-pipelining is turned on.
- -fshrink-wrap
- Emit function prologues only before parts of the function that need it, rather than at the top of the function. This flag is enabled by default at -O and higher.
- -fcaller-saves
-
Enable allocation of values to registers that are clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls. Such allocation is done only when it seems to result in better code.
- -fcombine-stack-adjustments
-
Tracks stack adjustments (pushes and pops) and stack memory references and then tries to find ways to combine them.
- -fconserve-stack
- Attempt to minimize stack usage. The compiler attempts to use less stack space, even if that makes the program slower. This option implies setting the large-stack-frame parameter to 100 and the large-stack-frame-growth parameter to 400.
- -ftree-reassoc
- Perform reassociation on trees. This flag is enabled by default at -O and higher.
- -ftree-pre
- Perform partial redundancy elimination (PRE) on trees. This flag is enabled by default at -O2 and -O3.
- -ftree-partial-pre
- Make partial redundancy elimination (PRE) more aggressive. This flag is enabled by default at -O3.
- -ftree-forwprop
- Perform forward propagation on trees. This flag is enabled by default at -O and higher.
- -ftree-fre
- Perform full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE only considers expressions that are computed on all paths leading to the redundant computation. This analysis is faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at -O and higher.
- -ftree-phiprop
- Perform hoisting of loads from conditional pointers on trees. This pass is enabled by default at -O and higher.
- -fhoist-adjacent-loads
- Speculatively hoist loads from both branches of an if-then-else if the loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction. This flag is enabled by default at -O2 and higher.
- -ftree-copy-prop
- Perform copy propagation on trees. This pass eliminates unnecessary copy operations. This flag is enabled by default at -O and higher.
- -fipa-pure-const
- Discover which functions are pure or constant. Enabled by default at -O and higher.
- -fipa-reference
- Discover which static variables do not escape the compilation unit. Enabled by default at -O and higher.
- -fipa-pta
- Perform interprocedural pointer analysis and interprocedural modification and reference analysis. This option can cause excessive memory and compile-time usage on large compilation units. It is not enabled by default at any optimization level.
- -fipa-profile
- Perform interprocedural profile propagation. The functions called only from cold functions are marked as cold. Also functions executed once (such as "cold", "noreturn", static constructors or destructors) are identified. Cold functions and loop less parts of functions executed once are then optimized for size. Enabled by default at -O and higher.
- -fipa-cp
- Perform interprocedural constant propagation. This optimization analyzes the program to determine when values passed to functions are constants and then optimizes accordingly. This optimization can substantially increase performance if the application has constants passed to functions. This flag is enabled by default at -O2, -Os and -O3.
- -fipa-cp-clone
- Perform function cloning to make interprocedural constant propagation stronger. When enabled, interprocedural constant propagation performs function cloning when externally visible function can be called with constant arguments. Because this optimization can create multiple copies of functions, it may significantly increase code size (see --param ipcp-unit-growth=value). This flag is enabled by default at -O3.
- -ftree-sink
- Perform forward store motion on trees. This flag is enabled by default at -O and higher.
- -ftree-bit-ccp
- Perform sparse conditional bit constant propagation on trees and propagate pointer alignment information. This pass only operates on local scalar variables and is enabled by default at -O and higher. It requires that -ftree-ccp is enabled.
- -ftree-ccp
- Perform sparse conditional constant propagation (CCP) on trees. This pass only operates on local scalar variables and is enabled by default at -O and higher.
- -ftree-switch-conversion
- Perform conversion of simple initializations in a switch to initializations from a scalar array. This flag is enabled by default at -O2 and higher.
- -ftree-tail-merge
- Look for identical code sequences. When found, replace one with a jump to the other. This optimization is known as tail merging or cross jumping. This flag is enabled by default at -O2 and higher. The compilation time in this pass can be limited using max-tail-merge-comparisons parameter and max-tail-merge-iterations parameter.
- -ftree-dce
- Perform dead code elimination (DCE) on trees. This flag is enabled by default at -O and higher.
- -ftree-builtin-call-dce
- Perform conditional dead code elimination (DCE) for calls to built-in functions that may set "errno" but are otherwise side-effect free. This flag is enabled by default at -O2 and higher if -Os is not also specified.
- -ftree-dominator-opts
- Perform a variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range propagation and expression simplification) based on a dominator tree traversal. This also performs jump threading (to reduce jumps to jumps). This flag is enabled by default at -O and higher.
- -ftree-dse
- Perform dead store elimination (DSE) on trees. A dead store is a store into a memory location that is later overwritten by another store without any intervening loads. In this case the earlier store can be deleted. This flag is enabled by default at -O and higher.
- -ftree-ch
- Perform loop header copying on trees. This is beneficial since it increases effectiveness of code motion optimizations. It also saves one jump. This flag is enabled by default at -O and higher. It is not enabled for -Os, since it usually increases code size.
- -ftree-loop-optimize
- Perform loop optimizations on trees. This flag is enabled by default at -O and higher.
- -ftree-loop-linear
- Perform loop interchange transformations on tree. Same as -floop-interchange. To use this code transformation, GCC has to be configured with --with-ppl and --with-cloog to enable the Graphite loop transformation infrastructure.
- -floop-interchange
-
Perform loop interchange transformations on loops. Interchanging two nested loops switches the inner and outer loops. For example, given a loop like:
DO J = 1, M
DO I = 1, N
A(J, I) = A(J, I) * C
ENDDO
ENDDO
DO I = 1, N
DO J = 1, M
A(J, I) = A(J, I) * C
ENDDO
ENDDO
- -floop-strip-mine
-
Perform loop strip mining transformations on loops. Strip mining splits a loop into two nested loops. The outer loop has strides equal to the strip size and the inner loop has strides of the original loop within a strip. The strip length can be changed using the loop-block-tile-size parameter. For example, given a loop like:
DO I = 1, N
A(I) = A(I) + C
ENDDO
DO II = 1, N, 51
DO I = II, min (II + 50, N)
A(I) = A(I) + C
ENDDO
ENDDO
- -floop-block
-
Perform loop blocking transformations on loops. Blocking strip mines each loop in the loop nest such that the memory accesses of the element loops fit inside caches. The strip length can be changed using the loop-block-tile-size parameter. For example, given a loop like:
DO I = 1, N
DO J = 1, M
A(J, I) = B(I) + C(J)
ENDDO
ENDDO
DO II = 1, N, 51
DO JJ = 1, M, 51
DO I = II, min (II + 50, N)
DO J = JJ, min (JJ + 50, M)
A(J, I) = B(I) + C(J)
ENDDO
ENDDO
ENDDO
ENDDO
- -fgraphite-identity
- Enable the identity transformation for graphite. For every SCoP we generate the polyhedral representation and transform it back to gimple. Using -fgraphite-identity we can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations are also performed by the code generator CLooG, like index splitting and dead code elimination in loops.
- -floop-nest-optimize
- Enable the ISL based loop nest optimizer. This is a generic loop nest optimizer based on the Pluto optimization algorithms. It calculates a loop structure optimized for data-locality and parallelism. This option is experimental.
- -floop-parallelize-all
- Use the Graphite data dependence analysis to identify loops that can be parallelized. Parallelize all the loops that can be analyzed to not contain loop carried dependences without checking that it is profitable to parallelize the loops.
- -fcheck-data-deps
- Compare the results of several data dependence analyzers. This option is used for debugging the data dependence analyzers.
- -ftree-loop-if-convert
- Attempt to transform conditional jumps in the innermost loops to branch-less equivalents. The intent is to remove control-flow from the innermost loops in order to improve the ability of the vectorization pass to handle these loops. This is enabled by default if vectorization is enabled.
- -ftree-loop-if-convert-stores
-
Attempt to also if-convert conditional jumps containing memory writes. This transformation can be unsafe for multi-threaded programs as it transforms conditional memory writes into unconditional memory writes. For example,
for (i = 0; i < N; i++)
if (cond)
A[i] = expr;
for (i = 0; i < N; i++)
A[i] = cond ? expr : A[i];
- -ftree-loop-distribution
-
Perform loop distribution. This flag can improve cache performance on big loop bodies and allow further loop optimizations, like parallelization or vectorization, to take place. For example, the loop
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDO
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDO
- -ftree-loop-distribute-patterns
-
Perform loop distribution of patterns that can be code generated with calls to a library. This flag is enabled by default at -O3.
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDO
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDO
- -ftree-loop-im
- Perform loop invariant motion on trees. This pass moves only invariants that are hard to handle at RTL level (function calls, operations that expand to nontrivial sequences of insns). With -funswitch-loops it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion.
- -ftree-loop-ivcanon
- Create a canonical counter for number of iterations in loops for which determining number of iterations requires complicated analysis. Later optimizations then may determine the number easily. Useful especially in connection with unrolling.
- -fivopts
- Perform induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees.
- -ftree-parallelize-loops=n
- Parallelize loops, i.e., split their iteration space to run in n threads. This is only possible for loops whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g. by memory bandwidth. This option implies -pthread, and thus is only supported on targets that have support for -pthread.
- -ftree-pta
- Perform function-local points-to analysis on trees. This flag is enabled by default at -O and higher.
- -ftree-sra
- Perform scalar replacement of aggregates. This pass replaces structure references with scalars to prevent committing structures to memory too early. This flag is enabled by default at -O and higher.
- -ftree-copyrename
- Perform copy renaming on trees. This pass attempts to rename compiler temporaries to other variables at copy locations, usually resulting in variable names which more closely resemble the original variables. This flag is enabled by default at -O and higher.
- -ftree-coalesce-inlined-vars
- Tell the copyrename pass (see -ftree-copyrename) to attempt to combine small user-defined variables too, but only if they were inlined from other functions. It is a more limited form of -ftree-coalesce-vars. This may harm debug information of such inlined variables, but it will keep variables of the inlined-into function apart from each other, such that they are more likely to contain the expected values in a debugging session. This was the default in GCC versions older than 4.7.
- -ftree-coalesce-vars
- Tell the copyrename pass (see -ftree-copyrename) to attempt to combine small user-defined variables too, instead of just compiler temporaries. This may severely limit the ability to debug an optimized program compiled with -fno-var-tracking-assignments. In the negated form, this flag prevents SSA coalescing of user variables, including inlined ones. This option is enabled by default.
- -ftree-ter
- Perform temporary expression replacement during the SSA->normal phase. Single use/single def temporaries are replaced at their use location with their defining expression. This results in non-GIMPLE code, but gives the expanders much more complex trees to work on resulting in better RTL generation. This is enabled by default at -O and higher.
- -ftree-slsr
- Perform straight-line strength reduction on trees. This recognizes related expressions involving multiplications and replaces them by less expensive calculations when possible. This is enabled by default at -O and higher.
- -ftree-vectorize
- Perform loop vectorization on trees. This flag is enabled by default at -O3.
- -ftree-slp-vectorize
- Perform basic block vectorization on trees. This flag is enabled by default at -O3 and when -ftree-vectorize is enabled.
- -ftree-vect-loop-version
- Perform loop versioning when doing loop vectorization on trees. When a loop appears to be vectorizable except that data alignment or data dependence cannot be determined at compile time, then vectorized and non-vectorized versions of the loop are generated along with run-time checks for alignment or dependence to control which version is executed. This option is enabled by default except at level -Os where it is disabled.
- -fvect-cost-model
- Enable cost model for vectorization. This option is enabled by default at -O3.
- -ftree-vrp
- Perform Value Range Propagation on trees. This is similar to the constant propagation pass, but instead of values, ranges of values are propagated. This allows the optimizers to remove unnecessary range checks like array bound checks and null pointer checks. This is enabled by default at -O2 and higher. Null pointer check elimination is only done if -fdelete-null-pointer-checks is enabled.
- -ftracer
- Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing other optimizations to do a better job.
- -funroll-loops
- Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. -funroll-loops implies -frerun-cse-after-loop. This option makes code larger, and may or may not make it run faster.
- -funroll-all-loops
- Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs run more slowly. -funroll-all-loops implies the same options as -funroll-loops,
- -fsplit-ivs-in-unroller
-
Enables expression of values of induction variables in later iterations of the unrolled loop using the value in the first iteration. This breaks long dependency chains, thus improving efficiency of the scheduling passes.
- -fvariable-expansion-in-unroller
- With this option, the compiler creates multiple copies of some local variables when unrolling a loop, which can result in superior code.
- -fpartial-inlining
-
Inline parts of functions. This option has any effect only when inlining itself is turned on by the -finline-functions or -finline-small-functions options.
- -fpredictive-commoning
-
Perform predictive commoning optimization, i.e., reusing computations (especially memory loads and stores) performed in previous iterations of loops.
- -fprefetch-loop-arrays
-
If supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays.
- -fno-peephole
- -fno-peephole2
-
Disable any machine-specific peephole optimizations. The difference between -fno-peephole and -fno-peephole2 is in how they are implemented in the compiler; some targets use one, some use the other, a few use both.
- -fno-guess-branch-probability
-
Do not guess branch probabilities using heuristics.
- -freorder-blocks
-
Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code locality.
- -freorder-blocks-and-partition
-
In addition to reordering basic blocks in the compiled function, in order to reduce number of taken branches, partitions hot and cold basic blocks into separate sections of the assembly and .o files, to improve paging and cache locality performance.
- -freorder-functions
-
Reorder functions in the object file in order to improve code locality. This is implemented by using special subsections ".text.hot" for most frequently executed functions and ".text.unlikely" for unlikely executed functions. Reordering is done by the linker so object file format must support named sections and linker must place them in a reasonable way.
- -fstrict-aliasing
-
Allow the compiler to assume the strictest aliasing rules applicable to the language being compiled. For C (and C++), this activates optimizations based on the type of expressions. In particular, an object of one type is assumed never to reside at the same address as an object of a different type, unless the types are almost the same. For example, an "unsigned int" can alias an "int", but not a "void*" or a "double". A character type may alias any other type.
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}
- -fstrict-overflow
-
Allow the compiler to assume strict signed overflow rules, depending on the language being compiled. For C (and C++) this means that overflow when doing arithmetic with signed numbers is undefined, which means that the compiler may assume that it does not happen. This permits various optimizations. For example, the compiler assumes that an expression like "i + 10 > i" is always true for signed "i". This assumption is only valid if signed overflow is undefined, as the expression is false if "i + 10" overflows when using twos complement arithmetic. When this option is in effect any attempt to determine whether an operation on signed numbers overflows must be written carefully to not actually involve overflow.
- -falign-functions
- -falign-functions=n
-
Align the start of functions to the next power-of-two greater than n, skipping up to n bytes. For instance, -falign-functions=32 aligns functions to the next 32-byte boundary, but -falign-functions=24 aligns to the next 32-byte boundary only if this can be done by skipping 23 bytes or less.
- -falign-labels
- -falign-labels=n
-
Align all branch targets to a power-of-two boundary, skipping up to n bytes like -falign-functions. This option can easily make code slower, because it must insert dummy operations for when the branch target is reached in the usual flow of the code.
- -falign-loops
- -falign-loops=n
-
Align loops to a power-of-two boundary, skipping up to n bytes like -falign-functions. If the loops are executed many times, this makes up for any execution of the dummy operations.
- -falign-jumps
- -falign-jumps=n
-
Align branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by jumping, skipping up to n bytes like -falign-functions. In this case, no dummy operations need be executed.
- -funit-at-a-time
-
This option is left for compatibility reasons. -funit-at-a-time has no effect, while -fno-unit-at-a-time implies -fno-toplevel-reorder and -fno-section-anchors.
- -fno-toplevel-reorder
-
Do not reorder top-level functions, variables, and "asm" statements. Output them in the same order that they appear in the input file. When this option is used, unreferenced static variables are not removed. This option is intended to support existing code that relies on a particular ordering. For new code, it is better to use attributes.
- -fweb
-
Constructs webs as commonly used for register allocation purposes and assign each web individual pseudo register. This allows the register allocation pass to operate on pseudos directly, but also strengthens several other optimization passes, such as CSE, loop optimizer and trivial dead code remover. It can, however, make debugging impossible, since variables no longer stay in a "home register".
- -fwhole-program
-
Assume that the current compilation unit represents the whole program being compiled. All public functions and variables with the exception of "main" and those merged by attribute "externally_visible" become static functions and in effect are optimized more aggressively by interprocedural optimizers.
- -flto[=n]
-
This option runs the standard link-time optimizer. When invoked with source code, it generates GIMPLE (one of GCC's internal representations) and writes it to special ELF sections in the object file. When the object files are linked together, all the function bodies are read from these ELF sections and instantiated as if they had been part of the same translation unit.
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o
gcc -o myprog -flto -O2 foo.c bar.c
gcc -c -O0 -flto foo.c
gcc -c -O0 -flto bar.c
gcc -o myprog -flto -O3 foo.o bar.o
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo
- -flto-partition=alg
- Specify the partitioning algorithm used by the link-time optimizer. The value is either "1to1" to specify a partitioning mirroring the original source files or "balanced" to specify partitioning into equally sized chunks (whenever possible) or "max" to create new partition for every symbol where possible. Specifying "none" as an algorithm disables partitioning and streaming completely. The default value is "balanced". While "1to1" can be used as an workaround for various code ordering issues, the "max" partitioning is intended for internal testing only.
- -flto-compression-level=n
- This option specifies the level of compression used for intermediate language written to LTO object files, and is only meaningful in conjunction with LTO mode ( -flto). Valid values are 0 (no compression) to 9 (maximum compression). Values outside this range are clamped to either 0 or 9. If the option is not given, a default balanced compression setting is used.
- -flto-report
-
Prints a report with internal details on the workings of the link-time optimizer. The contents of this report vary from version to version. It is meant to be useful to GCC developers when processing object files in LTO mode (via -flto).
- -fuse-linker-plugin
-
Enables the use of a linker plugin during link-time optimization. This option relies on plugin support in the linker, which is available in gold or in GNU ld 2.21 or newer.
- -ffat-lto-objects
-
Fat LTO objects are object files that contain both the intermediate language and the object code. This makes them usable for both LTO linking and normal linking. This option is effective only when compiling with -flto and is ignored at link time.
- -fcompare-elim
-
After register allocation and post-register allocation instruction splitting, identify arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic. If possible, eliminate the explicit comparison operation.
- -fuse-ld=bfd
- Use the bfd linker instead of the default linker.
- -fuse-ld=gold
- Use the gold linker instead of the default linker.
- -fcprop-registers
-
After register allocation and post-register allocation instruction splitting, perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy.
- -fprofile-correction
- Profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates. When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected.
- -fprofile-dir=path
- Set the directory to search for the profile data files in to path. This option affects only the profile data generated by -fprofile-generate, -ftest-coverage, -fprofile-arcs and used by -fprofile-use and -fbranch-probabilities and its related options. Both absolute and relative paths can be used. By default, GCC uses the current directory as path, thus the profile data file appears in the same directory as the object file.
- -fprofile-generate
- -fprofile-generate=path
-
Enable options usually used for instrumenting application to produce profile useful for later recompilation with profile feedback based optimization. You must use -fprofile-generate both when compiling and when linking your program.
- -fprofile-use
- -fprofile-use=path
-
Enable profile feedback directed optimizations, and optimizations generally profitable only with profile feedback available.
The following options control compiler behavior regarding floating-point arithmetic. These options trade off between speed and correctness. All must be specifically enabled.
- -ffloat-store
-
Do not store floating-point variables in registers, and inhibit other options that might change whether a floating-point value is taken from a register or memory.
- -fexcess-precision=style
-
This option allows further control over excess precision on machines where floating-point registers have more precision than the IEEE "float" and "double" types and the processor does not support operations rounding to those types. By default, -fexcess-precision=fast is in effect; this means that operations are carried out in the precision of the registers and that it is unpredictable when rounding to the types specified in the source code takes place. When compiling C, if -fexcess-precision=standard is specified then excess precision follows the rules specified in ISO C99; in particular, both casts and assignments cause values to be rounded to their semantic types (whereas -ffloat-store only affects assignments). This option is enabled by default for C if a strict conformance option such as -std=c99 is used.
- -ffast-math
-
Sets -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans and -fcx-limited-range.
- -fno-math-errno
-
Do not set "errno" after calling math functions that are executed with a single instruction, e.g., "sqrt". A program that relies on IEEE exceptions for math error handling may want to use this flag for speed while maintaining IEEE arithmetic compatibility.
- -funsafe-math-optimizations
-
Allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and (b) may violate IEEE or ANSI standards. When used at link-time, it may include libraries or startup files that change the default FPU control word or other similar optimizations.
- -fassociative-math
-
Allow re-association of operands in series of floating-point operations. This violates the ISO C and C++ language standard by possibly changing computation result. NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow (and thus cannot be used on code that relies on rounding behavior like "(x + 2**52) - 2**52". May also reorder floating-point comparisons and thus may not be used when ordered comparisons are required. This option requires that both -fno-signed-zeros and -fno-trapping-math be in effect. Moreover, it doesn't make much sense with -frounding-math. For Fortran the option is automatically enabled when both -fno-signed-zeros and -fno-trapping-math are in effect.
- -freciprocal-math
-
Allow the reciprocal of a value to be used instead of dividing by the value if this enables optimizations. For example "x / y" can be replaced with "x * (1/y)", which is useful if "(1/y)" is subject to common subexpression elimination. Note that this loses precision and increases the number of flops operating on the value.
- -ffinite-math-only
-
Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs.
- -fno-signed-zeros
-
Allow optimizations for floating-point arithmetic that ignore the signedness of zero. IEEE arithmetic specifies the behavior of distinct +0.0 and -0.0 values, which then prohibits simplification of expressions such as x+0.0 or 0.0*x (even with -ffinite-math-only). This option implies that the sign of a zero result isn't significant.
- -fno-trapping-math
-
Compile code assuming that floating-point operations cannot generate user-visible traps. These traps include division by zero, overflow, underflow, inexact result and invalid operation. This option requires that -fno-signaling-nans be in effect. Setting this option may allow faster code if one relies on "non-stop" IEEE arithmetic, for example.
- -frounding-math
-
Disable transformations and optimizations that assume default floating-point rounding behavior. This is round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic truncations. This option should be specified for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode. This option disables constant folding of floating-point expressions at compile time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes.
- -fsignaling-nans
-
Compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations. Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs. This option implies -ftrapping-math.
- -fsingle-precision-constant
- Treat floating-point constants as single precision instead of implicitly converting them to double-precision constants.
- -fcx-limited-range
-
When enabled, this option states that a range reduction step is not needed when performing complex division. Also, there is no checking whether the result of a complex multiplication or division is "NaN + I*NaN", with an attempt to rescue the situation in that case. The default is -fno-cx-limited-range, but is enabled by -ffast-math.
- -fcx-fortran-rules
-
Complex multiplication and division follow Fortran rules. Range reduction is done as part of complex division, but there is no checking whether the result of a complex multiplication or division is "NaN + I*NaN", with an attempt to rescue the situation in that case.
The following options control optimizations that may improve performance, but are not enabled by any -O options. This section includes experimental options that may produce broken code.
- -fbranch-probabilities
-
After running a program compiled with -fprofile-arcs, you can compile it a second time using -fbranch-probabilities, to improve optimizations based on the number of times each branch was taken. When a program compiled with -fprofile-arcs exits, it saves arc execution counts to a file called sourcename.gcda for each source file. The information in this data file is very dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations.
- -fprofile-values
-
If combined with -fprofile-arcs, it adds code so that some data about values of expressions in the program is gathered.
- -fvpt
-
If combined with -fprofile-arcs, this option instructs the compiler to add code to gather information about values of expressions.
- -frename-registers
-
Attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation. This optimization most benefits processors with lots of registers. Depending on the debug information format adopted by the target, however, it can make debugging impossible, since variables no longer stay in a "home register".
- -ftracer
-
Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing other optimizations to do a better job.
- -funroll-loops
-
Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. -funroll-loops implies -frerun-cse-after-loop, -fweb and -frename-registers. It also turns on complete loop peeling (i.e. complete removal of loops with a small constant number of iterations). This option makes code larger, and may or may not make it run faster.
- -funroll-all-loops
- Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs run more slowly. -funroll-all-loops implies the same options as -funroll-loops.
- -fpeel-loops
-
Peels loops for which there is enough information that they do not roll much (from profile feedback). It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations).
- -fmove-loop-invariants
- Enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1
- -funswitch-loops
- Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to result of the condition).
- -ffunction-sections
- -fdata-sections
-
Place each function or data item into its own section in the output file if the target supports arbitrary sections. The name of the function or the name of the data item determines the section's name in the output file.
- -fbranch-target-load-optimize
- Perform branch target register load optimization before prologue / epilogue threading. The use of target registers can typically be exposed only during reload, thus hoisting loads out of loops and doing inter-block scheduling needs a separate optimization pass.
- -fbranch-target-load-optimize2
- Perform branch target register load optimization after prologue / epilogue threading.
- -fbtr-bb-exclusive
- When performing branch target register load optimization, don't reuse branch target registers within any basic block.
- -fstack-protector
- Emit extra code to check for buffer overflows, such as stack smashing attacks. This is done by adding a guard variable to functions with vulnerable objects. This includes functions that call "alloca", and functions with buffers larger than 8 bytes. The guards are initialized when a function is entered and then checked when the function exits. If a guard check fails, an error message is printed and the program exits.
- -fstack-protector-all
- Like -fstack-protector except that all functions are protected.
- -fsection-anchors
-
Try to reduce the number of symbolic address calculations by using shared "anchor" symbols to address nearby objects. This transformation can help to reduce the number of GOT entries and GOT accesses on some targets.
static int a, b, c;
int foo (void) { return a + b + c; }
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}
- --param name=value
-
In some places, GCC uses various constants to control the amount of optimization that is done. For example, GCC does not inline functions that contain more than a certain number of instructions. You can control some of these constants on the command line using the --param option.
- predictable-branch-outcome
- When branch is predicted to be taken with probability lower than this threshold (in percent), then it is considered well predictable. The default is 10.
- max-crossjump-edges
- The maximum number of incoming edges to consider for cross-jumping. The algorithm used by -fcrossjumping is O(N^2) in the number of edges incoming to each block. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in executable size.
- min-crossjump-insns
- The minimum number of instructions that must be matched at the end of two blocks before cross-jumping is performed on them. This value is ignored in the case where all instructions in the block being cross-jumped from are matched. The default value is 5.
- max-grow-copy-bb-insns
- The maximum code size expansion factor when copying basic blocks instead of jumping. The expansion is relative to a jump instruction. The default value is 8.
- max-goto-duplication-insns
- The maximum number of instructions to duplicate to a block that jumps to a computed goto. To avoid O(N^2) behavior in a number of passes, GCC factors computed gotos early in the compilation process, and unfactors them as late as possible. Only computed jumps at the end of a basic blocks with no more than max-goto-duplication-insns are unfactored. The default value is 8.
- max-delay-slot-insn-search
- The maximum number of instructions to consider when looking for an instruction to fill a delay slot. If more than this arbitrary number of instructions are searched, the time savings from filling the delay slot are minimal, so stop searching. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in execution time.
- max-delay-slot-live-search
- When trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compilation time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph.
- max-gcse-memory
- The approximate maximum amount of memory that can be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization is not done.
- max-gcse-insertion-ratio
- If the ratio of expression insertions to deletions is larger than this value for any expression, then RTL PRE inserts or removes the expression and thus leaves partially redundant computations in the instruction stream. The default value is 20.
- max-pending-list-length
- The maximum number of pending dependencies scheduling allows before flushing the current state and starting over. Large functions with few branches or calls can create excessively large lists which needlessly consume memory and resources.
- max-modulo-backtrack-attempts
- The maximum number of backtrack attempts the scheduler should make when modulo scheduling a loop. Larger values can exponentially increase compilation time.
- max-inline-insns-single
- Several parameters control the tree inliner used in GCC. This number sets the maximum number of instructions (counted in GCC's internal representation) in a single function that the tree inliner considers for inlining. This only affects functions declared inline and methods implemented in a class declaration (C++). The default value is 400.
- max-inline-insns-auto
- When you use -finline-functions (included in -O3), a lot of functions that would otherwise not be considered for inlining by the compiler are investigated. To those functions, a different (more restrictive) limit compared to functions declared inline can be applied. The default value is 40.
- inline-min-speedup
- When estimated performance improvement of caller + callee runtime exceeds this threshold (in precent), the function can be inlined regardless the limit on --param max-inline-insns-single and --param max-inline-insns-auto.
- large-function-insns
- The limit specifying really large functions. For functions larger than this limit after inlining, inlining is constrained by --param large-function-growth. This parameter is useful primarily to avoid extreme compilation time caused by non-linear algorithms used by the back end. The default value is 2700.
- large-function-growth
- Specifies maximal growth of large function caused by inlining in percents. The default value is 100 which limits large function growth to 2.0 times the original size.
- large-unit-insns
- The limit specifying large translation unit. Growth caused by inlining of units larger than this limit is limited by --param inline-unit-growth. For small units this might be too tight. For example, consider a unit consisting of function A that is inline and B that just calls A three times. If B is small relative to A, the growth of unit is 300\% and yet such inlining is very sane. For very large units consisting of small inlineable functions, however, the overall unit growth limit is needed to avoid exponential explosion of code size. Thus for smaller units, the size is increased to --param large-unit-insns before applying --param inline-unit-growth. The default is 10000.
- inline-unit-growth
- Specifies maximal overall growth of the compilation unit caused by inlining. The default value is 30 which limits unit growth to 1.3 times the original size.
- ipcp-unit-growth
- Specifies maximal overall growth of the compilation unit caused by interprocedural constant propagation. The default value is 10 which limits unit growth to 1.1 times the original size.
- large-stack-frame
- The limit specifying large stack frames. While inlining the algorithm is trying to not grow past this limit too much. The default value is 256 bytes.
- large-stack-frame-growth
- Specifies maximal growth of large stack frames caused by inlining in percents. The default value is 1000 which limits large stack frame growth to 11 times the original size.
- max-inline-insns-recursive
- max-inline-insns-recursive-auto
-
Specifies the maximum number of instructions an out-of-line copy of a self-recursive inline function can grow into by performing recursive inlining.
- max-inline-recursive-depth
- max-inline-recursive-depth-auto
-
Specifies the maximum recursion depth used for recursive inlining.
- min-inline-recursive-probability
-
Recursive inlining is profitable only for function having deep recursion in average and can hurt for function having little recursion depth by increasing the prologue size or complexity of function body to other optimizers.
- early-inlining-insns
- Specify growth that the early inliner can make. In effect it increases the amount of inlining for code having a large abstraction penalty. The default value is 10.
- max-early-inliner-iterations
- max-early-inliner-iterations
- Limit of iterations of the early inliner. This basically bounds the number of nested indirect calls the early inliner can resolve. Deeper chains are still handled by late inlining.
- comdat-sharing-probability
- comdat-sharing-probability
- Probability (in percent) that C++ inline function with comdat visibility are shared across multiple compilation units. The default value is 20.
- min-vect-loop-bound
- The minimum number of iterations under which loops are not vectorized when -ftree-vectorize is used. The number of iterations after vectorization needs to be greater than the value specified by this option to allow vectorization. The default value is 0.
- gcse-cost-distance-ratio
- Scaling factor in calculation of maximum distance an expression can be moved by GCSE optimizations. This is currently supported only in the code hoisting pass. The bigger the ratio, the more aggressive code hoisting is with simple expressions, i.e., the expressions that have cost less than gcse-unrestricted-cost. Specifying 0 disables hoisting of simple expressions. The default value is 10.
- gcse-unrestricted-cost
- Cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations do not constrain the distance an expression can travel. This is currently supported only in the code hoisting pass. The lesser the cost, the more aggressive code hoisting is. Specifying 0 allows all expressions to travel unrestricted distances. The default value is 3.
- max-hoist-depth
- The depth of search in the dominator tree for expressions to hoist. This is used to avoid quadratic behavior in hoisting algorithm. The value of 0 does not limit on the search, but may slow down compilation of huge functions. The default value is 30.
- max-tail-merge-comparisons
- The maximum amount of similar bbs to compare a bb with. This is used to avoid quadratic behavior in tree tail merging. The default value is 10.
- max-tail-merge-iterations
- The maximum amount of iterations of the pass over the function. This is used to limit compilation time in tree tail merging. The default value is 2.
- max-unrolled-insns
- The maximum number of instructions that a loop may have to be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled.
- max-average-unrolled-insns
- The maximum number of instructions biased by probabilities of their execution that a loop may have to be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled.
- max-unroll-times
- The maximum number of unrollings of a single loop.
- max-peeled-insns
- The maximum number of instructions that a loop may have to be peeled. If a loop is peeled, this parameter also determines how many times the loop code is peeled.
- max-peel-times
- The maximum number of peelings of a single loop.
- max-peel-branches
- The maximum number of branches on the hot path through the peeled sequence.
- max-completely-peeled-insns
- The maximum number of insns of a completely peeled loop.
- max-completely-peel-times
- The maximum number of iterations of a loop to be suitable for complete peeling.
- max-completely-peel-loop-nest-depth
- The maximum depth of a loop nest suitable for complete peeling.
- max-unswitch-insns
- The maximum number of insns of an unswitched loop.
- max-unswitch-level
- The maximum number of branches unswitched in a single loop.
- lim-expensive
- The minimum cost of an expensive expression in the loop invariant motion.
- iv-consider-all-candidates-bound
- Bound on number of candidates for induction variables, below which all candidates are considered for each use in induction variable optimizations. If there are more candidates than this, only the most relevant ones are considered to avoid quadratic time complexity.
- iv-max-considered-uses
- The induction variable optimizations give up on loops that contain more induction variable uses.
- iv-always-prune-cand-set-bound
- If the number of candidates in the set is smaller than this value, always try to remove unnecessary ivs from the set when adding a new one.
- scev-max-expr-size
- Bound on size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer.
- scev-max-expr-complexity
- Bound on the complexity of the expressions in the scalar evolutions analyzer. Complex expressions slow the analyzer.
- omega-max-vars
- The maximum number of variables in an Omega constraint system. The default value is 128.
- omega-max-geqs
- The maximum number of inequalities in an Omega constraint system. The default value is 256.
- omega-max-eqs
- The maximum number of equalities in an Omega constraint system. The default value is 128.
- omega-max-wild-cards
- The maximum number of wildcard variables that the Omega solver is able to insert. The default value is 18.
- omega-hash-table-size
- The size of the hash table in the Omega solver. The default value is 550.
- omega-max-keys
- The maximal number of keys used by the Omega solver. The default value is 500.
- omega-eliminate-redundant-constraints
- When set to 1, use expensive methods to eliminate all redundant constraints. The default value is 0.
- vect-max-version-for-alignment-checks
- The maximum number of run-time checks that can be performed when doing loop versioning for alignment in the vectorizer. See option -ftree-vect-loop-version for more information.
- vect-max-version-for-alias-checks
- The maximum number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer. See option -ftree-vect-loop-version for more information.
- max-iterations-to-track
- The maximum number of iterations of a loop the brute-force algorithm for analysis of the number of iterations of the loop tries to evaluate.
- hot-bb-count-ws-permille
- A basic block profile count is considered hot if it contributes to the given permillage (i.e. 0...1000) of the entire profiled execution.
- hot-bb-frequency-fraction
- Select fraction of the entry block frequency of executions of basic block in function given basic block needs to have to be considered hot.
- max-predicted-iterations
- The maximum number of loop iterations we predict statically. This is useful in cases where a function contains a single loop with known bound and another loop with unknown bound. The known number of iterations is predicted correctly, while the unknown number of iterations average to roughly 10. This means that the loop without bounds appears artificially cold relative to the other one.
- align-threshold
- Select fraction of the maximal frequency of executions of a basic block in a function to align the basic block.
- align-loop-iterations
- A loop expected to iterate at least the selected number of iterations is aligned.
- tracer-dynamic-coverage
- tracer-dynamic-coverage-feedback
-
This value is used to limit superblock formation once the given percentage of executed instructions is covered. This limits unnecessary code size expansion.
- tracer-max-code-growth
- Stop tail duplication once code growth has reached given percentage. This is a rather artificial limit, as most of the duplicates are eliminated later in cross jumping, so it may be set to much higher values than is the desired code growth.
- tracer-min-branch-ratio
- Stop reverse growth when the reverse probability of best edge is less than this threshold (in percent).
- tracer-min-branch-ratio
- tracer-min-branch-ratio-feedback
-
Stop forward growth if the best edge has probability lower than this threshold.
- max-cse-path-length
- The maximum number of basic blocks on path that CSE considers. The default is 10.
- max-cse-insns
- The maximum number of instructions CSE processes before flushing. The default is 1000.
- ggc-min-expand
-
GCC uses a garbage collector to manage its own memory allocation. This parameter specifies the minimum percentage by which the garbage collector's heap should be allowed to expand between collections. Tuning this may improve compilation speed; it has no effect on code generation.
- ggc-min-heapsize
-
Minimum size of the garbage collector's heap before it begins bothering to collect garbage. The first collection occurs after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. Again, tuning this may improve compilation speed, and has no effect on code generation.
- max-reload-search-insns
- The maximum number of instruction reload should look backward for equivalent register. Increasing values mean more aggressive optimization, making the compilation time increase with probably slightly better performance. The default value is 100.
- max-cselib-memory-locations
- The maximum number of memory locations cselib should take into account. Increasing values mean more aggressive optimization, making the compilation time increase with probably slightly better performance. The default value is 500.
- reorder-blocks-duplicate
- reorder-blocks-duplicate-feedback
-
Used by the basic block reordering pass to decide whether to use unconditional branch or duplicate the code on its destination. Code is duplicated when its estimated size is smaller than this value multiplied by the estimated size of unconditional jump in the hot spots of the program.
- max-sched-ready-insns
- The maximum number of instructions ready to be issued the scheduler should consider at any given time during the first scheduling pass. Increasing values mean more thorough searches, making the compilation time increase with probably little benefit. The default value is 100.
- max-sched-region-blocks
- The maximum number of blocks in a region to be considered for interblock scheduling. The default value is 10.
- max-pipeline-region-blocks
- The maximum number of blocks in a region to be considered for pipelining in the selective scheduler. The default value is 15.
- max-sched-region-insns
- The maximum number of insns in a region to be considered for interblock scheduling. The default value is 100.
- max-pipeline-region-insns
- The maximum number of insns in a region to be considered for pipelining in the selective scheduler. The default value is 200.
- min-spec-prob
- The minimum probability (in percents) of reaching a source block for interblock speculative scheduling. The default value is 40.
- max-sched-extend-regions-iters
- The maximum number of iterations through CFG to extend regions. A value of 0 (the default) disables region extensions.
- max-sched-insn-conflict-delay
- The maximum conflict delay for an insn to be considered for speculative motion. The default value is 3.
- sched-spec-prob-cutoff
- The minimal probability of speculation success (in percents), so that speculative insns are scheduled. The default value is 40.
- sched-spec-state-edge-prob-cutoff
- The minimum probability an edge must have for the scheduler to save its state across it. The default value is 10.
- sched-mem-true-dep-cost
- Minimal distance (in CPU cycles) between store and load targeting same memory locations. The default value is 1.
- selsched-max-lookahead
- The maximum size of the lookahead window of selective scheduling. It is a depth of search for available instructions. The default value is 50.
- selsched-max-sched-times
- The maximum number of times that an instruction is scheduled during selective scheduling. This is the limit on the number of iterations through which the instruction may be pipelined. The default value is 2.
- selsched-max-insns-to-rename
- The maximum number of best instructions in the ready list that are considered for renaming in the selective scheduler. The default value is 2.
- sms-min-sc
- The minimum value of stage count that swing modulo scheduler generates. The default value is 2.
- max-last-value-rtl
- The maximum size measured as number of RTLs that can be recorded in an expression in combiner for a pseudo register as last known value of that register. The default is 10000.
- integer-share-limit
- Small integer constants can use a shared data structure, reducing the compiler's memory usage and increasing its speed. This sets the maximum value of a shared integer constant. The default value is 256.
- ssp-buffer-size
- The minimum size of buffers (i.e. arrays) that receive stack smashing protection when -fstack-protection is used.
- max-jump-thread-duplication-stmts
- Maximum number of statements allowed in a block that needs to be duplicated when threading jumps.
- max-fields-for-field-sensitive
- Maximum number of fields in a structure treated in a field sensitive manner during pointer analysis. The default is zero for -O0 and -O1, and 100 for -Os, -O2, and -O3.
- prefetch-latency
- Estimate on average number of instructions that are executed before prefetch finishes. The distance prefetched ahead is proportional to this constant. Increasing this number may also lead to less streams being prefetched (see simultaneous-prefetches).
- simultaneous-prefetches
- Maximum number of prefetches that can run at the same time.
- l1-cache-line-size
- The size of cache line in L1 cache, in bytes.
- l1-cache-size
- The size of L1 cache, in kilobytes.
- l2-cache-size
- The size of L2 cache, in kilobytes.
- min-insn-to-prefetch-ratio
- The minimum ratio between the number of instructions and the number of prefetches to enable prefetching in a loop.
- prefetch-min-insn-to-mem-ratio
- The minimum ratio between the number of instructions and the number of memory references to enable prefetching in a loop.
- use-canonical-types
- Whether the compiler should use the "canonical" type system. By default, this should always be 1, which uses a more efficient internal mechanism for comparing types in C++ and Objective-C++. However, if bugs in the canonical type system are causing compilation failures, set this value to 0 to disable canonical types.
- switch-conversion-max-branch-ratio
- Switch initialization conversion refuses to create arrays that are bigger than switch-conversion-max-branch-ratio times the number of branches in the switch.
- max-partial-antic-length
- Maximum length of the partial antic set computed during the tree partial redundancy elimination optimization ( -ftree-pre) when optimizing at -O3 and above. For some sorts of source code the enhanced partial redundancy elimination optimization can run away, consuming all of the memory available on the host machine. This parameter sets a limit on the length of the sets that are computed, which prevents the runaway behavior. Setting a value of 0 for this parameter allows an unlimited set length.
- sccvn-max-scc-size
- Maximum size of a strongly connected component (SCC) during SCCVN processing. If this limit is hit, SCCVN processing for the whole function is not done and optimizations depending on it are disabled. The default maximum SCC size is 10000.
- sccvn-max-alias-queries-per-access
- Maximum number of alias-oracle queries we perform when looking for redundancies for loads and stores. If this limit is hit the search is aborted and the load or store is not considered redundant. The number of queries is algorithmically limited to the number of stores on all paths from the load to the function entry. The default maxmimum number of queries is 1000.
- ira-max-loops-num
- IRA uses regional register allocation by default. If a function contains more loops than the number given by this parameter, only at most the given number of the most frequently-executed loops form regions for regional register allocation. The default value of the parameter is 100.
- ira-max-conflict-table-size
- Although IRA uses a sophisticated algorithm to compress the conflict table, the table can still require excessive amounts of memory for huge functions. If the conflict table for a function could be more than the size in MB given by this parameter, the register allocator instead uses a faster, simpler, and lower-quality algorithm that does not require building a pseudo-register conflict table. The default value of the parameter is 2000.
- ira-loop-reserved-regs
- IRA can be used to evaluate more accurate register pressure in loops for decisions to move loop invariants (see -O3). The number of available registers reserved for some other purposes is given by this parameter. The default value of the parameter is 2, which is the minimal number of registers needed by typical instructions. This value is the best found from numerous experiments.
- loop-invariant-max-bbs-in-loop
- Loop invariant motion can be very expensive, both in compilation time and in amount of needed compile-time memory, with very large loops. Loops with more basic blocks than this parameter won't have loop invariant motion optimization performed on them. The default value of the parameter is 1000 for -O1 and 10000 for -O2 and above.
- loop-max-datarefs-for-datadeps
- Building data dapendencies is expensive for very large loops. This parameter limits the number of data references in loops that are considered for data dependence analysis. These large loops are no handled by the optimizations using loop data dependencies. The default value is 1000.
- max-vartrack-size
- Sets a maximum number of hash table slots to use during variable tracking dataflow analysis of any function. If this limit is exceeded with variable tracking at assignments enabled, analysis for that function is retried without it, after removing all debug insns from the function. If the limit is exceeded even without debug insns, var tracking analysis is completely disabled for the function. Setting the parameter to zero makes it unlimited.
- max-vartrack-expr-depth
- Sets a maximum number of recursion levels when attempting to map variable names or debug temporaries to value expressions. This trades compilation time for more complete debug information. If this is set too low, value expressions that are available and could be represented in debug information may end up not being used; setting this higher may enable the compiler to find more complex debug expressions, but compile time and memory use may grow. The default is 12.
- min-nondebug-insn-uid
- Use uids starting at this parameter for nondebug insns. The range below the parameter is reserved exclusively for debug insns created by -fvar-tracking-assignments, but debug insns may get (non-overlapping) uids above it if the reserved range is exhausted.
- ipa-sra-ptr-growth-factor
- IPA-SRA replaces a pointer to an aggregate with one or more new parameters only when their cumulative size is less or equal to ipa-sra-ptr-growth-factor times the size of the original pointer parameter.
- tm-max-aggregate-size
- When making copies of thread-local variables in a transaction, this parameter specifies the size in bytes after which variables are saved with the logging functions as opposed to save/restore code sequence pairs. This option only applies when using -fgnu-tm.
- graphite-max-nb-scop-params
- To avoid exponential effects in the Graphite loop transforms, the number of parameters in a Static Control Part (SCoP) is bounded. The default value is 10 parameters. A variable whose value is unknown at compilation time and defined outside a SCoP is a parameter of the SCoP.
- graphite-max-bbs-per-function
- To avoid exponential effects in the detection of SCoPs, the size of the functions analyzed by Graphite is bounded. The default value is 100 basic blocks.
- loop-block-tile-size
- Loop blocking or strip mining transforms, enabled with -floop-block or -floop-strip-mine, strip mine each loop in the loop nest by a given number of iterations. The strip length can be changed using the loop-block-tile-size parameter. The default value is 51 iterations.
- ipa-cp-value-list-size
- IPA-CP attempts to track all possible values and types passed to a function's parameter in order to propagate them and perform devirtualization. ipa-cp-value-list-size is the maximum number of values and types it stores per one formal parameter of a function.
- lto-partitions
- Specify desired number of partitions produced during WHOPR compilation. The number of partitions should exceed the number of CPUs used for compilation. The default value is 32.
- lto-minpartition
- Size of minimal partition for WHOPR (in estimated instructions). This prevents expenses of splitting very small programs into too many partitions.
- cxx-max-namespaces-for-diagnostic-help
- The maximum number of namespaces to consult for suggestions when C++ name lookup fails for an identifier. The default is 1000.
- sink-frequency-threshold
- The maximum relative execution frequency (in percents) of the target block relative to a statement's original block to allow statement sinking of a statement. Larger numbers result in more aggressive statement sinking. The default value is 75. A small positive adjustment is applied for statements with memory operands as those are even more profitable so sink.
- max-stores-to-sink
- The maximum number of conditional stores paires that can be sunk. Set to 0 if either vectorization ( -ftree-vectorize) or if-conversion ( -ftree-loop-if-convert) is disabled. The default is 2.
- allow-load-data-races
- Allow optimizers to introduce new data races on loads. Set to 1 to allow, otherwise to 0. This option is enabled by default unless implicitly set by the -fmemory-model= option.
- allow-store-data-races
- Allow optimizers to introduce new data races on stores. Set to 1 to allow, otherwise to 0. This option is enabled by default unless implicitly set by the -fmemory-model= option.
- allow-packed-load-data-races
- Allow optimizers to introduce new data races on packed data loads. Set to 1 to allow, otherwise to 0. This option is enabled by default unless implicitly set by the -fmemory-model= option.
- allow-packed-store-data-races
- Allow optimizers to introduce new data races on packed data stores. Set to 1 to allow, otherwise to 0. This option is enabled by default unless implicitly set by the -fmemory-model= option.
- case-values-threshold
- The smallest number of different values for which it is best to use a jump-table instead of a tree of conditional branches. If the value is 0, use the default for the machine. The default is 0.
- tree-reassoc-width
- Set the maximum number of instructions executed in parallel in reassociated tree. This parameter overrides target dependent heuristics used by default if has non zero value.
- sched-pressure-algorithm
-
Choose between the two available implementations of -fsched-pressure. Algorithm 1 is the original implementation and is the more likely to prevent instructions from being reordered. Algorithm 2 was designed to be a compromise between the relatively conservative approach taken by algorithm 1 and the rather aggressive approach taken by the default scheduler. It relies more heavily on having a regular register file and accurate register pressure classes. See haifa-sched.c in the GCC sources for more details.
- max-slsr-cand-scan
- Set the maximum number of existing candidates that will be considered when seeking a basis for a new straight-line strength reduction candidate.
Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C source file before actual compilation.If you use the -E option, nothing is done except preprocessing. Some of these options make sense only together with -E because they cause the preprocessor output to be unsuitable for actual compilation.
- -Wp,option
- You can use -Wp,option to bypass the compiler driver and pass option directly through to the preprocessor. If option contains commas, it is split into multiple options at the commas. However, many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses this phase. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and let the driver handle the options instead.
- -Xpreprocessor option
-
Pass option as an option to the preprocessor. You can use this to supply system-specific preprocessor options that GCC does not recognize.
- -no-integrated-cpp
- Perform preprocessing as a separate pass before compilation. By default, GCC performs preprocessing as an integrated part of input tokenization and parsing. If this option is provided, the appropriate language front end ( cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively) is instead invoked twice, once for preprocessing only and once for actual compilation of the preprocessed input. This option may be useful in conjunction with the -B or -wrapper options to specify an alternate preprocessor or perform additional processing of the program source between normal preprocessing and compilation.
- -D name
- Predefine name as a macro, with definition 1.
- -D name=definition
-
The contents of definition are tokenized and processed as if they appeared during translation phase three in a #define directive. In particular, the definition will be truncated by embedded newline characters.
- -U name
- Cancel any previous definition of name, either built in or provided with a -D option.
- -undef
- Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined.
- -I dir
- Add the directory dir to the list of directories to be searched for header files. Directories named by -I are searched before the standard system include directories. If the directory dir is a standard system include directory, the option is ignored to ensure that the default search order for system directories and the special treatment of system headers are not defeated . If dir begins with "=", then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.
- -o file
- Write output to file. This is the same as specifying file as the second non-option argument to cpp. gcc has a different interpretation of a second non-option argument, so you must use -o to specify the output file.
- -Wall
- Turns on all optional warnings which are desirable for normal code. At present this is -Wcomment, -Wtrigraphs, -Wmultichar and a warning about integer promotion causing a change of sign in "#if" expressions. Note that many of the preprocessor's warnings are on by default and have no options to control them.
- -Wcomment
- -Wcomments
- Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline appears in a // comment. (Both forms have the same effect.)
- -Wtrigraphs
-
Most trigraphs in comments cannot affect the meaning of the program. However, a trigraph that would form an escaped newline ( ??/ at the end of a line) can, by changing where the comment begins or ends. Therefore, only trigraphs that would form escaped newlines produce warnings inside a comment.
- -Wtraditional
- Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C constructs that have no traditional C equivalent, and problematic constructs which should be avoided.
- -Wundef
- Warn whenever an identifier which is not a macro is encountered in an #if directive, outside of defined. Such identifiers are replaced with zero.
- -Wunused-macros
-
Warn about macros defined in the main file that are unused. A macro is used if it is expanded or tested for existence at least once. The preprocessor will also warn if the macro has not been used at the time it is redefined or undefined.
#if defined the_macro_causing_the_warning
#endif
- -Wendif-labels
-
Warn whenever an #else or an #endif are followed by text. This usually happens in code of the form
#if FOO
...
#else FOO
...
#endif FOO
- -Werror
- Make all warnings into hard errors. Source code which triggers warnings will be rejected.
- -Wsystem-headers
- Issue warnings for code in system headers. These are normally unhelpful in finding bugs in your own code, therefore suppressed. If you are responsible for the system library, you may want to see them.
- -w
- Suppress all warnings, including those which GNU CPP issues by default.
- -pedantic
- Issue all the mandatory diagnostics listed in the C standard. Some of them are left out by default, since they trigger frequently on harmless code.
- -pedantic-errors
- Issue all the mandatory diagnostics, and make all mandatory diagnostics into errors. This includes mandatory diagnostics that GCC issues without -pedantic but treats as warnings.
- -M
-
Instead of outputting the result of preprocessing, output a rule suitable for make describing the dependencies of the main source file. The preprocessor outputs one make rule containing the object file name for that source file, a colon, and the names of all the included files, including those coming from -include or -imacros command line options.
- -MM
-
Like -M but do not mention header files that are found in system header directories, nor header files that are included, directly or indirectly, from such a header.
- -MF file
-
When used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given the preprocessor sends the rules to the same place it would have sent preprocessed output.
- -MG
-
In conjunction with an option such as -M requesting dependency generation, -MG assumes missing header files are generated files and adds them to the dependency list without raising an error. The dependency filename is taken directly from the "#include" directive without prepending any path. -MG also suppresses preprocessed output, as a missing header file renders this useless.
- -MP
-
This option instructs CPP to add a phony target for each dependency other than the main file, causing each to depend on nothing. These dummy rules work around errors make gives if you remove header files without updating the Makefile to match.
test.o: test.c test.h
test.h:
- -MT target
-
Change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input file, deletes any directory components and any file suffix such as .c, and appends the platform's usual object suffix. The result is the target.
$(objpfx)foo.o: foo.c
- -MQ target
-
Same as -MT, but it quotes any characters which are special to Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
- -MD
-
-MD is equivalent to -M -MF file, except that -E is not implied. The driver determines file based on whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d, otherwise it takes the name of the input file, removes any directory components and suffix, and applies a .d suffix.
- -MMD
- Like -MD except mention only user header files, not system header files.
- -fpch-deps
- When using precompiled headers, this flag will cause the dependency-output flags to also list the files from the precompiled header's dependencies. If not specified only the precompiled header would be listed and not the files that were used to create it because those files are not consulted when a precompiled header is used.
- -fpch-preprocess
-
This option allows use of a precompiled header together with -E. It inserts a special "#pragma", "#pragma GCC pch_preprocess " filename"" in the output to mark the place where the precompiled header was found, and its filename. When -fpreprocessed is in use, GCC recognizes this "#pragma" and loads the PCH.
- -x c
- -x c++
- -x objective-c
- -x assembler-with-cpp
-
Specify the source language: C, C++, Objective-C, or assembly. This has nothing to do with standards conformance or extensions; it merely selects which base syntax to expect. If you give none of these options, cpp will deduce the language from the extension of the source file: .c, .cc, .m, or .S. Some other common extensions for C++ and assembly are also recognized. If cpp does not recognize the extension, it will treat the file as C; this is the most generic mode.
- -std=standard
- -ansi
-
Specify the standard to which the code should conform. Currently CPP knows about C and C++ standards; others may be added in the future.
- "c90"
- "c89"
- "iso9899:1990"
-
The ISO C standard from 1990. c90 is the customary shorthand for this version of the standard.
- "iso9899:199409"
- The 1990 C standard, as amended in 1994.
- "iso9899:1999"
- "c99"
- "iso9899:199x"
- "c9x"
- The revised ISO C standard, published in December 1999. Before publication, this was known as C9X.
- "iso9899:2011"
- "c11"
- "c1x"
- The revised ISO C standard, published in December 2011. Before publication, this was known as C1X.
- "gnu90"
- "gnu89"
- The 1990 C standard plus GNU extensions. This is the default.
- "gnu99"
- "gnu9x"
- The 1999 C standard plus GNU extensions.
- "gnu11"
- "gnu1x"
- The 2011 C standard plus GNU extensions.
- "c++98"
- The 1998 ISO C++ standard plus amendments.
- "gnu++98"
- The same as -std=c++98 plus GNU extensions. This is the default for C++ code.
- -I-
-
Split the include path. Any directories specified with -I options before -I- are searched only for headers requested with "#include " file""; they are not searched for "#include < file>". If additional directories are specified with -I options after the -I-, those directories are searched for all #include directives.
- -nostdinc
- Do not search the standard system directories for header files. Only the directories you have specified with -I options (and the directory of the current file, if appropriate) are searched.
- -nostdinc++
- Do not search for header files in the C++-specific standard directories, but do still search the other standard directories. (This option is used when building the C++ library.)
- -include file
-
Process file as if "#include "file"" appeared as the first line of the primary source file. However, the first directory searched for file is the preprocessor's working directory instead of the directory containing the main source file. If not found there, it is searched for in the remainder of the "#include "..."" search chain as normal.
- -imacros file
-
Exactly like -include, except that any output produced by scanning file is thrown away. Macros it defines remain defined. This allows you to acquire all the macros from a header without also processing its declarations.
- -idirafter dir
- Search dir for header files, but do it after all directories specified with -I and the standard system directories have been exhausted. dir is treated as a system include directory. If dir begins with "=", then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.
- -iprefix prefix
- Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you should include the final /.
- -iwithprefix dir
- -iwithprefixbefore dir
- Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. -iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it where -idirafter would.
- -isysroot dir
- This option is like the --sysroot option, but applies only to header files (except for Darwin targets, where it applies to both header files and libraries). See the --sysroot option for more information.
- -imultilib dir
- Use dir as a subdirectory of the directory containing target-specific C++ headers.
- -isystem dir
- Search dir for header files, after all directories specified by -I but before the standard system directories. Mark it as a system directory, so that it gets the same special treatment as is applied to the standard system directories. If dir begins with "=", then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.
- -cxx-isystem dir
- Search dir for C++ header files, after all directories specified by -I but before the standard system directories. Mark it as a system directory, so that it gets the same special treatment as is applied to the standard system directories.
- -iquote dir
- Search dir only for header files requested with "#include " file""; they are not searched for "#include < file>", before all directories specified by -I and before the standard system directories. If dir begins with "=", then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.
- -fdirectives-only
-
When preprocessing, handle directives, but do not expand macros.
- -iremap src:dst
- Replace the prefix src in __FILE__ with dst at expansion time. This option can be specified more than once. Processing stops at the first match.
- -fdollars-in-identifiers
- Accept $ in identifiers.
- -fextended-identifiers
- Accept universal character names in identifiers. This option is experimental; in a future version of GCC, it will be enabled by default for C99 and C++.
- -fno-canonical-system-headers
- When preprocessing, do not shorten system header paths with canonicalization.
- -fpreprocessed
-
Indicate to the preprocessor that the input file has already been preprocessed. This suppresses things like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes comments, so that you can pass a file preprocessed with -C to the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends.
- -ftabstop=width
- Set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is ignored. The default is 8.
- -fdebug-cpp
-
This option is only useful for debugging GCC. When used with -E, dumps debugging information about location maps. Every token in the output is preceded by the dump of the map its location belongs to. The dump of the map holding the location of a token would be:
{"P":F</file/path>;"F":F</includer/path>;"L":<line_num>;"C":<col_num>;"S":<system_header_p>;"M":<map_address>;"E":<macro_expansion_p>,"loc":<location>}
- -ftrack-macro-expansion[=level]
-
Track locations of tokens across macro expansions. This allows the compiler to emit diagnostic about the current macro expansion stack when a compilation error occurs in a macro expansion. Using this option makes the preprocessor and the compiler consume more memory. The level parameter can be used to choose the level of precision of token location tracking thus decreasing the memory consumption if necessary. Value 0 of level de-activates this option just as if no -ftrack-macro-expansion was present on the command line. Value 1 tracks tokens locations in a degraded mode for the sake of minimal memory overhead. In this mode all tokens resulting from the expansion of an argument of a function-like macro have the same location. Value 2 tracks tokens locations completely. This value is the most memory hungry. When this option is given no argument, the default parameter value is 2.
- -fexec-charset=charset
- Set the execution character set, used for string and character constants. The default is UTF-8. charset can be any encoding supported by the system's "iconv" library routine.
- -fwide-exec-charset=charset
- Set the wide execution character set, used for wide string and character constants. The default is UTF-32 or UTF-16, whichever corresponds to the width of "wchar_t". As with -fexec-charset, charset can be any encoding supported by the system's "iconv" library routine; however, you will have problems with encodings that do not fit exactly in "wchar_t".
- -finput-charset=charset
- Set the input character set, used for translation from the character set of the input file to the source character set used by GCC. If the locale does not specify, or GCC cannot get this information from the locale, the default is UTF-8. This can be overridden by either the locale or this command line option. Currently the command line option takes precedence if there's a conflict. charset can be any encoding supported by the system's "iconv" library routine.
- -fworking-directory
- Enable generation of linemarkers in the preprocessor output that will let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preprocessor will emit, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC will use this directory, when it's present in the preprocessed input, as the directory emitted as the current working directory in some debugging information formats. This option is implicitly enabled if debugging information is enabled, but this can be inhibited with the negated form -fno-working-directory. If the -P flag is present in the command line, this option has no effect, since no "#line" directives are emitted whatsoever.
- -fno-show-column
- Do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that does not understand the column numbers, such as dejagnu.
- -A predicate=answer
- Make an assertion with the predicate predicate and answer answer. This form is preferred to the older form -A predicate(answer), which is still supported, because it does not use shell special characters.
- -A -predicate=answer
- Cancel an assertion with the predicate predicate and answer answer.
- -dCHARS
- CHARS is a sequence of one or more of the following characters, and must not be preceded by a space. Other characters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify characters whose behavior conflicts, the result is undefined.
- M
-
Instead of the normal output, generate a list of #define directives for all the macros defined during the execution of the preprocessor, including predefined macros. This gives you a way of finding out what is predefined in your version of the preprocessor. Assuming you have no file foo.h, the command
touch foo.h; cpp -dM foo.h
- D
- Like M except in two respects: it does not include the predefined macros, and it outputs both the #define directives and the result of preprocessing. Both kinds of output go to the standard output file.
- N
- Like D, but emit only the macro names, not their expansions.
- I
- Output #include directives in addition to the result of preprocessing.
- U
- Like D except that only macros that are expanded, or whose definedness is tested in preprocessor directives, are output; the output is delayed until the use or test of the macro; and #undef directives are also output for macros tested but undefined at the time.
- -P
- Inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the preprocessor on something that is not C code, and will be sent to a program which might be confused by the linemarkers.
- -C
-
Do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive.
- -CC
-
Do not discard comments, including during macro expansion. This is like -C, except that comments contained within macros are also passed through to the output file where the macro is expanded.
- -traditional-cpp
- Try to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO C preprocessors.
- -trigraphs
-
Process trigraph sequences. These are three-character sequences, all starting with ??, that are defined by ISO C to stand for single characters. For example, ??/ stands for \, so '??/n' is a character constant for a newline. By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the -std and -ansi options.
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
- -remap
- Enable special code to work around file systems which only permit very short file names, such as MS-DOS.
- --help
- --target-help
- Print text describing all the command line options instead of preprocessing anything.
- -v
- Verbose mode. Print out GNU CPP's version number at the beginning of execution, and report the final form of the include path.
- -H
- Print the name of each header file used, in addition to other normal activities. Each name is indented to show how deep in the #include stack it is. Precompiled header files are also printed, even if they are found to be invalid; an invalid precompiled header file is printed with ...x and a valid one with ...! .
- -version
- --version
- Print out GNU CPP's version number. With one dash, proceed to preprocess as normal. With two dashes, exit immediately.
Passing Options to the Assembler
You can pass options to the assembler.- -Wa,option
- Pass option as an option to the assembler. If option contains commas, it is split into multiple options at the commas.
- -Xassembler option
-
Pass option as an option to the assembler. You can use this to supply system-specific assembler options that GCC does not recognize.
Options for Linking
These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step.- object-file-name
- A file name that does not end in a special recognized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file contents.) If linking is done, these object files are used as input to the linker.
- -c
- -S
- -E
- If any of these options is used, then the linker is not run, and object file names should not be used as arguments.
- -llibrary
- -l library
-
Search the library named library when linking. (The second alternative with the library as a separate argument is only for POSIX compliance and is not recommended.)
- -lobjc
- You need this special case of the -l option in order to link an Objective-C or Objective-C++ program.
- -nostartfiles
- Do not use the standard system startup files when linking. The standard system libraries are used normally, unless -nostdlib or -nodefaultlibs is used.
- -nodefaultlibs
-
Do not use the standard system libraries when linking. Only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as "-static-libgcc" or "-shared-libgcc", are ignored. The standard startup files are used normally, unless -nostartfiles is used.
- -nostdlib
-
Do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as "-static-libgcc" or "-shared-libgcc", are ignored.
- -pie
- Produce a position independent executable on targets that support it. For predictable results, you must also specify the same set of options used for compilation ( -fpie, -fPIE, or model suboptions) when you specify this linker option.
- -rdynamic
- Pass the flag -export-dynamic to the ELF linker, on targets that support it. This instructs the linker to add all symbols, not only used ones, to the dynamic symbol table. This option is needed for some uses of "dlopen" or to allow obtaining backtraces from within a program.
- -s
- Remove all symbol table and relocation information from the executable.
- -static
- On systems that support dynamic linking, this prevents linking with the shared libraries. On other systems, this option has no effect.
- -shared
- Produce a shared object which can then be linked with other objects to form an executable. Not all systems support this option. For predictable results, you must also specify the same set of options used for compilation ( -fpic, -fPIC, or model suboptions) when you specify this linker option.[1]
- -shared-libgcc
- -static-libgcc
-
On systems that provide libgcc as a shared library, these options force the use of either the shared or static version, respectively. If no shared version of libgcc was built when the compiler was configured, these options have no effect.
- -static-libasan
- When the -fsanitize=address option is used to link a program, the GCC driver automatically links against libasan. If libasan is available as a shared library, and the -static option is not used, then this links against the shared version of libasan. The -static-libasan option directs the GCC driver to link libasan statically, without necessarily linking other libraries statically.
- -static-libtsan
- When the -fsanitize=thread option is used to link a program, the GCC driver automatically links against libtsan. If libtsan is available as a shared library, and the -static option is not used, then this links against the shared version of libtsan. The -static-libtsan option directs the GCC driver to link libtsan statically, without necessarily linking other libraries statically.
- -static-libstdc++
- When the g++ program is used to link a C++ program, it normally automatically links against libstdc++. If libstdc++ is available as a shared library, and the -static option is not used, then this links against the shared version of libstdc++. That is normally fine. However, it is sometimes useful to freeze the version of libstdc++ used by the program without going all the way to a fully static link. The -static-libstdc++ option directs the g++ driver to link libstdc++ statically, without necessarily linking other libraries statically.
- -symbolic
- Bind references to global symbols when building a shared object. Warn about any unresolved references (unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this option.
- -T script
- Use script as the linker script. This option is supported by most systems using the GNU linker. On some targets, such as bare-board targets without an operating system, the -T option may be required when linking to avoid references to undefined symbols.
- -Xlinker option
-
Pass option as an option to the linker. You can use this to supply system-specific linker options that GCC does not recognize.
- -Wl,option
- Pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas. You can use this syntax to pass an argument to the option. For example, -Wl,-Map,output.map passes -Map output.map to the linker. When using the GNU linker, you can also get the same effect with -Wl,-Map=output.map.
- -u symbol
- Pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use -u multiple times with different symbols to force loading of additional library modules.
Options for Directory Search
These options specify directories to search for header files, for libraries and for parts of the compiler:- -Idir
-
Add the directory dir to the head of the list of directories to be searched for header files. This can be used to override a system header file, substituting your own version, since these directories are searched before the system header file directories. However, you should not use this option to add directories that contain vendor-supplied system header files (use -isystem for that). If you use more than one -I option, the directories are scanned in left-to-right order; the standard system directories come after.
- -iplugindir=dir
- Set the directory to search for plugins that are passed by -fplugin=name instead of -fplugin=path/name.so. This option is not meant to be used by the user, but only passed by the driver.
- -iquotedir
- Add the directory dir to the head of the list of directories to be searched for header files only for the case of #include "file"; they are not searched for #include <file>, otherwise just like -I.
- -iremap src:dst
- Replace the prefix src in __FILE__ with dst at expansion time. This option can be specified more than once. Processing stops at the first match.
- -Ldir
- Add directory dir to the list of directories to be searched for -l.
- -Bprefix
-
This option specifies where to find the executables, libraries, include files, and data files of the compiler itself.
- -specs=file
- Process file after the compiler reads in the standard specs file, in order to override the defaults which the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc. More than one -specs=file can be specified on the command line, and they are processed in order, from left to right.
- --sysroot=dir
-
Use dir as the logical root directory for headers and libraries. For example, if the compiler normally searches for headers in /usr/include and libraries in /usr/lib, it instead searches dir/usr/include and dir/usr/lib.
- --no-sysroot-suffix
- For some targets, a suffix is added to the root directory specified with --sysroot, depending on the other options used, so that headers may for example be found in dir/suffix/usr/include instead of dir/usr/include. This option disables the addition of such a suffix.
- -I-
-
This option has been deprecated. Please use -iquote instead for -I directories before the -I- and remove the -I-. Any directories you specify with -I options before the -I- option are searched only for the case of #include "file"; they are not searched for #include <file>.
Specifying Target Machine and Compiler Version
The usual way to run GCC is to run the executable called gcc, or machine-gcc when cross-compiling, or machine-gcc-version to run a version other than the one that was installed last.Hardware Models and Configurations
Each target machine types can have its own special options, starting with -m, to choose among various hardware models or configurations---for example, 68010 vs 68020, floating coprocessor or none. A single installed version of the compiler can compile for any model or configuration, according to the options specified.Some configurations of the compiler also support additional special options, usually for compatibility with other compilers on the same platform.
AArch64 Options
These options are defined for AArch64 implementations:
- -mbig-endian
- Generate big-endian code. This is the default when GCC is configured for an aarch64_be-*-* target.
- -mgeneral-regs-only
- Generate code which uses only the general registers.
- -mlittle-endian
- Generate little-endian code. This is the default when GCC is configured for an aarch64-*-* but not an aarch64_be-*-* target.
- -mcmodel=tiny
- Generate code for the tiny code model. The program and its statically defined symbols must be within 1GB of each other. Pointers are 64 bits. Programs can be statically or dynamically linked. This model is not fully implemented and mostly treated as small.
- -mcmodel=small
- Generate code for the small code model. The program and its statically defined symbols must be within 4GB of each other. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the default code model.
- -mcmodel=large
- Generate code for the large code model. This makes no assumptions about addresses and sizes of sections. Pointers are 64 bits. Programs can be statically linked only.
- -mstrict-align
- Do not assume that unaligned memory references will be handled by the system.
- -momit-leaf-frame-pointer
- -mno-omit-leaf-frame-pointer
- Omit or keep the frame pointer in leaf functions. The former behaviour is the default.
- -mtls-dialect=desc
- Use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default.
- -mtls-dialect=traditional
- Use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables.
- -march=name
-
Specify the name of the target architecture, optionally suffixed by one or more feature modifiers. This option has the form -march=arch{+[no]feature}*, where the only value for arch is armv8-a. The possible values for feature are documented in the sub-section below.
- -mcpu=name
-
Specify the name of the target processor, optionally suffixed by one or more feature modifiers. This option has the form -mcpu=cpu{+[no]feature}*, where the possible values for cpu are generic, large. The possible values for feature are documented in the sub-section below.
- -mtune=name
- Specify the name of the processor to tune the performance for. The code will be tuned as if the target processor were of the type specified in this option, but still using instructions compatible with the target processor specified by a -mcpu= option. This option cannot be suffixed by feature modifiers.
-march and -mcpu feature modifiers
Feature modifiers used with -march and -mcpu can be one the following:
- crypto
- Enable Crypto extension. This implies Advanced SIMD is enabled.
- fp
- Enable floating-point instructions.
- simd
- Enable Advanced SIMD instructions. This implies floating-point instructions are enabled. This is the default for all current possible values for options -march and -mcpu=.
Adapteva Epiphany Options
These -m options are defined for Adapteva Epiphany:
- -mhalf-reg-file
- Don't allocate any register in the range "r32"..."r63". That allows code to run on hardware variants that lack these registers.
- -mprefer-short-insn-regs
- Preferrentially allocate registers that allow short instruction generation. This can result in increased instruction count, so this may either reduce or increase overall code size.
- -mbranch-cost=num
- Set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases.
- -mcmove
- Enable the generation of conditional moves.
- -mnops=num
- Emit num NOPs before every other generated instruction.
- -mno-soft-cmpsf
- For single-precision floating-point comparisons, emit an "fsub" instruction and test the flags. This is faster than a software comparison, but can get incorrect results in the presence of NaNs, or when two different small numbers are compared such that their difference is calculated as zero. The default is -msoft-cmpsf, which uses slower, but IEEE-compliant, software comparisons.
- -mstack-offset=num
- Set the offset between the top of the stack and the stack pointer. E.g., a value of 8 means that the eight bytes in the range "sp+0...sp+7" can be used by leaf functions without stack allocation. Values other than 8 or 16 are untested and unlikely to work. Note also that this option changes the ABI; compiling a program with a different stack offset than the libraries have been compiled with generally does not work. This option can be useful if you want to evaluate if a different stack offset would give you better code, but to actually use a different stack offset to build working programs, it is recommended to configure the toolchain with the appropriate --with-stack-offset=num option.
- -mno-round-nearest
- Make the scheduler assume that the rounding mode has been set to truncating. The default is -mround-nearest.
- -mlong-calls
- If not otherwise specified by an attribute, assume all calls might be beyond the offset range of the "b" / "bl" instructions, and therefore load the function address into a register before performing a (otherwise direct) call. This is the default.
- -mshort-calls
- If not otherwise specified by an attribute, assume all direct calls are in the range of the "b" / "bl" instructions, so use these instructions for direct calls. The default is -mlong-calls.
- -msmall16
- Assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses for which -mlong-calls semantics are in effect.
- -mfp-mode=mode
-
Set the prevailing mode of the floating-point unit. This determines the floating-point mode that is provided and expected at function call and return time. Making this mode match the mode you predominantly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches.
- caller
- Any mode at function entry is valid, and retained or restored when the function returns, and when it calls other functions. This mode is useful for compiling libraries or other compilation units you might want to incorporate into different programs with different prevailing FPU modes, and the convenience of being able to use a single object file outweighs the size and speed overhead for any extra mode switching that might be needed, compared with what would be needed with a more specific choice of prevailing FPU mode.
- truncate
- This is the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode. That includes conversion from floating point to integer.
- round-nearest
- This is the mode used for floating-point calculations with round-to-nearest-or-even rounding mode.
- int
- This is the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate.
- -mnosplit-lohi
- -mno-postinc
- -mno-postmodify
- Code generation tweaks that disable, respectively, splitting of 32-bit loads, generation of post-increment addresses, and generation of post-modify addresses. The defaults are msplit-lohi, -mpost-inc, and -mpost-modify.
- -mnovect-double
- Change the preferred SIMD mode to SImode. The default is -mvect-double, which uses DImode as preferred SIMD mode.
- -max-vect-align=num
- The maximum alignment for SIMD vector mode types. num may be 4 or 8. The default is 8. Note that this is an ABI change, even though many library function interfaces are unaffected if they don't use SIMD vector modes in places that affect size and/or alignment of relevant types.
- -msplit-vecmove-early
- Split vector moves into single word moves before reload. In theory this can give better register allocation, but so far the reverse seems to be generally the case.
- -m1reg-reg
- Specify a register to hold the constant -1, which makes loading small negative constants and certain bitmasks faster. Allowable values for reg are r43 and r63, which specify use of that register as a fixed register, and none, which means that no register is used for this purpose. The default is -m1reg-none.
ARM Options
These -m options are defined for Advanced RISC Machines (ARM) architectures:
- -mabi=name
- Generate code for the specified ABI. Permissible values are: apcs-gnu, atpcs, aapcs, aapcs-linux and iwmmxt.
- -mapcs-frame
- Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this is not strictly necessary for correct execution of the code. Specifying -fomit-frame-pointer with this option causes the stack frames not to be generated for leaf functions. The default is -mno-apcs-frame.
- -mapcs
- This is a synonym for -mapcs-frame.
- -mthumb-interwork
- Generate code that supports calling between the ARM and Thumb instruction sets. Without this option, on pre-v5 architectures, the two instruction sets cannot be reliably used inside one program. The default is -mno-thumb-interwork, since slightly larger code is generated when -mthumb-interwork is specified. In AAPCS configurations this option is meaningless.
- -mno-sched-prolog
- Prevent the reordering of instructions in the function prologue, or the merging of those instruction with the instructions in the function's body. This means that all functions start with a recognizable set of instructions (or in fact one of a choice from a small set of different function prologues), and this information can be used to locate the start of functions inside an executable piece of code. The default is -msched-prolog.
- -mfloat-abi=name
-
Specifies which floating-point ABI to use. Permissible values are: soft, softfp and hard.
- -mlittle-endian
- Generate code for a processor running in little-endian mode. This is the default for all standard configurations.
- -mbig-endian
- Generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor.
- -mwords-little-endian
- This option only applies when generating code for big-endian processors. Generate code for a little-endian word order but a big-endian byte order. That is, a byte order of the form 32107654. Note: this option should only be used if you require compatibility with code for big-endian ARM processors generated by versions of the compiler prior to 2.8. This option is now deprecated.
- -mcpu=name
-
This specifies the name of the target ARM processor. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. Permissible names are: arm2, arm250, arm3, arm6, arm60, arm600, arm610, arm620, arm7, arm7m, arm7d, arm7dm, arm7di, arm7dmi, arm70, arm700, arm700i, arm710, arm710c, arm7100, arm720, arm7500, arm7500fe, arm7tdmi, arm7tdmi-s, arm710t, arm720t, arm740t, strongarm, strongarm110, strongarm1100, strongarm1110, arm8, arm810, arm9, arm9e, arm920, arm920t, arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t, arm9tdmi, arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e, arm1136j-s, arm1136jf-s, mpcore, mpcorenovfp, arm1156t2-s, arm1156t2f-s, arm1176jz-s, arm1176jzf-s, cortex-a5, cortex-a7, cortex-a8, cortex-a9, cortex-a15, cortex-r4, cortex-r4f, cortex-r5, cortex-m4, cortex-m3, cortex-m1, cortex-m0, cortex-m0plus, marvell-pj4, xscale, iwmmxt, iwmmxt2, ep9312, fa526, fa626, fa606te, fa626te, fmp626, fa726te.
- -mtune=name
-
This option is very similar to the -mcpu= option, except that instead of specifying the actual target processor type, and hence restricting which instructions can be used, it specifies that GCC should tune the performance of the code as if the target were of the type specified in this option, but still choosing the instructions it generates based on the CPU specified by a -mcpu= option. For some ARM implementations better performance can be obtained by using this option.
- -march=name
-
This specifies the name of the target ARM architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. This option can be used in conjunction with or instead of the -mcpu= option. Permissible names are: armv2, armv2a, armv3, armv3m, armv4, armv4t, armv5, armv5t, armv5e, armv5te, armv6, armv6j, armv6t2, armv6z, armv6zk, armv6-m, armv7, armv7-a, armv7-r, armv7-m, armv8-a, iwmmxt, iwmmxt2, ep9312.
- -mfpu=name
-
This specifies what floating-point hardware (or hardware emulation) is available on the target. Permissible names are: vfp, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd, vfpv3xd-fp16, neon, neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4, fp-armv8, neon-fp-armv8, and crypto-neon-fp-armv8.
- -mfp16-format=name
- Specify the format of the "__fp16" half-precision floating-point type. Permissible names are none, ieee, and alternative; the default is none, in which case the "__fp16" type is not defined.
- -mstructure-size-boundary=n
-
The sizes of all structures and unions are rounded up to a multiple of the number of bits set by this option. Permissible values are 8, 32 and 64. The default value varies for different toolchains. For the COFF targeted toolchain the default value is 8. A value of 64 is only allowed if the underlying ABI supports it.
- -mabort-on-noreturn
- Generate a call to the function "abort" at the end of a "noreturn" function. It is executed if the function tries to return.
- -mlong-calls
- -mno-long-calls
-
Tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function lies outside of the 64-megabyte addressing range of the offset-based version of subroutine call instruction.
- -msingle-pic-base
- Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins.
- -mpic-register=reg
- Specify the register to be used for PIC addressing. For standard PIC base case, the default will be any suitable register determined by compiler. For single PIC base case, the default is R9 if target is EABI based or stack-checking is enabled, otherwise the default is R10.
- -mpoke-function-name
-
Write the name of each function into the text section, directly preceding the function prologue. The generated code is similar to this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4
- -mthumb
- -marm
- Select between generating code that executes in ARM and Thumb states. The default for most configurations is to generate code that executes in ARM state, but the default can be changed by configuring GCC with the --with-mode=state configure option.
- -mtpcs-frame
- Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-tpcs-frame.
- -mtpcs-leaf-frame
- Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-apcs-leaf-frame.
- -mcallee-super-interworking
- Gives all externally visible functions in the file being compiled an ARM instruction set header which switches to Thumb mode before executing the rest of the function. This allows these functions to be called from non-interworking code. This option is not valid in AAPCS configurations because interworking is enabled by default.
- -mcaller-super-interworking
- Allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code has been compiled for interworking or not. There is a small overhead in the cost of executing a function pointer if this option is enabled. This option is not valid in AAPCS configurations because interworking is enabled by default.
- -mtp=name
- Specify the access model for the thread local storage pointer. The valid models are soft, which generates calls to "__aeabi_read_tp", cp15, which fetches the thread pointer from "cp15" directly (supported in the arm6k architecture), and auto, which uses the best available method for the selected processor. The default setting is auto.
- -mtls-dialect=dialect
- Specify the dialect to use for accessing thread local storage. Two dialects are supported---gnu and gnu2. The gnu dialect selects the original GNU scheme for supporting local and global dynamic TLS models. The gnu2 dialect selects the GNU descriptor scheme, which provides better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme.
- -mword-relocations
- Only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32). This is enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when -fpic or -fPIC is specified.
- -mfix-cortex-m3-ldrd
- Some Cortex-M3 cores can cause data corruption when "ldrd" instructions with overlapping destination and base registers are used. This option avoids generating these instructions. This option is enabled by default when -mcpu=cortex-m3 is specified.
- -munaligned-access
- -mno-unaligned-access
-
Enables (or disables) reading and writing of 16- and 32- bit values from addresses that are not 16- or 32- bit aligned. By default unaligned access is disabled for all pre-ARMv6 and all ARMv6-M architectures, and enabled for all other architectures. If unaligned access is not enabled then words in packed data structures will be accessed a byte at a time.
AVR Options
These options are defined for AVR implementations:
- -mmcu=mcu
-
Specify Atmel AVR instruction set architectures (ISA) or MCU type.
- "avr2"
- "Classic" devices with up to 8@tie{}KiB of program memory. mcu@tie{}= "attiny22", "attiny26", "at90c8534", "at90s2313", "at90s2323", "at90s2333", "at90s2343", "at90s4414", "at90s4433", "at90s4434", "at90s8515", "at90s8535".
- "avr25"
- "Classic" devices with up to 8@tie{}KiB of program memory and with the "MOVW" instruction. mcu@tie{}= "ata5272", "ata6289", "attiny13", "attiny13a", "attiny2313", "attiny2313a", "attiny24", "attiny24a", "attiny25", "attiny261", "attiny261a", "attiny43u", "attiny4313", "attiny44", "attiny44a", "attiny45", "attiny461", "attiny461a", "attiny48", "attiny84", "attiny84a", "attiny85", "attiny861", "attiny861a", "attiny87", "attiny88", "at86rf401".
- "avr3"
- "Classic" devices with 16@tie{}KiB up to 64@tie{}KiB of program memory. mcu@tie{}= "at43usb355", "at76c711".
- "avr31"
- "Classic" devices with 128@tie{}KiB of program memory. mcu@tie{}= "atmega103", "at43usb320".
- "avr35"
- "Classic" devices with 16@tie{}KiB up to 64@tie{}KiB of program memory and with the "MOVW" instruction. mcu@tie{}= "ata5505", "atmega16u2", "atmega32u2", "atmega8u2", "attiny1634", "attiny167", "at90usb162", "at90usb82".
- "avr4"
- "Enhanced" devices with up to 8@tie{}KiB of program memory. mcu@tie{}= "ata6285", "ata6286", "atmega48", "atmega48a", "atmega48p", "atmega48pa", "atmega8", "atmega8a", "atmega8hva", "atmega8515", "atmega8535", "atmega88", "atmega88a", "atmega88p", "atmega88pa", "at90pwm1", "at90pwm2", "at90pwm2b", "at90pwm3", "at90pwm3b", "at90pwm81".
- "avr5"
- "Enhanced" devices with 16@tie{}KiB up to 64@tie{}KiB of program memory. mcu@tie{}= "ata5790", "ata5790n", "ata5795", "atmega16", "atmega16a", "atmega16hva", "atmega16hva2", "atmega16hvb", "atmega16hvbrevb", "atmega16m1", "atmega16u4", "atmega161", "atmega162", "atmega163", "atmega164a", "atmega164p", "atmega164pa", "atmega165", "atmega165a", "atmega165p", "atmega165pa", "atmega168", "atmega168a", "atmega168p", "atmega168pa", "atmega169", "atmega169a", "atmega169p", "atmega169pa", "atmega26hvg", "atmega32", "atmega32a", "atmega32c1", "atmega32hvb", "atmega32hvbrevb", "atmega32m1", "atmega32u4", "atmega32u6", "atmega323", "atmega324a", "atmega324p", "atmega324pa", "atmega325", "atmega325a", "atmega325p", "atmega3250", "atmega3250a", "atmega3250p", "atmega3250pa", "atmega328", "atmega328p", "atmega329", "atmega329a", "atmega329p", "atmega329pa", "atmega3290", "atmega3290a", "atmega3290p", "atmega3290pa", "atmega406", "atmega48hvf", "atmega64", "atmega64a", "atmega64c1", "atmega64hve", "atmega64m1", "atmega64rfa2", "atmega64rfr2", "atmega640", "atmega644", "atmega644a", "atmega644p", "atmega644pa", "atmega645", "atmega645a", "atmega645p", "atmega6450", "atmega6450a", "atmega6450p", "atmega649", "atmega649a", "atmega649p", "atmega6490", "atmega6490a", "atmega6490p", "at90can32", "at90can64", "at90pwm161", "at90pwm216", "at90pwm316", "at90scr100", "at90usb646", "at90usb647", "at94k", "m3000".
- "avr51"
- "Enhanced" devices with 128@tie{}KiB of program memory. mcu@tie{}= "atmega128", "atmega128a", "atmega128rfa1", "atmega1280", "atmega1281", "atmega1284", "atmega1284p", "at90can128", "at90usb1286", "at90usb1287".
- "avr6"
- "Enhanced" devices with 3-byte PC, i.e. with more than 128@tie{}KiB of program memory. mcu@tie{}= "atmega2560", "atmega2561".
- "avrxmega2"
- "XMEGA" devices with more than 8@tie{}KiB and up to 64@tie{}KiB of program memory. mcu@tie{}= "atmxt112sl", "atmxt224", "atmxt224e", "atmxt336s", "atxmega16a4", "atxmega16a4u", "atxmega16c4", "atxmega16d4", "atxmega16x1", "atxmega32a4", "atxmega32a4u", "atxmega32c4", "atxmega32d4", "atxmega32e5", "atxmega32x1".
- "avrxmega4"
- "XMEGA" devices with more than 64@tie{}KiB and up to 128@tie{}KiB of program memory. mcu@tie{}= "atxmega64a3", "atxmega64a3u", "atxmega64a4u", "atxmega64b1", "atxmega64b3", "atxmega64c3", "atxmega64d3", "atxmega64d4".
- "avrxmega5"
- "XMEGA" devices with more than 64@tie{}KiB and up to 128@tie{}KiB of program memory and more than 64@tie{}KiB of RAM. mcu@tie{}= "atxmega64a1", "atxmega64a1u".
- "avrxmega6"
- "XMEGA" devices with more than 128@tie{}KiB of program memory. mcu@tie{}= "atmxt540s", "atmxt540sreva", "atxmega128a3", "atxmega128a3u", "atxmega128b1", "atxmega128b3", "atxmega128c3", "atxmega128d3", "atxmega128d4", "atxmega192a3", "atxmega192a3u", "atxmega192c3", "atxmega192d3", "atxmega256a3", "atxmega256a3b", "atxmega256a3bu", "atxmega256a3u", "atxmega256c3", "atxmega256d3", "atxmega384c3", "atxmega384d3".
- "avrxmega7"
- "XMEGA" devices with more than 128@tie{}KiB of program memory and more than 64@tie{}KiB of RAM. mcu@tie{}= "atxmega128a1", "atxmega128a1u", "atxmega128a4u".
- "avr1"
- This ISA is implemented by the minimal AVR core and supported for assembler only. mcu@tie{}= "attiny11", "attiny12", "attiny15", "attiny28", "at90s1200".
- -maccumulate-args
-
Accumulate outgoing function arguments and acquire/release the needed stack space for outgoing function arguments once in function prologue/epilogue. Without this option, outgoing arguments are pushed before calling a function and popped afterwards.
- -mbranch-cost=cost
- Set the branch costs for conditional branch instructions to cost. Reasonable values for cost are small, non-negative integers. The default branch cost is 0.
- -mcall-prologues
- Functions prologues/epilogues are expanded as calls to appropriate subroutines. Code size is smaller.
- -mint8
- Assume "int" to be 8-bit integer. This affects the sizes of all types: a "char" is 1 byte, an "int" is 1 byte, a "long" is 2 bytes, and "long long" is 4 bytes. Please note that this option does not conform to the C standards, but it results in smaller code size.
- -mno-interrupts
- Generated code is not compatible with hardware interrupts. Code size is smaller.
- -mrelax
-
Try to replace "CALL" resp. "JMP" instruction by the shorter "RCALL" resp. "RJMP" instruction if applicable. Setting "-mrelax" just adds the "--relax" option to the linker command line when the linker is called.
- -msp8
-
Treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. In general, you don't need to set this option by hand.
- -mstrict-X
-
Use address register "X" in a way proposed by the hardware. This means that "X" is only used in indirect, post-increment or pre-decrement addressing.
adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const
- -mtiny-stack
- Only change the lower 8@tie{}bits of the stack pointer.
- -Waddr-space-convert
- Warn about conversions between address spaces in the case where the resulting address space is not contained in the incoming address space.
"EIND" and Devices with more than 128 Ki Bytes of Flash
Pointers in the implementation are 16@tie{}bits wide. The address of a function or label is represented as word address so that indirect jumps and calls can target any code address in the range of 64@tie{}Ki words.
In order to facilitate indirect jump on devices with more than 128@tie{}Ki bytes of program memory space, there is a special function register called "EIND" that serves as most significant part of the target address when "EICALL" or "EIJMP" instructions are used.
Indirect jumps and calls on these devices are handled as follows by the compiler and are subject to some limitations:
- •
- The compiler never sets "EIND".
- •
- The compiler uses "EIND" implicitely in "EICALL"/"EIJMP" instructions or might read "EIND" directly in order to emulate an indirect call/jump by means of a "RET" instruction.
- •
- The compiler assumes that "EIND" never changes during the startup code or during the application. In particular, "EIND" is not saved/restored in function or interrupt service routine prologue/epilogue.
- •
- For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads sometimes also called trampolines. Thus, the indirect call/jump jumps to such a stub. The stub contains a direct jump to the desired address.
- •
- Linker relaxation must be turned on so that the linker will generate the stubs correctly an all situaltion. See the compiler option "-mrelax" and the linler option "--relax". There are corner cases where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error message.
- •
- The default linker script is arranged for code with "EIND = 0". If code is supposed to work for a setup with "EIND != 0", a custom linker script has to be used in order to place the sections whose name start with ".trampolines" into the segment where "EIND" points to.
- •
- The startup code from libgcc never sets "EIND". Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC on "EIND", see the AVR-LibC user manual ("http://nongnu.org/avr-libc/user-manual/").
- •
-
It is legitimate for user-specific startup code to set up "EIND" early, for example by means of initialization code located in section ".init3". Such code runs prior to general startup code that initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC that sets "EIND" to the segment where the vector table is located.
#include <avr/io.h>
static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}
- •
- Stubs are generated automatically by the linker if the following two conditions are met:
- -<The address of a label is taken by means of the "gs" modifier>
-
(short for generate stubs) like so:
LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))
- -<The final location of that label is in a code segment>
- outside the segment where the stubs are located.
- •
- The compiler emits such "gs" modifiers for code labels in the following situations:
- -<Taking address of a function or code label.>
- -<Computed goto.>
- -<If prologue-save function is used, see -mcall-prologues>
- command-line option.
- -<Switch/case dispatch tables. If you do not want such dispatch>
- tables you can specify the -fno-jump-tables command-line option.
- -<C and C++ constructors/destructors called during startup/shutdown.>
- -<If the tools hit a "gs()" modifier explained above.>
- •
-
Jumping to non-symbolic addresses like so is not supported:
int main (void)
{
/* Call function at word address 0x2 */
return ((int(*)(void)) 0x2)();
}
int main (void)
{
extern int func_4 (void);
/* Call function at byte address 0x4 */
return func_4();
}
Handling of the "RAMPD", "RAMPX", "RAMPY" and "RAMPZ" Special Function Registers
Some AVR devices support memories larger than the 64@tie{}KiB range that can be accessed with 16-bit pointers. To access memory locations outside this 64@tie{}KiB range, the contentent of a "RAMP" register is used as high part of the address: The "X", "Y", "Z" address register is concatenated with the "RAMPX", "RAMPY", "RAMPZ" special function register, respectively, to get a wide address. Similarly, "RAMPD" is used together with direct addressing.
- •
- The startup code initializes the "RAMP" special function registers with zero.
- •
- If a AVR Named Address Spaces,named address space other than generic or "__flash" is used, then "RAMPZ" is set as needed before the operation.
- •
- If the device supports RAM larger than 64@tie{KiB} and the compiler needs to change "RAMPZ" to accomplish an operation, "RAMPZ" is reset to zero after the operation.
- •
- If the device comes with a specific "RAMP" register, the ISR prologue/epilogue saves/restores that SFR and initializes it with zero in case the ISR code might (implicitly) use it.
- •
- RAM larger than 64@tie{KiB} is not supported by GCC for AVR targets. If you use inline assembler to read from locations outside the 16-bit address range and change one of the "RAMP" registers, you must reset it to zero after the access.
AVR Built-in Macros
GCC defines several built-in macros so that the user code can test for the presence or absence of features. Almost any of the following built-in macros are deduced from device capabilities and thus triggered by the "-mmcu=" command-line option.
For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR Built-in Functions.
- "__AVR_ARCH__"
-
Build-in macro that resolves to a decimal number that identifies the architecture and depends on the "-mmcu= mcu" option. Possible values are:
- "__AVR_Device__"
-
Setting "-mmcu=device" defines this built-in macro which reflects the device's name. For example, "-mmcu=atmega8" defines the built-in macro "__AVR_ATmega8__", "-mmcu=attiny261a" defines "__AVR_ATtiny261A__", etc.
- "__AVR_XMEGA__"
- The device / architecture belongs to the XMEGA family of devices.
- "__AVR_HAVE_ELPM__"
- The device has the the "ELPM" instruction.
- "__AVR_HAVE_ELPMX__"
- The device has the "ELPM Rn,Z" and "ELPM R n,Z+" instructions.
- "__AVR_HAVE_MOVW__"
- The device has the "MOVW" instruction to perform 16-bit register-register moves.
- "__AVR_HAVE_LPMX__"
- The device has the "LPM Rn,Z" and "LPM R n,Z+" instructions.
- "__AVR_HAVE_MUL__"
- The device has a hardware multiplier.
- "__AVR_HAVE_JMP_CALL__"
- The device has the "JMP" and "CALL" instructions. This is the case for devices with at least 16@tie{}KiB of program memory.
- "__AVR_HAVE_EIJMP_EICALL__"
- "__AVR_3_BYTE_PC__"
- The device has the "EIJMP" and "EICALL" instructions. This is the case for devices with more than 128@tie{}KiB of program memory. This also means that the program counter (PC) is 3@tie{}bytes wide.
- "__AVR_2_BYTE_PC__"
- The program counter (PC) is 2@tie{}bytes wide. This is the case for devices with up to 128@tie{}KiB of program memory.
- "__AVR_HAVE_8BIT_SP__"
- "__AVR_HAVE_16BIT_SP__"
- The stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the compiler. The definition of these macros is affected by "-mtiny-stack".
- "__AVR_HAVE_SPH__"
- "__AVR_SP8__"
- The device has the SPH (high part of stack pointer) special function register or has an 8-bit stack pointer, respectively. The definition of these macros is affected by "-mmcu=" and in the cases of "-mmcu=avr2" and "-mmcu=avr25" also by "-msp8".
- "__AVR_HAVE_RAMPD__"
- "__AVR_HAVE_RAMPX__"
- "__AVR_HAVE_RAMPY__"
- "__AVR_HAVE_RAMPZ__"
- The device has the "RAMPD", "RAMPX", "RAMPY", "RAMPZ" special function register, respectively.
- "__NO_INTERRUPTS__"
- This macro reflects the "-mno-interrupts" command line option.
- "__AVR_ERRATA_SKIP__"
- "__AVR_ERRATA_SKIP_JMP_CALL__"
- Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware erratum. Skip instructions are "SBRS", "SBRC", "SBIS", "SBIC" and "CPSE". The second macro is only defined if "__AVR_HAVE_JMP_CALL__" is also set.
- "__AVR_SFR_OFFSET__=offset"
- Instructions that can address I/O special function registers directly like "IN", "OUT", "SBI", etc. may use a different address as if addressed by an instruction to access RAM like "LD" or "STS". This offset depends on the device architecture and has to be subtracted from the RAM address in order to get the respective I/O@tie{}address.
- "__WITH_AVRLIBC__"
- The compiler is configured to be used together with AVR-Libc. See the "--with-avrlibc" configure option.
Blackfin Options
- -mcpu=cpu[-sirevision]
-
Specifies the name of the target Blackfin processor. Currently, cpu can be one of bf512, bf514, bf516, bf518, bf522, bf523, bf524, bf525, bf526, bf527, bf531, bf532, bf533, bf534, bf536, bf537, bf538, bf539, bf542, bf544, bf547, bf548, bf549, bf542m, bf544m, bf547m, bf548m, bf549m, bf561, bf592.
- -msim
- Specifies that the program will be run on the simulator. This causes the simulator BSP provided by libgloss to be linked in. This option has effect only for bfin-elf toolchain. Certain other options, such as -mid-shared-library and -mfdpic, imply -msim.
- -momit-leaf-frame-pointer
- Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers and makes an extra register available in leaf functions. The option -fomit-frame-pointer removes the frame pointer for all functions, which might make debugging harder.
- -mspecld-anomaly
- When enabled, the compiler ensures that the generated code does not contain speculative loads after jump instructions. If this option is used, "__WORKAROUND_SPECULATIVE_LOADS" is defined.
- -mno-specld-anomaly
- Don't generate extra code to prevent speculative loads from occurring.
- -mcsync-anomaly
- When enabled, the compiler ensures that the generated code does not contain CSYNC or SSYNC instructions too soon after conditional branches. If this option is used, "__WORKAROUND_SPECULATIVE_SYNCS" is defined.
- -mno-csync-anomaly
- Don't generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon after a conditional branch.
- -mlow-64k
- When enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of memory.
- -mno-low-64k
- Assume that the program is arbitrarily large. This is the default.
- -mstack-check-l1
- Do stack checking using information placed into L1 scratchpad memory by the uClinux kernel.
- -mid-shared-library
- Generate code that supports shared libraries via the library ID method. This allows for execute in place and shared libraries in an environment without virtual memory management. This option implies -fPIC. With a bfin-elf target, this option implies -msim.
- -mno-id-shared-library
- Generate code that doesn't assume ID-based shared libraries are being used. This is the default.
- -mleaf-id-shared-library
- Generate code that supports shared libraries via the library ID method, but assumes that this library or executable won't link against any other ID shared libraries. That allows the compiler to use faster code for jumps and calls.
- -mno-leaf-id-shared-library
- Do not assume that the code being compiled won't link against any ID shared libraries. Slower code is generated for jump and call insns.
- -mshared-library-id=n
- Specifies the identification number of the ID-based shared library being compiled. Specifying a value of 0 generates more compact code; specifying other values forces the allocation of that number to the current library but is no more space- or time-efficient than omitting this option.
- -msep-data
- Generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute in place in an environment without virtual memory management by eliminating relocations against the text section.
- -mno-sep-data
- Generate code that assumes that the data segment follows the text segment. This is the default.
- -mlong-calls
- -mno-long-calls
-
Tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function lies outside of the 24-bit addressing range of the offset-based version of subroutine call instruction.
- -mfast-fp
- Link with the fast floating-point library. This library relaxes some of the IEEE floating-point standard's rules for checking inputs against Not-a-Number (NAN), in the interest of performance.
- -minline-plt
- Enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without -mfdpic.
- -mmulticore
-
Build a standalone application for multicore Blackfin processors. This option causes proper start files and link scripts supporting multicore to be used, and defines the macro "__BFIN_MULTICORE". It can only be used with -mcpu=bf561[-sirevision].
- -mcorea
- Build a standalone application for Core A of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core A, and the macro "__BFIN_COREA" is defined. This option can only be used in conjunction with -mmulticore.
- -mcoreb
- Build a standalone application for Core B of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core B, and the macro "__BFIN_COREB" is defined. When this option is used, "coreb_main" should be used instead of "main". This option can only be used in conjunction with -mmulticore.
- -msdram
- Build a standalone application for SDRAM. Proper start files and link scripts are used to put the application into SDRAM, and the macro "__BFIN_SDRAM" is defined. The loader should initialize SDRAM before loading the application.
- -micplb
- Assume that ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For Linux targets, the default is to assume ICPLBs are enabled; for standalone applications the default is off.
C6X Options
- -march=name
- This specifies the name of the target architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. Permissible names are: c62x, c64x, c64x+, c67x, c67x+, c674x.
- -mbig-endian
- Generate code for a big-endian target.
- -mlittle-endian
- Generate code for a little-endian target. This is the default.
- -msim
- Choose startup files and linker script suitable for the simulator.
- -msdata=default
- Put small global and static data in the .neardata section, which is pointed to by register "B14". Put small uninitialized global and static data in the .bss section, which is adjacent to the .neardata section. Put small read-only data into the .rodata section. The corresponding sections used for large pieces of data are .fardata, .far and .const.
- -msdata=all
- Put all data, not just small objects, into the sections reserved for small data, and use addressing relative to the "B14" register to access them.
- -msdata=none
- Make no use of the sections reserved for small data, and use absolute addresses to access all data. Put all initialized global and static data in the .fardata section, and all uninitialized data in the .far section. Put all constant data into the .const section.
CRIS Options
These options are defined specifically for the CRIS ports.
- -march=architecture-type
- -mcpu=architecture-type
- Generate code for the specified architecture. The choices for architecture-type are v3, v8 and v10 for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0 except for cris-axis-linux-gnu, where the default is v10.
- -mtune=architecture-type
- Tune to architecture-type everything applicable about the generated code, except for the ABI and the set of available instructions. The choices for architecture-type are the same as for -march=architecture-type.
- -mmax-stack-frame=n
- Warn when the stack frame of a function exceeds n bytes.
- -metrax4
- -metrax100
- The options -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8 respectively.
- -mmul-bug-workaround
- -mno-mul-bug-workaround
- Work around a bug in the "muls" and "mulu" instructions for CPU models where it applies. This option is active by default.
- -mpdebug
- Enable CRIS-specific verbose debug-related information in the assembly code. This option also has the effect of turning off the #NO_APP formatted-code indicator to the assembler at the beginning of the assembly file.
- -mcc-init
- Do not use condition-code results from previous instruction; always emit compare and test instructions before use of condition codes.
- -mno-side-effects
- Do not emit instructions with side effects in addressing modes other than post-increment.
- -mstack-align
- -mno-stack-align
- -mdata-align
- -mno-data-align
- -mconst-align
- -mno-const-align
- These options (no- options) arrange (eliminate arrangements) for the stack frame, individual data and constants to be aligned for the maximum single data access size for the chosen CPU model. The default is to arrange for 32-bit alignment. ABI details such as structure layout are not affected by these options.
- -m32-bit
- -m16-bit
- -m8-bit
- Similar to the stack- data- and const-align options above, these options arrange for stack frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment.
- -mno-prologue-epilogue
- -mprologue-epilogue
- With -mno-prologue-epilogue, the normal function prologue and epilogue which set up the stack frame are omitted and no return instructions or return sequences are generated in the code. Use this option only together with visual inspection of the compiled code: no warnings or errors are generated when call-saved registers must be saved, or storage for local variables needs to be allocated.
- -mno-gotplt
- -mgotplt
- With -fpic and -fPIC, don't generate (do generate) instruction sequences that load addresses for functions from the PLT part of the GOT rather than (traditional on other architectures) calls to the PLT. The default is -mgotplt.
- -melf
- Legacy no-op option only recognized with the cris-axis-elf and cris-axis-linux-gnu targets.
- -mlinux
- Legacy no-op option only recognized with the cris-axis-linux-gnu target.
- -sim
- This option, recognized for the cris-axis-elf, arranges to link with input-output functions from a simulator library. Code, initialized data and zero-initialized data are allocated consecutively.
- -sim2
- Like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data at 0x80000000.
CR16 Options
These options are defined specifically for the CR16 ports.
- -mmac
- Enable the use of multiply-accumulate instructions. Disabled by default.
- -mcr16cplus
- -mcr16c
- Generate code for CR16C or CR16C+ architecture. CR16C+ architecture is default.
- -msim
- Links the library libsim.a which is in compatible with simulator. Applicable to ELF compiler only.
- -mint32
- Choose integer type as 32-bit wide.
- -mbit-ops
- Generates "sbit"/"cbit" instructions for bit manipulations.
- -mdata-model=model
- Choose a data model. The choices for model are near, far or medium. medium is default. However, far is not valid with -mcr16c, as the CR16C architecture does not support the far data model.
Darwin Options
These options are defined for all architectures running the Darwin operating system.
FSF GCC on Darwin does not create "fat" object files; it creates an object file for the single architecture that GCC was built to target. Apple's GCC on Darwin does create "fat" files if multiple -arch options are used; it does so by running the compiler or linker multiple times and joining the results together with lipo.
The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the ISA that GCC is targeting, like -mcpu or -march. The -force_cpusubtype_ALL option can be used to override this.
The Darwin tools vary in their behavior when presented with an ISA mismatch. The assembler, as, only permits instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit instructions in a ppc750 object file. The linker for shared libraries, /usr/bin/libtool, fails and prints an error if asked to create a shared library with a less restrictive subtype than its input files (for instance, trying to put a ppc970 object file in a ppc7400 library). The linker for executables, ld, quietly gives the executable the most restrictive subtype of any of its input files.
- -Fdir
-
Add the framework directory dir to the head of the list of directories to be searched for header files. These directories are interleaved with those specified by -I options and are scanned in a left-to-right order.
- -iframeworkdir
- Like -F except the directory is a treated as a system directory. The main difference between this -iframework and -F is that with -iframework the compiler does not warn about constructs contained within header files found via dir. This option is valid only for the C family of languages.
- -gused
- Emit debugging information for symbols that are used. For stabs debugging format, this enables -feliminate-unused-debug-symbols. This is by default ON.
- -gfull
- Emit debugging information for all symbols and types.
- -mmacosx-version-min=version
-
The earliest version of MacOS X that this executable will run on is version. Typical values of version include 10.1, 10.2, and 10.3.9.
- -mkernel
- Enable kernel development mode. The -mkernel option sets -static, -fno-common, -fno-cxa-atexit, -fno-exceptions, -fno-non-call-exceptions, -fapple-kext, -fno-weak and -fno-rtti where applicable. This mode also sets -mno-altivec, -msoft-float, -fno-builtin and -mlong-branch for PowerPC targets.
- -mone-byte-bool
-
Override the defaults for bool so that sizeof(bool)==1. By default sizeof(bool) is 4 when compiling for Darwin/PowerPC and 1 when compiling for Darwin/x86, so this option has no effect on x86.
- -mfix-and-continue
- -ffix-and-continue
- -findirect-data
- Generate code suitable for fast turnaround development, such as to allow GDB to dynamically load ".o" files into already-running programs. -findirect-data and -ffix-and-continue are provided for backwards compatibility.
- -all_load
- Loads all members of static archive libraries. See man ld(1) for more information.
- -arch_errors_fatal
- Cause the errors having to do with files that have the wrong architecture to be fatal.
- -bind_at_load
- Causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched.
- -bundle
- Produce a Mach-o bundle format file. See man ld(1) for more information.
- -bundle_loader executable
- This option specifies the executable that will load the build output file being linked. See man ld(1) for more information.
- -dynamiclib
- When passed this option, GCC produces a dynamic library instead of an executable when linking, using the Darwin libtool command.
- -force_cpusubtype_ALL
- This causes GCC's output file to have the ALL subtype, instead of one controlled by the -mcpu or -march option.
- -allowable_client client_name
- -client_name
- -compatibility_version
- -current_version
- -dead_strip
- -dependency-file
- -dylib_file
- -dylinker_install_name
- -dynamic
- -exported_symbols_list
- -filelist
- -flat_namespace
- -force_flat_namespace
- -headerpad_max_install_names
- -image_base
- -init
- -install_name
- -keep_private_externs
- -multi_module
- -multiply_defined
- -multiply_defined_unused
- -noall_load
- -no_dead_strip_inits_and_terms
- -nofixprebinding
- -nomultidefs
- -noprebind
- -noseglinkedit
- -pagezero_size
- -prebind
- -prebind_all_twolevel_modules
- -private_bundle
- -read_only_relocs
- -sectalign
- -sectobjectsymbols
- -whyload
- -seg1addr
- -sectcreate
- -sectobjectsymbols
- -sectorder
- -segaddr
- -segs_read_only_addr
- -segs_read_write_addr
- -seg_addr_table
- -seg_addr_table_filename
- -seglinkedit
- -segprot
- -segs_read_only_addr
- -segs_read_write_addr
- -single_module
- -static
- -sub_library
- -sub_umbrella
- -twolevel_namespace
- -umbrella
- -undefined
- -unexported_symbols_list
- -weak_reference_mismatches
- -whatsloaded
- These options are passed to the Darwin linker. The Darwin linker man page describes them in detail.
DEC Alpha Options
These -m options are defined for the DEC Alpha implementations:
- -mno-soft-float
- -msoft-float
-
Use (do not use) the hardware floating-point instructions for floating-point operations. When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulations routines, these routines issue floating-point operations. If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them.
- -mfp-reg
- -mno-fp-regs
-
Generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies -msoft-float. If the floating-point register set is not used, floating-point operands are passed in integer registers as if they were integers and floating-point results are passed in $0 instead of $f0. This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with -mno-fp-regs must also be compiled with that option.
- -mieee
- The Alpha architecture implements floating-point hardware optimized for maximum performance. It is mostly compliant with the IEEE floating-point standard. However, for full compliance, software assistance is required. This option generates code fully IEEE-compliant code except that the inexact-flag is not maintained (see below). If this option is turned on, the preprocessor macro "_IEEE_FP" is defined during compilation. The resulting code is less efficient but is able to correctly support denormalized numbers and exceptional IEEE values such as not-a-number and plus/minus infinity. Other Alpha compilers call this option -ieee_with_no_inexact.
- -mieee-with-inexact
- This is like -mieee except the generated code also maintains the IEEE inexact-flag. Turning on this option causes the generated code to implement fully-compliant IEEE math. In addition to "_IEEE_FP", "_IEEE_FP_EXACT" is defined as a preprocessor macro. On some Alpha implementations the resulting code may execute significantly slower than the code generated by default. Since there is very little code that depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call this option -ieee_with_inexact.
- -mfp-trap-mode=trap-mode
- This option controls what floating-point related traps are enabled. Other Alpha compilers call this option -fptm trap-mode. The trap mode can be set to one of four values:
- n
- This is the default (normal) setting. The only traps that are enabled are the ones that cannot be disabled in software (e.g., division by zero trap).
- u
- In addition to the traps enabled by n, underflow traps are enabled as well.
- su
- Like u, but the instructions are marked to be safe for software completion (see Alpha architecture manual for details).
- sui
- Like su, but inexact traps are enabled as well.
- -mfp-rounding-mode=rounding-mode
- Selects the IEEE rounding mode. Other Alpha compilers call this option -fprm rounding-mode. The rounding-mode can be one of:
- n
- Normal IEEE rounding mode. Floating-point numbers are rounded towards the nearest machine number or towards the even machine number in case of a tie.
- m
- Round towards minus infinity.
- c
- Chopped rounding mode. Floating-point numbers are rounded towards zero.
- d
- Dynamic rounding mode. A field in the floating-point control register ( fpcr, see Alpha architecture reference manual) controls the rounding mode in effect. The C library initializes this register for rounding towards plus infinity. Thus, unless your program modifies the fpcr, d corresponds to round towards plus infinity.
- -mtrap-precision=trap-precision
- In the Alpha architecture, floating-point traps are imprecise. This means without software assistance it is impossible to recover from a floating trap and program execution normally needs to be terminated. GCC can generate code that can assist operating system trap handlers in determining the exact location that caused a floating-point trap. Depending on the requirements of an application, different levels of precisions can be selected:
- p
- Program precision. This option is the default and means a trap handler can only identify which program caused a floating-point exception.
- f
- Function precision. The trap handler can determine the function that caused a floating-point exception.
- i
- Instruction precision. The trap handler can determine the exact instruction that caused a floating-point exception.
- -mieee-conformant
- This option marks the generated code as IEEE conformant. You must not use this option unless you also specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect is to emit the line .eflag 48 in the function prologue of the generated assembly file.
- -mbuild-constants
-
Normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions. If it cannot, it outputs the constant as a literal and generates code to load it from the data segment at run time.
- -mbwx
- -mno-bwx
- -mcix
- -mno-cix
- -mfix
- -mno-fix
- -mmax
- -mno-max
- Indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets. The default is to use the instruction sets supported by the CPU type specified via -mcpu= option or that of the CPU on which GCC was built if none is specified.
- -mfloat-vax
- -mfloat-ieee
- Generate code that uses (does not use) VAX F and G floating-point arithmetic instead of IEEE single and double precision.
- -mexplicit-relocs
- -mno-explicit-relocs
- Older Alpha assemblers provided no way to generate symbol relocations except via assembler macros. Use of these macros does not allow optimal instruction scheduling. GNU binutils as of version 2.12 supports a new syntax that allows the compiler to explicitly mark which relocations should apply to which instructions. This option is mostly useful for debugging, as GCC detects the capabilities of the assembler when it is built and sets the default accordingly.
- -msmall-data
- -mlarge-data
-
When -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations. When -msmall-data is used, objects 8 bytes long or smaller are placed in a small data area (the ".sdata" and ".sbss" sections) and are accessed via 16-bit relocations off of the $gp register. This limits the size of the small data area to 64KB, but allows the variables to be directly accessed via a single instruction.
- -msmall-text
- -mlarge-text
-
When -msmall-text is used, the compiler assumes that the code of the entire program (or shared library) fits in 4MB, and is thus reachable with a branch instruction. When -msmall-data is used, the compiler can assume that all local symbols share the same $gp value, and thus reduce the number of instructions required for a function call from 4 to 1.
- -mcpu=cpu_type
-
Set the instruction set and instruction scheduling parameters for machine type cpu_type. You can specify either the EV style name or the corresponding chip number. GCC supports scheduling parameters for the EV4, EV5 and EV6 family of processors and chooses the default values for the instruction set from the processor you specify. If you do not specify a processor type, GCC defaults to the processor on which the compiler was built.
- ev4
- ev45
- 21064
- Schedules as an EV4 and has no instruction set extensions.
- ev5
- 21164
- Schedules as an EV5 and has no instruction set extensions.
- ev56
- 21164a
- Schedules as an EV5 and supports the BWX extension.
- pca56
- 21164pc
- 21164PC
- Schedules as an EV5 and supports the BWX and MAX extensions.
- ev6
- 21264
- Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
- ev67
- 21264a
- Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
- -mtune=cpu_type
-
Set only the instruction scheduling parameters for machine type cpu_type. The instruction set is not changed.
- -mmemory-latency=time
-
Sets the latency the scheduler should assume for typical memory references as seen by the application. This number is highly dependent on the memory access patterns used by the application and the size of the external cache on the machine.
- number
- A decimal number representing clock cycles.
- L1
- L2
- L3
- main
- The compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that L3 is only valid for EV5.
FR30 Options
These options are defined specifically for the FR30 port.
- -msmall-model
- Use the small address space model. This can produce smaller code, but it does assume that all symbolic values and addresses fit into a 20-bit range.
- -mno-lsim
- Assume that runtime support has been provided and so there is no need to include the simulator library ( libsim.a) on the linker command line.
FRV Options
- -mgpr-32
- Only use the first 32 general-purpose registers.
- -mgpr-64
- Use all 64 general-purpose registers.
- -mfpr-32
- Use only the first 32 floating-point registers.
- -mfpr-64
- Use all 64 floating-point registers.
- -mhard-float
- Use hardware instructions for floating-point operations.
- -msoft-float
- Use library routines for floating-point operations.
- -malloc-cc
- Dynamically allocate condition code registers.
- -mfixed-cc
- Do not try to dynamically allocate condition code registers, only use "icc0" and "fcc0".
- -mdword
- Change ABI to use double word insns.
- -mno-dword
- Do not use double word instructions.
- -mdouble
- Use floating-point double instructions.
- -mno-double
- Do not use floating-point double instructions.
- -mmedia
- Use media instructions.
- -mno-media
- Do not use media instructions.
- -mmuladd
- Use multiply and add/subtract instructions.
- -mno-muladd
- Do not use multiply and add/subtract instructions.
- -mfdpic
- Select the FDPIC ABI, which uses function descriptors to represent pointers to functions. Without any PIC/PIE-related options, it implies -fPIE. With -fpic or -fpie, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with -fPIC or -fPIE, GOT offsets are computed with 32 bits. With a bfin-elf target, this option implies -msim.
- -minline-plt
- Enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without -mfdpic. It's enabled by default if optimizing for speed and compiling for shared libraries (i.e., -fPIC or -fpic), or when an optimization option such as -O3 or above is present in the command line.
- -mTLS
- Assume a large TLS segment when generating thread-local code.
- -mtls
- Do not assume a large TLS segment when generating thread-local code.
- -mgprel-ro
- Enable the use of "GPREL" relocations in the FDPIC ABI for data that is known to be in read-only sections. It's enabled by default, except for -fpic or -fpie: even though it may help make the global offset table smaller, it trades 1 instruction for 4. With -fPIC or -fPIE, it trades 3 instructions for 4, one of which may be shared by multiple symbols, and it avoids the need for a GOT entry for the referenced symbol, so it's more likely to be a win. If it is not, -mno-gprel-ro can be used to disable it.
- -multilib-library-pic
- Link with the (library, not FD) pic libraries. It's implied by -mlibrary-pic, as well as by -fPIC and -fpic without -mfdpic. You should never have to use it explicitly.
- -mlinked-fp
- Follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. This option is enabled by default and can be disabled with -mno-linked-fp.
- -mlong-calls
- Use indirect addressing to call functions outside the current compilation unit. This allows the functions to be placed anywhere within the 32-bit address space.
- -malign-labels
- Try to align labels to an 8-byte boundary by inserting NOPs into the previous packet. This option only has an effect when VLIW packing is enabled. It doesn't create new packets; it merely adds NOPs to existing ones.
- -mlibrary-pic
- Generate position-independent EABI code.
- -macc-4
- Use only the first four media accumulator registers.
- -macc-8
- Use all eight media accumulator registers.
- -mpack
- Pack VLIW instructions.
- -mno-pack
- Do not pack VLIW instructions.
- -mno-eflags
- Do not mark ABI switches in e_flags.
- -mcond-move
-
Enable the use of conditional-move instructions (default).
- -mno-cond-move
-
Disable the use of conditional-move instructions.
- -mscc
-
Enable the use of conditional set instructions (default).
- -mno-scc
-
Disable the use of conditional set instructions.
- -mcond-exec
-
Enable the use of conditional execution (default).
- -mno-cond-exec
-
Disable the use of conditional execution.
- -mvliw-branch
-
Run a pass to pack branches into VLIW instructions (default).
- -mno-vliw-branch
-
Do not run a pass to pack branches into VLIW instructions.
- -mmulti-cond-exec
-
Enable optimization of "&&" and "||" in conditional execution (default).
- -mno-multi-cond-exec
-
Disable optimization of "&&" and "||" in conditional execution.
- -mnested-cond-exec
-
Enable nested conditional execution optimizations (default).
- -mno-nested-cond-exec
-
Disable nested conditional execution optimizations.
- -moptimize-membar
- This switch removes redundant "membar" instructions from the compiler-generated code. It is enabled by default.
- -mno-optimize-membar
- This switch disables the automatic removal of redundant "membar" instructions from the generated code.
- -mtomcat-stats
- Cause gas to print out tomcat statistics.
- -mcpu=cpu
- Select the processor type for which to generate code. Possible values are frv, fr550, tomcat, fr500, fr450, fr405, fr400, fr300 and simple.
GNU/Linux Options
These -m options are defined for GNU/Linux targets:
- -mglibc
- Use the GNU C library. This is the default except on *-*-linux-*uclibc* and *-*-linux-*android* targets.
- -muclibc
- Use uClibc C library. This is the default on *-*-linux-*uclibc* targets.
- -mbionic
- Use Bionic C library. This is the default on *-*-linux-*android* targets.
- -mandroid
-
Compile code compatible with Android platform. This is the default on *-*-linux-*android* targets.
- -tno-android-cc
- Disable compilation effects of -mandroid, i.e., do not enable -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default.
- -tno-android-ld
- Disable linking effects of -mandroid, i.e., pass standard Linux linking options to the linker.
H8/300 Options
These -m options are defined for the H8/300 implementations:
- -mrelax
- Shorten some address references at link time, when possible; uses the linker option -relax.
- -mh
- Generate code for the H8/300H.
- -ms
- Generate code for the H8S.
- -mn
- Generate code for the H8S and H8/300H in the normal mode. This switch must be used either with -mh or -ms.
- -ms2600
- Generate code for the H8S/2600. This switch must be used with -ms.
- -mexr
- Extended registers are stored on stack before execution of function with monitor attribute. Default option is -mexr. This option is valid only for H8S targets.
- -mno-exr
- Extended registers are not stored on stack before execution of function with monitor attribute. Default option is -mno-exr. This option is valid only for H8S targets.
- -mint32
- Make "int" data 32 bits by default.
- -malign-300
- On the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and H8S is to align longs and floats on 4-byte boundaries. -malign-300 causes them to be aligned on 2-byte boundaries. This option has no effect on the H8/300.
HPPA Options
These -m options are defined for the HPPA family of computers:
- -march=architecture-type
- Generate code for the specified architecture. The choices for architecture-type are 1.0 for PA 1.0, 1.1 for PA 1.1, and 2.0 for PA 2.0 processors. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper architecture option for your machine. Code compiled for lower numbered architectures runs on higher numbered architectures, but not the other way around.
- -mpa-risc-1-0
- -mpa-risc-1-1
- -mpa-risc-2-0
- Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.
- -mbig-switch
- Generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out-of-range branches within a switch table.
- -mjump-in-delay
- Fill delay slots of function calls with unconditional jump instructions by modifying the return pointer for the function call to be the target of the conditional jump.
- -mdisable-fpregs
- Prevent floating-point registers from being used in any manner. This is necessary for compiling kernels that perform lazy context switching of floating-point registers. If you use this option and attempt to perform floating-point operations, the compiler aborts.
- -mdisable-indexing
- Prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH.
- -mno-space-regs
-
Generate code that assumes the target has no space registers. This allows GCC to generate faster indirect calls and use unscaled index address modes.
- -mfast-indirect-calls
-
Generate code that assumes calls never cross space boundaries. This allows GCC to emit code that performs faster indirect calls.
- -mfixed-range=register-range
- Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
- -mlong-load-store
- Generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is equivalent to the +k option to the HP compilers.
- -mportable-runtime
- Use the portable calling conventions proposed by HP for ELF systems.
- -mgas
- Enable the use of assembler directives only GAS understands.
- -mschedule=cpu-type
- Schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type are 700 7100, 7100LC, 7200, 7300 and 8000. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper scheduling option for your machine. The default scheduling is 8000.
- -mlinker-opt
- Enable the optimization pass in the HP-UX linker. Note this makes symbolic debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when linking some programs.
- -msoft-float
-
Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all HPPA targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.
- -msio
- Generate the predefine, "_SIO", for server IO. The default is -mwsio. This generates the predefines, "__hp9000s700", "__hp9000s700__" and "_WSIO", for workstation IO. These options are available under HP-UX and HI-UX.
- -mgnu-ld
- Use options specific to GNU ld. This passes -shared to ld when building a shared library. It is the default when GCC is configured, explicitly or implicitly, with the GNU linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which `gcc -print-prog-name=ld`. This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
- -mhp-ld
- Use options specific to HP ld. This passes -b to ld when building a shared library and passes +Accept TypeMismatch to ld on all links. It is the default when GCC is configured, explicitly or implicitly, with the HP linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which `gcc -print-prog-name=ld`. This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
- -mlong-calls
-
Generate code that uses long call sequences. This ensures that a call is always able to reach linker generated stubs. The default is to generate long calls only when the distance from the call site to the beginning of the function or translation unit, as the case may be, exceeds a predefined limit set by the branch type being used. The limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes.
- -munix=unix-std
-
Generate compiler predefines and select a startfile for the specified UNIX standard. The choices for unix-std are 93, 95 and 98. 93 is supported on all HP-UX versions. 95 is available on HP-UX 10.10 and later. 98 is available on HP-UX 11.11 and later. The default values are 93 for HP-UX 10.00, 95 for HP-UX 10.10 though to 11.00, and 98 for HP-UX 11.11 and later.
- -nolibdld
- Suppress the generation of link options to search libdld.sl when the -static option is specified on HP-UX 10 and later.
- -static
-
The HP-UX implementation of setlocale in libc has a dependency on libdld.sl. There isn't an archive version of libdld.sl. Thus, when the -static option is specified, special link options are needed to resolve this dependency.
- -threads
- Add support for multithreading with the dce thread library under HP-UX. This option sets flags for both the preprocessor and linker.
Intel 386 and AMD x86-64 Options
These -m options are defined for the i386 and x86-64 family of computers:
- -march=cpu-type
-
Generate instructions for the machine type cpu-type. In contrast to -mtune=cpu-type, which merely tunes the generated code for the specified cpu-type, -march=cpu-type allows GCC to generate code that may not run at all on processors other than the one indicated. Specifying -march=cpu-type implies -mtune=cpu-type.
- native
- This selects the CPU to generate code for at compilation time by determining the processor type of the compiling machine. Using -march=native enables all instruction subsets supported by the local machine (hence the result might not run on different machines). Using -mtune=native produces code optimized for the local machine under the constraints of the selected instruction set.
- i386
- Original Intel i386 CPU.
- i486
- Intel i486 CPU. (No scheduling is implemented for this chip.)
- i586
- pentium
- Intel Pentium CPU with no MMX support.
- pentium-mmx
- Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support.
- pentiumpro
- Intel Pentium Pro CPU.
- i686
- When used with -march, the Pentium Pro instruction set is used, so the code runs on all i686 family chips. When used with -mtune, it has the same meaning as generic.
- pentium2
- Intel Pentium II CPU, based on Pentium Pro core with MMX instruction set support.
- pentium3
- pentium3m
- Intel Pentium III CPU, based on Pentium Pro core with MMX and SSE instruction set support.
- pentium-m
- Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE and SSE2 instruction set support. Used by Centrino notebooks.
- pentium4
- pentium4m
- Intel Pentium 4 CPU with MMX, SSE and SSE2 instruction set support.
- prescott
- Improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support.
- nocona
- Improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2 and SSE3 instruction set support.
- core2
- Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.
- corei7
- Intel Core i7 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1 and SSE4.2 instruction set support.
- corei7-avx
- Intel Core i7 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AES and PCLMUL instruction set support.
- core-avx-i
- Intel Core CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AES, PCLMUL, FSGSBASE, RDRND and F16C instruction set support.
- core-avx2
- Intel Core CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2 and F16C instruction set support.
- atom
- Intel Atom CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.
- k6
- AMD K6 CPU with MMX instruction set support.
- k6-2
- k6-3
- Improved versions of AMD K6 CPU with MMX and 3DNow! instruction set support.
- athlon
- athlon-tbird
- AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions support.
- athlon-4
- athlon-xp
- athlon-mp
- Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and full SSE instruction set support.
- k8
- opteron
- athlon64
- athlon-fx
- Processors based on the AMD K8 core with x86-64 instruction set support, including the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (This supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and 64-bit instruction set extensions.)
- k8-sse3
- opteron-sse3
- athlon64-sse3
- Improved versions of AMD K8 cores with SSE3 instruction set support.
- amdfam10
- barcelona
- CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions.)
- bdver1
- CPUs based on AMD Family 15h cores with x86-64 instruction set support. (This supersets FMA4, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
- bdver2
- AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
- bdver3
- AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.
- btver1
- CPUs based on AMD Family 14h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A, CX16, ABM and 64-bit instruction set extensions.)
- btver2
- CPUs based on AMD Family 16h cores with x86-64 instruction set support. This includes MOVBE, F16C, BMI, AVX, PCL_MUL, AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and 64-bit instruction set extensions.
- winchip-c6
- IDT WinChip C6 CPU, dealt in same way as i486 with additional MMX instruction set support.
- winchip2
- IDT WinChip 2 CPU, dealt in same way as i486 with additional MMX and 3DNow! instruction set support.
- c3
- VIA C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)
- c3-2
- VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)
- geode
- AMD Geode embedded processor with MMX and 3DNow! instruction set support.
- -mtune=cpu-type
-
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. While picking a specific cpu-type schedules things appropriately for that particular chip, the compiler does not generate any code that cannot run on the default machine type unless you use a -march=cpu-type option. For example, if GCC is configured for i686-pc-linux-gnu then -mtune=pentium4 generates code that is tuned for Pentium 4 but still runs on i686 machines.
- generic
-
Produce code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=generic. But, if you do not know exactly what CPU users of your application will have, then you should use this option.
- -mcpu=cpu-type
- A deprecated synonym for -mtune.
- -mfpmath=unit
- Generate floating-point arithmetic for selected unit unit. The choices for unit are:
- 387
-
Use the standard 387 floating-point coprocessor present on the majority of chips and emulated otherwise. Code compiled with this option runs almost everywhere. The temporary results are computed in 80-bit precision instead of the precision specified by the type, resulting in slightly different results compared to most of other chips. See -ffloat-store for more detailed description.
- sse
-
Use scalar floating-point instructions present in the SSE instruction set. This instruction set is supported by Pentium III and newer chips, and in the AMD line by Athlon-4, Athlon XP and Athlon MP chips. The earlier version of the SSE instruction set supports only single-precision arithmetic, thus the double and extended-precision arithmetic are still done using 387. A later version, present only in Pentium 4 and AMD x86-64 chips, supports double-precision arithmetic too.
- sse,387
- sse+387
- both
- Attempt to utilize both instruction sets at once. This effectively doubles the amount of available registers, and on chips with separate execution units for 387 and SSE the execution resources too. Use this option with care, as it is still experimental, because the GCC register allocator does not model separate functional units well, resulting in unstable performance.
- -masm=dialect
- Output assembly instructions using selected dialect. Supported choices are intel or att (the default). Darwin does not support intel.
- -mieee-fp
- -mno-ieee-fp
- Control whether or not the compiler uses IEEE floating-point comparisons. These correctly handle the case where the result of a comparison is unordered.
- -msoft-float
-
Generate output containing library calls for floating point.
- -mno-fp-ret-in-387
-
Do not use the FPU registers for return values of functions.
- -mno-fancy-math-387
- Some 387 emulators do not support the "sin", "cos" and "sqrt" instructions for the 387. Specify this option to avoid generating those instructions. This option is the default on FreeBSD, OpenBSD and NetBSD. This option is overridden when -march indicates that the target CPU always has an FPU and so the instruction does not need emulation. These instructions are not generated unless you also use the -funsafe-math-optimizations switch.
- -malign-double
- -mno-align-double
-
Control whether GCC aligns "double", "long double", and "long long" variables on a two-word boundary or a one-word boundary. Aligning "double" variables on a two-word boundary produces code that runs somewhat faster on a Pentium at the expense of more memory.
- -m96bit-long-double
- -m128bit-long-double
-
These switches control the size of "long double" type. The i386 application binary interface specifies the size to be 96 bits, so -m96bit-long-double is the default in 32-bit mode.
- -mlong-double-64
- -mlong-double-80
-
These switches control the size of "long double" type. A size of 64 bits makes the "long double" type equivalent to the "double" type. This is the default for Bionic C library.
- -mlarge-data-threshold=threshold
- When -mcmodel=medium is specified, data objects larger than threshold are placed in the large data section. This value must be the same across all objects linked into the binary, and defaults to 65535.
- -mrtd
-
Use a different function-calling convention, in which functions that take a fixed number of arguments return with the "ret num" instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there.
- -mregparm=num
-
Control how many registers are used to pass integer arguments. By default, no registers are used to pass arguments, and at most 3 registers can be used. You can control this behavior for a specific function by using the function attribute regparm.
- -msseregparm
-
Use SSE register passing conventions for float and double arguments and return values. You can control this behavior for a specific function by using the function attribute sseregparm.
- -mvect8-ret-in-mem
- Return 8-byte vectors in memory instead of MMX registers. This is the default on Solaris@tie{}8 and 9 and VxWorks to match the ABI of the Sun Studio compilers until version 12. Later compiler versions (starting with Studio 12 Update@tie{}1) follow the ABI used by other x86 targets, which is the default on Solaris@tie{}10 and later. Only use this option if you need to remain compatible with existing code produced by those previous compiler versions or older versions of GCC.
- -mpc32
- -mpc64
- -mpc80
-
Set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32 is specified, the significands of results of floating-point operations are rounded to 24 bits (single precision); -mpc64 rounds the significands of results of floating-point operations to 53 bits (double precision) and -mpc80 rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default. When this option is used, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly.
- -mstackrealign
- Realign the stack at entry. On the Intel x86, the -mstackrealign option generates an alternate prologue and epilogue that realigns the run-time stack if necessary. This supports mixing legacy codes that keep 4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE compatibility. See also the attribute "force_align_arg_pointer", applicable to individual functions.
- -mpreferred-stack-boundary=num
-
Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits).
- -mincoming-stack-boundary=num
-
Assume the incoming stack is aligned to a 2 raised to num byte boundary. If -mincoming-stack-boundary is not specified, the one specified by -mpreferred-stack-boundary is used.
- -mmmx
- -mno-mmx
- -msse
- -mno-sse
- -msse2
- -mno-sse2
- -msse3
- -mno-sse3
- -mssse3
- -mno-ssse3
- -msse4.1
- -mno-sse4.1
- -msse4.2
- -mno-sse4.2
- -msse4
- -mno-sse4
- -mavx
- -mno-avx
- -mavx2
- -mno-avx2
- -maes
- -mno-aes
- -mpclmul
- -mno-pclmul
- -mfsgsbase
- -mno-fsgsbase
- -mrdrnd
- -mno-rdrnd
- -mf16c
- -mno-f16c
- -mfma
- -mno-fma
- -msse4a
- -mno-sse4a
- -mfma4
- -mno-fma4
- -mxop
- -mno-xop
- -mlwp
- -mno-lwp
- -m3dnow
- -mno-3dnow
- -mpopcnt
- -mno-popcnt
- -mabm
- -mno-abm
- -mbmi
- -mbmi2
- -mno-bmi
- -mno-bmi2
- -mlzcnt
- -mno-lzcnt
- -mrtm
- -mtbm
- -mno-tbm
-
These switches enable or disable the use of instructions in the MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, F16C, FMA, SSE4A, FMA4, XOP, LWP, ABM, BMI, BMI2, LZCNT, RTM or 3DNow! extended instruction sets. These extensions are also available as built-in functions: see X86 Built-in Functions, for details of the functions enabled and disabled by these switches.
- -mcld
- This option instructs GCC to emit a "cld" instruction in the prologue of functions that use string instructions. String instructions depend on the DF flag to select between autoincrement or autodecrement mode. While the ABI specifies the DF flag to be cleared on function entry, some operating systems violate this specification by not clearing the DF flag in their exception dispatchers. The exception handler can be invoked with the DF flag set, which leads to wrong direction mode when string instructions are used. This option can be enabled by default on 32-bit x86 targets by configuring GCC with the --enable-cld configure option. Generation of "cld" instructions can be suppressed with the -mno-cld compiler option in this case.
- -mvzeroupper
- This option instructs GCC to emit a "vzeroupper" instruction before a transfer of control flow out of the function to minimize the AVX to SSE transition penalty as well as remove unnecessary "zeroupper" intrinsics.
- -mprefer-avx128
- This option instructs GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer.
- -mcx16
- This option enables GCC to generate "CMPXCHG16B" instructions. "CMPXCHG16B" allows for atomic operations on 128-bit double quadword (or oword) data types. This is useful for high-resolution counters that can be updated by multiple processors (or cores). This instruction is generated as part of atomic built-in functions: see __sync Builtins or __atomic Builtins for details.
- -msahf
- This option enables generation of "SAHF" instructions in 64-bit code. Early Intel Pentium 4 CPUs with Intel 64 support, prior to the introduction of Pentium 4 G1 step in December 2005, lacked the "LAHF" and "SAHF" instructions which were supported by AMD64. These are load and store instructions, respectively, for certain status flags. In 64-bit mode, the "SAHF" instruction is used to optimize "fmod", "drem", and "remainder" built-in functions; see Other Builtins for details.
- -mmovbe
- This option enables use of the "movbe" instruction to implement "__builtin_bswap32" and "__builtin_bswap64".
- -mcrc32
- This option enables built-in functions "__builtin_ia32_crc32qi", "__builtin_ia32_crc32hi", "__builtin_ia32_crc32si" and "__builtin_ia32_crc32di" to generate the "crc32" machine instruction.
- -mrecip
-
This option enables use of "RCPSS" and "RSQRTSS" instructions (and their vectorized variants "RCPPS" and "RSQRTPS") with an additional Newton-Raphson step to increase precision instead of "DIVSS" and "SQRTSS" (and their vectorized variants) for single-precision floating-point arguments. These instructions are generated only when -funsafe-math-optimizations is enabled together with -finite-math-only and -fno-trapping-math. Note that while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994).
- -mrecip=opt
- This option controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a ! to invert the option:
- all
- Enable all estimate instructions.
- default
- Enable the default instructions, equivalent to -mrecip.
- none
- Disable all estimate instructions, equivalent to -mno-recip.
- div
- Enable the approximation for scalar division.
- vec-div
- Enable the approximation for vectorized division.
- sqrt
- Enable the approximation for scalar square root.
- vec-sqrt
- Enable the approximation for vectorized square root.
- -mveclibabi=type
-
Specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values for type are svml for the Intel short vector math library and acml for the AMD math core library. To use this option, both -ftree-vectorize and -funsafe-math-optimizations have to be enabled, and an SVML or ACML ABI-compatible library must be specified at link time.
- -mabi=name
- Generate code for the specified calling convention. Permissible values are sysv for the ABI used on GNU/Linux and other systems, and ms for the Microsoft ABI. The default is to use the Microsoft ABI when targeting Microsoft Windows and the SysV ABI on all other systems. You can control this behavior for a specific function by using the function attribute ms_abi/sysv_abi.
- -mtls-dialect=type
- Generate code to access thread-local storage using the gnu or gnu2 conventions. gnu is the conservative default; gnu2 is more efficient, but it may add compile- and run-time requirements that cannot be satisfied on all systems.
- -mpush-args
- -mno-push-args
- Use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as method using SUB/MOV operations and is enabled by default. In some cases disabling it may improve performance because of improved scheduling and reduced dependencies.
- -maccumulate-outgoing-args
- If enabled, the maximum amount of space required for outgoing arguments is computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when the preferred stack boundary is not equal to 2. The drawback is a notable increase in code size. This switch implies -mno-push-args.
- -mthreads
- Support thread-safe exception handling on MinGW. Programs that rely on thread-safe exception handling must compile and link all code with the -mthreads option. When compiling, -mthreads defines "-D_MT"; when linking, it links in a special thread helper library -lmingwthrd which cleans up per-thread exception-handling data.
- -mno-align-stringops
- Do not align the destination of inlined string operations. This switch reduces code size and improves performance in case the destination is already aligned, but GCC doesn't know about it.
- -minline-all-stringops
- By default GCC inlines string operations only when the destination is known to be aligned to least a 4-byte boundary. This enables more inlining and increases code size, but may improve performance of code that depends on fast "memcpy", "strlen", and "memset" for short lengths.
- -minline-stringops-dynamically
- For string operations of unknown size, use run-time checks with inline code for small blocks and a library call for large blocks.
- -mstringop-strategy=alg
- Override the internal decision heuristic for the particular algorithm to use for inlining string operations. The allowed values for alg are:
- rep_byte
- rep_4byte
- rep_8byte
- Expand using i386 "rep" prefix of the specified size.
- byte_loop
- loop
- unrolled_loop
- Expand into an inline loop.
- libcall
- Always use a library call.
- -momit-leaf-frame-pointer
- Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up, and restore frame pointers and makes an extra register available in leaf functions. The option -fomit-leaf-frame-pointer removes the frame pointer for leaf functions, which might make debugging harder.
- -mtls-direct-seg-refs
- -mno-tls-direct-seg-refs
-
Controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for 32-bit, %fs for 64-bit), or whether the thread base pointer must be added. Whether or not this is valid depends on the operating system, and whether it maps the segment to cover the entire TLS area.
- -msse2avx
- -mno-sse2avx
- Specify that the assembler should encode SSE instructions with VEX prefix. The option -mavx turns this on by default.
- -mfentry
- -mno-fentry
- If profiling is active (-pg), put the profiling counter call before the prologue. Note: On x86 architectures the attribute "ms_hook_prologue" isn't possible at the moment for -mfentry and -pg.
- -m8bit-idiv
- -mno-8bit-idiv
- On some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than 32-bit/64-bit integer divide. This option generates a run-time check. If both dividend and divisor are within range of 0 to 255, 8-bit unsigned integer divide is used instead of 32-bit/64-bit integer divide.
- -mavx256-split-unaligned-load
- -mavx256-split-unaligned-store
- Split 32-byte AVX unaligned load and store.
These -m switches are supported in addition to the above on x86-64 processors in 64-bit environments.
- -m32
- -m64
- -mx32
-
Generate code for a 32-bit or 64-bit environment. The -m32 option sets "int", "long", and pointer types to 32 bits, and generates code that runs on any i386 system.
- -mno-red-zone
- Do not use a so-called "red zone" for x86-64 code. The red zone is mandated by the x86-64 ABI; it is a 128-byte area beyond the location of the stack pointer that is not modified by signal or interrupt handlers and therefore can be used for temporary data without adjusting the stack pointer. The flag -mno-red-zone disables this red zone.
- -mcmodel=small
- Generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the default code model.
- -mcmodel=kernel
- Generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space. This model has to be used for Linux kernel code.
- -mcmodel=medium
- Generate code for the medium model: the program is linked in the lower 2 GB of the address space. Small symbols are also placed there. Symbols with sizes larger than -mlarge-data-threshold are put into large data or BSS sections and can be located above 2GB. Programs can be statically or dynamically linked.
- -mcmodel=large
- Generate code for the large model. This model makes no assumptions about addresses and sizes of sections.
- -maddress-mode=long
- Generate code for long address mode. This is only supported for 64-bit and x32 environments. It is the default address mode for 64-bit environments.
- -maddress-mode=short
- Generate code for short address mode. This is only supported for 32-bit and x32 environments. It is the default address mode for 32-bit and x32 environments.
i386 and x86-64 Windows Options
These additional options are available for Microsoft Windows targets:
- -mconsole
- This option specifies that a console application is to be generated, by instructing the linker to set the PE header subsystem type required for console applications. This option is available for Cygwin and MinGW targets and is enabled by default on those targets.
- -mdll
- This option is available for Cygwin and MinGW targets. It specifies that a DLL---a dynamic link library---is to be generated, enabling the selection of the required runtime startup object and entry point.
- -mnop-fun-dllimport
- This option is available for Cygwin and MinGW targets. It specifies that the "dllimport" attribute should be ignored.
- -mthread
- This option is available for MinGW targets. It specifies that MinGW-specific thread support is to be used.
- -municode
- This option is available for MinGW-w64 targets. It causes the "UNICODE" preprocessor macro to be predefined, and chooses Unicode-capable runtime startup code.
- -mwin32
- This option is available for Cygwin and MinGW targets. It specifies that the typical Microsoft Windows predefined macros are to be set in the pre-processor, but does not influence the choice of runtime library/startup code.
- -mwindows
- This option is available for Cygwin and MinGW targets. It specifies that a GUI application is to be generated by instructing the linker to set the PE header subsystem type appropriately.
- -fno-set-stack-executable
- This option is available for MinGW targets. It specifies that the executable flag for the stack used by nested functions isn't set. This is necessary for binaries running in kernel mode of Microsoft Windows, as there the User32 API, which is used to set executable privileges, isn't available.
- -fwritable-relocated-rdata
- This option is available for MinGW and Cygwin targets. It specifies that relocated-data in read-only section is put into .data section. This is a necessary for older runtimes not supporting modification of .rdata sections for pseudo-relocation.
- -mpe-aligned-commons
- This option is available for Cygwin and MinGW targets. It specifies that the GNU extension to the PE file format that permits the correct alignment of COMMON variables should be used when generating code. It is enabled by default if GCC detects that the target assembler found during configuration supports the feature.
See also under i386 and x86-64 Options for standard options.
IA-64 Options
These are the -m options defined for the Intel IA-64 architecture.
- -mbig-endian
- Generate code for a big-endian target. This is the default for HP-UX.
- -mlittle-endian
- Generate code for a little-endian target. This is the default for AIX5 and GNU/Linux.
- -mgnu-as
- -mno-gnu-as
- Generate (or don't) code for the GNU assembler. This is the default.
- -mgnu-ld
- -mno-gnu-ld
- Generate (or don't) code for the GNU linker. This is the default.
- -mno-pic
- Generate code that does not use a global pointer register. The result is not position independent code, and violates the IA-64 ABI.
- -mvolatile-asm-stop
- -mno-volatile-asm-stop
- Generate (or don't) a stop bit immediately before and after volatile asm statements.
- -mregister-names
- -mno-register-names
- Generate (or don't) in, loc, and out register names for the stacked registers. This may make assembler output more readable.
- -mno-sdata
- -msdata
- Disable (or enable) optimizations that use the small data section. This may be useful for working around optimizer bugs.
- -mconstant-gp
- Generate code that uses a single constant global pointer value. This is useful when compiling kernel code.
- -mauto-pic
- Generate code that is self-relocatable. This implies -mconstant-gp. This is useful when compiling firmware code.
- -minline-float-divide-min-latency
- Generate code for inline divides of floating-point values using the minimum latency algorithm.
- -minline-float-divide-max-throughput
- Generate code for inline divides of floating-point values using the maximum throughput algorithm.
- -mno-inline-float-divide
- Do not generate inline code for divides of floating-point values.
- -minline-int-divide-min-latency
- Generate code for inline divides of integer values using the minimum latency algorithm.
- -minline-int-divide-max-throughput
- Generate code for inline divides of integer values using the maximum throughput algorithm.
- -mno-inline-int-divide
- Do not generate inline code for divides of integer values.
- -minline-sqrt-min-latency
- Generate code for inline square roots using the minimum latency algorithm.
- -minline-sqrt-max-throughput
- Generate code for inline square roots using the maximum throughput algorithm.
- -mno-inline-sqrt
- Do not generate inline code for "sqrt".
- -mfused-madd
- -mno-fused-madd
- Do (don't) generate code that uses the fused multiply/add or multiply/subtract instructions. The default is to use these instructions.
- -mno-dwarf2-asm
- -mdwarf2-asm
- Don't (or do) generate assembler code for the DWARF 2 line number debugging info. This may be useful when not using the GNU assembler.
- -mearly-stop-bits
- -mno-early-stop-bits
- Allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit. This can improve instruction scheduling, but does not always do so.
- -mfixed-range=register-range
- Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
- -mtls-size=tls-size
- Specify bit size of immediate TLS offsets. Valid values are 14, 22, and 64.
- -mtune=cpu-type
- Tune the instruction scheduling for a particular CPU, Valid values are itanium, itanium1, merced, itanium2, and mckinley.
- -milp32
- -mlp64
- Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. These are HP-UX specific flags.
- -mno-sched-br-data-spec
- -msched-br-data-spec
- (Dis/En)able data speculative scheduling before reload. This results in generation of "ld.a" instructions and the corresponding check instructions ("ld.c" / "chk.a"). The default is 'disable'.
- -msched-ar-data-spec
- -mno-sched-ar-data-spec
- (En/Dis)able data speculative scheduling after reload. This results in generation of "ld.a" instructions and the corresponding check instructions ("ld.c" / "chk.a"). The default is 'enable'.
- -mno-sched-control-spec
- -msched-control-spec
- (Dis/En)able control speculative scheduling. This feature is available only during region scheduling (i.e. before reload). This results in generation of the "ld.s" instructions and the corresponding check instructions "chk.s". The default is 'disable'.
- -msched-br-in-data-spec
- -mno-sched-br-in-data-spec
- (En/Dis)able speculative scheduling of the instructions that are dependent on the data speculative loads before reload. This is effective only with -msched-br-data-spec enabled. The default is 'enable'.
- -msched-ar-in-data-spec
- -mno-sched-ar-in-data-spec
- (En/Dis)able speculative scheduling of the instructions that are dependent on the data speculative loads after reload. This is effective only with -msched-ar-data-spec enabled. The default is 'enable'.
- -msched-in-control-spec
- -mno-sched-in-control-spec
- (En/Dis)able speculative scheduling of the instructions that are dependent on the control speculative loads. This is effective only with -msched-control-spec enabled. The default is 'enable'.
- -mno-sched-prefer-non-data-spec-insns
- -msched-prefer-non-data-spec-insns
- If enabled, data-speculative instructions are chosen for schedule only if there are no other choices at the moment. This makes the use of the data speculation much more conservative. The default is 'disable'.
- -mno-sched-prefer-non-control-spec-insns
- -msched-prefer-non-control-spec-insns
- If enabled, control-speculative instructions are chosen for schedule only if there are no other choices at the moment. This makes the use of the control speculation much more conservative. The default is 'disable'.
- -mno-sched-count-spec-in-critical-path
- -msched-count-spec-in-critical-path
- If enabled, speculative dependencies are considered during computation of the instructions priorities. This makes the use of the speculation a bit more conservative. The default is 'disable'.
- -msched-spec-ldc
- Use a simple data speculation check. This option is on by default.
- -msched-control-spec-ldc
- Use a simple check for control speculation. This option is on by default.
- -msched-stop-bits-after-every-cycle
- Place a stop bit after every cycle when scheduling. This option is on by default.
- -msched-fp-mem-deps-zero-cost
- Assume that floating-point stores and loads are not likely to cause a conflict when placed into the same instruction group. This option is disabled by default.
- -msel-sched-dont-check-control-spec
- Generate checks for control speculation in selective scheduling. This flag is disabled by default.
- -msched-max-memory-insns=max-insns
- Limit on the number of memory insns per instruction group, giving lower priority to subsequent memory insns attempting to schedule in the same instruction group. Frequently useful to prevent cache bank conflicts. The default value is 1.
- -msched-max-memory-insns-hard-limit
- Makes the limit specified by msched-max-memory-insns a hard limit, disallowing more than that number in an instruction group. Otherwise, the limit is "soft", meaning that non-memory operations are preferred when the limit is reached, but memory operations may still be scheduled.
LM32 Options
These -m options are defined for the LatticeMico32 architecture:
- -mbarrel-shift-enabled
- Enable barrel-shift instructions.
- -mdivide-enabled
- Enable divide and modulus instructions.
- -mmultiply-enabled
- Enable multiply instructions.
- -msign-extend-enabled
- Enable sign extend instructions.
- -muser-enabled
- Enable user-defined instructions.
M32C Options
- -mcpu=name
- Select the CPU for which code is generated. name may be one of r8c for the R8C/Tiny series, m16c for the M16C (up to /60) series, m32cm for the M16C/80 series, or m32c for the M32C/80 series.
- -msim
- Specifies that the program will be run on the simulator. This causes an alternate runtime library to be linked in which supports, for example, file I/O. You must not use this option when generating programs that will run on real hardware; you must provide your own runtime library for whatever I/O functions are needed.
- -memregs=number
- Specifies the number of memory-based pseudo-registers GCC uses during code generation. These pseudo-registers are used like real registers, so there is a tradeoff between GCC's ability to fit the code into available registers, and the performance penalty of using memory instead of registers. Note that all modules in a program must be compiled with the same value for this option. Because of that, you must not use this option with GCC's default runtime libraries.
M32R/D Options
These -m options are defined for Renesas M32R/D architectures:
- -m32r2
- Generate code for the M32R/2.
- -m32rx
- Generate code for the M32R/X.
- -m32r
- Generate code for the M32R. This is the default.
- -mmodel=small
-
Assume all objects live in the lower 16MB of memory (so that their addresses can be loaded with the "ld24" instruction), and assume all subroutines are reachable with the "bl" instruction. This is the default.
- -mmodel=medium
- Assume objects may be anywhere in the 32-bit address space (the compiler generates "seth/add3" instructions to load their addresses), and assume all subroutines are reachable with the "bl" instruction.
- -mmodel=large
- Assume objects may be anywhere in the 32-bit address space (the compiler generates "seth/add3" instructions to load their addresses), and assume subroutines may not be reachable with the "bl" instruction (the compiler generates the much slower "seth/add3/jl" instruction sequence).
- -msdata=none
-
Disable use of the small data area. Variables are put into one of .data, .bss, or .rodata (unless the "section" attribute has been specified). This is the default.
- -msdata=sdata
- Put small global and static data in the small data area, but do not generate special code to reference them.
- -msdata=use
- Put small global and static data in the small data area, and generate special instructions to reference them.
- -G num
-
Put global and static objects less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS sections. The default value of num is 8. The -msdata option must be set to one of sdata or use for this option to have any effect.
- -mdebug
- Makes the M32R-specific code in the compiler display some statistics that might help in debugging programs.
- -malign-loops
- Align all loops to a 32-byte boundary.
- -mno-align-loops
- Do not enforce a 32-byte alignment for loops. This is the default.
- -missue-rate=number
- Issue number instructions per cycle. number can only be 1 or 2.
- -mbranch-cost=number
- number can only be 1 or 2. If it is 1 then branches are preferred over conditional code, if it is 2, then the opposite applies.
- -mflush-trap=number
- Specifies the trap number to use to flush the cache. The default is 12. Valid numbers are between 0 and 15 inclusive.
- -mno-flush-trap
- Specifies that the cache cannot be flushed by using a trap.
- -mflush-func=name
- Specifies the name of the operating system function to call to flush the cache. The default is _flush_cache, but a function call is only used if a trap is not available.
- -mno-flush-func
- Indicates that there is no OS function for flushing the cache.
M680x0 Options
These are the -m options defined for M680x0 and ColdFire processors. The default settings depend on which architecture was selected when the compiler was configured; the defaults for the most common choices are given below.
- -march=arch
-
Generate code for a specific M680x0 or ColdFire instruction set architecture. Permissible values of arch for M680x0 architectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. ColdFire architectures are selected according to Freescale's ISA classification and the permissible values are: isaa, isaaplus, isab and isac.
- -mcpu=cpu
- Generate code for a specific M680x0 or ColdFire processor. The M680x0 cpus are: 68000, 68010, 68020, 68030, 68040, 68060, 68302, 68332 and cpu32. The ColdFire cpus are given by the table below, which also classifies the CPUs into families:
- Family : -mcpu arguments
- 51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
- 5206 : 5202 5204 5206
- 5206e : 5206e
- 5208 : 5207 5208
- 5211a : 5210a 5211a
- 5213 : 5211 5212 5213
- 5216 : 5214 5216
- 52235 : 52230 52231 52232 52233 52234 52235
- 5225 : 5224 5225
- 52259 : 52252 52254 52255 52256 52258 52259
- 5235 : 5232 5233 5234 5235 523x
- 5249 : 5249
- 5250 : 5250
- 5271 : 5270 5271
- 5272 : 5272
- 5275 : 5274 5275
- 5282 : 5280 5281 5282 528x
- 53017 : 53011 53012 53013 53014 53015 53016 53017
- 5307 : 5307
- 5329 : 5327 5328 5329 532x
- 5373 : 5372 5373 537x
- 5407 : 5407
- 5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484 5485
- -mtune=tune
-
Tune the code for a particular microarchitecture within the constraints set by -march and -mcpu. The M680x0 microarchitectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. The ColdFire microarchitectures are: cfv1, cfv2, cfv3, cfv4 and cfv4e.
- -m68000
- -mc68000
-
Generate output for a 68000. This is the default when the compiler is configured for 68000-based systems. It is equivalent to -march=68000.
- -m68010
- Generate output for a 68010. This is the default when the compiler is configured for 68010-based systems. It is equivalent to -march=68010.
- -m68020
- -mc68020
- Generate output for a 68020. This is the default when the compiler is configured for 68020-based systems. It is equivalent to -march=68020.
- -m68030
- Generate output for a 68030. This is the default when the compiler is configured for 68030-based systems. It is equivalent to -march=68030.
- -m68040
-
Generate output for a 68040. This is the default when the compiler is configured for 68040-based systems. It is equivalent to -march=68040.
- -m68060
-
Generate output for a 68060. This is the default when the compiler is configured for 68060-based systems. It is equivalent to -march=68060.
- -mcpu32
-
Generate output for a CPU32. This is the default when the compiler is configured for CPU32-based systems. It is equivalent to -march=cpu32.
- -m5200
-
Generate output for a 520X ColdFire CPU. This is the default when the compiler is configured for 520X-based systems. It is equivalent to -mcpu=5206, and is now deprecated in favor of that option.
- -m5206e
- Generate output for a 5206e ColdFire CPU. The option is now deprecated in favor of the equivalent -mcpu=5206e.
- -m528x
- Generate output for a member of the ColdFire 528X family. The option is now deprecated in favor of the equivalent -mcpu=528x.
- -m5307
- Generate output for a ColdFire 5307 CPU. The option is now deprecated in favor of the equivalent -mcpu=5307.
- -m5407
- Generate output for a ColdFire 5407 CPU. The option is now deprecated in favor of the equivalent -mcpu=5407.
- -mcfv4e
- Generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of hardware floating-point instructions. The option is equivalent to -mcpu=547x, and is now deprecated in favor of that option.
- -m68020-40
-
Generate output for a 68040, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68040.
- -m68020-60
-
Generate output for a 68060, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68060.
- -mhard-float
- -m68881
- Generate floating-point instructions. This is the default for 68020 and above, and for ColdFire devices that have an FPU. It defines the macro __HAVE_68881__ on M680x0 targets and __mcffpu__ on ColdFire targets.
- -msoft-float
- Do not generate floating-point instructions; use library calls instead. This is the default for 68000, 68010, and 68832 targets. It is also the default for ColdFire devices that have no FPU.
- -mdiv
- -mno-div
-
Generate (do not generate) ColdFire hardware divide and remainder instructions. If -march is used without -mcpu, the default is "on" for ColdFire architectures and "off" for M680x0 architectures. Otherwise, the default is taken from the target CPU (either the default CPU, or the one specified by -mcpu). For example, the default is "off" for -mcpu=5206 and "on" for -mcpu=5206e.
- -mshort
- Consider type "int" to be 16 bits wide, like "short int". Additionally, parameters passed on the stack are also aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit.
- -mno-short
- Do not consider type "int" to be 16 bits wide. This is the default.
- -mnobitfield
- -mno-bitfield
- Do not use the bit-field instructions. The -m68000, -mcpu32 and -m5200 options imply -mnobitfield.
- -mbitfield
- Do use the bit-field instructions. The -m68020 option implies -mbitfield. This is the default if you use a configuration designed for a 68020.
- -mrtd
-
Use a different function-calling convention, in which functions that take a fixed number of arguments return with the "rtd" instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there.
- -mno-rtd
- Do not use the calling conventions selected by -mrtd. This is the default.
- -malign-int
- -mno-align-int
-
Control whether GCC aligns "int", "long", "long long", "float", "double", and "long double" variables on a 32-bit boundary ( -malign-int) or a 16-bit boundary (-mno-align-int). Aligning variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses at the expense of more memory.
- -mpcrel
- Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing. -fPIC is not presently supported with -mpcrel, though this could be supported for 68020 and higher processors.
- -mno-strict-align
- -mstrict-align
- Do not (do) assume that unaligned memory references are handled by the system.
- -msep-data
- Generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute-in-place in an environment without virtual memory management. This option implies -fPIC.
- -mno-sep-data
- Generate code that assumes that the data segment follows the text segment. This is the default.
- -mid-shared-library
- Generate code that supports shared libraries via the library ID method. This allows for execute-in-place and shared libraries in an environment without virtual memory management. This option implies -fPIC.
- -mno-id-shared-library
- Generate code that doesn't assume ID-based shared libraries are being used. This is the default.
- -mshared-library-id=n
- Specifies the identification number of the ID-based shared library being compiled. Specifying a value of 0 generates more compact code; specifying other values forces the allocation of that number to the current library, but is no more space- or time-efficient than omitting this option.
- -mxgot
- -mno-xgot
-
When generating position-independent code for ColdFire, generate code that works if the GOT has more than 8192 entries. This code is larger and slower than code generated without this option. On M680x0 processors, this option is not needed; -fPIC suffices.
relocation truncated to fit: R_68K_GOT16O foobar
MCore Options
These are the -m options defined for the Motorola M*Core processors.
- -mhardlit
- -mno-hardlit
- Inline constants into the code stream if it can be done in two instructions or less.
- -mdiv
- -mno-div
- Use the divide instruction. (Enabled by default).
- -mrelax-immediate
- -mno-relax-immediate
- Allow arbitrary-sized immediates in bit operations.
- -mwide-bitfields
- -mno-wide-bitfields
- Always treat bit-fields as "int"-sized.
- -m4byte-functions
- -mno-4byte-functions
- Force all functions to be aligned to a 4-byte boundary.
- -mcallgraph-data
- -mno-callgraph-data
- Emit callgraph information.
- -mslow-bytes
- -mno-slow-bytes
- Prefer word access when reading byte quantities.
- -mlittle-endian
- -mbig-endian
- Generate code for a little-endian target.
- -m210
- -m340
- Generate code for the 210 processor.
- -mno-lsim
- Assume that runtime support has been provided and so omit the simulator library ( libsim.a) from the linker command line.
- -mstack-increment=size
- Set the maximum amount for a single stack increment operation. Large values can increase the speed of programs that contain functions that need a large amount of stack space, but they can also trigger a segmentation fault if the stack is extended too much. The default value is 0x1000.
MeP Options
- -mabsdiff
- Enables the "abs" instruction, which is the absolute difference between two registers.
- -mall-opts
- Enables all the optional instructions---average, multiply, divide, bit operations, leading zero, absolute difference, min/max, clip, and saturation.
- -maverage
- Enables the "ave" instruction, which computes the average of two registers.
- -mbased=n
- Variables of size n bytes or smaller are placed in the ".based" section by default. Based variables use the $tp register as a base register, and there is a 128-byte limit to the ".based" section.
- -mbitops
- Enables the bit operation instructions---bit test ("btstm"), set ("bsetm"), clear ("bclrm"), invert ("bnotm"), and test-and-set ("tas").
- -mc=name
- Selects which section constant data is placed in. name may be "tiny", "near", or "far".
- -mclip
- Enables the "clip" instruction. Note that "-mclip" is not useful unless you also provide "-mminmax".
- -mconfig=name
- Selects one of the built-in core configurations. Each MeP chip has one or more modules in it; each module has a core CPU and a variety of coprocessors, optional instructions, and peripherals. The "MeP-Integrator" tool, not part of GCC, provides these configurations through this option; using this option is the same as using all the corresponding command-line options. The default configuration is "default".
- -mcop
- Enables the coprocessor instructions. By default, this is a 32-bit coprocessor. Note that the coprocessor is normally enabled via the "-mconfig=" option.
- -mcop32
- Enables the 32-bit coprocessor's instructions.
- -mcop64
- Enables the 64-bit coprocessor's instructions.
- -mivc2
- Enables IVC2 scheduling. IVC2 is a 64-bit VLIW coprocessor.
- -mdc
- Causes constant variables to be placed in the ".near" section.
- -mdiv
- Enables the "div" and "divu" instructions.
- -meb
- Generate big-endian code.
- -mel
- Generate little-endian code.
- -mio-volatile
- Tells the compiler that any variable marked with the "io" attribute is to be considered volatile.
- -ml
- Causes variables to be assigned to the ".far" section by default.
- -mleadz
- Enables the "leadz" (leading zero) instruction.
- -mm
- Causes variables to be assigned to the ".near" section by default.
- -mminmax
- Enables the "min" and "max" instructions.
- -mmult
- Enables the multiplication and multiply-accumulate instructions.
- -mno-opts
- Disables all the optional instructions enabled by "-mall-opts".
- -mrepeat
- Enables the "repeat" and "erepeat" instructions, used for low-overhead looping.
- -ms
- Causes all variables to default to the ".tiny" section. Note that there is a 65536-byte limit to this section. Accesses to these variables use the %gp base register.
- -msatur
- Enables the saturation instructions. Note that the compiler does not currently generate these itself, but this option is included for compatibility with other tools, like "as".
- -msdram
- Link the SDRAM-based runtime instead of the default ROM-based runtime.
- -msim
- Link the simulator runtime libraries.
- -msimnovec
- Link the simulator runtime libraries, excluding built-in support for reset and exception vectors and tables.
- -mtf
- Causes all functions to default to the ".far" section. Without this option, functions default to the ".near" section.
- -mtiny=n
- Variables that are n bytes or smaller are allocated to the ".tiny" section. These variables use the $gp base register. The default for this option is 4, but note that there's a 65536-byte limit to the ".tiny" section.
MicroBlaze Options
- -msoft-float
- Use software emulation for floating point (default).
- -mhard-float
- Use hardware floating-point instructions.
- -mmemcpy
- Do not optimize block moves, use "memcpy".
- -mno-clearbss
- This option is deprecated. Use -fno-zero-initialized-in-bss instead.
- -mcpu=cpu-type
- Use features of, and schedule code for, the given CPU. Supported values are in the format vX.YY.Z, where X is a major version, YY is the minor version, and Z is compatibility code. Example values are v3.00.a, v4.00.b, v5.00.a, v5.00.b, v5.00.b, v6.00.a.
- -mxl-soft-mul
- Use software multiply emulation (default).
- -mxl-soft-div
- Use software emulation for divides (default).
- -mxl-barrel-shift
- Use the hardware barrel shifter.
- -mxl-pattern-compare
- Use pattern compare instructions.
- -msmall-divides
- Use table lookup optimization for small signed integer divisions.
- -mxl-stack-check
- This option is deprecated. Use -fstack-check instead.
- -mxl-gp-opt
- Use GP-relative ".sdata"/".sbss" sections.
- -mxl-multiply-high
- Use multiply high instructions for high part of 32x32 multiply.
- -mxl-float-convert
- Use hardware floating-point conversion instructions.
- -mxl-float-sqrt
- Use hardware floating-point square root instruction.
- -mbig-endian
- Generate code for a big-endian target.
- -mlittle-endian
- Generate code for a little-endian target.
- -mxl-reorder
- Use reorder instructions (swap and byte reversed load/store).
- -mxl-mode-app-model
- Select application model app-model. Valid models are
- executable
- normal executable (default), uses startup code crt0.o.
- xmdstub
- for use with Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called xmdstub. This uses startup file crt1.o and sets the start address of the program to 0x800.
- bootstrap
- for applications that are loaded using a bootloader. This model uses startup file crt2.o which does not contain a processor reset vector handler. This is suitable for transferring control on a processor reset to the bootloader rather than the application.
- novectors
- for applications that do not require any of the MicroBlaze vectors. This option may be useful for applications running within a monitoring application. This model uses crt3.o as a startup file.
MIPS Options
- -EB
- Generate big-endian code.
- -EL
- Generate little-endian code. This is the default for mips*el-*-* configurations.
- -march=arch
-
Generate code that runs on arch, which can be the name of a generic MIPS ISA, or the name of a particular processor. The ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, mips64 and mips64r2. The processor names are: 4kc, 4km, 4kp, 4ksc, 4kec, 4kem, 4kep, 4ksd, 5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1, 24kec, 24kef2_1, 24kef1_1, 34kc, 34kf2_1, 34kf1_1, 34kn, 74kc, 74kf2_1, 74kf1_1, 74kf3_2, 1004kc, 1004kf2_1, 1004kf1_1, loongson2e, loongson2f, loongson3a, m4k, octeon, octeon+, octeon2, orion, r2000, r3000, r3900, r4000, r4400, r4600, r4650, r4700, r6000, r8000, rm7000, rm9000, r10000, r12000, r14000, r16000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130, vr4300, vr5000, vr5400, vr5500, xlr and xlp. The special value from-abi selects the most compatible architecture for the selected ABI (that is, mips1 for 32-bit ABIs and mips3 for 64-bit ABIs).
- -mtune=arch
-
Optimize for arch. Among other things, this option controls the way instructions are scheduled, and the perceived cost of arithmetic operations. The list of arch values is the same as for -march.
- -mips1
- Equivalent to -march=mips1.
- -mips2
- Equivalent to -march=mips2.
- -mips3
- Equivalent to -march=mips3.
- -mips4
- Equivalent to -march=mips4.
- -mips32
- Equivalent to -march=mips32.
- -mips32r2
- Equivalent to -march=mips32r2.
- -mips64
- Equivalent to -march=mips64.
- -mips64r2
- Equivalent to -march=mips64r2.
- -mips16
- -mno-mips16
-
Generate (do not generate) MIPS16 code. If GCC is targeting a MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE.
- -mflip-mips16
- Generate MIPS16 code on alternating functions. This option is provided for regression testing of mixed MIPS16/non-MIPS16 code generation, and is not intended for ordinary use in compiling user code.
- -minterlink-mips16
- -mno-interlink-mips16
-
Require (do not require) that non-MIPS16 code be link-compatible with MIPS16 code.
- -mabi=32
- -mabi=o64
- -mabi=n32
- -mabi=64
- -mabi=eabi
-
Generate code for the given ABI.
- -mabicalls
- -mno-abicalls
- Generate (do not generate) code that is suitable for SVR4-style dynamic objects. -mabicalls is the default for SVR4-based systems.
- -mshared
- -mno-shared
-
Generate (do not generate) code that is fully position-independent, and that can therefore be linked into shared libraries. This option only affects -mabicalls.
- -mplt
- -mno-plt
-
Assume (do not assume) that the static and dynamic linkers support PLTs and copy relocations. This option only affects -mno-shared -mabicalls. For the n64 ABI, this option has no effect without -msym32.
- -mxgot
- -mno-xgot
-
Lift (do not lift) the usual restrictions on the size of the global offset table.
relocation truncated to fit: R_MIPS_GOT16 foobar
- -mgp32
- Assume that general-purpose registers are 32 bits wide.
- -mgp64
- Assume that general-purpose registers are 64 bits wide.
- -mfp32
- Assume that floating-point registers are 32 bits wide.
- -mfp64
- Assume that floating-point registers are 64 bits wide.
- -mhard-float
- Use floating-point coprocessor instructions.
- -msoft-float
- Do not use floating-point coprocessor instructions. Implement floating-point calculations using library calls instead.
- -mno-float
- Equivalent to -msoft-float, but additionally asserts that the program being compiled does not perform any floating-point operations. This option is presently supported only by some bare-metal MIPS configurations, where it may select a special set of libraries that lack all floating-point support (including, for example, the floating-point "printf" formats). If code compiled with "-mno-float" accidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure.
- -msingle-float
- Assume that the floating-point coprocessor only supports single-precision operations.
- -mdouble-float
- Assume that the floating-point coprocessor supports double-precision operations. This is the default.
- -mllsc
- -mno-llsc
-
Use (do not use) ll, sc, and sync instructions to implement atomic memory built-in functions. When neither option is specified, GCC uses the instructions if the target architecture supports them.
- -mdsp
- -mno-dsp
-
Use (do not use) revision 1 of the MIPS DSP ASE.
This option defines the preprocessor macro __mips_dsp. It also defines __mips_dsp_rev to 1.
- -mdspr2
- -mno-dspr2
-
Use (do not use) revision 2 of the MIPS DSP ASE.
This option defines the preprocessor macros __mips_dsp and __mips_dspr2. It also defines __mips_dsp_rev to 2.
- -msmartmips
- -mno-smartmips
- Use (do not use) the MIPS SmartMIPS ASE.
- -mpaired-single
- -mno-paired-single
-
Use (do not use) paired-single floating-point instructions.
This option requires hardware floating-point support to be enabled.
- -mdmx
- -mno-mdmx
- Use (do not use) MIPS Digital Media Extension instructions. This option can only be used when generating 64-bit code and requires hardware floating-point support to be enabled.
- -mips3d
- -mno-mips3d
- Use (do not use) the MIPS-3D ASE. The option -mips3d implies -mpaired-single.
- -mmt
- -mno-mt
- Use (do not use) MT Multithreading instructions.
- -mmcu
- -mno-mcu
- Use (do not use) the MIPS MCU ASE instructions.
- -mlong64
- Force "long" types to be 64 bits wide. See -mlong32 for an explanation of the default and the way that the pointer size is determined.
- -mlong32
-
Force "long", "int", and pointer types to be 32 bits wide.
- -msym32
- -mno-sym32
- Assume (do not assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is useful in combination with -mabi=64 and -mno-abicalls because it allows GCC to generate shorter and faster references to symbolic addresses.
- -G num
-
Put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. GCC can then generate more efficient accesses to the data; see -mgpopt for details.
- -mlocal-sdata
- -mno-local-sdata
-
Extend (do not extend) the -G behavior to local data too, such as to static variables in C. -mlocal-sdata is the default for all configurations.
- -mextern-sdata
- -mno-extern-sdata
-
Assume (do not assume) that externally-defined data is in a small data section if the size of that data is within the -G limit. -mextern-sdata is the default for all configurations.
- -mgpopt
- -mno-gpopt
-
Use (do not use) GP-relative accesses for symbols that are known to be in a small data section; see -G, -mlocal-sdata and -mextern-sdata. -mgpopt is the default for all configurations.
- -membedded-data
- -mno-embedded-data
- Allocate variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data. This gives slightly slower code than the default, but reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems.
- -muninit-const-in-rodata
- -mno-uninit-const-in-rodata
- Put uninitialized "const" variables in the read-only data section. This option is only meaningful in conjunction with -membedded-data.
- -mcode-readable=setting
- Specify whether GCC may generate code that reads from executable sections. There are three possible settings:
- -mcode-readable=yes
- Instructions may freely access executable sections. This is the default setting.
- -mcode-readable=pcrel
- MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so. This option is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. It is also useful on processors that can be configured to have a dual instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads to the instruction RAM.
- -mcode-readable=no
- Instructions must not access executable sections. This option can be useful on targets that are configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM.
- -msplit-addresses
- -mno-split-addresses
- Enable (disable) use of the "%hi()" and "%lo()" assembler relocation operators. This option has been superseded by -mexplicit-relocs but is retained for backwards compatibility.
- -mexplicit-relocs
- -mno-explicit-relocs
-
Use (do not use) assembler relocation operators when dealing with symbolic addresses. The alternative, selected by -mno-explicit-relocs, is to use assembler macros instead.
- -mcheck-zero-division
- -mno-check-zero-division
-
Trap (do not trap) on integer division by zero.
- -mdivide-traps
- -mdivide-breaks
-
MIPS systems check for division by zero by generating either a conditional trap or a break instruction. Using traps results in smaller code, but is only supported on MIPS II and later. Also, some versions of the Linux kernel have a bug that prevents trap from generating the proper signal ("SIGFPE"). Use -mdivide-traps to allow conditional traps on architectures that support them and -mdivide-breaks to force the use of breaks.
- -mmemcpy
- -mno-memcpy
- Force (do not force) the use of "memcpy()" for non-trivial block moves. The default is -mno-memcpy, which allows GCC to inline most constant-sized copies.
- -mlong-calls
- -mno-long-calls
-
Disable (do not disable) use of the "jal" instruction. Calling functions using "jal" is more efficient but requires the caller and callee to be in the same 256 megabyte segment.
- -mmad
- -mno-mad
- Enable (disable) use of the "mad", "madu" and "mul" instructions, as provided by the R4650 ISA.
- -mfused-madd
- -mno-fused-madd
-
Enable (disable) use of the floating-point multiply-accumulate instructions, when they are available. The default is -mfused-madd.
- -nocpp
- Tell the MIPS assembler to not run its preprocessor over user assembler files (with a .s suffix) when assembling them.
- -mfix-24k
- -mno-fix-24k
- Work around the 24K E48 (lost data on stores during refill) errata. The workarounds are implemented by the assembler rather than by GCC.
- -mfix-r4000
- -mno-fix-r4000
- Work around certain R4000 CPU errata:
- -
- A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
- -
- A double-word or a variable shift may give an incorrect result if executed while an integer multiplication is in progress.
- -
- An integer division may give an incorrect result if started in a delay slot of a taken branch or a jump.
- -mfix-r4400
- -mno-fix-r4400
- Work around certain R4400 CPU errata:
- -
- A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
- -mfix-r10000
- -mno-fix-r10000
- Work around certain R10000 errata:
- -
- "ll"/"sc" sequences may not behave atomically on revisions prior to 3.0. They may deadlock on revisions 2.6 and earlier.
- -mfix-vr4120
- -mno-fix-vr4120
- Work around certain VR4120 errata:
- -
- "dmultu" does not always produce the correct result.
- -
- "div" and "ddiv" do not always produce the correct result if one of the operands is negative.
- -mfix-vr4130
- Work around the VR4130 "mflo"/"mfhi" errata. The workarounds are implemented by the assembler rather than by GCC, although GCC avoids using "mflo" and "mfhi" if the VR4130 "macc", "macchi", "dmacc" and "dmacchi" instructions are available instead.
- -mfix-sb1
- -mno-fix-sb1
- Work around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2 "F1" and "F2" floating-point errata.)
- -mr10k-cache-barrier=setting
-
Specify whether GCC should insert cache barriers to avoid the side-effects of speculation on R10K processors.
- 1.
- the memory occupied by the current function's stack frame;
- 2.
- the memory occupied by an incoming stack argument;
- 3.
- the memory occupied by an object with a link-time-constant address.
void foo (void);
- -mr10k-cache-barrier=load-store
- Insert a cache barrier before a load or store that might be speculatively executed and that might have side effects even if aborted.
- -mr10k-cache-barrier=store
- Insert a cache barrier before a store that might be speculatively executed and that might have side effects even if aborted.
- -mr10k-cache-barrier=none
- Disable the insertion of cache barriers. This is the default setting.
- -mflush-func=func
- -mno-flush-func
- Specifies the function to call to flush the I and D caches, or to not call any such function. If called, the function must take the same arguments as the common "_flush_func()", that is, the address of the memory range for which the cache is being flushed, the size of the memory range, and the number 3 (to flush both caches). The default depends on the target GCC was configured for, but commonly is either _flush_func or __cpu_flush.
- mbranch-cost=num
- Set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases. A zero cost redundantly selects the default, which is based on the -mtune setting.
- -mbranch-likely
- -mno-branch-likely
- Enable or disable use of Branch Likely instructions, regardless of the default for the selected architecture. By default, Branch Likely instructions may be generated if they are supported by the selected architecture. An exception is for the MIPS32 and MIPS64 architectures and processors that implement those architectures; for those, Branch Likely instructions are not be generated by default because the MIPS32 and MIPS64 architectures specifically deprecate their use.
- -mfp-exceptions
- -mno-fp-exceptions
-
Specifies whether FP exceptions are enabled. This affects how FP instructions are scheduled for some processors. The default is that FP exceptions are enabled.
- -mvr4130-align
- -mno-vr4130-align
-
The VR4130 pipeline is two-way superscalar, but can only issue two instructions together if the first one is 8-byte aligned. When this option is enabled, GCC aligns pairs of instructions that it thinks should execute in parallel.
- -msynci
- -mno-synci
-
Enable (disable) generation of "synci" instructions on architectures that support it. The "synci" instructions (if enabled) are generated when "__builtin___clear_cache()" is compiled.
- -mrelax-pic-calls
- -mno-relax-pic-calls
-
Try to turn PIC calls that are normally dispatched via register $25 into direct calls. This is only possible if the linker can resolve the destination at link-time and if the destination is within range for a direct call.
- -mmcount-ra-address
- -mno-mcount-ra-address
- Emit (do not emit) code that allows "_mcount" to modify the calling function's return address. When enabled, this option extends the usual "_mcount" interface with a new ra-address parameter, which has type "intptr_t *" and is passed in register $12. "_mcount" can then modify the return address by doing both of the following:
- •
- Returning the new address in register $31.
- •
- Storing the new address in "*ra-address", if ra-address is nonnull.
MMIX Options
These options are defined for the MMIX:
- -mlibfuncs
- -mno-libfuncs
- Specify that intrinsic library functions are being compiled, passing all values in registers, no matter the size.
- -mepsilon
- -mno-epsilon
- Generate floating-point comparison instructions that compare with respect to the "rE" epsilon register.
- -mabi=mmixware
- -mabi=gnu
- Generate code that passes function parameters and return values that (in the called function) are seen as registers $0 and up, as opposed to the GNU ABI which uses global registers $231 and up.
- -mzero-extend
- -mno-zero-extend
- When reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load instructions by default, rather than sign-extending ones.
- -mknuthdiv
- -mno-knuthdiv
- Make the result of a division yielding a remainder have the same sign as the divisor. With the default, -mno-knuthdiv, the sign of the remainder follows the sign of the dividend. Both methods are arithmetically valid, the latter being almost exclusively used.
- -mtoplevel-symbols
- -mno-toplevel-symbols
- Prepend (do not prepend) a : to all global symbols, so the assembly code can be used with the "PREFIX" assembly directive.
- -melf
- Generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator.
- -mbranch-predict
- -mno-branch-predict
- Use (do not use) the probable-branch instructions, when static branch prediction indicates a probable branch.
- -mbase-addresses
- -mno-base-addresses
- Generate (do not generate) code that uses base addresses. Using a base address automatically generates a request (handled by the assembler and the linker) for a constant to be set up in a global register. The register is used for one or more base address requests within the range 0 to 255 from the value held in the register. The generally leads to short and fast code, but the number of different data items that can be addressed is limited. This means that a program that uses lots of static data may require -mno-base-addresses.
- -msingle-exit
- -mno-single-exit
- Force (do not force) generated code to have a single exit point in each function.
MN10300 Options
These -m options are defined for Matsushita MN10300 architectures:
- -mmult-bug
- Generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default.
- -mno-mult-bug
- Do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.
- -mam33
- Generate code using features specific to the AM33 processor.
- -mno-am33
- Do not generate code using features specific to the AM33 processor. This is the default.
- -mam33-2
- Generate code using features specific to the AM33/2.0 processor.
- -mam34
- Generate code using features specific to the AM34 processor.
- -mtune=cpu-type
- Use the timing characteristics of the indicated CPU type when scheduling instructions. This does not change the targeted processor type. The CPU type must be one of mn10300, am33, am33-2 or am34.
- -mreturn-pointer-on-d0
- When generating a function that returns a pointer, return the pointer in both "a0" and "d0". Otherwise, the pointer is returned only in "a0", and attempts to call such functions without a prototype result in errors. Note that this option is on by default; use -mno-return-pointer-on-d0 to disable it.
- -mno-crt0
- Do not link in the C run-time initialization object file.
- -mrelax
-
Indicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This option only has an effect when used on the command line for the final link step.
- -mliw
- Allow the compiler to generate Long Instruction Word instructions if the target is the AM33 or later. This is the default. This option defines the preprocessor macro __LIW__.
- -mnoliw
- Do not allow the compiler to generate Long Instruction Word instructions. This option defines the preprocessor macro __NO_LIW__.
- -msetlb
- Allow the compiler to generate the SETLB and Lcc instructions if the target is the AM33 or later. This is the default. This option defines the preprocessor macro __SETLB__.
- -mnosetlb
- Do not allow the compiler to generate SETLB or Lcc instructions. This option defines the preprocessor macro __NO_SETLB__.
Moxie Options
- -meb
- Generate big-endian code. This is the default for moxie-*-* configurations.
- -mel
- Generate little-endian code.
- -mno-crt0
- Do not link in the C run-time initialization object file.
PDP-11 Options
These options are defined for the PDP-11:
- -mfpu
- Use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported.)
- -msoft-float
- Do not use hardware floating point.
- -mac0
- Return floating-point results in ac0 (fr0 in Unix assembler syntax).
- -mno-ac0
- Return floating-point results in memory. This is the default.
- -m40
- Generate code for a PDP-11/40.
- -m45
- Generate code for a PDP-11/45. This is the default.
- -m10
- Generate code for a PDP-11/10.
- -mbcopy-builtin
- Use inline "movmemhi" patterns for copying memory. This is the default.
- -mbcopy
- Do not use inline "movmemhi" patterns for copying memory.
- -mint16
- -mno-int32
- Use 16-bit "int". This is the default.
- -mint32
- -mno-int16
- Use 32-bit "int".
- -mfloat64
- -mno-float32
- Use 64-bit "float". This is the default.
- -mfloat32
- -mno-float64
- Use 32-bit "float".
- -mabshi
- Use "abshi2" pattern. This is the default.
- -mno-abshi
- Do not use "abshi2" pattern.
- -mbranch-expensive
- Pretend that branches are expensive. This is for experimenting with code generation only.
- -mbranch-cheap
- Do not pretend that branches are expensive. This is the default.
- -munix-asm
- Use Unix assembler syntax. This is the default when configured for pdp11-*-bsd.
- -mdec-asm
- Use DEC assembler syntax. This is the default when configured for any PDP-11 target other than pdp11-*-bsd.
picoChip Options
These -m options are defined for picoChip implementations:
- -mae=ae_type
-
Set the instruction set, register set, and instruction scheduling parameters for array element type ae_type. Supported values for ae_type are ANY, MUL, and MAC.
- -msymbol-as-address
- Enable the compiler to directly use a symbol name as an address in a load/store instruction, without first loading it into a register. Typically, the use of this option generates larger programs, which run faster than when the option isn't used. However, the results vary from program to program, so it is left as a user option, rather than being permanently enabled.
- -mno-inefficient-warnings
- Disables warnings about the generation of inefficient code. These warnings can be generated, for example, when compiling code that performs byte-level memory operations on the MAC AE type. The MAC AE has no hardware support for byte-level memory operations, so all byte load/stores must be synthesized from word load/store operations. This is inefficient and a warning is generated to indicate that you should rewrite the code to avoid byte operations, or to target an AE type that has the necessary hardware support. This option disables these warnings.
PowerPC Options
These are listed under
RL78 Options
- -msim
- Links in additional target libraries to support operation within a simulator.
- -mmul=none
- -mmul=g13
- -mmul=rl78
- Specifies the type of hardware multiplication support to be used. The default is "none", which uses software multiplication functions. The "g13" option is for the hardware multiply/divide peripheral only on the RL78/G13 targets. The "rl78" option is for the standard hardware multiplication defined in the RL78 software manual.
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
- -mpowerpc-gpopt
- -mno-powerpc-gpopt
- -mpowerpc-gfxopt
- -mno-powerpc-gfxopt
- -mpowerpc64
- -mno-powerpc64
- -mmfcrf
- -mno-mfcrf
- -mpopcntb
- -mno-popcntb
- -mpopcntd
- -mno-popcntd
- -mfprnd
- -mno-fprnd
- -mcmpb
- -mno-cmpb
- -mmfpgpr
- -mno-mfpgpr
- -mhard-dfp
- -mno-hard-dfp
-
You use these options to specify which instructions are available on the processor you are using. The default value of these options is determined when configuring GCC. Specifying the -mcpu=cpu_type overrides the specification of these options. We recommend you use the -mcpu=cpu_type option rather than the options listed above.
- -mcpu=cpu_type
-
Set architecture type, register usage, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are 401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476, 476fp, 505, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, a2, e300c2, e300c3, e500mc, e500mc64, e5500, e6500, ec603e, G3, G4, G5, titan, power3, power4, power5, power5+, power6, power6x, power7, power8, powerpc, powerpc64, and rs64.
- -mtune=cpu_type
- Set the instruction scheduling parameters for machine type cpu_type, but do not set the architecture type or register usage, as -mcpu=cpu_type does. The same values for cpu_type are used for -mtune as for -mcpu. If both are specified, the code generated uses the architecture and registers set by -mcpu, but the scheduling parameters set by -mtune.
- -mcmodel=small
- Generate PowerPC64 code for the small model: The TOC is limited to 64k.
- -mcmodel=medium
- Generate PowerPC64 code for the medium model: The TOC and other static data may be up to a total of 4G in size.
- -mcmodel=large
- Generate PowerPC64 code for the large model: The TOC may be up to 4G in size. Other data and code is only limited by the 64-bit address space.
- -maltivec
- -mno-altivec
- Generate code that uses (does not use) AltiVec instructions, and also enable the use of built-in functions that allow more direct access to the AltiVec instruction set. You may also need to set -mabi=altivec to adjust the current ABI with AltiVec ABI enhancements.
- -mvrsave
- -mno-vrsave
- Generate VRSAVE instructions when generating AltiVec code.
- -mgen-cell-microcode
- Generate Cell microcode instructions.
- -mwarn-cell-microcode
- Warn when a Cell microcode instruction is emitted. An example of a Cell microcode instruction is a variable shift.
- -msecure-plt
- Generate code that allows ld and ld.so to build executables and shared libraries with non-executable ".plt" and ".got" sections. This is a PowerPC 32-bit SYSV ABI option.
- -mbss-plt
- Generate code that uses a BSS ".plt" section that ld.so fills in, and requires ".plt" and ".got" sections that are both writable and executable. This is a PowerPC 32-bit SYSV ABI option.
- -misel
- -mno-isel
- This switch enables or disables the generation of ISEL instructions.
- -misel=yes/no
- This switch has been deprecated. Use -misel and -mno-isel instead.
- -mspe
- -mno-spe
- This switch enables or disables the generation of SPE simd instructions.
- -mpaired
- -mno-paired
- This switch enables or disables the generation of PAIRED simd instructions.
- -mspe=yes/no
- This option has been deprecated. Use -mspe and -mno-spe instead.
- -mvsx
- -mno-vsx
- Generate code that uses (does not use) vector/scalar (VSX) instructions, and also enable the use of built-in functions that allow more direct access to the VSX instruction set.
- -mfloat-gprs=yes/single/double/no
- -mfloat-gprs
-
This switch enables or disables the generation of floating-point operations on the general-purpose registers for architectures that support it.
- -m32
- -m64
- Generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including GNU/Linux). The 32-bit environment sets int, long and pointer to 32 bits and generates code that runs on any PowerPC variant. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits, and generates code for PowerPC64, as for -mpowerpc64.
- -mfull-toc
- -mno-fp-in-toc
- -mno-sum-in-toc
- -mminimal-toc
-
Modify generation of the TOC (Table Of Contents), which is created for every executable file. The -mfull-toc option is selected by default. In that case, GCC allocates at least one TOC entry for each unique non-automatic variable reference in your program. GCC also places floating-point constants in the TOC. However, only 16,384 entries are available in the TOC.
- -maix64
- -maix32
- Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit "long" type, and the infrastructure needed to support them. Specifying -maix64 implies -mpowerpc64, while -maix32 disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32.
- -mxl-compat
- -mno-xl-compat
-
Produce code that conforms more closely to IBM XL compiler semantics when using AIX-compatible ABI. Pass floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs. Do not assume that most significant double in 128-bit long double value is properly rounded when comparing values and converting to double. Use XL symbol names for long double support routines.
- -mpe
- Support IBM RS/6000 SP Parallel Environment (PE). Link an application written to use message passing with special startup code to enable the application to run. The system must have PE installed in the standard location ( /usr/lpp/ppe.poe/), or the specs file must be overridden with the -specs= option to specify the appropriate directory location. The Parallel Environment does not support threads, so the -mpe option and the -pthread option are incompatible.
- -malign-natural
- -malign-power
-
On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option -malign-natural overrides the ABI-defined alignment of larger types, such as floating-point doubles, on their natural size-based boundary. The option -malign-power instructs GCC to follow the ABI-specified alignment rules. GCC defaults to the standard alignment defined in the ABI.
- -msoft-float
- -mhard-float
- Generate code that does not use (uses) the floating-point register set. Software floating-point emulation is provided if you use the -msoft-float option, and pass the option to GCC when linking.
- -msingle-float
- -mdouble-float
- Generate code for single- or double-precision floating-point operations. -mdouble-float implies -msingle-float.
- -msimple-fpu
- Do not generate "sqrt" and "div" instructions for hardware floating-point unit.
- -mfpu=name
- Specify type of floating-point unit. Valid values for name are sp_lite (equivalent to -msingle-float -msimple-fpu), dp_lite (equivalent to -mdouble-float -msimple-fpu), sp_full (equivalent to -msingle-float), and dp_full (equivalent to -mdouble-float).
- -mxilinx-fpu
- Perform optimizations for the floating-point unit on Xilinx PPC 405/440.
- -mmultiple
- -mno-multiple
- Generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use -mmultiple on little-endian PowerPC systems, since those instructions do not work when the processor is in little-endian mode. The exceptions are PPC740 and PPC750 which permit these instructions in little-endian mode.
- -mstring
- -mno-string
- Generate code that uses (does not use) the load string instructions and the store string word instructions to save multiple registers and do small block moves. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use -mstring on little-endian PowerPC systems, since those instructions do not work when the processor is in little-endian mode. The exceptions are PPC740 and PPC750 which permit these instructions in little-endian mode.
- -mupdate
- -mno-update
- Generate code that uses (does not use) the load or store instructions that update the base register to the address of the calculated memory location. These instructions are generated by default. If you use -mno-update, there is a small window between the time that the stack pointer is updated and the address of the previous frame is stored, which means code that walks the stack frame across interrupts or signals may get corrupted data.
- -mavoid-indexed-addresses
- -mno-avoid-indexed-addresses
- Generate code that tries to avoid (not avoid) the use of indexed load or store instructions. These instructions can incur a performance penalty on Power6 processors in certain situations, such as when stepping through large arrays that cross a 16M boundary. This option is enabled by default when targeting Power6 and disabled otherwise.
- -mfused-madd
- -mno-fused-madd
- Generate code that uses (does not use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used. The machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off.
- -mmulhw
- -mno-mulhw
- Generate code that uses (does not use) the half-word multiply and multiply-accumulate instructions on the IBM 405, 440, 464 and 476 processors. These instructions are generated by default when targeting those processors.
- -mdlmzb
- -mno-dlmzb
- Generate code that uses (does not use) the string-search dlmzb instruction on the IBM 405, 440, 464 and 476 processors. This instruction is generated by default when targeting those processors.
- -mno-bit-align
- -mbit-align
-
On System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field.
- -mno-strict-align
- -mstrict-align
- On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references are handled by the system.
- -mrelocatable
- -mno-relocatable
- Generate code that allows (does not allow) a static executable to be relocated to a different address at run time. A simple embedded PowerPC system loader should relocate the entire contents of ".got2" and 4-byte locations listed in the ".fixup" section, a table of 32-bit addresses generated by this option. For this to work, all objects linked together must be compiled with -mrelocatable or -mrelocatable-lib. -mrelocatable code aligns the stack to an 8-byte boundary.
- -mrelocatable-lib
- -mno-relocatable-lib
- Like -mrelocatable, -mrelocatable-lib generates a ".fixup" section to allow static executables to be relocated at run time, but -mrelocatable-lib does not use the smaller stack alignment of -mrelocatable. Objects compiled with -mrelocatable-lib may be linked with objects compiled with any combination of the -mrelocatable options.
- -mno-toc
- -mtoc
- On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program.
- -mlittle
- -mlittle-endian
- On System V.4 and embedded PowerPC systems compile code for the processor in little-endian mode. The -mlittle-endian option is the same as -mlittle.
- -mbig
- -mbig-endian
- On System V.4 and embedded PowerPC systems compile code for the processor in big-endian mode. The -mbig-endian option is the same as -mbig.
- -mdynamic-no-pic
- On Darwin and Mac OS X systems, compile code so that it is not relocatable, but that its external references are relocatable. The resulting code is suitable for applications, but not shared libraries.
- -msingle-pic-base
- Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins.
- -mprioritize-restricted-insns=priority
- This option controls the priority that is assigned to dispatch-slot restricted instructions during the second scheduling pass. The argument priority takes the value 0, 1, or 2 to assign no, highest, or second-highest (respectively) priority to dispatch-slot restricted instructions.
- -msched-costly-dep=dependence_type
- This option controls which dependences are considered costly by the target during instruction scheduling. The argument dependence_type takes one of the following values:
- no
- No dependence is costly.
- all
- All dependences are costly.
- true_store_to_load
- A true dependence from store to load is costly.
- store_to_load
- Any dependence from store to load is costly.
- number
- Any dependence for which the latency is greater than or equal to number is costly.
- -minsert-sched-nops=scheme
- This option controls which NOP insertion scheme is used during the second scheduling pass. The argument scheme takes one of the following values:
- no
- Don't insert NOPs.
- pad
- Pad with NOPs any dispatch group that has vacant issue slots, according to the scheduler's grouping.
- regroup_exact
- Insert NOPs to force costly dependent insns into separate groups. Insert exactly as many NOPs as needed to force an insn to a new group, according to the estimated processor grouping.
- number
- Insert NOPs to force costly dependent insns into separate groups. Insert number NOPs to force an insn to a new group.
- -mcall-sysv
- On System V.4 and embedded PowerPC systems compile code using calling conventions that adhere to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC using powerpc-*-eabiaix.
- -mcall-sysv-eabi
- -mcall-eabi
- Specify both -mcall-sysv and -meabi options.
- -mcall-sysv-noeabi
- Specify both -mcall-sysv and -mno-eabi options.
- -mcall-aixdesc
- On System V.4 and embedded PowerPC systems compile code for the AIX operating system.
- -mcall-linux
- On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system.
- -mcall-freebsd
- On System V.4 and embedded PowerPC systems compile code for the FreeBSD operating system.
- -mcall-netbsd
- On System V.4 and embedded PowerPC systems compile code for the NetBSD operating system.
- -mcall-openbsd
- On System V.4 and embedded PowerPC systems compile code for the OpenBSD operating system.
- -maix-struct-return
- Return all structures in memory (as specified by the AIX ABI).
- -msvr4-struct-return
- Return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI).
- -mabi=abi-type
- Extend the current ABI with a particular extension, or remove such extension. Valid values are altivec, no-altivec, spe, no-spe, ibmlongdouble, ieeelongdouble.
- -mabi=spe
- Extend the current ABI with SPE ABI extensions. This does not change the default ABI, instead it adds the SPE ABI extensions to the current ABI.
- -mabi=no-spe
- Disable Book-E SPE ABI extensions for the current ABI.
- -mabi=ibmlongdouble
- Change the current ABI to use IBM extended-precision long double. This is a PowerPC 32-bit SYSV ABI option.
- -mabi=ieeelongdouble
- Change the current ABI to use IEEE extended-precision long double. This is a PowerPC 32-bit Linux ABI option.
- -mprototype
- -mno-prototype
- On System V.4 and embedded PowerPC systems assume that all calls to variable argument functions are properly prototyped. Otherwise, the compiler must insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register ( CR) to indicate whether floating-point values are passed in the floating-point registers in case the function takes variable arguments. With -mprototype, only calls to prototyped variable argument functions set or clear the bit.
- -msim
- On embedded PowerPC systems, assume that the startup module is called sim-crt0.o and that the standard C libraries are libsim.a and libc.a. This is the default for powerpc-*-eabisim configurations.
- -mmvme
- On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are libmvme.a and libc.a.
- -mads
- On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are libads.a and libc.a.
- -myellowknife
- On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are libyk.a and libc.a.
- -mvxworks
- On System V.4 and embedded PowerPC systems, specify that you are compiling for a VxWorks system.
- -memb
- On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags header to indicate that eabi extended relocations are used.
- -meabi
- -mno-eabi
- On System V.4 and embedded PowerPC systems do (do not) adhere to the Embedded Applications Binary Interface (EABI), which is a set of modifications to the System V.4 specifications. Selecting -meabi means that the stack is aligned to an 8-byte boundary, a function "__eabi" is called from "main" to set up the EABI environment, and the -msdata option can use both "r2" and "r13" to point to two separate small data areas. Selecting -mno-eabi means that the stack is aligned to a 16-byte boundary, no EABI initialization function is called from "main", and the -msdata option only uses "r13" to point to a single small data area. The -meabi option is on by default if you configured GCC using one of the powerpc*-*-eabi* options.
- -msdata=eabi
- On System V.4 and embedded PowerPC systems, put small initialized "const" global and static data in the .sdata2 section, which is pointed to by register "r2". Put small initialized non-"const" global and static data in the .sdata section, which is pointed to by register "r13". Put small uninitialized global and static data in the .sbss section, which is adjacent to the .sdata section. The -msdata=eabi option is incompatible with the -mrelocatable option. The -msdata=eabi option also sets the -memb option.
- -msdata=sysv
- On System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section, which is pointed to by register "r13". Put small uninitialized global and static data in the .sbss section, which is adjacent to the .sdata section. The -msdata=sysv option is incompatible with the -mrelocatable option.
- -msdata=default
- -msdata
- On System V.4 and embedded PowerPC systems, if -meabi is used, compile code the same as -msdata=eabi, otherwise compile code the same as -msdata=sysv.
- -msdata=data
- On System V.4 and embedded PowerPC systems, put small global data in the .sdata section. Put small uninitialized global data in the .sbss section. Do not use register "r13" to address small data however. This is the default behavior unless other -msdata options are used.
- -msdata=none
- -mno-sdata
- On embedded PowerPC systems, put all initialized global and static data in the .data section, and all uninitialized data in the .bss section.
- -mblock-move-inline-limit=num
- Inline all block moves (such as calls to "memcpy" or structure copies) less than or equal to num bytes. The minimum value for num is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets. The default value is target-specific.
- -G num
- On embedded PowerPC systems, put global and static items less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS section. By default, num is 8. The -G num switch is also passed to the linker. All modules should be compiled with the same -G num value.
- -mregnames
- -mno-regnames
- On System V.4 and embedded PowerPC systems do (do not) emit register names in the assembly language output using symbolic forms.
- -mlongcall
- -mno-longcall
-
By default assume that all calls are far away so that a longer and more expensive calling sequence is required. This is required for calls farther than 32 megabytes (33,554,432 bytes) from the current location. A short call is generated if the compiler knows the call cannot be that far away. This setting can be overridden by the "shortcall" function attribute, or by "#pragma longcall(0)".
- -mtls-markers
- -mno-tls-markers
- Mark (do not mark) calls to "__tls_get_addr" with a relocation specifying the function argument. The relocation allows the linker to reliably associate function call with argument setup instructions for TLS optimization, which in turn allows GCC to better schedule the sequence.
- -pthread
- Adds support for multithreading with the pthreads library. This option sets flags for both the preprocessor and linker.
- -mrecip
- -mno-recip
- This option enables use of the reciprocal estimate and reciprocal square root estimate instructions with additional Newton-Raphson steps to increase precision instead of doing a divide or square root and divide for floating-point arguments. You should use the -ffast-math option when using -mrecip (or at least -funsafe-math-optimizations, -finite-math-only, -freciprocal-math and -fno-trapping-math). Note that while the throughput of the sequence is generally higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994) for reciprocal square roots.
- -mrecip=opt
-
This option controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a "!" to invert the option: "all": enable all estimate instructions, "default": enable the default instructions, equivalent to -mrecip, "none": disable all estimate instructions, equivalent to -mno-recip; "div": enable the reciprocal approximation instructions for both single and double precision; "divf": enable the single-precision reciprocal approximation instructions; "divd": enable the double-precision reciprocal approximation instructions; "rsqrt": enable the reciprocal square root approximation instructions for both single and double precision; "rsqrtf": enable the single-precision reciprocal square root approximation instructions; "rsqrtd": enable the double-precision reciprocal square root approximation instructions;
- -mrecip-precision
- -mno-recip-precision
- Assume (do not assume) that the reciprocal estimate instructions provide higher-precision estimates than is mandated by the PowerPC ABI. Selecting -mcpu=power6, -mcpu=power7 or -mcpu=power8 automatically selects -mrecip-precision. The double-precision square root estimate instructions are not generated by default on low-precision machines, since they do not provide an estimate that converges after three steps.
- -mveclibabi=type
- Specifies the ABI type to use for vectorizing intrinsics using an external library. The only type supported at present is "mass", which specifies to use IBM's Mathematical Acceleration Subsystem (MASS) libraries for vectorizing intrinsics using external libraries. GCC currently emits calls to "acosd2", "acosf4", "acoshd2", "acoshf4", "asind2", "asinf4", "asinhd2", "asinhf4", "atan2d2", "atan2f4", "atand2", "atanf4", "atanhd2", "atanhf4", "cbrtd2", "cbrtf4", "cosd2", "cosf4", "coshd2", "coshf4", "erfcd2", "erfcf4", "erfd2", "erff4", "exp2d2", "exp2f4", "expd2", "expf4", "expm1d2", "expm1f4", "hypotd2", "hypotf4", "lgammad2", "lgammaf4", "log10d2", "log10f4", "log1pd2", "log1pf4", "log2d2", "log2f4", "logd2", "logf4", "powd2", "powf4", "sind2", "sinf4", "sinhd2", "sinhf4", "sqrtd2", "sqrtf4", "tand2", "tanf4", "tanhd2", and "tanhf4" when generating code for power7. Both -ftree-vectorize and -funsafe-math-optimizations must also be enabled. The MASS libraries must be specified at link time.
- -mfriz
- -mno-friz
- Generate (do not generate) the "friz" instruction when the -funsafe-math-optimizations option is used to optimize rounding of floating-point values to 64-bit integer and back to floating point. The "friz" instruction does not return the same value if the floating-point number is too large to fit in an integer.
- -mpointers-to-nested-functions
- -mno-pointers-to-nested-functions
- Generate (do not generate) code to load up the static chain register ( r11) when calling through a pointer on AIX and 64-bit Linux systems where a function pointer points to a 3-word descriptor giving the function address, TOC value to be loaded in register r2, and static chain value to be loaded in register r11. The -mpointers-to-nested-functions is on by default. You cannot call through pointers to nested functions or pointers to functions compiled in other languages that use the static chain if you use the -mno-pointers-to-nested-functions.
- -msave-toc-indirect
- -mno-save-toc-indirect
- Generate (do not generate) code to save the TOC value in the reserved stack location in the function prologue if the function calls through a pointer on AIX and 64-bit Linux systems. If the TOC value is not saved in the prologue, it is saved just before the call through the pointer. The -mno-save-toc-indirect option is the default.
RX Options
These command-line options are defined for RX targets:
- -m64bit-doubles
- -m32bit-doubles
- Make the "double" data type be 64 bits (-m64bit-doubles) or 32 bits ( -m32bit-doubles) in size. The default is -m32bit-doubles. Note RX floating-point hardware only works on 32-bit values, which is why the default is -m32bit-doubles.
- -fpu
- -nofpu
-
Enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware. The default is enabled for the RX600 series and disabled for the RX200 series.
- -mcpu=name
-
Selects the type of RX CPU to be targeted. Currently three types are supported, the generic RX600 and RX200 series hardware and the specific RX610 CPU. The default is RX600.
- -mbig-endian-data
- -mlittle-endian-data
- Store data (but not code) in the big-endian format. The default is -mlittle-endian-data, i.e. to store data in the little-endian format.
- -msmall-data-limit=N
-
Specifies the maximum size in bytes of global and static variables which can be placed into the small data area. Using the small data area can lead to smaller and faster code, but the size of area is limited and it is up to the programmer to ensure that the area does not overflow. Also when the small data area is used one of the RX's registers (usually "r13") is reserved for use pointing to this area, so it is no longer available for use by the compiler. This could result in slower and/or larger code if variables are pushed onto the stack instead of being held in this register.
- -msim
- -mno-sim
- Use the simulator runtime. The default is to use the libgloss board-specific runtime.
- -mas100-syntax
- -mno-as100-syntax
- When generating assembler output use a syntax that is compatible with Renesas's AS100 assembler. This syntax can also be handled by the GAS assembler, but it has some restrictions so it is not generated by default.
- -mmax-constant-size=N
-
Specifies the maximum size, in bytes, of a constant that can be used as an operand in a RX instruction. Although the RX instruction set does allow constants of up to 4 bytes in length to be used in instructions, a longer value equates to a longer instruction. Thus in some circumstances it can be beneficial to restrict the size of constants that are used in instructions. Constants that are too big are instead placed into a constant pool and referenced via register indirection.
- -mrelax
- Enable linker relaxation. Linker relaxation is a process whereby the linker attempts to reduce the size of a program by finding shorter versions of various instructions. Disabled by default.
- -mint-register=N
- Specify the number of registers to reserve for fast interrupt handler functions. The value N can be between 0 and 4. A value of 1 means that register "r13" is reserved for the exclusive use of fast interrupt handlers. A value of 2 reserves "r13" and "r12". A value of 3 reserves "r13", "r12" and "r11", and a value of 4 reserves "r13" through "r10". A value of 0, the default, does not reserve any registers.
- -msave-acc-in-interrupts
- Specifies that interrupt handler functions should preserve the accumulator register. This is only necessary if normal code might use the accumulator register, for example because it performs 64-bit multiplications. The default is to ignore the accumulator as this makes the interrupt handlers faster.
- -mpid
- -mno-pid
-
Enables the generation of position independent data. When enabled any access to constant data is done via an offset from a base address held in a register. This allows the location of constant data to be determined at run time without requiring the executable to be relocated, which is a benefit to embedded applications with tight memory constraints. Data that can be modified is not affected by this option.
- -mno-warn-multiple-fast-interrupts
- -mwarn-multiple-fast-interrupts
- Prevents GCC from issuing a warning message if it finds more than one fast interrupt handler when it is compiling a file. The default is to issue a warning for each extra fast interrupt handler found, as the RX only supports one such interrupt.
Note: The generic GCC command-line option -ffixed-reg has special significance to the RX port when used with the "interrupt" function attribute. This attribute indicates a function intended to process fast interrupts. GCC ensures that it only uses the registers "r10", "r11", "r12" and/or "r13" and only provided that the normal use of the corresponding registers have been restricted via the -ffixed-reg or -mint-register command-line options.
S/390 and zSeries Options
These are the -m options defined for the S/390 and zSeries architecture.
- -mhard-float
- -msoft-float
- Use (do not use) the hardware floating-point instructions and registers for floating-point operations. When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. When -mhard-float is specified, the compiler generates IEEE floating-point instructions. This is the default.
- -mhard-dfp
- -mno-hard-dfp
- Use (do not use) the hardware decimal-floating-point instructions for decimal-floating-point operations. When -mno-hard-dfp is specified, functions in libgcc.a are used to perform decimal-floating-point operations. When -mhard-dfp is specified, the compiler generates decimal-floating-point hardware instructions. This is the default for -march=z9-ec or higher.
- -mlong-double-64
- -mlong-double-128
- These switches control the size of "long double" type. A size of 64 bits makes the "long double" type equivalent to the "double" type. This is the default.
- -mbackchain
- -mno-backchain
-
Store (do not store) the address of the caller's frame as backchain pointer into the callee's stack frame. A backchain may be needed to allow debugging using tools that do not understand DWARF 2 call frame information. When -mno-packed-stack is in effect, the backchain pointer is stored at the bottom of the stack frame; when -mpacked-stack is in effect, the backchain is placed into the topmost word of the 96/160 byte register save area.
- -mpacked-stack
- -mno-packed-stack
-
Use (do not use) the packed stack layout. When -mno-packed-stack is specified, the compiler uses the all fields of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space. When -mpacked-stack is specified, register save slots are densely packed at the top of the register save area; unused space is reused for other purposes, allowing for more efficient use of the available stack space. However, when -mbackchain is also in effect, the topmost word of the save area is always used to store the backchain, and the return address register is always saved two words below the backchain.
- -msmall-exec
- -mno-small-exec
- Generate (or do not generate) code using the "bras" instruction to do subroutine calls. This only works reliably if the total executable size does not exceed 64k. The default is to use the "basr" instruction instead, which does not have this limitation.
- -m64
- -m31
- When -m31 is specified, generate code compliant to the GNU/Linux for S/390 ABI. When -m64 is specified, generate code compliant to the GNU/Linux for zSeries ABI. This allows GCC in particular to generate 64-bit instructions. For the s390 targets, the default is -m31, while the s390x targets default to -m64.
- -mzarch
- -mesa
- When -mzarch is specified, generate code using the instructions available on z/Architecture. When -mesa is specified, generate code using the instructions available on ESA/390. Note that -mesa is not possible with -m64. When generating code compliant to the GNU/Linux for S/390 ABI, the default is -mesa. When generating code compliant to the GNU/Linux for zSeries ABI, the default is -mzarch.
- -mmvcle
- -mno-mvcle
- Generate (or do not generate) code using the "mvcle" instruction to perform block moves. When -mno-mvcle is specified, use a "mvc" loop instead. This is the default unless optimizing for size.
- -mdebug
- -mno-debug
- Print (or do not print) additional debug information when compiling. The default is to not print debug information.
- -march=cpu-type
- Generate code that runs on cpu-type, which is the name of a system representing a certain processor type. Possible values for cpu-type are g5, g6, z900, z990, z9-109, z9-ec and z10. When generating code using the instructions available on z/Architecture, the default is -march=z900. Otherwise, the default is -march=g5.
- -mtune=cpu-type
- Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. The list of cpu-type values is the same as for -march. The default is the value used for -march.
- -mtpf-trace
- -mno-tpf-trace
- Generate code that adds (does not add) in TPF OS specific branches to trace routines in the operating system. This option is off by default, even when compiling for the TPF OS.
- -mfused-madd
- -mno-fused-madd
- Generate code that uses (does not use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used.
- -mwarn-framesize=framesize
- Emit a warning if the current function exceeds the given frame size. Because this is a compile-time check it doesn't need to be a real problem when the program runs. It is intended to identify functions that most probably cause a stack overflow. It is useful to be used in an environment with limited stack size e.g. the linux kernel.
- -mwarn-dynamicstack
- Emit a warning if the function calls "alloca" or uses dynamically-sized arrays. This is generally a bad idea with a limited stack size.
- -mstack-guard=stack-guard
- -mstack-size=stack-size
- If these options are provided the S/390 back end emits additional instructions in the function prologue that trigger a trap if the stack size is stack-guard bytes above the stack-size (remember that the stack on S/390 grows downward). If the stack-guard option is omitted the smallest power of 2 larger than the frame size of the compiled function is chosen. These options are intended to be used to help debugging stack overflow problems. The additionally emitted code causes only little overhead and hence can also be used in production-like systems without greater performance degradation. The given values have to be exact powers of 2 and stack-size has to be greater than stack-guard without exceeding 64k. In order to be efficient the extra code makes the assumption that the stack starts at an address aligned to the value given by stack-size. The stack-guard option can only be used in conjunction with stack-size.
- -mhotpatch[=halfwords]
- -mno-hotpatch
-
If the hotpatch option is enabled, a "hot-patching" function prologue is generated for all functions in the compilation unit. The funtion label is prepended with the given number of two-byte Nop instructions ( halfwords, maximum 1000000) or 12 Nop instructions if no argument is present. Functions with a hot-patching prologue are never inlined automatically, and a hot-patching prologue is never generated for functions functions that are explicitly inline.
Score Options
These options are defined for Score implementations:
- -meb
- Compile code for big-endian mode. This is the default.
- -mel
- Compile code for little-endian mode.
- -mnhwloop
- Disable generation of "bcnz" instructions.
- -muls
- Enable generation of unaligned load and store instructions.
- -mmac
- Enable the use of multiply-accumulate instructions. Disabled by default.
- -mscore5
- Specify the SCORE5 as the target architecture.
- -mscore5u
- Specify the SCORE5U of the target architecture.
- -mscore7
- Specify the SCORE7 as the target architecture. This is the default.
- -mscore7d
- Specify the SCORE7D as the target architecture.
SH Options
These -m options are defined for the SH implementations:
- -m1
- Generate code for the SH1.
- -m2
- Generate code for the SH2.
- -m2e
- Generate code for the SH2e.
- -m2a-nofpu
- Generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit is not used.
- -m2a-single-only
- Generate code for the SH2a-FPU, in such a way that no double-precision floating-point operations are used.
- -m2a-single
- Generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by default.
- -m2a
- Generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by default.
- -m3
- Generate code for the SH3.
- -m3e
- Generate code for the SH3e.
- -m4-nofpu
- Generate code for the SH4 without a floating-point unit.
- -m4-single-only
- Generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.
- -m4-single
- Generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
- -m4
- Generate code for the SH4.
- -m4a-nofpu
- Generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.
- -m4a-single-only
- Generate code for the SH4a, in such a way that no double-precision floating-point operations are used.
- -m4a-single
- Generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.
- -m4a
- Generate code for the SH4a.
- -m4al
- Same as -m4a-nofpu, except that it implicitly passes -dsp to the assembler. GCC doesn't generate any DSP instructions at the moment.
- -mb
- Compile code for the processor in big-endian mode.
- -ml
- Compile code for the processor in little-endian mode.
- -mdalign
- Align doubles at 64-bit boundaries. Note that this changes the calling conventions, and thus some functions from the standard C library do not work unless you recompile it first with -mdalign.
- -mrelax
- Shorten some address references at link time, when possible; uses the linker option -relax.
- -mbigtable
- Use 32-bit offsets in "switch" tables. The default is to use 16-bit offsets.
- -mbitops
- Enable the use of bit manipulation instructions on SH2A.
- -mfmovd
- Enable the use of the instruction "fmovd". Check -mdalign for alignment constraints.
- -mhitachi
- Comply with the calling conventions defined by Renesas.
- -mrenesas
- Comply with the calling conventions defined by Renesas.
- -mno-renesas
- Comply with the calling conventions defined for GCC before the Renesas conventions were available. This option is the default for all targets of the SH toolchain.
- -mnomacsave
- Mark the "MAC" register as call-clobbered, even if -mhitachi is given.
- -mieee
- -mno-ieee
- Control the IEEE compliance of floating-point comparisons, which affects the handling of cases where the result of a comparison is unordered. By default -mieee is implicitly enabled. If -ffinite-math-only is enabled -mno-ieee is implicitly set, which results in faster floating-point greater-equal and less-equal comparisons. The implcit settings can be overridden by specifying either -mieee or -mno-ieee.
- -minline-ic_invalidate
- Inline code to invalidate instruction cache entries after setting up nested function trampolines. This option has no effect if -musermode is in effect and the selected code generation option (e.g. -m4) does not allow the use of the "icbi" instruction. If the selected code generation option does not allow the use of the "icbi" instruction, and -musermode is not in effect, the inlined code manipulates the instruction cache address array directly with an associative write. This not only requires privileged mode at run time, but it also fails if the cache line had been mapped via the TLB and has become unmapped.
- -misize
- Dump instruction size and location in the assembly code.
- -mpadstruct
- This option is deprecated. It pads structures to multiple of 4 bytes, which is incompatible with the SH ABI.
- -matomic-model=model
- Sets the model of atomic operations and additional parameters as a comma separated list. For details on the atomic built-in functions see __atomic Builtins. The following models and parameters are supported:
- none
- Disable compiler generated atomic sequences and emit library calls for atomic operations. This is the default if the target is not "sh-*-linux*".
- soft-gusa
- Generate GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for SH3* and SH4* single-core systems. This option is enabled by default when the target is "sh-*-linux*" and SH3* or SH4*. When the target is SH4A, this option will also partially utilize the hardware atomic instructions "movli.l" and "movco.l" to create more efficient code, unless strict is specified.
- soft-tcb
- Generate software atomic sequences that use a variable in the thread control block. This is a variation of the gUSA sequences which can also be used on SH1* and SH2* targets. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for single-core systems. When using this model, the gbr-offset= parameter has to be specified as well.
- soft-imask
- Generate software atomic sequences that temporarily disable interrupts by setting "SR.IMASK = 1111". This model works only when the program runs in privileged mode and is only suitable for single-core systems. Additional support from the interrupt/exception handling code of the system is not required. This model is enabled by default when the target is "sh-*-linux*" and SH1* or SH2*.
- hard-llcs
- Generate hardware atomic sequences using the "movli.l" and "movco.l" instructions only. This is only available on SH4A and is suitable for multi-core systems. Since the hardware instructions support only 32 bit atomic variables access to 8 or 16 bit variables is emulated with 32 bit accesses. Code compiled with this option will also be compatible with other software atomic model interrupt/exception handling systems if executed on an SH4A system. Additional support from the interrupt/exception handling code of the system is not required for this model.
- gbr-offset=
- This parameter specifies the offset in bytes of the variable in the thread control block structure that should be used by the generated atomic sequences when the soft-tcb model has been selected. For other models this parameter is ignored. The specified value must be an integer multiple of four and in the range 0-1020.
- strict
- This parameter prevents mixed usage of multiple atomic models, even though they would be compatible, and will make the compiler generate atomic sequences of the specified model only.
- -mtas
- Generate the "tas.b" opcode for "__atomic_test_and_set". Notice that depending on the particular hardware and software configuration this can degrade overall performance due to the operand cache line flushes that are implied by the "tas.b" instruction. On multi-core SH4A processors the "tas.b" instruction must be used with caution since it can result in data corruption for certain cache configurations.
- -mspace
- Optimize for space instead of speed. Implied by -Os.
- -mprefergot
- When generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table.
- -musermode
- Don't generate privileged mode only code. This option implies -mno-inline-ic_invalidate if the inlined code would not work in user mode. This is the default when the target is "sh-*-linux*".
- -multcost=number
- Set the cost to assume for a multiply insn.
- -mdiv=strategy
- Set the division strategy to be used for integer division operations. For SHmedia strategy can be one of:
- fp
- Performs the operation in floating point. This has a very high latency, but needs only a few instructions, so it might be a good choice if your code has enough easily-exploitable ILP to allow the compiler to schedule the floating-point instructions together with other instructions. Division by zero causes a floating-point exception.
- inv
- Uses integer operations to calculate the inverse of the divisor, and then multiplies the dividend with the inverse. This strategy allows CSE and hoisting of the inverse calculation. Division by zero calculates an unspecified result, but does not trap.
- inv:minlat
- A variant of inv where, if no CSE or hoisting opportunities have been found, or if the entire operation has been hoisted to the same place, the last stages of the inverse calculation are intertwined with the final multiply to reduce the overall latency, at the expense of using a few more instructions, and thus offering fewer scheduling opportunities with other code.
- call
- Calls a library function that usually implements the inv:minlat strategy. This gives high code density for "m5-*media-nofpu" compilations.
- call2
- Uses a different entry point of the same library function, where it assumes that a pointer to a lookup table has already been set up, which exposes the pointer load to CSE and code hoisting optimizations.
- inv:call
- inv:call2
- inv:fp
- Use the inv algorithm for initial code generation, but if the code stays unoptimized, revert to the call, call2, or fp strategies, respectively. Note that the potentially-trapping side effect of division by zero is carried by a separate instruction, so it is possible that all the integer instructions are hoisted out, but the marker for the side effect stays where it is. A recombination to floating-point operations or a call is not possible in that case.
- inv20u
- inv20l
- Variants of the inv:minlat strategy. In the case that the inverse calculation is not separated from the multiply, they speed up division where the dividend fits into 20 bits (plus sign where applicable) by inserting a test to skip a number of operations in this case; this test slows down the case of larger dividends. inv20u assumes the case of a such a small dividend to be unlikely, and inv20l assumes it to be likely.
- call-div1
- Calls a library function that uses the single-step division instruction "div1" to perform the operation. Division by zero calculates an unspecified result and does not trap. This is the default except for SH4, SH2A and SHcompact.
- call-fp
- Calls a library function that performs the operation in double precision floating point. Division by zero causes a floating-point exception. This is the default for SHcompact with FPU. Specifying this for targets that do not have a double precision FPU will default to "call-div1".
- call-table
- Calls a library function that uses a lookup table for small divisors and the "div1" instruction with case distinction for larger divisors. Division by zero calculates an unspecified result and does not trap. This is the default for SH4. Specifying this for targets that do not have dynamic shift instructions will default to "call-div1".
- -maccumulate-outgoing-args
- Reserve space once for outgoing arguments in the function prologue rather than around each call. Generally beneficial for performance and size. Also needed for unwinding to avoid changing the stack frame around conditional code.
- -mdivsi3_libfunc=name
- Set the name of the library function used for 32-bit signed division to name. This only affects the name used in the call and inv:call division strategies, and the compiler still expects the same sets of input/output/clobbered registers as if this option were not present.
- -mfixed-range=register-range
- Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator can not use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
- -mindexed-addressing
- Enable the use of the indexed addressing mode for SHmedia32/SHcompact. This is only safe if the hardware and/or OS implement 32-bit wrap-around semantics for the indexed addressing mode. The architecture allows the implementation of processors with 64-bit MMU, which the OS could use to get 32-bit addressing, but since no current hardware implementation supports this or any other way to make the indexed addressing mode safe to use in the 32-bit ABI, the default is -mno-indexed-addressing.
- -mgettrcost=number
- Set the cost assumed for the "gettr" instruction to number. The default is 2 if -mpt-fixed is in effect, 100 otherwise.
- -mpt-fixed
-
Assume "pt*" instructions won't trap. This generally generates better-scheduled code, but is unsafe on current hardware. The current architecture definition says that "ptabs" and "ptrel" trap when the target anded with 3 is 3. This has the unintentional effect of making it unsafe to schedule these instructions before a branch, or hoist them out of a loop. For example, "__do_global_ctors", a part of libgcc that runs constructors at program startup, calls functions in a list which is delimited by -1. With the -mpt-fixed option, the "ptabs" is done before testing against -1. That means that all the constructors run a bit more quickly, but when the loop comes to the end of the list, the program crashes because "ptabs" loads -1 into a target register.
- -minvalid-symbols
- Assume symbols might be invalid. Ordinary function symbols generated by the compiler are always valid to load with "movi"/"shori"/"ptabs" or "movi"/"shori"/"ptrel", but with assembler and/or linker tricks it is possible to generate symbols that cause "ptabs" or "ptrel" to trap. This option is only meaningful when -mno-pt-fixed is in effect. It prevents cross-basic-block CSE, hoisting and most scheduling of symbol loads. The default is -mno-invalid-symbols.
- -mbranch-cost=num
- Assume num to be the cost for a branch instruction. Higher numbers make the compiler try to generate more branch-free code if possible. If not specified the value is selected depending on the processor type that is being compiled for.
- -mzdcbranch
- -mno-zdcbranch
- Assume (do not assume) that zero displacement conditional branch instructions "bt" and "bf" are fast. If -mzdcbranch is specified, the compiler will try to prefer zero displacement branch code sequences. This is enabled by default when generating code for SH4 and SH4A. It can be explicitly disabled by specifying -mno-zdcbranch.
- -mcbranchdi
- Enable the "cbranchdi4" instruction pattern.
- -mcmpeqdi
- Emit the "cmpeqdi_t" instruction pattern even when -mcbranchdi is in effect.
- -mfused-madd
- -mno-fused-madd
- Generate code that uses (does not use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used. The machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off.
- -mfsca
- -mno-fsca
- Allow or disallow the compiler to emit the "fsca" instruction for sine and cosine approximations. The option "-mfsca" must be used in combination with "-funsafe-math-optimizations". It is enabled by default when generating code for SH4A. Using "-mno-fsca" disables sine and cosine approximations even if "-funsafe-math-optimizations" is in effect.
- -mfsrra
- -mno-fsrra
- Allow or disallow the compiler to emit the "fsrra" instruction for reciprocal square root approximations. The option "-mfsrra" must be used in combination with "-funsafe-math-optimizations" and "-ffinite-math-only". It is enabled by default when generating code for SH4A. Using "-mno-fsrra" disables reciprocal square root approximations even if "-funsafe-math-optimizations" and "-ffinite-math-only" are in effect.
- -mpretend-cmove
- Prefer zero-displacement conditional branches for conditional move instruction patterns. This can result in faster code on the SH4 processor.
Solaris 2 Options
These -m options are supported on Solaris 2:
- -mimpure-text
-
-mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when linking a shared object. Using this option, you can link position-dependent code into a shared object.
These switches are supported in addition to the above on Solaris 2:
- -pthreads
- Add support for multithreading using the POSIX threads library. This option sets flags for both the preprocessor and linker. This option does not affect the thread safety of object code produced by the compiler or that of libraries supplied with it.
- -pthread
- This is a synonym for -pthreads.
SPARC Options
These -m options are supported on the SPARC:
- -mno-app-regs
- -mapp-regs
-
Specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications. This is the default.
- -mflat
- -mno-flat
-
With -mflat, the compiler does not generate save/restore instructions and uses a "flat" or single register window model. This model is compatible with the regular register window model. The local registers and the input registers (0--5) are still treated as "call-saved" registers and are saved on the stack as needed.
- -mfpu
- -mhard-float
- Generate output containing floating-point instructions. This is the default.
- -mno-fpu
- -msoft-float
-
Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets sparc-*-aout and sparclite-*-* do provide software floating-point support.
- -mhard-quad-float
- Generate output containing quad-word (long double) floating-point instructions.
- -msoft-quad-float
-
Generate output containing library calls for quad-word (long double) floating-point instructions. The functions called are those specified in the SPARC ABI. This is the default.
- -mno-unaligned-doubles
- -munaligned-doubles
-
Assume that doubles have 8-byte alignment. This is the default.
- -mno-faster-structs
- -mfaster-structs
- With -mfaster-structs, the compiler assumes that structures should have 8-byte alignment. This enables the use of pairs of "ldd" and "std" instructions for copies in structure assignment, in place of twice as many "ld" and "st" pairs. However, the use of this changed alignment directly violates the SPARC ABI. Thus, it's intended only for use on targets where the developer acknowledges that their resulting code is not directly in line with the rules of the ABI.
- -mcpu=cpu_type
-
Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are v7, cypress, v8, supersparc, hypersparc, leon, leon3, sparclite, f930, f934, sparclite86x, sparclet, tsc701, v9, ultrasparc, ultrasparc3, niagara, niagara2, niagara3 and niagara4.
- v7
- cypress
- v8
- supersparc, hypersparc, leon, leon3
- sparclite
- f930, f934, sparclite86x
- sparclet
- tsc701
- v9
- ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4
- -mtune=cpu_type
-
Set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that the option -mcpu=cpu_type does.
- -mv8plus
- -mno-v8plus
- With -mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI is that the global and out registers are considered 64 bits wide. This is enabled by default on Solaris in 32-bit mode for all SPARC-V9 processors.
- -mvis
- -mno-vis
- With -mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions. The default is -mno-vis.
- -mvis2
- -mno-vis2
- With -mvis2, GCC generates code that takes advantage of version 2.0 of the UltraSPARC Visual Instruction Set extensions. The default is -mvis2 when targeting a cpu that supports such instructions, such as UltraSPARC-III and later. Setting -mvis2 also sets -mvis.
- -mvis3
- -mno-vis3
- With -mvis3, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions. The default is -mvis3 when targeting a cpu that supports such instructions, such as niagara-3 and later. Setting -mvis3 also sets -mvis2 and -mvis.
- -mcbcond
- -mno-cbcond
- With -mcbcond, GCC generates code that takes advantage of compare-and-branch instructions, as defined in the Sparc Architecture 2011. The default is -mcbcond when targeting a cpu that supports such instructions, such as niagara-4 and later.
- -mpopc
- -mno-popc
- With -mpopc, GCC generates code that takes advantage of the UltraSPARC population count instruction. The default is -mpopc when targeting a cpu that supports such instructions, such as Niagara-2 and later.
- -mfmaf
- -mno-fmaf
- With -mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add Floating-point extensions. The default is -mfmaf when targeting a cpu that supports such instructions, such as Niagara-3 and later.
- -mfix-at697f
- Enable the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E processor).
- -mfix-ut699
- Enable the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor.
These -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments:
- -m32
- -m64
- Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
- -mcmodel=which
- Set the code model to one of
- medlow
- The Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. Programs can be statically or dynamically linked.
- medmid
- The Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
- medany
- The Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
- embmedany
- The Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data segments must be less than 2GB in size, both starting anywhere in memory (determined at link time). The global register %g4 points to the base of the data segment. Programs are statically linked and PIC is not supported.
- -mmemory-model=mem-model
- Set the memory model in force on the processor to one of
- default
- The default memory model for the processor and operating system.
- rmo
- Relaxed Memory Order
- pso
- Partial Store Order
- tso
- Total Store Order
- sc
- Sequential Consistency
- -mstack-bias
- -mno-stack-bias
- With -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by -2047 which must be added back when making stack frame references. This is the default in 64-bit mode. Otherwise, assume no such offset is present.
SPU Options
These -m options are supported on the SPU:
- -mwarn-reloc
- -merror-reloc
- The loader for SPU does not handle dynamic relocations. By default, GCC gives an error when it generates code that requires a dynamic relocation. -mno-error-reloc disables the error, -mwarn-reloc generates a warning instead.
- -msafe-dma
- -munsafe-dma
- Instructions that initiate or test completion of DMA must not be reordered with respect to loads and stores of the memory that is being accessed. With -munsafe-dma you must use the "volatile" keyword to protect memory accesses, but that can lead to inefficient code in places where the memory is known to not change. Rather than mark the memory as volatile, you can use -msafe-dma to tell the compiler to treat the DMA instructions as potentially affecting all memory.
- -mbranch-hints
- By default, GCC generates a branch hint instruction to avoid pipeline stalls for always-taken or probably-taken branches. A hint is not generated closer than 8 instructions away from its branch. There is little reason to disable them, except for debugging purposes, or to make an object a little bit smaller.
- -msmall-mem
- -mlarge-mem
- By default, GCC generates code assuming that addresses are never larger than 18 bits. With -mlarge-mem code is generated that assumes a full 32-bit address.
- -mstdmain
- By default, GCC links against startup code that assumes the SPU-style main function interface (which has an unconventional parameter list). With -mstdmain, GCC links your program against startup code that assumes a C99-style interface to "main", including a local copy of "argv" strings.
- -mfixed-range=register-range
- Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
- -mea32
- -mea64
- Compile code assuming that pointers to the PPU address space accessed via the "__ea" named address space qualifier are either 32 or 64 bits wide. The default is 32 bits. As this is an ABI-changing option, all object code in an executable must be compiled with the same setting.
- -maddress-space-conversion
- -mno-address-space-conversion
- Allow/disallow treating the "__ea" address space as superset of the generic address space. This enables explicit type casts between "__ea" and generic pointer as well as implicit conversions of generic pointers to "__ea" pointers. The default is to allow address space pointer conversions.
- -mcache-size=cache-size
- This option controls the version of libgcc that the compiler links to an executable and selects a software-managed cache for accessing variables in the "__ea" address space with a particular cache size. Possible options for cache-size are 8, 16, 32, 64 and 128. The default cache size is 64KB.
- -matomic-updates
- -mno-atomic-updates
- This option controls the version of libgcc that the compiler links to an executable and selects whether atomic updates to the software-managed cache of PPU-side variables are used. If you use atomic updates, changes to a PPU variable from SPU code using the "__ea" named address space qualifier do not interfere with changes to other PPU variables residing in the same cache line from PPU code. If you do not use atomic updates, such interference may occur; however, writing back cache lines is more efficient. The default behavior is to use atomic updates.
- -mdual-nops
- -mdual-nops=n
- By default, GCC inserts nops to increase dual issue when it expects it to increase performance. n can be a value from 0 to 10. A smaller n inserts fewer nops. 10 is the default, 0 is the same as -mno-dual-nops. Disabled with -Os.
- -mhint-max-nops=n
- Maximum number of nops to insert for a branch hint. A branch hint must be at least 8 instructions away from the branch it is affecting. GCC inserts up to n nops to enforce this, otherwise it does not generate the branch hint.
- -mhint-max-distance=n
- The encoding of the branch hint instruction limits the hint to be within 256 instructions of the branch it is affecting. By default, GCC makes sure it is within 125.
- -msafe-hints
- Work around a hardware bug that causes the SPU to stall indefinitely. By default, GCC inserts the "hbrp" instruction to make sure this stall won't happen.
Options for System V
These additional options are available on System V Release 4 for compatibility with other compilers on those systems:
- -G
- Create a shared object. It is recommended that -symbolic or -shared be used instead.
- -Qy
- Identify the versions of each tool used by the compiler, in a ".ident" assembler directive in the output.
- -Qn
- Refrain from adding ".ident" directives to the output file (this is the default).
- -YP,dirs
- Search the directories dirs, and no others, for libraries specified with -l.
- -Ym,dir
- Look in the directory dir to find the M4 preprocessor. The assembler uses this option.
TILE-Gx Options
These -m options are supported on the TILE-Gx:
- -mcmodel=small
- Generate code for the small model. The distance for direct calls is limited to 500M in either direction. PC-relative addresses are 32 bits. Absolute addresses support the full address range.
- -mcmodel=large
- Generate code for the large model. There is no limitation on call distance, pc-relative addresses, or absolute addresses.
- -mcpu=name
- Selects the type of CPU to be targeted. Currently the only supported type is tilegx.
- -m32
- -m64
- Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long, and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
TILEPro Options
These -m options are supported on the TILEPro:
- -mcpu=name
- Selects the type of CPU to be targeted. Currently the only supported type is tilepro.
- -m32
- Generate code for a 32-bit environment, which sets int, long, and pointer to 32 bits. This is the only supported behavior so the flag is essentially ignored.
V850 Options
These -m options are defined for V850 implementations:
- -mlong-calls
- -mno-long-calls
- Treat all calls as being far away (near). If calls are assumed to be far away, the compiler always loads the function's address into a register, and calls indirect through the pointer.
- -mno-ep
- -mep
- Do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer into the "ep" register, and use the shorter "sld" and "sst" instructions. The -mep option is on by default if you optimize.
- -mno-prolog-function
- -mprolog-function
- Do not use (do use) external functions to save and restore registers at the prologue and epilogue of a function. The external functions are slower, but use less code space if more than one function saves the same number of registers. The -mprolog-function option is on by default if you optimize.
- -mspace
- Try to make the code as small as possible. At present, this just turns on the -mep and -mprolog-function options.
- -mtda=n
- Put static or global variables whose size is n bytes or less into the tiny data area that register "ep" points to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte references).
- -msda=n
- Put static or global variables whose size is n bytes or less into the small data area that register "gp" points to. The small data area can hold up to 64 kilobytes.
- -mzda=n
- Put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory.
- -mv850
- Specify that the target processor is the V850.
- -mv850e3v5
- Specify that the target processor is the V850E3V5. The preprocessor constant __v850e3v5__ is defined if this option is used.
- -mv850e2v4
- Specify that the target processor is the V850E3V5. This is an alias for the -mv850e3v5 option.
- -mv850e2v3
- Specify that the target processor is the V850E2V3. The preprocessor constant __v850e2v3__ is defined if this option is used.
- -mv850e2
- Specify that the target processor is the V850E2. The preprocessor constant __v850e2__ is defined if this option is used.
- -mv850e1
- Specify that the target processor is the V850E1. The preprocessor constants __v850e1__ and __v850e__ are defined if this option is used.
- -mv850es
- Specify that the target processor is the V850ES. This is an alias for the -mv850e1 option.
- -mv850e
-
Specify that the target processor is the V850E. The preprocessor constant __v850e__ is defined if this option is used.
- -mdisable-callt
- -mno-disable-callt
-
This option suppresses generation of the "CALLT" instruction for the v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the v850 architecture.
- -mrelax
- -mno-relax
- Pass on (or do not pass on) the -mrelax command line option to the assembler.
- -mlong-jumps
- -mno-long-jumps
- Disable (or re-enable) the generation of PC-relative jump instructions.
- -msoft-float
- -mhard-float
- Disable (or re-enable) the generation of hardware floating point instructions. This option is only significant when the target architecture is V850E2V3 or higher. If hardware floating point instructions are being generated then the C preprocessor symbol "__FPU_OK__" will be defined, otherwise the symbol "__NO_FPU__" will be defined.
- -mloop
- Enables the use of the e3v5 LOOP instruction. The use of this instruction is not enabled by default when the e3v5 architecture is selected because its use is still experimental.
- -mrh850-abi
- -mghs
- Enables support for the RH850 version of the V850 ABI. This is the default. With this version of the ABI the following rules apply:
- •
- Integer sized structures and unions are returned via a memory pointer rather than a register.
- •
- Large structures and unions (more than 8 bytes in size) are passed by value.
- •
- Functions are aligned to 16-bit boundaries.
- •
- The -m8byte-align command line option is supported.
- •
- The -mdisable-callt command line option is enabled by default. The -mno-disable-callt command line option is not supported.
- -mgcc-abi
- Enables support for the old GCC version of the V850 ABI. With this version of the ABI the following rules apply:
- •
- Integer sized structures and unions are returned in register "r10".
- •
- Large structures and unions (more than 8 bytes in size) are passed by reference.
- •
- Functions are aligned to 32-bit boundaries, unless optimizing for size.
- •
- The -m8byte-align command line option is not supported.
- •
- The -mdisable-callt command line option is supported but not enabled by default.
- -m8byte-align
- -mno-8byte-align
- Enables support for "doubles" and "long long" types to be aligned on 8-byte boundaries. The default is to restrict the alignment of all objects to at most 4-bytes. When -m8byte-align is in effect the C preprocessor symbol "__V850_8BYTE_ALIGN__" will be defined.
- -mbig-switch
- Generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range branches within a switch table.
- -mapp-regs
- This option causes r2 and r5 to be used in the code generated by the compiler. This setting is the default.
- -mno-app-regs
- This option causes r2 and r5 to be treated as fixed registers.
VAX Options
These -m options are defined for the VAX:
- -munix
- Do not output certain jump instructions ("aobleq" and so on) that the Unix assembler for the VAX cannot handle across long ranges.
- -mgnu
- Do output those jump instructions, on the assumption that the GNU assembler is being used.
- -mg
- Output code for G-format floating-point numbers instead of D-format.
VMS Options
These -m options are defined for the VMS implementations:
- -mvms-return-codes
- Return VMS condition codes from "main". The default is to return POSIX-style condition (e.g. error) codes.
- -mdebug-main=prefix
- Flag the first routine whose name starts with prefix as the main routine for the debugger.
- -mmalloc64
- Default to 64-bit memory allocation routines.
- -mpointer-size=size
- Set the default size of pointers. Possible options for size are 32 or short for 32 bit pointers, 64 or long for 64 bit pointers, and no for supporting only 32 bit pointers. The later option disables "pragma pointer_size".
VxWorks Options
The options in this section are defined for all VxWorks targets. Options specific to the target hardware are listed with the other options for that target.
- -mrtp
- GCC can generate code for both VxWorks kernels and real time processes (RTPs). This option switches from the former to the latter. It also defines the preprocessor macro "__RTP__".
- -non-static
- Link an RTP executable against shared libraries rather than static libraries. The options -static and -shared can also be used for RTPs; -static is the default.
- -Bstatic
- -Bdynamic
- These options are passed down to the linker. They are defined for compatibility with Diab.
- -Xbind-lazy
- Enable lazy binding of function calls. This option is equivalent to -Wl,-z,now and is defined for compatibility with Diab.
- -Xbind-now
- Disable lazy binding of function calls. This option is the default and is defined for compatibility with Diab.
x86-64 Options
These are listed under
Xstormy16 Options
These options are defined for Xstormy16:
- -msim
- Choose startup files and linker script suitable for the simulator.
Xtensa Options
These options are supported for Xtensa targets:
- -mconst16
- -mno-const16
- Enable or disable use of "CONST16" instructions for loading constant values. The "CONST16" instruction is currently not a standard option from Tensilica. When enabled, "CONST16" instructions are always used in place of the standard "L32R" instructions. The use of "CONST16" is enabled by default only if the "L32R" instruction is not available.
- -mfused-madd
- -mno-fused-madd
- Enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option. This has no effect if the floating-point option is not also enabled. Disabling fused multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract operations. This may be desirable in some cases where strict IEEE 754-compliant results are required: the fused multiply add/subtract instructions do not round the intermediate result, thereby producing results with more bits of precision than specified by the IEEE standard. Disabling fused multiply add/subtract instructions also ensures that the program output is not sensitive to the compiler's ability to combine multiply and add/subtract operations.
- -mserialize-volatile
- -mno-serialize-volatile
- When this option is enabled, GCC inserts "MEMW" instructions before "volatile" memory references to guarantee sequential consistency. The default is -mserialize-volatile. Use -mno-serialize-volatile to omit the "MEMW" instructions.
- -mforce-no-pic
- For targets, like GNU/Linux, where all user-mode Xtensa code must be position-independent code (PIC), this option disables PIC for compiling kernel code.
- -mtext-section-literals
- -mno-text-section-literals
- Control the treatment of literal pools. The default is -mno-text-section-literals, which places literals in a separate section in the output file. This allows the literal pool to be placed in a data RAM/ROM, and it also allows the linker to combine literal pools from separate object files to remove redundant literals and improve code size. With -mtext-section-literals, the literals are interspersed in the text section in order to keep them as close as possible to their references. This may be necessary for large assembly files.
- -mtarget-align
- -mno-target-align
- When this option is enabled, GCC instructs the assembler to automatically align instructions to reduce branch penalties at the expense of some code density. The assembler attempts to widen density instructions to align branch targets and the instructions following call instructions. If there are not enough preceding safe density instructions to align a target, no widening is performed. The default is -mtarget-align. These options do not affect the treatment of auto-aligned instructions like "LOOP", which the assembler always aligns, either by widening density instructions or by inserting NOP instructions.
- -mlongcalls
- -mno-longcalls
- When this option is enabled, GCC instructs the assembler to translate direct calls to indirect calls unless it can determine that the target of a direct call is in the range allowed by the call instruction. This translation typically occurs for calls to functions in other source files. Specifically, the assembler translates a direct "CALL" instruction into an "L32R" followed by a "CALLX" instruction. The default is -mno-longcalls. This option should be used in programs where the call target can potentially be out of range. This option is implemented in the assembler, not the compiler, so the assembly code generated by GCC still shows direct call instructions---look at the disassembled object code to see the actual instructions. Note that the assembler uses an indirect call for every cross-file call, not just those that really are out of range.
zSeries Options
These are listed under
Options for Code Generation Conventions
These machine-independent options control the interface conventions used in code generation.Most of them have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed---the one that is not the default. You can figure out the other form by either removing no- or adding it.
- -fbounds-check
- For front ends that support it, generate additional code to check that indices used to access arrays are within the declared range. This is currently only supported by the Java and Fortran front ends, where this option defaults to true and false respectively.
- -fstack-reuse=reuse-level
-
This option controls stack space reuse for user declared local/auto variables and compiler generated temporaries. reuse_level can be all, named_vars, or none. all enables stack reuse for all local variables and temporaries, named_vars enables the reuse only for user defined local variables with names, and none disables stack reuse completely. The default value is all. The option is needed when the program extends the lifetime of a scoped local variable or a compiler generated temporary beyond the end point defined by the language. When a lifetime of a variable ends, and if the variable lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. Legacy code extending local lifetime will likely to break with the stack reuse optimization.
int *p;
{
int local1;
p = &local1;
local1 = 10;
....
}
{
int local2;
local2 = 20;
...
}
if (*p == 10) // out of scope use of local1
{
}
struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};
A *ap;
void foo(const A& ar)
{
ap = &ar;
}
void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns
{
A a(20);
....
}
ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
}
- -ftrapv
- This option generates traps for signed overflow on addition, subtraction, multiplication operations.
- -fwrapv
- This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation. This flag enables some optimizations and disables others. This option is enabled by default for the Java front end, as required by the Java language specification.
- -fexceptions
- Enable exception handling. Generates extra code needed to propagate exceptions. For some targets, this implies GCC generates frame unwind information for all functions, which can produce significant data size overhead, although it does not affect execution. If you do not specify this option, GCC enables it by default for languages like C++ that normally require exception handling, and disables it for languages like C that do not normally require it. However, you may need to enable this option when compiling C code that needs to interoperate properly with exception handlers written in C++. You may also wish to disable this option if you are compiling older C++ programs that don't use exception handling.
- -fnon-call-exceptions
- Generate code that allows trapping instructions to throw exceptions. Note that this requires platform-specific runtime support that does not exist everywhere. Moreover, it only allows trapping instructions to throw exceptions, i.e. memory references or floating-point instructions. It does not allow exceptions to be thrown from arbitrary signal handlers such as "SIGALRM".
- -fdelete-dead-exceptions
- Consider that instructions that may throw exceptions but don't otherwise contribute to the execution of the program can be optimized away. This option is enabled by default for the Ada front end, as permitted by the Ada language specification. Optimization passes that cause dead exceptions to be removed are enabled independently at different optimization levels.
- -funwind-tables
- Similar to -fexceptions, except that it just generates any needed static data, but does not affect the generated code in any other way. You normally do not need to enable this option; instead, a language processor that needs this handling enables it on your behalf.
- -fasynchronous-unwind-tables
- Generate unwind table in DWARF 2 format, if supported by target machine. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector).
- -fpcc-struct-return
-
Return "short" "struct" and "union" values in memory like longer ones, rather than in registers. This convention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files compiled with other compilers, particularly the Portable C Compiler (pcc).
- -freg-struct-return
-
Return "struct" and "union" values in registers when possible. This is more efficient for small structures than -fpcc-struct-return.
- -fshort-enums
-
Allocate to an "enum" type only as many bytes as it needs for the declared range of possible values. Specifically, the "enum" type is equivalent to the smallest integer type that has enough room.
- -fshort-double
-
Use the same size for "double" as for "float".
- -fshort-wchar
-
Override the underlying type for wchar_t to be short unsigned int instead of the default for the target. This option is useful for building programs to run under WINE.
- -fno-common
- In C code, controls the placement of uninitialized global variables. Unix C compilers have traditionally permitted multiple definitions of such variables in different compilation units by placing the variables in a common block. This is the behavior specified by -fcommon, and is the default for GCC on most targets. On the other hand, this behavior is not required by ISO C, and on some targets may carry a speed or code size penalty on variable references. The -fno-common option specifies that the compiler should place uninitialized global variables in the data section of the object file, rather than generating them as common blocks. This has the effect that if the same variable is declared (without "extern") in two different compilations, you get a multiple-definition error when you link them. In this case, you must compile with -fcommon instead. Compiling with -fno-common is useful on targets for which it provides better performance, or if you wish to verify that the program will work on other systems that always treat uninitialized variable declarations this way.
- -fno-ident
- Ignore the #ident directive.
- -finhibit-size-directive
- Don't output a ".size" assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory. This option is used when compiling crtstuff.c; you should not need to use it for anything else.
- -fverbose-asm
-
Put extra commentary information in the generated assembly code to make it more readable. This option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself).
- -frecord-gcc-switches
- This switch causes the command line used to invoke the compiler to be recorded into the object file that is being created. This switch is only implemented on some targets and the exact format of the recording is target and binary file format dependent, but it usually takes the form of a section containing ASCII text. This switch is related to the -fverbose-asm switch, but that switch only records information in the assembler output file as comments, so it never reaches the object file. See also -grecord-gcc-switches for another way of storing compiler options into the object file.
- -fpic
-
Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that -fpic does not work; in that case, recompile with -fPIC instead. (These maximums are 8k on the SPARC and 32k on the m68k and RS/6000. The 386 has no such limit.)
- -fPIC
-
If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on the m68k, PowerPC and SPARC.
- -fpie
- -fPIE
-
These options are similar to -fpic and -fPIC, but generated position independent code can be only linked into executables. Usually these options are used when -pie GCC option is used during linking.
- -fno-jump-tables
- Do not use jump tables for switch statements even where it would be more efficient than other code generation strategies. This option is of use in conjunction with -fpic or -fPIC for building code that forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables do not require a GOT and this option is not needed.
- -ffixed-reg
-
Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role).
- -fcall-used-reg
-
Treat the register named reg as an allocable register that is clobbered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way do not save and restore the register reg.
- -fcall-saved-reg
-
Treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way save and restore the register reg if they use it.
- -fpack-struct[=n]
-
Without a value specified, pack all structure members together without holes. When a value is specified (which must be a small power of two), pack structure members according to this value, representing the maximum alignment (that is, objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location.
- -finstrument-functions
-
Generate instrumentation calls for entry and exit to functions. Just after function entry and just before function exit, the following profiling functions are called with the address of the current function and its call site. (On some platforms, "__builtin_return_address" does not work beyond the current function, so the call site information may not be available to the profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
- -finstrument-functions-exclude-file-list=file,file,...
-
Set the list of functions that are excluded from instrumentation (see the description of "-finstrument-functions"). If the file that contains a function definition matches with one of file, then that function is not instrumented. The match is done on substrings: if the file parameter is a substring of the file name, it is considered to be a match.
-finstrument-functions-exclude-file-list=/bits/stl,include/sys
- -finstrument-functions-exclude-function-list=sym,sym,...
- This is similar to "-finstrument-functions-exclude-file-list", but this option sets the list of function names to be excluded from instrumentation. The function name to be matched is its user-visible name, such as "vector<int> blah(const vector<int> &)", not the internal mangled name (e.g., "_Z4blahRSt6vectorIiSaIiEE"). The match is done on substrings: if the sym parameter is a substring of the function name, it is considered to be a match. For C99 and C++ extended identifiers, the function name must be given in UTF-8, not using universal character names.
- -fstack-check
-
Generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag if you are running in an environment with multiple threads, but you only rarely need to specify it in a single-threaded environment since stack overflow is automatically detected on nearly all systems if there is only one stack.
- 1.
- Modified allocation strategy for large objects: they are always allocated dynamically if their size exceeds a fixed threshold.
- 2.
- Fixed limit on the size of the static frame of functions: when it is topped by a particular function, stack checking is not reliable and a warning is issued by the compiler.
- 3.
- Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance is hampered.
- -fstack-limit-register=reg
- -fstack-limit-symbol=sym
- -fno-stack-limit
-
Generate code to ensure that the stack does not grow beyond a certain value, either the value of a register or the address of a symbol. If a larger stack is required, a signal is raised at run time. For most targets, the signal is raised before the stack overruns the boundary, so it is possible to catch the signal without taking special precautions.
- -fsplit-stack
-
Generate code to automatically split the stack before it overflows. The resulting program has a discontiguous stack which can only overflow if the program is unable to allocate any more memory. This is most useful when running threaded programs, as it is no longer necessary to calculate a good stack size to use for each thread. This is currently only implemented for the i386 and x86_64 back ends running GNU/Linux.
- -fleading-underscore
-
This option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented in the object file. One use is to help link with legacy assembly code.
- -ftls-model=model
-
Alter the thread-local storage model to be used. The model argument should be one of "global-dynamic", "local-dynamic", "initial-exec" or "local-exec".
- -fvisibility=default|internal|hidden|protected
-
Set the default ELF image symbol visibility to the specified option---all symbols are marked with this unless overridden within the code. Using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent symbol clashes. It is strongly recommended that you use this in any shared objects you distribute.
- -fstrict-volatile-bitfields
-
This option should be used if accesses to volatile bit-fields (or other structure fields, although the compiler usually honors those types anyway) should use a single access of the width of the field's type, aligned to a natural alignment if possible. For example, targets with memory-mapped peripheral registers might require all such accesses to be 16 bits wide; with this flag you can declare all peripheral bit-fields as "unsigned short" (assuming short is 16 bits on these targets) to force GCC to use 16-bit accesses instead of, perhaps, a more efficient 32-bit access.
- -fsync-libcalls
-
This option controls whether any out-of-line instance of the "__sync" family of functions may be used to implement the C++11 "__atomic" family of functions.
ENVIRONMENT
This section describes several environment variables that affect how GCC operates. Some of them work by specifying directories or prefixes to use when searching for various kinds of files. Some are used to specify other aspects of the compilation environment.Note that you can also specify places to search using options such as -B, -I and -L. These take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GCC.
- LANG
- LC_CTYPE
- LC_MESSAGES
- LC_ALL
-
These environment variables control the way that GCC uses localization information which allows GCC to work with different national conventions. GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it has been configured to do so. These locale categories can be set to any value supported by your installation. A typical value is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8.
- TMPDIR
- If TMPDIR is set, it specifies the directory to use for temporary files. GCC uses temporary files to hold the output of one stage of compilation which is to be used as input to the next stage: for example, the output of the preprocessor, which is the input to the compiler proper.
- GCC_COMPARE_DEBUG
- Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. See the documentation of this option for more details.
- GCC_EXEC_PREFIX
-
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish.
- COMPILER_PATH
- The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the directories thus specified when searching for subprograms, if it can't find the subprograms using GCC_EXEC_PREFIX.
- LIBRARY_PATH
- The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it can't find them using GCC_EXEC_PREFIX. Linking using GCC also uses these directories when searching for ordinary libraries for the -l option (but directories specified with -L come first).
- LANG
- This variable is used to pass locale information to the compiler. One way in which this information is used is to determine the character set to be used when character literals, string literals and comments are parsed in C and C++. When the compiler is configured to allow multibyte characters, the following values for LANG are recognized:
- C-JIS
- Recognize JIS characters.
- C-SJIS
- Recognize SJIS characters.
- C-EUCJP
- Recognize EUCJP characters.
Some additional environment variables affect the behavior of the preprocessor.
- CPATH
- C_INCLUDE_PATH
- CPLUS_INCLUDE_PATH
- OBJC_INCLUDE_PATH
-
Each variable's value is a list of directories separated by a special character, much like PATH, in which to look for header files. The special character, "PATH_SEPARATOR", is target-dependent and determined at GCC build time. For Microsoft Windows-based targets it is a semicolon, and for almost all other targets it is a colon.
- DEPENDENCIES_OUTPUT
-
If this variable is set, its value specifies how to output dependencies for Make based on the non-system header files processed by the compiler. System header files are ignored in the dependency output.
- SUNPRO_DEPENDENCIES
- This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not ignored, so it implies -M rather than -MM. However, the dependence on the main input file is omitted.
- CPP_RESTRICTED
- If this variable is defined, cpp will skip any include file which is not a regular file, and will continue searching for the requested name (this is always done if the found file is a directory).
BUGS
For instructions on reporting bugs, see < http://www.NetBSD.org/Misc/send-pr.html>.FOOTNOTES
- 1.
- On some systems, gcc -shared needs to build supplementary stub code for constructors to work. On multi-libbed systems, gcc -shared must select the correct support libraries to link against. Failing to supply the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is innocuous.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1) and the Info entries for gcc, cpp, as, ld, binutils and gdb.AUTHOR
See the Info entry for gcc, or < http://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC.COPYRIGHT
Copyright (c) 1988-2013 Free Software Foundation, Inc.Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
2014-01-16 | gcc-4.8.3 |