GCC Command Options
*******************

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.

See Compiling C++ Programs, for a summary of special options for compiling C++ programs.

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: -dr is very different from -d -r.

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.

Many options have long names starting with -f or with -W---for example, -fforce-mem, -fstrength-reduce, -Wformat and so on.  Most of these have both positive and negative forms; the negative form of -ffoo would be -fno-foo.  This manual documents only one of these two forms, whichever one is not the default.

See Option Index, for an index to GCC's options.

Option Summary
==============

Here is a summary of all the options, grouped by type.  Explanations are in the following sections.

Overall Options
     See Options Controlling the Kind of Output. -c  -S  -E  -o FILE  -combine -pipe  -pass-exit-codes -ObjC (APPLE ONLY) -ObjC++ (APPLE ONLY) -arch ARCH (APPLE ONLY) -x LANGUAGE  -v  -###  --help  --target-help  --version

C Language Options
     See Options Controlling C Dialect. -ansi  -std=STANDARD  -aux-info FILENAME -faltivec (APPLE ONLY) -fasm-blocks (APPLE ONLY) -fno-asm  -fno-builtin  -fno-builtin-FUNCTION -fhosted  -ffreestanding  -fms-extensions -trigraphs  -no-integrated-cpp  -traditional  -traditional-cpp -fallow-single-precision  -fcond-mismatch -fconstant-cfstrings (APPLE ONLY) -fpch-preprocess (APPLE ONLY) -fsigned-bitfields  -fsigned-char -fpascal-strings (APPLE ONLY) -fcoalesce (APPLE ONLY) -fweak-coalesced (APPLE ONLY) -Wno-#warnings (APPLE ONLY) -Wextra-tokens (APPLE ONLY) -Wpragma-once (APPLE ONLY) -Wnewline-eof (APPLE ONLY) -Wno-altivec-long-deprecated (APPLE ONLY) -funsigned-bitfields  -funsigned-char -fwritable-strings

C++ Language Options
     See Options Controlling C++ Dialect. -fabi-version=N  -fno-access-control  -fcheck-new -fconserve-space  -fno-const-strings -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 -fcoalesce-templates (APPLE ONLY) -fms-extensions -fno-nonansi-builtins  -fno-operator-names -fno-optional-diags  -fpermissive -frepo  -fno-rtti  -fstats  -ftemplate-depth-N -fno-threadsafe-statics -fuse-cxa-atexit  -fno-weak  -nostdinc++ -fno-default-inline  -fvisibility-inlines-hidden -Wabi  -Wctor-dtor-privacy -Wnon-virtual-dtor  -Wreorder -Weffc++  -Wno-deprecated -Wno-non-template-friend  -Wold-style-cast -Woverloaded-virtual  -Wno-pmf-conversions -Wsign-promo  -Wsynth

Objective-C and Objective-C++ Language Options
     See Options Controlling Objective-C and Objective-C++ Dialects.

     -fconstant-string-class=CLASS-NAME -fgnu-runtime  -fnext-runtime -fno-nil-receivers -fobjc-call-cxx-cdtors (APPLE ONLY) -fobjc-exceptions -freplace-objc-classes -fzero-link -gen-decls -Wno-protocol  -Wselector -Wundeclared-selector

Language Independent Options
     See Options to Control Diagnostic Messages Formatting. -fmessage-length=N -fdiagnostics-show-location=[once|every-line]

Warning Options
     See Options to Request or Suppress Warnings. -fsyntax-only  -pedantic  -pedantic-errors -w  -Wextra  -Wall  -Waggregate-return -Wcast-align  -Wcast-qual  -Wchar-subscripts  -Wcomment -Wconversion  -Wno-deprecated-declarations -Wdisabled-optimization  -Wno-div-by-zero  -Wendif-labels -Werror  -Werror-implicit-function-declaration -Wfatal-errors  -Wfloat-equal  -Wformat  -Wformat=2 -Wno-format-extra-args -Wformat-nonliteral -Wformat-security  -Wformat-y2k -Wimplicit  -Wimplicit-function-declaration  -Wimplicit-int -Wimport  -Wno-import  -Winit-self  -Winline -Wno-invalid-offsetof  -Winvalid-pch -Wlarger-than-LEN  -Wlong-long -Wmain  -Wmissing-braces  -Wmissing-field-initializers -Wmissing-format-attribute  -Wmissing-include-dirs -Wmissing-noreturn -Wmost (APPLE ONLY) -Wno-multichar  -Wnonnull  -Wpacked  -Wpadded -Wparentheses  -Wpointer-arith  -Wredundant-decls -Wreturn-type  -Wsequence-point  -Wshadow -Wsign-compare  -Wstrict-aliasing -Wstrict-aliasing=2 -Wswitch  -Wswitch-default  -Wswitch-enum -Wsystem-headers  -Wtrigraphs  -Wundef  -Wuninitialized -Wunknown-pragmas  -Wunreachable-code -Wunused  -Wunused-function  -Wunused-label  -Wunused-parameter -Wunused-value  -Wunused-variable  -Wwrite-strings -Wvariadic-macros

C-only Warning Options
     -Wbad-function-cast  -Wmissing-declarations -Wmissing-prototypes  -Wnested-externs  -Wold-style-definition -Wstrict-prototypes  -Wtraditional -Wdeclaration-after-statement

Debugging Options
     See Options for Debugging Your Program or GCC. -dLETTERS  -dumpspecs  -dumpmachine  -dumpversion -fdump-unnumbered  -fdump-translation-unit[-N] -fdump-class-hierarchy[-N] -fdump-ipa-all -fdump-ipa-cgraph -fdump-tree-all -fdump-tree-original[-N] -fdump-tree-optimized[-N] -fdump-tree-inlined[-N] -fdump-tree-cfg -fdump-tree-vcg -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-scev [-N] -fdump-tree-ddall [-N] -fdump-tree-elck [-N] -fdump-tree-dom[-N] -fdump-tree-dse[-N] -fdump-tree-phiopt[-N] -fdump-tree-forwprop[-N] -fdump-tree-copyrename[-N] -fdump-tree-nrv -fdump-tree-vect -fdump-tree-sra[-N] -fdump-tree-fre[-N] -fdump-tree-loop[-N] -fdump-tree-vect[-N] -feliminate-dwarf2-dups -feliminate-unused-debug-types -feliminate-unused-debug-symbols -fmem-report -fprofile-arcs -ftree-based-profiling -frandom-seed=STRING -fsched-verbose=N -ftest-coverage  -ftime-report -fvar-tracking -g  -gLEVEL  -gcoff -gdwarf-2 -ggdb  -gstabs  -gstabs+  -gvms  -gxcoff  -gxcoff+ -p  -pg  -print-file-name=LIBRARY  -print-libgcc-file-name -print-multi-directory  -print-multi-lib -print-prog-name=PROGRAM  -print-search-dirs  -Q -save-temps  -time

Optimization Options
     See Options that Control Optimization. -falign-functions=N  -falign-jumps=N -falign-labels=N  -falign-loops=N -fbounds-check -fmudflap -fmudflapth -fmudflapir -fbranch-probabilities -fprofile-values -fvpt -fbranch-target-load-optimize -fbranch-target-load-optimize2 -fbtr-bb-exclusive -fcaller-saves  -fcprop-registers -fcreate-profile -fcse-follow-jumps  -fcse-skip-blocks  -fdata-sections -fdelayed-branch  -fdelete-null-pointer-checks -fexpensive-optimizations  -ffast-math  -ffloat-store -fforce-addr  -fforce-mem  -ffunction-sections -fgcse  -fgcse-lm  -fgcse-sm  -fgcse-las  -fgcse-after-reload -floop-optimize -fcrossjumping  -fif-conversion  -fif-conversion2 -finline-functions  -finline-limit=N  -fkeep-inline-functions -fkeep-static-consts  -fmerge-constants  -fmerge-all-constants -fmodulo-sched -fnew-ra  -fno-branch-count-reg -fno-default-inline  -fno-defer-pop -floop-optimize2 -fmove-loop-invariants -fno-function-cse  -fno-guess-branch-probability -fno-inline  -fno-math-errno  -fno-peephole  -fno-peephole2 -funsafe-math-optimizations  -ffinite-math-only -fno-trapping-math  -fno-zero-initialized-in-bss -fomit-frame-pointer  -foptimize-register-move -foptimize-sibling-calls  -fprefetch-loop-arrays -fprofile-generate -fprofile-use -fregmove  -frename-registers -freorder-blocks  -freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop  -frerun-loop-opt -frounding-math -fschedule-insns  -fschedule-insns2 -fno-sched-interblock  -fno-sched-spec  -fsched-spec-load -fsched-spec-load-dangerous -fsched-stalled-insns=N -sched-stalled-insns-dep=N -fsched2-use-superblocks -fsched2-use-traces -freschedule-modulo-scheduled-loops -fsignaling-nans -fsingle-precision-constant  -fspeculative-prefetching -fstrength-reduce  -fstrict-aliasing  -ftracer  -fthread-jumps -funroll-all-loops  -funroll-loops  -fpeel-loops -fsplit-ivs-in-unroller -funswitch-loops -ftree-pre  -ftree-ccp  -ftree-dce -ftree-loop-optimize -ftree-loop-linear -ftree-loop-im -ftree-loop-ivcanon -fivopts -ftree-dominator-opts -ftree-dse -ftree-copyrename -ftree-ch -ftree-sra -ftree-ter -ftree-lrs -ftree-fre -fscalar-evolutions -fall-data-deps -ftree-vectorize -fuse-profile --param NAME=VALUE -O  -O0  -O1  -O2  -O3  -Os -fast (APPLE ONLY)

Preprocessor Options
     See Options Controlling the Preprocessor. -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 -M  -MM  -MF  -MG  -MP  -MQ  -MT  -nostdinc -P  -fworking-directory  -remap -trigraphs  -undef  -UMACRO  -Wp,OPTION -Xpreprocessor OPTION

Assembler Option
     See Passing Options to the Assembler. -Wa,OPTION  -Xassembler OPTION

Linker Options
     See Options for Linking. OBJECT-FILE-NAME  -lLIBRARY -no-c++filt (APPLE ONLY) -nostartfiles  -nodefaultlibs  -nostdlib -pie -s  -static  -static-libgcc  -shared  -shared-libgcc  -symbolic -Wl,OPTION  -Xlinker OPTION -u SYMBOL

Directory Options
     See Options for Directory Search. -BPREFIX  -IDIR  -iquoteDIR  -LDIR  -specs=FILE  -I-

Target Options
     See Target Options. -V VERSION  -b MACHINE

Machine Dependent Options
     See Hardware Models and Configurations.

     Darwin Options -all_load  -allowable_client  -arch  -arch_errors_fatal -arch_only  -bind_at_load  -bundle  -bundle_loader -client_name  -compatibility_version  -current_version -dead_strip -dependency-file  -dylib_file  -dylinker_install_name -dynamic  -dynamiclib  -exported_symbols_list -filelist  -flat_namespace  -force_cpusubtype_ALL -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 -F -gused -gfull -mone-byte-bool

     i386 and x86-64 Options -mtune=CPU-TYPE  -march=CPU-TYPE -mfpmath=UNIT -masm=DIALECT  -mno-fancy-math-387 -mno-fp-ret-in-387  -msoft-float  -msvr3-shlib -mno-wide-multiply  -mrtd  -malign-double -mpreferred-stack-boundary=NUM -mmmx  -msse  -msse2 -msse3 -m3dnow -mthreads  -mno-align-stringops  -minline-all-stringops -mpush-args  -maccumulate-outgoing-args  -m128bit-long-double -m96bit-long-double  -mregparm=NUM  -momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs -mcmodel=CODE-MODEL -m32  -m64

     PowerPC Options See RS/6000 and PowerPC Options.

     RS/6000 and PowerPC Options -mcpu=CPU-TYPE -mtune=CPU-TYPE -mpower  -mno-power  -mpower2  -mno-power2 -mpowerpc  -mpowerpc64  -mno-powerpc -maltivec  -mno-altivec -mpowerpc-gpopt  -mno-powerpc-gpopt -mpowerpc-gfxopt  -mno-powerpc-gfxopt -mnew-mnemonics  -mold-mnemonics -mfull-toc   -mminimal-toc  -mno-fp-in-toc  -mno-sum-in-toc -m64  -m32  -mxl-call  -mno-xl-call  -mpe -malign-power  -malign-natural -msoft-float  -mhard-float  -mmultiple  -mno-multiple -mstring  -mno-string  -mupdate  -mno-update -mfused-madd  -mno-fused-madd  -mbit-align  -mno-bit-align -mstrict-align  -mno-strict-align  -mrelocatable -mno-relocatable  -mrelocatable-lib  -mno-relocatable-lib -mtoc  -mno-toc  -mlittle  -mlittle-endian  -mbig  -mbig-endian -mdynamic-no-pic -mprioritize-restricted-insns=PRIORITY -msched-costly-dep=DEPENDENCE_TYPE -minsert-sched-nops=SCHEME -mcall-sysv  -mcall-netbsd -maix-struct-return  -msvr4-struct-return -mabi=altivec  -mabi=no-altivec -mabi=spe  -mabi=no-spe -misel=yes  -misel=no -mspe=yes  -mspe=no -mfloat-gprs=yes  -mfloat-gprs=no -mprototype  -mno-prototype -msim  -mmvme  -mads  -myellowknife  -memb  -msdata -msdata=OPT  -mvxworks  -mwindiss  -G NUM  -pthread

Code Generation Options
     See Options for Code Generation Conventions. -fcall-saved-REG  -fcall-used-REG -ffixed-REG  -fexceptions -fnon-call-exceptions  -funwind-tables -fasynchronous-unwind-tables -finhibit-size-directive  -finstrument-functions -fno-common  -fno-ident -fpcc-struct-return  -fpic  -fPIC -fpie -fPIE -freg-struct-return  -fshared-data  -fshort-enums -fshort-double  -fshort-wchar -fverbose-asm  -fpack-struct[=N]  -fstack-check -fstack-limit-register=REG  -fstack-limit-symbol=SYM -fargument-alias  -fargument-noalias -fargument-noalias-global  -fleading-underscore -ftls-model=MODEL -ftrapv  -fwrapv  -fbounds-check -fvisibility

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 which must be preprocessed.

FILE.i
     C source code which should not be preprocessed.

FILE.ii
     C++ source code which 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 which 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 which should not be preprocessed.

FILE.h
     C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header.

FILE.cc
FILE.cp
FILE.cxx
FILE.cpp
FILE.CPP
FILE.c++
FILE.C
     C++ source code which 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 which must be preprocessed.  (APPLE ONLY)

FILE.mii
     Objective-C++ source code which should not be preprocessed.  (APPLE ONLY)

FILE.hh
FILE.H
     C++ header file to be turned into a precompiled header.

FILE.f
FILE.for
FILE.FOR
     Fortran source code which should not be preprocessed.

FILE.F
FILE.fpp
FILE.FPP
     Fortran source code which must be preprocessed (with the traditional preprocessor).

FILE.r
     Fortran source code which must be preprocessed with a RATFOR preprocessor (not included with GCC).

FILE.f90
FILE.f95
     Fortran 90/95 source code which should not be preprocessed.

FILE.ads
     Ada source code file which 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.  Apple's version of GCC runs the preprocessor on these files as well as those ending in .S.

FILE.S
     Assembler code which 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  c-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  ratfor
          f95
          java
          treelang

-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).

-ObjC

-ObjC++
     These are similar in effect to -x objective-c and -x objective-c++, but affect only the choice of compiler for files already identified as source files.  (APPLE ONLY)

-arch ARCH
     Compile for the specified target architecture ARCH.  The allowable values are i386 and ppc.  Multiple options work, and direct the compiler to produce ``fat'' binaries including object code for each architecture specified with -arch.  This option only works if assembler and libraries are available for each architecture specified.  (APPLE ONLY)

-pass-exit-codes
     Normally the gcc program will exit 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 will instead return with numerically highest error produced by any phase that returned an error indication.

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.

     By default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc., with .o.

     Unrecognized input files, not requiring compilation or assembly, are ignored.

-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.

     By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc., with .s.

     Input files that don't require compilation are ignored.

-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.

     Input files which don't require preprocessing are ignored.

-o FILE
     Place output in file FILE.  This applies regardless to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code.

     If -o is not specified, the default is to put an executable file in a.out, the object file for SOURCE.SUFFIX in SOURCE.o, its assembler file in SOURCE.s, a precompiled header file in SOURCE.SUFFIX.gch, and all preprocessed C source on standard output.

-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 all command arguments are quoted.  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.

-combine
     If you are compiling multiple source files, this option tells the driver to pass all the source files to the compiler at once (for those languages for which the compiler can handle this).  This will allow intermodule analysis (IMA) to be performed by the compiler.  Currently the only language for which this is supported is C.  If you pass source files for multiple languages to the driver, using this option, the driver will invoke the compiler(s) that support IMA once each, passing each compiler all the source files appropriate for it.  For those languages that do not support IMA this option will be ignored, and the compiler will be invoked once for each source file in that language.  If you use this option in conjunction with -save-temps, the compiler will generate multiple pre-processed files (one for each source file), but only one (combined) .o or .s file.

--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 will also be passed on to the various processes invoked by gcc, so that they can display the command line options they accept.  If the -Wextra option is also specified then command line options which have no documentation associated with them will also be displayed.

--target-help
     Print (on the standard output) a description of target specific command line options for each tool.

--version
     Display the version number and copyrights of the invoked GCC.

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 or .H; 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, C++ programs often require class libraries as well as a compiler that understands the C++ language---and under some circumstances, you might want to compile programs or header files from standard input, or otherwise without a suffix that flags them as C++ programs.  You might also like to precompile a C header file with a .h extension to be used in C++ compilations.  g++ is a program that calls GCC with the default language set to C++, and automatically specifies linking against the C++ library.  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. See Options Controlling C Dialect, for explanations of options for languages related to C. See Options Controlling C++ Dialect, for explanations of 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, support all ISO C90 programs.  In C++ mode, remove GNU extensions that conflict with ISO C++.

     This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of standard C++ (when compiling C++ code), such as the asm and typeof keywords, and predefined macros such as unix and vax that identify the type of system you are using.  It also enables the undesirable and rarely used ISO trigraph feature.  For the C compiler, it disables recognition of C++ style // comments as well as the inline keyword.

     The alternate keywords __asm__, __extension__, __inline__ and __typeof__ continue to work despite -ansi.  You would not want to use them in an ISO C program, of course, but it is useful to put them in header files that might be included in compilations done with -ansi.  Alternate predefined macros such as __unix__ and __vax__ are also available, with or without -ansi.

     The -ansi option does not cause non-ISO programs to be rejected gratuitously.  For that, -pedantic is required in addition to -ansi.  See Warning Options.

     The macro __STRICT_ANSI__ is predefined when the -ansi option is used.  Some header files may notice this macro and refrain from declaring certain functions or defining certain macros that the ISO standard doesn't call for; this is to avoid interfering with any programs that might use these names for other things.

     Functions which would normally be built in but do not have semantics defined by ISO C (such as alloca and ffs) are not built-in functions with -ansi is used.  See Other built-in functions provided by GCC, for details of the functions affected.

-std=
     Determine the language standard.  This option is currently only supported when compiling C or C++.  A value for this option must be provided; possible values are

     c89
     iso9899:1990
          ISO C90 (same as -ansi).

     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.

     gnu89
          Default, ISO C90 plus GNU extensions (including some C99 features).

     gnu99
     gnu9x
          ISO C99 plus GNU extensions.  When ISO C99 is fully implemented in GCC, this will become the default.  The name gnu9x is deprecated.

     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.

     Even when this option is not specified, you can still use some of the features of newer standards in so far as they do not conflict with previous C standards.  For example, you may use __restrict__ even when -std=c99 is not specified.

     The -std options specifying some version of ISO C have the same effects as -ansi, except that features that were not in ISO C90 but are in the specified version (for example, // comments and the inline keyword in ISO C99) are not disabled.

     See Language Standards Supported by GCC, for details of these standard versions.

-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.

     Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (I, N for new or O for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (C or F, respectively, in the following character).  In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration.

-faltivec
     Enable the AltiVec language extensions, as defined in Motorola's AltiVec PIM, for backwards compatibility with previous Apple versions of GCC.  This includes the recognition of vector and pixel as (context-dependent) keywords, the definition of built-in functions such as vec_add, and other extensions.  Note that unlike the option -maltivec, the extensions do not require the inclusion of any special header files.  (APPLE ONLY)

     Use of this flag should be avoided, and the functionality it supplies should be invoked by passing -maltivec or -mcpu=G4 and using #include <altivec.h> in your source file.

     This flag may disable some optimisations unless -maltivec or -mcpu=G4 is also passed.

     The flag brings the AltiVec intrinsics into the compilation by behaving as if -include altivec.h was passed.  A consequence of this is that if a precompiled header is to be used with this flag, the header should be included with -include before this flag appears on the command line.

-fasm-blocks
     Enable the use of blocks and entire functions of assembly code within a C or C++ file.  The syntax follows that used in CodeWarrior. (APPLE ONLY)

-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.

     In C++, this switch only affects the typeof keyword, since asm and inline are standard keywords.  You may want to use the -fno-gnu-keywords flag instead, which has the same effect.  In C99 mode (-std=c99 or -std=gnu99), this switch only affects the asm and typeof keywords, since inline is a standard keyword in ISO C99.

-fno-builtin
-fno-builtin-FUNCTION
     Don't recognize built-in functions that do not begin with __builtin_ as prefix.  See Other built-in functions provided by GCC, for details of the functions affected, including those which are not built-in functions when -ansi or -std options for strict ISO C conformance are used because they do not have an ISO standard meaning.

     GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to alloca may become single instructions that adjust the stack directly, and calls to memcpy may become inline copy loops.  The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library.  In addition, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function.  For example, warnings are given with -Wformat for bad calls to printf, when printf is built in, and strlen is known not to modify global memory.

     With the -fno-builtin-FUNCTION option only the built-in function FUNCTION is disabled.  FUNCTION must not begin with __builtin_.  If a function is named this is not built-in in this version of GCC, this option is ignored.  There is no corresponding -fbuiltin-FUNCTION option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:

          #define abs(n)          __builtin_abs ((n))
          #define strcpy(d, s)    __builtin_strcpy ((d), (s))

-fhosted

     Assert that compilation takes place in 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 takes place in 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.

     See Language Standards Supported by GCC, for details of freestanding and hosted environments.

-fms-extensions
     Accept some non-standard constructs used in Microsoft header files.

     Some cases of unnamed fields in structures and unions are only accepted with this option.  See Unnamed struct/union fields within structs/unions, for details.

-trigraphs
     Support ISO C trigraphs.  The -ansi option (and -std options for strict ISO C conformance) implies -trigraphs.

-no-integrated-cpp
     Performs a compilation in two passes: preprocessing and compiling.  This option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the -B option. The user supplied compilation step can then add in an additional preprocessing step after normal preprocessing but before compiling. The default is to use the integrated cpp (internal cpp)

     The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged.

-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++.

-fpch-preprocess
     Enable PCH processing even when -E or -save-temps is used.

-funsigned-char
     Let the type char be unsigned, like unsigned char.

     Each kind of machine has a default for what char should be.  It is either like unsigned char by default or like signed char by default.

     Ideally, a portable program should always use signed char or unsigned char when it depends on the signedness of an object. But many programs have been written to use plain char and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for.  This option, and its inverse, let you make such a program work with the opposite default.

     The type char is always a distinct type from each of signed char or unsigned char, even though its behavior is always just like one of those two.

-fsigned-char
     Let the type char be signed, like signed char.

     Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char.  Likewise, the option -fno-signed-char is equivalent to -funsigned-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.

-fconstant-cfstrings
     Enable the automatic creation of a CoreFoundation-type constant string whenever a special builtin __builtin__CFStringMakeConstantString is called on a literal string.  (APPLE ONLY)

-fpascal-strings
     Allow Pascal-style string literals to be constructed.  (APPLE ONLY)

     See Constructing String Literals with a Pascal-style Length Byte, for more information on the syntax and semantics of Pascal string literals.

-fcoalesce
     Coalesce duplicated functions and data. The linker will discard all but one, saving space.  Enabled by default.  (APPLE ONLY)

-fweak-coalesced
     Use the new OS X ``weak_definitions'' section attribute for coalesced items. A single ``normal'' definition will be chosen by the linker over any number of weakly-coalesced ones.  (APPLE ONLY)

-fwritable-strings
     Store string constants in the writable data segment and don't uniquize them.  This is for compatibility with old programs which assume they can write into string constants.

     Writing into string constants is a very bad idea; ``constants'' should be constant.

     This option is deprecated.

Options Controlling C++ Dialect
===============================

This section describes the command-line options that are only meaningful for C++ programs; but 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.  Version 2 is the version of the C++ ABI that first appeared in G++ 3.4.  Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.  Version 0 will always be the version that conforms most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change as ABI bugs are fixed.

     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 will only return 0 if it is declared throw(), in which case the compiler will always check 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).

-fconserve-space
     Put uninitialized or runtime-initialized global variables into the common segment, as C does.  This saves space in the executable at the cost of not diagnosing duplicate definitions.  If you compile with this flag and your program mysteriously crashes after main() has completed, you may have an object that is being destroyed twice because two definitions were merged.

     This option is no longer useful on most targets, now that support has been added for putting variables into BSS without making them common.

-fno-const-strings
     Give string constants type char * instead of type const char *.  By default, G++ uses type const char * as required by the standard.  Even if you use -fno-const-strings, you cannot actually modify the value of a string constant, unless you also use -fwritable-strings.

     This option might be removed in a future release of G++.  For maximum portability, you should structure your code so that it works with string constants that have type const char *.

-fno-elide-constructors
     The C++ standard allows an implementation to omit creating a temporary which 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 check for violation of exception specifications at runtime.  This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining NDEBUG.  The compiler will still optimize based on the exception specifications.

-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++.

     The default if neither flag is given to follow the standard, but to allow and give a warning for old-style code that would otherwise be invalid, or have different behavior.

-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 which are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. See Template Instantiation, for more information.

-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 will 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 will cause linker errors if these functions are not inlined everywhere they are called.

-fcoalesce-templates
     Mark instantiated templates as ``coalesced'': the linker will discard all but one, thus saving space.  (APPLE ONLY)

-fms-extensions
     Disable pedantic 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.

-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 will allow some nonconforming code to compile.

-frepo
     Enable automatic template instantiation at link time.  This option also implies -fno-implicit-templates.  See Template Instantiation, for more information.

-fno-rtti
     Disable generation of information about every class with virtual functions for use by the C++ runtime 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 it will generate it as needed.

-fstats
     Emit statistics about front-end processing at the end of the compilation. This information is generally only useful to the G++ development team.

-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.

-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 will only work if your C library supports __cxa_atexit. This option is not supported on Mac OS X.

-fvisibility-inlines-hidden
     Causes all inlined methods to be marked with __attribute__ ((visibility ("hidden"))) so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates. While it can cause bloating through duplication of code within each DSO where it is used, often the wastage is less than the considerable space occupied by a long symbol name in the export table which is typical when using templates and namespaces. For even more savings, combine with the -fvisibility=hidden switch.

-fno-weak
     Do not use weak symbol support, even if it is provided by the linker. By default, G++ will use weak symbols if they are available.  This option exists only for testing, and should not be used by end-users; it will result 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. See Options That Control Optimization.  Note that these functions will have linkage like inline functions; they just won't be inlined by default.

-Wabi (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 will be compatible.

     You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.

     The known incompatibilities at this point include:

       * 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; };

          In this case, G++ will place B::f2 into the same byte asA::f1; other compilers will not.  You can avoid this problem by explicitly padding A so that its size is a multiple of the byte size on your platform; that will cause G++ and other compilers to layout B identically.

       * 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 {};

          In this case, G++ will not place B into the tail-padding for A; other compilers will.  You can avoid this problem by explicitly padding A so that its size is a multiple of its alignment (ignoring virtual base classes); that will cause G++ and other compilers to layout C identically.

       * 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; };

          Assuming that an int does not have 4096 bits, G++ will make the union too small by the number of bits in an int.

       * 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 {};

          G++ will place the A base class of C at a nonzero offset; it should be placed at offset zero.  G++ mistakenly believes that the A data member of B is already at offset zero.

       * 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) {}

          Instantiations of these templates may be mangled incorrectly.

-Wctor-dtor-privacy (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.

-Wnon-virtual-dtor (C++ only)
     Warn when a class appears to be polymorphic, thereby requiring a virtual destructor, yet it declares a non-virtual one. This warning is enabled by -Wall.

-Wreorder (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) { }
          };

     The compiler will rearrange the member initializers for i and j to match the declaration order of the members, emitting a warning to that effect.  This warning is enabled by -Wall.

The following -W... options are not affected by -Wall.

-Weffc++ (C++ only)
     Warn about violations of the following style guidelines from Scott Meyers' Effective C++ 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.

     Also warn about violations of the following style guidelines from Scott Meyers' More Effective C++ book:

       * Item 6:  Distinguish between prefix and postfix forms of increment and decrement operators.

       * Item 7:  Never overload &&, ||, or ,.

     When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use grep -v to filter out those warnings.

-Wno-deprecated (C++ only)
     Do not warn about usage of deprecated features.  See Deprecated Features.

-Wno-non-template-friend (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++ only)
     Warn if an old-style (C-style) cast to a non-void type is used within a C++ program.  The new-style casts (static_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended effects and much easier to search for.

-Woverloaded-virtual (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);
          };

     the A class version of f is hidden in B, and code like:

          B* b;
          b->f();

     will fail to compile.

-Wno-pmf-conversions (C++ only)
     Disable the diagnostic for converting a bound pointer to member function to a plain pointer.

-Wsign-promo (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++ would try to preserve unsignedness, but the standard mandates the current behavior.

-Wsynth (C++ only)
     Warn when G++'s synthesis behavior does not match that of cfront.  For instance:

          struct A {
            operator int ();
            A& operator = (int);
          };

          main ()
          {
            A a,b;
            a = b;
          }

     In this example, G++ will synthesize a default A& operator = (const A&);, while cfront will use the user-defined operator =.

Options Controlling Objective-C and Objective-C++ Dialects
==========================================================

(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves.  See Language Standards Supported by GCC, for references.)

This section describes the command-line options that are only meaningful for Objective-C and Objective-C++ programs, but 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, will override 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 (e.g., [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.  Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.

-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 that will run 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 that will run all such default destructors, in reverse order.

     The - (id) .cxx_construct and/or - (void) .cxx_destruct methods thusly generated will only operate on instance variables declared in the current Objective-C class, and not those inherited from superclasses.  It is the responsibility of the Objective-C runtime to invoke all such methods in an object's inheritance hierarchy.  The - (id) .cxx_construct methods will be invoked by the runtime immediately after a new object instance is allocated; the - (void) .cxx_destruct methods will be invoked immediately before the runtime deallocates an object instance.

     As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the - (id) .cxx_construct and - (void) .cxx_destruct methods.

-fobjc-exceptions
     Enable syntactic support for structured exception handling in Objective-C, similar to what is offered by C++ and Java.  Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.

            @try {
              ...
                 @throw expr;
              ...
            }
            @catch (AnObjCClass *exc) {
              ...
                @throw expr;
              ...
                @throw;
              ...
            }
            @catch (AnotherClass *exc) {
              ...
            }
            @catch (id allOthers) {
              ...
            }
            @finally {
              ...
                @throw expr;
              ...
            }

     The @throw statement may appear anywhere in an Objective-C or Objective-C++ program; when used inside of a @catch block, the @throw may appear without an argument (as shown above), in which case the object caught by the @catch will be rethrown.

     Note that only (pointers to) Objective-C objects may be thrown and caught using this scheme.  When an object is thrown, it will be caught by the nearest @catch clause capable of handling objects of that type, analogously to how catch blocks work in C++ and Java.  A @catch(id ...) clause (as shown above) may also be provided to catch any and all Objective-C exceptions not caught by previous @catch clauses (if any).

     The @finally clause, if present, will be executed upon exit from the immediately preceding @try ... @catch section.  This will happen regardless of whether any exceptions are thrown, caught or rethrown inside the @try ... @catch section, analogously to the behavior of the finally clause in Java.

     There are several caveats to using the new exception mechanism:

       * Although currently designed to be binary compatible with NS_HANDLER-style idioms provided by the NSException class, the new exceptions can only be used on Mac OS X 10.3 (Panther) and later systems, due to additional functionality needed in the (NeXT) Objective-C runtime.

       * As mentioned above, the new exceptions do not support handling types other than Objective-C objects.   Furthermore, when used from Objective-C++, the Objective-C exception model does not interoperate with C++ exceptions at this time.  This means you cannot @throw an exception from Objective-C and catch it in C++, or vice versa (i.e., throw ... @catch).

     The -fobjc-exceptions switch also enables the use of synchronization blocks for thread-safe execution:

            @synchronized (ObjCClass *guard) {
              ...
            }

     Upon entering the @synchronized block, a thread of execution shall first check whether a lock has been placed on the corresponding guard object by another thread.  If it has, the current thread shall wait until the other thread relinquishes its lock.  Once guard becomes available, the current thread will place its own lock on it, execute the code contained in the @synchronized block, and finally relinquish the lock (thereby making guard available to other threads).

     Unlike Java, Objective-C does not allow for entire methods to be marked @synchronized.  Note that throwing exceptions out of @synchronized blocks is allowed, and will cause the guarding object to be unlocked properly.

-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.

-gen-decls
     Dump interface declarations for all classes seen in the source file to a file named SOURCENAME.decl.

-Wno-protocol
     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
     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.

-Wundeclared-selector
     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, ...).  The options described below can be used to control the diagnostic messages formatting algorithm, e.g. how many characters per line, how often source location information should be reported.  Right now, only the C++ front end can honor these options.  However it is expected, in the near future, that the remaining front ends would be able to digest them correctly.

-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 will be done; each error message will appear on a single line.

-fdiagnostics-show-location=once
     Only meaningful in line-wrapping mode.  Instructs the diagnostic messages reporter to emit once source location information; 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.

Options to Request or Suppress Warnings
=======================================

Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are risky or suggest there may have been an error.

You can request many specific warnings with options beginning -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.

The following options control the amount and kinds of warnings produced by GCC; for further, language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.

-fsyntax-only
     Check the code for syntax errors, but don't do anything beyond that.

-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.

     Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few will require -ansi or a -std option specifying the required version of ISO C).  However, without this option, certain GNU extensions and traditional C and C++ features are supported as well.  With this option, they are rejected.

     -pedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __.  Pedantic warnings are also disabled in the expression that follows __extension__.  However, only system header files should use these escape routes; application programs should avoid them. See Alternate Keywords.

     Some users try to use -pedantic to check programs for strict ISO C conformance.  They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all---only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added.

     A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable additional work and would be quite different from -pedantic.  We don't have plans to support such a feature in the near future.

     Where the standard specified with -std represents a GNU extended dialect of C, such as gnu89 or gnu99, there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based.  Warnings from -pedantic are given where they are required by the base standard.  (It would not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.)

-pedantic-errors
     Like -pedantic, except that errors are produced rather than warnings.

-w
     Inhibit all warning messages.

-Wno-import
     Inhibit warning messages about the use of #import.

-Wno-#warnings
     Inhibit warning messages issued by #warning.

-Wpragma-once
     Warn about the use of #pragma once.  (APPLE ONLY)

-Wextra-tokens
     Warn about extra tokens at the end of prepreprocessor directives.  (APPLE ONLY)

-Wnewline-eof
     Warn about files missing a newline at the end of the file.  (APPLE ONLY)

-Wno-altivec-long-deprecated
     Do not warn about the use of the deprecated 'long' keyword in AltiVec data types.  (APPLE ONLY)

-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.

-Wcomment
     Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a Backslash-Newline appears in a // comment.

-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.

-Wformat
     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 (see Function 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.

     The formats are checked against the format features supported by GNU libc version 2.2.  These include all ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU extensions.  Other library implementations may not support all these features; GCC does not support warning about features that go beyond a particular library's limitations.  However, if -pedantic is used with -Wformat, warnings will be given about format features not in the selected standard version (but not for strfmon formats, since those are not in any version of the C standard).  See Options Controlling C Dialect.

     Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull.

     -Wformat is included in -Wall.  For more control over some aspects of format checking, the options -Wformat-y2k, -Wno-format-extra-args, -Wno-format-zero-length, -Wformat-nonliteral, -Wformat-security, and -Wformat=2 are available, but are not included in -Wall.

-Wformat-y2k
     If -Wformat is specified, also warn about strftime formats which may yield only a two-digit year.

-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.

     Where the unused arguments lie between used arguments that are specified with $ operand number specifications, normally warnings are still given, since the implementation could not know what type to pass to va_arg to skip the unused arguments.  However, in the case of scanf formats, this option will suppress the warning if the unused arguments are all pointers, since the Single Unix Specification says that such unused arguments are allowed.

-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-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=2
     Enable -Wformat plus format checks not included in -Wformat.  Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k.

-Wnonnull
     Warn about passing a null pointer for arguments marked as requiring a non-null value by the nonnull function attribute.

     -Wnonnull is included in -Wall and -Wformat.  It can be disabled with the -Wno-nonnull option.

-Winit-self (C, C++, Objective-C and Objective-C++ only)
     Warn about uninitialized variables which are initialized with themselves. Note this option can only be used with the -Wuninitialized option, which in turn only works with -O1 and above.

     For example, GCC will warn about i being uninitialized in the following snippet only when -Winit-self has been specified:

          int f()
          {
            int i = i;
            return i;
          }

-Wimplicit-int
     Warn when a declaration does not specify a type.

-Wimplicit-function-declaration
-Werror-implicit-function-declaration
     Give a warning (or error) whenever a function is used before being declared.  The form -Wno-error-implicit-function-declaration is not supported.

-Wimplicit
     Same as -Wimplicit-int and -Wimplicit-function-declaration.

-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.

-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.

          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.  Only the warning for an assignment used as a truth value is supported when compiling C++; the other warnings are only supported when compiling C.

     Also warn if a comparison like x<=y<=z appears; this is equivalent to (x<=y ? 1 : 0) <= z, which is a different interpretation from that of ordinary mathematical notation.

     Also warn about constructions where there may be confusion to which if statement an else branch belongs.  Here is an example of such a case:

          {
            if (a)
              if (b)
                foo ();
            else
              bar ();
          }

     In C, every else branch belongs to the innermost possible if statement, which in this example is if (b).  This is often not what the programmer expected, as illustrated in the above example by indentation the programmer chose.  When there is the potential for this confusion, GCC will issue a warning when this flag is specified. To eliminate the warning, add explicit braces around the innermost if statement so there is no way the else could belong to the enclosing if.  The resulting code would look like this:

          {
            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 standard.

     The C standard defines the order in which expressions in a C program are evaluated in terms of sequence points, which represent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those executed after it.  These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a &&, ||, ? : or , (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is not specified.  All these rules describe only a partial order rather than a total order, since, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified.  However, the standards committee have ruled that function calls do not overlap.

     It is not specified when between sequence points modifications to the values of objects take effect.  Programs whose behavior depends on this have undefined behavior; the C standard specifies that ``Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression.  Furthermore, the prior value shall be read only to determine the value to be stored.''.  If a program breaks these rules, the results on any particular implementation are entirely unpredictable.

     Examples of code with undefined behavior are a = a++;, a[n] = b[n++] and a[i++] = i;.  Some more complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs.

     The present implementation of this option only works for C programs.  A future implementation may also work for C++ programs.

     The C standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at http://gcc.gnu.org/readings.html.

-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.

     For C, also warn if the return type of a function has a type qualifier such as const.  Such a type qualifier has no effect, since the value returned by a function is not an lvalue.  ISO C prohibits qualified void return types on function definitions, so such return types always receive a warning even without this option.

     For C++, a function without return type always produces a diagnostic message, even when -Wno-return-type is specified.  The only exceptions are main and functions defined in system headers.

-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.

-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.

-Wtrigraphs
     Warn if any trigraphs are encountered that might change the meaning of the program (trigraphs within comments are not warned about).

-Wunused-function
     Warn whenever a static function is declared but not defined or a non\-inline static function is unused.

-Wunused-label
     Warn whenever a label is declared but not used.

     To suppress this warning use the unused attribute (see Variable Attributes).

-Wunused-parameter
     Warn whenever a function parameter is unused aside from its declaration.

     To suppress this warning use the unused attribute (see Variable Attributes).

-Wunused-variable
     Warn whenever a local variable or non-constant static variable is unused aside from its declaration

     To suppress this warning use the unused attribute (see Variable Attributes).

-Wunused-value
     Warn whenever a statement computes a result that is explicitly not used.

     To suppress this warning cast the expression to void.

-Wunused
     All the above -Wunused options combined.

     In order to get a warning about an unused function parameter, you must either specify -Wextra -Wunused (note that -Wall implies -Wunused), or separately specify -Wunused-parameter.

-Wuninitialized
     Warn if an automatic variable is used without first being initialized or if a variable may be clobbered by a setjmp call.

     These warnings are possible only in optimizing compilation, because they require data flow information that is computed only when optimizing.  If you don't specify -O, you simply won't get these warnings.

     If you want to warn about code which uses the uninitialized value of the variable in its own initializer, use the -Winit-self option.

     These warnings occur only for variables that are candidates for register allocation.  Therefore, they do not occur for a variable that is declared volatile, or whose address is taken, or whose size is other than 1, 2, 4 or 8 bytes.  Also, they do not occur for structures, unions or arrays, even when they are in registers.

     Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed.

     These warnings are made optional because GCC is not smart enough to see all the reasons why the code might be correct despite 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);
          }

     If the value of y is always 1, 2 or 3, then x is always initialized, but GCC doesn't know this.  Here is another common case:

          {
            int save_y;
            if (change_y) save_y = y, y = new_y;
            ...
            if (change_y) y = save_y;
          }

     This has no bug because save_y is used only if it is set.

     This option also warns when a non-volatile automatic variable might be changed by a call to longjmp.  These warnings as well are possible only in optimizing compilation.

     The compiler sees only the calls to setjmp.  It cannot know where longjmp will be called; in fact, a signal handler could call it at any point in the code.  As a result, you may get a warning even when there is in fact no problem because longjmp cannot in fact be called at the place which would cause a problem.

     Some spurious warnings can be avoided if you declare all the functions you use that never return as noreturn.  See Function Attributes.

-Wunknown-pragmas
     Warn when a #pragma directive is encountered which is not understood by GCC.  If this command line option is used, warnings will even be issued for unknown pragmas in system header files.  This is not the case if the warnings were only enabled by the -Wall command line option.

-Wstrict-aliasing
     This option is only active when -fstrict-aliasing is active. It warns about code which 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.

-Wstrict-aliasing=2
     This option is only active when -fstrict-aliasing is active. It warns about all code which might break the strict aliasing rules that the compiler is using for optimization.  This warning catches all cases, but it will also give a warning for some ambiguous cases that are safe.

-Wall
     All of the above -W options combined.  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.

-Wmost
     This is equivalent to -Wall -Wno-parentheses.  (APPLE ONLY)

The following -W... options are not implied by -Wall. Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning.

-Wextra
     (This option used to be called -W.  The older name is still supported, but the newer name is more descriptive.)  Print extra warning messages for these events:

       * A function can return either with or without a value.  (Falling off the end of the function body is considered returning without a value.)  For example, this function would evoke such a warning:

               foo (a)
               {
                 if (a > 0)
                   return a;
               }

       * An expression-statement or the left-hand side of a comma expression contains no side effects. To suppress the warning, cast the unused expression to void. For example, an expression such as x[i,j] will cause a warning, but x[(void)i,j] will not.

       * An unsigned value is compared against zero with < or >=.

       * Storage-class specifiers like static are not the first things in a declaration.  According to the C Standard, this usage is obsolescent.

       * If -Wall or -Wunused is also specified, warn about unused arguments.

       * A comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. (But don't warn if -Wno-sign-compare is also specified.)

       * An aggregate has an initializer which does not initialize all members. This warning can be independently controlled by -Wmissing-field-initializers.

       * A function parameter is declared without a type specifier in K&R-style functions:

               void foo(bar) { }

       * An empty body occurs in an if or else statement.

       * A pointer is compared against integer zero with <, <=, >, or >=.

       * A variable might be changed by longjmp or vfork.

       * Any of several floating-point events that often indicate errors, such as overflow, underflow, loss of precision, etc.

       * (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.

       * (C++ only) A non-static reference or non-static const member appears in a class without constructors.

       * (C++ only) Ambiguous virtual bases.

       * (C++ only) Subscripting an array which has been declared register.

       * (C++ only) Taking the address of a variable which has been declared register.

       * (C++ only) A base class is not initialized in a derived class' copy constructor.

-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 will not warn about unknown pragmas in system headers---for that, -Wunknown-pragmas must also be used.

-Wfloat-equal
     Warn if floating point values are used in equality comparisons.

     The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers.  If you are doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem).  In particular, instead of testing for equality, you would check to see whether the two values have ranges that overlap; and this is done with the relational operators, so equality comparisons are probably mistaken.

-Wfour-char-constants
     Warn about four char constants, e.g. OSType 'APPL'.  This warning is disabled by default.

-Wtraditional (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 which 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 does not in ISO C.

       * In traditional C, some preprocessor directives did not exist. Traditional preprocessors would only consider 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 would ignore 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 would not recognize #elif, so it 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 would cause serious problems.  This is a subset of the possible conversion warnings, for the full set use -Wconversion.

       * Use of ISO C style function definitions.  This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features will 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.

-Wdeclaration-after-statement (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.  See Mixed Declarations.

-Wundef
     Warn if an undefined identifier is evaluated in an #if directive.

-Wendif-labels
     Warn whenever an #else or an #endif are followed by text.

-Wshadow
     Warn whenever a local variable shadows another local variable, parameter or global variable or whenever a built-in function is shadowed.

-Wlarger-than-LEN
     Warn whenever an object of larger than LEN bytes is defined.

-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.

-Wbad-function-cast (C only)
     Warn whenever a function call is cast to a non-matching type. For example, warn if int malloc() is cast to anything *.

-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 *.

-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 will get a warning; when compiling C++, warn about the deprecated conversion from string constants to char *. These warnings will 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 will just be a nuisance; this is why we did not make -Wall request these warnings.

-Wconversion
     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.

     Also, warn if a negative integer constant expression is implicitly converted to an unsigned type.  For example, warn about the assignment x = -1 if x is unsigned.  But do not warn about explicit casts like (unsigned) -1.

-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.

-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.)

-Wstrict-prototypes (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 which specifies the argument types.)

-Wold-style-definition (C only)
     Warn if an old-style function definition is used.  A warning is given even if there is a previous prototype.

-Wmissing-prototypes (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.  The aim is to detect global functions that fail to be declared in header files.

-Wmissing-declarations (C only)
     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.

-Wmissing-field-initializers
     Warn if a structure's initializer has some fields missing.  For example, the following code would cause such a warning, because x.h is implicitly zero:

          struct s { int f, g, h; };
          struct s x = { 3, 4 };

     This option does not warn about designated initializers, so the following modification would not trigger a warning:

          struct s { int f, g, h; };
          struct s x = { .f = 3, .g = 4 };

     This warning is included in -Wextra.  To get other -Wextra warnings without this one, use -Wextra -Wno-missing-field-initializers.

-Wmissing-noreturn
     Warn about functions which might be candidates for attribute noreturn. Note these are only possible candidates, not absolute ones.  Care should be taken to manually verify functions actually do not ever return before adding the noreturn attribute, otherwise subtle code generation bugs could be introduced.  You will not get a warning for main in hosted C environments.

-Wmissing-format-attribute
     If -Wformat is enabled, also warn about functions which might be candidates for format attributes.  Note these are only possible candidates, not absolute ones.  GCC will guess that format attributes might be appropriate for any function that calls a function like vprintf or vscanf, but this might not always be the case, and some functions for which format attributes are appropriate may not be detected.  This option has no effect unless -Wformat is enabled (possibly by -Wall).

-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. This flag does not control warning for a constant with four character, instead -Wfour-char-constants controls it.

-Wno-deprecated-declarations
     Do not warn about uses of functions, variables, and types marked as deprecated by using the deprecated attribute. (see Function Attributes, see Variable Attributes, see Type Attributes.)

-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 will be 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;
          };

-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 only)
     Warn if an extern declaration is encountered within a function.

-Wunreachable-code
     Warn if the compiler detects that code will never be executed.

     This option is intended to warn when the compiler detects that at least a whole line of source code will never be executed, because some condition is never satisfied or because it is after a procedure that never returns.

     It is possible for this option to produce a warning even though there are circumstances under which part of the affected line can be executed, so care should be taken when removing apparently-unreachable code.

     For instance, when a function is inlined, a warning may mean that the line is unreachable in only one inlined copy of the function.

     This option is not made part of -Wall because in a debugging version of a program there is often substantial code which checks correct functioning of the program and is, hopefully, unreachable because the program does work.  Another common use of unreachable code is to provide behavior which is selectable at compile-time.

-Winline
     Warn if a function can not be inlined and it was declared as inline. Even with this option, the compiler will not warn about failures to inline functions declared in system headers.

     The compiler uses a variety of heuristics to determine whether or not to inline a function.  For example, the compiler takes into account the size of the function being inlined and the the amount of inlining that has already been done in the current function.  Therefore, seemingly insignificant changes in the source program can cause the warnings produced by -Winline to appear or disappear.

-Wno-invalid-offsetof (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.

     The restrictions on offsetof may be relaxed in a future version of the C++ standard.

-Winvalid-pch
     Warn if a precompiled header (see Precompiled Headers) is found in the search path but can't be used.

-Wlong-long
     Warn if long long type is used.  This is default.  To inhibit the warning messages, use -Wno-long-long.  Flags -Wlong-long and -Wno-long-long are taken into account only when -pedantic flag is used.

-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.

-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 were unable to handle the code effectively.  Often, the problem is that your code is too big or too complex; GCC will refuse to optimize programs when the optimization itself is likely to take inordinate amounts of time.

-Werror
     Make all warnings into errors.

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.

     On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but will probably make other debuggers crash or refuse to read the program.  If you want to control for certain whether to generate the extra information, use -gstabs+ or -gstabs (see below).

     GCC allows you to use -g with -O.  The shortcuts taken by optimized code may occasionally produce surprising results: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values were already at hand; some statements may execute in different places because they were moved out of loops.

     Nevertheless it proves possible to debug optimized output.  This makes it reasonable to use the optimizer for programs that might have bugs.

     The following options are useful when GCC is generated with the capability for more than one debugging format.

-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.

-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 which 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.

-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.

-gdwarf-2
     Produce debugging information in DWARF version 2 format (if that is supported).  This is the format used by DBX on IRIX 6.  With this option, GCC uses features of DWARF version 3 when they are useful; version 3 is upward compatible with version 2, but may still cause problems for older debuggers.

     (Other debug formats, such as -gcoff, are not supported in Darwin or Mac OS X.)

-gLEVEL
-ggdbLEVEL
-gstabsLEVEL
     Request debugging information and also use LEVEL to specify how much information.  The default level is 2.

     Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug.  This includes descriptions of functions and external variables, but no information about local variables and no line numbers.

     Level 3 includes extra information, such as all the macro definitions present in the program.  Some debuggers support macro expansion when you use -g3.

     -gdwarf-2 does not accept a concatenated debug level, because GCC used to support an option -gdwarf that meant to generate debug information in version 1 of the DWARF format (which is very different from version 2), and it would have been too confusing.  That debug format is long obsolete, but the option cannot be changed now. Instead use an additional -gLEVEL option to change the debug level for DWARF2.

-feliminate-dwarf2-dups
     Compress DWARF2 debugging information by eliminating duplicated information about each symbol.  This option only makes sense when generating DWARF2 debugging information with -gdwarf-2.

-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.

-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).

       * 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 (see Options that Control Optimization).

       * 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.

     With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed.  When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code.

-ftree-based-profiling
     This option is used in addition to -fprofile-arcs or -fbranch-probabilities to control whether those optimizations are performed on a tree-based or rtl-based internal representation. If you use this option when compiling with -fprofile-arcs, you must also use it when compiling later with -fbranch-probabilities. Currently the tree-based optimization is in an early stage of development, and this option is recommended only for those people working on improving it.

-ftest-coverage
     Produce a notes file that the gcov code-coverage utility (see gcov---a Test Coverage Program) 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 will match the source files more closely, if you do not optimize.

-dLETTERS

-fdump-rtl-PASS
     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.  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.

     Most debug dumps can be enabled either passing a letter to the -d option, or with a long -fdump-rtl switch; here are the possible letters for use in LETTERS and PASS, and their meanings:

     -dA
          Annotate the assembler output with miscellaneous debugging information.

     -db
     -fdump-rtl-bp
          Dump after computing branch probabilities, to FILE.09.bp.

     -dB
     -fdump-rtl-bbro
          Dump after block reordering, to FILE.30.bbro.

     -dc
     -fdump-rtl-combine
          Dump after instruction combination, to the file FILE.17.combine.

     -dC
     -fdump-rtl-ce1
     -fdump-rtl-ce2
          -dC and -fdump-rtl-ce1 enable dumping after the first if conversion, to the file FILE.11.ce1.  -dC and -fdump-rtl-ce2 enable dumping after the second if conversion, to the file FILE.18.ce2.

     -dd
     -fdump-rtl-btl
     -fdump-rtl-dbr
          -dd and -fdump-rtl-btl enable dumping after branch target load optimization, to to FILE.31.btl.  -dd and -fdump-rtl-dbr enable dumping after delayed branch scheduling, to FILE.36.dbr.

     -dD
          Dump all macro definitions, at the end of preprocessing, in addition to normal output.

     -dE
     -fdump-rtl-ce3
          Dump after the third if conversion, to FILE.28.ce3.

     -df
     -fdump-rtl-cfg
     -fdump-rtl-life
          -df and -fdump-rtl-cfg enable dumping after control and data flow analysis, to FILE.08.cfg.  -df and -fdump-rtl-cfg enable dumping dump after life analysis, to FILE.16.life.

     -dg
     -fdump-rtl-greg
          Dump after global register allocation, to FILE.23.greg.

     -dG
     -fdump-rtl-gcse
     -fdump-rtl-bypass
          -dG and -fdump-rtl-gcse enable dumping after GCSE, to FILE.05.gcse.  -dG and -fdump-rtl-bypass enable dumping after jump bypassing and control flow optimizations, to FILE.07.bypass.

     -dh
     -fdump-rtl-eh
          Dump after finalization of EH handling code, to FILE.02.eh.

     -di
     -fdump-rtl-sibling
          Dump after sibling call optimizations, to FILE.01.sibling.

     -dj
     -fdump-rtl-jump
          Dump after the first jump optimization, to FILE.03.jump.

     -dk
     -fdump-rtl-stack
          Dump after conversion from registers to stack, to FILE.33.stack.

     -dl
     -fdump-rtl-lreg
          Dump after local register allocation, to FILE.22.lreg.

     -dL
     -fdump-rtl-loop
     -fdump-rtl-loop2
          -dL and -fdump-rtl-loop enable dumping after the first loop optimization pass, to FILE.06.loop.  -dL and -fdump-rtl-loop2 enable dumping after the second pass, to FILE.13.loop2.

     -dm
     -fdump-rtl-sms
          Dump after modulo scheduling, to FILE.20.sms.

     -dM
     -fdump-rtl-mach
          Dump after performing the machine dependent reorganization pass, to FILE.35.mach.

     -dn
     -fdump-rtl-rnreg
          Dump after register renumbering, to FILE.29.rnreg.

     -dN
     -fdump-rtl-regmove
          Dump after the register move pass, to FILE.19.regmove.

     -do
     -fdump-rtl-postreload
          Dump after post-reload optimizations, to FILE.24.postreload.

     -dr
     -fdump-rtl-expand
          Dump after RTL generation, to FILE.00.expand.

     -dR
     -fdump-rtl-sched2
          Dump after the second scheduling pass, to FILE.32.sched2.

     -ds
     -fdump-rtl-cse
          Dump after CSE (including the jump optimization that sometimes follows CSE), to FILE.04.cse.

     -dS
     -fdump-rtl-sched
          Dump after the first scheduling pass, to FILE.21.sched.

     -dt
     -fdump-rtl-cse2
          Dump after the second CSE pass (including the jump optimization that sometimes follows CSE), to FILE.15.cse2.

     -dT
     -fdump-rtl-tracer
          Dump after running tracer, to FILE.12.tracer.

     -dV
     -fdump-rtl-vpt
     -fdump-rtl-vartrack
          -dV and -fdump-rtl-vpt enable dumping after the value profile transformations, to FILE.10.vpt.  -dV and -fdump-rtl-vartrack enable dumping after variable tracking, to FILE.34.vartrack.

     -dw
     -fdump-rtl-flow2
          Dump after the second flow pass, to FILE.26.flow2.

     -dz
     -fdump-rtl-peephole2
          Dump after the peephole pass, to FILE.27.peephole2.

     -dZ
     -fdump-rtl-web
          Dump after live range splitting, to FILE.14.web.

     -da
     -fdump-rtl-all
          Produce all the dumps listed above.

     -dH
          Produce a core dump whenever an error occurs.

     -dm
          Print statistics on memory usage, at the end of the run, to standard error.

     -dp
          Annotate the assembler output with a comment indicating which pattern and alternative was 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.

     -dv
          For each of the other indicated dump files (either with -d or -fdump-rtl-PASS), dump a representation of the control flow graph suitable for viewing with VCG to FILE.PASS.vcg.

     -dx
          Just generate RTL for a function instead of compiling it.  Usually used with r (-fdump-rtl-expand).

     -dy
          Dump debugging information during parsing, to standard error.

-fdump-unnumbered
     When doing debugging dumps (see -d option above), suppress instruction numbers and line number note 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-translation-unit (C and C++ only)
-fdump-translation-unit-OPTIONS (C and 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.  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.  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. The following dumps are possible:

     all
          Enables all inter-procedural analysis dumps; currently the only produced dump is the cgraph dump.

     cgraph
          Dumps information about call-graph optimization, unused function removal, and inlining decisions.

-fdump-tree-SWITCH (C and C++ only)
-fdump-tree-SWITCH-OPTIONS (C and C++ only)
     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.  If the -OPTIONS form is used, OPTIONS is a list of - separated options that control the details of the dump. Not all options are applicable to all dumps, those which are not meaningful will be 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.

     slim
          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.  When dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.

     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).

     stats
          Enable dumping various statistics about the pass (not honored by every dump option).

     blocks
          Enable showing basic block boundaries (disabled in raw dumps).

     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.

     all
          Turn on all options, except raw, slim and lineno.

     The following tree dumps are possible:

     original
          Dump before any tree based optimization, to FILE.original.

     optimized
          Dump after all tree based optimization, to FILE.optimized.

     inlined
          Dump after function inlining, to FILE.inlined.

     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.

     vcg
          Dump the control flow graph of each function to a file in VCG format.  The file name is made by appending .vcg to the source file name.  Note that if the file contains more than one function, the generated file cannot be used directly by VCG.  You will need to cut and paste each function's graph into its own separate file first.

     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.

     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.

     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.

     scev
          Dump the information gathered by the scalar evolution analyzer. The file name is made by appending .scev to the source file name.

     ddall
          Dump all the data dependence relations. The file name is made by appending .ddall to the source file name.

     elck
          Dump each function after performing checks elimination based on scalar evolution informations.  The file name is made by appending .elck 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.

     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.

     loop
          Dump each function after applying tree-level loop optimizations.  The file name is made by appending .loop 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.

     all
          Enable all the available tree dumps with the flags provided in this option.

-frandom-seed=STRING
     This option provides a seed that GCC uses when it would otherwise use random numbers.  It is used to generate 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.

     The STRING should be different for every file you compile.

-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 -dS or -dR is specified, in which case it is output to the usual dump listing file, .sched or .sched2 respectively.  However for N greater than nine, the output is always printed to standard error.

     For N greater than zero, -fsched-verbose outputs the same information as -dRS.  For N greater than one, it also output basic block probabilities, detailed ready list information and unit/insn info.  For N greater than two, it includes RTL at abort point, control-flow and regions info.  And for N over four, -fsched-verbose also includes dependence info.

-save-temps
     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 would produce 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.

-time
     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).  The output looks like this:

          # cc1 0.12 0.01
          # as 0.00 0.01

     The first number on each line is the ``user time,'' that is time spent executing the program itself.  The second number is ``system time,'' time spent executing operating system routines on behalf of the program. Both numbers are in seconds.

-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).

     It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g) and the debug info format supports it.

-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-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.

     This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a.  You can do

          gcc -nostdlib FILES... `gcc -print-libgcc-file-name`

-print-search-dirs
     Print the name of the configured installation directory and a list of program and library directories gcc will search---and don't do anything else.

     This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such file or directory. To resolve this you either need to put cpp0 and the other compiler components where gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed them. Don't forget the trailing '/'. See Environment Variables.

-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.)  See Spec Files.

-feliminate-unused-debug-types
     Normally, when producing DWARF2 output, GCC will emit debugging information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit.  Sometimes this is useful, such as 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. With this option, GCC will avoid producing debug symbol output for types that are nowhere used in the source file being compiled.

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 would 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.  Optimization levels -O2 and above, in particular, enable unit-at-a-time mode, which allows the compiler to consider information gained from later functions in the file when compiling a function.  Compiling multiple files at once to a single output file in unit-at-a-time 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.

-O
-O1
     Optimize.  Optimizing compilation takes somewhat more time, and a lot more memory for a large function.

     With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.

     -O turns on the following optimization flags: -fdefer-pop -fmerge-constants -fthread-jumps -floop-optimize -fif-conversion -fif-conversion2 -fdelayed-branch -fguess-branch-probability -fcprop-registers

     -O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.

-O2
     Optimize even more.  GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff.  The compiler does not perform loop unrolling or function inlining when you specify -O2. As compared to -O, this option increases both compilation time and the performance of the generated code.

     -O2 turns on all optimization flags specified by -O.  It also turns on the following optimization flags: -fforce-mem -foptimize-sibling-calls -fstrength-reduce -fcse-follow-jumps  -fcse-skip-blocks -frerun-cse-after-loop  -frerun-loop-opt -fgcse  -fgcse-lm  -fgcse-sm  -fgcse-las -fdelete-null-pointer-checks -fexpensive-optimizations -fregmove -fschedule-insns  -fschedule-insns2 -fsched-interblock  -fsched-spec -fcaller-saves -fpeephole2 -freorder-blocks  -freorder-functions -fstrict-aliasing -funit-at-a-time -falign-functions  -falign-jumps -falign-loops  -falign-labels -fcrossjumping

     Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos.

     In Apple's version of GCC, -fstrict-aliasing, -freorder-blocks, and -fsched-interblock are disabled by default when optimizing.

-O3
     Optimize yet more.  -O3 turns on all optimizations specified by -O2 and also turns on the -finline-functions, -fweb and -fgcse-after-reload options.

-O0
     Do not optimize.  This is the default.

-fast
     Optimize for maximum performance. -fast changes the overall optimization strategy of GCC in order to produce the fastest possible running code for PPC7450 and G5 architectures. By default, -fast optimizes for G5. Programs optimized for G5 will not run on PPC7450. To optimize for PPC7450, add -mcpu=7450 on command line.

     -fast currently enables the following optimization flags (for G5 and PPC7450). These flags may change in the future. You cannot override any of these options if you use -fast except by setting -mcpu=7450.

     To build shared libraries with -fast, specify -fPIC on command line.

     -O3 -floop-transpose -fgcse-sm -funroll-loops -fstrict-aliasing -fsched-interblock -falign-loops=16 -falign-jumps=16 -falign-functions=16 -falign-jumps-max-skip=15 -falign-loops-max-skip=15 -malign-natural -ffast-math -freorder-blocks -freorder-blocks-and-partition -finline-floor -mdynamic-no-pic -mpowerpc-gpopt -force_cpusubtype_ALL -fstrict-aliasing -mtune=G5 -mcpu=G5 -mpowerpc64

     Important notes: -ffast-math results in code that is not necessarily IEEE-compliant.  -fstrict-aliasing is highly likely break non-standard-compliant programs.  -malign-natural only works properly if the entire program is compiled with it, and none of the standard headers/libraries contain any code that changes alignment when this option is used.

-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.

     -Os disables the following optimization flags: -falign-functions  -falign-jumps  -falign-loops -falign-labels  -freorder-blocks  -freorder-blocks-and-partition -fprefetch-loop-arrays

     When compiling for Apple powerPC targets, -Os disables use of the string instructions even though they would usually be smaller, because the kernel can't emulate them correctly in some rare cases. This behavior is not portable to any other gcc environment, and will not affect most programs at all.  If you really want the string instructions, use -mstring.

     If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.

Options of the form -fFLAG specify machine-independent flags.  Most flags have both positive and negative forms; the negative form of -ffoo would be -fno-foo.  In the table below, only one of the forms is listed---the one you typically will 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 which 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.

     Disabled at levels -O, -O2, -O3, -Os.

-fforce-mem
     Force memory operands to be copied into registers before doing arithmetic on them.  This produces better code by making all memory references potential common subexpressions.  When they are not common subexpressions, instruction combination should eliminate the separate register-load.

     Enabled at levels -O2, -O3, -Os.

-fforce-addr
     Force memory address constants to be copied into registers before doing arithmetic on them.  This may produce better code just as -fforce-mem may.

-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.

     On some machines, such as the VAX, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't exist.  The machine-description macro FRAME_POINTER_REQUIRED controls whether a target machine supports this flag.  See Register Usage (GNU Compiler Collection (GCC) Internals).

     Enabled at levels -O, -O2, -O3, -Os.

-foptimize-sibling-calls
     Optimize sibling and tail recursive calls.

     Enabled at levels -O2, -O3, -Os.

-fno-inline
     Don't pay attention to the inline keyword.  Normally this option is used to keep the compiler from expanding any functions inline. Note that if you are not optimizing, no functions can be expanded inline.

-finline-functions
     Integrate all simple functions into their callers.  The compiler heuristically decides which functions are simple enough to be worth integrating in this way.

     If all calls to a given function are integrated, and the function is declared static, then the function is normally not output as assembler code in its own right.

     Enabled at level -O3.

-finline-limit=N
     By default, GCC limits the size of functions that can be inlined.  This flag allows the control of this limit for functions that are explicitly marked as inline (i.e., marked with the inline keyword or defined within the class definition in c++).  N is the size of functions that can be inlined in number of pseudo instructions (not counting parameter handling).  The default value of N is 600. Increasing this value can result in more inlined code at the cost of compilation time and memory consumption.  Decreasing usually makes the compilation faster and less code will be inlined (which presumably means slower programs).  This option is particularly useful for programs that use inlining heavily such as those based on recursive templates with C++.

     Inlining is actually controlled by a number of parameters, which may be specified individually by using --param NAME=VALUE. The -finline-limit=N option sets some of these parameters as follows:

     max-inline-insns-single
          is set to N/2.

     max-inline-insns-auto
          is set to N/2.

     min-inline-insns
          is set to 130 or N/4, whichever is smaller.

     max-inline-insns-rtl
          is set to N.

     See below for a documentation of the individual parameters controlling inlining.

     Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size.  In no way, it represents a count of assembly instructions and as such its exact meaning might change from one release to an another.

-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 C.  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.

     GCC enables this option by default.  If you want to force the compiler to check if the variable was referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option.

-fmerge-constants
     Attempt to merge identical constants (string constants and floating point constants) across compilation units.

     This option is the default for optimized compilation if the assembler and linker support it.  Use -fno-merge-constants to inhibit this behavior.

     Enabled at levels -O, -O2, -O3, -Os.

-fmerge-all-constants
     Attempt to merge identical constants and identical variables.

     This option implies -fmerge-constants.  In addition to -fmerge-constants this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating point types.  Languages like C or C++ require each non-automatic variable to have distinct location, so using this option will result in non-conforming behavior.

-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.

-fnew-ra
     Use a graph coloring register allocator.  Currently this option is meant only for testing.  Users should not specify this option, since it is not yet ready for production use.

-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.

     The default is -fbranch-count-reg, enabled when -fstrength-reduce is enabled.

-fno-function-cse
     Do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly.

     This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used.

     The default is -ffunction-cse

-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.

     This option turns off this behavior because some programs explicitly rely on variables going to the data section.  E.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that.

     The default is -fzero-initialized-in-bss.

-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.

-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 will be 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.

     Use -fmudflapth instead of -fmudflap to compile and to link if your program is multi-threaded.  Use -fmudflapir, in addition to -fmudflap or -fmudflapth, if instrumentation should ignore pointer reads.  This produces less instrumentation (and therefore faster execution) and still provides some protection against outright memory corrupting writes, but allows erroneously read data to propagate within a program.

-fstrength-reduce
     Perform the optimizations of loop strength reduction and elimination of iteration variables.

     Enabled at levels -O2, -O3, -Os.

-fthread-jumps
     Perform optimizations where we 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.

     Enabled at levels -O, -O2, -O3, -Os.

-fcse-follow-jumps
     In common subexpression elimination, 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 will follow the jump when the condition tested is false.

     Enabled at levels -O2, -O3, -Os.

-fcse-skip-blocks
     This is similar to -fcse-follow-jumps, but causes CSE to follow jumps which 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.

     Enabled at levels -O2, -O3, -Os.

-frerun-cse-after-loop
     Re-run common subexpression elimination after loop optimizations has been performed.

     Enabled at levels -O2, -O3, -Os.

-frerun-loop-opt
     Run the loop optimizer twice.

     Enabled at levels -O2, -O3, -Os.

-fgcse
     Perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation.

     Note: When compiling a program using computed gotos, a GCC extension, you may get better runtime performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.

     Enabled at levels -O2, -O3, -Os.

-fgcse-lm
     When -fgcse-lm is enabled, global common subexpression elimination will attempt to move loads which 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.

     Enabled by default when gcse is enabled.

-fgcse-sm
     When -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination.  This pass will attempt 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.

     Enabled by default when gcse is enabled.

-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).

     Enabled by default when gcse is enabled.

-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 cleanup redundant spilling.

-floop-optimize
     Perform loop optimizations: move constant expressions out of loops, simplify exit test conditions and optionally do strength-reduction as well.

     Enabled at levels -O, -O2, -O3, -Os.

-floop-optimize2
     Perform loop optimizations using the new loop optimizer.  The optimizations (loop unrolling, peeling and unswitching, loop invariant motion) are enabled by separate flags.

-fcrossjumping
     Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The resulting code may or may not perform better than without cross-jumping.

     Enabled at levels -O, -O2, -O3, -Os.

-fif-conversion
     Attempt to transform conditional jumps into branch-less equivalents.  This include 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.

     Enabled at levels -O, -O2, -O3, -Os.

-fif-conversion2
     Use conditional execution (where available) to transform conditional jumps into branch-less equivalents.

     Enabled at levels -O, -O2, -O3, -Os.

-fdelete-null-pointer-checks
     Use global dataflow analysis to identify and eliminate useless checks for null pointers.  The compiler assumes that dereferencing a null pointer would have halted the program.  If a pointer is checked after it has already been dereferenced, it cannot be null.

     In some environments, this assumption is not true, and programs can safely dereference null pointers.  Use -fno-delete-null-pointer-checks to disable this optimization for programs which depend on that behavior.

     Enabled at levels -O2, -O3, -Os.

-fexpensive-optimizations
     Perform a number of minor optimizations that are relatively expensive.

     Enabled at levels -O2, -O3, -Os.

-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.

     Note -fregmove and -foptimize-register-move are the same optimization.

     Enabled at levels -O2, -O3, -Os.

-fdelayed-branch
     If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions.

     Enabled at levels -O, -O2, -O3, -Os.

-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.

     Enabled at levels -O2, -O3, -Os.

-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.

     Enabled at levels -O2, -O3, -Os.

-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-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=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.

-fsched-stalled-insns-dep=N
     Define how many insn groups (cycles) will be examined for a dependency on a stalled insn that is candidate for premature removal from the queue of stalled insns.  Has an effect only during the second scheduling pass, and only if -fsched-stalled-insns is used and its value is not zero.

-fsched2-use-superblocks
     When scheduling after register allocation, do use superblock scheduling algorithm.  Superblock scheduling allows motion across basic block boundaries resulting on 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.

     This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.

-fsched2-use-traces
     Use -fsched2-use-superblocks algorithm when scheduling after register allocation and additionally perform code duplication in order to increase the size of superblocks using tracer pass.  See -ftracer for details on trace formation.

     This mode should produce faster but significantly longer programs.  Also without -fbranch-probabilities the traces constructed may not match the reality and hurt the performance.  This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.

-freschedule-modulo-scheduled-loops
     The modulo scheduling comes before the traditional scheduling, if a loop was modulo scheduled we may want to prevent the later scheduling passes from changing its schedule, we use this option to control that.

-fcaller-saves
     Enable values to be allocated in registers that will be 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 than would otherwise be produced.

     This option is always enabled by default on certain machines, usually those which have no call-preserved registers to use instead.

     Enabled at levels -O2, -O3, -Os.

-ftree-pre
     Perform Partial Redundancy Elimination (PRE) 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 faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at -O and higher.

-ftree-ccp
     Perform sparse conditional constant propagation (CCP) on trees.  This flag is enabled by default at -O and higher.

-ftree-dce
     Perform dead code elimination (DCE) on trees.  This flag is enabled by default at -O and higher.

-ftree-dominator-opts
     Perform dead code elimination (DCE) on trees.  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-vectorize
     Perform loop vectorization on trees.  This flag is disabled by default.

-fscalar-evolutions
     Analyzes the scalar variables that determine the exit condition of a loop, and try to determine the number of iterations.  This pass is only used for verifying the correctness of the analyzer, and it is not enabled by default.

-fall-data-deps
     Perform the data dependence analysis on all the data references.  This pass is only used for verifying the correctness of the analyzer, and it is not enabled by default.

-ftree-elim-checks
     Perform elimination of checks based on scalar evolution informations. This flag is disabled by default.

-ftree-loop-optimize
     Perform loop optimizations on trees.  This flag is enabled by default at -O and higher.

-ftree-loop-linear
     Perform linear loop transformations on tree. This flag can improve cache performance and allow further loop optimizations to take place.

-ftree-lim
     Perform loop invariant motion on trees.  This pass moves only invartiants that would be hard to handle on 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.

-fivcanon
     Create a canonical counter for number of iterations in the loop for that 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-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-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-lrs
     Perform live range splitting during the SSA->normal phase.  Distinct live ranges of a variable are split into unique variables, allowing for better optimization later.  This is enabled by default at -O and higher.

-ftree-vectorize
     Perform loop vectorization on trees.

-ftracer
     Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing other optimizations to do 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 both -fstrength-reduce and -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 expressing 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 (for best results, -fweb should be used as well).

     Combination of -fweb and CSE is often sufficient to obtain the same effect. However in cases the loop body is more complicated than a single basic block, this is not reliable.  It also does not work at all on some of the architectures due to restrictions in the CSE pass.

     This optimization is enabled by default.

-fprefetch-loop-arrays
     If supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays.

     These options may generate better or worse code; results are highly dependent on the structure of loops within the source code.

-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.

     -fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.

-fno-guess-branch-probability
     Do not guess branch probabilities using a randomized model.

     Sometimes GCC will opt to use a randomized model to guess branch probabilities, when none are available from either profiling feedback (-fprofile-arcs) or __builtin_expect.  This means that different runs of the compiler on the same program may produce different object code.

     In a hard real-time system, people don't want different runs of the compiler to produce code that has different behavior; minimizing non-determinism is of paramount import.  This switch allows users to reduce non-determinism, possibly at the expense of inferior optimization.

     The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.

-freorder-blocks
     Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code locality.

     Enabled at levels -O2, -O3.

-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.

     This optimization is automatically turned off in the presence of exception handling, for linkonce sections, for functions with a user-defined section attribute and on any architecture that does not support named sections.

-freorder-functions
     Reorder basic blocks in the compiled function in order to reduce number of taken branches and 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.

     Also profile feedback must be available in to make this option effective.  See -fprofile-arcs for details.

     Enabled at levels -O2, -O3, -Os.

-fstrict-aliasing
     Allows 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.

     Pay special attention to code like this:

          union a_union {
            int i;
            double d;
          };

          int f() {
            a_union t;
            t.d = 3.0;
            return t.i;
          }

     The practice of reading from a different union member than the one most recently written to (called ``type-punning'') is common.  Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type.  So, the code above will work as expected.  However, this code might not:

          int f() {
            a_union t;
            int* ip;
            t.d = 3.0;
            ip = &t.i;
            return *ip;
          }

     Every language that wishes to perform language-specific alias analysis should define a function that computes, given an tree node, an alias set for the node.  Nodes in different alias sets are not allowed to alias.  For an example, see the C front-end function c_get_alias_set.

     Enabled at levels -O2, -O3, -Os.

-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 would align to the next 32-byte boundary only if this can be done by skipping 23 bytes or less.

     -fno-align-functions and -falign-functions=1 are equivalent and mean that functions will not be aligned.

     Some assemblers only support this flag when N is a power of two; in that case, it is rounded up.

     If N is not specified or is zero, use a machine-dependent default.

     Enabled at levels -O2, -O3.

-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.

     -fno-align-labels and -falign-labels=1 are equivalent and mean that labels will not be aligned.

     If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead.

     If N is not specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no alignment.

     Enabled at levels -O2, -O3.

-falign-loops
-falign-loops=N
     Align loops to a power-of-two boundary, skipping up to N bytes like -falign-functions.  The hope is that the loop will be executed many times, which will make up for any execution of the dummy operations.

     -fno-align-loops and -falign-loops=1 are equivalent and mean that loops will not be aligned.

     If N is not specified or is zero, use a machine-dependent default.

     Enabled at levels -O2, -O3.

-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.

     -fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops will not be aligned.

     If N is not specified or is zero, use a machine-dependent default.

     Enabled at levels -O2, -O3.

-funit-at-a-time
     Parse the whole compilation unit before starting to produce code. This allows some extra optimizations to take place but consumes more memory (in general).  There are some compatibility issues with unit-at-at-time mode:

       * enabling unit-at-a-time mode may change the order in which functions, variables, and top-level asm statements are emitted, and will likely break code relying on some particular ordering.  The majority of such top-level asm statements, though, can be replaced by section attributes.

       * unit-at-a-time mode removes unreferenced static variables and functions are removed.  This may result in undefined references when an asm statement refers directly to variables or functions that are otherwise unused.  In that case either the variable/function shall be listed as an operand of the asm statement operand or, in the case of top-level asm statements the attribute used shall be used on the declaration.

       * Static functions now can use non-standard passing conventions that may break asm statements calling functions directly. Again, attribute used will prevent this behavior.

     As a temporary workaround, -fno-unit-at-a-time can be used, but this scheme may not be supported by future releases of GCC.

     Enabled at levels -O2, -O3.

-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 will no longer stay in a ``home register''.

     Enabled at levels -O2, -O3, -Os, on targets where the default format for debugging information supports variable tracking.

-fno-cprop-registers
     After register allocation and post-register allocation instruction splitting, we perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy.

     Disabled at levels -O, -O2, -O3, -Os.

-fprofile-generate

     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.

     The following options are enabled: -fprofile-arcs, -fprofile-values, -fvpt.

-fprofile-use
     Enable profile feedback directed optimizations, and optimizations generally profitable only with profile feedback available.

     The following options are enabled: -fbranch-probabilities, -fvpt, -funroll-loops, -fpeel-loops, -ftracer.

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.

     This option prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a double is supposed to have.  Similarly for the x86 architecture.  For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point.  Use -ffloat-store for such programs, after modifying them to store all pertinent intermediate computations into variables.

-ffast-math
     Sets -fno-math-errno, -funsafe-math-optimizations,

     -fno-trapping-math, -ffinite-math-only, -fno-rounding-math and -fno-signaling-nans.

     This option causes the preprocessor macro __FAST_MATH__ to be defined.

     This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

-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.

     This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

     The default is -fmath-errno.

-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.

     This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

     The default is -fno-unsafe-math-optimizations.

-ffinite-math-only
     Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs.

     This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications.

     The default is -fno-finite-math-only.

-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 implies -fno-signaling-nans.  Setting this option may allow faster code if one relies on ``non-stop'' IEEE arithmetic, for example.

     This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

     The default is -ftrapping-math.

-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.

     The default is -fno-rounding-math.

     This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode. Future versions of GCC may provide finer control of this setting using C99's FENV_ACCESS pragma.  This command line option will be used to specify the default state for FENV_ACCESS.

-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.

     This option causes the preprocessor macro __SUPPORT_SNAN__ to be defined.

     The default is -fno-signaling-nans.

     This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.

-fsingle-precision-constant
     Treat floating point constant as single precision constant instead of implicitly converting it to double precision constant.

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 (see Options for Debugging Your Program or gcc), you can compile it a second time using -fbranch-probabilities, to improve optimizations based on the number of times each branch was taken.  When the 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.

     With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be used to improve optimization.  Currently, they are only used in one place: in reorg.c, instead of guessing which path a branch is mostly to take, the REG_BR_PROB values are used to exactly determine which path is taken more often.

-fprofile-values
     If combined with -fprofile-arcs, it adds code so that some data about values of expressions in the program is gathered.

     With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions and adds REG_VALUE_PROFILE notes to instructions for their later usage in optimizations.

     Enabled with -fprofile-generate and -fprofile-use.

-fvpt
     If combined with -fprofile-arcs, it instructs the compiler to add a code to gather information about values of expressions.

     With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based on them. Currently the optimizations include specialization of division operation using the knowledge about the value of the denominator.

-fspeculative-prefetching
     If combined with -fprofile-arcs, it instructs the compiler to add a code to gather information about addresses of memory references in the program.

     With -fbranch-probabilities, it reads back the data gathered and issues prefetch instructions according to them.  In addition to the opportunities noticed by -fprefetch-loop-arrays, it also notices more complicated memory access patterns -- for example accesses to the data stored in linked list whose elements are usually allocated sequentially.

     In order to prevent issuing double prefetches, usage of -fspeculative-prefetching implies -fno-prefetch-loop-arrays.

     Enabled with -fprofile-generate and -fprofile-use.

-frename-registers
     Attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation.  This optimization will most benefit processors with lots of registers.  Depending on the debug information format adopted by the target, however, it can make debugging impossible, since variables will no longer stay in a ``home register''.

     Not enabled by default at any level because it has known bugs.

-fnew-ra
     Use a graph coloring register allocator.  Currently this option is meant for testing, so we are interested to hear about miscompilations with -fnew-ra.

-ftracer
     Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing other optimizations to do better job.

     Enabled with -fprofile-use.

-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.  It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations). This option makes code larger, and may or may not make it run faster.

     Enabled with -fprofile-use.

-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 the loops for that 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).

     Enabled with -fprofile-use.

-fmove-loop-invariants
     Enables the loop invariant motion pass in the new 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).

-fprefetch-loop-arrays
     If supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays.

     Disabled at level -Os.

-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.

     Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space.  Most systems using the ELF object format and SPARC processors running Solaris 2 have linkers with such optimizations.  AIX may have these optimizations in the future.

     Only use these options when there are significant benefits from doing so.  When you specify these options, the assembler and linker will create larger object and executable files and will also be slower. You will not be able to use gprof on all systems if you specify this option and you may have problems with debugging if you specify both this option and -g.

-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 in within any basic block.

--param NAME=VALUE
     In some places, GCC uses various constants to control the amount of optimization that is done.  For example, GCC will not inline functions that contain more that a certain number of instructions.  You can control some of these constants on the command-line using the --param option.

     The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases.

     In each case, the VALUE is an integer.  The allowable choices for NAME are given in the following table:

     max-crossjump-edges
          The maximum number of incoming edges to consider for crossjumping. 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 compile time increase with probably small improvement in executable size.

     min-crossjump-insns
          The minimum number of instructions which must be matched at the end of two blocks before crossjumping will be performed on them.  This value is ignored in the case where all instructions in the block being crossjumped from are matched.  The default value is 5.

     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 is searched, the time savings from filling the delay slot will be minimal so stop searching.  Increasing values mean more aggressive optimization, making the compile time increase with probably small improvement in executable run 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 compile 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 will be allocated in order to perform the global common subexpression elimination optimization.  If more memory than specified is required, the optimization will not be done.

     max-gcse-passes
          The maximum number of passes of GCSE to run.  The default is 1.

     max-pending-list-length
          The maximum number of pending dependencies scheduling will allow 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-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 will consider for inlining.  This only affects functions declared inline and methods implemented in a class declaration (C++). The default value is 500.

     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 will be investigated.  To those functions, a different (more restrictive) limit compared to functions declared inline can be applied. The default value is 120.

     large-function-insns
          The limit specifying really large functions.  For functions greater than this limit 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 backend. This parameter is ignored when -funit-at-a-time is not used. The default value is 3000.

     large-function-growth
          Specifies maximal growth of large function caused by inlining in percents. This parameter is ignored when -funit-at-a-time is not used. The default value is 200.

     inline-unit-growth
          Specifies maximal overall growth of the compilation unit caused by inlining. This parameter is ignored when -funit-at-a-time is not used. The default value is 150.

     max-inline-insns-recursive
     max-inline-insns-recursive-auto
          Specifies maximum number of instructions out-of-line copy of self recursive inline function can grow into by performing recursive inlining.

          For functions declared inline --param max-inline-insns-recursive is taken into acount.  For function not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled and --param max-inline-insns-recursive-auto is used.  The default value is 500.

     max-inline-recursive-depth
     max-inline-recursive-depth-auto
          Specifies maximum recursion depth used by the recursive inlining.

          For functions declared inline --param max-inline-recursive-depth is taken into acount.  For function not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled and --param max-inline-recursive-depth-auto is used.  The default value is 500.

     max-inline-insns-rtl
          For languages that use the RTL inliner (this happens at a later stage than tree inlining), you can set the maximum allowable size (counted in RTL instructions) for the RTL inliner with this parameter. The default value is 600.

     max-unrolled-insns
          The maximum number of instructions that a loop should have if that loop is unrolled, and if the loop is unrolled, it 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 should have if that loop is unrolled, and if the loop is unrolled, it 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 should have if that loop is peeled, and if the loop is peeled, it determines how many times the loop code is peeled.

     max-peel-times
          The maximum number of peelings of a single loop.

     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-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 that all candidates are considered for each use in induction variable optimizations.  Only the most relevant candidates are considered if there are more candidates, to avoid quadratic time complexity.

     iv-max-considered-uses
          The induction variable optimizations give up on loops that contain more induction variable uses.

     max-iterations-to-track

          The maximum number of iterations of a loop the brute force algorithm for analysis of # of iterations of the loop tries to evaluate.

     hot-bb-count-fraction
          Select fraction of the maximal count of repetitions of basic block in program given basic block needs to have to be considered hot.

     hot-bb-frequency-fraction
          Select fraction of the maximal frequency of executions of basic block in function given basic block needs to have to be considered hot

     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.

          The tracer-dynamic-coverage-feedback is used only when profile feedback is available.  The real profiles (as opposed to statically estimated ones) are much less balanced allowing the threshold to be larger value.

     tracer-max-code-growth
          Stop tail duplication once code growth has reached given percentage.  This is rather hokey argument, as most of the duplicates will be 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 do have probability lower than this threshold.

          Similarly to tracer-dynamic-coverage two values are present, one for compilation for profile feedback and one for compilation without.  The value for compilation with profile feedback needs to be more conservative (higher) in order to make tracer effective.

     max-cse-path-length

          Maximum number of basic blocks on path that cse considers.  The default is 10.

     global-var-threshold

          Counts the number of function calls (N) and the number of call-clobbered variables (V).  If NxV is larger than this limit, a single artificial variable will be created to represent all the call-clobbered variables at function call sites.  This artificial variable will then be made to alias every call-clobbered variable. (done as int * size_t on the host machine; beware overflow).

     max-aliased-vops

          Maximum number of virtual operands allowed to represent aliases before triggering the alias grouping heuristic.  Alias grouping reduces compile times and memory consumption needed for aliasing at the expense of precision loss in alias information.

     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.

          The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB.  If getrlimit is available, the notion of "RAM" is the smallest of actual RAM and RLIMIT_DATA or RLIMIT_AS.  If GCC is not able to calculate RAM on a particular platform, the lower bound of 30% is used.  Setting this parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity.  This is extremely slow, but can be useful for debugging.

     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.

          The default is the smaller of RAM/8, RLIMIT_RSS, or a limit which tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes).  If GCC is not able to calculate RAM on a particular platform, the lower bound is used.  Setting this parameter very large effectively disables garbage collection.  Setting this parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity.

     max-reload-search-insns
          The maximum number of instruction reload should look backward for equivalent register.  Increasing values mean more aggressive optimization, making the compile time increase with probably slightly better performance.  The default value is 100.

     max-cselib-memory-location
          The maximum number of memory locations cselib should take into acount. Increasing values mean more aggressive optimization, making the compile time increase with probably slightly better performance.  The default value is 500.

     reorder-blocks-duplicate
     reorder-blocks-duplicate-feedback

          Used by 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.

          The reorder-block-duplicate-feedback is used only when profile feedback is available and may be set to higher values than reorder-block-duplicate since information about the hot spots is more accurate.

     max-sched-region-blocks
          The maximum number of blocks in a region to be considered for interblock scheduling.  The default value is 10.

     max-sched-region-insns
          The maximum number of insns in a region to be considered for interblock scheduling.  The default value is 100.

     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's.  The default value is 256.

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.

     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 which GCC does not know how to recognize.

     If you want to pass an option that takes an argument, you must use -Xpreprocessor twice, once for the option and once for the argument.

     [file cppopts.texi not included]

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 which GCC does not know how to recognize.

     If you want to pass an option that takes an argument, you must use -Xassembler twice, once for the option and once for the argument.

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.

In addition to the options listed below, Apple's GCC also accepts and passes nearly all of the options defined by the linker ld and by the library tool libtool.  Common options include -framework, -dynamic, -bundle, -flat_namespace, and so forth.  See the ld and libtool man pages for further details.

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.  See Overall Options.

-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.)

     It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified.  Thus, foo.o -lz bar.o searches library z after file foo.o but before bar.o.  If bar.o refers to functions in z, those functions may not be loaded.

     The linker searches a standard list of directories for the library, which is actually a file named libLIBRARY.a.  The linker then uses this file as if it had been specified precisely by name.

     The directories searched include several standard system directories plus any that you specify with -L.

     Normally the files found this way are library files---archive files whose members are object files.  The linker handles an archive file by scanning through it for members which define symbols that have so far been referenced but not defined.  But if the file that is found is an ordinary object file, it is linked in the usual fashion.  The only difference between using an -l option and specifying a file name is that -l surrounds LIBRARY with lib and .a and searches several directories.

-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 will be passed to the linker. The standard startup files are used normally, unless -nostartfiles is used.  The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc.  These entry points should be supplied through some other mechanism when this option is specified.

-no-c++filt
     By default all linker diagnostic output is piped through c++filt. This option suppresses that behavior.  (APPLE ONLY)

-nostdlib
     Do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify will be passed to the linker.  The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc.  These entry points should be supplied through some other mechanism when this option is specified.

     One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines that GCC uses to overcome shortcomings of particular machines, or special needs for some languages. (see Interfacing to GCC Output (GNU Compiler Collection (GCC) Internals), for more discussion of libgcc.a.) In most cases, you need libgcc.a even when you want to avoid other standard libraries.  In other words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that you have no unresolved references to internal GCC library subroutines.  (For example, __main, used to ensure C++ constructors will be called; see collect2 (GNU Compiler Collection (GCC) Internals).)

-pie
     Produce a position independent executable on targets which support it. For predictable results, you must also specify the same set of options that were used to generate code (-fpie, -fPIE, or model suboptions) when you specify this option.

-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.

     This option will not work on Mac OS X unless all libraries (including libgcc.a) have also been compiled with -static.  Since neither a static version of libSystem.dylib nor crt0.o are provided, this option is not useful to most people.

-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 that were used to generate code (-fpic, -fPIC, or model suboptions) when you specify this option. (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.)

     This option is not supported on Mac OS X.

-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.

     There are several situations in which an application should use the shared libgcc instead of the static version.  The most common of these is when the application wishes to throw and catch exceptions across different shared libraries.  In that case, each of the libraries as well as the application itself should use the shared libgcc.

     Therefore, the G++ and GCJ drivers automatically add -shared-libgcc whenever you build a shared library or a main executable, because C++ and Java programs typically use exceptions, so this is the right thing to do.

     If, instead, you use the GCC driver to create shared libraries, you may find that they will not always be linked with the shared libgcc. If GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that does not support option --eh-frame-hdr, it will link the shared version of libgcc into shared libraries by default.  Otherwise, it will take advantage of the linker and optimize away the linking with the shared version of libgcc, linking with the static version of libgcc by default.  This allows exceptions to propagate through such shared libraries, without incurring relocation costs at library load time.

     However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ or GCJ driver, as appropriate for the languages used in the program, or using the option -shared-libgcc, such that it is linked with the shared libgcc.

-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.

-Xlinker OPTION
     Pass OPTION as an option to the linker.  You can use this to supply system-specific linker options which GCC does not know how to recognize.

     If you want to pass an option that takes an argument, you must use -Xlinker twice, once for the option and once for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions.  It does not work to write -Xlinker "-assert definitions", because this passes the entire string as a single argument, which is not what the linker expects.

-Wl,OPTION
     Pass OPTION as an option to the linker.  If OPTION contains commas, it is split into multiple options at the commas.

-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.

     If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option will be ignored.  The directory will still be searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the include_next directive are not inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options.

-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.

-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.

     The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld.  It tries PREFIX as a prefix for each program it tries to run, both with and without MACHINE/VERSION/ (see Target Options).

     For each subprogram to be run, the compiler driver first tries the -B prefix, if any.  If that name is not found, or if -B was not specified, the driver tries two standard prefixes, which are /usr/lib/gcc/ and /usr/local/lib/gcc/.  If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your PATH environment variable.

     The compiler will check to see if the path provided by the -B refers to a directory, and if necessary it will add a directory separator character at the end of the path.

     -B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker.  They also apply to includes files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor.  In this case, the compiler appends include to the prefix.

     The run-time support file libgcc.a can also be searched for using the -B prefix, if needed.  If it is not found there, the two standard prefixes above are tried, and that is all.  The file is left out of the link if it is not found by those means.

     Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX.  See Environment Variables.

     As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it will be replaced by [dir/]include.  This is to help with boot-strapping the compiler.

-specs=FILE
     Process FILE after the compiler reads in the standard specs file, in order to override the defaults that 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.

-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>.

     If additional directories are specified with -I options after the -I-, these directories are searched for all #include directives.  (Ordinarily all -I directories are used this way.)

     In addition, the -I- option inhibits the use of the current directory (where the current input file came from) as the first search directory for #include "FILE".  There is no way to override this effect of -I-.  With -I. you can specify searching the directory which was current when the compiler was invoked.  That is not exactly the same as what the preprocessor does by default, but it is often satisfactory.

     -I- does not inhibit the use of the standard system directories for header files.  Thus, -I- and -nostdinc are independent.

Specifying subprocesses and the switches to pass to them
========================================================

gcc is a driver program.  It performs its job by invoking a sequence of other programs to do the work of compiling, assembling and linking.  GCC interprets its command-line parameters and uses these to deduce which programs it should invoke, and which command-line options it ought to place on their command lines.  This behavior is controlled by spec strings.  In most cases there is one spec string for each program that GCC can invoke, but a few programs have multiple spec strings to control their behavior.  The spec strings built into GCC can be overridden by using the -specs= command-line switch to specify a spec file.

Spec files are plaintext files that are used to construct spec strings.  They consist of a sequence of directives separated by blank lines.  The type of directive is determined by the first non-whitespace character on the line and it can be one of the following:

%COMMAND
     Issues a COMMAND to the spec file processor.  The commands that can appear here are:

     %include <FILE>
          Search for FILE and insert its text at the current point in the specs file.

     %include_noerr <FILE>
          Just like %include, but do not generate an error message if the include file cannot be found.

     %rename OLD_NAME NEW_NAME
          Rename the spec string OLD_NAME to NEW_NAME.

*[SPEC_NAME]:
     This tells the compiler to create, override or delete the named spec string.  All lines after this directive up to the next directive or blank line are considered to be the text for the spec string.  If this results in an empty string then the spec will be deleted.  (Or, if the spec did not exist, then nothing will happened.)  Otherwise, if the spec does not currently exist a new spec will be created.  If the spec does exist then its contents will be overridden by the text of this directive, unless the first character of that text is the + character, in which case the text will be appended to the spec.

[SUFFIX]:
     Creates a new [SUFFIX] spec pair.  All lines after this directive and up to the next directive or blank line are considered to make up the spec string for the indicated suffix.  When the compiler encounters an input file with the named suffix, it will processes the spec string in order to work out how to compile that file.  For example:

          .ZZ:
          z-compile -input %i

     This says that any input file whose name ends in .ZZ should be passed to the program z-compile, which should be invoked with the command-line switch -input and with the result of performing the %i substitution.  (See below.)

     As an alternative to providing a spec string, the text that follows a suffix directive can be one of the following:

     @LANGUAGE
          This says that the suffix is an alias for a known LANGUAGE.  This is similar to using the -x command-line switch to GCC to specify a language explicitly.  For example:

               .ZZ:
               @c++

          Says that .ZZ files are, in fact, C++ source files.

     #NAME
          This causes an error messages saying:

               NAME compiler not installed on this system.

     GCC already has an extensive list of suffixes built into it. This directive will add an entry to the end of the list of suffixes, but since the list is searched from the end backwards, it is effectively possible to override earlier entries using this technique.

GCC has the following spec strings built into it.  Spec files can override these strings or create their own.  Note that individual targets can also add their own spec strings to this list.

     asm          Options to pass to the assembler
     asm_final    Options to pass to the assembler post-processor
     cpp          Options to pass to the C preprocessor
     cc1          Options to pass to the C compiler
     cc1plus      Options to pass to the C++ compiler
     endfile      Object files to include at the end of the link
     link         Options to pass to the linker
     lib          Libraries to include on the command line to the linker
     libgcc       Decides which GCC support library to pass to the linker
     linker       Sets the name of the linker
     predefines   Defines to be passed to the C preprocessor
     signed_char  Defines to pass to CPP to say whether char is signed
                  by default
     startfile    Object files to include at the start of the link

Here is a small example of a spec file:

     %rename lib                 old_lib

     *lib:
     --start-group -lgcc -lc -leval1 --end-group %(old_lib)

This example renames the spec called lib to old_lib and then overrides the previous definition of lib with a new one. The new definition adds in some extra command-line options before including the text of the old definition.

Spec strings are a list of command-line options to be passed to their corresponding program.  In addition, the spec strings can contain %-prefixed sequences to substitute variable text or to conditionally insert text into the command line.  Using these constructs it is possible to generate quite complex command lines.

Here is a table of all defined %-sequences for spec strings.  Note that spaces are not generated automatically around the results of expanding these sequences.  Therefore you can concatenate them together or combine them with constant text in a single argument.

%%
     Substitute one % into the program name or argument.

%i
     Substitute the name of the input file being processed.

%b
     Substitute the basename of the input file being processed. This is the substring up to (and not including) the last period and not including the directory.

%B
     This is the same as %b, but include the file suffix (text after the last period).

%d
     Marks the argument containing or following the %d as a temporary file name, so that that file will be deleted if GCC exits successfully.  Unlike %g, this contributes no text to the argument.

%gSUFFIX
     Substitute a file name that has suffix SUFFIX and is chosen once per compilation, and mark the argument in the same way as %d.  To reduce exposure to denial-of-service attacks, the file name is now chosen in a way that is hard to predict even when previously chosen file names are known.  For example, %g.s ... %g.o ... %g.s might turn into ccUVUUAU.s ccXYAXZ12.o ccUVUUAU.s.  SUFFIX matches the regexp [.A-Za-z]* or the special string %O, which is treated exactly as if %O had been preprocessed.  Previously, %g was simply substituted with a file name chosen once per compilation, without regard to any appended suffix (which was therefore treated just like ordinary text), making such attacks more likely to succeed.

%uSUFFIX
     Like %g, but generates a new temporary file name even if %uSUFFIX was already seen.

%USUFFIX
     Substitutes the last file name generated with %uSUFFIX, generating a new one if there is no such last file name.  In the absence of any %uSUFFIX, this is just like %gSUFFIX, except they don't share the same suffix space, so %g.s ... %U.s ... %g.s ... %U.s would involve the generation of two distinct file names, one for each %g.s and another for each %U.s.  Previously, %U was simply substituted with a file name chosen for the previous %u, without regard to any appended suffix.

%jSUFFIX
     Substitutes the name of the HOST_BIT_BUCKET, if any, and if it is writable, and if save-temps is off; otherwise, substitute the name of a temporary file, just like %u.  This temporary file is not meant for communication between processes, but rather as a junk disposal mechanism.

%|SUFFIX
%mSUFFIX
     Like %g, except if -pipe is in effect.  In that case %| substitutes a single dash and %m substitutes nothing at all.  These are the two most common ways to instruct a program that it should read from standard input or write to standard output.  If you need something more elaborate you can use an %{pipe:X} construct: see for example f/lang-specs.h.

%.SUFFIX
     Substitutes .SUFFIX for the suffixes of a matched switch's args when it is subsequently output with %*.  SUFFIX is terminated by the next space or %.

%w
     Marks the argument containing or following the %w as the designated output file of this compilation.  This puts the argument into the sequence of arguments that %o will substitute later.

%o
     Substitutes the names of all the output files, with spaces automatically placed around them.  You should write spaces around the %o as well or the results are undefined. %o is for use in the specs for running the linker. Input files whose names have no recognized suffix are not compiled at all, but they are included among the output files, so they will be linked.

%O
     Substitutes the suffix for object files.  Note that this is handled specially when it immediately follows %g, %u, or %U, because of the need for those to form complete file names.  The handling is such that %O is treated exactly as if it had already been substituted, except that %g, %u, and %U do not currently support additional SUFFIX characters following %O as they would following, for example, .o.

%p
     Substitutes the standard macro predefinitions for the current target machine.  Use this when running cpp.

%P
     Like %p, but puts __ before and after the name of each predefined macro, except for macros that start with __ or with _L, where L is an uppercase letter.  This is for ISO C.

%I
     Substitute any of -iprefix (made from GCC_EXEC_PREFIX), -isysroot (made from TARGET_SYSTEM_ROOT), and -isystem (made from COMPILER_PATH and -B options) as necessary.

%s
     Current argument is the name of a library or startup file of some sort. Search for that file in a standard list of directories and substitute the full name found.

%eSTR
     Print STR as an error message.  STR is terminated by a newline. Use this when inconsistent options are detected.

%(NAME)
     Substitute the contents of spec string NAME at this point.

%[NAME]
     Like %(...) but put __ around -D arguments.

%x{OPTION}
     Accumulate an option for %X.

%X
     Output the accumulated linker options specified by -Wl or a %x spec string.

%Y
     Output the accumulated assembler options specified by -Wa.

%Z
     Output the accumulated preprocessor options specified by -Wp.

%a
     Process the asm spec.  This is used to compute the switches to be passed to the assembler.

%A
     Process the asm_final spec.  This is a spec string for passing switches to an assembler post-processor, if such a program is needed.

%l
     Process the link spec.  This is the spec for computing the command line passed to the linker.  Typically it will make use of the %L %G %S %D and %E sequences.

%D
     Dump out a -L option for each directory that GCC believes might contain startup files.  If the target supports multilibs then the current multilib directory will be prepended to each of these paths.

%M
     Output the multilib directory with directory separators replaced with _.  If multilib directories are not set, or the multilib directory is . then this option emits nothing.

%L
     Process the lib spec.  This is a spec string for deciding which libraries should be included on the command line to the linker.

%G
     Process the libgcc spec.  This is a spec string for deciding which GCC support library should be included on the command line to the linker.

%S
     Process the startfile spec.  This is a spec for deciding which object files should be the first ones passed to the linker.  Typically this might be a file named crt0.o.

%E
     Process the endfile spec.  This is a spec string that specifies the last object files that will be passed to the linker.

%C
     Process the cpp spec.  This is used to construct the arguments to be passed to the C preprocessor.

%1
     Process the cc1 spec.  This is used to construct the options to be passed to the actual C compiler (cc1).

%2
     Process the cc1plus spec.  This is used to construct the options to be passed to the actual C++ compiler (cc1plus).

%*
     Substitute the variable part of a matched option.  See below. Note that each comma in the substituted string is replaced by a single space.

%<S
     Remove all occurrences of -S from the command line.  Note---this command is position dependent.  % commands in the spec string before this one will see -S, % commands in the spec string after this one will not.

%:FUNCTION(ARGS)
     Call the named function FUNCTION, passing it ARGS. ARGS is first processed as a nested spec string, then split into an argument vector in the usual fashion.  The function returns a string which is processed as if it had appeared literally as part of the current spec.

     The following built-in spec functions are provided:

     if-exists
          The if-exists spec function takes one argument, an absolute pathname to a file.  If the file exists, if-exists returns the pathname.  Here is a small example of its usage:

               *startfile:
               crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s

     if-exists-else
          The if-exists-else spec function is similar to the if-exists spec function, except that it takes two arguments.  The first argument is an absolute pathname to a file.  If the file exists, if-exists-else returns the pathname.  If it does not exist, it returns the second argument. This way, if-exists-else can be used to select one file or another, based on the existence of the first.  Here is a small example of its usage:

               *startfile:
               crt0%O%s %:if-exists(crti%O%s) \
               %:if-exists-else(crtbeginT%O%s crtbegin%O%s)

     replace-outfile
          The replace-outfile spec function takes two arguments.  It looks for the first argument in the outfiles array and replaces it with the second argument.  Here is a small example of its usage:

               %{fgnu-runtime:%:replace-outfile(-lobjc -lobjc-gnu)}

%{S}
     Substitutes the -S switch, if that switch was given to GCC. If that switch was not specified, this substitutes nothing.  Note that the leading dash is omitted when specifying this option, and it is automatically inserted if the substitution is performed.  Thus the spec string %{foo} would match the command-line option -foo and would output the command line option -foo.

%W{S}
     Like %{S} but mark last argument supplied within as a file to be deleted on failure.

%{S*}
     Substitutes all the switches specified to GCC whose names start with -S, but which also take an argument.  This is used for switches like -o, -D, -I, etc. GCC considers -o foo as being one switch whose names starts with o.  %{o*} would substitute this text, including the space.  Thus two arguments would be generated.

%{S*&T*}
     Like %{S*}, but preserve order of S and T options (the order of S and T in the spec is not significant). There can be any number of ampersand-separated variables; for each the wild card is optional.  Useful for CPP as %{D*&U*&A*}.

%{S:X}
     Substitutes X, if the -S switch was given to GCC.

%{!S:X}
     Substitutes X, if the -S switch was not given to GCC.

%{S*:X}
     Substitutes X if one or more switches whose names start with -S are specified to GCC.  Normally X is substituted only once, no matter how many such switches appeared.  However, if %* appears somewhere in X, then X will be substituted once for each matching switch, with the %* replaced by the part of that switch that matched the *.

%{.S:X}
     Substitutes X, if processing a file with suffix S.

%{!.S:X}
     Substitutes X, if not processing a file with suffix S.

%{S|P:X}
     Substitutes X if either -S or -P was given to GCC. This may be combined with !, ., and * sequences as well, although they have a stronger binding than the |.  If %* appears in X, all of the alternatives must be starred, and only the first matching alternative is substituted.

     For example, a spec string like this:

          %{.c:-foo} %{!.c:-bar} %{.c|d:-baz} %{!.c|d:-boggle}

     will output the following command-line options from the following input command-line options:

          fred.c        -foo -baz
          jim.d         -bar -boggle
          -d fred.c     -foo -baz -boggle
          -d jim.d      -bar -baz -boggle

%{S:X; T:Y; :D}

     If S was given to GCC, substitutes X; else if T was given to GCC, substitutes Y; else substitutes D.  There can be as many clauses as you need.  This may be combined with ., !, |, and * as needed.

The conditional text X in a %{S:X} or similar construct may contain other nested % constructs or spaces, or even newlines.  They are processed as usual, as described above. Trailing white space in X is ignored.  White space may also appear anywhere on the left side of the colon in these constructs, except between . or * and the corresponding word.

The -O, -f, -m, and -W switches are handled specifically in these constructs.  If another value of -O or the negated form of a -f, -m, or -W switch is found later in the command line, the earlier switch value is ignored, except with {S*} where S is just one letter, which passes all matching options.

The character | at the beginning of the predicate text is used to indicate that a command should be piped to the following command, but only if -pipe is specified.

It is built into GCC which switches take arguments and which do not. (You might think it would be useful to generalize this to allow each compiler's spec to say which switches take arguments.  But this cannot be done in a consistent fashion.  GCC cannot even decide which input files have been specified without knowing which switches take arguments, and it must know which input files to compile in order to tell which compilers to run).

GCC also knows implicitly that arguments starting in -l are to be treated as compiler output files, and passed to the linker in their proper position among the other output files.

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.  Sometimes this is inconvenient, so GCC provides options that will switch to another cross-compiler or version.

-b MACHINE
     The argument MACHINE specifies the target machine for compilation.

     The value to use for MACHINE is the same as was specified as the machine type when configuring GCC as a cross-compiler.  For example, if a cross-compiler was configured with configure i386v, meaning to compile for an 80386 running System V, then you would specify -b i386v to run that cross compiler.

-V VERSION
     The argument VERSION specifies which version of GCC to run. This is useful when multiple versions are installed.  For example, VERSION might be 2.0, meaning to run GCC version 2.0.

The -V and -b options work by running the <machine>-gcc-<version> executable, so there's no real reason to use them if you can just run that directly.

Hardware Models and Configurations
==================================

Earlier we discussed the standard option -b which chooses among different installed compilers for completely different target machines, such as VAX vs. 68000 vs. 80386.

In addition, each of these 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.

These options are defined by the macro TARGET_SWITCHES in the machine description.  The default for the options is also defined by that macro, which enables you to change the defaults.

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 will create an object file for the single architecture that it was built to target.  Apple's GCC on Darwin does create ``fat'' files if multiple -arch options are used.

The subtype of the file created (PPC7400 or PPC970 or I686) is determined by the flag that specifies the ISA that GCC is targetting, -mcpu or -march.  The -force_cpusubtype_ALL option can be used to override this.

-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.

     A framework directory is a directory with frameworks in it.  A framework is a directory with a "Headers" and/or "PrivateHeaders" directory contained directly in it that ends in ".framework".  The name of a framework is the name of this directory excluding the ".framework".  Headers associated with the framework are found in one of those two directories, with "Headers" being searched first.  A subframework is a framework directory that is in a framework's "Frameworks" directory. Includes of subframework headers can only appear in a header of a framework that contains the subframework, or in a sibling subframework header.  Two subframeworks are siblings if they occur in the same framework.  A subframework should not have the same name as a framework, a warning will be issued if this is violated.  Currently a subframework cannot have subframeworks, in the future, the mechanism may be extended to support this.  The standard frameworks can be found in "/System/Library/Frameworks" and "/Library/Frameworks".  An example include looks like #include <Framework/header.h>, where Framework denotes the name of the framework and header.h is found in the "PrivateHeaders" or "Headers" directory.

-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.

-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.

     Warning: The -mone-byte-bool switch causes GCC to generate code that is not binary compatible with code generated without that switch.  Using this switch may require recompiling all other modules in a program, including system libraries.  Use this switch to conform to a non-default data model.

-mfix-and-continue
-ffix-and-continue
-findirect-data
     Generate code suitable for fast turn around development.  Needed to enable gdb to dynamically load .o files into already running programs.  -findirect-data and -ffix-and-continue are provided for backwards compatibility.

-fapple-kext
-findirect-virtual-calls
-fterminated-vtables
     Alter vtables, destructors, and other implementation details to more closely resemble the GCC 2.95 ABI.  This is to make kernel extensions loadable by Darwin kernels, and is required to build any Darwin kernel extension.  In addition, virtual calls are not made directly, instead, code is generated to always go through the virtual table, as virtual tables can be patched by the kernel module loader.  Vtables are altered by adding a zero word at the end of every vtable. -fno-exceptions and -static must also be used with this flag.  -findirect-virtual-calls and -fterminated-vtables are accepted for backwards compatibility but will be removed in the future.  (APPLE ONLY)

-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 be loading the build output file being linked. See man ld(1) for more information.

-dynamiclib
     When passed this option, GCC will produce 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.

Intel 386 and AMD x86-64 Options
--------------------------------

These -m options are defined for the i386 and x86-64 family of computers:

-mtune=CPU-TYPE
     Tune to CPU-TYPE everything applicable about the generated code, except for the ABI and the set of available instructions.  The choices for CPU-TYPE are:

     i386
          Original Intel's i386 CPU.

     i486
          Intel's i486 CPU.  (No scheduling is implemented for this chip.)

     i586, pentium
          Intel Pentium CPU with no MMX support.

     pentium-mmx
          Intel PentiumMMX CPU based on Pentium core with MMX instruction set support.

     i686, pentiumpro
          Intel PentiumPro CPU.

     pentium2
          Intel Pentium2 CPU based on PentiumPro core with MMX instruction set support.

     pentium3, pentium3m
          Intel Pentium3 CPU based on PentiumPro core with MMX and SSE instruction set support.

     pentium-m
          Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2 instruction set support.  Used by Centrino notebooks.

     pentium4, pentium4m
          Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set support.

     prescott
          Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support.

     nocona
          Improved version of Intel Pentium4 CPU with 64-bit extensions, MMX, SSE, SSE2 and SSE3 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
          AMD K8 core based CPUs with x86-64 instruction set support.  (This supersets MMX, SSE, SSE2, 3dNOW!, enhanced 3dNOW! 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 Winchip2 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 CPU with MMX and SSE instruction set support.  (No scheduling is implemented for this chip.)

     While picking a specific CPU-TYPE will schedule things appropriately for that particular chip, the compiler will not generate any code that does not run on the i386 without the -march=CPU-TYPE option being used.

-march=CPU-TYPE
     Generate instructions for the machine type CPU-TYPE.  The choices for CPU-TYPE are the same as for -mtune.  Moreover, specifying -march=CPU-TYPE implies -mtune=CPU-TYPE.

-mcpu=CPU-TYPE
     A deprecated synonym for -mtune.

-m386
-m486
-mpentium
-mpentiumpro
     These options are synonyms for -mtune=i386, -mtune=i486, -mtune=pentium, and -mtune=pentiumpro respectively. These synonyms are deprecated.

-mfpmath=UNIT
     Generate floating point arithmetics for selected unit UNIT.  The choices for UNIT are:

     387
          Use the standard 387 floating point coprocessor present majority of chips and emulated otherwise.  Code compiled with this option will run almost everywhere. The temporary results are computed in 80bit precision instead of precision specified by the type resulting in slightly different results compared to most of other chips. See -ffloat-store for more detailed description.

          This is the default choice for i386 compiler.

     sse
          Use scalar floating point instructions present in the SSE instruction set. This instruction set is supported by Pentium3 and newer chips, in the AMD line by Athlon-4, Athlon-xp and Athlon-mp chips.  The earlier version of SSE instruction set supports only single precision arithmetics, thus the double and extended precision arithmetics is still done using 387.  Later version, present only in Pentium4 and the future AMD x86-64 chips supports double precision arithmetics too.

          For i387 you need to use -march=CPU-TYPE, -msse or -msse2 switches to enable SSE extensions and make this option effective.  For x86-64 compiler, these extensions are enabled by default.

          The resulting code should be considerably faster in the majority of cases and avoid the numerical instability problems of 387 code, but may break some existing code that expects temporaries to be 80bit.

          This is the default choice for the x86-64 compiler.

     sse,387
          Attempt to utilize both instruction sets at once.  This effectively double 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 instable performance.

-masm=DIALECT
     Output asm instructions using selected DIALECT. Supported choices are intel or att (the default one).

-mieee-fp
-mno-ieee-fp
     Control whether or not the compiler uses IEEE floating point comparisons.  These handle correctly the case where the result of a comparison is unordered.

-msoft-float
     Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation.  You must make your own arrangements to provide suitable library functions for cross-compilation.

     On machines where a function returns floating point results in the 80387 register stack, some floating point opcodes may be emitted even if -msoft-float is used.

-mno-fp-ret-in-387
     Do not use the FPU registers for return values of functions.

     The usual calling convention has functions return values of types float and double in an FPU register, even if there is no FPU.  The idea is that the operating system should emulate an FPU.

     The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead.

-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 will always have an FPU and so the instruction will not need emulation.  As of revision 2.6.1, 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 will produce code that runs somewhat faster on a Pentium at the expense of more memory.

     Warning: if you use the -malign-double switch, structures containing the above types will be aligned differently than the published application binary interface specifications for the 386 and will not be binary compatible with structures in code compiled without that switch.

-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.

     Modern architectures (Pentium and newer) would prefer long double to be aligned to an 8 or 16 byte boundary.  In arrays or structures conforming to the ABI, this would not be possible.  So specifying a -m128bit-long-double will align long double to a 16 byte boundary by padding the long double with an additional 32 bit zero.

     In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that long double is to be aligned on 16 byte boundary.

     Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a long double.

     Warning: if you override the default value for your target ABI, the structures and arrays containing long double variables will change their size as well as function calling convention for function taking long double will be modified.  Hence they will not be binary compatible with arrays or structures in code compiled without that switch.

-msvr3-shlib
-mno-svr3-shlib
     Control whether GCC places uninitialized local variables into the bss or data segments.  -msvr3-shlib places them into bss.  These options are meaningful only on System V Release 3.

-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.

     You can specify that an individual function is called with this calling sequence with the function attribute stdcall.  You can also override the -mrtd option by using the function attribute cdecl.  See Function Attributes.

     Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.

     Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code will be generated for calls to those functions.

     In addition, seriously incorrect code will result if you call a function with too many arguments.  (Normally, extra arguments are harmlessly ignored.)

-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. See Function Attributes.

     Warning: if you use this switch, and NUM is nonzero, then you must build all modules with the same value, including any libraries.  This includes the system libraries and startup modules.

-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), except when optimizing for code size (-Os), in which case the default is the minimum correct alignment (4 bytes for x86, and 8 bytes for x86-64).

     On Pentium and PentiumPro, double and long double values should be aligned to an 8 byte boundary (see -malign-double) or suffer significant run time performance penalties.  On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 suffers similar penalties if it is not 16 byte aligned.

     To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned.  Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary will most likely misalign the stack.  It is recommended that libraries that use callbacks always use the default setting.

     This extra alignment does consume extra stack space, and generally increases code size.  Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to -mpreferred-stack-boundary=2.

-mmmx
-mno-mmx

-msse
-mno-sse

-msse2
-mno-sse2

-msse3
-mno-sse3

-m3dnow
-mno-3dnow
     These switches enable or disable the use of built-in functions that allow direct access to the MMX, SSE, SSE2, SSE3 and 3Dnow extensions of the instruction set.

     See X86 Built-in Functions, for details of the functions enabled and disabled by these switches.

     To have SSE/SSE2 instructions generated automatically from floating-point code, see -mfpmath=sse.

-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 will be computed in the function prologue.  This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when 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 Mingw32.  Code that relies 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 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 destination is known to be aligned at least to 4 byte boundary.  This enables more inlining, increase code size, but may improve performance of code that depends on fast memcpy, strlen and memset for short lengths.

-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.

-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 legal depends on the operating system, and whether it maps the segment to cover the entire TLS area.

     For systems that use GNU libc, the default is on.

These -m switches are supported in addition to the above on AMD x86-64 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 and generates code that runs on any i386 system. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits and generates code for AMD's x86-64 architecture.

-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 will not be 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 but symbols can be located anywhere in the address space.  Programs can be statically or dynamically linked, but building of shared libraries are not supported with the medium model.

-mcmodel=large
     Generate code for the large model: This model makes no assumptions about addresses and sizes of sections.  Currently GCC does not implement this model.

PowerPC Options
---------------

These are listed under see RS/6000 and PowerPC Options.

IBM RS/6000 and PowerPC Options
-------------------------------

These -m options are defined for the IBM RS/6000 and PowerPC:

-mpower
-mno-power
-mpower2
-mno-power2
-mpowerpc
-mno-powerpc
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
     GCC supports two related instruction set architectures for the RS/6000 and PowerPC.  The POWER instruction set are those instructions supported by the rios chip set used in the original RS/6000 systems and the PowerPC instruction set is the architecture of the Motorola MPC5xx, MPC6xx, MPC8xx microprocessors, and the IBM 4xx microprocessors.

     Neither architecture is a subset of the other.  However there is a large common subset of instructions supported by both.  An MQ register is included in processors supporting the POWER architecture.

     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.

     The -mpower option allows GCC to generate instructions that are found only in the POWER architecture and to use the MQ register. Specifying -mpower2 implies -power and also allows GCC to generate instructions that are present in the POWER2 architecture but not the original POWER architecture.

     The -mpowerpc option allows GCC to generate instructions that are found only in the 32-bit subset of the PowerPC architecture. Specifying -mpowerpc-gpopt implies -mpowerpc and also allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root.  Specifying -mpowerpc-gfxopt implies -mpowerpc and also allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select.

     The -mpowerpc64 option allows GCC to generate the additional 64-bit instructions that are found in the full PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities.  GCC defaults to -mno-powerpc64.

     If you specify both -mno-power and -mno-powerpc, GCC will use only the instructions in the common subset of both architectures plus some special AIX common-mode calls, and will not use the MQ register.  Specifying both -mpower and -mpowerpc permits GCC to use any instruction from either architecture and to allow use of the MQ register; specify this for the Motorola MPC601.

-mnew-mnemonics
-mold-mnemonics
     Select which mnemonics to use in the generated assembler code.  With -mnew-mnemonics, GCC uses the assembler mnemonics defined for the PowerPC architecture.  With -mold-mnemonics it uses the assembler mnemonics defined for the POWER architecture.  Instructions defined in only one architecture have only one mnemonic; GCC uses that mnemonic irrespective of which of these options is specified.

     GCC defaults to the mnemonics appropriate for the architecture in use.  Specifying -mcpu=CPU_TYPE sometimes overrides the value of these option.  Unless you are building a cross-compiler, you should normally not specify either -mnew-mnemonics or -mold-mnemonics, but should instead accept the default.

-mcpu=CPU_TYPE
     Set architecture type, register usage, choice of mnemonics, and instruction scheduling parameters for machine type CPU_TYPE. Supported values for CPU_TYPE are 401, 403, 405, 405fp, 440, 440fp, 505, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, common, ec603e, G3, G4, G5, power, power2, power3, power4, power5, powerpc, powerpc64, rios, rios1, rios2, rsc, and rs64a.

     -mcpu=common selects a completely generic processor.  Code generated under this option will run on any POWER or PowerPC processor. GCC will use only the instructions in the common subset of both architectures, and will not use the MQ register.  GCC assumes a generic processor model for scheduling purposes.

     -mcpu=power, -mcpu=power2, -mcpu=powerpc, and -mcpu=powerpc64 specify generic POWER, POWER2, pure 32-bit PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes.

     The other options specify a specific processor.  Code generated under those options will run best on that processor, and may not run at all on others.

     The -mcpu options automatically enable or disable the following options: -maltivec, -mhard-float, -mmfcrf, -mmultiple, -mnew-mnemonics, -mpower, -mpower2, -mpowerpc64, -mpowerpc-gpopt, -mpowerpc-gfxopt, -mstring.  The particular options set for any particular CPU will vary between compiler versions, depending on what setting seems to produce optimal code for that CPU; it doesn't necessarily reflect the actual hardware's capabilities.  If you wish to set an individual option to a particular value, you may specify it after the -mcpu option, like -mcpu=970 -mno-altivec.

     On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu option at present, since AIX does not have full support for these options.  You may still enable or disable them individually if you're sure it'll work in your environment.

-mtune=CPU_TYPE
     Set the instruction scheduling parameters for machine type CPU_TYPE, but do not set the architecture type, register usage, or choice of mnemonics, as -mcpu=CPU_TYPE would.  The same values for CPU_TYPE are used for -mtune as for -mcpu.  If both are specified, the code generated will use the architecture, registers, and mnemonics set by -mcpu, but the scheduling parameters set by -mtune.

-maltivec
-mno-altivec
     These switches enable or disable the use of built-in functions that allow access to the AltiVec instruction set.  You may also need to set -mabi=altivec to adjust the current ABI with AltiVec ABI enhancements.

-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 Booke SPE ABI extensions for the current ABI.

-misel=YES/NO
-misel
     This switch enables or disables the generation of ISEL instructions.

-mspe=YES/NO
-mspe
     This switch enables or disables the generation of SPE simd instructions.

-mfloat-gprs=YES/NO
-mfloat-gprs
     This switch enables or disables the generation of floating point operations on the general purpose registers for architectures that support it.  This option is currently only available on the MPC8540.

-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 will allocate at least one TOC entry for each unique non-automatic variable reference in your program.  GCC will also place floating-point constants in the TOC.  However, only 16,384 entries are available in the TOC.

     If you receive a linker error message that saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC and -mno-sum-in-toc forces GCC to generate code to calculate the sum of an address and a constant at run-time instead of putting that sum into the TOC.  You may specify one or both of these options.  Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space.

     If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc instead.  This option causes GCC to make only one TOC entry for every file.  When you specify this option, GCC will produce code that is slower and larger but which uses extremely little TOC space.  You may wish to use this option only on files that contain less frequently executed code.

-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 and -mpowerpc, while -maix32 disables the 64-bit ABI and implies -mno-powerpc64.  GCC defaults to -maix32.

-mxl-call
-mno-xl-call
     On AIX, pass floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs.  The AIX calling convention was extended but not initially documented to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than declared.  AIX XL compilers access floating point arguments which do not fit in the RSA from the stack when a subroutine is compiled without optimization.  Because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default and only is necessary when calling subroutines compiled by AIX XL compilers without optimization.

-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, 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.

-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 the instructions usage 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 the instructions usage 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.

-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 is used.

-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.

     For example, by default a structure containing nothing but 8 unsigned bit-fields of length 1 would be aligned to a 4 byte boundary and have a size of 4 bytes.  By using -mno-bit-align, the structure would be aligned to a 1 byte boundary and be one byte in size.

-mno-strict-align
-mstrict-align
     On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references will be handled by the system.

-mrelocatable
-mno-relocatable
     On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a different address at runtime.  If you use -mrelocatable on any module, all objects linked together must be compiled with -mrelocatable or -mrelocatable-lib.

-mrelocatable-lib
-mno-relocatable-lib
     On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a different address at runtime.  Modules compiled with -mrelocatable-lib can be linked with either modules compiled without -mrelocatable and -mrelocatable-lib or with modules compiled with 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.

-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 0/1/2 to assign NO/HIGHEST/SECOND-HIGHEST 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 which latency >= NUMBER is costly.

-minsert-sched-nops=SCHEME
     This option controls which nop insertion scheme will be 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 which 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 adheres 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
     Specify both -mcall-sysv and -meabi options.

-mcall-sysv-noeabi
     Specify both -mcall-sysv and -mno-eabi options.

-mcall-solaris
     On System V.4 and embedded PowerPC systems compile code for the Solaris operating system.

-mcall-linux
     On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system.

-mcall-gnu
     On System V.4 and embedded PowerPC systems compile code for the Hurd-based GNU system.

-mcall-netbsd
     On System V.4 and embedded PowerPC systems compile code for the NetBSD 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=altivec
     Extend the current ABI with AltiVec ABI extensions.  This does not change the default ABI, instead it adds the AltiVec ABI extensions to the current ABI.

-mabi=no-altivec
     Disable AltiVec ABI extensions for the current ABI.

-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 were passed in the floating point registers in case the function takes a variable arguments.  With -mprototype, only calls to prototyped variable argument functions will 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.

-mwindiss
     Specify that you are compiling for the WindISS simulation environment.

-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 to 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, do not call an initialization function from main, and the -msdata option will only use 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 and static data in the .sdata section.  Put small uninitialized global and static 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.

-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
     Default to making all function calls indirectly, using a register, so that functions which reside further than 32 megabytes (33,554,432 bytes) from the current location can be called.  This setting can be overridden by the shortcall function attribute, or by #pragma longcall(0).

     Some linkers are capable of detecting out-of-range calls and generating glue code on the fly.  On these systems, long calls are unnecessary and generate slower code.  As of this writing, the AIX linker can do this, as can the GNU linker for PowerPC/64.  It is planned to add this feature to the GNU linker for 32-bit PowerPC systems as well.

     On Darwin/PPC systems, #pragma longcall will generate ``jbsr callee, L42'', plus a ``branch island'' (glue code).  The two target addresses represent the callee and the ``branch island.'' The Darwin/PPC linker will prefer the first address and generate a ``bl callee'' if the PPC ``bl'' instruction will reach the callee directly; otherwise, the linker will generate ``bl L42'' to call the ``branch island.''  The ``branch island'' is appended to the body of the calling function; it computes the full 32-bit address of the callee and jumps to it.

     On Mach-O (Darwin) systems, this option directs the compiler emit to the glue for every direct call, and the Darwin linker decides whether to use or discard it.

     In the future, we may cause GCC to ignore all longcall specifications when the linker is known to generate glue.

-pthread
     Adds support for multithreading with the pthreads library. This option sets flags for both the preprocessor and linker.

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 would be -fno-foo.  In the table below, only one of the forms is listed---the one which 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 77 front-ends, where this option defaults to true and false respectively.

-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 other.  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 will generate 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 will enable it by default for languages like C++ which normally require exception handling, and disable 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.

-funwind-tables
     Similar to -fexceptions, except that it will just generate any needed static data, but will not affect the generated code in any other way. You will normally not enable this option; instead, a language processor that needs this handling would enable it on your behalf.

-fasynchronous-unwind-tables
     Generate unwind table in dwarf2 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).

     The precise convention for returning structures in memory depends on the target configuration macros.

     Short structures and unions are those whose size and alignment match that of some integer type.

     Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled with the -freg-struct-return switch. Use it to conform to a non-default application binary interface.

-freg-struct-return
     Return struct and union values in registers when possible. This is more efficient for small structures than -fpcc-struct-return.

     If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target.  If there is no standard convention, GCC defaults to -fpcc-struct-return, except on targets where GCC is the principal compiler.  In those cases, we can choose the standard, and we chose the more efficient register return alternative.

     Warning: code compiled with the -freg-struct-return switch is not binary compatible with code compiled with the -fpcc-struct-return switch. Use it to conform to a non-default application binary interface.

-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 will be equivalent to the smallest integer type which has enough room.

     Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

-fshort-double
     Use the same size for double as for float.

     Warning: the -fshort-double switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

-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.

     Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

-fshared-data
     Requests that the data and non-const variables of this compilation be shared data rather than private data.  The distinction makes sense only on certain operating systems, where shared data is shared between processes running the same program, while private data exists in one copy per process.

-fno-common
     In C, allocate even 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 will get an error when you link them. The only reason this might be useful is if you wish to verify that the program will work on other systems which always work 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).

     -fno-verbose-asm, the default, causes the extra information to be omitted and is useful when comparing two assembler files.

-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.)

     Position-independent code requires special support, and therefore works only on certain machines.  For the 386, GCC supports PIC for System V but not for the Sun 386i.  Code generated for the IBM RS/6000 is always position-independent.

     -fpic is not supported on Mac OS X.

-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 and the SPARC.

     Position-independent code requires special support, and therefore works only on certain machines.

     -fPIC is the default on Darwin and Mac OS X.

-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 will be used during linking.

-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).

     REG must be the name of a register.  The register names accepted are machine-specific and are defined in the REGISTER_NAMES macro in the machine description macro file.

     This flag does not have a negative form, because it specifies a three-way choice.

-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 will not save and restore the register REG.

     It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model will produce disastrous results.

     This flag does not have a negative form, because it specifies a three-way choice.

-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 will save and restore the register REG if they use it.

     It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model will produce disastrous results.

     A different sort of disaster will result from the use of this flag for a register in which function values may be returned.

     This flag does not have a negative form, because it specifies a three-way choice.

-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 will be output potentially unaligned at the next fitting location.

     Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary interface.

-finstrument-functions
     Generate instrumentation calls for entry and exit to functions.  Just after function entry and just before function exit, the following profiling functions will be 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);

     The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table.

     This instrumentation is also done for functions expanded inline in other functions.  The profiling calls will indicate where, conceptually, the inline function is entered and exited.  This means that addressable versions of such functions must be available.  If all your uses of a function are expanded inline, this may mean an additional expansion of code size.  If you use extern inline in your C code, an addressable version of such functions must be provided.  (This is normally the case anyways, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.)

     A function may be given the attribute no_instrument_function, in which case this instrumentation will not be done.  This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called (perhaps signal handlers, if the profiling routines generate output or allocate memory).

-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 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.

     Note that this switch does not actually cause checking to be done; the operating system must do that.  The switch causes generation of code to ensure that the operating system sees the stack being extended.

-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 the stack would grow beyond the value, a signal is raised.  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.

     For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB.  Note that this may only work with the GNU linker.

-fargument-alias
-fargument-noalias
-fargument-noalias-global
     Specify the possible relationships among parameters and between parameters and global data.

     -fargument-alias specifies that arguments (parameters) may alias each other and may alias global storage.

     -fargument-noalias specifies that arguments do not alias each other, but may alias global storage.

     -fargument-noalias-global specifies that arguments do not alias each other and do not alias global storage.

     Each language will automatically use whatever option is required by the language standard.  You should not need to use these options yourself.

-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.

     Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with code generated without that switch.  Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch.

-ftls-model=MODEL
     Alter the thread-local storage model to be used (see Thread-Local). The MODEL argument should be one of global-dynamic, local-dynamic, initial-exec or local-exec.

     The default without -fpic is initial-exec; with -fpic the default is global-dynamic.

-fvisibility=DEFAULT|INTERNAL|HIDDEN|PROTECTED
     Set the default ELF image symbol visibility to the specified option - all symbols will be 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.

     Despite the nomenclature, default always means public ie; available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage so the only other commonly used option will be hidden. The default if -fvisibility isn't specified is default ie; make every symbol public - this causes the same behavior as previous versions of GCC.

     A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by ``How To Write Shared Libraries'' by Ulrich Drepper (which can be found at http://people.redhat.com/~drepper/) - however a superior solution made possible by this option to marking things hidden when the default is public is to make the default hidden and mark things public. This is the norm with DLL's on Windows and with -fvisibility=hidden and __attribute__ ((visibility("default"))) instead of __declspec(dllexport) you get almost identical semantics with identical syntax. This is a great boon to those working with cross-platform projects.

     For those adding visibility support to existing code, you may find #pragma GCC visibility of use. This works by you enclosing the declarations you wish to set visibility for with (for example) #pragma GCC visibility push(hidden) and #pragma GCC visibility pop. These can be nested up to sixteen times. Bear in mind that symbol visibility should be viewed as part of the API interface contract and thus all new code should always specify visibility when it is not the default ie; declarations only for use within the local DSO should always be marked explicitly as hidden as so to avoid PLT indirection overheads - making this abundantly clear also aids readability and self-documentation of the code. Note that due to ISO C++ specification requirements, operator new and operator delete must always be of default visibility.

     An overview of these techniques, their benefits and how to use them is at http://www.nedprod.com/programs/gccvisibility.html.

Environment Variables Affecting GCC
===================================

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 (see Directory Options).  These take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GCC. See Controlling the Compilation Driver gcc (GNU Compiler Collection (GCC) Internals).

LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
     These environment variables control the way that GCC uses localization information that allow 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.

     The LC_CTYPE environment variable specifies character classification.  GCC uses it to determine the character boundaries in a string; this is needed for some multibyte encodings that contain quote and escape characters that would otherwise be interpreted as a string end or escape.

     The LC_MESSAGES environment variable specifies the language to use in diagnostic messages.

     If the LC_ALL environment variable is set, it overrides the value of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable.  If none of these variables are set, GCC defaults to traditional C English behavior.

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_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.

     If GCC_EXEC_PREFIX is not set, GCC will attempt to figure out an appropriate prefix to use based on the pathname it was invoked with.

     If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram.

     The default value of GCC_EXEC_PREFIX is PREFIX/lib/gcc/ where PREFIX is the value of prefix when you ran the configure script.

     Other prefixes specified with -B take precedence over this prefix.

     This prefix is also used for finding files such as crt0.o that are used for linking.

     In addition, the prefix is used in an unusual way in finding the directories to search for header files.  For each of the standard directories whose name normally begins with /usr/local/lib/gcc (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name.  Thus, with -Bfoo/, GCC will search foo/bar where it would normally search /usr/local/lib/bar. These alternate directories are searched first; the standard directories come next.

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.

     If LANG is not defined, or if it has some other value, then the compiler will use mblen and mbtowc as defined by the default locale to recognize and translate multibyte characters.

Some additional environments variables affect the behavior of the preprocessor.

     [file cppenv.texi not included]

Using Precompiled Headers
=========================

Often large projects have many header files that are included in every source file.  The time the compiler takes to process these header files over and over again can account for nearly all of the time required to build the project.  To make builds faster, GCC allows users to `precompile' a header file; then, if builds can use the precompiled header file they will be much faster.

Caution: There are a few known situations where GCC will crash when trying to use a precompiled header.  If you have trouble with a precompiled header, you should remove the precompiled header and compile without it.  In addition, please use GCC's on-line defect-tracking system to report any problems you encounter with precompiled headers.  See Bugs.

To create a precompiled header file, simply compile it as you would any other file, if necessary using the -x option to make the driver treat it as a C or C++ header file.  You will probably want to use a tool like make to keep the precompiled header up-to-date when the headers it contains change.

A precompiled header file will be searched for when #include is seen in the compilation.  As it searches for the included file (see Search Path (The C Preprocessor)) the compiler looks for a precompiled header in each directory just before it looks for the include file in that directory.  The name searched for is the name specified in the #include with .gch appended.  If the precompiled header file can't be used, it is ignored.

For instance, if you have #include "all.h", and you have all.h.gch in the same directory as all.h, then the precompiled header file will be used if possible, and the original header will be used otherwise.

Alternatively, you might decide to put the precompiled header file in a directory and use -I to ensure that directory is searched before (or instead of) the directory containing the original header. Then, if you want to check that the precompiled header file is always used, you can put a file of the same name as the original header in this directory containing an #error command.

This also works with -include.  So yet another way to use precompiled headers, good for projects not designed with precompiled header files in mind, is to simply take most of the header files used by a project, include them from another header file, precompile that header file, and -include the precompiled header.  If the header files have guards against multiple inclusion, they will be skipped because they've already been included (in the precompiled header).

If you need to precompile the same header file for different languages, targets, or compiler options, you can instead make a directory named like all.h.gch, and put each precompiled header in the directory, perhaps using -o.  It doesn't matter what you call the files in the directory, every precompiled header in the directory will be considered.  The first precompiled header encountered in the directory that is valid for this compilation will be used; they're searched in no particular order.

There are many other possibilities, limited only by your imagination, good sense, and the constraints of your build system.

A precompiled header file can be used only when these conditions apply:

  * Only one precompiled header can be used in a particular compilation.

  * A precompiled header can't be used once the first C token is seen.  You can have preprocessor directives before a precompiled header; you can even include a precompiled header from inside another header, so long as there are no C tokens before the #include.

  * The precompiled header file must be produced for the same language as the current compilation.  You can't use a C precompiled header for a C++ compilation.

  * The precompiled header file must be produced by the same compiler version and configuration as the current compilation is using. The easiest way to guarantee this is to use the same compiler binary for creating and using precompiled headers.

  * Any macros defined before the precompiled header is included must either be defined in the same way as when the precompiled header was generated, or must not affect the precompiled header, which usually means that the they don't appear in the precompiled header at all.

     The -D option is one way to define a macro before a precompiled header is included; using a #define can also do it. There are also some options that define macros implicitly, like -O and -Wdeprecated; the same rule applies to macros defined this way.

  * If debugging information is output when using the precompiled header, using -g or similar, the same kind of debugging information must have been output when building the precompiled header.  However, a precompiled header built using -g can be used in a compilation when no debugging information is being output.

  * The same -m options must generally be used when building and using the precompiled header.  See Submodel Options, for any cases where this rule is relaxed.

  * Each of the following options must be the same when building and using the precompiled header:

     -fexceptions -funit-at-a-time

  * Some other command-line options starting with -f, -p, or -O must be defined in the same way as when the precompiled header was generated.  At present, it's not clear which options are safe to change and which are not; the safest choice is to use exactly the same options when generating and using the precompiled header.  The following are known to be safe:

     -fpreprocessed -pedantic-errors

For all of these except the last, the compiler will automatically ignore the precompiled header if the conditions aren't met.  If you find an option combination that doesn't work and doesn't cause the precompiled header to be ignored, please consider filing a bug report, see Bugs.

If you do use differing options when generating and using the precompiled header, the actual behavior will be a mixture of the behavior for the options.  For instance, if you use -g to generate the precompiled header but not when using it, you may or may not get debugging information for routines in the precompiled header.

Running Protoize
================

The program protoize is an optional part of GCC.  You can use it to add prototypes to a program, thus converting the program to ISO C in one respect.  The companion program unprotoize does the reverse: it removes argument types from any prototypes that are found.

When you run these programs, you must specify a set of source files as command line arguments.  The conversion programs start out by compiling these files to see what functions they define.  The information gathered about a file FOO is saved in a file named FOO.X.

After scanning comes actual conversion.  The specified files are all eligible to be converted; any files they include (whether sources or just headers) are eligible as well.

But not all the eligible files are converted.  By default, protoize and unprotoize convert only source and header files in the current directory.  You can specify additional directories whose files should be converted with the -d DIRECTORY option.  You can also specify particular files to exclude with the -x FILE option.  A file is converted if it is eligible, its directory name matches one of the specified directory names, and its name within the directory has not been excluded.

Basic conversion with protoize consists of rewriting most function definitions and function declarations to specify the types of the arguments.  The only ones not rewritten are those for varargs functions.

protoize optionally inserts prototype declarations at the beginning of the source file, to make them available for any calls that precede the function's definition.  Or it can insert prototype declarations with block scope in the blocks where undeclared functions are called.

Basic conversion with unprotoize consists of rewriting most function declarations to remove any argument types, and rewriting function definitions to the old-style pre-ISO form.

Both conversion programs print a warning for any function declaration or definition that they can't convert.  You can suppress these warnings with -q.

The output from protoize or unprotoize replaces the original source file.  The original file is renamed to a name ending with .save (for DOS, the saved filename ends in .sav without the original .c suffix).  If the .save (.sav for DOS) file already exists, then the source file is simply discarded.

protoize and unprotoize both depend on GCC itself to scan the program and collect information about the functions it uses. So neither of these programs will work until GCC is installed.

Here is a table of the options you can use with protoize and unprotoize.  Each option works with both programs unless otherwise stated.

-B DIRECTORY
     Look for the file SYSCALLS.c.X in DIRECTORY, instead of the usual directory (normally /usr/local/lib).  This file contains prototype information about standard system functions.  This option applies only to protoize.

-c COMPILATION-OPTIONS
     Use COMPILATION-OPTIONS as the options when running gcc to produce the .X files.  The special option -aux-info is always passed in addition, to tell gcc to write a .X file.

     Note that the compilation options must be given as a single argument to protoize or unprotoize.  If you want to specify several gcc options, you must quote the entire set of compilation options to make them a single word in the shell.

     There are certain gcc arguments that you cannot use, because they would produce the wrong kind of output.  These include -g, -O, -c, -S, and -o If you include these in the COMPILATION-OPTIONS, they are ignored.

-C
     Rename files to end in .C (.cc for DOS-based file systems) instead of .c.  This is convenient if you are converting a C program to C++.  This option applies only to protoize.

-g
     Add explicit global declarations.  This means inserting explicit declarations at the beginning of each source file for each function that is called in the file and was not declared.  These declarations precede the first function definition that contains a call to an undeclared function.  This option applies only to protoize.

-i STRING
     Indent old-style parameter declarations with the string STRING. This option applies only to protoize.

     unprotoize converts prototyped function definitions to old-style function definitions, where the arguments are declared between the argument list and the initial {.  By default, unprotoize uses five spaces as the indentation.  If you want to indent with just one space instead, use -i " ".

-k
     Keep the .X files.  Normally, they are deleted after conversion is finished.

-l
     Add explicit local declarations.  protoize with -l inserts a prototype declaration for each function in each block which calls the function without any declaration.  This option applies only to protoize.

-n
     Make no real changes.  This mode just prints information about the conversions that would have been done without -n.

-N
     Make no .save files.  The original files are simply deleted. Use this option with caution.

-p PROGRAM
     Use the program PROGRAM as the compiler.  Normally, the name gcc is used.

-q
     Work quietly.  Most warnings are suppressed.

-v
     Print the version number, just like -v for gcc.

If you need special compiler options to compile one of your program's source files, then you should generate that file's .X file specially, by running gcc on that source file with the appropriate options and the option -aux-info.  Then run protoize on the entire set of files.  protoize will use the existing .X file because it is newer than the source file. For example:

     gcc -Dfoo=bar file1.c -aux-info file1.X
     protoize *.c

You need to include the special files along with the rest in the protoize command, even though their .X files already exist, because otherwise they won't get converted.

See Protoize Caveats, for more information on how to use protoize successfully.