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@c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999,
@c 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.
@c man begin COPYRIGHT
Copyright @copyright{} 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997,
1998, 1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``GNU General Public License'' and ``Funding
Free Software'', the Front-Cover texts being (a) (see below), and with
the Back-Cover Texts being (b) (see below). A copy of the license is
included in the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
@c man end
@c Set file name and title for the man page.
@setfilename gcc
@settitle GNU project C and C++ compiler
@c man begin SYNOPSIS
gcc [@option{-c}|@option{-S}|@option{-E}] [@option{-std=}@var{standard}]
[@option{-g}] [@option{-pg}] [@option{-O}@var{level}]
[@option{-W}@var{warn}@dots{}] [@option{-pedantic}]
[@option{-I}@var{dir}@dots{}] [@option{-L}@var{dir}@dots{}]
[@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}]
[@option{-f}@var{option}@dots{}] [@option{-m}@var{machine-option}@dots{}]
[@option{-o} @var{outfile}] @var{infile}@dots{}
Only the most useful options are listed here; see below for the
remainder. @samp{g++} accepts mostly the same options as @samp{gcc}.
@c man end
@c man begin SEEALSO
gpl(7), gfdl(7), fsf-funding(7),
cpp(1), gcov(1), g77(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1)
and the Info entries for @file{gcc}, @file{cpp}, @file{g77}, @file{as},
@file{ld}, @file{binutils} and @file{gdb}.
@c man end
@c man begin BUGS
For instructions on reporting bugs, see
@w{@uref{}}. Use of the @command{gccbug}
script to report bugs is recommended.
@c man end
@c man begin AUTHOR
See the Info entry for @command{gcc}, or
for contributors to GCC@.
@c man end
@end ignore
@node Invoking GCC
@chapter GCC Command Options
@cindex GCC command options
@cindex command options
@cindex options, GCC command
@c man begin DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation,
assembly and linking. The ``overall options'' allow you to stop this
process at an intermediate stage. For example, the @option{-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.
@cindex C compilation options
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.
@cindex C++ compilation options
@xref{Invoking G++,,Compiling C++ Programs}, for a summary of special
options for compiling C++ programs.
@cindex grouping options
@cindex options, grouping
The @command{gcc} program accepts options and file names as operands. Many
options have multi-letter names; therefore multiple single-letter options
may @emph{not} be grouped: @option{-dr} is very different from @w{@samp{-d
@cindex order of options
@cindex options, order
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 @option{-L} more than once,
the directories are searched in the order specified.
Many options have long names starting with @samp{-f} or with
@samp{-W}---for example, @option{-fforce-mem},
@option{-fstrength-reduce}, @option{-Wformat} and so on. Most of
these have both positive and negative forms; the negative form of
@option{-ffoo} would be @option{-fno-foo}. This manual documents
only one of these two forms, whichever one is not the default.
@c man end
@xref{Option Index}, for an index to GCC's options.
* Option Summary:: Brief list of all options, without explanations.
* Overall Options:: Controlling the kind of output:
an executable, object files, assembler files,
or preprocessed source.
* Invoking G++:: Compiling C++ programs.
* C Dialect Options:: Controlling the variant of C language compiled.
* C++ Dialect Options:: Variations on C++.
* Objective-C Dialect Options:: Variations on Objective-C.
* Language Independent Options:: Controlling how diagnostics should be
* Warning Options:: How picky should the compiler be?
* Debugging Options:: Symbol tables, measurements, and debugging dumps.
* Optimize Options:: How much optimization?
* Preprocessor Options:: Controlling header files and macro definitions.
Also, getting dependency information for Make.
* Assembler Options:: Passing options to the assembler.
* Link Options:: Specifying libraries and so on.
* Directory Options:: Where to find header files and libraries.
Where to find the compiler executable files.
* Spec Files:: How to pass switches to sub-processes.
* Target Options:: Running a cross-compiler, or an old version of GCC.
* Submodel Options:: Specifying minor hardware or convention variations,
such as 68010 vs 68020.
* Code Gen Options:: Specifying conventions for function calls, data layout
and register usage.
* Environment Variables:: Env vars that affect GCC.
* Precompiled Headers:: Compiling a header once, and using it many times.
* Running Protoize:: Automatically adding or removing function prototypes.
@end menu
@c man begin OPTIONS
@node Option Summary
@section Option Summary
Here is a summary of all the options, grouped by type. Explanations are
in the following sections.
@table @emph
@item Overall Options
@xref{Overall Options,,Options Controlling the Kind of Output}.
@gccoptlist{-c -S -E -o @var{file} -pipe -pass-exit-codes @gol
-x @var{language} -v -### --help --target-help --version}
@item C Language Options
@xref{C Dialect Options,,Options Controlling C Dialect}.
@gccoptlist{-ansi -std=@var{standard} -aux-info @var{filename} @gol
-fno-asm -fno-builtin -fno-builtin-@var{function} @gol
-fhosted -ffreestanding -fms-extensions @gol
-trigraphs -no-integrated-cpp -traditional -traditional-cpp @gol
-fallow-single-precision -fcond-mismatch @gol
-fsigned-bitfields -fsigned-char @gol
-funsigned-bitfields -funsigned-char @gol
@item C++ Language Options
@xref{C++ Dialect Options,,Options Controlling C++ Dialect}.
@gccoptlist{-fabi-version=@var{n} -fno-access-control -fcheck-new @gol
-fconserve-space -fno-const-strings @gol
-fno-elide-constructors @gol
-fno-enforce-eh-specs @gol
-ffor-scope -fno-for-scope -fno-gnu-keywords @gol
-fno-implicit-templates @gol
-fno-implicit-inline-templates @gol
-fno-implement-inlines -fms-extensions @gol
-fno-nonansi-builtins -fno-operator-names @gol
-fno-optional-diags -fpermissive @gol
-frepo -fno-rtti -fstats -ftemplate-depth-@var{n} @gol
-fuse-cxa-atexit -fno-weak -nostdinc++ @gol
-fno-default-inline -Wabi -Wctor-dtor-privacy @gol
-Wnon-virtual-dtor -Wreorder @gol
-Weffc++ -Wno-deprecated @gol
-Wno-non-template-friend -Wold-style-cast @gol
-Woverloaded-virtual -Wno-pmf-conversions @gol
-Wsign-promo -Wsynth}
@item Objective-C Language Options
@xref{Objective-C Dialect Options,,Options Controlling Objective-C Dialect}.
-fconstant-string-class=@var{class-name} @gol
-fgnu-runtime -fnext-runtime @gol
-fno-nil-receivers @gol
-fobjc-exceptions @gol
-freplace-objc-classes @gol
-fzero-link @gol
-gen-decls @gol
-Wno-protocol -Wselector -Wundeclared-selector}
@item Language Independent Options
@xref{Language Independent Options,,Options to Control Diagnostic Messages Formatting}.
@gccoptlist{-fmessage-length=@var{n} @gol
@item Warning Options
@xref{Warning Options,,Options to Request or Suppress Warnings}.
@gccoptlist{-fsyntax-only -pedantic -pedantic-errors @gol
-w -Wextra -Wall -Waggregate-return @gol
-Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment @gol
-Wconversion -Wno-deprecated-declarations @gol
-Wdisabled-optimization -Wno-div-by-zero -Wendif-labels @gol
-Werror -Werror-implicit-function-declaration @gol
-Wfloat-equal -Wformat -Wformat=2 @gol
-Wno-format-extra-args -Wformat-nonliteral @gol
-Wformat-security -Wformat-y2k @gol
-Wimplicit -Wimplicit-function-declaration -Wimplicit-int @gol
-Wimport -Wno-import -Winit-self -Winline @gol
-Wno-invalid-offsetof -Winvalid-pch @gol
-Wlarger-than-@var{len} -Wlong-long @gol
-Wmain -Wmissing-braces @gol
-Wmissing-format-attribute -Wmissing-noreturn @gol
-Wno-multichar -Wnonnull -Wpacked -Wpadded @gol
-Wparentheses -Wpointer-arith -Wredundant-decls @gol
-Wreturn-type -Wsequence-point -Wshadow @gol
-Wsign-compare -Wstrict-aliasing @gol
-Wswitch -Wswitch-default -Wswitch-enum @gol
-Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized @gol
-Wunknown-pragmas -Wunreachable-code @gol
-Wunused -Wunused-function -Wunused-label -Wunused-parameter @gol
-Wunused-value -Wunused-variable -Wwrite-strings}
@item C-only Warning Options
@gccoptlist{-Wbad-function-cast -Wmissing-declarations @gol
-Wmissing-prototypes -Wnested-externs -Wold-style-definition @gol
-Wstrict-prototypes -Wtraditional @gol
@item Debugging Options
@xref{Debugging Options,,Options for Debugging Your Program or GCC}.
@gccoptlist{-d@var{letters} -dumpspecs -dumpmachine -dumpversion @gol
-fdump-unnumbered -fdump-translation-unit@r{[}-@var{n}@r{]} @gol
-fdump-class-hierarchy@r{[}-@var{n}@r{]} @gol
-fdump-tree-original@r{[}-@var{n}@r{]} @gol
-fdump-tree-optimized@r{[}-@var{n}@r{]} @gol
-fdump-tree-inlined@r{[}-@var{n}@r{]} @gol
-feliminate-dwarf2-dups -feliminate-unused-debug-types @gol
-feliminate-unused-debug-symbols -fmem-report -fprofile-arcs @gol
-frandom-seed=@var{string} -fsched-verbose=@var{n} @gol
-ftest-coverage -ftime-report @gol
-g -g@var{level} -gcoff -gdwarf-2 @gol
-ggdb -gstabs -gstabs+ -gvms -gxcoff -gxcoff+ @gol
-p -pg -print-file-name=@var{library} -print-libgcc-file-name @gol
-print-multi-directory -print-multi-lib @gol
-print-prog-name=@var{program} -print-search-dirs -Q @gol
-save-temps -time}
@item Optimization Options
@xref{Optimize Options,,Options that Control Optimization}.
@gccoptlist{-falign-functions=@var{n} -falign-jumps=@var{n} @gol
-falign-labels=@var{n} -falign-loops=@var{n} @gol
-fbranch-probabilities -fprofile-values -fvpt -fbranch-target-load-optimize @gol
-fbranch-target-load-optimize2 -fcaller-saves -fcprop-registers @gol
-fcse-follow-jumps -fcse-skip-blocks -fdata-sections @gol
-fdelayed-branch -fdelete-null-pointer-checks @gol
-fexpensive-optimizations -ffast-math -ffloat-store @gol
-fforce-addr -fforce-mem -ffunction-sections @gol
-fgcse -fgcse-lm -fgcse-sm -fgcse-las -floop-optimize @gol
-fcrossjumping -fif-conversion -fif-conversion2 @gol
-finline-functions -finline-limit=@var{n} -fkeep-inline-functions @gol
-fkeep-static-consts -fmerge-constants -fmerge-all-constants @gol
-fmove-all-movables -fnew-ra -fno-branch-count-reg @gol
-fno-default-inline -fno-defer-pop @gol
-fno-function-cse -fno-guess-branch-probability @gol
-fno-inline -fno-math-errno -fno-peephole -fno-peephole2 @gol
-funsafe-math-optimizations -ffinite-math-only @gol
-fno-trapping-math -fno-zero-initialized-in-bss @gol
-fomit-frame-pointer -foptimize-register-move @gol
-foptimize-sibling-calls -fprefetch-loop-arrays @gol
-fprofile-generate -fprofile-use @gol
-freduce-all-givs -fregmove -frename-registers @gol
-freorder-blocks -freorder-functions @gol
-frerun-cse-after-loop -frerun-loop-opt @gol
-frounding-math -fschedule-insns -fschedule-insns2 @gol
-fno-sched-interblock -fno-sched-spec -fsched-spec-load @gol
-fsched-spec-load-dangerous @gol
-fsched-stalled-insns=@var{n} -sched-stalled-insns-dep=@var{n} @gol
-fsched2-use-superblocks @gol
-fsched2-use-traces -fsignaling-nans @gol
-fsingle-precision-constant @gol
-fstrength-reduce -fstrict-aliasing -ftracer -fthread-jumps @gol
-funroll-all-loops -funroll-loops -fpeel-loops @gol
-funswitch-loops -fold-unroll-loops -fold-unroll-all-loops @gol
--param @var{name}=@var{value}
-O -O0 -O1 -O2 -O3 -Os}
@item Preprocessor Options
@xref{Preprocessor Options,,Options Controlling the Preprocessor}.
@gccoptlist{-A@var{question}=@var{answer} @gol
-A-@var{question}@r{[}=@var{answer}@r{]} @gol
-C -dD -dI -dM -dN @gol
-D@var{macro}@r{[}=@var{defn}@r{]} -E -H @gol
-idirafter @var{dir} @gol
-include @var{file} -imacros @var{file} @gol
-iprefix @var{file} -iwithprefix @var{dir} @gol
-iwithprefixbefore @var{dir} -isystem @var{dir} @gol
-M -MM -MF -MG -MP -MQ -MT -nostdinc @gol
-P -fworking-directory -remap @gol
-trigraphs -undef -U@var{macro} -Wp,@var{option} @gol
-Xpreprocessor @var{option}}
@item Assembler Option
@xref{Assembler Options,,Passing Options to the Assembler}.
@gccoptlist{-Wa,@var{option} -Xassembler @var{option}}
@item Linker Options
@xref{Link Options,,Options for Linking}.
@gccoptlist{@var{object-file-name} -l@var{library} @gol
-nostartfiles -nodefaultlibs -nostdlib -pie @gol
-s -static -static-libgcc -shared -shared-libgcc -symbolic @gol
-Wl,@var{option} -Xlinker @var{option} @gol
-u @var{symbol}}
@item Directory Options
@xref{Directory Options,,Options for Directory Search}.
@gccoptlist{-B@var{prefix} -I@var{dir} -I- -L@var{dir} -specs=@var{file}}
@item Target Options
@c I wrote this xref this way to avoid overfull hbox. -- rms
@xref{Target Options}.
@gccoptlist{-V @var{version} -b @var{machine}}
@item Machine Dependent Options
@xref{Submodel Options,,Hardware Models and Configurations}.
@emph{M680x0 Options}
@gccoptlist{-m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 @gol
-m68060 -mcpu32 -m5200 -m68881 -mbitfield -mc68000 -mc68020 @gol
-mnobitfield -mrtd -mshort -msoft-float -mpcrel @gol
-malign-int -mstrict-align -msep-data -mno-sep-data @gol
-mshared-library-id=n -mid-shared-library -mno-id-shared-library}
@emph{M68hc1x Options}
@gccoptlist{-m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12 @gol
-mauto-incdec -minmax -mlong-calls -mshort @gol
@emph{VAX Options}
@gccoptlist{-mg -mgnu -munix}
@emph{SPARC Options}
@gccoptlist{-mcpu=@var{cpu-type} @gol
-mtune=@var{cpu-type} @gol
-mcmodel=@var{code-model} @gol
-m32 -m64 -mapp-regs -mno-app-regs @gol
-mfaster-structs -mno-faster-structs @gol
-mflat -mno-flat -mfpu -mno-fpu @gol
-mhard-float -msoft-float @gol
-mhard-quad-float -msoft-quad-float @gol
-mimpure-text -mno-impure-text -mlittle-endian @gol
-mstack-bias -mno-stack-bias @gol
-munaligned-doubles -mno-unaligned-doubles @gol
-mv8plus -mno-v8plus -mvis -mno-vis @gol
-mcypress -mf930 -mf934 @gol
-msparclite -msupersparc -mv8
-threads -pthreads}
@emph{ARM Options}
@gccoptlist{-mapcs-frame -mno-apcs-frame @gol
-mapcs-26 -mapcs-32 @gol
-mapcs-stack-check -mno-apcs-stack-check @gol
-mapcs-float -mno-apcs-float @gol
-mapcs-reentrant -mno-apcs-reentrant @gol
-msched-prolog -mno-sched-prolog @gol
-mlittle-endian -mbig-endian -mwords-little-endian @gol
-malignment-traps -mno-alignment-traps @gol
-msoft-float -mhard-float -mfpe @gol
-mthumb-interwork -mno-thumb-interwork @gol
-mcpu=@var{name} -march=@var{name} -mfpe=@var{name} @gol
-mstructure-size-boundary=@var{n} @gol
-mabort-on-noreturn @gol
-mlong-calls -mno-long-calls @gol
-msingle-pic-base -mno-single-pic-base @gol
-mpic-register=@var{reg} @gol
-mnop-fun-dllimport @gol
-mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns @gol
-mpoke-function-name @gol
-mthumb -marm @gol
-mtpcs-frame -mtpcs-leaf-frame @gol
-mcaller-super-interworking -mcallee-super-interworking}
@emph{MN10300 Options}
@gccoptlist{-mmult-bug -mno-mult-bug @gol
-mam33 -mno-am33 @gol
-mam33-2 -mno-am33-2 @gol
-mno-crt0 -mrelax}
@emph{M32R/D Options}
@gccoptlist{-m32r2 -m32rx -m32r @gol
-mdebug @gol
-malign-loops -mno-align-loops @gol
-missue-rate=@var{number} @gol
-mbranch-cost=@var{number} @gol
-mmodel=@var{code-size-model-type} @gol
-msdata=@var{sdata-type} @gol
-mno-flush-func -mflush-func=@var{name} @gol
-mno-flush-trap -mflush-trap=@var{number} @gol
-G @var{num}}
@emph{RS/6000 and PowerPC Options}
@gccoptlist{-mcpu=@var{cpu-type} @gol
-mtune=@var{cpu-type} @gol
-mpower -mno-power -mpower2 -mno-power2 @gol
-mpowerpc -mpowerpc64 -mno-powerpc @gol
-maltivec -mno-altivec @gol
-mpowerpc-gpopt -mno-powerpc-gpopt @gol
-mpowerpc-gfxopt -mno-powerpc-gfxopt @gol
-mnew-mnemonics -mold-mnemonics @gol
-mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc @gol
-m64 -m32 -mxl-call -mno-xl-call -mpe @gol
-malign-power -malign-natural @gol
-msoft-float -mhard-float -mmultiple -mno-multiple @gol
-mstring -mno-string -mupdate -mno-update @gol
-mfused-madd -mno-fused-madd -mbit-align -mno-bit-align @gol
-mstrict-align -mno-strict-align -mrelocatable @gol
-mno-relocatable -mrelocatable-lib -mno-relocatable-lib @gol
-mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian @gol
-mdynamic-no-pic @gol
-mprioritize-restricted-insns=@var{priority} @gol
-msched-costly-dep=@var{dependence_type} @gol
-minsert-sched-nops=@var{scheme} @gol
-mcall-sysv -mcall-netbsd @gol
-maix-struct-return -msvr4-struct-return @gol
-mabi=altivec -mabi=no-altivec @gol
-mabi=spe -mabi=no-spe @gol
-misel=yes -misel=no @gol
-mspe=yes -mspe=no @gol
-mfloat-gprs=yes -mfloat-gprs=no @gol
-mprototype -mno-prototype @gol
-msim -mmvme -mads -myellowknife -memb -msdata @gol
-msdata=@var{opt} -mvxworks -mwindiss -G @var{num} -pthread}
@emph{Darwin Options}
@gccoptlist{-all_load -allowable_client -arch -arch_errors_fatal @gol
-arch_only -bind_at_load -bundle -bundle_loader @gol
-client_name -compatibility_version -current_version @gol
-dependency-file -dylib_file -dylinker_install_name @gol
-dynamic -dynamiclib -exported_symbols_list @gol
-filelist -flat_namespace -force_cpusubtype_ALL @gol
-force_flat_namespace -headerpad_max_install_names @gol
-image_base -init -install_name -keep_private_externs @gol
-multi_module -multiply_defined -multiply_defined_unused @gol
-noall_load -nofixprebinding -nomultidefs -noprebind -noseglinkedit @gol
-pagezero_size -prebind -prebind_all_twolevel_modules @gol
-private_bundle -read_only_relocs -sectalign @gol
-sectobjectsymbols -whyload -seg1addr @gol
-sectcreate -sectobjectsymbols -sectorder @gol
-seg_addr_table -seg_addr_table_filename -seglinkedit @gol
-segprot -segs_read_only_addr -segs_read_write_addr @gol
-single_module -static -sub_library -sub_umbrella @gol
-twolevel_namespace -umbrella -undefined @gol
-unexported_symbols_list -weak_reference_mismatches @gol
@emph{MIPS Options}
@gccoptlist{-EL -EB -march=@var{arch} -mtune=@var{arch} @gol
-mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips64 @gol
-mips16 -mno-mips16 -mabi=@var{abi} -mabicalls -mno-abicalls @gol
-mxgot -mno-xgot -membedded-pic -mno-embedded-pic @gol
-mgp32 -mgp64 -mfp32 -mfp64 -mhard-float -msoft-float @gol
-msingle-float -mdouble-float -mint64 -mlong64 -mlong32 @gol
-G@var{num} -membedded-data -mno-embedded-data @gol
-muninit-const-in-rodata -mno-uninit-const-in-rodata @gol
-msplit-addresses -mno-split-addresses @gol
-mexplicit-relocs -mno-explicit-relocs @gol
-mrnames -mno-rnames @gol
-mcheck-zero-division -mno-check-zero-division @gol
-mmemcpy -mno-memcpy -mlong-calls -mno-long-calls @gol
-mmad -mno-mad -mfused-madd -mno-fused-madd -nocpp @gol
-mfix-sb1 -mno-fix-sb1 -mflush-func=@var{func} @gol
-mno-flush-func -mbranch-likely -mno-branch-likely}
@emph{i386 and x86-64 Options}
@gccoptlist{-mtune=@var{cpu-type} -march=@var{cpu-type} @gol
-mfpmath=@var{unit} @gol
-masm=@var{dialect} -mno-fancy-math-387 @gol
-mno-fp-ret-in-387 -msoft-float -msvr3-shlib @gol
-mno-wide-multiply -mrtd -malign-double @gol
-mpreferred-stack-boundary=@var{num} @gol
-mmmx -msse -msse2 -msse3 -m3dnow @gol
-mthreads -mno-align-stringops -minline-all-stringops @gol
-mpush-args -maccumulate-outgoing-args -m128bit-long-double @gol
-m96bit-long-double -mregparm=@var{num} -momit-leaf-frame-pointer @gol
-mno-red-zone -mno-tls-direct-seg-refs @gol
-mcmodel=@var{code-model} @gol
-m32 -m64}
@emph{HPPA Options}
@gccoptlist{-march=@var{architecture-type} @gol
-mbig-switch -mdisable-fpregs -mdisable-indexing @gol
-mfast-indirect-calls -mgas -mgnu-ld -mhp-ld @gol
-mjump-in-delay -mlinker-opt -mlong-calls @gol
-mlong-load-store -mno-big-switch -mno-disable-fpregs @gol
-mno-disable-indexing -mno-fast-indirect-calls -mno-gas @gol
-mno-jump-in-delay -mno-long-load-store @gol
-mno-portable-runtime -mno-soft-float @gol
-mno-space-regs -msoft-float -mpa-risc-1-0 @gol
-mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime @gol
-mschedule=@var{cpu-type} -mspace-regs -msio -mwsio @gol
-nolibdld -static -threads}
@emph{Intel 960 Options}
@gccoptlist{-m@var{cpu-type} -masm-compat -mclean-linkage @gol
-mcode-align -mcomplex-addr -mleaf-procedures @gol
-mic-compat -mic2.0-compat -mic3.0-compat @gol
-mintel-asm -mno-clean-linkage -mno-code-align @gol
-mno-complex-addr -mno-leaf-procedures @gol
-mno-old-align -mno-strict-align -mno-tail-call @gol
-mnumerics -mold-align -msoft-float -mstrict-align @gol
@emph{DEC Alpha Options}
@gccoptlist{-mno-fp-regs -msoft-float -malpha-as -mgas @gol
-mieee -mieee-with-inexact -mieee-conformant @gol
-mfp-trap-mode=@var{mode} -mfp-rounding-mode=@var{mode} @gol
-mtrap-precision=@var{mode} -mbuild-constants @gol
-mcpu=@var{cpu-type} -mtune=@var{cpu-type} @gol
-mbwx -mmax -mfix -mcix @gol
-mfloat-vax -mfloat-ieee @gol
-mexplicit-relocs -msmall-data -mlarge-data @gol
-msmall-text -mlarge-text @gol
@emph{DEC Alpha/VMS Options}
@emph{H8/300 Options}
@gccoptlist{-mrelax -mh -ms -mn -mint32 -malign-300}
@emph{SH Options}
@gccoptlist{-m1 -m2 -m2e -m3 -m3e @gol
-m4-nofpu -m4-single-only -m4-single -m4 @gol
-m5-64media -m5-64media-nofpu @gol
-m5-32media -m5-32media-nofpu @gol
-m5-compact -m5-compact-nofpu @gol
-mb -ml -mdalign -mrelax @gol
-mbigtable -mfmovd -mhitachi -mnomacsave @gol
-mieee -misize -mpadstruct -mspace @gol
-mprefergot -musermode}
@emph{System V Options}
@gccoptlist{-Qy -Qn -YP,@var{paths} -Ym,@var{dir}}
@emph{ARC Options}
@gccoptlist{-EB -EL @gol
-mmangle-cpu -mcpu=@var{cpu} -mtext=@var{text-section} @gol
-mdata=@var{data-section} -mrodata=@var{readonly-data-section}}
@emph{TMS320C3x/C4x Options}
@gccoptlist{-mcpu=@var{cpu} -mbig -msmall -mregparm -mmemparm @gol
-mfast-fix -mmpyi -mbk -mti -mdp-isr-reload @gol
-mrpts=@var{count} -mrptb -mdb -mloop-unsigned @gol
-mparallel-insns -mparallel-mpy -mpreserve-float}
@emph{V850 Options}
@gccoptlist{-mlong-calls -mno-long-calls -mep -mno-ep @gol
-mprolog-function -mno-prolog-function -mspace @gol
-mtda=@var{n} -msda=@var{n} -mzda=@var{n} @gol
-mapp-regs -mno-app-regs @gol
-mdisable-callt -mno-disable-callt @gol
-mv850e1 @gol
-mv850e @gol
-mv850 -mbig-switch}
@emph{NS32K Options}
@gccoptlist{-m32032 -m32332 -m32532 -m32081 -m32381 @gol
-mmult-add -mnomult-add -msoft-float -mrtd -mnortd @gol
-mregparam -mnoregparam -msb -mnosb @gol
-mbitfield -mnobitfield -mhimem -mnohimem}
@emph{AVR Options}
@gccoptlist{-mmcu=@var{mcu} -msize -minit-stack=@var{n} -mno-interrupts @gol
-mcall-prologues -mno-tablejump -mtiny-stack}
@emph{MCore Options}
@gccoptlist{-mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates @gol
-mno-relax-immediates -mwide-bitfields -mno-wide-bitfields @gol
-m4byte-functions -mno-4byte-functions -mcallgraph-data @gol
-mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim @gol
-mlittle-endian -mbig-endian -m210 -m340 -mstack-increment}
@emph{MMIX Options}
@gccoptlist{-mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu @gol
-mabi=mmixware -mzero-extend -mknuthdiv -mtoplevel-symbols @gol
-melf -mbranch-predict -mno-branch-predict -mbase-addresses @gol
-mno-base-addresses -msingle-exit -mno-single-exit}
@emph{IA-64 Options}
@gccoptlist{-mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic @gol
-mvolatile-asm-stop -mb-step -mregister-names -mno-sdata @gol
-mconstant-gp -mauto-pic -minline-float-divide-min-latency @gol
-minline-float-divide-max-throughput @gol
-minline-int-divide-min-latency @gol
-minline-int-divide-max-throughput -mno-dwarf2-asm @gol
@emph{D30V Options}
@gccoptlist{-mextmem -mextmemory -monchip -mno-asm-optimize @gol
-masm-optimize -mbranch-cost=@var{n} -mcond-exec=@var{n}}
@emph{S/390 and zSeries Options}
@gccoptlist{-mtune=@var{cpu-type} -march=@var{cpu-type} @gol
-mhard-float -msoft-float -mbackchain -mno-backchain @gol
-msmall-exec -mno-small-exec -mmvcle -mno-mvcle @gol
-m64 -m31 -mdebug -mno-debug -mesa -mzarch -mfused-madd -mno-fused-madd}
@emph{CRIS Options}
@gccoptlist{-mcpu=@var{cpu} -march=@var{cpu} -mtune=@var{cpu} @gol
-mmax-stack-frame=@var{n} -melinux-stacksize=@var{n} @gol
-metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects @gol
-mstack-align -mdata-align -mconst-align @gol
-m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt @gol
-melf -maout -melinux -mlinux -sim -sim2 @gol
-mmul-bug-workaround -mno-mul-bug-workaround}
@emph{PDP-11 Options}
@gccoptlist{-mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 @gol
-mbcopy -mbcopy-builtin -mint32 -mno-int16 @gol
-mint16 -mno-int32 -mfloat32 -mno-float64 @gol
-mfloat64 -mno-float32 -mabshi -mno-abshi @gol
-mbranch-expensive -mbranch-cheap @gol
-msplit -mno-split -munix-asm -mdec-asm}
@emph{Xstormy16 Options}
@emph{Xtensa Options}
@gccoptlist{-mconst16 -mno-const16 @gol
-mfused-madd -mno-fused-madd @gol
-mtext-section-literals -mno-text-section-literals @gol
-mtarget-align -mno-target-align @gol
-mlongcalls -mno-longcalls}
@emph{FRV Options}
@gccoptlist{-mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 @gol
-mhard-float -msoft-float @gol
-malloc-cc -mfixed-cc -mdword -mno-dword @gol
-mdouble -mno-double @gol
-mmedia -mno-media -mmuladd -mno-muladd @gol
-mlibrary-pic -macc-4 -macc-8 @gol
-mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move @gol
-mscc -mno-scc -mcond-exec -mno-cond-exec @gol
-mvliw-branch -mno-vliw-branch @gol
-mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec @gol
-mno-nested-cond-exec -mtomcat-stats @gol
@item Code Generation Options
@xref{Code Gen Options,,Options for Code Generation Conventions}.
@gccoptlist{-fcall-saved-@var{reg} -fcall-used-@var{reg} @gol
-ffixed-@var{reg} -fexceptions @gol
-fnon-call-exceptions -funwind-tables @gol
-fasynchronous-unwind-tables @gol
-finhibit-size-directive -finstrument-functions @gol
-fno-common -fno-ident @gol
-fpcc-struct-return -fpic -fPIC -fpie -fPIE @gol
-freg-struct-return -fshared-data -fshort-enums @gol
-fshort-double -fshort-wchar @gol
-fverbose-asm -fpack-struct -fstack-check @gol
-fstack-limit-register=@var{reg} -fstack-limit-symbol=@var{sym} @gol
-fargument-alias -fargument-noalias @gol
-fargument-noalias-global -fleading-underscore @gol
-ftls-model=@var{model} @gol
-ftrapv -fwrapv -fbounds-check}
@end table
* Overall Options:: Controlling the kind of output:
an executable, object files, assembler files,
or preprocessed source.
* C Dialect Options:: Controlling the variant of C language compiled.
* C++ Dialect Options:: Variations on C++.
* Objective-C Dialect Options:: Variations on Objective-C.
* Language Independent Options:: Controlling how diagnostics should be
* Warning Options:: How picky should the compiler be?
* Debugging Options:: Symbol tables, measurements, and debugging dumps.
* Optimize Options:: How much optimization?
* Preprocessor Options:: Controlling header files and macro definitions.
Also, getting dependency information for Make.
* Assembler Options:: Passing options to the assembler.
* Link Options:: Specifying libraries and so on.
* Directory Options:: Where to find header files and libraries.
Where to find the compiler executable files.
* Spec Files:: How to pass switches to sub-processes.
* Target Options:: Running a cross-compiler, or an old version of GCC.
@end menu
@node Overall Options
@section 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.
@cindex file name suffix
For any given input file, the file name suffix determines what kind of
compilation is done:
@table @gcctabopt
@item @var{file}.c
C source code which must be preprocessed.
@item @var{file}.i
C source code which should not be preprocessed.
@item @var{file}.ii
C++ source code which should not be preprocessed.
@item @var{file}.m
Objective-C source code. Note that you must link with the library
@file{libobjc.a} to make an Objective-C program work.
@item @var{file}.mi
Objective-C source code which should not be preprocessed.
@item @var{file}.h
C or C++ header file to be turned into a precompiled header.
@item @var{file}.cc
@itemx @var{file}.cp
@itemx @var{file}.cxx
@itemx @var{file}.cpp
@itemx @var{file}.CPP
@itemx @var{file}.c++
@itemx @var{file}.C
C++ source code which must be preprocessed. Note that in @samp{.cxx},
the last two letters must both be literally @samp{x}. Likewise,
@samp{.C} refers to a literal capital C@.
@item @var{file}.hh
@itemx @var{file}.H
C++ header file to be turned into a precompiled header.
@item @var{file}.f
@itemx @var{file}.for
@itemx @var{file}.FOR
Fortran source code which should not be preprocessed.
@item @var{file}.F
@itemx @var{file}.fpp
@itemx @var{file}.FPP
Fortran source code which must be preprocessed (with the traditional
@item @var{file}.r
Fortran source code which must be preprocessed with a RATFOR
preprocessor (not included with GCC)@.
@xref{Overall Options,,Options Controlling the Kind of Output, g77,
Using and Porting GNU Fortran}, for more details of the handling of
Fortran input files.
@c FIXME: Descriptions of Java file types.
@c @var{file}.java
@c @var{file}.class
@c @var{file}.zip
@c @var{file}.jar
@item @var{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 @dfn{specs}.
@itemx @var{file}.adb
Ada source code file containing a library unit body (a subprogram or
package body). Such files are also called @dfn{bodies}.
@c GCC also knows about some suffixes for languages not yet included:
@c Pascal:
@c @var{file}.p
@c @var{file}.pas
@item @var{file}.s
Assembler code.
@item @var{file}.S
Assembler code which must be preprocessed.
@item @var{other}
An object file to be fed straight into linking.
Any file name with no recognized suffix is treated this way.
@end table
@opindex x
You can specify the input language explicitly with the @option{-x} option:
@table @gcctabopt
@item -x @var{language}
Specify explicitly the @var{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 @option{-x} option. Possible values for @var{language} are:
c c-header cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objc-cpp-output
assembler assembler-with-cpp
f77 f77-cpp-input ratfor
@end smallexample
@item -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 @option{-x}
has not been used at all).
@item -pass-exit-codes
@opindex pass-exit-codes
Normally the @command{gcc} program will exit with the code of 1 if any
phase of the compiler returns a non-success return code. If you specify
@option{-pass-exit-codes}, the @command{gcc} program will instead return with
numerically highest error produced by any phase that returned an error
@end table
If you only want some of the stages of compilation, you can use
@option{-x} (or filename suffixes) to tell @command{gcc} where to start, and
one of the options @option{-c}, @option{-S}, or @option{-E} to say where
@command{gcc} is to stop. Note that some combinations (for example,
@samp{-x cpp-output -E}) instruct @command{gcc} to do nothing at all.
@table @gcctabopt
@item -c
@opindex 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 @samp{.c}, @samp{.i}, @samp{.s}, etc., with @samp{.o}.
Unrecognized input files, not requiring compilation or assembly, are
@item -S
@opindex 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 @samp{.c}, @samp{.i}, etc., with @samp{.s}.
Input files that don't require compilation are ignored.
@item -E
@opindex 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.
@cindex output file option
@item -o @var{file}
@opindex o
Place output in file @var{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 you specify @option{-o} when compiling more than one input file, or
you are producing an executable file as output, all the source files
on the command line will be compiled at once.
If @option{-o} is not specified, the default is to put an executable file
in @file{a.out}, the object file for @file{@var{source}.@var{suffix}} in
@file{@var{source}.o}, its assembler file in @file{@var{source}.s}, and
all preprocessed C source on standard output.
@item -v
@opindex 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.
@item -###
@opindex ###
Like @option{-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.
@item -pipe
@opindex 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.
@item --help
@opindex help
Print (on the standard output) a description of the command line options
understood by @command{gcc}. If the @option{-v} option is also specified
then @option{--help} will also be passed on to the various processes
invoked by @command{gcc}, so that they can display the command line options
they accept. If the @option{-Wextra} option is also specified then command
line options which have no documentation associated with them will also
be displayed.
@item --target-help
@opindex target-help
Print (on the standard output) a description of target specific command
line options for each tool.
@item --version
@opindex version
Display the version number and copyrights of the invoked GCC.
@end table
@node Invoking G++
@section Compiling C++ Programs
@cindex suffixes for C++ source
@cindex C++ source file suffixes
C++ source files conventionally use one of the suffixes @samp{.C},
@samp{.cc}, @samp{.cpp}, @samp{.CPP}, @samp{.c++}, @samp{.cp}, or
@samp{.cxx}; C++ header files often use @samp{.hh} or @samp{.H}; and
preprocessed C++ files use the suffix @samp{.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 @command{gcc}).
@findex g++
@findex c++
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
@samp{.h} extension to be used in C++ compilations. @command{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, @command{g++} is also installed with the name @command{c++}.
@cindex invoking @command{g++}
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.
@xref{C Dialect Options,,Options Controlling C Dialect}, for
explanations of options for languages related to C@.
@xref{C++ Dialect Options,,Options Controlling C++ Dialect}, for
explanations of options that are meaningful only for C++ programs.
@node C Dialect Options
@section Options Controlling C Dialect
@cindex dialect options
@cindex language dialect options
@cindex options, dialect
The following options control the dialect of C (or languages derived
from C, such as C++ and Objective-C) that the compiler accepts:
@table @gcctabopt
@cindex ANSI support
@cindex ISO support
@item -ansi
@opindex 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 @code{asm} and @code{typeof} keywords, and
predefined macros such as @code{unix} and @code{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 @samp{//} comments as well as
the @code{inline} keyword.
The alternate keywords @code{__asm__}, @code{__extension__},
@code{__inline__} and @code{__typeof__} continue to work despite
@option{-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 @option{-ansi}. Alternate predefined macros
such as @code{__unix__} and @code{__vax__} are also available, with or
without @option{-ansi}.
The @option{-ansi} option does not cause non-ISO programs to be
rejected gratuitously. For that, @option{-pedantic} is required in
addition to @option{-ansi}. @xref{Warning Options}.
The macro @code{__STRICT_ANSI__} is predefined when the @option{-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 @code{alloca} and @code{ffs}) are not built-in
functions with @option{-ansi} is used. @xref{Other Builtins,,Other
built-in functions provided by GCC}, for details of the functions
@item -std=
@opindex 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
@table @samp
@item c89
@itemx iso9899:1990
ISO C90 (same as @option{-ansi}).
@item iso9899:199409
ISO C90 as modified in amendment 1.
@item c99
@itemx c9x
@itemx iso9899:1999
@itemx iso9899:199x
ISO C99. Note that this standard is not yet fully supported; see
@w{@uref{}} for more information. The
names @samp{c9x} and @samp{iso9899:199x} are deprecated.
@item gnu89
Default, ISO C90 plus GNU extensions (including some C99 features).
@item gnu99
@itemx gnu9x
ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC,
this will become the default. The name @samp{gnu9x} is deprecated.
@item c++98
The 1998 ISO C++ standard plus amendments.
@item gnu++98
The same as @option{-std=c++98} plus GNU extensions. This is the
default for C++ code.
@end table
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 @code{__restrict__} even
when @option{-std=c99} is not specified.
The @option{-std} options specifying some version of ISO C have the same
effects as @option{-ansi}, except that features that were not in ISO C90
but are in the specified version (for example, @samp{//} comments and
the @code{inline} keyword in ISO C99) are not disabled.
@xref{Standards,,Language Standards Supported by GCC}, for details of
these standard versions.
@item -aux-info @var{filename}
@opindex aux-info
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 (@samp{I}, @samp{N} for new or
@samp{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 (@samp{C} or @samp{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.
@item -fno-asm
@opindex fno-asm
Do not recognize @code{asm}, @code{inline} or @code{typeof} as a
keyword, so that code can use these words as identifiers. You can use
the keywords @code{__asm__}, @code{__inline__} and @code{__typeof__}
instead. @option{-ansi} implies @option{-fno-asm}.
In C++, this switch only affects the @code{typeof} keyword, since
@code{asm} and @code{inline} are standard keywords. You may want to
use the @option{-fno-gnu-keywords} flag instead, which has the same
effect. In C99 mode (@option{-std=c99} or @option{-std=gnu99}), this
switch only affects the @code{asm} and @code{typeof} keywords, since
@code{inline} is a standard keyword in ISO C99.
@item -fno-builtin
@itemx -fno-builtin-@var{function}
@opindex fno-builtin
@cindex built-in functions
Don't recognize built-in functions that do not begin with
@samp{__builtin_} as prefix. @xref{Other Builtins,,Other built-in
functions provided by GCC}, for details of the functions affected,
including those which are not built-in functions when @option{-ansi} or
@option{-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 @code{alloca} may become single
instructions that adjust the stack directly, and calls to @code{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.
With the @option{-fno-builtin-@var{function}} option
only the built-in function @var{function} is
disabled. @var{function} must not begin with @samp{__builtin_}. If a
function is named this is not built-in in this version of GCC, this
option is ignored. There is no corresponding
@option{-fbuiltin-@var{function}} option; if you wish to enable
built-in functions selectively when using @option{-fno-builtin} or
@option{-ffreestanding}, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
@end smallexample
@item -fhosted
@opindex fhosted
@cindex hosted environment
Assert that compilation takes place in a hosted environment. This implies
@option{-fbuiltin}. A hosted environment is one in which the
entire standard library is available, and in which @code{main} has a return
type of @code{int}. Examples are nearly everything except a kernel.
This is equivalent to @option{-fno-freestanding}.
@item -ffreestanding
@opindex ffreestanding
@cindex hosted environment
Assert that compilation takes place in a freestanding environment. This
implies @option{-fno-builtin}. A freestanding environment
is one in which the standard library may not exist, and program startup may
not necessarily be at @code{main}. The most obvious example is an OS kernel.
This is equivalent to @option{-fno-hosted}.
@xref{Standards,,Language Standards Supported by GCC}, for details of
freestanding and hosted environments.
@item -fms-extensions
@opindex fms-extensions
Accept some non-standard constructs used in Microsoft header files.
@item -trigraphs
@opindex trigraphs
Support ISO C trigraphs. The @option{-ansi} option (and @option{-std}
options for strict ISO C conformance) implies @option{-trigraphs}.
@item -no-integrated-cpp
@opindex no-integrated-cpp
Performs a compilation in two passes: preprocessing and compiling. This
option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the
@option{-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.
@cindex traditional C language
@cindex C language, traditional
@item -traditional
@itemx -traditional-cpp
@opindex traditional-cpp
@opindex traditional
Formerly, these options caused GCC to attempt to emulate a pre-standard
C compiler. They are now only supported with the @option{-E} switch.
The preprocessor continues to support a pre-standard mode. See the GNU
CPP manual for details.
@item -fcond-mismatch
@opindex 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++.
@item -funsigned-char
@opindex funsigned-char
Let the type @code{char} be unsigned, like @code{unsigned char}.
Each kind of machine has a default for what @code{char} should
be. It is either like @code{unsigned char} by default or like
@code{signed char} by default.
Ideally, a portable program should always use @code{signed char} or
@code{unsigned char} when it depends on the signedness of an object.
But many programs have been written to use plain @code{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 @code{char} is always a distinct type from each of
@code{signed char} or @code{unsigned char}, even though its behavior
is always just like one of those two.
@item -fsigned-char
@opindex fsigned-char
Let the type @code{char} be signed, like @code{signed char}.
Note that this is equivalent to @option{-fno-unsigned-char}, which is
the negative form of @option{-funsigned-char}. Likewise, the option
@option{-fno-signed-char} is equivalent to @option{-funsigned-char}.
@item -fsigned-bitfields
@itemx -funsigned-bitfields
@itemx -fno-signed-bitfields
@itemx -fno-unsigned-bitfields
@opindex fsigned-bitfields
@opindex funsigned-bitfields
@opindex fno-signed-bitfields
@opindex fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned, when the
declaration does not use either @code{signed} or @code{unsigned}. By
default, such a bit-field is signed, because this is consistent: the
basic integer types such as @code{int} are signed types.
@item -fwritable-strings
@opindex 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.
@end table
@node C++ Dialect Options
@section Options Controlling C++ Dialect
@cindex compiler options, C++
@cindex C++ options, command line
@cindex options, C++
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 @code{firstClass.C} like this:
g++ -g -frepo -O -c firstClass.C
@end smallexample
In this example, only @option{-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 @emph{only} for compiling C++ programs:
@table @gcctabopt
@item -fabi-version=@var{n}
@opindex fabi-version
Use version @var{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.
@item -fno-access-control
@opindex fno-access-control
Turn off all access checking. This switch is mainly useful for working
around bugs in the access control code.
@item -fcheck-new
@opindex fcheck-new
Check that the pointer returned by @code{operator new} is non-null
before attempting to modify the storage allocated. This check is
normally unnecessary because the C++ standard specifies that
@code{operator new} will only return @code{0} if it is declared
@samp{throw()}, in which case the compiler will always check the
return value even without this option. In all other cases, when
@code{operator new} has a non-empty exception specification, memory
exhaustion is signalled by throwing @code{std::bad_alloc}. See also
@samp{new (nothrow)}.
@item -fconserve-space
@opindex 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 @code{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.
@item -fno-const-strings
@opindex fno-const-strings
Give string constants type @code{char *} instead of type @code{const
char *}. By default, G++ uses type @code{const char *} as required by
the standard. Even if you use @option{-fno-const-strings}, you cannot
actually modify the value of a string constant, unless you also use
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 @code{const char *}.
@item -fno-elide-constructors
@opindex 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.
@item -fno-enforce-eh-specs
@opindex 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 @samp{NDEBUG}. The compiler
will still optimize based on the exception specifications.
@item -ffor-scope
@itemx -fno-for-scope
@opindex ffor-scope
@opindex fno-for-scope
If @option{-ffor-scope} is specified, the scope of variables declared in
a @i{for-init-statement} is limited to the @samp{for} loop itself,
as specified by the C++ standard.
If @option{-fno-for-scope} is specified, the scope of variables declared in
a @i{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.
@item -fno-gnu-keywords
@opindex fno-gnu-keywords
Do not recognize @code{typeof} as a keyword, so that code can use this
word as an identifier. You can use the keyword @code{__typeof__} instead.
@option{-ansi} implies @option{-fno-gnu-keywords}.
@item -fno-implicit-templates
@opindex fno-implicit-templates
Never emit code for non-inline templates which are instantiated
implicitly (i.e.@: by use); only emit code for explicit instantiations.
@xref{Template Instantiation}, for more information.
@item -fno-implicit-inline-templates
@opindex 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.
@item -fno-implement-inlines
@opindex fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions
controlled by @samp{#pragma implementation}. This will cause linker
errors if these functions are not inlined everywhere they are called.
@item -fms-extensions
@opindex 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.
@item -fno-nonansi-builtins
@opindex fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by
ANSI/ISO C@. These include @code{ffs}, @code{alloca}, @code{_exit},
@code{index}, @code{bzero}, @code{conjf}, and other related functions.
@item -fno-operator-names
@opindex fno-operator-names
Do not treat the operator name keywords @code{and}, @code{bitand},
@code{bitor}, @code{compl}, @code{not}, @code{or} and @code{xor} as
synonyms as keywords.
@item -fno-optional-diags
@opindex 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.
@item -fpermissive
@opindex fpermissive
Downgrade some diagnostics about nonconformant code from errors to
warnings. Thus, using @option{-fpermissive} will allow some
nonconforming code to compile.
@item -frepo
@opindex frepo
Enable automatic template instantiation at link time. This option also
implies @option{-fno-implicit-templates}. @xref{Template
Instantiation}, for more information.
@item -fno-rtti
@opindex fno-rtti
Disable generation of information about every class with virtual
functions for use by the C++ runtime type identification features
(@samp{dynamic_cast} and @samp{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
@item -fstats
@opindex fstats
Emit statistics about front-end processing at the end of the compilation.
This information is generally only useful to the G++ development team.
@item -ftemplate-depth-@var{n}
@opindex ftemplate-depth
Set the maximum instantiation depth for template classes to @var{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.
@item -fuse-cxa-atexit
@opindex fuse-cxa-atexit
Register destructors for objects with static storage duration with the
@code{__cxa_atexit} function rather than the @code{atexit} function.
This option is required for fully standards-compliant handling of static
destructors, but will only work if your C library supports
@item -fno-weak
@opindex 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++.
@item -nostdinc++
@opindex 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.)
@end table
In addition, these optimization, warning, and code generation options
have meanings only for C++ programs:
@table @gcctabopt
@item -fno-default-inline
@opindex fno-default-inline
Do not assume @samp{inline} for functions defined inside a class scope.
@xref{Optimize Options,,Options That Control Optimization}. Note that these
functions will have linkage like inline functions; they just won't be
inlined by default.
@item -Wabi @r{(C++ only)}
@opindex Wabi
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:
@itemize @bullet
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; @};
@end smallexample
In this case, G++ will place @code{B::f2} into the same byte
as@code{A::f1}; other compilers will not. You can avoid this problem
by explicitly padding @code{A} so that its size is a multiple of the
byte size on your platform; that will cause G++ and other compilers to
layout @code{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 @{@};
@end smallexample
In this case, G++ will not place @code{B} into the tail-padding for
@code{A}; other compilers will. You can avoid this problem by
explicitly padding @code{A} so that its size is a multiple of its
alignment (ignoring virtual base classes); that will cause G++ and other
compilers to layout @code{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
union U @{ int i : 4096; @};
@end smallexample
Assuming that an @code{int} does not have 4096 bits, G++ will make the
union too small by the number of bits in an @code{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 @{@};
@end smallexample
G++ will place the @code{A} base class of @code{C} at a nonzero offset;
it should be placed at offset zero. G++ mistakenly believes that the
@code{A} data member of @code{B} is already at offset zero.
Names of template functions whose types involve @code{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) @{@}
@end smallexample
Instantiations of these templates may be mangled incorrectly.
@end itemize
@item -Wctor-dtor-privacy @r{(C++ only)}
@opindex Wctor-dtor-privacy
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.
@item -Wnon-virtual-dtor @r{(C++ only)}
@opindex Wnon-virtual-dtor
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 @option{-Wall}.
@item -Wreorder @r{(C++ only)}
@opindex Wreorder
@cindex reordering, warning
@cindex warning for reordering of member initializers
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) @{ @}
@end smallexample
The compiler will rearrange the member initializers for @samp{i}
and @samp{j} to match the declaration order of the members, emitting
a warning to that effect. This warning is enabled by @option{-Wall}.
@end table
The following @option{-W@dots{}} options are not affected by @option{-Wall}.
@table @gcctabopt
@item -Weffc++ @r{(C++ only)}
@opindex Weffc++
Warn about violations of the following style guidelines from Scott Meyers'
@cite{Effective C++} book:
@itemize @bullet
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 @code{operator=} return a reference to @code{*this}.
Item 23: Don't try to return a reference when you must return an object.
@end itemize
Also warn about violations of the following style guidelines from
Scott Meyers' @cite{More Effective C++} book:
@itemize @bullet
Item 6: Distinguish between prefix and postfix forms of increment and
decrement operators.
Item 7: Never overload @code{&&}, @code{||}, or @code{,}.
@end itemize
When selecting this option, be aware that the standard library
headers do not obey all of these guidelines; use @samp{grep -v}
to filter out those warnings.
@item -Wno-deprecated @r{(C++ only)}
@opindex Wno-deprecated
Do not warn about usage of deprecated features. @xref{Deprecated Features}.
@item -Wno-non-template-friend @r{(C++ only)}
@opindex Wno-non-template-friend
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.,
@samp{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++, @option{-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
@option{-Wno-non-template-friend} which keeps the conformant compiler code
but disables the helpful warning.
@item -Wold-style-cast @r{(C++ only)}
@opindex Wold-style-cast
Warn if an old-style (C-style) cast to a non-void type is used within
a C++ program. The new-style casts (@samp{static_cast},
@samp{reinterpret_cast}, and @samp{const_cast}) are less vulnerable to
unintended effects and much easier to search for.
@item -Woverloaded-virtual @r{(C++ only)}
@opindex Woverloaded-virtual
@cindex overloaded virtual fn, warning
@cindex warning for overloaded virtual fn
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);
@end smallexample
the @code{A} class version of @code{f} is hidden in @code{B}, and code
B* b;
@end smallexample
will fail to compile.
@item -Wno-pmf-conversions @r{(C++ only)}
@opindex Wno-pmf-conversions
Disable the diagnostic for converting a bound pointer to member function
to a plain pointer.
@item -Wsign-promo @r{(C++ only)}
@opindex Wsign-promo
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.
@item -Wsynth @r{(C++ only)}
@opindex Wsynth
@cindex warning for synthesized methods
@cindex synthesized methods, warning
Warn when G++'s synthesis behavior does not match that of cfront. For
struct A @{
operator int ();
A& operator = (int);
main ()
A a,b;
a = b;
@end smallexample
In this example, G++ will synthesize a default @samp{A& operator =
(const A&);}, while cfront will use the user-defined @samp{operator =}.
@end table
@node Objective-C Dialect Options
@section Options Controlling Objective-C Dialect
@cindex compiler options, Objective-C
@cindex Objective-C options, command line
@cindex options, Objective-C
(NOTE: This manual does not describe the Objective-C language itself. See
@w{@uref{}} for references.)
This section describes the command-line options that are only meaningful
for Objective-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 @code{some_class.m} like this:
gcc -g -fgnu-runtime -O -c some_class.m
@end smallexample
In this example, @option{-fgnu-runtime} is an option meant only for
Objective-C programs; you can use the other options with any language
supported by GCC@.
Here is a list of options that are @emph{only} for compiling Objective-C
@table @gcctabopt
@item -fconstant-string-class=@var{class-name}
@opindex fconstant-string-class
Use @var{class-name} as the name of the class to instantiate for each
literal string specified with the syntax @code{@@"@dots{}"}. The default
class name is @code{NXConstantString} if the GNU runtime is being used, and
@code{NSConstantString} if the NeXT runtime is being used (see below). The
@option{-fconstant-cfstrings} option, if also present, will override the
@option{-fconstant-string-class} setting and cause @code{@@"@dots{}"} literals
to be laid out as constant CoreFoundation strings.
@item -fgnu-runtime
@opindex fgnu-runtime
Generate object code compatible with the standard GNU Objective-C
runtime. This is the default for most types of systems.
@item -fnext-runtime
@opindex 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
@code{__NEXT_RUNTIME__} is predefined if (and only if) this option is
@item -fno-nil-receivers
@opindex fno-nil-receivers
Assume that all Objective-C message dispatches (e.g.,
@code{[receiver message:arg]}) in this translation unit ensure that the receiver
is not @code{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.
@item -fobjc-exceptions
@opindex 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;
@@catch (AnotherClass *exc) @{
@@catch (id allOthers) @{
@@finally @{
@@throw expr;
@end smallexample
The @code{@@throw} statement may appear anywhere in an Objective-C or
Objective-C++ program; when used inside of a @code{@@catch} block, the
@code{@@throw} may appear without an argument (as shown above), in which case
the object caught by the @code{@@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 @code{@@catch} clause capable of handling objects of that type,
analogously to how @code{catch} blocks work in C++ and Java. A
@code{@@catch(id @dots{})} clause (as shown above) may also be provided to catch
any and all Objective-C exceptions not caught by previous @code{@@catch}
clauses (if any).
The @code{@@finally} clause, if present, will be executed upon exit from the
immediately preceding @code{@@try @dots{} @@catch} section. This will happen
regardless of whether any exceptions are thrown, caught or rethrown
inside the @code{@@try @dots{} @@catch} section, analogously to the behavior
of the @code{finally} clause in Java.
There are several caveats to using the new exception mechanism:
@itemize @bullet
Although currently designed to be binary compatible with @code{NS_HANDLER}-style
idioms provided by the @code{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
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 @code{@@throw} an exception
from Objective-C and @code{catch} it in C++, or vice versa
(i.e., @code{throw @dots{} @@catch}).
@end itemize
The @option{-fobjc-exceptions} switch also enables the use of synchronization
blocks for thread-safe execution:
@@synchronized (ObjCClass *guard) @{
@end smallexample
Upon entering the @code{@@synchronized} block, a thread of execution shall
first check whether a lock has been placed on the corresponding @code{guard}
object by another thread. If it has, the current thread shall wait until
the other thread relinquishes its lock. Once @code{guard} becomes available,
the current thread will place its own lock on it, execute the code contained in
the @code{@@synchronized} block, and finally relinquish the lock (thereby
making @code{guard} available to other threads).
Unlike Java, Objective-C does not allow for entire methods to be marked
@code{@@synchronized}. Note that throwing exceptions out of
@code{@@synchronized} blocks is allowed, and will cause the guarding object
to be unlocked properly.
@item -freplace-objc-classes
@opindex freplace-objc-classes
Emit a special marker instructing @command{ld(1)} not to statically link in
the resulting object file, and allow @command{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.
@item -fzero-link
@opindex fzero-link
When compiling for the NeXT runtime, the compiler ordinarily replaces calls
to @code{objc_getClass("@dots{}")} (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 @option{-fzero-link} flag
suppresses this behavior and causes calls to @code{objc_getClass("@dots{}")}
to be retained. This is useful in Zero-Link debugging mode, since it allows
for individual class implementations to be modified during program execution.
@item -gen-decls
@opindex gen-decls
Dump interface declarations for all classes seen in the source file to a
file named @file{@var{sourcename}.decl}.
@item -Wno-protocol
@opindex 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 @code{-Wno-protocol} option, then
methods inherited from the superclass are considered to be implemented,
and no warning is issued for them.
@item -Wselector
@opindex 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 @code{@@selector(@dots{})}
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 @code{-fsyntax-only} option is
being used.
@item -Wundeclared-selector
@opindex Wundeclared-selector
Warn if a @code{@@selector(@dots{})} expression referring to an
undeclared selector is found. A selector is considered undeclared if no
method with that name has been declared before the
@code{@@selector(@dots{})} expression, either explicitly in an
@code{@@interface} or @code{@@protocol} declaration, or implicitly in
an @code{@@implementation} section. This option always performs its
checks as soon as a @code{@@selector(@dots{})} expression is found,
while @code{-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.
@item -print-objc-runtime-info
@opindex print-objc-runtime-info
Generate C header describing the largest structure that is passed by
value, if any.
@end table
@node Language Independent Options
@section Options to Control Diagnostic Messages Formatting
@cindex options to control diagnostics formatting
@cindex diagnostic messages
@cindex message formatting
Traditionally, diagnostic messages have been formatted irrespective of
the output device's aspect (e.g.@: its width, @dots{}). 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.
@table @gcctabopt
@item -fmessage-length=@var{n}
@opindex fmessage-length
Try to format error messages so that they fit on lines of about @var{n}
characters. The default is 72 characters for @command{g++} and 0 for the rest of
the front ends supported by GCC@. If @var{n} is zero, then no
line-wrapping will be done; each error message will appear on a single
@opindex fdiagnostics-show-location
@item -fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic messages
reporter to emit @emph{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
@item -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.
@end table
@node Warning Options
@section Options to Request or Suppress Warnings
@cindex options to control warnings
@cindex warning messages
@cindex messages, warning
@cindex suppressing 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 @samp{-W},
for example @option{-Wimplicit} to request warnings on implicit
declarations. Each of these specific warning options also has a
negative form beginning @samp{-Wno-} to turn off warnings;
for example, @option{-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
@ref{C++ Dialect Options} and @ref{Objective-C Dialect Options}.
@table @gcctabopt
@cindex syntax checking
@item -fsyntax-only
@opindex fsyntax-only
Check the code for syntax errors, but don't do anything beyond that.
@item -pedantic
@opindex 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 @option{-std} option used.
Valid ISO C and ISO C++ programs should compile properly with or without
this option (though a rare few will require @option{-ansi} or a
@option{-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.
@option{-pedantic} does not cause warning messages for use of the
alternate keywords whose names begin and end with @samp{__}. Pedantic
warnings are also disabled in the expression that follows
@code{__extension__}. However, only system header files should use
these escape routes; application programs should avoid them.
@xref{Alternate Keywords}.
Some users try to use @option{-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 @emph{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 @option{-pedantic}. We don't have plans to
support such a feature in the near future.
Where the standard specified with @option{-std} represents a GNU
extended dialect of C, such as @samp{gnu89} or @samp{gnu99}, there is a
corresponding @dfn{base standard}, the version of ISO C on which the GNU
extended dialect is based. Warnings from @option{-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.)
@item -pedantic-errors
@opindex pedantic-errors
Like @option{-pedantic}, except that errors are produced rather than
@item -w
@opindex w
Inhibit all warning messages.
@item -Wno-import
@opindex Wno-import
Inhibit warning messages about the use of @samp{#import}.
@item -Wchar-subscripts
@opindex Wchar-subscripts
Warn if an array subscript has type @code{char}. This is a common cause
of error, as programmers often forget that this type is signed on some
@item -Wcomment
@opindex Wcomment
Warn whenever a comment-start sequence @samp{/*} appears in a @samp{/*}
comment, or whenever a Backslash-Newline appears in a @samp{//} comment.
@item -Wformat
@opindex Wformat
Check calls to @code{printf} and @code{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 (@pxref{Function Attributes}), in the @code{printf},
@code{scanf}, @code{strftime} and @code{strfmon} (an X/Open extension,
not in the C standard) families.
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 @option{-pedantic} is used
with @option{-Wformat}, warnings will be given about format features not
in the selected standard version (but not for @code{strfmon} formats,
since those are not in any version of the C standard). @xref{C Dialect
Options,,Options Controlling C Dialect}.
Since @option{-Wformat} also checks for null format arguments for
several functions, @option{-Wformat} also implies @option{-Wnonnull}.
@option{-Wformat} is included in @option{-Wall}. For more control over some
aspects of format checking, the options @option{-Wformat-y2k},
@option{-Wno-format-extra-args}, @option{-Wno-format-zero-length},
@option{-Wformat-nonliteral}, @option{-Wformat-security}, and
@option{-Wformat=2} are available, but are not included in @option{-Wall}.
@item -Wformat-y2k
@opindex Wformat-y2k
If @option{-Wformat} is specified, also warn about @code{strftime}
formats which may yield only a two-digit year.
@item -Wno-format-extra-args
@opindex Wno-format-extra-args
If @option{-Wformat} is specified, do not warn about excess arguments to a
@code{printf} or @code{scanf} format function. The C standard specifies
that such arguments are ignored.
Where the unused arguments lie between used arguments that are
specified with @samp{$} operand number specifications, normally
warnings are still given, since the implementation could not know what
type to pass to @code{va_arg} to skip the unused arguments. However,
in the case of @code{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.
@item -Wno-format-zero-length
@opindex Wno-format-zero-length
If @option{-Wformat} is specified, do not warn about zero-length formats.
The C standard specifies that zero-length formats are allowed.
@item -Wformat-nonliteral
@opindex Wformat-nonliteral
If @option{-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 @code{va_list}.
@item -Wformat-security
@opindex Wformat-security
If @option{-Wformat} is specified, also warn about uses of format
functions that represent possible security problems. At present, this
warns about calls to @code{printf} and @code{scanf} functions where the
format string is not a string literal and there are no format arguments,
as in @code{printf (foo);}. This may be a security hole if the format
string came from untrusted input and contains @samp{%n}. (This is
currently a subset of what @option{-Wformat-nonliteral} warns about, but
in future warnings may be added to @option{-Wformat-security} that are not
included in @option{-Wformat-nonliteral}.)
@item -Wformat=2
@opindex Wformat=2
Enable @option{-Wformat} plus format checks not included in
@option{-Wformat}. Currently equivalent to @samp{-Wformat
-Wformat-nonliteral -Wformat-security -Wformat-y2k}.
@item -Wnonnull
@opindex Wnonnull
Warn about passing a null pointer for arguments marked as
requiring a non-null value by the @code{nonnull} function attribute.
@option{-Wnonnull} is included in @option{-Wall} and @option{-Wformat}. It
can be disabled with the @option{-Wno-nonnull} option.
@item -Winit-self @r{(C, C++, and Objective-C only)}
@opindex Winit-self
Warn about uninitialized variables which are initialized with themselves.
Note this option can only be used with the @option{-Wuninitialized} option,
which in turn only works with @option{-O1} and above.
For example, GCC will warn about @code{i} being uninitialized in the
following snippet only when @option{-Winit-self} has been specified:
int f()
int i = i;
return i;
@end group
@end smallexample
@item -Wimplicit-int
@opindex Wimplicit-int
Warn when a declaration does not specify a type.
@item -Wimplicit-function-declaration
@itemx -Werror-implicit-function-declaration
@opindex Wimplicit-function-declaration
@opindex Werror-implicit-function-declaration
Give a warning (or error) whenever a function is used before being
@item -Wimplicit
@opindex Wimplicit
Same as @option{-Wimplicit-int} and @option{-Wimplicit-function-declaration}.
@item -Wmain
@opindex Wmain
Warn if the type of @samp{main} is suspicious. @samp{main} should be a
function with external linkage, returning int, taking either zero
arguments, two, or three arguments of appropriate types.
@item -Wmissing-braces
@opindex Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed. In
the following example, the initializer for @samp{a} is not fully
bracketed, but that for @samp{b} is fully bracketed.
int a[2][2] = @{ 0, 1, 2, 3 @};
int b[2][2] = @{ @{ 0, 1 @}, @{ 2, 3 @} @};
@end smallexample
@item -Wparentheses
@opindex 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.
Also warn about constructions where there may be confusion to which
@code{if} statement an @code{else} branch belongs. Here is an example of
such a case:
if (a)
if (b)
foo ();
bar ();
@end group
@end smallexample
In C, every @code{else} branch belongs to the innermost possible @code{if}
statement, which in this example is @code{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
@code{if} statement so there is no way the @code{else} could belong to
the enclosing @code{if}. The resulting code would look like this:
if (a)
if (b)
foo ();
bar ();
@end group
@end smallexample
@item -Wsequence-point
@opindex 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 @dfn{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
@code{&&}, @code{||}, @code{? :} or @code{,} (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 @code{a = a++;}, @code{a[n]
= b[n++]} and @code{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
@item -Wreturn-type
@opindex Wreturn-type
Warn whenever a function is defined with a return-type that defaults to
@code{int}. Also warn about any @code{return} statement with no
return-value in a function whose return-type is not @code{void}.
For C++, a function without return type always produces a diagnostic
message, even when @option{-Wno-return-type} is specified. The only
exceptions are @samp{main} and functions defined in system headers.
@item -Wswitch
@opindex Wswitch
Warn whenever a @code{switch} statement has an index of enumerated type
and lacks a @code{case} for one or more of the named codes of that
enumeration. (The presence of a @code{default} label prevents this
warning.) @code{case} labels outside the enumeration range also
provoke warnings when this option is used.
@item -Wswitch-default
@opindex Wswitch-switch
Warn whenever a @code{switch} statement does not have a @code{default}
@item -Wswitch-enum
@opindex Wswitch-enum
Warn whenever a @code{switch} statement has an index of enumerated type
and lacks a @code{case} for one or more of the named codes of that
enumeration. @code{case} labels outside the enumeration range also
provoke warnings when this option is used.
@item -Wtrigraphs
@opindex Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning of
the program (trigraphs within comments are not warned about).
@item -Wunused-function
@opindex Wunused-function
Warn whenever a static function is declared but not defined or a
non\-inline static function is unused.
@item -Wunused-label
@opindex Wunused-label
Warn whenever a label is declared but not used.
To suppress this warning use the @samp{unused} attribute
(@pxref{Variable Attributes}).
@item -Wunused-parameter
@opindex Wunused-parameter
Warn whenever a function parameter is unused aside from its declaration.
To suppress this warning use the @samp{unused} attribute
(@pxref{Variable Attributes}).
@item -Wunused-variable
@opindex Wunused-variable
Warn whenever a local variable or non-constant static variable is unused
aside from its declaration
To suppress this warning use the @samp{unused} attribute
(@pxref{Variable Attributes}).
@item -Wunused-value
@opindex Wunused-value
Warn whenever a statement computes a result that is explicitly not used.
To suppress this warning cast the expression to @samp{void}.
@item -Wunused
@opindex Wunused
All the above @option{-Wunused} options combined.
In order to get a warning about an unused function parameter, you must
either specify @samp{-Wextra -Wunused} (note that @samp{-Wall} implies
@samp{-Wunused}), or separately specify @option{-Wunused-parameter}.
@item -Wuninitialized
@opindex Wuninitialized
Warn if an automatic variable is used without first being initialized or
if a variable may be clobbered by a @code{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 @option{-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 @option{-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 @code{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;
case 2: x = 4;
case 3: x = 5;
foo (x);
@end group
@end smallexample
If the value of @code{y} is always 1, 2 or 3, then @code{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;
@end smallexample
This has no bug because @code{save_y} is used only if it is set.
@cindex @code{longjmp} warnings
This option also warns when a non-volatile automatic variable might be
changed by a call to @code{longjmp}. These warnings as well are possible
only in optimizing compilation.
The compiler sees only the calls to @code{setjmp}. It cannot know
where @code{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 @code{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 @code{noreturn}. @xref{Function
@item -Wunknown-pragmas
@opindex Wunknown-pragmas
@cindex warning for unknown pragmas
@cindex unknown pragmas, warning
@cindex pragmas, warning of unknown
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 @option{-Wall} command line option.
@item -Wstrict-aliasing
@opindex Wstrict-aliasing
This option is only active when @option{-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 @option{-Wall}.
@item -Wall
@opindex Wall
All of the above @samp{-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 @ref{C++ Dialect Options} and
@ref{Objective-C Dialect Options}.
@end table
The following @option{-W@dots{}} options are not implied by @option{-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.
@table @gcctabopt
@item -Wextra
@opindex W
@opindex Wextra
(This option used to be called @option{-W}. The older name is still
supported, but the newer name is more descriptive.) Print extra warning
messages for these events:
@itemize @bullet
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
foo (a)
if (a > 0)
return a;
@end group
@end smallexample
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 @samp{x[i,j]} will cause a warning,
but @samp{x[(void)i,j]} will not.
An unsigned value is compared against zero with @samp{<} or @samp{>=}.
A comparison like @samp{x<=y<=z} appears; this is equivalent to
@samp{(x<=y ? 1 : 0) <= z}, which is a different interpretation from
that of ordinary mathematical notation.
Storage-class specifiers like @code{static} are not the first things in
a declaration. According to the C Standard, this usage is obsolescent.
The return type of a function has a type qualifier such as @code{const}.
Such a type qualifier has no effect, since the value returned by a
function is not an lvalue. (But don't warn about the GNU extension of
@code{volatile void} return types. That extension will be warned about
if @option{-pedantic} is specified.)
If @option{-Wall} or @option{-Wunused} is also specified, warn about unused
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 @option{-Wno-sign-compare} is also specified.)
An aggregate has an initializer which does not initialize all members.
For example, the following code would cause such a warning, because
@code{x.h} would be implicitly initialized to zero:
struct s @{ int f, g, h; @};
struct s x = @{ 3, 4 @};
@end smallexample
A function parameter is declared without a type specifier in K&R-style
void foo(bar) @{ @}
@end smallexample
An empty body occurs in an @samp{if} or @samp{else} statement.
A pointer is compared against integer zero with @samp{<}, @samp{<=},
@samp{>}, or @samp{>=}.
A variable might be changed by @samp{longjmp} or @samp{vfork}.
Any of several floating-point events that often indicate errors, such as
overflow, underflow, loss of precision, etc.
@item @r{(C++ only)}
An enumerator and a non-enumerator both appear in a conditional expression.
@item @r{(C++ only)}
A non-static reference or non-static @samp{const} member appears in a
class without constructors.
@item @r{(C++ only)}
Ambiguous virtual bases.
@item @r{(C++ only)}
Subscripting an array which has been declared @samp{register}.
@item @r{(C++ only)}
Taking the address of a variable which has been declared @samp{register}.
@item @r{(C++ only)}
A base class is not initialized in a derived class' copy constructor.
@end itemize
@item -Wno-div-by-zero
@opindex Wno-div-by-zero
@opindex Wdiv-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.
@item -Wsystem-headers
@opindex Wsystem-headers
@cindex warnings from system headers
@cindex system headers, warnings from
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 @option{-Wall} in conjunction with this
option will @emph{not} warn about unknown pragmas in system
headers---for that, @option{-Wunknown-pragmas} must also be used.
@item -Wfloat-equal
@opindex 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.
@item -Wtraditional @r{(C only)}
@opindex Wtraditional
Warn about certain constructs that behave differently in traditional and
ISO C@. Also warn about ISO C constructs that have no traditional C
equivalent, and/or problematic constructs which should be avoided.
@itemize @bullet
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 @samp{#} appeared in column 1 on the line. Therefore
@option{-Wtraditional} warns about directives that traditional C
understands but would ignore because the @samp{#} does not appear as the
first character on the line. It also suggests you hide directives like
@samp{#pragma} not understood by traditional C by indenting them. Some
traditional implementations would not recognize @samp{#elif}, so it
suggests avoiding it altogether.
A function-like macro that appears without arguments.
The unary plus operator.
The @samp{U} integer constant suffix, or the @samp{F} or @samp{L} floating point
constant suffixes. (Traditional C does support the @samp{L} suffix on integer
constants.) Note, these suffixes appear in macros defined in the system
headers of most modern systems, e.g.@: the @samp{_MIN}/@samp{_MAX} macros in @code{<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 @code{switch} statement has an operand of type @code{long}.
A non-@code{static} function declaration follows a @code{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.@: @code{__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 @option{-Wconversion}.
Use of ISO C style function definitions. This warning intentionally is
@emph{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, @code{PARAMS} and
@code{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.
@end itemize
@item -Wdeclaration-after-statement @r{(C only)}
@opindex Wdeclaration-after-statement
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. @xref{Mixed Declarations}.
@item -Wundef
@opindex Wundef
Warn if an undefined identifier is evaluated in an @samp{#if} directive.
@item -Wendif-labels
@opindex Wendif-labels
Warn whenever an @samp{#else} or an @samp{#endif} are followed by text.
@item -Wshadow
@opindex Wshadow
Warn whenever a local variable shadows another local variable, parameter or
global variable or whenever a built-in function is shadowed.
@item -Wlarger-than-@var{len}
@opindex Wlarger-than
Warn whenever an object of larger than @var{len} bytes is defined.
@item -Wpointer-arith
@opindex Wpointer-arith
Warn about anything that depends on the ``size of'' a function type or
of @code{void}. GNU C assigns these types a size of 1, for
convenience in calculations with @code{void *} pointers and pointers
to functions.
@item -Wbad-function-cast @r{(C only)}
@opindex Wbad-function-cast
Warn whenever a function call is cast to a non-matching type.
For example, warn if @code{int malloc()} is cast to @code{anything *}.
@item -Wcast-qual
@opindex Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier from
the target type. For example, warn if a @code{const char *} is cast
to an ordinary @code{char *}.
@item -Wcast-align
@opindex Wcast-align
Warn whenever a pointer is cast such that the required alignment of the
target is increased. For example, warn if a @code{char *} is cast to
an @code{int *} on machines where integers can only be accessed at
two- or four-byte boundaries.
@item -Wwrite-strings
@opindex Wwrite-strings
When compiling C, give string constants the type @code{const
char[@var{length}]} so that
copying the address of one into a non-@code{const} @code{char *}
pointer will get a warning; when compiling C++, warn about the
deprecated conversion from string constants to @code{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 @code{const} in
declarations and prototypes. Otherwise, it will just be a nuisance;
this is why we did not make @option{-Wall} request these warnings.
@item -Wconversion
@opindex 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
@code{x = -1} if @code{x} is unsigned. But do not warn about explicit
casts like @code{(unsigned) -1}.
@item -Wsign-compare
@opindex Wsign-compare
@cindex warning for comparison of signed and unsigned values
@cindex comparison of signed and unsigned values, warning
@cindex signed and unsigned values, comparison warning
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 @option{-Wextra}; to get the other warnings
of @option{-Wextra} without this warning, use @samp{-Wextra -Wno-sign-compare}.
@item -Waggregate-return
@opindex 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.)
@item -Wstrict-prototypes @r{(C only)}
@opindex Wstrict-prototypes
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
@item -Wold-style-definition @r{(C only)}
@opindex Wold-style-definition
Warn if an old-style function definition is used. A warning is given
even if there is a previous prototype.
@item -Wmissing-prototypes @r{(C only)}
@opindex Wmissing-prototypes
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.
@item -Wmissing-declarations @r{(C only)}
@opindex Wmissing-declarations
Warn if a global function is defined without a previous declaration.
Do so even if the definition itself provides a prototype.
Use this option to detect global functions that are not declared in
header files.
@item -Wmissing-noreturn
@opindex Wmissing-noreturn
Warn about functions which might be candidates for attribute @code{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 @code{noreturn} attribute, otherwise subtle code generation
bugs could be introduced. You will not get a warning for @code{main} in
hosted C environments.
@item -Wmissing-format-attribute
@opindex Wmissing-format-attribute
@opindex Wformat
If @option{-Wformat} is enabled, also warn about functions which might be
candidates for @code{format} attributes. Note these are only possible
candidates, not absolute ones. GCC will guess that @code{format}
attributes might be appropriate for any function that calls a function
like @code{vprintf} or @code{vscanf}, but this might not always be the
case, and some functions for which @code{format} attributes are
appropriate may not be detected. This option has no effect unless
@option{-Wformat} is enabled (possibly by @option{-Wall}).
@item -Wno-multichar
@opindex Wno-multichar
@opindex Wmultichar
Do not warn if a multicharacter constant (@samp{'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.
@item -Wno-deprecated-declarations
@opindex Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as
deprecated by using the @code{deprecated} attribute.
(@pxref{Function Attributes}, @pxref{Variable Attributes},
@pxref{Type Attributes}.)
@item -Wpacked
@opindex 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 @code{f.x} in @code{struct bar}
will be misaligned even though @code{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;
@end group
@end smallexample
@item -Wpadded
@opindex 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.
@item -Wredundant-decls
@opindex 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.
@item -Wnested-externs @r{(C only)}
@opindex Wnested-externs
Warn if an @code{extern} declaration is encountered within a function.
@item -Wunreachable-code
@opindex 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 @option{-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.
@item -Winline
@opindex 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 @option{-Winline} to appear or disappear.
@item -Wno-invalid-offsetof @r{(C++ only)}
@opindex Wno-invalid-offsetof
Suppress warnings from applying the @samp{offsetof} macro to a non-POD
type. According to the 1998 ISO C++ standard, applying @samp{offsetof}
to a non-POD type is undefined. In existing C++ implementations,
however, @samp{offsetof} typically gives meaningful results even when
applied to certain kinds of non-POD types. (Such as a simple
@samp{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 @samp{offsetof} may be relaxed in a future version
of the C++ standard.
@item -Winvalid-pch
@opindex Winvalid-pch
Warn if a precompiled header (@pxref{Precompiled Headers}) is found in
the search path but can't be used.
@item -Wlong-long
@opindex Wlong-long
@opindex Wno-long-long
Warn if @samp{long long} type is used. This is default. To inhibit
the warning messages, use @option{-Wno-long-long}. Flags
@option{-Wlong-long} and @option{-Wno-long-long} are taken into account
only when @option{-pedantic} flag is used.
@item -Wdisabled-optimization
@opindex 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.
@item -Werror
@opindex Werror
Make all warnings into errors.
@end table
@node Debugging Options
@section Options for Debugging Your Program or GCC
@cindex options, debugging
@cindex debugging information options
GCC has various special options that are used for debugging
either your program or GCC:
@table @gcctabopt
@item -g
@opindex g
Produce debugging information in the operating system's native format
(stabs, COFF, XCOFF, or DWARF)@. GDB can work with this debugging
On most systems that use stabs format, @option{-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 @option{-gstabs+}, @option{-gstabs},
@option{-gxcoff+}, @option{-gxcoff}, or @option{-gvms} (see below).
Unlike most other C compilers, GCC allows you to use @option{-g} with
@option{-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.
@item -ggdb
@opindex 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
@item -gstabs
@opindex 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.
@item -feliminate-unused-debug-symbols
@opindex feliminate-unused-debug-symbols
Produce debugging information in stabs format (if that is supported),
for only symbols that are actually used.
@item -gstabs+
@opindex 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.
@item -gcoff
@opindex gcoff
Produce debugging information in COFF format (if that is supported).
This is the format used by SDB on most System V systems prior to
System V Release 4.
@item -gxcoff
@opindex gxcoff
Produce debugging information in XCOFF format (if that is supported).
This is the format used by the DBX debugger on IBM RS/6000 systems.
@item -gxcoff+
@opindex gxcoff+
Produce debugging information in XCOFF format (if that is supported),
using GNU extensions understood only by the GNU debugger (GDB)@. The
use of these extensions is likely to make other debuggers crash or
refuse to read the program, and may cause assemblers other than the GNU
assembler (GAS) to fail with an error.
@item -gdwarf-2
@opindex gdwarf-2
Produce debugging information in DWARF version 2 format (if that is
supported). This is the format used by DBX on IRIX 6.
@item -gvms
@opindex gvms
Produce debugging information in VMS debug format (if that is
supported). This is the format used by DEBUG on VMS systems.
@item -g@var{level}
@itemx -ggdb@var{level}
@itemx -gstabs@var{level}
@itemx -gcoff@var{level}
@itemx -gxcoff@var{level}
@itemx -gvms@var{level}
Request debugging information and also use @var{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 @option{-g3}.
Note that in order to avoid confusion between DWARF1 debug level 2,
and DWARF2 @option{-gdwarf-2} does not accept a concatenated debug
level. Instead use an additional @option{-g@var{level}} option to
change the debug level for DWARF2.
@item -feliminate-dwarf2-dups
@opindex 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 @option{-gdwarf-2}.
@cindex @command{prof}
@item -p
@opindex p
Generate extra code to write profile information suitable for the
analysis program @command{prof}. You must use this option when compiling
the source files you want data about, and you must also use it when
@cindex @command{gprof}
@item -pg
@opindex pg
Generate extra code to write profile information suitable for the
analysis program @command{gprof}. You must use this option when compiling
the source files you want data about, and you must also use it when
@item -Q
@opindex Q
Makes the compiler print out each function name as it is compiled, and
print some statistics about each pass when it finishes.
@item -ftime-report
@opindex ftime-report
Makes the compiler print some statistics about the time consumed by each
pass when it finishes.
@item -fmem-report
@opindex fmem-report
Makes the compiler print some statistics about permanent memory
allocation when it finishes.
@item -fprofile-arcs
@opindex fprofile-arcs
Add code so that program flow @dfn{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
@file{@var{auxname}.gcda} for each source file. The data may be used for
profile-directed optimizations (@option{-fbranch-probabilities}), or for
test coverage analysis (@option{-ftest-coverage}). Each object file's
@var{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. @file{foo.gcda} for input file @file{dir/foo.c}, or
@file{dir/foo.gcda} for output file specified as @option{-o dir/foo.o}).
Compile the source files with @option{-fprofile-arcs} plus optimization
and code generation options. For test coverage analysis, use the
additional @option{-ftest-coverage} option. You do not need to profile
every source file in a program.
Link your object files with @option{-lgcov} or @option{-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
@code{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
@option{-fbranch-probabilities} (@pxref{Optimize Options,,Options that
Control Optimization}).
For test coverage analysis, use @command{gcov} to produce human readable
information from the @file{.gcno} and @file{.gcda} files. Refer to the
@command{gcov} documentation for further information.
@end itemize
With @option{-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.
@need 2000
@item -ftest-coverage
@opindex ftest-coverage
Produce a notes file that the @command{gcov} code-coverage utility
(@pxref{Gcov,, @command{gcov}---a Test Coverage Program}) can use to
show program coverage. Each source file's note file is called
@file{@var{auxname}.gcno}. Refer to the @option{-fprofile-arcs} option
above for a description of @var{auxname} and instructions on how to
generate test coverage data. Coverage data will match the source files
more closely, if you do not optimize.
@item -d@var{letters}
@opindex d
Says to make debugging dumps during compilation at times specified by
@var{letters}. This is used for debugging the compiler. The file names
for most of the dumps are made by appending a pass number and a word to
the @var{dumpname}. @var{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. In both cases any
suffix is removed (e.g. @file{foo.01.rtl} or @file{foo.02.sibling}).
Here are the possible letters for use in @var{letters}, and their
@table @samp
@item A
@opindex dA
Annotate the assembler output with miscellaneous debugging information.
@item b
@opindex db
Dump after computing branch probabilities, to @file{@var{file}.12.bp}.
@item B
@opindex dB
Dump after block reordering, to @file{@var{file}.31.bbro}.
@item c
@opindex dc
Dump after instruction combination, to the file @file{@var{file}.20.combine}.
@item C
@opindex dC
Dump after the first if conversion, to the file @file{@var{file}.14.ce1}.
Also dump after the second if conversion, to the file @file{@var{file}.21.ce2}.
@item d
@opindex dd
Dump after branch target load optimization, to to @file{@var{file}.32.btl}.
Also dump after delayed branch scheduling, to @file{@var{file}.36.dbr}.
@item D
@opindex dD
Dump all macro definitions, at the end of preprocessing, in addition to
normal output.
@item E
@opindex dE
Dump after the third if conversion, to @file{@var{file}.30.ce3}.
@item f
@opindex df
Dump after control and data flow analysis, to @file{@var{file}.11.cfg}.
Also dump after life analysis, to @file{@var{file}}.
@item F
@opindex dF
Dump after purging @code{ADDRESSOF} codes, to @file{@var{file}.07.addressof}.
@item g
@opindex dg
Dump after global register allocation, to @file{@var{file}.25.greg}.
@item G
@opindex dG
Dump after GCSE, to @file{@var{file}.08.gcse}.
Also dump after jump bypassing and control flow optimizations, to
@item h
@opindex dh
Dump after finalization of EH handling code, to @file{@var{file}}.
@item i
@opindex di
Dump after sibling call optimizations, to @file{@var{file}.02.sibling}.
@item j
@opindex dj
Dump after the first jump optimization, to @file{@var{file}.04.jump}.
@item k
@opindex dk
Dump after conversion from registers to stack, to @file{@var{file}.34.stack}.
@item l
@opindex dl
Dump after local register allocation, to @file{@var{file}.24.lreg}.
@item L
@opindex dL
Dump after loop optimization passes, to @file{@var{file}.09.loop} and
@item M
@opindex dM
Dump after performing the machine dependent reorganization pass, to
@item n
@opindex dn
Dump after register renumbering, to @file{@var{file}.29.rnreg}.
@item N
@opindex dN
Dump after the register move pass, to @file{@var{file}.22.regmove}.
@item o
@opindex do
Dump after post-reload optimizations, to @file{@var{file}.26.postreload}.
@item r
@opindex dr
Dump after RTL generation, to @file{@var{file}.01.rtl}.
@item R
@opindex dR
Dump after the second scheduling pass, to @file{@var{file}.33.sched2}.
@item s
@opindex ds
Dump after CSE (including the jump optimization that sometimes follows
CSE), to @file{@var{file}.06.cse}.
@item S
@opindex dS
Dump after the first scheduling pass, to @file{@var{file}.23.sched}.
@item t
@opindex dt
Dump after the second CSE pass (including the jump optimization that
sometimes follows CSE), to @file{@var{file}.18.cse2}.
@item T
@opindex dT
Dump after running tracer, to @file{@var{file}.15.tracer}.
@item u
@opindex du
Dump after null pointer elimination pass to @file{@var{file}.05.null}.
@item U
@opindex dU
Dump callgraph and unit-at-a-time optimization @file{@var{file}.00.unit}.
@item V
@opindex dV
Dump after the value profile transformations, to @file{@var{file}.13.vpt}.
@item w
@opindex dw
Dump after the second flow pass, to @file{@var{file}.27.flow2}.
@item z
@opindex dz
Dump after the peephole pass, to @file{@var{file}.28.peephole2}.
@item Z
@opindex dZ
Dump after constructing the web, to @file{@var{file}.17.web}.
@item a
@opindex da
Produce all the dumps listed above.
@item H
@opindex dH
Produce a core dump whenever an error occurs.
@item m
@opindex dm
Print statistics on memory usage, at the end of the run, to
standard error.
@item p
@opindex dp
Annotate the assembler output with a comment indicating which
pattern and alternative was used. The length of each instruction is
also printed.
@item P
@opindex dP
Dump the RTL in the assembler output as a comment before each instruction.
Also turns on @option{-dp} annotation.
@item v
@opindex dv
For each of the other indicated dump files (except for
@file{@var{file}.01.rtl}), dump a representation of the control flow graph
suitable for viewing with VCG to @file{@var{file}.@var{pass}.vcg}.
@item x
@opindex dx
Just generate RTL for a function instead of compiling it. Usually used
with @samp{r}.
@item y
@opindex dy
Dump debugging information during parsing, to standard error.
@end table
@item -fdump-unnumbered
@opindex fdump-unnumbered
When doing debugging dumps (see @option{-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 @option{-g}.
@item -fdump-translation-unit @r{(C and C++ only)}
@itemx -fdump-translation-unit-@var{options} @r{(C and C++ only)}
@opindex fdump-translation-unit
Dump a representation of the tree structure for the entire translation
unit to a file. The file name is made by appending @file{.tu} to the
source file name. If the @samp{-@var{options}} form is used, @var{options}
controls the details of the dump as described for the
@option{-fdump-tree} options.
@item -fdump-class-hierarchy @r{(C++ only)}
@itemx -fdump-class-hierarchy-@var{options} @r{(C++ only)}
@opindex fdump-class-hierarchy
Dump a representation of each class's hierarchy and virtual function
table layout to a file. The file name is made by appending @file{.class}
to the source file name. If the @samp{-@var{options}} form is used,
@var{options} controls the details of the dump as described for the
@option{-fdump-tree} options.
@item -fdump-tree-@var{switch} @r{(C++ only)}
@itemx -fdump-tree-@var{switch}-@var{options} @r{(C++ only)}
@opindex fdump-tree
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 @samp{-@var{options}}
form is used, @var{options} is a list of @samp{-} 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
@table @samp
@item 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.
@item 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.
@item all
Turn on all options.
@end table
The following tree dumps are possible:
@table @samp
@item original
Dump before any tree based optimization, to @file{@var{file}.original}.
@item optimized
Dump after all tree based optimization, to @file{@var{file}.optimized}.
@item inlined
Dump after function inlining, to @file{@var{file}.inlined}.
@end table
@item -frandom-seed=@var{string}
@opindex frandom-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 @option{-frandom-seed} option to produce
reproducibly identical object files.
The @var{string} should be different for every file you compile.
@item -fsched-verbose=@var{n}
@opindex fsched-verbose
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 @option{-dS} or @option{-dR} is
specified, in which case it is output to the usual dump
listing file, @file{.sched} or @file{.sched2} respectively. However
for @var{n} greater than nine, the output is always printed to standard
For @var{n} greater than zero, @option{-fsched-verbose} outputs the
same information as @option{-dRS}. For @var{n} greater than one, it
also output basic block probabilities, detailed ready list information
and unit/insn info. For @var{n} greater than two, it includes RTL
at abort point, control-flow and regions info. And for @var{n} over
four, @option{-fsched-verbose} also includes dependence info.
@item -save-temps
@opindex 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 @file{foo.c} with @samp{-c -save-temps} would produce files
@file{foo.i} and @file{foo.s}, as well as @file{foo.o}. This creates a
preprocessed @file{foo.i} output file even though the compiler now
normally uses an integrated preprocessor.
@item -time
@opindex 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
@end smallexample
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.
@item -print-file-name=@var{library}
@opindex print-file-name
Print the full absolute name of the library file @var{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.
@item -print-multi-directory
@opindex 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 @env{GCC_EXEC_PREFIX}.
@item -print-multi-lib
@opindex 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
@samp{;}, and each switch starts with an @samp{@@} instead of the
@samp{-}, without spaces between multiple switches. This is supposed to
ease shell-processing.
@item -print-prog-name=@var{program}
@opindex print-prog-name
Like @option{-print-file-name}, but searches for a program such as @samp{cpp}.
@item -print-libgcc-file-name
@opindex print-libgcc-file-name
Same as @option{-print-file-name=libgcc.a}.
This is useful when you use @option{-nostdlib} or @option{-nodefaultlibs}
but you do want to link with @file{libgcc.a}. You can do
gcc -nostdlib @var{files}@dots{} `gcc -print-libgcc-file-name`
@end smallexample
@item -print-search-dirs
@opindex print-search-dirs
Print the name of the configured installation directory and a list of
program and library directories @command{gcc} will search---and don't do anything else.
This is useful when @command{gcc} prints the error message
@samp{installation problem, cannot exec cpp0: No such file or directory}.
To resolve this you either need to put @file{cpp0} and the other compiler
components where @command{gcc} expects to find them, or you can set the environment
variable @env{GCC_EXEC_PREFIX} to the directory where you installed them.
Don't forget the trailing '/'.
@xref{Environment Variables}.
@item -dumpmachine
@opindex dumpmachine
Print the compiler's target machine (for example,
@samp{i686-pc-linux-gnu})---and don't do anything else.
@item -dumpversion
@opindex dumpversion
Print the compiler version (for example, @samp{3.0})---and don't do
anything else.
@item -dumpspecs
@opindex dumpspecs
Print the compiler's built-in specs---and don't do anything else. (This
is used when GCC itself is being built.) @xref{Spec Files}.
@item -feliminate-unused-debug-types
@opindex 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.
@end table
@node Optimize Options
@section Options That Control Optimization
@cindex optimize options
@cindex options, 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
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. Using the @option{-funit-at-a-time} flag will allow 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 (and using @option{-funit-at-a-time}) will allow
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.
@table @gcctabopt
@item -O
@itemx -O1
@opindex O
@opindex O1
Optimize. Optimizing compilation takes somewhat more time, and a lot
more memory for a large function.
With @option{-O}, the compiler tries to reduce code size and execution
time, without performing any optimizations that take a great deal of
compilation time.
@option{-O} turns on the following optimization flags:
@gccoptlist{-fdefer-pop @gol
-fmerge-constants @gol
-fthread-jumps @gol
-floop-optimize @gol
-fif-conversion @gol
-fif-conversion2 @gol
-fdelayed-branch @gol
-fguess-branch-probability @gol
@option{-O} also turns on @option{-fomit-frame-pointer} on machines
where doing so does not interfere with debugging.
@item -O2
@opindex 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 @option{-O2}.
As compared to @option{-O}, this option increases both compilation time
and the performance of the generated code.
@option{-O2} turns on all optimization flags specified by @option{-O}. It
also turns on the following optimization flags:
@gccoptlist{-fforce-mem @gol
-foptimize-sibling-calls @gol
-fstrength-reduce @gol
-fcse-follow-jumps -fcse-skip-blocks @gol
-frerun-cse-after-loop -frerun-loop-opt @gol
-fgcse -fgcse-lm -fgcse-sm -fgcse-las @gol
-fdelete-null-pointer-checks @gol
-fexpensive-optimizations @gol
-fregmove @gol
-fschedule-insns -fschedule-insns2 @gol
-fsched-interblock -fsched-spec @gol
-fcaller-saves @gol
-fpeephole2 @gol
-freorder-blocks -freorder-functions @gol
-fstrict-aliasing @gol
-funit-at-a-time @gol
-falign-functions -falign-jumps @gol
-falign-loops -falign-labels @gol
Please note the warning under @option{-fgcse} about
invoking @option{-O2} on programs that use computed gotos.
@item -O3
@opindex O3
Optimize yet more. @option{-O3} turns on all optimizations specified by
@option{-O2} and also turns on the @option{-finline-functions},
@option{-fweb} and @option{-frename-registers} options.
@item -O0
@opindex O0
Do not optimize. This is the default.
@item -Os
@opindex Os
Optimize for size. @option{-Os} enables all @option{-O2} optimizations that
do not typically increase code size. It also performs further
optimizations designed to reduce code size.
@option{-Os} disables the following optimization flags:
@gccoptlist{-falign-functions -falign-jumps -falign-loops @gol
-falign-labels -freorder-blocks -fprefetch-loop-arrays}
If you use multiple @option{-O} options, with or without level numbers,
the last such option is the one that is effective.
@end table
Options of the form @option{-f@var{flag}} specify machine-independent
flags. Most flags have both positive and negative forms; the negative
form of @option{-ffoo} would be @option{-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 @samp{no-}
or adding it.
The following options control specific optimizations. They are either
activated by @option{-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.
@table @gcctabopt
@item -fno-default-inline
@opindex 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
@w{@option{-O}}, member functions defined inside class scope are compiled
inline by default; i.e., you don't need to add @samp{inline} in front of
the member function name.
@item -fno-defer-pop
@opindex 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 @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
@item -fforce-mem
@opindex 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
Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
@item -fforce-addr
@opindex fforce-addr
Force memory address constants to be copied into registers before
doing arithmetic on them. This may produce better code just as
@option{-fforce-mem} may.
@item -fomit-frame-pointer
@opindex 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. @strong{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 @code{FRAME_POINTER_REQUIRED} controls
whether a target machine supports this flag. @xref{Registers,,Register
Usage, gccint, GNU Compiler Collection (GCC) Internals}.
Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
@item -foptimize-sibling-calls
@opindex foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
@item -fno-inline
@opindex fno-inline
Don't pay attention to the @code{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.
@item -finline-functions
@opindex 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 @code{static}, then the function is normally not output as
assembler code in its own right.
Enabled at level @option{-O3}.
@item -finline-limit=@var{n}
@opindex finline-limit
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++). @var{n} is the size of functions that can be inlined in
number of pseudo instructions (not counting parameter handling). The default
value of @var{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 @option{--param @var{name}=@var{value}}.
The @option{-finline-limit=@var{n}} option sets some of these parameters
as follows:
@table @gcctabopt
@item max-inline-insns-single
is set to @var{n}/2.
@item max-inline-insns-auto
is set to @var{n}/2.
@item min-inline-insns
is set to 130 or @var{n}/4, whichever is smaller.
@item max-inline-insns-rtl
is set to @var{n}.
@end table
See below for a documentation of the individual
parameters controlling inlining.
@emph{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.
@item -fkeep-inline-functions
@opindex fkeep-inline-functions
Even if all calls to a given function are integrated, and the function
is declared @code{static}, nevertheless output a separate run-time
callable version of the function. This switch does not affect
@code{extern inline} functions.
@item -fkeep-static-consts
@opindex fkeep-static-consts
Emit variables declared @code{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 @option{-fno-keep-static-consts} option.
@item -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 @option{-fno-merge-constants} to inhibit this
Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
@item -fmerge-all-constants
Attempt to merge identical constants and identical variables.
This option implies @option{-fmerge-constants}. In addition to
@option{-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
@item -fnew-ra
@opindex 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.
@item -fno-branch-count-reg
@opindex 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 @option{-fbranch-count-reg}, enabled when
@option{-fstrength-reduce} is enabled.
@item -fno-function-cse
@opindex 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 @option{-ffunction-cse}
@item -fno-zero-initialized-in-bss
@opindex 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
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 @option{-fzero-initialized-in-bss}.
@item -fstrength-reduce
@opindex fstrength-reduce
Perform the optimizations of loop strength reduction and
elimination of iteration variables.
Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
@item -fthread-jumps
@opindex 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 @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
@item -fcse-follow-jumps
@opindex 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 @code{if} statement with an
@code{else} clause, CSE will follow the jump when the condition
tested is false.
Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
@item -fcse-skip-blocks
@opindex fcse-skip-blocks
This is similar to @option{-fcse-follow-jumps}, but causes CSE to
follow jumps which conditionally skip over blocks. When CSE
encounters a simple @code{if} statement with no else clause,
@option{-fcse-skip-blocks} causes CSE to follow the jump around the
body of the @code{if}.
Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
@item -frerun-cse-after-loop
@opindex frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations has been
Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
@item -frerun-loop-opt
@opindex frerun-loop-opt
Run the loop optimizer twice.
Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
@item -fgcse
@opindex fgcse
Perform a global common subexpression elimination pass.
This pass also performs global constant and copy propagation.
@emph{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
@option{-fno-gcse} to the command line.
Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
@item -fgcse-lm
@opindex fgcse-lm
When @option{-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.
@item -fgcse-sm
@opindex fgcse-sm
When @option{-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 @option{-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.
@item -fgcse-las
@opindex fgcse-las
When @option{-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.
@item -floop-optimize
@opindex floop-optimize
Perform loop optimizations: move constant expressions out of loops, simplify
exit test conditions and optionally do strength-reduction and loop unrolling as
Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
@item -fcrossjumping
@opindex crossjumping
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 @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
@item -fif-conversion
@opindex if-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 @code{if-conversion2}.
Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
@item -fif-conversion2
@opindex if-conversion2
Use conditional execution (where available) to transform conditional jumps into
branch-less equivalents.
Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
@item -fdelete-null-pointer-checks
@opindex 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
@option{-fno-delete-null-pointer-checks} to disable this optimization
for programs which depend on that behavior.
Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
@item -fexpensive-optimizations
@opindex fexpensive-optimizations
Perform a number of minor optimizations that are relatively expensive.
Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
@item -foptimize-register-move
@itemx -fregmove
@opindex foptimize-register-move
@opindex 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
Note @option{-fregmove} and @option{-foptimize-register-move} are the same
Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
@item -fdelayed-branch
@opindex fdelayed-branch
If supported for the target machine, attempt to reorder instructions
to exploit instruction slots available after delayed branch
Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
@item -fschedule-insns
@opindex 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 @option{-O2}, @option{-O3}, @option{-Os}.
@item -fschedule-insns2
@opindex fschedule-insns2
Similar to @option{-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 @option{-O2}, @option{-O3}, @option{-Os}.
@item -fno-sched-interblock
@opindex fno-sched-interblock
Don't schedule instructions across basic blocks. This is normally
enabled by default when scheduling before register allocation, i.e.@:
with @option{-fschedule-insns} or at @option{-O2} or higher.
@item -fno-sched-spec
@opindex 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 @option{-fschedule-insns} or at @option{-O2} or higher.
@item -fsched-spec-load
@opindex fsched-spec-load
Allow speculative motion of some load instructions. This only makes
sense when scheduling before register allocation, i.e.@: with
@option{-fschedule-insns} or at @option{-O2} or higher.
@item -fsched-spec-load-dangerous
@opindex fsched-spec-load-dangerous
Allow speculative motion of more load instructions. This only makes
sense when scheduling before register allocation, i.e.@: with
@option{-fschedule-insns} or at @option{-O2} or higher.
@item -fsched-stalled-insns=@var{n}
@opindex fsched-stalled-insns
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.
@item -fsched-stalled-insns-dep=@var{n}
@opindex fsched-stalled-insns-dep
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 @option{-fsched-stalled-insns} is used and its value is not zero.
@item -fsched2-use-superblocks
@opindex 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
@option{-fschedule-insns2} or at @option{-O2} or higher.
@item -fsched2-use-traces
@opindex fsched2-use-traces
Use @option{-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 @option{-ftracer} for details on
trace formation.
This mode should produce faster but significantly longer programs. Also
without @code{-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
@option{-fschedule-insns2} or at @option{-O2} or higher.
@item -fcaller-saves
@opindex 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 @option{-O2}, @option{-O3}, @option{-Os}.
@item -fmove-all-movables
@opindex fmove-all-movables
Forces all invariant computations in loops to be moved
outside the loop.
@item -freduce-all-givs
@opindex freduce-all-givs
Forces all general-induction variables in loops to be
@emph{Note:} When compiling programs written in Fortran,
@option{-fmove-all-movables} and @option{-freduce-all-givs} are enabled
by default when you use the optimizer.
These options may generate better or worse code; results are highly
dependent on the structure of loops within the source code.
These two options are intended to be removed someday, once
they have helped determine the efficacy of various
approaches to improving loop optimizations.
Please contact @w{@email{}}, and describe how use of
these options affects the performance of your production code.
Examples of code that runs @emph{slower} when these options are
@emph{enabled} are very valuable.
@item -fno-peephole
@itemx -fno-peephole2
@opindex fno-peephole
@opindex fno-peephole2
Disable any machine-specific peephole optimizations. The difference
between @option{-fno-peephole} and @option{-fno-peephole2} is in how they
are implemented in the compiler; some targets use one, some use the
other, a few use both.
@option{-fpeephole} is enabled by default.
@option{-fpeephole2} enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
@item -fno-guess-branch-probability
@opindex 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
(@option{-fprofile-arcs}) or @samp{__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
The default is @option{-fguess-branch-probability} at levels
@option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
@item -freorder-blocks
@opindex freorder-blocks
Reorder basic blocks in the compiled function in order to reduce number of
taken branches and improve code locality.
Enabled at levels @option{-O2}, @option{-O3}.
@item -freorder-functions
@opindex 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 @code{} for most frequently executed functions and
@code{.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
@option{-fprofile-arcs} for details.
Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
@item -fstrict-aliasing
@opindex 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 @code{unsigned int} can alias an @code{int}, but not a
@code{void*} or a @code{double}. A character type may alias any other