1 @c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999,
2 @c 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation, Inc.
3 @c This is part of the GCC manual.
4 @c For copying conditions, see the file gcc.texi.
8 Copyright @copyright{} 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997,
9 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation, Inc.
11 Permission is granted to copy, distribute and/or modify this document
12 under the terms of the GNU Free Documentation License, Version 1.2 or
13 any later version published by the Free Software Foundation; with the
14 Invariant Sections being ``GNU General Public License'' and ``Funding
15 Free Software'', the Front-Cover texts being (a) (see below), and with
16 the Back-Cover Texts being (b) (see below). A copy of the license is
17 included in the gfdl(7) man page.
19 (a) The FSF's Front-Cover Text is:
23 (b) The FSF's Back-Cover Text is:
25 You have freedom to copy and modify this GNU Manual, like GNU
26 software. Copies published by the Free Software Foundation raise
27 funds for GNU development.
29 @c Set file name and title for the man page.
31 @settitle GNU project C and C++ compiler
33 gcc [@option{-c}|@option{-S}|@option{-E}] [@option{-std=}@var{standard}]
34 [@option{-g}] [@option{-pg}] [@option{-O}@var{level}]
35 [@option{-W}@var{warn}@dots{}] [@option{-pedantic}]
36 [@option{-I}@var{dir}@dots{}] [@option{-L}@var{dir}@dots{}]
37 [@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}]
38 [@option{-f}@var{option}@dots{}] [@option{-m}@var{machine-option}@dots{}]
39 [@option{-o} @var{outfile}] @var{infile}@dots{}
41 Only the most useful options are listed here; see below for the
42 remainder. @samp{g++} accepts mostly the same options as @samp{gcc}.
45 gpl(7), gfdl(7), fsf-funding(7),
46 cpp(1), gcov(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1)
47 and the Info entries for @file{gcc}, @file{cpp}, @file{as},
48 @file{ld}, @file{binutils} and @file{gdb}.
51 For instructions on reporting bugs, see
52 @w{@uref{http://gcc.gnu.org/bugs.html}}.
55 See the Info entry for @command{gcc}, or
56 @w{@uref{http://gcc.gnu.org/onlinedocs/gcc/Contributors.html}},
57 for contributors to GCC@.
62 @chapter GCC Command Options
63 @cindex GCC command options
64 @cindex command options
65 @cindex options, GCC command
67 @c man begin DESCRIPTION
68 When you invoke GCC, it normally does preprocessing, compilation,
69 assembly and linking. The ``overall options'' allow you to stop this
70 process at an intermediate stage. For example, the @option{-c} option
71 says not to run the linker. Then the output consists of object files
72 output by the assembler.
74 Other options are passed on to one stage of processing. Some options
75 control the preprocessor and others the compiler itself. Yet other
76 options control the assembler and linker; most of these are not
77 documented here, since you rarely need to use any of them.
79 @cindex C compilation options
80 Most of the command line options that you can use with GCC are useful
81 for C programs; when an option is only useful with another language
82 (usually C++), the explanation says so explicitly. If the description
83 for a particular option does not mention a source language, you can use
84 that option with all supported languages.
86 @cindex C++ compilation options
87 @xref{Invoking G++,,Compiling C++ Programs}, for a summary of special
88 options for compiling C++ programs.
90 @cindex grouping options
91 @cindex options, grouping
92 The @command{gcc} program accepts options and file names as operands. Many
93 options have multi-letter names; therefore multiple single-letter options
94 may @emph{not} be grouped: @option{-dr} is very different from @w{@samp{-d
97 @cindex order of options
98 @cindex options, order
99 You can mix options and other arguments. For the most part, the order
100 you use doesn't matter. Order does matter when you use several options
101 of the same kind; for example, if you specify @option{-L} more than once,
102 the directories are searched in the order specified.
104 Many options have long names starting with @samp{-f} or with
105 @samp{-W}---for example,
106 @option{-fstrength-reduce}, @option{-Wformat} and so on. Most of
107 these have both positive and negative forms; the negative form of
108 @option{-ffoo} would be @option{-fno-foo}. This manual documents
109 only one of these two forms, whichever one is not the default.
113 @xref{Option Index}, for an index to GCC's options.
116 * Option Summary:: Brief list of all options, without explanations.
117 * Overall Options:: Controlling the kind of output:
118 an executable, object files, assembler files,
119 or preprocessed source.
120 * Invoking G++:: Compiling C++ programs.
121 * C Dialect Options:: Controlling the variant of C language compiled.
122 * C++ Dialect Options:: Variations on C++.
123 * Objective-C and Objective-C++ Dialect Options:: Variations on Objective-C
125 * Language Independent Options:: Controlling how diagnostics should be
127 * Warning Options:: How picky should the compiler be?
128 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
129 * Optimize Options:: How much optimization?
130 * Preprocessor Options:: Controlling header files and macro definitions.
131 Also, getting dependency information for Make.
132 * Assembler Options:: Passing options to the assembler.
133 * Link Options:: Specifying libraries and so on.
134 * Directory Options:: Where to find header files and libraries.
135 Where to find the compiler executable files.
136 * Spec Files:: How to pass switches to sub-processes.
137 * Target Options:: Running a cross-compiler, or an old version of GCC.
138 * Submodel Options:: Specifying minor hardware or convention variations,
139 such as 68010 vs 68020.
140 * Code Gen Options:: Specifying conventions for function calls, data layout
142 * Environment Variables:: Env vars that affect GCC.
143 * Precompiled Headers:: Compiling a header once, and using it many times.
144 * Running Protoize:: Automatically adding or removing function prototypes.
150 @section Option Summary
152 Here is a summary of all the options, grouped by type. Explanations are
153 in the following sections.
156 @item Overall Options
157 @xref{Overall Options,,Options Controlling the Kind of Output}.
158 @gccoptlist{-c -S -E -o @var{file} -combine -pipe -pass-exit-codes @gol
159 -x @var{language} -v -### --help --target-help --version}
161 @item C Language Options
162 @xref{C Dialect Options,,Options Controlling C Dialect}.
163 @gccoptlist{-ansi -std=@var{standard} -aux-info @var{filename} @gol
164 -fno-asm -fno-builtin -fno-builtin-@var{function} @gol
165 -fhosted -ffreestanding -fms-extensions @gol
166 -trigraphs -no-integrated-cpp -traditional -traditional-cpp @gol
167 -fallow-single-precision -fcond-mismatch @gol
168 -fsigned-bitfields -fsigned-char @gol
169 -funsigned-bitfields -funsigned-char}
171 @item C++ Language Options
172 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}.
173 @gccoptlist{-fabi-version=@var{n} -fno-access-control -fcheck-new @gol
174 -fconserve-space -fno-const-strings @gol
175 -fno-elide-constructors @gol
176 -fno-enforce-eh-specs @gol
177 -ffor-scope -fno-for-scope -fno-gnu-keywords @gol
178 -fno-implicit-templates @gol
179 -fno-implicit-inline-templates @gol
180 -fno-implement-inlines -fms-extensions @gol
181 -fno-nonansi-builtins -fno-operator-names @gol
182 -fno-optional-diags -fpermissive @gol
183 -frepo -fno-rtti -fstats -ftemplate-depth-@var{n} @gol
184 -fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ @gol
185 -fno-default-inline -fvisibility-inlines-hidden @gol
186 -Wabi -Wctor-dtor-privacy @gol
187 -Wnon-virtual-dtor -Wreorder @gol
188 -Weffc++ -Wno-deprecated -Wstrict-null-sentinel @gol
189 -Wno-non-template-friend -Wold-style-cast @gol
190 -Woverloaded-virtual -Wno-pmf-conversions @gol
193 @item Objective-C and Objective-C++ Language Options
194 @xref{Objective-C and Objective-C++ Dialect Options,,Options Controlling
195 Objective-C and Objective-C++ Dialects}.
197 -fconstant-string-class=@var{class-name} @gol
198 -fgnu-runtime -fnext-runtime @gol
199 -fno-nil-receivers @gol
200 -fobjc-call-cxx-cdtors @gol
201 -fobjc-direct-dispatch @gol
202 -fobjc-exceptions @gol
204 -freplace-objc-classes @gol
207 -Wassign-intercept @gol
208 -Wno-protocol -Wselector @gol
209 -Wstrict-selector-match @gol
210 -Wundeclared-selector}
212 @item Language Independent Options
213 @xref{Language Independent Options,,Options to Control Diagnostic Messages Formatting}.
214 @gccoptlist{-fmessage-length=@var{n} @gol
215 -fdiagnostics-show-location=@r{[}once@r{|}every-line@r{]}} @gol
216 -fdiagnostics-show-options
218 @item Warning Options
219 @xref{Warning Options,,Options to Request or Suppress Warnings}.
220 @gccoptlist{-fsyntax-only -pedantic -pedantic-errors @gol
221 -w -Wextra -Wall -Waggregate-return -Wno-attributes @gol
222 -Wc++-compat -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment @gol
223 -Wconversion -Wno-deprecated-declarations @gol
224 -Wdisabled-optimization -Wno-div-by-zero -Wno-endif-labels @gol
225 -Werror -Werror-implicit-function-declaration @gol
226 -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 @gol
227 -Wno-format-extra-args -Wformat-nonliteral @gol
228 -Wformat-security -Wformat-y2k @gol
229 -Wimplicit -Wimplicit-function-declaration -Wimplicit-int @gol
230 -Wimport -Wno-import -Winit-self -Winline @gol
231 -Wno-int-to-pointer-cast @gol
232 -Wno-invalid-offsetof -Winvalid-pch @gol
233 -Wlarger-than-@var{len} -Wunsafe-loop-optimizations -Wlong-long @gol
234 -Wmain -Wmissing-braces -Wmissing-field-initializers @gol
235 -Wmissing-format-attribute -Wmissing-include-dirs @gol
236 -Wmissing-noreturn @gol
237 -Wno-multichar -Wnonnull -Wpacked -Wpadded @gol
238 -Wparentheses -Wpointer-arith -Wno-pointer-to-int-cast @gol
239 -Wredundant-decls @gol
240 -Wreturn-type -Wsequence-point -Wshadow @gol
241 -Wsign-compare -Wstrict-aliasing -Wstrict-aliasing=2 @gol
242 -Wswitch -Wswitch-default -Wswitch-enum @gol
243 -Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized @gol
244 -Wunknown-pragmas -Wno-pragmas -Wunreachable-code @gol
245 -Wunused -Wunused-function -Wunused-label -Wunused-parameter @gol
246 -Wunused-value -Wunused-variable -Wwrite-strings @gol
249 @item C-only Warning Options
250 @gccoptlist{-Wbad-function-cast -Wmissing-declarations @gol
251 -Wmissing-prototypes -Wnested-externs -Wold-style-definition @gol
252 -Wstrict-prototypes -Wtraditional @gol
253 -Wdeclaration-after-statement -Wno-pointer-sign}
255 @item Debugging Options
256 @xref{Debugging Options,,Options for Debugging Your Program or GCC}.
257 @gccoptlist{-d@var{letters} -dumpspecs -dumpmachine -dumpversion @gol
258 -fdump-unnumbered -fdump-translation-unit@r{[}-@var{n}@r{]} @gol
259 -fdump-class-hierarchy@r{[}-@var{n}@r{]} @gol
260 -fdump-ipa-all -fdump-ipa-cgraph @gol
262 -fdump-tree-original@r{[}-@var{n}@r{]} @gol
263 -fdump-tree-optimized@r{[}-@var{n}@r{]} @gol
264 -fdump-tree-inlined@r{[}-@var{n}@r{]} @gol
265 -fdump-tree-cfg -fdump-tree-vcg -fdump-tree-alias @gol
267 -fdump-tree-ssa@r{[}-@var{n}@r{]} -fdump-tree-pre@r{[}-@var{n}@r{]} @gol
268 -fdump-tree-ccp@r{[}-@var{n}@r{]} -fdump-tree-dce@r{[}-@var{n}@r{]} @gol
269 -fdump-tree-gimple@r{[}-raw@r{]} -fdump-tree-mudflap@r{[}-@var{n}@r{]} @gol
270 -fdump-tree-dom@r{[}-@var{n}@r{]} @gol
271 -fdump-tree-dse@r{[}-@var{n}@r{]} @gol
272 -fdump-tree-phiopt@r{[}-@var{n}@r{]} @gol
273 -fdump-tree-forwprop@r{[}-@var{n}@r{]} @gol
274 -fdump-tree-copyrename@r{[}-@var{n}@r{]} @gol
275 -fdump-tree-nrv -fdump-tree-vect @gol
276 -fdump-tree-sink @gol
277 -fdump-tree-sra@r{[}-@var{n}@r{]} @gol
278 -fdump-tree-salias @gol
279 -fdump-tree-fre@r{[}-@var{n}@r{]} @gol
280 -fdump-tree-vrp@r{[}-@var{n}@r{]} @gol
281 -ftree-vectorizer-verbose=@var{n} @gol
282 -fdump-tree-storeccp@r{[}-@var{n}@r{]} @gol
283 -feliminate-dwarf2-dups -feliminate-unused-debug-types @gol
284 -feliminate-unused-debug-symbols -fmem-report -fprofile-arcs -ftree-based-profiling @gol
285 -frandom-seed=@var{string} -fsched-verbose=@var{n} @gol
286 -ftest-coverage -ftime-report -fvar-tracking @gol
287 -g -g@var{level} -gcoff -gdwarf-2 @gol
288 -ggdb -gstabs -gstabs+ -gvms -gxcoff -gxcoff+ @gol
289 -p -pg -print-file-name=@var{library} -print-libgcc-file-name @gol
290 -print-multi-directory -print-multi-lib @gol
291 -print-prog-name=@var{program} -print-search-dirs -Q @gol
294 @item Optimization Options
295 @xref{Optimize Options,,Options that Control Optimization}.
296 @gccoptlist{-falign-functions=@var{n} -falign-jumps=@var{n} @gol
297 -falign-labels=@var{n} -falign-loops=@var{n} @gol
298 -fbounds-check -fmudflap -fmudflapth -fmudflapir @gol
299 -fbranch-probabilities -fprofile-values -fvpt -fbranch-target-load-optimize @gol
300 -fbranch-target-load-optimize2 -fbtr-bb-exclusive @gol
301 -fcaller-saves -fcprop-registers -fcse-follow-jumps @gol
302 -fcse-skip-blocks -fcx-limited-range -fdata-sections @gol
303 -fdelayed-branch -fdelete-null-pointer-checks -fearly-inlining @gol
304 -fexpensive-optimizations -ffast-math -ffloat-store @gol
305 -fforce-addr -ffunction-sections @gol
306 -fgcse -fgcse-lm -fgcse-sm -fgcse-las -fgcse-after-reload @gol
307 -floop-optimize -fcrossjumping -fif-conversion -fif-conversion2 @gol
308 -finline-functions -finline-limit=@var{n} -fkeep-inline-functions @gol
309 -fkeep-static-consts -fmerge-constants -fmerge-all-constants @gol
310 -fmodulo-sched -fno-branch-count-reg @gol
311 -fno-default-inline -fno-defer-pop -floop-optimize2 -fmove-loop-invariants @gol
312 -fno-function-cse -fno-guess-branch-probability @gol
313 -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 @gol
314 -funsafe-math-optimizations -funsafe-loop-optimizations -ffinite-math-only @gol
315 -fno-trapping-math -fno-zero-initialized-in-bss @gol
316 -fomit-frame-pointer -foptimize-register-move @gol
317 -foptimize-sibling-calls -fprefetch-loop-arrays @gol
318 -fprofile-generate -fprofile-use @gol
319 -fregmove -frename-registers @gol
320 -freorder-blocks -freorder-blocks-and-partition -freorder-functions @gol
321 -frerun-cse-after-loop -frerun-loop-opt @gol
322 -frounding-math -fschedule-insns -fschedule-insns2 @gol
323 -fno-sched-interblock -fno-sched-spec -fsched-spec-load @gol
324 -fsched-spec-load-dangerous @gol
325 -fsched-stalled-insns=@var{n} -sched-stalled-insns-dep=@var{n} @gol
326 -fsched2-use-superblocks @gol
327 -fsched2-use-traces -freschedule-modulo-scheduled-loops @gol
328 -fsignaling-nans -fsingle-precision-constant -fspeculative-prefetching @gol
329 -fstrength-reduce -fstrict-aliasing -ftracer -fthread-jumps @gol
330 -funroll-all-loops -funroll-loops -fpeel-loops @gol
331 -fsplit-ivs-in-unroller -funswitch-loops @gol
332 -fvariable-expansion-in-unroller @gol
333 -ftree-pre -ftree-ccp -ftree-dce -ftree-loop-optimize @gol
334 -ftree-loop-linear -ftree-loop-im -ftree-loop-ivcanon -fivopts @gol
335 -ftree-dominator-opts -ftree-dse -ftree-copyrename -ftree-sink @gol
336 -ftree-ch -ftree-sra -ftree-ter -ftree-lrs -ftree-fre -ftree-vectorize @gol
337 -ftree-salias -fweb @gol
338 -ftree-copy-prop -ftree-store-ccp -ftree-store-copy-prop -fwhole-program @gol
339 --param @var{name}=@var{value}
340 -O -O0 -O1 -O2 -O3 -Os}
342 @item Preprocessor Options
343 @xref{Preprocessor Options,,Options Controlling the Preprocessor}.
344 @gccoptlist{-A@var{question}=@var{answer} @gol
345 -A-@var{question}@r{[}=@var{answer}@r{]} @gol
346 -C -dD -dI -dM -dN @gol
347 -D@var{macro}@r{[}=@var{defn}@r{]} -E -H @gol
348 -idirafter @var{dir} @gol
349 -include @var{file} -imacros @var{file} @gol
350 -iprefix @var{file} -iwithprefix @var{dir} @gol
351 -iwithprefixbefore @var{dir} -isystem @var{dir} @gol
352 -isysroot @var{dir} @gol
353 -M -MM -MF -MG -MP -MQ -MT -nostdinc @gol
354 -P -fworking-directory -remap @gol
355 -trigraphs -undef -U@var{macro} -Wp,@var{option} @gol
356 -Xpreprocessor @var{option}}
358 @item Assembler Option
359 @xref{Assembler Options,,Passing Options to the Assembler}.
360 @gccoptlist{-Wa,@var{option} -Xassembler @var{option}}
363 @xref{Link Options,,Options for Linking}.
364 @gccoptlist{@var{object-file-name} -l@var{library} @gol
365 -nostartfiles -nodefaultlibs -nostdlib -pie @gol
366 -s -static -static-libgcc -shared -shared-libgcc -symbolic @gol
367 -Wl,@var{option} -Xlinker @var{option} @gol
370 @item Directory Options
371 @xref{Directory Options,,Options for Directory Search}.
372 @gccoptlist{-B@var{prefix} -I@var{dir} -iquote@var{dir} -L@var{dir}
373 -specs=@var{file} -I- --sysroot=@var{dir}}
376 @c I wrote this xref this way to avoid overfull hbox. -- rms
377 @xref{Target Options}.
378 @gccoptlist{-V @var{version} -b @var{machine}}
380 @item Machine Dependent Options
381 @xref{Submodel Options,,Hardware Models and Configurations}.
382 @c This list is ordered alphanumerically by subsection name.
383 @c Try and put the significant identifier (CPU or system) first,
384 @c so users have a clue at guessing where the ones they want will be.
387 @gccoptlist{-EB -EL @gol
388 -mmangle-cpu -mcpu=@var{cpu} -mtext=@var{text-section} @gol
389 -mdata=@var{data-section} -mrodata=@var{readonly-data-section}}
392 @gccoptlist{-mapcs-frame -mno-apcs-frame @gol
393 -mabi=@var{name} @gol
394 -mapcs-stack-check -mno-apcs-stack-check @gol
395 -mapcs-float -mno-apcs-float @gol
396 -mapcs-reentrant -mno-apcs-reentrant @gol
397 -msched-prolog -mno-sched-prolog @gol
398 -mlittle-endian -mbig-endian -mwords-little-endian @gol
399 -mfloat-abi=@var{name} -msoft-float -mhard-float -mfpe @gol
400 -mthumb-interwork -mno-thumb-interwork @gol
401 -mcpu=@var{name} -march=@var{name} -mfpu=@var{name} @gol
402 -mstructure-size-boundary=@var{n} @gol
403 -mabort-on-noreturn @gol
404 -mlong-calls -mno-long-calls @gol
405 -msingle-pic-base -mno-single-pic-base @gol
406 -mpic-register=@var{reg} @gol
407 -mnop-fun-dllimport @gol
408 -mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns @gol
409 -mpoke-function-name @gol
411 -mtpcs-frame -mtpcs-leaf-frame @gol
412 -mcaller-super-interworking -mcallee-super-interworking}
415 @gccoptlist{-mmcu=@var{mcu} -msize -minit-stack=@var{n} -mno-interrupts @gol
416 -mcall-prologues -mno-tablejump -mtiny-stack -mint8}
418 @emph{Blackfin Options}
419 @gccoptlist{-momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer @gol
420 -mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly -mno-csync-anomaly @gol
421 -mlow-64k -mno-low64k -mid-shared-library @gol
422 -mno-id-shared-library -mshared-library-id=@var{n} @gol
423 -mlong-calls -mno-long-calls}
426 @gccoptlist{-mcpu=@var{cpu} -march=@var{cpu} -mtune=@var{cpu} @gol
427 -mmax-stack-frame=@var{n} -melinux-stacksize=@var{n} @gol
428 -metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects @gol
429 -mstack-align -mdata-align -mconst-align @gol
430 -m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt @gol
431 -melf -maout -melinux -mlinux -sim -sim2 @gol
432 -mmul-bug-workaround -mno-mul-bug-workaround}
434 @emph{Darwin Options}
435 @gccoptlist{-all_load -allowable_client -arch -arch_errors_fatal @gol
436 -arch_only -bind_at_load -bundle -bundle_loader @gol
437 -client_name -compatibility_version -current_version @gol
439 -dependency-file -dylib_file -dylinker_install_name @gol
440 -dynamic -dynamiclib -exported_symbols_list @gol
441 -filelist -flat_namespace -force_cpusubtype_ALL @gol
442 -force_flat_namespace -headerpad_max_install_names @gol
443 -image_base -init -install_name -keep_private_externs @gol
444 -multi_module -multiply_defined -multiply_defined_unused @gol
445 -noall_load -no_dead_strip_inits_and_terms @gol
446 -nofixprebinding -nomultidefs -noprebind -noseglinkedit @gol
447 -pagezero_size -prebind -prebind_all_twolevel_modules @gol
448 -private_bundle -read_only_relocs -sectalign @gol
449 -sectobjectsymbols -whyload -seg1addr @gol
450 -sectcreate -sectobjectsymbols -sectorder @gol
451 -segaddr -segs_read_only_addr -segs_read_write_addr @gol
452 -seg_addr_table -seg_addr_table_filename -seglinkedit @gol
453 -segprot -segs_read_only_addr -segs_read_write_addr @gol
454 -single_module -static -sub_library -sub_umbrella @gol
455 -twolevel_namespace -umbrella -undefined @gol
456 -unexported_symbols_list -weak_reference_mismatches @gol
457 -whatsloaded -F -gused -gfull -mmacosx-min-version=@var{version} @gol
460 @emph{DEC Alpha Options}
461 @gccoptlist{-mno-fp-regs -msoft-float -malpha-as -mgas @gol
462 -mieee -mieee-with-inexact -mieee-conformant @gol
463 -mfp-trap-mode=@var{mode} -mfp-rounding-mode=@var{mode} @gol
464 -mtrap-precision=@var{mode} -mbuild-constants @gol
465 -mcpu=@var{cpu-type} -mtune=@var{cpu-type} @gol
466 -mbwx -mmax -mfix -mcix @gol
467 -mfloat-vax -mfloat-ieee @gol
468 -mexplicit-relocs -msmall-data -mlarge-data @gol
469 -msmall-text -mlarge-text @gol
470 -mmemory-latency=@var{time}}
472 @emph{DEC Alpha/VMS Options}
473 @gccoptlist{-mvms-return-codes}
476 @gccoptlist{-mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 @gol
477 -mhard-float -msoft-float @gol
478 -malloc-cc -mfixed-cc -mdword -mno-dword @gol
479 -mdouble -mno-double @gol
480 -mmedia -mno-media -mmuladd -mno-muladd @gol
481 -mfdpic -minline-plt -mgprel-ro -multilib-library-pic @gol
482 -mlinked-fp -mlong-calls -malign-labels @gol
483 -mlibrary-pic -macc-4 -macc-8 @gol
484 -mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move @gol
485 -moptimize-membar -mno-optimize-membar @gol
486 -mscc -mno-scc -mcond-exec -mno-cond-exec @gol
487 -mvliw-branch -mno-vliw-branch @gol
488 -mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec @gol
489 -mno-nested-cond-exec -mtomcat-stats @gol
493 @emph{H8/300 Options}
494 @gccoptlist{-mrelax -mh -ms -mn -mint32 -malign-300}
497 @gccoptlist{-march=@var{architecture-type} @gol
498 -mbig-switch -mdisable-fpregs -mdisable-indexing @gol
499 -mfast-indirect-calls -mgas -mgnu-ld -mhp-ld @gol
500 -mfixed-range=@var{register-range} @gol
501 -mjump-in-delay -mlinker-opt -mlong-calls @gol
502 -mlong-load-store -mno-big-switch -mno-disable-fpregs @gol
503 -mno-disable-indexing -mno-fast-indirect-calls -mno-gas @gol
504 -mno-jump-in-delay -mno-long-load-store @gol
505 -mno-portable-runtime -mno-soft-float @gol
506 -mno-space-regs -msoft-float -mpa-risc-1-0 @gol
507 -mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime @gol
508 -mschedule=@var{cpu-type} -mspace-regs -msio -mwsio @gol
509 -munix=@var{unix-std} -nolibdld -static -threads}
511 @emph{i386 and x86-64 Options}
512 @gccoptlist{-mtune=@var{cpu-type} -march=@var{cpu-type} @gol
513 -mfpmath=@var{unit} @gol
514 -masm=@var{dialect} -mno-fancy-math-387 @gol
515 -mno-fp-ret-in-387 -msoft-float -msvr3-shlib @gol
516 -mno-wide-multiply -mrtd -malign-double @gol
517 -mpreferred-stack-boundary=@var{num} @gol
518 -mmmx -msse -msse2 -msse3 -m3dnow @gol
519 -mthreads -mno-align-stringops -minline-all-stringops @gol
520 -mpush-args -maccumulate-outgoing-args -m128bit-long-double @gol
521 -m96bit-long-double -mregparm=@var{num} -msseregparm @gol
522 -momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs @gol
523 -mcmodel=@var{code-model} @gol
527 @gccoptlist{-mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic @gol
528 -mvolatile-asm-stop -mregister-names -mno-sdata @gol
529 -mconstant-gp -mauto-pic -minline-float-divide-min-latency @gol
530 -minline-float-divide-max-throughput @gol
531 -minline-int-divide-min-latency @gol
532 -minline-int-divide-max-throughput @gol
533 -minline-sqrt-min-latency -minline-sqrt-max-throughput @gol
534 -mno-dwarf2-asm -mearly-stop-bits @gol
535 -mfixed-range=@var{register-range} -mtls-size=@var{tls-size} @gol
536 -mtune=@var{cpu-type} -mt -pthread -milp32 -mlp64}
538 @emph{M32R/D Options}
539 @gccoptlist{-m32r2 -m32rx -m32r @gol
541 -malign-loops -mno-align-loops @gol
542 -missue-rate=@var{number} @gol
543 -mbranch-cost=@var{number} @gol
544 -mmodel=@var{code-size-model-type} @gol
545 -msdata=@var{sdata-type} @gol
546 -mno-flush-func -mflush-func=@var{name} @gol
547 -mno-flush-trap -mflush-trap=@var{number} @gol
551 @gccoptlist{-mcpu=@var{cpu} -msim -memregs=@var{number}}
553 @emph{M680x0 Options}
554 @gccoptlist{-m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 @gol
555 -m68060 -mcpu32 -m5200 -m68881 -mbitfield -mc68000 -mc68020 @gol
556 -mnobitfield -mrtd -mshort -msoft-float -mpcrel @gol
557 -malign-int -mstrict-align -msep-data -mno-sep-data @gol
558 -mshared-library-id=n -mid-shared-library -mno-id-shared-library}
560 @emph{M68hc1x Options}
561 @gccoptlist{-m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12 @gol
562 -mauto-incdec -minmax -mlong-calls -mshort @gol
563 -msoft-reg-count=@var{count}}
566 @gccoptlist{-mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates @gol
567 -mno-relax-immediates -mwide-bitfields -mno-wide-bitfields @gol
568 -m4byte-functions -mno-4byte-functions -mcallgraph-data @gol
569 -mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim @gol
570 -mlittle-endian -mbig-endian -m210 -m340 -mstack-increment}
573 @gccoptlist{-EL -EB -march=@var{arch} -mtune=@var{arch} @gol
574 -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips64 @gol
575 -mips16 -mno-mips16 -mabi=@var{abi} -mabicalls -mno-abicalls @gol
576 -mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfp64 @gol
577 -mhard-float -msoft-float -msingle-float -mdouble-float @gol
578 -mdsp -mpaired-single -mips3d @gol
579 -mlong64 -mlong32 -msym32 -mno-sym32 @gol
580 -G@var{num} -membedded-data -mno-embedded-data @gol
581 -muninit-const-in-rodata -mno-uninit-const-in-rodata @gol
582 -msplit-addresses -mno-split-addresses @gol
583 -mexplicit-relocs -mno-explicit-relocs @gol
584 -mcheck-zero-division -mno-check-zero-division @gol
585 -mdivide-traps -mdivide-breaks @gol
586 -mmemcpy -mno-memcpy -mlong-calls -mno-long-calls @gol
587 -mmad -mno-mad -mfused-madd -mno-fused-madd -nocpp @gol
588 -mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400 @gol
589 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130 @gol
590 -mfix-sb1 -mno-fix-sb1 @gol
591 -mflush-func=@var{func} -mno-flush-func @gol
592 -mbranch-likely -mno-branch-likely @gol
593 -mfp-exceptions -mno-fp-exceptions @gol
594 -mvr4130-align -mno-vr4130-align}
597 @gccoptlist{-mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu @gol
598 -mabi=mmixware -mzero-extend -mknuthdiv -mtoplevel-symbols @gol
599 -melf -mbranch-predict -mno-branch-predict -mbase-addresses @gol
600 -mno-base-addresses -msingle-exit -mno-single-exit}
602 @emph{MN10300 Options}
603 @gccoptlist{-mmult-bug -mno-mult-bug @gol
604 -mam33 -mno-am33 @gol
605 -mam33-2 -mno-am33-2 @gol
608 @emph{PDP-11 Options}
609 @gccoptlist{-mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 @gol
610 -mbcopy -mbcopy-builtin -mint32 -mno-int16 @gol
611 -mint16 -mno-int32 -mfloat32 -mno-float64 @gol
612 -mfloat64 -mno-float32 -mabshi -mno-abshi @gol
613 -mbranch-expensive -mbranch-cheap @gol
614 -msplit -mno-split -munix-asm -mdec-asm}
616 @emph{PowerPC Options}
617 See RS/6000 and PowerPC Options.
619 @emph{RS/6000 and PowerPC Options}
620 @gccoptlist{-mcpu=@var{cpu-type} @gol
621 -mtune=@var{cpu-type} @gol
622 -mpower -mno-power -mpower2 -mno-power2 @gol
623 -mpowerpc -mpowerpc64 -mno-powerpc @gol
624 -maltivec -mno-altivec @gol
625 -mpowerpc-gpopt -mno-powerpc-gpopt @gol
626 -mpowerpc-gfxopt -mno-powerpc-gfxopt @gol
627 -mnew-mnemonics -mold-mnemonics @gol
628 -mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc @gol
629 -m64 -m32 -mxl-compat -mno-xl-compat -mpe @gol
630 -malign-power -malign-natural @gol
631 -msoft-float -mhard-float -mmultiple -mno-multiple @gol
632 -mstring -mno-string -mupdate -mno-update @gol
633 -mfused-madd -mno-fused-madd -mbit-align -mno-bit-align @gol
634 -mstrict-align -mno-strict-align -mrelocatable @gol
635 -mno-relocatable -mrelocatable-lib -mno-relocatable-lib @gol
636 -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian @gol
637 -mdynamic-no-pic -maltivec -mswdiv @gol
638 -mprioritize-restricted-insns=@var{priority} @gol
639 -msched-costly-dep=@var{dependence_type} @gol
640 -minsert-sched-nops=@var{scheme} @gol
641 -mcall-sysv -mcall-netbsd @gol
642 -maix-struct-return -msvr4-struct-return @gol
643 -mabi=@var{abi-type} -msecure-plt -mbss-plt @gol
644 -misel -mno-isel @gol
645 -misel=yes -misel=no @gol
647 -mspe=yes -mspe=no @gol
648 -mvrsave -mno-vrsave @gol
649 -mfloat-gprs=yes -mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double @gol
650 -mprototype -mno-prototype @gol
651 -msim -mmvme -mads -myellowknife -memb -msdata @gol
652 -msdata=@var{opt} -mvxworks -mwindiss -G @var{num} -pthread}
654 @emph{S/390 and zSeries Options}
655 @gccoptlist{-mtune=@var{cpu-type} -march=@var{cpu-type} @gol
656 -mhard-float -msoft-float -mbackchain -mno-backchain @gol
657 -mpacked-stack -mno-packed-stack @gol
658 -msmall-exec -mno-small-exec -mmvcle -mno-mvcle @gol
659 -m64 -m31 -mdebug -mno-debug -mesa -mzarch @gol
660 -mtpf-trace -mno-tpf-trace -mfused-madd -mno-fused-madd @gol
661 -mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard}
664 @gccoptlist{-m1 -m2 -m2e -m3 -m3e @gol
665 -m4-nofpu -m4-single-only -m4-single -m4 @gol
666 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al @gol
667 -m5-64media -m5-64media-nofpu @gol
668 -m5-32media -m5-32media-nofpu @gol
669 -m5-compact -m5-compact-nofpu @gol
670 -mb -ml -mdalign -mrelax @gol
671 -mbigtable -mfmovd -mhitachi -mrenesas -mno-renesas -mnomacsave @gol
672 -mieee -misize -mpadstruct -mspace @gol
673 -mprefergot -musermode -multcost=@var{number} -mdiv=@var{strategy} @gol
674 -mdivsi3_libfunc=@var{name} @gol
675 -madjust-unroll -mindexed-addressing -mgettrcost=@var{number} -mpt-fixed @gol
679 @gccoptlist{-mcpu=@var{cpu-type} @gol
680 -mtune=@var{cpu-type} @gol
681 -mcmodel=@var{code-model} @gol
682 -m32 -m64 -mapp-regs -mno-app-regs @gol
683 -mfaster-structs -mno-faster-structs @gol
684 -mfpu -mno-fpu -mhard-float -msoft-float @gol
685 -mhard-quad-float -msoft-quad-float @gol
686 -mimpure-text -mno-impure-text -mlittle-endian @gol
687 -mstack-bias -mno-stack-bias @gol
688 -munaligned-doubles -mno-unaligned-doubles @gol
689 -mv8plus -mno-v8plus -mvis -mno-vis
692 @emph{System V Options}
693 @gccoptlist{-Qy -Qn -YP,@var{paths} -Ym,@var{dir}}
695 @emph{TMS320C3x/C4x Options}
696 @gccoptlist{-mcpu=@var{cpu} -mbig -msmall -mregparm -mmemparm @gol
697 -mfast-fix -mmpyi -mbk -mti -mdp-isr-reload @gol
698 -mrpts=@var{count} -mrptb -mdb -mloop-unsigned @gol
699 -mparallel-insns -mparallel-mpy -mpreserve-float}
702 @gccoptlist{-mlong-calls -mno-long-calls -mep -mno-ep @gol
703 -mprolog-function -mno-prolog-function -mspace @gol
704 -mtda=@var{n} -msda=@var{n} -mzda=@var{n} @gol
705 -mapp-regs -mno-app-regs @gol
706 -mdisable-callt -mno-disable-callt @gol
712 @gccoptlist{-mg -mgnu -munix}
714 @emph{x86-64 Options}
715 See i386 and x86-64 Options.
717 @emph{Xstormy16 Options}
720 @emph{Xtensa Options}
721 @gccoptlist{-mconst16 -mno-const16 @gol
722 -mfused-madd -mno-fused-madd @gol
723 -mtext-section-literals -mno-text-section-literals @gol
724 -mtarget-align -mno-target-align @gol
725 -mlongcalls -mno-longcalls}
727 @emph{zSeries Options}
728 See S/390 and zSeries Options.
730 @item Code Generation Options
731 @xref{Code Gen Options,,Options for Code Generation Conventions}.
732 @gccoptlist{-fcall-saved-@var{reg} -fcall-used-@var{reg} @gol
733 -ffixed-@var{reg} -fexceptions @gol
734 -fnon-call-exceptions -funwind-tables @gol
735 -fasynchronous-unwind-tables @gol
736 -finhibit-size-directive -finstrument-functions @gol
737 -fno-common -fno-ident @gol
738 -fpcc-struct-return -fpic -fPIC -fpie -fPIE @gol
739 -fno-jump-tables @gol
740 -freg-struct-return -fshared-data -fshort-enums @gol
741 -fshort-double -fshort-wchar @gol
742 -fverbose-asm -fpack-struct[=@var{n}] -fstack-check @gol
743 -fstack-limit-register=@var{reg} -fstack-limit-symbol=@var{sym} @gol
744 -fargument-alias -fargument-noalias @gol
745 -fargument-noalias-global -fleading-underscore @gol
746 -ftls-model=@var{model} @gol
747 -ftrapv -fwrapv -fbounds-check @gol
752 * Overall Options:: Controlling the kind of output:
753 an executable, object files, assembler files,
754 or preprocessed source.
755 * C Dialect Options:: Controlling the variant of C language compiled.
756 * C++ Dialect Options:: Variations on C++.
757 * Objective-C and Objective-C++ Dialect Options:: Variations on Objective-C
759 * Language Independent Options:: Controlling how diagnostics should be
761 * Warning Options:: How picky should the compiler be?
762 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
763 * Optimize Options:: How much optimization?
764 * Preprocessor Options:: Controlling header files and macro definitions.
765 Also, getting dependency information for Make.
766 * Assembler Options:: Passing options to the assembler.
767 * Link Options:: Specifying libraries and so on.
768 * Directory Options:: Where to find header files and libraries.
769 Where to find the compiler executable files.
770 * Spec Files:: How to pass switches to sub-processes.
771 * Target Options:: Running a cross-compiler, or an old version of GCC.
774 @node Overall Options
775 @section Options Controlling the Kind of Output
777 Compilation can involve up to four stages: preprocessing, compilation
778 proper, assembly and linking, always in that order. GCC is capable of
779 preprocessing and compiling several files either into several
780 assembler input files, or into one assembler input file; then each
781 assembler input file produces an object file, and linking combines all
782 the object files (those newly compiled, and those specified as input)
783 into an executable file.
785 @cindex file name suffix
786 For any given input file, the file name suffix determines what kind of
791 C source code which must be preprocessed.
794 C source code which should not be preprocessed.
797 C++ source code which should not be preprocessed.
800 Objective-C source code. Note that you must link with the @file{libobjc}
801 library to make an Objective-C program work.
804 Objective-C source code which should not be preprocessed.
808 Objective-C++ source code. Note that you must link with the @file{libobjc}
809 library to make an Objective-C++ program work. Note that @samp{.M} refers
810 to a literal capital M@.
813 Objective-C++ source code which should not be preprocessed.
816 C, C++, Objective-C or Objective-C++ header file to be turned into a
821 @itemx @var{file}.cxx
822 @itemx @var{file}.cpp
823 @itemx @var{file}.CPP
824 @itemx @var{file}.c++
826 C++ source code which must be preprocessed. Note that in @samp{.cxx},
827 the last two letters must both be literally @samp{x}. Likewise,
828 @samp{.C} refers to a literal capital C@.
832 Objective-C++ source code which must be preprocessed.
835 Objective-C++ source code which should not be preprocessed.
839 C++ header file to be turned into a precompiled header.
842 @itemx @var{file}.for
843 @itemx @var{file}.FOR
844 Fortran source code which should not be preprocessed.
847 @itemx @var{file}.fpp
848 @itemx @var{file}.FPP
849 Fortran source code which must be preprocessed (with the traditional
853 Fortran source code which must be preprocessed with a RATFOR
854 preprocessor (not included with GCC)@.
857 @itemx @var{file}.f95
858 Fortran 90/95 source code which should not be preprocessed.
860 @c FIXME: Descriptions of Java file types.
867 Ada source code file which contains a library unit declaration (a
868 declaration of a package, subprogram, or generic, or a generic
869 instantiation), or a library unit renaming declaration (a package,
870 generic, or subprogram renaming declaration). Such files are also
873 @itemx @var{file}.adb
874 Ada source code file containing a library unit body (a subprogram or
875 package body). Such files are also called @dfn{bodies}.
877 @c GCC also knows about some suffixes for languages not yet included:
886 Assembler code which must be preprocessed.
889 An object file to be fed straight into linking.
890 Any file name with no recognized suffix is treated this way.
894 You can specify the input language explicitly with the @option{-x} option:
897 @item -x @var{language}
898 Specify explicitly the @var{language} for the following input files
899 (rather than letting the compiler choose a default based on the file
900 name suffix). This option applies to all following input files until
901 the next @option{-x} option. Possible values for @var{language} are:
903 c c-header c-cpp-output
904 c++ c++-header c++-cpp-output
905 objective-c objective-c-header objective-c-cpp-output
906 objective-c++ objective-c++-header objective-c++-cpp-output
907 assembler assembler-with-cpp
909 f77 f77-cpp-input ratfor
916 Turn off any specification of a language, so that subsequent files are
917 handled according to their file name suffixes (as they are if @option{-x}
918 has not been used at all).
920 @item -pass-exit-codes
921 @opindex pass-exit-codes
922 Normally the @command{gcc} program will exit with the code of 1 if any
923 phase of the compiler returns a non-success return code. If you specify
924 @option{-pass-exit-codes}, the @command{gcc} program will instead return with
925 numerically highest error produced by any phase that returned an error
929 If you only want some of the stages of compilation, you can use
930 @option{-x} (or filename suffixes) to tell @command{gcc} where to start, and
931 one of the options @option{-c}, @option{-S}, or @option{-E} to say where
932 @command{gcc} is to stop. Note that some combinations (for example,
933 @samp{-x cpp-output -E}) instruct @command{gcc} to do nothing at all.
938 Compile or assemble the source files, but do not link. The linking
939 stage simply is not done. The ultimate output is in the form of an
940 object file for each source file.
942 By default, the object file name for a source file is made by replacing
943 the suffix @samp{.c}, @samp{.i}, @samp{.s}, etc., with @samp{.o}.
945 Unrecognized input files, not requiring compilation or assembly, are
950 Stop after the stage of compilation proper; do not assemble. The output
951 is in the form of an assembler code file for each non-assembler input
954 By default, the assembler file name for a source file is made by
955 replacing the suffix @samp{.c}, @samp{.i}, etc., with @samp{.s}.
957 Input files that don't require compilation are ignored.
961 Stop after the preprocessing stage; do not run the compiler proper. The
962 output is in the form of preprocessed source code, which is sent to the
965 Input files which don't require preprocessing are ignored.
967 @cindex output file option
970 Place output in file @var{file}. This applies regardless to whatever
971 sort of output is being produced, whether it be an executable file,
972 an object file, an assembler file or preprocessed C code.
974 If @option{-o} is not specified, the default is to put an executable
975 file in @file{a.out}, the object file for
976 @file{@var{source}.@var{suffix}} in @file{@var{source}.o}, its
977 assembler file in @file{@var{source}.s}, a precompiled header file in
978 @file{@var{source}.@var{suffix}.gch}, and all preprocessed C source on
983 Print (on standard error output) the commands executed to run the stages
984 of compilation. Also print the version number of the compiler driver
985 program and of the preprocessor and the compiler proper.
989 Like @option{-v} except the commands are not executed and all command
990 arguments are quoted. This is useful for shell scripts to capture the
991 driver-generated command lines.
995 Use pipes rather than temporary files for communication between the
996 various stages of compilation. This fails to work on some systems where
997 the assembler is unable to read from a pipe; but the GNU assembler has
1002 If you are compiling multiple source files, this option tells the driver
1003 to pass all the source files to the compiler at once (for those
1004 languages for which the compiler can handle this). This will allow
1005 intermodule analysis (IMA) to be performed by the compiler. Currently the only
1006 language for which this is supported is C@. If you pass source files for
1007 multiple languages to the driver, using this option, the driver will invoke
1008 the compiler(s) that support IMA once each, passing each compiler all the
1009 source files appropriate for it. For those languages that do not support
1010 IMA this option will be ignored, and the compiler will be invoked once for
1011 each source file in that language. If you use this option in conjunction
1012 with @option{-save-temps}, the compiler will generate multiple
1014 (one for each source file), but only one (combined) @file{.o} or
1019 Print (on the standard output) a description of the command line options
1020 understood by @command{gcc}. If the @option{-v} option is also specified
1021 then @option{--help} will also be passed on to the various processes
1022 invoked by @command{gcc}, so that they can display the command line options
1023 they accept. If the @option{-Wextra} option is also specified then command
1024 line options which have no documentation associated with them will also
1028 @opindex target-help
1029 Print (on the standard output) a description of target specific command
1030 line options for each tool.
1034 Display the version number and copyrights of the invoked GCC@.
1038 @section Compiling C++ Programs
1040 @cindex suffixes for C++ source
1041 @cindex C++ source file suffixes
1042 C++ source files conventionally use one of the suffixes @samp{.C},
1043 @samp{.cc}, @samp{.cpp}, @samp{.CPP}, @samp{.c++}, @samp{.cp}, or
1044 @samp{.cxx}; C++ header files often use @samp{.hh} or @samp{.H}; and
1045 preprocessed C++ files use the suffix @samp{.ii}. GCC recognizes
1046 files with these names and compiles them as C++ programs even if you
1047 call the compiler the same way as for compiling C programs (usually
1048 with the name @command{gcc}).
1052 However, C++ programs often require class libraries as well as a
1053 compiler that understands the C++ language---and under some
1054 circumstances, you might want to compile programs or header files from
1055 standard input, or otherwise without a suffix that flags them as C++
1056 programs. You might also like to precompile a C header file with a
1057 @samp{.h} extension to be used in C++ compilations. @command{g++} is a
1058 program that calls GCC with the default language set to C++, and
1059 automatically specifies linking against the C++ library. On many
1060 systems, @command{g++} is also installed with the name @command{c++}.
1062 @cindex invoking @command{g++}
1063 When you compile C++ programs, you may specify many of the same
1064 command-line options that you use for compiling programs in any
1065 language; or command-line options meaningful for C and related
1066 languages; or options that are meaningful only for C++ programs.
1067 @xref{C Dialect Options,,Options Controlling C Dialect}, for
1068 explanations of options for languages related to C@.
1069 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}, for
1070 explanations of options that are meaningful only for C++ programs.
1072 @node C Dialect Options
1073 @section Options Controlling C Dialect
1074 @cindex dialect options
1075 @cindex language dialect options
1076 @cindex options, dialect
1078 The following options control the dialect of C (or languages derived
1079 from C, such as C++, Objective-C and Objective-C++) that the compiler
1083 @cindex ANSI support
1087 In C mode, support all ISO C90 programs. In C++ mode,
1088 remove GNU extensions that conflict with ISO C++.
1090 This turns off certain features of GCC that are incompatible with ISO
1091 C90 (when compiling C code), or of standard C++ (when compiling C++ code),
1092 such as the @code{asm} and @code{typeof} keywords, and
1093 predefined macros such as @code{unix} and @code{vax} that identify the
1094 type of system you are using. It also enables the undesirable and
1095 rarely used ISO trigraph feature. For the C compiler,
1096 it disables recognition of C++ style @samp{//} comments as well as
1097 the @code{inline} keyword.
1099 The alternate keywords @code{__asm__}, @code{__extension__},
1100 @code{__inline__} and @code{__typeof__} continue to work despite
1101 @option{-ansi}. You would not want to use them in an ISO C program, of
1102 course, but it is useful to put them in header files that might be included
1103 in compilations done with @option{-ansi}. Alternate predefined macros
1104 such as @code{__unix__} and @code{__vax__} are also available, with or
1105 without @option{-ansi}.
1107 The @option{-ansi} option does not cause non-ISO programs to be
1108 rejected gratuitously. For that, @option{-pedantic} is required in
1109 addition to @option{-ansi}. @xref{Warning Options}.
1111 The macro @code{__STRICT_ANSI__} is predefined when the @option{-ansi}
1112 option is used. Some header files may notice this macro and refrain
1113 from declaring certain functions or defining certain macros that the
1114 ISO standard doesn't call for; this is to avoid interfering with any
1115 programs that might use these names for other things.
1117 Functions which would normally be built in but do not have semantics
1118 defined by ISO C (such as @code{alloca} and @code{ffs}) are not built-in
1119 functions with @option{-ansi} is used. @xref{Other Builtins,,Other
1120 built-in functions provided by GCC}, for details of the functions
1125 Determine the language standard. This option is currently only
1126 supported when compiling C or C++. A value for this option must be
1127 provided; possible values are
1132 ISO C90 (same as @option{-ansi}).
1134 @item iso9899:199409
1135 ISO C90 as modified in amendment 1.
1141 ISO C99. Note that this standard is not yet fully supported; see
1142 @w{@uref{http://gcc.gnu.org/c99status.html}} for more information. The
1143 names @samp{c9x} and @samp{iso9899:199x} are deprecated.
1146 Default, ISO C90 plus GNU extensions (including some C99 features).
1150 ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC,
1151 this will become the default. The name @samp{gnu9x} is deprecated.
1154 The 1998 ISO C++ standard plus amendments.
1157 The same as @option{-std=c++98} plus GNU extensions. This is the
1158 default for C++ code.
1161 Even when this option is not specified, you can still use some of the
1162 features of newer standards in so far as they do not conflict with
1163 previous C standards. For example, you may use @code{__restrict__} even
1164 when @option{-std=c99} is not specified.
1166 The @option{-std} options specifying some version of ISO C have the same
1167 effects as @option{-ansi}, except that features that were not in ISO C90
1168 but are in the specified version (for example, @samp{//} comments and
1169 the @code{inline} keyword in ISO C99) are not disabled.
1171 @xref{Standards,,Language Standards Supported by GCC}, for details of
1172 these standard versions.
1174 @item -aux-info @var{filename}
1176 Output to the given filename prototyped declarations for all functions
1177 declared and/or defined in a translation unit, including those in header
1178 files. This option is silently ignored in any language other than C@.
1180 Besides declarations, the file indicates, in comments, the origin of
1181 each declaration (source file and line), whether the declaration was
1182 implicit, prototyped or unprototyped (@samp{I}, @samp{N} for new or
1183 @samp{O} for old, respectively, in the first character after the line
1184 number and the colon), and whether it came from a declaration or a
1185 definition (@samp{C} or @samp{F}, respectively, in the following
1186 character). In the case of function definitions, a K&R-style list of
1187 arguments followed by their declarations is also provided, inside
1188 comments, after the declaration.
1192 Do not recognize @code{asm}, @code{inline} or @code{typeof} as a
1193 keyword, so that code can use these words as identifiers. You can use
1194 the keywords @code{__asm__}, @code{__inline__} and @code{__typeof__}
1195 instead. @option{-ansi} implies @option{-fno-asm}.
1197 In C++, this switch only affects the @code{typeof} keyword, since
1198 @code{asm} and @code{inline} are standard keywords. You may want to
1199 use the @option{-fno-gnu-keywords} flag instead, which has the same
1200 effect. In C99 mode (@option{-std=c99} or @option{-std=gnu99}), this
1201 switch only affects the @code{asm} and @code{typeof} keywords, since
1202 @code{inline} is a standard keyword in ISO C99.
1205 @itemx -fno-builtin-@var{function}
1206 @opindex fno-builtin
1207 @cindex built-in functions
1208 Don't recognize built-in functions that do not begin with
1209 @samp{__builtin_} as prefix. @xref{Other Builtins,,Other built-in
1210 functions provided by GCC}, for details of the functions affected,
1211 including those which are not built-in functions when @option{-ansi} or
1212 @option{-std} options for strict ISO C conformance are used because they
1213 do not have an ISO standard meaning.
1215 GCC normally generates special code to handle certain built-in functions
1216 more efficiently; for instance, calls to @code{alloca} may become single
1217 instructions that adjust the stack directly, and calls to @code{memcpy}
1218 may become inline copy loops. The resulting code is often both smaller
1219 and faster, but since the function calls no longer appear as such, you
1220 cannot set a breakpoint on those calls, nor can you change the behavior
1221 of the functions by linking with a different library. In addition,
1222 when a function is recognized as a built-in function, GCC may use
1223 information about that function to warn about problems with calls to
1224 that function, or to generate more efficient code, even if the
1225 resulting code still contains calls to that function. For example,
1226 warnings are given with @option{-Wformat} for bad calls to
1227 @code{printf}, when @code{printf} is built in, and @code{strlen} is
1228 known not to modify global memory.
1230 With the @option{-fno-builtin-@var{function}} option
1231 only the built-in function @var{function} is
1232 disabled. @var{function} must not begin with @samp{__builtin_}. If a
1233 function is named this is not built-in in this version of GCC, this
1234 option is ignored. There is no corresponding
1235 @option{-fbuiltin-@var{function}} option; if you wish to enable
1236 built-in functions selectively when using @option{-fno-builtin} or
1237 @option{-ffreestanding}, you may define macros such as:
1240 #define abs(n) __builtin_abs ((n))
1241 #define strcpy(d, s) __builtin_strcpy ((d), (s))
1246 @cindex hosted environment
1248 Assert that compilation takes place in a hosted environment. This implies
1249 @option{-fbuiltin}. A hosted environment is one in which the
1250 entire standard library is available, and in which @code{main} has a return
1251 type of @code{int}. Examples are nearly everything except a kernel.
1252 This is equivalent to @option{-fno-freestanding}.
1254 @item -ffreestanding
1255 @opindex ffreestanding
1256 @cindex hosted environment
1258 Assert that compilation takes place in a freestanding environment. This
1259 implies @option{-fno-builtin}. A freestanding environment
1260 is one in which the standard library may not exist, and program startup may
1261 not necessarily be at @code{main}. The most obvious example is an OS kernel.
1262 This is equivalent to @option{-fno-hosted}.
1264 @xref{Standards,,Language Standards Supported by GCC}, for details of
1265 freestanding and hosted environments.
1267 @item -fms-extensions
1268 @opindex fms-extensions
1269 Accept some non-standard constructs used in Microsoft header files.
1271 Some cases of unnamed fields in structures and unions are only
1272 accepted with this option. @xref{Unnamed Fields,,Unnamed struct/union
1273 fields within structs/unions}, for details.
1277 Support ISO C trigraphs. The @option{-ansi} option (and @option{-std}
1278 options for strict ISO C conformance) implies @option{-trigraphs}.
1280 @item -no-integrated-cpp
1281 @opindex no-integrated-cpp
1282 Performs a compilation in two passes: preprocessing and compiling. This
1283 option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the
1284 @option{-B} option. The user supplied compilation step can then add in
1285 an additional preprocessing step after normal preprocessing but before
1286 compiling. The default is to use the integrated cpp (internal cpp)
1288 The semantics of this option will change if "cc1", "cc1plus", and
1289 "cc1obj" are merged.
1291 @cindex traditional C language
1292 @cindex C language, traditional
1294 @itemx -traditional-cpp
1295 @opindex traditional-cpp
1296 @opindex traditional
1297 Formerly, these options caused GCC to attempt to emulate a pre-standard
1298 C compiler. They are now only supported with the @option{-E} switch.
1299 The preprocessor continues to support a pre-standard mode. See the GNU
1300 CPP manual for details.
1302 @item -fcond-mismatch
1303 @opindex fcond-mismatch
1304 Allow conditional expressions with mismatched types in the second and
1305 third arguments. The value of such an expression is void. This option
1306 is not supported for C++.
1308 @item -funsigned-char
1309 @opindex funsigned-char
1310 Let the type @code{char} be unsigned, like @code{unsigned char}.
1312 Each kind of machine has a default for what @code{char} should
1313 be. It is either like @code{unsigned char} by default or like
1314 @code{signed char} by default.
1316 Ideally, a portable program should always use @code{signed char} or
1317 @code{unsigned char} when it depends on the signedness of an object.
1318 But many programs have been written to use plain @code{char} and
1319 expect it to be signed, or expect it to be unsigned, depending on the
1320 machines they were written for. This option, and its inverse, let you
1321 make such a program work with the opposite default.
1323 The type @code{char} is always a distinct type from each of
1324 @code{signed char} or @code{unsigned char}, even though its behavior
1325 is always just like one of those two.
1328 @opindex fsigned-char
1329 Let the type @code{char} be signed, like @code{signed char}.
1331 Note that this is equivalent to @option{-fno-unsigned-char}, which is
1332 the negative form of @option{-funsigned-char}. Likewise, the option
1333 @option{-fno-signed-char} is equivalent to @option{-funsigned-char}.
1335 @item -fsigned-bitfields
1336 @itemx -funsigned-bitfields
1337 @itemx -fno-signed-bitfields
1338 @itemx -fno-unsigned-bitfields
1339 @opindex fsigned-bitfields
1340 @opindex funsigned-bitfields
1341 @opindex fno-signed-bitfields
1342 @opindex fno-unsigned-bitfields
1343 These options control whether a bit-field is signed or unsigned, when the
1344 declaration does not use either @code{signed} or @code{unsigned}. By
1345 default, such a bit-field is signed, because this is consistent: the
1346 basic integer types such as @code{int} are signed types.
1349 @node C++ Dialect Options
1350 @section Options Controlling C++ Dialect
1352 @cindex compiler options, C++
1353 @cindex C++ options, command line
1354 @cindex options, C++
1355 This section describes the command-line options that are only meaningful
1356 for C++ programs; but you can also use most of the GNU compiler options
1357 regardless of what language your program is in. For example, you
1358 might compile a file @code{firstClass.C} like this:
1361 g++ -g -frepo -O -c firstClass.C
1365 In this example, only @option{-frepo} is an option meant
1366 only for C++ programs; you can use the other options with any
1367 language supported by GCC@.
1369 Here is a list of options that are @emph{only} for compiling C++ programs:
1373 @item -fabi-version=@var{n}
1374 @opindex fabi-version
1375 Use version @var{n} of the C++ ABI@. Version 2 is the version of the
1376 C++ ABI that first appeared in G++ 3.4. Version 1 is the version of
1377 the C++ ABI that first appeared in G++ 3.2. Version 0 will always be
1378 the version that conforms most closely to the C++ ABI specification.
1379 Therefore, the ABI obtained using version 0 will change as ABI bugs
1382 The default is version 2.
1384 @item -fno-access-control
1385 @opindex fno-access-control
1386 Turn off all access checking. This switch is mainly useful for working
1387 around bugs in the access control code.
1391 Check that the pointer returned by @code{operator new} is non-null
1392 before attempting to modify the storage allocated. This check is
1393 normally unnecessary because the C++ standard specifies that
1394 @code{operator new} will only return @code{0} if it is declared
1395 @samp{throw()}, in which case the compiler will always check the
1396 return value even without this option. In all other cases, when
1397 @code{operator new} has a non-empty exception specification, memory
1398 exhaustion is signalled by throwing @code{std::bad_alloc}. See also
1399 @samp{new (nothrow)}.
1401 @item -fconserve-space
1402 @opindex fconserve-space
1403 Put uninitialized or runtime-initialized global variables into the
1404 common segment, as C does. This saves space in the executable at the
1405 cost of not diagnosing duplicate definitions. If you compile with this
1406 flag and your program mysteriously crashes after @code{main()} has
1407 completed, you may have an object that is being destroyed twice because
1408 two definitions were merged.
1410 This option is no longer useful on most targets, now that support has
1411 been added for putting variables into BSS without making them common.
1413 @item -fno-const-strings
1414 @opindex fno-const-strings
1415 Give string constants type @code{char *} instead of type @code{const
1416 char *}. By default, G++ uses type @code{const char *} as required by
1417 the standard. Even if you use @option{-fno-const-strings}, you cannot
1418 actually modify the value of a string constant.
1420 This option might be removed in a future release of G++. For maximum
1421 portability, you should structure your code so that it works with
1422 string constants that have type @code{const char *}.
1424 @item -fno-elide-constructors
1425 @opindex fno-elide-constructors
1426 The C++ standard allows an implementation to omit creating a temporary
1427 which is only used to initialize another object of the same type.
1428 Specifying this option disables that optimization, and forces G++ to
1429 call the copy constructor in all cases.
1431 @item -fno-enforce-eh-specs
1432 @opindex fno-enforce-eh-specs
1433 Don't check for violation of exception specifications at runtime. This
1434 option violates the C++ standard, but may be useful for reducing code
1435 size in production builds, much like defining @samp{NDEBUG}. The compiler
1436 will still optimize based on the exception specifications.
1439 @itemx -fno-for-scope
1441 @opindex fno-for-scope
1442 If @option{-ffor-scope} is specified, the scope of variables declared in
1443 a @i{for-init-statement} is limited to the @samp{for} loop itself,
1444 as specified by the C++ standard.
1445 If @option{-fno-for-scope} is specified, the scope of variables declared in
1446 a @i{for-init-statement} extends to the end of the enclosing scope,
1447 as was the case in old versions of G++, and other (traditional)
1448 implementations of C++.
1450 The default if neither flag is given to follow the standard,
1451 but to allow and give a warning for old-style code that would
1452 otherwise be invalid, or have different behavior.
1454 @item -fno-gnu-keywords
1455 @opindex fno-gnu-keywords
1456 Do not recognize @code{typeof} as a keyword, so that code can use this
1457 word as an identifier. You can use the keyword @code{__typeof__} instead.
1458 @option{-ansi} implies @option{-fno-gnu-keywords}.
1460 @item -fno-implicit-templates
1461 @opindex fno-implicit-templates
1462 Never emit code for non-inline templates which are instantiated
1463 implicitly (i.e.@: by use); only emit code for explicit instantiations.
1464 @xref{Template Instantiation}, for more information.
1466 @item -fno-implicit-inline-templates
1467 @opindex fno-implicit-inline-templates
1468 Don't emit code for implicit instantiations of inline templates, either.
1469 The default is to handle inlines differently so that compiles with and
1470 without optimization will need the same set of explicit instantiations.
1472 @item -fno-implement-inlines
1473 @opindex fno-implement-inlines
1474 To save space, do not emit out-of-line copies of inline functions
1475 controlled by @samp{#pragma implementation}. This will cause linker
1476 errors if these functions are not inlined everywhere they are called.
1478 @item -fms-extensions
1479 @opindex fms-extensions
1480 Disable pedantic warnings about constructs used in MFC, such as implicit
1481 int and getting a pointer to member function via non-standard syntax.
1483 @item -fno-nonansi-builtins
1484 @opindex fno-nonansi-builtins
1485 Disable built-in declarations of functions that are not mandated by
1486 ANSI/ISO C@. These include @code{ffs}, @code{alloca}, @code{_exit},
1487 @code{index}, @code{bzero}, @code{conjf}, and other related functions.
1489 @item -fno-operator-names
1490 @opindex fno-operator-names
1491 Do not treat the operator name keywords @code{and}, @code{bitand},
1492 @code{bitor}, @code{compl}, @code{not}, @code{or} and @code{xor} as
1493 synonyms as keywords.
1495 @item -fno-optional-diags
1496 @opindex fno-optional-diags
1497 Disable diagnostics that the standard says a compiler does not need to
1498 issue. Currently, the only such diagnostic issued by G++ is the one for
1499 a name having multiple meanings within a class.
1502 @opindex fpermissive
1503 Downgrade some diagnostics about nonconformant code from errors to
1504 warnings. Thus, using @option{-fpermissive} will allow some
1505 nonconforming code to compile.
1509 Enable automatic template instantiation at link time. This option also
1510 implies @option{-fno-implicit-templates}. @xref{Template
1511 Instantiation}, for more information.
1515 Disable generation of information about every class with virtual
1516 functions for use by the C++ runtime type identification features
1517 (@samp{dynamic_cast} and @samp{typeid}). If you don't use those parts
1518 of the language, you can save some space by using this flag. Note that
1519 exception handling uses the same information, but it will generate it as
1524 Emit statistics about front-end processing at the end of the compilation.
1525 This information is generally only useful to the G++ development team.
1527 @item -ftemplate-depth-@var{n}
1528 @opindex ftemplate-depth
1529 Set the maximum instantiation depth for template classes to @var{n}.
1530 A limit on the template instantiation depth is needed to detect
1531 endless recursions during template class instantiation. ANSI/ISO C++
1532 conforming programs must not rely on a maximum depth greater than 17.
1534 @item -fno-threadsafe-statics
1535 @opindex fno-threadsafe-statics
1536 Do not emit the extra code to use the routines specified in the C++
1537 ABI for thread-safe initialization of local statics. You can use this
1538 option to reduce code size slightly in code that doesn't need to be
1541 @item -fuse-cxa-atexit
1542 @opindex fuse-cxa-atexit
1543 Register destructors for objects with static storage duration with the
1544 @code{__cxa_atexit} function rather than the @code{atexit} function.
1545 This option is required for fully standards-compliant handling of static
1546 destructors, but will only work if your C library supports
1547 @code{__cxa_atexit}.
1549 @item -fvisibility-inlines-hidden
1550 @opindex fvisibility-inlines-hidden
1551 Causes all inlined methods to be marked with
1552 @code{__attribute__ ((visibility ("hidden")))} so that they do not
1553 appear in the export table of a DSO and do not require a PLT indirection
1554 when used within the DSO@. Enabling this option can have a dramatic effect
1555 on load and link times of a DSO as it massively reduces the size of the
1556 dynamic export table when the library makes heavy use of templates. While
1557 it can cause bloating through duplication of code within each DSO where
1558 it is used, often the wastage is less than the considerable space occupied
1559 by a long symbol name in the export table which is typical when using
1560 templates and namespaces. For even more savings, combine with the
1561 @option{-fvisibility=hidden} switch.
1565 Do not use weak symbol support, even if it is provided by the linker.
1566 By default, G++ will use weak symbols if they are available. This
1567 option exists only for testing, and should not be used by end-users;
1568 it will result in inferior code and has no benefits. This option may
1569 be removed in a future release of G++.
1573 Do not search for header files in the standard directories specific to
1574 C++, but do still search the other standard directories. (This option
1575 is used when building the C++ library.)
1578 In addition, these optimization, warning, and code generation options
1579 have meanings only for C++ programs:
1582 @item -fno-default-inline
1583 @opindex fno-default-inline
1584 Do not assume @samp{inline} for functions defined inside a class scope.
1585 @xref{Optimize Options,,Options That Control Optimization}. Note that these
1586 functions will have linkage like inline functions; they just won't be
1589 @item -Wabi @r{(C++ only)}
1591 Warn when G++ generates code that is probably not compatible with the
1592 vendor-neutral C++ ABI@. Although an effort has been made to warn about
1593 all such cases, there are probably some cases that are not warned about,
1594 even though G++ is generating incompatible code. There may also be
1595 cases where warnings are emitted even though the code that is generated
1598 You should rewrite your code to avoid these warnings if you are
1599 concerned about the fact that code generated by G++ may not be binary
1600 compatible with code generated by other compilers.
1602 The known incompatibilities at this point include:
1607 Incorrect handling of tail-padding for bit-fields. G++ may attempt to
1608 pack data into the same byte as a base class. For example:
1611 struct A @{ virtual void f(); int f1 : 1; @};
1612 struct B : public A @{ int f2 : 1; @};
1616 In this case, G++ will place @code{B::f2} into the same byte
1617 as@code{A::f1}; other compilers will not. You can avoid this problem
1618 by explicitly padding @code{A} so that its size is a multiple of the
1619 byte size on your platform; that will cause G++ and other compilers to
1620 layout @code{B} identically.
1623 Incorrect handling of tail-padding for virtual bases. G++ does not use
1624 tail padding when laying out virtual bases. For example:
1627 struct A @{ virtual void f(); char c1; @};
1628 struct B @{ B(); char c2; @};
1629 struct C : public A, public virtual B @{@};
1633 In this case, G++ will not place @code{B} into the tail-padding for
1634 @code{A}; other compilers will. You can avoid this problem by
1635 explicitly padding @code{A} so that its size is a multiple of its
1636 alignment (ignoring virtual base classes); that will cause G++ and other
1637 compilers to layout @code{C} identically.
1640 Incorrect handling of bit-fields with declared widths greater than that
1641 of their underlying types, when the bit-fields appear in a union. For
1645 union U @{ int i : 4096; @};
1649 Assuming that an @code{int} does not have 4096 bits, G++ will make the
1650 union too small by the number of bits in an @code{int}.
1653 Empty classes can be placed at incorrect offsets. For example:
1663 struct C : public B, public A @{@};
1667 G++ will place the @code{A} base class of @code{C} at a nonzero offset;
1668 it should be placed at offset zero. G++ mistakenly believes that the
1669 @code{A} data member of @code{B} is already at offset zero.
1672 Names of template functions whose types involve @code{typename} or
1673 template template parameters can be mangled incorrectly.
1676 template <typename Q>
1677 void f(typename Q::X) @{@}
1679 template <template <typename> class Q>
1680 void f(typename Q<int>::X) @{@}
1684 Instantiations of these templates may be mangled incorrectly.
1688 @item -Wctor-dtor-privacy @r{(C++ only)}
1689 @opindex Wctor-dtor-privacy
1690 Warn when a class seems unusable because all the constructors or
1691 destructors in that class are private, and it has neither friends nor
1692 public static member functions.
1694 @item -Wnon-virtual-dtor @r{(C++ only)}
1695 @opindex Wnon-virtual-dtor
1696 Warn when a class appears to be polymorphic, thereby requiring a virtual
1697 destructor, yet it declares a non-virtual one.
1698 This warning is enabled by @option{-Wall}.
1700 @item -Wreorder @r{(C++ only)}
1702 @cindex reordering, warning
1703 @cindex warning for reordering of member initializers
1704 Warn when the order of member initializers given in the code does not
1705 match the order in which they must be executed. For instance:
1711 A(): j (0), i (1) @{ @}
1715 The compiler will rearrange the member initializers for @samp{i}
1716 and @samp{j} to match the declaration order of the members, emitting
1717 a warning to that effect. This warning is enabled by @option{-Wall}.
1720 The following @option{-W@dots{}} options are not affected by @option{-Wall}.
1723 @item -Weffc++ @r{(C++ only)}
1725 Warn about violations of the following style guidelines from Scott Meyers'
1726 @cite{Effective C++} book:
1730 Item 11: Define a copy constructor and an assignment operator for classes
1731 with dynamically allocated memory.
1734 Item 12: Prefer initialization to assignment in constructors.
1737 Item 14: Make destructors virtual in base classes.
1740 Item 15: Have @code{operator=} return a reference to @code{*this}.
1743 Item 23: Don't try to return a reference when you must return an object.
1747 Also warn about violations of the following style guidelines from
1748 Scott Meyers' @cite{More Effective C++} book:
1752 Item 6: Distinguish between prefix and postfix forms of increment and
1753 decrement operators.
1756 Item 7: Never overload @code{&&}, @code{||}, or @code{,}.
1760 When selecting this option, be aware that the standard library
1761 headers do not obey all of these guidelines; use @samp{grep -v}
1762 to filter out those warnings.
1764 @item -Wno-deprecated @r{(C++ only)}
1765 @opindex Wno-deprecated
1766 Do not warn about usage of deprecated features. @xref{Deprecated Features}.
1768 @item -Wstrict-null-sentinel @r{(C++ only)}
1769 @opindex Wstrict-null-sentinel
1770 Warn also about the use of an uncasted @code{NULL} as sentinel. When
1771 compiling only with GCC this is a valid sentinel, as @code{NULL} is defined
1772 to @code{__null}. Although it is a null pointer constant not a null pointer,
1773 it is guaranteed to of the same size as a pointer. But this use is
1774 not portable across different compilers.
1776 @item -Wno-non-template-friend @r{(C++ only)}
1777 @opindex Wno-non-template-friend
1778 Disable warnings when non-templatized friend functions are declared
1779 within a template. Since the advent of explicit template specification
1780 support in G++, if the name of the friend is an unqualified-id (i.e.,
1781 @samp{friend foo(int)}), the C++ language specification demands that the
1782 friend declare or define an ordinary, nontemplate function. (Section
1783 14.5.3). Before G++ implemented explicit specification, unqualified-ids
1784 could be interpreted as a particular specialization of a templatized
1785 function. Because this non-conforming behavior is no longer the default
1786 behavior for G++, @option{-Wnon-template-friend} allows the compiler to
1787 check existing code for potential trouble spots and is on by default.
1788 This new compiler behavior can be turned off with
1789 @option{-Wno-non-template-friend} which keeps the conformant compiler code
1790 but disables the helpful warning.
1792 @item -Wold-style-cast @r{(C++ only)}
1793 @opindex Wold-style-cast
1794 Warn if an old-style (C-style) cast to a non-void type is used within
1795 a C++ program. The new-style casts (@samp{static_cast},
1796 @samp{reinterpret_cast}, and @samp{const_cast}) are less vulnerable to
1797 unintended effects and much easier to search for.
1799 @item -Woverloaded-virtual @r{(C++ only)}
1800 @opindex Woverloaded-virtual
1801 @cindex overloaded virtual fn, warning
1802 @cindex warning for overloaded virtual fn
1803 Warn when a function declaration hides virtual functions from a
1804 base class. For example, in:
1811 struct B: public A @{
1816 the @code{A} class version of @code{f} is hidden in @code{B}, and code
1824 will fail to compile.
1826 @item -Wno-pmf-conversions @r{(C++ only)}
1827 @opindex Wno-pmf-conversions
1828 Disable the diagnostic for converting a bound pointer to member function
1831 @item -Wsign-promo @r{(C++ only)}
1832 @opindex Wsign-promo
1833 Warn when overload resolution chooses a promotion from unsigned or
1834 enumerated type to a signed type, over a conversion to an unsigned type of
1835 the same size. Previous versions of G++ would try to preserve
1836 unsignedness, but the standard mandates the current behavior.
1841 A& operator = (int);
1851 In this example, G++ will synthesize a default @samp{A& operator =
1852 (const A&);}, while cfront will use the user-defined @samp{operator =}.
1855 @node Objective-C and Objective-C++ Dialect Options
1856 @section Options Controlling Objective-C and Objective-C++ Dialects
1858 @cindex compiler options, Objective-C and Objective-C++
1859 @cindex Objective-C and Objective-C++ options, command line
1860 @cindex options, Objective-C and Objective-C++
1861 (NOTE: This manual does not describe the Objective-C and Objective-C++
1862 languages themselves. See @xref{Standards,,Language Standards
1863 Supported by GCC}, for references.)
1865 This section describes the command-line options that are only meaningful
1866 for Objective-C and Objective-C++ programs, but you can also use most of
1867 the language-independent GNU compiler options.
1868 For example, you might compile a file @code{some_class.m} like this:
1871 gcc -g -fgnu-runtime -O -c some_class.m
1875 In this example, @option{-fgnu-runtime} is an option meant only for
1876 Objective-C and Objective-C++ programs; you can use the other options with
1877 any language supported by GCC@.
1879 Note that since Objective-C is an extension of the C language, Objective-C
1880 compilations may also use options specific to the C front-end (e.g.,
1881 @option{-Wtraditional}). Similarly, Objective-C++ compilations may use
1882 C++-specific options (e.g., @option{-Wabi}).
1884 Here is a list of options that are @emph{only} for compiling Objective-C
1885 and Objective-C++ programs:
1888 @item -fconstant-string-class=@var{class-name}
1889 @opindex fconstant-string-class
1890 Use @var{class-name} as the name of the class to instantiate for each
1891 literal string specified with the syntax @code{@@"@dots{}"}. The default
1892 class name is @code{NXConstantString} if the GNU runtime is being used, and
1893 @code{NSConstantString} if the NeXT runtime is being used (see below). The
1894 @option{-fconstant-cfstrings} option, if also present, will override the
1895 @option{-fconstant-string-class} setting and cause @code{@@"@dots{}"} literals
1896 to be laid out as constant CoreFoundation strings.
1899 @opindex fgnu-runtime
1900 Generate object code compatible with the standard GNU Objective-C
1901 runtime. This is the default for most types of systems.
1903 @item -fnext-runtime
1904 @opindex fnext-runtime
1905 Generate output compatible with the NeXT runtime. This is the default
1906 for NeXT-based systems, including Darwin and Mac OS X@. The macro
1907 @code{__NEXT_RUNTIME__} is predefined if (and only if) this option is
1910 @item -fno-nil-receivers
1911 @opindex fno-nil-receivers
1912 Assume that all Objective-C message dispatches (e.g.,
1913 @code{[receiver message:arg]}) in this translation unit ensure that the receiver
1914 is not @code{nil}. This allows for more efficient entry points in the runtime
1915 to be used. Currently, this option is only available in conjunction with
1916 the NeXT runtime on Mac OS X 10.3 and later.
1918 @item -fobjc-call-cxx-cdtors
1919 @opindex fobjc-call-cxx-cdtors
1920 For each Objective-C class, check if any of its instance variables is a
1921 C++ object with a non-trivial default constructor. If so, synthesize a
1922 special @code{- (id) .cxx_construct} instance method that will run
1923 non-trivial default constructors on any such instance variables, in order,
1924 and then return @code{self}. Similarly, check if any instance variable
1925 is a C++ object with a non-trivial destructor, and if so, synthesize a
1926 special @code{- (void) .cxx_destruct} method that will run
1927 all such default destructors, in reverse order.
1929 The @code{- (id) .cxx_construct} and/or @code{- (void) .cxx_destruct} methods
1930 thusly generated will only operate on instance variables declared in the
1931 current Objective-C class, and not those inherited from superclasses. It
1932 is the responsibility of the Objective-C runtime to invoke all such methods
1933 in an object's inheritance hierarchy. The @code{- (id) .cxx_construct} methods
1934 will be invoked by the runtime immediately after a new object
1935 instance is allocated; the @code{- (void) .cxx_destruct} methods will
1936 be invoked immediately before the runtime deallocates an object instance.
1938 As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has
1939 support for invoking the @code{- (id) .cxx_construct} and
1940 @code{- (void) .cxx_destruct} methods.
1942 @item -fobjc-direct-dispatch
1943 @opindex fobjc-direct-dispatch
1944 Allow fast jumps to the message dispatcher. On Darwin this is
1945 accomplished via the comm page.
1947 @item -fobjc-exceptions
1948 @opindex fobjc-exceptions
1949 Enable syntactic support for structured exception handling in Objective-C,
1950 similar to what is offered by C++ and Java. Currently, this option is only
1951 available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
1959 @@catch (AnObjCClass *exc) @{
1966 @@catch (AnotherClass *exc) @{
1969 @@catch (id allOthers) @{
1979 The @code{@@throw} statement may appear anywhere in an Objective-C or
1980 Objective-C++ program; when used inside of a @code{@@catch} block, the
1981 @code{@@throw} may appear without an argument (as shown above), in which case
1982 the object caught by the @code{@@catch} will be rethrown.
1984 Note that only (pointers to) Objective-C objects may be thrown and
1985 caught using this scheme. When an object is thrown, it will be caught
1986 by the nearest @code{@@catch} clause capable of handling objects of that type,
1987 analogously to how @code{catch} blocks work in C++ and Java. A
1988 @code{@@catch(id @dots{})} clause (as shown above) may also be provided to catch
1989 any and all Objective-C exceptions not caught by previous @code{@@catch}
1992 The @code{@@finally} clause, if present, will be executed upon exit from the
1993 immediately preceding @code{@@try @dots{} @@catch} section. This will happen
1994 regardless of whether any exceptions are thrown, caught or rethrown
1995 inside the @code{@@try @dots{} @@catch} section, analogously to the behavior
1996 of the @code{finally} clause in Java.
1998 There are several caveats to using the new exception mechanism:
2002 Although currently designed to be binary compatible with @code{NS_HANDLER}-style
2003 idioms provided by the @code{NSException} class, the new
2004 exceptions can only be used on Mac OS X 10.3 (Panther) and later
2005 systems, due to additional functionality needed in the (NeXT) Objective-C
2009 As mentioned above, the new exceptions do not support handling
2010 types other than Objective-C objects. Furthermore, when used from
2011 Objective-C++, the Objective-C exception model does not interoperate with C++
2012 exceptions at this time. This means you cannot @code{@@throw} an exception
2013 from Objective-C and @code{catch} it in C++, or vice versa
2014 (i.e., @code{throw @dots{} @@catch}).
2017 The @option{-fobjc-exceptions} switch also enables the use of synchronization
2018 blocks for thread-safe execution:
2021 @@synchronized (ObjCClass *guard) @{
2026 Upon entering the @code{@@synchronized} block, a thread of execution shall
2027 first check whether a lock has been placed on the corresponding @code{guard}
2028 object by another thread. If it has, the current thread shall wait until
2029 the other thread relinquishes its lock. Once @code{guard} becomes available,
2030 the current thread will place its own lock on it, execute the code contained in
2031 the @code{@@synchronized} block, and finally relinquish the lock (thereby
2032 making @code{guard} available to other threads).
2034 Unlike Java, Objective-C does not allow for entire methods to be marked
2035 @code{@@synchronized}. Note that throwing exceptions out of
2036 @code{@@synchronized} blocks is allowed, and will cause the guarding object
2037 to be unlocked properly.
2041 Enable garbage collection (GC) in Objective-C and Objective-C++ programs.
2043 @item -freplace-objc-classes
2044 @opindex freplace-objc-classes
2045 Emit a special marker instructing @command{ld(1)} not to statically link in
2046 the resulting object file, and allow @command{dyld(1)} to load it in at
2047 run time instead. This is used in conjunction with the Fix-and-Continue
2048 debugging mode, where the object file in question may be recompiled and
2049 dynamically reloaded in the course of program execution, without the need
2050 to restart the program itself. Currently, Fix-and-Continue functionality
2051 is only available in conjunction with the NeXT runtime on Mac OS X 10.3
2056 When compiling for the NeXT runtime, the compiler ordinarily replaces calls
2057 to @code{objc_getClass("@dots{}")} (when the name of the class is known at
2058 compile time) with static class references that get initialized at load time,
2059 which improves run-time performance. Specifying the @option{-fzero-link} flag
2060 suppresses this behavior and causes calls to @code{objc_getClass("@dots{}")}
2061 to be retained. This is useful in Zero-Link debugging mode, since it allows
2062 for individual class implementations to be modified during program execution.
2066 Dump interface declarations for all classes seen in the source file to a
2067 file named @file{@var{sourcename}.decl}.
2069 @item -Wassign-intercept
2070 @opindex Wassign-intercept
2071 Warn whenever an Objective-C assignment is being intercepted by the
2075 @opindex Wno-protocol
2076 If a class is declared to implement a protocol, a warning is issued for
2077 every method in the protocol that is not implemented by the class. The
2078 default behavior is to issue a warning for every method not explicitly
2079 implemented in the class, even if a method implementation is inherited
2080 from the superclass. If you use the @option{-Wno-protocol} option, then
2081 methods inherited from the superclass are considered to be implemented,
2082 and no warning is issued for them.
2086 Warn if multiple methods of different types for the same selector are
2087 found during compilation. The check is performed on the list of methods
2088 in the final stage of compilation. Additionally, a check is performed
2089 for each selector appearing in a @code{@@selector(@dots{})}
2090 expression, and a corresponding method for that selector has been found
2091 during compilation. Because these checks scan the method table only at
2092 the end of compilation, these warnings are not produced if the final
2093 stage of compilation is not reached, for example because an error is
2094 found during compilation, or because the @option{-fsyntax-only} option is
2097 @item -Wstrict-selector-match
2098 @opindex Wstrict-selector-match
2099 Warn if multiple methods with differing argument and/or return types are
2100 found for a given selector when attempting to send a message using this
2101 selector to a receiver of type @code{id} or @code{Class}. When this flag
2102 is off (which is the default behavior), the compiler will omit such warnings
2103 if any differences found are confined to types which share the same size
2106 @item -Wundeclared-selector
2107 @opindex Wundeclared-selector
2108 Warn if a @code{@@selector(@dots{})} expression referring to an
2109 undeclared selector is found. A selector is considered undeclared if no
2110 method with that name has been declared before the
2111 @code{@@selector(@dots{})} expression, either explicitly in an
2112 @code{@@interface} or @code{@@protocol} declaration, or implicitly in
2113 an @code{@@implementation} section. This option always performs its
2114 checks as soon as a @code{@@selector(@dots{})} expression is found,
2115 while @option{-Wselector} only performs its checks in the final stage of
2116 compilation. This also enforces the coding style convention
2117 that methods and selectors must be declared before being used.
2119 @item -print-objc-runtime-info
2120 @opindex print-objc-runtime-info
2121 Generate C header describing the largest structure that is passed by
2126 @node Language Independent Options
2127 @section Options to Control Diagnostic Messages Formatting
2128 @cindex options to control diagnostics formatting
2129 @cindex diagnostic messages
2130 @cindex message formatting
2132 Traditionally, diagnostic messages have been formatted irrespective of
2133 the output device's aspect (e.g.@: its width, @dots{}). The options described
2134 below can be used to control the diagnostic messages formatting
2135 algorithm, e.g.@: how many characters per line, how often source location
2136 information should be reported. Right now, only the C++ front end can
2137 honor these options. However it is expected, in the near future, that
2138 the remaining front ends would be able to digest them correctly.
2141 @item -fmessage-length=@var{n}
2142 @opindex fmessage-length
2143 Try to format error messages so that they fit on lines of about @var{n}
2144 characters. The default is 72 characters for @command{g++} and 0 for the rest of
2145 the front ends supported by GCC@. If @var{n} is zero, then no
2146 line-wrapping will be done; each error message will appear on a single
2149 @opindex fdiagnostics-show-location
2150 @item -fdiagnostics-show-location=once
2151 Only meaningful in line-wrapping mode. Instructs the diagnostic messages
2152 reporter to emit @emph{once} source location information; that is, in
2153 case the message is too long to fit on a single physical line and has to
2154 be wrapped, the source location won't be emitted (as prefix) again,
2155 over and over, in subsequent continuation lines. This is the default
2158 @item -fdiagnostics-show-location=every-line
2159 Only meaningful in line-wrapping mode. Instructs the diagnostic
2160 messages reporter to emit the same source location information (as
2161 prefix) for physical lines that result from the process of breaking
2162 a message which is too long to fit on a single line.
2164 @item -fdiagnostics-show-options
2165 @opindex fdiagnostics-show-options
2166 This option instructs the diagnostic machinery to add text to each
2167 diagnostic emitted, which indicates which command line option directly
2168 controls that diagnostic, when such an option is known to the
2169 diagnostic machinery.
2173 @node Warning Options
2174 @section Options to Request or Suppress Warnings
2175 @cindex options to control warnings
2176 @cindex warning messages
2177 @cindex messages, warning
2178 @cindex suppressing warnings
2180 Warnings are diagnostic messages that report constructions which
2181 are not inherently erroneous but which are risky or suggest there
2182 may have been an error.
2184 You can request many specific warnings with options beginning @samp{-W},
2185 for example @option{-Wimplicit} to request warnings on implicit
2186 declarations. Each of these specific warning options also has a
2187 negative form beginning @samp{-Wno-} to turn off warnings;
2188 for example, @option{-Wno-implicit}. This manual lists only one of the
2189 two forms, whichever is not the default.
2191 The following options control the amount and kinds of warnings produced
2192 by GCC; for further, language-specific options also refer to
2193 @ref{C++ Dialect Options} and @ref{Objective-C and Objective-C++ Dialect
2197 @cindex syntax checking
2199 @opindex fsyntax-only
2200 Check the code for syntax errors, but don't do anything beyond that.
2204 Issue all the warnings demanded by strict ISO C and ISO C++;
2205 reject all programs that use forbidden extensions, and some other
2206 programs that do not follow ISO C and ISO C++. For ISO C, follows the
2207 version of the ISO C standard specified by any @option{-std} option used.
2209 Valid ISO C and ISO C++ programs should compile properly with or without
2210 this option (though a rare few will require @option{-ansi} or a
2211 @option{-std} option specifying the required version of ISO C)@. However,
2212 without this option, certain GNU extensions and traditional C and C++
2213 features are supported as well. With this option, they are rejected.
2215 @option{-pedantic} does not cause warning messages for use of the
2216 alternate keywords whose names begin and end with @samp{__}. Pedantic
2217 warnings are also disabled in the expression that follows
2218 @code{__extension__}. However, only system header files should use
2219 these escape routes; application programs should avoid them.
2220 @xref{Alternate Keywords}.
2222 Some users try to use @option{-pedantic} to check programs for strict ISO
2223 C conformance. They soon find that it does not do quite what they want:
2224 it finds some non-ISO practices, but not all---only those for which
2225 ISO C @emph{requires} a diagnostic, and some others for which
2226 diagnostics have been added.
2228 A feature to report any failure to conform to ISO C might be useful in
2229 some instances, but would require considerable additional work and would
2230 be quite different from @option{-pedantic}. We don't have plans to
2231 support such a feature in the near future.
2233 Where the standard specified with @option{-std} represents a GNU
2234 extended dialect of C, such as @samp{gnu89} or @samp{gnu99}, there is a
2235 corresponding @dfn{base standard}, the version of ISO C on which the GNU
2236 extended dialect is based. Warnings from @option{-pedantic} are given
2237 where they are required by the base standard. (It would not make sense
2238 for such warnings to be given only for features not in the specified GNU
2239 C dialect, since by definition the GNU dialects of C include all
2240 features the compiler supports with the given option, and there would be
2241 nothing to warn about.)
2243 @item -pedantic-errors
2244 @opindex pedantic-errors
2245 Like @option{-pedantic}, except that errors are produced rather than
2250 Inhibit all warning messages.
2254 Inhibit warning messages about the use of @samp{#import}.
2256 @item -Wchar-subscripts
2257 @opindex Wchar-subscripts
2258 Warn if an array subscript has type @code{char}. This is a common cause
2259 of error, as programmers often forget that this type is signed on some
2261 This warning is enabled by @option{-Wall}.
2265 Warn whenever a comment-start sequence @samp{/*} appears in a @samp{/*}
2266 comment, or whenever a Backslash-Newline appears in a @samp{//} comment.
2267 This warning is enabled by @option{-Wall}.
2269 @item -Wfatal-errors
2270 @opindex Wfatal-errors
2271 This option causes the compiler to abort compilation on the first error
2272 occurred rather than trying to keep going and printing further error
2277 @opindex ffreestanding
2278 @opindex fno-builtin
2279 Check calls to @code{printf} and @code{scanf}, etc., to make sure that
2280 the arguments supplied have types appropriate to the format string
2281 specified, and that the conversions specified in the format string make
2282 sense. This includes standard functions, and others specified by format
2283 attributes (@pxref{Function Attributes}), in the @code{printf},
2284 @code{scanf}, @code{strftime} and @code{strfmon} (an X/Open extension,
2285 not in the C standard) families (or other target-specific families).
2286 Which functions are checked without format attributes having been
2287 specified depends on the standard version selected, and such checks of
2288 functions without the attribute specified are disabled by
2289 @option{-ffreestanding} or @option{-fno-builtin}.
2291 The formats are checked against the format features supported by GNU
2292 libc version 2.2. These include all ISO C90 and C99 features, as well
2293 as features from the Single Unix Specification and some BSD and GNU
2294 extensions. Other library implementations may not support all these
2295 features; GCC does not support warning about features that go beyond a
2296 particular library's limitations. However, if @option{-pedantic} is used
2297 with @option{-Wformat}, warnings will be given about format features not
2298 in the selected standard version (but not for @code{strfmon} formats,
2299 since those are not in any version of the C standard). @xref{C Dialect
2300 Options,,Options Controlling C Dialect}.
2302 Since @option{-Wformat} also checks for null format arguments for
2303 several functions, @option{-Wformat} also implies @option{-Wnonnull}.
2305 @option{-Wformat} is included in @option{-Wall}. For more control over some
2306 aspects of format checking, the options @option{-Wformat-y2k},
2307 @option{-Wno-format-extra-args}, @option{-Wno-format-zero-length},
2308 @option{-Wformat-nonliteral}, @option{-Wformat-security}, and
2309 @option{-Wformat=2} are available, but are not included in @option{-Wall}.
2312 @opindex Wformat-y2k
2313 If @option{-Wformat} is specified, also warn about @code{strftime}
2314 formats which may yield only a two-digit year.
2316 @item -Wno-format-extra-args
2317 @opindex Wno-format-extra-args
2318 If @option{-Wformat} is specified, do not warn about excess arguments to a
2319 @code{printf} or @code{scanf} format function. The C standard specifies
2320 that such arguments are ignored.
2322 Where the unused arguments lie between used arguments that are
2323 specified with @samp{$} operand number specifications, normally
2324 warnings are still given, since the implementation could not know what
2325 type to pass to @code{va_arg} to skip the unused arguments. However,
2326 in the case of @code{scanf} formats, this option will suppress the
2327 warning if the unused arguments are all pointers, since the Single
2328 Unix Specification says that such unused arguments are allowed.
2330 @item -Wno-format-zero-length
2331 @opindex Wno-format-zero-length
2332 If @option{-Wformat} is specified, do not warn about zero-length formats.
2333 The C standard specifies that zero-length formats are allowed.
2335 @item -Wformat-nonliteral
2336 @opindex Wformat-nonliteral
2337 If @option{-Wformat} is specified, also warn if the format string is not a
2338 string literal and so cannot be checked, unless the format function
2339 takes its format arguments as a @code{va_list}.
2341 @item -Wformat-security
2342 @opindex Wformat-security
2343 If @option{-Wformat} is specified, also warn about uses of format
2344 functions that represent possible security problems. At present, this
2345 warns about calls to @code{printf} and @code{scanf} functions where the
2346 format string is not a string literal and there are no format arguments,
2347 as in @code{printf (foo);}. This may be a security hole if the format
2348 string came from untrusted input and contains @samp{%n}. (This is
2349 currently a subset of what @option{-Wformat-nonliteral} warns about, but
2350 in future warnings may be added to @option{-Wformat-security} that are not
2351 included in @option{-Wformat-nonliteral}.)
2355 Enable @option{-Wformat} plus format checks not included in
2356 @option{-Wformat}. Currently equivalent to @samp{-Wformat
2357 -Wformat-nonliteral -Wformat-security -Wformat-y2k}.
2361 Warn about passing a null pointer for arguments marked as
2362 requiring a non-null value by the @code{nonnull} function attribute.
2364 @option{-Wnonnull} is included in @option{-Wall} and @option{-Wformat}. It
2365 can be disabled with the @option{-Wno-nonnull} option.
2367 @item -Winit-self @r{(C, C++, Objective-C and Objective-C++ only)}
2369 Warn about uninitialized variables which are initialized with themselves.
2370 Note this option can only be used with the @option{-Wuninitialized} option,
2371 which in turn only works with @option{-O1} and above.
2373 For example, GCC will warn about @code{i} being uninitialized in the
2374 following snippet only when @option{-Winit-self} has been specified:
2385 @item -Wimplicit-int
2386 @opindex Wimplicit-int
2387 Warn when a declaration does not specify a type.
2388 This warning is enabled by @option{-Wall}.
2390 @item -Wimplicit-function-declaration
2391 @itemx -Werror-implicit-function-declaration
2392 @opindex Wimplicit-function-declaration
2393 @opindex Werror-implicit-function-declaration
2394 Give a warning (or error) whenever a function is used before being
2395 declared. The form @option{-Wno-error-implicit-function-declaration}
2397 This warning is enabled by @option{-Wall} (as a warning, not an error).
2401 Same as @option{-Wimplicit-int} and @option{-Wimplicit-function-declaration}.
2402 This warning is enabled by @option{-Wall}.
2406 Warn if the type of @samp{main} is suspicious. @samp{main} should be a
2407 function with external linkage, returning int, taking either zero
2408 arguments, two, or three arguments of appropriate types.
2409 This warning is enabled by @option{-Wall}.
2411 @item -Wmissing-braces
2412 @opindex Wmissing-braces
2413 Warn if an aggregate or union initializer is not fully bracketed. In
2414 the following example, the initializer for @samp{a} is not fully
2415 bracketed, but that for @samp{b} is fully bracketed.
2418 int a[2][2] = @{ 0, 1, 2, 3 @};
2419 int b[2][2] = @{ @{ 0, 1 @}, @{ 2, 3 @} @};
2422 This warning is enabled by @option{-Wall}.
2424 @item -Wmissing-include-dirs @r{(C, C++, Objective-C and Objective-C++ only)}
2425 @opindex Wmissing-include-dirs
2426 Warn if a user-supplied include directory does not exist.
2429 @opindex Wparentheses
2430 Warn if parentheses are omitted in certain contexts, such
2431 as when there is an assignment in a context where a truth value
2432 is expected, or when operators are nested whose precedence people
2433 often get confused about. Only the warning for an assignment used as
2434 a truth value is supported when compiling C++; the other warnings are
2435 only supported when compiling C@.
2437 Also warn if a comparison like @samp{x<=y<=z} appears; this is
2438 equivalent to @samp{(x<=y ? 1 : 0) <= z}, which is a different
2439 interpretation from that of ordinary mathematical notation.
2441 Also warn about constructions where there may be confusion to which
2442 @code{if} statement an @code{else} branch belongs. Here is an example of
2457 In C, every @code{else} branch belongs to the innermost possible @code{if}
2458 statement, which in this example is @code{if (b)}. This is often not
2459 what the programmer expected, as illustrated in the above example by
2460 indentation the programmer chose. When there is the potential for this
2461 confusion, GCC will issue a warning when this flag is specified.
2462 To eliminate the warning, add explicit braces around the innermost
2463 @code{if} statement so there is no way the @code{else} could belong to
2464 the enclosing @code{if}. The resulting code would look like this:
2480 This warning is enabled by @option{-Wall}.
2482 @item -Wsequence-point
2483 @opindex Wsequence-point
2484 Warn about code that may have undefined semantics because of violations
2485 of sequence point rules in the C standard.
2487 The C standard defines the order in which expressions in a C program are
2488 evaluated in terms of @dfn{sequence points}, which represent a partial
2489 ordering between the execution of parts of the program: those executed
2490 before the sequence point, and those executed after it. These occur
2491 after the evaluation of a full expression (one which is not part of a
2492 larger expression), after the evaluation of the first operand of a
2493 @code{&&}, @code{||}, @code{? :} or @code{,} (comma) operator, before a
2494 function is called (but after the evaluation of its arguments and the
2495 expression denoting the called function), and in certain other places.
2496 Other than as expressed by the sequence point rules, the order of
2497 evaluation of subexpressions of an expression is not specified. All
2498 these rules describe only a partial order rather than a total order,
2499 since, for example, if two functions are called within one expression
2500 with no sequence point between them, the order in which the functions
2501 are called is not specified. However, the standards committee have
2502 ruled that function calls do not overlap.
2504 It is not specified when between sequence points modifications to the
2505 values of objects take effect. Programs whose behavior depends on this
2506 have undefined behavior; the C standard specifies that ``Between the
2507 previous and next sequence point an object shall have its stored value
2508 modified at most once by the evaluation of an expression. Furthermore,
2509 the prior value shall be read only to determine the value to be
2510 stored.''. If a program breaks these rules, the results on any
2511 particular implementation are entirely unpredictable.
2513 Examples of code with undefined behavior are @code{a = a++;}, @code{a[n]
2514 = b[n++]} and @code{a[i++] = i;}. Some more complicated cases are not
2515 diagnosed by this option, and it may give an occasional false positive
2516 result, but in general it has been found fairly effective at detecting
2517 this sort of problem in programs.
2519 The present implementation of this option only works for C programs. A
2520 future implementation may also work for C++ programs.
2522 The C standard is worded confusingly, therefore there is some debate
2523 over the precise meaning of the sequence point rules in subtle cases.
2524 Links to discussions of the problem, including proposed formal
2525 definitions, may be found on the GCC readings page, at
2526 @w{@uref{http://gcc.gnu.org/readings.html}}.
2528 This warning is enabled by @option{-Wall}.
2531 @opindex Wreturn-type
2532 Warn whenever a function is defined with a return-type that defaults to
2533 @code{int}. Also warn about any @code{return} statement with no
2534 return-value in a function whose return-type is not @code{void}.
2536 For C, also warn if the return type of a function has a type qualifier
2537 such as @code{const}. Such a type qualifier has no effect, since the
2538 value returned by a function is not an lvalue. ISO C prohibits
2539 qualified @code{void} return types on function definitions, so such
2540 return types always receive a warning even without this option.
2542 For C++, a function without return type always produces a diagnostic
2543 message, even when @option{-Wno-return-type} is specified. The only
2544 exceptions are @samp{main} and functions defined in system headers.
2546 This warning is enabled by @option{-Wall}.
2550 Warn whenever a @code{switch} statement has an index of enumerated type
2551 and lacks a @code{case} for one or more of the named codes of that
2552 enumeration. (The presence of a @code{default} label prevents this
2553 warning.) @code{case} labels outside the enumeration range also
2554 provoke warnings when this option is used.
2555 This warning is enabled by @option{-Wall}.
2557 @item -Wswitch-default
2558 @opindex Wswitch-switch
2559 Warn whenever a @code{switch} statement does not have a @code{default}
2563 @opindex Wswitch-enum
2564 Warn whenever a @code{switch} statement has an index of enumerated type
2565 and lacks a @code{case} for one or more of the named codes of that
2566 enumeration. @code{case} labels outside the enumeration range also
2567 provoke warnings when this option is used.
2571 Warn if any trigraphs are encountered that might change the meaning of
2572 the program (trigraphs within comments are not warned about).
2573 This warning is enabled by @option{-Wall}.
2575 @item -Wunused-function
2576 @opindex Wunused-function
2577 Warn whenever a static function is declared but not defined or a
2578 non\-inline static function is unused.
2579 This warning is enabled by @option{-Wall}.
2581 @item -Wunused-label
2582 @opindex Wunused-label
2583 Warn whenever a label is declared but not used.
2584 This warning is enabled by @option{-Wall}.
2586 To suppress this warning use the @samp{unused} attribute
2587 (@pxref{Variable Attributes}).
2589 @item -Wunused-parameter
2590 @opindex Wunused-parameter
2591 Warn whenever a function parameter is unused aside from its declaration.
2593 To suppress this warning use the @samp{unused} attribute
2594 (@pxref{Variable Attributes}).
2596 @item -Wunused-variable
2597 @opindex Wunused-variable
2598 Warn whenever a local variable or non-constant static variable is unused
2599 aside from its declaration
2600 This warning is enabled by @option{-Wall}.
2602 To suppress this warning use the @samp{unused} attribute
2603 (@pxref{Variable Attributes}).
2605 @item -Wunused-value
2606 @opindex Wunused-value
2607 Warn whenever a statement computes a result that is explicitly not used.
2608 This warning is enabled by @option{-Wall}.
2610 To suppress this warning cast the expression to @samp{void}.
2614 All the above @option{-Wunused} options combined.
2616 In order to get a warning about an unused function parameter, you must
2617 either specify @samp{-Wextra -Wunused} (note that @samp{-Wall} implies
2618 @samp{-Wunused}), or separately specify @option{-Wunused-parameter}.
2620 @item -Wuninitialized
2621 @opindex Wuninitialized
2622 Warn if an automatic variable is used without first being initialized or
2623 if a variable may be clobbered by a @code{setjmp} call.
2625 These warnings are possible only in optimizing compilation,
2626 because they require data flow information that is computed only
2627 when optimizing. If you don't specify @option{-O}, you simply won't
2630 If you want to warn about code which uses the uninitialized value of the
2631 variable in its own initializer, use the @option{-Winit-self} option.
2633 These warnings occur for individual uninitialized or clobbered
2634 elements of structure, union or array variables as well as for
2635 variables which are uninitialized or clobbered as a whole. They do
2636 not occur for variables or elements declared @code{volatile}. Because
2637 these warnings depend on optimization, the exact variables or elements
2638 for which there are warnings will depend on the precise optimization
2639 options and version of GCC used.
2641 Note that there may be no warning about a variable that is used only
2642 to compute a value that itself is never used, because such
2643 computations may be deleted by data flow analysis before the warnings
2646 These warnings are made optional because GCC is not smart
2647 enough to see all the reasons why the code might be correct
2648 despite appearing to have an error. Here is one example of how
2669 If the value of @code{y} is always 1, 2 or 3, then @code{x} is
2670 always initialized, but GCC doesn't know this. Here is
2671 another common case:
2676 if (change_y) save_y = y, y = new_y;
2678 if (change_y) y = save_y;
2683 This has no bug because @code{save_y} is used only if it is set.
2685 @cindex @code{longjmp} warnings
2686 This option also warns when a non-volatile automatic variable might be
2687 changed by a call to @code{longjmp}. These warnings as well are possible
2688 only in optimizing compilation.
2690 The compiler sees only the calls to @code{setjmp}. It cannot know
2691 where @code{longjmp} will be called; in fact, a signal handler could
2692 call it at any point in the code. As a result, you may get a warning
2693 even when there is in fact no problem because @code{longjmp} cannot
2694 in fact be called at the place which would cause a problem.
2696 Some spurious warnings can be avoided if you declare all the functions
2697 you use that never return as @code{noreturn}. @xref{Function
2700 This warning is enabled by @option{-Wall}.
2702 @item -Wunknown-pragmas
2703 @opindex Wunknown-pragmas
2704 @cindex warning for unknown pragmas
2705 @cindex unknown pragmas, warning
2706 @cindex pragmas, warning of unknown
2707 Warn when a #pragma directive is encountered which is not understood by
2708 GCC@. If this command line option is used, warnings will even be issued
2709 for unknown pragmas in system header files. This is not the case if
2710 the warnings were only enabled by the @option{-Wall} command line option.
2713 @opindex Wno-pragmas
2715 Do not warn about misuses of pragmas, such as incorrect parameters,
2716 invalid syntax, or conflicts between pragmas. See also
2717 @samp{-Wunknown-pragmas}.
2719 @item -Wstrict-aliasing
2720 @opindex Wstrict-aliasing
2721 This option is only active when @option{-fstrict-aliasing} is active.
2722 It warns about code which might break the strict aliasing rules that the
2723 compiler is using for optimization. The warning does not catch all
2724 cases, but does attempt to catch the more common pitfalls. It is
2725 included in @option{-Wall}.
2727 @item -Wstrict-aliasing=2
2728 @opindex Wstrict-aliasing=2
2729 This option is only active when @option{-fstrict-aliasing} is active.
2730 It warns about all code which might break the strict aliasing rules that the
2731 compiler is using for optimization. This warning catches all cases, but
2732 it will also give a warning for some ambiguous cases that are safe.
2736 All of the above @samp{-W} options combined. This enables all the
2737 warnings about constructions that some users consider questionable, and
2738 that are easy to avoid (or modify to prevent the warning), even in
2739 conjunction with macros. This also enables some language-specific
2740 warnings described in @ref{C++ Dialect Options} and
2741 @ref{Objective-C and Objective-C++ Dialect Options}.
2744 The following @option{-W@dots{}} options are not implied by @option{-Wall}.
2745 Some of them warn about constructions that users generally do not
2746 consider questionable, but which occasionally you might wish to check
2747 for; others warn about constructions that are necessary or hard to avoid
2748 in some cases, and there is no simple way to modify the code to suppress
2755 (This option used to be called @option{-W}. The older name is still
2756 supported, but the newer name is more descriptive.) Print extra warning
2757 messages for these events:
2761 A function can return either with or without a value. (Falling
2762 off the end of the function body is considered returning without
2763 a value.) For example, this function would evoke such a
2777 An expression-statement or the left-hand side of a comma expression
2778 contains no side effects.
2779 To suppress the warning, cast the unused expression to void.
2780 For example, an expression such as @samp{x[i,j]} will cause a warning,
2781 but @samp{x[(void)i,j]} will not.
2784 An unsigned value is compared against zero with @samp{<} or @samp{>=}.
2787 Storage-class specifiers like @code{static} are not the first things in
2788 a declaration. According to the C Standard, this usage is obsolescent.
2791 If @option{-Wall} or @option{-Wunused} is also specified, warn about unused
2795 A comparison between signed and unsigned values could produce an
2796 incorrect result when the signed value is converted to unsigned.
2797 (But don't warn if @option{-Wno-sign-compare} is also specified.)
2800 An aggregate has an initializer which does not initialize all members.
2801 This warning can be independently controlled by
2802 @option{-Wmissing-field-initializers}.
2805 A function parameter is declared without a type specifier in K&R-style
2813 An empty body occurs in an @samp{if} or @samp{else} statement.
2816 A pointer is compared against integer zero with @samp{<}, @samp{<=},
2817 @samp{>}, or @samp{>=}.
2820 A variable might be changed by @samp{longjmp} or @samp{vfork}.
2823 Any of several floating-point events that often indicate errors, such as
2824 overflow, underflow, loss of precision, etc.
2826 @item @r{(C++ only)}
2827 An enumerator and a non-enumerator both appear in a conditional expression.
2829 @item @r{(C++ only)}
2830 A non-static reference or non-static @samp{const} member appears in a
2831 class without constructors.
2833 @item @r{(C++ only)}
2834 Ambiguous virtual bases.
2836 @item @r{(C++ only)}
2837 Subscripting an array which has been declared @samp{register}.
2839 @item @r{(C++ only)}
2840 Taking the address of a variable which has been declared @samp{register}.
2842 @item @r{(C++ only)}
2843 A base class is not initialized in a derived class' copy constructor.
2846 @item -Wno-div-by-zero
2847 @opindex Wno-div-by-zero
2848 @opindex Wdiv-by-zero
2849 Do not warn about compile-time integer division by zero. Floating point
2850 division by zero is not warned about, as it can be a legitimate way of
2851 obtaining infinities and NaNs.
2853 @item -Wsystem-headers
2854 @opindex Wsystem-headers
2855 @cindex warnings from system headers
2856 @cindex system headers, warnings from
2857 Print warning messages for constructs found in system header files.
2858 Warnings from system headers are normally suppressed, on the assumption
2859 that they usually do not indicate real problems and would only make the
2860 compiler output harder to read. Using this command line option tells
2861 GCC to emit warnings from system headers as if they occurred in user
2862 code. However, note that using @option{-Wall} in conjunction with this
2863 option will @emph{not} warn about unknown pragmas in system
2864 headers---for that, @option{-Wunknown-pragmas} must also be used.
2867 @opindex Wfloat-equal
2868 Warn if floating point values are used in equality comparisons.
2870 The idea behind this is that sometimes it is convenient (for the
2871 programmer) to consider floating-point values as approximations to
2872 infinitely precise real numbers. If you are doing this, then you need
2873 to compute (by analyzing the code, or in some other way) the maximum or
2874 likely maximum error that the computation introduces, and allow for it
2875 when performing comparisons (and when producing output, but that's a
2876 different problem). In particular, instead of testing for equality, you
2877 would check to see whether the two values have ranges that overlap; and
2878 this is done with the relational operators, so equality comparisons are
2881 @item -Wtraditional @r{(C only)}
2882 @opindex Wtraditional
2883 Warn about certain constructs that behave differently in traditional and
2884 ISO C@. Also warn about ISO C constructs that have no traditional C
2885 equivalent, and/or problematic constructs which should be avoided.
2889 Macro parameters that appear within string literals in the macro body.
2890 In traditional C macro replacement takes place within string literals,
2891 but does not in ISO C@.
2894 In traditional C, some preprocessor directives did not exist.
2895 Traditional preprocessors would only consider a line to be a directive
2896 if the @samp{#} appeared in column 1 on the line. Therefore
2897 @option{-Wtraditional} warns about directives that traditional C
2898 understands but would ignore because the @samp{#} does not appear as the
2899 first character on the line. It also suggests you hide directives like
2900 @samp{#pragma} not understood by traditional C by indenting them. Some
2901 traditional implementations would not recognize @samp{#elif}, so it
2902 suggests avoiding it altogether.
2905 A function-like macro that appears without arguments.
2908 The unary plus operator.
2911 The @samp{U} integer constant suffix, or the @samp{F} or @samp{L} floating point
2912 constant suffixes. (Traditional C does support the @samp{L} suffix on integer
2913 constants.) Note, these suffixes appear in macros defined in the system
2914 headers of most modern systems, e.g.@: the @samp{_MIN}/@samp{_MAX} macros in @code{<limits.h>}.
2915 Use of these macros in user code might normally lead to spurious
2916 warnings, however GCC's integrated preprocessor has enough context to
2917 avoid warning in these cases.
2920 A function declared external in one block and then used after the end of
2924 A @code{switch} statement has an operand of type @code{long}.
2927 A non-@code{static} function declaration follows a @code{static} one.
2928 This construct is not accepted by some traditional C compilers.
2931 The ISO type of an integer constant has a different width or
2932 signedness from its traditional type. This warning is only issued if
2933 the base of the constant is ten. I.e.@: hexadecimal or octal values, which
2934 typically represent bit patterns, are not warned about.
2937 Usage of ISO string concatenation is detected.
2940 Initialization of automatic aggregates.
2943 Identifier conflicts with labels. Traditional C lacks a separate
2944 namespace for labels.
2947 Initialization of unions. If the initializer is zero, the warning is
2948 omitted. This is done under the assumption that the zero initializer in
2949 user code appears conditioned on e.g.@: @code{__STDC__} to avoid missing
2950 initializer warnings and relies on default initialization to zero in the
2954 Conversions by prototypes between fixed/floating point values and vice
2955 versa. The absence of these prototypes when compiling with traditional
2956 C would cause serious problems. This is a subset of the possible
2957 conversion warnings, for the full set use @option{-Wconversion}.
2960 Use of ISO C style function definitions. This warning intentionally is
2961 @emph{not} issued for prototype declarations or variadic functions
2962 because these ISO C features will appear in your code when using
2963 libiberty's traditional C compatibility macros, @code{PARAMS} and
2964 @code{VPARAMS}. This warning is also bypassed for nested functions
2965 because that feature is already a GCC extension and thus not relevant to
2966 traditional C compatibility.
2969 @item -Wdeclaration-after-statement @r{(C only)}
2970 @opindex Wdeclaration-after-statement
2971 Warn when a declaration is found after a statement in a block. This
2972 construct, known from C++, was introduced with ISO C99 and is by default
2973 allowed in GCC@. It is not supported by ISO C90 and was not supported by
2974 GCC versions before GCC 3.0. @xref{Mixed Declarations}.
2978 Warn if an undefined identifier is evaluated in an @samp{#if} directive.
2980 @item -Wno-endif-labels
2981 @opindex Wno-endif-labels
2982 @opindex Wendif-labels
2983 Do not warn whenever an @samp{#else} or an @samp{#endif} are followed by text.
2987 Warn whenever a local variable shadows another local variable, parameter or
2988 global variable or whenever a built-in function is shadowed.
2990 @item -Wlarger-than-@var{len}
2991 @opindex Wlarger-than
2992 Warn whenever an object of larger than @var{len} bytes is defined.
2994 @item -Wunsafe-loop-optimizations
2995 @opindex Wunsafe-loop-optimizations
2996 Warn if the loop cannot be optimized because the compiler could not
2997 assume anything on the bounds of the loop indices. With
2998 @option{-funsafe-loop-optimizations} warn if the compiler made
3001 @item -Wpointer-arith
3002 @opindex Wpointer-arith
3003 Warn about anything that depends on the ``size of'' a function type or
3004 of @code{void}. GNU C assigns these types a size of 1, for
3005 convenience in calculations with @code{void *} pointers and pointers
3008 @item -Wbad-function-cast @r{(C only)}
3009 @opindex Wbad-function-cast
3010 Warn whenever a function call is cast to a non-matching type.
3011 For example, warn if @code{int malloc()} is cast to @code{anything *}.
3014 Warn about ISO C constructs that are outside of the common subset of
3015 ISO C and ISO C++, e.g.@: request for implicit conversion from
3016 @code{void *} to a pointer to non-@code{void} type.
3020 Warn whenever a pointer is cast so as to remove a type qualifier from
3021 the target type. For example, warn if a @code{const char *} is cast
3022 to an ordinary @code{char *}.
3025 @opindex Wcast-align
3026 Warn whenever a pointer is cast such that the required alignment of the
3027 target is increased. For example, warn if a @code{char *} is cast to
3028 an @code{int *} on machines where integers can only be accessed at
3029 two- or four-byte boundaries.
3031 @item -Wwrite-strings
3032 @opindex Wwrite-strings
3033 When compiling C, give string constants the type @code{const
3034 char[@var{length}]} so that
3035 copying the address of one into a non-@code{const} @code{char *}
3036 pointer will get a warning; when compiling C++, warn about the
3037 deprecated conversion from string constants to @code{char *}.
3038 These warnings will help you find at
3039 compile time code that can try to write into a string constant, but
3040 only if you have been very careful about using @code{const} in
3041 declarations and prototypes. Otherwise, it will just be a nuisance;
3042 this is why we did not make @option{-Wall} request these warnings.
3045 @opindex Wconversion
3046 Warn if a prototype causes a type conversion that is different from what
3047 would happen to the same argument in the absence of a prototype. This
3048 includes conversions of fixed point to floating and vice versa, and
3049 conversions changing the width or signedness of a fixed point argument
3050 except when the same as the default promotion.
3052 Also, warn if a negative integer constant expression is implicitly
3053 converted to an unsigned type. For example, warn about the assignment
3054 @code{x = -1} if @code{x} is unsigned. But do not warn about explicit
3055 casts like @code{(unsigned) -1}.
3057 @item -Wsign-compare
3058 @opindex Wsign-compare
3059 @cindex warning for comparison of signed and unsigned values
3060 @cindex comparison of signed and unsigned values, warning
3061 @cindex signed and unsigned values, comparison warning
3062 Warn when a comparison between signed and unsigned values could produce
3063 an incorrect result when the signed value is converted to unsigned.
3064 This warning is also enabled by @option{-Wextra}; to get the other warnings
3065 of @option{-Wextra} without this warning, use @samp{-Wextra -Wno-sign-compare}.
3067 @item -Waggregate-return
3068 @opindex Waggregate-return
3069 Warn if any functions that return structures or unions are defined or
3070 called. (In languages where you can return an array, this also elicits
3073 @item -Wno-attributes
3074 @opindex Wno-attributes
3075 @opindex Wattributes
3076 Do not warn if an unexpected @code{__attribute__} is used, such as
3077 unrecognized attributes, function attributes applied to variables,
3078 etc. This will not stop errors for incorrect use of supported
3081 @item -Wstrict-prototypes @r{(C only)}
3082 @opindex Wstrict-prototypes
3083 Warn if a function is declared or defined without specifying the
3084 argument types. (An old-style function definition is permitted without
3085 a warning if preceded by a declaration which specifies the argument
3088 @item -Wold-style-definition @r{(C only)}
3089 @opindex Wold-style-definition
3090 Warn if an old-style function definition is used. A warning is given
3091 even if there is a previous prototype.
3093 @item -Wmissing-prototypes @r{(C only)}
3094 @opindex Wmissing-prototypes
3095 Warn if a global function is defined without a previous prototype
3096 declaration. This warning is issued even if the definition itself
3097 provides a prototype. The aim is to detect global functions that fail
3098 to be declared in header files.
3100 @item -Wmissing-declarations @r{(C only)}
3101 @opindex Wmissing-declarations
3102 Warn if a global function is defined without a previous declaration.
3103 Do so even if the definition itself provides a prototype.
3104 Use this option to detect global functions that are not declared in
3107 @item -Wmissing-field-initializers
3108 @opindex Wmissing-field-initializers
3111 Warn if a structure's initializer has some fields missing. For
3112 example, the following code would cause such a warning, because
3113 @code{x.h} is implicitly zero:
3116 struct s @{ int f, g, h; @};
3117 struct s x = @{ 3, 4 @};
3120 This option does not warn about designated initializers, so the following
3121 modification would not trigger a warning:
3124 struct s @{ int f, g, h; @};
3125 struct s x = @{ .f = 3, .g = 4 @};
3128 This warning is included in @option{-Wextra}. To get other @option{-Wextra}
3129 warnings without this one, use @samp{-Wextra -Wno-missing-field-initializers}.
3131 @item -Wmissing-noreturn
3132 @opindex Wmissing-noreturn
3133 Warn about functions which might be candidates for attribute @code{noreturn}.
3134 Note these are only possible candidates, not absolute ones. Care should
3135 be taken to manually verify functions actually do not ever return before
3136 adding the @code{noreturn} attribute, otherwise subtle code generation
3137 bugs could be introduced. You will not get a warning for @code{main} in
3138 hosted C environments.
3140 @item -Wmissing-format-attribute
3141 @opindex Wmissing-format-attribute
3143 Warn about function pointers which might be candidates for @code{format}
3144 attributes. Note these are only possible candidates, not absolute ones.
3145 GCC will guess that function pointers with @code{format} attributes that
3146 are used in assignment, initialization, parameter passing or return
3147 statements should have a corresponding @code{format} attribute in the
3148 resulting type. I.e.@: the left-hand side of the assignment or
3149 initialization, the type of the parameter variable, or the return type
3150 of the containing function respectively should also have a @code{format}
3151 attribute to avoid the warning.
3153 GCC will also warn about function definitions which might be
3154 candidates for @code{format} attributes. Again, these are only
3155 possible candidates. GCC will guess that @code{format} attributes
3156 might be appropriate for any function that calls a function like
3157 @code{vprintf} or @code{vscanf}, but this might not always be the
3158 case, and some functions for which @code{format} attributes are
3159 appropriate may not be detected.
3161 @item -Wno-multichar
3162 @opindex Wno-multichar
3164 Do not warn if a multicharacter constant (@samp{'FOOF'}) is used.
3165 Usually they indicate a typo in the user's code, as they have
3166 implementation-defined values, and should not be used in portable code.
3168 @item -Wnormalized=<none|id|nfc|nfkc>
3169 @opindex Wnormalized
3172 @cindex character set, input normalization
3173 In ISO C and ISO C++, two identifiers are different if they are
3174 different sequences of characters. However, sometimes when characters
3175 outside the basic ASCII character set are used, you can have two
3176 different character sequences that look the same. To avoid confusion,
3177 the ISO 10646 standard sets out some @dfn{normalization rules} which
3178 when applied ensure that two sequences that look the same are turned into
3179 the same sequence. GCC can warn you if you are using identifiers which
3180 have not been normalized; this option controls that warning.
3182 There are four levels of warning that GCC supports. The default is
3183 @option{-Wnormalized=nfc}, which warns about any identifier which is
3184 not in the ISO 10646 ``C'' normalized form, @dfn{NFC}. NFC is the
3185 recommended form for most uses.
3187 Unfortunately, there are some characters which ISO C and ISO C++ allow
3188 in identifiers that when turned into NFC aren't allowable as
3189 identifiers. That is, there's no way to use these symbols in portable
3190 ISO C or C++ and have all your identifiers in NFC.
3191 @option{-Wnormalized=id} suppresses the warning for these characters.
3192 It is hoped that future versions of the standards involved will correct
3193 this, which is why this option is not the default.
3195 You can switch the warning off for all characters by writing
3196 @option{-Wnormalized=none}. You would only want to do this if you
3197 were using some other normalization scheme (like ``D''), because
3198 otherwise you can easily create bugs that are literally impossible to see.
3200 Some characters in ISO 10646 have distinct meanings but look identical
3201 in some fonts or display methodologies, especially once formatting has
3202 been applied. For instance @code{\u207F}, ``SUPERSCRIPT LATIN SMALL
3203 LETTER N'', will display just like a regular @code{n} which has been
3204 placed in a superscript. ISO 10646 defines the @dfn{NFKC}
3205 normalisation scheme to convert all these into a standard form as
3206 well, and GCC will warn if your code is not in NFKC if you use
3207 @option{-Wnormalized=nfkc}. This warning is comparable to warning
3208 about every identifier that contains the letter O because it might be
3209 confused with the digit 0, and so is not the default, but may be
3210 useful as a local coding convention if the programming environment is
3211 unable to be fixed to display these characters distinctly.
3213 @item -Wno-deprecated-declarations
3214 @opindex Wno-deprecated-declarations
3215 Do not warn about uses of functions, variables, and types marked as
3216 deprecated by using the @code{deprecated} attribute.
3217 (@pxref{Function Attributes}, @pxref{Variable Attributes},
3218 @pxref{Type Attributes}.)
3222 Warn if a structure is given the packed attribute, but the packed
3223 attribute has no effect on the layout or size of the structure.
3224 Such structures may be mis-aligned for little benefit. For
3225 instance, in this code, the variable @code{f.x} in @code{struct bar}
3226 will be misaligned even though @code{struct bar} does not itself
3227 have the packed attribute:
3234 @} __attribute__((packed));
3244 Warn if padding is included in a structure, either to align an element
3245 of the structure or to align the whole structure. Sometimes when this
3246 happens it is possible to rearrange the fields of the structure to
3247 reduce the padding and so make the structure smaller.
3249 @item -Wredundant-decls
3250 @opindex Wredundant-decls
3251 Warn if anything is declared more than once in the same scope, even in
3252 cases where multiple declaration is valid and changes nothing.
3254 @item -Wnested-externs @r{(C only)}
3255 @opindex Wnested-externs
3256 Warn if an @code{extern} declaration is encountered within a function.
3258 @item -Wunreachable-code
3259 @opindex Wunreachable-code
3260 Warn if the compiler detects that code will never be executed.
3262 This option is intended to warn when the compiler detects that at
3263 least a whole line of source code will never be executed, because
3264 some condition is never satisfied or because it is after a
3265 procedure that never returns.
3267 It is possible for this option to produce a warning even though there
3268 are circumstances under which part of the affected line can be executed,
3269 so care should be taken when removing apparently-unreachable code.
3271 For instance, when a function is inlined, a warning may mean that the
3272 line is unreachable in only one inlined copy of the function.
3274 This option is not made part of @option{-Wall} because in a debugging
3275 version of a program there is often substantial code which checks
3276 correct functioning of the program and is, hopefully, unreachable
3277 because the program does work. Another common use of unreachable
3278 code is to provide behavior which is selectable at compile-time.
3282 Warn if a function can not be inlined and it was declared as inline.
3283 Even with this option, the compiler will not warn about failures to
3284 inline functions declared in system headers.
3286 The compiler uses a variety of heuristics to determine whether or not
3287 to inline a function. For example, the compiler takes into account
3288 the size of the function being inlined and the amount of inlining
3289 that has already been done in the current function. Therefore,
3290 seemingly insignificant changes in the source program can cause the
3291 warnings produced by @option{-Winline} to appear or disappear.
3293 @item -Wno-invalid-offsetof @r{(C++ only)}
3294 @opindex Wno-invalid-offsetof
3295 Suppress warnings from applying the @samp{offsetof} macro to a non-POD
3296 type. According to the 1998 ISO C++ standard, applying @samp{offsetof}
3297 to a non-POD type is undefined. In existing C++ implementations,
3298 however, @samp{offsetof} typically gives meaningful results even when
3299 applied to certain kinds of non-POD types. (Such as a simple
3300 @samp{struct} that fails to be a POD type only by virtue of having a
3301 constructor.) This flag is for users who are aware that they are
3302 writing nonportable code and who have deliberately chosen to ignore the
3305 The restrictions on @samp{offsetof} may be relaxed in a future version
3306 of the C++ standard.
3308 @item -Wno-int-to-pointer-cast @r{(C only)}
3309 @opindex Wno-int-to-pointer-cast
3310 Suppress warnings from casts to pointer type of an integer of a
3313 @item -Wno-pointer-to-int-cast @r{(C only)}
3314 @opindex Wno-pointer-to-int-cast
3315 Suppress warnings from casts from a pointer to an integer type of a
3319 @opindex Winvalid-pch
3320 Warn if a precompiled header (@pxref{Precompiled Headers}) is found in
3321 the search path but can't be used.
3325 @opindex Wno-long-long
3326 Warn if @samp{long long} type is used. This is default. To inhibit
3327 the warning messages, use @option{-Wno-long-long}. Flags
3328 @option{-Wlong-long} and @option{-Wno-long-long} are taken into account
3329 only when @option{-pedantic} flag is used.
3331 @item -Wvariadic-macros
3332 @opindex Wvariadic-macros
3333 @opindex Wno-variadic-macros
3334 Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU
3335 alternate syntax when in pedantic ISO C99 mode. This is default.
3336 To inhibit the warning messages, use @option{-Wno-variadic-macros}.
3338 @item -Wdisabled-optimization
3339 @opindex Wdisabled-optimization
3340 Warn if a requested optimization pass is disabled. This warning does
3341 not generally indicate that there is anything wrong with your code; it
3342 merely indicates that GCC's optimizers were unable to handle the code
3343 effectively. Often, the problem is that your code is too big or too
3344 complex; GCC will refuse to optimize programs when the optimization
3345 itself is likely to take inordinate amounts of time.
3347 @item -Wno-pointer-sign
3348 @opindex Wno-pointer-sign
3349 Don't warn for pointer argument passing or assignment with different signedness.
3350 Only useful in the negative form since this warning is enabled by default.
3351 This option is only supported for C and Objective-C@.
3355 Make all warnings into errors.
3358 @node Debugging Options
3359 @section Options for Debugging Your Program or GCC
3360 @cindex options, debugging
3361 @cindex debugging information options
3363 GCC has various special options that are used for debugging
3364 either your program or GCC:
3369 Produce debugging information in the operating system's native format
3370 (stabs, COFF, XCOFF, or DWARF 2)@. GDB can work with this debugging
3373 On most systems that use stabs format, @option{-g} enables use of extra
3374 debugging information that only GDB can use; this extra information
3375 makes debugging work better in GDB but will probably make other debuggers
3377 refuse to read the program. If you want to control for certain whether
3378 to generate the extra information, use @option{-gstabs+}, @option{-gstabs},
3379 @option{-gxcoff+}, @option{-gxcoff}, or @option{-gvms} (see below).
3381 GCC allows you to use @option{-g} with
3382 @option{-O}. The shortcuts taken by optimized code may occasionally
3383 produce surprising results: some variables you declared may not exist
3384 at all; flow of control may briefly move where you did not expect it;
3385 some statements may not be executed because they compute constant
3386 results or their values were already at hand; some statements may
3387 execute in different places because they were moved out of loops.
3389 Nevertheless it proves possible to debug optimized output. This makes
3390 it reasonable to use the optimizer for programs that might have bugs.
3392 The following options are useful when GCC is generated with the
3393 capability for more than one debugging format.
3397 Produce debugging information for use by GDB@. This means to use the
3398 most expressive format available (DWARF 2, stabs, or the native format
3399 if neither of those are supported), including GDB extensions if at all
3404 Produce debugging information in stabs format (if that is supported),
3405 without GDB extensions. This is the format used by DBX on most BSD
3406 systems. On MIPS, Alpha and System V Release 4 systems this option
3407 produces stabs debugging output which is not understood by DBX or SDB@.
3408 On System V Release 4 systems this option requires the GNU assembler.
3410 @item -feliminate-unused-debug-symbols
3411 @opindex feliminate-unused-debug-symbols
3412 Produce debugging information in stabs format (if that is supported),
3413 for only symbols that are actually used.
3417 Produce debugging information in stabs format (if that is supported),
3418 using GNU extensions understood only by the GNU debugger (GDB)@. The
3419 use of these extensions is likely to make other debuggers crash or
3420 refuse to read the program.
3424 Produce debugging information in COFF format (if that is supported).
3425 This is the format used by SDB on most System V systems prior to
3430 Produce debugging information in XCOFF format (if that is supported).
3431 This is the format used by the DBX debugger on IBM RS/6000 systems.
3435 Produce debugging information in XCOFF format (if that is supported),
3436 using GNU extensions understood only by the GNU debugger (GDB)@. The
3437 use of these extensions is likely to make other debuggers crash or
3438 refuse to read the program, and may cause assemblers other than the GNU
3439 assembler (GAS) to fail with an error.
3443 Produce debugging information in DWARF version 2 format (if that is
3444 supported). This is the format used by DBX on IRIX 6. With this
3445 option, GCC uses features of DWARF version 3 when they are useful;
3446 version 3 is upward compatible with version 2, but may still cause
3447 problems for older debuggers.
3451 Produce debugging information in VMS debug format (if that is
3452 supported). This is the format used by DEBUG on VMS systems.
3455 @itemx -ggdb@var{level}
3456 @itemx -gstabs@var{level}
3457 @itemx -gcoff@var{level}
3458 @itemx -gxcoff@var{level}
3459 @itemx -gvms@var{level}
3460 Request debugging information and also use @var{level} to specify how
3461 much information. The default level is 2.
3463 Level 1 produces minimal information, enough for making backtraces in
3464 parts of the program that you don't plan to debug. This includes
3465 descriptions of functions and external variables, but no information
3466 about local variables and no line numbers.
3468 Level 3 includes extra information, such as all the macro definitions
3469 present in the program. Some debuggers support macro expansion when
3470 you use @option{-g3}.
3472 @option{-gdwarf-2} does not accept a concatenated debug level, because
3473 GCC used to support an option @option{-gdwarf} that meant to generate
3474 debug information in version 1 of the DWARF format (which is very
3475 different from version 2), and it would have been too confusing. That
3476 debug format is long obsolete, but the option cannot be changed now.
3477 Instead use an additional @option{-g@var{level}} option to change the
3478 debug level for DWARF2.
3480 @item -feliminate-dwarf2-dups
3481 @opindex feliminate-dwarf2-dups
3482 Compress DWARF2 debugging information by eliminating duplicated
3483 information about each symbol. This option only makes sense when
3484 generating DWARF2 debugging information with @option{-gdwarf-2}.
3486 @cindex @command{prof}
3489 Generate extra code to write profile information suitable for the
3490 analysis program @command{prof}. You must use this option when compiling
3491 the source files you want data about, and you must also use it when
3494 @cindex @command{gprof}
3497 Generate extra code to write profile information suitable for the
3498 analysis program @command{gprof}. You must use this option when compiling
3499 the source files you want data about, and you must also use it when
3504 Makes the compiler print out each function name as it is compiled, and
3505 print some statistics about each pass when it finishes.
3508 @opindex ftime-report
3509 Makes the compiler print some statistics about the time consumed by each
3510 pass when it finishes.
3513 @opindex fmem-report
3514 Makes the compiler print some statistics about permanent memory
3515 allocation when it finishes.
3517 @item -fprofile-arcs
3518 @opindex fprofile-arcs
3519 Add code so that program flow @dfn{arcs} are instrumented. During
3520 execution the program records how many times each branch and call is
3521 executed and how many times it is taken or returns. When the compiled
3522 program exits it saves this data to a file called
3523 @file{@var{auxname}.gcda} for each source file. The data may be used for
3524 profile-directed optimizations (@option{-fbranch-probabilities}), or for
3525 test coverage analysis (@option{-ftest-coverage}). Each object file's
3526 @var{auxname} is generated from the name of the output file, if
3527 explicitly specified and it is not the final executable, otherwise it is
3528 the basename of the source file. In both cases any suffix is removed
3529 (e.g.@: @file{foo.gcda} for input file @file{dir/foo.c}, or
3530 @file{dir/foo.gcda} for output file specified as @option{-o dir/foo.o}).
3531 @xref{Cross-profiling}.
3533 @cindex @command{gcov}
3537 This option is used to compile and link code instrumented for coverage
3538 analysis. The option is a synonym for @option{-fprofile-arcs}
3539 @option{-ftest-coverage} (when compiling) and @option{-lgcov} (when
3540 linking). See the documentation for those options for more details.
3545 Compile the source files with @option{-fprofile-arcs} plus optimization
3546 and code generation options. For test coverage analysis, use the
3547 additional @option{-ftest-coverage} option. You do not need to profile
3548 every source file in a program.
3551 Link your object files with @option{-lgcov} or @option{-fprofile-arcs}
3552 (the latter implies the former).
3555 Run the program on a representative workload to generate the arc profile
3556 information. This may be repeated any number of times. You can run
3557 concurrent instances of your program, and provided that the file system
3558 supports locking, the data files will be correctly updated. Also
3559 @code{fork} calls are detected and correctly handled (double counting
3563 For profile-directed optimizations, compile the source files again with
3564 the same optimization and code generation options plus
3565 @option{-fbranch-probabilities} (@pxref{Optimize Options,,Options that
3566 Control Optimization}).
3569 For test coverage analysis, use @command{gcov} to produce human readable
3570 information from the @file{.gcno} and @file{.gcda} files. Refer to the
3571 @command{gcov} documentation for further information.
3575 With @option{-fprofile-arcs}, for each function of your program GCC
3576 creates a program flow graph, then finds a spanning tree for the graph.
3577 Only arcs that are not on the spanning tree have to be instrumented: the
3578 compiler adds code to count the number of times that these arcs are
3579 executed. When an arc is the only exit or only entrance to a block, the
3580 instrumentation code can be added to the block; otherwise, a new basic
3581 block must be created to hold the instrumentation code.
3583 @item -ftree-based-profiling
3584 @opindex ftree-based-profiling
3585 This option is used in addition to @option{-fprofile-arcs} or
3586 @option{-fbranch-probabilities} to control whether those optimizations
3587 are performed on a tree-based or rtl-based internal representation.
3588 If you use this option when compiling with @option{-fprofile-arcs},
3589 you must also use it when compiling later with @option{-fbranch-probabilities}.
3590 Currently the tree-based optimization is in an early stage of
3591 development, and this option is recommended only for those people
3592 working on improving it.
3595 @item -ftest-coverage
3596 @opindex ftest-coverage
3597 Produce a notes file that the @command{gcov} code-coverage utility
3598 (@pxref{Gcov,, @command{gcov}---a Test Coverage Program}) can use to
3599 show program coverage. Each source file's note file is called
3600 @file{@var{auxname}.gcno}. Refer to the @option{-fprofile-arcs} option
3601 above for a description of @var{auxname} and instructions on how to
3602 generate test coverage data. Coverage data will match the source files
3603 more closely, if you do not optimize.
3605 @item -d@var{letters}
3606 @item -fdump-rtl-@var{pass}
3608 Says to make debugging dumps during compilation at times specified by
3609 @var{letters}. This is used for debugging the RTL-based passes of the
3610 compiler. The file names for most of the dumps are made by appending a
3611 pass number and a word to the @var{dumpname}. @var{dumpname} is generated
3612 from the name of the output file, if explicitly specified and it is not
3613 an executable, otherwise it is the basename of the source file.
3615 Most debug dumps can be enabled either passing a letter to the @option{-d}
3616 option, or with a long @option{-fdump-rtl} switch; here are the possible
3617 letters for use in @var{letters} and @var{pass}, and their meanings:
3622 Annotate the assembler output with miscellaneous debugging information.
3625 @itemx -fdump-rtl-bp
3627 @opindex fdump-rtl-bp
3628 Dump after computing branch probabilities, to @file{@var{file}.09.bp}.
3631 @itemx -fdump-rtl-bbro
3633 @opindex fdump-rtl-bbro
3634 Dump after block reordering, to @file{@var{file}.30.bbro}.
3637 @itemx -fdump-rtl-combine
3639 @opindex fdump-rtl-combine
3640 Dump after instruction combination, to the file @file{@var{file}.17.combine}.
3643 @itemx -fdump-rtl-ce1
3644 @itemx -fdump-rtl-ce2
3646 @opindex fdump-rtl-ce1
3647 @opindex fdump-rtl-ce2
3648 @option{-dC} and @option{-fdump-rtl-ce1} enable dumping after the
3649 first if conversion, to the file @file{@var{file}.11.ce1}. @option{-dC}
3650 and @option{-fdump-rtl-ce2} enable dumping after the second if
3651 conversion, to the file @file{@var{file}.18.ce2}.
3654 @itemx -fdump-rtl-btl
3655 @itemx -fdump-rtl-dbr
3657 @opindex fdump-rtl-btl
3658 @opindex fdump-rtl-dbr
3659 @option{-dd} and @option{-fdump-rtl-btl} enable dumping after branch
3660 target load optimization, to @file{@var{file}.31.btl}. @option{-dd}
3661 and @option{-fdump-rtl-dbr} enable dumping after delayed branch
3662 scheduling, to @file{@var{file}.36.dbr}.
3666 Dump all macro definitions, at the end of preprocessing, in addition to
3670 @itemx -fdump-rtl-ce3
3672 @opindex fdump-rtl-ce3
3673 Dump after the third if conversion, to @file{@var{file}.28.ce3}.
3676 @itemx -fdump-rtl-cfg
3677 @itemx -fdump-rtl-life
3679 @opindex fdump-rtl-cfg
3680 @opindex fdump-rtl-life
3681 @option{-df} and @option{-fdump-rtl-cfg} enable dumping after control
3682 and data flow analysis, to @file{@var{file}.08.cfg}. @option{-df}
3683 and @option{-fdump-rtl-cfg} enable dumping dump after life analysis,
3684 to @file{@var{file}.16.life}.
3687 @itemx -fdump-rtl-greg
3689 @opindex fdump-rtl-greg
3690 Dump after global register allocation, to @file{@var{file}.23.greg}.
3693 @itemx -fdump-rtl-gcse
3694 @itemx -fdump-rtl-bypass
3696 @opindex fdump-rtl-gcse
3697 @opindex fdump-rtl-bypass
3698 @option{-dG} and @option{-fdump-rtl-gcse} enable dumping after GCSE, to
3699 @file{@var{file}.05.gcse}. @option{-dG} and @option{-fdump-rtl-bypass}
3700 enable dumping after jump bypassing and control flow optimizations, to
3701 @file{@var{file}.07.bypass}.
3704 @itemx -fdump-rtl-eh
3706 @opindex fdump-rtl-eh
3707 Dump after finalization of EH handling code, to @file{@var{file}.02.eh}.
3710 @itemx -fdump-rtl-sibling
3712 @opindex fdump-rtl-sibling
3713 Dump after sibling call optimizations, to @file{@var{file}.01.sibling}.
3716 @itemx -fdump-rtl-jump
3718 @opindex fdump-rtl-jump
3719 Dump after the first jump optimization, to @file{@var{file}.03.jump}.
3722 @itemx -fdump-rtl-stack
3724 @opindex fdump-rtl-stack
3725 Dump after conversion from registers to stack, to @file{@var{file}.33.stack}.
3728 @itemx -fdump-rtl-lreg
3730 @opindex fdump-rtl-lreg
3731 Dump after local register allocation, to @file{@var{file}.22.lreg}.
3734 @itemx -fdump-rtl-loop
3735 @itemx -fdump-rtl-loop2
3737 @opindex fdump-rtl-loop
3738 @opindex fdump-rtl-loop2
3739 @option{-dL} and @option{-fdump-rtl-loop} enable dumping after the first
3740 loop optimization pass, to @file{@var{file}.06.loop}. @option{-dL} and
3741 @option{-fdump-rtl-loop2} enable dumping after the second pass, to
3742 @file{@var{file}.13.loop2}.
3745 @itemx -fdump-rtl-sms
3747 @opindex fdump-rtl-sms
3748 Dump after modulo scheduling, to @file{@var{file}.20.sms}.
3751 @itemx -fdump-rtl-mach
3753 @opindex fdump-rtl-mach
3754 Dump after performing the machine dependent reorganization pass, to
3755 @file{@var{file}.35.mach}.
3758 @itemx -fdump-rtl-rnreg
3760 @opindex fdump-rtl-rnreg
3761 Dump after register renumbering, to @file{@var{file}.29.rnreg}.
3764 @itemx -fdump-rtl-regmove
3766 @opindex fdump-rtl-regmove
3767 Dump after the register move pass, to @file{@var{file}.19.regmove}.
3770 @itemx -fdump-rtl-postreload
3772 @opindex fdump-rtl-postreload
3773 Dump after post-reload optimizations, to @file{@var{file}.24.postreload}.
3776 @itemx -fdump-rtl-expand
3778 @opindex fdump-rtl-expand
3779 Dump after RTL generation, to @file{@var{file}.00.expand}.
3782 @itemx -fdump-rtl-sched2
3784 @opindex fdump-rtl-sched2
3785 Dump after the second scheduling pass, to @file{@var{file}.32.sched2}.
3788 @itemx -fdump-rtl-cse
3790 @opindex fdump-rtl-cse
3791 Dump after CSE (including the jump optimization that sometimes follows
3792 CSE), to @file{@var{file}.04.cse}.
3795 @itemx -fdump-rtl-sched
3797 @opindex fdump-rtl-sched
3798 Dump after the first scheduling pass, to @file{@var{file}.21.sched}.
3801 @itemx -fdump-rtl-cse2
3803 @opindex fdump-rtl-cse2
3804 Dump after the second CSE pass (including the jump optimization that
3805 sometimes follows CSE), to @file{@var{file}.15.cse2}.
3808 @itemx -fdump-rtl-tracer
3810 @opindex fdump-rtl-tracer
3811 Dump after running tracer, to @file{@var{file}.12.tracer}.
3814 @itemx -fdump-rtl-vpt
3815 @itemx -fdump-rtl-vartrack
3817 @opindex fdump-rtl-vpt
3818 @opindex fdump-rtl-vartrack
3819 @option{-dV} and @option{-fdump-rtl-vpt} enable dumping after the value
3820 profile transformations, to @file{@var{file}.10.vpt}. @option{-dV}
3821 and @option{-fdump-rtl-vartrack} enable dumping after variable tracking,
3822 to @file{@var{file}.34.vartrack}.
3825 @itemx -fdump-rtl-flow2
3827 @opindex fdump-rtl-flow2
3828 Dump after the second flow pass, to @file{@var{file}.26.flow2}.
3831 @itemx -fdump-rtl-peephole2
3833 @opindex fdump-rtl-peephole2
3834 Dump after the peephole pass, to @file{@var{file}.27.peephole2}.
3837 @itemx -fdump-rtl-web
3839 @opindex fdump-rtl-web
3840 Dump after live range splitting, to @file{@var{file}.14.web}.
3843 @itemx -fdump-rtl-all
3845 @opindex fdump-rtl-all
3846 Produce all the dumps listed above.
3850 Produce a core dump whenever an error occurs.
3854 Print statistics on memory usage, at the end of the run, to
3859 Annotate the assembler output with a comment indicating which
3860 pattern and alternative was used. The length of each instruction is
3865 Dump the RTL in the assembler output as a comment before each instruction.
3866 Also turns on @option{-dp} annotation.
3870 For each of the other indicated dump files (either with @option{-d} or
3871 @option{-fdump-rtl-@var{pass}}), dump a representation of the control flow
3872 graph suitable for viewing with VCG to @file{@var{file}.@var{pass}.vcg}.
3876 Just generate RTL for a function instead of compiling it. Usually used
3877 with @samp{r} (@option{-fdump-rtl-expand}).
3881 Dump debugging information during parsing, to standard error.
3884 @item -fdump-unnumbered
3885 @opindex fdump-unnumbered
3886 When doing debugging dumps (see @option{-d} option above), suppress instruction
3887 numbers and line number note output. This makes it more feasible to
3888 use diff on debugging dumps for compiler invocations with different
3889 options, in particular with and without @option{-g}.
3891 @item -fdump-translation-unit @r{(C++ only)}
3892 @itemx -fdump-translation-unit-@var{options} @r{(C++ only)}
3893 @opindex fdump-translation-unit
3894 Dump a representation of the tree structure for the entire translation
3895 unit to a file. The file name is made by appending @file{.tu} to the
3896 source file name. If the @samp{-@var{options}} form is used, @var{options}
3897 controls the details of the dump as described for the
3898 @option{-fdump-tree} options.
3900 @item -fdump-class-hierarchy @r{(C++ only)}
3901 @itemx -fdump-class-hierarchy-@var{options} @r{(C++ only)}
3902 @opindex fdump-class-hierarchy
3903 Dump a representation of each class's hierarchy and virtual function
3904 table layout to a file. The file name is made by appending @file{.class}
3905 to the source file name. If the @samp{-@var{options}} form is used,
3906 @var{options} controls the details of the dump as described for the
3907 @option{-fdump-tree} options.
3909 @item -fdump-ipa-@var{switch}
3911 Control the dumping at various stages of inter-procedural analysis
3912 language tree to a file. The file name is generated by appending a switch
3913 specific suffix to the source file name. The following dumps are possible:
3917 Enables all inter-procedural analysis dumps; currently the only produced
3918 dump is the @samp{cgraph} dump.
3921 Dumps information about call-graph optimization, unused function removal,
3922 and inlining decisions.
3925 @item -fdump-tree-@var{switch} @r{(C and C++ only)}
3926 @itemx -fdump-tree-@var{switch}-@var{options} @r{(C and C++ only)}
3928 Control the dumping at various stages of processing the intermediate
3929 language tree to a file. The file name is generated by appending a switch
3930 specific suffix to the source file name. If the @samp{-@var{options}}
3931 form is used, @var{options} is a list of @samp{-} separated options that
3932 control the details of the dump. Not all options are applicable to all
3933 dumps, those which are not meaningful will be ignored. The following
3934 options are available
3938 Print the address of each node. Usually this is not meaningful as it
3939 changes according to the environment and source file. Its primary use
3940 is for tying up a dump file with a debug environment.
3942 Inhibit dumping of members of a scope or body of a function merely
3943 because that scope has been reached. Only dump such items when they
3944 are directly reachable by some other path. When dumping pretty-printed
3945 trees, this option inhibits dumping the bodies of control structures.
3947 Print a raw representation of the tree. By default, trees are
3948 pretty-printed into a C-like representation.
3950 Enable more detailed dumps (not honored by every dump option).
3952 Enable dumping various statistics about the pass (not honored by every dump
3955 Enable showing basic block boundaries (disabled in raw dumps).
3957 Enable showing virtual operands for every statement.
3959 Enable showing line numbers for statements.
3961 Enable showing the unique ID (@code{DECL_UID}) for each variable.
3963 Turn on all options, except @option{raw}, @option{slim} and @option{lineno}.
3966 The following tree dumps are possible:
3970 Dump before any tree based optimization, to @file{@var{file}.original}.
3973 Dump after all tree based optimization, to @file{@var{file}.optimized}.
3976 Dump after function inlining, to @file{@var{file}.inlined}.
3979 @opindex fdump-tree-gimple
3980 Dump each function before and after the gimplification pass to a file. The
3981 file name is made by appending @file{.gimple} to the source file name.
3984 @opindex fdump-tree-cfg
3985 Dump the control flow graph of each function to a file. The file name is
3986 made by appending @file{.cfg} to the source file name.
3989 @opindex fdump-tree-vcg
3990 Dump the control flow graph of each function to a file in VCG format. The
3991 file name is made by appending @file{.vcg} to the source file name. Note
3992 that if the file contains more than one function, the generated file cannot
3993 be used directly by VCG@. You will need to cut and paste each function's
3994 graph into its own separate file first.
3997 @opindex fdump-tree-ch
3998 Dump each function after copying loop headers. The file name is made by
3999 appending @file{.ch} to the source file name.
4002 @opindex fdump-tree-ssa
4003 Dump SSA related information to a file. The file name is made by appending
4004 @file{.ssa} to the source file name.
4007 @opindex fdump-tree-salias
4008 Dump structure aliasing variable information to a file. This file name
4009 is made by appending @file{.salias} to the source file name.
4012 @opindex fdump-tree-alias
4013 Dump aliasing information for each function. The file name is made by
4014 appending @file{.alias} to the source file name.
4017 @opindex fdump-tree-ccp
4018 Dump each function after CCP@. The file name is made by appending
4019 @file{.ccp} to the source file name.
4022 @opindex fdump-tree-storeccp
4023 Dump each function after STORE-CCP. The file name is made by appending
4024 @file{.storeccp} to the source file name.
4027 @opindex fdump-tree-pre
4028 Dump trees after partial redundancy elimination. The file name is made
4029 by appending @file{.pre} to the source file name.
4032 @opindex fdump-tree-fre
4033 Dump trees after full redundancy elimination. The file name is made
4034 by appending @file{.fre} to the source file name.
4037 @opindex fdump-tree-copyprop
4038 Dump trees after copy propagation. The file name is made
4039 by appending @file{.copyprop} to the source file name.
4041 @item store_copyprop
4042 @opindex fdump-tree-store_copyprop
4043 Dump trees after store copy-propagation. The file name is made
4044 by appending @file{.store_copyprop} to the source file name.
4047 @opindex fdump-tree-dce
4048 Dump each function after dead code elimination. The file name is made by
4049 appending @file{.dce} to the source file name.
4052 @opindex fdump-tree-mudflap
4053 Dump each function after adding mudflap instrumentation. The file name is
4054 made by appending @file{.mudflap} to the source file name.
4057 @opindex fdump-tree-sra
4058 Dump each function after performing scalar replacement of aggregates. The
4059 file name is made by appending @file{.sra} to the source file name.
4062 @opindex fdump-tree-sink
4063 Dump each function after performing code sinking. The file name is made
4064 by appending @file{.sink} to the source file name.
4067 @opindex fdump-tree-dom
4068 Dump each function after applying dominator tree optimizations. The file
4069 name is made by appending @file{.dom} to the source file name.
4072 @opindex fdump-tree-dse
4073 Dump each function after applying dead store elimination. The file
4074 name is made by appending @file{.dse} to the source file name.
4077 @opindex fdump-tree-phiopt
4078 Dump each function after optimizing PHI nodes into straightline code. The file
4079 name is made by appending @file{.phiopt} to the source file name.
4082 @opindex fdump-tree-forwprop
4083 Dump each function after forward propagating single use variables. The file
4084 name is made by appending @file{.forwprop} to the source file name.
4087 @opindex fdump-tree-copyrename
4088 Dump each function after applying the copy rename optimization. The file
4089 name is made by appending @file{.copyrename} to the source file name.
4092 @opindex fdump-tree-nrv
4093 Dump each function after applying the named return value optimization on
4094 generic trees. The file name is made by appending @file{.nrv} to the source
4098 @opindex fdump-tree-vect
4099 Dump each function after applying vectorization of loops. The file name is
4100 made by appending @file{.vect} to the source file name.
4103 @opindex fdump-tree-vrp
4104 Dump each function after Value Range Propagation (VRP). The file name
4105 is made by appending @file{.vrp} to the source file name.
4108 @opindex fdump-tree-all
4109 Enable all the available tree dumps with the flags provided in this option.
4112 @item -ftree-vectorizer-verbose=@var{n}
4113 @opindex ftree-vectorizer-verbose
4114 This option controls the amount of debugging output the vectorizer prints.
4115 This information is written to standard error, unless @option{-fdump-tree-all}
4116 or @option{-fdump-tree-vect} is specified, in which case it is output to the
4117 usual dump listing file, @file{.vect}.
4119 @item -frandom-seed=@var{string}
4120 @opindex frandom-string
4121 This option provides a seed that GCC uses when it would otherwise use
4122 random numbers. It is used to generate certain symbol names
4123 that have to be different in every compiled file. It is also used to
4124 place unique stamps in coverage data files and the object files that
4125 produce them. You can use the @option{-frandom-seed} option to produce
4126 reproducibly identical object files.
4128 The @var{string} should be different for every file you compile.
4130 @item -fsched-verbose=@var{n}
4131 @opindex fsched-verbose
4132 On targets that use instruction scheduling, this option controls the
4133 amount of debugging output the scheduler prints. This information is
4134 written to standard error, unless @option{-dS} or @option{-dR} is
4135 specified, in which case it is output to the usual dump
4136 listing file, @file{.sched} or @file{.sched2} respectively. However
4137 for @var{n} greater than nine, the output is always printed to standard
4140 For @var{n} greater than zero, @option{-fsched-verbose} outputs the
4141 same information as @option{-dRS}. For @var{n} greater than one, it
4142 also output basic block probabilities, detailed ready list information
4143 and unit/insn info. For @var{n} greater than two, it includes RTL
4144 at abort point, control-flow and regions info. And for @var{n} over
4145 four, @option{-fsched-verbose} also includes dependence info.
4149 Store the usual ``temporary'' intermediate files permanently; place them
4150 in the current directory and name them based on the source file. Thus,
4151 compiling @file{foo.c} with @samp{-c -save-temps} would produce files
4152 @file{foo.i} and @file{foo.s}, as well as @file{foo.o}. This creates a
4153 preprocessed @file{foo.i} output file even though the compiler now
4154 normally uses an integrated preprocessor.
4156 When used in combination with the @option{-x} command line option,
4157 @option{-save-temps} is sensible enough to avoid over writing an
4158 input source file with the same extension as an intermediate file.
4159 The corresponding intermediate file may be obtained by renaming the
4160 source file before using @option{-save-temps}.
4164 Report the CPU time taken by each subprocess in the compilation
4165 sequence. For C source files, this is the compiler proper and assembler
4166 (plus the linker if linking is done). The output looks like this:
4173 The first number on each line is the ``user time'', that is time spent
4174 executing the program itself. The second number is ``system time'',
4175 time spent executing operating system routines on behalf of the program.
4176 Both numbers are in seconds.
4178 @item -fvar-tracking
4179 @opindex fvar-tracking
4180 Run variable tracking pass. It computes where variables are stored at each
4181 position in code. Better debugging information is then generated
4182 (if the debugging information format supports this information).
4184 It is enabled by default when compiling with optimization (@option{-Os},
4185 @option{-O}, @option{-O2}, ...), debugging information (@option{-g}) and
4186 the debug info format supports it.
4188 @item -print-file-name=@var{library}
4189 @opindex print-file-name
4190 Print the full absolute name of the library file @var{library} that
4191 would be used when linking---and don't do anything else. With this
4192 option, GCC does not compile or link anything; it just prints the
4195 @item -print-multi-directory
4196 @opindex print-multi-directory
4197 Print the directory name corresponding to the multilib selected by any
4198 other switches present in the command line. This directory is supposed
4199 to exist in @env{GCC_EXEC_PREFIX}.
4201 @item -print-multi-lib
4202 @opindex print-multi-lib
4203 Print the mapping from multilib directory names to compiler switches
4204 that enable them. The directory name is separated from the switches by
4205 @samp{;}, and each switch starts with an @samp{@@} instead of the
4206 @samp{-}, without spaces between multiple switches. This is supposed to
4207 ease shell-processing.
4209 @item -print-prog-name=@var{program}
4210 @opindex print-prog-name
4211 Like @option{-print-file-name}, but searches for a program such as @samp{cpp}.
4213 @item -print-libgcc-file-name
4214 @opindex print-libgcc-file-name
4215 Same as @option{-print-file-name=libgcc.a}.
4217 This is useful when you use @option{-nostdlib} or @option{-nodefaultlibs}
4218 but you do want to link with @file{libgcc.a}. You can do
4221 gcc -nostdlib @var{files}@dots{} `gcc -print-libgcc-file-name`
4224 @item -print-search-dirs
4225 @opindex print-search-dirs
4226 Print the name of the configured installation directory and a list of
4227 program and library directories @command{gcc} will search---and don't do anything else.
4229 This is useful when @command{gcc} prints the error message
4230 @samp{installation problem, cannot exec cpp0: No such file or directory}.
4231 To resolve this you either need to put @file{cpp0} and the other compiler
4232 components where @command{gcc} expects to find them, or you can set the environment
4233 variable @env{GCC_EXEC_PREFIX} to the directory where you installed them.
4234 Don't forget the trailing @samp{/}.
4235 @xref{Environment Variables}.
4238 @opindex dumpmachine
4239 Print the compiler's target machine (for example,
4240 @samp{i686-pc-linux-gnu})---and don't do anything else.
4243 @opindex dumpversion
4244 Print the compiler version (for example, @samp{3.0})---and don't do
4249 Print the compiler's built-in specs---and don't do anything else. (This
4250 is used when GCC itself is being built.) @xref{Spec Files}.
4252 @item -feliminate-unused-debug-types
4253 @opindex feliminate-unused-debug-types
4254 Normally, when producing DWARF2 output, GCC will emit debugging
4255 information for all types declared in a compilation
4256 unit, regardless of whether or not they are actually used
4257 in that compilation unit. Sometimes this is useful, such as
4258 if, in the debugger, you want to cast a value to a type that is
4259 not actually used in your program (but is declared). More often,
4260 however, this results in a significant amount of wasted space.
4261 With this option, GCC will avoid producing debug symbol output
4262 for types that are nowhere used in the source file being compiled.
4265 @node Optimize Options
4266 @section Options That Control Optimization
4267 @cindex optimize options
4268 @cindex options, optimization
4270 These options control various sorts of optimizations.
4272 Without any optimization option, the compiler's goal is to reduce the
4273 cost of compilation and to make debugging produce the expected
4274 results. Statements are independent: if you stop the program with a
4275 breakpoint between statements, you can then assign a new value to any
4276 variable or change the program counter to any other statement in the
4277 function and get exactly the results you would expect from the source
4280 Turning on optimization flags makes the compiler attempt to improve
4281 the performance and/or code size at the expense of compilation time
4282 and possibly the ability to debug the program.
4284 The compiler performs optimization based on the knowledge it has of
4285 the program. Optimization levels @option{-O2} and above, in
4286 particular, enable @emph{unit-at-a-time} mode, which allows the
4287 compiler to consider information gained from later functions in
4288 the file when compiling a function. Compiling multiple files at
4289 once to a single output file in @emph{unit-at-a-time} mode allows
4290 the compiler to use information gained from all of the files when
4291 compiling each of them.
4293 Not all optimizations are controlled directly by a flag. Only
4294 optimizations that have a flag are listed.
4301 Optimize. Optimizing compilation takes somewhat more time, and a lot
4302 more memory for a large function.
4304 With @option{-O}, the compiler tries to reduce code size and execution
4305 time, without performing any optimizations that take a great deal of
4308 @option{-O} turns on the following optimization flags:
4309 @gccoptlist{-fdefer-pop @gol
4310 -fdelayed-branch @gol
4311 -fguess-branch-probability @gol
4312 -fcprop-registers @gol
4313 -floop-optimize @gol
4314 -fif-conversion @gol
4315 -fif-conversion2 @gol
4318 -ftree-dominator-opts @gol
4323 -ftree-copyrename @gol
4328 @option{-O} also turns on @option{-fomit-frame-pointer} on machines
4329 where doing so does not interfere with debugging.
4333 Optimize even more. GCC performs nearly all supported optimizations
4334 that do not involve a space-speed tradeoff. The compiler does not
4335 perform loop unrolling or function inlining when you specify @option{-O2}.
4336 As compared to @option{-O}, this option increases both compilation time
4337 and the performance of the generated code.
4339 @option{-O2} turns on all optimization flags specified by @option{-O}. It
4340 also turns on the following optimization flags:
4341 @gccoptlist{-fthread-jumps @gol
4343 -foptimize-sibling-calls @gol
4344 -fcse-follow-jumps -fcse-skip-blocks @gol
4345 -fgcse -fgcse-lm @gol
4346 -fexpensive-optimizations @gol
4347 -fstrength-reduce @gol
4348 -frerun-cse-after-loop -frerun-loop-opt @gol
4351 -fschedule-insns -fschedule-insns2 @gol
4352 -fsched-interblock -fsched-spec @gol
4354 -fstrict-aliasing @gol
4355 -fdelete-null-pointer-checks @gol
4356 -freorder-blocks -freorder-functions @gol
4357 -funit-at-a-time @gol
4358 -falign-functions -falign-jumps @gol
4359 -falign-loops -falign-labels @gol
4363 Please note the warning under @option{-fgcse} about
4364 invoking @option{-O2} on programs that use computed gotos.
4368 Optimize yet more. @option{-O3} turns on all optimizations specified by
4369 @option{-O2} and also turns on the @option{-finline-functions},
4370 @option{-funswitch-loops} and @option{-fgcse-after-reload} options.
4374 Do not optimize. This is the default.
4378 Optimize for size. @option{-Os} enables all @option{-O2} optimizations that
4379 do not typically increase code size. It also performs further
4380 optimizations designed to reduce code size.
4382 @option{-Os} disables the following optimization flags:
4383 @gccoptlist{-falign-functions -falign-jumps -falign-loops @gol
4384 -falign-labels -freorder-blocks -freorder-blocks-and-partition -fprefetch-loop-arrays}
4386 If you use multiple @option{-O} options, with or without level numbers,
4387 the last such option is the one that is effective.
4390 Options of the form @option{-f@var{flag}} specify machine-independent
4391 flags. Most flags have both positive and negative forms; the negative
4392 form of @option{-ffoo} would be @option{-fno-foo}. In the table
4393 below, only one of the forms is listed---the one you typically will
4394 use. You can figure out the other form by either removing @samp{no-}
4397 The following options control specific optimizations. They are either
4398 activated by @option{-O} options or are related to ones that are. You
4399 can use the following flags in the rare cases when ``fine-tuning'' of
4400 optimizations to be performed is desired.
4403 @item -fno-default-inline
4404 @opindex fno-default-inline
4405 Do not make member functions inline by default merely because they are
4406 defined inside the class scope (C++ only). Otherwise, when you specify
4407 @w{@option{-O}}, member functions defined inside class scope are compiled
4408 inline by default; i.e., you don't need to add @samp{inline} in front of
4409 the member function name.
4411 @item -fno-defer-pop
4412 @opindex fno-defer-pop
4413 Always pop the arguments to each function call as soon as that function
4414 returns. For machines which must pop arguments after a function call,
4415 the compiler normally lets arguments accumulate on the stack for several
4416 function calls and pops them all at once.
4418 Disabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4422 Force memory operands to be copied into registers before doing
4423 arithmetic on them. This produces better code by making all memory
4424 references potential common subexpressions. When they are not common
4425 subexpressions, instruction combination should eliminate the separate
4426 register-load. This option is now a nop and will be removed in 4.2.
4429 @opindex fforce-addr
4430 Force memory address constants to be copied into registers before
4431 doing arithmetic on them.
4433 @item -fomit-frame-pointer
4434 @opindex fomit-frame-pointer
4435 Don't keep the frame pointer in a register for functions that
4436 don't need one. This avoids the instructions to save, set up and
4437 restore frame pointers; it also makes an extra register available
4438 in many functions. @strong{It also makes debugging impossible on
4441 On some machines, such as the VAX, this flag has no effect, because
4442 the standard calling sequence automatically handles the frame pointer
4443 and nothing is saved by pretending it doesn't exist. The
4444 machine-description macro @code{FRAME_POINTER_REQUIRED} controls
4445 whether a target machine supports this flag. @xref{Registers,,Register
4446 Usage, gccint, GNU Compiler Collection (GCC) Internals}.
4448 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4450 @item -foptimize-sibling-calls
4451 @opindex foptimize-sibling-calls
4452 Optimize sibling and tail recursive calls.
4454 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4458 Don't pay attention to the @code{inline} keyword. Normally this option
4459 is used to keep the compiler from expanding any functions inline.
4460 Note that if you are not optimizing, no functions can be expanded inline.
4462 @item -finline-functions
4463 @opindex finline-functions
4464 Integrate all simple functions into their callers. The compiler
4465 heuristically decides which functions are simple enough to be worth
4466 integrating in this way.
4468 If all calls to a given function are integrated, and the function is
4469 declared @code{static}, then the function is normally not output as
4470 assembler code in its own right.
4472 Enabled at level @option{-O3}.
4474 @item -fearly-inlining
4475 @opindex fearly-inlining
4476 Inline functions marked by @code{always_inline} and functions whose body seems
4477 smaller than the function call overhead early before doing
4478 @option{-fprofile-generate} instrumentation and real inlining pass. Doing so
4479 makes profiling significantly cheaper and usually inlining faster on programs
4480 having large chains of nested wrapper functions.
4484 @item -finline-limit=@var{n}
4485 @opindex finline-limit
4486 By default, GCC limits the size of functions that can be inlined. This flag
4487 allows the control of this limit for functions that are explicitly marked as
4488 inline (i.e., marked with the inline keyword or defined within the class
4489 definition in c++). @var{n} is the size of functions that can be inlined in
4490 number of pseudo instructions (not counting parameter handling). The default
4491 value of @var{n} is 600.
4492 Increasing this value can result in more inlined code at
4493 the cost of compilation time and memory consumption. Decreasing usually makes
4494 the compilation faster and less code will be inlined (which presumably
4495 means slower programs). This option is particularly useful for programs that
4496 use inlining heavily such as those based on recursive templates with C++.
4498 Inlining is actually controlled by a number of parameters, which may be
4499 specified individually by using @option{--param @var{name}=@var{value}}.
4500 The @option{-finline-limit=@var{n}} option sets some of these parameters
4504 @item max-inline-insns-single
4505 is set to @var{n}/2.
4506 @item max-inline-insns-auto
4507 is set to @var{n}/2.
4508 @item min-inline-insns
4509 is set to 130 or @var{n}/4, whichever is smaller.
4510 @item max-inline-insns-rtl
4514 See below for a documentation of the individual
4515 parameters controlling inlining.
4517 @emph{Note:} pseudo instruction represents, in this particular context, an
4518 abstract measurement of function's size. In no way does it represent a count
4519 of assembly instructions and as such its exact meaning might change from one
4520 release to an another.
4522 @item -fkeep-inline-functions
4523 @opindex fkeep-inline-functions
4524 In C, emit @code{static} functions that are declared @code{inline}
4525 into the object file, even if the function has been inlined into all
4526 of its callers. This switch does not affect functions using the
4527 @code{extern inline} extension in GNU C@. In C++, emit any and all
4528 inline functions into the object file.
4530 @item -fkeep-static-consts
4531 @opindex fkeep-static-consts
4532 Emit variables declared @code{static const} when optimization isn't turned
4533 on, even if the variables aren't referenced.
4535 GCC enables this option by default. If you want to force the compiler to
4536 check if the variable was referenced, regardless of whether or not
4537 optimization is turned on, use the @option{-fno-keep-static-consts} option.
4539 @item -fmerge-constants
4540 Attempt to merge identical constants (string constants and floating point
4541 constants) across compilation units.
4543 This option is the default for optimized compilation if the assembler and
4544 linker support it. Use @option{-fno-merge-constants} to inhibit this
4547 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4549 @item -fmerge-all-constants
4550 Attempt to merge identical constants and identical variables.
4552 This option implies @option{-fmerge-constants}. In addition to
4553 @option{-fmerge-constants} this considers e.g.@: even constant initialized
4554 arrays or initialized constant variables with integral or floating point
4555 types. Languages like C or C++ require each non-automatic variable to
4556 have distinct location, so using this option will result in non-conforming
4559 @item -fmodulo-sched
4560 @opindex fmodulo-sched
4561 Perform swing modulo scheduling immediately before the first scheduling
4562 pass. This pass looks at innermost loops and reorders their
4563 instructions by overlapping different iterations.
4565 @item -fno-branch-count-reg
4566 @opindex fno-branch-count-reg
4567 Do not use ``decrement and branch'' instructions on a count register,
4568 but instead generate a sequence of instructions that decrement a
4569 register, compare it against zero, then branch based upon the result.
4570 This option is only meaningful on architectures that support such
4571 instructions, which include x86, PowerPC, IA-64 and S/390.
4573 The default is @option{-fbranch-count-reg}, enabled when
4574 @option{-fstrength-reduce} is enabled.
4576 @item -fno-function-cse
4577 @opindex fno-function-cse
4578 Do not put function addresses in registers; make each instruction that
4579 calls a constant function contain the function's address explicitly.
4581 This option results in less efficient code, but some strange hacks
4582 that alter the assembler output may be confused by the optimizations
4583 performed when this option is not used.
4585 The default is @option{-ffunction-cse}
4587 @item -fno-zero-initialized-in-bss
4588 @opindex fno-zero-initialized-in-bss
4589 If the target supports a BSS section, GCC by default puts variables that
4590 are initialized to zero into BSS@. This can save space in the resulting
4593 This option turns off this behavior because some programs explicitly
4594 rely on variables going to the data section. E.g., so that the
4595 resulting executable can find the beginning of that section and/or make
4596 assumptions based on that.
4598 The default is @option{-fzero-initialized-in-bss}.
4600 @item -fbounds-check
4601 @opindex fbounds-check
4602 For front-ends that support it, generate additional code to check that
4603 indices used to access arrays are within the declared range. This is
4604 currently only supported by the Java and Fortran front-ends, where
4605 this option defaults to true and false respectively.
4607 @item -fmudflap -fmudflapth -fmudflapir
4611 @cindex bounds checking
4613 For front-ends that support it (C and C++), instrument all risky
4614 pointer/array dereferencing operations, some standard library
4615 string/heap functions, and some other associated constructs with
4616 range/validity tests. Modules so instrumented should be immune to
4617 buffer overflows, invalid heap use, and some other classes of C/C++
4618 programming errors. The instrumentation relies on a separate runtime
4619 library (@file{libmudflap}), which will be linked into a program if
4620 @option{-fmudflap} is given at link time. Run-time behavior of the
4621 instrumented program is controlled by the @env{MUDFLAP_OPTIONS}
4622 environment variable. See @code{env MUDFLAP_OPTIONS=-help a.out}
4625 Use @option{-fmudflapth} instead of @option{-fmudflap} to compile and to
4626 link if your program is multi-threaded. Use @option{-fmudflapir}, in
4627 addition to @option{-fmudflap} or @option{-fmudflapth}, if
4628 instrumentation should ignore pointer reads. This produces less
4629 instrumentation (and therefore faster execution) and still provides
4630 some protection against outright memory corrupting writes, but allows
4631 erroneously read data to propagate within a program.
4633 @item -fstrength-reduce
4634 @opindex fstrength-reduce
4635 Perform the optimizations of loop strength reduction and
4636 elimination of iteration variables.
4638 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4640 @item -fthread-jumps
4641 @opindex fthread-jumps
4642 Perform optimizations where we check to see if a jump branches to a
4643 location where another comparison subsumed by the first is found. If
4644 so, the first branch is redirected to either the destination of the
4645 second branch or a point immediately following it, depending on whether
4646 the condition is known to be true or false.
4648 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4650 @item -fcse-follow-jumps
4651 @opindex fcse-follow-jumps
4652 In common subexpression elimination, scan through jump instructions
4653 when the target of the jump is not reached by any other path. For
4654 example, when CSE encounters an @code{if} statement with an
4655 @code{else} clause, CSE will follow the jump when the condition
4658 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4660 @item -fcse-skip-blocks
4661 @opindex fcse-skip-blocks
4662 This is similar to @option{-fcse-follow-jumps}, but causes CSE to
4663 follow jumps which conditionally skip over blocks. When CSE
4664 encounters a simple @code{if} statement with no else clause,
4665 @option{-fcse-skip-blocks} causes CSE to follow the jump around the
4666 body of the @code{if}.
4668 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4670 @item -frerun-cse-after-loop
4671 @opindex frerun-cse-after-loop
4672 Re-run common subexpression elimination after loop optimizations has been
4675 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4677 @item -frerun-loop-opt
4678 @opindex frerun-loop-opt
4679 Run the loop optimizer twice.
4681 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4685 Perform a global common subexpression elimination pass.
4686 This pass also performs global constant and copy propagation.
4688 @emph{Note:} When compiling a program using computed gotos, a GCC
4689 extension, you may get better runtime performance if you disable
4690 the global common subexpression elimination pass by adding
4691 @option{-fno-gcse} to the command line.
4693 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4697 When @option{-fgcse-lm} is enabled, global common subexpression elimination will
4698 attempt to move loads which are only killed by stores into themselves. This
4699 allows a loop containing a load/store sequence to be changed to a load outside
4700 the loop, and a copy/store within the loop.
4702 Enabled by default when gcse is enabled.
4706 When @option{-fgcse-sm} is enabled, a store motion pass is run after
4707 global common subexpression elimination. This pass will attempt to move
4708 stores out of loops. When used in conjunction with @option{-fgcse-lm},
4709 loops containing a load/store sequence can be changed to a load before
4710 the loop and a store after the loop.
4712 Not enabled at any optimization level.
4716 When @option{-fgcse-las} is enabled, the global common subexpression
4717 elimination pass eliminates redundant loads that come after stores to the
4718 same memory location (both partial and full redundancies).
4720 Not enabled at any optimization level.
4722 @item -fgcse-after-reload
4723 @opindex fgcse-after-reload
4724 When @option{-fgcse-after-reload} is enabled, a redundant load elimination
4725 pass is performed after reload. The purpose of this pass is to cleanup
4728 @item -floop-optimize
4729 @opindex floop-optimize
4730 Perform loop optimizations: move constant expressions out of loops, simplify
4731 exit test conditions and optionally do strength-reduction as well.
4733 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4735 @item -floop-optimize2
4736 @opindex floop-optimize2
4737 Perform loop optimizations using the new loop optimizer. The optimizations
4738 (loop unrolling, peeling and unswitching, loop invariant motion) are enabled
4741 @item -funsafe-loop-optimizations
4742 @opindex funsafe-loop-optimizations
4743 If given, the loop optimizer will assume that loop indices do not
4744 overflow, and that the loops with nontrivial exit condition are not
4745 infinite. This enables a wider range of loop optimizations even if
4746 the loop optimizer itself cannot prove that these assumptions are valid.
4747 Using @option{-Wunsafe-loop-optimizations}, the compiler will warn you
4748 if it finds this kind of loop.
4750 @item -fcrossjumping
4751 @opindex crossjumping
4752 Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The
4753 resulting code may or may not perform better than without cross-jumping.
4755 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4757 @item -fif-conversion
4758 @opindex if-conversion
4759 Attempt to transform conditional jumps into branch-less equivalents. This
4760 include use of conditional moves, min, max, set flags and abs instructions, and
4761 some tricks doable by standard arithmetics. The use of conditional execution
4762 on chips where it is available is controlled by @code{if-conversion2}.
4764 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4766 @item -fif-conversion2
4767 @opindex if-conversion2
4768 Use conditional execution (where available) to transform conditional jumps into
4769 branch-less equivalents.
4771 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4773 @item -fdelete-null-pointer-checks
4774 @opindex fdelete-null-pointer-checks
4775 Use global dataflow analysis to identify and eliminate useless checks
4776 for null pointers. The compiler assumes that dereferencing a null
4777 pointer would have halted the program. If a pointer is checked after
4778 it has already been dereferenced, it cannot be null.
4780 In some environments, this assumption is not true, and programs can
4781 safely dereference null pointers. Use
4782 @option{-fno-delete-null-pointer-checks} to disable this optimization
4783 for programs which depend on that behavior.
4785 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4787 @item -fexpensive-optimizations
4788 @opindex fexpensive-optimizations
4789 Perform a number of minor optimizations that are relatively expensive.
4791 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4793 @item -foptimize-register-move
4795 @opindex foptimize-register-move
4797 Attempt to reassign register numbers in move instructions and as
4798 operands of other simple instructions in order to maximize the amount of
4799 register tying. This is especially helpful on machines with two-operand
4802 Note @option{-fregmove} and @option{-foptimize-register-move} are the same
4805 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4807 @item -fdelayed-branch
4808 @opindex fdelayed-branch
4809 If supported for the target machine, attempt to reorder instructions
4810 to exploit instruction slots available after delayed branch
4813 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4815 @item -fschedule-insns
4816 @opindex fschedule-insns
4817 If supported for the target machine, attempt to reorder instructions to
4818 eliminate execution stalls due to required data being unavailable. This
4819 helps machines that have slow floating point or memory load instructions
4820 by allowing other instructions to be issued until the result of the load
4821 or floating point instruction is required.
4823 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4825 @item -fschedule-insns2
4826 @opindex fschedule-insns2
4827 Similar to @option{-fschedule-insns}, but requests an additional pass of
4828 instruction scheduling after register allocation has been done. This is
4829 especially useful on machines with a relatively small number of
4830 registers and where memory load instructions take more than one cycle.
4832 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4834 @item -fno-sched-interblock
4835 @opindex fno-sched-interblock
4836 Don't schedule instructions across basic blocks. This is normally
4837 enabled by default when scheduling before register allocation, i.e.@:
4838 with @option{-fschedule-insns} or at @option{-O2} or higher.
4840 @item -fno-sched-spec
4841 @opindex fno-sched-spec
4842 Don't allow speculative motion of non-load instructions. This is normally
4843 enabled by default when scheduling before register allocation, i.e.@:
4844 with @option{-fschedule-insns} or at @option{-O2} or higher.
4846 @item -fsched-spec-load
4847 @opindex fsched-spec-load
4848 Allow speculative motion of some load instructions. This only makes
4849 sense when scheduling before register allocation, i.e.@: with
4850 @option{-fschedule-insns} or at @option{-O2} or higher.
4852 @item -fsched-spec-load-dangerous
4853 @opindex fsched-spec-load-dangerous
4854 Allow speculative motion of more load instructions. This only makes
4855 sense when scheduling before register allocation, i.e.@: with
4856 @option{-fschedule-insns} or at @option{-O2} or higher.
4858 @item -fsched-stalled-insns=@var{n}
4859 @opindex fsched-stalled-insns
4860 Define how many insns (if any) can be moved prematurely from the queue
4861 of stalled insns into the ready list, during the second scheduling pass.
4863 @item -fsched-stalled-insns-dep=@var{n}
4864 @opindex fsched-stalled-insns-dep
4865 Define how many insn groups (cycles) will be examined for a dependency
4866 on a stalled insn that is candidate for premature removal from the queue
4867 of stalled insns. Has an effect only during the second scheduling pass,
4868 and only if @option{-fsched-stalled-insns} is used and its value is not zero.
4870 @item -fsched2-use-superblocks
4871 @opindex fsched2-use-superblocks
4872 When scheduling after register allocation, do use superblock scheduling
4873 algorithm. Superblock scheduling allows motion across basic block boundaries
4874 resulting on faster schedules. This option is experimental, as not all machine
4875 descriptions used by GCC model the CPU closely enough to avoid unreliable
4876 results from the algorithm.
4878 This only makes sense when scheduling after register allocation, i.e.@: with
4879 @option{-fschedule-insns2} or at @option{-O2} or higher.
4881 @item -fsched2-use-traces
4882 @opindex fsched2-use-traces
4883 Use @option{-fsched2-use-superblocks} algorithm when scheduling after register
4884 allocation and additionally perform code duplication in order to increase the
4885 size of superblocks using tracer pass. See @option{-ftracer} for details on
4888 This mode should produce faster but significantly longer programs. Also
4889 without @option{-fbranch-probabilities} the traces constructed may not
4890 match the reality and hurt the performance. This only makes
4891 sense when scheduling after register allocation, i.e.@: with
4892 @option{-fschedule-insns2} or at @option{-O2} or higher.
4894 @item -freschedule-modulo-scheduled-loops
4895 @opindex fscheduling-in-modulo-scheduled-loops
4896 The modulo scheduling comes before the traditional scheduling, if a loop was modulo scheduled
4897 we may want to prevent the later scheduling passes from changing its schedule, we use this
4898 option to control that.
4900 @item -fcaller-saves
4901 @opindex fcaller-saves
4902 Enable values to be allocated in registers that will be clobbered by
4903 function calls, by emitting extra instructions to save and restore the
4904 registers around such calls. Such allocation is done only when it
4905 seems to result in better code than would otherwise be produced.
4907 This option is always enabled by default on certain machines, usually
4908 those which have no call-preserved registers to use instead.
4910 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4913 Perform Partial Redundancy Elimination (PRE) on trees. This flag is
4914 enabled by default at @option{-O2} and @option{-O3}.
4917 Perform Full Redundancy Elimination (FRE) on trees. The difference
4918 between FRE and PRE is that FRE only considers expressions
4919 that are computed on all paths leading to the redundant computation.
4920 This analysis faster than PRE, though it exposes fewer redundancies.
4921 This flag is enabled by default at @option{-O} and higher.
4923 @item -ftree-copy-prop
4924 Perform copy propagation on trees. This pass eliminates unnecessary
4925 copy operations. This flag is enabled by default at @option{-O} and
4928 @item -ftree-store-copy-prop
4929 Perform copy propagation of memory loads and stores. This pass
4930 eliminates unnecessary copy operations in memory references
4931 (structures, global variables, arrays, etc). This flag is enabled by
4932 default at @option{-O2} and higher.
4935 Perform structural alias analysis on trees. This flag
4936 is enabled by default at @option{-O} and higher.
4939 Perform forward store motion on trees. This flag is
4940 enabled by default at @option{-O} and higher.
4943 Perform sparse conditional constant propagation (CCP) on trees. This
4944 pass only operates on local scalar variables and is enabled by default
4945 at @option{-O} and higher.
4947 @item -ftree-store-ccp
4948 Perform sparse conditional constant propagation (CCP) on trees. This
4949 pass operates on both local scalar variables and memory stores and
4950 loads (global variables, structures, arrays, etc). This flag is
4951 enabled by default at @option{-O2} and higher.
4954 Perform dead code elimination (DCE) on trees. This flag is enabled by
4955 default at @option{-O} and higher.
4957 @item -ftree-dominator-opts
4958 Perform dead code elimination (DCE) on trees. This flag is enabled by
4959 default at @option{-O} and higher.
4962 Perform loop header copying on trees. This is beneficial since it increases
4963 effectiveness of code motion optimizations. It also saves one jump. This flag
4964 is enabled by default at @option{-O} and higher. It is not enabled
4965 for @option{-Os}, since it usually increases code size.
4967 @item -ftree-loop-optimize
4968 Perform loop optimizations on trees. This flag is enabled by default
4969 at @option{-O} and higher.
4971 @item -ftree-loop-linear
4972 Perform linear loop transformations on tree. This flag can improve cache
4973 performance and allow further loop optimizations to take place.
4975 @item -ftree-loop-im
4976 Perform loop invariant motion on trees. This pass moves only invariants that
4977 would be hard to handle at RTL level (function calls, operations that expand to
4978 nontrivial sequences of insns). With @option{-funswitch-loops} it also moves
4979 operands of conditions that are invariant out of the loop, so that we can use
4980 just trivial invariantness analysis in loop unswitching. The pass also includes
4983 @item -ftree-loop-ivcanon
4984 Create a canonical counter for number of iterations in the loop for that
4985 determining number of iterations requires complicated analysis. Later
4986 optimizations then may determine the number easily. Useful especially
4987 in connection with unrolling.
4990 Perform induction variable optimizations (strength reduction, induction
4991 variable merging and induction variable elimination) on trees.
4994 Perform scalar replacement of aggregates. This pass replaces structure
4995 references with scalars to prevent committing structures to memory too
4996 early. This flag is enabled by default at @option{-O} and higher.
4998 @item -ftree-copyrename
4999 Perform copy renaming on trees. This pass attempts to rename compiler
5000 temporaries to other variables at copy locations, usually resulting in
5001 variable names which more closely resemble the original variables. This flag
5002 is enabled by default at @option{-O} and higher.
5005 Perform temporary expression replacement during the SSA->normal phase. Single
5006 use/single def temporaries are replaced at their use location with their
5007 defining expression. This results in non-GIMPLE code, but gives the expanders
5008 much more complex trees to work on resulting in better RTL generation. This is
5009 enabled by default at @option{-O} and higher.
5012 Perform live range splitting during the SSA->normal phase. Distinct live
5013 ranges of a variable are split into unique variables, allowing for better
5014 optimization later. This is enabled by default at @option{-O} and higher.
5016 @item -ftree-vectorize
5017 Perform loop vectorization on trees.
5020 Perform Value Range Propagation on trees. This is similar to the
5021 constant propagation pass, but instead of values, ranges of values are
5022 propagated. This allows the optimizers to remove unnecessary range
5023 checks like array bound checks and null pointer checks. This is
5024 enabled by default at @option{-O2} and higher. Null pointer check
5025 elimination is only done if @option{-fdelete-null-pointer-checks} is
5030 Perform tail duplication to enlarge superblock size. This transformation
5031 simplifies the control flow of the function allowing other optimizations to do
5034 @item -funroll-loops
5035 @opindex funroll-loops
5036 Unroll loops whose number of iterations can be determined at compile
5037 time or upon entry to the loop. @option{-funroll-loops} implies both
5038 @option{-fstrength-reduce} and @option{-frerun-cse-after-loop}. This
5039 option makes code larger, and may or may not make it run faster.
5041 @item -funroll-all-loops
5042 @opindex funroll-all-loops
5043 Unroll all loops, even if their number of iterations is uncertain when
5044 the loop is entered. This usually makes programs run more slowly.
5045 @option{-funroll-all-loops} implies the same options as
5046 @option{-funroll-loops},
5048 @item -fsplit-ivs-in-unroller
5049 @opindex -fsplit-ivs-in-unroller
5050 Enables expressing of values of induction variables in later iterations
5051 of the unrolled loop using the value in the first iteration. This breaks
5052 long dependency chains, thus improving efficiency of the scheduling passes.
5054 Combination of @option{-fweb} and CSE is often sufficient to obtain the
5055 same effect. However in cases the loop body is more complicated than
5056 a single basic block, this is not reliable. It also does not work at all
5057 on some of the architectures due to restrictions in the CSE pass.
5059 This optimization is enabled by default.
5061 @item -fvariable-expansion-in-unroller
5062 @opindex -fvariable-expansion-in-unroller
5063 With this option, the compiler will create multiple copies of some
5064 local variables when unrolling a loop which can result in superior code.
5066 @item -fprefetch-loop-arrays
5067 @opindex fprefetch-loop-arrays
5068 If supported by the target machine, generate instructions to prefetch
5069 memory to improve the performance of loops that access large arrays.
5071 These options may generate better or worse code; results are highly
5072 dependent on the structure of loops within the source code.
5075 @itemx -fno-peephole2
5076 @opindex fno-peephole
5077 @opindex fno-peephole2
5078 Disable any machine-specific peephole optimizations. The difference
5079 between @option{-fno-peephole} and @option{-fno-peephole2} is in how they
5080 are implemented in the compiler; some targets use one, some use the
5081 other, a few use both.
5083 @option{-fpeephole} is enabled by default.
5084 @option{-fpeephole2} enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5086 @item -fno-guess-branch-probability
5087 @opindex fno-guess-branch-probability
5088 Do not guess branch probabilities using heuristics.
5090 GCC will use heuristics to guess branch probabilities if they are
5091 not provided by profiling feedback (@option{-fprofile-arcs}). These
5092 heuristics are based on the control flow graph. If some branch probabilities
5093 are specified by @samp{__builtin_expect}, then the heuristics will be
5094 used to guess branch probabilities for the rest of the control flow graph,
5095 taking the @samp{__builtin_expect} info into account. The interactions
5096 between the heuristics and @samp{__builtin_expect} can be complex, and in
5097 some cases, it may be useful to disable the heuristics so that the effects
5098 of @samp{__builtin_expect} are easier to understand.
5100 The default is @option{-fguess-branch-probability} at levels
5101 @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
5103 @item -freorder-blocks
5104 @opindex freorder-blocks
5105 Reorder basic blocks in the compiled function in order to reduce number of
5106 taken branches and improve code locality.
5108 Enabled at levels @option{-O2}, @option{-O3}.
5110 @item -freorder-blocks-and-partition
5111 @opindex freorder-blocks-and-partition
5112 In addition to reordering basic blocks in the compiled function, in order
5113 to reduce number of taken branches, partitions hot and cold basic blocks
5114 into separate sections of the assembly and .o files, to improve
5115 paging and cache locality performance.
5117 This optimization is automatically turned off in the presence of
5118 exception handling, for linkonce sections, for functions with a user-defined
5119 section attribute and on any architecture that does not support named
5122 @item -freorder-functions
5123 @opindex freorder-functions
5124 Reorder functions in the object file in order to
5125 improve code locality. This is implemented by using special
5126 subsections @code{.text.hot} for most frequently executed functions and
5127 @code{.text.unlikely} for unlikely executed functions. Reordering is done by
5128 the linker so object file format must support named sections and linker must
5129 place them in a reasonable way.
5131 Also profile feedback must be available in to make this option effective. See
5132 @option{-fprofile-arcs} for details.
5134 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5136 @item -fstrict-aliasing
5137 @opindex fstrict-aliasing
5138 Allows the compiler to assume the strictest aliasing rules applicable to
5139 the language being compiled. For C (and C++), this activates
5140 optimizations based on the type of expressions. In particular, an
5141 object of one type is assumed never to reside at the same address as an
5142 object of a different type, unless the types are almost the same. For
5143 example, an @code{unsigned int} can alias an @code{int}, but not a
5144 @code{void*} or a @code{double}. A character type may alias any other
5147 Pay special attention to code like this:
5160 The practice of reading from a different union member than the one most
5161 recently written to (called ``type-punning'') is common. Even with
5162 @option{-fstrict-aliasing}, type-punning is allowed, provided the memory
5163 is accessed through the union type. So, the code above will work as
5164 expected. However, this code might not:
5175 Every language that wishes to perform language-specific alias analysis
5176 should define a function that computes, given an @code{tree}
5177 node, an alias set for the node. Nodes in different alias sets are not
5178 allowed to alias. For an example, see the C front-end function
5179 @code{c_get_alias_set}.
5181 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5183 @item -falign-functions
5184 @itemx -falign-functions=@var{n}
5185 @opindex falign-functions
5186 Align the start of functions to the next power-of-two greater than
5187 @var{n}, skipping up to @var{n} bytes. For instance,
5188 @option{-falign-functions=32} aligns functions to the next 32-byte
5189 boundary, but @option{-falign-functions=24} would align to the next
5190 32-byte boundary only if this can be done by skipping 23 bytes or less.
5192 @option{-fno-align-functions} and @option{-falign-functions=1} are
5193 equivalent and mean that functions will not be aligned.
5195 Some assemblers only support this flag when @var{n} is a power of two;
5196 in that case, it is rounded up.
5198 If @var{n} is not specified or is zero, use a machine-dependent default.
5200 Enabled at levels @option{-O2}, @option{-O3}.
5202 @item -falign-labels
5203 @itemx -falign-labels=@var{n}
5204 @opindex falign-labels
5205 Align all branch targets to a power-of-two boundary, skipping up to
5206 @var{n} bytes like @option{-falign-functions}. This option can easily
5207 make code slower, because it must insert dummy operations for when the
5208 branch target is reached in the usual flow of the code.
5210 @option{-fno-align-labels} and @option{-falign-labels=1} are
5211 equivalent and mean that labels will not be aligned.
5213 If @option{-falign-loops} or @option{-falign-jumps} are applicable and
5214 are greater than this value, then their values are used instead.
5216 If @var{n} is not specified or is zero, use a machine-dependent default
5217 which is very likely to be @samp{1}, meaning no alignment.
5219 Enabled at levels @option{-O2}, @option{-O3}.
5222 @itemx -falign-loops=@var{n}
5223 @opindex falign-loops
5224 Align loops to a power-of-two boundary, skipping up to @var{n} bytes
5225 like @option{-falign-functions}. The hope is that the loop will be
5226 executed many times, which will make up for any execution of the dummy
5229 @option{-fno-align-loops} and @option{-falign-loops=1} are
5230 equivalent and mean that loops will not be aligned.
5232 If @var{n} is not specified or is zero, use a machine-dependent default.
5234 Enabled at levels @option{-O2}, @option{-O3}.
5237 @itemx -falign-jumps=@var{n}
5238 @opindex falign-jumps
5239 Align branch targets to a power-of-two boundary, for branch targets
5240 where the targets can only be reached by jumping, skipping up to @var{n}
5241 bytes like @option{-falign-functions}. In this case, no dummy operations
5244 @option{-fno-align-jumps} and @option{-falign-jumps=1} are
5245 equivalent and mean that loops will not be aligned.
5247 If @var{n} is not specified or is zero, use a machine-dependent default.
5249 Enabled at levels @option{-O2}, @option{-O3}.
5251 @item -funit-at-a-time
5252 @opindex funit-at-a-time
5253 Parse the whole compilation unit before starting to produce code.
5254 This allows some extra optimizations to take place but consumes
5255 more memory (in general). There are some compatibility issues
5256 with @emph{unit-at-at-time} mode:
5259 enabling @emph{unit-at-a-time} mode may change the order
5260 in which functions, variables, and top-level @code{asm} statements
5261 are emitted, and will likely break code relying on some particular
5262 ordering. The majority of such top-level @code{asm} statements,
5263 though, can be replaced by @code{section} attributes.
5266 @emph{unit-at-a-time} mode removes unreferenced static variables
5267 and functions. This may result in undefined references
5268 when an @code{asm} statement refers directly to variables or functions
5269 that are otherwise unused. In that case either the variable/function
5270 shall be listed as an operand of the @code{asm} statement operand or,
5271 in the case of top-level @code{asm} statements the attribute @code{used}
5272 shall be used on the declaration.
5275 Static functions now can use non-standard passing conventions that
5276 may break @code{asm} statements calling functions directly. Again,
5277 attribute @code{used} will prevent this behavior.
5280 As a temporary workaround, @option{-fno-unit-at-a-time} can be used,
5281 but this scheme may not be supported by future releases of GCC@.
5283 Enabled at levels @option{-O2}, @option{-O3}.
5287 Constructs webs as commonly used for register allocation purposes and assign
5288 each web individual pseudo register. This allows the register allocation pass
5289 to operate on pseudos directly, but also strengthens several other optimization
5290 passes, such as CSE, loop optimizer and trivial dead code remover. It can,
5291 however, make debugging impossible, since variables will no longer stay in a
5294 Enabled by default with @option{-funroll-loops}.
5296 @item -fwhole-program
5297 @opindex fwhole-program
5298 Assume that the current compilation unit represents whole program being
5299 compiled. All public functions and variables with the exception of @code{main}
5300 and those merged by attribute @code{externally_visible} become static functions
5301 and in a affect gets more aggressively optimized by interprocedural optimizers.
5302 While this option is equivalent to proper use of @code{static} keyword for
5303 programs consisting of single file, in combination with option
5304 @option{--combine} this flag can be used to compile most of smaller scale C
5305 programs since the functions and variables become local for the whole combined
5306 compilation unit, not for the single source file itself.
5309 @item -fno-cprop-registers
5310 @opindex fno-cprop-registers
5311 After register allocation and post-register allocation instruction splitting,
5312 we perform a copy-propagation pass to try to reduce scheduling dependencies
5313 and occasionally eliminate the copy.
5315 Disabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
5317 @item -fprofile-generate
5318 @opindex fprofile-generate
5320 Enable options usually used for instrumenting application to produce
5321 profile useful for later recompilation with profile feedback based
5322 optimization. You must use @option{-fprofile-generate} both when
5323 compiling and when linking your program.
5325 The following options are enabled: @code{-fprofile-arcs}, @code{-fprofile-values}, @code{-fvpt}.
5328 @opindex fprofile-use
5329 Enable profile feedback directed optimizations, and optimizations
5330 generally profitable only with profile feedback available.
5332 The following options are enabled: @code{-fbranch-probabilities},
5333 @code{-fvpt}, @code{-funroll-loops}, @code{-fpeel-loops}, @code{-ftracer}.
5337 The following options control compiler behavior regarding floating
5338 point arithmetic. These options trade off between speed and
5339 correctness. All must be specifically enabled.
5343 @opindex ffloat-store
5344 Do not store floating point variables in registers, and inhibit other
5345 options that might change whether a floating point value is taken from a
5348 @cindex floating point precision
5349 This option prevents undesirable excess precision on machines such as
5350 the 68000 where the floating registers (of the 68881) keep more
5351 precision than a @code{double} is supposed to have. Similarly for the
5352 x86 architecture. For most programs, the excess precision does only
5353 good, but a few programs rely on the precise definition of IEEE floating
5354 point. Use @option{-ffloat-store} for such programs, after modifying
5355 them to store all pertinent intermediate computations into variables.
5359 Sets @option{-fno-math-errno}, @option{-funsafe-math-optimizations}, @*
5360 @option{-fno-trapping-math}, @option{-ffinite-math-only},
5361 @option{-fno-rounding-math}, @option{-fno-signaling-nans}
5362 and @option{fcx-limited-range}.
5364 This option causes the preprocessor macro @code{__FAST_MATH__} to be defined.
5366 This option should never be turned on by any @option{-O} option since
5367 it can result in incorrect output for programs which depend on
5368 an exact implementation of IEEE or ISO rules/specifications for
5371 @item -fno-math-errno
5372 @opindex fno-math-errno
5373 Do not set ERRNO after calling math functions that are executed
5374 with a single instruction, e.g., sqrt. A program that relies on
5375 IEEE exceptions for math error handling may want to use this flag
5376 for speed while maintaining IEEE arithmetic compatibility.
5378 This option should never be turned on by any @option{-O} option since
5379 it can result in incorrect output for programs which depend on
5380 an exact implementation of IEEE or ISO rules/specifications for
5383 The default is @option{-fmath-errno}.
5385 On Darwin systems, the math library never sets @code{errno}. There is therefore
5386 no reason for the compiler to consider the possibility that it might,
5387 and @option{-fno-math-errno} is the default.
5389 @item -funsafe-math-optimizations
5390 @opindex funsafe-math-optimizations
5391 Allow optimizations for floating-point arithmetic that (a) assume
5392 that arguments and results are valid and (b) may violate IEEE or
5393 ANSI standards. When used at link-time, it may include libraries
5394 or startup files that change the default FPU control word or other
5395 similar optimizations.
5397 This option should never be turned on by any @option{-O} option since
5398 it can result in incorrect output for programs which depend on
5399 an exact implementation of IEEE or ISO rules/specifications for
5402 The default is @option{-fno-unsafe-math-optimizations}.
5404 @item -ffinite-math-only
5405 @opindex ffinite-math-only
5406 Allow optimizations for floating-point arithmetic that assume
5407 that arguments and results are not NaNs or +-Infs.
5409 This option should never be turned on by any @option{-O} option since
5410 it can result in incorrect output for programs which depend on
5411 an exact implementation of IEEE or ISO rules/specifications.
5413 The default is @option{-fno-finite-math-only}.
5415 @item -fno-trapping-math
5416 @opindex fno-trapping-math
5417 Compile code assuming that floating-point operations cannot generate
5418 user-visible traps. These traps include division by zero, overflow,
5419 underflow, inexact result and invalid operation. This option implies
5420 @option{-fno-signaling-nans}. Setting this option may allow faster
5421 code if one relies on ``non-stop'' IEEE arithmetic, for example.
5423 This option should never be turned on by any @option{-O} option since
5424 it can result in incorrect output for programs which depend on
5425 an exact implementation of IEEE or ISO rules/specifications for
5428 The default is @option{-ftrapping-math}.
5430 @item -frounding-math
5431 @opindex frounding-math
5432 Disable transformations and optimizations that assume default floating
5433 point rounding behavior. This is round-to-zero for all floating point
5434 to integer conversions, and round-to-nearest for all other arithmetic
5435 truncations. This option should be specified for programs that change
5436 the FP rounding mode dynamically, or that may be executed with a
5437 non-default rounding mode. This option disables constant folding of
5438 floating point expressions at compile-time (which may be affected by
5439 rounding mode) and arithmetic transformations that are unsafe in the
5440 presence of sign-dependent rounding modes.
5442 The default is @option{-fno-rounding-math}.
5444 This option is experimental and does not currently guarantee to
5445 disable all GCC optimizations that are affected by rounding mode.
5446 Future versions of GCC may provide finer control of this setting
5447 using C99's @code{FENV_ACCESS} pragma. This command line option
5448 will be used to specify the default state for @code{FENV_ACCESS}.
5450 @item -fsignaling-nans
5451 @opindex fsignaling-nans
5452 Compile code assuming that IEEE signaling NaNs may generate user-visible
5453 traps during floating-point operations. Setting this option disables
5454 optimizations that may change the number of exceptions visible with
5455 signaling NaNs. This option implies @option{-ftrapping-math}.
5457 This option causes the preprocessor macro @code{__SUPPORT_SNAN__} to
5460 The default is @option{-fno-signaling-nans}.
5462 This option is experimental and does not currently guarantee to
5463 disable all GCC optimizations that affect signaling NaN behavior.
5465 @item -fsingle-precision-constant
5466 @opindex fsingle-precision-constant
5467 Treat floating point constant as single precision constant instead of
5468 implicitly converting it to double precision constant.
5470 @item -fcx-limited-range
5471 @itemx -fno-cx-limited-range
5472 @opindex fcx-limited-range
5473 @opindex fno-cx-limited-range
5474 When enabled, this option states that a range reduction step is not
5475 needed when performing complex division. The default is
5476 @option{-fno-cx-limited-range}, but is enabled by @option{-ffast-math}.
5478 This option controls the default setting of the ISO C99
5479 @code{CX_LIMITED_RANGE} pragma. Nevertheless, the option applies to
5484 The following options control optimizations that may improve
5485 performance, but are not enabled by any @option{-O} options. This
5486 section includes experimental options that may produce broken code.
5489 @item -fbranch-probabilities
5490 @opindex fbranch-probabilities
5491 After running a program compiled with @option{-fprofile-arcs}
5492 (@pxref{Debugging Options,, Options for Debugging Your Program or
5493 @command{gcc}}), you can compile it a second time using
5494 @option{-fbranch-probabilities}, to improve optimizations based on
5495 the number of times each branch was taken. When the program
5496 compiled with @option{-fprofile-arcs} exits it saves arc execution
5497 counts to a file called @file{@var{sourcename}.gcda} for each source
5498 file The information in this data file is very dependent on the
5499 structure of the generated code, so you must use the same source code
5500 and the same optimization options for both compilations.
5502 With @option{-fbranch-probabilities}, GCC puts a
5503 @samp{REG_BR_PROB} note on each @samp{JUMP_INSN} and @samp{CALL_INSN}.
5504 These can be used to improve optimization. Currently, they are only
5505 used in one place: in @file{reorg.c}, instead of guessing which path a
5506 branch is mostly to take, the @samp{REG_BR_PROB} values are used to
5507 exactly determine which path is taken more often.
5509 @item -fprofile-values
5510 @opindex fprofile-values
5511 If combined with @option{-fprofile-arcs}, it adds code so that some
5512 data about values of expressions in the program is gathered.
5514 With @option{-fbranch-probabilities}, it reads back the data gathered
5515 from profiling values of expressions and adds @samp{REG_VALUE_PROFILE}
5516 notes to instructions for their later usage in optimizations.
5518 Enabled with @option{-fprofile-generate} and @option{-fprofile-use}.
5522 If combined with @option{-fprofile-arcs}, it instructs the compiler to add
5523 a code to gather information about values of expressions.
5525 With @option{-fbranch-probabilities}, it reads back the data gathered
5526 and actually performs the optimizations based on them.
5527 Currently the optimizations include specialization of division operation
5528 using the knowledge about the value of the denominator.
5530 @item -fspeculative-prefetching
5531 @opindex fspeculative-prefetching
5532 If combined with @option{-fprofile-arcs}, it instructs the compiler to add
5533 a code to gather information about addresses of memory references in the
5536 With @option{-fbranch-probabilities}, it reads back the data gathered
5537 and issues prefetch instructions according to them. In addition to the opportunities
5538 noticed by @option{-fprefetch-loop-arrays}, it also notices more complicated
5539 memory access patterns---for example accesses to the data stored in linked
5540 list whose elements are usually allocated sequentially.
5542 In order to prevent issuing double prefetches, usage of
5543 @option{-fspeculative-prefetching} implies @option{-fno-prefetch-loop-arrays}.
5545 Enabled with @option{-fprofile-generate} and @option{-fprofile-use}.
5547 @item -frename-registers
5548 @opindex frename-registers
5549 Attempt to avoid false dependencies in scheduled code by making use
5550 of registers left over after register allocation. This optimization
5551 will most benefit processors with lots of registers. Depending on the
5552 debug information format adopted by the target, however, it can
5553 make debugging impossible, since variables will no longer stay in
5554 a ``home register''.
5556 Enabled by default with @option{-funroll-loops}.
5560 Perform tail duplication to enlarge superblock size. This transformation
5561 simplifies the control flow of the function allowing other optimizations to do
5564 Enabled with @option{-fprofile-use}.
5566 @item -funroll-loops
5567 @opindex funroll-loops
5568 Unroll loops whose number of iterations can be determined at compile time or
5569 upon entry to the loop. @option{-funroll-loops} implies
5570 @option{-frerun-cse-after-loop}, @option{-fweb} and @option{-frename-registers}.
5571 It also turns on complete loop peeling (i.e.@: complete removal of loops with
5572 small constant number of iterations). This option makes code larger, and may
5573 or may not make it run faster.
5575 Enabled with @option{-fprofile-use}.
5577 @item -funroll-all-loops
5578 @opindex funroll-all-loops
5579 Unroll all loops, even if their number of iterations is uncertain when
5580 the loop is entered. This usually makes programs run more slowly.
5581 @option{-funroll-all-loops} implies the same options as
5582 @option{-funroll-loops}.
5585 @opindex fpeel-loops
5586 Peels the loops for that there is enough information that they do not
5587 roll much (from profile feedback). It also turns on complete loop peeling
5588 (i.e.@: complete removal of loops with small constant number of iterations).
5590 Enabled with @option{-fprofile-use}.
5592 @item -fmove-loop-invariants
5593 @opindex fmove-loop-invariants
5594 Enables the loop invariant motion pass in the new loop optimizer. Enabled
5595 at level @option{-O1}
5597 @item -funswitch-loops
5598 @opindex funswitch-loops
5599 Move branches with loop invariant conditions out of the loop, with duplicates
5600 of the loop on both branches (modified according to result of the condition).
5602 @item -fprefetch-loop-arrays
5603 @opindex fprefetch-loop-arrays
5604 If supported by the target machine, generate instructions to prefetch
5605 memory to improve the performance of loops that access large arrays.
5607 Disabled at level @option{-Os}.
5609 @item -ffunction-sections
5610 @itemx -fdata-sections
5611 @opindex ffunction-sections
5612 @opindex fdata-sections
5613 Place each function or data item into its own section in the output
5614 file if the target supports arbitrary sections. The name of the
5615 function or the name of the data item determines the section's name
5618 Use these options on systems where the linker can perform optimizations
5619 to improve locality of reference in the instruction space. Most systems
5620 using the ELF object format and SPARC processors running Solaris 2 have
5621 linkers with such optimizations. AIX may have these optimizations in
5624 Only use these options when there are significant benefits from doing
5625 so. When you specify these options, the assembler and linker will
5626 create larger object and executable files and will also be slower.
5627 You will not be able to use @code{gprof} on all systems if you
5628 specify this option and you may have problems with debugging if
5629 you specify both this option and @option{-g}.
5631 @item -fbranch-target-load-optimize
5632 @opindex fbranch-target-load-optimize
5633 Perform branch target register load optimization before prologue / epilogue
5635 The use of target registers can typically be exposed only during reload,
5636 thus hoisting loads out of loops and doing inter-block scheduling needs
5637 a separate optimization pass.
5639 @item -fbranch-target-load-optimize2
5640 @opindex fbranch-target-load-optimize2
5641 Perform branch target register load optimization after prologue / epilogue
5644 @item -fbtr-bb-exclusive
5645 @opindex fbtr-bb-exclusive
5646 When performing branch target register load optimization, don't reuse
5647 branch target registers in within any basic block.
5649 @item --param @var{name}=@var{value}
5651 In some places, GCC uses various constants to control the amount of
5652 optimization that is done. For example, GCC will not inline functions
5653 that contain more that a certain number of instructions. You can
5654 control some of these constants on the command-line using the
5655 @option{--param} option.
5657 The names of specific parameters, and the meaning of the values, are
5658 tied to the internals of the compiler, and are subject to change
5659 without notice in future releases.
5661 In each case, the @var{value} is an integer. The allowable choices for
5662 @var{name} are given in the following table:
5665 @item salias-max-implicit-fields
5666 The maximum number of fields in a variable without direct
5667 structure accesses for which structure aliasing will consider trying
5668 to track each field. The default is 5
5670 @item sra-max-structure-size
5671 The maximum structure size, in bytes, at which the scalar replacement
5672 of aggregates (SRA) optimization will perform block copies. The
5673 default value, 0, implies that GCC will select the most appropriate
5676 @item sra-field-structure-ratio
5677 The threshold ratio (as a percentage) between instantiated fields and
5678 the complete structure size. We say that if the ratio of the number
5679 of bytes in instantiated fields to the number of bytes in the complete
5680 structure exceeds this parameter, then block copies are not used. The
5683 @item max-crossjump-edges
5684 The maximum number of incoming edges to consider for crossjumping.
5685 The algorithm used by @option{-fcrossjumping} is @math{O(N^2)} in
5686 the number of edges incoming to each block. Increasing values mean
5687 more aggressive optimization, making the compile time increase with
5688 probably small improvement in executable size.
5690 @item min-crossjump-insns
5691 The minimum number of instructions which must be matched at the end
5692 of two blocks before crossjumping will be performed on them. This
5693 value is ignored in the case where all instructions in the block being
5694 crossjumped from are matched. The default value is 5.
5696 @item max-goto-duplication-insns
5697 The maximum number of instructions to duplicate to a block that jumps
5698 to a computed goto. To avoid @math{O(N^2)} behavior in a number of
5699 passes, GCC factors computed gotos early in the compilation process,
5700 and unfactors them as late as possible. Only computed jumps at the
5701 end of a basic blocks with no more than max-goto-duplication-insns are
5702 unfactored. The default value is 8.
5704 @item max-delay-slot-insn-search
5705 The maximum number of instructions to consider when looking for an
5706 instruction to fill a delay slot. If more than this arbitrary number of
5707 instructions is searched, the time savings from filling the delay slot
5708 will be minimal so stop searching. Increasing values mean more
5709 aggressive optimization, making the compile time increase with probably
5710 small improvement in executable run time.
5712 @item max-delay-slot-live-search
5713 When trying to fill delay slots, the maximum number of instructions to
5714 consider when searching for a block with valid live register
5715 information. Increasing this arbitrarily chosen value means more
5716 aggressive optimization, increasing the compile time. This parameter
5717 should be removed when the delay slot code is rewritten to maintain the
5720 @item max-gcse-memory
5721 The approximate maximum amount of memory that will be allocated in
5722 order to perform the global common subexpression elimination
5723 optimization. If more memory than specified is required, the
5724 optimization will not be done.
5726 @item max-gcse-passes
5727 The maximum number of passes of GCSE to run. The default is 1.
5729 @item max-pending-list-length
5730 The maximum number of pending dependencies scheduling will allow
5731 before flushing the current state and starting over. Large functions
5732 with few branches or calls can create excessively large lists which
5733 needlessly consume memory and resources.
5735 @item max-inline-insns-single
5736 Several parameters control the tree inliner used in gcc.
5737 This number sets the maximum number of instructions (counted in GCC's
5738 internal representation) in a single function that the tree inliner
5739 will consider for inlining. This only affects functions declared
5740 inline and methods implemented in a class declaration (C++).
5741 The default value is 450.
5743 @item max-inline-insns-auto
5744 When you use @option{-finline-functions} (included in @option{-O3}),
5745 a lot of functions that would otherwise not be considered for inlining
5746 by the compiler will be investigated. To those functions, a different
5747 (more restrictive) limit compared to functions declared inline can
5749 The default value is 90.
5751 @item large-function-insns
5752 The limit specifying really large functions. For functions larger than this
5753 limit after inlining inlining is constrained by
5754 @option{--param large-function-growth}. This parameter is useful primarily
5755 to avoid extreme compilation time caused by non-linear algorithms used by the
5757 This parameter is ignored when @option{-funit-at-a-time} is not used.
5758 The default value is 2700.
5760 @item large-function-growth
5761 Specifies maximal growth of large function caused by inlining in percents.
5762 This parameter is ignored when @option{-funit-at-a-time} is not used.
5763 The default value is 100 which limits large function growth to 2.0 times
5766 @item inline-unit-growth
5767 Specifies maximal overall growth of the compilation unit caused by inlining.
5768 This parameter is ignored when @option{-funit-at-a-time} is not used.
5769 The default value is 50 which limits unit growth to 1.5 times the original
5772 @item max-inline-insns-recursive
5773 @itemx max-inline-insns-recursive-auto
5774 Specifies maximum number of instructions out-of-line copy of self recursive inline
5775 function can grow into by performing recursive inlining.
5777 For functions declared inline @option{--param max-inline-insns-recursive} is
5778 taken into acount. For function not declared inline, recursive inlining
5779 happens only when @option{-finline-functions} (included in @option{-O3}) is
5780 enabled and @option{--param max-inline-insns-recursive-auto} is used. The
5781 default value is 450.
5783 @item max-inline-recursive-depth
5784 @itemx max-inline-recursive-depth-auto
5785 Specifies maximum recursion depth used by the recursive inlining.
5787 For functions declared inline @option{--param max-inline-recursive-depth} is
5788 taken into acount. For function not declared inline, recursive inlining
5789 happens only when @option{-finline-functions} (included in @option{-O3}) is
5790 enabled and @option{--param max-inline-recursive-depth-auto} is used. The
5791 default value is 450.
5793 @item min-inline-recursive-probability
5794 Recursive inlining is profitable only for function having deep recursion
5795 in average and can hurt for function having little recursion depth by
5796 increasing the prologue size or complexity of function body to other
5799 When profile feedback is available (see @option{-fprofile-generate}) the actual
5800 recursion depth can be guessed from probability that function will recurse via
5801 given call expression. This parameter limits inlining only to call expression
5802 whose probability exceeds given threshold (in percents). The default value is
5805 @item inline-call-cost
5806 Specify cost of call instruction relative to simple arithmetics operations
5807 (having cost of 1). Increasing this cost disqualifies inlining of non-leaf
5808 functions and at the same time increases size of leaf function that is believed to
5809 reduce function size by being inlined. In effect it increases amount of
5810 inlining for code having large abstraction penalty (many functions that just
5811 pass the arguments to other functions) and decrease inlining for code with low
5812 abstraction penalty. The default value is 16.
5814 @item max-unrolled-insns
5815 The maximum number of instructions that a loop should have if that loop
5816 is unrolled, and if the loop is unrolled, it determines how many times
5817 the loop code is unrolled.
5819 @item max-average-unrolled-insns
5820 The maximum number of instructions biased by probabilities of their execution
5821 that a loop should have if that loop is unrolled, and if the loop is unrolled,
5822 it determines how many times the loop code is unrolled.
5824 @item max-unroll-times
5825 The maximum number of unrollings of a single loop.
5827 @item max-peeled-insns
5828 The maximum number of instructions that a loop should have if that loop
5829 is peeled, and if the loop is peeled, it determines how many times
5830 the loop code is peeled.
5832 @item max-peel-times
5833 The maximum number of peelings of a single loop.
5835 @item max-completely-peeled-insns
5836 The maximum number of insns of a completely peeled loop.
5838 @item max-completely-peel-times
5839 The maximum number of iterations of a loop to be suitable for complete peeling.
5841 @item max-unswitch-insns
5842 The maximum number of insns of an unswitched loop.
5844 @item max-unswitch-level
5845 The maximum number of branches unswitched in a single loop.
5848 The minimum cost of an expensive expression in the loop invariant motion.
5850 @item iv-consider-all-candidates-bound
5851 Bound on number of candidates for induction variables below that
5852 all candidates are considered for each use in induction variable
5853 optimizations. Only the most relevant candidates are considered
5854 if there are more candidates, to avoid quadratic time complexity.
5856 @item iv-max-considered-uses
5857 The induction variable optimizations give up on loops that contain more
5858 induction variable uses.
5860 @item iv-always-prune-cand-set-bound
5861 If number of candidates in the set is smaller than this value,
5862 we always try to remove unnecessary ivs from the set during its
5863 optimization when a new iv is added to the set.
5865 @item scev-max-expr-size
5866 Bound on size of expressions used in the scalar evolutions analyzer.
5867 Large expressions slow the analyzer.
5869 @item max-iterations-to-track
5871 The maximum number of iterations of a loop the brute force algorithm
5872 for analysis of # of iterations of the loop tries to evaluate.
5874 @item hot-bb-count-fraction
5875 Select fraction of the maximal count of repetitions of basic block in program
5876 given basic block needs to have to be considered hot.
5878 @item hot-bb-frequency-fraction
5879 Select fraction of the maximal frequency of executions of basic block in
5880 function given basic block needs to have to be considered hot
5882 @item tracer-dynamic-coverage
5883 @itemx tracer-dynamic-coverage-feedback
5885 This value is used to limit superblock formation once the given percentage of
5886 executed instructions is covered. This limits unnecessary code size
5889 The @option{tracer-dynamic-coverage-feedback} is used only when profile
5890 feedback is available. The real profiles (as opposed to statically estimated
5891 ones) are much less balanced allowing the threshold to be larger value.
5893 @item tracer-max-code-growth
5894 Stop tail duplication once code growth has reached given percentage. This is
5895 rather hokey argument, as most of the duplicates will be eliminated later in
5896 cross jumping, so it may be set to much higher values than is the desired code
5899 @item tracer-min-branch-ratio
5901 Stop reverse growth when the reverse probability of best edge is less than this
5902 threshold (in percent).
5904 @item tracer-min-branch-ratio
5905 @itemx tracer-min-branch-ratio-feedback
5907 Stop forward growth if the best edge do have probability lower than this
5910 Similarly to @option{tracer-dynamic-coverage} two values are present, one for
5911 compilation for profile feedback and one for compilation without. The value
5912 for compilation with profile feedback needs to be more conservative (higher) in
5913 order to make tracer effective.
5915 @item max-cse-path-length
5917 Maximum number of basic blocks on path that cse considers. The default is 10.
5919 @item global-var-threshold
5921 Counts the number of function calls (@var{n}) and the number of
5922 call-clobbered variables (@var{v}). If @var{n}x@var{v} is larger than this limit, a
5923 single artificial variable will be created to represent all the
5924 call-clobbered variables at function call sites. This artificial
5925 variable will then be made to alias every call-clobbered variable.
5926 (done as @code{int * size_t} on the host machine; beware overflow).
5928 @item max-aliased-vops
5930 Maximum number of virtual operands allowed to represent aliases
5931 before triggering the alias grouping heuristic. Alias grouping
5932 reduces compile times and memory consumption needed for aliasing at
5933 the expense of precision loss in alias information.
5935 @item ggc-min-expand
5937 GCC uses a garbage collector to manage its own memory allocation. This
5938 parameter specifies the minimum percentage by which the garbage
5939 collector's heap should be allowed to expand between collections.
5940 Tuning this may improve compilation speed; it has no effect on code
5943 The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when
5944 RAM >= 1GB@. If @code{getrlimit} is available, the notion of "RAM" is
5945 the smallest of actual RAM and @code{RLIMIT_DATA} or @code{RLIMIT_AS}. If
5946 GCC is not able to calculate RAM on a particular platform, the lower
5947 bound of 30% is used. Setting this parameter and
5948 @option{ggc-min-heapsize} to zero causes a full collection to occur at
5949 every opportunity. This is extremely slow, but can be useful for
5952 @item ggc-min-heapsize
5954 Minimum size of the garbage collector's heap before it begins bothering
5955 to collect garbage. The first collection occurs after the heap expands
5956 by @option{ggc-min-expand}% beyond @option{ggc-min-heapsize}. Again,
5957 tuning this may improve compilation speed, and has no effect on code
5960 The default is the smaller of RAM/8, RLIMIT_RSS, or a limit which
5961 tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but
5962 with a lower bound of 4096 (four megabytes) and an upper bound of
5963 131072 (128 megabytes). If GCC is not able to calculate RAM on a
5964 particular platform, the lower bound is used. Setting this parameter
5965 very large effectively disables garbage collection. Setting this
5966 parameter and @option{ggc-min-expand} to zero causes a full collection
5967 to occur at every opportunity.
5969 @item max-reload-search-insns
5970 The maximum number of instruction reload should look backward for equivalent
5971 register. Increasing values mean more aggressive optimization, making the
5972 compile time increase with probably slightly better performance. The default
5975 @item max-cselib-memory-location
5976 The maximum number of memory locations cselib should take into acount.
5977 Increasing values mean more aggressive optimization, making the compile time
5978 increase with probably slightly better performance. The default value is 500.
5980 @item reorder-blocks-duplicate
5981 @itemx reorder-blocks-duplicate-feedback
5983 Used by basic block reordering pass to decide whether to use unconditional
5984 branch or duplicate the code on its destination. Code is duplicated when its
5985 estimated size is smaller than this value multiplied by the estimated size of
5986 unconditional jump in the hot spots of the program.
5988 The @option{reorder-block-duplicate-feedback} is used only when profile
5989 feedback is available and may be set to higher values than
5990 @option{reorder-block-duplicate} since information about the hot spots is more
5993 @item max-sched-region-blocks
5994 The maximum number of blocks in a region to be considered for
5995 interblock scheduling. The default value is 10.
5997 @item max-sched-region-insns
5998 The maximum number of insns in a region to be considered for
5999 interblock scheduling. The default value is 100.
6001 @item max-last-value-rtl
6003 The maximum size measured as number of RTLs that can be recorded in an expression
6004 in combiner for a pseudo register as last known value of that register. The default
6007 @item integer-share-limit
6008 Small integer constants can use a shared data structure, reducing the
6009 compiler's memory usage and increasing its speed. This sets the maximum
6010 value of a shared integer constant's. The default value is 256.
6012 @item min-virtual-mappings
6013 Specifies the minimum number of virtual mappings in the incremental
6014 SSA updater that should be registered to trigger the virtual mappings
6015 heuristic defined by virtual-mappings-ratio. The default value is
6018 @item virtual-mappings-ratio
6019 If the number of virtual mappings is virtual-mappings-ratio bigger
6020 than the number of virtual symbols to be updated, then the incremental
6021 SSA updater switches to a full update for those symbols. The default
6027 @node Preprocessor Options
6028 @section Options Controlling the Preprocessor
6029 @cindex preprocessor options
6030 @cindex options, preprocessor
6032 These options control the C preprocessor, which is run on each C source
6033 file before actual compilation.
6035 If you use the @option{-E} option, nothing is done except preprocessing.
6036 Some of these options make sense only together with @option{-E} because
6037 they cause the preprocessor output to be unsuitable for actual
6042 You can use @option{-Wp,@var{option}} to bypass the compiler driver
6043 and pass @var{option} directly through to the preprocessor. If
6044 @var{option} contains commas, it is split into multiple options at the
6045 commas. However, many options are modified, translated or interpreted
6046 by the compiler driver before being passed to the preprocessor, and
6047 @option{-Wp} forcibly bypasses this phase. The preprocessor's direct
6048 interface is undocumented and subject to change, so whenever possible
6049 you should avoid using @option{-Wp} and let the driver handle the
6052 @item -Xpreprocessor @var{option}
6053 @opindex preprocessor
6054 Pass @var{option} as an option to the preprocessor. You can use this to
6055 supply system-specific preprocessor options which GCC does not know how to
6058 If you want to pass an option that takes an argument, you must use
6059 @option{-Xpreprocessor} twice, once for the option and once for the argument.
6062 @include cppopts.texi
6064 @node Assembler Options
6065 @section Passing Options to the Assembler
6067 @c prevent bad page break with this line
6068 You can pass options to the assembler.
6071 @item -Wa,@var{option}
6073 Pass @var{option} as an option to the assembler. If @var{option}
6074 contains commas, it is split into multiple options at the commas.
6076 @item -Xassembler @var{option}
6078 Pass @var{option} as an option to the assembler. You can use this to
6079 supply system-specific assembler options which GCC does not know how to
6082 If you want to pass an option that takes an argument, you must use
6083 @option{-Xassembler} twice, once for the option and once for the argument.
6088 @section Options for Linking
6089 @cindex link options
6090 @cindex options, linking
6092 These options come into play when the compiler links object files into
6093 an executable output file. They are meaningless if the compiler is
6094 not doing a link step.
6098 @item @var{object-file-name}
6099 A file name that does not end in a special recognized suffix is
6100 considered to name an object file or library. (Object files are
6101 distinguished from libraries by the linker according to the file
6102 contents.) If linking is done, these object files are used as input
6111 If any of these options is used, then the linker is not run, and
6112 object file names should not be used as arguments. @xref{Overall
6116 @item -l@var{library}
6117 @itemx -l @var{library}
6119 Search the library named @var{library} when linking. (The second
6120 alternative with the library as a separate argument is only for
6121 POSIX compliance and is not recommended.)
6123 It makes a difference where in the command you write this option; the
6124 linker searches and processes libraries and object files in the order they
6125 are specified. Thus, @samp{foo.o -lz bar.o} searches library @samp{z}
6126 after file @file{foo.o} but before @file{bar.o}. If @file{bar.o} refers
6127 to functions in @samp{z}, those functions may not be loaded.
6129 The linker searches a standard list of directories for the library,
6130 which is actually a file named @file{lib@var{library}.a}. The linker
6131 then uses this file as if it had been specified precisely by name.
6133 The directories searched include several standard system directories
6134 plus any that you specify with @option{-L}.
6136 Normally the files found this way are library files---archive files
6137 whose members are object files. The linker handles an archive file by
6138 scanning through it for members which define symbols that have so far
6139 been referenced but not defined. But if the file that is found is an
6140 ordinary object file, it is linked in the usual fashion. The only
6141 difference between using an @option{-l} option and specifying a file name
6142 is that @option{-l} surrounds @var{library} with @samp{lib} and @samp{.a}
6143 and searches several directories.
6147 You need this special case of the @option{-l} option in order to
6148 link an Objective-C or Objective-C++ program.
6151 @opindex nostartfiles
6152 Do not use the standard system startup files when linking.
6153 The standard system libraries are used normally, unless @option{-nostdlib}
6154 or @option{-nodefaultlibs} is used.
6156 @item -nodefaultlibs
6157 @opindex nodefaultlibs
6158 Do not use the standard system libraries when linking.
6159 Only the libraries you specify will be passed to the linker.
6160 The standard startup files are used normally, unless @option{-nostartfiles}
6161 is used. The compiler may generate calls to @code{memcmp},
6162 @code{memset}, @code{memcpy} and @code{memmove}.
6163 These entries are usually resolved by entries in
6164 libc. These entry points should be supplied through some other
6165 mechanism when this option is specified.
6169 Do not use the standard system startup files or libraries when linking.
6170 No startup files and only the libraries you specify will be passed to
6171 the linker. The compiler may generate calls to @code{memcmp}, @code{memset},
6172 @code{memcpy} and @code{memmove}.
6173 These entries are usually resolved by entries in
6174 libc. These entry points should be supplied through some other
6175 mechanism when this option is specified.
6177 @cindex @option{-lgcc}, use with @option{-nostdlib}
6178 @cindex @option{-nostdlib} and unresolved references
6179 @cindex unresolved references and @option{-nostdlib}
6180 @cindex @option{-lgcc}, use with @option{-nodefaultlibs}
6181 @cindex @option{-nodefaultlibs} and unresolved references
6182 @cindex unresolved references and @option{-nodefaultlibs}
6183 One of the standard libraries bypassed by @option{-nostdlib} and
6184 @option{-nodefaultlibs} is @file{libgcc.a}, a library of internal subroutines
6185 that GCC uses to overcome shortcomings of particular machines, or special
6186 needs for some languages.
6187 (@xref{Interface,,Interfacing to GCC Output,gccint,GNU Compiler
6188 Collection (GCC) Internals},
6189 for more discussion of @file{libgcc.a}.)
6190 In most cases, you need @file{libgcc.a} even when you want to avoid
6191 other standard libraries. In other words, when you specify @option{-nostdlib}
6192 or @option{-nodefaultlibs} you should usually specify @option{-lgcc} as well.
6193 This ensures that you have no unresolved references to internal GCC
6194 library subroutines. (For example, @samp{__main}, used to ensure C++
6195 constructors will be called; @pxref{Collect2,,@code{collect2}, gccint,
6196 GNU Compiler Collection (GCC) Internals}.)
6200 Produce a position independent executable on targets which support it.
6201 For predictable results, you must also specify the same set of options
6202 that were used to generate code (@option{-fpie}, @option{-fPIE},
6203 or model suboptions) when you specify this option.
6207 Remove all symbol table and relocation information from the executable.
6211 On systems that support dynamic linking, this prevents linking with the shared
6212 libraries. On other systems, this option has no effect.
6216 Produce a shared object which can then be linked with other objects to
6217 form an executable. Not all systems support this option. For predictable
6218 results, you must also specify the same set of options that were used to
6219 generate code (@option{-fpic}, @option{-fPIC}, or model suboptions)
6220 when you specify this option.@footnote{On some systems, @samp{gcc -shared}
6221 needs to build supplementary stub code for constructors to work. On
6222 multi-libbed systems, @samp{gcc -shared} must select the correct support
6223 libraries to link against. Failing to supply the correct flags may lead
6224 to subtle defects. Supplying them in cases where they are not necessary
6227 @item -shared-libgcc
6228 @itemx -static-libgcc
6229 @opindex shared-libgcc
6230 @opindex static-libgcc
6231 On systems that provide @file{libgcc} as a shared library, these options
6232 force the use of either the shared or static version respectively.
6233 If no shared version of @file{libgcc} was built when the compiler was
6234 configured, these options have no effect.
6236 There are several situations in which an application should use the
6237 shared @file{libgcc} instead of the static version. The most common
6238 of these is when the application wishes to throw and catch exceptions
6239 across different shared libraries. In that case, each of the libraries
6240 as well as the application itself should use the shared @file{libgcc}.
6242 Therefore, the G++ and GCJ drivers automatically add
6243 @option{-shared-libgcc} whenever you build a shared library or a main
6244 executable, because C++ and Java programs typically use exceptions, so
6245 this is the right thing to do.
6247 If, instead, you use the GCC driver to create shared libraries, you may
6248 find that they will not always be linked with the shared @file{libgcc}.
6249 If GCC finds, at its configuration time, that you have a non-GNU linker
6250 or a GNU linker that does not support option @option{--eh-frame-hdr},
6251 it will link the shared version of @file{libgcc} into shared libraries
6252 by default. Otherwise, it will take advantage of the linker and optimize
6253 away the linking with the shared version of @file{libgcc}, linking with
6254 the static version of libgcc by default. This allows exceptions to
6255 propagate through such shared libraries, without incurring relocation
6256 costs at library load time.
6258 However, if a library or main executable is supposed to throw or catch
6259 exceptions, you must link it using the G++ or GCJ driver, as appropriate
6260 for the languages used in the program, or using the option
6261 @option{-shared-libgcc}, such that it is linked with the shared
6266 Bind references to global symbols when building a shared object. Warn
6267 about any unresolved references (unless overridden by the link editor
6268 option @samp{-Xlinker -z -Xlinker defs}). Only a few systems support
6271 @item -Xlinker @var{option}
6273 Pass @var{option} as an option to the linker. You can use this to
6274 supply system-specific linker options which GCC does not know how to
6277 If you want to pass an option that takes an argument, you must use
6278 @option{-Xlinker} twice, once for the option and once for the argument.
6279 For example, to pass @option{-assert definitions}, you must write
6280 @samp{-Xlinker -assert -Xlinker definitions}. It does not work to write
6281 @option{-Xlinker "-assert definitions"}, because this passes the entire
6282 string as a single argument, which is not what the linker expects.
6284 @item -Wl,@var{option}
6286 Pass @var{option} as an option to the linker. If @var{option} contains
6287 commas, it is split into multiple options at the commas.
6289 @item -u @var{symbol}
6291 Pretend the symbol @var{symbol} is undefined, to force linking of
6292 library modules to define it. You can use @option{-u} multiple times with
6293 different symbols to force loading of additional library modules.
6296 @node Directory Options
6297 @section Options for Directory Search
6298 @cindex directory options
6299 @cindex options, directory search
6302 These options specify directories to search for header files, for
6303 libraries and for parts of the compiler:
6308 Add the directory @var{dir} to the head of the list of directories to be
6309 searched for header files. This can be used to override a system header
6310 file, substituting your own version, since these directories are
6311 searched before the system header file directories. However, you should
6312 not use this option to add directories that contain vendor-supplied
6313 system header files (use @option{-isystem} for that). If you use more than
6314 one @option{-I} option, the directories are scanned in left-to-right
6315 order; the standard system directories come after.
6317 If a standard system include directory, or a directory specified with
6318 @option{-isystem}, is also specified with @option{-I}, the @option{-I}
6319 option will be ignored. The directory will still be searched but as a
6320 system directory at its normal position in the system include chain.
6321 This is to ensure that GCC's procedure to fix buggy system headers and
6322 the ordering for the include_next directive are not inadvertently changed.
6323 If you really need to change the search order for system directories,
6324 use the @option{-nostdinc} and/or @option{-isystem} options.
6326 @item -iquote@var{dir}
6328 Add the directory @var{dir} to the head of the list of directories to
6329 be searched for header files only for the case of @samp{#include
6330 "@var{file}"}; they are not searched for @samp{#include <@var{file}>},
6331 otherwise just like @option{-I}.
6335 Add directory @var{dir} to the list of directories to be searched
6338 @item -B@var{prefix}
6340 This option specifies where to find the executables, libraries,
6341 include files, and data files of the compiler itself.
6343 The compiler driver program runs one or more of the subprograms
6344 @file{cpp}, @file{cc1}, @file{as} and @file{ld}. It tries
6345 @var{prefix} as a prefix for each program it tries to run, both with and
6346 without @samp{@var{machine}/@var{version}/} (@pxref{Target Options}).
6348 For each subprogram to be run, the compiler driver first tries the
6349 @option{-B} prefix, if any. If that name is not found, or if @option{-B}
6350 was not specified, the driver tries two standard prefixes, which are
6351 @file{/usr/lib/gcc/} and @file{/usr/local/lib/gcc/}. If neither of
6352 those results in a file name that is found, the unmodified program
6353 name is searched for using the directories specified in your
6354 @env{PATH} environment variable.
6356 The compiler will check to see if the path provided by the @option{-B}
6357 refers to a directory, and if necessary it will add a directory
6358 separator character at the end of the path.
6360 @option{-B} prefixes that effectively specify directory names also apply
6361 to libraries in the linker, because the compiler translates these
6362 options into @option{-L} options for the linker. They also apply to
6363 includes files in the preprocessor, because the compiler translates these
6364 options into @option{-isystem} options for the preprocessor. In this case,
6365 the compiler appends @samp{include} to the prefix.
6367 The run-time support file @file{libgcc.a} can also be searched for using
6368 the @option{-B} prefix, if needed. If it is not found there, the two
6369 standard prefixes above are tried, and that is all. The file is left
6370 out of the link if it is not found by those means.
6372 Another way to specify a prefix much like the @option{-B} prefix is to use
6373 the environment variable @env{GCC_EXEC_PREFIX}. @xref{Environment
6376 As a special kludge, if the path provided by @option{-B} is
6377 @file{[dir/]stage@var{N}/}, where @var{N} is a number in the range 0 to
6378 9, then it will be replaced by @file{[dir/]include}. This is to help
6379 with boot-strapping the compiler.
6381 @item -specs=@var{file}
6383 Process @var{file} after the compiler reads in the standard @file{specs}
6384 file, in order to override the defaults that the @file{gcc} driver
6385 program uses when determining what switches to pass to @file{cc1},
6386 @file{cc1plus}, @file{as}, @file{ld}, etc. More than one
6387 @option{-specs=@var{file}} can be specified on the command line, and they
6388 are processed in order, from left to right.
6390 @item --sysroot=@var{dir}
6392 Use @var{dir} as the logical root directory for headers and libraries.
6393 For example, if the compiler would normally search for headers in
6394 @file{/usr/include} and libraries in @file{/usr/lib}, it will instead
6395 search @file{@var{dir}/usr/include} and @file{@var{dir}/usr/lib}.
6397 If you use both this option and the @option{-isysroot} option, then
6398 the @option{--sysroot} option will apply to libraries, but the
6399 @option{-isysroot} option will apply to header files.
6401 The GNU linker (beginning with version 2.16) has the necessary support
6402 for this option. If your linker does not support this option, the
6403 header file aspect of @option{--sysroot} will still work, but the
6404 library aspect will not.
6408 This option has been deprecated. Please use @option{-iquote} instead for
6409 @option{-I} directories before the @option{-I-} and remove the @option{-I-}.
6410 Any directories you specify with @option{-I} options before the @option{-I-}
6411 option are searched only for the case of @samp{#include "@var{file}"};
6412 they are not searched for @samp{#include <@var{file}>}.
6414 If additional directories are specified with @option{-I} options after
6415 the @option{-I-}, these directories are searched for all @samp{#include}
6416 directives. (Ordinarily @emph{all} @option{-I} directories are used
6419 In addition, the @option{-I-} option inhibits the use of the current
6420 directory (where the current input file came from) as the first search
6421 directory for @samp{#include "@var{file}"}. There is no way to
6422 override this effect of @option{-I-}. With @option{-I.} you can specify
6423 searching the directory which was current when the compiler was
6424 invoked. That is not exactly the same as what the preprocessor does
6425 by default, but it is often satisfactory.
6427 @option{-I-} does not inhibit the use of the standard system directories
6428 for header files. Thus, @option{-I-} and @option{-nostdinc} are
6435 @section Specifying subprocesses and the switches to pass to them
6438 @command{gcc} is a driver program. It performs its job by invoking a
6439 sequence of other programs to do the work of compiling, assembling and
6440 linking. GCC interprets its command-line parameters and uses these to
6441 deduce which programs it should invoke, and which command-line options
6442 it ought to place on their command lines. This behavior is controlled
6443 by @dfn{spec strings}. In most cases there is one spec string for each
6444 program that GCC can invoke, but a few programs have multiple spec
6445 strings to control their behavior. The spec strings built into GCC can
6446 be overridden by using the @option{-specs=} command-line switch to specify
6449 @dfn{Spec files} are plaintext files that are used to construct spec
6450 strings. They consist of a sequence of directives separated by blank
6451 lines. The type of directive is determined by the first non-whitespace
6452 character on the line and it can be one of the following:
6455 @item %@var{command}
6456 Issues a @var{command} to the spec file processor. The commands that can
6460 @item %include <@var{file}>
6462 Search for @var{file} and insert its text at the current point in the
6465 @item %include_noerr <@var{file}>
6466 @cindex %include_noerr
6467 Just like @samp{%include}, but do not generate an error message if the include
6468 file cannot be found.
6470 @item %rename @var{old_name} @var{new_name}
6472 Rename the spec string @var{old_name} to @var{new_name}.
6476 @item *[@var{spec_name}]:
6477 This tells the compiler to create, override or delete the named spec
6478 string. All lines after this directive up to the next directive or
6479 blank line are considered to be the text for the spec string. If this
6480 results in an empty string then the spec will be deleted. (Or, if the
6481 spec did not exist, then nothing will happened.) Otherwise, if the spec
6482 does not currently exist a new spec will be created. If the spec does
6483 exist then its contents will be overridden by the text of this
6484 directive, unless the first character of that text is the @samp{+}
6485 character, in which case the text will be appended to the spec.
6487 @item [@var{suffix}]:
6488 Creates a new @samp{[@var{suffix}] spec} pair. All lines after this directive
6489 and up to the next directive or blank line are considered to make up the
6490 spec string for the indicated suffix. When the compiler encounters an
6491 input file with the named suffix, it will processes the spec string in
6492 order to work out how to compile that file. For example:
6499 This says that any input file whose name ends in @samp{.ZZ} should be
6500 passed to the program @samp{z-compile}, which should be invoked with the
6501 command-line switch @option{-input} and with the result of performing the
6502 @samp{%i} substitution. (See below.)
6504 As an alternative to providing a spec string, the text that follows a
6505 suffix directive can be one of the following:
6508 @item @@@var{language}
6509 This says that the suffix is an alias for a known @var{language}. This is
6510 similar to using the @option{-x} command-line switch to GCC to specify a
6511 language explicitly. For example:
6518 Says that .ZZ files are, in fact, C++ source files.
6521 This causes an error messages saying:
6524 @var{name} compiler not installed on this system.
6528 GCC already has an extensive list of suffixes built into it.
6529 This directive will add an entry to the end of the list of suffixes, but
6530 since the list is searched from the end backwards, it is effectively
6531 possible to override earlier entries using this technique.
6535 GCC has the following spec strings built into it. Spec files can
6536 override these strings or create their own. Note that individual
6537 targets can also add their own spec strings to this list.
6540 asm Options to pass to the assembler
6541 asm_final Options to pass to the assembler post-processor
6542 cpp Options to pass to the C preprocessor
6543 cc1 Options to pass to the C compiler
6544 cc1plus Options to pass to the C++ compiler
6545 endfile Object files to include at the end of the link
6546 link Options to pass to the linker
6547 lib Libraries to include on the command line to the linker
6548 libgcc Decides which GCC support library to pass to the linker
6549 linker Sets the name of the linker
6550 predefines Defines to be passed to the C preprocessor
6551 signed_char Defines to pass to CPP to say whether @code{char} is signed
6553 startfile Object files to include at the start of the link
6556 Here is a small example of a spec file:
6562 --start-group -lgcc -lc -leval1 --end-group %(old_lib)
6565 This example renames the spec called @samp{lib} to @samp{old_lib} and
6566 then overrides the previous definition of @samp{lib} with a new one.
6567 The new definition adds in some extra command-line options before
6568 including the text of the old definition.
6570 @dfn{Spec strings} are a list of command-line options to be passed to their
6571 corresponding program. In addition, the spec strings can contain
6572 @samp{%}-prefixed sequences to substitute variable text or to
6573 conditionally insert text into the command line. Using these constructs
6574 it is possible to generate quite complex command lines.
6576 Here is a table of all defined @samp{%}-sequences for spec
6577 strings. Note that spaces are not generated automatically around the
6578 results of expanding these sequences. Therefore you can concatenate them
6579 together or combine them with constant text in a single argument.
6583 Substitute one @samp{%} into the program name or argument.
6586 Substitute the name of the input file being processed.
6589 Substitute the basename of the input file being processed.
6590 This is the substring up to (and not including) the last period
6591 and not including the directory.
6594 This is the same as @samp{%b}, but include the file suffix (text after
6598 Marks the argument containing or following the @samp{%d} as a
6599 temporary file name, so that that file will be deleted if GCC exits
6600 successfully. Unlike @samp{%g}, this contributes no text to the
6603 @item %g@var{suffix}
6604 Substitute a file name that has suffix @var{suffix} and is chosen
6605 once per compilation, and mark the argument in the same way as
6606 @samp{%d}. To reduce exposure to denial-of-service attacks, the file
6607 name is now chosen in a way that is hard to predict even when previously
6608 chosen file names are known. For example, @samp{%g.s @dots{} %g.o @dots{} %g.s}
6609 might turn into @samp{ccUVUUAU.s ccXYAXZ12.o ccUVUUAU.s}. @var{suffix} matches
6610 the regexp @samp{[.A-Za-z]*} or the special string @samp{%O}, which is
6611 treated exactly as if @samp{%O} had been preprocessed. Previously, @samp{%g}
6612 was simply substituted with a file name chosen once per compilation,
6613 without regard to any appended suffix (which was therefore treated
6614 just like ordinary text), making such attacks more likely to succeed.
6616 @item %u@var{suffix}
6617 Like @samp{%g}, but generates a new temporary file name even if
6618 @samp{%u@var{suffix}} was already seen.
6620 @item %U@var{suffix}
6621 Substitutes the last file name generated with @samp{%u@var{suffix}}, generating a
6622 new one if there is no such last file name. In the absence of any
6623 @samp{%u@var{suffix}}, this is just like @samp{%g@var{suffix}}, except they don't share
6624 the same suffix @emph{space}, so @samp{%g.s @dots{} %U.s @dots{} %g.s @dots{} %U.s}
6625 would involve the generation of two distinct file names, one
6626 for each @samp{%g.s} and another for each @samp{%U.s}. Previously, @samp{%U} was
6627 simply substituted with a file name chosen for the previous @samp{%u},
6628 without regard to any appended suffix.
6630 @item %j@var{suffix}
6631 Substitutes the name of the @code{HOST_BIT_BUCKET}, if any, and if it is
6632 writable, and if save-temps is off; otherwise, substitute the name
6633 of a temporary file, just like @samp{%u}. This temporary file is not
6634 meant for communication between processes, but rather as a junk
6637 @item %|@var{suffix}
6638 @itemx %m@var{suffix}
6639 Like @samp{%g}, except if @option{-pipe} is in effect. In that case
6640 @samp{%|} substitutes a single dash and @samp{%m} substitutes nothing at
6641 all. These are the two most common ways to instruct a program that it
6642 should read from standard input or write to standard output. If you
6643 need something more elaborate you can use an @samp{%@{pipe:@code{X}@}}
6644 construct: see for example @file{f/lang-specs.h}.
6646 @item %.@var{SUFFIX}
6647 Substitutes @var{.SUFFIX} for the suffixes of a matched switch's args
6648 when it is subsequently output with @samp{%*}. @var{SUFFIX} is
6649 terminated by the next space or %.
6652 Marks the argument containing or following the @samp{%w} as the
6653 designated output file of this compilation. This puts the argument
6654 into the sequence of arguments that @samp{%o} will substitute later.
6657 Substitutes the names of all the output files, with spaces
6658 automatically placed around them. You should write spaces
6659 around the @samp{%o} as well or the results are undefined.
6660 @samp{%o} is for use in the specs for running the linker.
6661 Input files whose names have no recognized suffix are not compiled
6662 at all, but they are included among the output files, so they will
6666 Substitutes the suffix for object files. Note that this is
6667 handled specially when it immediately follows @samp{%g, %u, or %U},
6668 because of the need for those to form complete file names. The
6669 handling is such that @samp{%O} is treated exactly as if it had already
6670 been substituted, except that @samp{%g, %u, and %U} do not currently
6671 support additional @var{suffix} characters following @samp{%O} as they would
6672 following, for example, @samp{.o}.
6675 Substitutes the standard macro predefinitions for the
6676 current target machine. Use this when running @code{cpp}.
6679 Like @samp{%p}, but puts @samp{__} before and after the name of each
6680 predefined macro, except for macros that start with @samp{__} or with
6681 @samp{_@var{L}}, where @var{L} is an uppercase letter. This is for ISO
6685 Substitute any of @option{-iprefix} (made from @env{GCC_EXEC_PREFIX}),
6686 @option{-isysroot} (made from @env{TARGET_SYSTEM_ROOT}), and
6687 @option{-isystem} (made from @env{COMPILER_PATH} and @option{-B} options)
6691 Current argument is the name of a library or startup file of some sort.
6692 Search for that file in a standard list of directories and substitute
6693 the full name found.
6696 Print @var{str} as an error message. @var{str} is terminated by a newline.
6697 Use this when inconsistent options are detected.
6700 Substitute the contents of spec string @var{name} at this point.
6703 Like @samp{%(@dots{})} but put @samp{__} around @option{-D} arguments.
6705 @item %x@{@var{option}@}
6706 Accumulate an option for @samp{%X}.
6709 Output the accumulated linker options specified by @option{-Wl} or a @samp{%x}
6713 Output the accumulated assembler options specified by @option{-Wa}.
6716 Output the accumulated preprocessor options specified by @option{-Wp}.
6719 Process the @code{asm} spec. This is used to compute the
6720 switches to be passed to the assembler.
6723 Process the @code{asm_final} spec. This is a spec string for
6724 passing switches to an assembler post-processor, if such a program is
6728 Process the @code{link} spec. This is the spec for computing the
6729 command line passed to the linker. Typically it will make use of the
6730 @samp{%L %G %S %D and %E} sequences.
6733 Dump out a @option{-L} option for each directory that GCC believes might
6734 contain startup files. If the target supports multilibs then the
6735 current multilib directory will be prepended to each of these paths.
6738 Process the @code{lib} spec. This is a spec string for deciding which
6739 libraries should be included on the command line to the linker.
6742 Process the @code{libgcc} spec. This is a spec string for deciding
6743 which GCC support library should be included on the command line to the linker.
6746 Process the @code{startfile} spec. This is a spec for deciding which
6747 object files should be the first ones passed to the linker. Typically
6748 this might be a file named @file{crt0.o}.
6751 Process the @code{endfile} spec. This is a spec string that specifies
6752 the last object files that will be passed to the linker.
6755 Process the @code{cpp} spec. This is used to construct the arguments
6756 to be passed to the C preprocessor.
6759 Process the @code{cc1} spec. This is used to construct the options to be
6760 passed to the actual C compiler (@samp{cc1}).
6763 Process the @code{cc1plus} spec. This is used to construct the options to be
6764 passed to the actual C++ compiler (@samp{cc1plus}).
6767 Substitute the variable part of a matched option. See below.
6768 Note that each comma in the substituted string is replaced by
6772 Remove all occurrences of @code{-S} from the command line. Note---this
6773 command is position dependent. @samp{%} commands in the spec string
6774 before this one will see @code{-S}, @samp{%} commands in the spec string
6775 after this one will not.
6777 @item %:@var{function}(@var{args})
6778 Call the named function @var{function}, passing it @var{args}.
6779 @var{args} is first processed as a nested spec string, then split
6780 into an argument vector in the usual fashion. The function returns
6781 a string which is processed as if it had appeared literally as part
6782 of the current spec.
6784 The following built-in spec functions are provided:
6787 @item @code{if-exists}
6788 The @code{if-exists} spec function takes one argument, an absolute
6789 pathname to a file. If the file exists, @code{if-exists} returns the
6790 pathname. Here is a small example of its usage:
6794 crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s
6797 @item @code{if-exists-else}
6798 The @code{if-exists-else} spec function is similar to the @code{if-exists}
6799 spec function, except that it takes two arguments. The first argument is
6800 an absolute pathname to a file. If the file exists, @code{if-exists-else}
6801 returns the pathname. If it does not exist, it returns the second argument.
6802 This way, @code{if-exists-else} can be used to select one file or another,
6803 based on the existence of the first. Here is a small example of its usage:
6807 crt0%O%s %:if-exists(crti%O%s) \
6808 %:if-exists-else(crtbeginT%O%s crtbegin%O%s)
6811 @item @code{replace-outfile}
6812 The @code{replace-outfile} spec function takes two arguments. It looks for the
6813 first argument in the outfiles array and replaces it with the second argument. Here
6814 is a small example of its usage:
6817 %@{fgnu-runtime:%:replace-outfile(-lobjc -lobjc-gnu)@}
6823 Substitutes the @code{-S} switch, if that switch was given to GCC@.
6824 If that switch was not specified, this substitutes nothing. Note that
6825 the leading dash is omitted when specifying this option, and it is
6826 automatically inserted if the substitution is performed. Thus the spec
6827 string @samp{%@{foo@}} would match the command-line option @option{-foo}
6828 and would output the command line option @option{-foo}.
6830 @item %W@{@code{S}@}
6831 Like %@{@code{S}@} but mark last argument supplied within as a file to be
6834 @item %@{@code{S}*@}
6835 Substitutes all the switches specified to GCC whose names start
6836 with @code{-S}, but which also take an argument. This is used for
6837 switches like @option{-o}, @option{-D}, @option{-I}, etc.
6838 GCC considers @option{-o foo} as being
6839 one switch whose names starts with @samp{o}. %@{o*@} would substitute this
6840 text, including the space. Thus two arguments would be generated.
6842 @item %@{@code{S}*&@code{T}*@}
6843 Like %@{@code{S}*@}, but preserve order of @code{S} and @code{T} options
6844 (the order of @code{S} and @code{T} in the spec is not significant).
6845 There can be any number of ampersand-separated variables; for each the
6846 wild card is optional. Useful for CPP as @samp{%@{D*&U*&A*@}}.
6848 @item %@{@code{S}:@code{X}@}
6849 Substitutes @code{X}, if the @samp{-S} switch was given to GCC@.
6851 @item %@{!@code{S}:@code{X}@}
6852 Substitutes @code{X}, if the @samp{-S} switch was @emph{not} given to GCC@.
6854 @item %@{@code{S}*:@code{X}@}
6855 Substitutes @code{X} if one or more switches whose names start with
6856 @code{-S} are specified to GCC@. Normally @code{X} is substituted only
6857 once, no matter how many such switches appeared. However, if @code{%*}
6858 appears somewhere in @code{X}, then @code{X} will be substituted once
6859 for each matching switch, with the @code{%*} replaced by the part of
6860 that switch that matched the @code{*}.
6862 @item %@{.@code{S}:@code{X}@}
6863 Substitutes @code{X}, if processing a file with suffix @code{S}.
6865 @item %@{!.@code{S}:@code{X}@}
6866 Substitutes @code{X}, if @emph{not} processing a file with suffix @code{S}.
6868 @item %@{@code{S}|@code{P}:@code{X}@}
6869 Substitutes @code{X} if either @code{-S} or @code{-P} was given to GCC@.
6870 This may be combined with @samp{!}, @samp{.}, and @code{*} sequences as well,
6871 although they have a stronger binding than the @samp{|}. If @code{%*}
6872 appears in @code{X}, all of the alternatives must be starred, and only
6873 the first matching alternative is substituted.
6875 For example, a spec string like this:
6878 %@{.c:-foo@} %@{!.c:-bar@} %@{.c|d:-baz@} %@{!.c|d:-boggle@}
6881 will output the following command-line options from the following input
6882 command-line options:
6887 -d fred.c -foo -baz -boggle
6888 -d jim.d -bar -baz -boggle
6891 @item %@{S:X; T:Y; :D@}
6893 If @code{S} was given to GCC, substitutes @code{X}; else if @code{T} was
6894 given to GCC, substitutes @code{Y}; else substitutes @code{D}. There can
6895 be as many clauses as you need. This may be combined with @code{.},
6896 @code{!}, @code{|}, and @code{*} as needed.
6901 The conditional text @code{X} in a %@{@code{S}:@code{X}@} or similar
6902 construct may contain other nested @samp{%} constructs or spaces, or
6903 even newlines. They are processed as usual, as described above.
6904 Trailing white space in @code{X} is ignored. White space may also
6905 appear anywhere on the left side of the colon in these constructs,
6906 except between @code{.} or @code{*} and the corresponding word.
6908 The @option{-O}, @option{-f}, @option{-m}, and @option{-W} switches are
6909 handled specifically in these constructs. If another value of
6910 @option{-O} or the negated form of a @option{-f}, @option{-m}, or
6911 @option{-W} switch is found later in the command line, the earlier
6912 switch value is ignored, except with @{@code{S}*@} where @code{S} is
6913 just one letter, which passes all matching options.
6915 The character @samp{|} at the beginning of the predicate text is used to
6916 indicate that a command should be piped to the following command, but
6917 only if @option{-pipe} is specified.
6919 It is built into GCC which switches take arguments and which do not.
6920 (You might think it would be useful to generalize this to allow each
6921 compiler's spec to say which switches take arguments. But this cannot
6922 be done in a consistent fashion. GCC cannot even decide which input
6923 files have been specified without knowing which switches take arguments,
6924 and it must know which input files to compile in order to tell which
6927 GCC also knows implicitly that arguments starting in @option{-l} are to be
6928 treated as compiler output files, and passed to the linker in their
6929 proper position among the other output files.
6931 @c man begin OPTIONS
6933 @node Target Options
6934 @section Specifying Target Machine and Compiler Version
6935 @cindex target options
6936 @cindex cross compiling
6937 @cindex specifying machine version
6938 @cindex specifying compiler version and target machine
6939 @cindex compiler version, specifying
6940 @cindex target machine, specifying
6942 The usual way to run GCC is to run the executable called @file{gcc}, or
6943 @file{<machine>-gcc} when cross-compiling, or
6944 @file{<machine>-gcc-<version>} to run a version other than the one that
6945 was installed last. Sometimes this is inconvenient, so GCC provides
6946 options that will switch to another cross-compiler or version.
6949 @item -b @var{machine}
6951 The argument @var{machine} specifies the target machine for compilation.
6953 The value to use for @var{machine} is the same as was specified as the
6954 machine type when configuring GCC as a cross-compiler. For
6955 example, if a cross-compiler was configured with @samp{configure
6956 i386v}, meaning to compile for an 80386 running System V, then you
6957 would specify @option{-b i386v} to run that cross compiler.
6959 @item -V @var{version}
6961 The argument @var{version} specifies which version of GCC to run.
6962 This is useful when multiple versions are installed. For example,
6963 @var{version} might be @samp{2.0}, meaning to run GCC version 2.0.
6966 The @option{-V} and @option{-b} options work by running the
6967 @file{<machine>-gcc-<version>} executable, so there's no real reason to
6968 use them if you can just run that directly.
6970 @node Submodel Options
6971 @section Hardware Models and Configurations
6972 @cindex submodel options
6973 @cindex specifying hardware config
6974 @cindex hardware models and configurations, specifying
6975 @cindex machine dependent options
6977 Earlier we discussed the standard option @option{-b} which chooses among
6978 different installed compilers for completely different target
6979 machines, such as VAX vs.@: 68000 vs.@: 80386.
6981 In addition, each of these target machine types can have its own
6982 special options, starting with @samp{-m}, to choose among various
6983 hardware models or configurations---for example, 68010 vs 68020,
6984 floating coprocessor or none. A single installed version of the
6985 compiler can compile for any model or configuration, according to the
6988 Some configurations of the compiler also support additional special
6989 options, usually for compatibility with other compilers on the same
6992 @c This list is ordered alphanumerically by subsection name.
6993 @c It should be the same order and spelling as these options are listed
6994 @c in Machine Dependent Options
7000 * Blackfin Options::
7003 * DEC Alpha Options::
7004 * DEC Alpha/VMS Options::
7008 * i386 and x86-64 Options::
7020 * RS/6000 and PowerPC Options::
7021 * S/390 and zSeries Options::
7024 * System V Options::
7025 * TMS320C3x/C4x Options::
7029 * Xstormy16 Options::
7035 @subsection ARC Options
7038 These options are defined for ARC implementations:
7043 Compile code for little endian mode. This is the default.
7047 Compile code for big endian mode.
7050 @opindex mmangle-cpu
7051 Prepend the name of the cpu to all public symbol names.
7052 In multiple-processor systems, there are many ARC variants with different
7053 instruction and register set characteristics. This flag prevents code
7054 compiled for one cpu to be linked with code compiled for another.
7055 No facility exists for handling variants that are ``almost identical''.
7056 This is an all or nothing option.
7058 @item -mcpu=@var{cpu}
7060 Compile code for ARC variant @var{cpu}.
7061 Which variants are supported depend on the configuration.
7062 All variants support @option{-mcpu=base}, this is the default.
7064 @item -mtext=@var{text-section}
7065 @itemx -mdata=@var{data-section}
7066 @itemx -mrodata=@var{readonly-data-section}
7070 Put functions, data, and readonly data in @var{text-section},
7071 @var{data-section}, and @var{readonly-data-section} respectively
7072 by default. This can be overridden with the @code{section} attribute.
7073 @xref{Variable Attributes}.
7078 @subsection ARM Options
7081 These @samp{-m} options are defined for Advanced RISC Machines (ARM)
7085 @item -mabi=@var{name}
7087 Generate code for the specified ABI@. Permissible values are: @samp{apcs-gnu},
7088 @samp{atpcs}, @samp{aapcs} and @samp{iwmmxt}.
7091 @opindex mapcs-frame
7092 Generate a stack frame that is compliant with the ARM Procedure Call
7093 Standard for all functions, even if this is not strictly necessary for
7094 correct execution of the code. Specifying @option{-fomit-frame-pointer}
7095 with this option will cause the stack frames not to be generated for
7096 leaf functions. The default is @option{-mno-apcs-frame}.
7100 This is a synonym for @option{-mapcs-frame}.
7103 @c not currently implemented
7104 @item -mapcs-stack-check
7105 @opindex mapcs-stack-check
7106 Generate code to check the amount of stack space available upon entry to
7107 every function (that actually uses some stack space). If there is
7108 insufficient space available then either the function
7109 @samp{__rt_stkovf_split_small} or @samp{__rt_stkovf_split_big} will be
7110 called, depending upon the amount of stack space required. The run time
7111 system is required to provide these functions. The default is
7112 @option{-mno-apcs-stack-check}, since this produces smaller code.
7114 @c not currently implemented
7116 @opindex mapcs-float
7117 Pass floating point arguments using the float point registers. This is
7118 one of the variants of the APCS@. This option is recommended if the
7119 target hardware has a floating point unit or if a lot of floating point
7120 arithmetic is going to be performed by the code. The default is
7121 @option{-mno-apcs-float}, since integer only code is slightly increased in
7122 size if @option{-mapcs-float} is used.
7124 @c not currently implemented
7125 @item -mapcs-reentrant
7126 @opindex mapcs-reentrant
7127 Generate reentrant, position independent code. The default is
7128 @option{-mno-apcs-reentrant}.
7131 @item -mthumb-interwork
7132 @opindex mthumb-interwork
7133 Generate code which supports calling between the ARM and Thumb
7134 instruction sets. Without this option the two instruction sets cannot
7135 be reliably used inside one program. The default is
7136 @option{-mno-thumb-interwork}, since slightly larger code is generated
7137 when @option{-mthumb-interwork} is specified.
7139 @item -mno-sched-prolog
7140 @opindex mno-sched-prolog
7141 Prevent the reordering of instructions in the function prolog, or the
7142 merging of those instruction with the instructions in the function's
7143 body. This means that all functions will start with a recognizable set
7144 of instructions (or in fact one of a choice from a small set of
7145 different function prologues), and this information can be used to
7146 locate the start if functions inside an executable piece of code. The
7147 default is @option{-msched-prolog}.
7150 @opindex mhard-float
7151 Generate output containing floating point instructions. This is the
7155 @opindex msoft-float
7156 Generate output containing library calls for floating point.
7157 @strong{Warning:} the requisite libraries are not available for all ARM
7158 targets. Normally the facilities of the machine's usual C compiler are
7159 used, but this cannot be done directly in cross-compilation. You must make
7160 your own arrangements to provide suitable library functions for
7163 @option{-msoft-float} changes the calling convention in the output file;
7164 therefore, it is only useful if you compile @emph{all} of a program with
7165 this option. In particular, you need to compile @file{libgcc.a}, the
7166 library that comes with GCC, with @option{-msoft-float} in order for
7169 @item -mfloat-abi=@var{name}
7171 Specifies which ABI to use for floating point values. Permissible values
7172 are: @samp{soft}, @samp{softfp} and @samp{hard}.
7174 @samp{soft} and @samp{hard} are equivalent to @option{-msoft-float}
7175 and @option{-mhard-float} respectively. @samp{softfp} allows the generation
7176 of floating point instructions, but still uses the soft-float calling
7179 @item -mlittle-endian
7180 @opindex mlittle-endian
7181 Generate code for a processor running in little-endian mode. This is
7182 the default for all standard configurations.
7185 @opindex mbig-endian
7186 Generate code for a processor running in big-endian mode; the default is
7187 to compile code for a little-endian processor.
7189 @item -mwords-little-endian
7190 @opindex mwords-little-endian
7191 This option only applies when generating code for big-endian processors.
7192 Generate code for a little-endian word order but a big-endian byte
7193 order. That is, a byte order of the form @samp{32107654}. Note: this
7194 option should only be used if you require compatibility with code for
7195 big-endian ARM processors generated by versions of the compiler prior to
7198 @item -mcpu=@var{name}
7200 This specifies the name of the target ARM processor. GCC uses this name
7201 to determine what kind of instructions it can emit when generating
7202 assembly code. Permissible names are: @samp{arm2}, @samp{arm250},
7203 @samp{arm3}, @samp{arm6}, @samp{arm60}, @samp{arm600}, @samp{arm610},
7204 @samp{arm620}, @samp{arm7}, @samp{arm7m}, @samp{arm7d}, @samp{arm7dm},
7205 @samp{arm7di}, @samp{arm7dmi}, @samp{arm70}, @samp{arm700},
7206 @samp{arm700i}, @samp{arm710}, @samp{arm710c}, @samp{arm7100},
7207 @samp{arm7500}, @samp{arm7500fe}, @samp{arm7tdmi}, @samp{arm7tdmi-s},
7208 @samp{arm8}, @samp{strongarm}, @samp{strongarm110}, @samp{strongarm1100},
7209 @samp{arm8}, @samp{arm810}, @samp{arm9}, @samp{arm9e}, @samp{arm920},
7210 @samp{arm920t}, @samp{arm922t}, @samp{arm946e-s}, @samp{arm966e-s},
7211 @samp{arm968e-s}, @samp{arm926ej-s}, @samp{arm940t}, @samp{arm9tdmi},
7212 @samp{arm10tdmi}, @samp{arm1020t}, @samp{arm1026ej-s},
7213 @samp{arm10e}, @samp{arm1020e}, @samp{arm1022e},
7214 @samp{arm1136j-s}, @samp{arm1136jf-s}, @samp{mpcore}, @samp{mpcorenovfp},
7215 @samp{arm1176jz-s}, @samp{arm1176jzf-s}, @samp{xscale}, @samp{iwmmxt},
7218 @itemx -mtune=@var{name}
7220 This option is very similar to the @option{-mcpu=} option, except that
7221 instead of specifying the actual target processor type, and hence
7222 restricting which instructions can be used, it specifies that GCC should
7223 tune the performance of the code as if the target were of the type
7224 specified in this option, but still choosing the instructions that it
7225 will generate based on the cpu specified by a @option{-mcpu=} option.
7226 For some ARM implementations better performance can be obtained by using
7229 @item -march=@var{name}
7231 This specifies the name of the target ARM architecture. GCC uses this
7232 name to determine what kind of instructions it can emit when generating
7233 assembly code. This option can be used in conjunction with or instead
7234 of the @option{-mcpu=} option. Permissible names are: @samp{armv2},
7235 @samp{armv2a}, @samp{armv3}, @samp{armv3m}, @samp{armv4}, @samp{armv4t},
7236 @samp{armv5}, @samp{armv5t}, @samp{armv5te}, @samp{armv6}, @samp{armv6j},
7237 @samp{iwmmxt}, @samp{ep9312}.
7239 @item -mfpu=@var{name}
7240 @itemx -mfpe=@var{number}
7241 @itemx -mfp=@var{number}
7245 This specifies what floating point hardware (or hardware emulation) is
7246 available on the target. Permissible names are: @samp{fpa}, @samp{fpe2},
7247 @samp{fpe3}, @samp{maverick}, @samp{vfp}. @option{-mfp} and @option{-mfpe}
7248 are synonyms for @option{-mfpu}=@samp{fpe}@var{number}, for compatibility
7249 with older versions of GCC@.
7251 If @option{-msoft-float} is specified this specifies the format of
7252 floating point values.
7254 @item -mstructure-size-boundary=@var{n}
7255 @opindex mstructure-size-boundary
7256 The size of all structures and unions will be rounded up to a multiple
7257 of the number of bits set by this option. Permissible values are 8, 32
7258 and 64. The default value varies for different toolchains. For the COFF
7259 targeted toolchain the default value is 8. A value of 64 is only allowed
7260 if the underlying ABI supports it.
7262 Specifying the larger number can produce faster, more efficient code, but
7263 can also increase the size of the program. Different values are potentially
7264 incompatible. Code compiled with one value cannot necessarily expect to
7265 work with code or libraries compiled with another value, if they exchange
7266 information using structures or unions.
7268 @item -mabort-on-noreturn
7269 @opindex mabort-on-noreturn
7270 Generate a call to the function @code{abort} at the end of a
7271 @code{noreturn} function. It will be executed if the function tries to
7275 @itemx -mno-long-calls
7276 @opindex mlong-calls
7277 @opindex mno-long-calls
7278 Tells the compiler to perform function calls by first loading the
7279 address of the function into a register and then performing a subroutine
7280 call on this register. This switch is needed if the target function
7281 will lie outside of the 64 megabyte addressing range of the offset based
7282 version of subroutine call instruction.
7284 Even if this switch is enabled, not all function calls will be turned
7285 into long calls. The heuristic is that static functions, functions
7286 which have the @samp{short-call} attribute, functions that are inside
7287 the scope of a @samp{#pragma no_long_calls} directive and functions whose
7288 definitions have already been compiled within the current compilation
7289 unit, will not be turned into long calls. The exception to this rule is
7290 that weak function definitions, functions with the @samp{long-call}
7291 attribute or the @samp{section} attribute, and functions that are within
7292 the scope of a @samp{#pragma long_calls} directive, will always be
7293 turned into long calls.
7295 This feature is not enabled by default. Specifying
7296 @option{-mno-long-calls} will restore the default behavior, as will
7297 placing the function calls within the scope of a @samp{#pragma
7298 long_calls_off} directive. Note these switches have no effect on how
7299 the compiler generates code to handle function calls via function
7302 @item -mnop-fun-dllimport
7303 @opindex mnop-fun-dllimport
7304 Disable support for the @code{dllimport} attribute.
7306 @item -msingle-pic-base
7307 @opindex msingle-pic-base
7308 Treat the register used for PIC addressing as read-only, rather than
7309 loading it in the prologue for each function. The run-time system is
7310 responsible for initializing this register with an appropriate value
7311 before execution begins.
7313 @item -mpic-register=@var{reg}
7314 @opindex mpic-register
7315 Specify the register to be used for PIC addressing. The default is R10
7316 unless stack-checking is enabled, when R9 is used.
7318 @item -mcirrus-fix-invalid-insns
7319 @opindex mcirrus-fix-invalid-insns
7320 @opindex mno-cirrus-fix-invalid-insns
7321 Insert NOPs into the instruction stream to in order to work around
7322 problems with invalid Maverick instruction combinations. This option
7323 is only valid if the @option{-mcpu=ep9312} option has been used to
7324 enable generation of instructions for the Cirrus Maverick floating
7325 point co-processor. This option is not enabled by default, since the
7326 problem is only present in older Maverick implementations. The default
7327 can be re-enabled by use of the @option{-mno-cirrus-fix-invalid-insns}
7330 @item -mpoke-function-name
7331 @opindex mpoke-function-name
7332 Write the name of each function into the text section, directly
7333 preceding the function prologue. The generated code is similar to this:
7337 .ascii "arm_poke_function_name", 0
7340 .word 0xff000000 + (t1 - t0)
7341 arm_poke_function_name
7343 stmfd sp!, @{fp, ip, lr, pc@}
7347 When performing a stack backtrace, code can inspect the value of
7348 @code{pc} stored at @code{fp + 0}. If the trace function then looks at
7349 location @code{pc - 12} and the top 8 bits are set, then we know that
7350 there is a function name embedded immediately preceding this location
7351 and has length @code{((pc[-3]) & 0xff000000)}.
7355 Generate code for the 16-bit Thumb instruction set. The default is to
7356 use the 32-bit ARM instruction set.
7359 @opindex mtpcs-frame
7360 Generate a stack frame that is compliant with the Thumb Procedure Call
7361 Standard for all non-leaf functions. (A leaf function is one that does
7362 not call any other functions.) The default is @option{-mno-tpcs-frame}.
7364 @item -mtpcs-leaf-frame
7365 @opindex mtpcs-leaf-frame
7366 Generate a stack frame that is compliant with the Thumb Procedure Call
7367 Standard for all leaf functions. (A leaf function is one that does
7368 not call any other functions.) The default is @option{-mno-apcs-leaf-frame}.
7370 @item -mcallee-super-interworking
7371 @opindex mcallee-super-interworking
7372 Gives all externally visible functions in the file being compiled an ARM
7373 instruction set header which switches to Thumb mode before executing the
7374 rest of the function. This allows these functions to be called from
7375 non-interworking code.
7377 @item -mcaller-super-interworking
7378 @opindex mcaller-super-interworking
7379 Allows calls via function pointers (including virtual functions) to
7380 execute correctly regardless of whether the target code has been
7381 compiled for interworking or not. There is a small overhead in the cost
7382 of executing a function pointer if this option is enabled.
7387 @subsection AVR Options
7390 These options are defined for AVR implementations:
7393 @item -mmcu=@var{mcu}
7395 Specify ATMEL AVR instruction set or MCU type.
7397 Instruction set avr1 is for the minimal AVR core, not supported by the C
7398 compiler, only for assembler programs (MCU types: at90s1200, attiny10,
7399 attiny11, attiny12, attiny15, attiny28).
7401 Instruction set avr2 (default) is for the classic AVR core with up to
7402 8K program memory space (MCU types: at90s2313, at90s2323, attiny22,
7403 at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515,
7404 at90c8534, at90s8535).
7406 Instruction set avr3 is for the classic AVR core with up to 128K program
7407 memory space (MCU types: atmega103, atmega603, at43usb320, at76c711).
7409 Instruction set avr4 is for the enhanced AVR core with up to 8K program
7410 memory space (MCU types: atmega8, atmega83, atmega85).
7412 Instruction set avr5 is for the enhanced AVR core with up to 128K program
7413 memory space (MCU types: atmega16, atmega161, atmega163, atmega32, atmega323,
7414 atmega64, atmega128, at43usb355, at94k).
7418 Output instruction sizes to the asm file.
7420 @item -minit-stack=@var{N}
7421 @opindex minit-stack
7422 Specify the initial stack address, which may be a symbol or numeric value,
7423 @samp{__stack} is the default.
7425 @item -mno-interrupts
7426 @opindex mno-interrupts
7427 Generated code is not compatible with hardware interrupts.
7428 Code size will be smaller.
7430 @item -mcall-prologues
7431 @opindex mcall-prologues
7432 Functions prologues/epilogues expanded as call to appropriate
7433 subroutines. Code size will be smaller.
7435 @item -mno-tablejump
7436 @opindex mno-tablejump
7437 Do not generate tablejump insns which sometimes increase code size.
7440 @opindex mtiny-stack
7441 Change only the low 8 bits of the stack pointer.
7445 Assume int to be 8 bit integer. This affects the sizes of all types: A
7446 char will be 1 byte, an int will be 1 byte, an long will be 2 bytes
7447 and long long will be 4 bytes. Please note that this option does not
7448 comply to the C standards, but it will provide you with smaller code
7452 @node Blackfin Options
7453 @subsection Blackfin Options
7454 @cindex Blackfin Options
7457 @item -momit-leaf-frame-pointer
7458 @opindex momit-leaf-frame-pointer
7459 Don't keep the frame pointer in a register for leaf functions. This
7460 avoids the instructions to save, set up and restore frame pointers and
7461 makes an extra register available in leaf functions. The option
7462 @option{-fomit-frame-pointer} removes the frame pointer for all functions
7463 which might make debugging harder.
7465 @item -mspecld-anomaly
7466 @opindex mspecld-anomaly
7467 When enabled, the compiler will ensure that the generated code does not
7468 contain speculative loads after jump instructions. This option is enabled
7471 @item -mno-specld-anomaly
7472 @opindex mno-specld-anomaly
7473 Don't generate extra code to prevent speculative loads from occurring.
7475 @item -mcsync-anomaly
7476 @opindex mcsync-anomaly
7477 When enabled, the compiler will ensure that the generated code does not
7478 contain CSYNC or SSYNC instructions too soon after conditional branches.
7479 This option is enabled by default.
7481 @item -mno-csync-anomaly
7482 @opindex mno-csync-anomaly
7483 Don't generate extra code to prevent CSYNC or SSYNC instructions from
7484 occurring too soon after a conditional branch.
7488 When enabled, the compiler is free to take advantage of the knowledge that
7489 the entire program fits into the low 64k of memory.
7492 @opindex mno-low-64k
7493 Assume that the program is arbitrarily large. This is the default.
7495 @item -mid-shared-library
7496 @opindex mid-shared-library
7497 Generate code that supports shared libraries via the library ID method.
7498 This allows for execute in place and shared libraries in an environment
7499 without virtual memory management. This option implies @option{-fPIC}.
7501 @item -mno-id-shared-library
7502 @opindex mno-id-shared-library
7503 Generate code that doesn't assume ID based shared libraries are being used.
7504 This is the default.
7506 @item -mshared-library-id=n
7507 @opindex mshared-library-id
7508 Specified the identification number of the ID based shared library being
7509 compiled. Specifying a value of 0 will generate more compact code, specifying
7510 other values will force the allocation of that number to the current
7511 library but is no more space or time efficient than omitting this option.
7514 @itemx -mno-long-calls
7515 @opindex mlong-calls
7516 @opindex mno-long-calls
7517 Tells the compiler to perform function calls by first loading the
7518 address of the function into a register and then performing a subroutine
7519 call on this register. This switch is needed if the target function
7520 will lie outside of the 24 bit addressing range of the offset based
7521 version of subroutine call instruction.
7523 This feature is not enabled by default. Specifying
7524 @option{-mno-long-calls} will restore the default behavior. Note these
7525 switches have no effect on how the compiler generates code to handle
7526 function calls via function pointers.
7530 @subsection CRIS Options
7531 @cindex CRIS Options
7533 These options are defined specifically for the CRIS ports.
7536 @item -march=@var{architecture-type}
7537 @itemx -mcpu=@var{architecture-type}
7540 Generate code for the specified architecture. The choices for
7541 @var{architecture-type} are @samp{v3}, @samp{v8} and @samp{v10} for
7542 respectively ETRAX@w{ }4, ETRAX@w{ }100, and ETRAX@w{ }100@w{ }LX@.
7543 Default is @samp{v0} except for cris-axis-linux-gnu, where the default is
7546 @item -mtune=@var{architecture-type}
7548 Tune to @var{architecture-type} everything applicable about the generated
7549 code, except for the ABI and the set of available instructions. The
7550 choices for @var{architecture-type} are the same as for
7551 @option{-march=@var{architecture-type}}.
7553 @item -mmax-stack-frame=@var{n}
7554 @opindex mmax-stack-frame
7555 Warn when the stack frame of a function exceeds @var{n} bytes.
7557 @item -melinux-stacksize=@var{n}
7558 @opindex melinux-stacksize
7559 Only available with the @samp{cris-axis-aout} target. Arranges for
7560 indications in the program to the kernel loader that the stack of the
7561 program should be set to @var{n} bytes.
7567 The options @option{-metrax4} and @option{-metrax100} are synonyms for
7568 @option{-march=v3} and @option{-march=v8} respectively.
7570 @item -mmul-bug-workaround
7571 @itemx -mno-mul-bug-workaround
7572 @opindex mmul-bug-workaround
7573 @opindex mno-mul-bug-workaround
7574 Work around a bug in the @code{muls} and @code{mulu} instructions for CPU
7575 models where it applies. This option is active by default.
7579 Enable CRIS-specific verbose debug-related information in the assembly
7580 code. This option also has the effect to turn off the @samp{#NO_APP}
7581 formatted-code indicator to the assembler at the beginning of the
7586 Do not use condition-code results from previous instruction; always emit
7587 compare and test instructions before use of condition codes.
7589 @item -mno-side-effects
7590 @opindex mno-side-effects
7591 Do not emit instructions with side-effects in addressing modes other than
7595 @itemx -mno-stack-align
7597 @itemx -mno-data-align
7598 @itemx -mconst-align
7599 @itemx -mno-const-align
7600 @opindex mstack-align
7601 @opindex mno-stack-align
7602 @opindex mdata-align
7603 @opindex mno-data-align
7604 @opindex mconst-align
7605 @opindex mno-const-align
7606 These options (no-options) arranges (eliminate arrangements) for the
7607 stack-frame, individual data and constants to be aligned for the maximum
7608 single data access size for the chosen CPU model. The default is to
7609 arrange for 32-bit alignment. ABI details such as structure layout are
7610 not affected by these options.
7618 Similar to the stack- data- and const-align options above, these options
7619 arrange for stack-frame, writable data and constants to all be 32-bit,
7620 16-bit or 8-bit aligned. The default is 32-bit alignment.
7622 @item -mno-prologue-epilogue
7623 @itemx -mprologue-epilogue
7624 @opindex mno-prologue-epilogue
7625 @opindex mprologue-epilogue
7626 With @option{-mno-prologue-epilogue}, the normal function prologue and
7627 epilogue that sets up the stack-frame are omitted and no return
7628 instructions or return sequences are generated in the code. Use this
7629 option only together with visual inspection of the compiled code: no
7630 warnings or errors are generated when call-saved registers must be saved,
7631 or storage for local variable needs to be allocated.
7637 With @option{-fpic} and @option{-fPIC}, don't generate (do generate)
7638 instruction sequences that load addresses for functions from the PLT part
7639 of the GOT rather than (traditional on other architectures) calls to the
7640 PLT@. The default is @option{-mgotplt}.
7644 Legacy no-op option only recognized with the cris-axis-aout target.
7648 Legacy no-op option only recognized with the cris-axis-elf and
7649 cris-axis-linux-gnu targets.
7653 Only recognized with the cris-axis-aout target, where it selects a
7654 GNU/linux-like multilib, include files and instruction set for
7659 Legacy no-op option only recognized with the cris-axis-linux-gnu target.
7663 This option, recognized for the cris-axis-aout and cris-axis-elf arranges
7664 to link with input-output functions from a simulator library. Code,
7665 initialized data and zero-initialized data are allocated consecutively.
7669 Like @option{-sim}, but pass linker options to locate initialized data at
7670 0x40000000 and zero-initialized data at 0x80000000.
7673 @node Darwin Options
7674 @subsection Darwin Options
7675 @cindex Darwin options
7677 These options are defined for all architectures running the Darwin operating
7680 FSF GCC on Darwin does not create ``fat'' object files; it will create
7681 an object file for the single architecture that it was built to
7682 target. Apple's GCC on Darwin does create ``fat'' files if multiple
7683 @option{-arch} options are used; it does so by running the compiler or
7684 linker multiple times and joining the results together with
7687 The subtype of the file created (like @samp{ppc7400} or @samp{ppc970} or
7688 @samp{i686}) is determined by the flags that specify the ISA
7689 that GCC is targetting, like @option{-mcpu} or @option{-march}. The
7690 @option{-force_cpusubtype_ALL} option can be used to override this.
7692 The Darwin tools vary in their behavior when presented with an ISA
7693 mismatch. The assembler, @file{as}, will only permit instructions to
7694 be used that are valid for the subtype of the file it is generating,
7695 so you cannot put 64-bit instructions in an @samp{ppc750} object file.
7696 The linker for shared libraries, @file{/usr/bin/libtool}, will fail
7697 and print an error if asked to create a shared library with a less
7698 restrictive subtype than its input files (for instance, trying to put
7699 a @samp{ppc970} object file in a @samp{ppc7400} library). The linker
7700 for executables, @file{ld}, will quietly give the executable the most
7701 restrictive subtype of any of its input files.
7706 Add the framework directory @var{dir} to the head of the list of
7707 directories to be searched for header files. These directories are
7708 interleaved with those specified by @option{-I} options and are
7709 scanned in a left-to-right order.
7711 A framework directory is a directory with frameworks in it. A
7712 framework is a directory with a @samp{"Headers"} and/or
7713 @samp{"PrivateHeaders"} directory contained directly in it that ends
7714 in @samp{".framework"}. The name of a framework is the name of this
7715 directory excluding the @samp{".framework"}. Headers associated with
7716 the framework are found in one of those two directories, with
7717 @samp{"Headers"} being searched first. A subframework is a framework
7718 directory that is in a framework's @samp{"Frameworks"} directory.
7719 Includes of subframework headers can only appear in a header of a
7720 framework that contains the subframework, or in a sibling subframework
7721 header. Two subframeworks are siblings if they occur in the same
7722 framework. A subframework should not have the same name as a
7723 framework, a warning will be issued if this is violated. Currently a
7724 subframework cannot have subframeworks, in the future, the mechanism
7725 may be extended to support this. The standard frameworks can be found
7726 in @samp{"/System/Library/Frameworks"} and
7727 @samp{"/Library/Frameworks"}. An example include looks like
7728 @code{#include <Framework/header.h>}, where @samp{Framework} denotes
7729 the name of the framework and header.h is found in the
7730 @samp{"PrivateHeaders"} or @samp{"Headers"} directory.
7734 Emit debugging information for symbols that are used. For STABS
7735 debugging format, this enables @option{-feliminate-unused-debug-symbols}.
7736 This is by default ON@.
7740 Emit debugging information for all symbols and types.
7742 @item -mmacosx-version-min=@var{version}
7743 The earliest version of MacOS X that this executable will run on
7744 is @var{version}. Typical values of @var{version} include @code{10.1},
7745 @code{10.2}, and @code{10.3.9}.
7747 The default for this option is to make choices that seem to be most
7750 @item -mone-byte-bool
7751 @opindex -mone-byte-bool
7752 Override the defaults for @samp{bool} so that @samp{sizeof(bool)==1}.
7753 By default @samp{sizeof(bool)} is @samp{4} when compiling for
7754 Darwin/PowerPC and @samp{1} when compiling for Darwin/x86, so this
7755 option has no effect on x86.
7757 @strong{Warning:} The @option{-mone-byte-bool} switch causes GCC
7758 to generate code that is not binary compatible with code generated
7759 without that switch. Using this switch may require recompiling all
7760 other modules in a program, including system libraries. Use this
7761 switch to conform to a non-default data model.
7763 @item -mfix-and-continue
7764 @itemx -ffix-and-continue
7765 @itemx -findirect-data
7766 @opindex mfix-and-continue
7767 @opindex ffix-and-continue
7768 @opindex findirect-data
7769 Generate code suitable for fast turn around development. Needed to
7770 enable gdb to dynamically load @code{.o} files into already running
7771 programs. @option{-findirect-data} and @option{-ffix-and-continue}
7772 are provided for backwards compatibility.
7776 Loads all members of static archive libraries.
7777 See man ld(1) for more information.
7779 @item -arch_errors_fatal
7780 @opindex arch_errors_fatal
7781 Cause the errors having to do with files that have the wrong architecture
7785 @opindex bind_at_load
7786 Causes the output file to be marked such that the dynamic linker will
7787 bind all undefined references when the file is loaded or launched.
7791 Produce a Mach-o bundle format file.
7792 See man ld(1) for more information.
7794 @item -bundle_loader @var{executable}
7795 @opindex bundle_loader
7796 This option specifies the @var{executable} that will be loading the build
7797 output file being linked. See man ld(1) for more information.
7800 @opindex -dynamiclib
7801 When passed this option, GCC will produce a dynamic library instead of
7802 an executable when linking, using the Darwin @file{libtool} command.
7804 @item -force_cpusubtype_ALL
7805 @opindex -force_cpusubtype_ALL
7806 This causes GCC's output file to have the @var{ALL} subtype, instead of
7807 one controlled by the @option{-mcpu} or @option{-march} option.
7809 @item -allowable_client @var{client_name}
7811 @itemx -compatibility_version
7812 @itemx -current_version
7814 @itemx -dependency-file
7816 @itemx -dylinker_install_name
7818 @itemx -exported_symbols_list
7820 @itemx -flat_namespace
7821 @itemx -force_flat_namespace
7822 @itemx -headerpad_max_install_names
7825 @itemx -install_name
7826 @itemx -keep_private_externs
7827 @itemx -multi_module
7828 @itemx -multiply_defined
7829 @itemx -multiply_defined_unused
7831 @itemx -no_dead_strip_inits_and_terms
7832 @itemx -nofixprebinding
7835 @itemx -noseglinkedit
7836 @itemx -pagezero_size
7838 @itemx -prebind_all_twolevel_modules
7839 @itemx -private_bundle
7840 @itemx -read_only_relocs
7842 @itemx -sectobjectsymbols
7846 @itemx -sectobjectsymbols
7849 @itemx -segs_read_only_addr
7850 @itemx -segs_read_write_addr
7851 @itemx -seg_addr_table
7852 @itemx -seg_addr_table_filename
7855 @itemx -segs_read_only_addr
7856 @itemx -segs_read_write_addr
7857 @itemx -single_module
7860 @itemx -sub_umbrella
7861 @itemx -twolevel_namespace
7864 @itemx -unexported_symbols_list
7865 @itemx -weak_reference_mismatches
7868 @opindex allowable_client
7869 @opindex client_name
7870 @opindex compatibility_version
7871 @opindex current_version
7873 @opindex dependency-file
7875 @opindex dylinker_install_name
7877 @opindex exported_symbols_list
7879 @opindex flat_namespace
7880 @opindex force_flat_namespace
7881 @opindex headerpad_max_install_names
7884 @opindex install_name
7885 @opindex keep_private_externs
7886 @opindex multi_module
7887 @opindex multiply_defined
7888 @opindex multiply_defined_unused
7890 @opindex no_dead_strip_inits_and_terms
7891 @opindex nofixprebinding
7892 @opindex nomultidefs
7894 @opindex noseglinkedit
7895 @opindex pagezero_size
7897 @opindex prebind_all_twolevel_modules
7898 @opindex private_bundle
7899 @opindex read_only_relocs
7901 @opindex sectobjectsymbols
7905 @opindex sectobjectsymbols
7908 @opindex segs_read_only_addr
7909 @opindex segs_read_write_addr
7910 @opindex seg_addr_table
7911 @opindex seg_addr_table_filename
7912 @opindex seglinkedit
7914 @opindex segs_read_only_addr
7915 @opindex segs_read_write_addr
7916 @opindex single_module
7918 @opindex sub_library
7919 @opindex sub_umbrella
7920 @opindex twolevel_namespace
7923 @opindex unexported_symbols_list
7924 @opindex weak_reference_mismatches
7925 @opindex whatsloaded
7927 These options are passed to the Darwin linker. The Darwin linker man page
7928 describes them in detail.
7931 @node DEC Alpha Options
7932 @subsection DEC Alpha Options
7934 These @samp{-m} options are defined for the DEC Alpha implementations:
7937 @item -mno-soft-float
7939 @opindex mno-soft-float
7940 @opindex msoft-float
7941 Use (do not use) the hardware floating-point instructions for
7942 floating-point operations. When @option{-msoft-float} is specified,
7943 functions in @file{libgcc.a} will be used to perform floating-point
7944 operations. Unless they are replaced by routines that emulate the
7945 floating-point operations, or compiled in such a way as to call such
7946 emulations routines, these routines will issue floating-point
7947 operations. If you are compiling for an Alpha without floating-point
7948 operations, you must ensure that the library is built so as not to call
7951 Note that Alpha implementations without floating-point operations are
7952 required to have floating-point registers.
7957 @opindex mno-fp-regs
7958 Generate code that uses (does not use) the floating-point register set.
7959 @option{-mno-fp-regs} implies @option{-msoft-float}. If the floating-point
7960 register set is not used, floating point operands are passed in integer
7961 registers as if they were integers and floating-point results are passed
7962 in @code{$0} instead of @code{$f0}. This is a non-standard calling sequence,
7963 so any function with a floating-point argument or return value called by code
7964 compiled with @option{-mno-fp-regs} must also be compiled with that
7967 A typical use of this option is building a kernel that does not use,
7968 and hence need not save and restore, any floating-point registers.
7972 The Alpha architecture implements floating-point hardware optimized for
7973 maximum performance. It is mostly compliant with the IEEE floating
7974 point standard. However, for full compliance, software assistance is
7975 required. This option generates code fully IEEE compliant code
7976 @emph{except} that the @var{inexact-flag} is not maintained (see below).
7977 If this option is turned on, the preprocessor macro @code{_IEEE_FP} is
7978 defined during compilation. The resulting code is less efficient but is
7979 able to correctly support denormalized numbers and exceptional IEEE
7980 values such as not-a-number and plus/minus infinity. Other Alpha
7981 compilers call this option @option{-ieee_with_no_inexact}.
7983 @item -mieee-with-inexact
7984 @opindex mieee-with-inexact
7985 This is like @option{-mieee} except the generated code also maintains
7986 the IEEE @var{inexact-flag}. Turning on this option causes the
7987 generated code to implement fully-compliant IEEE math. In addition to
7988 @code{_IEEE_FP}, @code{_IEEE_FP_EXACT} is defined as a preprocessor
7989 macro. On some Alpha implementations the resulting code may execute
7990 significantly slower than the code generated by default. Since there is
7991 very little code that depends on the @var{inexact-flag}, you should
7992 normally not specify this option. Other Alpha compilers call this
7993 option @option{-ieee_with_inexact}.
7995 @item -mfp-trap-mode=@var{trap-mode}
7996 @opindex mfp-trap-mode
7997 This option controls what floating-point related traps are enabled.
7998 Other Alpha compilers call this option @option{-fptm @var{trap-mode}}.
7999 The trap mode can be set to one of four values:
8003 This is the default (normal) setting. The only traps that are enabled
8004 are the ones that cannot be disabled in software (e.g., division by zero
8008 In addition to the traps enabled by @samp{n}, underflow traps are enabled
8012 Like @samp{su}, but the instructions are marked to be safe for software
8013 completion (see Alpha architecture manual for details).
8016 Like @samp{su}, but inexact traps are enabled as well.
8019 @item -mfp-rounding-mode=@var{rounding-mode}
8020 @opindex mfp-rounding-mode
8021 Selects the IEEE rounding mode. Other Alpha compilers call this option
8022 @option{-fprm @var{rounding-mode}}. The @var{rounding-mode} can be one
8027 Normal IEEE rounding mode. Floating point numbers are rounded towards
8028 the nearest machine number or towards the even machine number in case
8032 Round towards minus infinity.
8035 Chopped rounding mode. Floating point numbers are rounded towards zero.
8038 Dynamic rounding mode. A field in the floating point control register
8039 (@var{fpcr}, see Alpha architecture reference manual) controls the
8040 rounding mode in effect. The C library initializes this register for
8041 rounding towards plus infinity. Thus, unless your program modifies the
8042 @var{fpcr}, @samp{d} corresponds to round towards plus infinity.
8045 @item -mtrap-precision=@var{trap-precision}
8046 @opindex mtrap-precision
8047 In the Alpha architecture, floating point traps are imprecise. This
8048 means without software assistance it is impossible to recover from a
8049 floating trap and program execution normally needs to be terminated.
8050 GCC can generate code that can assist operating system trap handlers
8051 in determining the exact location that caused a floating point trap.
8052 Depending on the requirements of an application, different levels of
8053 precisions can be selected:
8057 Program precision. This option is the default and means a trap handler
8058 can only identify which program caused a floating point exception.
8061 Function precision. The trap handler can determine the function that
8062 caused a floating point exception.
8065 Instruction precision. The trap handler can determine the exact
8066 instruction that caused a floating point exception.
8069 Other Alpha compilers provide the equivalent options called
8070 @option{-scope_safe} and @option{-resumption_safe}.
8072 @item -mieee-conformant
8073 @opindex mieee-conformant
8074 This option marks the generated code as IEEE conformant. You must not
8075 use this option unless you also specify @option{-mtrap-precision=i} and either
8076 @option{-mfp-trap-mode=su} or @option{-mfp-trap-mode=sui}. Its only effect
8077 is to emit the line @samp{.eflag 48} in the function prologue of the
8078 generated assembly file. Under DEC Unix, this has the effect that
8079 IEEE-conformant math library routines will be linked in.
8081 @item -mbuild-constants
8082 @opindex mbuild-constants
8083 Normally GCC examines a 32- or 64-bit integer constant to
8084 see if it can construct it from smaller constants in two or three
8085 instructions. If it cannot, it will output the constant as a literal and
8086 generate code to load it from the data segment at runtime.
8088 Use this option to require GCC to construct @emph{all} integer constants
8089 using code, even if it takes more instructions (the maximum is six).
8091 You would typically use this option to build a shared library dynamic
8092 loader. Itself a shared library, it must relocate itself in memory
8093 before it can find the variables and constants in its own data segment.
8099 Select whether to generate code to be assembled by the vendor-supplied
8100 assembler (@option{-malpha-as}) or by the GNU assembler @option{-mgas}.
8118 Indicate whether GCC should generate code to use the optional BWX,
8119 CIX, FIX and MAX instruction sets. The default is to use the instruction
8120 sets supported by the CPU type specified via @option{-mcpu=} option or that
8121 of the CPU on which GCC was built if none was specified.
8126 @opindex mfloat-ieee
8127 Generate code that uses (does not use) VAX F and G floating point
8128 arithmetic instead of IEEE single and double precision.
8130 @item -mexplicit-relocs
8131 @itemx -mno-explicit-relocs
8132 @opindex mexplicit-relocs
8133 @opindex mno-explicit-relocs
8134 Older Alpha assemblers provided no way to generate symbol relocations
8135 except via assembler macros. Use of these macros does not allow
8136 optimal instruction scheduling. GNU binutils as of version 2.12
8137 supports a new syntax that allows the compiler to explicitly mark
8138 which relocations should apply to which instructions. This option
8139 is mostly useful for debugging, as GCC detects the capabilities of
8140 the assembler when it is built and sets the default accordingly.
8144 @opindex msmall-data
8145 @opindex mlarge-data
8146 When @option{-mexplicit-relocs} is in effect, static data is
8147 accessed via @dfn{gp-relative} relocations. When @option{-msmall-data}
8148 is used, objects 8 bytes long or smaller are placed in a @dfn{small data area}
8149 (the @code{.sdata} and @code{.sbss} sections) and are accessed via
8150 16-bit relocations off of the @code{$gp} register. This limits the
8151 size of the small data area to 64KB, but allows the variables to be
8152 directly accessed via a single instruction.
8154 The default is @option{-mlarge-data}. With this option the data area
8155 is limited to just below 2GB@. Programs that require more than 2GB of
8156 data must use @code{malloc} or @code{mmap} to allocate the data in the
8157 heap instead of in the program's data segment.
8159 When generating code for shared libraries, @option{-fpic} implies
8160 @option{-msmall-data} and @option{-fPIC} implies @option{-mlarge-data}.
8164 @opindex msmall-text
8165 @opindex mlarge-text
8166 When @option{-msmall-text} is used, the compiler assumes that the
8167 code of the entire program (or shared library) fits in 4MB, and is
8168 thus reachable with a branch instruction. When @option{-msmall-data}
8169 is used, the compiler can assume that all local symbols share the
8170 same @code{$gp} value, and thus reduce the number of instructions
8171 required for a function call from 4 to 1.
8173 The default is @option{-mlarge-text}.
8175 @item -mcpu=@var{cpu_type}
8177 Set the instruction set and instruction scheduling parameters for
8178 machine type @var{cpu_type}. You can specify either the @samp{EV}
8179 style name or the corresponding chip number. GCC supports scheduling
8180 parameters for the EV4, EV5 and EV6 family of processors and will
8181 choose the default values for the instruction set from the processor
8182 you specify. If you do not specify a processor type, GCC will default
8183 to the processor on which the compiler was built.
8185 Supported values for @var{cpu_type} are
8191 Schedules as an EV4 and has no instruction set extensions.
8195 Schedules as an EV5 and has no instruction set extensions.
8199 Schedules as an EV5 and supports the BWX extension.
8204 Schedules as an EV5 and supports the BWX and MAX extensions.
8208 Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
8212 Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
8215 @item -mtune=@var{cpu_type}
8217 Set only the instruction scheduling parameters for machine type
8218 @var{cpu_type}. The instruction set is not changed.
8220 @item -mmemory-latency=@var{time}
8221 @opindex mmemory-latency
8222 Sets the latency the scheduler should assume for typical memory
8223 references as seen by the application. This number is highly
8224 dependent on the memory access patterns used by the application
8225 and the size of the external cache on the machine.
8227 Valid options for @var{time} are
8231 A decimal number representing clock cycles.
8237 The compiler contains estimates of the number of clock cycles for
8238 ``typical'' EV4 & EV5 hardware for the Level 1, 2 & 3 caches
8239 (also called Dcache, Scache, and Bcache), as well as to main memory.
8240 Note that L3 is only valid for EV5.
8245 @node DEC Alpha/VMS Options
8246 @subsection DEC Alpha/VMS Options
8248 These @samp{-m} options are defined for the DEC Alpha/VMS implementations:
8251 @item -mvms-return-codes
8252 @opindex mvms-return-codes
8253 Return VMS condition codes from main. The default is to return POSIX
8254 style condition (e.g.@ error) codes.
8258 @subsection FRV Options
8265 Only use the first 32 general purpose registers.
8270 Use all 64 general purpose registers.
8275 Use only the first 32 floating point registers.
8280 Use all 64 floating point registers
8283 @opindex mhard-float
8285 Use hardware instructions for floating point operations.
8288 @opindex msoft-float
8290 Use library routines for floating point operations.
8295 Dynamically allocate condition code registers.
8300 Do not try to dynamically allocate condition code registers, only
8301 use @code{icc0} and @code{fcc0}.
8306 Change ABI to use double word insns.
8311 Do not use double word instructions.
8316 Use floating point double instructions.
8321 Do not use floating point double instructions.
8326 Use media instructions.
8331 Do not use media instructions.
8336 Use multiply and add/subtract instructions.
8341 Do not use multiply and add/subtract instructions.
8346 Select the FDPIC ABI, that uses function descriptors to represent
8347 pointers to functions. Without any PIC/PIE-related options, it
8348 implies @option{-fPIE}. With @option{-fpic} or @option{-fpie}, it
8349 assumes GOT entries and small data are within a 12-bit range from the
8350 GOT base address; with @option{-fPIC} or @option{-fPIE}, GOT offsets
8351 are computed with 32 bits.
8354 @opindex minline-plt
8356 Enable inlining of PLT entries in function calls to functions that are
8357 not known to bind locally. It has no effect without @option{-mfdpic}.
8358 It's enabled by default if optimizing for speed and compiling for
8359 shared libraries (i.e., @option{-fPIC} or @option{-fpic}), or when an
8360 optimization option such as @option{-O3} or above is present in the
8366 Assume a large TLS segment when generating thread-local code.
8371 Do not assume a large TLS segment when generating thread-local code.
8376 Enable the use of @code{GPREL} relocations in the FDPIC ABI for data
8377 that is known to be in read-only sections. It's enabled by default,
8378 except for @option{-fpic} or @option{-fpie}: even though it may help
8379 make the global offset table smaller, it trades 1 instruction for 4.
8380 With @option{-fPIC} or @option{-fPIE}, it trades 3 instructions for 4,
8381 one of which may be shared by multiple symbols, and it avoids the need
8382 for a GOT entry for the referenced symbol, so it's more likely to be a
8383 win. If it is not, @option{-mno-gprel-ro} can be used to disable it.
8385 @item -multilib-library-pic
8386 @opindex multilib-library-pic
8388 Link with the (library, not FD) pic libraries. It's implied by
8389 @option{-mlibrary-pic}, as well as by @option{-fPIC} and
8390 @option{-fpic} without @option{-mfdpic}. You should never have to use
8396 Follow the EABI requirement of always creating a frame pointer whenever
8397 a stack frame is allocated. This option is enabled by default and can
8398 be disabled with @option{-mno-linked-fp}.
8401 @opindex mlong-calls
8403 Use indirect addressing to call functions outside the current
8404 compilation unit. This allows the functions to be placed anywhere
8405 within the 32-bit address space.
8407 @item -malign-labels
8408 @opindex malign-labels
8410 Try to align labels to an 8-byte boundary by inserting nops into the
8411 previous packet. This option only has an effect when VLIW packing
8412 is enabled. It doesn't create new packets; it merely adds nops to
8416 @opindex mlibrary-pic
8418 Generate position-independent EABI code.
8423 Use only the first four media accumulator registers.
8428 Use all eight media accumulator registers.
8433 Pack VLIW instructions.
8438 Do not pack VLIW instructions.
8443 Do not mark ABI switches in e_flags.
8448 Enable the use of conditional-move instructions (default).
8450 This switch is mainly for debugging the compiler and will likely be removed
8451 in a future version.
8453 @item -mno-cond-move
8454 @opindex mno-cond-move
8456 Disable the use of conditional-move instructions.
8458 This switch is mainly for debugging the compiler and will likely be removed
8459 in a future version.
8464 Enable the use of conditional set instructions (default).
8466 This switch is mainly for debugging the compiler and will likely be removed
8467 in a future version.
8472 Disable the use of conditional set instructions.
8474 This switch is mainly for debugging the compiler and will likely be removed
8475 in a future version.
8480 Enable the use of conditional execution (default).
8482 This switch is mainly for debugging the compiler and will likely be removed
8483 in a future version.
8485 @item -mno-cond-exec
8486 @opindex mno-cond-exec
8488 Disable the use of conditional execution.
8490 This switch is mainly for debugging the compiler and will likely be removed
8491 in a future version.
8494 @opindex mvliw-branch
8496 Run a pass to pack branches into VLIW instructions (default).
8498 This switch is mainly for debugging the compiler and will likely be removed
8499 in a future version.
8501 @item -mno-vliw-branch
8502 @opindex mno-vliw-branch
8504 Do not run a pass to pack branches into VLIW instructions.
8506 This switch is mainly for debugging the compiler and will likely be removed
8507 in a future version.
8509 @item -mmulti-cond-exec
8510 @opindex mmulti-cond-exec
8512 Enable optimization of @code{&&} and @code{||} in conditional execution
8515 This switch is mainly for debugging the compiler and will likely be removed
8516 in a future version.
8518 @item -mno-multi-cond-exec
8519 @opindex mno-multi-cond-exec
8521 Disable optimization of @code{&&} and @code{||} in conditional execution.
8523 This switch is mainly for debugging the compiler and will likely be removed
8524 in a future version.
8526 @item -mnested-cond-exec
8527 @opindex mnested-cond-exec
8529 Enable nested conditional execution optimizations (default).
8531 This switch is mainly for debugging the compiler and will likely be removed
8532 in a future version.
8534 @item -mno-nested-cond-exec
8535 @opindex mno-nested-cond-exec
8537 Disable nested conditional execution optimizations.
8539 This switch is mainly for debugging the compiler and will likely be removed
8540 in a future version.
8542 @item -moptimize-membar
8543 @opindex moptimize-membar
8545 This switch removes redundant @code{membar} instructions from the
8546 compiler generated code. It is enabled by default.
8548 @item -mno-optimize-membar
8549 @opindex mno-optimize-membar
8551 This switch disables the automatic removal of redundant @code{membar}
8552 instructions from the generated code.
8554 @item -mtomcat-stats
8555 @opindex mtomcat-stats
8557 Cause gas to print out tomcat statistics.
8559 @item -mcpu=@var{cpu}
8562 Select the processor type for which to generate code. Possible values are
8563 @samp{frv}, @samp{fr550}, @samp{tomcat}, @samp{fr500}, @samp{fr450},
8564 @samp{fr405}, @samp{fr400}, @samp{fr300} and @samp{simple}.
8568 @node H8/300 Options
8569 @subsection H8/300 Options
8571 These @samp{-m} options are defined for the H8/300 implementations:
8576 Shorten some address references at link time, when possible; uses the
8577 linker option @option{-relax}. @xref{H8/300,, @code{ld} and the H8/300,
8578 ld, Using ld}, for a fuller description.
8582 Generate code for the H8/300H@.
8586 Generate code for the H8S@.
8590 Generate code for the H8S and H8/300H in the normal mode. This switch
8591 must be used either with @option{-mh} or @option{-ms}.
8595 Generate code for the H8S/2600. This switch must be used with @option{-ms}.
8599 Make @code{int} data 32 bits by default.
8603 On the H8/300H and H8S, use the same alignment rules as for the H8/300.
8604 The default for the H8/300H and H8S is to align longs and floats on 4
8606 @option{-malign-300} causes them to be aligned on 2 byte boundaries.
8607 This option has no effect on the H8/300.
8611 @subsection HPPA Options
8612 @cindex HPPA Options
8614 These @samp{-m} options are defined for the HPPA family of computers:
8617 @item -march=@var{architecture-type}
8619 Generate code for the specified architecture. The choices for
8620 @var{architecture-type} are @samp{1.0} for PA 1.0, @samp{1.1} for PA
8621 1.1, and @samp{2.0} for PA 2.0 processors. Refer to
8622 @file{/usr/lib/sched.models} on an HP-UX system to determine the proper
8623 architecture option for your machine. Code compiled for lower numbered
8624 architectures will run on higher numbered architectures, but not the
8628 @itemx -mpa-risc-1-1
8629 @itemx -mpa-risc-2-0
8630 @opindex mpa-risc-1-0
8631 @opindex mpa-risc-1-1
8632 @opindex mpa-risc-2-0
8633 Synonyms for @option{-march=1.0}, @option{-march=1.1}, and @option{-march=2.0} respectively.
8636 @opindex mbig-switch
8637 Generate code suitable for big switch tables. Use this option only if
8638 the assembler/linker complain about out of range branches within a switch
8641 @item -mjump-in-delay
8642 @opindex mjump-in-delay
8643 Fill delay slots of function calls with unconditional jump instructions
8644 by modifying the return pointer for the function call to be the target
8645 of the conditional jump.
8647 @item -mdisable-fpregs
8648 @opindex mdisable-fpregs
8649 Prevent floating point registers from being used in any manner. This is
8650 necessary for compiling kernels which perform lazy context switching of
8651 floating point registers. If you use this option and attempt to perform
8652 floating point operations, the compiler will abort.
8654 @item -mdisable-indexing
8655 @opindex mdisable-indexing
8656 Prevent the compiler from using indexing address modes. This avoids some
8657 rather obscure problems when compiling MIG generated code under MACH@.
8659 @item -mno-space-regs
8660 @opindex mno-space-regs
8661 Generate code that assumes the target has no space registers. This allows
8662 GCC to generate faster indirect calls and use unscaled index address modes.
8664 Such code is suitable for level 0 PA systems and kernels.
8666 @item -mfast-indirect-calls
8667 @opindex mfast-indirect-calls
8668 Generate code that assumes calls never cross space boundaries. This
8669 allows GCC to emit code which performs faster indirect calls.
8671 This option will not work in the presence of shared libraries or nested
8674 @item -mfixed-range=@var{register-range}
8675 @opindex mfixed-range
8676 Generate code treating the given register range as fixed registers.
8677 A fixed register is one that the register allocator can not use. This is
8678 useful when compiling kernel code. A register range is specified as
8679 two registers separated by a dash. Multiple register ranges can be
8680 specified separated by a comma.
8682 @item -mlong-load-store
8683 @opindex mlong-load-store
8684 Generate 3-instruction load and store sequences as sometimes required by
8685 the HP-UX 10 linker. This is equivalent to the @samp{+k} option to
8688 @item -mportable-runtime
8689 @opindex mportable-runtime
8690 Use the portable calling conventions proposed by HP for ELF systems.
8694 Enable the use of assembler directives only GAS understands.
8696 @item -mschedule=@var{cpu-type}
8698 Schedule code according to the constraints for the machine type
8699 @var{cpu-type}. The choices for @var{cpu-type} are @samp{700}
8700 @samp{7100}, @samp{7100LC}, @samp{7200}, @samp{7300} and @samp{8000}. Refer
8701 to @file{/usr/lib/sched.models} on an HP-UX system to determine the
8702 proper scheduling option for your machine. The default scheduling is
8706 @opindex mlinker-opt
8707 Enable the optimization pass in the HP-UX linker. Note this makes symbolic
8708 debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9
8709 linkers in which they give bogus error messages when linking some programs.
8712 @opindex msoft-float
8713 Generate output containing library calls for floating point.
8714 @strong{Warning:} the requisite libraries are not available for all HPPA
8715 targets. Normally the facilities of the machine's usual C compiler are
8716 used, but this cannot be done directly in cross-compilation. You must make
8717 your own arrangements to provide suitable library functions for
8718 cross-compilation. The embedded target @samp{hppa1.1-*-pro}
8719 does provide software floating point support.
8721 @option{-msoft-float} changes the calling convention in the output file;
8722 therefore, it is only useful if you compile @emph{all} of a program with
8723 this option. In particular, you need to compile @file{libgcc.a}, the
8724 library that comes with GCC, with @option{-msoft-float} in order for
8729 Generate the predefine, @code{_SIO}, for server IO@. The default is
8730 @option{-mwsio}. This generates the predefines, @code{__hp9000s700},
8731 @code{__hp9000s700__} and @code{_WSIO}, for workstation IO@. These
8732 options are available under HP-UX and HI-UX@.
8736 Use GNU ld specific options. This passes @option{-shared} to ld when
8737 building a shared library. It is the default when GCC is configured,
8738 explicitly or implicitly, with the GNU linker. This option does not
8739 have any affect on which ld is called, it only changes what parameters
8740 are passed to that ld. The ld that is called is determined by the
8741 @option{--with-ld} configure option, GCC's program search path, and
8742 finally by the user's @env{PATH}. The linker used by GCC can be printed
8743 using @samp{which `gcc -print-prog-name=ld`}.
8747 Use HP ld specific options. This passes @option{-b} to ld when building
8748 a shared library and passes @option{+Accept TypeMismatch} to ld on all
8749 links. It is the default when GCC is configured, explicitly or
8750 implicitly, with the HP linker. This option does not have any affect on
8751 which ld is called, it only changes what parameters are passed to that
8752 ld. The ld that is called is determined by the @option{--with-ld}
8753 configure option, GCC's program search path, and finally by the user's
8754 @env{PATH}. The linker used by GCC can be printed using @samp{which
8755 `gcc -print-prog-name=ld`}.
8758 @opindex mno-long-calls
8759 Generate code that uses long call sequences. This ensures that a call
8760 is always able to reach linker generated stubs. The default is to generate
8761 long calls only when the distance from the call site to the beginning
8762 of the function or translation unit, as the case may be, exceeds a
8763 predefined limit set by the branch type being used. The limits for
8764 normal calls are 7,600,000 and 240,000 bytes, respectively for the
8765 PA 2.0 and PA 1.X architectures. Sibcalls are always limited at
8768 Distances are measured from the beginning of functions when using the
8769 @option{-ffunction-sections} option, or when using the @option{-mgas}
8770 and @option{-mno-portable-runtime} options together under HP-UX with
8773 It is normally not desirable to use this option as it will degrade
8774 performance. However, it may be useful in large applications,
8775 particularly when partial linking is used to build the application.
8777 The types of long calls used depends on the capabilities of the
8778 assembler and linker, and the type of code being generated. The
8779 impact on systems that support long absolute calls, and long pic
8780 symbol-difference or pc-relative calls should be relatively small.
8781 However, an indirect call is used on 32-bit ELF systems in pic code
8782 and it is quite long.
8784 @item -munix=@var{unix-std}
8786 Generate compiler predefines and select a startfile for the specified
8787 UNIX standard. The choices for @var{unix-std} are @samp{93}, @samp{95}
8788 and @samp{98}. @samp{93} is supported on all HP-UX versions. @samp{95}
8789 is available on HP-UX 10.10 and later. @samp{98} is available on HP-UX
8790 11.11 and later. The default values are @samp{93} for HP-UX 10.00,
8791 @samp{95} for HP-UX 10.10 though to 11.00, and @samp{98} for HP-UX 11.11
8794 @option{-munix=93} provides the same predefines as GCC 3.3 and 3.4.
8795 @option{-munix=95} provides additional predefines for @code{XOPEN_UNIX}
8796 and @code{_XOPEN_SOURCE_EXTENDED}, and the startfile @file{unix95.o}.
8797 @option{-munix=98} provides additional predefines for @code{_XOPEN_UNIX},
8798 @code{_XOPEN_SOURCE_EXTENDED}, @code{_INCLUDE__STDC_A1_SOURCE} and
8799 @code{_INCLUDE_XOPEN_SOURCE_500}, and the startfile @file{unix98.o}.
8801 It is @emph{important} to note that this option changes the interfaces
8802 for various library routines. It also affects the operational behavior
8803 of the C library. Thus, @emph{extreme} care is needed in using this
8806 Library code that is intended to operate with more than one UNIX
8807 standard must test, set and restore the variable @var{__xpg4_extended_mask}
8808 as appropriate. Most GNU software doesn't provide this capability.
8812 Suppress the generation of link options to search libdld.sl when the
8813 @option{-static} option is specified on HP-UX 10 and later.
8817 The HP-UX implementation of setlocale in libc has a dependency on
8818 libdld.sl. There isn't an archive version of libdld.sl. Thus,
8819 when the @option{-static} option is specified, special link options
8820 are needed to resolve this dependency.
8822 On HP-UX 10 and later, the GCC driver adds the necessary options to
8823 link with libdld.sl when the @option{-static} option is specified.
8824 This causes the resulting binary to be dynamic. On the 64-bit port,
8825 the linkers generate dynamic binaries by default in any case. The
8826 @option{-nolibdld} option can be used to prevent the GCC driver from
8827 adding these link options.
8831 Add support for multithreading with the @dfn{dce thread} library
8832 under HP-UX@. This option sets flags for both the preprocessor and
8836 @node i386 and x86-64 Options
8837 @subsection Intel 386 and AMD x86-64 Options
8838 @cindex i386 Options
8839 @cindex x86-64 Options
8840 @cindex Intel 386 Options
8841 @cindex AMD x86-64 Options
8843 These @samp{-m} options are defined for the i386 and x86-64 family of
8847 @item -mtune=@var{cpu-type}
8849 Tune to @var{cpu-type} everything applicable about the generated code, except
8850 for the ABI and the set of available instructions. The choices for
8854 Original Intel's i386 CPU@.
8856 Intel's i486 CPU@. (No scheduling is implemented for this chip.)
8858 Intel Pentium CPU with no MMX support.
8860 Intel PentiumMMX CPU based on Pentium core with MMX instruction set support.
8861 @item i686, pentiumpro
8862 Intel PentiumPro CPU@.
8864 Intel Pentium2 CPU based on PentiumPro core with MMX instruction set support.
8865 @item pentium3, pentium3m
8866 Intel Pentium3 CPU based on PentiumPro core with MMX and SSE instruction set
8869 Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2 instruction set
8870 support. Used by Centrino notebooks.
8871 @item pentium4, pentium4m
8872 Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set support.
8874 Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and SSE3 instruction
8877 Improved version of Intel Pentium4 CPU with 64-bit extensions, MMX, SSE,
8878 SSE2 and SSE3 instruction set support.
8880 AMD K6 CPU with MMX instruction set support.
8882 Improved versions of AMD K6 CPU with MMX and 3dNOW! instruction set support.
8883 @item athlon, athlon-tbird
8884 AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and SSE prefetch instructions
8886 @item athlon-4, athlon-xp, athlon-mp
8887 Improved AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and full SSE
8888 instruction set support.
8889 @item k8, opteron, athlon64, athlon-fx
8890 AMD K8 core based CPUs with x86-64 instruction set support. (This supersets
8891 MMX, SSE, SSE2, 3dNOW!, enhanced 3dNOW! and 64-bit instruction set extensions.)
8893 IDT Winchip C6 CPU, dealt in same way as i486 with additional MMX instruction
8896 IDT Winchip2 CPU, dealt in same way as i486 with additional MMX and 3dNOW!
8897 instruction set support.
8899 Via C3 CPU with MMX and 3dNOW! instruction set support. (No scheduling is
8900 implemented for this chip.)
8902 Via C3-2 CPU with MMX and SSE instruction set support. (No scheduling is
8903 implemented for this chip.)
8906 While picking a specific @var{cpu-type} will schedule things appropriately
8907 for that particular chip, the compiler will not generate any code that
8908 does not run on the i386 without the @option{-march=@var{cpu-type}} option
8911 @item -march=@var{cpu-type}
8913 Generate instructions for the machine type @var{cpu-type}. The choices
8914 for @var{cpu-type} are the same as for @option{-mtune}. Moreover,
8915 specifying @option{-march=@var{cpu-type}} implies @option{-mtune=@var{cpu-type}}.
8917 @item -mcpu=@var{cpu-type}
8919 A deprecated synonym for @option{-mtune}.
8928 @opindex mpentiumpro
8929 These options are synonyms for @option{-mtune=i386}, @option{-mtune=i486},
8930 @option{-mtune=pentium}, and @option{-mtune=pentiumpro} respectively.
8931 These synonyms are deprecated.
8933 @item -mfpmath=@var{unit}
8935 Generate floating point arithmetics for selected unit @var{unit}. The choices
8940 Use the standard 387 floating point coprocessor present majority of chips and
8941 emulated otherwise. Code compiled with this option will run almost everywhere.
8942 The temporary results are computed in 80bit precision instead of precision
8943 specified by the type resulting in slightly different results compared to most
8944 of other chips. See @option{-ffloat-store} for more detailed description.
8946 This is the default choice for i386 compiler.
8949 Use scalar floating point instructions present in the SSE instruction set.
8950 This instruction set is supported by Pentium3 and newer chips, in the AMD line
8951 by Athlon-4, Athlon-xp and Athlon-mp chips. The earlier version of SSE
8952 instruction set supports only single precision arithmetics, thus the double and
8953 extended precision arithmetics is still done using 387. Later version, present
8954 only in Pentium4 and the future AMD x86-64 chips supports double precision
8957 For the i386 compiler, you need to use @option{-march=@var{cpu-type}}, @option{-msse}
8958 or @option{-msse2} switches to enable SSE extensions and make this option
8959 effective. For the x86-64 compiler, these extensions are enabled by default.
8961 The resulting code should be considerably faster in the majority of cases and avoid
8962 the numerical instability problems of 387 code, but may break some existing
8963 code that expects temporaries to be 80bit.
8965 This is the default choice for the x86-64 compiler.
8968 Attempt to utilize both instruction sets at once. This effectively double the
8969 amount of available registers and on chips with separate execution units for
8970 387 and SSE the execution resources too. Use this option with care, as it is
8971 still experimental, because the GCC register allocator does not model separate
8972 functional units well resulting in instable performance.
8975 @item -masm=@var{dialect}
8976 @opindex masm=@var{dialect}
8977 Output asm instructions using selected @var{dialect}. Supported choices are
8978 @samp{intel} or @samp{att} (the default one).
8983 @opindex mno-ieee-fp
8984 Control whether or not the compiler uses IEEE floating point
8985 comparisons. These handle correctly the case where the result of a
8986 comparison is unordered.
8989 @opindex msoft-float
8990 Generate output containing library calls for floating point.
8991 @strong{Warning:} the requisite libraries are not part of GCC@.
8992 Normally the facilities of the machine's usual C compiler are used, but
8993 this can't be done directly in cross-compilation. You must make your
8994 own arrangements to provide suitable library functions for
8997 On machines where a function returns floating point results in the 80387
8998 register stack, some floating point opcodes may be emitted even if
8999 @option{-msoft-float} is used.
9001 @item -mno-fp-ret-in-387
9002 @opindex mno-fp-ret-in-387
9003 Do not use the FPU registers for return values of functions.
9005 The usual calling convention has functions return values of types
9006 @code{float} and @code{double} in an FPU register, even if there
9007 is no FPU@. The idea is that the operating system should emulate
9010 The option @option{-mno-fp-ret-in-387} causes such values to be returned
9011 in ordinary CPU registers instead.
9013 @item -mno-fancy-math-387
9014 @opindex mno-fancy-math-387
9015 Some 387 emulators do not support the @code{sin}, @code{cos} and
9016 @code{sqrt} instructions for the 387. Specify this option to avoid
9017 generating those instructions. This option is the default on FreeBSD,
9018 OpenBSD and NetBSD@. This option is overridden when @option{-march}
9019 indicates that the target cpu will always have an FPU and so the
9020 instruction will not need emulation. As of revision 2.6.1, these
9021 instructions are not generated unless you also use the
9022 @option{-funsafe-math-optimizations} switch.
9024 @item -malign-double
9025 @itemx -mno-align-double
9026 @opindex malign-double
9027 @opindex mno-align-double
9028 Control whether GCC aligns @code{double}, @code{long double}, and
9029 @code{long long} variables on a two word boundary or a one word
9030 boundary. Aligning @code{double} variables on a two word boundary will
9031 produce code that runs somewhat faster on a @samp{Pentium} at the
9032 expense of more memory.
9034 @strong{Warning:} if you use the @option{-malign-double} switch,
9035 structures containing the above types will be aligned differently than
9036 the published application binary interface specifications for the 386
9037 and will not be binary compatible with structures in code compiled
9038 without that switch.
9040 @item -m96bit-long-double
9041 @itemx -m128bit-long-double
9042 @opindex m96bit-long-double
9043 @opindex m128bit-long-double
9044 These switches control the size of @code{long double} type. The i386
9045 application binary interface specifies the size to be 96 bits,
9046 so @option{-m96bit-long-double} is the default in 32 bit mode.
9048 Modern architectures (Pentium and newer) would prefer @code{long double}
9049 to be aligned to an 8 or 16 byte boundary. In arrays or structures
9050 conforming to the ABI, this would not be possible. So specifying a
9051 @option{-m128bit-long-double} will align @code{long double}
9052 to a 16 byte boundary by padding the @code{long double} with an additional
9055 In the x86-64 compiler, @option{-m128bit-long-double} is the default choice as
9056 its ABI specifies that @code{long double} is to be aligned on 16 byte boundary.
9058 Notice that neither of these options enable any extra precision over the x87
9059 standard of 80 bits for a @code{long double}.
9061 @strong{Warning:} if you override the default value for your target ABI, the
9062 structures and arrays containing @code{long double} variables will change
9063 their size as well as function calling convention for function taking
9064 @code{long double} will be modified. Hence they will not be binary
9065 compatible with arrays or structures in code compiled without that switch.
9069 @itemx -mno-svr3-shlib
9070 @opindex msvr3-shlib
9071 @opindex mno-svr3-shlib
9072 Control whether GCC places uninitialized local variables into the
9073 @code{bss} or @code{data} segments. @option{-msvr3-shlib} places them
9074 into @code{bss}. These options are meaningful only on System V Release 3.
9078 Use a different function-calling convention, in which functions that
9079 take a fixed number of arguments return with the @code{ret} @var{num}
9080 instruction, which pops their arguments while returning. This saves one
9081 instruction in the caller since there is no need to pop the arguments
9084 You can specify that an individual function is called with this calling
9085 sequence with the function attribute @samp{stdcall}. You can also
9086 override the @option{-mrtd} option by using the function attribute
9087 @samp{cdecl}. @xref{Function Attributes}.
9089 @strong{Warning:} this calling convention is incompatible with the one
9090 normally used on Unix, so you cannot use it if you need to call
9091 libraries compiled with the Unix compiler.
9093 Also, you must provide function prototypes for all functions that
9094 take variable numbers of arguments (including @code{printf});
9095 otherwise incorrect code will be generated for calls to those
9098 In addition, seriously incorrect code will result if you call a
9099 function with too many arguments. (Normally, extra arguments are
9100 harmlessly ignored.)
9102 @item -mregparm=@var{num}
9104 Control how many registers are used to pass integer arguments. By
9105 default, no registers are used to pass arguments, and at most 3
9106 registers can be used. You can control this behavior for a specific
9107 function by using the function attribute @samp{regparm}.
9108 @xref{Function Attributes}.
9110 @strong{Warning:} if you use this switch, and
9111 @var{num} is nonzero, then you must build all modules with the same
9112 value, including any libraries. This includes the system libraries and
9116 @opindex msseregparm
9117 Use SSE register passing conventions for float and double arguments
9118 and return values. You can control this behavior for a specific
9119 function by using the function attribute @samp{sseregparm}.
9120 @xref{Function Attributes}.
9122 @strong{Warning:} if you use this switch then you must build all
9123 modules with the same value, including any libraries. This includes
9124 the system libraries and startup modules.
9126 @item -mpreferred-stack-boundary=@var{num}
9127 @opindex mpreferred-stack-boundary
9128 Attempt to keep the stack boundary aligned to a 2 raised to @var{num}
9129 byte boundary. If @option{-mpreferred-stack-boundary} is not specified,
9130 the default is 4 (16 bytes or 128 bits), except when optimizing for code
9131 size (@option{-Os}), in which case the default is the minimum correct
9132 alignment (4 bytes for x86, and 8 bytes for x86-64).
9134 On Pentium and PentiumPro, @code{double} and @code{long double} values
9135 should be aligned to an 8 byte boundary (see @option{-malign-double}) or
9136 suffer significant run time performance penalties. On Pentium III, the
9137 Streaming SIMD Extension (SSE) data type @code{__m128} suffers similar
9138 penalties if it is not 16 byte aligned.
9140 To ensure proper alignment of this values on the stack, the stack boundary
9141 must be as aligned as that required by any value stored on the stack.
9142 Further, every function must be generated such that it keeps the stack
9143 aligned. Thus calling a function compiled with a higher preferred
9144 stack boundary from a function compiled with a lower preferred stack
9145 boundary will most likely misalign the stack. It is recommended that
9146 libraries that use callbacks always use the default setting.
9148 This extra alignment does consume extra stack space, and generally
9149 increases code size. Code that is sensitive to stack space usage, such
9150 as embedded systems and operating system kernels, may want to reduce the
9151 preferred alignment to @option{-mpreferred-stack-boundary=2}.
9169 These switches enable or disable the use of built-in functions that allow
9170 direct access to the MMX, SSE, SSE2, SSE3 and 3Dnow extensions of the
9173 @xref{X86 Built-in Functions}, for details of the functions enabled
9174 and disabled by these switches.
9176 To have SSE/SSE2 instructions generated automatically from floating-point
9177 code, see @option{-mfpmath=sse}.
9180 @itemx -mno-push-args
9182 @opindex mno-push-args
9183 Use PUSH operations to store outgoing parameters. This method is shorter
9184 and usually equally fast as method using SUB/MOV operations and is enabled
9185 by default. In some cases disabling it may improve performance because of
9186 improved scheduling and reduced dependencies.
9188 @item -maccumulate-outgoing-args
9189 @opindex maccumulate-outgoing-args
9190 If enabled, the maximum amount of space required for outgoing arguments will be
9191 computed in the function prologue. This is faster on most modern CPUs
9192 because of reduced dependencies, improved scheduling and reduced stack usage
9193 when preferred stack boundary is not equal to 2. The drawback is a notable
9194 increase in code size. This switch implies @option{-mno-push-args}.
9198 Support thread-safe exception handling on @samp{Mingw32}. Code that relies
9199 on thread-safe exception handling must compile and link all code with the
9200 @option{-mthreads} option. When compiling, @option{-mthreads} defines
9201 @option{-D_MT}; when linking, it links in a special thread helper library
9202 @option{-lmingwthrd} which cleans up per thread exception handling data.
9204 @item -mno-align-stringops
9205 @opindex mno-align-stringops
9206 Do not align destination of inlined string operations. This switch reduces
9207 code size and improves performance in case the destination is already aligned,
9208 but GCC doesn't know about it.
9210 @item -minline-all-stringops
9211 @opindex minline-all-stringops
9212 By default GCC inlines string operations only when destination is known to be
9213 aligned at least to 4 byte boundary. This enables more inlining, increase code
9214 size, but may improve performance of code that depends on fast memcpy, strlen
9215 and memset for short lengths.
9217 @item -momit-leaf-frame-pointer
9218 @opindex momit-leaf-frame-pointer
9219 Don't keep the frame pointer in a register for leaf functions. This
9220 avoids the instructions to save, set up and restore frame pointers and
9221 makes an extra register available in leaf functions. The option
9222 @option{-fomit-frame-pointer} removes the frame pointer for all functions
9223 which might make debugging harder.
9225 @item -mtls-direct-seg-refs
9226 @itemx -mno-tls-direct-seg-refs
9227 @opindex mtls-direct-seg-refs
9228 Controls whether TLS variables may be accessed with offsets from the
9229 TLS segment register (@code{%gs} for 32-bit, @code{%fs} for 64-bit),
9230 or whether the thread base pointer must be added. Whether or not this
9231 is legal depends on the operating system, and whether it maps the
9232 segment to cover the entire TLS area.
9234 For systems that use GNU libc, the default is on.
9237 These @samp{-m} switches are supported in addition to the above
9238 on AMD x86-64 processors in 64-bit environments.
9245 Generate code for a 32-bit or 64-bit environment.
9246 The 32-bit environment sets int, long and pointer to 32 bits and
9247 generates code that runs on any i386 system.
9248 The 64-bit environment sets int to 32 bits and long and pointer
9249 to 64 bits and generates code for AMD's x86-64 architecture.
9252 @opindex no-red-zone
9253 Do not use a so called red zone for x86-64 code. The red zone is mandated
9254 by the x86-64 ABI, it is a 128-byte area beyond the location of the
9255 stack pointer that will not be modified by signal or interrupt handlers
9256 and therefore can be used for temporary data without adjusting the stack
9257 pointer. The flag @option{-mno-red-zone} disables this red zone.
9259 @item -mcmodel=small
9260 @opindex mcmodel=small
9261 Generate code for the small code model: the program and its symbols must
9262 be linked in the lower 2 GB of the address space. Pointers are 64 bits.
9263 Programs can be statically or dynamically linked. This is the default
9266 @item -mcmodel=kernel
9267 @opindex mcmodel=kernel
9268 Generate code for the kernel code model. The kernel runs in the
9269 negative 2 GB of the address space.
9270 This model has to be used for Linux kernel code.
9272 @item -mcmodel=medium
9273 @opindex mcmodel=medium
9274 Generate code for the medium model: The program is linked in the lower 2
9275 GB of the address space but symbols can be located anywhere in the
9276 address space. Programs can be statically or dynamically linked, but
9277 building of shared libraries are not supported with the medium model.
9279 @item -mcmodel=large
9280 @opindex mcmodel=large
9281 Generate code for the large model: This model makes no assumptions
9282 about addresses and sizes of sections. Currently GCC does not implement
9287 @subsection IA-64 Options
9288 @cindex IA-64 Options
9290 These are the @samp{-m} options defined for the Intel IA-64 architecture.
9294 @opindex mbig-endian
9295 Generate code for a big endian target. This is the default for HP-UX@.
9297 @item -mlittle-endian
9298 @opindex mlittle-endian
9299 Generate code for a little endian target. This is the default for AIX5
9306 Generate (or don't) code for the GNU assembler. This is the default.
9307 @c Also, this is the default if the configure option @option{--with-gnu-as}
9314 Generate (or don't) code for the GNU linker. This is the default.
9315 @c Also, this is the default if the configure option @option{--with-gnu-ld}
9320 Generate code that does not use a global pointer register. The result
9321 is not position independent code, and violates the IA-64 ABI@.
9323 @item -mvolatile-asm-stop
9324 @itemx -mno-volatile-asm-stop
9325 @opindex mvolatile-asm-stop
9326 @opindex mno-volatile-asm-stop
9327 Generate (or don't) a stop bit immediately before and after volatile asm
9330 @item -mregister-names
9331 @itemx -mno-register-names
9332 @opindex mregister-names
9333 @opindex mno-register-names
9334 Generate (or don't) @samp{in}, @samp{loc}, and @samp{out} register names for
9335 the stacked registers. This may make assembler output more readable.
9341 Disable (or enable) optimizations that use the small data section. This may
9342 be useful for working around optimizer bugs.
9345 @opindex mconstant-gp
9346 Generate code that uses a single constant global pointer value. This is
9347 useful when compiling kernel code.
9351 Generate code that is self-relocatable. This implies @option{-mconstant-gp}.
9352 This is useful when compiling firmware code.
9354 @item -minline-float-divide-min-latency
9355 @opindex minline-float-divide-min-latency
9356 Generate code for inline divides of floating point values
9357 using the minimum latency algorithm.
9359 @item -minline-float-divide-max-throughput
9360 @opindex minline-float-divide-max-throughput
9361 Generate code for inline divides of floating point values
9362 using the maximum throughput algorithm.
9364 @item -minline-int-divide-min-latency
9365 @opindex minline-int-divide-min-latency
9366 Generate code for inline divides of integer values
9367 using the minimum latency algorithm.
9369 @item -minline-int-divide-max-throughput
9370 @opindex minline-int-divide-max-throughput
9371 Generate code for inline divides of integer values
9372 using the maximum throughput algorithm.
9374 @item -minline-sqrt-min-latency
9375 @opindex minline-sqrt-min-latency
9376 Generate code for inline square roots
9377 using the minimum latency algorithm.
9379 @item -minline-sqrt-max-throughput
9380 @opindex minline-sqrt-max-throughput
9381 Generate code for inline square roots
9382 using the maximum throughput algorithm.
9384 @item -mno-dwarf2-asm
9386 @opindex mno-dwarf2-asm
9387 @opindex mdwarf2-asm
9388 Don't (or do) generate assembler code for the DWARF2 line number debugging
9389 info. This may be useful when not using the GNU assembler.
9391 @item -mearly-stop-bits
9392 @itemx -mno-early-stop-bits
9393 @opindex mearly-stop-bits
9394 @opindex mno-early-stop-bits
9395 Allow stop bits to be placed earlier than immediately preceding the
9396 instruction that triggered the stop bit. This can improve instruction
9397 scheduling, but does not always do so.
9399 @item -mfixed-range=@var{register-range}
9400 @opindex mfixed-range
9401 Generate code treating the given register range as fixed registers.
9402 A fixed register is one that the register allocator can not use. This is
9403 useful when compiling kernel code. A register range is specified as
9404 two registers separated by a dash. Multiple register ranges can be
9405 specified separated by a comma.
9407 @item -mtls-size=@var{tls-size}
9409 Specify bit size of immediate TLS offsets. Valid values are 14, 22, and
9412 @item -mtune-arch=@var{cpu-type}
9414 Tune the instruction scheduling for a particular CPU, Valid values are
9415 itanium, itanium1, merced, itanium2, and mckinley.
9421 Add support for multithreading using the POSIX threads library. This
9422 option sets flags for both the preprocessor and linker. It does
9423 not affect the thread safety of object code produced by the compiler or
9424 that of libraries supplied with it. These are HP-UX specific flags.
9430 Generate code for a 32-bit or 64-bit environment.
9431 The 32-bit environment sets int, long and pointer to 32 bits.
9432 The 64-bit environment sets int to 32 bits and long and pointer
9433 to 64 bits. These are HP-UX specific flags.
9438 @subsection M32C Options
9439 @cindex M32C options
9442 @item -mcpu=@var{name}
9444 Select the CPU for which code is generated. @var{name} may be one of
9445 @samp{r8c} for the R8C/Tiny series, @samp{m16c} for the M16C (up to
9446 /60) series, @samp{m32cm} for the M16C/80 series, or @samp{m32c} for
9451 Specifies that the program will be run on the simulator. This causes
9452 an alternate runtime library to be linked in which supports, for
9453 example, file I/O. You must not use this option when generating
9454 programs that will run on real hardware; you must provide your own
9455 runtime library for whatever I/O functions are needed.
9457 @item -memregs=@var{number}
9459 Specifies the number of memory-based pseudo-registers GCC will use
9460 during code generation. These pseudo-registers will be used like real
9461 registers, so there is a tradeoff between GCC's ability to fit the
9462 code into available registers, and the performance penalty of using
9463 memory instead of registers. Note that all modules in a program must
9464 be compiled with the same value for this option. Because of that, you
9465 must not use this option with the default runtime libraries gcc
9470 @node M32R/D Options
9471 @subsection M32R/D Options
9472 @cindex M32R/D options
9474 These @option{-m} options are defined for Renesas M32R/D architectures:
9479 Generate code for the M32R/2@.
9483 Generate code for the M32R/X@.
9487 Generate code for the M32R@. This is the default.
9490 @opindex mmodel=small
9491 Assume all objects live in the lower 16MB of memory (so that their addresses
9492 can be loaded with the @code{ld24} instruction), and assume all subroutines
9493 are reachable with the @code{bl} instruction.
9494 This is the default.
9496 The addressability of a particular object can be set with the
9497 @code{model} attribute.
9499 @item -mmodel=medium
9500 @opindex mmodel=medium
9501 Assume objects may be anywhere in the 32-bit address space (the compiler
9502 will generate @code{seth/add3} instructions to load their addresses), and
9503 assume all subroutines are reachable with the @code{bl} instruction.
9506 @opindex mmodel=large
9507 Assume objects may be anywhere in the 32-bit address space (the compiler
9508 will generate @code{seth/add3} instructions to load their addresses), and
9509 assume subroutines may not be reachable with the @code{bl} instruction
9510 (the compiler will generate the much slower @code{seth/add3/jl}
9511 instruction sequence).
9514 @opindex msdata=none
9515 Disable use of the small data area. Variables will be put into
9516 one of @samp{.data}, @samp{bss}, or @samp{.rodata} (unless the
9517 @code{section} attribute has been specified).
9518 This is the default.
9520 The small data area consists of sections @samp{.sdata} and @samp{.sbss}.
9521 Objects may be explicitly put in the small data area with the
9522 @code{section} attribute using one of these sections.
9525 @opindex msdata=sdata
9526 Put small global and static data in the small data area, but do not
9527 generate special code to reference them.
9531 Put small global and static data in the small data area, and generate
9532 special instructions to reference them.
9536 @cindex smaller data references
9537 Put global and static objects less than or equal to @var{num} bytes
9538 into the small data or bss sections instead of the normal data or bss
9539 sections. The default value of @var{num} is 8.
9540 The @option{-msdata} option must be set to one of @samp{sdata} or @samp{use}
9541 for this option to have any effect.
9543 All modules should be compiled with the same @option{-G @var{num}} value.
9544 Compiling with different values of @var{num} may or may not work; if it
9545 doesn't the linker will give an error message---incorrect code will not be
9550 Makes the M32R specific code in the compiler display some statistics
9551 that might help in debugging programs.
9554 @opindex malign-loops
9555 Align all loops to a 32-byte boundary.
9557 @item -mno-align-loops
9558 @opindex mno-align-loops
9559 Do not enforce a 32-byte alignment for loops. This is the default.
9561 @item -missue-rate=@var{number}
9562 @opindex missue-rate=@var{number}
9563 Issue @var{number} instructions per cycle. @var{number} can only be 1
9566 @item -mbranch-cost=@var{number}
9567 @opindex mbranch-cost=@var{number}
9568 @var{number} can only be 1 or 2. If it is 1 then branches will be
9569 preferred over conditional code, if it is 2, then the opposite will
9572 @item -mflush-trap=@var{number}
9573 @opindex mflush-trap=@var{number}
9574 Specifies the trap number to use to flush the cache. The default is
9575 12. Valid numbers are between 0 and 15 inclusive.
9577 @item -mno-flush-trap
9578 @opindex mno-flush-trap
9579 Specifies that the cache cannot be flushed by using a trap.
9581 @item -mflush-func=@var{name}
9582 @opindex mflush-func=@var{name}
9583 Specifies the name of the operating system function to call to flush
9584 the cache. The default is @emph{_flush_cache}, but a function call
9585 will only be used if a trap is not available.
9587 @item -mno-flush-func
9588 @opindex mno-flush-func
9589 Indicates that there is no OS function for flushing the cache.
9593 @node M680x0 Options
9594 @subsection M680x0 Options
9595 @cindex M680x0 options
9597 These are the @samp{-m} options defined for the 68000 series. The default
9598 values for these options depends on which style of 68000 was selected when
9599 the compiler was configured; the defaults for the most common choices are
9607 Generate output for a 68000. This is the default
9608 when the compiler is configured for 68000-based systems.
9610 Use this option for microcontrollers with a 68000 or EC000 core,
9611 including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
9617 Generate output for a 68020. This is the default
9618 when the compiler is configured for 68020-based systems.
9622 Generate output containing 68881 instructions for floating point.
9623 This is the default for most 68020 systems unless @option{--nfp} was
9624 specified when the compiler was configured.
9628 Generate output for a 68030. This is the default when the compiler is
9629 configured for 68030-based systems.
9633 Generate output for a 68040. This is the default when the compiler is
9634 configured for 68040-based systems.
9636 This option inhibits the use of 68881/68882 instructions that have to be
9637 emulated by software on the 68040. Use this option if your 68040 does not
9638 have code to emulate those instructions.
9642 Generate output for a 68060. This is the default when the compiler is
9643 configured for 68060-based systems.
9645 This option inhibits the use of 68020 and 68881/68882 instructions that
9646 have to be emulated by software on the 68060. Use this option if your 68060
9647 does not have code to emulate those instructions.
9651 Generate output for a CPU32. This is the default
9652 when the compiler is configured for CPU32-based systems.
9654 Use this option for microcontrollers with a
9655 CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334,
9656 68336, 68340, 68341, 68349 and 68360.
9660 Generate output for a 520X ``coldfire'' family cpu. This is the default
9661 when the compiler is configured for 520X-based systems.
9663 Use this option for microcontroller with a 5200 core, including
9664 the MCF5202, MCF5203, MCF5204 and MCF5202.
9669 Generate output for a 68040, without using any of the new instructions.
9670 This results in code which can run relatively efficiently on either a
9671 68020/68881 or a 68030 or a 68040. The generated code does use the
9672 68881 instructions that are emulated on the 68040.
9676 Generate output for a 68060, without using any of the new instructions.
9677 This results in code which can run relatively efficiently on either a
9678 68020/68881 or a 68030 or a 68040. The generated code does use the
9679 68881 instructions that are emulated on the 68060.
9682 @opindex msoft-float
9683 Generate output containing library calls for floating point.
9684 @strong{Warning:} the requisite libraries are not available for all m68k
9685 targets. Normally the facilities of the machine's usual C compiler are
9686 used, but this can't be done directly in cross-compilation. You must
9687 make your own arrangements to provide suitable library functions for
9688 cross-compilation. The embedded targets @samp{m68k-*-aout} and
9689 @samp{m68k-*-coff} do provide software floating point support.
9693 Consider type @code{int} to be 16 bits wide, like @code{short int}.
9694 Additionally, parameters passed on the stack are also aligned to a
9695 16-bit boundary even on targets whose API mandates promotion to 32-bit.
9698 @opindex mnobitfield
9699 Do not use the bit-field instructions. The @option{-m68000}, @option{-mcpu32}
9700 and @option{-m5200} options imply @w{@option{-mnobitfield}}.
9704 Do use the bit-field instructions. The @option{-m68020} option implies
9705 @option{-mbitfield}. This is the default if you use a configuration
9706 designed for a 68020.
9710 Use a different function-calling convention, in which functions
9711 that take a fixed number of arguments return with the @code{rtd}
9712 instruction, which pops their arguments while returning. This
9713 saves one instruction in the caller since there is no need to pop
9714 the arguments there.
9716 This calling convention is incompatible with the one normally
9717 used on Unix, so you cannot use it if you need to call libraries
9718 compiled with the Unix compiler.
9720 Also, you must provide function prototypes for all functions that
9721 take variable numbers of arguments (including @code{printf});
9722 otherwise incorrect code will be generated for calls to those
9725 In addition, seriously incorrect code will result if you call a
9726 function with too many arguments. (Normally, extra arguments are
9727 harmlessly ignored.)
9729 The @code{rtd} instruction is supported by the 68010, 68020, 68030,
9730 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
9733 @itemx -mno-align-int
9735 @opindex mno-align-int
9736 Control whether GCC aligns @code{int}, @code{long}, @code{long long},
9737 @code{float}, @code{double}, and @code{long double} variables on a 32-bit
9738 boundary (@option{-malign-int}) or a 16-bit boundary (@option{-mno-align-int}).
9739 Aligning variables on 32-bit boundaries produces code that runs somewhat
9740 faster on processors with 32-bit busses at the expense of more memory.
9742 @strong{Warning:} if you use the @option{-malign-int} switch, GCC will
9743 align structures containing the above types differently than
9744 most published application binary interface specifications for the m68k.
9748 Use the pc-relative addressing mode of the 68000 directly, instead of
9749 using a global offset table. At present, this option implies @option{-fpic},
9750 allowing at most a 16-bit offset for pc-relative addressing. @option{-fPIC} is
9751 not presently supported with @option{-mpcrel}, though this could be supported for
9752 68020 and higher processors.
9754 @item -mno-strict-align
9755 @itemx -mstrict-align
9756 @opindex mno-strict-align
9757 @opindex mstrict-align
9758 Do not (do) assume that unaligned memory references will be handled by
9762 Generate code that allows the data segment to be located in a different
9763 area of memory from the text segment. This allows for execute in place in
9764 an environment without virtual memory management. This option implies
9768 Generate code that assumes that the data segment follows the text segment.
9769 This is the default.
9771 @item -mid-shared-library
9772 Generate code that supports shared libraries via the library ID method.
9773 This allows for execute in place and shared libraries in an environment
9774 without virtual memory management. This option implies @option{-fPIC}.
9776 @item -mno-id-shared-library
9777 Generate code that doesn't assume ID based shared libraries are being used.
9778 This is the default.
9780 @item -mshared-library-id=n
9781 Specified the identification number of the ID based shared library being
9782 compiled. Specifying a value of 0 will generate more compact code, specifying
9783 other values will force the allocation of that number to the current
9784 library but is no more space or time efficient than omitting this option.
9788 @node M68hc1x Options
9789 @subsection M68hc1x Options
9790 @cindex M68hc1x options
9792 These are the @samp{-m} options defined for the 68hc11 and 68hc12
9793 microcontrollers. The default values for these options depends on
9794 which style of microcontroller was selected when the compiler was configured;
9795 the defaults for the most common choices are given below.
9802 Generate output for a 68HC11. This is the default
9803 when the compiler is configured for 68HC11-based systems.
9809 Generate output for a 68HC12. This is the default
9810 when the compiler is configured for 68HC12-based systems.
9816 Generate output for a 68HCS12.
9819 @opindex mauto-incdec
9820 Enable the use of 68HC12 pre and post auto-increment and auto-decrement
9827 Enable the use of 68HC12 min and max instructions.
9830 @itemx -mno-long-calls
9831 @opindex mlong-calls
9832 @opindex mno-long-calls
9833 Treat all calls as being far away (near). If calls are assumed to be
9834 far away, the compiler will use the @code{call} instruction to
9835 call a function and the @code{rtc} instruction for returning.
9839 Consider type @code{int} to be 16 bits wide, like @code{short int}.
9841 @item -msoft-reg-count=@var{count}
9842 @opindex msoft-reg-count
9843 Specify the number of pseudo-soft registers which are used for the
9844 code generation. The maximum number is 32. Using more pseudo-soft
9845 register may or may not result in better code depending on the program.
9846 The default is 4 for 68HC11 and 2 for 68HC12.
9851 @subsection MCore Options
9852 @cindex MCore options
9854 These are the @samp{-m} options defined for the Motorola M*Core
9862 @opindex mno-hardlit
9863 Inline constants into the code stream if it can be done in two
9864 instructions or less.
9870 Use the divide instruction. (Enabled by default).
9872 @item -mrelax-immediate
9873 @itemx -mno-relax-immediate
9874 @opindex mrelax-immediate
9875 @opindex mno-relax-immediate
9876 Allow arbitrary sized immediates in bit operations.
9878 @item -mwide-bitfields
9879 @itemx -mno-wide-bitfields
9880 @opindex mwide-bitfields
9881 @opindex mno-wide-bitfields
9882 Always treat bit-fields as int-sized.
9884 @item -m4byte-functions
9885 @itemx -mno-4byte-functions
9886 @opindex m4byte-functions
9887 @opindex mno-4byte-functions
9888 Force all functions to be aligned to a four byte boundary.
9890 @item -mcallgraph-data
9891 @itemx -mno-callgraph-data
9892 @opindex mcallgraph-data
9893 @opindex mno-callgraph-data
9894 Emit callgraph information.
9897 @itemx -mno-slow-bytes
9898 @opindex mslow-bytes
9899 @opindex mno-slow-bytes
9900 Prefer word access when reading byte quantities.
9902 @item -mlittle-endian
9904 @opindex mlittle-endian
9905 @opindex mbig-endian
9906 Generate code for a little endian target.
9912 Generate code for the 210 processor.
9916 @subsection MIPS Options
9917 @cindex MIPS options
9923 Generate big-endian code.
9927 Generate little-endian code. This is the default for @samp{mips*el-*-*}
9930 @item -march=@var{arch}
9932 Generate code that will run on @var{arch}, which can be the name of a
9933 generic MIPS ISA, or the name of a particular processor.
9935 @samp{mips1}, @samp{mips2}, @samp{mips3}, @samp{mips4},
9936 @samp{mips32}, @samp{mips32r2}, and @samp{mips64}.
9937 The processor names are:
9938 @samp{4kc}, @samp{4kp}, @samp{5kc}, @samp{20kc},
9940 @samp{r2000}, @samp{r3000}, @samp{r3900}, @samp{r4000}, @samp{r4400},
9941 @samp{r4600}, @samp{r4650}, @samp{r6000}, @samp{r8000}, @samp{rm7000},
9945 @samp{vr4100}, @samp{vr4111}, @samp{vr4120}, @samp{vr4130}, @samp{vr4300},
9946 @samp{vr5000}, @samp{vr5400} and @samp{vr5500}.
9947 The special value @samp{from-abi} selects the
9948 most compatible architecture for the selected ABI (that is,
9949 @samp{mips1} for 32-bit ABIs and @samp{mips3} for 64-bit ABIs)@.
9951 In processor names, a final @samp{000} can be abbreviated as @samp{k}
9952 (for example, @samp{-march=r2k}). Prefixes are optional, and
9953 @samp{vr} may be written @samp{r}.
9955 GCC defines two macros based on the value of this option. The first
9956 is @samp{_MIPS_ARCH}, which gives the name of target architecture, as
9957 a string. The second has the form @samp{_MIPS_ARCH_@var{foo}},
9958 where @var{foo} is the capitalized value of @samp{_MIPS_ARCH}@.
9959 For example, @samp{-march=r2000} will set @samp{_MIPS_ARCH}
9960 to @samp{"r2000"} and define the macro @samp{_MIPS_ARCH_R2000}.
9962 Note that the @samp{_MIPS_ARCH} macro uses the processor names given
9963 above. In other words, it will have the full prefix and will not
9964 abbreviate @samp{000} as @samp{k}. In the case of @samp{from-abi},
9965 the macro names the resolved architecture (either @samp{"mips1"} or
9966 @samp{"mips3"}). It names the default architecture when no
9967 @option{-march} option is given.
9969 @item -mtune=@var{arch}
9971 Optimize for @var{arch}. Among other things, this option controls
9972 the way instructions are scheduled, and the perceived cost of arithmetic
9973 operations. The list of @var{arch} values is the same as for
9976 When this option is not used, GCC will optimize for the processor
9977 specified by @option{-march}. By using @option{-march} and
9978 @option{-mtune} together, it is possible to generate code that will
9979 run on a family of processors, but optimize the code for one
9980 particular member of that family.
9982 @samp{-mtune} defines the macros @samp{_MIPS_TUNE} and
9983 @samp{_MIPS_TUNE_@var{foo}}, which work in the same way as the
9984 @samp{-march} ones described above.
9988 Equivalent to @samp{-march=mips1}.
9992 Equivalent to @samp{-march=mips2}.
9996 Equivalent to @samp{-march=mips3}.
10000 Equivalent to @samp{-march=mips4}.
10004 Equivalent to @samp{-march=mips32}.
10008 Equivalent to @samp{-march=mips32r2}.
10012 Equivalent to @samp{-march=mips64}.
10017 @opindex mno-mips16
10018 Generate (do not generate) MIPS16 code. If GCC is targetting a
10019 MIPS32 or MIPS64 architecture, it will make use of the MIPS16e ASE@.
10031 Generate code for the given ABI@.
10033 Note that the EABI has a 32-bit and a 64-bit variant. GCC normally
10034 generates 64-bit code when you select a 64-bit architecture, but you
10035 can use @option{-mgp32} to get 32-bit code instead.
10037 For information about the O64 ABI, see
10038 @w{@uref{http://gcc.gnu.org/projects/mipso64-abi.html}}.
10041 @itemx -mno-abicalls
10043 @opindex mno-abicalls
10044 Generate (do not generate) SVR4-style position-independent code.
10045 @option{-mabicalls} is the default for SVR4-based systems.
10051 Lift (do not lift) the usual restrictions on the size of the global
10054 GCC normally uses a single instruction to load values from the GOT@.
10055 While this is relatively efficient, it will only work if the GOT
10056 is smaller than about 64k. Anything larger will cause the linker
10057 to report an error such as:
10059 @cindex relocation truncated to fit (MIPS)
10061 relocation truncated to fit: R_MIPS_GOT16 foobar
10064 If this happens, you should recompile your code with @option{-mxgot}.
10065 It should then work with very large GOTs, although it will also be
10066 less efficient, since it will take three instructions to fetch the
10067 value of a global symbol.
10069 Note that some linkers can create multiple GOTs. If you have such a
10070 linker, you should only need to use @option{-mxgot} when a single object
10071 file accesses more than 64k's worth of GOT entries. Very few do.
10073 These options have no effect unless GCC is generating position
10078 Assume that general-purpose registers are 32 bits wide.
10082 Assume that general-purpose registers are 64 bits wide.
10086 Assume that floating-point registers are 32 bits wide.
10090 Assume that floating-point registers are 64 bits wide.
10093 @opindex mhard-float
10094 Use floating-point coprocessor instructions.
10097 @opindex msoft-float
10098 Do not use floating-point coprocessor instructions. Implement
10099 floating-point calculations using library calls instead.
10101 @item -msingle-float
10102 @opindex msingle-float
10103 Assume that the floating-point coprocessor only supports single-precision
10106 @itemx -mdouble-float
10107 @opindex mdouble-float
10108 Assume that the floating-point coprocessor supports double-precision
10109 operations. This is the default.
10115 Use (do not use) the MIPS DSP ASE. @xref{MIPS DSP Built-in Functions}.
10117 @itemx -mpaired-single
10118 @itemx -mno-paired-single
10119 @opindex mpaired-single
10120 @opindex mno-paired-single
10121 Use (do not use) paired-single floating-point instructions.
10122 @xref{MIPS Paired-Single Support}. This option can only be used
10123 when generating 64-bit code and requires hardware floating-point
10124 support to be enabled.
10129 @opindex mno-mips3d
10130 Use (do not use) the MIPS-3D ASE@. @xref{MIPS-3D Built-in Functions}.
10131 The option @option{-mips3d} implies @option{-mpaired-single}.
10135 Force @code{long} types to be 64 bits wide. See @option{-mlong32} for
10136 an explanation of the default and the way that the pointer size is
10141 Force @code{long}, @code{int}, and pointer types to be 32 bits wide.
10143 The default size of @code{int}s, @code{long}s and pointers depends on
10144 the ABI@. All the supported ABIs use 32-bit @code{int}s. The n64 ABI
10145 uses 64-bit @code{long}s, as does the 64-bit EABI; the others use
10146 32-bit @code{long}s. Pointers are the same size as @code{long}s,
10147 or the same size as integer registers, whichever is smaller.
10153 Assume (do not assume) that all symbols have 32-bit values, regardless
10154 of the selected ABI@. This option is useful in combination with
10155 @option{-mabi=64} and @option{-mno-abicalls} because it allows GCC
10156 to generate shorter and faster references to symbolic addresses.
10160 @cindex smaller data references (MIPS)
10161 @cindex gp-relative references (MIPS)
10162 Put global and static items less than or equal to @var{num} bytes into
10163 the small data or bss section instead of the normal data or bss section.
10164 This allows the data to be accessed using a single instruction.
10166 All modules should be compiled with the same @option{-G @var{num}}
10169 @item -membedded-data
10170 @itemx -mno-embedded-data
10171 @opindex membedded-data
10172 @opindex mno-embedded-data
10173 Allocate variables to the read-only data section first if possible, then
10174 next in the small data section if possible, otherwise in data. This gives
10175 slightly slower code than the default, but reduces the amount of RAM required
10176 when executing, and thus may be preferred for some embedded systems.
10178 @item -muninit-const-in-rodata
10179 @itemx -mno-uninit-const-in-rodata
10180 @opindex muninit-const-in-rodata
10181 @opindex mno-uninit-const-in-rodata
10182 Put uninitialized @code{const} variables in the read-only data section.
10183 This option is only meaningful in conjunction with @option{-membedded-data}.
10185 @item -msplit-addresses
10186 @itemx -mno-split-addresses
10187 @opindex msplit-addresses
10188 @opindex mno-split-addresses
10189 Enable (disable) use of the @code{%hi()} and @code{%lo()} assembler
10190 relocation operators. This option has been superseded by
10191 @option{-mexplicit-relocs} but is retained for backwards compatibility.
10193 @item -mexplicit-relocs
10194 @itemx -mno-explicit-relocs
10195 @opindex mexplicit-relocs
10196 @opindex mno-explicit-relocs
10197 Use (do not use) assembler relocation operators when dealing with symbolic
10198 addresses. The alternative, selected by @option{-mno-explicit-relocs},
10199 is to use assembler macros instead.
10201 @option{-mexplicit-relocs} is the default if GCC was configured
10202 to use an assembler that supports relocation operators.
10204 @item -mcheck-zero-division
10205 @itemx -mno-check-zero-division
10206 @opindex mcheck-zero-division
10207 @opindex mno-check-zero-division
10208 Trap (do not trap) on integer division by zero. The default is
10209 @option{-mcheck-zero-division}.
10211 @item -mdivide-traps
10212 @itemx -mdivide-breaks
10213 @opindex mdivide-traps
10214 @opindex mdivide-breaks
10215 MIPS systems check for division by zero by generating either a
10216 conditional trap or a break instruction. Using traps results in
10217 smaller code, but is only supported on MIPS II and later. Also, some
10218 versions of the Linux kernel have a bug that prevents trap from
10219 generating the proper signal (@code{SIGFPE}). Use @option{-mdivide-traps} to
10220 allow conditional traps on architectures that support them and
10221 @option{-mdivide-breaks} to force the use of breaks.
10223 The default is usually @option{-mdivide-traps}, but this can be
10224 overridden at configure time using @option{--with-divide=breaks}.
10225 Divide-by-zero checks can be completely disabled using
10226 @option{-mno-check-zero-division}.
10231 @opindex mno-memcpy
10232 Force (do not force) the use of @code{memcpy()} for non-trivial block
10233 moves. The default is @option{-mno-memcpy}, which allows GCC to inline
10234 most constant-sized copies.
10237 @itemx -mno-long-calls
10238 @opindex mlong-calls
10239 @opindex mno-long-calls
10240 Disable (do not disable) use of the @code{jal} instruction. Calling
10241 functions using @code{jal} is more efficient but requires the caller
10242 and callee to be in the same 256 megabyte segment.
10244 This option has no effect on abicalls code. The default is
10245 @option{-mno-long-calls}.
10251 Enable (disable) use of the @code{mad}, @code{madu} and @code{mul}
10252 instructions, as provided by the R4650 ISA@.
10255 @itemx -mno-fused-madd
10256 @opindex mfused-madd
10257 @opindex mno-fused-madd
10258 Enable (disable) use of the floating point multiply-accumulate
10259 instructions, when they are available. The default is
10260 @option{-mfused-madd}.
10262 When multiply-accumulate instructions are used, the intermediate
10263 product is calculated to infinite precision and is not subject to
10264 the FCSR Flush to Zero bit. This may be undesirable in some
10269 Tell the MIPS assembler to not run its preprocessor over user
10270 assembler files (with a @samp{.s} suffix) when assembling them.
10273 @itemx -mno-fix-r4000
10274 @opindex mfix-r4000
10275 @opindex mno-fix-r4000
10276 Work around certain R4000 CPU errata:
10279 A double-word or a variable shift may give an incorrect result if executed
10280 immediately after starting an integer division.
10282 A double-word or a variable shift may give an incorrect result if executed
10283 while an integer multiplication is in progress.
10285 An integer division may give an incorrect result if started in a delay slot
10286 of a taken branch or a jump.
10290 @itemx -mno-fix-r4400
10291 @opindex mfix-r4400
10292 @opindex mno-fix-r4400
10293 Work around certain R4400 CPU errata:
10296 A double-word or a variable shift may give an incorrect result if executed
10297 immediately after starting an integer division.
10301 @itemx -mno-fix-vr4120
10302 @opindex mfix-vr4120
10303 Work around certain VR4120 errata:
10306 @code{dmultu} does not always produce the correct result.
10308 @code{div} and @code{ddiv} do not always produce the correct result if one
10309 of the operands is negative.
10311 The workarounds for the division errata rely on special functions in
10312 @file{libgcc.a}. At present, these functions are only provided by
10313 the @code{mips64vr*-elf} configurations.
10315 Other VR4120 errata require a nop to be inserted between certain pairs of
10316 instructions. These errata are handled by the assembler, not by GCC itself.
10319 @opindex mfix-vr4130
10320 Work around the VR4130 @code{mflo}/@code{mfhi} errata. The
10321 workarounds are implemented by the assembler rather than by GCC,
10322 although GCC will avoid using @code{mflo} and @code{mfhi} if the
10323 VR4130 @code{macc}, @code{macchi}, @code{dmacc} and @code{dmacchi}
10324 instructions are available instead.
10327 @itemx -mno-fix-sb1
10329 Work around certain SB-1 CPU core errata.
10330 (This flag currently works around the SB-1 revision 2
10331 ``F1'' and ``F2'' floating point errata.)
10333 @item -mflush-func=@var{func}
10334 @itemx -mno-flush-func
10335 @opindex mflush-func
10336 Specifies the function to call to flush the I and D caches, or to not
10337 call any such function. If called, the function must take the same
10338 arguments as the common @code{_flush_func()}, that is, the address of the
10339 memory range for which the cache is being flushed, the size of the
10340 memory range, and the number 3 (to flush both caches). The default
10341 depends on the target GCC was configured for, but commonly is either
10342 @samp{_flush_func} or @samp{__cpu_flush}.
10344 @item -mbranch-likely
10345 @itemx -mno-branch-likely
10346 @opindex mbranch-likely
10347 @opindex mno-branch-likely
10348 Enable or disable use of Branch Likely instructions, regardless of the
10349 default for the selected architecture. By default, Branch Likely
10350 instructions may be generated if they are supported by the selected
10351 architecture. An exception is for the MIPS32 and MIPS64 architectures
10352 and processors which implement those architectures; for those, Branch
10353 Likely instructions will not be generated by default because the MIPS32
10354 and MIPS64 architectures specifically deprecate their use.
10356 @item -mfp-exceptions
10357 @itemx -mno-fp-exceptions
10358 @opindex mfp-exceptions
10359 Specifies whether FP exceptions are enabled. This affects how we schedule
10360 FP instructions for some processors. The default is that FP exceptions are
10363 For instance, on the SB-1, if FP exceptions are disabled, and we are emitting
10364 64-bit code, then we can use both FP pipes. Otherwise, we can only use one
10367 @item -mvr4130-align
10368 @itemx -mno-vr4130-align
10369 @opindex mvr4130-align
10370 The VR4130 pipeline is two-way superscalar, but can only issue two
10371 instructions together if the first one is 8-byte aligned. When this
10372 option is enabled, GCC will align pairs of instructions that it
10373 thinks should execute in parallel.
10375 This option only has an effect when optimizing for the VR4130.
10376 It normally makes code faster, but at the expense of making it bigger.
10377 It is enabled by default at optimization level @option{-O3}.
10381 @subsection MMIX Options
10382 @cindex MMIX Options
10384 These options are defined for the MMIX:
10388 @itemx -mno-libfuncs
10390 @opindex mno-libfuncs
10391 Specify that intrinsic library functions are being compiled, passing all
10392 values in registers, no matter the size.
10395 @itemx -mno-epsilon
10397 @opindex mno-epsilon
10398 Generate floating-point comparison instructions that compare with respect
10399 to the @code{rE} epsilon register.
10401 @item -mabi=mmixware
10403 @opindex mabi-mmixware
10405 Generate code that passes function parameters and return values that (in
10406 the called function) are seen as registers @code{$0} and up, as opposed to
10407 the GNU ABI which uses global registers @code{$231} and up.
10409 @item -mzero-extend
10410 @itemx -mno-zero-extend
10411 @opindex mzero-extend
10412 @opindex mno-zero-extend
10413 When reading data from memory in sizes shorter than 64 bits, use (do not
10414 use) zero-extending load instructions by default, rather than
10415 sign-extending ones.
10418 @itemx -mno-knuthdiv
10420 @opindex mno-knuthdiv
10421 Make the result of a division yielding a remainder have the same sign as
10422 the divisor. With the default, @option{-mno-knuthdiv}, the sign of the
10423 remainder follows the sign of the dividend. Both methods are
10424 arithmetically valid, the latter being almost exclusively used.
10426 @item -mtoplevel-symbols
10427 @itemx -mno-toplevel-symbols
10428 @opindex mtoplevel-symbols
10429 @opindex mno-toplevel-symbols
10430 Prepend (do not prepend) a @samp{:} to all global symbols, so the assembly
10431 code can be used with the @code{PREFIX} assembly directive.
10435 Generate an executable in the ELF format, rather than the default
10436 @samp{mmo} format used by the @command{mmix} simulator.
10438 @item -mbranch-predict
10439 @itemx -mno-branch-predict
10440 @opindex mbranch-predict
10441 @opindex mno-branch-predict
10442 Use (do not use) the probable-branch instructions, when static branch
10443 prediction indicates a probable branch.
10445 @item -mbase-addresses
10446 @itemx -mno-base-addresses
10447 @opindex mbase-addresses
10448 @opindex mno-base-addresses
10449 Generate (do not generate) code that uses @emph{base addresses}. Using a
10450 base address automatically generates a request (handled by the assembler
10451 and the linker) for a constant to be set up in a global register. The
10452 register is used for one or more base address requests within the range 0
10453 to 255 from the value held in the register. The generally leads to short
10454 and fast code, but the number of different data items that can be
10455 addressed is limited. This means that a program that uses lots of static
10456 data may require @option{-mno-base-addresses}.
10458 @item -msingle-exit
10459 @itemx -mno-single-exit
10460 @opindex msingle-exit
10461 @opindex mno-single-exit
10462 Force (do not force) generated code to have a single exit point in each
10466 @node MN10300 Options
10467 @subsection MN10300 Options
10468 @cindex MN10300 options
10470 These @option{-m} options are defined for Matsushita MN10300 architectures:
10475 Generate code to avoid bugs in the multiply instructions for the MN10300
10476 processors. This is the default.
10478 @item -mno-mult-bug
10479 @opindex mno-mult-bug
10480 Do not generate code to avoid bugs in the multiply instructions for the
10481 MN10300 processors.
10485 Generate code which uses features specific to the AM33 processor.
10489 Do not generate code which uses features specific to the AM33 processor. This
10494 Do not link in the C run-time initialization object file.
10498 Indicate to the linker that it should perform a relaxation optimization pass
10499 to shorten branches, calls and absolute memory addresses. This option only
10500 has an effect when used on the command line for the final link step.
10502 This option makes symbolic debugging impossible.
10505 @node PDP-11 Options
10506 @subsection PDP-11 Options
10507 @cindex PDP-11 Options
10509 These options are defined for the PDP-11:
10514 Use hardware FPP floating point. This is the default. (FIS floating
10515 point on the PDP-11/40 is not supported.)
10518 @opindex msoft-float
10519 Do not use hardware floating point.
10523 Return floating-point results in ac0 (fr0 in Unix assembler syntax).
10527 Return floating-point results in memory. This is the default.
10531 Generate code for a PDP-11/40.
10535 Generate code for a PDP-11/45. This is the default.
10539 Generate code for a PDP-11/10.
10541 @item -mbcopy-builtin
10542 @opindex bcopy-builtin
10543 Use inline @code{movmemhi} patterns for copying memory. This is the
10548 Do not use inline @code{movmemhi} patterns for copying memory.
10554 Use 16-bit @code{int}. This is the default.
10560 Use 32-bit @code{int}.
10563 @itemx -mno-float32
10565 @opindex mno-float32
10566 Use 64-bit @code{float}. This is the default.
10569 @itemx -mno-float64
10571 @opindex mno-float64
10572 Use 32-bit @code{float}.
10576 Use @code{abshi2} pattern. This is the default.
10580 Do not use @code{abshi2} pattern.
10582 @item -mbranch-expensive
10583 @opindex mbranch-expensive
10584 Pretend that branches are expensive. This is for experimenting with
10585 code generation only.
10587 @item -mbranch-cheap
10588 @opindex mbranch-cheap
10589 Do not pretend that branches are expensive. This is the default.
10593 Generate code for a system with split I&D@.
10597 Generate code for a system without split I&D@. This is the default.
10601 Use Unix assembler syntax. This is the default when configured for
10602 @samp{pdp11-*-bsd}.
10606 Use DEC assembler syntax. This is the default when configured for any
10607 PDP-11 target other than @samp{pdp11-*-bsd}.
10610 @node PowerPC Options
10611 @subsection PowerPC Options
10612 @cindex PowerPC options
10614 These are listed under @xref{RS/6000 and PowerPC Options}.
10616 @node RS/6000 and PowerPC Options
10617 @subsection IBM RS/6000 and PowerPC Options
10618 @cindex RS/6000 and PowerPC Options
10619 @cindex IBM RS/6000 and PowerPC Options
10621 These @samp{-m} options are defined for the IBM RS/6000 and PowerPC:
10628 @itemx -mno-powerpc
10629 @itemx -mpowerpc-gpopt
10630 @itemx -mno-powerpc-gpopt
10631 @itemx -mpowerpc-gfxopt
10632 @itemx -mno-powerpc-gfxopt
10634 @itemx -mno-powerpc64
10638 @opindex mno-power2
10640 @opindex mno-powerpc
10641 @opindex mpowerpc-gpopt
10642 @opindex mno-powerpc-gpopt
10643 @opindex mpowerpc-gfxopt
10644 @opindex mno-powerpc-gfxopt
10645 @opindex mpowerpc64
10646 @opindex mno-powerpc64
10647 GCC supports two related instruction set architectures for the
10648 RS/6000 and PowerPC@. The @dfn{POWER} instruction set are those
10649 instructions supported by the @samp{rios} chip set used in the original
10650 RS/6000 systems and the @dfn{PowerPC} instruction set is the
10651 architecture of the Motorola MPC5xx, MPC6xx, MPC8xx microprocessors, and
10652 the IBM 4xx microprocessors.
10654 Neither architecture is a subset of the other. However there is a
10655 large common subset of instructions supported by both. An MQ
10656 register is included in processors supporting the POWER architecture.
10658 You use these options to specify which instructions are available on the
10659 processor you are using. The default value of these options is
10660 determined when configuring GCC@. Specifying the
10661 @option{-mcpu=@var{cpu_type}} overrides the specification of these
10662 options. We recommend you use the @option{-mcpu=@var{cpu_type}} option
10663 rather than the options listed above.
10665 The @option{-mpower} option allows GCC to generate instructions that
10666 are found only in the POWER architecture and to use the MQ register.
10667 Specifying @option{-mpower2} implies @option{-power} and also allows GCC
10668 to generate instructions that are present in the POWER2 architecture but
10669 not the original POWER architecture.
10671 The @option{-mpowerpc} option allows GCC to generate instructions that
10672 are found only in the 32-bit subset of the PowerPC architecture.
10673 Specifying @option{-mpowerpc-gpopt} implies @option{-mpowerpc} and also allows
10674 GCC to use the optional PowerPC architecture instructions in the
10675 General Purpose group, including floating-point square root. Specifying
10676 @option{-mpowerpc-gfxopt} implies @option{-mpowerpc} and also allows GCC to
10677 use the optional PowerPC architecture instructions in the Graphics
10678 group, including floating-point select.
10680 The @option{-mpowerpc64} option allows GCC to generate the additional
10681 64-bit instructions that are found in the full PowerPC64 architecture
10682 and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to
10683 @option{-mno-powerpc64}.
10685 If you specify both @option{-mno-power} and @option{-mno-powerpc}, GCC
10686 will use only the instructions in the common subset of both
10687 architectures plus some special AIX common-mode calls, and will not use
10688 the MQ register. Specifying both @option{-mpower} and @option{-mpowerpc}
10689 permits GCC to use any instruction from either architecture and to
10690 allow use of the MQ register; specify this for the Motorola MPC601.
10692 @item -mnew-mnemonics
10693 @itemx -mold-mnemonics
10694 @opindex mnew-mnemonics
10695 @opindex mold-mnemonics
10696 Select which mnemonics to use in the generated assembler code. With
10697 @option{-mnew-mnemonics}, GCC uses the assembler mnemonics defined for
10698 the PowerPC architecture. With @option{-mold-mnemonics} it uses the
10699 assembler mnemonics defined for the POWER architecture. Instructions
10700 defined in only one architecture have only one mnemonic; GCC uses that
10701 mnemonic irrespective of which of these options is specified.
10703 GCC defaults to the mnemonics appropriate for the architecture in
10704 use. Specifying @option{-mcpu=@var{cpu_type}} sometimes overrides the
10705 value of these option. Unless you are building a cross-compiler, you
10706 should normally not specify either @option{-mnew-mnemonics} or
10707 @option{-mold-mnemonics}, but should instead accept the default.
10709 @item -mcpu=@var{cpu_type}
10711 Set architecture type, register usage, choice of mnemonics, and
10712 instruction scheduling parameters for machine type @var{cpu_type}.
10713 Supported values for @var{cpu_type} are @samp{401}, @samp{403},
10714 @samp{405}, @samp{405fp}, @samp{440}, @samp{440fp}, @samp{505},
10715 @samp{601}, @samp{602}, @samp{603}, @samp{603e}, @samp{604},
10716 @samp{604e}, @samp{620}, @samp{630}, @samp{740}, @samp{7400},
10717 @samp{7450}, @samp{750}, @samp{801}, @samp{821}, @samp{823},
10718 @samp{860}, @samp{970}, @samp{8540}, @samp{common}, @samp{ec603e}, @samp{G3},
10719 @samp{G4}, @samp{G5}, @samp{power}, @samp{power2}, @samp{power3},
10720 @samp{power4}, @samp{power5}, @samp{powerpc}, @samp{powerpc64},
10721 @samp{rios}, @samp{rios1}, @samp{rios2}, @samp{rsc}, and @samp{rs64}.
10723 @option{-mcpu=common} selects a completely generic processor. Code
10724 generated under this option will run on any POWER or PowerPC processor.
10725 GCC will use only the instructions in the common subset of both
10726 architectures, and will not use the MQ register. GCC assumes a generic
10727 processor model for scheduling purposes.
10729 @option{-mcpu=power}, @option{-mcpu=power2}, @option{-mcpu=powerpc}, and
10730 @option{-mcpu=powerpc64} specify generic POWER, POWER2, pure 32-bit
10731 PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine
10732 types, with an appropriate, generic processor model assumed for
10733 scheduling purposes.
10735 The other options specify a specific processor. Code generated under
10736 those options will run best on that processor, and may not run at all on
10739 The @option{-mcpu} options automatically enable or disable the
10740 following options: @option{-maltivec}, @option{-mhard-float},
10741 @option{-mmfcrf}, @option{-mmultiple}, @option{-mnew-mnemonics},
10742 @option{-mpower}, @option{-mpower2}, @option{-mpowerpc64},
10743 @option{-mpowerpc-gpopt}, @option{-mpowerpc-gfxopt},
10744 @option{-mstring}. The particular options set for any particular CPU
10745 will vary between compiler versions, depending on what setting seems
10746 to produce optimal code for that CPU; it doesn't necessarily reflect
10747 the actual hardware's capabilities. If you wish to set an individual
10748 option to a particular value, you may specify it after the
10749 @option{-mcpu} option, like @samp{-mcpu=970 -mno-altivec}.
10751 On AIX, the @option{-maltivec} and @option{-mpowerpc64} options are
10752 not enabled or disabled by the @option{-mcpu} option at present, since
10753 AIX does not have full support for these options. You may still
10754 enable or disable them individually if you're sure it'll work in your
10757 @item -mtune=@var{cpu_type}
10759 Set the instruction scheduling parameters for machine type
10760 @var{cpu_type}, but do not set the architecture type, register usage, or
10761 choice of mnemonics, as @option{-mcpu=@var{cpu_type}} would. The same
10762 values for @var{cpu_type} are used for @option{-mtune} as for
10763 @option{-mcpu}. If both are specified, the code generated will use the
10764 architecture, registers, and mnemonics set by @option{-mcpu}, but the
10765 scheduling parameters set by @option{-mtune}.
10771 Generate code to compute division as reciprocal estimate and iterative
10772 refinement, creating opportunities for increased throughput. This
10773 feature requires: optional PowerPC Graphics instruction set for single
10774 precision and FRE instruction for double precision, assuming divides
10775 cannot generate user-visible traps, and the domain values not include
10776 Infinities, denormals or zero denominator.
10779 @itemx -mno-altivec
10781 @opindex mno-altivec
10782 Generate code that uses (does not use) AltiVec instructions, and also
10783 enable the use of built-in functions that allow more direct access to
10784 the AltiVec instruction set. You may also need to set
10785 @option{-mabi=altivec} to adjust the current ABI with AltiVec ABI
10791 @opindex mno-vrsave
10792 Generate VRSAVE instructions when generating AltiVec code.
10796 Extend the current ABI with SPE ABI extensions. This does not change
10797 the default ABI, instead it adds the SPE ABI extensions to the current
10801 @opindex mabi=no-spe
10802 Disable Booke SPE ABI extensions for the current ABI@.
10805 @opindex msecure-plt
10806 Generate code that allows ld and ld.so to build executables and shared
10807 libraries with non-exec .plt and .got sections. This is a PowerPC
10808 32-bit SYSV ABI option.
10812 Generate code that uses a BSS .plt section that ld.so fills in, and
10813 requires .plt and .got sections that are both writable and executable.
10814 This is a PowerPC 32-bit SYSV ABI option.
10820 This switch enables or disables the generation of ISEL instructions.
10822 @item -misel=@var{yes/no}
10823 This switch has been deprecated. Use @option{-misel} and
10824 @option{-mno-isel} instead.
10830 This switch enables or disables the generation of SPE simd
10833 @item -mspe=@var{yes/no}
10834 This option has been deprecated. Use @option{-mspe} and
10835 @option{-mno-spe} instead.
10837 @item -mfloat-gprs=@var{yes/single/double/no}
10838 @itemx -mfloat-gprs
10839 @opindex mfloat-gprs
10840 This switch enables or disables the generation of floating point
10841 operations on the general purpose registers for architectures that
10844 The argument @var{yes} or @var{single} enables the use of
10845 single-precision floating point operations.
10847 The argument @var{double} enables the use of single and
10848 double-precision floating point operations.
10850 The argument @var{no} disables floating point operations on the
10851 general purpose registers.
10853 This option is currently only available on the MPC854x.
10859 Generate code for 32-bit or 64-bit environments of Darwin and SVR4
10860 targets (including GNU/Linux). The 32-bit environment sets int, long
10861 and pointer to 32 bits and generates code that runs on any PowerPC
10862 variant. The 64-bit environment sets int to 32 bits and long and
10863 pointer to 64 bits, and generates code for PowerPC64, as for
10864 @option{-mpowerpc64}.
10867 @itemx -mno-fp-in-toc
10868 @itemx -mno-sum-in-toc
10869 @itemx -mminimal-toc
10871 @opindex mno-fp-in-toc
10872 @opindex mno-sum-in-toc
10873 @opindex mminimal-toc
10874 Modify generation of the TOC (Table Of Contents), which is created for
10875 every executable file. The @option{-mfull-toc} option is selected by
10876 default. In that case, GCC will allocate at least one TOC entry for
10877 each unique non-automatic variable reference in your program. GCC
10878 will also place floating-point constants in the TOC@. However, only
10879 16,384 entries are available in the TOC@.
10881 If you receive a linker error message that saying you have overflowed
10882 the available TOC space, you can reduce the amount of TOC space used
10883 with the @option{-mno-fp-in-toc} and @option{-mno-sum-in-toc} options.
10884 @option{-mno-fp-in-toc} prevents GCC from putting floating-point
10885 constants in the TOC and @option{-mno-sum-in-toc} forces GCC to
10886 generate code to calculate the sum of an address and a constant at
10887 run-time instead of putting that sum into the TOC@. You may specify one
10888 or both of these options. Each causes GCC to produce very slightly
10889 slower and larger code at the expense of conserving TOC space.
10891 If you still run out of space in the TOC even when you specify both of
10892 these options, specify @option{-mminimal-toc} instead. This option causes
10893 GCC to make only one TOC entry for every file. When you specify this
10894 option, GCC will produce code that is slower and larger but which
10895 uses extremely little TOC space. You may wish to use this option
10896 only on files that contain less frequently executed code.
10902 Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit
10903 @code{long} type, and the infrastructure needed to support them.
10904 Specifying @option{-maix64} implies @option{-mpowerpc64} and
10905 @option{-mpowerpc}, while @option{-maix32} disables the 64-bit ABI and
10906 implies @option{-mno-powerpc64}. GCC defaults to @option{-maix32}.
10909 @itemx -mno-xl-compat
10910 @opindex mxl-compat
10911 @opindex mno-xl-compat
10912 Produce code that conforms more closely to IBM XLC semantics when using
10913 AIX-compatible ABI. Pass floating-point arguments to prototyped
10914 functions beyond the register save area (RSA) on the stack in addition
10915 to argument FPRs. Do not assume that most significant double in 128
10916 bit long double value is properly rounded when comparing values.
10918 The AIX calling convention was extended but not initially documented to
10919 handle an obscure K&R C case of calling a function that takes the
10920 address of its arguments with fewer arguments than declared. AIX XL
10921 compilers access floating point arguments which do not fit in the
10922 RSA from the stack when a subroutine is compiled without
10923 optimization. Because always storing floating-point arguments on the
10924 stack is inefficient and rarely needed, this option is not enabled by
10925 default and only is necessary when calling subroutines compiled by AIX
10926 XL compilers without optimization.
10930 Support @dfn{IBM RS/6000 SP} @dfn{Parallel Environment} (PE)@. Link an
10931 application written to use message passing with special startup code to
10932 enable the application to run. The system must have PE installed in the
10933 standard location (@file{/usr/lpp/ppe.poe/}), or the @file{specs} file
10934 must be overridden with the @option{-specs=} option to specify the
10935 appropriate directory location. The Parallel Environment does not
10936 support threads, so the @option{-mpe} option and the @option{-pthread}
10937 option are incompatible.
10939 @item -malign-natural
10940 @itemx -malign-power
10941 @opindex malign-natural
10942 @opindex malign-power
10943 On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option
10944 @option{-malign-natural} overrides the ABI-defined alignment of larger
10945 types, such as floating-point doubles, on their natural size-based boundary.
10946 The option @option{-malign-power} instructs GCC to follow the ABI-specified
10947 alignment rules. GCC defaults to the standard alignment defined in the ABI@.
10949 On 64-bit Darwin, natural alignment is the default, and @option{-malign-power}
10953 @itemx -mhard-float
10954 @opindex msoft-float
10955 @opindex mhard-float
10956 Generate code that does not use (uses) the floating-point register set.
10957 Software floating point emulation is provided if you use the
10958 @option{-msoft-float} option, and pass the option to GCC when linking.
10961 @itemx -mno-multiple
10963 @opindex mno-multiple
10964 Generate code that uses (does not use) the load multiple word
10965 instructions and the store multiple word instructions. These
10966 instructions are generated by default on POWER systems, and not
10967 generated on PowerPC systems. Do not use @option{-mmultiple} on little
10968 endian PowerPC systems, since those instructions do not work when the
10969 processor is in little endian mode. The exceptions are PPC740 and
10970 PPC750 which permit the instructions usage in little endian mode.
10975 @opindex mno-string
10976 Generate code that uses (does not use) the load string instructions
10977 and the store string word instructions to save multiple registers and
10978 do small block moves. These instructions are generated by default on
10979 POWER systems, and not generated on PowerPC systems. Do not use
10980 @option{-mstring} on little endian PowerPC systems, since those
10981 instructions do not work when the processor is in little endian mode.
10982 The exceptions are PPC740 and PPC750 which permit the instructions
10983 usage in little endian mode.
10988 @opindex mno-update
10989 Generate code that uses (does not use) the load or store instructions
10990 that update the base register to the address of the calculated memory
10991 location. These instructions are generated by default. If you use
10992 @option{-mno-update}, there is a small window between the time that the
10993 stack pointer is updated and the address of the previous frame is
10994 stored, which means code that walks the stack frame across interrupts or
10995 signals may get corrupted data.
10998 @itemx -mno-fused-madd
10999 @opindex mfused-madd
11000 @opindex mno-fused-madd
11001 Generate code that uses (does not use) the floating point multiply and
11002 accumulate instructions. These instructions are generated by default if
11003 hardware floating is used.
11005 @item -mno-bit-align
11007 @opindex mno-bit-align
11008 @opindex mbit-align
11009 On System V.4 and embedded PowerPC systems do not (do) force structures
11010 and unions that contain bit-fields to be aligned to the base type of the
11013 For example, by default a structure containing nothing but 8
11014 @code{unsigned} bit-fields of length 1 would be aligned to a 4 byte
11015 boundary and have a size of 4 bytes. By using @option{-mno-bit-align},
11016 the structure would be aligned to a 1 byte boundary and be one byte in
11019 @item -mno-strict-align
11020 @itemx -mstrict-align
11021 @opindex mno-strict-align
11022 @opindex mstrict-align
11023 On System V.4 and embedded PowerPC systems do not (do) assume that
11024 unaligned memory references will be handled by the system.
11026 @item -mrelocatable
11027 @itemx -mno-relocatable
11028 @opindex mrelocatable
11029 @opindex mno-relocatable
11030 On embedded PowerPC systems generate code that allows (does not allow)
11031 the program to be relocated to a different address at runtime. If you
11032 use @option{-mrelocatable} on any module, all objects linked together must
11033 be compiled with @option{-mrelocatable} or @option{-mrelocatable-lib}.
11035 @item -mrelocatable-lib
11036 @itemx -mno-relocatable-lib
11037 @opindex mrelocatable-lib
11038 @opindex mno-relocatable-lib
11039 On embedded PowerPC systems generate code that allows (does not allow)
11040 the program to be relocated to a different address at runtime. Modules
11041 compiled with @option{-mrelocatable-lib} can be linked with either modules
11042 compiled without @option{-mrelocatable} and @option{-mrelocatable-lib} or
11043 with modules compiled with the @option{-mrelocatable} options.
11049 On System V.4 and embedded PowerPC systems do not (do) assume that
11050 register 2 contains a pointer to a global area pointing to the addresses
11051 used in the program.
11054 @itemx -mlittle-endian
11056 @opindex mlittle-endian
11057 On System V.4 and embedded PowerPC systems compile code for the
11058 processor in little endian mode. The @option{-mlittle-endian} option is
11059 the same as @option{-mlittle}.
11062 @itemx -mbig-endian
11064 @opindex mbig-endian
11065 On System V.4 and embedded PowerPC systems compile code for the
11066 processor in big endian mode. The @option{-mbig-endian} option is
11067 the same as @option{-mbig}.
11069 @item -mdynamic-no-pic
11070 @opindex mdynamic-no-pic
11071 On Darwin and Mac OS X systems, compile code so that it is not
11072 relocatable, but that its external references are relocatable. The
11073 resulting code is suitable for applications, but not shared
11076 @item -mprioritize-restricted-insns=@var{priority}
11077 @opindex mprioritize-restricted-insns
11078 This option controls the priority that is assigned to
11079 dispatch-slot restricted instructions during the second scheduling
11080 pass. The argument @var{priority} takes the value @var{0/1/2} to assign
11081 @var{no/highest/second-highest} priority to dispatch slot restricted
11084 @item -msched-costly-dep=@var{dependence_type}
11085 @opindex msched-costly-dep
11086 This option controls which dependences are considered costly
11087 by the target during instruction scheduling. The argument
11088 @var{dependence_type} takes one of the following values:
11089 @var{no}: no dependence is costly,
11090 @var{all}: all dependences are costly,
11091 @var{true_store_to_load}: a true dependence from store to load is costly,
11092 @var{store_to_load}: any dependence from store to load is costly,
11093 @var{number}: any dependence which latency >= @var{number} is costly.
11095 @item -minsert-sched-nops=@var{scheme}
11096 @opindex minsert-sched-nops
11097 This option controls which nop insertion scheme will be used during
11098 the second scheduling pass. The argument @var{scheme} takes one of the
11100 @var{no}: Don't insert nops.
11101 @var{pad}: Pad with nops any dispatch group which has vacant issue slots,
11102 according to the scheduler's grouping.
11103 @var{regroup_exact}: Insert nops to force costly dependent insns into
11104 separate groups. Insert exactly as many nops as needed to force an insn
11105 to a new group, according to the estimated processor grouping.
11106 @var{number}: Insert nops to force costly dependent insns into
11107 separate groups. Insert @var{number} nops to force an insn to a new group.
11110 @opindex mcall-sysv
11111 On System V.4 and embedded PowerPC systems compile code using calling
11112 conventions that adheres to the March 1995 draft of the System V
11113 Application Binary Interface, PowerPC processor supplement. This is the
11114 default unless you configured GCC using @samp{powerpc-*-eabiaix}.
11116 @item -mcall-sysv-eabi
11117 @opindex mcall-sysv-eabi
11118 Specify both @option{-mcall-sysv} and @option{-meabi} options.
11120 @item -mcall-sysv-noeabi
11121 @opindex mcall-sysv-noeabi
11122 Specify both @option{-mcall-sysv} and @option{-mno-eabi} options.
11124 @item -mcall-solaris
11125 @opindex mcall-solaris
11126 On System V.4 and embedded PowerPC systems compile code for the Solaris
11130 @opindex mcall-linux
11131 On System V.4 and embedded PowerPC systems compile code for the
11132 Linux-based GNU system.
11136 On System V.4 and embedded PowerPC systems compile code for the
11137 Hurd-based GNU system.
11139 @item -mcall-netbsd
11140 @opindex mcall-netbsd
11141 On System V.4 and embedded PowerPC systems compile code for the
11142 NetBSD operating system.
11144 @item -maix-struct-return
11145 @opindex maix-struct-return
11146 Return all structures in memory (as specified by the AIX ABI)@.
11148 @item -msvr4-struct-return
11149 @opindex msvr4-struct-return
11150 Return structures smaller than 8 bytes in registers (as specified by the
11153 @item -mabi=@var{abi-type}
11155 Extend the current ABI with a particular extension, or remove such extension.
11156 Valid values are @var{altivec}, @var{no-altivec}, @var{spe},
11160 @itemx -mno-prototype
11161 @opindex mprototype
11162 @opindex mno-prototype
11163 On System V.4 and embedded PowerPC systems assume that all calls to
11164 variable argument functions are properly prototyped. Otherwise, the
11165 compiler must insert an instruction before every non prototyped call to
11166 set or clear bit 6 of the condition code register (@var{CR}) to
11167 indicate whether floating point values were passed in the floating point
11168 registers in case the function takes a variable arguments. With
11169 @option{-mprototype}, only calls to prototyped variable argument functions
11170 will set or clear the bit.
11174 On embedded PowerPC systems, assume that the startup module is called
11175 @file{sim-crt0.o} and that the standard C libraries are @file{libsim.a} and
11176 @file{libc.a}. This is the default for @samp{powerpc-*-eabisim}.
11181 On embedded PowerPC systems, assume that the startup module is called
11182 @file{crt0.o} and the standard C libraries are @file{libmvme.a} and
11187 On embedded PowerPC systems, assume that the startup module is called
11188 @file{crt0.o} and the standard C libraries are @file{libads.a} and
11191 @item -myellowknife
11192 @opindex myellowknife
11193 On embedded PowerPC systems, assume that the startup module is called
11194 @file{crt0.o} and the standard C libraries are @file{libyk.a} and
11199 On System V.4 and embedded PowerPC systems, specify that you are
11200 compiling for a VxWorks system.
11204 Specify that you are compiling for the WindISS simulation environment.
11208 On embedded PowerPC systems, set the @var{PPC_EMB} bit in the ELF flags
11209 header to indicate that @samp{eabi} extended relocations are used.
11215 On System V.4 and embedded PowerPC systems do (do not) adhere to the
11216 Embedded Applications Binary Interface (eabi) which is a set of
11217 modifications to the System V.4 specifications. Selecting @option{-meabi}
11218 means that the stack is aligned to an 8 byte boundary, a function
11219 @code{__eabi} is called to from @code{main} to set up the eabi
11220 environment, and the @option{-msdata} option can use both @code{r2} and
11221 @code{r13} to point to two separate small data areas. Selecting
11222 @option{-mno-eabi} means that the stack is aligned to a 16 byte boundary,
11223 do not call an initialization function from @code{main}, and the
11224 @option{-msdata} option will only use @code{r13} to point to a single
11225 small data area. The @option{-meabi} option is on by default if you
11226 configured GCC using one of the @samp{powerpc*-*-eabi*} options.
11229 @opindex msdata=eabi
11230 On System V.4 and embedded PowerPC systems, put small initialized
11231 @code{const} global and static data in the @samp{.sdata2} section, which
11232 is pointed to by register @code{r2}. Put small initialized
11233 non-@code{const} global and static data in the @samp{.sdata} section,
11234 which is pointed to by register @code{r13}. Put small uninitialized
11235 global and static data in the @samp{.sbss} section, which is adjacent to
11236 the @samp{.sdata} section. The @option{-msdata=eabi} option is
11237 incompatible with the @option{-mrelocatable} option. The
11238 @option{-msdata=eabi} option also sets the @option{-memb} option.
11241 @opindex msdata=sysv
11242 On System V.4 and embedded PowerPC systems, put small global and static
11243 data in the @samp{.sdata} section, which is pointed to by register
11244 @code{r13}. Put small uninitialized global and static data in the
11245 @samp{.sbss} section, which is adjacent to the @samp{.sdata} section.
11246 The @option{-msdata=sysv} option is incompatible with the
11247 @option{-mrelocatable} option.
11249 @item -msdata=default
11251 @opindex msdata=default
11253 On System V.4 and embedded PowerPC systems, if @option{-meabi} is used,
11254 compile code the same as @option{-msdata=eabi}, otherwise compile code the
11255 same as @option{-msdata=sysv}.
11258 @opindex msdata-data
11259 On System V.4 and embedded PowerPC systems, put small global and static
11260 data in the @samp{.sdata} section. Put small uninitialized global and
11261 static data in the @samp{.sbss} section. Do not use register @code{r13}
11262 to address small data however. This is the default behavior unless
11263 other @option{-msdata} options are used.
11267 @opindex msdata=none
11269 On embedded PowerPC systems, put all initialized global and static data
11270 in the @samp{.data} section, and all uninitialized data in the
11271 @samp{.bss} section.
11275 @cindex smaller data references (PowerPC)
11276 @cindex .sdata/.sdata2 references (PowerPC)
11277 On embedded PowerPC systems, put global and static items less than or
11278 equal to @var{num} bytes into the small data or bss sections instead of
11279 the normal data or bss section. By default, @var{num} is 8. The
11280 @option{-G @var{num}} switch is also passed to the linker.
11281 All modules should be compiled with the same @option{-G @var{num}} value.
11284 @itemx -mno-regnames
11286 @opindex mno-regnames
11287 On System V.4 and embedded PowerPC systems do (do not) emit register
11288 names in the assembly language output using symbolic forms.
11291 @itemx -mno-longcall
11293 @opindex mno-longcall
11294 Default to making all function calls indirectly, using a register, so
11295 that functions which reside further than 32 megabytes (33,554,432
11296 bytes) from the current location can be called. This setting can be
11297 overridden by the @code{shortcall} function attribute, or by
11298 @code{#pragma longcall(0)}.
11300 Some linkers are capable of detecting out-of-range calls and generating
11301 glue code on the fly. On these systems, long calls are unnecessary and
11302 generate slower code. As of this writing, the AIX linker can do this,
11303 as can the GNU linker for PowerPC/64. It is planned to add this feature
11304 to the GNU linker for 32-bit PowerPC systems as well.
11306 On Darwin/PPC systems, @code{#pragma longcall} will generate ``jbsr
11307 callee, L42'', plus a ``branch island'' (glue code). The two target
11308 addresses represent the callee and the ``branch island''. The
11309 Darwin/PPC linker will prefer the first address and generate a ``bl
11310 callee'' if the PPC ``bl'' instruction will reach the callee directly;
11311 otherwise, the linker will generate ``bl L42'' to call the ``branch
11312 island''. The ``branch island'' is appended to the body of the
11313 calling function; it computes the full 32-bit address of the callee
11316 On Mach-O (Darwin) systems, this option directs the compiler emit to
11317 the glue for every direct call, and the Darwin linker decides whether
11318 to use or discard it.
11320 In the future, we may cause GCC to ignore all longcall specifications
11321 when the linker is known to generate glue.
11325 Adds support for multithreading with the @dfn{pthreads} library.
11326 This option sets flags for both the preprocessor and linker.
11330 @node S/390 and zSeries Options
11331 @subsection S/390 and zSeries Options
11332 @cindex S/390 and zSeries Options
11334 These are the @samp{-m} options defined for the S/390 and zSeries architecture.
11338 @itemx -msoft-float
11339 @opindex mhard-float
11340 @opindex msoft-float
11341 Use (do not use) the hardware floating-point instructions and registers
11342 for floating-point operations. When @option{-msoft-float} is specified,
11343 functions in @file{libgcc.a} will be used to perform floating-point
11344 operations. When @option{-mhard-float} is specified, the compiler
11345 generates IEEE floating-point instructions. This is the default.
11348 @itemx -mno-backchain
11349 @opindex mbackchain
11350 @opindex mno-backchain
11351 Store (do not store) the address of the caller's frame as backchain pointer
11352 into the callee's stack frame.
11353 A backchain may be needed to allow debugging using tools that do not understand
11354 DWARF-2 call frame information.
11355 When @option{-mno-packed-stack} is in effect, the backchain pointer is stored
11356 at the bottom of the stack frame; when @option{-mpacked-stack} is in effect,
11357 the backchain is placed into the topmost word of the 96/160 byte register
11360 In general, code compiled with @option{-mbackchain} is call-compatible with
11361 code compiled with @option{-mmo-backchain}; however, use of the backchain
11362 for debugging purposes usually requires that the whole binary is built with
11363 @option{-mbackchain}. Note that the combination of @option{-mbackchain},
11364 @option{-mpacked-stack} and @option{-mhard-float} is not supported. In order
11365 to build a linux kernel use @option{-msoft-float}.
11367 The default is to not maintain the backchain.
11369 @item -mpacked-stack
11370 @item -mno-packed-stack
11371 @opindex mpacked-stack
11372 @opindex mno-packed-stack
11373 Use (do not use) the packed stack layout. When @option{-mno-packed-stack} is
11374 specified, the compiler uses the all fields of the 96/160 byte register save
11375 area only for their default purpose; unused fields still take up stack space.
11376 When @option{-mpacked-stack} is specified, register save slots are densely
11377 packed at the top of the register save area; unused space is reused for other
11378 purposes, allowing for more efficient use of the available stack space.
11379 However, when @option{-mbackchain} is also in effect, the topmost word of
11380 the save area is always used to store the backchain, and the return address
11381 register is always saved two words below the backchain.
11383 As long as the stack frame backchain is not used, code generated with
11384 @option{-mpacked-stack} is call-compatible with code generated with
11385 @option{-mno-packed-stack}. Note that some non-FSF releases of GCC 2.95 for
11386 S/390 or zSeries generated code that uses the stack frame backchain at run
11387 time, not just for debugging purposes. Such code is not call-compatible
11388 with code compiled with @option{-mpacked-stack}. Also, note that the
11389 combination of @option{-mbackchain},
11390 @option{-mpacked-stack} and @option{-mhard-float} is not supported. In order
11391 to build a linux kernel use @option{-msoft-float}.
11393 The default is to not use the packed stack layout.
11396 @itemx -mno-small-exec
11397 @opindex msmall-exec
11398 @opindex mno-small-exec
11399 Generate (or do not generate) code using the @code{bras} instruction
11400 to do subroutine calls.
11401 This only works reliably if the total executable size does not
11402 exceed 64k. The default is to use the @code{basr} instruction instead,
11403 which does not have this limitation.
11409 When @option{-m31} is specified, generate code compliant to the
11410 GNU/Linux for S/390 ABI@. When @option{-m64} is specified, generate
11411 code compliant to the GNU/Linux for zSeries ABI@. This allows GCC in
11412 particular to generate 64-bit instructions. For the @samp{s390}
11413 targets, the default is @option{-m31}, while the @samp{s390x}
11414 targets default to @option{-m64}.
11420 When @option{-mzarch} is specified, generate code using the
11421 instructions available on z/Architecture.
11422 When @option{-mesa} is specified, generate code using the
11423 instructions available on ESA/390. Note that @option{-mesa} is
11424 not possible with @option{-m64}.
11425 When generating code compliant to the GNU/Linux for S/390 ABI,
11426 the default is @option{-mesa}. When generating code compliant
11427 to the GNU/Linux for zSeries ABI, the default is @option{-mzarch}.
11433 Generate (or do not generate) code using the @code{mvcle} instruction
11434 to perform block moves. When @option{-mno-mvcle} is specified,
11435 use a @code{mvc} loop instead. This is the default unless optimizing for
11442 Print (or do not print) additional debug information when compiling.
11443 The default is to not print debug information.
11445 @item -march=@var{cpu-type}
11447 Generate code that will run on @var{cpu-type}, which is the name of a system
11448 representing a certain processor type. Possible values for
11449 @var{cpu-type} are @samp{g5}, @samp{g6}, @samp{z900}, and @samp{z990}.
11450 When generating code using the instructions available on z/Architecture,
11451 the default is @option{-march=z900}. Otherwise, the default is
11452 @option{-march=g5}.
11454 @item -mtune=@var{cpu-type}
11456 Tune to @var{cpu-type} everything applicable about the generated code,
11457 except for the ABI and the set of available instructions.
11458 The list of @var{cpu-type} values is the same as for @option{-march}.
11459 The default is the value used for @option{-march}.
11462 @itemx -mno-tpf-trace
11463 @opindex mtpf-trace
11464 @opindex mno-tpf-trace
11465 Generate code that adds (does not add) in TPF OS specific branches to trace
11466 routines in the operating system. This option is off by default, even
11467 when compiling for the TPF OS@.
11470 @itemx -mno-fused-madd
11471 @opindex mfused-madd
11472 @opindex mno-fused-madd
11473 Generate code that uses (does not use) the floating point multiply and
11474 accumulate instructions. These instructions are generated by default if
11475 hardware floating point is used.
11477 @item -mwarn-framesize=@var{framesize}
11478 @opindex mwarn-framesize
11479 Emit a warning if the current function exceeds the given frame size. Because
11480 this is a compile time check it doesn't need to be a real problem when the program
11481 runs. It is intended to identify functions which most probably cause
11482 a stack overflow. It is useful to be used in an environment with limited stack
11483 size e.g.@: the linux kernel.
11485 @item -mwarn-dynamicstack
11486 @opindex mwarn-dynamicstack
11487 Emit a warning if the function calls alloca or uses dynamically
11488 sized arrays. This is generally a bad idea with a limited stack size.
11490 @item -mstack-guard=@var{stack-guard}
11491 @item -mstack-size=@var{stack-size}
11492 @opindex mstack-guard
11493 @opindex mstack-size
11494 These arguments always have to be used in conjunction. If they are present the s390
11495 back end emits additional instructions in the function prologue which trigger a trap
11496 if the stack size is @var{stack-guard} bytes above the @var{stack-size}
11497 (remember that the stack on s390 grows downward). These options are intended to
11498 be used to help debugging stack overflow problems. The additionally emitted code
11499 cause only little overhead and hence can also be used in production like systems
11500 without greater performance degradation. The given values have to be exact
11501 powers of 2 and @var{stack-size} has to be greater than @var{stack-guard}.
11502 In order to be efficient the extra code makes the assumption that the stack starts
11503 at an address aligned to the value given by @var{stack-size}.
11507 @subsection SH Options
11509 These @samp{-m} options are defined for the SH implementations:
11514 Generate code for the SH1.
11518 Generate code for the SH2.
11521 Generate code for the SH2e.
11525 Generate code for the SH3.
11529 Generate code for the SH3e.
11533 Generate code for the SH4 without a floating-point unit.
11535 @item -m4-single-only
11536 @opindex m4-single-only
11537 Generate code for the SH4 with a floating-point unit that only
11538 supports single-precision arithmetic.
11542 Generate code for the SH4 assuming the floating-point unit is in
11543 single-precision mode by default.
11547 Generate code for the SH4.
11551 Generate code for the SH4al-dsp, or for a SH4a in such a way that the
11552 floating-point unit is not used.
11554 @item -m4a-single-only
11555 @opindex m4a-single-only
11556 Generate code for the SH4a, in such a way that no double-precision
11557 floating point operations are used.
11560 @opindex m4a-single
11561 Generate code for the SH4a assuming the floating-point unit is in
11562 single-precision mode by default.
11566 Generate code for the SH4a.
11570 Same as @option{-m4a-nofpu}, except that it implicitly passes
11571 @option{-dsp} to the assembler. GCC doesn't generate any DSP
11572 instructions at the moment.
11576 Compile code for the processor in big endian mode.
11580 Compile code for the processor in little endian mode.
11584 Align doubles at 64-bit boundaries. Note that this changes the calling
11585 conventions, and thus some functions from the standard C library will
11586 not work unless you recompile it first with @option{-mdalign}.
11590 Shorten some address references at link time, when possible; uses the
11591 linker option @option{-relax}.
11595 Use 32-bit offsets in @code{switch} tables. The default is to use
11600 Enable the use of the instruction @code{fmovd}.
11604 Comply with the calling conventions defined by Renesas.
11608 Comply with the calling conventions defined by Renesas.
11612 Comply with the calling conventions defined for GCC before the Renesas
11613 conventions were available. This option is the default for all
11614 targets of the SH toolchain except for @samp{sh-symbianelf}.
11617 @opindex mnomacsave
11618 Mark the @code{MAC} register as call-clobbered, even if
11619 @option{-mhitachi} is given.
11623 Increase IEEE-compliance of floating-point code.
11624 At the moment, this is equivalent to @option{-fno-finite-math-only}.
11625 When generating 16 bit SH opcodes, getting IEEE-conforming results for
11626 comparisons of NANs / infinities incurs extra overhead in every
11627 floating point comparison, therefore the default is set to
11628 @option{-ffinite-math-only}.
11632 Dump instruction size and location in the assembly code.
11635 @opindex mpadstruct
11636 This option is deprecated. It pads structures to multiple of 4 bytes,
11637 which is incompatible with the SH ABI@.
11641 Optimize for space instead of speed. Implied by @option{-Os}.
11644 @opindex mprefergot
11645 When generating position-independent code, emit function calls using
11646 the Global Offset Table instead of the Procedure Linkage Table.
11650 Generate a library function call to invalidate instruction cache
11651 entries, after fixing up a trampoline. This library function call
11652 doesn't assume it can write to the whole memory address space. This
11653 is the default when the target is @code{sh-*-linux*}.
11655 @item -multcost=@var{number}
11656 @opindex multcost=@var{number}
11657 Set the cost to assume for a multiply insn.
11659 @item -mdiv=@var{strategy}
11660 @opindex mdiv=@var{strategy}
11661 Set the division strategy to use for SHmedia code. @var{strategy} must be
11662 one of: call, call2, fp, inv, inv:minlat, inv20u, inv20l, inv:call,
11663 inv:call2, inv:fp .
11664 "fp" performs the operation in floating point. This has a very high latency,
11665 but needs only a few instructions, so it might be a good choice if
11666 your code has enough easily exploitable ILP to allow the compiler to
11667 schedule the floating point instructions together with other instructions.
11668 Division by zero causes a floating point exception.
11669 "inv" uses integer operations to calculate the inverse of the divisor,
11670 and then multiplies the dividend with the inverse. This strategy allows
11671 cse and hoisting of the inverse calculation. Division by zero calculates
11672 an unspecified result, but does not trap.
11673 "inv:minlat" is a variant of "inv" where if no cse / hoisting opportunities
11674 have been found, or if the entire operation has been hoisted to the same
11675 place, the last stages of the inverse calculation are intertwined with the
11676 final multiply to reduce the overall latency, at the expense of using a few
11677 more instructions, and thus offering fewer scheduling opportunities with
11679 "call" calls a library function that usually implements the inv:minlat
11681 This gives high code density for m5-*media-nofpu compilations.
11682 "call2" uses a different entry point of the same library function, where it
11683 assumes that a pointer to a lookup table has already been set up, which
11684 exposes the pointer load to cse / code hoisting optimizations.
11685 "inv:call", "inv:call2" and "inv:fp" all use the "inv" algorithm for initial
11686 code generation, but if the code stays unoptimized, revert to the "call",
11687 "call2", or "fp" strategies, respectively. Note that the
11688 potentially-trapping side effect of division by zero is carried by a
11689 separate instruction, so it is possible that all the integer instructions
11690 are hoisted out, but the marker for the side effect stays where it is.
11691 A recombination to fp operations or a call is not possible in that case.
11692 "inv20u" and "inv20l" are variants of the "inv:minlat" strategy. In the case
11693 that the inverse calculation was nor separated from the multiply, they speed
11694 up division where the dividend fits into 20 bits (plus sign where applicable),
11695 by inserting a test to skip a number of operations in this case; this test
11696 slows down the case of larger dividends. inv20u assumes the case of a such
11697 a small dividend to be unlikely, and inv20l assumes it to be likely.
11699 @item -mdivsi3_libfunc=@var{name}
11700 @opindex mdivsi3_libfunc=@var{name}
11701 Set the name of the library function used for 32 bit signed division to
11702 @var{name}. This only affect the name used in the call and inv:call
11703 division strategies, and the compiler will still expect the same
11704 sets of input/output/clobbered registers as if this option was not present.
11706 @item -madjust-unroll
11707 @opindex madjust-unroll
11708 Throttle unrolling to avoid thrashing target registers.
11709 This option only has an effect if the gcc code base supports the
11710 TARGET_ADJUST_UNROLL_MAX target hook.
11712 @item -mindexed-addressing
11713 @opindex mindexed-addressing
11714 Enable the use of the indexed addressing mode for SHmedia32/SHcompact.
11715 This is only safe if the hardware and/or OS implement 32 bit wrap-around
11716 semantics for the indexed addressing mode. The architecture allows the
11717 implementation of processors with 64 bit MMU, which the OS could use to
11718 get 32 bit addressing, but since no current hardware implementation supports
11719 this or any other way to make the indexed addressing mode safe to use in
11720 the 32 bit ABI, the default is -mno-indexed-addressing.
11722 @item -mgettrcost=@var{number}
11723 @opindex mgettrcost=@var{number}
11724 Set the cost assumed for the gettr instruction to @var{number}.
11725 The default is 2 if @option{-mpt-fixed} is in effect, 100 otherwise.
11729 Assume pt* instructions won't trap. This will generally generate better
11730 scheduled code, but is unsafe on current hardware. The current architecture
11731 definition says that ptabs and ptrel trap when the target anded with 3 is 3.
11732 This has the unintentional effect of making it unsafe to schedule ptabs /
11733 ptrel before a branch, or hoist it out of a loop. For example,
11734 __do_global_ctors, a part of libgcc that runs constructors at program
11735 startup, calls functions in a list which is delimited by -1. With the
11736 -mpt-fixed option, the ptabs will be done before testing against -1.
11737 That means that all the constructors will be run a bit quicker, but when
11738 the loop comes to the end of the list, the program crashes because ptabs
11739 loads -1 into a target register. Since this option is unsafe for any
11740 hardware implementing the current architecture specification, the default
11741 is -mno-pt-fixed. Unless the user specifies a specific cost with
11742 @option{-mgettrcost}, -mno-pt-fixed also implies @option{-mgettrcost=100};
11743 this deters register allocation using target registers for storing
11746 @item -minvalid-symbols
11747 @opindex minvalid-symbols
11748 Assume symbols might be invalid. Ordinary function symbols generated by
11749 the compiler will always be valid to load with movi/shori/ptabs or
11750 movi/shori/ptrel, but with assembler and/or linker tricks it is possible
11751 to generate symbols that will cause ptabs / ptrel to trap.
11752 This option is only meaningful when @option{-mno-pt-fixed} is in effect.
11753 It will then prevent cross-basic-block cse, hoisting and most scheduling
11754 of symbol loads. The default is @option{-mno-invalid-symbols}.
11757 @node SPARC Options
11758 @subsection SPARC Options
11759 @cindex SPARC options
11761 These @samp{-m} options are supported on the SPARC:
11764 @item -mno-app-regs
11766 @opindex mno-app-regs
11768 Specify @option{-mapp-regs} to generate output using the global registers
11769 2 through 4, which the SPARC SVR4 ABI reserves for applications. This
11772 To be fully SVR4 ABI compliant at the cost of some performance loss,
11773 specify @option{-mno-app-regs}. You should compile libraries and system
11774 software with this option.
11777 @itemx -mhard-float
11779 @opindex mhard-float
11780 Generate output containing floating point instructions. This is the
11784 @itemx -msoft-float
11786 @opindex msoft-float
11787 Generate output containing library calls for floating point.
11788 @strong{Warning:} the requisite libraries are not available for all SPARC
11789 targets. Normally the facilities of the machine's usual C compiler are
11790 used, but this cannot be done directly in cross-compilation. You must make
11791 your own arrangements to provide suitable library functions for
11792 cross-compilation. The embedded targets @samp{sparc-*-aout} and
11793 @samp{sparclite-*-*} do provide software floating point support.
11795 @option{-msoft-float} changes the calling convention in the output file;
11796 therefore, it is only useful if you compile @emph{all} of a program with
11797 this option. In particular, you need to compile @file{libgcc.a}, the
11798 library that comes with GCC, with @option{-msoft-float} in order for
11801 @item -mhard-quad-float
11802 @opindex mhard-quad-float
11803 Generate output containing quad-word (long double) floating point
11806 @item -msoft-quad-float
11807 @opindex msoft-quad-float
11808 Generate output containing library calls for quad-word (long double)
11809 floating point instructions. The functions called are those specified
11810 in the SPARC ABI@. This is the default.
11812 As of this writing, there are no SPARC implementations that have hardware
11813 support for the quad-word floating point instructions. They all invoke
11814 a trap handler for one of these instructions, and then the trap handler
11815 emulates the effect of the instruction. Because of the trap handler overhead,
11816 this is much slower than calling the ABI library routines. Thus the
11817 @option{-msoft-quad-float} option is the default.
11819 @item -mno-unaligned-doubles
11820 @itemx -munaligned-doubles
11821 @opindex mno-unaligned-doubles
11822 @opindex munaligned-doubles
11823 Assume that doubles have 8 byte alignment. This is the default.
11825 With @option{-munaligned-doubles}, GCC assumes that doubles have 8 byte
11826 alignment only if they are contained in another type, or if they have an
11827 absolute address. Otherwise, it assumes they have 4 byte alignment.
11828 Specifying this option avoids some rare compatibility problems with code
11829 generated by other compilers. It is not the default because it results
11830 in a performance loss, especially for floating point code.
11832 @item -mno-faster-structs
11833 @itemx -mfaster-structs
11834 @opindex mno-faster-structs
11835 @opindex mfaster-structs
11836 With @option{-mfaster-structs}, the compiler assumes that structures
11837 should have 8 byte alignment. This enables the use of pairs of
11838 @code{ldd} and @code{std} instructions for copies in structure
11839 assignment, in place of twice as many @code{ld} and @code{st} pairs.
11840 However, the use of this changed alignment directly violates the SPARC
11841 ABI@. Thus, it's intended only for use on targets where the developer
11842 acknowledges that their resulting code will not be directly in line with
11843 the rules of the ABI@.
11845 @item -mimpure-text
11846 @opindex mimpure-text
11847 @option{-mimpure-text}, used in addition to @option{-shared}, tells
11848 the compiler to not pass @option{-z text} to the linker when linking a
11849 shared object. Using this option, you can link position-dependent
11850 code into a shared object.
11852 @option{-mimpure-text} suppresses the ``relocations remain against
11853 allocatable but non-writable sections'' linker error message.
11854 However, the necessary relocations will trigger copy-on-write, and the
11855 shared object is not actually shared across processes. Instead of
11856 using @option{-mimpure-text}, you should compile all source code with
11857 @option{-fpic} or @option{-fPIC}.
11859 This option is only available on SunOS and Solaris.
11861 @item -mcpu=@var{cpu_type}
11863 Set the instruction set, register set, and instruction scheduling parameters
11864 for machine type @var{cpu_type}. Supported values for @var{cpu_type} are
11865 @samp{v7}, @samp{cypress}, @samp{v8}, @samp{supersparc}, @samp{sparclite},
11866 @samp{f930}, @samp{f934}, @samp{hypersparc}, @samp{sparclite86x},
11867 @samp{sparclet}, @samp{tsc701}, @samp{v9}, @samp{ultrasparc}, and
11868 @samp{ultrasparc3}.
11870 Default instruction scheduling parameters are used for values that select
11871 an architecture and not an implementation. These are @samp{v7}, @samp{v8},
11872 @samp{sparclite}, @samp{sparclet}, @samp{v9}.
11874 Here is a list of each supported architecture and their supported
11879 v8: supersparc, hypersparc
11880 sparclite: f930, f934, sparclite86x
11882 v9: ultrasparc, ultrasparc3
11885 By default (unless configured otherwise), GCC generates code for the V7
11886 variant of the SPARC architecture. With @option{-mcpu=cypress}, the compiler
11887 additionally optimizes it for the Cypress CY7C602 chip, as used in the
11888 SPARCStation/SPARCServer 3xx series. This is also appropriate for the older
11889 SPARCStation 1, 2, IPX etc.
11891 With @option{-mcpu=v8}, GCC generates code for the V8 variant of the SPARC
11892 architecture. The only difference from V7 code is that the compiler emits
11893 the integer multiply and integer divide instructions which exist in SPARC-V8
11894 but not in SPARC-V7. With @option{-mcpu=supersparc}, the compiler additionally
11895 optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and
11898 With @option{-mcpu=sparclite}, GCC generates code for the SPARClite variant of
11899 the SPARC architecture. This adds the integer multiply, integer divide step
11900 and scan (@code{ffs}) instructions which exist in SPARClite but not in SPARC-V7.
11901 With @option{-mcpu=f930}, the compiler additionally optimizes it for the
11902 Fujitsu MB86930 chip, which is the original SPARClite, with no FPU@. With
11903 @option{-mcpu=f934}, the compiler additionally optimizes it for the Fujitsu
11904 MB86934 chip, which is the more recent SPARClite with FPU@.
11906 With @option{-mcpu=sparclet}, GCC generates code for the SPARClet variant of
11907 the SPARC architecture. This adds the integer multiply, multiply/accumulate,
11908 integer divide step and scan (@code{ffs}) instructions which exist in SPARClet
11909 but not in SPARC-V7. With @option{-mcpu=tsc701}, the compiler additionally
11910 optimizes it for the TEMIC SPARClet chip.
11912 With @option{-mcpu=v9}, GCC generates code for the V9 variant of the SPARC
11913 architecture. This adds 64-bit integer and floating-point move instructions,
11914 3 additional floating-point condition code registers and conditional move
11915 instructions. With @option{-mcpu=ultrasparc}, the compiler additionally
11916 optimizes it for the Sun UltraSPARC I/II chips. With
11917 @option{-mcpu=ultrasparc3}, the compiler additionally optimizes it for the
11918 Sun UltraSPARC III chip.
11920 @item -mtune=@var{cpu_type}
11922 Set the instruction scheduling parameters for machine type
11923 @var{cpu_type}, but do not set the instruction set or register set that the
11924 option @option{-mcpu=@var{cpu_type}} would.
11926 The same values for @option{-mcpu=@var{cpu_type}} can be used for
11927 @option{-mtune=@var{cpu_type}}, but the only useful values are those
11928 that select a particular cpu implementation. Those are @samp{cypress},
11929 @samp{supersparc}, @samp{hypersparc}, @samp{f930}, @samp{f934},
11930 @samp{sparclite86x}, @samp{tsc701}, @samp{ultrasparc}, and
11931 @samp{ultrasparc3}.
11936 @opindex mno-v8plus
11937 With @option{-mv8plus}, GCC generates code for the SPARC-V8+ ABI@. The
11938 difference from the V8 ABI is that the global and out registers are
11939 considered 64-bit wide. This is enabled by default on Solaris in 32-bit
11940 mode for all SPARC-V9 processors.
11946 With @option{-mvis}, GCC generates code that takes advantage of the UltraSPARC
11947 Visual Instruction Set extensions. The default is @option{-mno-vis}.
11950 These @samp{-m} options are supported in addition to the above
11951 on SPARC-V9 processors in 64-bit environments:
11954 @item -mlittle-endian
11955 @opindex mlittle-endian
11956 Generate code for a processor running in little-endian mode. It is only
11957 available for a few configurations and most notably not on Solaris and Linux.
11963 Generate code for a 32-bit or 64-bit environment.
11964 The 32-bit environment sets int, long and pointer to 32 bits.
11965 The 64-bit environment sets int to 32 bits and long and pointer
11968 @item -mcmodel=medlow
11969 @opindex mcmodel=medlow
11970 Generate code for the Medium/Low code model: 64-bit addresses, programs
11971 must be linked in the low 32 bits of memory. Programs can be statically
11972 or dynamically linked.
11974 @item -mcmodel=medmid
11975 @opindex mcmodel=medmid
11976 Generate code for the Medium/Middle code model: 64-bit addresses, programs
11977 must be linked in the low 44 bits of memory, the text and data segments must
11978 be less than 2GB in size and the data segment must be located within 2GB of
11981 @item -mcmodel=medany
11982 @opindex mcmodel=medany
11983 Generate code for the Medium/Anywhere code model: 64-bit addresses, programs
11984 may be linked anywhere in memory, the text and data segments must be less
11985 than 2GB in size and the data segment must be located within 2GB of the
11988 @item -mcmodel=embmedany
11989 @opindex mcmodel=embmedany
11990 Generate code for the Medium/Anywhere code model for embedded systems:
11991 64-bit addresses, the text and data segments must be less than 2GB in
11992 size, both starting anywhere in memory (determined at link time). The
11993 global register %g4 points to the base of the data segment. Programs
11994 are statically linked and PIC is not supported.
11997 @itemx -mno-stack-bias
11998 @opindex mstack-bias
11999 @opindex mno-stack-bias
12000 With @option{-mstack-bias}, GCC assumes that the stack pointer, and
12001 frame pointer if present, are offset by @minus{}2047 which must be added back
12002 when making stack frame references. This is the default in 64-bit mode.
12003 Otherwise, assume no such offset is present.
12006 These switches are supported in addition to the above on Solaris:
12011 Add support for multithreading using the Solaris threads library. This
12012 option sets flags for both the preprocessor and linker. This option does
12013 not affect the thread safety of object code produced by the compiler or
12014 that of libraries supplied with it.
12018 Add support for multithreading using the POSIX threads library. This
12019 option sets flags for both the preprocessor and linker. This option does
12020 not affect the thread safety of object code produced by the compiler or
12021 that of libraries supplied with it.
12024 @node System V Options
12025 @subsection Options for System V
12027 These additional options are available on System V Release 4 for
12028 compatibility with other compilers on those systems:
12033 Create a shared object.
12034 It is recommended that @option{-symbolic} or @option{-shared} be used instead.
12038 Identify the versions of each tool used by the compiler, in a
12039 @code{.ident} assembler directive in the output.
12043 Refrain from adding @code{.ident} directives to the output file (this is
12046 @item -YP,@var{dirs}
12048 Search the directories @var{dirs}, and no others, for libraries
12049 specified with @option{-l}.
12051 @item -Ym,@var{dir}
12053 Look in the directory @var{dir} to find the M4 preprocessor.
12054 The assembler uses this option.
12055 @c This is supposed to go with a -Yd for predefined M4 macro files, but
12056 @c the generic assembler that comes with Solaris takes just -Ym.
12059 @node TMS320C3x/C4x Options
12060 @subsection TMS320C3x/C4x Options
12061 @cindex TMS320C3x/C4x Options
12063 These @samp{-m} options are defined for TMS320C3x/C4x implementations:
12067 @item -mcpu=@var{cpu_type}
12069 Set the instruction set, register set, and instruction scheduling
12070 parameters for machine type @var{cpu_type}. Supported values for
12071 @var{cpu_type} are @samp{c30}, @samp{c31}, @samp{c32}, @samp{c40}, and
12072 @samp{c44}. The default is @samp{c40} to generate code for the
12077 @itemx -msmall-memory
12079 @opindex mbig-memory
12081 @opindex msmall-memory
12083 Generates code for the big or small memory model. The small memory
12084 model assumed that all data fits into one 64K word page. At run-time
12085 the data page (DP) register must be set to point to the 64K page
12086 containing the .bss and .data program sections. The big memory model is
12087 the default and requires reloading of the DP register for every direct
12094 Allow (disallow) allocation of general integer operands into the block
12095 count register BK@.
12101 Enable (disable) generation of code using decrement and branch,
12102 DBcond(D), instructions. This is enabled by default for the C4x. To be
12103 on the safe side, this is disabled for the C3x, since the maximum
12104 iteration count on the C3x is @math{2^{23} + 1} (but who iterates loops more than
12105 @math{2^{23}} times on the C3x?). Note that GCC will try to reverse a loop so
12106 that it can utilize the decrement and branch instruction, but will give
12107 up if there is more than one memory reference in the loop. Thus a loop
12108 where the loop counter is decremented can generate slightly more
12109 efficient code, in cases where the RPTB instruction cannot be utilized.
12111 @item -mdp-isr-reload
12113 @opindex mdp-isr-reload
12115 Force the DP register to be saved on entry to an interrupt service
12116 routine (ISR), reloaded to point to the data section, and restored on
12117 exit from the ISR@. This should not be required unless someone has
12118 violated the small memory model by modifying the DP register, say within
12125 For the C3x use the 24-bit MPYI instruction for integer multiplies
12126 instead of a library call to guarantee 32-bit results. Note that if one
12127 of the operands is a constant, then the multiplication will be performed
12128 using shifts and adds. If the @option{-mmpyi} option is not specified for the C3x,
12129 then squaring operations are performed inline instead of a library call.
12132 @itemx -mno-fast-fix
12134 @opindex mno-fast-fix
12135 The C3x/C4x FIX instruction to convert a floating point value to an
12136 integer value chooses the nearest integer less than or equal to the
12137 floating point value rather than to the nearest integer. Thus if the
12138 floating point number is negative, the result will be incorrectly
12139 truncated an additional code is necessary to detect and correct this
12140 case. This option can be used to disable generation of the additional
12141 code required to correct the result.
12147 Enable (disable) generation of repeat block sequences using the RPTB
12148 instruction for zero overhead looping. The RPTB construct is only used
12149 for innermost loops that do not call functions or jump across the loop
12150 boundaries. There is no advantage having nested RPTB loops due to the
12151 overhead required to save and restore the RC, RS, and RE registers.
12152 This is enabled by default with @option{-O2}.
12154 @item -mrpts=@var{count}
12158 Enable (disable) the use of the single instruction repeat instruction
12159 RPTS@. If a repeat block contains a single instruction, and the loop
12160 count can be guaranteed to be less than the value @var{count}, GCC will
12161 emit a RPTS instruction instead of a RPTB@. If no value is specified,
12162 then a RPTS will be emitted even if the loop count cannot be determined
12163 at compile time. Note that the repeated instruction following RPTS does
12164 not have to be reloaded from memory each iteration, thus freeing up the
12165 CPU buses for operands. However, since interrupts are blocked by this
12166 instruction, it is disabled by default.
12168 @item -mloop-unsigned
12169 @itemx -mno-loop-unsigned
12170 @opindex mloop-unsigned
12171 @opindex mno-loop-unsigned
12172 The maximum iteration count when using RPTS and RPTB (and DB on the C40)
12173 is @math{2^{31} + 1} since these instructions test if the iteration count is
12174 negative to terminate the loop. If the iteration count is unsigned
12175 there is a possibility than the @math{2^{31} + 1} maximum iteration count may be
12176 exceeded. This switch allows an unsigned iteration count.
12180 Try to emit an assembler syntax that the TI assembler (asm30) is happy
12181 with. This also enforces compatibility with the API employed by the TI
12182 C3x C compiler. For example, long doubles are passed as structures
12183 rather than in floating point registers.
12189 Generate code that uses registers (stack) for passing arguments to functions.
12190 By default, arguments are passed in registers where possible rather
12191 than by pushing arguments on to the stack.
12193 @item -mparallel-insns
12194 @itemx -mno-parallel-insns
12195 @opindex mparallel-insns
12196 @opindex mno-parallel-insns
12197 Allow the generation of parallel instructions. This is enabled by
12198 default with @option{-O2}.
12200 @item -mparallel-mpy
12201 @itemx -mno-parallel-mpy
12202 @opindex mparallel-mpy
12203 @opindex mno-parallel-mpy
12204 Allow the generation of MPY||ADD and MPY||SUB parallel instructions,
12205 provided @option{-mparallel-insns} is also specified. These instructions have
12206 tight register constraints which can pessimize the code generation
12207 of large functions.
12212 @subsection V850 Options
12213 @cindex V850 Options
12215 These @samp{-m} options are defined for V850 implementations:
12219 @itemx -mno-long-calls
12220 @opindex mlong-calls
12221 @opindex mno-long-calls
12222 Treat all calls as being far away (near). If calls are assumed to be
12223 far away, the compiler will always load the functions address up into a
12224 register, and call indirect through the pointer.
12230 Do not optimize (do optimize) basic blocks that use the same index
12231 pointer 4 or more times to copy pointer into the @code{ep} register, and
12232 use the shorter @code{sld} and @code{sst} instructions. The @option{-mep}
12233 option is on by default if you optimize.
12235 @item -mno-prolog-function
12236 @itemx -mprolog-function
12237 @opindex mno-prolog-function
12238 @opindex mprolog-function
12239 Do not use (do use) external functions to save and restore registers
12240 at the prologue and epilogue of a function. The external functions
12241 are slower, but use less code space if more than one function saves
12242 the same number of registers. The @option{-mprolog-function} option
12243 is on by default if you optimize.
12247 Try to make the code as small as possible. At present, this just turns
12248 on the @option{-mep} and @option{-mprolog-function} options.
12250 @item -mtda=@var{n}
12252 Put static or global variables whose size is @var{n} bytes or less into
12253 the tiny data area that register @code{ep} points to. The tiny data
12254 area can hold up to 256 bytes in total (128 bytes for byte references).
12256 @item -msda=@var{n}
12258 Put static or global variables whose size is @var{n} bytes or less into
12259 the small data area that register @code{gp} points to. The small data
12260 area can hold up to 64 kilobytes.
12262 @item -mzda=@var{n}
12264 Put static or global variables whose size is @var{n} bytes or less into
12265 the first 32 kilobytes of memory.
12269 Specify that the target processor is the V850.
12272 @opindex mbig-switch
12273 Generate code suitable for big switch tables. Use this option only if
12274 the assembler/linker complain about out of range branches within a switch
12279 This option will cause r2 and r5 to be used in the code generated by
12280 the compiler. This setting is the default.
12282 @item -mno-app-regs
12283 @opindex mno-app-regs
12284 This option will cause r2 and r5 to be treated as fixed registers.
12288 Specify that the target processor is the V850E1. The preprocessor
12289 constants @samp{__v850e1__} and @samp{__v850e__} will be defined if
12290 this option is used.
12294 Specify that the target processor is the V850E@. The preprocessor
12295 constant @samp{__v850e__} will be defined if this option is used.
12297 If neither @option{-mv850} nor @option{-mv850e} nor @option{-mv850e1}
12298 are defined then a default target processor will be chosen and the
12299 relevant @samp{__v850*__} preprocessor constant will be defined.
12301 The preprocessor constants @samp{__v850} and @samp{__v851__} are always
12302 defined, regardless of which processor variant is the target.
12304 @item -mdisable-callt
12305 @opindex mdisable-callt
12306 This option will suppress generation of the CALLT instruction for the
12307 v850e and v850e1 flavors of the v850 architecture. The default is
12308 @option{-mno-disable-callt} which allows the CALLT instruction to be used.
12313 @subsection VAX Options
12314 @cindex VAX options
12316 These @samp{-m} options are defined for the VAX:
12321 Do not output certain jump instructions (@code{aobleq} and so on)
12322 that the Unix assembler for the VAX cannot handle across long
12327 Do output those jump instructions, on the assumption that you
12328 will assemble with the GNU assembler.
12332 Output code for g-format floating point numbers instead of d-format.
12335 @node x86-64 Options
12336 @subsection x86-64 Options
12337 @cindex x86-64 options
12339 These are listed under @xref{i386 and x86-64 Options}.
12341 @node Xstormy16 Options
12342 @subsection Xstormy16 Options
12343 @cindex Xstormy16 Options
12345 These options are defined for Xstormy16:
12350 Choose startup files and linker script suitable for the simulator.
12353 @node Xtensa Options
12354 @subsection Xtensa Options
12355 @cindex Xtensa Options
12357 These options are supported for Xtensa targets:
12361 @itemx -mno-const16
12363 @opindex mno-const16
12364 Enable or disable use of @code{CONST16} instructions for loading
12365 constant values. The @code{CONST16} instruction is currently not a
12366 standard option from Tensilica. When enabled, @code{CONST16}
12367 instructions are always used in place of the standard @code{L32R}
12368 instructions. The use of @code{CONST16} is enabled by default only if
12369 the @code{L32R} instruction is not available.
12372 @itemx -mno-fused-madd
12373 @opindex mfused-madd
12374 @opindex mno-fused-madd
12375 Enable or disable use of fused multiply/add and multiply/subtract
12376 instructions in the floating-point option. This has no effect if the
12377 floating-point option is not also enabled. Disabling fused multiply/add
12378 and multiply/subtract instructions forces the compiler to use separate
12379 instructions for the multiply and add/subtract operations. This may be
12380 desirable in some cases where strict IEEE 754-compliant results are
12381 required: the fused multiply add/subtract instructions do not round the
12382 intermediate result, thereby producing results with @emph{more} bits of
12383 precision than specified by the IEEE standard. Disabling fused multiply
12384 add/subtract instructions also ensures that the program output is not
12385 sensitive to the compiler's ability to combine multiply and add/subtract
12388 @item -mtext-section-literals
12389 @itemx -mno-text-section-literals
12390 @opindex mtext-section-literals
12391 @opindex mno-text-section-literals
12392 Control the treatment of literal pools. The default is
12393 @option{-mno-text-section-literals}, which places literals in a separate
12394 section in the output file. This allows the literal pool to be placed
12395 in a data RAM/ROM, and it also allows the linker to combine literal
12396 pools from separate object files to remove redundant literals and
12397 improve code size. With @option{-mtext-section-literals}, the literals
12398 are interspersed in the text section in order to keep them as close as
12399 possible to their references. This may be necessary for large assembly
12402 @item -mtarget-align
12403 @itemx -mno-target-align
12404 @opindex mtarget-align
12405 @opindex mno-target-align
12406 When this option is enabled, GCC instructs the assembler to
12407 automatically align instructions to reduce branch penalties at the
12408 expense of some code density. The assembler attempts to widen density
12409 instructions to align branch targets and the instructions following call
12410 instructions. If there are not enough preceding safe density
12411 instructions to align a target, no widening will be performed. The
12412 default is @option{-mtarget-align}. These options do not affect the
12413 treatment of auto-aligned instructions like @code{LOOP}, which the
12414 assembler will always align, either by widening density instructions or
12415 by inserting no-op instructions.
12418 @itemx -mno-longcalls
12419 @opindex mlongcalls
12420 @opindex mno-longcalls
12421 When this option is enabled, GCC instructs the assembler to translate
12422 direct calls to indirect calls unless it can determine that the target
12423 of a direct call is in the range allowed by the call instruction. This
12424 translation typically occurs for calls to functions in other source
12425 files. Specifically, the assembler translates a direct @code{CALL}
12426 instruction into an @code{L32R} followed by a @code{CALLX} instruction.
12427 The default is @option{-mno-longcalls}. This option should be used in
12428 programs where the call target can potentially be out of range. This
12429 option is implemented in the assembler, not the compiler, so the
12430 assembly code generated by GCC will still show direct call
12431 instructions---look at the disassembled object code to see the actual
12432 instructions. Note that the assembler will use an indirect call for
12433 every cross-file call, not just those that really will be out of range.
12436 @node zSeries Options
12437 @subsection zSeries Options
12438 @cindex zSeries options
12440 These are listed under @xref{S/390 and zSeries Options}.
12442 @node Code Gen Options
12443 @section Options for Code Generation Conventions
12444 @cindex code generation conventions
12445 @cindex options, code generation
12446 @cindex run-time options
12448 These machine-independent options control the interface conventions
12449 used in code generation.
12451 Most of them have both positive and negative forms; the negative form
12452 of @option{-ffoo} would be @option{-fno-foo}. In the table below, only
12453 one of the forms is listed---the one which is not the default. You
12454 can figure out the other form by either removing @samp{no-} or adding
12458 @item -fbounds-check
12459 @opindex fbounds-check
12460 For front-ends that support it, generate additional code to check that
12461 indices used to access arrays are within the declared range. This is
12462 currently only supported by the Java and Fortran 77 front-ends, where
12463 this option defaults to true and false respectively.
12467 This option generates traps for signed overflow on addition, subtraction,
12468 multiplication operations.
12472 This option instructs the compiler to assume that signed arithmetic
12473 overflow of addition, subtraction and multiplication wraps around
12474 using twos-complement representation. This flag enables some optimizations
12475 and disables other. This option is enabled by default for the Java
12476 front-end, as required by the Java language specification.
12479 @opindex fexceptions
12480 Enable exception handling. Generates extra code needed to propagate
12481 exceptions. For some targets, this implies GCC will generate frame
12482 unwind information for all functions, which can produce significant data
12483 size overhead, although it does not affect execution. If you do not
12484 specify this option, GCC will enable it by default for languages like
12485 C++ which normally require exception handling, and disable it for
12486 languages like C that do not normally require it. However, you may need
12487 to enable this option when compiling C code that needs to interoperate
12488 properly with exception handlers written in C++. You may also wish to
12489 disable this option if you are compiling older C++ programs that don't
12490 use exception handling.
12492 @item -fnon-call-exceptions
12493 @opindex fnon-call-exceptions
12494 Generate code that allows trapping instructions to throw exceptions.
12495 Note that this requires platform-specific runtime support that does
12496 not exist everywhere. Moreover, it only allows @emph{trapping}
12497 instructions to throw exceptions, i.e.@: memory references or floating
12498 point instructions. It does not allow exceptions to be thrown from
12499 arbitrary signal handlers such as @code{SIGALRM}.
12501 @item -funwind-tables
12502 @opindex funwind-tables
12503 Similar to @option{-fexceptions}, except that it will just generate any needed
12504 static data, but will not affect the generated code in any other way.
12505 You will normally not enable this option; instead, a language processor
12506 that needs this handling would enable it on your behalf.
12508 @item -fasynchronous-unwind-tables
12509 @opindex fasynchronous-unwind-tables
12510 Generate unwind table in dwarf2 format, if supported by target machine. The
12511 table is exact at each instruction boundary, so it can be used for stack
12512 unwinding from asynchronous events (such as debugger or garbage collector).
12514 @item -fpcc-struct-return
12515 @opindex fpcc-struct-return
12516 Return ``short'' @code{struct} and @code{union} values in memory like
12517 longer ones, rather than in registers. This convention is less
12518 efficient, but it has the advantage of allowing intercallability between
12519 GCC-compiled files and files compiled with other compilers, particularly
12520 the Portable C Compiler (pcc).
12522 The precise convention for returning structures in memory depends
12523 on the target configuration macros.
12525 Short structures and unions are those whose size and alignment match
12526 that of some integer type.
12528 @strong{Warning:} code compiled with the @option{-fpcc-struct-return}
12529 switch is not binary compatible with code compiled with the
12530 @option{-freg-struct-return} switch.
12531 Use it to conform to a non-default application binary interface.
12533 @item -freg-struct-return
12534 @opindex freg-struct-return
12535 Return @code{struct} and @code{union} values in registers when possible.
12536 This is more efficient for small structures than
12537 @option{-fpcc-struct-return}.
12539 If you specify neither @option{-fpcc-struct-return} nor
12540 @option{-freg-struct-return}, GCC defaults to whichever convention is
12541 standard for the target. If there is no standard convention, GCC
12542 defaults to @option{-fpcc-struct-return}, except on targets where GCC is
12543 the principal compiler. In those cases, we can choose the standard, and
12544 we chose the more efficient register return alternative.
12546 @strong{Warning:} code compiled with the @option{-freg-struct-return}
12547 switch is not binary compatible with code compiled with the
12548 @option{-fpcc-struct-return} switch.
12549 Use it to conform to a non-default application binary interface.
12551 @item -fshort-enums
12552 @opindex fshort-enums
12553 Allocate to an @code{enum} type only as many bytes as it needs for the
12554 declared range of possible values. Specifically, the @code{enum} type
12555 will be equivalent to the smallest integer type which has enough room.
12557 @strong{Warning:} the @option{-fshort-enums} switch causes GCC to generate
12558 code that is not binary compatible with code generated without that switch.
12559 Use it to conform to a non-default application binary interface.
12561 @item -fshort-double
12562 @opindex fshort-double
12563 Use the same size for @code{double} as for @code{float}.
12565 @strong{Warning:} the @option{-fshort-double} switch causes GCC to generate
12566 code that is not binary compatible with code generated without that switch.
12567 Use it to conform to a non-default application binary interface.
12569 @item -fshort-wchar
12570 @opindex fshort-wchar
12571 Override the underlying type for @samp{wchar_t} to be @samp{short
12572 unsigned int} instead of the default for the target. This option is
12573 useful for building programs to run under WINE@.
12575 @strong{Warning:} the @option{-fshort-wchar} switch causes GCC to generate
12576 code that is not binary compatible with code generated without that switch.
12577 Use it to conform to a non-default application binary interface.
12579 @item -fshared-data
12580 @opindex fshared-data
12581 Requests that the data and non-@code{const} variables of this
12582 compilation be shared data rather than private data. The distinction
12583 makes sense only on certain operating systems, where shared data is
12584 shared between processes running the same program, while private data
12585 exists in one copy per process.
12588 @opindex fno-common
12589 In C, allocate even uninitialized global variables in the data section of the
12590 object file, rather than generating them as common blocks. This has the
12591 effect that if the same variable is declared (without @code{extern}) in
12592 two different compilations, you will get an error when you link them.
12593 The only reason this might be useful is if you wish to verify that the
12594 program will work on other systems which always work this way.
12598 Ignore the @samp{#ident} directive.
12600 @item -finhibit-size-directive
12601 @opindex finhibit-size-directive
12602 Don't output a @code{.size} assembler directive, or anything else that
12603 would cause trouble if the function is split in the middle, and the
12604 two halves are placed at locations far apart in memory. This option is
12605 used when compiling @file{crtstuff.c}; you should not need to use it
12608 @item -fverbose-asm
12609 @opindex fverbose-asm
12610 Put extra commentary information in the generated assembly code to
12611 make it more readable. This option is generally only of use to those
12612 who actually need to read the generated assembly code (perhaps while
12613 debugging the compiler itself).
12615 @option{-fno-verbose-asm}, the default, causes the
12616 extra information to be omitted and is useful when comparing two assembler
12621 @cindex global offset table
12623 Generate position-independent code (PIC) suitable for use in a shared
12624 library, if supported for the target machine. Such code accesses all
12625 constant addresses through a global offset table (GOT)@. The dynamic
12626 loader resolves the GOT entries when the program starts (the dynamic
12627 loader is not part of GCC; it is part of the operating system). If
12628 the GOT size for the linked executable exceeds a machine-specific
12629 maximum size, you get an error message from the linker indicating that
12630 @option{-fpic} does not work; in that case, recompile with @option{-fPIC}
12631 instead. (These maximums are 8k on the SPARC and 32k
12632 on the m68k and RS/6000. The 386 has no such limit.)
12634 Position-independent code requires special support, and therefore works
12635 only on certain machines. For the 386, GCC supports PIC for System V
12636 but not for the Sun 386i. Code generated for the IBM RS/6000 is always
12637 position-independent.
12641 If supported for the target machine, emit position-independent code,
12642 suitable for dynamic linking and avoiding any limit on the size of the
12643 global offset table. This option makes a difference on the m68k,
12644 PowerPC and SPARC@.
12646 Position-independent code requires special support, and therefore works
12647 only on certain machines.
12653 These options are similar to @option{-fpic} and @option{-fPIC}, but
12654 generated position independent code can be only linked into executables.
12655 Usually these options are used when @option{-pie} GCC option will be
12656 used during linking.
12658 @item -fno-jump-tables
12659 @opindex fno-jump-tables
12660 Do not use jump tables for switch statements even where it would be
12661 more efficient than other code generation strategies. This option is
12662 of use in conjunction with @option{-fpic} or @option{-fPIC} for
12663 building code which forms part of a dynamic linker and cannot
12664 reference the address of a jump table. On some targets, jump tables
12665 do not require a GOT and this option is not needed.
12667 @item -ffixed-@var{reg}
12669 Treat the register named @var{reg} as a fixed register; generated code
12670 should never refer to it (except perhaps as a stack pointer, frame
12671 pointer or in some other fixed role).
12673 @var{reg} must be the name of a register. The register names accepted
12674 are machine-specific and are defined in the @code{REGISTER_NAMES}
12675 macro in the machine description macro file.
12677 This flag does not have a negative form, because it specifies a
12680 @item -fcall-used-@var{reg}
12681 @opindex fcall-used
12682 Treat the register named @var{reg} as an allocable register that is
12683 clobbered by function calls. It may be allocated for temporaries or
12684 variables that do not live across a call. Functions compiled this way
12685 will not save and restore the register @var{reg}.
12687 It is an error to used this flag with the frame pointer or stack pointer.
12688 Use of this flag for other registers that have fixed pervasive roles in
12689 the machine's execution model will produce disastrous results.
12691 This flag does not have a negative form, because it specifies a
12694 @item -fcall-saved-@var{reg}
12695 @opindex fcall-saved
12696 Treat the register named @var{reg} as an allocable register saved by
12697 functions. It may be allocated even for temporaries or variables that
12698 live across a call. Functions compiled this way will save and restore
12699 the register @var{reg} if they use it.
12701 It is an error to used this flag with the frame pointer or stack pointer.
12702 Use of this flag for other registers that have fixed pervasive roles in
12703 the machine's execution model will produce disastrous results.
12705 A different sort of disaster will result from the use of this flag for
12706 a register in which function values may be returned.
12708 This flag does not have a negative form, because it specifies a
12711 @item -fpack-struct[=@var{n}]
12712 @opindex fpack-struct
12713 Without a value specified, pack all structure members together without
12714 holes. When a value is specified (which must be a small power of two), pack
12715 structure members according to this value, representing the maximum
12716 alignment (that is, objects with default alignment requirements larger than
12717 this will be output potentially unaligned at the next fitting location.
12719 @strong{Warning:} the @option{-fpack-struct} switch causes GCC to generate
12720 code that is not binary compatible with code generated without that switch.
12721 Additionally, it makes the code suboptimal.
12722 Use it to conform to a non-default application binary interface.
12724 @item -finstrument-functions
12725 @opindex finstrument-functions
12726 Generate instrumentation calls for entry and exit to functions. Just
12727 after function entry and just before function exit, the following
12728 profiling functions will be called with the address of the current
12729 function and its call site. (On some platforms,
12730 @code{__builtin_return_address} does not work beyond the current
12731 function, so the call site information may not be available to the
12732 profiling functions otherwise.)
12735 void __cyg_profile_func_enter (void *this_fn,
12737 void __cyg_profile_func_exit (void *this_fn,
12741 The first argument is the address of the start of the current function,
12742 which may be looked up exactly in the symbol table.
12744 This instrumentation is also done for functions expanded inline in other
12745 functions. The profiling calls will indicate where, conceptually, the
12746 inline function is entered and exited. This means that addressable
12747 versions of such functions must be available. If all your uses of a
12748 function are expanded inline, this may mean an additional expansion of
12749 code size. If you use @samp{extern inline} in your C code, an
12750 addressable version of such functions must be provided. (This is
12751 normally the case anyways, but if you get lucky and the optimizer always
12752 expands the functions inline, you might have gotten away without
12753 providing static copies.)
12755 A function may be given the attribute @code{no_instrument_function}, in
12756 which case this instrumentation will not be done. This can be used, for
12757 example, for the profiling functions listed above, high-priority
12758 interrupt routines, and any functions from which the profiling functions
12759 cannot safely be called (perhaps signal handlers, if the profiling
12760 routines generate output or allocate memory).
12762 @item -fstack-check
12763 @opindex fstack-check
12764 Generate code to verify that you do not go beyond the boundary of the
12765 stack. You should specify this flag if you are running in an
12766 environment with multiple threads, but only rarely need to specify it in
12767 a single-threaded environment since stack overflow is automatically
12768 detected on nearly all systems if there is only one stack.
12770 Note that this switch does not actually cause checking to be done; the
12771 operating system must do that. The switch causes generation of code
12772 to ensure that the operating system sees the stack being extended.
12774 @item -fstack-limit-register=@var{reg}
12775 @itemx -fstack-limit-symbol=@var{sym}
12776 @itemx -fno-stack-limit
12777 @opindex fstack-limit-register
12778 @opindex fstack-limit-symbol
12779 @opindex fno-stack-limit
12780 Generate code to ensure that the stack does not grow beyond a certain value,
12781 either the value of a register or the address of a symbol. If the stack
12782 would grow beyond the value, a signal is raised. For most targets,
12783 the signal is raised before the stack overruns the boundary, so
12784 it is possible to catch the signal without taking special precautions.
12786 For instance, if the stack starts at absolute address @samp{0x80000000}
12787 and grows downwards, you can use the flags
12788 @option{-fstack-limit-symbol=__stack_limit} and
12789 @option{-Wl,--defsym,__stack_limit=0x7ffe0000} to enforce a stack limit
12790 of 128KB@. Note that this may only work with the GNU linker.
12792 @cindex aliasing of parameters
12793 @cindex parameters, aliased
12794 @item -fargument-alias
12795 @itemx -fargument-noalias
12796 @itemx -fargument-noalias-global
12797 @opindex fargument-alias
12798 @opindex fargument-noalias
12799 @opindex fargument-noalias-global
12800 Specify the possible relationships among parameters and between
12801 parameters and global data.
12803 @option{-fargument-alias} specifies that arguments (parameters) may
12804 alias each other and may alias global storage.@*
12805 @option{-fargument-noalias} specifies that arguments do not alias
12806 each other, but may alias global storage.@*
12807 @option{-fargument-noalias-global} specifies that arguments do not
12808 alias each other and do not alias global storage.
12810 Each language will automatically use whatever option is required by
12811 the language standard. You should not need to use these options yourself.
12813 @item -fleading-underscore
12814 @opindex fleading-underscore
12815 This option and its counterpart, @option{-fno-leading-underscore}, forcibly
12816 change the way C symbols are represented in the object file. One use
12817 is to help link with legacy assembly code.
12819 @strong{Warning:} the @option{-fleading-underscore} switch causes GCC to
12820 generate code that is not binary compatible with code generated without that
12821 switch. Use it to conform to a non-default application binary interface.
12822 Not all targets provide complete support for this switch.
12824 @item -ftls-model=@var{model}
12825 Alter the thread-local storage model to be used (@pxref{Thread-Local}).
12826 The @var{model} argument should be one of @code{global-dynamic},
12827 @code{local-dynamic}, @code{initial-exec} or @code{local-exec}.
12829 The default without @option{-fpic} is @code{initial-exec}; with
12830 @option{-fpic} the default is @code{global-dynamic}.
12832 @item -fvisibility=@var{default|internal|hidden|protected}
12833 @opindex fvisibility
12834 Set the default ELF image symbol visibility to the specified option---all
12835 symbols will be marked with this unless overridden within the code.
12836 Using this feature can very substantially improve linking and
12837 load times of shared object libraries, produce more optimized
12838 code, provide near-perfect API export and prevent symbol clashes.
12839 It is @strong{strongly} recommended that you use this in any shared objects
12842 Despite the nomenclature, @code{default} always means public ie;
12843 available to be linked against from outside the shared object.
12844 @code{protected} and @code{internal} are pretty useless in real-world
12845 usage so the only other commonly used option will be @code{hidden}.
12846 The default if @option{-fvisibility} isn't specified is
12847 @code{default}, i.e., make every
12848 symbol public---this causes the same behavior as previous versions of
12851 A good explanation of the benefits offered by ensuring ELF
12852 symbols have the correct visibility is given by ``How To Write
12853 Shared Libraries'' by Ulrich Drepper (which can be found at
12854 @w{@uref{http://people.redhat.com/~drepper/}})---however a superior
12855 solution made possible by this option to marking things hidden when
12856 the default is public is to make the default hidden and mark things
12857 public. This is the norm with DLL's on Windows and with @option{-fvisibility=hidden}
12858 and @code{__attribute__ ((visibility("default")))} instead of
12859 @code{__declspec(dllexport)} you get almost identical semantics with
12860 identical syntax. This is a great boon to those working with
12861 cross-platform projects.
12863 For those adding visibility support to existing code, you may find
12864 @samp{#pragma GCC visibility} of use. This works by you enclosing
12865 the declarations you wish to set visibility for with (for example)
12866 @samp{#pragma GCC visibility push(hidden)} and
12867 @samp{#pragma GCC visibility pop}.
12868 Bear in mind that symbol visibility should be viewed @strong{as
12869 part of the API interface contract} and thus all new code should
12870 always specify visibility when it is not the default ie; declarations
12871 only for use within the local DSO should @strong{always} be marked explicitly
12872 as hidden as so to avoid PLT indirection overheads---making this
12873 abundantly clear also aids readability and self-documentation of the code.
12874 Note that due to ISO C++ specification requirements, operator new and
12875 operator delete must always be of default visibility.
12877 An overview of these techniques, their benefits and how to use them
12878 is at @w{@uref{http://gcc.gnu.org/wiki/Visibility}}.
12884 @node Environment Variables
12885 @section Environment Variables Affecting GCC
12886 @cindex environment variables
12888 @c man begin ENVIRONMENT
12889 This section describes several environment variables that affect how GCC
12890 operates. Some of them work by specifying directories or prefixes to use
12891 when searching for various kinds of files. Some are used to specify other
12892 aspects of the compilation environment.
12894 Note that you can also specify places to search using options such as
12895 @option{-B}, @option{-I} and @option{-L} (@pxref{Directory Options}). These
12896 take precedence over places specified using environment variables, which
12897 in turn take precedence over those specified by the configuration of GCC@.
12898 @xref{Driver,, Controlling the Compilation Driver @file{gcc}, gccint,
12899 GNU Compiler Collection (GCC) Internals}.
12904 @c @itemx LC_COLLATE
12906 @c @itemx LC_MONETARY
12907 @c @itemx LC_NUMERIC
12912 @c @findex LC_COLLATE
12913 @findex LC_MESSAGES
12914 @c @findex LC_MONETARY
12915 @c @findex LC_NUMERIC
12919 These environment variables control the way that GCC uses
12920 localization information that allow GCC to work with different
12921 national conventions. GCC inspects the locale categories
12922 @env{LC_CTYPE} and @env{LC_MESSAGES} if it has been configured to do
12923 so. These locale categories can be set to any value supported by your
12924 installation. A typical value is @samp{en_GB.UTF-8} for English in the United
12925 Kingdom encoded in UTF-8.
12927 The @env{LC_CTYPE} environment variable specifies character
12928 classification. GCC uses it to determine the character boundaries in
12929 a string; this is needed for some multibyte encodings that contain quote
12930 and escape characters that would otherwise be interpreted as a string
12933 The @env{LC_MESSAGES} environment variable specifies the language to
12934 use in diagnostic messages.
12936 If the @env{LC_ALL} environment variable is set, it overrides the value
12937 of @env{LC_CTYPE} and @env{LC_MESSAGES}; otherwise, @env{LC_CTYPE}
12938 and @env{LC_MESSAGES} default to the value of the @env{LANG}
12939 environment variable. If none of these variables are set, GCC
12940 defaults to traditional C English behavior.
12944 If @env{TMPDIR} is set, it specifies the directory to use for temporary
12945 files. GCC uses temporary files to hold the output of one stage of
12946 compilation which is to be used as input to the next stage: for example,
12947 the output of the preprocessor, which is the input to the compiler
12950 @item GCC_EXEC_PREFIX
12951 @findex GCC_EXEC_PREFIX
12952 If @env{GCC_EXEC_PREFIX} is set, it specifies a prefix to use in the
12953 names of the subprograms executed by the compiler. No slash is added
12954 when this prefix is combined with the name of a subprogram, but you can
12955 specify a prefix that ends with a slash if you wish.
12957 If @env{GCC_EXEC_PREFIX} is not set, GCC will attempt to figure out
12958 an appropriate prefix to use based on the pathname it was invoked with.
12960 If GCC cannot find the subprogram using the specified prefix, it
12961 tries looking in the usual places for the subprogram.
12963 The default value of @env{GCC_EXEC_PREFIX} is
12964 @file{@var{prefix}/lib/gcc/} where @var{prefix} is the value
12965 of @code{prefix} when you ran the @file{configure} script.
12967 Other prefixes specified with @option{-B} take precedence over this prefix.
12969 This prefix is also used for finding files such as @file{crt0.o} that are
12972 In addition, the prefix is used in an unusual way in finding the
12973 directories to search for header files. For each of the standard
12974 directories whose name normally begins with @samp{/usr/local/lib/gcc}
12975 (more precisely, with the value of @env{GCC_INCLUDE_DIR}), GCC tries
12976 replacing that beginning with the specified prefix to produce an
12977 alternate directory name. Thus, with @option{-Bfoo/}, GCC will search
12978 @file{foo/bar} where it would normally search @file{/usr/local/lib/bar}.
12979 These alternate directories are searched first; the standard directories
12982 @item COMPILER_PATH
12983 @findex COMPILER_PATH
12984 The value of @env{COMPILER_PATH} is a colon-separated list of
12985 directories, much like @env{PATH}. GCC tries the directories thus
12986 specified when searching for subprograms, if it can't find the
12987 subprograms using @env{GCC_EXEC_PREFIX}.
12990 @findex LIBRARY_PATH
12991 The value of @env{LIBRARY_PATH} is a colon-separated list of
12992 directories, much like @env{PATH}. When configured as a native compiler,
12993 GCC tries the directories thus specified when searching for special
12994 linker files, if it can't find them using @env{GCC_EXEC_PREFIX}. Linking
12995 using GCC also uses these directories when searching for ordinary
12996 libraries for the @option{-l} option (but directories specified with
12997 @option{-L} come first).
13001 @cindex locale definition
13002 This variable is used to pass locale information to the compiler. One way in
13003 which this information is used is to determine the character set to be used
13004 when character literals, string literals and comments are parsed in C and C++.
13005 When the compiler is configured to allow multibyte characters,
13006 the following values for @env{LANG} are recognized:
13010 Recognize JIS characters.
13012 Recognize SJIS characters.
13014 Recognize EUCJP characters.
13017 If @env{LANG} is not defined, or if it has some other value, then the
13018 compiler will use mblen and mbtowc as defined by the default locale to
13019 recognize and translate multibyte characters.
13023 Some additional environments variables affect the behavior of the
13026 @include cppenv.texi
13030 @node Precompiled Headers
13031 @section Using Precompiled Headers
13032 @cindex precompiled headers
13033 @cindex speed of compilation
13035 Often large projects have many header files that are included in every
13036 source file. The time the compiler takes to process these header files
13037 over and over again can account for nearly all of the time required to
13038 build the project. To make builds faster, GCC allows users to
13039 `precompile' a header file; then, if builds can use the precompiled
13040 header file they will be much faster.
13042 To create a precompiled header file, simply compile it as you would any
13043 other file, if necessary using the @option{-x} option to make the driver
13044 treat it as a C or C++ header file. You will probably want to use a
13045 tool like @command{make} to keep the precompiled header up-to-date when
13046 the headers it contains change.
13048 A precompiled header file will be searched for when @code{#include} is
13049 seen in the compilation. As it searches for the included file
13050 (@pxref{Search Path,,Search Path,cpp,The C Preprocessor}) the
13051 compiler looks for a precompiled header in each directory just before it
13052 looks for the include file in that directory. The name searched for is
13053 the name specified in the @code{#include} with @samp{.gch} appended. If
13054 the precompiled header file can't be used, it is ignored.
13056 For instance, if you have @code{#include "all.h"}, and you have
13057 @file{all.h.gch} in the same directory as @file{all.h}, then the
13058 precompiled header file will be used if possible, and the original
13059 header will be used otherwise.
13061 Alternatively, you might decide to put the precompiled header file in a
13062 directory and use @option{-I} to ensure that directory is searched
13063 before (or instead of) the directory containing the original header.
13064 Then, if you want to check that the precompiled header file is always
13065 used, you can put a file of the same name as the original header in this
13066 directory containing an @code{#error} command.
13068 This also works with @option{-include}. So yet another way to use
13069 precompiled headers, good for projects not designed with precompiled
13070 header files in mind, is to simply take most of the header files used by
13071 a project, include them from another header file, precompile that header
13072 file, and @option{-include} the precompiled header. If the header files
13073 have guards against multiple inclusion, they will be skipped because
13074 they've already been included (in the precompiled header).
13076 If you need to precompile the same header file for different
13077 languages, targets, or compiler options, you can instead make a
13078 @emph{directory} named like @file{all.h.gch}, and put each precompiled
13079 header in the directory, perhaps using @option{-o}. It doesn't matter
13080 what you call the files in the directory, every precompiled header in
13081 the directory will be considered. The first precompiled header
13082 encountered in the directory that is valid for this compilation will
13083 be used; they're searched in no particular order.
13085 There are many other possibilities, limited only by your imagination,
13086 good sense, and the constraints of your build system.
13088 A precompiled header file can be used only when these conditions apply:
13092 Only one precompiled header can be used in a particular compilation.
13095 A precompiled header can't be used once the first C token is seen. You
13096 can have preprocessor directives before a precompiled header; you can
13097 even include a precompiled header from inside another header, so long as
13098 there are no C tokens before the @code{#include}.
13101 The precompiled header file must be produced for the same language as
13102 the current compilation. You can't use a C precompiled header for a C++
13106 The precompiled header file must have been produced by the same compiler
13107 binary as the current compilation is using.
13110 Any macros defined before the precompiled header is included must
13111 either be defined in the same way as when the precompiled header was
13112 generated, or must not affect the precompiled header, which usually
13113 means that they don't appear in the precompiled header at all.
13115 The @option{-D} option is one way to define a macro before a
13116 precompiled header is included; using a @code{#define} can also do it.
13117 There are also some options that define macros implicitly, like
13118 @option{-O} and @option{-Wdeprecated}; the same rule applies to macros
13121 @item If debugging information is output when using the precompiled
13122 header, using @option{-g} or similar, the same kind of debugging information
13123 must have been output when building the precompiled header. However,
13124 a precompiled header built using @option{-g} can be used in a compilation
13125 when no debugging information is being output.
13127 @item The same @option{-m} options must generally be used when building
13128 and using the precompiled header. @xref{Submodel Options},
13129 for any cases where this rule is relaxed.
13131 @item Each of the following options must be the same when building and using
13132 the precompiled header:
13134 @gccoptlist{-fexceptions -funit-at-a-time}
13137 Some other command-line options starting with @option{-f},
13138 @option{-p}, or @option{-O} must be defined in the same way as when
13139 the precompiled header was generated. At present, it's not clear
13140 which options are safe to change and which are not; the safest choice
13141 is to use exactly the same options when generating and using the
13142 precompiled header. The following are known to be safe:
13144 @gccoptlist{-fpreprocessed
13145 -fsched-interblock -fsched-spec -fsched-spec-load -fsched-spec-load-dangerous
13146 -fsched-verbose=<number> -fschedule-insns
13151 For all of these except the last, the compiler will automatically
13152 ignore the precompiled header if the conditions aren't met. If you
13153 find an option combination that doesn't work and doesn't cause the
13154 precompiled header to be ignored, please consider filing a bug report,
13157 If you do use differing options when generating and using the
13158 precompiled header, the actual behavior will be a mixture of the
13159 behavior for the options. For instance, if you use @option{-g} to
13160 generate the precompiled header but not when using it, you may or may
13161 not get debugging information for routines in the precompiled header.
13163 @node Running Protoize
13164 @section Running Protoize
13166 The program @code{protoize} is an optional part of GCC@. You can use
13167 it to add prototypes to a program, thus converting the program to ISO
13168 C in one respect. The companion program @code{unprotoize} does the
13169 reverse: it removes argument types from any prototypes that are found.
13171 When you run these programs, you must specify a set of source files as
13172 command line arguments. The conversion programs start out by compiling
13173 these files to see what functions they define. The information gathered
13174 about a file @var{foo} is saved in a file named @file{@var{foo}.X}.
13176 After scanning comes actual conversion. The specified files are all
13177 eligible to be converted; any files they include (whether sources or
13178 just headers) are eligible as well.
13180 But not all the eligible files are converted. By default,
13181 @code{protoize} and @code{unprotoize} convert only source and header
13182 files in the current directory. You can specify additional directories
13183 whose files should be converted with the @option{-d @var{directory}}
13184 option. You can also specify particular files to exclude with the
13185 @option{-x @var{file}} option. A file is converted if it is eligible, its
13186 directory name matches one of the specified directory names, and its
13187 name within the directory has not been excluded.
13189 Basic conversion with @code{protoize} consists of rewriting most
13190 function definitions and function declarations to specify the types of
13191 the arguments. The only ones not rewritten are those for varargs
13194 @code{protoize} optionally inserts prototype declarations at the
13195 beginning of the source file, to make them available for any calls that
13196 precede the function's definition. Or it can insert prototype
13197 declarations with block scope in the blocks where undeclared functions
13200 Basic conversion with @code{unprotoize} consists of rewriting most
13201 function declarations to remove any argument types, and rewriting
13202 function definitions to the old-style pre-ISO form.
13204 Both conversion programs print a warning for any function declaration or
13205 definition that they can't convert. You can suppress these warnings
13208 The output from @code{protoize} or @code{unprotoize} replaces the
13209 original source file. The original file is renamed to a name ending
13210 with @samp{.save} (for DOS, the saved filename ends in @samp{.sav}
13211 without the original @samp{.c} suffix). If the @samp{.save} (@samp{.sav}
13212 for DOS) file already exists, then the source file is simply discarded.
13214 @code{protoize} and @code{unprotoize} both depend on GCC itself to
13215 scan the program and collect information about the functions it uses.
13216 So neither of these programs will work until GCC is installed.
13218 Here is a table of the options you can use with @code{protoize} and
13219 @code{unprotoize}. Each option works with both programs unless
13223 @item -B @var{directory}
13224 Look for the file @file{SYSCALLS.c.X} in @var{directory}, instead of the
13225 usual directory (normally @file{/usr/local/lib}). This file contains
13226 prototype information about standard system functions. This option
13227 applies only to @code{protoize}.
13229 @item -c @var{compilation-options}
13230 Use @var{compilation-options} as the options when running @command{gcc} to
13231 produce the @samp{.X} files. The special option @option{-aux-info} is
13232 always passed in addition, to tell @command{gcc} to write a @samp{.X} file.
13234 Note that the compilation options must be given as a single argument to
13235 @code{protoize} or @code{unprotoize}. If you want to specify several
13236 @command{gcc} options, you must quote the entire set of compilation options
13237 to make them a single word in the shell.
13239 There are certain @command{gcc} arguments that you cannot use, because they
13240 would produce the wrong kind of output. These include @option{-g},
13241 @option{-O}, @option{-c}, @option{-S}, and @option{-o} If you include these in
13242 the @var{compilation-options}, they are ignored.
13245 Rename files to end in @samp{.C} (@samp{.cc} for DOS-based file
13246 systems) instead of @samp{.c}. This is convenient if you are converting
13247 a C program to C++. This option applies only to @code{protoize}.
13250 Add explicit global declarations. This means inserting explicit
13251 declarations at the beginning of each source file for each function
13252 that is called in the file and was not declared. These declarations
13253 precede the first function definition that contains a call to an
13254 undeclared function. This option applies only to @code{protoize}.
13256 @item -i @var{string}
13257 Indent old-style parameter declarations with the string @var{string}.
13258 This option applies only to @code{protoize}.
13260 @code{unprotoize} converts prototyped function definitions to old-style
13261 function definitions, where the arguments are declared between the
13262 argument list and the initial @samp{@{}. By default, @code{unprotoize}
13263 uses five spaces as the indentation. If you want to indent with just
13264 one space instead, use @option{-i " "}.
13267 Keep the @samp{.X} files. Normally, they are deleted after conversion
13271 Add explicit local declarations. @code{protoize} with @option{-l} inserts
13272 a prototype declaration for each function in each block which calls the
13273 function without any declaration. This option applies only to
13277 Make no real changes. This mode just prints information about the conversions
13278 that would have been done without @option{-n}.
13281 Make no @samp{.save} files. The original files are simply deleted.
13282 Use this option with caution.
13284 @item -p @var{program}
13285 Use the program @var{program} as the compiler. Normally, the name
13286 @file{gcc} is used.
13289 Work quietly. Most warnings are suppressed.
13292 Print the version number, just like @option{-v} for @command{gcc}.
13295 If you need special compiler options to compile one of your program's
13296 source files, then you should generate that file's @samp{.X} file
13297 specially, by running @command{gcc} on that source file with the
13298 appropriate options and the option @option{-aux-info}. Then run
13299 @code{protoize} on the entire set of files. @code{protoize} will use
13300 the existing @samp{.X} file because it is newer than the source file.
13304 gcc -Dfoo=bar file1.c -aux-info file1.X
13309 You need to include the special files along with the rest in the
13310 @code{protoize} command, even though their @samp{.X} files already
13311 exist, because otherwise they won't get converted.
13313 @xref{Protoize Caveats}, for more information on how to use
13314 @code{protoize} successfully.