invoke.texi

早期freebsd实现

second branch or a point immediately following it, depending on whether
the condition is known to be true or false.

@item -fcse-follow-jumps
In common subexpression elimination, scan through jump instructions
when the target of the jump is not reached by any other path.  For
example, when CSE encounters an @code{if} statement with an
@code{else} clause, CSE will follow the jump when the condition
tested is false.

@item -fcse-skip-blocks
This is similar to @samp{-fcse-follow-jumps}, but causes CSE to
follow jumps which conditionally skip over blocks.  When CSE
encounters a simple @code{if} statement with no else clause,
@samp{-fcse-skip-blocks} causes CSE to follow the jump around the
body of the @code{if}.

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

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

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

@item -fschedule-insns
If supported for the target machine, attempt to reorder instructions to
eliminate execution stalls due to required data being unavailable.  This
helps machines that have slow floating point or memory load instructions
by allowing other instructions to be issued until the result of the load
or floating point instruction is required.

@item -fschedule-insns2
Similar to @samp{-fschedule-insns}, but requests an additional pass of
instruction scheduling after register allocation has been done.  This is
especially useful on machines with a relatively small number of
registers and where memory load instructions take more than one cycle.

@item -fcaller-saves
Enable values to be allocated in registers that will be clobbered by
function calls, by emitting extra instructions to save and restore the
registers around such calls.  Such allocation is done only when it
seems to result in better code than would otherwise be produced.

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

@item -funroll-loops
Perform the optimization of loop unrolling.  This is only done for loops
whose number of iterations can be determined at compile time or run time.
@samp{-funroll-loop} implies @samp{-fstrength-reduce} and
@samp{-frerun-cse-after-loop}.

@item -funroll-all-loops
Perform the optimization of loop unrolling.  This is done for all loops
and usually makes programs run more slowly.  @samp{-funroll-all-loops}
implies @samp{-fstrength-reduce} and @samp{-frerun-cse-after-loop}.

@item -fno-peephole
Disable any machine-specific peephole optimizations.
@end table

@node Preprocessor Options
@section Options Controlling the Preprocessor
@cindex preprocessor options
@cindex options, preprocessor

These options control the C preprocessor, which is run on each C source
file before actual compilation.

If you use the @samp{-E} option, nothing is done except preprocessing.
Some of these options make sense only together with @samp{-E} because
they cause the preprocessor output to be unsuitable for actual
compilation.

@table @code
@item -include @var{file}
Process @var{file} as input before processing the regular input file.
In effect, the contents of @var{file} are compiled first.  Any @samp{-D}
and @samp{-U} options on the command line are always processed before
@samp{-include @var{file}}, regardless of the order in which they are
written.  All the @samp{-include} and @samp{-imacros} options are
processed in the order in which they are written.

@item -imacros @var{file}
Process @var{file} as input, discarding the resulting output, before
processing the regular input file.  Because the output generated from
@var{file} is discarded, the only effect of @samp{-imacros @var{file}}
is to make the macros defined in @var{file} available for use in the
main input.

Any @samp{-D} and @samp{-U} options on the command line are always
processed before @samp{-imacros @var{file}}, regardless of the order in
which they are written.  All the @samp{-include} and @samp{-imacros}
options are processed in the order in which they are written.

@item -nostdinc
Do not search the standard system directories for header files.  Only
the directories you have specified with @samp{-I} options (and the
current directory, if appropriate) are searched.  @xref{Directory
Options}, for information on @samp{-I}.

By using both @samp{-nostdinc} and @samp{-I-}, you can limit the include-file
search path to only those directories you specify explicitly.

@item -nostdinc++
Do not search for header files in the C++-specific standard directories,
but do still search the other standard directories.
(This option is used when building @samp{libg++}.)

@item -undef
Do not predefine any nonstandard macros.  (Including architecture flags).

@item -E
Run only the C preprocessor.  Preprocess all the C source files
specified and output the results to standard output or to the
specified output file.

@item -C
Tell the preprocessor not to discard comments.  Used with the
@samp{-E} option.

@item -P
Tell the preprocessor not to generate @samp{#line} commands.
Used with the @samp{-E} option.

@cindex make
@cindex dependencies, make
@item -M
Tell the preprocessor to output a rule suitable for @code{make}
describing the dependencies of each object file.  For each source file,
the preprocessor outputs one @code{make}-rule whose target is the object
file name for that source file and whose dependencies are all the files
@samp{#include}d in it.  This rule may be a single line or may be
continued with @samp{\}-newline if it is long.  The list of rules is
printed on standard output instead of the preprocessed C program.

@samp{-M} implies @samp{-E}.

Another way to specify output of a @code{make} rule is by setting
the environment variable @code{DEPENDENCIES_OUTPUT} (@pxref{Environment
Variables}).

@item -MM
Like @samp{-M} but the output mentions only the user header files
included with @samp{#include "@var{file}"}.  System header files
included with @samp{#include <@var{file}>} are omitted.

@item -MD
Like @samp{-M} but the dependency information is written to files with
names made by replacing @samp{.o} with @samp{.d} at the end of the
output file names.  This is in addition to compiling the input files as
specified---@samp{-MD} does not inhibit ordinary compilation the way
@samp{-M} does.

The Mach utility @samp{md} can be used to merge the @samp{.d} files
into a single dependency file suitable for using with the @samp{make}
command.

@item -MMD
Like @samp{-MD} except mention only user header files, not system
header files.

@item -H
Print the name of each header file used, in addition to other normal
activities.

@item -A@var{question}(@var{answer})
Assert the answer @var{answer} for @var{question}, in case it is tested
with a preprocessor conditional such as @samp{#if
#@var{question}(@var{answer})}.  @samp{-A-} disables the standard
assertions that normally describe the target machine.

@item -D@var{macro}
Define macro @var{macro} with the string @samp{1} as its definition.

@item -D@var{macro}=@var{defn}
Define macro @var{macro} as @var{defn}.  All instances of @samp{-D} on
the command line are processed before any @samp{-U} options.

@item -U@var{macro}
Undefine macro @var{macro}.  @samp{-U} options are evaluated after all
@samp{-D} options, but before any @samp{-include} and @samp{-imacros}
options.

@item -dM
Tell the preprocessor to output only a list of the macro definitions
that are in effect at the end of preprocessing.  Used with the @samp{-E}
option.

@item -dD
Tell the preprocessing to pass all macro definitions into the output, in
their proper sequence in the rest of the output.

@item -dN
Like @samp{-dD} except that the macro arguments and contents are omitted.
Only @samp{#define @var{name}} is included in the output.

@item -trigraphs
Support ANSI C trigraphs.  You don't want to know about this
brain-damage.  The @samp{-ansi} option also has this effect.
@end table

@node Assembler Options
@section Passing Options to the Assembler

@table @samp
@item -Wa,@var{option}
Pass @var{option} as an option to the assembler.  If @var{option}
contains commas, it is split into multiple options at the commas.
@end table

@node Link Options
@section Options for Linking
@cindex link options
@cindex options, linking

These options come into play when the compiler links object files into
an executable output file.  They are meaningless if the compiler is
not doing a link step.

@table @code
@cindex file names
@item @var{object-file-name}
A file name that does not end in a special recognized suffix is
considered to name an object file or library.  (Object files are
distinguished from libraries by the linker according to the file
contents.)  If linking is done, these object files are used as input
to the linker.

@item -c
@itemx -S
@itemx -E
If any of these options is used, then the linker is not run, and
object file names should not be used as arguments.  @xref{Overall
Options}.

@cindex Libraries
@item -l@var{library}
Search the library named @var{library} when linking.

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

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

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

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

@item -lobjc
This special case of the @samp{-l} option is what you need to do when you
link an Objective C program.

@item -nostdlib
Don't use the standard system libraries and startup files when linking.
Only the files you specify will be passed to the linker.

@item -static
On systems that support dynamic linking, this prevents linking with the shared
libraries.  On other systems, this
option has no effect.

@item -shared
Produce a shared object which can then be linked with other objects to
form an executable.  Only a few systems support this option.

@item -symbolic
Bind references to global symbols when building a shared object.  Warn
about any unresolved references (unless overridden by the link editor
option @samp{-Xlinker -z -Xlinker defs}).  Only a few systems support
this option.

@item -Xlinker @var{option}
Pass @var{option} as an option to the linker.  You can use this to
supply system-specific linker options which GNU CC does not know how to
recognize.

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

@item -Wl,@var{option}
Pass @var{option} as an option to the linker.  If @var{option} contains
commas, it is split into multiple options at the commas.

@item -u @var{symbol}
Pretend the symbol @var{symbol} is undefined, to force linking of
library modules to define it.  You can use @samp{-u} multiple times with
different symbols to force loading of additional library modules.
@end table

@node Directory Options
@section Options for Directory Search
@cindex directory options
@cindex options, directory search
@cindex search path

These options specify directories to search for header files, for
libraries and for parts of the compiler:

@table @code
@item -I@var{dir}
Append directory @var{dir} to the list of directories searched for
include files.

@item -I-
Any directories you specify with @samp{-I} options before the @samp{-I-}
option are searched only for the case of @samp{#include "@var{file}"};
they are not searched for @samp{#include <@var{file}>}.

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

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

@samp{-I-} does not inhibit