-x language
Specify explicitly the language for the following input files (rather than letting the compiler
choose a default based on the file name suffix). This option applies to all following input files
until the next -x option. Possible values for language are:
c c-header cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objc-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input ratfor
java
treelang
|
-x none
Turn off any specification of a language, so that subsequent files are handled according to their
file name suffixes (as they are if -x has not been used at all).
|
-pass-exit-codes
Normally the gcc program will exit with the code of 1 if any phase of the compiler returns a non-
success return code. If you specify -pass-exit-codes, the gcc program will instead return with
numerically highest error produced by any phase that returned an error indication.
If you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc
where to start, and one of the options -c, -S, or -E to say where gcc is to stop. Note that some
combinations (for example, -x cpp-output -E) instruct gcc to do nothing at all.
|
-c Compile or assemble the source files, but do not link. The linking stage simply is not done. The
ultimate output is in the form of an object file for each source file.
By default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc.,
with .o.
Unrecognized input files, not requiring compilation or assembly, are ignored.
|
-S Stop after the stage of compilation proper; do not assemble. The output is in the form of an
assembler code file for each non-assembler input file specified.
By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc.,
with .s.
Input files that don't require compilation are ignored.
|
-E Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of
preprocessed source code, which is sent to the standard output.
Input files which don't require preprocessing are ignored.
|
-o file
Place output in file file. This applies regardless to whatever sort of output is being produced,
whether it be an executable file, an object file, an assembler file or preprocessed C code.
If you specify -o when compiling more than one input file, or you are producing an executable file as
output, all the source files on the command line will be compiled at once.
If -o is not specified, the default is to put an executable file in a.out, the object file for
source.suffix in source.o, its assembler file in source.s, and all preprocessed C source on standard
output.
|
-v Print (on standard error output) the commands executed to run the stages of compilation. Also print
the version number of the compiler driver program and of the preprocessor and the compiler proper.
|
-pipe
Use pipes rather than temporary files for communication between the various stages of compilation.
This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU
assembler has no trouble.
|
--help
Print (on the standard output) a description of the command line options understood by gcc. If the
-v option is also specified then --help will also be passed on to the various processes invoked by
gcc, so that they can display the command line options they accept. If the -Wextra option is also
specified then command line options which have no documentation associated with them will also be
displayed.
|
--target-help
Print (on the standard output) a description of target specific command line options for each tool.
|
--version
Display the version number and copyrights of the invoked GCC.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++
header files often use .hh or .H; and preprocessed C++ files use the suffix .ii. GCC recognizes files
with these names and compiles them as C++ programs even if you call the compiler the same way as for
compiling C programs (usually with the name gcc).
However, C++ programs often require class libraries as well as a compiler that understands the C++
language---and under some circumstances, you might want to compile programs or header files from standard
input, or otherwise without a suffix that flags them as C++ programs. You might also like to precompile
a C header file with a .h extension to be used in C++ compilations. g++ is a program that calls GCC with
the default language set to C++, and automatically specifies linking against the C++ library. On many
systems, g++ is also installed with the name c++.
When you compile C++ programs, you may specify many of the same command-line options that you use for
compiling programs in any language; or command-line options meaningful for C and related languages; or
options that are meaningful only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived from C, such as C++ and Objective-C)
that the compiler accepts:
|
-ansi
In C mode, support all ISO C90 programs. In C++ mode, remove GNU extensions that conflict with ISO
C++.
|
-std=
Determine the language standard. This option is currently only supported when compiling C or C++. A
value for this option must be provided; possible values are
|
-aux-info filename
Output to the given filename prototyped declarations for all functions declared and/or defined in a
translation unit, including those in header files. This option is silently ignored in any language
other than C.
Besides declarations, the file indicates, in comments, the origin of each declaration (source file
and line), whether the declaration was implicit, prototyped or unprototyped (I, N for new or O for
old, respectively, in the first character after the line number and the colon), and whether it came
from a declaration or a definition (C or F, respectively, in the following character). In the case
of function definitions, a K&R-style list of arguments followed by their declarations is also
provided, inside comments, after the declaration.
|
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so that code can use these words as
identifiers. You can use the keywords "__asm__", "__inline__" and "__typeof__" instead. -ansi
implies -fno-asm.
In C++, this switch only affects the "typeof" keyword, since "asm" and "inline" are standard
keywords. You may want to use the -fno-gnu-keywords flag instead, which has the same effect. In C99
mode (-std=c99 or -std=gnu99), this switch only affects the "asm" and "typeof" keywords, since
"inline" is a standard keyword in ISO C99.
|
-fno-builtin
-fno-builtin-function
Don't recognize built-in functions that do not begin with __builtin_ as prefix.
GCC normally generates special code to handle certain built-in functions more efficiently; for
instance, calls to "alloca" may become single instructions that adjust the stack directly, and calls
to "memcpy" may become inline copy loops. The resulting code is often both smaller and faster, but
since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor
can you change the behavior of the functions by linking with a different library.
With the -fno-builtin-function option only the built-in function function is disabled. function must
not begin with __builtin_. If a function is named this is not built-in in this version of GCC, this
option is ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-
in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
|
-fhosted
Assert that compilation takes place in a hosted environment. This implies -fbuiltin. A hosted
environment is one in which the entire standard library is available, and in which "main" has a
return type of "int". Examples are nearly everything except a kernel. This is equivalent to
-fno-freestanding.
|
-ffreestanding
Assert that compilation takes place in a freestanding environment. This implies -fno-builtin. A
freestanding environment is one in which the standard library may not exist, and program startup may
not necessarily be at "main". The most obvious example is an OS kernel. This is equivalent to
-fno-hosted.
|
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
|
-trigraphs
Support ISO C trigraphs. The -ansi option (and -std options for strict ISO C conformance) implies
-trigraphs.
|
-no-integrated-cpp
Performs a compilation in two passes: preprocessing and compiling. This option allows a user
supplied "cc1", "cc1plus", or "cc1obj" via the -B option. The user supplied compilation step can then
add in an additional preprocessing step after normal preprocessing but before compiling. The default
is to use the integrated cpp (internal cpp)
The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged.
|
-traditional
-traditional-cpp
Formerly, these options caused GCC to attempt to emulate a pre-standard C compiler. They are now
only supported with the -E switch. The preprocessor continues to support a pre-standard mode. See
the GNU CPP manual for details.
|
-fcond-mismatch
Allow conditional expressions with mismatched types in the second and third arguments. The value of
such an expression is void. This option is not supported for C++.
|
-funsigned-char
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should be. It is either like "unsigned char" by
default or like "signed char" by default.
Ideally, a portable program should always use "signed char" or "unsigned char" when it depends on the
signedness of an object. But many programs have been written to use plain "char" and expect it to be
signed, or expect it to be unsigned, depending on the machines they were written for. This option,
and its inverse, let you make such a program work with the opposite default.
The type "char" is always a distinct type from each of "signed char" or "unsigned char", even though
its behavior is always just like one of those two.
|
-fsigned-char
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char.
Likewise, the option -fno-signed-char is equivalent to -funsigned-char.
|
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned, when the declaration does not use
either "signed" or "unsigned". By default, such a bit-field is signed, because this is consistent:
the basic integer types such as "int" are signed types.
|
-fwritable-strings
Store string constants in the writable data segment and don't uniquize them. This is for
compatibility with old programs which assume they can write into string constants.
|
-fabi-version=n
Use version n of the C++ ABI. Version 2 is the version of the C++ ABI that first appeared in G++
3.4. Version 1 is the version of the C++ ABI that first appeared in G++ 3.2. Version 0 will always
be the version that conforms most closely to the C++ ABI specification. Therefore, the ABI obtained
using version 0 will change as ABI bugs are fixed.
The default is version 2.
|
-fno-access-control
Turn off all access checking. This switch is mainly useful for working around bugs in the access
control code.
|
-fcheck-new
Check that the pointer returned by "operator new" is non-null before attempting to modify the storage
allocated. This check is normally unnecessary because the C++ standard specifies that "operator new"
will only return 0 if it is declared throw(), in which case the compiler will always check the return
value even without this option. In all other cases, when "operator new" has a non-empty exception
specification, memory exhaustion is signalled by throwing "std::bad_alloc". See also new (nothrow).
|
-fconserve-space
Put uninitialized or runtime-initialized global variables into the common segment, as C does. This
saves space in the executable at the cost of not diagnosing duplicate definitions. If you compile
with this flag and your program mysteriously crashes after "main()" has completed, you may have an
object that is being destroyed twice because two definitions were merged.
This option is no longer useful on most targets, now that support has been added for putting
variables into BSS without making them common.
|
-fno-const-strings
Give string constants type "char *" instead of type "const char *". By default, G++ uses type "const
char *" as required by the standard. Even if you use -fno-const-strings, you cannot actually modify
the value of a string constant, unless you also use -fwritable-strings.
This option might be removed in a future release of G++. For maximum portability, you should
structure your code so that it works with string constants that have type "const char *".
|
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a temporary which is only used to
initialize another object of the same type. Specifying this option disables that optimization, and
forces G++ to call the copy constructor in all cases.
|
-fno-enforce-eh-specs
Don't check for violation of exception specifications at runtime. This option violates the C++
standard, but may be useful for reducing code size in production builds, much like defining NDEBUG.
The compiler will still optimize based on the exception specifications.
|
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a for-init-statement is limited to
the for loop itself, as specified by the C++ standard. If -fno-for-scope is specified, the scope of
variables declared in a for-init-statement extends to the end of the enclosing scope, as was the case
in old versions of G++, and other (traditional) implementations of C++.
The default if neither flag is given to follow the standard, but to allow and give a warning for old-
style code that would otherwise be invalid, or have different behavior.
|
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this word as an identifier. You can use
the keyword "__typeof__" instead. -ansi implies -fno-gnu-keywords.
|
-fno-implicit-templates
Never emit code for non-inline templates which are instantiated implicitly (i.e. by use); only emit
code for explicit instantiations.
|
-fno-implicit-inline-templates
Don't emit code for implicit instantiations of inline templates, either. The default is to handle
inlines differently so that compiles with and without optimization will need the same set of explicit
instantiations.
|
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions controlled by #pragma
implementation. This will cause linker errors if these functions are not inlined everywhere they are
called.
|
-fms-extensions
Disable pedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to
member function via non-standard syntax.
|
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include "ffs",
"alloca", "_exit", "index", "bzero", "conjf", and other related functions.
|
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor", "compl", "not", "or" and "xor" as
synonyms as keywords.
|
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only
such diagnostic issued by G++ is the one for a name having multiple meanings within a class.
|
-fpermissive
Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using
-fpermissive will allow some nonconforming code to compile.
|
-frepo
Enable automatic template instantiation at link time. This option also implies
-fno-implicit-templates.
|
-fno-rtti
Disable generation of information about every class with virtual functions for use by the C++ runtime
type identification features (dynamic_cast and typeid). If you don't use those parts of the
language, you can save some space by using this flag. Note that exception handling uses the same
information, but it will generate it as needed.
|
-fstats
Emit statistics about front-end processing at the end of the compilation. This information is
generally only useful to the G++ development team.
|
-ftemplate-depth-n
Set the maximum instantiation depth for template classes to n. A limit on the template instantiation
depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++
conforming programs must not rely on a maximum depth greater than 17.
|
-fuse-cxa-atexit
Register destructors for objects with static storage duration with the "__cxa_atexit" function rather
than the "atexit" function. This option is required for fully standards-compliant handling of static
destructors, but will only work if your C library supports "__cxa_atexit".
|
-fno-weak
Do not use weak symbol support, even if it is provided by the linker. By default, G++ will use weak
symbols if they are available. This option exists only for testing, and should not be used by end-
users; it will result in inferior code and has no benefits. This option may be removed in a future
release of G++.
|
-fno-default-inline
Do not assume inline for functions defined inside a class scope.
Note that these functions will have linkage like inline functions; they just won't be inlined by
default.
|
-Wabi (C++ only)
Warn when G++ generates code that is probably not compatible with the vendor-neutral C++ ABI.
Although an effort has been made to warn about all such cases, there are probably some cases that are
not warned about, even though G++ is generating incompatible code. There may also be cases where
warnings are emitted even though the code that is generated will be compatible.
|
-Wctor-dtor-privacy (C++ only)
Warn when a class seems unusable because all the constructors or destructors in that class are
private, and it has neither friends nor public static member functions.
|
-Wnon-virtual-dtor (C++ only)
Warn when a class appears to be polymorphic, thereby requiring a virtual destructor, yet it declares
a non-virtual one. This warning is enabled by -Wall.
|
-Wreorder (C++ only)
Warn when the order of member initializers given in the code does not match the order in which they
must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler will rearrange the member initializers for i and j to match the declaration order of the
members, emitting a warning to that effect. This warning is enabled by -Wall.
The following -W... options are not affected by -Wall.
|
-Wno-deprecated (C++ only)
Do not warn about usage of deprecated features.
|
-Wno-non-template-friend (C++ only)
Disable warnings when non-templatized friend functions are declared within a template. Since the
advent of explicit template specification support in G++, if the name of the friend is an
unqualified-id (i.e., friend foo(int)), the C++ language specification demands that the friend
declare or define an ordinary, nontemplate function. (Section 14.5.3). Before G++ implemented
explicit specification, unqualified-ids could be interpreted as a particular specialization of a
templatized function. Because this non-conforming behavior is no longer the default behavior for
G++, -Wnon-template-friend allows the compiler to check existing code for potential trouble spots and
is on by default. This new compiler behavior can be turned off with -Wno-non-template-friend which
keeps the conformant compiler code but disables the helpful warning.
|
-Wold-style-cast (C++ only)
Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style
casts (static_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended effects and
much easier to search for.
|
-Woverloaded-virtual (C++ only)
Warn when a function declaration hides virtual functions from a base class. For example, in:
|
-Wno-pmf-conversions (C++ only)
Disable the diagnostic for converting a bound pointer to member function to a plain pointer.
|
-Wsign-promo (C++ only)
Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type,
over a conversion to an unsigned type of the same size. Previous versions of G++ would try to
preserve unsignedness, but the standard mandates the current behavior.
|
-fconstant-string-class=class-name
Use class-name as the name of the class to instantiate for each literal string specified with the
syntax "@"..."". The default class name is "NXConstantString" if the GNU runtime is being used, and
"NSConstantString" if the NeXT runtime is being used (see below). The -fconstant-cfstrings option,
if also present, will override the -fconstant-string-class setting and cause "@"..."" literals to be
laid out as constant CoreFoundation strings.
|
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C runtime. This is the default for
most types of systems.
|
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the default for NeXT-based systems,
including Darwin and Mac OS X. The macro "__NEXT_RUNTIME__" is predefined if (and only if) this
option is used.
|
-fno-nil-receivers
Assume that all Objective-C message dispatches (e.g., "[receiver message:arg]") in this translation
unit ensure that the receiver is not "nil". This allows for more efficient entry points in the
runtime to be used. Currently, this option is only available in conjunction with the NeXT runtime on
Mac OS X 10.3 and later.
|
-fobjc-exceptions
Enable syntactic support for structured exception handling in Objective-C, similar to what is offered
by C++ and Java. Currently, this option is only available in conjunction with the NeXT runtime on
Mac OS X 10.3 and later.
|
-freplace-objc-classes
Emit a special marker instructing ld(1) not to statically link in the resulting object file, and
allow dyld(1) to load it in at run time instead. This is used in conjunction with the Fix-and-
Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded
in the course of program execution, without the need to restart the program itself. Currently, Fix-
and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3
and later.
|
-fzero-link
When compiling for the NeXT runtime, the compiler ordinarily replaces calls to "objc_getClass("...")"
(when the name of the class is known at compile time) with static class references that get
initialized at load time, which improves run-time performance. Specifying the -fzero-link flag
suppresses this behavior and causes calls to "objc_getClass("...")" to be retained. This is useful
in Zero-Link debugging mode, since it allows for individual class implementations to be modified
during program execution.
|
-gen-decls
Dump interface declarations for all classes seen in the source file to a file named sourcename.decl.
|
-Wno-protocol
If a class is declared to implement a protocol, a warning is issued for every method in the protocol
that is not implemented by the class. The default behavior is to issue a warning for every method
not explicitly implemented in the class, even if a method implementation is inherited from the
superclass. If you use the "-Wno-protocol" option, then methods inherited from the superclass are
considered to be implemented, and no warning is issued for them.
|
-Wselector
Warn if multiple methods of different types for the same selector are found during compilation. The
check is performed on the list of methods in the final stage of compilation. Additionally, a check
is performed for each selector appearing in a "@selector(...)" expression, and a corresponding
method for that selector has been found during compilation. Because these checks scan the method
table only at the end of compilation, these warnings are not produced if the final stage of
compilation is not reached, for example because an error is found during compilation, or because the
"-fsyntax-only" option is being used.
|
-Wundeclared-selector
Warn if a "@selector(...)" expression referring to an undeclared selector is found. A selector is
considered undeclared if no method with that name has been declared before the "@selector(...)"
expression, either explicitly in an @interface or @protocol declaration, or implicitly in an
@implementation section. This option always performs its checks as soon as a "@selector(...)"
expression is found, while "-Wselector" only performs its checks in the final stage of compilation.
This also enforces the coding style convention that methods and selectors must be declared before
being used.
|
-print-objc-runtime-info
Generate C header describing the largest structure that is passed by value, if any.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g.
its width, ...). The options described below can be used to control the diagnostic messages formatting
algorithm, e.g. how many characters per line, how often source location information should be reported.
Right now, only the C++ front end can honor these options. However it is expected, in the near future,
that the remaining front ends would be able to digest them correctly.
|
-fmessage-length=n
Try to format error messages so that they fit on lines of about n characters. The default is 72
characters for g++ and 0 for the rest of the front ends supported by GCC. If n is zero, then no
line-wrapping will be done; each error message will appear on a single line.
|
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit once
source location information; that is, in case the message is too long to fit on a single physical
line and has to be wrapped, the source location won't be emitted (as prefix) again, over and over, in
subsequent continuation lines. This is the default behavior.
|
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the same
source location information (as prefix) for physical lines that result from the process of breaking a
message which is too long to fit on a single line.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions which are not inherently erroneous but which
are risky or suggest there may have been an error.
You can request many specific warnings with options beginning -W, for example -Wimplicit to request
warnings on implicit declarations. Each of these specific warning options also has a negative form
beginning -Wno- to turn off warnings; for example, -Wno-implicit. This manual lists only one of the two
forms, whichever is not the default.
The following options control the amount and kinds of warnings produced by GCC; for further, language-
specific options also refer to C++ Dialect Options and Objective-C Dialect Options.
|
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond that.
|
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden
extensions, and some other programs that do not follow ISO C and ISO C++. For ISO C, follows the
version of the ISO C standard specified by any -std option used.
|
-pedantic-errors
Like -pedantic, except that errors are produced rather than warnings.
|
-w Inhibit all warning messages.
|
-Wno-import
Inhibit warning messages about the use of #import.
|
-Wchar-subscripts
Warn if an array subscript has type "char". This is a common cause of error, as programmers often
forget that this type is signed on some machines.
|
-Wcomment
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a Backslash-Newline
appears in a // comment.
|
-Wformat
Check calls to "printf" and "scanf", etc., to make sure that the arguments supplied have types
appropriate to the format string specified, and that the conversions specified in the format string
make sense. This includes standard functions, and others specified by format attributes, in the
"printf", "scanf", "strftime" and "strfmon" (an X/Open extension, not in the C standard) families.
The formats are checked against the format features supported by GNU libc version 2.2. These include
all ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and
GNU extensions. Other library implementations may not support all these features; GCC does not
support warning about features that go beyond a particular library's limitations. However, if
-pedantic is used with -Wformat, warnings will be given about format features not in the selected
standard version (but not for "strfmon" formats, since those are not in any version of the C
standard).
Since -Wformat also checks for null format arguments for several functions, -Wformat also implies
-Wnonnull.
|
-Wformat is included in -Wall. For more control over some aspects of format checking, the options
-Wformat-y2k, -Wno-format-extra-args, -Wno-format-zero-length, -Wformat-nonliteral,
-Wformat-security, and -Wformat=2 are available, but are not included in -Wall.
|
-Wformat-y2k
If -Wformat is specified, also warn about "strftime" formats which may yield only a two-digit year.
|
-Wno-format-extra-args
If -Wformat is specified, do not warn about excess arguments to a "printf" or "scanf" format
function. The C standard specifies that such arguments are ignored.
Where the unused arguments lie between used arguments that are specified with $ operand number
specifications, normally warnings are still given, since the implementation could not know what type
to pass to "va_arg" to skip the unused arguments. However, in the case of "scanf" formats, this
option will suppress the warning if the unused arguments are all pointers, since the Single Unix
Specification says that such unused arguments are allowed.
|
-Wno-format-zero-length
If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-
length formats are allowed.
|
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not a string literal and so cannot be
checked, unless the format function takes its format arguments as a "va_list".
|
-Wformat-security
If -Wformat is specified, also warn about uses of format functions that represent possible security
problems. At present, this warns about calls to "printf" and "scanf" functions where the format
string is not a string literal and there are no format arguments, as in "printf (foo);". This may be
a security hole if the format string came from untrusted input and contains %n. (This is currently a
subset of what -Wformat-nonliteral warns about, but in future warnings may be added to
-Wformat-security that are not included in -Wformat-nonliteral.)
|
-Wnonnull
Warn about passing a null pointer for arguments marked as requiring a non-null value by the "nonnull"
function attribute.
|
-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option.
-Winit-self (C, C++, and Objective-C only)
Warn about uninitialized variables which are initialized with themselves. Note this option can only
be used with the -Wuninitialized option, which in turn only works with -O1 and above.
For example, GCC will warn about "i" being uninitialized in the following snippet only when
-Winit-self has been specified:
int f()
{
int i = i;
return i;
}
|
-Wimplicit-int
Warn when a declaration does not specify a type.
|
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
Give a warning (or error) whenever a function is used before being declared.
|
-Wimplicit
Same as -Wimplicit-int and -Wimplicit-function-declaration.
|
-Wmain
Warn if the type of main is suspicious. main should be a function with external linkage, returning
int, taking either zero arguments, two, or three arguments of appropriate types.
|
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed. In the following example, the
initializer for a is not fully bracketed, but that for b is fully bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
|
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context
where a truth value is expected, or when operators are nested whose precedence people often get
confused about.
Also warn about constructions where there may be confusion to which "if" statement an "else" branch
belongs. Here is an example of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C, every "else" branch belongs to the innermost possible "if" statement, which in this example is
"if (b)". This is often not what the programmer expected, as illustrated in the above example by
indentation the programmer chose. When there is the potential for this confusion, GCC will issue a
warning when this flag is specified. To eliminate the warning, add explicit braces around the
innermost "if" statement so there is no way the "else" could belong to the enclosing "if". The
resulting code would look like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
|
-Wsequence-point
Warn about code that may have undefined semantics because of violations of sequence point rules in
the C standard.
|
-Wreturn-type
Warn whenever a function is defined with a return-type that defaults to "int". Also warn about any
"return" statement with no return-value in a function whose return-type is not "void".
For C++, a function without return type always produces a diagnostic message, even when
-Wno-return-type is specified. The only exceptions are main and functions defined in system headers.
|
-Wswitch
Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more
of the named codes of that enumeration. (The presence of a "default" label prevents this warning.)
"case" labels outside the enumeration range also provoke warnings when this option is used.
|
-Wswitch-default
Warn whenever a "switch" statement does not have a "default" case.
|
-Wswitch-enum
Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more
of the named codes of that enumeration. "case" labels outside the enumeration range also provoke
warnings when this option is used.
|
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning of the program (trigraphs within
comments are not warned about).
|
-Wunused-function
Warn whenever a static function is declared but not defined or a non\-inline static function is
unused.
|
-Wunused-label
Warn whenever a label is declared but not used.
To suppress this warning use the unused attribute.
|
-Wunused-parameter
Warn whenever a function parameter is unused aside from its declaration.
To suppress this warning use the unused attribute.
|
-Wunused-variable
Warn whenever a local variable or non-constant static variable is unused aside from its declaration
To suppress this warning use the unused attribute.
|
-Wunused-value
Warn whenever a statement computes a result that is explicitly not used.
To suppress this warning cast the expression to void.
|
-Wunused
All the above -Wunused options combined.
In order to get a warning about an unused function parameter, you must either specify -Wextra
-Wunused (note that -Wall implies -Wunused), or separately specify -Wunused-parameter.
|
-Wuninitialized
Warn if an automatic variable is used without first being initialized or if a variable may be
clobbered by a "setjmp" call.
|
-Wunknown-pragmas
Warn when a #pragma directive is encountered which is not understood by GCC. If this command line
option is used, warnings will even be issued for unknown pragmas in system header files. This is not
the case if the warnings were only enabled by the -Wall command line option.
|
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active. It warns about code which might break
the strict aliasing rules that the compiler is using for optimization. The warning does not catch all
cases, but does attempt to catch the more common pitfalls. It is included in -Wall.
|
-Wall
All of the above -W options combined. This enables all the warnings about constructions that some
users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in
conjunction with macros. This also enables some language-specific warnings described in C++ Dialect
Options and Objective-C Dialect Options.
The following -W... options are not implied by -Wall. Some of them warn about constructions that users
generally do not consider questionable, but which occasionally you might wish to check for; others warn
about constructions that are necessary or hard to avoid in some cases, and there is no simple way to
modify the code to suppress the warning.
|
-Wextra
(This option used to be called -W. The older name is still supported, but the newer name is more
descriptive.) Print extra warning messages for these events:
|
-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating point division by zero is not
warned about, as it can be a legitimate way of obtaining infinities and NaNs.
|
-Wsystem-headers
Print warning messages for constructs found in system header files. Warnings from system headers are
normally suppressed, on the assumption that they usually do not indicate real problems and would only
make the compiler output harder to read. Using this command line option tells GCC to emit warnings
from system headers as if they occurred in user code. However, note that using -Wall in conjunction
with this option will not warn about unknown pragmas in system headers---for that, -Wunknown-pragmas
must also be used.
|
-Wfloat-equal
Warn if floating point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-
point values as approximations to infinitely precise real numbers. If you are doing this, then you
need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error
that the computation introduces, and allow for it when performing comparisons (and when producing
output, but that's a different problem). In particular, instead of testing for equality, you would
check to see whether the two values have ranges that overlap; and this is done with the relational
operators, so equality comparisons are probably mistaken.
|
-Wtraditional (C only)
Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO
C constructs that have no traditional C equivalent, and/or problematic constructs which should be
avoided.
|
-Wdeclaration-after-statement (C only)
Warn when a declaration is found after a statement in a block. This construct, known from C++, was
introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90 and was not
supported by GCC versions before GCC 3.0.
|
-Wundef
Warn if an undefined identifier is evaluated in an #if directive.
|
-Wendif-labels
Warn whenever an #else or an #endif are followed by text.
|
-Wshadow
Warn whenever a local variable shadows another local variable, parameter or global variable or
whenever a built-in function is shadowed.
|
-Wlarger-than-len
Warn whenever an object of larger than len bytes is defined.
|
-Wpointer-arith
Warn about anything that depends on the ``size of'' a function type or of "void". GNU C assigns
these types a size of 1, for convenience in calculations with "void *" pointers and pointers to
functions.
|
-Wbad-function-cast (C only)
Warn whenever a function call is cast to a non-matching type. For example, warn if "int malloc()" is
cast to "anything *".
|
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier from the target type. For example,
warn if a "const char *" is cast to an ordinary "char *".
|
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of the target is increased. For
example, warn if a "char *" is cast to an "int *" on machines where integers can only be accessed at
two- or four-byte boundaries.
|
-Wwrite-strings
When compiling C, give string constants the type "const char[length]" so that copying the address of
one into a non-"const" "char *" pointer will get a warning; when compiling C++, warn about the
deprecated conversion from string constants to "char *". These warnings will help you find at
compile time code that can try to write into a string constant, but only if you have been very
careful about using "const" in declarations and prototypes. Otherwise, it will just be a nuisance;
this is why we did not make -Wall request these warnings.
|
-Wconversion
Warn if a prototype causes a type conversion that is different from what would happen to the same
argument in the absence of a prototype. This includes conversions of fixed point to floating and
vice versa, and conversions changing the width or signedness of a fixed point argument except when
the same as the default promotion.
Also, warn if a negative integer constant expression is implicitly converted to an unsigned type.
For example, warn about the assignment "x = -1" if "x" is unsigned. But do not warn about explicit
casts like "(unsigned) -1".
|
-Wsign-compare
Warn when a comparison between signed and unsigned values could produce an incorrect result when the
signed value is converted to unsigned. This warning is also enabled by -Wextra; to get the other
warnings of -Wextra without this warning, use -Wextra -Wno-sign-compare.
|
-Waggregate-return
Warn if any functions that return structures or unions are defined or called. (In languages where
you can return an array, this also elicits a warning.)
|
-Wstrict-prototypes (C only)
Warn if a function is declared or defined without specifying the argument types. (An old-style
function definition is permitted without a warning if preceded by a declaration which specifies the
argument types.)
|
-Wold-style-definition (C only)
Warn if an old-style function definition is used. A warning is given even if there is a previous
prototype.
|
-Wmissing-prototypes (C only)
Warn if a global function is defined without a previous prototype declaration. This warning is
issued even if the definition itself provides a prototype. The aim is to detect global functions
that fail to be declared in header files.
|
-Wmissing-declarations (C only)
Warn if a global function is defined without a previous declaration. Do so even if the definition
itself provides a prototype. Use this option to detect global functions that are not declared in
header files.
|
-Wmissing-noreturn
Warn about functions which might be candidates for attribute "noreturn". Note these are only
possible candidates, not absolute ones. Care should be taken to manually verify functions actually
do not ever return before adding the "noreturn" attribute, otherwise subtle code generation bugs
could be introduced. You will not get a warning for "main" in hosted C environments.
|
-Wmissing-format-attribute
If -Wformat is enabled, also warn about functions which might be candidates for "format" attributes.
Note these are only possible candidates, not absolute ones. GCC will guess that "format" attributes
might be appropriate for any function that calls a function like "vprintf" or "vscanf", but this
might not always be the case, and some functions for which "format" attributes are appropriate may
not be detected. This option has no effect unless -Wformat is enabled (possibly by -Wall).
|
-Wno-multichar
Do not warn if a multicharacter constant ('FOOF') is used. Usually they indicate a typo in the
user's code, as they have implementation-defined values, and should not be used in portable code.
|
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as deprecated by using the
"deprecated" attribute. (@pxref{Function Attributes}, @pxref{Variable Attributes}, @pxref{Type
Attributes}.)
|
-Wpacked
Warn if a structure is given the packed attribute, but the packed attribute has no effect on the
layout or size of the structure. Such structures may be mis-aligned for little benefit. For
instance, in this code, the variable "f.x" in "struct bar" will be misaligned even though "struct
bar" does not itself have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
|
-Wpadded
Warn if padding is included in a structure, either to align an element of the structure or to align
the whole structure. Sometimes when this happens it is possible to rearrange the fields of the
structure to reduce the padding and so make the structure smaller.
|
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even in cases where multiple
declaration is valid and changes nothing.
|
-Wnested-externs (C only)
Warn if an "extern" declaration is encountered within a function.
|
-Wunreachable-code
Warn if the compiler detects that code will never be executed.
This option is intended to warn when the compiler detects that at least a whole line of source code
will never be executed, because some condition is never satisfied or because it is after a procedure
that never returns.
It is possible for this option to produce a warning even though there are circumstances under which
part of the affected line can be executed, so care should be taken when removing apparently-
unreachable code.
For instance, when a function is inlined, a warning may mean that the line is unreachable in only one
inlined copy of the function.
This option is not made part of -Wall because in a debugging version of a program there is often
substantial code which checks correct functioning of the program and is, hopefully, unreachable
because the program does work. Another common use of unreachable code is to provide behavior which
is selectable at compile-time.
|
-Winline
Warn if a function can not be inlined and it was declared as inline. Even with this option, the
compiler will not warn about failures to inline functions declared in system headers.
The compiler uses a variety of heuristics to determine whether or not to inline a function. For
example, the compiler takes into account the size of the function being inlined and the the amount of
inlining that has already been done in the current function. Therefore, seemingly insignificant
changes in the source program can cause the warnings produced by -Winline to appear or disappear.
|
-Wno-invalid-offsetof (C++ only)
Suppress warnings from applying the offsetof macro to a non-POD type. According to the 1998 ISO C++
standard, applying offsetof to a non-POD type is undefined. In existing C++ implementations,
however, offsetof typically gives meaningful results even when applied to certain kinds of non-POD
types. (Such as a simple struct that fails to be a POD type only by virtue of having a constructor.)
This flag is for users who are aware that they are writing nonportable code and who have deliberately
chosen to ignore the warning about it.
The restrictions on offsetof may be relaxed in a future version of the C++ standard.
|
-Winvalid-pch
Warn if a precompiled header is found in the search path but can't be used.
|
-Wlong-long
Warn if long long type is used. This is default. To inhibit the warning messages, use
-Wno-long-long. Flags -Wlong-long and -Wno-long-long are taken into account only when -pedantic flag
is used.
|
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning does not generally indicate that
there is anything wrong with your code; it merely indicates that GCC's optimizers were unable to
handle the code effectively. Often, the problem is that your code is too big or too complex; GCC
will refuse to optimize programs when the optimization itself is likely to take inordinate amounts of
time.
|
-Werror
Make all warnings into errors.
Options for Debugging Your Program or GCC
GCC has various special options that are used for debugging either your program or GCC:
|
-g Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF).
GDB can work with this debugging information.
On most systems that use stabs format, -g enables use of extra debugging information that only GDB
can use; this extra information makes debugging work better in GDB but will probably make other
debuggers crash or refuse to read the program. If you want to control for certain whether to
generate the extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).
Unlike most other C compilers, GCC allows you to use -g with -O. The shortcuts taken by optimized
code may occasionally produce surprising results: some variables you declared may not exist at all;
flow of control may briefly move where you did not expect it; some statements may not be executed
because they compute constant results or their values were already at hand; some statements may
execute in different places because they were moved out of loops.
Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the
optimizer for programs that might have bugs.
The following options are useful when GCC is generated with the capability for more than one
debugging format.
|
-ggdb
Produce debugging information for use by GDB. This means to use the most expressive format available
(DWARF 2, stabs, or the native format if neither of those are supported), including GDB extensions if
at all possible.
|
-gstabs
Produce debugging information in stabs format (if that is supported), without GDB extensions. This
is the format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this
option produces stabs debugging output which is not understood by DBX or SDB. On System V Release 4
systems this option requires the GNU assembler.
|
-feliminate-unused-debug-symbols
Produce debugging information in stabs format (if that is supported), for only symbols that are
actually used.
|
-gcoff
Produce debugging information in COFF format (if that is supported). This is the format used by SDB
on most System V systems prior to System V Release 4.
|
-gxcoff
Produce debugging information in XCOFF format (if that is supported). This is the format used by the
DBX debugger on IBM RS/6000 systems.
|
-gdwarf-2
Produce debugging information in DWARF version 2 format (if that is supported). This is the format
used by DBX on IRIX 6.
|
-gvms
Produce debugging information in VMS debug format (if that is supported). This is the format used by
DEBUG on VMS systems.
|
-glevel
-ggdblevel
-gstabslevel
-gcofflevel
-gxcofflevel
-gvmslevel
Request debugging information and also use level to specify how much information. The default level
is 2.
Level 1 produces minimal information, enough for making backtraces in parts of the program that you
don't plan to debug. This includes descriptions of functions and external variables, but no
information about local variables and no line numbers.
Level 3 includes extra information, such as all the macro definitions present in the program. Some
debuggers support macro expansion when you use -g3.
Note that in order to avoid confusion between DWARF1 debug level 2, and DWARF2 -gdwarf-2 does not
accept a concatenated debug level. Instead use an additional -glevel option to change the debug
level for DWARF2.
|
-feliminate-dwarf2-dups
Compress DWARF2 debugging information by eliminating duplicated information about each symbol. This
option only makes sense when generating DWARF2 debugging information with -gdwarf-2.
|
-p Generate extra code to write profile information suitable for the analysis program prof. You must
use this option when compiling the source files you want data about, and you must also use it when
linking.
-pg Generate extra code to write profile information suitable for the analysis program gprof. You must
use this option when compiling the source files you want data about, and you must also use it when
linking.
|
-Q Makes the compiler print out each function name as it is compiled, and print some statistics about
each pass when it finishes.
|
-ftime-report
Makes the compiler print some statistics about the time consumed by each pass when it finishes.
|
-fmem-report
Makes the compiler print some statistics about permanent memory allocation when it finishes.
|
-fprofile-arcs
Add code so that program flow arcs are instrumented. During execution the program records how many
times each branch and call is executed and how many times it is taken or returns. When the compiled
program exits it saves this data to a file called auxname.gcda for each source file. The data may be
used for profile-directed optimizations (-fbranch-probabilities), or for test coverage analysis
(-ftest-coverage). Each object file's auxname is generated from the name of the output file, if
explicitly specified and it is not the final executable, otherwise it is the basename of the source
file. In both cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda
for output file specified as -o dir/foo.o).
|
-ftest-coverage
Produce a notes file that the gcov code-coverage utility can use to show program coverage. Each
source file's note file is called auxname.gcno. Refer to the -fprofile-arcs option above for a
description of auxname and instructions on how to generate test coverage data. Coverage data will
match the source files more closely, if you do not optimize.
|
-dletters
Says to make debugging dumps during compilation at times specified by letters. This is used for
debugging the compiler. The file names for most of the dumps are made by appending a pass number and
a word to the dumpname. dumpname is generated from the name of the output file, if explicitly
specified and it is not an executable, otherwise it is the basename of the source file. In both cases
any suffix is removed (e.g. foo.01.rtl or foo.02.sibling). Here are the possible letters for use in
letters, and their meanings:
|
-fdump-unnumbered
When doing debugging dumps (see -d option above), suppress instruction numbers and line number note
output. This makes it more feasible to use diff on debugging dumps for compiler invocations with
different options, in particular with and without -g.
-fdump-translation-unit (C and C++ only)
-fdump-translation-unit-options (C and C++ only)
Dump a representation of the tree structure for the entire translation unit to a file. The file name
is made by appending .tu to the source file name. If the -options form is used, options controls the
details of the dump as described for the -fdump-tree options.
|
-fdump-class-hierarchy (C++ only)
-fdump-class-hierarchy-options (C++ only)
Dump a representation of each class's hierarchy and virtual function table layout to a file. The
file name is made by appending .class to the source file name. If the -options form is used, options
controls the details of the dump as described for the -fdump-tree options.
|
-fdump-tree-switch (C++ only)
-fdump-tree-switch-options (C++ only)
Control the dumping at various stages of processing the intermediate language tree to a file. The
file name is generated by appending a switch specific suffix to the source file name. If the
-options form is used, options is a list of - separated options that control the details of the dump.
Not all options are applicable to all dumps, those which are not meaningful will be ignored. The
following options are available
|
-frandom-seed=string
This option provides a seed that GCC uses when it would otherwise use random numbers. It is used to
generate certain symbol names that have to be different in every compiled file. It is also used to
place unique stamps in coverage data files and the object files that produce them. You can use the
-frandom-seed option to produce reproducibly identical object files.
The string should be different for every file you compile.
|
-fsched-verbose=n
On targets that use instruction scheduling, this option controls the amount of debugging output the
scheduler prints. This information is written to standard error, unless -dS or -dR is specified, in
which case it is output to the usual dump listing file, .sched or .sched2 respectively. However for
n greater than nine, the output is always printed to standard error.
For n greater than zero, -fsched-verbose outputs the same information as -dRS. For n greater than
one, it also output basic block probabilities, detailed ready list information and unit/insn info.
For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n
over four, -fsched-verbose also includes dependence info.
|
-save-temps
Store the usual ``temporary'' intermediate files permanently; place them in the current directory and
name them based on the source file. Thus, compiling foo.c with -c -save-temps would produce files
foo.i and foo.s, as well as foo.o. This creates a preprocessed foo.i output file even though the
compiler now normally uses an integrated preprocessor.
|
-time
Report the CPU time taken by each subprocess in the compilation sequence. For C source files, this
is the compiler proper and assembler (plus the linker if linking is done). The output looks like
this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the ``user time,'' that is time spent executing the program itself.
The second number is ``system time,'' time spent executing operating system routines on behalf of the
program. Both numbers are in seconds.
|
-print-file-name=library
Print the full absolute name of the library file library that would be used when linking---and don't
do anything else. With this option, GCC does not compile or link anything; it just prints the file
name.
|
-print-multi-directory
Print the directory name corresponding to the multilib selected by any other switches present in the
command line. This directory is supposed to exist in GCC_EXEC_PREFIX.
|
-print-multi-lib
Print the mapping from multilib directory names to compiler switches that enable them. The directory
name is separated from the switches by ;, and each switch starts with an @} instead of the @samp{-,
without spaces between multiple switches. This is supposed to ease shell-processing.
|
-print-prog-name=program
Like -print-file-name, but searches for a program such as cpp.
|
-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You
can do
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
|
-print-search-dirs
Print the name of the configured installation directory and a list of program and library directories
gcc will search---and don't do anything else.
This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such file
or directory. To resolve this you either need to put cpp0 and the other compiler components where
gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory
where you installed them. Don't forget the trailing '/'.
|
-dumpmachine
Print the compiler's target machine (for example, i686-pc-linux-gnu)---and don't do anything else.
|
-dumpversion
Print the compiler version (for example, 3.0)---and don't do anything else.
|
-dumpspecs
Print the compiler's built-in specs---and don't do anything else. (This is used when GCC itself is
being built.)
|
-feliminate-unused-debug-types
Normally, when producing DWARF2 output, GCC will emit debugging information for all types declared in
a compilation unit, regardless of whether or not they are actually used in that compilation unit.
Sometimes this is useful, such as if, in the debugger, you want to cast a value to a type that is not
actually used in your program (but is declared). More often, however, this results in a significant
amount of wasted space. With this option, GCC will avoid producing debug symbol output for types
that are nowhere used in the source file being compiled.
|
-O
-O1 Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large
function.
With -O, the compiler tries to reduce code size and execution time, without performing any
optimizations that take a great deal of compilation time.
-O turns on the following optimization flags: -fdefer-pop -fmerge-constants -fthread-jumps
-floop-optimize -fif-conversion -fif-conversion2 -fdelayed-branch -fguess-branch-probability
-fcprop-registers
-O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.
|
-O2 Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-
speed tradeoff. The compiler does not perform loop unrolling or function inlining when you specify
-O2. As compared to -O, this option increases both compilation time and the performance of the
generated code.
-O2 turns on all optimization flags specified by -O. It also turns on the following optimization
flags: -fforce-mem -foptimize-sibling-calls -fstrength-reduce -fcse-follow-jumps -fcse-skip-blocks
-frerun-cse-after-loop -frerun-loop-opt -fgcse -fgcse-lm -fgcse-sm -fgcse-las
-fdelete-null-pointer-checks -fexpensive-optimizations -fregmove -fschedule-insns -fschedule-insns2
-fsched-interblock -fsched-spec -fcaller-saves -fpeephole2 -freorder-blocks -freorder-functions
-fstrict-aliasing -funit-at-a-time -falign-functions -falign-jumps -falign-loops -falign-labels
-fcrossjumping
Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos.
|
-O3 Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the
-finline-functions, -fweb, -frename-registers and -funswitch-loops options.
|
-O0 Do not optimize. This is the default.
|
-Os Optimize for size. -Os enables all -O2 optimizations that do not typically increase code size. It
also performs further optimizations designed to reduce code size.
-Os disables the following optimization flags: -falign-functions -falign-jumps -falign-loops
-falign-labels -freorder-blocks -fprefetch-loop-arrays
If you use multiple -O options, with or without level numbers, the last such option is the one that
is effective.
Options of the form -fflag specify machine-independent flags. Most flags have both positive and negative
forms; the negative form of -ffoo would be -fno-foo. In the table below, only one of the forms is
listed---the one you typically will use. You can figure out the other form by either removing no- or
adding it.
The following options control specific optimizations. They are either activated by -O options or are
related to ones that are. You can use the following flags in the rare cases when ``fine-tuning'' of
optimizations to be performed is desired.
|
-fno-default-inline
Do not make member functions inline by default merely because they are defined inside the class scope
(C++ only). Otherwise, when you specify -O, member functions defined inside class scope are compiled
inline by default; i.e., you don't need to add inline in front of the member function name.
|
-fno-defer-pop
Always pop the arguments to each function call as soon as that function returns. For machines which
must pop arguments after a function call, the compiler normally lets arguments accumulate on the
stack for several function calls and pops them all at once.
Disabled at levels -O, -O2, -O3, -Os.
|
-fforce-mem
Force memory operands to be copied into registers before doing arithmetic on them. This produces
better code by making all memory references potential common subexpressions. When they are not
common subexpressions, instruction combination should eliminate the separate register-load.
Enabled at levels -O2, -O3, -Os.
|
-fforce-addr
Force memory address constants to be copied into registers before doing arithmetic on them. This may
produce better code just as -fforce-mem may.
|
-fomit-frame-pointer
Don't keep the frame pointer in a register for functions that don't need one. This avoids the
instructions to save, set up and restore frame pointers; it also makes an extra register available in
many functions. It also makes debugging impossible on some machines.
On some machines, such as the VAX, this flag has no effect, because the standard calling sequence
automatically handles the frame pointer and nothing is saved by pretending it doesn't exist. The
machine-description macro "FRAME_POINTER_REQUIRED" controls whether a target machine supports this
flag.
Enabled at levels -O, -O2, -O3, -Os.
|
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
|
-fno-inline
Don't pay attention to the "inline" keyword. Normally this option is used to keep the compiler from
expanding any functions inline. Note that if you are not optimizing, no functions can be expanded
inline.
|
-finline-functions
Integrate all simple functions into their callers. The compiler heuristically decides which
functions are simple enough to be worth integrating in this way.
If all calls to a given function are integrated, and the function is declared "static", then the
function is normally not output as assembler code in its own right.
Enabled at level -O3.
|
-finline-limit=n
By default, GCC limits the size of functions that can be inlined. This flag allows the control of
this limit for functions that are explicitly marked as inline (i.e., marked with the inline keyword
or defined within the class definition in c++). n is the size of functions that can be inlined in
number of pseudo instructions (not counting parameter handling). The default value of n is 600.
Increasing this value can result in more inlined code at the cost of compilation time and memory
consumption. Decreasing usually makes the compilation faster and less code will be inlined (which
presumably means slower programs). This option is particularly useful for programs that use inlining
heavily such as those based on recursive templates with C++.
Inlining is actually controlled by a number of parameters, which may be specified individually by
using --param name=value. The -finline-limit=n option sets some of these parameters as follows:
@item max-inline-insns-single
is set to I<n>/2.
@item max-inline-insns-auto
is set to I<n>/2.
@item min-inline-insns
is set to 130 or I<n>/4, whichever is smaller.
@item max-inline-insns-rtl
is set to I<n>.
See below for a documentation of the individual parameters controlling inlining.
Note: pseudo instruction represents, in this particular context, an abstract measurement of
function's size. In no way, it represents a count of assembly instructions and as such its exact
meaning might change from one release to an another.
|
-fkeep-inline-functions
Even if all calls to a given function are integrated, and the function is declared "static",
nevertheless output a separate run-time callable version of the function. This switch does not
affect "extern inline" functions.
|
-fkeep-static-consts
Emit variables declared "static const" when optimization isn't turned on, even if the variables
aren't referenced.
GCC enables this option by default. If you want to force the compiler to check if the variable was
referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts
option.
|
-fmerge-constants
Attempt to merge identical constants (string constants and floating point constants) across
compilation units.
This option is the default for optimized compilation if the assembler and linker support it. Use
-fno-merge-constants to inhibit this behavior.
Enabled at levels -O, -O2, -O3, -Os.
|
-fmerge-all-constants
Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to -fmerge-constants this considers e.g. even
constant initialized arrays or initialized constant variables with integral or floating point types.
Languages like C or C++ require each non-automatic variable to have distinct location, so using this
option will result in non-conforming behavior.
|
-fnew-ra
Use a graph coloring register allocator. Currently this option is meant only for testing. Users
should not specify this option, since it is not yet ready for production use.
|
-fno-branch-count-reg
Do not use ``decrement and branch'' instructions on a count register, but instead generate a sequence
of instructions that decrement a register, compare it against zero, then branch based upon the
result. This option is only meaningful on architectures that support such instructions, which
include x86, PowerPC, IA-64 and S/390.
The default is -fbranch-count-reg, enabled when -fstrength-reduce is enabled.
|
-fno-function-cse
Do not put function addresses in registers; make each instruction that calls a constant function
contain the function's address explicitly.
This option results in less efficient code, but some strange hacks that alter the assembler output
may be confused by the optimizations performed when this option is not used.
The default is -ffunction-cse
|
-fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default puts variables that are initialized to zero into
BSS. This can save space in the resulting code.
This option turns off this behavior because some programs explicitly rely on variables going to the
data section. E.g., so that the resulting executable can find the beginning of that section and/or
make assumptions based on that.
The default is -fzero-initialized-in-bss.
|
-fstrength-reduce
Perform the optimizations of loop strength reduction and elimination of iteration variables.
Enabled at levels -O2, -O3, -Os.
|
-fthread-jumps
Perform optimizations where we check to see if a jump branches to a location where another comparison
subsumed by the first is found. If so, the first branch is redirected to either the destination of
the second branch or a point immediately following it, depending on whether the condition is known to
be true or false.
Enabled at levels -O, -O2, -O3, -Os.
|
-fcse-follow-jumps
In common subexpression elimination, scan through jump instructions when the target of the jump is
not reached by any other path. For example, when CSE encounters an "if" statement with an "else"
clause, CSE will follow the jump when the condition tested is false.
Enabled at levels -O2, -O3, -Os.
|
-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to follow jumps which conditionally skip over
blocks. When CSE encounters a simple "if" statement with no else clause, -fcse-skip-blocks causes
CSE to follow the jump around the body of the "if".
Enabled at levels -O2, -O3, -Os.
|
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations has been performed.
Enabled at levels -O2, -O3, -Os.
|
-frerun-loop-opt
Run the loop optimizer twice.
Enabled at levels -O2, -O3, -Os.
|
-fgcse
Perform a global common subexpression elimination pass. This pass also performs global constant and
copy propagation.
Note: When compiling a program using computed gotos, a GCC extension, you may get better runtime
performance if you disable the global common subexpression elimination pass by adding -fno-gcse to
the command line.
Enabled at levels -O2, -O3, -Os.
|
-fgcse-lm
When -fgcse-lm is enabled, global common subexpression elimination will attempt to move loads which
are only killed by stores into themselves. This allows a loop containing a load/store sequence to be
changed to a load outside the loop, and a copy/store within the loop.
Enabled by default when gcse is enabled.
|
-fgcse-sm
When -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination.
This pass will attempt to move stores out of loops. When used in conjunction with -fgcse-lm, loops
containing a load/store sequence can be changed to a load before the loop and a store after the loop.
Enabled by default when gcse is enabled.
|
-fgcse-las
When -fgcse-las is enabled, the global common subexpression elimination pass eliminates redundant
loads that come after stores to the same memory location (both partial and full redundancies).
Enabled by default when gcse is enabled.
|
-floop-optimize
Perform loop optimizations: move constant expressions out of loops, simplify exit test conditions and
optionally do strength-reduction and loop unrolling as well.
Enabled at levels -O, -O2, -O3, -Os.
|
-fcrossjumping
Perform cross-jumping transformation. This transformation unifies equivalent code and save code size.
The resulting code may or may not perform better than without cross-jumping.
Enabled at levels -O, -O2, -O3, -Os.
|
-fif-conversion
Attempt to transform conditional jumps into branch-less equivalents. This include use of conditional
moves, min, max, set flags and abs instructions, and some tricks doable by standard arithmetics. The
use of conditional execution on chips where it is available is controlled by "if-conversion2".
Enabled at levels -O, -O2, -O3, -Os.
|
-fif-conversion2
Use conditional execution (where available) to transform conditional jumps into branch-less
equivalents.
Enabled at levels -O, -O2, -O3, -Os.
|
-fdelete-null-pointer-checks
Use global dataflow analysis to identify and eliminate useless checks for null pointers. The
compiler assumes that dereferencing a null pointer would have halted the program. If a pointer is
checked after it has already been dereferenced, it cannot be null.
In some environments, this assumption is not true, and programs can safely dereference null pointers.
Use -fno-delete-null-pointer-checks to disable this optimization for programs which depend on that
behavior.
Enabled at levels -O2, -O3, -Os.
|
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively expensive.
Enabled at levels -O2, -O3, -Os.
|
-foptimize-register-move
-fregmove
Attempt to reassign register numbers in move instructions and as operands of other simple
instructions in order to maximize the amount of register tying. This is especially helpful on
machines with two-operand instructions.
Note -fregmove and -foptimize-register-move are the same optimization.
Enabled at levels -O2, -O3, -Os.
|
-fdelayed-branch
If supported for the target machine, attempt to reorder instructions to exploit instruction slots
available after delayed branch instructions.
Enabled at levels -O, -O2, -O3, -Os.
|
-fschedule-insns
If supported for the target machine, attempt to reorder instructions to eliminate execution stalls
due to required data being unavailable. This helps machines that have slow floating point or memory
load instructions by allowing other instructions to be issued until the result of the load or
floating point instruction is required.
Enabled at levels -O2, -O3, -Os.
|
-fschedule-insns2
Similar to -fschedule-insns, but requests an additional pass of instruction scheduling after register
allocation has been done. This is especially useful on machines with a relatively small number of
registers and where memory load instructions take more than one cycle.
Enabled at levels -O2, -O3, -Os.
|
-fno-sched-interblock
Don't schedule instructions across basic blocks. This is normally enabled by default when scheduling
before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
|
-fno-sched-spec
Don't allow speculative motion of non-load instructions. This is normally enabled by default when
scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
|
-fsched-spec-load
Allow speculative motion of some load instructions. This only makes sense when scheduling before
register allocation, i.e. with -fschedule-insns or at -O2 or higher.
|
-fsched-spec-load-dangerous
Allow speculative motion of more load instructions. This only makes sense when scheduling before
register allocation, i.e. with -fschedule-insns or at -O2 or higher.
|
-fsched-stalled-insns=n
Define how many insns (if any) can be moved prematurely from the queue of stalled insns into the
ready list, during the second scheduling pass.
|
-fsched-stalled-insns-dep=n
Define how many insn groups (cycles) will be examined for a dependency on a stalled insn that is
candidate for premature removal from the queue of stalled insns. Has an effect only during the
second scheduling pass, and only if -fsched-stalled-insns is used and its value is not zero.
|
-fsched2-use-superblocks
When scheduling after register allocation, do use superblock scheduling algorithm. Superblock
scheduling allows motion across basic block boundaries resulting on faster schedules. This option is
experimental, as not all machine descriptions used by GCC model the CPU closely enough to avoid
unreliable results from the algorithm.
This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at
-O2 or higher.
|
-fsched2-use-traces
Use -fsched2-use-superblocks algorithm when scheduling after register allocation and additionally
perform code duplication in order to increase the size of superblocks using tracer pass. See
-ftracer for details on trace formation.
This mode should produce faster but significantly longer programs. Also without
"-fbranch-probabilities" the traces constructed may not match the reality and hurt the performance.
This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at
-O2 or higher.
|
-fcaller-saves
Enable values to be allocated in registers that will be clobbered by function calls, by emitting
extra instructions to save and restore the registers around such calls. Such allocation is done only
when it seems to result in better code than would otherwise be produced.
This option is always enabled by default on certain machines, usually those which have no call-
preserved registers to use instead.
Enabled at levels -O2, -O3, -Os.
|
-fmove-all-movables
Forces all invariant computations in loops to be moved outside the loop.
|
-freduce-all-givs
Forces all general-induction variables in loops to be strength-reduced.
Note: When compiling programs written in Fortran, -fmove-all-movables and -freduce-all-givs are
enabled by default when you use the optimizer.
These options may generate better or worse code; results are highly dependent on the structure of
loops within the source code.
These two options are intended to be removed someday, once they have helped determine the efficacy of
various approaches to improving loop optimizations.
Please contact <gcc@gcc.gnu.org>, and describe how use of these options affects the performance of
your production code. Examples of code that runs slower when these options are enabled are very
valuable.
|
-fno-peephole
-fno-peephole2
Disable any machine-specific peephole optimizations. The difference between -fno-peephole and
-fno-peephole2 is in how they are implemented in the compiler; some targets use one, some use the
other, a few use both.
|
-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.
|
-fno-guess-branch-probability
Do not guess branch probabilities using a randomized model.
Sometimes GCC will opt to use a randomized model to guess branch probabilities, when none are
available from either profiling feedback (-fprofile-arcs) or __builtin_expect. This means that
different runs of the compiler on the same program may produce different object code.
In a hard real-time system, people don't want different runs of the compiler to produce code that has
different behavior; minimizing non-determinism is of paramount import. This switch allows users to
reduce non-determinism, possibly at the expense of inferior optimization.
The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.
|
-freorder-blocks
Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve
code locality.
Enabled at levels -O2, -O3.
|
-freorder-functions
Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve
code locality. This is implemented by using special subsections ".text.hot" for most frequently
executed functions and ".text.unlikely" for unlikely executed functions. Reordering is done by the
linker so object file format must support named sections and linker must place them in a reasonable
way.
Also profile feedback must be available in to make this option effective. See -fprofile-arcs for
details.
Enabled at levels -O2, -O3, -Os.
|
-fstrict-aliasing
Allows the compiler to assume the strictest aliasing rules applicable to the language being compiled.
For C (and C++), this activates optimizations based on the type of expressions. In particular, an
object of one type is assumed never to reside at the same address as an object of a different type,
unless the types are almost the same. For example, an "unsigned int" can alias an "int", but not a
"void*" or a "double". A character type may alias any other type.
|
-falign-functions
-falign-functions=n
Align the start of functions to the next power-of-two greater than n, skipping up to n bytes. For
instance, -falign-functions=32 aligns functions to the next 32-byte boundary, but
-falign-functions=24 would align to the next 32-byte boundary only if this can be done by skipping 23
bytes or less.
|
-fno-align-functions and -falign-functions=1 are equivalent and mean that functions will not be
aligned.
Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
|
-falign-labels
-falign-labels=n
Align all branch targets to a power-of-two boundary, skipping up to n bytes like -falign-functions.
This option can easily make code slower, because it must insert dummy operations for when the branch
target is reached in the usual flow of the code.
|
-fno-align-labels and -falign-labels=1 are equivalent and mean that labels will not be aligned.
If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values
are used instead.
If n is not specified or is zero, use a machine-dependent default which is very likely to be 1,
meaning no alignment.
Enabled at levels -O2, -O3.
|
-falign-loops
-falign-loops=n
Align loops to a power-of-two boundary, skipping up to n bytes like -falign-functions. The hope is
that the loop will be executed many times, which will make up for any execution of the dummy
operations.
|
-fno-align-loops and -falign-loops=1 are equivalent and mean that loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
|
-falign-jumps
-falign-jumps=n
Align branch targets to a power-of-two boundary, for branch targets where the targets can only be
reached by jumping, skipping up to n bytes like -falign-functions. In this case, no dummy operations
need be executed.
|
-fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
|
-frename-registers
Attempt to avoid false dependencies in scheduled code by making use of registers left over after
register allocation. This optimization will most benefit processors with lots of registers. It can,
however, make debugging impossible, since variables will no longer stay in a ``home register''.
|
-fweb
Constructs webs as commonly used for register allocation purposes and assign each web individual
pseudo register. This allows the register allocation pass to operate on pseudos directly, but also
strengthens several other optimization passes, such as CSE, loop optimizer and trivial dead code
remover. It can, however, make debugging impossible, since variables will no longer stay in a ``home
register''.
Enabled at levels -O3.
|
-fno-cprop-registers
After register allocation and post-register allocation instruction splitting, we perform a copy-
propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy.
Disabled at levels -O, -O2, -O3, -Os.
|
-fprofile-generate
Enable options usually used for instrumenting application to produce profile useful for later
recompilation with profile feedback based optimization. You must use "-fprofile-generate" both when
compiling and when linking your program.
The following options are enabled: "-fprofile-arcs", "-fprofile-values", "-fvpt".
|
-fprofile-use
Enable profile feedback directed optimizations, and optimizations generally profitable only with
profile feedback available.
The following options are enabled: "-fbranch-probabilities", "-fvpt", "-funroll-loops",
"-fpeel-loops", "-ftracer".
The following options control compiler behavior regarding floating point arithmetic. These options trade
off between speed and correctness. All must be specifically enabled.
|
-ffloat-store
Do not store floating point variables in registers, and inhibit other options that might change
whether a floating point value is taken from a register or memory.
This option prevents undesirable excess precision on machines such as the 68000 where the floating
registers (of the 68881) keep more precision than a "double" is supposed to have. Similarly for the
x86 architecture. For most programs, the excess precision does only good, but a few programs rely on
the precise definition of IEEE floating point. Use -ffloat-store for such programs, after modifying
them to store all pertinent intermediate computations into variables.
|
-ffast-math
Sets -fno-math-errno, -funsafe-math-optimizations, -fno-trapping-math, -ffinite-math-only,
-fno-rounding-math and -fno-signaling-nans.
This option causes the preprocessor macro "__FAST_MATH__" to be defined.
This option should never be turned on by any -O option since it can result in incorrect output for
programs which depend on an exact implementation of IEEE or ISO rules/specifications for math
functions.
|
-fno-math-errno
Do not set ERRNO after calling math functions that are executed with a single instruction, e.g.,
sqrt. A program that relies on IEEE exceptions for math error handling may want to use this flag for
speed while maintaining IEEE arithmetic compatibility.
This option should never be turned on by any -O option since it can result in incorrect output for
programs which depend on an exact implementation of IEEE or ISO rules/specifications for math
functions.
The default is -fmath-errno.
|
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a) assume that arguments and results are
valid and (b) may violate IEEE or ANSI standards. When used at link-time, it may include libraries
or startup files that change the default FPU control word or other similar optimizations.
This option should never be turned on by any -O option since it can result in incorrect output for
programs which depend on an exact implementation of IEEE or ISO rules/specifications for math
functions.
The default is -fno-unsafe-math-optimizations.
|
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs
or +-Infs.
This option should never be turned on by any -O option since it can result in incorrect output for
programs which depend on an exact implementation of IEEE or ISO rules/specifications.
The default is -fno-finite-math-only.
|
-fno-trapping-math
Compile code assuming that floating-point operations cannot generate user-visible traps. These traps
include division by zero, overflow, underflow, inexact result and invalid operation. This option
implies -fno-signaling-nans. Setting this option may allow faster code if one relies on ``non-stop''
IEEE arithmetic, for example.
This option should never be turned on by any -O option since it can result in incorrect output for
programs which depend on an exact implementation of IEEE or ISO rules/specifications for math
functions.
The default is -ftrapping-math.
|
-frounding-math
Disable transformations and optimizations that assume default floating point rounding behavior. This
is round-to-zero for all floating point to integer conversions, and round-to-nearest for all other
arithmetic truncations. This option should be specified for programs that change the FP rounding
mode dynamically, or that may be executed with a non-default rounding mode. This option disables
constant folding of floating point expressions at compile-time (which may be affected by rounding
mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding
modes.
The default is -fno-rounding-math.
This option is experimental and does not currently guarantee to disable all GCC optimizations that
are affected by rounding mode. Future versions of GCC may provide finer control of this setting
using C99's "FENV_ACCESS" pragma. This command line option will be used to specify the default state
for "FENV_ACCESS".
|
-fsignaling-nans
Compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point
operations. Setting this option disables optimizations that may change the number of exceptions
visible with signaling NaNs. This option implies -ftrapping-math.
This option causes the preprocessor macro "__SUPPORT_SNAN__" to be defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to disable all GCC optimizations that
affect signaling NaN behavior.
|
-fsingle-precision-constant
Treat floating point constant as single precision constant instead of implicitly converting it to
double precision constant.
The following options control optimizations that may improve performance, but are not enabled by any -O
options. This section includes experimental options that may produce broken code.
|
-fbranch-probabilities
After running a program compiled with -fprofile-arcs, you can compile it a second time using
-fbranch-probabilities, to improve optimizations based on the number of times each branch was taken.
When the program compiled with -fprofile-arcs exits it saves arc execution counts to a file called
sourcename.gcda for each source file The information in this data file is very dependent on the
structure of the generated code, so you must use the same source code and the same optimization
options for both compilations.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can
be used to improve optimization. Currently, they are only used in one place: in reorg.c, instead of
guessing which path a branch is mostly to take, the REG_BR_PROB values are used to exactly determine
which path is taken more often.
|
-fprofile-values
If combined with -fprofile-arcs, it adds code so that some data about values of expressions in the
program is gathered.
With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions and
adds REG_VALUE_PROFILE notes to instructions for their later usage in optimizations.
|
-fvpt
If combined with -fprofile-arcs, it instructs the compiler to add a code to gather information about
values of expressions.
With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations
based on them. Currently the optimizations include specialization of division operation using the
knowledge about the value of the denominator.
|
-fnew-ra
Use a graph coloring register allocator. Currently this option is meant for testing, so we are
interested to hear about miscompilations with -fnew-ra.
|
-ftracer
Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow
of the function allowing other optimizations to do better job.
|
-funit-at-a-time
Parse the whole compilation unit before starting to produce code. This allows some extra
optimizations to take place but consumes more memory.
|
-funroll-loops
Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop.
-funroll-loops implies -frerun-cse-after-loop. It also turns on complete loop peeling (i.e. complete
removal of loops with small constant number of iterations). This option makes code larger, and may
or may not make it run faster.
|
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This
usually makes programs run more slowly. -funroll-all-loops implies the same options as
-funroll-loops.
|
-fpeel-loops
Peels the loops for that there is enough information that they do not roll much (from profile
feedback). It also turns on complete loop peeling (i.e. complete removal of loops with small
constant number of iterations).
|
-funswitch-loops
Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both
branches (modified according to result of the condition).
|
-fold-unroll-loops
Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop,
using the old loop unroller whose loop recognition is based on notes from frontend.
-fold-unroll-loops implies both -fstrength-reduce and -frerun-cse-after-loop. This option makes code
larger, and may or may not make it run faster.
|
-fold-unroll-all-loops
Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This is
done using the old loop unroller whose loop recognition is based on notes from frontend. This
usually makes programs run more slowly. -fold-unroll-all-loops implies the same options as
-fold-unroll-loops.
|
-funswitch-loops
Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both
branches (modified according to result of the condition).
|
-funswitch-loops
Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both
branches (modified according to result of the condition).
|
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to prefetch memory to improve the
performance of loops that access large arrays.
Disabled at level -Os.
|
-ffunction-sections
-fdata-sections
Place each function or data item into its own section in the output file if the target supports
arbitrary sections. The name of the function or the name of the data item determines the section's
name in the output file.
Use these options on systems where the linker can perform optimizations to improve locality of
reference in the instruction space. Most systems using the ELF object format and SPARC processors
running Solaris 2 have linkers with such optimizations. AIX may have these optimizations in the
future.
Only use these options when there are significant benefits from doing so. When you specify these
options, the assembler and linker will create larger object and executable files and will also be
slower. You will not be able to use "gprof" on all systems if you specify this option and you may
have problems with debugging if you specify both this option and -g.
|
-fbranch-target-load-optimize
Perform branch target register load optimization before prologue / epilogue threading. The use of
target registers can typically be exposed only during reload, thus hoisting loads out of loops and
doing inter-block scheduling needs a separate optimization pass.
|
-fbranch-target-load-optimize2
Perform branch target register load optimization after prologue / epilogue threading.
|
--param name=value
In some places, GCC uses various constants to control the amount of optimization that is done. For
example, GCC will not inline functions that contain more that a certain number of instructions. You
can control some of these constants on the command-line using the --param option.
|
-Xpreprocessor option
Pass option as an option to the preprocessor. You can use this to supply system-specific
preprocessor options which GCC does not know how to recognize.
If you want to pass an option that takes an argument, you must use -Xpreprocessor twice, once for the
option and once for the argument.
|
-D name
Predefine name as a macro, with definition 1.
|
-D name=definition
Predefine name as a macro, with definition definition. The contents of definition are tokenized and
processed as if they appeared during translation phase three in a #define directive. In particular,
the definition will be truncated by embedded newline characters.
If you are invoking the preprocessor from a shell or shell-like program you may need to use the
shell's quoting syntax to protect characters such as spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line, write its argument list with
surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells,
so you will need to quote the option. With sh and csh, -D'name(args...)=definition' works.
-D and -U options are processed in the order they are given on the command line. All -imacros file
and -include file options are processed after all -D and -U options.
|
-U name
Cancel any previous definition of name, either built in or provided with a -D option.
|
-undef
Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain
defined.
|
-I dir
Add the directory dir to the list of directories to be searched for header files. Directories named
by -I are searched before the standard system include directories. If the directory dir is a
standard system include directory, the option is ignored to ensure that the default search order for
system directories and the special treatment of system headers are not defeated .
|
-o file
Write output to file. This is the same as specifying file as the second non-option argument to cpp.
gcc has a different interpretation of a second non-option argument, so you must use -o to specify the
output file.
|
-Wall
Turns on all optional warnings which are desirable for normal code. At present this is -Wcomment,
-Wtrigraphs, -Wmultichar and a warning about integer promotion causing a change of sign in "#if"
expressions. Note that many of the preprocessor's warnings are on by default and have no options to
control them.
|
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline
appears in a // comment. (Both forms have the same effect.)
|
-Wtrigraphs
@anchor{Wtrigraphs} Most trigraphs in comments cannot affect the meaning of the program. However, a
trigraph that would form an escaped newline (??/ at the end of a line) can, by changing where the
comment begins or ends. Therefore, only trigraphs that would form escaped newlines produce warnings
inside a comment.
This option is implied by -Wall. If -Wall is not given, this option is still enabled unless
trigraphs are enabled. To get trigraph conversion without warnings, but get the other -Wall
warnings, use -trigraphs -Wall -Wno-trigraphs.
|
-Wtraditional
Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO
C constructs that have no traditional C equivalent, and problematic constructs which should be
avoided.
|
-Wimport
Warn the first time #import is used.
|
-Wundef
Warn whenever an identifier which is not a macro is encountered in an #if directive, outside of
defined. Such identifiers are replaced with zero.
|
-Wunused-macros
Warn about macros defined in the main file that are unused. A macro is used if it is expanded or
tested for existence at least once. The preprocessor will also warn if the macro has not been used
at the time it is redefined or undefined.
Built-in macros, macros defined on the command line, and macros defined in include files are not
warned about.
Note: If a macro is actually used, but only used in skipped conditional blocks, then CPP will report
it as unused. To avoid the warning in such a case, you might improve the scope of the macro's
definition by, for example, moving it into the first skipped block. Alternatively, you could provide
a dummy use with something like:
#if defined the_macro_causing_the_warning
#endif
|
-Wendif-labels
Warn whenever an #else or an #endif are followed by text. This usually happens in code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments, but often are not in older programs. This warning
is on by default.
|
-Werror
Make all warnings into hard errors. Source code which triggers warnings will be rejected.
|
-Wsystem-headers
Issue warnings for code in system headers. These are normally unhelpful in finding bugs in your own
code, therefore suppressed. If you are responsible for the system library, you may want to see them.
|
-w Suppress all warnings, including those which GNU CPP issues by default.
|
-pedantic
Issue all the mandatory diagnostics listed in the C standard. Some of them are left out by default,
since they trigger frequently on harmless code.
|
-pedantic-errors
Issue all the mandatory diagnostics, and make all mandatory diagnostics into errors. This includes
mandatory diagnostics that GCC issues without -pedantic but treats as warnings.
|
-M Instead of outputting the result of preprocessing, output a rule suitable for make describing the
dependencies of the main source file. The preprocessor outputs one make rule containing the object
file name for that source file, a colon, and the names of all the included files, including those
coming from -include or -imacros command line options.
|
-MF file
When used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given
the preprocessor sends the rules to the same place it would have sent preprocessed output.
|
-MT target
Change the target of the rule emitted by dependency generation. By default CPP takes the name of the
main input file, including any path, deletes any file suffix such as .c, and appends the platform's
usual object suffix. The result is the target.
An -MT option will set the target to be exactly the string you specify. If you want multiple
targets, you can specify them as a single argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
|
-MQ target
Same as -MT, but it quotes any characters which are special to Make. -MQ '$(objpfx)foo.o' gives
|
-MMD
Like -MD except mention only user header files, not system -header files.
|
-fpch-deps
When using precompiled headers, this flag will cause the dependency-output flags to also list the
files from the precompiled header's dependencies. If not specified only the precompiled header would
be listed and not the files that were used to create it because those files are not consulted when a
precompiled header is used.
|
-x c
-x c++
-x objective-c
-x assembler-with-cpp
Specify the source language: C, C++, Objective-C, or assembly. This has nothing to do with standards
conformance or extensions; it merely selects which base syntax to expect. If you give none of these
options, cpp will deduce the language from the extension of the source file: .c, .cc, .m, or .S.
Some other common extensions for C++ and assembly are also recognized. If cpp does not recognize the
extension, it will treat the file as C; this is the most generic mode.
Note: Previous versions of cpp accepted a -lang option which selected both the language and the
standards conformance level. This option has been removed, because it conflicts with the -l option.
|
-std=standard
-ansi
Specify the standard to which the code should conform. Currently CPP knows about C and C++
standards; others may be added in the future.
|
-nostdinc
Do not search the standard system directories for header files. Only the directories you have
specified with -I options (and the directory of the current file, if appropriate) are searched.
|
-include file
Process file as if "#include "file"" appeared as the first line of the primary source file. However,
the first directory searched for file is the preprocessor's working directory instead of the
directory containing the main source file. If not found there, it is searched for in the remainder
of the "#include "..."" search chain as normal.
If multiple -include options are given, the files are included in the order they appear on the
command line.
|
-imacros file
Exactly like -include, except that any output produced by scanning file is thrown away. Macros it
defines remain defined. This allows you to acquire all the macros from a header without also
processing its declarations.
All files specified by -imacros are processed before all files specified by -include.
|
-idirafter dir
Search dir for header files, but do it after all directories specified with -I and the standard
system directories have been exhausted. dir is treated as a system include directory.
|
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a
directory, you should include the final /.
|
-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the
include search path. -iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it
where -idirafter would.
|
-isystem dir
Search dir for header files, after all directories specified by -I but before the standard system
directories. Mark it as a system directory, so that it gets the same special treatment as is applied
to the standard system directories.
|
-fdollars-in-identifiers
@anchor{fdollars-in-identifiers} Accept $ in identifiers.
|
-fpreprocessed
Indicate to the preprocessor that the input file has already been preprocessed. This suppresses
things like macro expansion, trigraph conversion, escaped newline splicing, and processing of most
directives. The preprocessor still recognizes and removes comments, so that you can pass a file
preprocessed with -C to the compiler without problems. In this mode the integrated preprocessor is
little more than a tokenizer for the front ends.
-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. These are the
extensions that GCC uses for preprocessed files created by -save-temps.
|
-ftabstop=width
Set the distance between tab stops. This helps the preprocessor report correct column numbers in
warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than
100, the option is ignored. The default is 8.
|
-fexec-charset=charset
Set the execution character set, used for string and character constants. The default is UTF-8.
charset can be any encoding supported by the system's "iconv" library routine.
|
-fwide-exec-charset=charset
Set the wide execution character set, used for wide string and character constants. The default is
UTF-32 or UTF-16, whichever corresponds to the width of "wchar_t". As with -ftarget-charset, charset
can be any encoding supported by the system's "iconv" library routine; however, you will have
problems with encodings that do not fit exactly in "wchar_t".
|
-finput-charset=charset
Set the input character set, used for translation from the character set of the input file to the
source character set used by GCC. If the locale does not specify, or GCC cannot get this information
from the locale, the default is UTF-8. This can be overridden by either the locale or this command
line option. Currently the command line option takes precedence if there's a conflict. charset can be
any encoding supported by the system's "iconv" library routine.
|
-fworking-directory
Enable generation of linemarkers in the preprocessor output that will let the compiler know the
current working directory at the time of preprocessing. When this option is enabled, the
preprocessor will emit, after the initial linemarker, a second linemarker with the current working
directory followed by two slashes. GCC will use this directory, when it's present in the
preprocessed input, as the directory emitted as the current working directory in some debugging
information formats. This option is implicitly enabled if debugging information is enabled, but this
can be inhibited with the negated form -fno-working-directory. If the -P flag is present in the
command line, this option has no effect, since no "#line" directives are emitted whatsoever.
|
-fno-show-column
Do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned
by a program that does not understand the column numbers, such as dejagnu.
|
-A predicate=answer
Make an assertion with the predicate predicate and answer answer. This form is preferred to the
older form -A predicate(answer), which is still supported, because it does not use shell special
characters.
|
-A -predicate=answer
Cancel an assertion with the predicate predicate and answer answer.
|
-dCHARS
CHARS is a sequence of one or more of the following characters, and must not be preceded by a space.
Other characters are interpreted by the compiler proper, or reserved for future versions of GCC, and
so are silently ignored. If you specify characters whose behavior conflicts, the result is
undefined.
|
-P Inhibit generation of linemarkers in the output from the preprocessor. This might be useful when
running the preprocessor on something that is not C code, and will be sent to a program which might
be confused by the linemarkers.
|
-C Do not discard comments. All comments are passed through to the output file, except for comments in
processed directives, which are deleted along with the directive.
You should be prepared for side effects when using -C; it causes the preprocessor to treat comments
as tokens in their own right. For example, comments appearing at the start of what would be a
directive line have the effect of turning that line into an ordinary source line, since the first
token on the line is no longer a #.
-CC Do not discard comments, including during macro expansion. This is like -C, except that comments
contained within macros are also passed through to the output file where the macro is expanded.
In addition to the side-effects of the -C option, the -CC option causes all C++-style comments inside
a macro to be converted to C-style comments. This is to prevent later use of that macro from
inadvertently commenting out the remainder of the source line.
The -CC option is generally used to support lint comments.
|
-traditional-cpp
Try to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO C preprocessors.
|
-trigraphs
Process trigraph sequences. These are three-character sequences, all starting with ??, that are
defined by ISO C to stand for single characters. For example, ??/ stands for \, so '??/n' is a
character constant for a newline. By default, GCC ignores trigraphs, but in standard-conforming
modes it converts them. See the -std and -ansi options.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
|
-remap
Enable special code to work around file systems which only permit very short file names, such as MS-
DOS.
|
--help
--target-help
Print text describing all the command line options instead of preprocessing anything.
|
-v Verbose mode. Print out GNU CPP's version number at the beginning of execution, and report the final
form of the include path.
|
-H Print the name of each header file used, in addition to other normal activities. Each name is
indented to show how deep in the #include stack it is. Precompiled header files are also printed,
even if they are found to be invalid; an invalid precompiled header file is printed with ...x and a
valid one with ...! .
|
-version
--version
Print out GNU CPP's version number. With one dash, proceed to preprocess as normal. With two
dashes, exit immediately.
Passing Options to the Assembler
You can pass options to the assembler.
|
-Wa,option
Pass option as an option to the assembler. If option contains commas, it is split into multiple
options at the commas.
|
-Xassembler option
Pass option as an option to the assembler. You can use this to supply system-specific assembler
options which GCC does not know how to recognize.
If you want to pass an option that takes an argument, you must use -Xassembler twice, once for the
option and once for the argument.
Options for Linking
These options come into play when the compiler links object files into an executable output file. They
are meaningless if the compiler is not doing a link step.
object-file-name
A file name that does not end in a special recognized suffix is considered to name an object file or
library. (Object files are distinguished from libraries by the linker according to the file
contents.) If linking is done, these object files are used as input to the linker.
|
-c
-S
-E If any of these options is used, then the linker is not run, and object file names should not be used
as arguments.
|
-llibrary
-l library
Search the library named library when linking. (The second alternative with the library as a
separate argument is only for POSIX compliance and is not recommended.)
It makes a difference where in the command you write this option; the linker searches and processes
libraries and object files in the order they are specified. Thus, foo.o -lz bar.o searches library z
after file foo.o but before bar.o. If bar.o refers to functions in z, those functions may not be
loaded.
The linker searches a standard list of directories for the library, which is actually a file named
liblibrary.a. The linker then uses this file as if it had been specified precisely by name.
The directories searched include several standard system directories plus any that you specify with
-L.
Normally the files found this way are library files---archive files whose members are object files.
The linker handles an archive file by scanning through it for members which define symbols that have
so far been referenced but not defined. But if the file that is found is an ordinary object file, it
is linked in the usual fashion. The only difference between using an -l option and specifying a file
name is that -l surrounds library with lib and .a and searches several directories.
|
-lobjc
You need this special case of the -l option in order to link an Objective-C program.
|
-nostartfiles
Do not use the standard system startup files when linking. The standard system libraries are used
normally, unless -nostdlib or -nodefaultlibs is used.
|
-nodefaultlibs
Do not use the standard system libraries when linking. Only the libraries you specify will be passed
to the linker. The standard startup files are used normally, unless -nostartfiles is used. The
compiler may generate calls to memcmp, memset, and memcpy for System V (and ISO C) environments or to
bcopy and bzero for BSD environments. These entries are usually resolved by entries in libc. These
entry points should be supplied through some other mechanism when this option is specified.
|
-nostdlib
Do not use the standard system startup files or libraries when linking. No startup files and only
the libraries you specify will be passed to the linker. The compiler may generate calls to memcmp,
memset, and memcpy for System V (and ISO C) environments or to bcopy and bzero for BSD environments.
These entries are usually resolved by entries in libc. These entry points should be supplied through
some other mechanism when this option is specified.
One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of
internal subroutines that GCC uses to overcome shortcomings of particular machines, or special needs
for some languages.
In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other
words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This
ensures that you have no unresolved references to internal GCC library subroutines. (For example,
__main, used to ensure C++ constructors will be called.)
|
-pie
Produce a position independent executable on targets which support it. For predictable results, you
must also specify the same set of options that were used to generate code (-fpie, -fPIE, or model
suboptions) when you specify this option.
|
-s Remove all symbol table and relocation information from the executable.
|
-static
On systems that support dynamic linking, this prevents linking with the shared libraries. On other
systems, this option has no effect.
|
-shared
Produce a shared object which can then be linked with other objects to form an executable. Not all
systems support this option. For predictable results, you must also specify the same set of options
that were used to generate code (-fpic, -fPIC, or model suboptions) when you specify this option.[1]
|
-shared-libgcc
-static-libgcc
On systems that provide libgcc as a shared library, these options force the use of either the shared
or static version respectively. If no shared version of libgcc was built when the compiler was
configured, these options have no effect.
There are several situations in which an application should use the shared libgcc instead of the
static version. The most common of these is when the application wishes to throw and catch
exceptions across different shared libraries. In that case, each of the libraries as well as the
application itself should use the shared libgcc.
Therefore, the G++ and GCJ drivers automatically add -shared-libgcc whenever you build a shared
library or a main executable, because C++ and Java programs typically use exceptions, so this is the
right thing to do.
If, instead, you use the GCC driver to create shared libraries, you may find that they will not
always be linked with the shared libgcc. If GCC finds, at its configuration time, that you have a
non-GNU linker or a GNU linker that does not support option --eh-frame-hdr, it will link the shared
version of libgcc into shared libraries by default. Otherwise, it will take advantage of the linker
and optimize away the linking with the shared version of libgcc, linking with the static version of
libgcc by default. This allows exceptions to propagate through such shared libraries, without
incurring relocation costs at library load time.
However, if a library or main executable is supposed to throw or catch exceptions, you must link it
using the G++ or GCJ driver, as appropriate for the languages used in the program, or using the
option -shared-libgcc, such that it is linked with the shared libgcc.
|
-symbolic
Bind references to global symbols when building a shared object. Warn about any unresolved
references (unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few
systems support this option.
|
-Xlinker option
Pass option as an option to the linker. You can use this to supply system-specific linker options
which GCC does not know how to recognize.
If you want to pass an option that takes an argument, you must use -Xlinker twice, once for the
option and once for the argument. For example, to pass -assert definitions, you must write -Xlinker
-assert -Xlinker definitions. It does not work to write -Xlinker "-assert definitions", because this
passes the entire string as a single argument, which is not what the linker expects.
|
-Wl,option
Pass option as an option to the linker. If option contains commas, it is split into multiple options
at the commas.
|
-u symbol
Pretend the symbol symbol is undefined, to force linking of library modules to define it. You can
use -u multiple times with different symbols to force loading of additional library modules.
Options for Directory Search
These options specify directories to search for header files, for libraries and for parts of the
compiler:
|
-Idir
Add the directory dir to the head of the list of directories to be searched for header files. This
can be used to override a system header file, substituting your own version, since these directories
are searched before the system header file directories. However, you should not use this option to
add directories that contain vendor-supplied system header files (use -isystem for that). If you use
more than one -I option, the directories are scanned in left-to-right order; the standard system
directories come after.
|
-Ldir
Add directory dir to the list of directories to be searched for -l.
|
-Bprefix
This option specifies where to find the executables, libraries, include files, and data files of the
compiler itself.
|
-specs=file
Process file after the compiler reads in the standard specs file, in order to override the defaults
that the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc.
More than one -specs=file can be specified on the command line, and they are processed in order, from
left to right.
Specifying Target Machine and Compiler Version
The usual way to run GCC is to run the executable called gcc, or <machine>-gcc when cross-compiling, or
<machine>-gcc-<version> to run a version other than the one that was installed last. Sometimes this is
inconvenient, so GCC provides options that will switch to another cross-compiler or version.
|
-b machine
The argument machine specifies the target machine for compilation.
The value to use for machine is the same as was specified as the machine type when configuring GCC as
a cross-compiler. For example, if a cross-compiler was configured with configure i386v, meaning to
compile for an 80386 running System V, then you would specify -b i386v to run that cross compiler.
|
-V version
The argument version specifies which version of GCC to run. This is useful when multiple versions
are installed. For example, version might be 2.0, meaning to run GCC version 2.0.
|
-m68000
-mc68000
Generate output for a 68000. This is the default when the compiler is configured for 68000-based
systems.
Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306,
68307, 68322, 68328 and 68356.
|
-m68020
-mc68020
Generate output for a 68020. This is the default when the compiler is configured for 68020-based
systems.
|
-m68881
Generate output containing 68881 instructions for floating point. This is the default for most 68020
systems unless --nfp was specified when the compiler was configured.
|
-m68030
Generate output for a 68030. This is the default when the compiler is configured for 68030-based
systems.
|
-m68040
Generate output for a 68040. This is the default when the compiler is configured for 68040-based
systems.
This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the
68040. Use this option if your 68040 does not have code to emulate those instructions.
|
-m68060
Generate output for a 68060. This is the default when the compiler is configured for 68060-based
systems.
This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by
software on the 68060. Use this option if your 68060 does not have code to emulate those
instructions.
|
-mcpu32
Generate output for a CPU32. This is the default when the compiler is configured for CPU32-based
systems.
Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332,
68333, 68334, 68336, 68340, 68341, 68349 and 68360.
|
-m5200
Generate output for a 520X ``coldfire'' family cpu. This is the default when the compiler is
configured for 520X-based systems.
Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and
MCF5202.
|
-m68020-40
Generate output for a 68040, without using any of the new instructions. This results in code which
can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code
does use the 68881 instructions that are emulated on the 68040.
|
-m68020-60
Generate output for a 68060, without using any of the new instructions. This results in code which
can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code
does use the 68881 instructions that are emulated on the 68060.
|
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are
not available for all m68k targets. Normally the facilities of the machine's usual C compiler are
used, but this can't be done directly in cross-compilation. You must make your own arrangements to
provide suitable library functions for cross-compilation. The embedded targets m68k-*-aout and
m68k-*-coff do provide software floating point support.
|
-mshort
Consider type "int" to be 16 bits wide, like "short int".
|
-mnobitfield
Do not use the bit-field instructions. The -m68000, -mcpu32 and -m5200 options imply -mnobitfield.
|
-mbitfield
Do use the bit-field instructions. The -m68020 option implies -mbitfield. This is the default if
you use a configuration designed for a 68020.
|
-mrtd
Use a different function-calling convention, in which functions that take a fixed number of arguments
return with the "rtd" instruction, which pops their arguments while returning. This saves one
instruction in the caller since there is no need to pop the arguments there.
This calling convention is incompatible with the one normally used on Unix, so you cannot use it if
you need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of arguments
(including "printf"); otherwise incorrect code will be generated for calls to those functions.
In addition, seriously incorrect code will result if you call a function with too many arguments.
(Normally, extra arguments are harmlessly ignored.)
The "rtd" instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but
not by the 68000 or 5200.
|
-malign-int
-mno-align-int
Control whether GCC aligns "int", "long", "long long", "float", "double", and "long double" variables
on a 32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Aligning variables on
32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses at the
expense of more memory.
Warning: if you use the -malign-int switch, GCC will align structures containing the above types
differently than most published application binary interface specifications for the m68k.
|
-mpcrel
Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table.
At present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing.
-fPIC is not presently supported with -mpcrel, though this could be supported for 68020 and higher
processors.
|
-mno-strict-align
-mstrict-align
Do not (do) assume that unaligned memory references will be handled by the system.
|
-msep-data
Generate code that allows the data segment to be located in a different area of memory from the text
segment. This allows for execute in place in an environment without virtual memory management. This
option implies -fPIC.
|
-mno-sep-data
Generate code that assumes that the data segment follows the text segment. This is the default.
|
-mid-shared-library
Generate code that supports shared libraries via the library ID method. This allows for execute in
place and shared libraries in an environment without virtual memory management. This option implies
-fPIC.
|
-mno-id-shared-library
Generate code that doesn't assume ID based shared libraries are being used. This is the default.
|
-mshared-library-id=n
Specified the identification number of the ID based shared library being compiled. Specifying a
value of 0 will generate more compact code, specifying other values will force the allocation of that
number to the current library but is no more space or time efficient than omitting this option.
M68hc1x Options
These are the -m options defined for the 68hc11 and 68hc12 microcontrollers. The default values for
these options depends on which style of microcontroller was selected when the compiler was configured;
the defaults for the most common choices are given below.
|
-m6811
-m68hc11
Generate output for a 68HC11. This is the default when the compiler is configured for 68HC11-based
systems.
|
-m6812
-m68hc12
Generate output for a 68HC12. This is the default when the compiler is configured for 68HC12-based
systems.
|
-m68S12
-m68hcs12
Generate output for a 68HCS12.
|
-mauto-incdec
Enable the use of 68HC12 pre and post auto-increment and auto-decrement addressing modes.
|
-minmax
-nominmax
Enable the use of 68HC12 min and max instructions.
|
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are assumed to be far away, the compiler will use
the "call" instruction to call a function and the "rtc" instruction for returning.
|
-mshort
Consider type "int" to be 16 bits wide, like "short int".
|
-msoft-reg-count=count
Specify the number of pseudo-soft registers which are used for the code generation. The maximum
number is 32. Using more pseudo-soft register may or may not result in better code depending on the
program. The default is 4 for 68HC11 and 2 for 68HC12.
VAX Options
These -m options are defined for the VAX:
|
-munix
Do not output certain jump instructions ("aobleq" and so on) that the Unix assembler for the VAX
cannot handle across long ranges.
|
-mgnu
Do output those jump instructions, on the assumption that you will assemble with the GNU assembler.
-mg Output code for g-format floating point numbers instead of d-format.
SPARC Options
These -m options are supported on the SPARC:
|
-mno-app-regs
-mapp-regs
Specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4
ABI reserves for applications. This is the default, except on Solaris.
To be fully SVR4 ABI compliant at the cost of some performance loss, specify -mno-app-regs. You
should compile libraries and system software with this option.
|
-mfpu
-mhard-float
Generate output containing floating point instructions. This is the default.
|
-mno-fpu
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are
not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are
used, but this cannot be done directly in cross-compilation. You must make your own arrangements to
provide suitable library functions for cross-compilation. The embedded targets sparc-*-aout and
sparclite-*-* do provide software floating point support.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you
compile all of a program with this option. In particular, you need to compile libgcc.a, the library
that comes with GCC, with -msoft-float in order for this to work.
|
-mhard-quad-float
Generate output containing quad-word (long double) floating point instructions.
|
-msoft-quad-float
Generate output containing library calls for quad-word (long double) floating point instructions.
The functions called are those specified in the SPARC ABI. This is the default.
As of this writing, there are no SPARC implementations that have hardware support for the quad-word
floating point instructions. They all invoke a trap handler for one of these instructions, and then
the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this
is much slower than calling the ABI library routines. Thus the -msoft-quad-float option is the
default.
|
-mno-flat
-mflat
With -mflat, the compiler does not generate save/restore instructions and will use a ``flat'' or
single register window calling convention. This model uses %i7 as the frame pointer and is
compatible with the normal register window model. Code from either may be intermixed. The local
registers and the input registers (0--5) are still treated as ``call saved'' registers and will be
saved on the stack as necessary.
With -mno-flat (the default), the compiler emits save/restore instructions (except for leaf
functions) and is the normal mode of operation.
These options are deprecated and will be deleted in a future GCC release.
|
-mno-unaligned-doubles
-munaligned-doubles
Assume that doubles have 8 byte alignment. This is the default.
With -munaligned-doubles, GCC assumes that doubles have 8 byte alignment only if they are contained
in another type, or if they have an absolute address. Otherwise, it assumes they have 4 byte
alignment. Specifying this option avoids some rare compatibility problems with code generated by
other compilers. It is not the default because it results in a performance loss, especially for
floating point code.
|
-mno-faster-structs
-mfaster-structs
With -mfaster-structs, the compiler assumes that structures should have 8 byte alignment. This
enables the use of pairs of "ldd" and "std" instructions for copies in structure assignment, in place
of twice as many "ld" and "st" pairs. However, the use of this changed alignment directly violates
the SPARC ABI. Thus, it's intended only for use on targets where the developer acknowledges that
their resulting code will not be directly in line with the rules of the ABI.
|
-mimpure-text
-mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when
linking a shared object. Using this option, you can link position-dependent code into a shared
object.
-mimpure-text suppresses the ``relocations remain against allocatable but non-writable sections''
linker error message. However, the necessary relocations will trigger copy-on-write, and the shared
object is not actually shared across processes. Instead of using -mimpure-text, you should compile
all source code with -fpic or -fPIC.
This option is only available on SunOS and Solaris.
|
-mv8
-msparclite
These two options select variations on the SPARC architecture. These options are deprecated and will
be deleted in a future GCC release. They have been replaced with -mcpu=xxx.
|
-mcypress
-msupersparc
-mf930
-mf934
These four options select the processor for which the code is optimized. These options are
deprecated and will be deleted in a future GCC release. They have been replaced with -mcpu=xxx.
|
-mv8plus
-mno-v8plus
With -mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI is that the
global and out registers are considered 64-bit wide. This is enabled by default on Solaris in 32-bit
mode for all SPARC-V9 processors.
|
-mvis
-mno-vis
With -mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set
extensions. The default is -mno-vis.
These -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments:
|
-mlittle-endian
Generate code for a processor running in little-endian mode. It is only available for a few
configurations and most notably not on Solaris and Linux.
|
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer
to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
|
-mcmodel=medlow
Generate code for the Medium/Low code model: 64-bit addresses, programs must be linked in the low 32
bits of memory. Programs can be statically or dynamically linked.
|
-mcmodel=medmid
Generate code for the Medium/Middle code model: 64-bit addresses, programs must be linked in the low
44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must
be located within 2GB of the text segment.
|
-mcmodel=medany
Generate code for the Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere
in memory, the text and data segments must be less than 2GB in size and the data segment must be
located within 2GB of the text segment.
|
-mcmodel=embmedany
Generate code for the Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and
data segments must be less than 2GB in size, both starting anywhere in memory (determined at link
time). The global register %g4 points to the base of the data segment. Programs are statically
linked and PIC is not supported.
|
-mstack-bias
-mno-stack-bias
With -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by
-2047 which must be added back when making stack frame references. This is the default in 64-bit
mode. Otherwise, assume no such offset is present.
These switches are supported in addition to the above on Solaris:
|
-threads
Add support for multithreading using the Solaris threads library. This option sets flags for both
the preprocessor and linker. This option does not affect the thread safety of object code produced
by the compiler or that of libraries supplied with it.
|
-pthreads
Add support for multithreading using the POSIX threads library. This option sets flags for both the
preprocessor and linker. This option does not affect the thread safety of object code produced by
the compiler or that of libraries supplied with it.
ARM Options
These -m options are defined for Advanced RISC Machines (ARM) architectures:
|
-mapcs-frame
Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even
if this is not strictly necessary for correct execution of the code. Specifying -fomit-frame-pointer
with this option will cause the stack frames not to be generated for leaf functions. The default is
-mno-apcs-frame.
|
-mapcs
This is a synonym for -mapcs-frame.
|
-mapcs-26
Generate code for a processor running with a 26-bit program counter, and conforming to the function
calling standards for the APCS 26-bit option.
This option is deprecated. Future releases of the GCC will only support generating code that runs in
apcs-32 mode.
|
-mapcs-32
Generate code for a processor running with a 32-bit program counter, and conforming to the function
calling standards for the APCS 32-bit option.
This flag is deprecated. Future releases of GCC will make this flag unconditional.
|
-mthumb-interwork
Generate code which supports calling between the ARM and Thumb instruction sets. Without this option
the two instruction sets cannot be reliably used inside one program. The default is
-mno-thumb-interwork, since slightly larger code is generated when -mthumb-interwork is specified.
|
-mno-sched-prolog
Prevent the reordering of instructions in the function prolog, or the merging of those instruction
with the instructions in the function's body. This means that all functions will start with a
recognizable set of instructions (or in fact one of a choice from a small set of different function
prologues), and this information can be used to locate the start if functions inside an executable
piece of code. The default is -msched-prolog.
|
-mhard-float
Generate output containing floating point instructions. This is the default.
|
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are
not available for all ARM targets. Normally the facilities of the machine's usual C compiler are
used, but this cannot be done directly in cross-compilation. You must make your own arrangements to
provide suitable library functions for cross-compilation.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you
compile all of a program with this option. In particular, you need to compile libgcc.a, the library
that comes with GCC, with -msoft-float in order for this to work.
|
-mlittle-endian
Generate code for a processor running in little-endian mode. This is the default for all standard
configurations.
|
-mbig-endian
Generate code for a processor running in big-endian mode; the default is to compile code for a
little-endian processor.
|
-mwords-little-endian
This option only applies when generating code for big-endian processors. Generate code for a little-
endian word order but a big-endian byte order. That is, a byte order of the form 32107654. Note:
this option should only be used if you require compatibility with code for big-endian ARM processors
generated by versions of the compiler prior to 2.8.
|
-malignment-traps
Generate code that will not trap if the MMU has alignment traps enabled. On ARM architectures prior
to ARMv4, there were no instructions to access half-word objects stored in memory. However, when
reading from memory a feature of the ARM architecture allows a word load to be used, even if the
address is unaligned, and the processor core will rotate the data as it is being loaded. This option
tells the compiler that such misaligned accesses will cause a MMU trap and that it should instead
synthesize the access as a series of byte accesses. The compiler can still use word accesses to load
half-word data if it knows that the address is aligned to a word boundary.
This option has no effect when compiling for ARM architecture 4 or later, since these processors have
instructions to directly access half-word objects in memory.
|
-mno-alignment-traps
Generate code that assumes that the MMU will not trap unaligned accesses. This produces better code
when the target instruction set does not have half-word memory operations (i.e. implementations prior
to ARMv4).
Note that you cannot use this option to access unaligned word objects, since the processor will only
fetch one 32-bit aligned object from memory.
The default setting is -malignment-traps, since this produces code that will also run on processors
implementing ARM architecture version 6 or later.
This option is deprecated and will be removed in the next release of GCC.
|
-mcpu=name
This specifies the name of the target ARM processor. GCC uses this name to determine what kind of
instructions it can emit when generating assembly code. Permissible names are: arm2, arm250, arm3,
arm6, arm60, arm600, arm610, arm620, arm7, arm7m, arm7d, arm7dm, arm7di, arm7dmi, arm70, arm700,
arm700i, arm710, arm710c, arm7100, arm7500, arm7500fe, arm7tdmi, arm8, strongarm, strongarm110,
strongarm1100, arm8, arm810, arm9, arm9e, arm920, arm920t, arm926ejs, arm940t, arm9tdmi, arm10tdmi,
arm1020t, arm1026ejs, arm1136js, arm1136jfs ,xscale, iwmmxt, ep9312.
|
-mtune=name
This option is very similar to the -mcpu= option, except that instead of specifying the actual target
processor type, and hence restricting which instructions can be used, it specifies that GCC should
tune the performance of the code as if the target were of the type specified in this option, but
still choosing the instructions that it will generate based on the cpu specified by a -mcpu= option.
For some ARM implementations better performance can be obtained by using this option.
|
-march=name
This specifies the name of the target ARM architecture. GCC uses this name to determine what kind of
instructions it can emit when generating assembly code. This option can be used in conjunction with
or instead of the -mcpu= option. Permissible names are: armv2, armv2a, armv3, armv3m, armv4, armv4t,
armv5, armv5t, armv5te, armv6j, iwmmxt, ep9312.
|
-mfpe=number
-mfp=number
This specifies the version of the floating point emulation available on the target. Permissible
values are 2 and 3. -mfp= is a synonym for -mfpe=, for compatibility with older versions of GCC.
|
-mstructure-size-boundary=n
The size of all structures and unions will be rounded up to a multiple of the number of bits set by
this option. Permissible values are 8 and 32. The default value varies for different toolchains.
For the COFF targeted toolchain the default value is 8. Specifying the larger number can produce
faster, more efficient code, but can also increase the size of the program. The two values are
potentially incompatible. Code compiled with one value cannot necessarily expect to work with code
or libraries compiled with the other value, if they exchange information using structures or unions.
|
-mabort-on-noreturn
Generate a call to the function "abort" at the end of a "noreturn" function. It will be executed if
the function tries to return.
|
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the address of the function into a
register and then performing a subroutine call on this register. This switch is needed if the target
function will lie outside of the 64 megabyte addressing range of the offset based version of
subroutine call instruction.
Even if this switch is enabled, not all function calls will be turned into long calls. The heuristic
is that static functions, functions which have the short-call attribute, functions that are inside
the scope of a #pragma no_long_calls directive and functions whose definitions have already been
compiled within the current compilation unit, will not be turned into long calls. The exception to
this rule is that weak function definitions, functions with the long-call attribute or the section
attribute, and functions that are within the scope of a #pragma long_calls directive, will always be
turned into long calls.
This feature is not enabled by default. Specifying -mno-long-calls will restore the default
behavior, as will placing the function calls within the scope of a #pragma long_calls_off directive.
Note these switches have no effect on how the compiler generates code to handle function calls via
function pointers.
|
-mnop-fun-dllimport
Disable support for the "dllimport" attribute.
|
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for
each function. The run-time system is responsible for initializing this register with an appropriate
value before execution begins.
|
-mpic-register=reg
Specify the register to be used for PIC addressing. The default is R10 unless stack-checking is
enabled, when R9 is used.
|
-mcirrus-fix-invalid-insns
Insert NOPs into the instruction stream to in order to work around problems with invalid Maverick
instruction combinations. This option is only valid if the -mcpu=ep9312 option has been used to
enable generation of instructions for the Cirrus Maverick floating point co-processor. This option
is not enabled by default, since the problem is only present in older Maverick implementations. The
default can be re-enabled by use of the -mno-cirrus-fix-invalid-insns switch.
|
-mpoke-function-name
Write the name of each function into the text section, directly preceding the function prologue. The
generated code is similar to this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4
When performing a stack backtrace, code can inspect the value of "pc" stored at "fp + 0". If the
trace function then looks at location "pc - 12" and the top 8 bits are set, then we know that there
is a function name embedded immediately preceding this location and has length "((pc[-3]) &
0xff000000)".
|
-mthumb
Generate code for the 16-bit Thumb instruction set. The default is to use the 32-bit ARM instruction
set.
|
-mtpcs-frame
Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf
functions. (A leaf function is one that does not call any other functions.) The default is
-mno-tpcs-frame.
|
-mtpcs-leaf-frame
Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf
functions. (A leaf function is one that does not call any other functions.) The default is
-mno-apcs-leaf-frame.
|
-mcallee-super-interworking
Gives all externally visible functions in the file being compiled an ARM instruction set header which
switches to Thumb mode before executing the rest of the function. This allows these functions to be
called from non-interworking code.
|
-mcaller-super-interworking
Allows calls via function pointers (including virtual functions) to execute correctly regardless of
whether the target code has been compiled for interworking or not. There is a small overhead in the
cost of executing a function pointer if this option is enabled.
MN10300 Options
These -m options are defined for Matsushita MN10300 architectures:
|
-mmult-bug
Generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the
default.
|
-mno-mult-bug
Do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.
|
-mam33
Generate code which uses features specific to the AM33 processor.
|
-mno-am33
Do not generate code which uses features specific to the AM33 processor. This is the default.
|
-mno-crt0
Do not link in the C run-time initialization object file.
|
-mrelax
Indicate to the linker that it should perform a relaxation optimization pass to shorten branches,
calls and absolute memory addresses. This option only has an effect when used on the command line
for the final link step.
This option makes symbolic debugging impossible.
M32R/D Options
These -m options are defined for Renesas M32R/D architectures:
|
-m32r2
Generate code for the M32R/2.
|
-m32rx
Generate code for the M32R/X.
|
-m32r
Generate code for the M32R. This is the default.
|
-mmodel=small
Assume all objects live in the lower 16MB of memory (so that their addresses can be loaded with the
"ld24" instruction), and assume all subroutines are reachable with the "bl" instruction. This is the
default.
The addressability of a particular object can be set with the "model" attribute.
|
-mmodel=medium
Assume objects may be anywhere in the 32-bit address space (the compiler will generate "seth/add3"
instructions to load their addresses), and assume all subroutines are reachable with the "bl"
instruction.
|
-mmodel=large
Assume objects may be anywhere in the 32-bit address space (the compiler will generate "seth/add3"
instructions to load their addresses), and assume subroutines may not be reachable with the "bl"
instruction (the compiler will generate the much slower "seth/add3/jl" instruction sequence).
|
-msdata=none
Disable use of the small data area. Variables will be put into one of .data, bss, or .rodata (unless
the "section" attribute has been specified). This is the default.
The small data area consists of sections .sdata and .sbss. Objects may be explicitly put in the
small data area with the "section" attribute using one of these sections.
|
-msdata=sdata
Put small global and static data in the small data area, but do not generate special code to
reference them.
|
-msdata=use
Put small global and static data in the small data area, and generate special instructions to
reference them.
|
-G num
Put global and static objects less than or equal to num bytes into the small data or bss sections
instead of the normal data or bss sections. The default value of num is 8. The -msdata option must
be set to one of sdata or use for this option to have any effect.
All modules should be compiled with the same -G num value. Compiling with different values of num
may or may not work; if it doesn't the linker will give an error message---incorrect code will not be
generated.
|
-mdebug
Makes the M32R specific code in the compiler display some statistics that might help in debugging
programs.
|
-malign-loops
Align all loops to a 32-byte boundary.
|
-mno-align-loops
Do not enforce a 32-byte alignment for loops. This is the default.
|
-missue-rate=number
Issue number instructions per cycle. number can only be 1 or 2.
|
-mbranch-cost=number
number can only be 1 or 2. If it is 1 then branches will be preferred over conditional code, if it
is 2, then the opposite will apply.
|
-mflush-trap=number
Specifies the trap number to use to flush the cache. The default is 12. Valid numbers are between 0
and 15 inclusive.
|
-mno-flush-trap
Specifies that the cache cannot be flushed by using a trap.
|
-mflush-func=name
Specifies the name of the operating system function to call to flush the cache. The default is
_flush_cache, but a function call will only be used if a trap is not available.
|
-mno-flush-func
Indicates that there is no OS function for flushing the cache.
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
|
-mpower
-mno-power
-mpower2
-mno-power2
-mpowerpc
-mno-powerpc
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
GCC supports two related instruction set architectures for the RS/6000 and PowerPC. The POWER
instruction set are those instructions supported by the rios chip set used in the original RS/6000
systems and the PowerPC instruction set is the architecture of the Motorola MPC5xx, MPC6xx, MPC8xx
microprocessors, and the IBM 4xx microprocessors.
|
-mnew-mnemonics
-mold-mnemonics
Select which mnemonics to use in the generated assembler code. With -mnew-mnemonics, GCC uses the
assembler mnemonics defined for the PowerPC architecture. With -mold-mnemonics it uses the assembler
mnemonics defined for the POWER architecture. Instructions defined in only one architecture have
only one mnemonic; GCC uses that mnemonic irrespective of which of these options is specified.
GCC defaults to the mnemonics appropriate for the architecture in use. Specifying -mcpu=cpu_type
sometimes overrides the value of these option. Unless you are building a cross-compiler, you should
normally not specify either -mnew-mnemonics or -mold-mnemonics, but should instead accept the
default.
|
-mcpu=common selects a completely generic processor. Code generated under this option will run on
any POWER or PowerPC processor. GCC will use only the instructions in the common subset of both
architectures, and will not use the MQ register. GCC assumes a generic processor model for
scheduling purposes.
|
-mcpu=power, -mcpu=power2, -mcpu=powerpc, and -mcpu=powerpc64 specify generic POWER, POWER2, pure
32-bit PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine types, with an
appropriate, generic processor model assumed for scheduling purposes.
The other options specify a specific processor. Code generated under those options will run best on
that processor, and may not run at all on others.
The -mcpu options automatically enable or disable the following options: -maltivec, -mhard-float,
-mmfcrf, -mmultiple, -mnew-mnemonics, -mpower, -mpower2, -mpowerpc64, -mpowerpc-gpopt,
-mpowerpc-gfxopt, -mstring. The particular options set for any particular CPU will vary between
compiler versions, depending on what setting seems to produce optimal code for that CPU; it doesn't
necessarily reflect the actual hardware's capabilities. If you wish to set an individual option to a
particular value, you may specify it after the -mcpu option, like -mcpu=970 -mno-altivec.
On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu option at
present, since AIX does not have full support for these options. You may still enable or disable
them individually if you're sure it'll work in your environment.
|
-maltivec
-mno-altivec
These switches enable or disable the use of built-in functions that allow access to the AltiVec
instruction set. You may also need to set -mabi=altivec to adjust the current ABI with AltiVec ABI
enhancements.
|
-mabi=spe
Extend the current ABI with SPE ABI extensions. This does not change the default ABI, instead it
adds the SPE ABI extensions to the current ABI.
|
-mabi=no-spe
Disable Booke SPE ABI extensions for the current ABI.
|
-misel=yes/no
-misel
This switch enables or disables the generation of ISEL instructions.
|
-mspe=yes/no
-mspe
This switch enables or disables the generation of SPE simd instructions.
|
-mfloat-gprs=yes/no
-mfloat-gprs
This switch enables or disables the generation of floating point operations on the general purpose
registers for architectures that support it. This option is currently only available on the MPC8540.
|
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is created for every executable file. The
-mfull-toc option is selected by default. In that case, GCC will allocate at least one TOC entry for
each unique non-automatic variable reference in your program. GCC will also place floating-point
constants in the TOC. However, only 16,384 entries are available in the TOC.
If you receive a linker error message that saying you have overflowed the available TOC space, you
can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options.
-mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC and -mno-sum-in-toc
forces GCC to generate code to calculate the sum of an address and a constant at run-time instead of
putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to
produce very slightly slower and larger code at the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both of these options, specify
-mminimal-toc instead. This option causes GCC to make only one TOC entry for every file. When you
specify this option, GCC will produce code that is slower and larger but which uses extremely little
TOC space. You may wish to use this option only on files that contain less frequently executed code.
|
-maix64
-maix32
Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit "long" type, and the
infrastructure needed to support them. Specifying -maix64 implies -mpowerpc64 and -mpowerpc, while
-maix32 disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32.
|
-mxl-compat
-mno-xl-compat
Produce code that conforms more closely to IBM XLC semantics when using AIX-compatible ABI. Pass
floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in
addition to argument FPRs. Do not assume that most significant double in 128 bit long double value
is properly rounded when comparing values.
The AIX calling convention was extended but not initially documented to handle an obscure K&R C case
of calling a function that takes the address of its arguments with fewer arguments than declared.
AIX XL compilers access floating point arguments which do not fit in the RSA from the stack when a
subroutine is compiled without optimization. Because always storing floating-point arguments on the
stack is inefficient and rarely needed, this option is not enabled by default and only is necessary
when calling subroutines compiled by AIX XL compilers without optimization.
|
-mpe
Support IBM RS/6000 SP Parallel Environment (PE). Link an application written to use message passing
with special startup code to enable the application to run. The system must have PE installed in the
standard location (/usr/lpp/ppe.poe/), or the specs file must be overridden with the -specs= option
to specify the appropriate directory location. The Parallel Environment does not support threads, so
the -mpe option and the -pthread option are incompatible.
|
-malign-natural
-malign-power
On AIX, Darwin, and 64-bit PowerPC GNU/Linux, the option -malign-natural overrides the ABI-defined
alignment of larger types, such as floating-point doubles, on their natural size-based boundary. The
option -malign-power instructs GCC to follow the ABI-specified alignment rules. GCC defaults to the
standard alignment defined in the ABI.
|
-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-point register set. Software floating point
emulation is provided if you use the -msoft-float option, and pass the option to GCC when linking.
|
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple word instructions and the store multiple
word instructions. These instructions are generated by default on POWER systems, and not generated
on PowerPC systems. Do not use -mmultiple on little endian PowerPC systems, since those instructions
do not work when the processor is in little endian mode. The exceptions are PPC740 and PPC750 which
permit the instructions usage in little endian mode.
|
-mstring
-mno-string
Generate code that uses (does not use) the load string instructions and the store string word
instructions to save multiple registers and do small block moves. These instructions are generated
by default on POWER systems, and not generated on PowerPC systems. Do not use -mstring on little
endian PowerPC systems, since those instructions do not work when the processor is in little endian
mode. The exceptions are PPC740 and PPC750 which permit the instructions usage in little endian
mode.
|
-mupdate
-mno-update
Generate code that uses (does not use) the load or store instructions that update the base register
to the address of the calculated memory location. These instructions are generated by default. If
you use -mno-update, there is a small window between the time that the stack pointer is updated and
the address of the previous frame is stored, which means code that walks the stack frame across
interrupts or signals may get corrupted data.
|
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating point multiply and accumulate instructions.
These instructions are generated by default if hardware floating is used.
|
-mno-bit-align
-mbit-align
On System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain bit-
fields to be aligned to the base type of the bit-field.
For example, by default a structure containing nothing but 8 "unsigned" bit-fields of length 1 would
be aligned to a 4 byte boundary and have a size of 4 bytes. By using -mno-bit-align, the structure
would be aligned to a 1 byte boundary and be one byte in size.
|
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references will
be handled by the system.
|
-mrelocatable
-mno-relocatable
On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to
a different address at runtime. If you use -mrelocatable on any module, all objects linked together
must be compiled with -mrelocatable or -mrelocatable-lib.
|
-mrelocatable-lib
-mno-relocatable-lib
On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to
a different address at runtime. Modules compiled with -mrelocatable-lib can be linked with either
modules compiled without -mrelocatable and -mrelocatable-lib or with modules compiled with the
-mrelocatable options.
|
-mno-toc
-mtoc
On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a
global area pointing to the addresses used in the program.
|
-mlittle
-mlittle-endian
On System V.4 and embedded PowerPC systems compile code for the processor in little endian mode. The
-mlittle-endian option is the same as -mlittle.
|
-mbig
-mbig-endian
On System V.4 and embedded PowerPC systems compile code for the processor in big endian mode. The
-mbig-endian option is the same as -mbig.
|
-mdynamic-no-pic
On Darwin and Mac OS X systems, compile code so that it is not relocatable, but that its external
references are relocatable. The resulting code is suitable for applications, but not shared
libraries.
|
-mprioritize-restricted-insns=priority
This option controls the priority that is assigned to dispatch-slot restricted instructions during
the second scheduling pass. The argument priority takes the value 0/1/2 to assign
no/highest/second-highest priority to dispatch slot restricted instructions.
|
-minsert-sched-nops=scheme
This option controls which nop insertion scheme will be used during the second scheduling pass. The
argument scheme takes one of the following values: no: Don't insert nops. pad: Pad with nops any
dispatch group which has vacant issue slots, according to the scheduler's grouping. regroup_exact:
Insert nops to force costly dependent insns into separate groups. Insert exactly as many nops as
needed to force an insn to a new group, according to the estimated processor grouping. number:
Insert nops to force costly dependent insns into separate groups. Insert number nops to force an
insn to a new group.
|
-mcall-sysv
On System V.4 and embedded PowerPC systems compile code using calling conventions that adheres to the
March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is
the default unless you configured GCC using powerpc-*-eabiaix.
|
-mcall-sysv-eabi
Specify both -mcall-sysv and -meabi options.
|
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
|
-mcall-solaris
On System V.4 and embedded PowerPC systems compile code for the Solaris operating system.
|
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system.
|
-mcall-gnu
On System V.4 and embedded PowerPC systems compile code for the Hurd-based GNU system.
|
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the NetBSD operating system.
|
-maix-struct-return
Return all structures in memory (as specified by the AIX ABI).
|
-msvr4-struct-return
Return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI).
|
-mabi=altivec
Extend the current ABI with AltiVec ABI extensions. This does not change the default ABI, instead it
adds the AltiVec ABI extensions to the current ABI.
|
-mabi=no-altivec
Disable AltiVec ABI extensions for the current ABI.
|
-mprototype
-mno-prototype
On System V.4 and embedded PowerPC systems assume that all calls to variable argument functions are
properly prototyped. Otherwise, the compiler must insert an instruction before every non prototyped
call to set or clear bit 6 of the condition code register (CR) to indicate whether floating point
values were passed in the floating point registers in case the function takes a variable arguments.
With -mprototype, only calls to prototyped variable argument functions will set or clear the bit.
|
-msim
On embedded PowerPC systems, assume that the startup module is called sim-crt0.o and that the
standard C libraries are libsim.a and libc.a. This is the default for powerpc-*-eabisim.
configurations.
|
-mmvme
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C
libraries are libmvme.a and libc.a.
|
-mads
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C
libraries are libads.a and libc.a.
|
-myellowknife
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C
libraries are libyk.a and libc.a.
|
-mvxworks
On System V.4 and embedded PowerPC systems, specify that you are compiling for a VxWorks system.
|
-mwindiss
Specify that you are compiling for the WindISS simulation environment.
|
-memb
On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags header to indicate that eabi
extended relocations are used.
|
-meabi
-mno-eabi
On System V.4 and embedded PowerPC systems do (do not) adhere to the Embedded Applications Binary
Interface (eabi) which is a set of modifications to the System V.4 specifications. Selecting -meabi
means that the stack is aligned to an 8 byte boundary, a function "__eabi" is called to from "main"
to set up the eabi environment, and the -msdata option can use both "r2" and "r13" to point to two
separate small data areas. Selecting -mno-eabi means that the stack is aligned to a 16 byte
boundary, do not call an initialization function from "main", and the -msdata option will only use
"r13" to point to a single small data area. The -meabi option is on by default if you configured GCC
using one of the powerpc*-*-eabi* options.
|
-msdata=eabi
On System V.4 and embedded PowerPC systems, put small initialized "const" global and static data in
the .sdata2 section, which is pointed to by register "r2". Put small initialized non-"const" global
and static data in the .sdata section, which is pointed to by register "r13". Put small
uninitialized global and static data in the .sbss section, which is adjacent to the .sdata section.
The -msdata=eabi option is incompatible with the -mrelocatable option. The -msdata=eabi option also
sets the -memb option.
|
-msdata=sysv
On System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section,
which is pointed to by register "r13". Put small uninitialized global and static data in the .sbss
section, which is adjacent to the .sdata section. The -msdata=sysv option is incompatible with the
-mrelocatable option.
|
-msdata=default
-msdata
On System V.4 and embedded PowerPC systems, if -meabi is used, compile code the same as -msdata=eabi,
otherwise compile code the same as -msdata=sysv.
|
-msdata-data
On System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section.
Put small uninitialized global and static data in the .sbss section. Do not use register "r13" to
address small data however. This is the default behavior unless other -msdata options are used.
|
-msdata=none
-mno-sdata
On embedded PowerPC systems, put all initialized global and static data in the .data section, and all
uninitialized data in the .bss section.
|
-G num
On embedded PowerPC systems, put global and static items less than or equal to num bytes into the
small data or bss sections instead of the normal data or bss section. By default, num is 8. The -G
num switch is also passed to the linker. All modules should be compiled with the same -G num value.
|
-mregnames
-mno-regnames
On System V.4 and embedded PowerPC systems do (do not) emit register names in the assembly language
output using symbolic forms.
|
-mlongcall
-mno-longcall
Default to making all function calls via pointers, so that functions which reside further than 64
megabytes (67,108,864 bytes) from the current location can be called. This setting can be overridden
by the "shortcall" function attribute, or by "#pragma longcall(0)".
Some linkers are capable of detecting out-of-range calls and generating glue code on the fly. On
these systems, long calls are unnecessary and generate slower code. As of this writing, the AIX
linker can do this, as can the GNU linker for PowerPC/64. It is planned to add this feature to the
GNU linker for 32-bit PowerPC systems as well.
On Mach-O (Darwin) systems, this option directs the compiler emit to the glue for every direct call,
and the Darwin linker decides whether to use or discard it.
In the future, we may cause GCC to ignore all longcall specifications when the linker is known to
generate glue.
|
-pthread
Adds support for multithreading with the pthreads library. This option sets flags for both the
preprocessor and linker.
Darwin Options
These options are defined for all architectures running the Darwin operating system. They are useful for
compatibility with other Mac OS compilers.
|
-all_load
Loads all members of static archive libraries. See man ld(1) for more information.
|
-arch_errors_fatal
Cause the errors having to do with files that have the wrong architecture to be fatal.
|
-bind_at_load
Causes the output file to be marked such that the dynamic linker will bind all undefined references
when the file is loaded or launched.
|
-bundle
Produce a Mach-o bundle format file. See man ld(1) for more information.
|
-bundle_loader executable
This specifies the executable that will be loading the build output file being linked. See man ld(1)
for more information.
|
-allowable_client client_name
-arch_only
-client_name
-compatibility_version
-current_version
-dependency-file
-dylib_file
-dylinker_install_name
-dynamic
-dynamiclib
-exported_symbols_list
-filelist
-flat_namespace
-force_cpusubtype_ALL
-force_flat_namespace
-headerpad_max_install_names
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-nofixprebinding
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-seg_addr_table
-seg_addr_table_filename
-seglinkedit
-segprot
-segs_read_only_addr
-segs_read_write_addr
-single_module
-static
-sub_library
-sub_umbrella
-twolevel_namespace
-umbrella
-undefined
-unexported_symbols_list
-weak_reference_mismatches
-whatsloaded
These options are available for Darwin linker. Darwin linker man page describes them in detail.
MIPS Options
|
-EB Generate big-endian code.
|
-EL Generate little-endian code. This is the default for mips*el-*-* configurations.
|
-march=arch
Generate code that will run on arch, which can be the name of a generic MIPS ISA, or the name of a
particular processor. The ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, and mips64.
The processor names are: 4kc, 4kp, 5kc, 20kc, m4k, r2000, r3000, r3900, r4000, r4400, r4600, r4650,
r6000, r8000, rm7000, rm9000, orion, sb1, vr4100, vr4111, vr4120, vr4300, vr5000, vr5400 and vr5500.
The special value from-abi selects the most compatible architecture for the selected ABI (that is,
mips1 for 32-bit ABIs and mips3 for 64-bit ABIs).
In processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are
optional, and vr may be written r.
GCC defines two macros based on the value of this option. The first is _MIPS_ARCH, which gives the
name of target architecture, as a string. The second has the form _MIPS_ARCH_foo, where foo is the
capitalized value of _MIPS_ARCH. For example, -march=r2000 will set _MIPS_ARCH to "r2000" and define
the macro _MIPS_ARCH_R2000.
Note that the _MIPS_ARCH macro uses the processor names given above. In other words, it will have
the full prefix and will not abbreviate 000 as k. In the case of from-abi, the macro names the
resolved architecture (either "mips1" or "mips3"). It names the default architecture when no -march
option is given.
|
-mtune=arch
Optimize for arch. Among other things, this option controls the way instructions are scheduled, and
the perceived cost of arithmetic operations. The list of arch values is the same as for -march.
When this option is not used, GCC will optimize for the processor specified by -march. By using
-march and -mtune together, it is possible to generate code that will run on a family of processors,
but optimize the code for one particular member of that family.
-mtune defines the macros _MIPS_TUNE and _MIPS_TUNE_foo, which work in the same way as the -march
ones described above.
|
-mips1
Equivalent to -march=mips1.
|
-mips2
Equivalent to -march=mips2.
|
-mips3
Equivalent to -march=mips3.
|
-mips4
Equivalent to -march=mips4.
|
-mips32
Equivalent to -march=mips32.
|
-mips32r2
Equivalent to -march=mips32r2.
|
-mips64
Equivalent to -march=mips64.
|
-mips16
-mno-mips16
Use (do not use) the MIPS16 ISA.
|
-mabicalls
-mno-abicalls
Generate (do not generate) SVR4-style position-independent code. -mabicalls is the default for
SVR4-based systems.
|
-mxgot
-mno-xgot
Lift (do not lift) the usual restrictions on the size of the global offset table.
|
-membedded-pic
-mno-embedded-pic
Generate (do not generate) position-independent code suitable for some embedded systems. All calls
are made using PC relative addresses, and all data is addressed using the $gp register. No more than
65536 bytes of global data may be used. This requires GNU as and GNU ld, which do most of the work.
|
-mgp32
Assume that general-purpose registers are 32 bits wide.
|
-mgp64
Assume that general-purpose registers are 64 bits wide.
|
-mfp32
Assume that floating-point registers are 32 bits wide.
|
-mfp64
Assume that floating-point registers are 64 bits wide.
|
-mhard-float
Use floating-point coprocessor instructions.
|
-msoft-float
Do not use floating-point coprocessor instructions. Implement floating-point calculations using
library calls instead.
|
-msingle-float
Assume that the floating-point coprocessor only supports single-precision operations.
|
-mdouble-float
Assume that the floating-point coprocessor supports double-precision operations. This is the
default.
|
-mint64
Force "int" and "long" types to be 64 bits wide. See -mlong32 for an explanation of the default and
the way that the pointer size is determined.
|
-mlong64
Force "long" types to be 64 bits wide. See -mlong32 for an explanation of the default and the way
that the pointer size is determined.
|
-mlong32
Force "long", "int", and pointer types to be 32 bits wide.
The default size of "int"s, "long"s and pointers depends on the ABI. All the supported ABIs use
32-bit "int"s. The n64 ABI uses 64-bit "long"s, as does the 64-bit EABI; the others use 32-bit
"long"s. Pointers are the same size as "long"s, or the same size as integer registers, whichever is
smaller.
|
-G num
Put global and static items less than or equal to num bytes into the small data or bss section
instead of the normal data or bss section. This allows the data to be accessed using a single
instruction.
All modules should be compiled with the same -G num value.
|
-membedded-data
-mno-embedded-data
Allocate variables to the read-only data section first if possible, then next in the small data
section if possible, otherwise in data. This gives slightly slower code than the default, but
reduces the amount of RAM required when executing, and thus may be preferred for some embedded
systems.
|
-muninit-const-in-rodata
-mno-uninit-const-in-rodata
Put uninitialized "const" variables in the read-only data section. This option is only meaningful in
conjunction with -membedded-data.
|
-msplit-addresses
-mno-split-addresses
Enable (disable) use of the "%hi()" and "%lo()" assembler relocation operators. This option has been
superceded by -mexplicit-relocs but is retained for backwards compatibility.
|
-mexplicit-relocs
-mno-explicit-relocs
Use (do not use) assembler relocation operators when dealing with symbolic addresses. The
alternative, selected by -mno-explicit-relocs, is to use assembler macros instead.
-mexplicit-relocs is usually the default if GCC was configured to use an assembler that supports
relocation operators. However, there are two exceptions:
GCC is not yet able to generate explicit relocations for the combination of -mabi=64 and
-mno-abicalls. This will be addressed in a future release.
The combination of -mabicalls and -fno-unit-at-a-time implies -mno-explicit-relocs unless
explicitly overridden. This is because, when generating abicalls, the choice of relocation
depends on whether a symbol is local or global. In some rare cases, GCC will not be able to
decide this until the whole compilation unit has been read.
|
-mrnames
-mno-rnames
Generate (do not generate) code that refers to registers using their software names. The default is
-mno-rnames, which tells GCC to use hardware names like $4 instead of software names like a0. The
only assembler known to support -rnames is the Algorithmics assembler.
|
-mcheck-zero-division
-mno-check-zero-division
Trap (do not trap) on integer division by zero. The default is -mcheck-zero-division.
|
-mmemcpy
-mno-memcpy
Force (do not force) the use of "memcpy()" for non-trivial block moves. The default is -mno-memcpy,
which allows GCC to inline most constant-sized copies.
|
-mlong-calls
-mno-long-calls
Disable (do not disable) use of the "jal" instruction. Calling functions using "jal" is more
efficient but requires the caller and callee to be in the same 256 megabyte segment.
This option has no effect on abicalls code. The default is -mno-long-calls.
|
-mmad
-mno-mad
Enable (disable) use of the "mad", "madu" and "mul" instructions, as provided by the R4650 ISA.
|
-mfused-madd
-mno-fused-madd
Enable (disable) use of the floating point multiply-accumulate instructions, when they are available.
The default is -mfused-madd.
When multiply-accumulate instructions are used, the intermediate product is calculated to infinite
precision and is not subject to the FCSR Flush to Zero bit. This may be undesirable in some
circumstances.
|
-nocpp
Tell the MIPS assembler to not run its preprocessor over user assembler files (with a .s suffix) when
assembling them.
|
-mfix-sb1
-mno-fix-sb1
Work around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2
``F1'' and ``F2'' floating point errata.)
|
-mflush-func=func
-mno-flush-func
Specifies the function to call to flush the I and D caches, or to not call any such function. If
called, the function must take the same arguments as the common "_flush_func()", that is, the address
of the memory range for which the cache is being flushed, the size of the memory range, and the
number 3 (to flush both caches). The default depends on the target GCC was configured for, but
commonly is either _flush_func or __cpu_flush.
|
-mbranch-likely
-mno-branch-likely
Enable or disable use of Branch Likely instructions, regardless of the default for the selected
architecture. By default, Branch Likely instructions may be generated if they are supported by the
selected architecture. An exception is for the MIPS32 and MIPS64 architectures and processors which
implement those architectures; for those, Branch Likely instructions will not be generated by default
because the MIPS32 and MIPS64 architectures specifically deprecate their use.
Intel 386 and AMD x86-64 Options
These -m options are defined for the i386 and x86-64 family of computers:
|
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of
available instructions. The choices for cpu-type are:
|
-march=cpu-type
Generate instructions for the machine type cpu-type. The choices for cpu-type are the same as for
-mtune. Moreover, specifying -march=cpu-type implies -mtune=cpu-type.
|
-mcpu=cpu-type
A deprecated synonym for -mtune.
|
-m386
-m486
-mpentium
-mpentiumpro
These options are synonyms for -mtune=i386, -mtune=i486, -mtune=pentium, and -mtune=pentiumpro
respectively. These synonyms are deprecated.
|
-mfpmath=unit
Generate floating point arithmetics for selected unit unit. The choices for unit are:
|
-masm=dialect
Output asm instructions using selected dialect. Supported choices are intel or att (the default one).
|
-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating point comparisons. These handle correctly the
case where the result of a comparison is unordered.
|
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are
not part of GCC. Normally the facilities of the machine's usual C compiler are used, but this can't
be done directly in cross-compilation. You must make your own arrangements to provide suitable
library functions for cross-compilation.
On machines where a function returns floating point results in the 80387 register stack, some
floating point opcodes may be emitted even if -msoft-float is used.
|
-mno-fp-ret-in-387
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types "float" and "double" in an FPU
register, even if there is no FPU. The idea is that the operating system should emulate an FPU.
The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead.
|
-mno-fancy-math-387
Some 387 emulators do not support the "sin", "cos" and "sqrt" instructions for the 387. Specify this
option to avoid generating those instructions. This option is the default on FreeBSD, OpenBSD and
NetBSD. This option is overridden when -march indicates that the target cpu will always have an FPU
and so the instruction will not need emulation. As of revision 2.6.1, these instructions are not
generated unless you also use the -funsafe-math-optimizations switch.
|
-malign-double
-mno-align-double
Control whether GCC aligns "double", "long double", and "long long" variables on a two word boundary
or a one word boundary. Aligning "double" variables on a two word boundary will produce code that
runs somewhat faster on a Pentium at the expense of more memory.
Warning: if you use the -malign-double switch, structures containing the above types will be aligned
differently than the published application binary interface specifications for the 386 and will not
be binary compatible with structures in code compiled without that switch.
|
-m96bit-long-double
-m128bit-long-double
These switches control the size of "long double" type. The i386 application binary interface
specifies the size to be 96 bits, so -m96bit-long-double is the default in 32 bit mode.
Modern architectures (Pentium and newer) would prefer "long double" to be aligned to an 8 or 16 byte
boundary. In arrays or structures conforming to the ABI, this would not be possible. So specifying
a -m128bit-long-double will align "long double" to a 16 byte boundary by padding the "long double"
with an additional 32 bit zero.
In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that "long
double" is to be aligned on 16 byte boundary.
Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for
a "long double".
Warning: if you override the default value for your target ABI, the structures and arrays containing
"long double" variables will change their size as well as function calling convention for function
taking "long double" will be modified. Hence they will not be binary compatible with arrays or
structures in code compiled without that switch.
|
-msvr3-shlib
-mno-svr3-shlib
Control whether GCC places uninitialized local variables into the "bss" or "data" segments.
-msvr3-shlib places them into "bss". These options are meaningful only on System V Release 3.
|
-mrtd
Use a different function-calling convention, in which functions that take a fixed number of arguments
return with the "ret" num instruction, which pops their arguments while returning. This saves one
instruction in the caller since there is no need to pop the arguments there.
You can specify that an individual function is called with this calling sequence with the function
attribute stdcall. You can also override the -mrtd option by using the function attribute cdecl.
Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot
use it if you need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of arguments
(including "printf"); otherwise incorrect code will be generated for calls to those functions.
In addition, seriously incorrect code will result if you call a function with too many arguments.
(Normally, extra arguments are harmlessly ignored.)
|
-mregparm=num
Control how many registers are used to pass integer arguments. By default, no registers are used to
pass arguments, and at most 3 registers can be used. You can control this behavior for a specific
function by using the function attribute regparm.
Warning: if you use this switch, and num is nonzero, then you must build all modules with the same
value, including any libraries. This includes the system libraries and startup modules.
|
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If
-mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits), except when
optimizing for code size (-Os), in which case the default is the minimum correct alignment (4 bytes
for x86, and 8 bytes for x86-64).
On Pentium and PentiumPro, "double" and "long double" values should be aligned to an 8 byte boundary
(see -malign-double) or suffer significant run time performance penalties. On Pentium III, the
Streaming SIMD Extension (SSE) data type "__m128" suffers similar penalties if it is not 16 byte
aligned.
To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that
required by any value stored on the stack. Further, every function must be generated such that it
keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary
from a function compiled with a lower preferred stack boundary will most likely misalign the stack.
It is recommended that libraries that use callbacks always use the default setting.
This extra alignment does consume extra stack space, and generally increases code size. Code that is
sensitive to stack space usage, such as embedded systems and operating system kernels, may want to
reduce the preferred alignment to -mpreferred-stack-boundary=2.
|
-mmmx
-mno-mmx
-msse
-mno-sse
-msse2
-mno-sse2
-msse3
-mno-sse3
-m3dnow
-mno-3dnow
These switches enable or disable the use of built-in functions that allow direct access to the MMX,
SSE, SSE2, SSE3 and 3Dnow extensions of the instruction set.
To have SSE/SSE2 instructions generated automatically from floating-point code, see -mfpmath=sse.
|
-mpush-args
-mno-push-args
Use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as
method using SUB/MOV operations and is enabled by default. In some cases disabling it may improve
performance because of improved scheduling and reduced dependencies.
|
-maccumulate-outgoing-args
If enabled, the maximum amount of space required for outgoing arguments will be computed in the
function prologue. This is faster on most modern CPUs because of reduced dependencies, improved
scheduling and reduced stack usage when preferred stack boundary is not equal to 2. The drawback is
a notable increase in code size. This switch implies -mno-push-args.
|
-mthreads
Support thread-safe exception handling on Mingw32. Code that relies on thread-safe exception
handling must compile and link all code with the -mthreads option. When compiling, -mthreads defines
-D_MT; when linking, it links in a special thread helper library -lmingwthrd which cleans up per
thread exception handling data.
|
-mno-align-stringops
Do not align destination of inlined string operations. This switch reduces code size and improves
performance in case the destination is already aligned, but GCC doesn't know about it.
|
-minline-all-stringops
By default GCC inlines string operations only when destination is known to be aligned at least to 4
byte boundary. This enables more inlining, increase code size, but may improve performance of code
that depends on fast memcpy, strlen and memset for short lengths.
|
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save,
set up and restore frame pointers and makes an extra register available in leaf functions. The
option -fomit-frame-pointer removes the frame pointer for all functions which might make debugging
harder.
|
-mtls-direct-seg-refs
-mno-tls-direct-seg-refs
Controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for
32-bit, %fs for 64-bit), or whether the thread base pointer must be added. Whether or not this is
legal depends on the operating system, and whether it maps the segment to cover the entire TLS area.
For systems that use GNU libc, the default is on.
These -m switches are supported in addition to the above on AMD x86-64 processors in 64-bit environments.
|
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer
to 32 bits and generates code that runs on any i386 system. The 64-bit environment sets int to 32
bits and long and pointer to 64 bits and generates code for AMD's x86-64 architecture.
|
-mno-red-zone
Do not use a so called red zone for x86-64 code. The red zone is mandated by the x86-64 ABI, it is a
128-byte area beyond the location of the stack pointer that will not be modified by signal or
interrupt handlers and therefore can be used for temporary data without adjusting the stack pointer.
The flag -mno-red-zone disables this red zone.
|
-mcmodel=small
Generate code for the small code model: the program and its symbols must be linked in the lower 2 GB
of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked. This
is the default code model.
|
-mcmodel=kernel
Generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space.
This model has to be used for Linux kernel code.
|
-mcmodel=medium
Generate code for the medium model: The program is linked in the lower 2 GB of the address space but
symbols can be located anywhere in the address space. Programs can be statically or dynamically
linked, but building of shared libraries are not supported with the medium model.
|
-mcmodel=large
Generate code for the large model: This model makes no assumptions about addresses and sizes of
sections. Currently GCC does not implement this model.
HPPA Options
These -m options are defined for the HPPA family of computers:
|
-march=architecture-type
Generate code for the specified architecture. The choices for architecture-type are 1.0 for PA 1.0,
1.1 for PA 1.1, and 2.0 for PA 2.0 processors. Refer to /usr/lib/sched.models on an HP-UX system to
determine the proper architecture option for your machine. Code compiled for lower numbered
architectures will run on higher numbered architectures, but not the other way around.
PA 2.0 support currently requires gas snapshot 19990413 or later. The next release of binutils
(current is 2.9.1) will probably contain PA 2.0 support.
|
-mpa-risc-1-0
-mpa-risc-1-1
-mpa-risc-2-0
Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.
|
-mbig-switch
Generate code suitable for big switch tables. Use this option only if the assembler/linker complain
about out of range branches within a switch table.
|
-mjump-in-delay
Fill delay slots of function calls with unconditional jump instructions by modifying the return
pointer for the function call to be the target of the conditional jump.
|
-mdisable-fpregs
Prevent floating point registers from being used in any manner. This is necessary for compiling
kernels which perform lazy context switching of floating point registers. If you use this option and
attempt to perform floating point operations, the compiler will abort.
|
-mdisable-indexing
Prevent the compiler from using indexing address modes. This avoids some rather obscure problems
when compiling MIG generated code under MACH.
|
-mno-space-regs
Generate code that assumes the target has no space registers. This allows GCC to generate faster
indirect calls and use unscaled index address modes.
Such code is suitable for level 0 PA systems and kernels.
|
-mfast-indirect-calls
Generate code that assumes calls never cross space boundaries. This allows GCC to emit code which
performs faster indirect calls.
This option will not work in the presence of shared libraries or nested functions.
|
-mlong-load-store
Generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This
is equivalent to the +k option to the HP compilers.
|
-mportable-runtime
Use the portable calling conventions proposed by HP for ELF systems.
|
-mgas
Enable the use of assembler directives only GAS understands.
|
-mschedule=cpu-type
Schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type
are 700 7100, 7100LC, 7200, 7300 and 8000. Refer to /usr/lib/sched.models on an HP-UX system to
determine the proper scheduling option for your machine. The default scheduling is 8000.
|
-mlinker-opt
Enable the optimization pass in the HP-UX linker. Note this makes symbolic debugging impossible. It
also triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when
linking some programs.
|
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are
not available for all HPPA targets. Normally the facilities of the machine's usual C compiler are
used, but this cannot be done directly in cross-compilation. You must make your own arrangements to
provide suitable library functions for cross-compilation. The embedded target hppa1.1-*-pro does
provide software floating point support.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you
compile all of a program with this option. In particular, you need to compile libgcc.a, the library
that comes with GCC, with -msoft-float in order for this to work.
|
-msio
Generate the predefine, "_SIO", for server IO. The default is -mwsio. This generates the
predefines, "__hp9000s700", "__hp9000s700__" and "_WSIO", for workstation IO. These options are
available under HP-UX and HI-UX.
|
-mgnu-ld
Use GNU ld specific options. This passes -shared to ld when building a shared library. It is the
default when GCC is configured, explicitly or implicitly, with the GNU linker. This option does not
have any affect on which ld is called, it only changes what parameters are passed to that ld. The ld
that is called is determined by the --with-ld configure option, GCC's program search path, and
finally by the user's PATH. The linker used by GCC can be printed using which `gcc
-print-prog-name=ld`. This option is only available on the 64 bit HP-UX GCC, i.e. configured with
hppa*64*-*-hpux*.
|
-mhp-ld
Use HP ld specific options. This passes -b to ld when building a shared library and passes +Accept
TypeMismatch to ld on all links. It is the default when GCC is configured, explicitly or implicitly,
with the HP linker. This option does not have any affect on which ld is called, it only changes what
parameters are passed to that ld. The ld that is called is determined by the --with-ld configure
option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be
printed using which `gcc -print-prog-name=ld`. This option is only available on the 64 bit HP-UX
GCC, i.e. configured with hppa*64*-*-hpux*.
|
-mlong-calls
Generate code that uses long call sequences. This ensures that a call is always able to reach linker
generated stubs. The default is to generate long calls only when the distance from the call site to
the beginning of the function or translation unit, as the case may be, exceeds a predefined limit set
by the branch type being used. The limits for normal calls are 7,600,000 and 240,000 bytes,
respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes.
Distances are measured from the beginning of functions when using the -ffunction-sections option, or
when using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.
It is normally not desirable to use this option as it will degrade performance. However, it may be
useful in large applications, particularly when partial linking is used to build the application.
The types of long calls used depends on the capabilities of the assembler and linker, and the type of
code being generated. The impact on systems that support long absolute calls, and long pic symbol-
difference or pc-relative calls should be relatively small. However, an indirect call is used on
32-bit ELF systems in pic code and it is quite long.
|
-nolibdld
Suppress the generation of link options to search libdld.sl when the -static option is specified on
HP-UX 10 and later.
|
-static
The HP-UX implementation of setlocale in libc has a dependency on libdld.sl. There isn't an archive
version of libdld.sl. Thus, when the -static option is specified, special link options are needed to
resolve this dependency.
On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when the
-static option is specified. This causes the resulting binary to be dynamic. On the 64-bit port,
the linkers generate dynamic binaries by default in any case. The -nolibdld option can be used to
prevent the GCC driver from adding these link options.
|
-threads
Add support for multithreading with the dce thread library under HP-UX. This option sets flags for
both the preprocessor and linker.
Intel 960 Options
These -m options are defined for the Intel 960 implementations:
|
-mcpu-type
Assume the defaults for the machine type cpu-type for some of the other options, including
instruction scheduling, floating point support, and addressing modes. The choices for cpu-type are
ka, kb, mc, ca, cf, sa, and sb. The default is kb.
|
-mnumerics
-msoft-float
The -mnumerics option indicates that the processor does support floating-point instructions. The
-msoft-float option indicates that floating-point support should not be assumed.
|
-mleaf-procedures
-mno-leaf-procedures
Do (or do not) attempt to alter leaf procedures to be callable with the "bal" instruction as well as
"call". This will result in more efficient code for explicit calls when the "bal" instruction can be
substituted by the assembler or linker, but less efficient code in other cases, such as calls via
function pointers, or using a linker that doesn't support this optimization.
|
-mtail-call
-mno-tail-call
Do (or do not) make additional attempts (beyond those of the machine-independent portions of the
compiler) to optimize tail-recursive calls into branches. You may not want to do this because the
detection of cases where this is not valid is not totally complete. The default is -mno-tail-call.
|
-mcomplex-addr
-mno-complex-addr
Assume (or do not assume) that the use of a complex addressing mode is a win on this implementation
of the i960. Complex addressing modes may not be worthwhile on the K-series, but they definitely are
on the C-series. The default is currently -mcomplex-addr for all processors except the CB and CC.
|
-mcode-align
-mno-code-align
Align code to 8-byte boundaries for faster fetching (or don't bother). Currently turned on by
default for C-series implementations only.
|
-mic-compat
-mic2.0-compat
-mic3.0-compat
Enable compatibility with iC960 v2.0 or v3.0.
|
-masm-compat
-mintel-asm
Enable compatibility with the iC960 assembler.
|
-mstrict-align
-mno-strict-align
Do not permit (do permit) unaligned accesses.
|
-mold-align
Enable structure-alignment compatibility with Intel's gcc release version 1.3 (based on gcc 1.37).
This option implies -mstrict-align.
|
-mlong-double-64
Implement type long double as 64-bit floating point numbers. Without the option long double is
implemented by 80-bit floating point numbers. The only reason we have it because there is no 128-bit
long double support in fp-bit.c yet. So it is only useful for people using soft-float targets.
Otherwise, we should recommend against use of it.
DEC Alpha Options
These -m options are defined for the DEC Alpha implementations:
|
-mno-soft-float
-msoft-float
Use (do not use) the hardware floating-point instructions for floating-point operations. When
-msoft-float is specified, functions in libgcc.a will be used to perform floating-point operations.
Unless they are replaced by routines that emulate the floating-point operations, or compiled in such
a way as to call such emulations routines, these routines will issue floating-point operations. If
you are compiling for an Alpha without floating-point operations, you must ensure that the library is
built so as not to call them.
Note that Alpha implementations without floating-point operations are required to have floating-point
registers.
|
-mfp-reg
-mno-fp-regs
Generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies
-msoft-float. If the floating-point register set is not used, floating point operands are passed in
integer registers as if they were integers and floating-point results are passed in $0 instead of
$f0. This is a non-standard calling sequence, so any function with a floating-point argument or
return value called by code compiled with -mno-fp-regs must also be compiled with that option.
A typical use of this option is building a kernel that does not use, and hence need not save and
restore, any floating-point registers.
|
-mieee
The Alpha architecture implements floating-point hardware optimized for maximum performance. It is
mostly compliant with the IEEE floating point standard. However, for full compliance, software
assistance is required. This option generates code fully IEEE compliant code except that the
inexact-flag is not maintained (see below). If this option is turned on, the preprocessor macro
"_IEEE_FP" is defined during compilation. The resulting code is less efficient but is able to
correctly support denormalized numbers and exceptional IEEE values such as not-a-number and
plus/minus infinity. Other Alpha compilers call this option -ieee_with_no_inexact.
DEBIAN SPECIFIC: This option is on by default, unless -ffinite-math-only (which is part of the
-ffast-math set) is specified, because the software functions in the GNU libc math libraries generate
denormalized numbers, NaNs, and infs (all of which will cause a programs to SIGFPE when it attempts
to use the results without -mieee).
|
-mieee-with-inexact
This is like -mieee except the generated code also maintains the IEEE inexact-flag. Turning on this
option causes the generated code to implement fully-compliant IEEE math. In addition to "_IEEE_FP",
"_IEEE_FP_EXACT" is defined as a preprocessor macro. On some Alpha implementations the resulting
code may execute significantly slower than the code generated by default. Since there is very little
code that depends on the inexact-flag, you should normally not specify this option. Other Alpha
compilers call this option -ieee_with_inexact.
|
-mfp-trap-mode=trap-mode
This option controls what floating-point related traps are enabled. Other Alpha compilers call this
option -fptm trap-mode. The trap mode can be set to one of four values:
n This is the default (normal) setting. The only traps that are enabled are the ones that cannot
be disabled in software (e.g., division by zero trap).
u In addition to the traps enabled by n, underflow traps are enabled as well.
su Like su, but the instructions are marked to be safe for software completion (see Alpha
architecture manual for details).
sui Like su, but inexact traps are enabled as well.
|
-mfp-rounding-mode=rounding-mode
Selects the IEEE rounding mode. Other Alpha compilers call this option -fprm rounding-mode. The
rounding-mode can be one of:
n Normal IEEE rounding mode. Floating point numbers are rounded towards the nearest machine number
or towards the even machine number in case of a tie.
m Round towards minus infinity.
c Chopped rounding mode. Floating point numbers are rounded towards zero.
d Dynamic rounding mode. A field in the floating point control register (fpcr, see Alpha
architecture reference manual) controls the rounding mode in effect. The C library initializes
this register for rounding towards plus infinity. Thus, unless your program modifies the fpcr, d
corresponds to round towards plus infinity.
|
-mtrap-precision=trap-precision
In the Alpha architecture, floating point traps are imprecise. This means without software
assistance it is impossible to recover from a floating trap and program execution normally needs to
be terminated. GCC can generate code that can assist operating system trap handlers in determining
the exact location that caused a floating point trap. Depending on the requirements of an
application, different levels of precisions can be selected:
p Program precision. This option is the default and means a trap handler can only identify which
program caused a floating point exception.
f Function precision. The trap handler can determine the function that caused a floating point
exception.
i Instruction precision. The trap handler can determine the exact instruction that caused a
floating point exception.
Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.
|
-mieee-conformant
This option marks the generated code as IEEE conformant. You must not use this option unless you
also specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect
is to emit the line .eflag 48 in the function prologue of the generated assembly file. Under DEC
Unix, this has the effect that IEEE-conformant math library routines will be linked in.
|
-mbuild-constants
Normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller
constants in two or three instructions. If it cannot, it will output the constant as a literal and
generate code to load it from the data segment at runtime.
Use this option to require GCC to construct all integer constants using code, even if it takes more
instructions (the maximum is six).
You would typically use this option to build a shared library dynamic loader. Itself a shared
library, it must relocate itself in memory before it can find the variables and constants in its own
data segment.
|
-malpha-as
-mgas
Select whether to generate code to be assembled by the vendor-supplied assembler (-malpha-as) or by
the GNU assembler -mgas.
|
-mbwx
-mno-bwx
-mcix
-mno-cix
-mfix
-mno-fix
-mmax
-mno-max
Indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets.
The default is to use the instruction sets supported by the CPU type specified via -mcpu= option or
that of the CPU on which GCC was built if none was specified.
|
-mfloat-vax
-mfloat-ieee
Generate code that uses (does not use) VAX F and G floating point arithmetic instead of IEEE single
and double precision.
|
-mexplicit-relocs
-mno-explicit-relocs
Older Alpha assemblers provided no way to generate symbol relocations except via assembler macros.
Use of these macros does not allow optimal instruction scheduling. GNU binutils as of version 2.12
supports a new syntax that allows the compiler to explicitly mark which relocations should apply to
which instructions. This option is mostly useful for debugging, as GCC detects the capabilities of
the assembler when it is built and sets the default accordingly.
|
-msmall-data
-mlarge-data
When -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations. When
-msmall-data is used, objects 8 bytes long or smaller are placed in a small data area (the ".sdata"
and ".sbss" sections) and are accessed via 16-bit relocations off of the $gp register. This limits
the size of the small data area to 64KB, but allows the variables to be directly accessed via a
single instruction.
The default is -mlarge-data. With this option the data area is limited to just below 2GB. Programs
that require more than 2GB of data must use "malloc" or "mmap" to allocate the data in the heap
instead of in the program's data segment.
When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.
|
-msmall-text
-mlarge-text
When -msmall-text is used, the compiler assumes that the code of the entire program (or shared
library) fits in 4MB, and is thus reachable with a branch instruction. When -msmall-data is used,
the compiler can assume that all local symbols share the same $gp value, and thus reduce the number
of instructions required for a function call from 4 to 1.
The default is -mlarge-text.
|
-mmemory-latency=time
Sets the latency the scheduler should assume for typical memory references as seen by the
application. This number is highly dependent on the memory access patterns used by the application
and the size of the external cache on the machine.
|
-mvms-return-codes
Return VMS condition codes from main. The default is to return POSIX style condition (e.g. error)
codes.
H8/300 Options
These -m options are defined for the H8/300 implementations:
|
-mrelax
Shorten some address references at link time, when possible; uses the linker option -relax.
|
-mh Generate code for the H8/300H.
|
-ms Generate code for the H8S.
-mn Generate code for the H8S and H8/300H in the normal mode. This switch must be used either with -mh
or -ms.
|
-ms2600
Generate code for the H8S/2600. This switch must be used with -ms.
|
-mint32
Make "int" data 32 bits by default.
|
-malign-300
On the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H
and H8S is to align longs and floats on 4 byte boundaries. -malign-300 causes them to be aligned on
2 byte boundaries. This option has no effect on the H8/300.
SH Options
These -m options are defined for the SH implementations:
|
-m1 Generate code for the SH1.
|
-m2 Generate code for the SH2.
|
-m2e
Generate code for the SH2e.
|
-m3 Generate code for the SH3.
|
-m3e
Generate code for the SH3e.
|
-m4-nofpu
Generate code for the SH4 without a floating-point unit.
|
-m4-single-only
Generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.
|
-m4-single
Generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
|
-m4 Generate code for the SH4.
-mb Compile code for the processor in big endian mode.
-ml Compile code for the processor in little endian mode.
|
-mdalign
Align doubles at 64-bit boundaries. Note that this changes the calling conventions, and thus some
functions from the standard C library will not work unless you recompile it first with -mdalign.
|
-mrelax
Shorten some address references at link time, when possible; uses the linker option -relax.
|
-mbigtable
Use 32-bit offsets in "switch" tables. The default is to use 16-bit offsets.
|
-mfmovd
Enable the use of the instruction "fmovd".
|
-mhitachi
Comply with the calling conventions defined by Renesas.
|
-mnomacsave
Mark the "MAC" register as call-clobbered, even if -mhitachi is given.
|
-mieee
Increase IEEE-compliance of floating-point code.
|
-misize
Dump instruction size and location in the assembly code.
|
-mpadstruct
This option is deprecated. It pads structures to multiple of 4 bytes, which is incompatible with the
SH ABI.
|
-mspace
Optimize for space instead of speed. Implied by -Os.
|
-mprefergot
When generating position-independent code, emit function calls using the Global Offset Table instead
of the Procedure Linkage Table.
|
-musermode
Generate a library function call to invalidate instruction cache entries, after fixing up a
trampoline. This library function call doesn't assume it can write to the whole memory address
space. This is the default when the target is "sh-*-linux*".
Options for System V
These additional options are available on System V Release 4 for compatibility with other compilers on
those systems:
|
-G Create a shared object. It is recommended that -symbolic or -shared be used instead.
-Qy Identify the versions of each tool used by the compiler, in a ".ident" assembler directive in the
output.
-Qn Refrain from adding ".ident" directives to the output file (this is the default).
|
-YP,dirs
Search the directories dirs, and no others, for libraries specified with -l.
|
-Ym,dir
Look in the directory dir to find the M4 preprocessor. The assembler uses this option.
TMS320C3x/C4x Options
These -m options are defined for TMS320C3x/C4x implementations:
|
-mbig-memory
-mbig
-msmall-memory
-msmall
Generates code for the big or small memory model. The small memory model assumed that all data fits
into one 64K word page. At run-time the data page (DP) register must be set to point to the 64K page
containing the .bss and .data program sections. The big memory model is the default and requires
reloading of the DP register for every direct memory access.
|
-mbk
-mno-bk
Allow (disallow) allocation of general integer operands into the block count register BK.
|
-mdb
-mno-db
Enable (disable) generation of code using decrement and branch, DBcond(D), instructions. This is
enabled by default for the C4x. To be on the safe side, this is disabled for the C3x, since the
maximum iteration count on the C3x is 2^{23 + 1} (but who iterates loops more than 2^{23} times on
the C3x?). Note that GCC will try to reverse a loop so that it can utilize the decrement and branch
instruction, but will give up if there is more than one memory reference in the loop. Thus a loop
where the loop counter is decremented can generate slightly more efficient code, in cases where the
RPTB instruction cannot be utilized.
|
-mdp-isr-reload
-mparanoid
Force the DP register to be saved on entry to an interrupt service routine (ISR), reloaded to point
to the data section, and restored on exit from the ISR. This should not be required unless someone
has violated the small memory model by modifying the DP register, say within an object library.
|
-mmpyi
-mno-mpyi
For the C3x use the 24-bit MPYI instruction for integer multiplies instead of a library call to
guarantee 32-bit results. Note that if one of the operands is a constant, then the multiplication
will be performed using shifts and adds. If the -mmpyi option is not specified for the C3x, then
squaring operations are performed inline instead of a library call.
|
-mfast-fix
-mno-fast-fix
The C3x/C4x FIX instruction to convert a floating point value to an integer value chooses the nearest
integer less than or equal to the floating point value rather than to the nearest integer. Thus if
the floating point number is negative, the result will be incorrectly truncated an additional code is
necessary to detect and correct this case. This option can be used to disable generation of the
additional code required to correct the result.
|
-mrptb
-mno-rptb
Enable (disable) generation of repeat block sequences using the RPTB instruction for zero overhead
looping. The RPTB construct is only used for innermost loops that do not call functions or jump
across the loop boundaries. There is no advantage having nested RPTB loops due to the overhead
required to save and restore the RC, RS, and RE registers. This is enabled by default with -O2.
|
-mrpts=count
-mno-rpts
Enable (disable) the use of the single instruction repeat instruction RPTS. If a repeat block
contains a single instruction, and the loop count can be guaranteed to be less than the value count,
GCC will emit a RPTS instruction instead of a RPTB. If no value is specified, then a RPTS will be
emitted even if the loop count cannot be determined at compile time. Note that the repeated
instruction following RPTS does not have to be reloaded from memory each iteration, thus freeing up
the CPU buses for operands. However, since interrupts are blocked by this instruction, it is
disabled by default.
|
-mloop-unsigned
-mno-loop-unsigned
The maximum iteration count when using RPTS and RPTB (and DB on the C40) is 2^{31 + 1} since these
instructions test if the iteration count is negative to terminate the loop. If the iteration count
is unsigned there is a possibility than the 2^{31 + 1} maximum iteration count may be exceeded. This
switch allows an unsigned iteration count.
|
-mti
Try to emit an assembler syntax that the TI assembler (asm30) is happy with. This also enforces
compatibility with the API employed by the TI C3x C compiler. For example, long doubles are passed
as structures rather than in floating point registers.
|
-mregparm
-mmemparm
Generate code that uses registers (stack) for passing arguments to functions. By default, arguments
are passed in registers where possible rather than by pushing arguments on to the stack.
|
-mparallel-insns
-mno-parallel-insns
Allow the generation of parallel instructions. This is enabled by default with -O2.
|
-mparallel-mpy
-mno-parallel-mpy
Allow the generation of MPY||ADD and MPY||SUB parallel instructions, provided -mparallel-insns is
also specified. These instructions have tight register constraints which can pessimize the code
generation of large functions.
V850 Options
These -m options are defined for V850 implementations:
|
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are assumed to be far away, the compiler will
always load the functions address up into a register, and call indirect through the pointer.
|
-mno-ep
-mep
Do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy
pointer into the "ep" register, and use the shorter "sld" and "sst" instructions. The -mep option is
on by default if you optimize.
|
-mno-prolog-function
-mprolog-function
Do not use (do use) external functions to save and restore registers at the prologue and epilogue of
a function. The external functions are slower, but use less code space if more than one function
saves the same number of registers. The -mprolog-function option is on by default if you optimize.
|
-mspace
Try to make the code as small as possible. At present, this just turns on the -mep and
-mprolog-function options.
|
-mtda=n
Put static or global variables whose size is n bytes or less into the tiny data area that register
"ep" points to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte
references).
|
-msda=n
Put static or global variables whose size is n bytes or less into the small data area that register
"gp" points to. The small data area can hold up to 64 kilobytes.
|
-mzda=n
Put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory.
|
-mv850
Specify that the target processor is the V850.
|
-mbig-switch
Generate code suitable for big switch tables. Use this option only if the assembler/linker complain
about out of range branches within a switch table.
|
-mapp-regs
This option will cause r2 and r5 to be used in the code generated by the compiler. This setting is
the default.
|
-mno-app-regs
This option will cause r2 and r5 to be treated as fixed registers.
|
-mv850e1
Specify that the target processor is the V850E1. The preprocessor constants __v850e1__ and __v850e__
will be defined if this option is used.
|
-mv850e
Specify that the target processor is the V850E. The preprocessor constant __v850e__ will be defined
if this option is used.
If neither -mv850 nor -mv850e nor -mv850e1 are defined then a default target processor will be chosen
and the relevant __v850*__ preprocessor constant will be defined.
The preprocessor constants __v850 and __v851__ are always defined, regardless of which processor
variant is the target.
|
-mdisable-callt
This option will suppress generation of the CALLT instruction for the v850e and v850e1 flavors of the
v850 architecture. The default is -mno-disable-callt which allows the CALLT instruction to be used.
ARC Options
These options are defined for ARC implementations:
|
-EL Compile code for little endian mode. This is the default.
-EB Compile code for big endian mode.
|
-mmangle-cpu
Prepend the name of the cpu to all public symbol names. In multiple-processor systems, there are
many ARC variants with different instruction and register set characteristics. This flag prevents
code compiled for one cpu to be linked with code compiled for another. No facility exists for
handling variants that are ``almost identical''. This is an all or nothing option.
|
-mcpu=cpu
Compile code for ARC variant cpu. Which variants are supported depend on the configuration. All
variants support -mcpu=base, this is the default.
|
-mtext=text-section
-mdata=data-section
-mrodata=readonly-data-section
Put functions, data, and readonly data in text-section, data-section, and readonly-data-section
respectively by default. This can be overridden with the "section" attribute.
NS32K Options
These are the -m options defined for the 32000 series. The default values for these options depends on
which style of 32000 was selected when the compiler was configured; the defaults for the most common
choices are given below.
|
-m32032
-m32032
Generate output for a 32032. This is the default when the compiler is configured for 32032 and 32016
based systems.
|
-m32332
-m32332
Generate output for a 32332. This is the default when the compiler is configured for 32332-based
systems.
|
-m32532
-m32532
Generate output for a 32532. This is the default when the compiler is configured for 32532-based
systems.
|
-m32081
Generate output containing 32081 instructions for floating point. This is the default for all
systems.
|
-m32381
Generate output containing 32381 instructions for floating point. This also implies -m32081. The
32381 is only compatible with the 32332 and 32532 cpus. This is the default for the pc532-netbsd
configuration.
|
-mmulti-add
Try and generate multiply-add floating point instructions "polyF" and "dotF". This option is only
available if the -m32381 option is in effect. Using these instructions requires changes to register
allocation which generally has a negative impact on performance. This option should only be enabled
when compiling code particularly likely to make heavy use of multiply-add instructions.
|
-mnomulti-add
Do not try and generate multiply-add floating point instructions "polyF" and "dotF". This is the
default on all platforms.
|
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries may
not be available.
|
-mieee-compare
-mno-ieee-compare
Control whether or not the compiler uses IEEE floating point comparisons. These handle correctly the
case where the result of a comparison is unordered. Warning: the requisite kernel support may not be
available.
|
-mnobitfield
Do not use the bit-field instructions. On some machines it is faster to use shifting and masking
operations. This is the default for the pc532.
|
-mbitfield
Do use the bit-field instructions. This is the default for all platforms except the pc532.
|
-mrtd
Use a different function-calling convention, in which functions that take a fixed number of arguments
return pop their arguments on return with the "ret" instruction.
This calling convention is incompatible with the one normally used on Unix, so you cannot use it if
you need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of arguments
(including "printf"); otherwise incorrect code will be generated for calls to those functions.
In addition, seriously incorrect code will result if you call a function with too many arguments.
(Normally, extra arguments are harmlessly ignored.)
This option takes its name from the 680x0 "rtd" instruction.
|
-mregparam
Use a different function-calling convention where the first two arguments are passed in registers.
This calling convention is incompatible with the one normally used on Unix, so you cannot use it if
you need to call libraries compiled with the Unix compiler.
|
-mnoregparam
Do not pass any arguments in registers. This is the default for all targets.
|
-msb
It is OK to use the sb as an index register which is always loaded with zero. This is the default
for the pc532-netbsd target.
|
-mnosb
The sb register is not available for use or has not been initialized to zero by the run time system.
This is the default for all targets except the pc532-netbsd. It is also implied whenever -mhimem or
-fpic is set.
|
-mhimem
Many ns32000 series addressing modes use displacements of up to 512MB. If an address is above 512MB
then displacements from zero can not be used. This option causes code to be generated which can be
loaded above 512MB. This may be useful for operating systems or ROM code.
|
-mnohimem
Assume code will be loaded in the first 512MB of virtual address space. This is the default for all
platforms.
AVR Options
These options are defined for AVR implementations:
|
-mmcu=mcu
Specify ATMEL AVR instruction set or MCU type.
|
-msize
Output instruction sizes to the asm file.
|
-minit-stack=N
Specify the initial stack address, which may be a symbol or numeric value, __stack is the default.
|
-mno-interrupts
Generated code is not compatible with hardware interrupts. Code size will be smaller.
|
-mcall-prologues
Functions prologues/epilogues expanded as call to appropriate subroutines. Code size will be
smaller.
|
-mno-tablejump
Do not generate tablejump insns which sometimes increase code size.
|
-mtiny-stack
Change only the low 8 bits of the stack pointer.
MCore Options
These are the -m options defined for the Motorola M*Core processors.
|
-mhardlit
-mno-hardlit
Inline constants into the code stream if it can be done in two instructions or less.
|
-mdiv
-mno-div
Use the divide instruction. (Enabled by default).
|
-mrelax-immediate
-mno-relax-immediate
Allow arbitrary sized immediates in bit operations.
|
-mwide-bitfields
-mno-wide-bitfields
Always treat bit-fields as int-sized.
|
-m4byte-functions
-mno-4byte-functions
Force all functions to be aligned to a four byte boundary.
|
-mcallgraph-data
-mno-callgraph-data
Emit callgraph information.
|
-mslow-bytes
-mno-slow-bytes
Prefer word access when reading byte quantities.
|
-mlittle-endian
-mbig-endian
Generate code for a little endian target.
|
-m210
-m340
Generate code for the 210 processor.
IA-64 Options
These are the -m options defined for the Intel IA-64 architecture.
|
-mbig-endian
Generate code for a big endian target. This is the default for HP-UX.
|
-mlittle-endian
Generate code for a little endian target. This is the default for AIX5 and GNU/Linux.
|
-mgnu-as
-mno-gnu-as
Generate (or don't) code for the GNU assembler. This is the default.
|
-mgnu-ld
-mno-gnu-ld
Generate (or don't) code for the GNU linker. This is the default.
|
-mno-pic
Generate code that does not use a global pointer register. The result is not position independent
code, and violates the IA-64 ABI.
|
-mvolatile-asm-stop
-mno-volatile-asm-stop
Generate (or don't) a stop bit immediately before and after volatile asm statements.
|
-mb-step
Generate code that works around Itanium B step errata.
|
-mregister-names
-mno-register-names
Generate (or don't) in, loc, and out register names for the stacked registers. This may make
assembler output more readable.
|
-mno-sdata
-msdata
Disable (or enable) optimizations that use the small data section. This may be useful for working
around optimizer bugs.
|
-mconstant-gp
Generate code that uses a single constant global pointer value. This is useful when compiling kernel
code.
|
-mauto-pic
Generate code that is self-relocatable. This implies -mconstant-gp. This is useful when compiling
firmware code.
|
-minline-float-divide-min-latency
Generate code for inline divides of floating point values using the minimum latency algorithm.
|
-minline-float-divide-max-throughput
Generate code for inline divides of floating point values using the maximum throughput algorithm.
|
-minline-int-divide-min-latency
Generate code for inline divides of integer values using the minimum latency algorithm.
|
-minline-int-divide-max-throughput
Generate code for inline divides of integer values using the maximum throughput algorithm.
|
-minline-sqrt-min-latency
Generate code for inline square roots using the minimum latency algorithm.
|
-minline-sqrt-max-throughput
Generate code for inline square roots using the maximum throughput algorithm.
|
-mno-dwarf2-asm
-mdwarf2-asm
Don't (or do) generate assembler code for the DWARF2 line number debugging info. This may be useful
when not using the GNU assembler.
|
-mearly-stop-bits
-mno-early-stop-bits
Allow stop bits to be placed earlier than immediately preceding the instruction that triggered the
stop bit. This can improve instruction scheduling, but does not always do so.
|
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the
register allocator can not use. This is useful when compiling kernel code. A register range is
specified as two registers separated by a dash. Multiple register ranges can be specified separated
by a comma.
|
-mtls-size=tls-size
Specify bit size of immediate TLS offsets. Valid values are 14, 22, and 64.
|
-mtune=cpu-type
Tune the instruction scheduling for a particular CPU, Valid values are itanium, itanium1, merced,
itanium2, and mckinley.
|
-mt
-pthread
Add support for multithreading using the POSIX threads library. This option sets flags for both the
preprocessor and linker. It does not affect the thread safety of object code produced by the
compiler or that of libraries supplied with it. These are HP-UX specific flags.
|
-milp32
-mlp64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer
to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. These are
HP-UX specific flags.
D30V Options
These -m options are defined for D30V implementations:
|
-mextmem
Link the .text, .data, .bss, .strings, .rodata, .rodata1, .data1 sections into external memory, which
starts at location 0x80000000.
|
-mextmemory
Same as the -mextmem switch.
|
-monchip
Link the .text section into onchip text memory, which starts at location 0x0. Also link .data, .bss,
.strings, .rodata, .rodata1, .data1 sections into onchip data memory, which starts at location
0x20000000.
|
-mno-asm-optimize
-masm-optimize
Disable (enable) passing -O to the assembler when optimizing. The assembler uses the -O option to
automatically parallelize adjacent short instructions where possible.
|
-mbranch-cost=n
Increase the internal costs of branches to n. Higher costs means that the compiler will issue more
instructions to avoid doing a branch. The default is 2.
|
-mcond-exec=n
Specify the maximum number of conditionally executed instructions that replace a branch. The default
is 4.
S/390 and zSeries Options
These are the -m options defined for the S/390 and zSeries architecture.
|
-mhard-float
-msoft-float
Use (do not use) the hardware floating-point instructions and registers for floating-point
operations. When -msoft-float is specified, functions in libgcc.a will be used to perform floating-
point operations. When -mhard-float is specified, the compiler generates IEEE floating-point
instructions. This is the default.
|
-mbackchain
-mno-backchain
Generate (or do not generate) code which maintains an explicit backchain within the stack frame that
points to the caller's frame. This may be needed to allow debugging using tools that do not
understand DWARF-2 call frame information. The default is not to generate the backchain.
|
-msmall-exec
-mno-small-exec
Generate (or do not generate) code using the "bras" instruction to do subroutine calls. This only
works reliably if the total executable size does not exceed 64k. The default is to use the "basr"
instruction instead, which does not have this limitation.
|
-m64
-m31
When -m31 is specified, generate code compliant to the GNU/Linux for S/390 ABI. When -m64 is
specified, generate code compliant to the GNU/Linux for zSeries ABI. This allows GCC in particular
to generate 64-bit instructions. For the s390 targets, the default is -m31, while the s390x targets
default to -m64.
|
-mzarch
-mesa
When -mzarch is specified, generate code using the instructions available on z/Architecture. When
-mesa is specified, generate code using the instructions available on ESA/390. Note that -mesa is not
possible with -m64. When generating code compliant to the GNU/Linux for S/390 ABI, the default is
-mesa. When generating code compliant to the GNU/Linux for zSeries ABI, the default is -mzarch.
|
-mmvcle
-mno-mvcle
Generate (or do not generate) code using the "mvcle" instruction to perform block moves. When
-mno-mvcle is specified, use a "mvc" loop instead. This is the default.
|
-mdebug
-mno-debug
Print (or do not print) additional debug information when compiling. The default is to not print
debug information.
|
-march=cpu-type
Generate code that will run on cpu-type, which is the name of a system representing a certain
processor type. Possible values for cpu-type are g5, g6, z900, and z990. When generating code using
the instructions available on z/Architecture, the default is -march=z900. Otherwise, the default is
-march=g5.
|
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of
available instructions. The list of cpu-type values is the same as for -march. The default is the
value used for -march.
|
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating point multiply and accumulate instructions.
These instructions are generated by default if hardware floating point is used.
CRIS Options
These options are defined specifically for the CRIS ports.
|
-march=architecture-type
-mcpu=architecture-type
Generate code for the specified architecture. The choices for architecture-type are v3, v8 and v10
for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0 except for cris-axis-linux-gnu,
where the default is v10.
|
-mtune=architecture-type
Tune to architecture-type everything applicable about the generated code, except for the ABI and the
set of available instructions. The choices for architecture-type are the same as for
-march=architecture-type.
|
-mmax-stack-frame=n
Warn when the stack frame of a function exceeds n bytes.
|
-melinux-stacksize=n
Only available with the cris-axis-aout target. Arranges for indications in the program to the kernel
loader that the stack of the program should be set to n bytes.
|
-metrax4
-metrax100
The options -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8 respectively.
|
-mmul-bug-workaround
-mno-mul-bug-workaround
Work around a bug in the "muls" and "mulu" instructions for CPU models where it applies. This option
is active by default.
|
-mpdebug
Enable CRIS-specific verbose debug-related information in the assembly code. This option also has
the effect to turn off the #NO_APP formatted-code indicator to the assembler at the beginning of the
assembly file.
|
-mcc-init
Do not use condition-code results from previous instruction; always emit compare and test
instructions before use of condition codes.
|
-mno-side-effects
Do not emit instructions with side-effects in addressing modes other than post-increment.
|
-mstack-align
-mno-stack-align
-mdata-align
-mno-data-align
-mconst-align
-mno-const-align
These options (no-options) arranges (eliminate arrangements) for the stack-frame, individual data and
constants to be aligned for the maximum single data access size for the chosen CPU model. The
default is to arrange for 32-bit alignment. ABI details such as structure layout are not affected by
these options.
|
-m32-bit
-m16-bit
-m8-bit
Similar to the stack- data- and const-align options above, these options arrange for stack-frame,
writable data and constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit
alignment.
|
-mno-prologue-epilogue
-mprologue-epilogue
With -mno-prologue-epilogue, the normal function prologue and epilogue that sets up the stack-frame
are omitted and no return instructions or return sequences are generated in the code. Use this
option only together with visual inspection of the compiled code: no warnings or errors are generated
when call-saved registers must be saved, or storage for local variable needs to be allocated.
|
-mno-gotplt
-mgotplt
With -fpic and -fPIC, don't generate (do generate) instruction sequences that load addresses for
functions from the PLT part of the GOT rather than (traditional on other architectures) calls to the
PLT. The default is -mgotplt.
|
-maout
Legacy no-op option only recognized with the cris-axis-aout target.
|
-melf
Legacy no-op option only recognized with the cris-axis-elf and cris-axis-linux-gnu targets.
|
-melinux
Only recognized with the cris-axis-aout target, where it selects a GNU/linux-like multilib, include
files and instruction set for -march=v8.
|
-mlinux
Legacy no-op option only recognized with the cris-axis-linux-gnu target.
|
-sim
This option, recognized for the cris-axis-aout and cris-axis-elf arranges to link with input-output
functions from a simulator library. Code, initialized data and zero-initialized data are allocated
consecutively.
|
-sim2
Like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data
at 0x80000000.
MMIX Options
These options are defined for the MMIX:
|
-mlibfuncs
-mno-libfuncs
Specify that intrinsic library functions are being compiled, passing all values in registers, no
matter the size.
|
-mepsilon
-mno-epsilon
Generate floating-point comparison instructions that compare with respect to the "rE" epsilon
register.
|
-mabi=mmixware
-mabi=gnu
Generate code that passes function parameters and return values that (in the called function) are
seen as registers $0 and up, as opposed to the GNU ABI which uses global registers $231 and up.
|
-mzero-extend
-mno-zero-extend
When reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load
instructions by default, rather than sign-extending ones.
|
-mknuthdiv
-mno-knuthdiv
Make the result of a division yielding a remainder have the same sign as the divisor. With the
default, -mno-knuthdiv, the sign of the remainder follows the sign of the dividend. Both methods are
arithmetically valid, the latter being almost exclusively used.
|
-mtoplevel-symbols
-mno-toplevel-symbols
Prepend (do not prepend) a : to all global symbols, so the assembly code can be used with the
"PREFIX" assembly directive.
|
-melf
Generate an executable in the ELF format, rather than the default mmo format used by the mmix
simulator.
|
-mbranch-predict
-mno-branch-predict
Use (do not use) the probable-branch instructions, when static branch prediction indicates a probable
branch.
|
-mbase-addresses
-mno-base-addresses
Generate (do not generate) code that uses base addresses. Using a base address automatically
generates a request (handled by the assembler and the linker) for a constant to be set up in a global
register. The register is used for one or more base address requests within the range 0 to 255 from
the value held in the register. The generally leads to short and fast code, but the number of
different data items that can be addressed is limited. This means that a program that uses lots of
static data may require -mno-base-addresses.
|
-msingle-exit
-mno-single-exit
Force (do not force) generated code to have a single exit point in each function.
PDP-11 Options
These options are defined for the PDP-11:
|
-mfpu
Use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not
supported.)
|
-msoft-float
Do not use hardware floating point.
|
-mac0
Return floating-point results in ac0 (fr0 in Unix assembler syntax).
|
-mno-ac0
Return floating-point results in memory. This is the default.
|
-m40
Generate code for a PDP-11/40.
|
-m45
Generate code for a PDP-11/45. This is the default.
|
-m10
Generate code for a PDP-11/10.
|
-mbcopy-builtin
Use inline "movstrhi" patterns for copying memory. This is the default.
|
-mbcopy
Do not use inline "movstrhi" patterns for copying memory.
|
-mint16
-mno-int32
Use 16-bit "int". This is the default.
|
-mint32
-mno-int16
Use 32-bit "int".
|
-mfloat64
-mno-float32
Use 64-bit "float". This is the default.
|
-mfloat32
-mno-float64
Use 32-bit "float".
|
-mabshi
Use "abshi2" pattern. This is the default.
|
-mno-abshi
Do not use "abshi2" pattern.
|
-mbranch-expensive
Pretend that branches are expensive. This is for experimenting with code generation only.
|
-mbranch-cheap
Do not pretend that branches are expensive. This is the default.
|
-msplit
Generate code for a system with split I&D.
|
-mno-split
Generate code for a system without split I&D. This is the default.
|
-munix-asm
Use Unix assembler syntax. This is the default when configured for pdp11-*-bsd.
|
-mdec-asm
Use DEC assembler syntax. This is the default when configured for any PDP-11 target other than
pdp11-*-bsd.
Xstormy16 Options
These options are defined for Xstormy16:
|
-msim
Choose startup files and linker script suitable for the simulator.
FRV Options
|
-mgpr-32
Only use the first 32 general purpose registers.
|
-mgpr-64
Use all 64 general purpose registers.
|
-mfpr-32
Use only the first 32 floating point registers.
|
-mfpr-64
Use all 64 floating point registers
|
-mhard-float
Use hardware instructions for floating point operations.
|
-msoft-float
Use library routines for floating point operations.
|
-malloc-cc
Dynamically allocate condition code registers.
|
-mfixed-cc
Do not try to dynamically allocate condition code registers, only use "icc0" and "fcc0".
|
-mdword
Change ABI to use double word insns.
|
-mno-dword
Do not use double word instructions.
|
-mdouble
Use floating point double instructions.
|
-mno-double
Do not use floating point double instructions.
|
-mmedia
Use media instructions.
|
-mno-media
Do not use media instructions.
|
-mmuladd
Use multiply and add/subtract instructions.
|
-mno-muladd
Do not use multiply and add/subtract instructions.
|
-mlibrary-pic
Enable PIC support for building libraries
|
-macc-4
Use only the first four media accumulator registers.
|
-macc-8
Use all eight media accumulator registers.
|
-mpack
Pack VLIW instructions.
|
-mno-pack
Do not pack VLIW instructions.
|
-mno-eflags
Do not mark ABI switches in e_flags.
|
-mcond-move
Enable the use of conditional-move instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
|
-mno-cond-move
Disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
|
-mscc
Enable the use of conditional set instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
|
-mno-scc
Disable the use of conditional set instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
|
-mcond-exec
Enable the use of conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
|
-mno-cond-exec
Disable the use of conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
|
-mvliw-branch
Run a pass to pack branches into VLIW instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
|
-mno-vliw-branch
Do not run a pass to pack branches into VLIW instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
|
-mmulti-cond-exec
Enable optimization of "&&" and "||" in conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
|
-mno-multi-cond-exec
Disable optimization of "&&" and "||" in conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
|
-mnested-cond-exec
Enable nested conditional execution optimizations (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
|
-mno-nested-cond-exec
Disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
|
-mtomcat-stats
Cause gas to print out tomcat statistics.
|
-mcpu=cpu
Select the processor type for which to generate code. Possible values are simple, tomcat, fr500,
fr400, fr300, frv.
Xtensa Options
These options are supported for Xtensa targets:
|
-mconst16
-mno-const16
Enable or disable use of "CONST16" instructions for loading constant values. The "CONST16"
instruction is currently not a standard option from Tensilica. When enabled, "CONST16" instructions
are always used in place of the standard "L32R" instructions. The use of "CONST16" is enabled by
default only if the "L32R" instruction is not available.
|
-mfused-madd
-mno-fused-madd
Enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point
option. This has no effect if the floating-point option is not also enabled. Disabling fused
multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for
the multiply and add/subtract operations. This may be desirable in some cases where strict IEEE
754-compliant results are required: the fused multiply add/subtract instructions do not round the
intermediate result, thereby producing results with more bits of precision than specified by the IEEE
standard. Disabling fused multiply add/subtract instructions also ensures that the program output is
not sensitive to the compiler's ability to combine multiply and add/subtract operations.
|
-mtext-section-literals
-mno-text-section-literals
Control the treatment of literal pools. The default is -mno-text-section-literals, which places
literals in a separate section in the output file. This allows the literal pool to be placed in a
data RAM/ROM, and it also allows the linker to combine literal pools from separate object files to
remove redundant literals and improve code size. With -mtext-section-literals, the literals are
interspersed in the text section in order to keep them as close as possible to their references.
This may be necessary for large assembly files.
|
-mtarget-align
-mno-target-align
When this option is enabled, GCC instructs the assembler to automatically align instructions to
reduce branch penalties at the expense of some code density. The assembler attempts to widen density
instructions to align branch targets and the instructions following call instructions. If there are
not enough preceding safe density instructions to align a target, no widening will be performed. The
default is -mtarget-align. These options do not affect the treatment of auto-aligned instructions
like "LOOP", which the assembler will always align, either by widening density instructions or by
inserting no-op instructions.
|
-mlongcalls
-mno-longcalls
When this option is enabled, GCC instructs the assembler to translate direct calls to indirect calls
unless it can determine that the target of a direct call is in the range allowed by the call
instruction. This translation typically occurs for calls to functions in other source files.
Specifically, the assembler translates a direct "CALL" instruction into an "L32R" followed by a
"CALLX" instruction. The default is -mno-longcalls. This option should be used in programs where
the call target can potentially be out of range. This option is implemented in the assembler, not
the compiler, so the assembly code generated by GCC will still show direct call instructions---look
at the disassembled object code to see the actual instructions. Note that the assembler will use an
indirect call for every cross-file call, not just those that really will be out of range.
Options for Code Generation Conventions
These machine-independent options control the interface conventions used in code generation.
Most of them have both positive and negative forms; the negative form of -ffoo would be -fno-foo. In the
table below, only one of the forms is listed---the one which is not the default. You can figure out the
other form by either removing no- or adding it.
|
-fbounds-check
For front-ends that support it, generate additional code to check that indices used to access arrays
are within the declared range. This is currently only supported by the Java and Fortran 77 front-
ends, where this option defaults to true and false respectively.
|
-ftrapv
This option generates traps for signed overflow on addition, subtraction, multiplication operations.
|
-fwrapv
This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction
and multiplication wraps around using twos-complement representation. This flag enables some
optimizations and disables other. This option is enabled by default for the Java front-end, as
required by the Java language specification.
|
-fexceptions
Enable exception handling. Generates extra code needed to propagate exceptions. For some targets,
this implies GCC will generate frame unwind information for all functions, which can produce
significant data size overhead, although it does not affect execution. If you do not specify this
option, GCC will enable it by default for languages like C++ which normally require exception
handling, and disable it for languages like C that do not normally require it. However, you may need
to enable this option when compiling C code that needs to interoperate properly with exception
handlers written in C++. You may also wish to disable this option if you are compiling older C++
programs that don't use exception handling.
|
-fnon-call-exceptions
Generate code that allows trapping instructions to throw exceptions. Note that this requires
platform-specific runtime support that does not exist everywhere. Moreover, it only allows trapping
instructions to throw exceptions, i.e. memory references or floating point instructions. It does not
allow exceptions to be thrown from arbitrary signal handlers such as "SIGALRM".
|
-funwind-tables
Similar to -fexceptions, except that it will just generate any needed static data, but will not
affect the generated code in any other way. You will normally not enable this option; instead, a
language processor that needs this handling would enable it on your behalf.
|
-fasynchronous-unwind-tables
Generate unwind table in dwarf2 format, if supported by target machine. The table is exact at each
instruction boundary, so it can be used for stack unwinding from asynchronous events (such as
debugger or garbage collector).
|
-fpcc-struct-return
Return ``short'' "struct" and "union" values in memory like longer ones, rather than in registers.
This convention is less efficient, but it has the advantage of allowing intercallability between GCC-
compiled files and files compiled with other compilers, particularly the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends on the target configuration macros.
Short structures and unions are those whose size and alignment match that of some integer type.
Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code
compiled with the -freg-struct-return switch. Use it to conform to a non-default application binary
interface.
|
-freg-struct-return
Return "struct" and "union" values in registers when possible. This is more efficient for small
structures than -fpcc-struct-return.
If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever
convention is standard for the target. If there is no standard convention, GCC defaults to
-fpcc-struct-return, except on targets where GCC is the principal compiler. In those cases, we can
choose the standard, and we chose the more efficient register return alternative.
Warning: code compiled with the -freg-struct-return switch is not binary compatible with code
compiled with the -fpcc-struct-return switch. Use it to conform to a non-default application binary
interface.
|
-fshort-enums
Allocate to an "enum" type only as many bytes as it needs for the declared range of possible values.
Specifically, the "enum" type will be equivalent to the smallest integer type which has enough room.
Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with code
generated without that switch. Use it to conform to a non-default application binary interface.
|
-fshort-double
Use the same size for "double" as for "float".
Warning: the -fshort-double switch causes GCC to generate code that is not binary compatible with
code generated without that switch. Use it to conform to a non-default application binary interface.
|
-fshort-wchar
Override the underlying type for wchar_t to be short unsigned int instead of the default for the
target. This option is useful for building programs to run under WINE.
Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with code
generated without that switch. Use it to conform to a non-default application binary interface.
|
-fshared-data
Requests that the data and non-"const" variables of this compilation be shared data rather than
private data. The distinction makes sense only on certain operating systems, where shared data is
shared between processes running the same program, while private data exists in one copy per process.
|
-fno-common
In C, allocate even uninitialized global variables in the data section of the object file, rather
than generating them as common blocks. This has the effect that if the same variable is declared
(without "extern") in two different compilations, you will get an error when you link them. The only
reason this might be useful is if you wish to verify that the program will work on other systems
which always work this way.
|
-fno-ident
Ignore the #ident directive.
|
-finhibit-size-directive
Don't output a ".size" assembler directive, or anything else that would cause trouble if the function
is split in the middle, and the two halves are placed at locations far apart in memory. This option
is used when compiling crtstuff.c; you should not need to use it for anything else.
|
-fverbose-asm
Put extra commentary information in the generated assembly code to make it more readable. This
option is generally only of use to those who actually need to read the generated assembly code
(perhaps while debugging the compiler itself).
-fno-verbose-asm, the default, causes the extra information to be omitted and is useful when
comparing two assembler files.
|
-fpic
Generate position-independent code (PIC) suitable for use in a shared library, if supported for the
target machine. Such code accesses all constant addresses through a global offset table (GOT). The
dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of
GCC; it is part of the operating system). If the GOT size for the linked executable exceeds a
machine-specific maximum size, you get an error message from the linker indicating that -fpic does
not work; in that case, recompile with -fPIC instead. (These maximums are 8k on the SPARC and 32k on
the m68k and RS/6000. The 386 has no such limit.)
Position-independent code requires special support, and therefore works only on certain machines.
For the 386, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM
RS/6000 is always position-independent.
|
-fPIC
If supported for the target machine, emit position-independent code, suitable for dynamic linking and
avoiding any limit on the size of the global offset table. This option makes a difference on the
m68k and the SPARC.
Position-independent code requires special support, and therefore works only on certain machines.
|
-fpie
-fPIE
These options are similar to -fpic and -fPIC, but generated position independent code can be only
linked into executables. Usually these options are used when -pie GCC option will be used during
linking.
|
-ffixed-reg
Treat the register named reg as a fixed register; generated code should never refer to it (except
perhaps as a stack pointer, frame pointer or in some other fixed role).
reg must be the name of a register. The register names accepted are machine-specific and are defined
in the "REGISTER_NAMES" macro in the machine description macro file.
This flag does not have a negative form, because it specifies a three-way choice.
|
-fcall-used-reg
Treat the register named reg as an allocable register that is clobbered by function calls. It may be
allocated for temporaries or variables that do not live across a call. Functions compiled this way
will not save and restore the register reg.
It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other
registers that have fixed pervasive roles in the machine's execution model will produce disastrous
results.
This flag does not have a negative form, because it specifies a three-way choice.
|
-fcall-saved-reg
Treat the register named reg as an allocable register saved by functions. It may be allocated even
for temporaries or variables that live across a call. Functions compiled this way will save and
restore the register reg if they use it.
It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other
registers that have fixed pervasive roles in the machine's execution model will produce disastrous
results.
A different sort of disaster will result from the use of this flag for a register in which function
values may be returned.
This flag does not have a negative form, because it specifies a three-way choice.
|
-fpack-struct
Pack all structure members together without holes.
Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code
generated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a
non-default application binary interface.
|
-finstrument-functions
Generate instrumentation calls for entry and exit to functions. Just after function entry and just
before function exit, the following profiling functions will be called with the address of the
current function and its call site. (On some platforms, "__builtin_return_address" does not work
beyond the current function, so the call site information may not be available to the profiling
functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the current function, which may be looked up
exactly in the symbol table.
This currently disables function inlining. This restriction is expected to be removed in future
releases.
A function may be given the attribute "no_instrument_function", in which case this instrumentation
will not be done. This can be used, for example, for the profiling functions listed above, high-
priority interrupt routines, and any functions from which the profiling functions cannot safely be
called (perhaps signal handlers, if the profiling routines generate output or allocate memory).
|
-fstack-check
Generate code to verify that you do not go beyond the boundary of the stack. You should specify this
flag if you are running in an environment with multiple threads, but only rarely need to specify it
in a single-threaded environment since stack overflow is automatically detected on nearly all systems
if there is only one stack.
Note that this switch does not actually cause checking to be done; the operating system must do that.
The switch causes generation of code to ensure that the operating system sees the stack being
extended.
|
-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
Generate code to ensure that the stack does not grow beyond a certain value, either the value of a
register or the address of a symbol. If the stack would grow beyond the value, a signal is raised.
For most targets, the signal is raised before the stack overruns the boundary, so it is possible to
catch the signal without taking special precautions.
For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the
flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack
limit of 128KB. Note that this may only work with the GNU linker.
|
-fargument-alias
-fargument-noalias
-fargument-noalias-global
Specify the possible relationships among parameters and between parameters and global data.
-fargument-alias specifies that arguments (parameters) may alias each other and may alias global
storage.-fargument-noalias specifies that arguments do not alias each other, but may alias global
storage.-fargument-noalias-global specifies that arguments do not alias each other and do not alias
global storage.
Each language will automatically use whatever option is required by the language standard. You
should not need to use these options yourself.
|
-fleading-underscore
This option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are
represented in the object file. One use is to help link with legacy assembly code.
Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible
with code generated without that switch. Use it to conform to a non-default application binary
interface. Not all targets provide complete support for this switch.
|
-ftls-model=model
Alter the thread-local storage model to be used. The model argument should be one of
"global-dynamic", "local-dynamic", "initial-exec" or "local-exec".
|