-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 objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input f95 f95-cpp-input
go
java
|
-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. The C, C++, and
Fortran frontends return 4, if an internal compiler error is encountered.
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 that don't require preprocessing are ignored.
|
-o file
Place output in file file. This applies regardless to whatever sort of output is being produced,
whether it be an executable file, an object file, an assembler file or preprocessed C code.
If -o is not specified, the default is to put an executable file in a.out, the object file for
source.suffix in source.o, its assembler file in source.s, a precompiled header file in
source.suffix.gch, and all preprocessed C source on standard output.
|
-v Print (on standard error output) the commands executed to run the stages of compilation. Also print
the version number of the compiler driver program and of the preprocessor and the compiler proper.
|
-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 has also
been specified (prior to the --help option), then command-line options that 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.
For some targets extra target-specific information may also be printed.
|
--help=target,undocumented
The sense of a qualifier can be inverted by prefixing it with the ^ character, so for example to
display all binary warning options (i.e., ones that are either on or off and that do not take an
argument) that have a description, use:
|
--help=warnings,^joined,^undocumented
|
--help=target,optimizers
The --help= option can be repeated on the command line. Each successive use will display its
requested class of options, skipping those that have already been displayed.
If the -Q option appears on the command line before the --help= option, then the descriptive text
displayed by --help= is changed. Instead of describing the displayed options, an indication is given
as to whether the option is enabled, disabled or set to a specific value (assuming that the compiler
knows this at the point where the --help= option is used).
Here is a truncated example from the ARM port of gcc:
|
-Q -O2 --help=optimizers
Alternatively you can discover which binary optimizations are enabled by -O3 by using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
|
-no-canonical-prefixes
Do not expand any symbolic links, resolve references to /../ or /./, or make the path absolute when
generating a relative prefix.
|
--version
Display the version number and copyrights of the invoked GCC.
|
-wrapper
Invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters
are passed as a comma separated list.
gcc -c t.c -wrapper gdb,--args
This will invoke all subprograms of gcc under gdb --args, thus the invocation of cc1 will be gdb
--args cc1 ....
|
-fplugin-arg-name-key=value
Define an argument called key with a value of value for the plugin called name.
|
-fdump-ada-spec[-slim]
For C and C++ source and include files, generate corresponding Ada specs.
|
-fdump-go-spec=file
For input files in any language, generate corresponding Go declarations in file. This generates Go
"const", "type", "var", and "func" declarations which may be a useful way to start writing a Go
interface to code written in some other language.
|
-ansi
In C mode, this is equivalent to -std=c90. In C++ mode, it is equivalent to -std=c++98.
|
-std=
Determine the language standard. This option is currently only supported when compiling C or C++.
|
-fgnu89-inline
The option -fgnu89-inline tells GCC to use the traditional GNU semantics for "inline" functions when
in C99 mode.
This option is accepted and ignored by GCC versions 4.1.3 up to but not including 4.3. In GCC
versions 4.3 and later it changes the behavior of GCC in C99 mode. Using this option is roughly
equivalent to adding the "gnu_inline" function attribute to all inline functions.
The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for "inline" when in C99
or gnu99 mode (i.e., it specifies the default behavior). This option was first supported in GCC 4.3.
This option is not supported in -std=c90 or -std=gnu90 mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be used to check which
semantics are in effect for "inline" functions.
|
-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.
|
-fallow-parameterless-variadic-functions
Accept variadic functions without named parameters.
Although it is possible to define such a function, this is not very useful as it is not possible to
read the arguments. This is only supported for C as this construct is allowed by C++.
|
-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 which adjust the stack directly, and calls
to "memcpy" may become inline copy loops. The resulting code is often both smaller and faster, but
since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor
can you change the behavior of the functions by linking with a different library. In addition, when
a function is recognized as a built-in function, GCC may use information about that function to warn
about problems with calls to that function, or to generate more efficient code, even if the resulting
code still contains calls to that function. For example, warnings are given with -Wformat for bad
calls to "printf", when "printf" is built in, and "strlen" is known not to modify global memory.
With the -fno-builtin-function option only the built-in function function is disabled. function must
not begin with __builtin_. If a function is named that 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.
|
-fopenmp
Enable handling of OpenMP directives "#pragma omp" in C/C++ and "!$omp" in Fortran. When -fopenmp is
specified, the compiler generates parallel code according to the OpenMP Application Program Interface
v3.0 <http://www.openmp.org/>. This option implies -pthread, and thus is only supported on targets
that have support for -pthread.
|
-fgnu-tm
When the option -fgnu-tm is specified, the compiler will generate code for the Linux variant of
Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is
an experimental feature whose interface may change in future versions of GCC, as the official
specification changes. Please note that not all architectures are supported for this feature.
For more information on GCC's support for transactional memory,
Note that the transactional memory feature is not supported with non-call exceptions
(-fnon-call-exceptions).
|
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
In C++ code, this allows member names in structures to be similar to previous types declarations.
typedef int UOW;
struct ABC {
UOW UOW;
};
Some cases of unnamed fields in structures and unions are only accepted with this option.
|
-fplan9-extensions
Accept some non-standard constructs used in Plan 9 code.
This enables -fms-extensions, permits passing pointers to structures with anonymous fields to
functions that expect pointers to elements of the type of the field, and permits referring to
anonymous fields declared using a typedef. This is only supported for C, not C++.
|
-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++.
|
-flax-vector-conversions
Allow implicit conversions between vectors with differing numbers of elements and/or incompatible
element types. This option should not be used for new code.
|
-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.
Options Controlling C++ Dialect
This section describes the command-line options that are only meaningful for C++ programs; but you can
also use most of the GNU compiler options regardless of what language your program is in. For example,
you might compile a file "firstClass.C" like this:
g++ -g -frepo -O -c firstClass.C
In this example, only -frepo is an option meant only for C++ programs; you can use the other options with
any language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
|
-fabi-version=n
Use version n of the C++ ABI. Version 2 is the version of the C++ ABI that first appeared in G++
3.4. Version 1 is the version of the C++ ABI that first appeared in G++ 3.2. Version 0 will always
be the version that conforms most closely to the C++ ABI specification. Therefore, the ABI obtained
using version 0 will change as ABI bugs are fixed.
|
-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 run-time-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.
|
-fconstexpr-depth=n
Set the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed to
detect endless recursion during constant expression evaluation. The minimum specified by the
standard is 512.
|
-fdeduce-init-list
Enable deduction of a template type parameter as std::initializer_list from a brace-enclosed
initializer list, i.e.
template <class T> auto forward(T t) -> decltype (realfn (t))
{
return realfn (t);
}
void f()
{
forward({1,2}); // call forward<std::initializer_list<int>>
}
This deduction was implemented as a possible extension to the originally proposed semantics for the
C++11 standard, but was not part of the final standard, so it is disabled by default. This option is
deprecated, and may be removed in a future version of G++.
|
-ffriend-injection
Inject friend functions into the enclosing namespace, so that they are visible outside the scope of
the class in which they are declared. Friend functions were documented to work this way in the old
Annotated C++ Reference Manual, and versions of G++ before 4.1 always worked that way. However, in
ISO C++ a friend function that is not declared in an enclosing scope can only be found using argument
dependent lookup. This option causes friends to be injected as they were in earlier releases.
This option is for compatibility, and may be removed in a future release of G++.
|
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a temporary that 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 generate code to check for violation of exception specifications at run time. This option
violates the C++ standard, but may be useful for reducing code size in production builds, much like
defining NDEBUG. This does not give user code permission to throw exceptions in violation of the
exception specifications; the compiler will still optimize based on the specifications, so throwing
an unexpected exception will result in undefined behavior.
|
-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 that 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.
|
-fnothrow-opt
Treat a "throw()" exception specification as though it were a "noexcept" specification to reduce or
eliminate the text size overhead relative to a function with no exception specification. If the
function has local variables of types with non-trivial destructors, the exception specification will
actually make the function smaller because the EH cleanups for those variables can be optimized away.
The semantic effect is that an exception thrown out of a function with such an exception
specification will result in a call to "terminate" rather than "unexpected".
|
-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.
|
-fno-pretty-templates
When an error message refers to a specialization of a function template, the compiler will normally
print the signature of the template followed by the template arguments and any typedefs or typenames
in the signature (e.g. "void f(T) [with T = int]" rather than "void f(int)") so that it's clear which
template is involved. When an error message refers to a specialization of a class template, the
compiler will omit any template arguments that match the default template arguments for that
template. If either of these behaviors make it harder to understand the error message rather than
easier, using -fno-pretty-templates will disable them.
|
-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++ run-
time 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. The dynamic_cast operator can still be used for casts
that do not require run-time type information, i.e. casts to "void *" or to unambiguous base classes.
|
-fstats
Emit statistics about front-end processing at the end of the compilation. This information is
generally only useful to the G++ development team.
|
-fstrict-enums
Allow the compiler to optimize using the assumption that a value of enumerated type can only be one
of the values of the enumeration (as defined in the C++ standard; basically, a value that can be
represented in the minimum number of bits needed to represent all the enumerators). This assumption
may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated
type.
|
-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 (changed to 1024 in C++11). The
default value is 900, as the compiler can run out of stack space before hitting 1024 in some
situations.
|
-fno-threadsafe-statics
Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe
initialization of local statics. You can use this option to reduce code size slightly in code that
doesn't need to be thread-safe.
|
-fuse-cxa-atexit
Register destructors for objects with static storage duration with the "__cxa_atexit" function rather
than the "atexit" function. This option is required for fully standards-compliant handling of static
destructors, but will only work if your C library supports "__cxa_atexit".
|
-fno-use-cxa-get-exception-ptr
Don't use the "__cxa_get_exception_ptr" runtime routine. This will cause "std::uncaught_exception"
to be incorrect, but is necessary if the runtime routine is not available.
|
-fvisibility-inlines-hidden
This switch declares that the user does not attempt to compare pointers to inline functions or
methods where the addresses of the two functions were taken in different shared objects.
The effect of this is that GCC may, effectively, mark inline methods with "__attribute__ ((visibility
("hidden")))" so that they do not appear in the export table of a DSO and do not require a PLT
indirection when used within the DSO. Enabling this option can have a dramatic effect on load and
link times of a DSO as it massively reduces the size of the dynamic export table when the library
makes heavy use of templates.
The behavior of this switch is not quite the same as marking the methods as hidden directly, because
it does not affect static variables local to the function or cause the compiler to deduce that the
function is defined in only one shared object.
You may mark a method as having a visibility explicitly to negate the effect of the switch for that
method. For example, if you do want to compare pointers to a particular inline method, you might
mark it as having default visibility. Marking the enclosing class with explicit visibility will have
no effect.
Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise
cross a shared library boundary.
|
-fvisibility-ms-compat
This flag attempts to use visibility settings to make GCC's C++ linkage model compatible with that of
Microsoft Visual Studio.
The flag makes these changes to GCC's linkage model:
|
-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.
|
-Wreorder (C++ and Objective-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:
|
-Wno-non-template-friend (C++ and Objective-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.
|
-Wsign-promo (C++ and Objective-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 ("[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. This option is only available in conjunction with the NeXT runtime and ABI version 0 or 1.
|
-fobjc-abi-version=n
Use version n of the Objective-C ABI for the selected runtime. This option is currently supported
only for the NeXT runtime. In that case, Version 0 is the traditional (32-bit) ABI without support
for properties and other Objective-C 2.0 additions. Version 1 is the traditional (32-bit) ABI with
support for properties and other Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI.
If nothing is specified, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit
target machines.
|
-fobjc-call-cxx-cdtors
For each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial
default constructor. If so, synthesize a special "- (id) .cxx_construct" instance method which will
run non-trivial default constructors on any such instance variables, in order, and then return
"self". Similarly, check if any instance variable is a C++ object with a non-trivial destructor, and
if so, synthesize a special "- (void) .cxx_destruct" method which will run all such default
destructors, in reverse order.
The "- (id) .cxx_construct" and "- (void) .cxx_destruct" methods thusly generated will only operate
on instance variables declared in the current Objective-C class, and not those inherited from
superclasses. It is the responsibility of the Objective-C runtime to invoke all such methods in an
object's inheritance hierarchy. The "- (id) .cxx_construct" methods will be invoked by the runtime
immediately after a new object instance is allocated; the "- (void) .cxx_destruct" methods will be
invoked immediately before the runtime deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the "-
(id) .cxx_construct" and "- (void) .cxx_destruct" methods.
|
-fobjc-direct-dispatch
Allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page.
|
-fobjc-exceptions
Enable syntactic support for structured exception handling in Objective-C, similar to what is offered
by C++ and Java. This option is required to use the Objective-C keywords @try, @throw, @catch,
@finally and @synchronized. This option is available with both the GNU runtime and the NeXT runtime
(but not available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier).
|
-fobjc-gc
Enable garbage collection (GC) in Objective-C and Objective-C++ programs. This option is only
available with the NeXT runtime; the GNU runtime has a different garbage collection implementation
that does not require special compiler flags.
|
-fobjc-nilcheck
For the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before
doing the actual method call. This is the default and can be disabled using -fno-objc-nilcheck.
Class methods and super calls are never checked for nil in this way no matter what this flag is set
to. Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime
ABI, is used.
|
-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. The GNU runtime currently always retains calls to "objc_get_class("...")"
regardless of command-line options.
|
-gen-decls
Dump interface declarations for all classes seen in the source file to a file named sourcename.decl.
|
-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.
|
-fno-diagnostics-show-option
By default, each diagnostic emitted includes text indicating the command-line option that directly
controls the diagnostic (if such an option is known to the diagnostic machinery). Specifying the
-fno-diagnostics-show-option flag suppresses that behavior.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are
risky or suggest there may have been an error.
The following language-independent options do not enable specific warnings but control the kinds of
diagnostics produced by GCC.
|
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond that.
|
-fmax-errors=n
Limits the maximum number of error messages to n, at which point GCC bails out rather than attempting
to continue processing the source code. If n is 0 (the default), there is no limit on the number of
error messages produced. If -Wfatal-errors is also specified, then -Wfatal-errors takes precedence
over this option.
|
-w Inhibit all warning messages.
|
-Werror
Make all warnings into errors.
|
-Werror=
Make the specified warning into an error. The specifier for a warning is appended, for example
-Werror=switch turns the warnings controlled by -Wswitch into errors. This switch takes a negative
form, to be used to negate -Werror for specific warnings, for example -Wno-error=switch makes
-Wswitch warnings not be errors, even when -Werror is in effect.
The warning message for each controllable warning includes the option that controls the warning.
That option can then be used with -Werror= and -Wno-error= as described above. (Printing of the
option in the warning message can be disabled using the -fno-diagnostics-show-option flag.)
Note that specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply
anything.
|
-Wfatal-errors
This option causes the compiler to abort compilation on the first error occurred rather than trying
to keep going and printing further error messages.
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. For further, language-specific options also refer to C++ Dialect
Options and Objective-C and Objective-C++ Dialect Options.
When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC will emit a diagnostic
stating that the option is not recognized. However, if the -Wno- form is used, the behavior is slightly
different: No diagnostic will be produced for -Wno-unknown-warning unless other diagnostics are being
produced. This allows the use of new -Wno- options with old compilers, but if something goes wrong, the
compiler will warn that an unrecognized option was used.
|
-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.
|
-Wall
This enables all the warnings about constructions that some users consider questionable, and that are
easy to avoid (or modify to prevent the warning), even in conjunction with macros. This also enables
some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++
Dialect Options.
-Wall turns on the following warning flags:
-Waddress -Warray-bounds (only with -O2) -Wc++11-compat -Wchar-subscripts -Wenum-compare (in C/Objc;
this is on by default in C++) -Wimplicit-int (C and Objective-C only) -Wimplicit-function-declaration
(C and Objective-C only) -Wcomment -Wformat -Wmain (only for C/ObjC and unless -ffreestanding)
-Wmaybe-uninitialized -Wmissing-braces -Wnonnull -Wparentheses -Wpointer-sign -Wreorder -Wreturn-type
-Wsequence-point -Wsign-compare (only in C++) -Wstrict-aliasing -Wstrict-overflow=1 -Wswitch
-Wtrigraphs -Wuninitialized -Wunknown-pragmas -Wunused-function -Wunused-label -Wunused-value
-Wunused-variable -Wvolatile-register-var
Note that some warning flags 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. Some of them are enabled by -Wextra but many
of them must be enabled individually.
|
-Wextra
This enables some extra warning flags that are not enabled by -Wall. (This option used to be called
-W. The older name is still supported, but the newer name is more descriptive.)
|
-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. This warning is enabled by -Wall.
|
-Wcomment
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a Backslash-Newline
appears in a // comment. This warning is enabled by -Wall.
|
-Wno-coverage-mismatch
Warn if feedback profiles do not match when using the -fprofile-use option. If a source file was
changed between -fprofile-gen and -fprofile-use, the files with the profile feedback can fail to
match the source file and GCC cannot use the profile feedback information. By default, this warning
is enabled and is treated as an error. -Wno-coverage-mismatch can be used to disable the warning or
-Wno-error=coverage-mismatch can be used to disable the error. Disabling the error for this warning
can result in poorly optimized code and is useful only in the case of very minor changes such as bug
fixes to an existing code-base. Completely disabling the warning is not recommended.
|
-Wno-cpp
(C, Objective-C, C++, Objective-C++ and Fortran only)
|
-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 (or
other target-specific families). Which functions are checked without format attributes having been
specified depends on the standard version selected, and such checks of functions without the
attribute specified are disabled by -ffreestanding or -fno-builtin.
The formats are checked against the format features supported by GNU libc version 2.2. These include
all ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and
GNU extensions. Other library implementations may not support all these features; GCC does not
support warning about features that go beyond a particular library's limitations. However, if
-pedantic is used with -Wformat, warnings will be given about format features not in the selected
standard version (but not for "strfmon" formats, since those are not in any version of the C
standard).
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 that may yield only a two-digit year.
|
-Wno-format-contains-nul
If -Wformat is specified, do not warn about format strings that contain NUL bytes.
|
-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.
|
-Wimplicit (C and Objective-C only)
Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall.
-Wignored-qualifiers (C and C++ only)
Warn if the return type of a function has a type qualifier such as "const". For ISO C such a type
qualifier has no effect, since the value returned by a function is not an lvalue. For C++, the
warning is only emitted for scalar types or "void". ISO C prohibits qualified "void" return types on
function definitions, so such return types always receive a warning even without this option.
This warning is also enabled by -Wextra.
|
-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. This warning is
enabled by default in C++ and is enabled by either -Wall or -pedantic.
|
-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 } };
This warning is enabled by -Wall.
-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
Warn if a user-supplied include directory does not exist.
|
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context
where a truth value is expected, or when operators are nested whose precedence people often get
confused about.
|
-Wsequence-point
Warn about code that may have undefined semantics because of violations of sequence point rules in
the C and C++ standards.
|
-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" (falling off
the end of the function body is considered returning without a value), and about a "return" statement
with an expression in a function whose return-type is "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.
This warning is enabled by -Wall.
|
-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 (even if
there is a "default" label). This warning is enabled by -Wall.
|
-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. The only difference between -Wswitch and this option is that this
option gives a warning about an omitted enumeration code even if there is a "default" label.
|
-Wsync-nand (C and C++ only)
Warn when "__sync_fetch_and_nand" and "__sync_nand_and_fetch" built-in functions are used. These
functions changed semantics in GCC 4.4.
|
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning of the program (trigraphs within
comments are not warned about). This warning is enabled by -Wall.
|
-Wunused-but-set-parameter
Warn whenever a function parameter is assigned to, but otherwise unused (aside from its declaration).
To suppress this warning use the unused attribute.
This warning is also enabled by -Wunused together with -Wextra.
|
-Wunused-but-set-variable
Warn whenever a local variable is assigned to, but otherwise unused (aside from its declaration).
This warning is enabled by -Wall.
To suppress this warning use the unused attribute.
This warning is also enabled by -Wunused, which is enabled by -Wall.
|
-Wunused-function
Warn whenever a static function is declared but not defined or a non-inline static function is
unused. This warning is enabled by -Wall.
|
-Wunused-label
Warn whenever a label is declared but not used. This warning is enabled by -Wall.
To suppress this warning use the unused attribute.
-Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only)
Warn when a typedef locally defined in a function is not used.
|
-Wunused-parameter
Warn whenever a function parameter is unused aside from its declaration.
To suppress this warning use the unused attribute.
|
-Wno-unused-result
Do not warn if a caller of a function marked with attribute "warn_unused_result" does not use its
return value. The default is -Wunused-result.
|
-Wunused-variable
Warn whenever a local variable or non-constant static variable is unused aside from its declaration.
This warning is enabled by -Wall.
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 unused expression to void. This includes an expression-statement or the left-hand side of a
comma expression that contains no side effects. For example, an expression such as x[i,j] will cause
a warning, while x[(void)i,j] will not.
This warning is enabled by -Wall.
|
-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. In C++, warn if a non-static reference or non-static const member
appears in a class without constructors.
If you want to warn about code that uses the uninitialized value of the variable in its own
initializer, use the -Winit-self option.
These warnings occur for individual uninitialized or clobbered elements of structure, union or array
variables as well as for variables that are uninitialized or clobbered as a whole. They do not occur
for variables or elements declared "volatile". Because these warnings depend on optimization, the
exact variables or elements for which there are warnings will depend on the precise optimization
options and version of GCC used.
Note that there may be no warning about a variable that is used only to compute a value that itself
is never used, because such computations may be deleted by data flow analysis before the warnings are
printed.
|
-Wmaybe-uninitialized
For an automatic variable, if there exists a path from the function entry to a use of the variable
that is initialized, but there exist some other paths the variable is not initialized, the compiler
will emit a warning if it can not prove the uninitialized paths do not happen at run time. These
warnings are made optional because GCC is not smart enough to see all the reasons why the code might
be correct despite appearing to have an error. Here is one example of how this can happen:
|
-Wunknown-pragmas
Warn when a "#pragma" directive is encountered that 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.
|
-Wno-pragmas
Do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts
between pragmas. See also -Wunknown-pragmas.
|
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active. It warns about code that 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. It is
equivalent to -Wstrict-aliasing=3
|
-Wstrict-aliasing=n
This option is only active when -fstrict-aliasing is active. It warns about code that might break
the strict aliasing rules that the compiler is using for optimization. Higher levels correspond to
higher accuracy (fewer false positives). Higher levels also correspond to more effort, similar to
the way -O works. -Wstrict-aliasing is equivalent to -Wstrict-aliasing=n, with n=3.
Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but
-fstrict-aliasing still breaks the code, as it has very few false negatives. However, it has many
false positives. Warns for all pointer conversions between possibly incompatible types, even if
never dereferenced. Runs in the front end only.
Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as
level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only
warns when an address is taken. Warns about incomplete types. Runs in the front end only.
Level 3 (default for -Wstrict-aliasing): Should have very few false positives and few false
negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the
common pun+dereference pattern in the front end: "*(int*)&some_float". If optimization is enabled,
it also runs in the back end, where it deals with multiple statement cases using flow-sensitive
points-to information. Only warns when the converted pointer is dereferenced. Does not warn about
incomplete types.
|
-Wstrict-overflow
-Wstrict-overflow=n
This option is only active when -fstrict-overflow is active. It warns about cases where the compiler
optimizes based on the assumption that signed overflow does not occur. Note that it does not warn
about all cases where the code might overflow: it only warns about cases where the compiler
implements some optimization. Thus this warning depends on the optimization level.
An optimization that assumes that signed overflow does not occur is perfectly safe if the values of
the variables involved are such that overflow never does, in fact, occur. Therefore this warning can
easily give a false positive: a warning about code that is not actually a problem. To help focus on
important issues, several warning levels are defined. No warnings are issued for the use of
undefined signed overflow when estimating how many iterations a loop will require, in particular when
determining whether a loop will be executed at all.
|
-Wsuggest-attribute=[pure|const|noreturn]
Warn for cases where adding an attribute may be beneficial. The attributes currently supported are
listed below.
|
-Wsuggest-attribute=pure
-Wsuggest-attribute=const
-Wsuggest-attribute=noreturn
Warn about functions that might be candidates for attributes "pure", "const" or "noreturn". The
compiler only warns for functions visible in other compilation units or (in the case of "pure"
and "const") if it cannot prove that the function returns normally. A function returns normally
if it doesn't contain an infinite loop nor returns abnormally by throwing, calling "abort()" or
trapping. This analysis requires option -fipa-pure-const, which is enabled by default at -O and
higher. Higher optimization levels improve the accuracy of the analysis.
|
-Warray-bounds
This option is only active when -ftree-vrp is active (default for -O2 and above). It warns about
subscripts to arrays that are always out of bounds. This warning is enabled by -Wall.
|
-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.
|
-Wtrampolines
Warn about trampolines generated for pointers to nested functions.
A trampoline is a small piece of data or code that is created at run
time on the stack when the address of a nested function is taken, and
is used to call the nested function indirectly. For some targets, it
is made up of data only and thus requires no special treatment. But,
for most targets, it is made up of code and thus requires the stack
to be made executable in order for the program to work properly.
|
-Wfloat-equal
Warn if floating-point values are used in equality comparisons.
|
-Wundef
Warn if an undefined identifier is evaluated in an #if directive.
|
-Wno-endif-labels
Do not warn whenever an #else or an #endif are followed by text.
|
-Wshadow
Warn whenever a local variable or type declaration shadows another variable, parameter, type, or
class member (in C++), or whenever a built-in function is shadowed. Note that in C++, the compiler
will not warn if a local variable shadows a struct/class/enum, but will warn if it shadows an
explicit typedef.
|
-Wlarger-than=len
Warn whenever an object of larger than len bytes is defined.
|
-Wframe-larger-than=len
Warn if the size of a function frame is larger than len bytes. The computation done to determine the
stack frame size is approximate and not conservative. The actual requirements may be somewhat
greater than len even if you do not get a warning. In addition, any space allocated via "alloca",
variable-length arrays, or related constructs is not included by the compiler when determining
whether or not to issue a warning.
|
-Wno-free-nonheap-object
Do not warn when attempting to free an object that was not allocated on the heap.
|
-Wstack-usage=len
Warn if the stack usage of a function might be larger than len bytes. The computation done to
determine the stack usage is conservative. Any space allocated via "alloca", variable-length arrays,
or related constructs is included by the compiler when determining whether or not to issue a warning.
|
-Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler could not assume anything on the bounds of
the loop indices. With -funsafe-loop-optimizations warn if the compiler made such assumptions.
|
-Wno-pedantic-ms-format (MinGW targets only)
Disables the warnings about non-ISO "printf" / "scanf" format width specifiers "I32", "I64", and "I"
used on Windows targets depending on the MS runtime, when you are using the options -Wformat and
-pedantic without gnu-extensions.
|
-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.
In C++, warn also when an arithmetic operation involves "NULL". This warning is also enabled by
-pedantic.
|
-Wtype-limits
Warn if a comparison is always true or always false due to the limited range of the data type, but do
not warn for constant expressions. For example, warn if an unsigned variable is compared against
zero with < or >=. This warning is also enabled by -Wextra.
|
-Wbad-function-cast (C and Objective-C only)
Warn whenever a function call is cast to a non-matching type. For example, warn if "int malloc()" is
cast to "anything *".
-Wc++-compat (C and Objective-C only)
Warn about ISO C constructs that are outside of the common subset of ISO C and ISO C++, e.g. request
for implicit conversion from "void *" to a pointer to non-"void" type.
-Wc++11-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g.,
identifiers in ISO C++ 1998 that are keywords in ISO C++ 2011. This warning turns on -Wnarrowing and
is enabled by -Wall.
|
-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 *".
Also warn when making a cast that introduces a type qualifier in an unsafe way. For example, casting
"char **" to "const char **" is unsafe, as in this example:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';
|
-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. 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.
When compiling C++, warn about the deprecated conversion from string literals to "char *". This
warning is enabled by default for C++ programs.
|
-Wclobbered
Warn for variables that might be changed by longjmp or vfork. This warning is also enabled by
-Wextra.
|
-Wconversion
Warn for implicit conversions that may alter a value. This includes conversions between real and
integer, like "abs (x)" when "x" is "double"; conversions between signed and unsigned, like "unsigned
ui = -1"; and conversions to smaller types, like "sqrtf (M_PI)". Do not warn for explicit casts like
"abs ((int) x)" and "ui = (unsigned) -1", or if the value is not changed by the conversion like in
"abs (2.0)". Warnings about conversions between signed and unsigned integers can be disabled by
using -Wno-sign-conversion.
For C++, also warn for confusing overload resolution for user-defined conversions; and conversions
that will never use a type conversion operator: conversions to "void", the same type, a base class or
a reference to them. Warnings about conversions between signed and unsigned integers are disabled by
default in C++ unless -Wsign-conversion is explicitly enabled.
-Wno-conversion-null (C++ and Objective-C++ only)
Do not warn for conversions between "NULL" and non-pointer types. -Wconversion-null is enabled by
default.
-Wzero-as-null-pointer-constant (C++ and Objective-C++ only)
Warn when a literal '0' is used as null pointer constant. This can be useful to facilitate the
conversion to "nullptr" in C++11.
|
-Wempty-body
Warn if an empty body occurs in an if, else or do while statement. This warning is also enabled by
-Wextra.
|
-Wenum-compare
Warn about a comparison between values of different enumerated types. In C++ this warning is enabled
by default. In C this warning is enabled by -Wall.
|
-Wjump-misses-init (C, Objective-C only)
Warn if a "goto" statement or a "switch" statement jumps forward across the initialization of a
variable, or jumps backward to a label after the variable has been initialized. This only warns
about variables that are initialized when they are declared. This warning is only supported for C
and Objective-C; in C++ this sort of branch is an error in any case.
-Wjump-misses-init is included in -Wc++-compat. It can be disabled with the -Wno-jump-misses-init
option.
|
-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.
|
-Wsign-conversion
Warn for implicit conversions that may change the sign of an integer value, like assigning a signed
integer expression to an unsigned integer variable. An explicit cast silences the warning. In C, this
option is enabled also by -Wconversion.
|
-Waddress
Warn about suspicious uses of memory addresses. These include using the address of a function in a
conditional expression, such as "void func(void); if (func)", and comparisons against the memory
address of a string literal, such as "if (x == "abc")". Such uses typically indicate a programmer
error: the address of a function always evaluates to true, so their use in a conditional usually
indicate that the programmer forgot the parentheses in a function call; and comparisons against
string literals result in unspecified behavior and are not portable in C, so they usually indicate
that the programmer intended to use "strcmp". This warning is enabled by -Wall.
|
-Wlogical-op
Warn about suspicious uses of logical operators in expressions. This includes using logical
operators in contexts where a bit-wise operator is likely to be expected.
|
-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.)
|
-Wno-attributes
Do not warn if an unexpected "__attribute__" is used, such as unrecognized attributes, function
attributes applied to variables, etc. This will not stop errors for incorrect use of supported
attributes.
|
-Wno-builtin-macro-redefined
Do not warn if certain built-in macros are redefined. This suppresses warnings for redefinition of
"__TIMESTAMP__", "__TIME__", "__DATE__", "__FILE__", and "__BASE_FILE__".
|
-Wmissing-declarations
Warn if a global function is defined without a previous declaration. Do so even if the definition
itself provides a prototype. Use this option to detect global functions that are not declared in
header files. In C++, no warnings are issued for function templates, or for inline functions, or for
functions in anonymous namespaces.
|
-Wmissing-field-initializers
Warn if a structure's initializer has some fields missing. For example, the following code would
cause such a warning, because "x.h" is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
This option does not warn about designated initializers, so the following modification would not
trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra
-Wno-missing-field-initializers.
|
-Wmissing-format-attribute
Warn about function pointers that might be candidates for "format" attributes. Note these are only
possible candidates, not absolute ones. GCC will guess that function pointers with "format"
attributes that are used in assignment, initialization, parameter passing or return statements should
have a corresponding "format" attribute in the resulting type. I.e. the left-hand side of the
assignment or initialization, the type of the parameter variable, or the return type of the
containing function respectively should also have a "format" attribute to avoid the warning.
GCC will also warn about function definitions that might be candidates for "format" attributes.
Again, these are only possible candidates. 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.
|
-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.
|
-Wnormalized=<none|id|nfc|nfkc>
In ISO C and ISO C++, two identifiers are different if they are different sequences of characters.
However, sometimes when characters outside the basic ASCII character set are used, you can have two
different character sequences that look the same. To avoid confusion, the ISO 10646 standard sets
out some normalization rules which when applied ensure that two sequences that look the same are
turned into the same sequence. GCC can warn you if you are using identifiers that have not been
normalized; this option controls that warning.
There are four levels of warning supported by GCC. The default is -Wnormalized=nfc, which warns
about any identifier that is not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended
form for most uses.
Unfortunately, there are some characters allowed in identifiers by ISO C and ISO C++ that, when
turned into NFC, are not allowed in identifiers. That is, there's no way to use these symbols in
portable ISO C or C++ and have all your identifiers in NFC. -Wnormalized=id suppresses the warning
for these characters. It is hoped that future versions of the standards involved will correct this,
which is why this option is not the default.
You can switch the warning off for all characters by writing -Wnormalized=none. You would only want
to do this if you were using some other normalization scheme (like "D"), because otherwise you can
easily create bugs that are literally impossible to see.
Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display
methodologies, especially once formatting has been applied. For instance "\u207F", "SUPERSCRIPT
LATIN SMALL LETTER N", will display just like a regular "n" that has been placed in a superscript.
ISO 10646 defines the NFKC normalization scheme to convert all these into a standard form as well,
and GCC will warn if your code is not in NFKC if you use -Wnormalized=nfkc. This warning is
comparable to warning about every identifier that contains the letter O because it might be confused
with the digit 0, and so is not the default, but may be useful as a local coding convention if the
programming environment is unable to be fixed to display these characters distinctly.
|
-Wno-deprecated
Do not warn about usage of deprecated features.
|
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as deprecated by using the
"deprecated" attribute.
|
-Wno-overflow
Do not warn about compile-time overflow in constant expressions.
-Woverride-init (C and Objective-C only)
Warn if an initialized field without side effects is overridden when using designated initializers.
This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra
-Wno-override-init.
|
-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;
};
|
-Wpacked-bitfield-compat
The 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute on bit-fields of type "char". This
has been fixed in GCC 4.4 but the change can lead to differences in the structure layout. GCC
informs you when the offset of such a field has changed in GCC 4.4. For example there is no longer a
4-bit padding between field "a" and "b" in this structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));
This warning is enabled by default. Use -Wno-packed-bitfield-compat to disable this warning.
|
-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 and Objective-C only)
Warn if an "extern" declaration is encountered within a function.
|
-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 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++ and Objective-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.
|
-Wno-int-to-pointer-cast
Suppress warnings from casts to pointer type of an integer of a different size. In C++, casting to a
pointer type of smaller size is an error. Wint-to-pointer-cast is enabled by default.
-Wno-pointer-to-int-cast (C and Objective-C only)
Suppress warnings from casts from a pointer to an integer type of a different size.
|
-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 enabled by either -pedantic or -Wtraditional in ISO C90 and
C++98 modes. To inhibit the warning messages, use -Wno-long-long.
|
-Wvariadic-macros
Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU alternate syntax when in
pedantic ISO C99 mode. This is default. To inhibit the warning messages, use -Wno-variadic-macros.
|
-Wvector-operation-performance
Warn if vector operation is not implemented via SIMD capabilities of the architecture. Mainly useful
for the performance tuning. Vector operation can be implemented "piecewise", which means that the
scalar operation is performed on every vector element; "in parallel", which means that the vector
operation is implemented using scalars of wider type, which normally is more performance efficient;
and "as a single scalar", which means that vector fits into a scalar type.
|
-Wvla
Warn if variable length array is used in the code. -Wno-vla will prevent the -pedantic warning of
the variable length array.
|
-Wvolatile-register-var
Warn if a register variable is declared volatile. The volatile modifier does not inhibit all
optimizations that may eliminate reads and/or writes to register variables. This warning is enabled
by -Wall.
|
-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.
-Wpointer-sign (C and Objective-C only)
Warn for pointer argument passing or assignment with different signedness. This option is only
supported for C and Objective-C. It is implied by -Wall and by -pedantic, which can be disabled with
-Wno-pointer-sign.
|
-Wstack-protector
This option is only active when -fstack-protector is active. It warns about functions that will not
be protected against stack smashing.
|
-Wno-mudflap
Suppress warnings about constructs that cannot be instrumented by -fmudflap.
|
-Woverlength-strings
Warn about string constants that are longer than the "minimum maximum" length specified in the C
standard. Modern compilers generally allow string constants that are much longer than the standard's
minimum limit, but very portable programs should avoid using longer strings.
The limit applies after string constant concatenation, and does not count the trailing NUL. In C90,
the limit was 509 characters; in C99, it was raised to 4095. C++98 does not specify a normative
minimum maximum, so we do not diagnose overlength strings in C++.
This option is implied by -pedantic, and can be disabled with -Wno-overlength-strings.
-Wunsuffixed-float-constants (C and Objective-C only)
GCC will issue a warning for any floating constant that does not have a suffix. When used together
with -Wsystem-headers it will warn about such constants in system header files. This can be useful
when preparing code to use with the "FLOAT_CONST_DECIMAL64" pragma from the decimal floating-point
extension to C99.
Options for Debugging Your Program or GCC
GCC has various special options that are used for debugging either your program or GCC:
|
-g Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF
2). GDB can work with this debugging information.
On most systems that use stabs format, -g enables use of extra debugging information that only GDB
can use; this extra information makes debugging work better in GDB but will probably make other
debuggers crash or refuse to read the program. If you want to control for certain whether to
generate the extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).
GCC allows you to use -g with -O. The shortcuts taken by optimized code may occasionally produce
surprising results: some variables you declared may not exist at all; flow of control may briefly
move where you did not expect it; some statements may not be executed because they compute constant
results or their values were already at hand; some statements may execute in different places because
they were moved out of loops.
Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the
optimizer for programs that might have bugs.
The following options are useful when GCC is generated with the capability for more than one
debugging format.
|
-ggdb
Produce debugging information for use by GDB. This means to use the most expressive format available
(DWARF 2, stabs, or the native format if neither of those are supported), including GDB extensions if
at all possible.
|
-gstabs
Produce debugging information in stabs format (if that is supported), without GDB extensions. This
is the format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this
option produces stabs debugging output that 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.
|
-femit-class-debug-always
Instead of emitting debugging information for a C++ class in only one object file, emit it in all
object files using the class. This option should be used only with debuggers that are unable to
handle the way GCC normally emits debugging information for classes because using this option will
increase the size of debugging information by as much as a factor of two.
|
-fno-debug-types-section
By default when using DWARF v4 or higher type DIEs will be put into their own .debug_types section
instead of making them part of the .debug_info section. It is more efficient to put them in a
separate comdat sections since the linker will then be able to remove duplicates. But not all DWARF
consumers support .debug_types sections yet.
|
-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-version
Produce debugging information in DWARF format (if that is supported). This is the format used by DBX
on IRIX 6. The value of version may be either 2, 3 or 4; the default version is 2.
Note that with DWARF version 2 some ports require, and will always use, some non-conflicting DWARF 3
extensions in the unwind tables.
Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit.
|
-grecord-gcc-switches
This switch causes the command-line options used to invoke the compiler that may affect code
generation to be appended to the DW_AT_producer attribute in DWARF debugging information. The
options are concatenated with spaces separating them from each other and from the compiler version.
See also -frecord-gcc-switches for another way of storing compiler options into the object file.
|
-gno-record-gcc-switches
Disallow appending command-line options to the DW_AT_producer attribute in DWARF debugging
information. This is the default.
|
-gstrict-dwarf
Disallow using extensions of later DWARF standard version than selected with -gdwarf-version. On
most targets using non-conflicting DWARF extensions from later standard versions is allowed.
|
-gno-strict-dwarf
Allow using extensions of later DWARF standard version than selected with -gdwarf-version.
|
-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 0 produces no debug information at all. Thus, -g0 negates -g.
Level 1 produces minimal information, enough for making backtraces in parts of the program that you
don't plan to debug. This includes descriptions of functions and external variables, but no
information about local variables and no line numbers.
Level 3 includes extra information, such as all the macro definitions present in the program. Some
debuggers support macro expansion when you use -g3.
-gdwarf-2 does not accept a concatenated debug level, because GCC used to support an option -gdwarf
that meant to generate debug information in version 1 of the DWARF format (which is very different
from version 2), and it would have been too confusing. That debug format is long obsolete, but the
option cannot be changed now. Instead use an additional -glevel option to change the debug level for
DWARF.
|
-gtoggle
Turn off generation of debug info, if leaving out this option would have generated it, or turn it on
at level 2 otherwise. The position of this argument in the command line does not matter, it takes
effect after all other options are processed, and it does so only once, no matter how many times it
is given. This is mainly intended to be used with -fcompare-debug.
|
-fdump-final-insns[=file]
Dump the final internal representation (RTL) to file. If the optional argument is omitted (or if
file is "."), the name of the dump file will be determined by appending ".gkd" to the compilation
output file name.
|
-fcompare-debug[=opts]
If no error occurs during compilation, run the compiler a second time, adding opts and
-fcompare-debug-second to the arguments passed to the second compilation. Dump the final internal
representation in both compilations, and print an error if they differ.
If the equal sign is omitted, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and nonzero, implicitly enables
-fcompare-debug. If GCC_COMPARE_DEBUG is defined to a string starting with a dash, then it is used
for opts, otherwise the default -gtoggle is used.
|
-fcompare-debug-second
This option is implicitly passed to the compiler for the second compilation requested by
-fcompare-debug, along with options to silence warnings, and omitting other options that would cause
side-effect compiler outputs to files or to the standard output. Dump files and preserved temporary
files are renamed so as to contain the ".gk" additional extension during the second compilation, to
avoid overwriting those generated by the first.
When this option is passed to the compiler driver, it causes the first compilation to be skipped,
which makes it useful for little other than debugging the compiler proper.
|
-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.
|
-femit-struct-debug-baseonly
Emit debug information for struct-like types only when the base name of the compilation source file
matches the base name of file in which the struct was defined.
This option substantially reduces the size of debugging information, but at significant potential
loss in type information to the debugger. See -femit-struct-debug-reduced for a less aggressive
option. See -femit-struct-debug-detailed for more detailed control.
This option works only with DWARF 2.
|
-femit-struct-debug-reduced
Emit debug information for struct-like types only when the base name of the compilation source file
matches the base name of file in which the type was defined, unless the struct is a template or
defined in a system header.
This option significantly reduces the size of debugging information, with some potential loss in type
information to the debugger. See -femit-struct-debug-baseonly for a more aggressive option. See
-femit-struct-debug-detailed for more detailed control.
This option works only with DWARF 2.
|
-femit-struct-debug-detailed[=spec-list]
Specify the struct-like types for which the compiler will generate debug information. The intent is
to reduce duplicate struct debug information between different object files within the same program.
|
-fno-merge-debug-strings
Direct the linker to not merge together strings in the debugging information that are identical in
different object files. Merging is not supported by all assemblers or linkers. Merging decreases
the size of the debug information in the output file at the cost of increasing link processing time.
Merging is enabled by default.
|
-fno-dwarf2-cfi-asm
Emit DWARF 2 unwind info as compiler generated ".eh_frame" section instead of using GAS ".cfi_*"
directives.
|
-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.
|
-fpre-ipa-mem-report
-fpost-ipa-mem-report
Makes the compiler print some statistics about permanent memory allocation before or after
interprocedural optimization.
|
-fstack-usage
Makes the compiler output stack usage information for the program, on a per-function basis. The
filename for the dump is made by appending .su to the auxname. auxname 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. An entry is made up of three fields:
|
-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).
|
--coverage
This option is used to compile and link code instrumented for coverage analysis. The option is a
synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the
documentation for those options for more details.
|
-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.
|
-fdbg-cnt-list
Print the name and the counter upper bound for all debug counters.
|
-fdbg-cnt=counter-value-list
Set the internal debug counter upper bound. counter-value-list is a comma-separated list of
name:value pairs which sets the upper bound of each debug counter name to value. All debug counters
have the initial upper bound of UINT_MAX, thus dbg_cnt() returns true always unless the upper bound
is set by this option. e.g. With -fdbg-cnt=dce:10,tail_call:0 dbg_cnt(dce) will return true only for
first 10 invocations
|
-fenable-kind-pass
-fdisable-kind-pass=range-list
This is a set of debugging options that are used to explicitly disable/enable optimization passes.
For compiler users, regular options for enabling/disabling passes should be used instead.
|
-dletters
-fdump-rtl-pass
Says to make debugging dumps during compilation at times specified by letters. This is used for
debugging the RTL-based passes of the compiler. The file names for most of the dumps are made by
appending a pass number and a word to the dumpname, and the files are created in the directory of the
output file. Note that the pass number is computed statically as passes get registered into the pass
manager. Thus the numbering is not related to the dynamic order of execution of passes. In
particular, a pass installed by a plugin could have a number over 200 even if it executed quite
early. 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. These switches may have different
effects when -E is used for preprocessing.
Debug dumps can be enabled with a -fdump-rtl switch or some -d option letters. Here are the possible
letters for use in pass and letters, and their meanings:
|
-fdump-rtl-alignments
Dump after branch alignments have been computed.
|
-fdump-rtl-asmcons
Dump after fixing rtl statements that have unsatisfied in/out constraints.
|
-fdump-rtl-auto_inc_dec
Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or
auto dec instructions.
|
-fdump-rtl-barriers
Dump after cleaning up the barrier instructions.
|
-fdump-rtl-bbpart
Dump after partitioning hot and cold basic blocks.
|
-fdump-rtl-bbro
Dump after block reordering.
|
-fdump-rtl-btl1
-fdump-rtl-btl2
-fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two branch target load optimization
passes.
|
-fdump-rtl-bypass
Dump after jump bypassing and control flow optimizations.
|
-fdump-rtl-combine
Dump after the RTL instruction combination pass.
|
-fdump-rtl-compgotos
Dump after duplicating the computed gotos.
|
-fdump-rtl-ce1
-fdump-rtl-ce2
-fdump-rtl-ce3
-fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if conversion
passes.
|
-fdump-rtl-cprop_hardreg
Dump after hard register copy propagation.
|
-fdump-rtl-csa
Dump after combining stack adjustments.
|
-fdump-rtl-cse1
-fdump-rtl-cse2
-fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common sub-expression
elimination passes.
|
-fdump-rtl-dce
Dump after the standalone dead code elimination passes.
|
-fdump-rtl-dbr
Dump after delayed branch scheduling.
|
-fdump-rtl-dce1
-fdump-rtl-dce2
-fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store elimination passes.
|
-fdump-rtl-eh
Dump after finalization of EH handling code.
|
-fdump-rtl-eh_ranges
Dump after conversion of EH handling range regions.
|
-fdump-rtl-expand
Dump after RTL generation.
|
-fdump-rtl-fwprop1
-fdump-rtl-fwprop2
-fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward propagation
passes.
|
-fdump-rtl-gcse1
-fdump-rtl-gcse2
-fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common subexpression
elimination.
|
-fdump-rtl-init-regs
Dump after the initialization of the registers.
|
-fdump-rtl-initvals
Dump after the computation of the initial value sets.
|
-fdump-rtl-into_cfglayout
Dump after converting to cfglayout mode.
|
-fdump-rtl-ira
Dump after iterated register allocation.
|
-fdump-rtl-jump
Dump after the second jump optimization.
|
-fdump-rtl-loop2
-fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.
|
-fdump-rtl-mach
Dump after performing the machine dependent reorganization pass, if that pass exists.
|
-fdump-rtl-mode_sw
Dump after removing redundant mode switches.
|
-fdump-rtl-rnreg
Dump after register renumbering.
|
-fdump-rtl-outof_cfglayout
Dump after converting from cfglayout mode.
|
-fdump-rtl-peephole2
Dump after the peephole pass.
|
-fdump-rtl-postreload
Dump after post-reload optimizations.
|
-fdump-rtl-pro_and_epilogue
Dump after generating the function prologues and epilogues.
|
-fdump-rtl-regmove
Dump after the register move pass.
|
-fdump-rtl-sched1
-fdump-rtl-sched2
-fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic block scheduling passes.
|
-fdump-rtl-see
Dump after sign extension elimination.
|
-fdump-rtl-seqabstr
Dump after common sequence discovery.
|
-fdump-rtl-shorten
Dump after shortening branches.
|
-fdump-rtl-sibling
Dump after sibling call optimizations.
|
-fdump-rtl-split1
-fdump-rtl-split2
-fdump-rtl-split3
-fdump-rtl-split4
-fdump-rtl-split5
-fdump-rtl-split1, -fdump-rtl-split2, -fdump-rtl-split3, -fdump-rtl-split4 and -fdump-rtl-split5
enable dumping after five rounds of instruction splitting.
|
-fdump-rtl-sms
Dump after modulo scheduling. This pass is only run on some architectures.
|
-fdump-rtl-stack
Dump after conversion from GCC's "flat register file" registers to the x87's stack-like
registers. This pass is only run on x86 variants.
|
-fdump-rtl-subreg1
-fdump-rtl-subreg2
-fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after the two subreg expansion passes.
|
-fdump-rtl-unshare
Dump after all rtl has been unshared.
|
-fdump-rtl-vartrack
Dump after variable tracking.
|
-fdump-rtl-vregs
Dump after converting virtual registers to hard registers.
|
-fdump-rtl-web
Dump after live range splitting.
|
-fdump-rtl-regclass
-fdump-rtl-subregs_of_mode_init
-fdump-rtl-subregs_of_mode_finish
-fdump-rtl-dfinit
-fdump-rtl-dfinish
These dumps are defined but always produce empty files.
|
-fdump-rtl-all
Produce all the dumps listed above.
|
-fdump-noaddr
When doing debugging dumps, suppress address output. This makes it more feasible to use diff on
debugging dumps for compiler invocations with different compiler binaries and/or different text / bss
/ data / heap / stack / dso start locations.
|
-fdump-unnumbered
When doing debugging dumps, suppress instruction numbers and address 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-unnumbered-links
When doing debugging dumps (see -d option above), suppress instruction numbers for the links to the
previous and next instructions in a sequence.
|
-fdump-translation-unit (C++ only)
-fdump-translation-unit-options (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, and the file is created in the same directory as
the output file. 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, and the file is created in the same
directory as the output file. If the -options form is used, options controls the details of the dump
as described for the -fdump-tree options.
|
-fdump-ipa-switch
Control the dumping at various stages of inter-procedural analysis language tree to a file. The file
name is generated by appending a switch specific suffix to the source file name, and the file is
created in the same directory as the output file. The following dumps are possible:
all Enables all inter-procedural analysis dumps.
cgraph
Dumps information about call-graph optimization, unused function removal, and inlining decisions.
inline
Dump after function inlining.
|
-fdump-passes
Dump the list of optimization passes that are turned on and off by the current command-line options.
|
-fdump-statistics-option
Enable and control dumping of pass statistics in a separate file. The file name is generated by
appending a suffix ending in .statistics to the source file name, and the file is created in the same
directory as the output file. If the -option form is used, -stats will cause counters to be summed
over the whole compilation unit while -details will dump every event as the passes generate them.
The default with no option is to sum counters for each function compiled.
|
-fdump-tree-switch
-fdump-tree-switch-options
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, and the file is
created in the same directory as the output file. If the -options form is used, options is a list of
- separated options which control the details of the dump. Not all options are applicable to all
dumps; those that are not meaningful will be ignored. The following options are available
|
-ftree-vectorizer-verbose=n
This option controls the amount of debugging output the vectorizer prints. This information is
written to standard error, unless -fdump-tree-all or -fdump-tree-vect is specified, in which case it
is output to the usual dump listing file, .vect. For n=0 no diagnostic information is reported. If
n=1 the vectorizer reports each loop that got vectorized, and the total number of loops that got
vectorized. If n=2 the vectorizer also reports non-vectorized loops that passed the first analysis
phase (vect_analyze_loop_form) - i.e. countable, inner-most, single-bb, single-entry/exit loops.
This is the same verbosity level that -fdump-tree-vect-stats uses. Higher verbosity levels mean
either more information dumped for each reported loop, or same amount of information reported for
more loops: if n=3, vectorizer cost model information is reported. If n=4, alignment related
information is added to the reports. If n=5, data-references related information (e.g. memory
dependences, memory access-patterns) is added to the reports. If n=6, the vectorizer reports also
non-vectorized inner-most loops that did not pass the first analysis phase (i.e., may not be
countable, or may have complicated control-flow). If n=7, the vectorizer reports also non-vectorized
nested loops. If n=8, SLP related information is added to the reports. For n=9, all the information
the vectorizer generates during its analysis and transformation is reported. This is the same
verbosity level that -fdump-tree-vect-details uses.
|
-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 -fdump-rtl-sched1 or
-fdump-rtl-sched2 is specified, in which case it is output to the usual dump listing file, .sched1 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 -fdump-rtl-sched1 and
-fdump-rtl-sched2. 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
-save-temps=cwd
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.
When used in combination with the -x command-line option, -save-temps is sensible enough to avoid
over writing an input source file with the same extension as an intermediate file. The corresponding
intermediate file may be obtained by renaming the source file before using -save-temps.
If you invoke GCC in parallel, compiling several different source files that share a common base name
in different subdirectories or the same source file compiled for multiple output destinations, it is
likely that the different parallel compilers will interfere with each other, and overwrite the
temporary files. For instance:
gcc -save-temps -o outdir1/foo.o indir1/foo.c&
gcc -save-temps -o outdir2/foo.o indir2/foo.c&
may result in foo.i and foo.o being written to simultaneously by both compilers.
|
-save-temps=obj
Store the usual "temporary" intermediate files permanently. If the -o option is used, the temporary
files are based on the object file. If the -o option is not used, the -save-temps=obj switch behaves
like -save-temps.
For example:
|
-time[=file]
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).
|
-fvar-tracking
Run variable tracking pass. It computes where variables are stored at each position in code. Better
debugging information is then generated (if the debugging information format supports this
information).
It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information
(-g) and the debug info format supports it.
|
-fvar-tracking-assignments
Annotate assignments to user variables early in the compilation and attempt to carry the annotations
over throughout the compilation all the way to the end, in an attempt to improve debug information
while optimizing. Use of -gdwarf-4 is recommended along with it.
It can be enabled even if var-tracking is disabled, in which case annotations will be created and
maintained, but discarded at the end.
|
-fvar-tracking-assignments-toggle
Toggle -fvar-tracking-assignments, in the same way that -gtoggle toggles -g.
|
-print-file-name=library
Print the full absolute name of the library file library that would be used when linking---and don't
do anything else. With this option, GCC does not compile or link anything; it just prints the file
name.
|
-print-multi-directory
Print the directory name corresponding to the multilib selected by any other switches present in the
command line. This directory is supposed to exist in GCC_EXEC_PREFIX.
|
-print-multi-lib
Print the mapping from multilib directory names to compiler switches that enable them. The directory
name is separated from the switches by ;, and each switch starts with an @ instead of the -, without
spaces between multiple switches. This is supposed to ease shell-processing.
|
-print-multi-os-directory
Print the path to OS libraries for the selected multilib, relative to some lib subdirectory. If OS
libraries are present in the lib subdirectory and no multilibs are used, this is usually just ., if
OS libraries are present in libsuffix sibling directories this prints e.g. ../lib64, ../lib or
../lib32, or if OS libraries are present in lib/subdir subdirectories it prints e.g. amd64, sparcv9
or ev6.
|
-print-multiarch
Print the path to OS libraries for the selected multiarch, relative to some lib subdirectory.
|
-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 /.
|
-print-sysroot
Print the target sysroot directory that will be used during compilation. This is the target sysroot
specified either at configure time or using the --sysroot option, possibly with an extra suffix that
depends on compilation options. If no target sysroot is specified, the option prints nothing.
|
-print-sysroot-headers-suffix
Print the suffix added to the target sysroot when searching for headers, or give an error if the
compiler is not configured with such a suffix---and don't do anything else.
|
-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:
-fauto-inc-dec -fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch -fdse
-fguess-branch-probability -fif-conversion2 -fif-conversion -fipa-pure-const -fipa-profile
-fipa-reference -fmerge-constants -fsplit-wide-types -ftree-bit-ccp -ftree-builtin-call-dce
-ftree-ccp -ftree-ch -ftree-copyrename -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop
-ftree-fre -ftree-phiprop -ftree-sra -ftree-pta -ftree-ter -funit-at-a-time
-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. 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: -fthread-jumps -falign-functions -falign-jumps -falign-loops -falign-labels -fcaller-saves
-fcrossjumping -fcse-follow-jumps -fcse-skip-blocks -fdelete-null-pointer-checks -fdevirtualize
-fexpensive-optimizations -fgcse -fgcse-lm -finline-small-functions -findirect-inlining -fipa-sra
-foptimize-sibling-calls -fpartial-inlining -fpeephole2 -fregmove -freorder-blocks
-freorder-functions -frerun-cse-after-loop -fsched-interblock -fsched-spec -fschedule-insns
-fschedule-insns2 -fstrict-aliasing -fstrict-overflow -ftree-switch-conversion -ftree-tail-merge
-ftree-pre -ftree-vrp
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, -funswitch-loops, -fpredictive-commoning, -fgcse-after-reload, -ftree-vectorize
and -fipa-cp-clone options.
-O0 Reduce compilation time and make debugging produce the expected results. 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 -freorder-blocks-and-partition -fprefetch-loop-arrays
-ftree-vect-loop-version
|
-Ofast
Disregard strict standards compliance. -Ofast enables all -O3 optimizations. It also enables
optimizations that are not valid for all standard compliant programs. It turns on -ffast-math and
the Fortran-specific -fno-protect-parens and -fstack-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 that
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.
|
-fforward-propagate
Perform a forward propagation pass on RTL. The pass tries to combine two instructions and checks if
the result can be simplified. If loop unrolling is active, two passes are performed and the second
is scheduled after loop unrolling.
This option is enabled by default at optimization levels -O, -O2, -O3, -Os.
|
-ffp-contract=style
-ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables
floating-point expression contraction such as forming of fused multiply-add operations if the target
has native support for them. -ffp-contract=on enables floating-point expression contraction if
allowed by the language standard. This is currently not implemented and treated equal to
-ffp-contract=off.
|
-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.
Starting with GCC version 4.6, the default setting (when not optimizing for size) for 32-bit Linux
x86 and 32-bit Darwin x86 targets has been changed to -fomit-frame-pointer. The default can be
reverted to -fno-omit-frame-pointer by configuring GCC with the --enable-frame-pointer configure
option.
Enabled at levels -O, -O2, -O3, -Os.
|
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
|
-fno-inline
Do not expand any functions inline apart from those marked with the "always_inline" attribute. This
is the default when not optimizing.
Single functions can be exempted from inlining by marking them with the "noinline" attribute.
|
-finline-small-functions
Integrate functions into their callers when their body is smaller than expected function call code
(so overall size of program gets smaller). The compiler heuristically decides which functions are
simple enough to be worth integrating in this way. This inlining applies to all functions, even
those not declared inline.
Enabled at level -O2.
|
-findirect-inlining
Inline also indirect calls that are discovered to be known at compile time thanks to previous
inlining. This option has any effect only when inlining itself is turned on by the
-finline-functions or -finline-small-functions options.
Enabled at level -O2.
|
-finline-functions
Consider all functions for inlining, even if they are not declared inline. The compiler
heuristically decides which functions are 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-functions-called-once
Consider all "static" functions called once for inlining into their caller even if they are not
marked "inline". If a call to a given function is integrated, then the function is not output as
assembler code in its own right.
Enabled at levels -O1, -O2, -O3 and -Os.
|
-fearly-inlining
Inline functions marked by "always_inline" and functions whose body seems smaller than the function
call overhead early before doing -fprofile-generate instrumentation and real inlining pass. Doing so
makes profiling significantly cheaper and usually inlining faster on programs having large chains of
nested wrapper functions.
Enabled by default.
|
-fipa-sra
Perform interprocedural scalar replacement of aggregates, removal of unused parameters and
replacement of parameters passed by reference by parameters passed by value.
Enabled at levels -O2, -O3 and -Os.
|
-finline-limit=n
By default, GCC limits the size of functions that can be inlined. This flag allows coarse control of
this limit. n is the size of functions that can be inlined in number of pseudo instructions.
|
-fno-keep-inline-dllexport
This is a more fine-grained version of -fkeep-inline-functions, which applies only to functions that
are declared using the "dllexport" attribute or declspec
|
-fkeep-inline-functions
In C, emit "static" functions that are declared "inline" into the object file, even if the function
has been inlined into all of its callers. This switch does not affect functions using the "extern
inline" extension in GNU C90. In C++, emit any and all inline functions into the object file.
|
-fkeep-static-consts
Emit variables declared "static const" when optimization isn't turned on, even if the variables
aren't referenced.
GCC enables this option by default. If you want to force the compiler to check if the variable was
referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts
option.
|
-fmerge-constants
Attempt to merge identical constants (string constants and floating-point constants) across
compilation units.
This option is the default for optimized compilation if the assembler and linker support it. Use
-fno-merge-constants to inhibit this behavior.
Enabled at levels -O, -O2, -O3, -Os.
|
-fmerge-all-constants
Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to -fmerge-constants this considers e.g. even
constant initialized arrays or initialized constant variables with integral or floating-point types.
Languages like C or C++ require each variable, including multiple instances of the same variable in
recursive calls, to have distinct locations, so using this option will result in non-conforming
behavior.
|
-fmodulo-sched
Perform swing modulo scheduling immediately before the first scheduling pass. This pass looks at
innermost loops and reorders their instructions by overlapping different iterations.
|
-fmodulo-sched-allow-regmoves
Perform more aggressive SMS based modulo scheduling with register moves allowed. By setting this
flag certain anti-dependences edges will be deleted which will trigger the generation of reg-moves
based on the life-range analysis. This option is effective only with -fmodulo-sched enabled.
|
-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.
|
-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.
|
-fmudflap -fmudflapth -fmudflapir
For front-ends that support it (C and C++), instrument all risky pointer/array dereferencing
operations, some standard library string/heap functions, and some other associated constructs with
range/validity tests. Modules so instrumented should be immune to buffer overflows, invalid heap
use, and some other classes of C/C++ programming errors. The instrumentation relies on a separate
runtime library (libmudflap), which will be linked into a program if -fmudflap is given at link time.
Run-time behavior of the instrumented program is controlled by the MUDFLAP_OPTIONS environment
variable. See "env MUDFLAP_OPTIONS=-help a.out" for its options.
Use -fmudflapth instead of -fmudflap to compile and to link if your program is multi-threaded. Use
-fmudflapir, in addition to -fmudflap or -fmudflapth, if instrumentation should ignore pointer reads.
This produces less instrumentation (and therefore faster execution) and still provides some
protection against outright memory corrupting writes, but allows erroneously read data to propagate
within a program.
|
-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 -O2, -O3, -Os.
|
-fsplit-wide-types
When using a type that occupies multiple registers, such as "long long" on a 32-bit system, split the
registers apart and allocate them independently. This normally generates better code for those
types, but may make debugging more difficult.
Enabled at levels -O, -O2, -O3, -Os.
|
-fcse-follow-jumps
In common subexpression elimination (CSE), 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 that 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.
|
-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 run-time
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 that
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.
Not enabled at any optimization level.
|
-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).
Not enabled at any optimization level.
|
-fgcse-after-reload
When -fgcse-after-reload is enabled, a redundant load elimination pass is performed after reload.
The purpose of this pass is to cleanup redundant spilling.
|
-funsafe-loop-optimizations
If given, the loop optimizer will assume that loop indices do not overflow, and that the loops with
nontrivial exit condition are not infinite. This enables a wider range of loop optimizations even if
the loop optimizer itself cannot prove that these assumptions are valid. Using
-Wunsafe-loop-optimizations, the compiler will warn you if it finds this kind of loop.
|
-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 -O2, -O3, -Os.
|
-fauto-inc-dec
Combine increments or decrements of addresses with memory accesses. This pass is always skipped on
architectures that do not have instructions to support this. Enabled by default at -O and higher on
architectures that support this.
|
-fdce
Perform dead code elimination (DCE) on RTL. Enabled by default at -O and higher.
|
-fdse
Perform dead store elimination (DSE) on RTL. Enabled by default at -O and higher.
|
-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
Assume that programs cannot safely dereference null pointers, and that no code or data element
resides there. This enables simple constant folding optimizations at all optimization levels. In
addition, other optimization passes in GCC use this flag to control global dataflow analyses that
eliminate useless checks for null pointers; these assume that if a pointer is checked after it has
already been dereferenced, it cannot be null.
Note however that in some environments this assumption is not true. Use
-fno-delete-null-pointer-checks to disable this optimization for programs that depend on that
behavior.
Some targets, especially embedded ones, disable this option at all levels. Otherwise it is enabled
at all levels: -O0, -O1, -O2, -O3, -Os. Passes that use the information are enabled independently at
different optimization levels.
|
-fdevirtualize
Attempt to convert calls to virtual functions to direct calls. This is done both within a procedure
and interprocedurally as part of indirect inlining ("-findirect-inlining") and interprocedural
constant propagation (-fipa-cp). Enabled at levels -O2, -O3, -Os.
|
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively expensive.
Enabled at levels -O2, -O3, -Os.
|
-free
Attempt to remove redundant extension instructions. This is especially helpful for the x86-64
architecture which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit
half.
Enabled for x86 at levels -O2, -O3.
|
-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.
|
-fira-algorithm=algorithm
Use the specified coloring algorithm for the integrated register allocator. The algorithm argument
can be priority, which specifies Chow's priority coloring, or CB, which specifies Chaitin-Briggs
coloring. Chaitin-Briggs coloring is not implemented for all architectures, but for those targets
that do support it, it is the default because it generates better code.
|
-fira-region=region
Use specified regions for the integrated register allocator. The region argument should be one of
the following:
all Use all loops as register allocation regions. This can give the best results for machines with a
small and/or irregular register set.
mixed
Use all loops except for loops with small register pressure as the regions. This value usually
gives the best results in most cases and for most architectures, and is enabled by default when
compiling with optimization for speed (-O, -O2, ...).
one Use all functions as a single region. This typically results in the smallest code size, and is
enabled by default for -Os or -O0.
|
-fira-loop-pressure
Use IRA to evaluate register pressure in loops for decisions to move loop invariants. This option
usually results in generation of faster and smaller code on machines with large register files (>= 32
registers), but it can slow the compiler down.
This option is enabled at level -O3 for some targets.
|
-fno-ira-share-save-slots
Disable sharing of stack slots used for saving call-used hard registers living through a call. Each
hard register gets a separate stack slot, and as a result function stack frames are larger.
|
-fno-ira-share-spill-slots
Disable sharing of stack slots allocated for pseudo-registers. Each pseudo-register that does not
get a hard register gets a separate stack slot, and as a result function stack frames are larger.
|
-fira-verbose=n
Control the verbosity of the dump file for the integrated register allocator. The default value is
5. If the value n is greater or equal to 10, the dump output is sent to stderr using the same format
as n minus 10.
|
-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.
|
-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-pressure
Enable register pressure sensitive insn scheduling before the register allocation. This only makes
sense when scheduling before register allocation is enabled, i.e. with -fschedule-insns or at -O2 or
higher. Usage of this option can improve the generated code and decrease its size by preventing
register pressure increase above the number of available hard registers and as a consequence register
spills in the register allocation.
|
-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
-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. -fno-sched-stalled-insns means that no insns will be
moved prematurely, -fsched-stalled-insns=0 means there is no limit on how many queued insns can be
moved prematurely. -fsched-stalled-insns without a value is equivalent to -fsched-stalled-insns=1.
|
-fsched-stalled-insns-dep
-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. This has an effect only during the
second scheduling pass, and only if -fsched-stalled-insns is used. -fno-sched-stalled-insns-dep is
equivalent to -fsched-stalled-insns-dep=0. -fsched-stalled-insns-dep without a value is equivalent
to -fsched-stalled-insns-dep=1.
|
-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.
|
-fsched-group-heuristic
Enable the group heuristic in the scheduler. This heuristic favors the instruction that belongs to a
schedule group. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
|
-fsched-critical-path-heuristic
Enable the critical-path heuristic in the scheduler. This heuristic favors instructions on the
critical path. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
|
-fsched-spec-insn-heuristic
Enable the speculative instruction heuristic in the scheduler. This heuristic favors speculative
instructions with greater dependency weakness. This is enabled by default when scheduling is
enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
|
-fsched-rank-heuristic
Enable the rank heuristic in the scheduler. This heuristic favors the instruction belonging to a
basic block with greater size or frequency. This is enabled by default when scheduling is enabled,
i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
|
-fsched-last-insn-heuristic
Enable the last-instruction heuristic in the scheduler. This heuristic favors the instruction that
is less dependent on the last instruction scheduled. This is enabled by default when scheduling is
enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
|
-fsched-dep-count-heuristic
Enable the dependent-count heuristic in the scheduler. This heuristic favors the instruction that
has more instructions depending on it. This is enabled by default when scheduling is enabled, i.e.
with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
|
-freschedule-modulo-scheduled-loops
The modulo scheduling comes before the traditional scheduling, if a loop was modulo scheduled we may
want to prevent the later scheduling passes from changing its schedule, we use this option to control
that.
|
-fselective-scheduling
Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the
first scheduler pass.
|
-fselective-scheduling2
Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the
second scheduler pass.
|
-fsel-sched-pipelining
Enable software pipelining of innermost loops during selective scheduling. This option has no effect
until one of -fselective-scheduling or -fselective-scheduling2 is turned on.
|
-fsel-sched-pipelining-outer-loops
When pipelining loops during selective scheduling, also pipeline outer loops. This option has no
effect until -fsel-sched-pipelining is turned on.
|
-fshrink-wrap
Emit function prologues only before parts of the function that need it, rather than at the top of the
function. This flag is enabled by default at -O and 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.
|
-fcombine-stack-adjustments
Tracks stack adjustments (pushes and pops) and stack memory references and then tries to find ways to
combine them.
Enabled by default at -O1 and higher.
|
-fconserve-stack
Attempt to minimize stack usage. The compiler will attempt to use less stack space, even if that
makes the program slower. This option implies setting the large-stack-frame parameter to 100 and the
large-stack-frame-growth parameter to 400.
|
-ftree-reassoc
Perform reassociation on trees. This flag is enabled by default at -O and higher.
|
-ftree-pre
Perform partial redundancy elimination (PRE) on trees. This flag is enabled by default at -O2 and
-O3.
|
-ftree-forwprop
Perform forward propagation on trees. This flag is enabled by default at -O and higher.
|
-ftree-fre
Perform full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE
only considers expressions that are computed on all paths leading to the redundant computation. This
analysis is faster than PRE, though it exposes fewer redundancies. This flag is enabled by default
at -O and higher.
|
-ftree-phiprop
Perform hoisting of loads from conditional pointers on trees. This pass is enabled by default at -O
and higher.
|
-ftree-copy-prop
Perform copy propagation on trees. This pass eliminates unnecessary copy operations. This flag is
enabled by default at -O and higher.
|
-fipa-pure-const
Discover which functions are pure or constant. Enabled by default at -O and higher.
|
-fipa-reference
Discover which static variables do not escape cannot escape the compilation unit. Enabled by default
at -O and higher.
|
-fipa-pta
Perform interprocedural pointer analysis and interprocedural modification and reference analysis.
This option can cause excessive memory and compile-time usage on large compilation units. It is not
enabled by default at any optimization level.
|
-fipa-profile
Perform interprocedural profile propagation. The functions called only from cold functions are
marked as cold. Also functions executed once (such as "cold", "noreturn", static constructors or
destructors) are identified. Cold functions and loop less parts of functions executed once are then
optimized for size. Enabled by default at -O and higher.
|
-fipa-cp
Perform interprocedural constant propagation. This optimization analyzes the program to determine
when values passed to functions are constants and then optimizes accordingly. This optimization can
substantially increase performance if the application has constants passed to functions. This flag
is enabled by default at -O2, -Os and -O3.
|
-fipa-cp-clone
Perform function cloning to make interprocedural constant propagation stronger. When enabled,
interprocedural constant propagation will perform function cloning when externally visible function
can be called with constant arguments. Because this optimization can create multiple copies of
functions, it may significantly increase code size (see --param ipcp-unit-growth=value). This flag
is enabled by default at -O3.
|
-fipa-matrix-reorg
Perform matrix flattening and transposing. Matrix flattening tries to replace an m-dimensional
matrix with its equivalent n-dimensional matrix, where n < m. This reduces the level of indirection
needed for accessing the elements of the matrix. The second optimization is matrix transposing, which
attempts to change the order of the matrix's dimensions in order to improve cache locality. Both
optimizations need the -fwhole-program flag. Transposing is enabled only if profiling information is
available.
|
-ftree-sink
Perform forward store motion on trees. This flag is enabled by default at -O and higher.
|
-ftree-bit-ccp
Perform sparse conditional bit constant propagation on trees and propagate pointer alignment
information. This pass only operates on local scalar variables and is enabled by default at -O and
higher. It requires that -ftree-ccp is enabled.
|
-ftree-ccp
Perform sparse conditional constant propagation (CCP) on trees. This pass only operates on local
scalar variables and is enabled by default at -O and higher.
|
-ftree-switch-conversion
Perform conversion of simple initializations in a switch to initializations from a scalar array.
This flag is enabled by default at -O2 and higher.
|
-ftree-tail-merge
Look for identical code sequences. When found, replace one with a jump to the other. This
optimization is known as tail merging or cross jumping. This flag is enabled by default at -O2 and
higher. The compilation time in this pass can be limited using max-tail-merge-comparisons parameter
and max-tail-merge-iterations parameter.
|
-ftree-dce
Perform dead code elimination (DCE) on trees. This flag is enabled by default at -O and higher.
|
-ftree-builtin-call-dce
Perform conditional dead code elimination (DCE) for calls to builtin functions that may set "errno"
but are otherwise side-effect free. This flag is enabled by default at -O2 and higher if -Os is not
also specified.
|
-ftree-dominator-opts
Perform a variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range
propagation and expression simplification) based on a dominator tree traversal. This also performs
jump threading (to reduce jumps to jumps). This flag is enabled by default at -O and higher.
|
-ftree-dse
Perform dead store elimination (DSE) on trees. A dead store is a store into a memory location that
is later overwritten by another store without any intervening loads. In this case the earlier store
can be deleted. This flag is enabled by default at -O and higher.
|
-ftree-ch
Perform loop header copying on trees. This is beneficial since it increases effectiveness of code
motion optimizations. It also saves one jump. This flag is enabled by default at -O and higher. It
is not enabled for -Os, since it usually increases code size.
|
-ftree-loop-optimize
Perform loop optimizations on trees. This flag is enabled by default at -O and higher.
|
-ftree-loop-linear
Perform loop interchange transformations on tree. Same as -floop-interchange. To use this code
transformation, GCC has to be configured with --with-ppl and --with-cloog to enable the Graphite loop
transformation infrastructure.
|
-floop-interchange
Perform loop interchange transformations on loops. Interchanging two nested loops switches the inner
and outer loops. For example, given a loop like:
|
-floop-strip-mine
Perform loop strip mining transformations on loops. Strip mining splits a loop into two nested
loops. The outer loop has strides equal to the strip size and the inner loop has strides of the
original loop within a strip. The strip length can be changed using the loop-block-tile-size
parameter. For example, given a loop like:
|
-floop-block
Perform loop blocking transformations on loops. Blocking strip mines each loop in the loop nest such
that the memory accesses of the element loops fit inside caches. The strip length can be changed
using the loop-block-tile-size parameter. For example, given a loop like:
|
-fgraphite-identity
Enable the identity transformation for graphite. For every SCoP we generate the polyhedral
representation and transform it back to gimple. Using -fgraphite-identity we can check the costs or
benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations are also
performed by the code generator CLooG, like index splitting and dead code elimination in loops.
|
-floop-flatten
Removes the loop nesting structure: transforms the loop nest into a single loop. This transformation
can be useful as an enablement transform for vectorization and parallelization. This feature is
experimental. To use this code transformation, GCC has to be configured with --with-ppl and
--with-cloog to enable the Graphite loop transformation infrastructure.
|
-floop-parallelize-all
Use the Graphite data dependence analysis to identify loops that can be parallelized. Parallelize
all the loops that can be analyzed to not contain loop carried dependences without checking that it
is profitable to parallelize the loops.
|
-fcheck-data-deps
Compare the results of several data dependence analyzers. This option is used for debugging the data
dependence analyzers.
|
-ftree-loop-if-convert
Attempt to transform conditional jumps in the innermost loops to branch-less equivalents. The intent
is to remove control-flow from the innermost loops in order to improve the ability of the
vectorization pass to handle these loops. This is enabled by default if vectorization is enabled.
|
-ftree-loop-if-convert-stores
Attempt to also if-convert conditional jumps containing memory writes. This transformation can be
unsafe for multi-threaded programs as it transforms conditional memory writes into unconditional
memory writes. For example,
for (i = 0; i < N; i++)
if (cond)
A[i] = expr;
would be transformed to
|
-ftree-loop-distribution
Perform loop distribution. This flag can improve cache performance on big loop bodies and allow
further loop optimizations, like parallelization or vectorization, to take place. For example, the
loop
|
-ftree-loop-distribute-patterns
Perform loop distribution of patterns that can be code generated with calls to a library. This flag
is enabled by default at -O3.
This pass distributes the initialization loops and generates a call to memset zero. For example, the
loop
|
-ftree-loop-im
Perform loop invariant motion on trees. This pass moves only invariants that would be hard to handle
at RTL level (function calls, operations that expand to nontrivial sequences of insns). With
-funswitch-loops it also moves operands of conditions that are invariant out of the loop, so that we
can use just trivial invariantness analysis in loop unswitching. The pass also includes store
motion.
|
-ftree-loop-ivcanon
Create a canonical counter for number of iterations in loops for which determining number of
iterations requires complicated analysis. Later optimizations then may determine the number easily.
Useful especially in connection with unrolling.
|
-fivopts
Perform induction variable optimizations (strength reduction, induction variable merging and
induction variable elimination) on trees.
|
-ftree-parallelize-loops=n
Parallelize loops, i.e., split their iteration space to run in n threads. This is only possible for
loops whose iterations are independent and can be arbitrarily reordered. The optimization is only
profitable on multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g.
by memory bandwidth. This option implies -pthread, and thus is only supported on targets that have
support for -pthread.
|
-ftree-pta
Perform function-local points-to analysis on trees. This flag is enabled by default at -O and
higher.
|
-ftree-sra
Perform scalar replacement of aggregates. This pass replaces structure references with scalars to
prevent committing structures to memory too early. This flag is enabled by default at -O and higher.
|
-ftree-copyrename
Perform copy renaming on trees. This pass attempts to rename compiler temporaries to other variables
at copy locations, usually resulting in variable names which more closely resemble the original
variables. This flag is enabled by default at -O and higher.
|
-ftree-ter
Perform temporary expression replacement during the SSA->normal phase. Single use/single def
temporaries are replaced at their use location with their defining expression. This results in non-
GIMPLE code, but gives the expanders much more complex trees to work on resulting in better RTL
generation. This is enabled by default at -O and higher.
|
-ftree-vectorize
Perform loop vectorization on trees. This flag is enabled by default at -O3.
|
-ftree-slp-vectorize
Perform basic block vectorization on trees. This flag is enabled by default at -O3 and when
-ftree-vectorize is enabled.
|
-ftree-vect-loop-version
Perform loop versioning when doing loop vectorization on trees. When a loop appears to be
vectorizable except that data alignment or data dependence cannot be determined at compile time, then
vectorized and non-vectorized versions of the loop are generated along with run-time checks for
alignment or dependence to control which version is executed. This option is enabled by default
except at level -Os where it is disabled.
|
-fvect-cost-model
Enable cost model for vectorization.
|
-ftree-vrp
Perform Value Range Propagation on trees. This is similar to the constant propagation pass, but
instead of values, ranges of values are propagated. This allows the optimizers to remove unnecessary
range checks like array bound checks and null pointer checks. This is enabled by default at -O2 and
higher. Null pointer check elimination is only done if -fdelete-null-pointer-checks is enabled.
|
-ftracer
Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow
of the function allowing other optimizations to do better job.
|
-funroll-loops
Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop.
-funroll-loops implies -frerun-cse-after-loop. This option makes code larger, and may or may not
make it run faster.
|
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This
usually makes programs run more slowly. -funroll-all-loops implies the same options as
-funroll-loops,
|
-fsplit-ivs-in-unroller
Enables expressing of values of induction variables in later iterations of the unrolled loop using
the value in the first iteration. This breaks long dependency chains, thus improving efficiency of
the scheduling passes.
Combination of -fweb and CSE is often sufficient to obtain the same effect. However in cases the
loop body is more complicated than a single basic block, this is not reliable. It also does not work
at all on some of the architectures due to restrictions in the CSE pass.
This optimization is enabled by default.
|
-fvariable-expansion-in-unroller
With this option, the compiler will create multiple copies of some local variables when unrolling a
loop which can result in superior code.
|
-fpartial-inlining
Inline parts of functions. This option has any effect only when inlining itself is turned on by the
-finline-functions or -finline-small-functions options.
Enabled at level -O2.
|
-fpredictive-commoning
Perform predictive commoning optimization, i.e., reusing computations (especially memory loads and
stores) performed in previous iterations of loops.
This option is enabled at level -O3.
|
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to prefetch memory to improve the
performance of loops that access large arrays.
This option may generate better or worse code; results are highly dependent on the structure of loops
within the source code.
Disabled at level -Os.
|
-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.
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-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.
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-fno-guess-branch-probability
Do not guess branch probabilities using heuristics.
GCC will use heuristics to guess branch probabilities if they are not provided by profiling feedback
(-fprofile-arcs). These heuristics are based on the control flow graph. If some branch
probabilities are specified by __builtin_expect, then the heuristics will be used to guess branch
probabilities for the rest of the control flow graph, taking the __builtin_expect info into account.
The interactions between the heuristics and __builtin_expect can be complex, and in some cases, it
may be useful to disable the heuristics so that the effects of __builtin_expect are easier to
understand.
The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.
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-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.
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-freorder-blocks-and-partition
In addition to reordering basic blocks in the compiled function, in order to reduce number of taken
branches, partitions hot and cold basic blocks into separate sections of the assembly and .o files,
to improve paging and cache locality performance.
This optimization is automatically turned off in the presence of exception handling, for linkonce
sections, for functions with a user-defined section attribute and on any architecture that does not
support named sections.
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-freorder-functions
Reorder functions in the object file in order to 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.
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-fstrict-aliasing
Allow 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.
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-fstrict-overflow
Allow the compiler to assume strict signed overflow rules, depending on the language being compiled.
For C (and C++) this means that overflow when doing arithmetic with signed numbers is undefined,
which means that the compiler may assume that it will not happen. This permits various
optimizations. For example, the compiler will assume that an expression like "i + 10 > i" will
always be true for signed "i". This assumption is only valid if signed overflow is undefined, as the
expression is false if "i + 10" overflows when using twos complement arithmetic. When this option is
in effect any attempt to determine whether an operation on signed numbers will overflow must be
written carefully to not actually involve overflow.
This option also allows the compiler to assume strict pointer semantics: given a pointer to an
object, if adding an offset to that pointer does not produce a pointer to the same object, the
addition is undefined. This permits the compiler to conclude that "p + u > p" is always true for a
pointer "p" and unsigned integer "u". This assumption is only valid because pointer wraparound is
undefined, as the expression is false if "p + u" overflows using twos complement arithmetic.
See also the -fwrapv option. Using -fwrapv means that integer signed overflow is fully defined: it
wraps. When -fwrapv is used, there is no difference between -fstrict-overflow and
-fno-strict-overflow for integers. With -fwrapv certain types of overflow are permitted. For
example, if the compiler gets an overflow when doing arithmetic on constants, the overflowed value
can still be used with -fwrapv, but not otherwise.
The -fstrict-overflow option is enabled at levels -O2, -O3, -Os.
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-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.
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-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.
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-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.
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-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.
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-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.
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-fno-align-loops and -falign-loops=1 are equivalent and mean that loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
|
-falign-jumps
-falign-jumps=n
Align branch targets to a power-of-two boundary, for branch targets where the targets can only be
reached by jumping, skipping up to n bytes like -falign-functions. In this case, no dummy operations
need be executed.
|
-fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
|
-funit-at-a-time
This option is left for compatibility reasons. -funit-at-a-time has no effect, while
-fno-unit-at-a-time implies -fno-toplevel-reorder and -fno-section-anchors.
Enabled by default.
|
-fno-toplevel-reorder
Do not reorder top-level functions, variables, and "asm" statements. Output them in the same order
that they appear in the input file. When this option is used, unreferenced static variables will not
be removed. This option is intended to support existing code that relies on a particular ordering.
For new code, it is better to use attributes.
Enabled at level -O0. When disabled explicitly, it also implies -fno-section-anchors, which is
otherwise enabled at -O0 on some targets.
|
-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 by default with -funroll-loops.
|
-fwhole-program
Assume that the current compilation unit represents the whole program being compiled. All public
functions and variables with the exception of "main" and those merged by attribute
"externally_visible" become static functions and in effect are optimized more aggressively by
interprocedural optimizers. If gold is used as the linker plugin, "externally_visible" attributes are
automatically added to functions (not variable yet due to a current gold issue) that are accessed
outside of LTO objects according to resolution file produced by gold. For other linkers that cannot
generate resolution file, explicit "externally_visible" attributes are still necessary. While this
option is equivalent to proper use of the "static" keyword for programs consisting of a single file,
in combination with option -flto this flag can be used to compile many smaller scale programs since
the functions and variables become local for the whole combined compilation unit, not for the single
source file itself.
This option implies -fwhole-file for Fortran programs.
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-flto[=n]
This option runs the standard link-time optimizer. When invoked with source code, it generates
GIMPLE (one of GCC's internal representations) and writes it to special ELF sections in the object
file. When the object files are linked together, all the function bodies are read from these ELF
sections and instantiated as if they had been part of the same translation unit.
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-flto-partition=alg
Specify the partitioning algorithm used by the link-time optimizer. The value is either "1to1" to
specify a partitioning mirroring the original source files or "balanced" to specify partitioning into
equally sized chunks (whenever possible). Specifying "none" as an algorithm disables partitioning
and streaming completely. The default value is "balanced".
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-flto-compression-level=n
This option specifies the level of compression used for intermediate language written to LTO object
files, and is only meaningful in conjunction with LTO mode (-flto). Valid values are 0 (no
compression) to 9 (maximum compression). Values outside this range are clamped to either 0 or 9. If
the option is not given, a default balanced compression setting is used.
|
-flto-report
Prints a report with internal details on the workings of the link-time optimizer. The contents of
this report vary from version to version. It is meant to be useful to GCC developers when processing
object files in LTO mode (via -flto).
Disabled by default.
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-fuse-linker-plugin
Enables the use of a linker plugin during link-time optimization. This option relies on plugin
support in the linker, which is available in gold or in GNU ld 2.21 or newer.
This option enables the extraction of object files with GIMPLE bytecode out of library archives. This
improves the quality of optimization by exposing more code to the link-time optimizer. This
information specifies what symbols can be accessed externally (by non-LTO object or during dynamic
linking). Resulting code quality improvements on binaries (and shared libraries that use hidden
visibility) are similar to "-fwhole-program". See -flto for a description of the effect of this flag
and how to use it.
This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use
with a linker supporting plugins (GNU ld 2.21 or newer or gold).
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-ffat-lto-objects
Fat LTO objects are object files that contain both the intermediate language and the object code.
This makes them usable for both LTO linking and normal linking. This option is effective only when
compiling with -flto and is ignored at link time.
-fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete toolchain to
be aware of LTO. It requires a linker with linker plugin support for basic functionality.
Additionally, nm, ar and ranlib need to support linker plugins to allow a full-featured build
environment (capable of building static libraries etc).
The default is -ffat-lto-objects but this default is intended to change in future releases when
linker plugin enabled environments become more common.
|
-fcompare-elim
After register allocation and post-register allocation instruction splitting, identify arithmetic
instructions that compute processor flags similar to a comparison operation based on that arithmetic.
If possible, eliminate the explicit comparison operation.
This pass only applies to certain targets that cannot explicitly represent the comparison operation
before register allocation is complete.
Enabled at levels -O, -O2, -O3, -Os.
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-fcprop-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.
Enabled at levels -O, -O2, -O3, -Os.
|
-fprofile-correction
Profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due
to missed counter updates. When this option is specified, GCC will use heuristics to correct or
smooth out such inconsistencies. By default, GCC will emit an error message when an inconsistent
profile is detected.
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-fprofile-dir=path
Set the directory to search for the profile data files in to path. This option affects only the
profile data generated by -fprofile-generate, -ftest-coverage, -fprofile-arcs and used by
-fprofile-use and -fbranch-probabilities and its related options. Both absolute and relative paths
can be used. By default, GCC will use the current directory as path, thus the profile data file will
appear in the same directory as the object file.
|
-fprofile-generate
-fprofile-generate=path
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".
If path is specified, GCC will look at the path to find the profile feedback data files. See
-fprofile-dir.
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-fprofile-use
-fprofile-use=path
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"
By default, GCC emits an error message if the feedback profiles do not match the source code. This
error can be turned into a warning by using -Wcoverage-mismatch. Note this may result in poorly
optimized code.
If path is specified, GCC will look at the path to find the profile feedback data files. See
-fprofile-dir.
The following options control compiler behavior regarding floating-point arithmetic. These options trade
off between speed and correctness. All must be specifically enabled.
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-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.
|
-fexcess-precision=style
This option allows further control over excess precision on machines where floating-point registers
have more precision than the IEEE "float" and "double" types and the processor does not support
operations rounding to those types. By default, -fexcess-precision=fast is in effect; this means
that operations are carried out in the precision of the registers and that it is unpredictable when
rounding to the types specified in the source code takes place. When compiling C, if
-fexcess-precision=standard is specified then excess precision will follow the rules specified in ISO
C99; in particular, both casts and assignments cause values to be rounded to their semantic types
(whereas -ffloat-store only affects assignments). This option is enabled by default for C if a
strict conformance option such as -std=c99 is used.
-fexcess-precision=standard is not implemented for languages other than C, and has no effect if
-funsafe-math-optimizations or -ffast-math is specified. On the x86, it also has no effect if
-mfpmath=sse or -mfpmath=sse+387 is specified; in the former case, IEEE semantics apply without
excess precision, and in the latter, rounding is unpredictable.
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-ffast-math
Sets -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math,
-fno-signaling-nans and -fcx-limited-range.
This option causes the preprocessor macro "__FAST_MATH__" to be defined.
This option is not turned on by any -O option besides -Ofast since it can result in incorrect output
for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that do not require the guarantees of
these specifications.
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-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 is not turned on by any -O option since it can result in incorrect output for programs
that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It
may, however, yield faster code for programs that do not require the guarantees of these
specifications.
The default is -fmath-errno.
On Darwin systems, the math library never sets "errno". There is therefore no reason for the
compiler to consider the possibility that it might, and -fno-math-errno is the default.
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-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 is not turned on by any -O option since it can result in incorrect output for programs
that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It
may, however, yield faster code for programs that do not require the guarantees of these
specifications. Enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and
-freciprocal-math.
The default is -fno-unsafe-math-optimizations.
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-fassociative-math
Allow re-association of operands in series of floating-point operations. This violates the ISO C and
C++ language standard by possibly changing computation result. NOTE: re-ordering may change the sign
of zero as well as ignore NaNs and inhibit or create underflow or overflow (and thus cannot be used
on code that relies on rounding behavior like "(x + 2**52) - 2**52". May also reorder floating-point
comparisons and thus may not be used when ordered comparisons are required. This option requires
that both -fno-signed-zeros and -fno-trapping-math be in effect. Moreover, it doesn't make much
sense with -frounding-math. For Fortran the option is automatically enabled when both
-fno-signed-zeros and -fno-trapping-math are in effect.
The default is -fno-associative-math.
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-freciprocal-math
Allow the reciprocal of a value to be used instead of dividing by the value if this enables
optimizations. For example "x / y" can be replaced with "x * (1/y)", which is useful if "(1/y)" is
subject to common subexpression elimination. Note that this loses precision and increases the number
of flops operating on the value.
The default is -fno-reciprocal-math.
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-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs
or +-Infs.
This option is not turned on by any -O option since it can result in incorrect output for programs
that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It
may, however, yield faster code for programs that do not require the guarantees of these
specifications.
The default is -fno-finite-math-only.
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-fno-signed-zeros
Allow optimizations for floating-point arithmetic that ignore the signedness of zero. IEEE
arithmetic specifies the behavior of distinct +0.0 and -0.0 values, which then prohibits
simplification of expressions such as x+0.0 or 0.0*x (even with -ffinite-math-only). This option
implies that the sign of a zero result isn't significant.
The default is -fsigned-zeros.
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-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
requires that -fno-signaling-nans be in effect. 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 that depend on an exact implementation of IEEE or ISO rules/specifications for math
functions.
The default is -ftrapping-math.
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-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 constants as single precision instead of implicitly converting them to double-
precision constants.
|
-fcx-limited-range
When enabled, this option states that a range reduction step is not needed when performing complex
division. Also, there is no checking whether the result of a complex multiplication or division is
"NaN + I*NaN", with an attempt to rescue the situation in that case. The default is
-fno-cx-limited-range, but is enabled by -ffast-math.
This option controls the default setting of the ISO C99 "CX_LIMITED_RANGE" pragma. Nevertheless, the
option applies to all languages.
|
-fcx-fortran-rules
Complex multiplication and division follow Fortran rules. Range reduction is done as part of complex
division, but there is no checking whether the result of a complex multiplication or division is "NaN
+ I*NaN", with an attempt to rescue the situation in that case.
The default is -fno-cx-fortran-rules.
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 most likely 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 for
usage in optimizations.
Enabled with -fprofile-generate and -fprofile-use.
|
-fvpt
If combined with -fprofile-arcs, it instructs the compiler to add a code to gather information about
values of expressions.
With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations
based on them. Currently the optimizations include specialization of division operation using the
knowledge about the value of the denominator.
|
-frename-registers
Attempt to avoid false dependencies in scheduled code by making use of registers left over after
register allocation. This optimization will most benefit processors with lots of registers.
Depending on the debug information format adopted by the target, however, it can make debugging
impossible, since variables will no longer stay in a "home register".
Enabled by default with -funroll-loops and -fpeel-loops.
|
-ftracer
Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow
of the function allowing other optimizations to do better job.
Enabled with -fprofile-use.
|
-funroll-loops
Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop.
-funroll-loops implies -frerun-cse-after-loop, -fweb and -frename-registers. It also turns on
complete loop peeling (i.e. complete removal of loops with small constant number of iterations).
This option makes code larger, and may or may not make it run faster.
Enabled with -fprofile-use.
|
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This
usually makes programs run more slowly. -funroll-all-loops implies the same options as
-funroll-loops.
|
-fpeel-loops
Peels loops for which there is enough information that they do not roll much (from profile feedback).
It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of
iterations).
Enabled with -fprofile-use.
|
-fmove-loop-invariants
Enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1
|
-funswitch-loops
Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both
branches (modified according to result of the condition).
|
-ffunction-sections
-fdata-sections
Place each function or data item into its own section in the output file if the target supports
arbitrary sections. The name of the function or the name of the data item determines the section's
name in the output file.
Use these options on systems where the linker can perform optimizations to improve locality of
reference in the instruction space. Most systems using the ELF object format and SPARC processors
running Solaris 2 have linkers with such optimizations. AIX may have these optimizations in the
future.
Only use these options when there are significant benefits from doing so. When you specify these
options, the assembler and linker will create larger object and executable files and will also be
slower. You will not be able to use "gprof" on all systems if you specify this option and you may
have problems with debugging if you specify both this option and -g.
|
-fbranch-target-load-optimize
Perform branch target register load optimization before prologue / epilogue threading. The use of
target registers can typically be exposed only during reload, thus hoisting loads out of loops and
doing inter-block scheduling needs a separate optimization pass.
|
-fbranch-target-load-optimize2
Perform branch target register load optimization after prologue / epilogue threading.
|
-fbtr-bb-exclusive
When performing branch target register load optimization, don't reuse branch target registers in
within any basic block.
|
-fstack-protector
Emit extra code to check for buffer overflows, such as stack smashing attacks. This is done by
adding a guard variable to functions with vulnerable objects. This includes functions that call
alloca, and functions with buffers larger than 8 bytes. The guards are initialized when a function
is entered and then checked when the function exits. If a guard check fails, an error message is
printed and the program exits.
|
-fstack-protector-all
Like -fstack-protector except that all functions are protected.
|
-fsection-anchors
Try to reduce the number of symbolic address calculations by using shared "anchor" symbols to address
nearby objects. This transformation can help to reduce the number of GOT entries and GOT accesses on
some targets.
|
--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 than a certain number of instructions. You
can control some of these constants on the command line using the --param option.
|
-Wp,option
You can use -Wp,option to bypass the compiler driver and pass option directly through to the
preprocessor. If option contains commas, it is split into multiple options at the commas. However,
many options are modified, translated or interpreted by the compiler driver before being passed to
the preprocessor, and -Wp forcibly bypasses this phase. The preprocessor's direct interface is
undocumented and subject to change, so whenever possible you should avoid using -Wp and let the
driver handle the options instead.
|
-Xpreprocessor option
Pass option as an option to the preprocessor. You can use this to supply system-specific
preprocessor options that 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
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 . If dir begins with
"=", then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.
|
-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
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.
|
-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, deletes any directory components and 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.
|
-fpch-preprocess
This option allows use of a precompiled header together with -E. It inserts a special "#pragma",
"#pragma GCC pch_preprocess "filename"" in the output to mark the place where the precompiled header
was found, and its filename. When -fpreprocessed is in use, GCC recognizes this "#pragma" and loads
the PCH.
This option is off by default, because the resulting preprocessed output is only really suitable as
input to GCC. It is switched on by -save-temps.
You should not write this "#pragma" in your own code, but it is safe to edit the filename if the PCH
file is available in a different location. The filename may be absolute or it may be relative to
GCC's current directory.
|
-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. If dir begins
with "=", then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.
|
-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.
|
-isysroot dir
This option is like the --sysroot option, but applies only to header files (except for Darwin
targets, where it applies to both header files and libraries). See the --sysroot option for more
information.
|
-imultilib dir
Use dir as a subdirectory of the directory containing target-specific C++ headers.
|
-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. If dir begins with "=", then the "=" will be replaced by the
sysroot prefix; see --sysroot and -isysroot.
|
-iquote dir
Search dir only for header files requested with "#include "file""; they are not searched for
"#include <file>", before all directories specified by -I and before the standard system directories.
If dir begins with "=", then the "=" will be replaced by the sysroot prefix; see --sysroot and
-isysroot.
|
-fdirectives-only
When preprocessing, handle directives, but do not expand macros.
The option's behavior depends on the -E and -fpreprocessed options.
With -E, preprocessing is limited to the handling of directives such as "#define", "#ifdef", and
"#error". Other preprocessor operations, such as macro expansion and trigraph conversion are not
performed. In addition, the -dD option is implicitly enabled.
With -fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros such
as "__LINE__", which are contextually dependent, are handled normally. This enables compilation of
files previously preprocessed with "-E -fdirectives-only".
With both -E and -fpreprocessed, the rules for -fpreprocessed take precedence. This enables full
preprocessing of files previously preprocessed with "-E -fdirectives-only".
|
-fdollars-in-identifiers
Accept $ in identifiers.
|
-fextended-identifiers
Accept universal character names in identifiers. This option is experimental; in a future version of
GCC, it will be enabled by default for C99 and C++.
|
-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.
|
-fdebug-cpp
This option is only useful for debugging GCC. When used with -E, dumps debugging information about
location maps. Every token in the output is preceded by the dump of the map its location belongs to.
The dump of the map holding the location of a token would be:
{"P":F</file/path>;"F":F</includer/path>;"L":<line_num>;"C":<col_num>;"S":<system_header_p>;"M":<map_address>;"E":<macro_expansion_p>,"loc":<location>}
When used without -E, this option has no effect.
|
-ftrack-macro-expansion[=level]
Track locations of tokens across macro expansions. This allows the compiler to emit diagnostic about
the current macro expansion stack when a compilation error occurs in a macro expansion. Using this
option makes the preprocessor and the compiler consume more memory. The level parameter can be used
to choose the level of precision of token location tracking thus decreasing the memory consumption if
necessary. Value 0 of level de-activates this option just as if no -ftrack-macro-expansion was
present on the command line. Value 1 tracks tokens locations in a degraded mode for the sake of
minimal memory overhead. In this mode all tokens resulting from the expansion of an argument of a
function-like macro have the same location. Value 2 tracks tokens locations completely. This value is
the most memory hungry. When this option is given no argument, the default parameter value is 2.
|
-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 -fexec-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 that 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 or Objective-C++ program.
|
-nostartfiles
Do not use the standard system startup files when linking. The standard system libraries are used
normally, unless -nostdlib or -nodefaultlibs is used.
|
-nodefaultlibs
Do not use the standard system libraries when linking. Only the libraries you specify will be passed
to the linker, options specifying linkage of the system libraries, such as "-static-libgcc" or
"-shared-libgcc", will be ignored. The standard startup files are used normally, unless
-nostartfiles is used. The compiler may generate calls to "memcmp", "memset", "memcpy" and
"memmove". These entries are usually resolved by entries in libc. These entry points should be
supplied through some other mechanism when this option is specified.
|
-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, options specifying linkage of the system
libraries, such as "-static-libgcc" or "-shared-libgcc", will be ignored. The compiler may generate
calls to "memcmp", "memset", "memcpy" and "memmove". These entries are usually resolved by entries
in libc. These entry points should be supplied through some other mechanism when this option is
specified.
One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of
internal subroutines which 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 that 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.
|
-rdynamic
Pass the flag -export-dynamic to the ELF linker, on targets that support it. This instructs the
linker to add all symbols, not only used ones, to the dynamic symbol table. This option is needed for
some uses of "dlopen" or to allow obtaining backtraces from within a program.
|
-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.
|
-T script
Use script as the linker script. This option is supported by most systems using the GNU linker. On
some targets, such as bare-board targets without an operating system, the -T option may be required
when linking to avoid references to undefined symbols.
|
-Xlinker option
Pass option as an option to the linker. You can use this to supply system-specific linker options
that GCC does not recognize.
If you want to pass an option that takes a separate 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.
When using the GNU linker, it is usually more convenient to pass arguments to linker options using
the option=value syntax than as separate arguments. For example, you can specify -Xlinker
-Map=output.map rather than -Xlinker -Map -Xlinker output.map. Other linkers may not support this
syntax for command-line options.
|
-Wl,option
Pass option as an option to the linker. If option contains commas, it is split into multiple options
at the commas. You can use this syntax to pass an argument to the option. For example,
-Wl,-Map,output.map passes -Map output.map to the linker. When using the GNU linker, you can also
get the same effect with -Wl,-Map=output.map.
|
-u symbol
Pretend the symbol symbol is undefined, to force linking of library modules to define it. You can
use -u multiple times with different symbols to force loading of additional library modules.
Options for Directory Search
These options specify directories to search for header files, for libraries and for parts of the
compiler:
|
-Idir
Add the directory dir to the head of the list of directories to be searched for header files. This
can be used to override a system header file, substituting your own version, since these directories
are searched before the system header file directories. However, you should not use this option to
add directories that contain vendor-supplied system header files (use -isystem for that). If you use
more than one -I option, the directories are scanned in left-to-right order; the standard system
directories come after.
If a standard system include directory, or a directory specified with -isystem, is also specified
with -I, the -I option will be ignored. The directory will still be searched but as a system
directory at its normal position in the system include chain. This is to ensure that GCC's procedure
to fix buggy system headers and the ordering for the include_next directive are not inadvertently
changed. If you really need to change the search order for system directories, use the -nostdinc
and/or -isystem options.
|
-iplugindir=dir
Set the directory to search for plugins that are passed by -fplugin=name instead of
-fplugin=path/name.so. This option is not meant to be used by the user, but only passed by the
driver.
|
-iquotedir
Add the directory dir to the head of the list of directories to be searched for header files only for
the case of #include "file"; they are not searched for #include <file>, otherwise just like -I.
|
-Ldir
Add directory dir to the list of directories to be searched for -l.
|
-Bprefix
This option specifies where to find the executables, libraries, include files, and data files of the
compiler itself.
|
-specs=file
Process file after the compiler reads in the standard specs file, in order to override the defaults
which 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.
|
--sysroot=dir
Use dir as the logical root directory for headers and libraries. For example, if the compiler would
normally search for headers in /usr/include and libraries in /usr/lib, it will instead search
dir/usr/include and dir/usr/lib.
|
-mhalf-reg-file
Don't allocate any register in the range "r32"..."r63". That allows code to run on hardware variants
that lack these registers.
|
-mprefer-short-insn-regs
Preferrentially allocate registers that allow short instruction generation. This can result in
increasesd instruction count, so if this reduces or increases code size might vary from case to case.
|
-mbranch-cost=num
Set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is
not guaranteed to produce consistent results across releases.
|
-mcmove
Enable the generation of conditional moves.
|
-mnops=num
Emit num nops before every other generated instruction.
|
-mno-soft-cmpsf
For single-precision floating-point comparisons, emit an fsub instruction and test the flags. This
is faster than a software comparison, but can get incorrect results in the presence of NaNs, or when
two different small numbers are compared such that their difference is calculated as zero. The
default is -msoft-cmpsf, which uses slower, but IEEE-compliant, software comparisons.
|
-mstack-offset=num
Set the offset between the top of the stack and the stack pointer. E.g., a value of 8 means that the
eight bytes in the range sp+0...sp+7 can be used by leaf functions without stack allocation. Values
other than 8 or 16 are untested and unlikely to work. Note also that this option changes the ABI,
compiling a program with a different stack offset than the libraries have been compiled with will
generally not work. This option can be useful if you want to evaluate if a different stack offset
would give you better code, but to actually use a different stack offset to build working programs,
it is recommended to configure the toolchain with the appropriate --with-stack-offset=num option.
|
-mno-round-nearest
Make the scheduler assume that the rounding mode has been set to truncating. The default is
-mround-nearest.
|
-mlong-calls
If not otherwise specified by an attribute, assume all calls might be beyond the offset range of the
b / bl instructions, and therefore load the function address into a register before performing a
(otherwise direct) call. This is the default.
|
-mshort-calls
If not otherwise specified by an attribute, assume all direct calls are in the range of the b / bl
instructions, so use these instructions for direct calls. The default is -mlong-calls.
|
-msmall16
Assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses
for which -mlong-calls semantics are in effect.
|
-mfp-mode=mode
Set the prevailing mode of the floating-point unit. This determines the floating-point mode that is
provided and expected at function call and return time. Making this mode match the mode you
predominantly need at function start can make your programs smaller and faster by avoiding
unnecessary mode switches.
|
-mnosplit-lohi
-mno-postinc
-mno-postmodify
Code generation tweaks that disable, respectively, splitting of 32-bit loads, generation of post-
increment addresses, and generation of post-modify addresses. The defaults are msplit-lohi,
-mpost-inc, and -mpost-modify.
|
-mnovect-double
Change the preferred SIMD mode to SImode. The default is -mvect-double, which uses DImode as
preferred SIMD mode.
|
-max-vect-align=num
The maximum alignment for SIMD vector mode types. num may be 4 or 8. The default is 8. Note that
this is an ABI change, even though many library function interfaces will be unaffected, if they don't
use SIMD vector modes in places where they affect size and/or alignment of relevant types.
|
-msplit-vecmove-early
Split vector moves into single word moves before reload. In theory this could give better register
allocation, but so far the reverse seems to be generally the case.
|
-m1reg-reg
Specify a register to hold the constant -1, which makes loading small negative constants and certain
bitmasks faster. Allowable values for reg are r43 and r63, which specify to use that register as a
fixed register, and none, which means that no register is used for this purpose. The default is
-m1reg-none.
ARM Options
These -m options are defined for Advanced RISC Machines (ARM) architectures:
|
-mabi=name
Generate code for the specified ABI. Permissible values are: apcs-gnu, atpcs, aapcs, aapcs-linux and
iwmmxt.
|
-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.
|
-mthumb-interwork
Generate code that supports calling between the ARM and Thumb instruction sets. Without this option,
on pre-v5 architectures, 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. In AAPCS configurations this option is meaningless.
|
-mno-sched-prolog
Prevent the reordering of instructions in the function prologue, 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.
|
-mfloat-abi=name
Specifies which floating-point ABI to use. Permissible values are: soft, softfp and hard.
Specifying soft causes GCC to generate output containing library calls for floating-point operations.
softfp allows the generation of code using hardware floating-point instructions, but still uses the
soft-float calling conventions. hard allows generation of floating-point instructions and uses FPU-
specific calling conventions.
The default depends on the specific target configuration. Note that the hard-float and soft-float
ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a
compatible set of libraries.
|
-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. This option is now deprecated.
|
-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, arm720, arm7500, arm7500fe, arm7tdmi, arm7tdmi-s, arm710t,
arm720t, arm740t, strongarm, strongarm110, strongarm1100, strongarm1110, arm8, arm810, arm9, arm9e,
arm920, arm920t, arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t, arm9tdmi, arm10tdmi,
arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e, arm1136j-s, arm1136jf-s, mpcore, mpcorenovfp,
arm1156t2-s, arm1156t2f-s, arm1176jz-s, arm1176jzf-s, cortex-a5, cortex-a7, cortex-a8, cortex-a9,
cortex-a15, cortex-r4, cortex-r4f, cortex-r5, cortex-m4, cortex-m3, cortex-m1, cortex-m0, xscale,
iwmmxt, iwmmxt2, ep9312, fa526, fa626, fa606te, fa626te, fmp626, fa726te.
-mcpu=generic-arch is also permissible, and is equivalent to -march=arch -mtune=generic-arch. See
-mtune for more information.
-mcpu=native causes the compiler to auto-detect the CPU of the build computer. At present, this
feature is only supported on Linux, and not all architectures are recognized. If the auto-detect is
unsuccessful the option has no effect.
|
-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.
|
-mtune=generic-arch specifies that GCC should tune the performance for a blend of processors within
architecture arch. The aim is to generate code that run well on the current most popular processors,
balancing between optimizations that benefit some CPUs in the range, and avoiding performance
pitfalls of other CPUs. The effects of this option may change in future GCC versions as CPU models
come and go.
-mtune=native causes the compiler to auto-detect the CPU of the build computer. At present, this
feature is only supported on Linux, and not all architectures are recognized. If the auto-detect is
unsuccessful the option has no effect.
|
-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, armv5e, armv5te, armv6, armv6j, armv6t2, armv6z, armv6zk, armv6-m, armv7, armv7-a,
armv7-r, armv7-m, iwmmxt, iwmmxt2, ep9312.
-march=native causes the compiler to auto-detect the architecture of the build computer. At present,
this feature is only supported on Linux, and not all architectures are recognized. If the auto-
detect is unsuccessful the option has no effect.
|
-mfpu=name
-mfpe=number
-mfp=number
This specifies what floating-point hardware (or hardware emulation) is available on the target.
Permissible names are: fpa, fpe2, fpe3, maverick, vfp, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16,
vfpv3xd, vfpv3xd-fp16, neon, neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16 and neon-vfpv4. -mfp and -mfpe
are synonyms for -mfpu=fpenumber, for compatibility with older versions of GCC.
If -msoft-float is specified this specifies the format of floating-point values.
If the selected floating-point hardware includes the NEON extension (e.g. -mfpu=neon), note that
floating-point operations will not be used by GCC's auto-vectorization pass unless
-funsafe-math-optimizations is also specified. This is because NEON hardware does not fully
implement the IEEE 754 standard for floating-point arithmetic (in particular denormal values are
treated as zero), so the use of NEON instructions may lead to a loss of precision.
|
-mfp16-format=name
Specify the format of the "__fp16" half-precision floating-point type. Permissible names are none,
ieee, and alternative; the default is none, in which case the "__fp16" type is not defined.
|
-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, 32 and 64. The default value varies for different
toolchains. For the COFF targeted toolchain the default value is 8. A value of 64 is only allowed
if the underlying ABI supports it.
Specifying the larger number can produce faster, more efficient code, but can also increase the size
of the program. Different values are potentially incompatible. Code compiled with one value cannot
necessarily expect to work with code or libraries compiled with another 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 that 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.
|
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for
each function. The runtime 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
-marm
Select between generating code that executes in ARM and Thumb states. The default for most
configurations is to generate code that executes in ARM state, but the default can be changed by
configuring GCC with the --with-mode=state configure option.
|
-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. This option is not valid in AAPCS configurations because
interworking is enabled by default.
|
-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. This option is not valid in AAPCS
configurations because interworking is enabled by default.
|
-mtp=name
Specify the access model for the thread local storage pointer. The valid models are soft, which
generates calls to "__aeabi_read_tp", cp15, which fetches the thread pointer from "cp15" directly
(supported in the arm6k architecture), and auto, which uses the best available method for the
selected processor. The default setting is auto.
|
-mtls-dialect=dialect
Specify the dialect to use for accessing thread local storage. Two dialects are supported --- gnu
and gnu2. The gnu dialect selects the original GNU scheme for supporting local and global dynamic
TLS models. The gnu2 dialect selects the GNU descriptor scheme, which provides better performance
for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but does
require new assembler, linker and library support. Initial and local exec TLS models are unaffected
by this option and always use the original scheme.
|
-mword-relocations
Only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32). This is enabled by
default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when
-fpic or -fPIC is specified.
|
-mfix-cortex-m3-ldrd
Some Cortex-M3 cores can cause data corruption when "ldrd" instructions with overlapping destination
and base registers are used. This option avoids generating these instructions. This option is
enabled by default when -mcpu=cortex-m3 is specified.
AVR Options
These options are defined for AVR implementations:
|
-mmcu=mcu
Specify Atmel AVR instruction set architectures (ISA) or MCU type.
|
-maccumulate-args
Accumulate outgoing function arguments and acquire/release the needed stack space for outgoing
function arguments once in function prologue/epilogue. Without this option, outgoing arguments are
pushed before calling a function and popped afterwards.
Popping the arguments after the function call can be expensive on AVR so that accumulating the stack
space might lead to smaller executables because arguments need not to be removed from the stack after
such a function call.
This option can lead to reduced code size for functions that perform several calls to functions that
get their arguments on the stack like calls to printf-like functions.
|
-mbranch-cost=cost
Set the branch costs for conditional branch instructions to cost. Reasonable values for cost are
small, non-negative integers. The default branch cost is 0.
|
-mcall-prologues
Functions prologues/epilogues expanded as call to appropriate subroutines. Code size will be
smaller.
|
-mint8
Assume int to be 8-bit integer. This affects the sizes of all types: a char will be 1 byte, an int
will be 1 byte, a long will be 2 bytes and long long will be 4 bytes. Please note that this option
does not comply to the C standards, but it will provide you with smaller code size.
|
-mno-interrupts
Generated code is not compatible with hardware interrupts. Code size will be smaller.
|
-mrelax
Try to replace "CALL" resp. "JMP" instruction by the shorter "RCALL" resp. "RJMP" instruction if
applicable. Setting "-mrelax" just adds the "--relax" option to the linker command line when the
linker is called.
Jump relaxing is performed by the linker because jump offsets are not known before code is located.
Therefore, the assembler code generated by the compiler will be the same, but the instructions in the
executable may differ from instructions in the assembler code.
|
-mshort-calls
Use "RCALL"/"RJMP" instructions even on devices with 16@tie{}KiB or more of program memory, i.e. on
devices that have the "CALL" and "JMP" instructions. See also the "-mrelax" command line option.
|
-mstrict-X
Use address register "X" in a way proposed by the hardware. This means that "X" will only be used in
indirect, post-increment or pre-decrement addressing.
Without this option, the "X" register may be used in the same way as "Y" or "Z" which then is
emulated by additional instructions. For example, loading a value with "X+const" addressing with a
small non-negative "const < 64" to a register Rn will be performed as
|
-mtiny-stack
Only use the lower 8@tie{}bits of the stack pointer and assume that the high byte of SP is always
zero.
|
-msim
Specifies that the program will be run on the simulator. This causes the simulator BSP provided by
libgloss to be linked in. This option has effect only for bfin-elf toolchain. Certain other
options, such as -mid-shared-library and -mfdpic, imply -msim.
|
-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.
|
-mspecld-anomaly
When enabled, the compiler will ensure that the generated code does not contain speculative loads
after jump instructions. If this option is used, "__WORKAROUND_SPECULATIVE_LOADS" is defined.
|
-mno-specld-anomaly
Don't generate extra code to prevent speculative loads from occurring.
|
-mcsync-anomaly
When enabled, the compiler will ensure that the generated code does not contain CSYNC or SSYNC
instructions too soon after conditional branches. If this option is used,
"__WORKAROUND_SPECULATIVE_SYNCS" is defined.
|
-mno-csync-anomaly
Don't generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon after a
conditional branch.
|
-mlow-64k
When enabled, the compiler is free to take advantage of the knowledge that the entire program fits
into the low 64k of memory.
|
-mno-low-64k
Assume that the program is arbitrarily large. This is the default.
|
-mstack-check-l1
Do stack checking using information placed into L1 scratchpad memory by the uClinux kernel.
|
-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. With a bfin-elf target, this option implies -msim.
|
-mno-id-shared-library
Generate code that doesn't assume ID based shared libraries are being used. This is the default.
|
-mleaf-id-shared-library
Generate code that supports shared libraries via the library ID method, but assumes that this library
or executable won't link against any other ID shared libraries. That allows the compiler to use
faster code for jumps and calls.
|
-mno-leaf-id-shared-library
Do not assume that the code being compiled won't link against any ID shared libraries. Slower code
will be generated for jump and call insns.
|
-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.
|
-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 by
eliminating relocations against the text section.
|
-mno-sep-data
Generate code that assumes that the data segment follows the text segment. This is the default.
|
-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 lies outside of the 24-bit addressing range of the offset-based version of subroutine call
instruction.
This feature is not enabled by default. Specifying -mno-long-calls will restore the default
behavior. Note these switches have no effect on how the compiler generates code to handle function
calls via function pointers.
|
-mfast-fp
Link with the fast floating-point library. This library relaxes some of the IEEE floating-point
standard's rules for checking inputs against Not-a-Number (NAN), in the interest of performance.
|
-minline-plt
Enable inlining of PLT entries in function calls to functions that are not known to bind locally. It
has no effect without -mfdpic.
|
-mmulticore
Build standalone application for multicore Blackfin processor. Proper start files and link scripts
will be used to support multicore. This option defines "__BFIN_MULTICORE". It can only be used with
-mcpu=bf561[-sirevision]. It can be used with -mcorea or -mcoreb. If it's used without -mcorea or
-mcoreb, single application/dual core programming model is used. In this model, the main function of
Core B should be named as coreb_main. If it's used with -mcorea or -mcoreb, one application per core
programming model is used. If this option is not used, single core application programming model is
used.
|
-mcorea
Build standalone application for Core A of BF561 when using one application per core programming
model. Proper start files and link scripts will be used to support Core A. This option defines
"__BFIN_COREA". It must be used with -mmulticore.
|
-mcoreb
Build standalone application for Core B of BF561 when using one application per core programming
model. Proper start files and link scripts will be used to support Core B. This option defines
"__BFIN_COREB". When this option is used, coreb_main should be used instead of main. It must be used
with -mmulticore.
|
-msdram
Build standalone application for SDRAM. Proper start files and link scripts will be used to put the
application into SDRAM. Loader should initialize SDRAM before loading the application into SDRAM.
This option defines "__BFIN_SDRAM".
|
-micplb
Assume that ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For
Linux targets, the default is to assume ICPLBs are enabled; for standalone applications the default
is off.
C6X Options
|
-march=name
This specifies the name of the target architecture. GCC uses this name to determine what kind of
instructions it can emit when generating assembly code. Permissible names are: c62x, c64x, c64x+,
c67x, c67x+, c674x.
|
-mbig-endian
Generate code for a big-endian target.
|
-mlittle-endian
Generate code for a little-endian target. This is the default.
|
-msim
Choose startup files and linker script suitable for the simulator.
|
-msdata=default
Put small global and static data in the .neardata section, which is pointed to by register "B14".
Put small uninitialized global and static data in the .bss section, which is adjacent to the
.neardata section. Put small read-only data into the .rodata section. The corresponding sections
used for large pieces of data are .fardata, .far and .const.
|
-msdata=all
Put all data, not just small objets, into the sections reserved for small data, and use addressing
relative to the "B14" register to access them.
|
-msdata=none
Make no use of the sections reserved for small data, and use absolute addresses to access all data.
Put all initialized global and static data in the .fardata section, and all uninitialized data in the
.far section. Put all constant data into the .const section.
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.
|
-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 which set 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.
|
-melf
Legacy no-op option only recognized with the cris-axis-elf and cris-axis-linux-gnu targets.
|
-mlinux
Legacy no-op option only recognized with the cris-axis-linux-gnu target.
|
-sim
This option, recognized for the 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.
CR16 Options
These options are defined specifically for the CR16 ports.
|
-mmac
Enable the use of multiply-accumulate instructions. Disabled by default.
|
-mcr16cplus
-mcr16c
Generate code for CR16C or CR16C+ architecture. CR16C+ architecture is default.
|
-msim
Links the library libsim.a which is in compatible with simulator. Applicable to elf compiler only.
|
-mint32
Choose integer type as 32-bit wide.
|
-mbit-ops
Generates sbit/cbit instructions for bit manipulations.
|
-mdata-model=model
Choose a data model. The choices for model are near, far or medium. medium is default. However, far
is not valid when -mcr16c option is chosen as CR16C architecture does not support far data model.
|
-Fdir
Add the framework directory dir to the head of the list of directories to be searched for header
files. These directories are interleaved with those specified by -I options and are scanned in a
left-to-right order.
A framework directory is a directory with frameworks in it. A framework is a directory with a
"Headers" and/or "PrivateHeaders" directory contained directly in it that ends in ".framework". The
name of a framework is the name of this directory excluding the ".framework". Headers associated
with the framework are found in one of those two directories, with "Headers" being searched first. A
subframework is a framework directory that is in a framework's "Frameworks" directory. Includes of
subframework headers can only appear in a header of a framework that contains the subframework, or in
a sibling subframework header. Two subframeworks are siblings if they occur in the same framework.
A subframework should not have the same name as a framework, a warning will be issued if this is
violated. Currently a subframework cannot have subframeworks, in the future, the mechanism may be
extended to support this. The standard frameworks can be found in "/System/Library/Frameworks" and
"/Library/Frameworks". An example include looks like "#include <Framework/header.h>", where
Framework denotes the name of the framework and header.h is found in the "PrivateHeaders" or
"Headers" directory.
|
-iframeworkdir
Like -F except the directory is a treated as a system directory. The main difference between this
-iframework and -F is that with -iframework the compiler does not warn about constructs contained
within header files found via dir. This option is valid only for the C family of languages.
|
-gused
Emit debugging information for symbols that are used. For STABS debugging format, this enables
-feliminate-unused-debug-symbols. This is by default ON.
|
-gfull
Emit debugging information for all symbols and types.
|
-mmacosx-version-min=version
The earliest version of MacOS X that this executable will run on is version. Typical values of
version include 10.1, 10.2, and 10.3.9.
If the compiler was built to use the system's headers by default, then the default for this option is
the system version on which the compiler is running, otherwise the default is to make choices that
are compatible with as many systems and code bases as possible.
|
-mkernel
Enable kernel development mode. The -mkernel option sets -static, -fno-common, -fno-cxa-atexit,
-fno-exceptions, -fno-non-call-exceptions, -fapple-kext, -fno-weak and -fno-rtti where applicable.
This mode also sets -mno-altivec, -msoft-float, -fno-builtin and -mlong-branch for PowerPC targets.
|
-mone-byte-bool
Override the defaults for bool so that sizeof(bool)==1. By default sizeof(bool) is 4 when compiling
for Darwin/PowerPC and 1 when compiling for Darwin/x86, so this option has no effect on x86.
Warning: The -mone-byte-bool switch causes GCC to generate code that is not binary compatible with
code generated without that switch. Using this switch may require recompiling all other modules in a
program, including system libraries. Use this switch to conform to a non-default data model.
|
-mfix-and-continue
-ffix-and-continue
-findirect-data
Generate code suitable for fast turn around development. Needed to enable gdb to dynamically load
".o" files into already running programs. -findirect-data and -ffix-and-continue are provided for
backwards compatibility.
|
-all_load
Loads all members of static archive libraries. See man ld(1) for more information.
|
-arch_errors_fatal
Cause the errors having to do with files that have the wrong architecture to be fatal.
|
-bind_at_load
Causes the output file to be marked such that the dynamic linker will bind all undefined references
when the file is loaded or launched.
|
-bundle
Produce a Mach-o bundle format file. See man ld(1) for more information.
|
-bundle_loader executable
This option specifies the executable that will be loading the build output file being linked. See
man ld(1) for more information.
|
-dynamiclib
When passed this option, GCC will produce a dynamic library instead of an executable when linking,
using the Darwin libtool command.
|
-force_cpusubtype_ALL
This causes GCC's output file to have the ALL subtype, instead of one controlled by the -mcpu or
-march option.
|
-allowable_client client_name
-client_name
-compatibility_version
-current_version
-dead_strip
-dependency-file
-dylib_file
-dylinker_install_name
-dynamic
-exported_symbols_list
-filelist
-flat_namespace
-force_flat_namespace
-headerpad_max_install_names
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-no_dead_strip_inits_and_terms
-nofixprebinding
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-segaddr
-segs_read_only_addr
-segs_read_write_addr
-seg_addr_table
-seg_addr_table_filename
-seglinkedit
-segprot
-segs_read_only_addr
-segs_read_write_addr
-single_module
-static
-sub_library
-sub_umbrella
-twolevel_namespace
-umbrella
-undefined
-unexported_symbols_list
-weak_reference_mismatches
-whatsloaded
These options are passed to the Darwin linker. The Darwin linker man page describes them in detail.
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.
|
-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 u, 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 run time.
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.
|
-mdebug-main=prefix
Flag the first routine whose name starts with prefix as the main routine for the debugger.
|
-mmalloc64
Default to 64-bit memory allocation routines.
FR30 Options
These options are defined specifically for the FR30 port.
|
-msmall-model
Use the small address space model. This can produce smaller code, but it does assume that all
symbolic values and addresses will fit into a 20-bit range.
|
-mno-lsim
Assume that runtime support has been provided and so there is no need to include the simulator
library (libsim.a) on the linker command line.
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.
|
-mfdpic
Select the FDPIC ABI, which uses function descriptors to represent pointers to functions. Without
any PIC/PIE-related options, it implies -fPIE. With -fpic or -fpie, it assumes GOT entries and small
data are within a 12-bit range from the GOT base address; with -fPIC or -fPIE, GOT offsets are
computed with 32 bits. With a bfin-elf target, this option implies -msim.
|
-minline-plt
Enable inlining of PLT entries in function calls to functions that are not known to bind locally. It
has no effect without -mfdpic. It's enabled by default if optimizing for speed and compiling for
shared libraries (i.e., -fPIC or -fpic), or when an optimization option such as -O3 or above is
present in the command line.
|
-mTLS
Assume a large TLS segment when generating thread-local code.
|
-mtls
Do not assume a large TLS segment when generating thread-local code.
|
-mgprel-ro
Enable the use of "GPREL" relocations in the FDPIC ABI for data that is known to be in read-only
sections. It's enabled by default, except for -fpic or -fpie: even though it may help make the
global offset table smaller, it trades 1 instruction for 4. With -fPIC or -fPIE, it trades 3
instructions for 4, one of which may be shared by multiple symbols, and it avoids the need for a GOT
entry for the referenced symbol, so it's more likely to be a win. If it is not, -mno-gprel-ro can be
used to disable it.
|
-multilib-library-pic
Link with the (library, not FD) pic libraries. It's implied by -mlibrary-pic, as well as by -fPIC
and -fpic without -mfdpic. You should never have to use it explicitly.
|
-mlinked-fp
Follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated.
This option is enabled by default and can be disabled with -mno-linked-fp.
|
-mlong-calls
Use indirect addressing to call functions outside the current compilation unit. This allows the
functions to be placed anywhere within the 32-bit address space.
|
-malign-labels
Try to align labels to an 8-byte boundary by inserting nops into the previous packet. This option
only has an effect when VLIW packing is enabled. It doesn't create new packets; it merely adds nops
to existing ones.
|
-mlibrary-pic
Generate position-independent EABI code.
|
-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.
|
-moptimize-membar
This switch removes redundant "membar" instructions from the compiler generated code. It is enabled
by default.
|
-mno-optimize-membar
This switch disables the automatic removal of redundant "membar" instructions from the generated
code.
|
-mtomcat-stats
Cause gas to print out tomcat statistics.
|
-mcpu=cpu
Select the processor type for which to generate code. Possible values are frv, fr550, tomcat, fr500,
fr450, fr405, fr400, fr300 and simple.
GNU/Linux Options
These -m options are defined for GNU/Linux targets:
|
-mglibc
Use the GNU C library. This is the default except on *-*-linux-*uclibc* and *-*-linux-*android*
targets.
|
-muclibc
Use uClibc C library. This is the default on *-*-linux-*uclibc* targets.
|
-mbionic
Use Bionic C library. This is the default on *-*-linux-*android* targets.
|
-mandroid
Compile code compatible with Android platform. This is the default on *-*-linux-*android* targets.
When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. When
linking, this option makes the GCC driver pass Android-specific options to the linker. Finally, this
option causes the preprocessor macro "__ANDROID__" to be defined.
|
-tno-android-cc
Disable compilation effects of -mandroid, i.e., do not enable -mbionic, -fPIC, -fno-exceptions and
-fno-rtti by default.
|
-tno-android-ld
Disable linking effects of -mandroid, i.e., pass standard Linux linking options to the linker.
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.
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.
|
-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 that perform lazy context switching of floating-point registers. If you use this option and
attempt to perform floating-point operations, the compiler aborts.
|
-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 that
performs faster indirect calls.
This option will not work in the presence of shared libraries or nested functions.
|
-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.
|
-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.
-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.
|
-munix=unix-std
Generate compiler predefines and select a startfile for the specified UNIX standard. The choices for
unix-std are 93, 95 and 98. 93 is supported on all HP-UX versions. 95 is available on HP-UX 10.10
and later. 98 is available on HP-UX 11.11 and later. The default values are 93 for HP-UX 10.00, 95
for HP-UX 10.10 though to 11.00, and 98 for HP-UX 11.11 and later.
|
-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 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.
|
-mfpmath=unit
Generate floating-point arithmetic 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). Darwin does not support intel.
|
-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 produces code that runs
somewhat faster on a Pentium at the expense of more memory.
On x86-64, -malign-double is enabled by default.
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) prefer "long double" to be aligned to an 8- or 16-byte
boundary. In arrays or structures conforming to the ABI, this is not possible. So specifying
-m128bit-long-double aligns "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.
|
-mlarge-data-threshold=number
When -mcmodel=medium is specified, the data greater than threshold are placed in large data section.
This value must be the same across all object linked into the binary and defaults to 65535.
|
-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.
|
-msseregparm
Use SSE register passing conventions for float and double arguments and return values. You can
control this behavior for a specific function by using the function attribute sseregparm.
Warning: if you use this switch then you must build all modules with the same value, including any
libraries. This includes the system libraries and startup modules.
|
-mvect8-ret-in-mem
Return 8-byte vectors in memory instead of MMX registers. This is the default on Solaris@tie{}8 and
9 and VxWorks to match the ABI of the Sun Studio compilers until version 12. Later compiler versions
(starting with Studio 12 Update@tie{}1) follow the ABI used by other x86 targets, which is the
default on Solaris@tie{}10 and later. Only use this option if you need to remain compatible with
existing code produced by those previous compiler versions or older versions of GCC.
|
-mpc32
-mpc64
-mpc80
Set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32 is specified, the significands
of results of floating-point operations are rounded to 24 bits (single precision); -mpc64 rounds the
significands of results of floating-point operations to 53 bits (double precision) and -mpc80 rounds
the significands of results of floating-point operations to 64 bits (extended double precision),
which is the default. When this option is used, floating-point operations in higher precisions are
not available to the programmer without setting the FPU control word explicitly.
Setting the rounding of floating-point operations to less than the default 80 bits can speed some
programs by 2% or more. Note that some mathematical libraries assume that extended-precision
(80-bit) floating-point operations are enabled by default; routines in such libraries could suffer
significant loss of accuracy, typically through so-called "catastrophic cancellation", when this
option is used to set the precision to less than extended precision.
|
-mstackrealign
Realign the stack at entry. On the Intel x86, the -mstackrealign option will generate an alternate
prologue and epilogue that realigns the run-time stack if necessary. This supports mixing legacy
codes that keep a 4-byte aligned stack with modern codes that keep a 16-byte stack for SSE
compatibility. See also the attribute "force_align_arg_pointer", applicable to individual functions.
|
-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).
|
-mincoming-stack-boundary=num
Assume the incoming stack is aligned to a 2 raised to num byte boundary. If
-mincoming-stack-boundary is not specified, the one specified by -mpreferred-stack-boundary will be
used.
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" may not work properly 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
-mssse3
-mno-ssse3
-msse4.1
-mno-sse4.1
-msse4.2
-mno-sse4.2
-msse4
-mno-sse4
-mavx
-mno-avx
-mavx2
-mno-avx2
-maes
-mno-aes
-mpclmul
-mno-pclmul
-mfsgsbase
-mno-fsgsbase
-mrdrnd
-mno-rdrnd
-mf16c
-mno-f16c
-mfma
-mno-fma
-msse4a
-mno-sse4a
-mfma4
-mno-fma4
-mxop
-mno-xop
-mlwp
-mno-lwp
-m3dnow
-mno-3dnow
-mpopcnt
-mno-popcnt
-mabm
-mno-abm
-mbmi
-mbmi2
-mno-bmi
-mno-bmi2
-mlzcnt
-mno-lzcnt
-mtbm
-mno-tbm
These switches enable or disable the use of instructions in the MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, F16C, FMA, SSE4A, FMA4, XOP, LWP, ABM, BMI, BMI2, LZCNT or
3DNow!
extended instruction sets. These extensions are also available as built-in functions: see X86
Built-in Functions, for details of the functions enabled and disabled by these switches.
To have SSE/SSE2 instructions generated automatically from floating-point code (as opposed to 387
instructions), see -mfpmath=sse.
GCC depresses SSEx instructions when -mavx is used. Instead, it generates new AVX instructions or AVX
equivalence for all SSEx instructions when needed.
These options will enable GCC to use these extended instructions in generated code, even without
-mfpmath=sse. Applications that perform run-time CPU detection must compile separate files for each
supported architecture, using the appropriate flags. In particular, the file containing the CPU
detection code should be compiled without these options.
|
-mcld
This option instructs GCC to emit a "cld" instruction in the prologue of functions that use string
instructions. String instructions depend on the DF flag to select between autoincrement or
autodecrement mode. While the ABI specifies the DF flag to be cleared on function entry, some
operating systems violate this specification by not clearing the DF flag in their exception
dispatchers. The exception handler can be invoked with the DF flag set, which leads to wrong
direction mode when string instructions are used. This option can be enabled by default on 32-bit
x86 targets by configuring GCC with the --enable-cld configure option. Generation of "cld"
instructions can be suppressed with the -mno-cld compiler option in this case.
|
-mvzeroupper
This option instructs GCC to emit a "vzeroupper" instruction before a transfer of control flow out of
the function to minimize AVX to SSE transition penalty as well as remove unnecessary zeroupper
intrinsics.
|
-mcx16
This option will enable GCC to use CMPXCHG16B instruction in generated code. CMPXCHG16B allows for
atomic operations on 128-bit double quadword (or oword) data types. This is useful for high
resolution counters that could be updated by multiple processors (or cores). This instruction is
generated as part of atomic built-in functions: see __sync Builtins or __atomic Builtins for details.
|
-msahf
This option will enable GCC to use SAHF instruction in generated 64-bit code. Early Intel CPUs with
Intel 64 lacked LAHF and SAHF instructions supported by AMD64 until introduction of Pentium 4 G1 step
in December 2005. LAHF and SAHF are load and store instructions, respectively, for certain status
flags. In 64-bit mode, SAHF instruction is used to optimize "fmod", "drem" or "remainder" built-in
functions: see Other Builtins for details.
|
-mmovbe
This option will enable GCC to use movbe instruction to implement "__builtin_bswap32" and
"__builtin_bswap64".
|
-mcrc32
This option will enable built-in functions, "__builtin_ia32_crc32qi", "__builtin_ia32_crc32hi".
"__builtin_ia32_crc32si" and "__builtin_ia32_crc32di" to generate the crc32 machine instruction.
|
-mrecip
This option will enable GCC to use RCPSS and RSQRTSS instructions (and their vectorized variants
RCPPS and RSQRTPS) with an additional Newton-Raphson step to increase precision instead of DIVSS and
SQRTSS (and their vectorized variants) for single-precision floating-point arguments. These
instructions are generated only when -funsafe-math-optimizations is enabled together with
-finite-math-only and -fno-trapping-math. Note that while the throughput of the sequence is higher
than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased
by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994).
Note that GCC implements "1.0f/sqrtf(x)" in terms of RSQRTSS (or RSQRTPS) already with -ffast-math
(or the above option combination), and doesn't need -mrecip.
Also note that GCC emits the above sequence with additional Newton-Raphson step for vectorized
single-float division and vectorized "sqrtf(x)" already with -ffast-math (or the above option
combination), and doesn't need -mrecip.
|
-mrecip=opt
This option allows to control which reciprocal estimate instructions may be used. opt is a comma
separated list of options, which may be preceded by a "!" to invert the option: "all": enable all
estimate instructions, "default": enable the default instructions, equivalent to -mrecip, "none":
disable all estimate instructions, equivalent to -mno-recip, "div": enable the approximation for
scalar division, "vec-div": enable the approximation for vectorized division, "sqrt": enable the
approximation for scalar square root, "vec-sqrt": enable the approximation for vectorized square
root.
So for example, -mrecip=all,!sqrt would enable all of the reciprocal approximations, except for
square root.
|
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics using an external library. Supported types
are "svml" for the Intel short vector math library and "acml" for the AMD math core library style of
interfacing. GCC will currently emit calls to "vmldExp2", "vmldLn2", "vmldLog102", "vmldLog102",
"vmldPow2", "vmldTanh2", "vmldTan2", "vmldAtan2", "vmldAtanh2", "vmldCbrt2", "vmldSinh2", "vmldSin2",
"vmldAsinh2", "vmldAsin2", "vmldCosh2", "vmldCos2", "vmldAcosh2", "vmldAcos2", "vmlsExp4", "vmlsLn4",
"vmlsLog104", "vmlsLog104", "vmlsPow4", "vmlsTanh4", "vmlsTan4", "vmlsAtan4", "vmlsAtanh4",
"vmlsCbrt4", "vmlsSinh4", "vmlsSin4", "vmlsAsinh4", "vmlsAsin4", "vmlsCosh4", "vmlsCos4",
"vmlsAcosh4" and "vmlsAcos4" for corresponding function type when -mveclibabi=svml is used and
"__vrd2_sin", "__vrd2_cos", "__vrd2_exp", "__vrd2_log", "__vrd2_log2", "__vrd2_log10", "__vrs4_sinf",
"__vrs4_cosf", "__vrs4_expf", "__vrs4_logf", "__vrs4_log2f", "__vrs4_log10f" and "__vrs4_powf" for
corresponding function type when -mveclibabi=acml is used. Both -ftree-vectorize and
-funsafe-math-optimizations have to be enabled. A SVML or ACML ABI compatible library will have to be
specified at link time.
|
-mabi=name
Generate code for the specified calling convention. Permissible values are: sysv for the ABI used on
GNU/Linux and other systems and ms for the Microsoft ABI. The default is to use the Microsoft ABI
when targeting Windows. On all other systems, the default is the SYSV ABI. You can control this
behavior for a specific function by using the function attribute ms_abi/sysv_abi.
|
-mtls-dialect=type
Generate code to access thread-local storage using the gnu or gnu2 conventions. gnu is the
conservative default; gnu2 is more efficient, but it may add compile- and run-time requirements that
cannot be satisfied on all systems.
|
-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 the destination is known to be aligned to least a
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.
|
-minline-stringops-dynamically
For string operations of unknown size, use run-time checks with inline code for small blocks and a
library call for large blocks.
|
-mstringop-strategy=alg
Overwrite internal decision heuristic about particular algorithm to inline string operation with.
The allowed values are "rep_byte", "rep_4byte", "rep_8byte" for expanding using i386 "rep" prefix of
specified size, "byte_loop", "loop", "unrolled_loop" for expanding inline loop, "libcall" for always
expanding library call.
|
-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.
|
-msse2avx
-mno-sse2avx
Specify that the assembler should encode SSE instructions with VEX prefix. The option -mavx turns
this on by default.
|
-mfentry
-mno-fentry
If profiling is active -pg put the profiling counter call before prologue. Note: On x86
architectures the attribute "ms_hook_prologue" isn't possible at the moment for -mfentry and -pg.
|
-m8bit-idiv
-mno-8bit-idiv
On some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than 32-bit/64-bit
integer divide. This option generates a run-time check. If both dividend and divisor are within
range of 0 to 255, 8-bit unsigned integer divide is used instead of 32-bit/64-bit integer divide.
|
-mavx256-split-unaligned-load
-mavx256-split-unaligned-store
Split 32-byte AVX unaligned load and store.
These -m switches are supported in addition to the above on AMD x86-64 processors in 64-bit environments.
|
-m32
-m64
-mx32
Generate code for a 32-bit or 64-bit environment. The -m32 option sets int, long and pointer to 32
bits and generates code that runs on any i386 system. The -m64 option sets int to 32 bits and long
and pointer to 64 bits and generates code for AMD's x86-64 architecture. The -mx32 option sets int,
long and pointer to 32 bits and generates code for AMD's x86-64 architecture. For darwin only the
-m64 option turns off the -fno-pic and -mdynamic-no-pic options.
|
-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.
Small symbols are also placed there. Symbols with sizes larger than -mlarge-data-threshold are put
into large data or bss sections and can be located above 2GB. Programs can be statically or
dynamically linked.
|
-mcmodel=large
Generate code for the large model: This model makes no assumptions about addresses and sizes of
sections.
i386 and x86-64 Windows Options
These additional options are available for Windows targets:
|
-mconsole
This option is available for Cygwin and MinGW targets. It specifies that a console application is to
be generated, by instructing the linker to set the PE header subsystem type required for console
applications. This is the default behavior for Cygwin and MinGW targets.
|
-mdll
This option is available for Cygwin and MinGW targets. It specifies that a DLL - a dynamic link
library - is to be generated, enabling the selection of the required runtime startup object and entry
point.
|
-mnop-fun-dllimport
This option is available for Cygwin and MinGW targets. It specifies that the dllimport attribute
should be ignored.
|
-mthread
This option is available for MinGW targets. It specifies that MinGW-specific thread support is to be
used.
|
-municode
This option is available for mingw-w64 targets. It specifies that the UNICODE macro is getting pre-
defined and that the unicode capable runtime startup code is chosen.
|
-mwin32
This option is available for Cygwin and MinGW targets. It specifies that the typical Windows pre-
defined macros are to be set in the pre-processor, but does not influence the choice of runtime
library/startup code.
|
-mwindows
This option is available for Cygwin and MinGW targets. It specifies that a GUI application is to be
generated by instructing the linker to set the PE header subsystem type appropriately.
|
-fno-set-stack-executable
This option is available for MinGW targets. It specifies that the executable flag for stack used by
nested functions isn't set. This is necessary for binaries running in kernel mode of Windows, as
there the user32 API, which is used to set executable privileges, isn't available.
|
-mpe-aligned-commons
This option is available for Cygwin and MinGW targets. It specifies that the GNU extension to the PE
file format that permits the correct alignment of COMMON variables should be used when generating
code. It will be enabled by default if GCC detects that the target assembler found during
configuration supports the feature.
See also under i386 and x86-64 Options for standard options.
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.
|
-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.
|
-mno-inline-float-divide
Do not generate inline code for divides of floating-point values.
|
-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.
|
-mno-inline-int-divide
Do not generate inline code for divides of integer values.
|
-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-inline-sqrt
Do not generate inline code for sqrt.
|
-mfused-madd
-mno-fused-madd
Do (don't) generate code that uses the fused multiply/add or multiply/subtract instructions. The
default is to use these instructions.
|
-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.
|
-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.
|
-mno-sched-br-data-spec
-msched-br-data-spec
(Dis/En)able data speculative scheduling before reload. This will result in generation of the ld.a
instructions and the corresponding check instructions (ld.c / chk.a). The default is 'disable'.
|
-msched-ar-data-spec
-mno-sched-ar-data-spec
(En/Dis)able data speculative scheduling after reload. This will result in generation of the ld.a
instructions and the corresponding check instructions (ld.c / chk.a). The default is 'enable'.
|
-mno-sched-control-spec
-msched-control-spec
(Dis/En)able control speculative scheduling. This feature is available only during region scheduling
(i.e. before reload). This will result in generation of the ld.s instructions and the corresponding
check instructions chk.s . The default is 'disable'.
|
-msched-br-in-data-spec
-mno-sched-br-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the data speculative
loads before reload. This is effective only with -msched-br-data-spec enabled. The default is
'enable'.
|
-msched-ar-in-data-spec
-mno-sched-ar-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the data speculative
loads after reload. This is effective only with -msched-ar-data-spec enabled. The default is
'enable'.
|
-msched-in-control-spec
-mno-sched-in-control-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the control speculative
loads. This is effective only with -msched-control-spec enabled. The default is 'enable'.
|
-mno-sched-prefer-non-data-spec-insns
-msched-prefer-non-data-spec-insns
If enabled, data speculative instructions will be chosen for schedule only if there are no other
choices at the moment. This will make the use of the data speculation much more conservative. The
default is 'disable'.
|
-mno-sched-prefer-non-control-spec-insns
-msched-prefer-non-control-spec-insns
If enabled, control speculative instructions will be chosen for schedule only if there are no other
choices at the moment. This will make the use of the control speculation much more conservative.
The default is 'disable'.
|
-mno-sched-count-spec-in-critical-path
-msched-count-spec-in-critical-path
If enabled, speculative dependencies will be considered during computation of the instructions
priorities. This will make the use of the speculation a bit more conservative. The default is
'disable'.
|
-msched-spec-ldc
Use a simple data speculation check. This option is on by default.
|
-msched-control-spec-ldc
Use a simple check for control speculation. This option is on by default.
|
-msched-stop-bits-after-every-cycle
Place a stop bit after every cycle when scheduling. This option is on by default.
|
-msched-fp-mem-deps-zero-cost
Assume that floating-point stores and loads are not likely to cause a conflict when placed into the
same instruction group. This option is disabled by default.
|
-msel-sched-dont-check-control-spec
Generate checks for control speculation in selective scheduling. This flag is disabled by default.
|
-msched-max-memory-insns=max-insns
Limit on the number of memory insns per instruction group, giving lower priority to subsequent memory
insns attempting to schedule in the same instruction group. Frequently useful to prevent cache bank
conflicts. The default value is 1.
|
-msched-max-memory-insns-hard-limit
Disallow more than `msched-max-memory-insns' in instruction group. Otherwise, limit is `soft'
meaning that we would prefer non-memory operations when limit is reached but may still schedule
memory operations.
IA-64/VMS Options
These -m options are defined for the IA-64/VMS implementations:
|
-mvms-return-codes
Return VMS condition codes from main. The default is to return POSIX style condition (e.g. error)
codes.
|
-mdebug-main=prefix
Flag the first routine whose name starts with prefix as the main routine for the debugger.
|
-mmalloc64
Default to 64-bit memory allocation routines.
LM32 Options
These -m options are defined for the Lattice Mico32 architecture:
|
-mbarrel-shift-enabled
Enable barrel-shift instructions.
|
-mdivide-enabled
Enable divide and modulus instructions.
|
-mmultiply-enabled
Enable multiply instructions.
|
-msign-extend-enabled
Enable sign extend instructions.
|
-muser-enabled
Enable user-defined instructions.
M32C Options
|
-mcpu=name
Select the CPU for which code is generated. name may be one of r8c for the R8C/Tiny series, m16c for
the M16C (up to /60) series, m32cm for the M16C/80 series, or m32c for the M32C/80 series.
|
-msim
Specifies that the program will be run on the simulator. This causes an alternate runtime library to
be linked in which supports, for example, file I/O. You must not use this option when generating
programs that will run on real hardware; you must provide your own runtime library for whatever I/O
functions are needed.
|
-memregs=number
Specifies the number of memory-based pseudo-registers GCC will use during code generation. These
pseudo-registers will be used like real registers, so there is a tradeoff between GCC's ability to
fit the code into available registers, and the performance penalty of using memory instead of
registers. Note that all modules in a program must be compiled with the same value for this option.
Because of that, you must not use this option with the default runtime libraries gcc builds.
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.
M680x0 Options
These are the -m options defined for M680x0 and ColdFire processors. The default settings depend on
which architecture was selected when the compiler was configured; the defaults for the most common
choices are given below.
|
-march=arch
Generate code for a specific M680x0 or ColdFire instruction set architecture. Permissible values of
arch for M680x0 architectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. ColdFire
architectures are selected according to Freescale's ISA classification and the permissible values
are: isaa, isaaplus, isab and isac.
gcc defines a macro __mcfarch__ whenever it is generating code for a ColdFire target. The arch in
this macro is one of the -march arguments given above.
When used together, -march and -mtune select code that runs on a family of similar processors but
that is optimized for a particular microarchitecture.
|
-mcpu=cpu
Generate code for a specific M680x0 or ColdFire processor. The M680x0 cpus are: 68000, 68010, 68020,
68030, 68040, 68060, 68302, 68332 and cpu32. The ColdFire cpus are given by the table below, which
also classifies the CPUs into families:
Family : -mcpu arguments
51 : 51 51ac 51cn 51em 51qe
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484 5485
|
-mcpu=cpu overrides -march=arch if arch is compatible with cpu. Other combinations of -mcpu and
-march are rejected.
gcc defines the macro __mcf_cpu_cpu when ColdFire target cpu is selected. It also defines
__mcf_family_family, where the value of family is given by the table above.
|
-mtune=tune
Tune the code for a particular microarchitecture, within the constraints set by -march and -mcpu.
The M680x0 microarchitectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. The ColdFire
microarchitectures are: cfv1, cfv2, cfv3, cfv4 and cfv4e.
You can also use -mtune=68020-40 for code that needs to run relatively well on 68020, 68030 and 68040
targets. -mtune=68020-60 is similar but includes 68060 targets as well. These two options select
the same tuning decisions as -m68020-40 and -m68020-60 respectively.
gcc defines the macros __mcarch and __mcarch__ when tuning for 680x0 architecture arch. It also
defines mcarch unless either -ansi or a non-GNU -std option is used. If gcc is tuning for a range of
architectures, as selected by -mtune=68020-40 or -mtune=68020-60, it defines the macros for every
architecture in the range.
gcc also defines the macro __muarch__ when tuning for ColdFire microarchitecture uarch, where uarch
is one of the arguments given above.
|
-m68000
-mc68000
Generate output for a 68000. This is the default when the compiler is configured for 68000-based
systems. It is equivalent to -march=68000.
Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306,
68307, 68322, 68328 and 68356.
|
-m68010
Generate output for a 68010. This is the default when the compiler is configured for 68010-based
systems. It is equivalent to -march=68010.
|
-m68020
-mc68020
Generate output for a 68020. This is the default when the compiler is configured for 68020-based
systems. It is equivalent to -march=68020.
|
-m68030
Generate output for a 68030. This is the default when the compiler is configured for 68030-based
systems. It is equivalent to -march=68030.
|
-m68040
Generate output for a 68040. This is the default when the compiler is configured for 68040-based
systems. It is equivalent to -march=68040.
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. It is equivalent to -march=68060.
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. It is equivalent to -march=cpu32.
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 CPU. This is the default when the compiler is configured for
520X-based systems. It is equivalent to -mcpu=5206, and is now deprecated in favor of that option.
Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and
MCF5206.
|
-m5206e
Generate output for a 5206e ColdFire CPU. The option is now deprecated in favor of the equivalent
-mcpu=5206e.
|
-m528x
Generate output for a member of the ColdFire 528X family. The option is now deprecated in favor of
the equivalent -mcpu=528x.
|
-m5307
Generate output for a ColdFire 5307 CPU. The option is now deprecated in favor of the equivalent
-mcpu=5307.
|
-m5407
Generate output for a ColdFire 5407 CPU. The option is now deprecated in favor of the equivalent
-mcpu=5407.
|
-mcfv4e
Generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of hardware
floating-point instructions. The option is equivalent to -mcpu=547x, and is now deprecated in favor
of that option.
|
-m68020-40
Generate output for a 68040, without using any of the new instructions. This results in code that
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.
The option is equivalent to -march=68020 -mtune=68020-40.
|
-m68020-60
Generate output for a 68060, without using any of the new instructions. This results in code that
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.
The option is equivalent to -march=68020 -mtune=68020-60.
|
-mhard-float
-m68881
Generate floating-point instructions. This is the default for 68020 and above, and for ColdFire
devices that have an FPU. It defines the macro __HAVE_68881__ on M680x0 targets and __mcffpu__ on
ColdFire targets.
|
-msoft-float
Do not generate floating-point instructions; use library calls instead. This is the default for
68000, 68010, and 68832 targets. It is also the default for ColdFire devices that have no FPU.
|
-mdiv
-mno-div
Generate (do not generate) ColdFire hardware divide and remainder instructions. If -march is used
without -mcpu, the default is "on" for ColdFire architectures and "off" for M680x0 architectures.
Otherwise, the default is taken from the target CPU (either the default CPU, or the one specified by
-mcpu). For example, the default is "off" for -mcpu=5206 and "on" for -mcpu=5206e.
gcc defines the macro __mcfhwdiv__ when this option is enabled.
|
-mshort
Consider type "int" to be 16 bits wide, like "short int". Additionally, parameters passed on the
stack are also aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit.
|
-mno-short
Do not consider type "int" to be 16 bits wide. This is the default.
|
-mnobitfield
-mno-bitfield
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.
|
-mno-rtd
Do not use the calling conventions selected by -mrtd. This is the default.
|
-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.
|
-mxgot
-mno-xgot
When generating position-independent code for ColdFire, generate code that works if the GOT has more
than 8192 entries. This code is larger and slower than code generated without this option. On
M680x0 processors, this option is not needed; -fPIC suffices.
|
-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 4-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.
|
-mno-lsim
Assume that runtime support has been provided and so omit the simulator library (libsim.a) from the
linker command line.
|
-mstack-increment=size
Set the maximum amount for a single stack increment operation. Large values can increase the speed
of programs that contain functions that need a large amount of stack space, but they can also trigger
a segmentation fault if the stack is extended too much. The default value is 0x1000.
MeP Options
|
-mabsdiff
Enables the "abs" instruction, which is the absolute difference between two registers.
|
-mall-opts
Enables all the optional instructions - average, multiply, divide, bit operations, leading zero,
absolute difference, min/max, clip, and saturation.
|
-maverage
Enables the "ave" instruction, which computes the average of two registers.
|
-mbased=n
Variables of size n bytes or smaller will be placed in the ".based" section by default. Based
variables use the $tp register as a base register, and there is a 128-byte limit to the ".based"
section.
|
-mbitops
Enables the bit operation instructions - bit test ("btstm"), set ("bsetm"), clear ("bclrm"), invert
("bnotm"), and test-and-set ("tas").
|
-mc=name
Selects which section constant data will be placed in. name may be "tiny", "near", or "far".
|
-mclip
Enables the "clip" instruction. Note that "-mclip" is not useful unless you also provide "-mminmax".
|
-mconfig=name
Selects one of the build-in core configurations. Each MeP chip has one or more modules in it; each
module has a core CPU and a variety of coprocessors, optional instructions, and peripherals. The
"MeP-Integrator" tool, not part of GCC, provides these configurations through this option; using this
option is the same as using all the corresponding command-line options. The default configuration is
"default".
|
-mcop
Enables the coprocessor instructions. By default, this is a 32-bit coprocessor. Note that the
coprocessor is normally enabled via the "-mconfig=" option.
|
-mcop32
Enables the 32-bit coprocessor's instructions.
|
-mcop64
Enables the 64-bit coprocessor's instructions.
|
-mivc2
Enables IVC2 scheduling. IVC2 is a 64-bit VLIW coprocessor.
|
-mdc
Causes constant variables to be placed in the ".near" section.
|
-mdiv
Enables the "div" and "divu" instructions.
|
-meb
Generate big-endian code.
|
-mel
Generate little-endian code.
|
-mio-volatile
Tells the compiler that any variable marked with the "io" attribute is to be considered volatile.
-ml Causes variables to be assigned to the ".far" section by default.
|
-mleadz
Enables the "leadz" (leading zero) instruction.
-mm Causes variables to be assigned to the ".near" section by default.
|
-mminmax
Enables the "min" and "max" instructions.
|
-mmult
Enables the multiplication and multiply-accumulate instructions.
|
-mno-opts
Disables all the optional instructions enabled by "-mall-opts".
|
-mrepeat
Enables the "repeat" and "erepeat" instructions, used for low-overhead looping.
-ms Causes all variables to default to the ".tiny" section. Note that there is a 65536-byte limit to
this section. Accesses to these variables use the %gp base register.
|
-msatur
Enables the saturation instructions. Note that the compiler does not currently generate these
itself, but this option is included for compatibility with other tools, like "as".
|
-msdram
Link the SDRAM-based runtime instead of the default ROM-based runtime.
|
-msim
Link the simulator runtime libraries.
|
-msimnovec
Link the simulator runtime libraries, excluding built-in support for reset and exception vectors and
tables.
|
-mtf
Causes all functions to default to the ".far" section. Without this option, functions default to the
".near" section.
|
-mtiny=n
Variables that are n bytes or smaller will be allocated to the ".tiny" section. These variables use
the $gp base register. The default for this option is 4, but note that there's a 65536-byte limit to
the ".tiny" section.
MicroBlaze Options
|
-msoft-float
Use software emulation for floating point (default).
|
-mhard-float
Use hardware floating-point instructions.
|
-mmemcpy
Do not optimize block moves, use "memcpy".
|
-mno-clearbss
This option is deprecated. Use -fno-zero-initialized-in-bss instead.
|
-mcpu=cpu-type
Use features of and schedule code for given CPU. Supported values are in the format vX.YY.Z, where X
is a major version, YY is the minor version, and Z is compatibility code. Example values are
v3.00.a, v4.00.b, v5.00.a, v5.00.b, v5.00.b, v6.00.a.
|
-mxl-soft-mul
Use software multiply emulation (default).
|
-mxl-soft-div
Use software emulation for divides (default).
|
-mxl-barrel-shift
Use the hardware barrel shifter.
|
-mxl-pattern-compare
Use pattern compare instructions.
|
-msmall-divides
Use table lookup optimization for small signed integer divisions.
|
-mxl-stack-check
This option is deprecated. Use -fstack-check instead.
|
-mxl-gp-opt
Use GP relative sdata/sbss sections.
|
-mxl-multiply-high
Use multiply high instructions for high part of 32x32 multiply.
|
-mxl-float-convert
Use hardware floating-point conversion instructions.
|
-mxl-float-sqrt
Use hardware floating-point square root instruction.
|
-mxl-mode-app-model
Select application model app-model. Valid models are
|
-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, mips64 and
mips64r2. The processor names are: 4kc, 4km, 4kp, 4ksc, 4kec, 4kem, 4kep, 4ksd, 5kc, 5kf, 20kc,
24kc, 24kf2_1, 24kf1_1, 24kec, 24kef2_1, 24kef1_1, 34kc, 34kf2_1, 34kf1_1, 74kc, 74kf2_1, 74kf1_1,
74kf3_2, 1004kc, 1004kf2_1, 1004kf1_1, loongson2e, loongson2f, loongson3a, m4k, octeon, octeon+,
octeon2, orion, r2000, r3000, r3900, r4000, r4400, r4600, r4650, r6000, r8000, rm7000, rm9000,
r10000, r12000, r14000, r16000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130, vr4300, vr5000, vr5400,
vr5500 and xlr. 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).
|
-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.
|
-mips64r2
Equivalent to -march=mips64r2.
|
-mips16
-mno-mips16
Generate (do not generate) MIPS16 code. If GCC is targetting a MIPS32 or MIPS64 architecture, it
will make use of the MIPS16e ASE.
MIPS16 code generation can also be controlled on a per-function basis by means of "mips16" and
"nomips16" attributes.
|
-mflip-mips16
Generate MIPS16 code on alternating functions. This option is provided for regression testing of
mixed MIPS16/non-MIPS16 code generation, and is not intended for ordinary use in compiling user code.
|
-minterlink-mips16
-mno-interlink-mips16
Require (do not require) that non-MIPS16 code be link-compatible with MIPS16 code.
For example, non-MIPS16 code cannot jump directly to MIPS16 code; it must either use a call or an
indirect jump. -minterlink-mips16 therefore disables direct jumps unless GCC knows that the target
of the jump is not MIPS16.
|
-mabicalls
-mno-abicalls
Generate (do not generate) code that is suitable for SVR4-style dynamic objects. -mabicalls is the
default for SVR4-based systems.
|
-mshared
-mno-shared
Generate (do not generate) code that is fully position-independent, and that can therefore be linked
into shared libraries. This option only affects -mabicalls.
All -mabicalls code has traditionally been position-independent, regardless of options like -fPIC and
-fpic. However, as an extension, the GNU toolchain allows executables to use absolute accesses for
locally-binding symbols. It can also use shorter GP initialization sequences and generate direct
calls to locally-defined functions. This mode is selected by -mno-shared.
-mno-shared depends on binutils 2.16 or higher and generates objects that can only be linked by the
GNU linker. However, the option does not affect the ABI of the final executable; it only affects the
ABI of relocatable objects. Using -mno-shared will generally make executables both smaller and
quicker.
|
-mshared is the default.
|
-mplt
-mno-plt
Assume (do not assume) that the static and dynamic linkers support PLTs and copy relocations. This
option only affects -mno-shared -mabicalls. For the n64 ABI, this option has no effect without
-msym32.
You can make -mplt the default by configuring GCC with --with-mips-plt. The default is -mno-plt
otherwise.
|
-mxgot
-mno-xgot
Lift (do not lift) the usual restrictions on the size of the global offset table.
|
-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.
|
-mllsc
-mno-llsc
Use (do not use) ll, sc, and sync instructions to implement atomic memory built-in functions. When
neither option is specified, GCC will use the instructions if the target architecture supports them.
-mllsc is useful if the runtime environment can emulate the instructions and -mno-llsc can be useful
when compiling for nonstandard ISAs. You can make either option the default by configuring GCC with
--with-llsc and --without-llsc respectively. --with-llsc is the default for some configurations; see
the installation documentation for details.
|
-mdsp
-mno-dsp
Use (do not use) revision 1 of the MIPS DSP ASE.
This option defines the preprocessor macro __mips_dsp. It also defines __mips_dsp_rev to 1.
|
-mdspr2
-mno-dspr2
Use (do not use) revision 2 of the MIPS DSP ASE.
This option defines the preprocessor macros __mips_dsp and __mips_dspr2. It also defines
__mips_dsp_rev to 2.
|
-msmartmips
-mno-smartmips
Use (do not use) the MIPS SmartMIPS ASE.
|
-mpaired-single
-mno-paired-single
Use (do not use) paired-single floating-point instructions.
This option requires hardware floating-point support to be enabled.
|
-mdmx
-mno-mdmx
Use (do not use) MIPS Digital Media Extension instructions. This option can only be used when
generating 64-bit code and requires hardware floating-point support to be enabled.
|
-mips3d
-mno-mips3d
Use (do not use) the MIPS-3D ASE. The option -mips3d implies -mpaired-single.
|
-mmt
-mno-mt
Use (do not use) MT Multithreading instructions.
|
-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.
|
-msym32
-mno-sym32
Assume (do not assume) that all symbols have 32-bit values, regardless of the selected ABI. This
option is useful in combination with -mabi=64 and -mno-abicalls because it allows GCC to generate
shorter and faster references to symbolic addresses.
|
-G num
Put definitions of externally-visible data in a small data section if that data is no bigger than num
bytes. GCC can then access the data more efficiently; see -mgpopt for details.
The default -G option depends on the configuration.
|
-mlocal-sdata
-mno-local-sdata
Extend (do not extend) the -G behavior to local data too, such as to static variables in C.
-mlocal-sdata is the default for all configurations.
If the linker complains that an application is using too much small data, you might want to try
rebuilding the less performance-critical parts with -mno-local-sdata. You might also want to build
large libraries with -mno-local-sdata, so that the libraries leave more room for the main program.
|
-mextern-sdata
-mno-extern-sdata
Assume (do not assume) that externally-defined data will be in a small data section if that data is
within the -G limit. -mextern-sdata is the default for all configurations.
If you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var
that is no bigger than num bytes, you must make sure that Var is placed in a small data section. If
Var is defined by another module, you must either compile that module with a high-enough -G setting
or attach a "section" attribute to Var's definition. If Var is common, you must link the application
with a high-enough -G setting.
The easiest way of satisfying these restrictions is to compile and link every module with the same -G
option. However, you may wish to build a library that supports several different small data limits.
You can do this by compiling the library with the highest supported -G setting and additionally using
-mno-extern-sdata to stop the library from making assumptions about externally-defined data.
|
-mgpopt
-mno-gpopt
Use (do not use) GP-relative accesses for symbols that are known to be in a small data section; see
-G, -mlocal-sdata and -mextern-sdata. -mgpopt is the default for all configurations.
-mno-gpopt is useful for cases where the $gp register might not hold the value of "_gp". For
example, if the code is part of a library that might be used in a boot monitor, programs that call
boot monitor routines will pass an unknown value in $gp. (In such situations, the boot monitor
itself would usually be compiled with -G0.)
-mno-gpopt implies -mno-local-sdata and -mno-extern-sdata.
|
-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.
|
-mcode-readable=setting
Specify whether GCC may generate code that reads from executable sections. There are three possible
settings:
|
-mcode-readable=yes
Instructions may freely access executable sections. This is the default setting.
|
-mcode-readable=pcrel
MIPS16 PC-relative load instructions can access executable sections, but other instructions must
not do so. This option is useful on 4KSc and 4KSd processors when the code TLBs have the Read
Inhibit bit set. It is also useful on processors that can be configured to have a dual
instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads
to the instruction RAM.
|
-mcode-readable=no
Instructions must not access executable sections. This option can be useful on targets that are
configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not
automatically redirect PC-relative loads to the instruction RAM.
|
-msplit-addresses
-mno-split-addresses
Enable (disable) use of the "%hi()" and "%lo()" assembler relocation operators. This option has been
superseded 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 the default if GCC was configured to use an assembler that supports relocation
operators.
|
-mcheck-zero-division
-mno-check-zero-division
Trap (do not trap) on integer division by zero.
The default is -mcheck-zero-division.
|
-mdivide-traps
-mdivide-breaks
MIPS systems check for division by zero by generating either a conditional trap or a break
instruction. Using traps results in smaller code, but is only supported on MIPS II and later. Also,
some versions of the Linux kernel have a bug that prevents trap from generating the proper signal
("SIGFPE"). Use -mdivide-traps to allow conditional traps on architectures that support them and
-mdivide-breaks to force the use of breaks.
The default is usually -mdivide-traps, but this can be overridden at configure time using
--with-divide=breaks. Divide-by-zero checks can be completely disabled using
-mno-check-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-24k
-mno-fix-24k
Work around the 24K E48 (lost data on stores during refill) errata. The workarounds are implemented
by the assembler rather than by GCC.
|
-mfix-r4000
-mno-fix-r4000
Work around certain R4000 CPU errata:
- A double-word or a variable shift may give an incorrect result if executed immediately after
starting an integer division.
- A double-word or a variable shift may give an incorrect result if executed while an integer
multiplication is in progress.
- An integer division may give an incorrect result if started in a delay slot of a taken branch or
a jump.
|
-mfix-r4400
-mno-fix-r4400
Work around certain R4400 CPU errata:
- A double-word or a variable shift may give an incorrect result if executed immediately after
starting an integer division.
|
-mfix-r10000
-mno-fix-r10000
Work around certain R10000 errata:
- "ll"/"sc" sequences may not behave atomically on revisions prior to 3.0. They may deadlock on
revisions 2.6 and earlier.
This option can only be used if the target architecture supports branch-likely instructions.
-mfix-r10000 is the default when -march=r10000 is used; -mno-fix-r10000 is the default otherwise.
|
-mfix-vr4120
-mno-fix-vr4120
Work around certain VR4120 errata:
- "dmultu" does not always produce the correct result.
- "div" and "ddiv" do not always produce the correct result if one of the operands is negative.
The workarounds for the division errata rely on special functions in libgcc.a. At present, these
functions are only provided by the "mips64vr*-elf" configurations.
Other VR4120 errata require a nop to be inserted between certain pairs of instructions. These errata
are handled by the assembler, not by GCC itself.
|
-mfix-vr4130
Work around the VR4130 "mflo"/"mfhi" errata. The workarounds are implemented by the assembler rather
than by GCC, although GCC will avoid using "mflo" and "mfhi" if the VR4130 "macc", "macchi", "dmacc"
and "dmacchi" instructions are available instead.
|
-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.)
|
-mr10k-cache-barrier=setting
Specify whether GCC should insert cache barriers to avoid the side-effects of speculation on R10K
processors.
|
-mr10k-cache-barrier=load-store
Insert a cache barrier before a load or store that might be speculatively executed and that might
have side effects even if aborted.
|
-mr10k-cache-barrier=store
Insert a cache barrier before a store that might be speculatively executed and that might have
side effects even if aborted.
|
-mr10k-cache-barrier=none
Disable the insertion of cache barriers. This is the default setting.
|
-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-cost=num
Set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is
not guaranteed to produce consistent results across releases. A zero cost redundantly selects the
default, which is based on the -mtune setting.
|
-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 that
implement those architectures; for those, Branch Likely instructions will not be generated by default
because the MIPS32 and MIPS64 architectures specifically deprecate their use.
|
-mfp-exceptions
-mno-fp-exceptions
Specifies whether FP exceptions are enabled. This affects how we schedule FP instructions for some
processors. The default is that FP exceptions are enabled.
For instance, on the SB-1, if FP exceptions are disabled, and we are emitting 64-bit code, then we
can use both FP pipes. Otherwise, we can only use one FP pipe.
|
-mvr4130-align
-mno-vr4130-align
The VR4130 pipeline is two-way superscalar, but can only issue two instructions together if the first
one is 8-byte aligned. When this option is enabled, GCC will align pairs of instructions that it
thinks should execute in parallel.
This option only has an effect when optimizing for the VR4130. It normally makes code faster, but at
the expense of making it bigger. It is enabled by default at optimization level -O3.
|
-msynci
-mno-synci
Enable (disable) generation of "synci" instructions on architectures that support it. The "synci"
instructions (if enabled) will be generated when "__builtin___clear_cache()" is compiled.
This option defaults to "-mno-synci", but the default can be overridden by configuring with
"--with-synci".
When compiling code for single processor systems, it is generally safe to use "synci". However, on
many multi-core (SMP) systems, it will not invalidate the instruction caches on all cores and may
lead to undefined behavior.
|
-mrelax-pic-calls
-mno-relax-pic-calls
Try to turn PIC calls that are normally dispatched via register $25 into direct calls. This is only
possible if the linker can resolve the destination at link-time and if the destination is within
range for a direct call.
-mrelax-pic-calls is the default if GCC was configured to use an assembler and a linker that supports
the ".reloc" assembly directive and "-mexplicit-relocs" is in effect. With "-mno-explicit-relocs",
this optimization can be performed by the assembler and the linker alone without help from the
compiler.
|
-mmcount-ra-address
-mno-mcount-ra-address
Emit (do not emit) code that allows "_mcount" to modify the calling function's return address. When
enabled, this option extends the usual "_mcount" interface with a new ra-address parameter, which has
type "intptr_t *" and is passed in register $12. "_mcount" can then modify the return address by
doing both of the following:
|
-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.
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 using features specific to the AM33 processor.
|
-mno-am33
Do not generate code using features specific to the AM33 processor. This is the default.
|
-mam33-2
Generate code using features specific to the AM33/2.0 processor.
|
-mam34
Generate code using features specific to the AM34 processor.
|
-mtune=cpu-type
Use the timing characteristics of the indicated CPU type when scheduling instructions. This does not
change the targeted processor type. The CPU type must be one of mn10300, am33, am33-2 or am34.
|
-mreturn-pointer-on-d0
When generating a function that returns a pointer, return the pointer in both "a0" and "d0".
Otherwise, the pointer is returned only in a0, and attempts to call such functions without a
prototype would result in errors. Note that this option is on by default; use
-mno-return-pointer-on-d0 to disable it.
|
-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.
|
-mliw
Allow the compiler to generate Long Instruction Word instructions if the target is the AM33 or later.
This is the default. This option defines the preprocessor macro __LIW__.
|
-mnoliw
Do not allow the compiler to generate Long Instruction Word instructions. This option defines the
preprocessor macro __NO_LIW__.
|
-msetlb
Allow the compiler to generate the SETLB and Lcc instructions if the target is the AM33 or later.
This is the default. This option defines the preprocessor macro __SETLB__.
|
-mnosetlb
Do not allow the compiler to generate SETLB or Lcc instructions. This option defines the
preprocessor macro __NO_SETLB__.
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 "movmemhi" patterns for copying memory. This is the default.
|
-mbcopy
Do not use inline "movmemhi" 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.
|
-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.
picoChip Options
These -m options are defined for picoChip implementations:
|
-mae=ANY selects a completely generic AE type. Code generated with this option will run on any of
the other AE types. The code will not be as efficient as it would be if compiled for a specific AE
type, and some types of operation (e.g., multiplication) will not work properly on all types of AE.
|
-mae=MUL selects a MUL AE type. This is the most useful AE type for compiled code, and is the
default.
|
-mae=MAC selects a DSP-style MAC AE. Code compiled with this option may suffer from poor performance
of byte (char) manipulation, since the DSP AE does not provide hardware support for byte load/stores.
|
-msymbol-as-address
Enable the compiler to directly use a symbol name as an address in a load/store instruction, without
first loading it into a register. Typically, the use of this option will generate larger programs,
which run faster than when the option isn't used. However, the results vary from program to program,
so it is left as a user option, rather than being permanently enabled.
|
-mno-inefficient-warnings
Disables warnings about the generation of inefficient code. These warnings can be generated, for
example, when compiling code that performs byte-level memory operations on the MAC AE type. The MAC
AE has no hardware support for byte-level memory operations, so all byte load/stores must be
synthesized from word load/store operations. This is inefficient and a warning will be generated
indicating to the programmer that they should rewrite the code to avoid byte operations, or to target
an AE type that has the necessary hardware support. This option enables the warning to be turned
off.
PowerPC Options
These are listed under
RL78 Options
|
-msim
Links in additional target libraries to support operation within a simulator.
|
-mmul=none
-mmul=g13
-mmul=rl78
Specifies the type of hardware multiplication support to be used. The default is "none", which uses
software multiplication functions. The "g13" option is for the hardware multiply/divide peripheral
only on the RL78/G13 targets. The "rl78" option is for the standard hardware multiplication defined
in the RL78 software manual.
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
-mmfcrf
-mno-mfcrf
-mpopcntb
-mno-popcntb
-mpopcntd
-mno-popcntd
-mfprnd
-mno-fprnd
-mcmpb
-mno-cmpb
-mmfpgpr
-mno-mfpgpr
-mhard-dfp
-mno-hard-dfp
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 Freescale MPC5xx, MPC6xx, MPC8xx
microprocessors, and the IBM 4xx, 6xx, and follow-on 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 -mfprnd -mhard-float -mmfcrf -mmultiple -mnew-mnemonics -mpopcntb -mpopcntd -mpower
-mpower2 -mpowerpc64 -mpowerpc-gpopt -mpowerpc-gfxopt -msingle-float -mdouble-float -msimple-fpu
-mstring -mmulhw -mdlmzb -mmfpgpr -mvsx
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 because 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.
|
-mcmodel=small
Generate PowerPC64 code for the small model: The TOC is limited to 64k.
|
-mcmodel=medium
Generate PowerPC64 code for the medium model: The TOC and other static data may be up to a total of
4G in size.
|
-mcmodel=large
Generate PowerPC64 code for the large model: The TOC may be up to 4G in size. Other data and code is
only limited by the 64-bit address space.
|
-maltivec
-mno-altivec
Generate code that uses (does not use) AltiVec instructions, and also enable the use of built-in
functions that allow more direct access to the AltiVec instruction set. You may also need to set
-mabi=altivec to adjust the current ABI with AltiVec ABI enhancements.
|
-mvrsave
-mno-vrsave
Generate VRSAVE instructions when generating AltiVec code.
|
-mgen-cell-microcode
Generate Cell microcode instructions
|
-mwarn-cell-microcode
Warning when a Cell microcode instruction is going to emitted. An example of a Cell microcode
instruction is a variable shift.
|
-msecure-plt
Generate code that allows ld and ld.so to build executables and shared libraries with non-exec .plt
and .got sections. This is a PowerPC 32-bit SYSV ABI option.
|
-mbss-plt
Generate code that uses a BSS .plt section that ld.so fills in, and requires .plt and .got sections
that are both writable and executable. This is a PowerPC 32-bit SYSV ABI option.
|
-misel
-mno-isel
This switch enables or disables the generation of ISEL instructions.
|
-misel=yes/no
This switch has been deprecated. Use -misel and -mno-isel instead.
|
-mspe
-mno-spe
This switch enables or disables the generation of SPE simd instructions.
|
-mpaired
-mno-paired
This switch enables or disables the generation of PAIRED simd instructions.
|
-mspe=yes/no
This option has been deprecated. Use -mspe and -mno-spe instead.
|
-mvsx
-mno-vsx
Generate code that uses (does not use) vector/scalar (VSX) instructions, and also enable the use of
built-in functions that allow more direct access to the VSX instruction set.
|
-mfloat-gprs=yes/single/double/no
-mfloat-gprs
This switch enables or disables the generation of floating-point operations on the general-purpose
registers for architectures that support it.
The argument yes or single enables the use of single-precision floating-point operations.
The argument double enables the use of single and double-precision floating-point operations.
The argument no disables floating-point operations on the general-purpose registers.
This option is currently only available on the MPC854x.
|
-m32
-m64
Generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including GNU/Linux).
The 32-bit environment sets int, long and pointer to 32 bits and generates code that runs on any
PowerPC variant. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits, and
generates code for PowerPC64, as for -mpowerpc64.
|
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is created for every executable file. The
-mfull-toc option is selected by default. In that case, GCC will allocate at least one TOC entry for
each unique non-automatic variable reference in your program. GCC will also place floating-point
constants in the TOC. However, only 16,384 entries are available in the TOC.
If you receive a linker error message that saying you have overflowed the available TOC space, you
can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options.
-mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC and -mno-sum-in-toc
forces GCC to generate code to calculate the sum of an address and a constant at run time instead of
putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to
produce very slightly slower and larger code at the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both of these options, specify
-mminimal-toc instead. This option causes GCC to make only one TOC entry for every file. When you
specify this option, GCC will produce code that is slower and larger but which uses extremely little
TOC space. You may wish to use this option only on files that contain less frequently executed code.
|
-maix64
-maix32
Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit "long" type, and the
infrastructure needed to support them. Specifying -maix64 implies -mpowerpc64 and -mpowerpc, while
-maix32 disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32.
|
-mxl-compat
-mno-xl-compat
Produce code that conforms more closely to IBM XL compiler 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 and converting to double. Use XL symbol names
for long double support routines.
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.
IBM XL compilers access floating-point arguments that 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 IBM 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, 32-bit 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.
On 64-bit Darwin, natural alignment is the default, and -malign-power is not supported.
|
-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.
|
-msingle-float
-mdouble-float
Generate code for single- or double-precision floating-point operations. -mdouble-float implies
-msingle-float.
|
-msimple-fpu
Do not generate sqrt and div instructions for hardware floating-point unit.
|
-mfpu
Specify type of floating-point unit. Valid values are sp_lite (equivalent to -msingle-float
-msimple-fpu), dp_lite (equivalent to -mdouble-float -msimple-fpu), sp_full (equivalent to
-msingle-float), and dp_full (equivalent to -mdouble-float).
|
-mxilinx-fpu
Perform optimizations for the floating-point unit on Xilinx PPC 405/440.
|
-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 these instructions 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 these instructions 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.
|
-mavoid-indexed-addresses
-mno-avoid-indexed-addresses
Generate code that tries to avoid (not avoid) the use of indexed load or store instructions. These
instructions can incur a performance penalty on Power6 processors in certain situations, such as when
stepping through large arrays that cross a 16M boundary. This option is enabled by default when
targetting Power6 and disabled otherwise.
|
-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. The machine-
dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and
-mno-fused-madd is mapped to -ffp-contract=off.
|
-mmulhw
-mno-mulhw
Generate code that uses (does not use) the half-word multiply and multiply-accumulate instructions on
the IBM 405, 440, 464 and 476 processors. These instructions are generated by default when
targetting those processors.
|
-mdlmzb
-mno-dlmzb
Generate code that uses (does not use) the string-search dlmzb instruction on the IBM 405, 440, 464
and 476 processors. This instruction is generated by default when targetting those processors.
|
-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 is
aligned to a 4-byte boundary and has a size of 4 bytes. By using -mno-bit-align, the structure is
aligned to a 1-byte boundary and is 1 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
Generate code that allows (does not allow) a static executable to be relocated to a different address
at run time. A simple embedded PowerPC system loader should relocate the entire contents of ".got2"
and 4-byte locations listed in the ".fixup" section, a table of 32-bit addresses generated by this
option. For this to work, all objects linked together must be compiled with -mrelocatable or
-mrelocatable-lib. -mrelocatable code aligns the stack to an 8-byte boundary.
|
-mrelocatable-lib
-mno-relocatable-lib
Like -mrelocatable, -mrelocatable-lib generates a ".fixup" section to allow static executables to be
relocated at run time, but -mrelocatable-lib does not use the smaller stack alignment of
-mrelocatable. Objects compiled with -mrelocatable-lib may be linked with objects compiled with any
combination of 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.
|
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for
each function. The runtime system is responsible for initializing this register with an appropriate
value before execution begins.
|
-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 that 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
-mcall-eabi
Specify both -mcall-sysv and -meabi options.
|
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
|
-mcall-aixdesc
On System V.4 and embedded PowerPC systems compile code for the AIX operating system.
|
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system.
|
-mcall-freebsd
On System V.4 and embedded PowerPC systems compile code for the FreeBSD operating system.
|
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the NetBSD operating system.
|
-mcall-openbsd
On System V.4 and embedded PowerPC systems compile code for the OpenBSD 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=abi-type
Extend the current ABI with a particular extension, or remove such extension. Valid values are
altivec, no-altivec, spe, no-spe, ibmlongdouble, ieeelongdouble.
|
-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.
|
-mabi=ibmlongdouble
Change the current ABI to use IBM extended-precision long double. This is a PowerPC 32-bit SYSV ABI
option.
|
-mabi=ieeelongdouble
Change the current ABI to use IEEE extended-precision long double. This is a PowerPC 32-bit Linux
ABI option.
|
-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 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.
|
-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 data in the .sdata section. Put small
uninitialized global 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.
|
-mblock-move-inline-limit=num
Inline all block moves (such as calls to "memcpy" or structure copies) less than or equal to num
bytes. The minimum value for num is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets. The
default value is target-specific.
|
-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
By default assume that all calls are far away so that a longer more expensive calling sequence is
required. This is required for calls further than 32 megabytes (33,554,432 bytes) from the current
location. A short call will be generated if the compiler knows the call cannot be that far away.
This setting can be overridden by the "shortcall" function attribute, or by "#pragma longcall(0)".
Some linkers are capable of detecting out-of-range calls and generating glue code on the fly. On
these systems, long calls are unnecessary and generate slower code. As of this writing, the AIX
linker can do this, as can the GNU linker for PowerPC/64. It is planned to add this feature to the
GNU linker for 32-bit PowerPC systems as well.
On Darwin/PPC systems, "#pragma longcall" will generate "jbsr callee, L42", plus a "branch island"
(glue code). The two target addresses represent the callee and the "branch island". The Darwin/PPC
linker will prefer the first address and generate a "bl callee" if the PPC "bl" instruction will
reach the callee directly; otherwise, the linker will generate "bl L42" to call the "branch island".
The "branch island" is appended to the body of the calling function; it computes the full 32-bit
address of the callee and jumps to it.
On Mach-O (Darwin) systems, this option directs the compiler emit to the glue for every direct call,
and the Darwin linker decides whether to use or discard it.
In the future, we may cause GCC to ignore all longcall specifications when the linker is known to
generate glue.
|
-mtls-markers
-mno-tls-markers
Mark (do not mark) calls to "__tls_get_addr" with a relocation specifying the function argument. The
relocation allows ld to reliably associate function call with argument setup instructions for TLS
optimization, which in turn allows gcc to better schedule the sequence.
|
-pthread
Adds support for multithreading with the pthreads library. This option sets flags for both the
preprocessor and linker.
|
-mrecip
-mno-recip
This option will enable GCC to use the reciprocal estimate and reciprocal square root estimate
instructions with additional Newton-Raphson steps to increase precision instead of doing a divide or
square root and divide for floating-point arguments. You should use the -ffast-math option when
using -mrecip (or at least -funsafe-math-optimizations, -finite-math-only, -freciprocal-math and
-fno-trapping-math). Note that while the throughput of the sequence is generally higher than the
throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to
2 ulp (i.e. the inverse of 1.0 equals 0.99999994) for reciprocal square roots.
|
-mrecip=opt
This option allows to control which reciprocal estimate instructions may be used. opt is a comma
separated list of options, which may be preceded by a "!" to invert the option: "all": enable all
estimate instructions, "default": enable the default instructions, equivalent to -mrecip, "none":
disable all estimate instructions, equivalent to -mno-recip; "div": enable the reciprocal
approximation instructions for both single and double precision; "divf": enable the single-precision
reciprocal approximation instructions; "divd": enable the double-precision reciprocal approximation
instructions; "rsqrt": enable the reciprocal square root approximation instructions for both single
and double precision; "rsqrtf": enable the single-precision reciprocal square root approximation
instructions; "rsqrtd": enable the double-precision reciprocal square root approximation
instructions;
So for example, -mrecip=all,!rsqrtd would enable the all of the reciprocal estimate instructions,
except for the "FRSQRTE", "XSRSQRTEDP", and "XVRSQRTEDP" instructions which handle the double-
precision reciprocal square root calculations.
|
-mrecip-precision
-mno-recip-precision
Assume (do not assume) that the reciprocal estimate instructions provide higher-precision estimates
than is mandated by the PowerPC ABI. Selecting -mcpu=power6 or -mcpu=power7 automatically selects
-mrecip-precision. The double-precision square root estimate instructions are not generated by
default on low-precision machines, since they do not provide an estimate that converges after three
steps.
|
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics using an external library. The only type
supported at present is "mass", which specifies to use IBM's Mathematical Acceleration Subsystem
(MASS) libraries for vectorizing intrinsics using external libraries. GCC will currently emit calls
to "acosd2", "acosf4", "acoshd2", "acoshf4", "asind2", "asinf4", "asinhd2", "asinhf4", "atan2d2",
"atan2f4", "atand2", "atanf4", "atanhd2", "atanhf4", "cbrtd2", "cbrtf4", "cosd2", "cosf4", "coshd2",
"coshf4", "erfcd2", "erfcf4", "erfd2", "erff4", "exp2d2", "exp2f4", "expd2", "expf4", "expm1d2",
"expm1f4", "hypotd2", "hypotf4", "lgammad2", "lgammaf4", "log10d2", "log10f4", "log1pd2", "log1pf4",
"log2d2", "log2f4", "logd2", "logf4", "powd2", "powf4", "sind2", "sinf4", "sinhd2", "sinhf4",
"sqrtd2", "sqrtf4", "tand2", "tanf4", "tanhd2", and "tanhf4" when generating code for power7. Both
-ftree-vectorize and -funsafe-math-optimizations have to be enabled. The MASS libraries will have to
be specified at link time.
|
-mfriz
-mno-friz
Generate (do not generate) the "friz" instruction when the -funsafe-math-optimizations option is used
to optimize rounding of floating-point values to 64-bit integer and back to floating point. The
"friz" instruction does not return the same value if the floating-point number is too large to fit in
an integer.
|
-mpointers-to-nested-functions
-mno-pointers-to-nested-functions
Generate (do not generate) code to load up the static chain register (r11) when calling through a
pointer on AIX and 64-bit Linux systems where a function pointer points to a 3-word descriptor giving
the function address, TOC value to be loaded in register r2, and static chain value to be loaded in
register r11. The -mpointers-to-nested-functions is on by default. You will not be able to call
through pointers to nested functions or pointers to functions compiled in other languages that use
the static chain if you use the -mno-pointers-to-nested-functions.
|
-msave-toc-indirect
-mno-save-toc-indirect
Generate (do not generate) code to save the TOC value in the reserved stack location in the function
prologue if the function calls through a pointer on AIX and 64-bit Linux systems. If the TOC value
is not saved in the prologue, it is saved just before the call through the pointer. The
-mno-save-toc-indirect option is the default.
RX Options
These command-line options are defined for RX targets:
|
-m64bit-doubles
-m32bit-doubles
Make the "double" data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The
default is -m32bit-doubles. Note RX floating-point hardware only works on 32-bit values, which is
why the default is -m32bit-doubles.
|
-fpu
-nofpu
Enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware. The default is enabled
for the RX600 series and disabled for the RX200 series.
Floating-point instructions will only be generated for 32-bit floating-point values however, so if
the -m64bit-doubles option is in use then the FPU hardware will not be used for doubles.
Note If the -fpu option is enabled then -funsafe-math-optimizations is also enabled automatically.
This is because the RX FPU instructions are themselves unsafe.
|
-mcpu=name
Selects the type of RX CPU to be targeted. Currently three types are supported, the generic RX600
and RX200 series hardware and the specific RX610 CPU. The default is RX600.
The only difference between RX600 and RX610 is that the RX610 does not support the "MVTIPL"
instruction.
The RX200 series does not have a hardware floating-point unit and so -nofpu is enabled by default
when this type is selected.
|
-mbig-endian-data
-mlittle-endian-data
Store data (but not code) in the big-endian format. The default is -mlittle-endian-data, i.e. to
store data in the little-endian format.
|
-msmall-data-limit=N
Specifies the maximum size in bytes of global and static variables which can be placed into the small
data area. Using the small data area can lead to smaller and faster code, but the size of area is
limited and it is up to the programmer to ensure that the area does not overflow. Also when the
small data area is used one of the RX's registers (usually "r13") is reserved for use pointing to
this area, so it is no longer available for use by the compiler. This could result in slower and/or
larger code if variables which once could have been held in the reserved register are now pushed onto
the stack.
Note, common variables (variables that have not been initialized) and constants are not placed into
the small data area as they are assigned to other sections in the output executable.
The default value is zero, which disables this feature. Note, this feature is not enabled by default
with higher optimization levels (-O2 etc) because of the potentially detrimental effects of reserving
a register. It is up to the programmer to experiment and discover whether this feature is of benefit
to their program. See the description of the -mpid option for a description of how the actual
register to hold the small data area pointer is chosen.
|
-msim
-mno-sim
Use the simulator runtime. The default is to use the libgloss board specific runtime.
|
-mas100-syntax
-mno-as100-syntax
When generating assembler output use a syntax that is compatible with Renesas's AS100 assembler.
This syntax can also be handled by the GAS assembler but it has some restrictions so generating it is
not the default option.
|
-mmax-constant-size=N
Specifies the maximum size, in bytes, of a constant that can be used as an operand in a RX
instruction. Although the RX instruction set does allow constants of up to 4 bytes in length to be
used in instructions, a longer value equates to a longer instruction. Thus in some circumstances it
can be beneficial to restrict the size of constants that are used in instructions. Constants that
are too big are instead placed into a constant pool and referenced via register indirection.
The value N can be between 0 and 4. A value of 0 (the default) or 4 means that constants of any size
are allowed.
|
-mrelax
Enable linker relaxation. Linker relaxation is a process whereby the linker will attempt to reduce
the size of a program by finding shorter versions of various instructions. Disabled by default.
|
-mint-register=N
Specify the number of registers to reserve for fast interrupt handler functions. The value N can be
between 0 and 4. A value of 1 means that register "r13" will be reserved for the exclusive use of
fast interrupt handlers. A value of 2 reserves "r13" and "r12". A value of 3 reserves "r13", "r12"
and "r11", and a value of 4 reserves "r13" through "r10". A value of 0, the default, does not
reserve any registers.
|
-msave-acc-in-interrupts
Specifies that interrupt handler functions should preserve the accumulator register. This is only
necessary if normal code might use the accumulator register, for example because it performs 64-bit
multiplications. The default is to ignore the accumulator as this makes the interrupt handlers
faster.
|
-mpid
-mno-pid
Enables the generation of position independent data. When enabled any access to constant data will
done via an offset from a base address held in a register. This allows the location of constant data
to be determined at run time without requiring the executable to be relocated, which is a benefit to
embedded applications with tight memory constraints. Data that can be modified is not affected by
this option.
|
-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.
|
-mhard-dfp
-mno-hard-dfp
Use (do not use) the hardware decimal-floating-point instructions for decimal-floating-point
operations. When -mno-hard-dfp is specified, functions in libgcc.a will be used to perform decimal-
floating-point operations. When -mhard-dfp is specified, the compiler generates decimal-floating-
point hardware instructions. This is the default for -march=z9-ec or higher.
|
-mlong-double-64
-mlong-double-128
These switches control the size of "long double" type. A size of 64 bits makes the "long double" type
equivalent to the "double" type. This is the default.
|
-mbackchain
-mno-backchain
Store (do not store) the address of the caller's frame as backchain pointer into the callee's stack
frame. A backchain may be needed to allow debugging using tools that do not understand DWARF-2 call
frame information. When -mno-packed-stack is in effect, the backchain pointer is stored at the
bottom of the stack frame; when -mpacked-stack is in effect, the backchain is placed into the topmost
word of the 96/160 byte register save area.
In general, code compiled with -mbackchain is call-compatible with code compiled with -mmo-backchain;
however, use of the backchain for debugging purposes usually requires that the whole binary is built
with -mbackchain. Note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not
supported. In order to build a linux kernel use -msoft-float.
The default is to not maintain the backchain.
|
-mpacked-stack
-mno-packed-stack
Use (do not use) the packed stack layout. When -mno-packed-stack is specified, the compiler uses the
all fields of the 96/160 byte register save area only for their default purpose; unused fields still
take up stack space. When -mpacked-stack is specified, register save slots are densely packed at the
top of the register save area; unused space is reused for other purposes, allowing for more efficient
use of the available stack space. However, when -mbackchain is also in effect, the topmost word of
the save area is always used to store the backchain, and the return address register is always saved
two words below the backchain.
As long as the stack frame backchain is not used, code generated with -mpacked-stack is call-
compatible with code generated with -mno-packed-stack. Note that some non-FSF releases of GCC 2.95
for S/390 or zSeries generated code that uses the stack frame backchain at run time, not just for
debugging purposes. Such code is not call-compatible with code compiled with -mpacked-stack. Also,
note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order
to build a linux kernel use -msoft-float.
The default is to not use the packed stack layout.
|
-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 unless optimizing for size.
|
-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, z990, z9-109, z9-ec and z10. 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.
|
-mtpf-trace
-mno-tpf-trace
Generate code that adds (does not add) in TPF OS specific branches to trace routines in the operating
system. This option is off by default, even when compiling for the TPF OS.
|
-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.
|
-mwarn-framesize=framesize
Emit a warning if the current function exceeds the given frame size. Because this is a compile-time
check it doesn't need to be a real problem when the program runs. It is intended to identify
functions that most probably cause a stack overflow. It is useful to be used in an environment with
limited stack size e.g. the linux kernel.
|
-mwarn-dynamicstack
Emit a warning if the function calls alloca or uses dynamically sized arrays. This is generally a
bad idea with a limited stack size.
|
-mstack-guard=stack-guard
-mstack-size=stack-size
If these options are provided the s390 back end emits additional instructions in the function
prologue which trigger a trap if the stack size is stack-guard bytes above the stack-size (remember
that the stack on s390 grows downward). If the stack-guard option is omitted the smallest power of 2
larger than the frame size of the compiled function is chosen. These options are intended to be used
to help debugging stack overflow problems. The additionally emitted code causes only little overhead
and hence can also be used in production like systems without greater performance degradation. The
given values have to be exact powers of 2 and stack-size has to be greater than stack-guard without
exceeding 64k. In order to be efficient the extra code makes the assumption that the stack starts at
an address aligned to the value given by stack-size. The stack-guard option can only be used in
conjunction with stack-size.
Score Options
These options are defined for Score implementations:
|
-meb
Compile code for big-endian mode. This is the default.
|
-mel
Compile code for little-endian mode.
|
-mnhwloop
Disable generate bcnz instruction.
|
-muls
Enable generate unaligned load and store instruction.
|
-mmac
Enable the use of multiply-accumulate instructions. Disabled by default.
|
-mscore5
Specify the SCORE5 as the target architecture.
|
-mscore5u
Specify the SCORE5U of the target architecture.
|
-mscore7
Specify the SCORE7 as the target architecture. This is the default.
|
-mscore7d
Specify the SCORE7D as the target architecture.
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.
|
-m2a-nofpu
Generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit
is not used.
|
-m2a-single-only
Generate code for the SH2a-FPU, in such a way that no double-precision floating-point operations are
used.
|
-m2a-single
Generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by
default.
|
-m2a
Generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by
default.
|
-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.
|
-m4a-nofpu
Generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not
used.
|
-m4a-single-only
Generate code for the SH4a, in such a way that no double-precision floating-point operations are
used.
|
-m4a-single
Generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.
|
-m4a
Generate code for the SH4a.
|
-m4al
Same as -m4a-nofpu, except that it implicitly passes -dsp to the assembler. GCC doesn't generate any
DSP instructions at the moment.
-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.
|
-mbitops
Enable the use of bit manipulation instructions on SH2A.
|
-mfmovd
Enable the use of the instruction "fmovd". Check -mdalign for alignment constraints.
|
-mhitachi
Comply with the calling conventions defined by Renesas.
|
-mrenesas
Comply with the calling conventions defined by Renesas.
|
-mno-renesas
Comply with the calling conventions defined for GCC before the Renesas conventions were available.
This option is the default for all targets of the SH toolchain.
|
-mnomacsave
Mark the "MAC" register as call-clobbered, even if -mhitachi is given.
|
-mieee
Increase IEEE compliance of floating-point code. At the moment, this is equivalent to
-fno-finite-math-only. When generating 16-bit SH opcodes, getting IEEE-conforming results for
comparisons of NANs / infinities incurs extra overhead in every floating-point comparison, therefore
the default is set to -ffinite-math-only.
|
-minline-ic_invalidate
Inline code to invalidate instruction cache entries after setting up nested function trampolines.
This option has no effect if -musermode is in effect and the selected code generation option (e.g.
-m4) does not allow the use of the icbi instruction. If the selected code generation option does not
allow the use of the icbi instruction, and -musermode is not in effect, the inlined code will
manipulate the instruction cache address array directly with an associative write. This not only
requires privileged mode, but it will also fail if the cache line had been mapped via the TLB and has
become unmapped.
|
-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.
|
-msoft-atomic
Generate GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions. The
generated atomic sequences require support from the interrupt / exception handling code of the system
and are only suitable for single-core systems. They will not perform correctly on multi-core
systems. This option is enabled by default when the target is "sh-*-linux*". For details on the
atomic built-in functions see __atomic Builtins.
|
-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
Don't generate privileged mode only code; implies -mno-inline-ic_invalidate if the inlined code would
not work in user mode. This is the default when the target is "sh-*-linux*".
|
-multcost=number
Set the cost to assume for a multiply insn.
|
-mdiv=strategy
Set the division strategy to use for SHmedia code. strategy must be one of: call, call2, fp, inv,
inv:minlat, inv20u, inv20l, inv:call, inv:call2, inv:fp . "fp" performs the operation in floating
point. This has a very high latency, but needs only a few instructions, so it might be a good choice
if your code has enough easily-exploitable ILP to allow the compiler to schedule the floating-point
instructions together with other instructions. Division by zero causes a floating-point exception.
"inv" uses integer operations to calculate the inverse of the divisor, and then multiplies the
dividend with the inverse. This strategy allows cse and hoisting of the inverse calculation.
Division by zero calculates an unspecified result, but does not trap. "inv:minlat" is a variant of
"inv" where if no cse / hoisting opportunities have been found, or if the entire operation has been
hoisted to the same place, the last stages of the inverse calculation are intertwined with the final
multiply to reduce the overall latency, at the expense of using a few more instructions, and thus
offering fewer scheduling opportunities with other code. "call" calls a library function that
usually implements the inv:minlat strategy. This gives high code density for m5-*media-nofpu
compilations. "call2" uses a different entry point of the same library function, where it assumes
that a pointer to a lookup table has already been set up, which exposes the pointer load to cse /
code hoisting optimizations. "inv:call", "inv:call2" and "inv:fp" all use the "inv" algorithm for
initial code generation, but if the code stays unoptimized, revert to the "call", "call2", or "fp"
strategies, respectively. Note that the potentially-trapping side effect of division by zero is
carried by a separate instruction, so it is possible that all the integer instructions are hoisted
out, but the marker for the side effect stays where it is. A recombination to fp operations or a
call is not possible in that case. "inv20u" and "inv20l" are variants of the "inv:minlat" strategy.
In the case that the inverse calculation was nor separated from the multiply, they speed up division
where the dividend fits into 20 bits (plus sign where applicable), by inserting a test to skip a
number of operations in this case; this test slows down the case of larger dividends. inv20u assumes
the case of a such a small dividend to be unlikely, and inv20l assumes it to be likely.
|
-maccumulate-outgoing-args
Reserve space once for outgoing arguments in the function prologue rather than around each call.
Generally beneficial for performance and size. Also needed for unwinding to avoid changing the stack
frame around conditional code.
|
-mdivsi3_libfunc=name
Set the name of the library function used for 32-bit signed division to name. This only affect the
name used in the call and inv:call division strategies, and the compiler will still expect the same
sets of input/output/clobbered registers as if this option was not present.
|
-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.
|
-madjust-unroll
Throttle unrolling to avoid thrashing target registers. This option only has an effect if the gcc
code base supports the TARGET_ADJUST_UNROLL_MAX target hook.
|
-mindexed-addressing
Enable the use of the indexed addressing mode for SHmedia32/SHcompact. This is only safe if the
hardware and/or OS implement 32-bit wrap-around semantics for the indexed addressing mode. The
architecture allows the implementation of processors with 64-bit MMU, which the OS could use to get
32-bit addressing, but since no current hardware implementation supports this or any other way to
make the indexed addressing mode safe to use in the 32-bit ABI, the default is
-mno-indexed-addressing.
|
-mgettrcost=number
Set the cost assumed for the gettr instruction to number. The default is 2 if -mpt-fixed is in
effect, 100 otherwise.
|
-mpt-fixed
Assume pt* instructions won't trap. This will generally generate better scheduled code, but is
unsafe on current hardware. The current architecture definition says that ptabs and ptrel trap when
the target anded with 3 is 3. This has the unintentional effect of making it unsafe to schedule
ptabs / ptrel before a branch, or hoist it out of a loop. For example, __do_global_ctors, a part of
libgcc that runs constructors at program startup, calls functions in a list which is delimited by -1.
With the -mpt-fixed option, the ptabs will be done before testing against -1. That means that all
the constructors will be run a bit quicker, but when the loop comes to the end of the list, the
program crashes because ptabs loads -1 into a target register. Since this option is unsafe for any
hardware implementing the current architecture specification, the default is -mno-pt-fixed. Unless
the user specifies a specific cost with -mgettrcost, -mno-pt-fixed also implies -mgettrcost=100; this
deters register allocation using target registers for storing ordinary integers.
|
-minvalid-symbols
Assume symbols might be invalid. Ordinary function symbols generated by the compiler will always be
valid to load with movi/shori/ptabs or movi/shori/ptrel, but with assembler and/or linker tricks it
is possible to generate symbols that will cause ptabs / ptrel to trap. This option is only
meaningful when -mno-pt-fixed is in effect. It will then prevent cross-basic-block cse, hoisting and
most scheduling of symbol loads. The default is -mno-invalid-symbols.
|
-mbranch-cost=num
Assume num to be the cost for a branch instruction. Higher numbers will make the compiler try to
generate more branch-free code if possible. If not specified the value is selected depending on the
processor type that is being compiled for.
|
-mcbranchdi
Enable the "cbranchdi4" instruction pattern.
|
-mcmpeqdi
Emit the "cmpeqdi_t" instruction pattern even when -mcbranchdi is in effect.
|
-mfused-madd
Allow the usage of the "fmac" instruction (floating-point multiply-accumulate) if the processor type
supports it. Enabling this option might generate code that produces different numeric floating-point
results compared to strict IEEE 754 arithmetic.
|
-mpretend-cmove
Prefer zero-displacement conditional branches for conditional move instruction patterns. This can
result in faster code on the SH4 processor.
Solaris 2 Options
These -m options are supported on Solaris 2:
|
-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.
These switches are supported in addition to the above on Solaris 2:
|
-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.
|
-pthread
This is a synonym for -pthreads.
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.
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.
|
-mflat
-mno-flat
With -mflat, the compiler does not generate save/restore instructions and uses a "flat" or single
register window model. This model is compatible with the regular register window model. The local
registers and the input registers (0--5) are still treated as "call-saved" registers and will be
saved on the stack as needed.
With -mno-flat (the default), the compiler generates save/restore instructions (except for leaf
functions). This is the normal operating mode.
|
-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-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.
|
-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 bits 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.
|
-mvis2
-mno-vis2
With -mvis2, GCC generates code that takes advantage of version 2.0 of the UltraSPARC Visual
Instruction Set extensions. The default is -mvis2 when targetting a cpu that supports such
instructions, such as UltraSPARC-III and later. Setting -mvis2 also sets -mvis.
|
-mvis3
-mno-vis3
With -mvis3, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual
Instruction Set extensions. The default is -mvis3 when targetting a cpu that supports such
instructions, such as niagara-3 and later. Setting -mvis3 also sets -mvis2 and -mvis.
|
-mpopc
-mno-popc
With -mpopc, GCC generates code that takes advantage of the UltraSPARC population count instruction.
The default is -mpopc when targetting a cpu that supports such instructions, such as Niagara-2 and
later.
|
-mfmaf
-mno-fmaf
With -mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add Floating-
point extensions. The default is -mfmaf when targetting a cpu that supports such instructions, such
as Niagara-3 and later.
|
-mfix-at697f
Enable the documented workaround for the single erratum of the Atmel AT697F processor (which
corresponds to erratum #13 of the AT697E processor).
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=which
Set the code model to one of
medlow
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.
medmid
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.
medany
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.
embmedany
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.
|
-mmemory-model=mem-model
Set the memory model in force on the processor to one of
|
-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.
SPU Options
These -m options are supported on the SPU:
|
-mwarn-reloc
-merror-reloc
The loader for SPU does not handle dynamic relocations. By default, GCC will give an error when it
generates code that requires a dynamic relocation. -mno-error-reloc disables the error, -mwarn-reloc
will generate a warning instead.
|
-msafe-dma
-munsafe-dma
Instructions that initiate or test completion of DMA must not be reordered with respect to loads and
stores of the memory that is being accessed. Users typically address this problem using the volatile
keyword, but that can lead to inefficient code in places where the memory is known to not change.
Rather than mark the memory as volatile we treat the DMA instructions as potentially effecting all
memory. With -munsafe-dma users must use the volatile keyword to protect memory accesses.
|
-mbranch-hints
By default, GCC will generate a branch hint instruction to avoid pipeline stalls for always taken or
probably taken branches. A hint will not be generated closer than 8 instructions away from its
branch. There is little reason to disable them, except for debugging purposes, or to make an object
a little bit smaller.
|
-msmall-mem
-mlarge-mem
By default, GCC generates code assuming that addresses are never larger than 18 bits. With
-mlarge-mem code is generated that assumes a full 32-bit address.
|
-mstdmain
By default, GCC links against startup code that assumes the SPU-style main function interface (which
has an unconventional parameter list). With -mstdmain, GCC will link your program against startup
code that assumes a C99-style interface to "main", including a local copy of "argv" strings.
|
-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.
|
-mea32
-mea64
Compile code assuming that pointers to the PPU address space accessed via the "__ea" named address
space qualifier are either 32 or 64 bits wide. The default is 32 bits. As this is an ABI changing
option, all object code in an executable must be compiled with the same setting.
|
-maddress-space-conversion
-mno-address-space-conversion
Allow/disallow treating the "__ea" address space as superset of the generic address space. This
enables explicit type casts between "__ea" and generic pointer as well as implicit conversions of
generic pointers to "__ea" pointers. The default is to allow address space pointer conversions.
|
-mcache-size=cache-size
This option controls the version of libgcc that the compiler links to an executable and selects a
software-managed cache for accessing variables in the "__ea" address space with a particular cache
size. Possible options for cache-size are 8, 16, 32, 64 and 128. The default cache size is 64KB.
|
-matomic-updates
-mno-atomic-updates
This option controls the version of libgcc that the compiler links to an executable and selects
whether atomic updates to the software-managed cache of PPU-side variables are used. If you use
atomic updates, changes to a PPU variable from SPU code using the "__ea" named address space
qualifier will not interfere with changes to other PPU variables residing in the same cache line from
PPU code. If you do not use atomic updates, such interference may occur; however, writing back cache
lines will be more efficient. The default behavior is to use atomic updates.
|
-mdual-nops
-mdual-nops=n
By default, GCC will insert nops to increase dual issue when it expects it to increase performance.
n can be a value from 0 to 10. A smaller n will insert fewer nops. 10 is the default, 0 is the same
as -mno-dual-nops. Disabled with -Os.
|
-mhint-max-nops=n
Maximum number of nops to insert for a branch hint. A branch hint must be at least 8 instructions
away from the branch it is effecting. GCC will insert up to n nops to enforce this, otherwise it
will not generate the branch hint.
|
-mhint-max-distance=n
The encoding of the branch hint instruction limits the hint to be within 256 instructions of the
branch it is effecting. By default, GCC makes sure it is within 125.
|
-msafe-hints
Work around a hardware bug that causes the SPU to stall indefinitely. By default, GCC will insert
the "hbrp" instruction to make sure this stall won't happen.
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.
TILE-Gx Options
These -m options are supported on the TILE-Gx:
|
-mcpu=name
Selects the type of CPU to be targeted. Currently the only supported type is tilegx.
|
-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.
TILEPro Options
These -m options are supported on the TILEPro:
|
-mcpu=name
Selects the type of CPU to be targeted. Currently the only supported type is tilepro.
|
-m32
Generate code for a 32-bit environment, which sets int, long, and pointer to 32 bits. This is the
only supported behavior so the flag is essentially ignored.
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.
|
-mv850e2v3
Specify that the target processor is the V850E2V3. The preprocessor constants __v850e2v3__ will be
defined if this option is used.
|
-mv850e2
Specify that the target processor is the V850E2. The preprocessor constants __v850e2__ will be
defined if this option is used.
|
-mv850e1
Specify that the target processor is the V850E1. The preprocessor constants __v850e1__ and __v850e__
will be defined if this option is used.
|
-mv850es
Specify that the target processor is the V850ES. This is an alias for the -mv850e1 option.
|
-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 nor -mv850e2 nor -mv850e2v3 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, v850e1, v850e2 and
v850e2v3 flavors of the v850 architecture. The default is -mno-disable-callt which allows the CALLT
instruction to be used.
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.
VxWorks Options
The options in this section are defined for all VxWorks targets. Options specific to the target hardware
are listed with the other options for that target.
|
-mrtp
GCC can generate code for both VxWorks kernels and real time processes (RTPs). This option switches
from the former to the latter. It also defines the preprocessor macro "__RTP__".
|
-non-static
Link an RTP executable against shared libraries rather than static libraries. The options -static
and -shared can also be used for RTPs; -static is the default.
|
-Bstatic
-Bdynamic
These options are passed down to the linker. They are defined for compatibility with Diab.
|
-Xbind-lazy
Enable lazy binding of function calls. This option is equivalent to -Wl,-z,now and is defined for
compatibility with Diab.
|
-Xbind-now
Disable lazy binding of function calls. This option is the default and is defined for compatibility
with Diab.
x86-64 Options
These are listed under
Xstormy16 Options
These options are defined for Xstormy16:
|
-msim
Choose startup files and linker script suitable for the simulator.
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.
|
-mserialize-volatile
-mno-serialize-volatile
When this option is enabled, GCC inserts "MEMW" instructions before "volatile" memory references to
guarantee sequential consistency. The default is -mserialize-volatile. Use -mno-serialize-volatile
to omit the "MEMW" instructions.
|
-mforce-no-pic
For targets, like GNU/Linux, where all user-mode Xtensa code must be position-independent code (PIC),
this option disables PIC for compiling kernel code.
|
-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.
zSeries Options
These are listed under
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 that 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 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 others. 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++ that 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 that 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.
|
-fno-common
In C code, controls the placement of uninitialized global variables. Unix C compilers have
traditionally permitted multiple definitions of such variables in different compilation units by
placing the variables in a common block. This is the behavior specified by -fcommon, and is the
default for GCC on most targets. On the other hand, this behavior is not required by ISO C, and on
some targets may carry a speed or code size penalty on variable references. The -fno-common option
specifies that the compiler should place 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 a multiple-
definition error when you link them. In this case, you must compile with -fcommon instead.
Compiling with -fno-common is useful on targets for which it provides better performance, or if you
wish to verify that the program will work on other systems that always treat uninitialized variable
declarations 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.
|
-frecord-gcc-switches
This switch causes the command line that was used to invoke the compiler to be recorded into the
object file that is being created. This switch is only implemented on some targets and the exact
format of the recording is target and binary file format dependent, but it usually takes the form of
a section containing ASCII text. This switch is related to the -fverbose-asm switch, but that switch
only records information in the assembler output file as comments, so it never reaches the object
file. See also -grecord-gcc-switches for another way of storing compiler options into the object
file.
|
-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.
When this flag is set, the macros "__pic__" and "__PIC__" are defined to 1.
|
-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, PowerPC and SPARC.
Position-independent code requires special support, and therefore works only on certain machines.
When this flag is set, the macros "__pic__" and "__PIC__" are defined to 2.
|
-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.
-fpie and -fPIE both define the macros "__pie__" and "__PIE__". The macros have the value 1 for
-fpie and 2 for -fPIE.
|
-fno-jump-tables
Do not use jump tables for switch statements even where it would be more efficient than other code
generation strategies. This option is of use in conjunction with -fpic or -fPIC for building code
that forms part of a dynamic linker and cannot reference the address of a jump table. On some
targets, jump tables do not require a GOT and this option is not needed.
|
-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.
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-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.
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-fpack-struct[=n]
Without a value specified, pack all structure members together without holes. When a value is
specified (which must be a small power of two), pack structure members according to this value,
representing the maximum alignment (that is, objects with default alignment requirements larger than
this will be output potentially unaligned at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code
generated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a
non-default application binary interface.
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-finstrument-functions
Generate instrumentation calls for entry and exit to functions. Just after function entry and just
before function exit, the following profiling functions will be called with the address of the
current function and its call site. (On some platforms, "__builtin_return_address" does not work
beyond the current function, so the call site information may not be available to the profiling
functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the current function, which may be looked up
exactly in the symbol table.
This instrumentation is also done for functions expanded inline in other functions. The profiling
calls will indicate where, conceptually, the inline function is entered and exited. This means that
addressable versions of such functions must be available. If all your uses of a function are
expanded inline, this may mean an additional expansion of code size. If you use extern inline in
your C code, an addressable version of such functions must be provided. (This is normally the case
anyways, but if you get lucky and the optimizer always expands the functions inline, you might have
gotten away without providing static copies.)
A function may be given the attribute "no_instrument_function", in which case this instrumentation
will not be done. This can be used, for example, for the profiling functions listed above, high-
priority interrupt routines, and any functions from which the profiling functions cannot safely be
called (perhaps signal handlers, if the profiling routines generate output or allocate memory).
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-finstrument-functions-exclude-file-list=file,file,...
Set the list of functions that are excluded from instrumentation (see the description of
"-finstrument-functions"). If the file that contains a function definition matches with one of file,
then that function is not instrumented. The match is done on substrings: if the file parameter is a
substring of the file name, it is considered to be a match.
For example:
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-finstrument-functions-exclude-function-list=sym,sym,...
This is similar to "-finstrument-functions-exclude-file-list", but this option sets the list of
function names to be excluded from instrumentation. The function name to be matched is its user-
visible name, such as "vector<int> blah(const vector<int> &)", not the internal mangled name (e.g.,
"_Z4blahRSt6vectorIiSaIiEE"). The match is done on substrings: if the sym parameter is a substring
of the function name, it is considered to be a match. For C99 and C++ extended identifiers, the
function name must be given in UTF-8, not using universal character names.
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-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.
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-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.
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-fsplit-stack
Generate code to automatically split the stack before it overflows. The resulting program has a
discontiguous stack which can only overflow if the program is unable to allocate any more memory.
This is most useful when running threaded programs, as it is no longer necessary to calculate a good
stack size to use for each thread. This is currently only implemented for the i386 and x86_64 back
ends running GNU/Linux.
When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be
much stack space available for the latter code to run. If compiling all code, including library
code, with -fsplit-stack is not an option, then the linker can fix up these calls so that the code
compiled without -fsplit-stack always has a large stack. Support for this is implemented in the gold
linker in GNU binutils release 2.21 and later.
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-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.
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-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".
The default without -fpic is "initial-exec"; with -fpic the default is "global-dynamic".
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-fvisibility=default|internal|hidden|protected
Set the default ELF image symbol visibility to the specified option---all symbols will be marked with
this unless overridden within the code. Using this feature can very substantially improve linking
and load times of shared object libraries, produce more optimized code, provide near-perfect API
export and prevent symbol clashes. It is strongly recommended that you use this in any shared
objects you distribute.
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-fstrict-volatile-bitfields
This option should be used if accesses to volatile bit-fields (or other structure fields, although
the compiler usually honors those types anyway) should use a single access of the width of the
field's type, aligned to a natural alignment if possible. For example, targets with memory-mapped
peripheral registers might require all such accesses to be 16 bits wide; with this flag the user
could declare all peripheral bit-fields as "unsigned short" (assuming short is 16 bits on these
targets) to force GCC to use 16-bit accesses instead of, perhaps, a more efficient 32-bit access.
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