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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 is -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-fstack-reuse=reuse-levelFor example,
int *p;
{
int local1;
p = &local1;
local1 = 10;
....
}
{
int local2;
local2 = 20;
...
}
if (*p == 10) // out of scope use of local1
{
}
Another example:
struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};
A *ap;
void foo(const A& ar)
{
ap = &ar;
}
void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns
{
A a(20);
....
}
ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
}
The lifetime of a compiler generated temporary is well defined by the C++
standard. When a lifetime of a temporary ends, and if the temporary lives
in memory, the optimizing compiler has the freedom to reuse its stack
space with other temporaries or scoped local variables whose live range
does not overlap with it. However some of the legacy code relies on
the behavior of older compilers in which temporaries' stack space is
not reused, the aggressive stack reuse can lead to runtime errors. This
option is used to control the temporary stack reuse optimization.
-ftrapv-fwrapv-fexceptions-fnon-call-exceptionsSIGALRM.
-fdelete-dead-exceptions-funwind-tables-fasynchronous-unwind-tables-fno-gnu-uniqueSTB_GNU_UNIQUE binding to make sure that definitions
of template static data members and static local variables in inline
functions are unique even in the presence of RTLD_LOCAL; this
is necessary to avoid problems with a library used by two different
RTLD_LOCAL plugins depending on a definition in one of them and
therefore disagreeing with the other one about the binding of the
symbol. But this causes dlclose to be ignored for affected
DSOs; if your program relies on reinitialization of a DSO via
dlclose and dlopen, you can use
-fno-gnu-unique.
-fpcc-struct-returnstruct 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-returnstruct 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-enumsenum type only as many bytes as it needs for the
declared range of possible values. Specifically, the enum type
is 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-doubledouble 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-wcharwchar_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-commonextern) in two different compilations,
you 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#ident directive.
-finhibit-size-directive.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-fno-verbose-asm, the default, causes the
extra information to be omitted and is useful when comparing two assembler
files.
-frecord-gcc-switches-fpicPosition-independent code requires special support, and therefore works only on certain machines. For the x86, 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.
-fPICPosition-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-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-ffixed-regreg 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-regIt is an error to use 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 produces disastrous results.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-saved-regIt is an error to use 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 produces disastrous results.
A different sort of disaster results from the use of this flag for a register in which function values may be returned.
This flag does not have a negative form, because it specifies a
three-way choice.
-fpack-struct[=n]Warning: the -fpack-struct switch causes GCC to generate
code that is not binary compatible with code generated without that switch.
Additionally, it makes the code suboptimal.
Use it to conform to a non-default application binary interface.
-finstrument-functions__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 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 anyway, 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 is not 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).
-finstrument-functions-exclude-file-list=file,file,...For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sys
excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys.
If, for some reason, you want to include letter ‘,’ in one of
sym, write ‘\,’. For example,
-finstrument-functions-exclude-file-list='\,\,tmp'
(note the single quote surrounding the option).
-finstrument-functions-exclude-function-list=sym,sym,...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.
-fstack-checkNote that this switch does not actually cause checking to be done; the operating system or the language runtime must do that. The switch causes generation of code to ensure that they see the stack being extended.
You can additionally specify a string parameter: ‘no’ means no checking, ‘generic’ means force the use of old-style checking, ‘specific’ means use the best checking method and is equivalent to bare -fstack-check.
Old-style checking is a generic mechanism that requires no specific target support in the compiler but comes with the following drawbacks:
Note that old-style stack checking is also the fallback method for
‘specific’ if no target support has been added in the compiler.
-fstack-limit-register=reg-fstack-limit-symbol=sym-fno-stack-limitFor 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.
-fsplit-stackWhen 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.
-fleading-underscoreWarning: the -fleading-underscore switch causes GCC to
generate code that is not binary compatible with code generated without that
switch. Use it to conform to a non-default application binary interface.
Not all targets provide complete support for this switch.
-ftls-model=modelThe default without -fpic is ‘initial-exec’; with
-fpic the default is ‘global-dynamic’.
-fvisibility=[default|internal|hidden|protected]Despite the nomenclature, ‘default’ always means public; i.e., available to be linked against from outside the shared object. ‘protected’ and ‘internal’ are pretty useless in real-world usage so the only other commonly used option is ‘hidden’. The default if -fvisibility isn't specified is ‘default’, i.e., make every symbol public.
A good explanation of the benefits offered by ensuring ELF
symbols have the correct visibility is given by “How To Write
Shared Libraries” by Ulrich Drepper (which can be found at
http://www.akkadia.org/drepper/)—however a superior
solution made possible by this option to marking things hidden when
the default is public is to make the default hidden and mark things
public. This is the norm with DLLs on Windows and with -fvisibility=hidden
and __attribute__ ((visibility("default"))) instead of
__declspec(dllexport) you get almost identical semantics with
identical syntax. This is a great boon to those working with
cross-platform projects.
For those adding visibility support to existing code, you may find
#pragma GCC visibility of use. This works by you enclosing
the declarations you wish to set visibility for with (for example)
#pragma GCC visibility push(hidden) and
#pragma GCC visibility pop.
Bear in mind that symbol visibility should be viewed as
part of the API interface contract and thus all new code should
always specify visibility when it is not the default; i.e., declarations
only for use within the local DSO should always be marked explicitly
as hidden as so to avoid PLT indirection overheads—making this
abundantly clear also aids readability and self-documentation of the code.
Note that due to ISO C++ specification requirements, operator new and
operator delete must always be of default visibility.
Be aware that headers from outside your project, in particular system
headers and headers from any other library you use, may not be
expecting to be compiled with visibility other than the default. You
may need to explicitly say #pragma GCC visibility push(default)
before including any such headers.
extern declarations are not affected by -fvisibility, so
a lot of code can be recompiled with -fvisibility=hidden with
no modifications. However, this means that calls to extern
functions with no explicit visibility use the PLT, so it is more
effective to use __attribute ((visibility)) and/or
#pragma GCC visibility to tell the compiler which extern
declarations should be treated as hidden.
Note that -fvisibility does affect C++ vague linkage entities. This means that, for instance, an exception class that is be thrown between DSOs must be explicitly marked with default visibility so that the ‘type_info’ nodes are unified between the DSOs.
An overview of these techniques, their benefits and how to use them
is at http://gcc.gnu.org/wiki/Visibility.
-fstrict-volatile-bitfieldsunsigned 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.
If this option is disabled, the compiler uses the most efficient instruction. In the previous example, that might be a 32-bit load instruction, even though that accesses bytes that do not contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated.
In some cases, such as when the packed attribute is applied to a
structure field, it may not be possible to access the field with a single
read or write that is correctly aligned for the target machine. In this
case GCC falls back to generating multiple accesses rather than code that
will fault or truncate the result at run time.
Note: Due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members. It is therefore recommended to define all bits of the field's type as bit-field members.
The default value of this option is determined by the application binary
interface for the target processor.
-fsync-libcalls__sync
family of functions may be used to implement the C++11 __atomic
family of functions.
The default value of this option is enabled, thus the only useful form of the option is -fno-sync-libcalls. This option is used in the implementation of the libatomic runtime library.