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.
-fstack-reuse=reuse-level ¶This option controls stack space reuse for user declared local/auto variables and compiler generated temporaries. reuse_level can be ‘all’, ‘named_vars’, or ‘none’. ‘all’ enables stack reuse for all local variables and temporaries, ‘named_vars’ enables the reuse only for user defined local variables with names, and ‘none’ disables stack reuse completely. The default value is ‘all’. The option is needed when the program extends the lifetime of a scoped local variable or a compiler generated temporary beyond the end point defined by the language. When a lifetime of a variable ends, and if the variable 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. Legacy code extending local lifetime is likely to break with the stack reuse optimization.
For 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 ¶This option generates traps for signed overflow on addition, subtraction, multiplication operations. The options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being effective.
-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. The options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being effective.
-fexceptions ¶Enable exception handling. Generates extra code needed to propagate exceptions. For some targets, this implies GCC generates 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 enables it by default for languages like C++ that normally require exception handling, and disables 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.
-fdelete-dead-exceptions ¶Consider that instructions that may throw exceptions but don’t otherwise contribute to the execution of the program can be optimized away. This option is enabled by default for the Ada front end, as permitted by the Ada language specification. Optimization passes that cause dead exceptions to be removed are enabled independently at different optimization levels.
-funwind-tables ¶Similar to -fexceptions, except that it just generates any needed static data, but does not affect the generated code in any other way. You normally do not need to enable this option; instead, a language processor that needs this handling enables it on your behalf.
-fasynchronous-unwind-tables ¶Generate unwind table in DWARF 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).
-fno-gnu-unique ¶On systems with recent GNU assembler and C library, the C++ compiler
uses the STB_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-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
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-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 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 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, 28k on AArch64 and 32k on the m68k and RS/6000. The x86 has no such limit.)
Position-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.
-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 AArch64, 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 ¶-fPIEThese 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 is 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-plt ¶Do not use the PLT for external function calls in position-independent code. Instead, load the callee address at call sites from the GOT and branch to it. This leads to more efficient code by eliminating PLT stubs and exposing GOT loads to optimizations. On architectures such as 32-bit x86 where PLT stubs expect the GOT pointer in a specific register, this gives more register allocation freedom to the compiler. Lazy binding requires use of the PLT; with -fno-plt all external symbols are resolved at load time.
Alternatively, the function attribute noplt can be used to avoid calls
through the PLT for specific external functions.
In position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT to use the GOT instead.
-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 do not save and restore the register reg.
It 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-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 save and restore the register reg if they use it.
It 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] ¶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 are 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.
-fleading-underscore ¶This option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented in the object file. One use is to help link with legacy assembly code.
Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch.
-ftls-model=model ¶Alter the thread-local storage model to be used (see Thread-Local Storage). The model argument should be one of ‘global-dynamic’, ‘local-dynamic’, ‘initial-exec’ or ‘local-exec’. Note that the choice is subject to optimization: the compiler may use a more efficient model for symbols not visible outside of the translation unit, or if -fpic is not given on the command line.
The default without -fpic is ‘initial-exec’; with -fpic the default is ‘global-dynamic’.
-fvisibility=[default|internal|hidden|protected] ¶Set the default ELF image symbol visibility to the specified option—all symbols are 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.
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-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 you can
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.
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 ¶This option controls whether any out-of-line instance of the __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.