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### 3.18 Options for Code Generation Conventions

These machine-independent options control the interface conventions used in code generation.

Most of them have both positive and negative forms; the negative form of `-ffoo` would be `-fno-foo`. In the table below, only one of the forms is listed—the one which is not the default. You can figure out the other form by either removing ``no-`' or adding it.

`-fbounds-check`
  For front-ends that support it, generate additional code to check that indices used to access arrays are within the declared range. This is currently only supported by the Java and Fortran 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++ which normally require exception handling, and disable it for languages like C that do not normally require it. However, you may need to enable this option when compiling C code that needs to interoperate properly with exception handlers written in C++. You may also wish to disable this option if you are compiling older C++ programs that don't use exception handling.
`-fnon-call-exceptions`
  Generate code that allows trapping instructions to throw exceptions. Note that this requires platform-specific runtime support that does not exist everywhere. Moreover, it only allows trapping instructions to throw exceptions, i.e. memory references or floating point instructions. It does not allow exceptions to be thrown from arbitrary signal handlers such as `SIGALRM`.
`-funwind-tables`
  Similar to `-fexceptions`, except that it will just generate any needed static data, but will not affect the generated code in any other way. You will normally not enable this option; instead, a language processor that needs this handling would enable it on your behalf.
`-fasynchronous-unwind-tables`
  Generate unwind table in dwarf2 format, if supported by target machine. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector).
`-fpcc-struct-return`
  Return “short” `struct` and `union` values in memory like longer ones, rather than in registers. This convention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files compiled with other compilers, particularly the Portable C Compiler (pcc).

  The precise convention for returning structures in memory depends on the target configuration macros.

  Short structures and unions are those whose size and alignment match that of some integer type.

  Warning: code compiled with the `-fpcc-struct-return` switch is not binary compatible with code compiled with the `-freg-struct-return` switch. Use it to conform to a non-default application binary interface.

`-freg-struct-return`
  Return `struct` and `union` values in registers when possible. This is more efficient for small structures than `-fpcc-struct-return`.

  If you specify neither `-fpcc-struct-return` nor `-freg-struct-return`, GCC defaults to whichever convention is standard for the target. If there is no standard convention, GCC defaults to `-fpcc-struct-return`, except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative.

  Warning: code compiled with the `-freg-struct-return` switch is not binary compatible with code compiled with the `-fpcc-struct-return` switch. Use it to conform to a non-default application binary interface.

`-fshort-enums`
  Allocate to an `enum` type only as many bytes as it needs for the declared range of possible values. Specifically, the `enum` type will be equivalent to the smallest integer type which has enough room.

  Warning: the `-fshort-enums` switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

`-fshort-double`
  Use the same size for `double` as for `float`.

  Warning: the `-fshort-double` switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

`-fshort-wchar`
  Override the underlying type for ``wchar_t`' to be ``short unsigned int`' instead of the default for the target. This option is useful for building programs to run under WINE.

  Warning: the `-fshort-wchar` switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

`-fno-common`
  In C, allocate even uninitialized global variables in the data section of the object file, rather than generating them as common blocks. This has the effect that if the same variable is declared (without `extern`) in two different compilations, you will get an error when you link them. The only reason this might be useful is if you wish to verify that the program will work on other systems which always work this way.
`-fno-ident`
  Ignore the ``#ident`' directive.
`-finhibit-size-directive`
  Don't output a `.size` assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory. This option is used when compiling `crtstuff.c`; you should not need to use it for anything else.
`-fverbose-asm`
  Put extra commentary information in the generated assembly code to make it more readable. This option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself).

  `-fno-verbose-asm`, the default, causes the extra information to be omitted and is useful when comparing two assembler files.

`-fpic`
  Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that `-fpic` does not work; in that case, recompile with `-fPIC` instead. (These maximums are 8k on the SPARC and 32k on the m68k and RS/6000. The 386 has no such limit.)

  Position-independent code requires special support, and therefore works only on certain machines. For the 386, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent.

  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.
`-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 which 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.

`-fcall-saved-`reg
  Treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way will save and restore the register reg if they use it.

  It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model will produce disastrous results.

  A different sort of disaster will result from the use of this flag for a register in which function values may be returned.

  This flag does not have a negative form, because it specifies a three-way choice.

`-fpack-struct[=`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.

`-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).

`-fstack-check`
  Generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag if you are running in an environment with multiple threads, but only rarely need to specify it in a single-threaded environment since stack overflow is automatically detected on nearly all systems if there is only one stack.

  Note that this switch does not actually cause checking to be done; the operating system must do that. The switch causes generation of code to ensure that the operating system sees the stack being extended.

`-fstack-limit-register=`reg
`-fstack-limit-symbol=`sym
`-fno-stack-limit`
  Generate code to ensure that the stack does not grow beyond a certain value, either the value of a register or the address of a symbol. If the stack would grow beyond the value, a signal is raised. For most targets, the signal is raised before the stack overruns the boundary, so it is possible to catch the signal without taking special precautions.

  For instance, if the stack starts at absolute address ``0x80000000`' and grows downwards, you can use the flags `-fstack-limit-symbol=__stack_limit` and `-Wl,--defsym,__stack_limit=0x7ffe0000` to enforce a stack limit of 128KB. Note that this may only work with the GNU linker.

`-fargument-alias`
`-fargument-noalias`
`-fargument-noalias-global`
`-fargument-noalias-anything`
  Specify the possible relationships among parameters and between parameters and global data.

  `-fargument-alias` specifies that arguments (parameters) may alias each other and may alias global storage.
  `-fargument-noalias` specifies that arguments do not alias each other, but may alias global storage.
  `-fargument-noalias-global` specifies that arguments do not alias each other and do not alias global storage. `-fargument-noalias-anything` specifies that arguments do not alias any other storage.

  Each language will automatically use whatever option is required by the language standard. You should not need to use these options yourself.

`-fleading-underscore`
  This option and its counterpart, `-fno-leading-underscore`, forcibly change the way C symbols are represented in the object file. One use is to help link with legacy assembly code.

  Warning: the `-fleading-underscore` switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch.

`-ftls-model=`model
  Alter the thread-local storage model to be used (see Thread-Local). 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`.

`-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.

  Despite the nomenclature, `default` always means public ie; 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 will be `hidden`. The default if `-fvisibility` isn't specified is `default`, i.e., make every symbol public—this causes the same behavior as previous versions of GCC.

  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://people.redhat.com/~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 DLL's 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 ie; 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 will 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 will be thrown between DSOs must be explicitly marked with default visibility so that the ``type_info`' nodes will be unified between the DSOs.

  An overview of these techniques, their benefits and how to use them is at http://gcc.gnu.org/wiki/Visibility.