Using and Porting GNU CC - 17. Target Description Macros

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# 17. Target Description Macros

In addition to the file ``machine.md'`, a machine description includes a C header file conventionally given the name ``machine.h'`. This header file defines numerous macros that convey the information about the target machine that does not fit into the scheme of the ``.md'` file. The file ``tm.h'` should be a link to ``machine.h'`. The header file ``config.h'` includes ``tm.h'` and most compiler source files include ``config.h'`.

## 17.1 Controlling the Compilation Driver, ``gcc'`

You can control the compilation driver.

`SWITCH_TAKES_ARG (char)`
  A C expression which determines whether the option ``-char'` takes arguments. The value should be the number of arguments that option takes--zero, for many options. By default, this macro is defined as `DEFAULT_SWITCH_TAKES_ARG`, which handles the standard options properly. You need not define `SWITCH_TAKES_ARG` unless you wish to add additional options which take arguments. Any redefinition should call `DEFAULT_SWITCH_TAKES_ARG` and then check for additional options.
`WORD_SWITCH_TAKES_ARG (name)`
  A C expression which determines whether the option ``-name'` takes arguments. The value should be the number of arguments that option takes--zero, for many options. This macro rather than `SWITCH_TAKES_ARG` is used for multi-character option names. By default, this macro is defined as `DEFAULT_WORD_SWITCH_TAKES_ARG`, which handles the standard options properly. You need not define `WORD_SWITCH_TAKES_ARG` unless you wish to add additional options which take arguments. Any redefinition should call `DEFAULT_WORD_SWITCH_TAKES_ARG` and then check for additional options.
`SWITCH_CURTAILS_COMPILATION (char)`
  A C expression which determines whether the option ``-char'` stops compilation before the generation of an executable. The value is boolean, non-zero if the option does stop an executable from being generated, zero otherwise. By default, this macro is defined as `DEFAULT_SWITCH_CURTAILS_COMPILATION`, which handles the standard options properly. You need not define `SWITCH_CURTAILS_COMPILATION` unless you wish to add additional options which affect the generation of an executable. Any redefinition should call `DEFAULT_SWITCH_CURTAILS_COMPILATION` and then check for additional options.
`SWITCHES_NEED_SPACES`
  A string-valued C expression which enumerates the options for which the linker needs a space between the option and its argument. If this macro is not defined, the default value is `""`.
`CPP_SPEC`
  A C string constant that tells the GNU CC driver program options to pass to CPP. It can also specify how to translate options you give to GNU CC into options for GNU CC to pass to the CPP. Do not define this macro if it does not need to do anything.
`NO_BUILTIN_SIZE_TYPE`
  If this macro is defined, the preprocessor will not define the builtin macro `__SIZE_TYPE__`. The macro `__SIZE_TYPE__` must then be defined by `CPP_SPEC` instead. This should be defined if `SIZE_TYPE` depends on target dependent flags which are not accessible to the preprocessor. Otherwise, it should not be defined.
`NO_BUILTIN_PTRDIFF_TYPE`
  If this macro is defined, the preprocessor will not define the builtin macro `__PTRDIFF_TYPE__`. The macro `__PTRDIFF_TYPE__` must then be defined by `CPP_SPEC` instead. This should be defined if `PTRDIFF_TYPE` depends on target dependent flags which are not accessible to the preprocessor. Otherwise, it should not be defined.
`SIGNED_CHAR_SPEC`
  A C string constant that tells the GNU CC driver program options to pass to CPP. By default, this macro is defined to pass the option ``-D__CHAR_UNSIGNED__'` to CPP if `char` will be treated as `unsigned char` by `cc1`. Do not define this macro unless you need to override the default definition.
`CC1_SPEC`
  A C string constant that tells the GNU CC driver program options to pass to `cc1`. It can also specify how to translate options you give to GNU CC into options for GNU CC to pass to the `cc1`. Do not define this macro if it does not need to do anything.
`CC1PLUS_SPEC`
  A C string constant that tells the GNU CC driver program options to pass to `cc1plus`. It can also specify how to translate options you give to GNU CC into options for GNU CC to pass to the `cc1plus`. Note that everything defined in CC1_SPEC is already passed to `cc1plus` so there is no need to duplicate the contents of CC1_SPEC in CC1PLUS_SPEC. Do not define this macro if it does not need to do anything.
`ASM_SPEC`
  A C string constant that tells the GNU CC driver program options to pass to the assembler. It can also specify how to translate options you give to GNU CC into options for GNU CC to pass to the assembler. See the file ``sun3.h'` for an example of this. Do not define this macro if it does not need to do anything.
`ASM_FINAL_SPEC`
  A C string constant that tells the GNU CC driver program how to run any programs which cleanup after the normal assembler. Normally, this is not needed. See the file ``mips.h'` for an example of this. Do not define this macro if it does not need to do anything.
`LINK_SPEC`
  A C string constant that tells the GNU CC driver program options to pass to the linker. It can also specify how to translate options you give to GNU CC into options for GNU CC to pass to the linker. Do not define this macro if it does not need to do anything.
`LIB_SPEC`
  Another C string constant used much like `LINK_SPEC`. The difference between the two is that `LIB_SPEC` is used at the end of the command given to the linker. If this macro is not defined, a default is provided that loads the standard C library from the usual place. See ``gcc.c'`.
`LIBGCC_SPEC`
  Another C string constant that tells the GNU CC driver program how and when to place a reference to ``libgcc.a'` into the linker command line. This constant is placed both before and after the value of `LIB_SPEC`. If this macro is not defined, the GNU CC driver provides a default that passes the string ``-lgcc'` to the linker unless the ``-shared'` option is specified.
`STARTFILE_SPEC`
  Another C string constant used much like `LINK_SPEC`. The difference between the two is that `STARTFILE_SPEC` is used at the very beginning of the command given to the linker. If this macro is not defined, a default is provided that loads the standard C startup file from the usual place. See ``gcc.c'`.
`ENDFILE_SPEC`
  Another C string constant used much like `LINK_SPEC`. The difference between the two is that `ENDFILE_SPEC` is used at the very end of the command given to the linker. Do not define this macro if it does not need to do anything.
`EXTRA_SPECS`
  Define this macro to provide additional specifications to put in the ``specs'` file that can be used in various specifications like `CC1_SPEC`. The definition should be an initializer for an array of structures, containing a string constant, that defines the specification name, and a string constant that provides the specification. Do not define this macro if it does not need to do anything. `EXTRA_SPECS` is useful when an architecture contains several related targets, which have various `..._SPECS` which are similar to each other, and the maintainer would like one central place to keep these definitions. For example, the PowerPC System V.4 targets use `EXTRA_SPECS` to define either `_CALL_SYSV` when the System V calling sequence is used or `_CALL_AIX` when the older AIX-based calling sequence is used. The ``config/rs6000/rs6000.h'` target file defines:

      #define EXTRA_SPECS \
        { "cpp_sysv_default", CPP_SYSV_DEFAULT },

      #define CPP_SYS_DEFAULT ""

  The ``config/rs6000/sysv.h'` target file defines:

      #undef CPP_SPEC
      #define CPP_SPEC \
      "%{posix: -D_POSIX_SOURCE } \
      %{mcall-sysv: -D_CALL_SYSV } %{mcall-aix: -D_CALL_AIX } \
      %{!mcall-sysv: %{!mcall-aix: %(cpp_sysv_default) }} \
      %{msoft-float: -D_SOFT_FLOAT} %{mcpu=403: -D_SOFT_FLOAT}"

      #undef CPP_SYSV_DEFAULT
      #define CPP_SYSV_DEFAULT "-D_CALL_SYSV"

  while the ``config/rs6000/eabiaix.h'` target file defines `CPP_SYSV_DEFAULT` as:

      #undef CPP_SYSV_DEFAULT
      #define CPP_SYSV_DEFAULT "-D_CALL_AIX"

`LINK_LIBGCC_SPECIAL`
  Define this macro if the driver program should find the library ``libgcc.a'` itself and should not pass ``-L'` options to the linker. If you do not define this macro, the driver program will pass the argument ``-lgcc'` to tell the linker to do the search and will pass ``-L'` options to it.
`LINK_LIBGCC_SPECIAL_1`
  Define this macro if the driver program should find the library ``libgcc.a'`. If you do not define this macro, the driver program will pass the argument ``-lgcc'` to tell the linker to do the search. This macro is similar to `LINK_LIBGCC_SPECIAL`, except that it does not affect ``-L'` options.
`LINK_COMMAND_SPEC`
  A C string constant giving the complete command line need to execute the linker. When you do this, you will need to update your port each time a change is made to the link command line within ``gcc.c'`. Therefore, define this macro only if you need to completely redefine the command line for invoking the linker and there is no other way to accomplish the effect you need.
`MULTILIB_DEFAULTS`
  Define this macro as a C expression for the initializer of an array of string to tell the driver program which options are defaults for this target and thus do not need to be handled specially when using `MULTILIB_OPTIONS`. Do not define this macro if `MULTILIB_OPTIONS` is not defined in the target makefile fragment or if none of the options listed in `MULTILIB_OPTIONS` are set by default. See section 19.1 The Target Makefile Fragment.
`RELATIVE_PREFIX_NOT_LINKDIR`
  Define this macro to tell `gcc` that it should only translate a ``-B'` prefix into a ``-L'` linker option if the prefix indicates an absolute file name.
`STANDARD_EXEC_PREFIX`
  Define this macro as a C string constant if you wish to override the standard choice of ``/usr/local/lib/gcc-lib/'` as the default prefix to try when searching for the executable files of the compiler.
`MD_EXEC_PREFIX`
  If defined, this macro is an additional prefix to try after `STANDARD_EXEC_PREFIX`. `MD_EXEC_PREFIX` is not searched when the ``-b'` option is used, or the compiler is built as a cross compiler.
`STANDARD_STARTFILE_PREFIX`
  Define this macro as a C string constant if you wish to override the standard choice of ``/usr/local/lib/'` as the default prefix to try when searching for startup files such as ``crt0.o'`.
`MD_STARTFILE_PREFIX`
  If defined, this macro supplies an additional prefix to try after the standard prefixes. `MD_EXEC_PREFIX` is not searched when the ``-b'` option is used, or when the compiler is built as a cross compiler.
`MD_STARTFILE_PREFIX_1`
  If defined, this macro supplies yet another prefix to try after the standard prefixes. It is not searched when the ``-b'` option is used, or when the compiler is built as a cross compiler.
`INIT_ENVIRONMENT`
  Define this macro as a C string constant if you wish to set environment variables for programs called by the driver, such as the assembler and loader. The driver passes the value of this macro to `putenv` to initialize the necessary environment variables.
`LOCAL_INCLUDE_DIR`
  Define this macro as a C string constant if you wish to override the standard choice of ``/usr/local/include'` as the default prefix to try when searching for local header files. `LOCAL_INCLUDE_DIR` comes before `SYSTEM_INCLUDE_DIR` in the search order. Cross compilers do not use this macro and do not search either ``/usr/local/include'` or its replacement.
`SYSTEM_INCLUDE_DIR`
  Define this macro as a C string constant if you wish to specify a system-specific directory to search for header files before the standard directory. `SYSTEM_INCLUDE_DIR` comes before `STANDARD_INCLUDE_DIR` in the search order. Cross compilers do not use this macro and do not search the directory specified.
`STANDARD_INCLUDE_DIR`
  Define this macro as a C string constant if you wish to override the standard choice of ``/usr/include'` as the default prefix to try when searching for header files. Cross compilers do not use this macro and do not search either ``/usr/include'` or its replacement.
`STANDARD_INCLUDE_COMPONENT`
  The "component" corresponding to `STANDARD_INCLUDE_DIR`. See `INCLUDE_DEFAULTS`, below, for the description of components. If you do not define this macro, no component is used.
`INCLUDE_DEFAULTS`
  Define this macro if you wish to override the entire default search path for include files. For a native compiler, the default search path usually consists of `GCC_INCLUDE_DIR`, `LOCAL_INCLUDE_DIR`, `SYSTEM_INCLUDE_DIR`, `GPLUSPLUS_INCLUDE_DIR`, and `STANDARD_INCLUDE_DIR`. In addition, `GPLUSPLUS_INCLUDE_DIR` and `GCC_INCLUDE_DIR` are defined automatically by ``Makefile'`, and specify private search areas for GCC. The directory `GPLUSPLUS_INCLUDE_DIR` is used only for C++ programs. The definition should be an initializer for an array of structures. Each array element should have four elements: the directory name (a string constant), the component name, and flag for C++-only directories, and a flag showing that the includes in the directory don't need to be wrapped in `extern ``C'`` when compiling C++. Mark the end of the array with a null element. The component name denotes what GNU package the include file is part of, if any, in all upper-case letters. For example, it might be ``GCC'` or ``BINUTILS'`. If the package is part of the a vendor-supplied operating system, code the component name as ``0'`. For example, here is the definition used for VAX/VMS:

      #define INCLUDE_DEFAULTS \
      {                                       \
        { "GNU_GXX_INCLUDE:", "G++", 1, 1},   \
        { "GNU_CC_INCLUDE:", "GCC", 0, 0},    \
        { "SYS$SYSROOT:[SYSLIB.]", 0, 0, 0},  \
        { ".", 0, 0, 0},                      \
        { 0, 0, 0, 0}                         \
      }

Here is the order of prefixes tried for exec files:

1. Any prefixes specified by the user with ``-B'`.
2. The environment variable `GCC_EXEC_PREFIX`, if any.
3. The directories specified by the environment variable `COMPILER_PATH`.
4. The macro `STANDARD_EXEC_PREFIX`.
5. ``/usr/lib/gcc/'`.
6. The macro `MD_EXEC_PREFIX`, if any.

Here is the order of prefixes tried for startfiles:

1. Any prefixes specified by the user with ``-B'`.
2. The environment variable `GCC_EXEC_PREFIX`, if any.
3. The directories specified by the environment variable `LIBRARY_PATH` (native only, cross compilers do not use this).
4. The macro `STANDARD_EXEC_PREFIX`.
5. ``/usr/lib/gcc/'`.
6. The macro `MD_EXEC_PREFIX`, if any.
7. The macro `MD_STARTFILE_PREFIX`, if any.
8. The macro `STANDARD_STARTFILE_PREFIX`.
9. ``/lib/'`.
10. ``/usr/lib/'`.

## 17.2 Run-time Target Specification

Here are run-time target specifications.

`CPP_PREDEFINES`
  Define this to be a string constant containing ``-D'` options to define the predefined macros that identify this machine and system. These macros will be predefined unless the ``-ansi'` option is specified. In addition, a parallel set of macros are predefined, whose names are made by appending ``__'` at the beginning and at the end. These ``__'` macros are permitted by the ANSI standard, so they are predefined regardless of whether ``-ansi'` is specified. For example, on the Sun, one can use the following value:

      "-Dmc68000 -Dsun -Dunix"

  The result is to define the macros `__mc68000__`, `__sun__` and `__unix__` unconditionally, and the macros `mc68000`, `sun` and `unix` provided ``-ansi'` is not specified.
`extern int target_flags;`
  This declaration should be present.
`TARGET_...`
  This series of macros is to allow compiler command arguments to enable or disable the use of optional features of the target machine. For example, one machine description serves both the 68000 and the 68020; a command argument tells the compiler whether it should use 68020-only instructions or not. This command argument works by means of a macro `TARGET_68020` that tests a bit in `target_flags`. Define a macro `TARGET_featurename` for each such option. Its definition should test a bit in `target_flags`; for example:

      #define TARGET_68020 (target_flags & 1)

  One place where these macros are used is in the condition-expressions of instruction patterns. Note how `TARGET_68020` appears frequently in the 68000 machine description file, ``m68k.md'`. Another place they are used is in the definitions of the other macros in the ``machine.h'` file.
`TARGET_SWITCHES`
  This macro defines names of command options to set and clear bits in `target_flags`. Its definition is an initializer with a subgrouping for each command option. Each subgrouping contains a string constant, that defines the option name, a number, which contains the bits to set in `target_flags`, and a second string which is the description displayed by --help. If the number is negative then the bits specified by the number are cleared instead of being set. If the description string is present but empty, then no help information will be displayed for that option, but it will not count as an undocumented option. The actual option name is made by appending ``-m'` to the specified name. One of the subgroupings should have a null string. The number in this grouping is the default value for `target_flags`. Any target options act starting with that value. Here is an example which defines ``-m68000'` and ``-m68020'` with opposite meanings, and picks the latter as the default:

      #define TARGET_SWITCHES \
        { { "68020", 1, "" },      \
          { "68000", -1, "Compile for the 68000" }, \
          { "", 1, "" }}

`TARGET_OPTIONS`
  This macro is similar to `TARGET_SWITCHES` but defines names of command options that have values. Its definition is an initializer with a subgrouping for each command option. Each subgrouping contains a string constant, that defines the fixed part of the option name, the address of a variable, and a description string. The variable, type `char *`, is set to the variable part of the given option if the fixed part matches. The actual option name is made by appending ``-m'` to the specified name. Here is an example which defines ``-mshort-data-number'`. If the given option is ``-mshort-data-512'`, the variable `m88k_short_data` will be set to the string `"512"`.

      extern char *m88k_short_data;
      #define TARGET_OPTIONS \
       { { "short-data-", &m88k_short_data, "Specify the size of the short data section" } }

`TARGET_VERSION`
  This macro is a C statement to print on `stderr` a string describing the particular machine description choice. Every machine description should define `TARGET_VERSION`. For example:

      #ifdef MOTOROLA
      #define TARGET_VERSION \
        fprintf (stderr, " (68k, Motorola syntax)");
      #else
      #define TARGET_VERSION \
        fprintf (stderr, " (68k, MIT syntax)");
      #endif

`OVERRIDE_OPTIONS`
  Sometimes certain combinations of command options do not make sense on a particular target machine. You can define a macro `OVERRIDE_OPTIONS` to take account of this. This macro, if defined, is executed once just after all the command options have been parsed. Don't use this macro to turn on various extra optimizations for ``-O'`. That is what `OPTIMIZATION_OPTIONS` is for.
`OPTIMIZATION_OPTIONS (level, size)`
  Some machines may desire to change what optimizations are performed for various optimization levels. This macro, if defined, is executed once just after the optimization level is determined and before the remainder of the command options have been parsed. Values set in this macro are used as the default values for the other command line options. level is the optimization level specified; 2 if ``-O2'` is specified, 1 if ``-O'` is specified, and 0 if neither is specified. size is non-zero if ``-Os'` is specified and zero otherwise. You should not use this macro to change options that are not machine-specific. These should uniformly selected by the same optimization level on all supported machines. Use this macro to enable machine-specific optimizations. Do not examine `write_symbols` in this macro! The debugging options are not supposed to alter the generated code.
`CAN_DEBUG_WITHOUT_FP`
  Define this macro if debugging can be performed even without a frame pointer. If this macro is defined, GNU CC will turn on the ``-fomit-frame-pointer'` option whenever ``-O'` is specified.

## 17.3 Storage Layout

Note that the definitions of the macros in this table which are sizes or alignments measured in bits do not need to be constant. They can be C expressions that refer to static variables, such as the `target_flags`. See section 17.2 Run-time Target Specification.

`BITS_BIG_ENDIAN`
  Define this macro to have the value 1 if the most significant bit in a byte has the lowest number; otherwise define it to have the value zero. This means that bit-field instructions count from the most significant bit. If the machine has no bit-field instructions, then this must still be defined, but it doesn't matter which value it is defined to. This macro need not be a constant. This macro does not affect the way structure fields are packed into bytes or words; that is controlled by `BYTES_BIG_ENDIAN`.
`BYTES_BIG_ENDIAN`
  Define this macro to have the value 1 if the most significant byte in a word has the lowest number. This macro need not be a constant.
`WORDS_BIG_ENDIAN`
  Define this macro to have the value 1 if, in a multiword object, the most significant word has the lowest number. This applies to both memory locations and registers; GNU CC fundamentally assumes that the order of words in memory is the same as the order in registers. This macro need not be a constant.
`LIBGCC2_WORDS_BIG_ENDIAN`
  Define this macro if WORDS_BIG_ENDIAN is not constant. This must be a constant value with the same meaning as WORDS_BIG_ENDIAN, which will be used only when compiling libgcc2.c. Typically the value will be set based on preprocessor defines.
`FLOAT_WORDS_BIG_ENDIAN`
  Define this macro to have the value 1 if `DFmode`, `XFmode` or `TFmode` floating point numbers are stored in memory with the word containing the sign bit at the lowest address; otherwise define it to have the value 0. This macro need not be a constant. You need not define this macro if the ordering is the same as for multi-word integers.
`BITS_PER_UNIT`
  Define this macro to be the number of bits in an addressable storage unit (byte); normally 8.
`BITS_PER_WORD`
  Number of bits in a word; normally 32.
`MAX_BITS_PER_WORD`
  Maximum number of bits in a word. If this is undefined, the default is `BITS_PER_WORD`. Otherwise, it is the constant value that is the largest value that `BITS_PER_WORD` can have at run-time.
`UNITS_PER_WORD`
  Number of storage units in a word; normally 4.
`MIN_UNITS_PER_WORD`
  Minimum number of units in a word. If this is undefined, the default is `UNITS_PER_WORD`. Otherwise, it is the constant value that is the smallest value that `UNITS_PER_WORD` can have at run-time.
`POINTER_SIZE`
  Width of a pointer, in bits. You must specify a value no wider than the width of `Pmode`. If it is not equal to the width of `Pmode`, you must define `POINTERS_EXTEND_UNSIGNED`.
`POINTERS_EXTEND_UNSIGNED`
  A C expression whose value is nonzero if pointers that need to be extended from being `POINTER_SIZE` bits wide to `Pmode` are to be zero-extended and zero if they are to be sign-extended. You need not define this macro if the `POINTER_SIZE` is equal to the width of `Pmode`.
`PROMOTE_MODE (m, unsignedp, type)`
  A macro to update m and unsignedp when an object whose type is type and which has the specified mode and signedness is to be stored in a register. This macro is only called when type is a scalar type. On most RISC machines, which only have operations that operate on a full register, define this macro to set m to `word_mode` if m is an integer mode narrower than `BITS_PER_WORD`. In most cases, only integer modes should be widened because wider-precision floating-point operations are usually more expensive than their narrower counterparts. For most machines, the macro definition does not change unsignedp. However, some machines, have instructions that preferentially handle either signed or unsigned quantities of certain modes. For example, on the DEC Alpha, 32-bit loads from memory and 32-bit add instructions sign-extend the result to 64 bits. On such machines, set unsignedp according to which kind of extension is more efficient. Do not define this macro if it would never modify m.
`PROMOTE_FUNCTION_ARGS`
  Define this macro if the promotion described by `PROMOTE_MODE` should also be done for outgoing function arguments.
`PROMOTE_FUNCTION_RETURN`
  Define this macro if the promotion described by `PROMOTE_MODE` should also be done for the return value of functions. If this macro is defined, `FUNCTION_VALUE` must perform the same promotions done by `PROMOTE_MODE`.
`PROMOTE_FOR_CALL_ONLY`
  Define this macro if the promotion described by `PROMOTE_MODE` should only be performed for outgoing function arguments or function return values, as specified by `PROMOTE_FUNCTION_ARGS` and `PROMOTE_FUNCTION_RETURN`, respectively.
`PARM_BOUNDARY`
  Normal alignment required for function parameters on the stack, in bits. All stack parameters receive at least this much alignment regardless of data type. On most machines, this is the same as the size of an integer.
`STACK_BOUNDARY`
  Define this macro if there is a guaranteed alignment for the stack pointer on this machine. The definition is a C expression for the desired alignment (measured in bits). This value is used as a default if PREFERRED_STACK_BOUNDARY is not defined.
`PREFERRED_STACK_BOUNDARY`
  Define this macro if you wish to preserve a certain alignment for the stack pointer. The definition is a C expression for the desired alignment (measured in bits). If STACK_BOUNDARY is also defined, this macro must evaluate to a value equal to or larger than STACK_BOUNDARY. If `PUSH_ROUNDING` is not defined, the stack will always be aligned to the specified boundary. If `PUSH_ROUNDING` is defined and specifies a less strict alignment than `PREFERRED_STACK_BOUNDARY`, the stack may be momentarily unaligned while pushing arguments.
`FUNCTION_BOUNDARY`
  Alignment required for a function entry point, in bits.
`FUNCTION_BOUNDARY_MAX_SKIP (max_skip)`
  If defined, the maximum number of padding bytes to insert to force the start a function to a particular alignment. If max_skip is smaller than the number of bytes needed to satisfy an alignment request, then no alignment is made. The compiler will choose a reasonable default value if you do not define this macro. This macro has no effct if `ASM_OUTPUT_MAX_SKIP_ALIGN` is not defined.
`BIGGEST_ALIGNMENT`
  Biggest alignment that any data type can require on this machine, in bits.
`MINIMUM_ATOMIC_ALIGNMENT`
  If defined, the smallest alignment, in bits, that can be given to an object that can be referenced in one operation, without disturbing any nearby object. Normally, this is `BITS_PER_UNIT`, but may be larger on machines that don't have byte or half-word store operations.
`BIGGEST_FIELD_ALIGNMENT`
  Biggest alignment that any structure field can require on this machine, in bits. If defined, this overrides `BIGGEST_ALIGNMENT` for structure fields only.
`ADJUST_FIELD_ALIGN (field, computed)`
  An expression for the alignment of a structure field field if the alignment computed in the usual way is computed. GNU CC uses this value instead of the value in `BIGGEST_ALIGNMENT` or `BIGGEST_FIELD_ALIGNMENT`, if defined, for structure fields only.
`MAX_OFILE_ALIGNMENT`
  Biggest alignment supported by the object file format of this machine. Use this macro to limit the alignment which can be specified using the `__attribute__ ((aligned (n)))` construct. If not defined, the default value is `BIGGEST_ALIGNMENT`.
`DATA_ALIGNMENT (type, basic-align)`
  If defined, a C expression to compute the alignment for a variables in the static store. type is the data type, and basic-align is the alignment that the object would ordinarily have. The value of this macro is used instead of that alignment to align the object. If this macro is not defined, then basic-align is used. One use of this macro is to increase alignment of medium-size data to make it all fit in fewer cache lines. Another is to cause character arrays to be word-aligned so that `strcpy` calls that copy constants to character arrays can be done inline.
`CONSTANT_ALIGNMENT (constant, basic-align)`
  If defined, a C expression to compute the alignment given to a constant that is being placed in memory. constant is the constant and basic-align is the alignment that the object would ordinarily have. The value of this macro is used instead of that alignment to align the object. If this macro is not defined, then basic-align is used. The typical use of this macro is to increase alignment for string constants to be word aligned so that `strcpy` calls that copy constants can be done inline.
`EMPTY_FIELD_BOUNDARY`
  Alignment in bits to be given to a structure bit field that follows an empty field such as `int : 0;`. Note that `PCC_BITFIELD_TYPE_MATTERS` also affects the alignment that results from an empty field.
`STRUCTURE_SIZE_BOUNDARY`
  Number of bits which any structure or union's size must be a multiple of. Each structure or union's size is rounded up to a multiple of this. If you do not define this macro, the default is the same as `BITS_PER_UNIT`.
`STRICT_ALIGNMENT`
  Define this macro to be the value 1 if instructions will fail to work if given data not on the nominal alignment. If instructions will merely go slower in that case, define this macro as 0.
`PCC_BITFIELD_TYPE_MATTERS`
  Define this if you wish to imitate the way many other C compilers handle alignment of bitfields and the structures that contain them. The behavior is that the type written for a bitfield (`int`, `short`, or other integer type) imposes an alignment for the entire structure, as if the structure really did contain an ordinary field of that type. In addition, the bitfield is placed within the structure so that it would fit within such a field, not crossing a boundary for it. Thus, on most machines, a bitfield whose type is written as `int` would not cross a four-byte boundary, and would force four-byte alignment for the whole structure. (The alignment used may not be four bytes; it is controlled by the other alignment parameters.) If the macro is defined, its definition should be a C expression; a nonzero value for the expression enables this behavior. Note that if this macro is not defined, or its value is zero, some bitfields may cross more than one alignment boundary. The compiler can support such references if there are ``insv'`, ``extv'`, and ``extzv'` insns that can directly reference memory. The other known way of making bitfields work is to define `STRUCTURE_SIZE_BOUNDARY` as large as `BIGGEST_ALIGNMENT`. Then every structure can be accessed with fullwords. Unless the machine has bitfield instructions or you define `STRUCTURE_SIZE_BOUNDARY` that way, you must define `PCC_BITFIELD_TYPE_MATTERS` to have a nonzero value. If your aim is to make GNU CC use the same conventions for laying out bitfields as are used by another compiler, here is how to investigate what the other compiler does. Compile and run this program:

      struct foo1
      {
        char x;
        char :0;
        char y;
      };

      struct foo2
      {
        char x;
        int :0;
        char y;
      };

      main ()
      {
        printf ("Size of foo1 is %d\n",
                sizeof (struct foo1));
        printf ("Size of foo2 is %d\n",
                sizeof (struct foo2));
        exit (0);
      }

  If this prints 2 and 5, then the compiler's behavior is what you would get from `PCC_BITFIELD_TYPE_MATTERS`.
`BITFIELD_NBYTES_LIMITED`
  Like PCC_BITFIELD_TYPE_MATTERS except that its effect is limited to aligning a bitfield within the structure.
`ROUND_TYPE_SIZE (type, computed, specified)`
  Define this macro as an expression for the overall size of a type (given by type as a tree node) when the size computed in the usual way is computed and the alignment is specified. The default is to round computed up to a multiple of specified.
`ROUND_TYPE_ALIGN (type, computed, specified)`
  Define this macro as an expression for the alignment of a type (given by type as a tree node) if the alignment computed in the usual way is computed and the alignment explicitly specified was specified. The default is to use specified if it is larger; otherwise, use the smaller of computed and `BIGGEST_ALIGNMENT`
`MAX_FIXED_MODE_SIZE`
  An integer expression for the size in bits of the largest integer machine mode that should actually be used. All integer machine modes of this size or smaller can be used for structures and unions with the appropriate sizes. If this macro is undefined, `GET_MODE_BITSIZE (DImode)` is assumed.
`STACK_SAVEAREA_MODE (save_level)`
  If defined, an expression of type `enum machine_mode` that specifies the mode of the save area operand of a `save_stack_level` named pattern (see section 16.7 Standard Pattern Names For Generation). save_level is one of `SAVE_BLOCK`, `SAVE_FUNCTION`, or `SAVE_NONLOCAL` and selects which of the three named patterns is having its mode specified. You need not define this macro if it always returns `Pmode`. You would most commonly define this macro if the `save_stack_level` patterns need to support both a 32- and a 64-bit mode.
`STACK_SIZE_MODE`
  If defined, an expression of type `enum machine_mode` that specifies the mode of the size increment operand of an `allocate_stack` named pattern (see section 16.7 Standard Pattern Names For Generation). You need not define this macro if it always returns `word_mode`. You would most commonly define this macro if the `allocate_stack` pattern needs to support both a 32- and a 64-bit mode.
`CHECK_FLOAT_VALUE (mode, value, overflow)`
  A C statement to validate the value value (of type `double`) for mode mode. This means that you check whether value fits within the possible range of values for mode mode on this target machine. The mode mode is always a mode of class `MODE_FLOAT`. overflow is nonzero if the value is already known to be out of range. If value is not valid or if overflow is nonzero, you should set overflow to 1 and then assign some valid value to value. Allowing an invalid value to go through the compiler can produce incorrect assembler code which may even cause Unix assemblers to crash. This macro need not be defined if there is no work for it to do.
`TARGET_FLOAT_FORMAT`
  A code distinguishing the floating point format of the target machine. There are three defined values:
  `IEEE_FLOAT_FORMAT`
    This code indicates IEEE floating point. It is the default; there is no need to define this macro when the format is IEEE.
  `VAX_FLOAT_FORMAT`
    This code indicates the peculiar format used on the Vax.
  `UNKNOWN_FLOAT_FORMAT`
    This code indicates any other format.
  The value of this macro is compared with `HOST_FLOAT_FORMAT` (see section 18. The Configuration File) to determine whether the target machine has the same format as the host machine. If any other formats are actually in use on supported machines, new codes should be defined for them. The ordering of the component words of floating point values stored in memory is controlled by `FLOAT_WORDS_BIG_ENDIAN` for the target machine and `HOST_FLOAT_WORDS_BIG_ENDIAN` for the host.
`DEFAULT_VTABLE_THUNKS`
  GNU CC supports two ways of implementing C++ vtables: traditional or with so-called "thunks". The flag ``-fvtable-thunk'` chooses between them. Define this macro to be a C expression for the default value of that flag. If `DEFAULT_VTABLE_THUNKS` is 0, GNU CC uses the traditional implementation by default. The "thunk" implementation is more efficient (especially if you have provided an implementation of `ASM_OUTPUT_MI_THUNK`, see section 17.7.10 Function Entry and Exit), but is not binary compatible with code compiled using the traditional implementation. If you are writing a new ports, define `DEFAULT_VTABLE_THUNKS` to 1. If you do not define this macro, the default for ``-fvtable-thunk'` is 0.

## 17.4 Layout of Source Language Data Types

These macros define the sizes and other characteristics of the standard basic data types used in programs being compiled. Unlike the macros in the previous section, these apply to specific features of C and related languages, rather than to fundamental aspects of storage layout.

`INT_TYPE_SIZE`
  A C expression for the size in bits of the type `int` on the target machine. If you don't define this, the default is one word.
`MAX_INT_TYPE_SIZE`
  Maximum number for the size in bits of the type `int` on the target machine. If this is undefined, the default is `INT_TYPE_SIZE`. Otherwise, it is the constant value that is the largest value that `INT_TYPE_SIZE` can have at run-time. This is used in `cpp`.
`SHORT_TYPE_SIZE`
  A C expression for the size in bits of the type `short` on the target machine. If you don't define this, the default is half a word. (If this would be less than one storage unit, it is rounded up to one unit.)
`LONG_TYPE_SIZE`
  A C expression for the size in bits of the type `long` on the target machine. If you don't define this, the default is one word.
`MAX_LONG_TYPE_SIZE`
  Maximum number for the size in bits of the type `long` on the target machine. If this is undefined, the default is `LONG_TYPE_SIZE`. Otherwise, it is the constant value that is the largest value that `LONG_TYPE_SIZE` can have at run-time. This is used in `cpp`.
`LONG_LONG_TYPE_SIZE`
  A C expression for the size in bits of the type `long long` on the target machine. If you don't define this, the default is two words. If you want to support GNU Ada on your machine, the value of macro must be at least 64.
`CHAR_TYPE_SIZE`
  A C expression for the size in bits of the type `char` on the target machine. If you don't define this, the default is one quarter of a word. (If this would be less than one storage unit, it is rounded up to one unit.)
`MAX_CHAR_TYPE_SIZE`
  Maximum number for the size in bits of the type `char` on the target machine. If this is undefined, the default is `CHAR_TYPE_SIZE`. Otherwise, it is the constant value that is the largest value that `CHAR_TYPE_SIZE` can have at run-time. This is used in `cpp`.
`FLOAT_TYPE_SIZE`
  A C expression for the size in bits of the type `float` on the target machine. If you don't define this, the default is one word.
`DOUBLE_TYPE_SIZE`
  A C expression for the size in bits of the type `double` on the target machine. If you don't define this, the default is two words.
`LONG_DOUBLE_TYPE_SIZE`
  A C expression for the size in bits of the type `long double` on the target machine. If you don't define this, the default is two words.
`WIDEST_HARDWARE_FP_SIZE`
  A C expression for the size in bits of the widest floating-point format supported by the hardware. If you define this macro, you must specify a value less than or equal to the value of `LONG_DOUBLE_TYPE_SIZE`. If you do not define this macro, the value of `LONG_DOUBLE_TYPE_SIZE` is the default.
`DEFAULT_SIGNED_CHAR`
  An expression whose value is 1 or 0, according to whether the type `char` should be signed or unsigned by default. The user can always override this default with the options ``-fsigned-char'` and ``-funsigned-char'`.
`DEFAULT_SHORT_ENUMS`
  A C expression to determine whether to give an `enum` type only as many bytes as it takes to represent the range of possible values of that type. A nonzero value means to do that; a zero value means all `enum` types should be allocated like `int`. If you don't define the macro, the default is 0.
`SIZE_TYPE`
  A C expression for a string describing the name of the data type to use for size values. The typedef name `size_t` is defined using the contents of the string. The string can contain more than one keyword. If so, separate them with spaces, and write first any length keyword, then `unsigned` if appropriate, and finally `int`. The string must exactly match one of the data type names defined in the function `init_decl_processing` in the file ``c-decl.c'`. You may not omit `int` or change the order--that would cause the compiler to crash on startup. If you don't define this macro, the default is `"long unsigned int"`.
`PTRDIFF_TYPE`
  A C expression for a string describing the name of the data type to use for the result of subtracting two pointers. The typedef name `ptrdiff_t` is defined using the contents of the string. See `SIZE_TYPE` above for more information. If you don't define this macro, the default is `"long int"`.
`WCHAR_TYPE`
  A C expression for a string describing the name of the data type to use for wide characters. The typedef name `wchar_t` is defined using the contents of the string. See `SIZE_TYPE` above for more information. If you don't define this macro, the default is `"int"`.
`WCHAR_TYPE_SIZE`
  A C expression for the size in bits of the data type for wide characters. This is used in `cpp`, which cannot make use of `WCHAR_TYPE`.
`MAX_WCHAR_TYPE_SIZE`
  Maximum number for the size in bits of the data type for wide characters. If this is undefined, the default is `WCHAR_TYPE_SIZE`. Otherwise, it is the constant value that is the largest value that `WCHAR_TYPE_SIZE` can have at run-time. This is used in `cpp`.
`OBJC_INT_SELECTORS`
  Define this macro if the type of Objective C selectors should be `int`. If this macro is not defined, then selectors should have the type `struct objc_selector *`.
`OBJC_SELECTORS_WITHOUT_LABELS`
  Define this macro if the compiler can group all the selectors together into a vector and use just one label at the beginning of the vector. Otherwise, the compiler must give each selector its own assembler label. On certain machines, it is important to have a separate label for each selector because this enables the linker to eliminate duplicate selectors.
`TARGET_BELL`
  A C constant expression for the integer value for escape sequence ``\a'`.
`TARGET_BS`
`TARGET_TAB`
`TARGET_NEWLINE`
  C constant expressions for the integer values for escape sequences ``\b'`, ``\t'` and ``\n'`.
`TARGET_VT`
`TARGET_FF`
`TARGET_CR`
  C constant expressions for the integer values for escape sequences ``\v'`, ``\f'` and ``\r'`.

## 17.5 Register Usage

This section explains how to describe what registers the target machine has, and how (in general) they can be used.

The description of which registers a specific instruction can use is done with register classes; see section 17.6 Register Classes. For information on using registers to access a stack frame, see section 17.7.3 Registers That Address the Stack Frame. For passing values in registers, see section 17.7.6 Passing Arguments in Registers. For returning values in registers, see section 17.7.7 How Scalar Function Values Are Returned.

### 17.5.1 Basic Characteristics of Registers

Registers have various characteristics.

`FIRST_PSEUDO_REGISTER`
  Number of hardware registers known to the compiler. They receive numbers 0 through `FIRST_PSEUDO_REGISTER-1`; thus, the first pseudo register's number really is assigned the number `FIRST_PSEUDO_REGISTER`.
`FIXED_REGISTERS`
  An initializer that says which registers are used for fixed purposes all throughout the compiled code and are therefore not available for general allocation. These would include the stack pointer, the frame pointer (except on machines where that can be used as a general register when no frame pointer is needed), the program counter on machines where that is considered one of the addressable registers, and any other numbered register with a standard use. This information is expressed as a sequence of numbers, separated by commas and surrounded by braces. The nth number is 1 if register n is fixed, 0 otherwise. The table initialized from this macro, and the table initialized by the following one, may be overridden at run time either automatically, by the actions of the macro `CONDITIONAL_REGISTER_USAGE`, or by the user with the command options ``-ffixed-reg'`, ``-fcall-used-reg'` and ``-fcall-saved-reg'`.
`CALL_USED_REGISTERS`
  Like `FIXED_REGISTERS` but has 1 for each register that is clobbered (in general) by function calls as well as for fixed registers. This macro therefore identifies the registers that are not available for general allocation of values that must live across function calls. If a register has 0 in `CALL_USED_REGISTERS`, the compiler automatically saves it on function entry and restores it on function exit, if the register is used within the function.
`HARD_REGNO_CALL_PART_CLOBBERED (regno, mode)`
  A C expression that is non-zero if it is not permissible to store a value of mode mode in hard register number regno across a call without some part of it being clobbered. For most machines this macro need not be defined. It is only required for machines that do not preserve the entire contents of a register across a call.
`CONDITIONAL_REGISTER_USAGE`
  Zero or more C statements that may conditionally modify four variables `fixed_regs`, `call_used_regs`, `global_regs` (these three are of type `char []`) and `reg_class_contents` (of type `HARD_REG_SET`). Before the macro is called `fixed_regs`, `call_used_regs` and `reg_class_contents` have been initialized from `FIXED_REGISTERS`, `CALL_USED_REGISTERS` and `REG_CLASS_CONTENTS`, respectively, `global_regs` has been cleared, and any ``-ffixed-reg'`, ``-fcall-used-reg'` and ``-fcall-saved-reg'` command options have been applied. This is necessary in case the fixed or call-clobbered registers depend on target flags. You need not define this macro if it has no work to do. If the usage of an entire class of registers depends on the target flags, you may indicate this to GCC by using this macro to modify `fixed_regs` and `call_used_regs` to 1 for each of the registers in the classes which should not be used by GCC. Also define the macro `REG_CLASS_FROM_LETTER` to return `NO_REGS` if it is called with a letter for a class that shouldn't be used. (However, if this class is not included in `GENERAL_REGS` and all of the insn patterns whose constraints permit this class are controlled by target switches, then GCC will automatically avoid using these registers when the target switches are opposed to them.)
`NON_SAVING_SETJMP`
  If this macro is defined and has a nonzero value, it means that `setjmp` and related functions fail to save the registers, or that `longjmp` fails to restore them. To compensate, the compiler avoids putting variables in registers in functions that use `setjmp`.
`INCOMING_REGNO (out)`
  Define this macro if the target machine has register windows. This C expression returns the register number as seen by the called function corresponding to the register number out as seen by the calling function. Return out if register number out is not an outbound register.
`OUTGOING_REGNO (in)`
  Define this macro if the target machine has register windows. This C expression returns the register number as seen by the calling function corresponding to the register number in as seen by the called function. Return in if register number in is not an inbound register.

### 17.5.2 Order of Allocation of Registers

Registers are allocated in order.

`REG_ALLOC_ORDER`
  If defined, an initializer for a vector of integers, containing the numbers of hard registers in the order in which GNU CC should prefer to use them (from most preferred to least). If this macro is not defined, registers are used lowest numbered first (all else being equal). One use of this macro is on machines where the highest numbered registers must always be saved and the save-multiple-registers instruction supports only sequences of consecutive registers. On such machines, define `REG_ALLOC_ORDER` to be an initializer that lists the highest numbered allocable register first.
`ORDER_REGS_FOR_LOCAL_ALLOC`
  A C statement (sans semicolon) to choose the order in which to allocate hard registers for pseudo-registers local to a basic block. Store the desired register order in the array `reg_alloc_order`. Element 0 should be the register to allocate first; element 1, the next register; and so on. The macro body should not assume anything about the contents of `reg_alloc_order` before execution of the macro. On most machines, it is not necessary to define this macro.

### 17.5.3 How Values Fit in Registers

This section discusses the macros that describe which kinds of values (specifically, which machine modes) each register can hold, and how many consecutive registers are needed for a given mode.

`HARD_REGNO_NREGS (regno, mode)`
  A C expression for the number of consecutive hard registers, starting at register number regno, required to hold a value of mode mode. On a machine where all registers are exactly one word, a suitable definition of this macro is

      #define HARD_REGNO_NREGS(REGNO, MODE)            \
         ((GET_MODE_SIZE (MODE) + UNITS_PER_WORD - 1)  \
          / UNITS_PER_WORD))

`ALTER_HARD_SUBREG (tgt_mode, word, src_mode, regno)`
  A C expression that returns an adjusted hard register number for

      (subreg:tgt_mode (reg:src_mode regno) word)

  This may be needed if the target machine has mixed sized big-endian registers, like Sparc v9.
`HARD_REGNO_MODE_OK (regno, mode)`
  A C expression that is nonzero if it is permissible to store a value of mode mode in hard register number regno (or in several registers starting with that one). For a machine where all registers are equivalent, a suitable definition is

      #define HARD_REGNO_MODE_OK(REGNO, MODE) 1

  You need not include code to check for the numbers of fixed registers, because the allocation mechanism considers them to be always occupied. On some machines, double-precision values must be kept in even/odd register pairs. You can implement that by defining this macro to reject odd register numbers for such modes. The minimum requirement for a mode to be OK in a register is that the ``movmode'` instruction pattern support moves between the register and other hard register in the same class and that moving a value into the register and back out not alter it. Since the same instruction used to move `word_mode` will work for all narrower integer modes, it is not necessary on any machine for `HARD_REGNO_MODE_OK` to distinguish between these modes, provided you define patterns ``movhi'`, etc., to take advantage of this. This is useful because of the interaction between `HARD_REGNO_MODE_OK` and `MODES_TIEABLE_P`; it is very desirable for all integer modes to be tieable. Many machines have special registers for floating point arithmetic. Often people assume that floating point machine modes are allowed only in floating point registers. This is not true. Any registers that can hold integers can safely hold a floating point machine mode, whether or not floating arithmetic can be done on it in those registers. Integer move instructions can be used to move the values. On some machines, though, the converse is true: fixed-point machine modes may not go in floating registers. This is true if the floating registers normalize any value stored in them, because storing a non-floating value there would garble it. In this case, `HARD_REGNO_MODE_OK` should reject fixed-point machine modes in floating registers. But if the floating registers do not automatically normalize, if you can store any bit pattern in one and retrieve it unchanged without a trap, then any machine mode may go in a floating register, so you can define this macro to say so. The primary significance of special floating registers is rather that they are the registers acceptable in floating point arithmetic instructions. However, this is of no concern to `HARD_REGNO_MODE_OK`. You handle it by writing the proper constraints for those instructions. On some machines, the floating registers are especially slow to access, so that it is better to store a value in a stack frame than in such a register if floating point arithmetic is not being done. As long as the floating registers are not in class `GENERAL_REGS`, they will not be used unless some pattern's constraint asks for one.
`MODES_TIEABLE_P (mode1, mode2)`
  A C expression that is nonzero if a value of mode mode1 is accessible in mode mode2 without copying. If `HARD_REGNO_MODE_OK (r, mode1)` and `HARD_REGNO_MODE_OK (r, mode2)` are always the same for any r, then `MODES_TIEABLE_P (mode1, mode2)` should be nonzero. If they differ for any r, you should define this macro to return zero unless some other mechanism ensures the accessibility of the value in a narrower mode. You should define this macro to return nonzero in as many cases as possible since doing so will allow GNU CC to perform better register allocation.
`AVOID_CCMODE_COPIES`
  Define this macro if the compiler should avoid copies to/from `CCmode` registers. You should only define this macro if support fo copying to/from `CCmode` is incomplete.

### 17.5.4 Handling Leaf Functions

On some machines, a leaf function (i.e., one which makes no calls) can run more efficiently if it does not make its own register window. Often this means it is required to receive its arguments in the registers where they are passed by the caller, instead of the registers where they would normally arrive.

The special treatment for leaf functions generally applies only when other conditions are met; for example, often they may use only those registers for its own variables and temporaries. We use the term "leaf function" to mean a function that is suitable for this special handling, so that functions with no calls are not necessarily "leaf functions".

GNU CC assigns register numbers before it knows whether the function is suitable for leaf function treatment. So it needs to renumber the registers in order to output a leaf function. The following macros accomplish this.

`LEAF_REGISTERS`
  A C initializer for a vector, indexed by hard register number, which contains 1 for a register that is allowable in a candidate for leaf function treatment. If leaf function treatment involves renumbering the registers, then the registers marked here should be the ones before renumbering--those that GNU CC would ordinarily allocate. The registers which will actually be used in the assembler code, after renumbering, should not be marked with 1 in this vector. Define this macro only if the target machine offers a way to optimize the treatment of leaf functions.
`LEAF_REG_REMAP (regno)`
  A C expression whose value is the register number to which regno should be renumbered, when a function is treated as a leaf function. If regno is a register number which should not appear in a leaf function before renumbering, then the expression should yield -1, which will cause the compiler to abort. Define this macro only if the target machine offers a way to optimize the treatment of leaf functions, and registers need to be renumbered to do this.

Normally, `FUNCTION_PROLOGUE` and `FUNCTION_EPILOGUE` must treat leaf functions specially. It can test the C variable `leaf_function` which is nonzero for leaf functions. (The variable `leaf_function` is defined only if `LEAF_REGISTERS` is defined.)

### 17.5.5 Registers That Form a Stack

There are special features to handle computers where some of the "registers" form a stack, as in the 80387 coprocessor for the 80386. Stack registers are normally written by pushing onto the stack, and are numbered relative to the top of the stack.

Currently, GNU CC can only handle one group of stack-like registers, and they must be consecutively numbered.

`STACK_REGS`
  Define this if the machine has any stack-like registers.
`FIRST_STACK_REG`
  The number of the first stack-like register. This one is the top of the stack.
`LAST_STACK_REG`
  The number of the last stack-like register. This one is the bottom of the stack.

### 17.5.6 Obsolete Macros for Controlling Register Usage

These features do not work very well. They exist because they used to be required to generate correct code for the 80387 coprocessor of the 80386. They are no longer used by that machine description and may be removed in a later version of the compiler. Don't use them!

`OVERLAPPING_REGNO_P (regno)`
  If defined, this is a C expression whose value is nonzero if hard register number regno is an overlapping register. This means a hard register which overlaps a hard register with a different number. (Such overlap is undesirable, but occasionally it allows a machine to be supported which otherwise could not be.) This macro must return nonzero for all the registers which overlap each other. GNU CC can use an overlapping register only in certain limited ways. It can be used for allocation within a basic block, and may be spilled for reloading; that is all. If this macro is not defined, it means that none of the hard registers overlap each other. This is the usual situation.
`INSN_CLOBBERS_REGNO_P (insn, regno)`
  If defined, this is a C expression whose value should be nonzero if the insn insn has the effect of mysteriously clobbering the contents of hard register number regno. By "mysterious" we mean that the insn's RTL expression doesn't describe such an effect. If this macro is not defined, it means that no insn clobbers registers mysteriously. This is the usual situation; all else being equal, it is best for the RTL expression to show all the activity.

## 17.6 Register Classes

On many machines, the numbered registers are not all equivalent. For example, certain registers may not be allowed for indexed addressing; certain registers may not be allowed in some instructions. These machine restrictions are described to the compiler using register classes.

You define a number of register classes, giving each one a name and saying which of the registers belong to it. Then you can specify register classes that are allowed as operands to particular instruction patterns.

In general, each register will belong to several classes. In fact, one class must be named `ALL_REGS` and contain all the registers. Another class must be named `NO_REGS` and contain no registers. Often the union of two classes will be another class; however, this is not required.

One of the classes must be named `GENERAL_REGS`. There is nothing terribly special about the name, but the operand constraint letters ``r'` and ``g'` specify this class. If `GENERAL_REGS` is the same as `ALL_REGS`, just define it as a macro which expands to `ALL_REGS`.

Order the classes so that if class x is contained in class y then x has a lower class number than y.

The way classes other than `GENERAL_REGS` are specified in operand constraints is through machine-dependent operand constraint letters. You can define such letters to correspond to various classes, then use them in operand constraints.

You should define a class for the union of two classes whenever some instruction allows both classes. For example, if an instruction allows either a floating point (coprocessor) register or a general register for a certain operand, you should define a class `FLOAT_OR_GENERAL_REGS` which includes both of them. Otherwise you will get suboptimal code.

You must also specify certain redundant information about the register classes: for each class, which classes contain it and which ones are contained in it; for each pair of classes, the largest class contained in their union.

When a value occupying several consecutive registers is expected in a certain class, all the registers used must belong to that class. Therefore, register classes cannot be used to enforce a requirement for a register pair to start with an even-numbered register. The way to specify this requirement is with `HARD_REGNO_MODE_OK`.

Register classes used for input-operands of bitwise-and or shift instructions have a special requirement: each such class must have, for each fixed-point machine mode, a subclass whose registers can transfer that mode to or from memory. For example, on some machines, the operations for single-byte values (`QImode`) are limited to certain registers. When this is so, each register class that is used in a bitwise-and or shift instruction must have a subclass consisting of registers from which single-byte values can be loaded or stored. This is so that `PREFERRED_RELOAD_CLASS` can always have a possible value to return.

`enum reg_class`
  An enumeral type that must be defined with all the register class names as enumeral values. `NO_REGS` must be first. `ALL_REGS` must be the last register class, followed by one more enumeral value, `LIM_REG_CLASSES`, which is not a register class but rather tells how many classes there are. Each register class has a number, which is the value of casting the class name to type `int`. The number serves as an index in many of the tables described below.
`N_REG_CLASSES`
  The number of distinct register classes, defined as follows:

      #define N_REG_CLASSES (int) LIM_REG_CLASSES

`REG_CLASS_NAMES`
  An initializer containing the names of the register classes as C string constants. These names are used in writing some of the debugging dumps.
`REG_CLASS_CONTENTS`
  An initializer containing the contents of the register classes, as integers which are bit masks. The nth integer specifies the contents of class n. The way the integer mask is interpreted is that register r is in the class if `mask & (1 << r)` is 1. When the machine has more than 32 registers, an integer does not suffice. Then the integers are replaced by sub-initializers, braced groupings containing several integers. Each sub-initializer must be suitable as an initializer for the type `HARD_REG_SET` which is defined in ``hard-reg-set.h'`.
`REGNO_REG_CLASS (regno)`
  A C expression whose value is a register class containing hard register regno. In general there is more than one such class; choose a class which is minimal, meaning that no smaller class also contains the register.
`BASE_REG_CLASS`
  A macro whose definition is the name of the class to which a valid base register must belong. A base register is one used in an address which is the register value plus a displacement.
`INDEX_REG_CLASS`
  A macro whose definition is the name of the class to which a valid index register must belong. An index register is one used in an address where its value is either multiplied by a scale factor or added to another register (as well as added to a displacement).
`REG_CLASS_FROM_LETTER (char)`
  A C expression which defines the machine-dependent operand constraint letters for register classes. If char is such a letter, the value should be the register class corresponding to it. Otherwise, the value should be `NO_REGS`. The register letter ``r'`, corresponding to class `GENERAL_REGS`, will not be passed to this macro; you do not need to handle it.
`REGNO_OK_FOR_BASE_P (num)`
  A C expression which is nonzero if register number num is suitable for use as a base register in operand addresses. It may be either a suitable hard register or a pseudo register that has been allocated such a hard register.
`REGNO_MODE_OK_FOR_BASE_P (num, mode)`
  A C expression that is just like `REGNO_OK_FOR_BASE_P`, except that that expression may examine the mode of the memory reference in mode. You should define this macro if the mode of the memory reference affects whether a register may be used as a base register. If you define this macro, the compiler will use it instead of `REGNO_OK_FOR_BASE_P`.
`REGNO_OK_FOR_INDEX_P (num)`
  A C expression which is nonzero if register number num is suitable for use as an index register in operand addresses. It may be either a suitable hard register or a pseudo register that has been allocated such a hard register. The difference between an index register and a base register is that the index register may be scaled. If an address involves the sum of two registers, neither one of them scaled, then either one may be labeled the "base" and the other the "index"; but whichever labeling is used must fit the machine's constraints of which registers may serve in each capacity. The compiler will try both labelings, looking for one that is valid, and will reload one or both registers only if neither labeling works.
`PREFERRED_RELOAD_CLASS (x, class)`
  A C expression that places additional restrictions on the register class to use when it is necessary to copy value x into a register in class class. The value is a register class; perhaps class, or perhaps another, smaller class. On many machines, the following definition is safe:

      #define PREFERRED_RELOAD_CLASS(X,CLASS) CLASS

  Sometimes returning a more restrictive class makes better code. For example, on the 68000, when x is an integer constant that is in range for a ``moveq'` instruction, the value of this macro is always `DATA_REGS` as long as class includes the data registers. Requiring a data register guarantees that a ``moveq'` will be used. If x is a `const_double`, by returning `NO_REGS` you can force x into a memory constant. This is useful on certain machines where immediate floating values cannot be loaded into certain kinds of registers.
`PREFERRED_OUTPUT_RELOAD_CLASS (x, class)`
  Like `PREFERRED_RELOAD_CLASS`, but for output reloads instead of input reloads. If you don't define this macro, the default is to use class, unchanged.
`LIMIT_RELOAD_CLASS (mode, class)`
  A C expression that places additional restrictions on the register class to use when it is necessary to be able to hold a value of mode mode in a reload register for which class class would ordinarily be used. Unlike `PREFERRED_RELOAD_CLASS`, this macro should be used when there are certain modes that simply can't go in certain reload classes. The value is a register class; perhaps class, or perhaps another, smaller class. Don't define this macro unless the target machine has limitations which require the macro to do something nontrivial.
`SECONDARY_RELOAD_CLASS (class, mode, x)`
`SECONDARY_INPUT_RELOAD_CLASS (class, mode, x)`
`SECONDARY_OUTPUT_RELOAD_CLASS (class, mode, x)`
  Many machines have some registers that cannot be copied directly to or from memory or even from other types of registers. An example is the ``MQ'` register, which on most machines, can only be copied to or from general registers, but not memory. Some machines allow copying all registers to and from memory, but require a scratch register for stores to some memory locations (e.g., those with symbolic address on the RT, and those with certain symbolic address on the Sparc when compiling PIC). In some cases, both an intermediate and a scratch register are required. You should define these macros to indicate to the reload phase that it may need to allocate at least one register for a reload in addition to the register to contain the data. Specifically, if copying x to a register class in mode requires an intermediate register, you should define `SECONDARY_INPUT_RELOAD_CLASS` to return the largest register class all of whose registers can be used as intermediate registers or scratch registers. If copying a register class in mode to x requires an intermediate or scratch register, `SECONDARY_OUTPUT_RELOAD_CLASS` should be defined to return the largest register class required. If the requirements for input and output reloads are the same, the macro `SECONDARY_RELOAD_CLASS` should be used instead of defining both macros identically. The values returned by these macros are often `GENERAL_REGS`. Return `NO_REGS` if no spare register is needed; i.e., if x can be directly copied to or from a register of class in mode without requiring a scratch register. Do not define this macro if it would always return `NO_REGS`. If a scratch register is required (either with or without an intermediate register), you should define patterns for ``reload_inm'` or ``reload_outm'`, as required (see section 16.7 Standard Pattern Names For Generation. These patterns, which will normally be implemented with a `define_expand`, should be similar to the ``movm'` patterns, except that operand 2 is the scratch register. Define constraints for the reload register and scratch register that contain a single register class. If the original reload register (whose class is class) can meet the constraint given in the pattern, the value returned by these macros is used for the class of the scratch register. Otherwise, two additional reload registers are required. Their classes are obtained from the constraints in the insn pattern. x might be a pseudo-register or a `subreg` of a pseudo-register, which could either be in a hard register or in memory. Use `true_regnum` to find out; it will return -1 if the pseudo is in memory and the hard register number if it is in a register. These macros should not be used in the case where a particular class of registers can only be copied to memory and not to another class of registers. In that case, secondary reload registers are not needed and would not be helpful. Instead, a stack location must be used to perform the copy and the `movm` pattern should use memory as a intermediate storage. This case often occurs between floating-point and general registers.
`SECONDARY_MEMORY_NEEDED (class1, class2, m)`
  Certain machines have the property that some registers cannot be copied to some other registers without using memory. Define this macro on those machines to be a C expression that is non-zero if objects of mode m in registers of class1 can only be copied to registers of class class2 by storing a register of class1 into memory and loading that memory location into a register of class2. Do not define this macro if its value would always be zero.
`SECONDARY_MEMORY_NEEDED_RTX (mode)`
  Normally when `SECONDARY_MEMORY_NEEDED` is defined, the compiler allocates a stack slot for a memory location needed for register copies. If this macro is defined, the compiler instead uses the memory location defined by this macro. Do not define this macro if you do not define `SECONDARY_MEMORY_NEEDED`.
`SECONDARY_MEMORY_NEEDED_MODE (mode)`
  When the compiler needs a secondary memory location to copy between two registers of mode mode, it normally allocates sufficient memory to hold a quantity of `BITS_PER_WORD` bits and performs the store and load operations in a mode that many bits wide and whose class is the same as that of mode. This is right thing to do on most machines because it ensures that all bits of the register are copied and prevents accesses to the registers in a narrower mode, which some machines prohibit for floating-point registers. However, this default behavior is not correct on some machines, such as the DEC Alpha, that store short integers in floating-point registers differently than in integer registers. On those machines, the default widening will not work correctly and you must define this macro to suppress that widening in some cases. See the file ``alpha.h'` for details. Do not define this macro if you do not define `SECONDARY_MEMORY_NEEDED` or if widening mode to a mode that is `BITS_PER_WORD` bits wide is correct for your machine.
`SMALL_REGISTER_CLASSES`
  On some machines, it is risky to let hard registers live across arbitrary insns. Typically, these machines have instructions that require values to be in specific registers (like an accumulator), and reload will fail if the required hard register is used for another purpose across such an insn. Define `SMALL_REGISTER_CLASSES` to be an expression with a non-zero value on these machines. When this macro has a non-zero value, the compiler will try to minimize the lifetime of hard registers. It is always safe to define this macro with a non-zero value, but if you unnecessarily define it, you will reduce the amount of optimizations that can be performed in some cases. If you do not define this macro with a non-zero value when it is required, the compiler will run out of spill registers and print a fatal error message. For most machines, you should not define this macro at all.
`CLASS_LIKELY_SPILLED_P (class)`
  A C expression whose value is nonzero if pseudos that have been assigned to registers of class class would likely be spilled because registers of class are needed for spill registers. The default value of this macro returns 1 if class has exactly one register and zero otherwise. On most machines, this default should be used. Only define this macro to some other expression if pseudos allocated by ``local-alloc.c'` end up in memory because their hard registers were needed for spill registers. If this macro returns nonzero for those classes, those pseudos will only be allocated by ``global.c'`, which knows how to reallocate the pseudo to another register. If there would not be another register available for reallocation, you should not change the definition of this macro since the only effect of such a definition would be to slow down register allocation.
`CLASS_MAX_NREGS (class, mode)`
  A C expression for the maximum number of consecutive registers of class class needed to hold a value of mode mode. This is closely related to the macro `HARD_REGNO_NREGS`. In fact, the value of the macro `CLASS_MAX_NREGS (class, mode)` should be the maximum value of `HARD_REGNO_NREGS (regno, mode)` for all regno values in the class class. This macro helps control the handling of multiple-word values in the reload pass.
`CLASS_CANNOT_CHANGE_SIZE`
  If defined, a C expression for a class that contains registers which the compiler must always access in a mode that is the same size as the mode in which it loaded the register. For the example, loading 32-bit integer or floating-point objects into floating-point registers on the Alpha extends them to 64-bits. Therefore loading a 64-bit object and then storing it as a 32-bit object does not store the low-order 32-bits, as would be the case for a normal register. Therefore, ``alpha.h'` defines this macro as `FLOAT_REGS`.

Three other special macros describe which operands fit which constraint letters.

`CONST_OK_FOR_LETTER_P (value, c)`
  A C expression that defines the machine-dependent operand constraint letters (``I'`, ``J'`, ``K'`, ... ``P'`) that specify particular ranges of integer values. If c is one of those letters, the expression should check that value, an integer, is in the appropriate range and return 1 if so, 0 otherwise. If c is not one of those letters, the value should be 0 regardless of value.
`CONST_DOUBLE_OK_FOR_LETTER_P (value, c)`
  A C expression that defines the machine-dependent operand constraint letters that specify particular ranges of `const_double` values (``G'` or ``H'`). If c is one of those letters, the expression should check that value, an RTX of code `const_double`, is in the appropriate range and return 1 if so, 0 otherwise. If c is not one of those letters, the value should be 0 regardless of value. `const_double` is used for all floating-point constants and for `DImode` fixed-point constants. A given letter can accept either or both kinds of values. It can use `GET_MODE` to distinguish between these kinds.
`EXTRA_CONSTRAINT (value, c)`
  A C expression that defines the optional machine-dependent constraint letters (``Q'`, ``R'`, ``S'`, ``T'`, ``U'`) that can be used to segregate specific types of operands, usually memory references, for the target machine. Normally this macro will not be defined. If it is required for a particular target machine, it should return 1 if value corresponds to the operand type represented by the constraint letter c. If c is not defined as an extra constraint, the value returned should be 0 regardless of value. For example, on the ROMP, load instructions cannot have their output in r0 if the memory reference contains a symbolic address. Constraint letter ``Q'` is defined as representing a memory address that does not contain a symbolic address. An alternative is specified with a ``Q'` constraint on the input and ``r'` on the output. The next alternative specifies ``m'` on the input and a register class that does not include r0 on the output.

## 17.7 Stack Layout and Calling Conventions

This describes the stack layout and calling conventions.

### 17.7.1 Basic Stack Layout

Here is the basic stack layout.

`STACK_GROWS_DOWNWARD`
  Define this macro if pushing a word onto the stack moves the stack pointer to a smaller address. When we say, "define this macro if ...," it means that the compiler checks this macro only with `#ifdef` so the precise definition used does not matter.
`FRAME_GROWS_DOWNWARD`
  Define this macro if the addresses of local variable slots are at negative offsets from the frame pointer.
`ARGS_GROW_DOWNWARD`
  Define this macro if successive arguments to a function occupy decreasing addresses on the stack.
`STARTING_FRAME_OFFSET`
  Offset from the frame pointer to the first local variable slot to be allocated. If `FRAME_GROWS_DOWNWARD`, find the next slot's offset by subtracting the first slot's length from `STARTING_FRAME_OFFSET`. Otherwise, it is found by adding the length of the first slot to the value `STARTING_FRAME_OFFSET`.
`STACK_POINTER_OFFSET`
  Offset from the stack pointer register to the first location at which outgoing arguments are placed. If not specified, the default value of zero is used. This is the proper value for most machines. If `ARGS_GROW_DOWNWARD`, this is the offset to the location above the first location at which outgoing arguments are placed.
`FIRST_PARM_OFFSET (fundecl)`
  Offset from the argument pointer register to the first argument's address. On some machines it may depend on the data type of the function. If `ARGS_GROW_DOWNWARD`, this is the offset to the location above the first argument's address.
`STACK_DYNAMIC_OFFSET (fundecl)`
  Offset from the stack pointer register to an item dynamically allocated on the stack, e.g., by `alloca`. The default value for this macro is `STACK_POINTER_OFFSET` plus the length of the outgoing arguments. The default is correct for most machines. See ``function.c'` for details.
`DYNAMIC_CHAIN_ADDRESS (frameaddr)`
  A C expression whose value is RTL representing the address in a stack frame where the pointer to the caller's frame is stored. Assume that frameaddr is an RTL expression for the address of the stack frame itself. If you don't define this macro, the default is to return the value of frameaddr---that is, the stack frame address is also the address of the stack word that points to the previous frame.
`SETUP_FRAME_ADDRESSES`
  If defined, a C expression that produces the machine-specific code to setup the stack so that arbitrary frames can be accessed. For example, on the Sparc, we must flush all of the register windows to the stack before we can access arbitrary stack frames. You will seldom need to define this macro.
`BUILTIN_SETJMP_FRAME_VALUE`
  If defined, a C expression that contains an rtx that is used to store the address of the current frame into the built in `setjmp` buffer. The default value, `virtual_stack_vars_rtx`, is correct for most machines. One reason you may need to define this macro is if `hard_frame_pointer_rtx` is the appropriate value on your machine.
`RETURN_ADDR_RTX (count, frameaddr)`
  A C expression whose value is RTL representing the value of the return address for the frame count steps up from the current frame, after the prologue. frameaddr is the frame pointer of the count frame, or the frame pointer of the count - 1 frame if `RETURN_ADDR_IN_PREVIOUS_FRAME` is defined. The value of the expression must always be the correct address when count is zero, but may be `NULL_RTX` if there is not way to determine the return address of other frames.
`RETURN_ADDR_IN_PREVIOUS_FRAME`
  Define this if the return address of a particular stack frame is accessed from the frame pointer of the previous stack frame.
`INCOMING_RETURN_ADDR_RTX`
  A C expression whose value is RTL representing the location of the incoming return address at the beginning of any function, before the prologue. This RTL is either a `REG`, indicating that the return value is saved in ``REG'`, or a `MEM` representing a location in the stack. You only need to define this macro if you want to support call frame debugging information like that provided by DWARF 2.
`INCOMING_FRAME_SP_OFFSET`
  A C expression whose value is an integer giving the offset, in bytes, from the value of the stack pointer register to the top of the stack frame at the beginning of any function, before the prologue. The top of the frame is defined to be the value of the stack pointer in the previous frame, just before the call instruction. You only need to define this macro if you want to support call frame debugging information like that provided by DWARF 2.
`ARG_POINTER_CFA_OFFSET`
  A C expression whose value is an integer giving the offset, in bytes, from the argument pointer to the canonical frame address (cfa). The final value should coincide with that calculated by `INCOMING_FRAME_SP_OFFSET`. Which is unfortunately not usable during virtual register instantiation. You only need to define this macro if you want to support call frame debugging information like that provided by DWARF 2.
`SMALL_STACK`
  Define this macro if the stack size for the target is very small. This has the effect of disabling gcc's builtin ``alloca'` support.

### 17.7.2 Specifying How Stack Checking is Done

GNU CC will check that stack references are within the boundaries of the stack, if the ``-fstack-check'` is specified, in one of three ways:

1. If the value of the `STACK_CHECK_BUILTIN` macro is nonzero, GNU CC will assume that you have arranged for stack checking to be done at appropriate places in the configuration files, e.g., in `FUNCTION_PROLOGUE`. GNU CC will do not other special processing.
2. If `STACK_CHECK_BUILTIN` is zero and you defined a named pattern called `check_stack` in your ``md'` file, GNU CC will call that pattern with one argument which is the address to compare the stack value against. You must arrange for this pattern to report an error if the stack pointer is out of range.
3. If neither of the above are true, GNU CC will generate code to periodically "probe" the stack pointer using the values of the macros defined below.

Normally, you will use the default values of these macros, so GNU CC will use the third approach.

`STACK_CHECK_BUILTIN`
  A nonzero value if stack checking is done by the configuration files in a machine-dependent manner. You should define this macro if stack checking is require by the ABI of your machine or if you would like to have to stack checking in some more efficient way than GNU CC's portable approach. The default value of this macro is zero.
`STACK_CHECK_PROBE_INTERVAL`
  An integer representing the interval at which GNU CC must generate stack probe instructions. You will normally define this macro to be no larger than the size of the "guard pages" at the end of a stack area. The default value of 4096 is suitable for most systems.
`STACK_CHECK_PROBE_LOAD`
  A integer which is nonzero if GNU CC should perform the stack probe as a load instruction and zero if GNU CC should use a store instruction. The default is zero, which is the most efficient choice on most systems.
`STACK_CHECK_PROTECT`
  The number of bytes of stack needed to recover from a stack overflow, for languages where such a recovery is supported. The default value of 75 words should be adequate for most machines.
`STACK_CHECK_MAX_FRAME_SIZE`
  The maximum size of a stack frame, in bytes. GNU CC will generate probe instructions in non-leaf functions to ensure at least this many bytes of stack are available. If a stack frame is larger than this size, stack checking will not be reliable and GNU CC will issue a warning. The default is chosen so that GNU CC only generates one instruction on most systems. You should normally not change the default value of this macro.
`STACK_CHECK_FIXED_FRAME_SIZE`
  GNU CC uses this value to generate the above warning message. It represents the amount of fixed frame used by a function, not including space for any callee-saved registers, temporaries and user variables. You need only specify an upper bound for this amount and will normally use the default of four words.
`STACK_CHECK_MAX_VAR_SIZE`
  The maximum size, in bytes, of an object that GNU CC will place in the fixed area of the stack frame when the user specifies ``-fstack-check'`. GNU CC computed the default from the values of the above macros and you will normally not need to override that default.

### 17.7.3 Registers That Address the Stack Frame

This discusses registers that address the stack frame.

`STACK_POINTER_REGNUM`
  The register number of the stack pointer register, which must also be a fixed register according to `FIXED_REGISTERS`. On most machines, the hardware determines which register this is.
`FRAME_POINTER_REGNUM`
  The register number of the frame pointer register, which is used to access automatic variables in the stack frame. On some machines, the hardware determines which register this is. On other machines, you can choose any register you wish for this purpose.
`HARD_FRAME_POINTER_REGNUM`
  On some machines the offset between the frame pointer and starting offset of the automatic variables is not known until after register allocation has been done (for example, because the saved registers are between these two locations). On those machines, define `FRAME_POINTER_REGNUM` the number of a special, fixed register to be used internally until the offset is known, and define `HARD_FRAME_POINTER_REGNUM` to be the actual hard register number used for the frame pointer. You should define this macro only in the very rare circumstances when it is not possible to calculate the offset between the frame pointer and the automatic variables until after register allocation has been completed. When this macro is defined, you must also indicate in your definition of `ELIMINABLE_REGS` how to eliminate `FRAME_POINTER_REGNUM` into either `HARD_FRAME_POINTER_REGNUM` or `STACK_POINTER_REGNUM`. Do not define this macro if it would be the same as `FRAME_POINTER_REGNUM`.
`ARG_POINTER_REGNUM`
  The register number of the arg pointer register, which is used to access the function's argument list. On some machines, this is the same as the frame pointer register. On some machines, the hardware determines which register this is. On other machines, you can choose any register you wish for this purpose. If this is not the same register as the frame pointer register, then you must mark it as a fixed register according to `FIXED_REGISTERS`, or arrange to be able to eliminate it (see section 17.7.4 Eliminating Frame Pointer and Arg Pointer).
`RETURN_ADDRESS_POINTER_REGNUM`
  The register number of the return address pointer register, which is used to access the current function's return address from the stack. On some machines, the return address is not at a fixed offset from the frame pointer or stack pointer or argument pointer. This register can be defined to point to the return address on the stack, and then be converted by `ELIMINABLE_REGS` into either the frame pointer or stack pointer. Do not define this macro unless there is no other way to get the return address from the stack.
`STATIC_CHAIN_REGNUM`
`STATIC_CHAIN_INCOMING_REGNUM`
  Register numbers used for passing a function's static chain pointer. If register windows are used, the register number as seen by the called function is `STATIC_CHAIN_INCOMING_REGNUM`, while the register number as seen by the calling function is `STATIC_CHAIN_REGNUM`. If these registers are the same, `STATIC_CHAIN_INCOMING_REGNUM` need not be defined. The static chain register need not be a fixed register. If the static chain is passed in memory, these macros should not be defined; instead, the next two macros should be defined.
`STATIC_CHAIN`
`STATIC_CHAIN_INCOMING`
  If the static chain is passed in memory, these macros provide rtx giving `mem` expressions that denote where they are stored. `STATIC_CHAIN` and `STATIC_CHAIN_INCOMING` give the locations as seen by the calling and called functions, respectively. Often the former will be at an offset from the stack pointer and the latter at an offset from the frame pointer. The variables `stack_pointer_rtx`, `frame_pointer_rtx`, and `arg_pointer_rtx` will have been initialized prior to the use of these macros and should be used to refer to those items. If the static chain is passed in a register, the two previous macros should be defined instead.

### 17.7.4 Eliminating Frame Pointer and Arg Pointer

This is about eliminating the frame pointer and arg pointer.

`FRAME_POINTER_REQUIRED`
  A C expression which is nonzero if a function must have and use a frame pointer. This expression is evaluated in the reload pass. If its value is nonzero the function will have a frame pointer. The expression can in principle examine the current function and decide according to the facts, but on most machines the constant 0 or the constant 1 suffices. Use 0 when the machine allows code to be generated with no frame pointer, and doing so saves some time or space. Use 1 when there is no possible advantage to avoiding a frame pointer. In certain cases, the compiler does not know how to produce valid code without a frame pointer. The compiler recognizes those cases and automatically gives the function a frame pointer regardless of what `FRAME_POINTER_REQUIRED` says. You don't need to worry about them. In a function that does not require a frame pointer, the frame pointer register can be allocated for ordinary usage, unless you mark it as a fixed register. See `FIXED_REGISTERS` for more information.
`INITIAL_FRAME_POINTER_OFFSET (depth-var)`
  A C statement to store in the variable depth-var the difference between the frame pointer and the stack pointer values immediately after the function prologue. The value would be computed from information such as the result of `get_frame_size ()` and the tables of registers `regs_ever_live` and `call_used_regs`. If `ELIMINABLE_REGS` is defined, this macro will be not be used and need not be defined. Otherwise, it must be defined even if `FRAME_POINTER_REQUIRED` is defined to always be true; in that case, you may set depth-var to anything.
`ELIMINABLE_REGS`
  If defined, this macro specifies a table of register pairs used to eliminate unneeded registers that point into the stack frame. If it is not defined, the only elimination attempted by the compiler is to replace references to the frame pointer with references to the stack pointer. The definition of this macro is a list of structure initializations, each of which specifies an original and replacement register. On some machines, the position of the argument pointer is not known until the compilation is completed. In such a case, a separate hard register must be used for the argument pointer. This register can be eliminated by replacing it with either the frame pointer or the argument pointer, depending on whether or not the frame pointer has been eliminated. In this case, you might specify:

      #define ELIMINABLE_REGS  \
      {{ARG_POINTER_REGNUM, STACK_POINTER_REGNUM}, \
       {ARG_POINTER_REGNUM, FRAME_POINTER_REGNUM}, \
       {FRAME_POINTER_REGNUM, STACK_POINTER_REGNUM}}

  Note that the elimination of the argument pointer with the stack pointer is specified first since that is the preferred elimination.
`CAN_ELIMINATE (from-reg, to-reg)`
  A C expression that returns non-zero if the compiler is allowed to try to replace register number from-reg with register number to-reg. This macro need only be defined if `ELIMINABLE_REGS` is defined, and will usually be the constant 1, since most of the cases preventing register elimination are things that the compiler already knows about.
`INITIAL_ELIMINATION_OFFSET (from-reg, to-reg, offset-var)`
  This macro is similar to `INITIAL_FRAME_POINTER_OFFSET`. It specifies the initial difference between the specified pair of registers. This macro must be defined if `ELIMINABLE_REGS` is defined.
`LONGJMP_RESTORE_FROM_STACK`
  Define this macro if the `longjmp` function restores registers from the stack frames, rather than from those saved specifically by `setjmp`. Certain quantities must not be kept in registers across a call to `setjmp` on such machines.

### 17.7.5 Passing Function Arguments on the Stack

The macros in this section control how arguments are passed on the stack. See the following section for other macros that control passing certain arguments in registers.

`PROMOTE_PROTOTYPES`
  Define this macro if an argument declared in a prototype as an integral type smaller than `int` should actually be passed as an `int`. In addition to avoiding errors in certain cases of mismatch, it also makes for better code on certain machines.
`PUSH_ROUNDING (npushed)`
  A C expression that is the number of bytes actually pushed onto the stack when an instruction attempts to push npushed bytes. If the target machine does not have a push instruction, do not define this macro. That directs GNU CC to use an alternate strategy: to allocate the entire argument block and then store the arguments into it. On some machines, the definition

      #define PUSH_ROUNDING(BYTES) (BYTES)

  will suffice. But on other machines, instructions that appear to push one byte actually push two bytes in an attempt to maintain alignment. Then the definition should be

      #define PUSH_ROUNDING(BYTES) (((BYTES) + 1) & ~1)

`ACCUMULATE_OUTGOING_ARGS`
  If defined, the maximum amount of space required for outgoing arguments will be computed and placed into the variable `current_function_outgoing_args_size`. No space will be pushed onto the stack for each call; instead, the function prologue should increase the stack frame size by this amount. Defining both `PUSH_ROUNDING` and `ACCUMULATE_OUTGOING_ARGS` is not proper.
`REG_PARM_STACK_SPACE (fndecl)`
  Define this macro if functions should assume that stack space has been allocated for arguments even when their values are passed in registers. The value of this macro is the size, in bytes, of the area reserved for arguments passed in registers for the function represented by fndecl. This space can be allocated by the caller, or be a part of the machine-dependent stack frame: `OUTGOING_REG_PARM_STACK_SPACE` says which.
`MAYBE_REG_PARM_STACK_SPACE`
`FINAL_REG_PARM_STACK_SPACE (const_size, var_size)`
  Define these macros in addition to the one above if functions might allocate stack space for arguments even when their values are passed in registers. These should be used when the stack space allocated for arguments in registers is not a simple constant independent of the function declaration. The value of the first macro is the size, in bytes, of the area that we should initially assume would be reserved for arguments passed in registers. The value of the second macro is the actual size, in bytes, of the area that will be reserved for arguments passed in registers. This takes two arguments: an integer representing the number of bytes of fixed sized arguments on the stack, and a tree representing the number of bytes of variable sized arguments on the stack. When these macros are defined, `REG_PARM_STACK_SPACE` will only be called for libcall functions, the current function, or for a function being called when it is known that such stack space must be allocated. In each case this value can be easily computed. When deciding whether a called function needs such stack space, and how much space to reserve, GNU CC uses these two macros instead of `REG_PARM_STACK_SPACE`.
`OUTGOING_REG_PARM_STACK_SPACE`
  Define this if it is the responsibility of the caller to allocate the area reserved for arguments passed in registers. If `ACCUMULATE_OUTGOING_ARGS` is defined, this macro controls whether the space for these arguments counts in the value of `current_function_outgoing_args_size`.
`STACK_PARMS_IN_REG_PARM_AREA`
  Define this macro if `REG_PARM_STACK_SPACE` is defined, but the stack parameters don't skip the area specified by it. Normally, when a parameter is not passed in registers, it is placed on the stack beyond the `REG_PARM_STACK_SPACE` area. Defining this macro suppresses this behavior and causes the parameter to be passed on the stack in its natural location.
`RETURN_POPS_ARGS (fundecl, funtype, stack-size)`
  A C expression that should indicate the number of bytes of its own arguments that a function pops on returning, or 0 if the function pops no arguments and the caller must therefore pop them all after the function returns. fundecl is a C variable whose value is a tree node that describes the function in question. Normally it is a node of type `FUNCTION_DECL` that describes the declaration of the function. From this you can obtain the DECL_MACHINE_ATTRIBUTES of the function. funtype is a C variable whose value is a tree node that describes the function in question. Normally it is a node of type `FUNCTION_TYPE` that describes the data type of the function. From this it is possible to obtain the data types of the value and arguments (if known). When a call to a library function is being considered, fundecl will contain an identifier node for the library function. Thus, if you need to distinguish among various library functions, you can do so by their names. Note that "library function" in this context means a function used to perform arithmetic, whose name is known specially in the compiler and was not mentioned in the C code being compiled. stack-size is the number of bytes of arguments passed on the stack. If a variable number of bytes is passed, it is zero, and argument popping will always be the responsibility of the calling function. On the Vax, all functions always pop their arguments, so the definition of this macro is stack-size. On the 68000, using the standard calling convention, no functions pop their arguments, so the value of the macro is always 0 in this case. But an alternative calling convention is available in which functions that take a fixed number of arguments pop them but other functions (such as `printf`) pop nothing (the caller pops all). When this convention is in use, funtype is examined to determine whether a function takes a fixed number of arguments.

### 17.7.6 Passing Arguments in Registers

This section describes the macros which let you control how various types of arguments are passed in registers or how they are arranged in the stack.

`FUNCTION_ARG (cum, mode, type, named)`
  A C expression that controls whether a function argument is passed in a register, and which register. The arguments are cum, which summarizes all the previous arguments; mode, the machine mode of the argument; type, the data type of the argument as a tree node or 0 if that is not known (which happens for C support library functions); and named, which is 1 for an ordinary argument and 0 for nameless arguments that correspond to ``...'` in the called function's prototype. The value of the expression is usually either a `reg` RTX for the hard register in which to pass the argument, or zero to pass the argument on the stack. For machines like the Vax and 68000, where normally all arguments are pushed, zero suffices as a definition. The value of the expression can also be a `parallel` RTX. This is used when an argument is passed in multiple locations. The mode of the of the `parallel` should be the mode of the entire argument. The `parallel` holds any number of `expr_list` pairs; each one describes where part of the argument is passed. In each `expr_list`, the first operand can be either a `reg` RTX for the hard register in which to pass this part of the argument, or zero to pass the argument on the stack. If this operand is a `reg`, then the mode indicates how large this part of the argument is. The second operand of the `expr_list` is a `const_int` which gives the offset in bytes into the entire argument where this part starts. The usual way to make the ANSI library ``stdarg.h'` work on a machine where some arguments are usually passed in registers, is to cause nameless arguments to be passed on the stack instead. This is done by making `FUNCTION_ARG` return 0 whenever named is 0. You may use the macro `MUST_PASS_IN_STACK (mode, type)` in the definition of this macro to determine if this argument is of a type that must be passed in the stack. If `REG_PARM_STACK_SPACE` is not defined and `FUNCTION_ARG` returns non-zero for such an argument, the compiler will abort. If `REG_PARM_STACK_SPACE` is defined, the argument will be computed in the stack and then loaded into a register.
`MUST_PASS_IN_STACK (mode, type)`
  Define as a C expression that evaluates to nonzero if we do not know how to pass TYPE solely in registers. The file ``expr.h'` defines a definition that is usually appropriate, refer to ``expr.h'` for additional documentation.
`FUNCTION_INCOMING_ARG (cum, mode, type, named)`
  Define this macro if the target machine has "register windows", so that the register in which a function sees an arguments is not necessarily the same as the one in which the caller passed the argument. For such machines, `FUNCTION_ARG` computes the register in which the caller passes the value, and `FUNCTION_INCOMING_ARG` should be defined in a similar fashion to tell the function being called where the arguments will arrive. If `FUNCTION_INCOMING_ARG` is not defined, `FUNCTION_ARG` serves both purposes.
`FUNCTION_ARG_PARTIAL_NREGS (cum, mode, type, named)`
  A C expression for the number of words, at the beginning of an argument, must be put in registers. The value must be zero for arguments that are passed entirely in registers or that are entirely pushed on the stack. On some machines, certain arguments must be passed partially in registers and partially in memory. On these machines, typically the first n words of arguments are passed in registers, and the rest on the stack. If a multi-word argument (a `double` or a structure) crosses that boundary, its first few words must be passed in registers and the rest must be pushed. This macro tells the compiler when this occurs, and how many of the words should go in registers. `FUNCTION_ARG` for these arguments should return the first register to be used by the caller for this argument; likewise `FUNCTION_INCOMING_ARG`, for the called function.
`FUNCTION_ARG_PASS_BY_REFERENCE (cum, mode, type, named)`
  A C expression that indicates when an argument must be passed by reference. If nonzero for an argument, a copy of that argument is made in memory and a pointer to the argument is passed instead of the argument itself. The pointer is passed in whatever way is appropriate for passing a pointer to that type. On machines where `REG_PARM_STACK_SPACE` is not defined, a suitable definition of this macro might be

      #define FUNCTION_ARG_PASS_BY_REFERENCE\
      (CUM, MODE, TYPE, NAMED)  \
        MUST_PASS_IN_STACK (MODE, TYPE)

`FUNCTION_ARG_CALLEE_COPIES (cum, mode, type, named)`
  If defined, a C expression that indicates when it is the called function's responsibility to make a copy of arguments passed by invisible reference. Normally, the caller makes a copy and passes the address of the copy to the routine being called. When FUNCTION_ARG_CALLEE_COPIES is defined and is nonzero, the caller does not make a copy. Instead, it passes a pointer to the "live" value. The called function must not modify this value. If it can be determined that the value won't be modified, it need not make a copy; otherwise a copy must be made.
`FUNCTION_ARG_KEEP_AS_REFERENCE (cum, mode, type, named)`
  If defined, a C expression that indicates when it is more desirable to keep an argument passed by invisible reference as a reference, rather than copying it to a pseudo register.
`CUMULATIVE_ARGS`
  A C type for declaring a variable that is used as the first argument of `FUNCTION_ARG` and other related values. For some target machines, the type `int` suffices and can hold the number of bytes of argument so far. There is no need to record in `CUMULATIVE_ARGS` anything about the arguments that have been passed on the stack. The compiler has other variables to keep track of that. For target machines on which all arguments are passed on the stack, there is no need to store anything in `CUMULATIVE_ARGS`; however, the data structure must exist and should not be empty, so use `int`.
`INIT_CUMULATIVE_ARGS (cum, fntype, libname, indirect)`
  A C statement (sans semicolon) for initializing the variable cum for the state at the beginning of the argument list. The variable has type `CUMULATIVE_ARGS`. The value of fntype is the tree node for the data type of the function which will receive the args, or 0 if the args are to a compiler support library function. The value of indirect is nonzero when processing an indirect call, for example a call through a function pointer. The value of indirect is zero for a call to an explicitly named function, a library function call, or when `INIT_CUMULATIVE_ARGS` is used to find arguments for the function being compiled. When processing a call to a compiler support library function, libname identifies which one. It is a `symbol_ref` rtx which contains the name of the function, as a string. libname is 0 when an ordinary C function call is being processed. Thus, each time this macro is called, either libname or fntype is nonzero, but never both of them at once.
`INIT_CUMULATIVE_INCOMING_ARGS (cum, fntype, libname)`
  Like `INIT_CUMULATIVE_ARGS` but overrides it for the purposes of finding the arguments for the function being compiled. If this macro is undefined, `INIT_CUMULATIVE_ARGS` is used instead. The value passed for libname is always 0, since library routines with special calling conventions are never compiled with GNU CC. The argument libname exists for symmetry with `INIT_CUMULATIVE_ARGS`.
`FUNCTION_ARG_ADVANCE (cum, mode, type, named)`
  A C statement (sans semicolon) to update the summarizer variable cum to advance past an argument in the argument list. The values mode, type and named describe that argument. Once this is done, the variable cum is suitable for analyzing the following argument with `FUNCTION_ARG`, etc. This macro need not do anything if the argument in question was passed on the stack. The compiler knows how to track the amount of stack space used for arguments without any special help.
`FUNCTION_ARG_PADDING (mode, type)`
  If defined, a C expression which determines whether, and in which direction, to pad out an argument with extra space. The value should be of type `enum direction`: either `upward` to pad above the argument, `downward` to pad below, or `none` to inhibit padding. The amount of padding is always just enough to reach the next multiple of `FUNCTION_ARG_BOUNDARY`; this macro does not control it. This macro has a default definition which is right for most systems. For little-endian machines, the default is to pad upward. For big-endian machines, the default is to pad downward for an argument of constant size shorter than an `int`, and upward otherwise.
`FUNCTION_ARG_BOUNDARY (mode, type)`
  If defined, a C expression that gives the alignment boundary, in bits, of an argument with the specified mode and type. If it is not defined, `PARM_BOUNDARY` is used for all arguments.
`FUNCTION_ARG_REGNO_P (regno)`
  A C expression that is nonzero if regno is the number of a hard register in which function arguments are sometimes passed. This does not include implicit arguments such as the static chain and the structure-value address. On many machines, no registers can be used for this purpose since all function arguments are pushed on the stack.
`LOAD_ARGS_REVERSED`
  If defined, the order in which arguments are loaded into their respective argument registers is reversed so that the last argument is loaded first. This macro only effects arguments passed in registers.

### 17.7.7 How Scalar Function Values Are Returned

This section discusses the macros that control returning scalars as values--values that can fit in registers.

`TRADITIONAL_RETURN_FLOAT`
  Define this macro if ``-traditional'` should not cause functions declared to return `float` to convert the value to `double`.
`FUNCTION_VALUE (valtype, func)`
  A C expression to create an RTX representing the place where a function returns a value of data type valtype. valtype is a tree node representing a data type. Write `TYPE_MODE (valtype)` to get the machine mode used to represent that type. On many machines, only the mode is relevant. (Actually, on most machines, scalar values are returned in the same place regardless of mode). The value of the expression is usually a `reg` RTX for the hard register where the return value is stored. The value can also be a `parallel` RTX, if the return value is in multiple places. See `FUNCTION_ARG` for an explanation of the `parallel` form. If `PROMOTE_FUNCTION_RETURN` is defined, you must apply the same promotion rules specified in `PROMOTE_MODE` if valtype is a scalar type. If the precise function being called is known, func is a tree node (`FUNCTION_DECL`) for it; otherwise, func is a null pointer. This makes it possible to use a different value-returning convention for specific functions when all their calls are known. `FUNCTION_VALUE` is not used for return vales with aggregate data types, because these are returned in another way. See `STRUCT_VALUE_REGNUM` and related macros, below.
`FUNCTION_OUTGOING_VALUE (valtype, func)`
  Define this macro if the target machine has "register windows" so that the register in which a function returns its value is not the same as the one in which the caller sees the value. For such machines, `FUNCTION_VALUE` computes the register in which the caller will see the value. `FUNCTION_OUTGOING_VALUE` should be defined in a similar fashion to tell the function where to put the value. If `FUNCTION_OUTGOING_VALUE` is not defined, `FUNCTION_VALUE` serves both purposes. `FUNCTION_OUTGOING_VALUE` is not used for return vales with aggregate data types, because these are returned in another way. See `STRUCT_VALUE_REGNUM` and related macros, below.
`LIBCALL_VALUE (mode)`
  A C expression to create an RTX representing the place where a library function returns a value of mode mode. If the precise function being called is known, func is a tree node (`FUNCTION_DECL`) for it; otherwise, func is a null pointer. This makes it possible to use a different value-returning convention for specific functions when all their calls are known. Note that "library function" in this context means a compiler support routine, used to perform arithmetic, whose name is known specially by the compiler and was not mentioned in the C code being compiled. The definition of `LIBRARY_VALUE` need not be concerned aggregate data types, because none of the library functions returns such types.
`FUNCTION_VALUE_REGNO_P (regno)`
  A C expression that is nonzero if regno is the number of a hard register in which the values of called function may come back. A register whose use for returning values is limited to serving as the second of a pair (for a value of type `double`, say) need not be recognized by this macro. So for most machines, this definition suffices:

      #define FUNCTION_VALUE_REGNO_P(N) ((N) == 0)

  If the machine has register windows, so that the caller and the called function use different registers for the return value, this macro should recognize only the caller's register numbers.
`APPLY_RESULT_SIZE`
  Define this macro if ``untyped_call'` and ``untyped_return'` need more space than is implied by `FUNCTION_VALUE_REGNO_P` for saving and restoring an arbitrary return value.

### 17.7.8 How Large Values Are Returned

When a function value's mode is `BLKmode` (and in some other cases), the value is not returned according to `FUNCTION_VALUE` (see section 17.7.7 How Scalar Function Values Are Returned). Instead, the caller passes the address of a block of memory in which the value should be stored. This address is called the structure value address.

This section describes how to control returning structure values in memory.

`RETURN_IN_MEMORY (type)`
  A C expression which can inhibit the returning of certain function values in registers, based on the type of value. A nonzero value says to return the function value in memory, just as large structures are always returned. Here type will be a C expression of type `tree`, representing the data type of the value. Note that values of mode `BLKmode` must be explicitly handled by this macro. Also, the option ``-fpcc-struct-return'` takes effect regardless of this macro. On most systems, it is possible to leave the macro undefined; this causes a default definition to be used, whose value is the constant 1 for `BLKmode` values, and 0 otherwise. Do not use this macro to indicate that structures and unions should always be returned in memory. You should instead use `DEFAULT_PCC_STRUCT_RETURN` to indicate this.
`DEFAULT_PCC_STRUCT_RETURN`
  Define this macro to be 1 if all structure and union return values must be in memory. Since this results in slower code, this should be defined only if needed for compatibility with other compilers or with an ABI. If you define this macro to be 0, then the conventions used for structure and union return values are decided by the `RETURN_IN_MEMORY` macro. If not defined, this defaults to the value 1.
`STRUCT_VALUE_REGNUM`
  If the structure value address is passed in a register, then `STRUCT_VALUE_REGNUM` should be the number of that register.
`STRUCT_VALUE`
  If the structure value address is not passed in a register, define `STRUCT_VALUE` as an expression returning an RTX for the place where the address is passed. If it returns 0, the address is passed as an "invisible" first argument.
`STRUCT_VALUE_INCOMING_REGNUM`
  On some architectures the place where the structure value address is found by the called function is not the same place that the caller put it. This can be due to register windows, or it could be because the function prologue moves it to a different place. If the incoming location of the structure value address is in a register, define this macro as the register number.
`STRUCT_VALUE_INCOMING`
  If the incoming location is not a register, then you should define `STRUCT_VALUE_INCOMING` as an expression for an RTX for where the called function should find the value. If it should find the value on the stack, define this to create a `mem` which refers to the frame pointer. A definition of 0 means that the address is passed as an "invisible" first argument.
`PCC_STATIC_STRUCT_RETURN`
  Define this macro if the usual system convention on the target machine for returning structures and unions is for the called function to return the address of a static variable containing the value. Do not define this if the usual system convention is for the caller to pass an address to the subroutine. This macro has effect in ``-fpcc-struct-return'` mode, but it does nothing when you use ``-freg-struct-return'` mode.

### 17.7.9 Caller-Saves Register Allocation

If you enable it, GNU CC can save registers around function calls. This makes it possible to use call-clobbered registers to hold variables that must live across calls.

`DEFAULT_CALLER_SAVES`
  Define this macro if function calls on the target machine do not preserve any registers; in other words, if `CALL_USED_REGISTERS` has 1 for all registers. When defined, this macro enables ``-fcaller-saves'` by default for all optimization levels. It has no effect for optimization levels 2 and higher, where ``-fcaller-saves'` is the default.
`CALLER_SAVE_PROFITABLE (refs, calls)`
  A C expression to determine whether it is worthwhile to consider placing a pseudo-register in a call-clobbered hard register and saving and restoring it around each function call. The expression should be 1 when this is worth doing, and 0 otherwise. If you don't define this macro, a default is used which is good on most machines: `4 * calls < refs`.
`HARD_REGNO_CALLER_SAVE_MODE (regno, nregs)`
  A C expression specifying which mode is required for saving nregs of a pseudo-register in call-clobbered hard register regno. If regno is unsuitable for caller save, `VOIDmode` should be returned. For most machines this macro need not be defined since GCC will select the smallest suitable mode.

### 17.7.10 Function Entry and Exit

This section describes the macros that output function entry (prologue) and exit (epilogue) code.

`FUNCTION_PROLOGUE (file, size)`
  A C compound statement that outputs the assembler code for entry to a function. The prologue is responsible for setting up the stack frame, initializing the frame pointer register, saving registers that must be saved, and allocating size additional bytes of storage for the local variables. size is an integer. file is a stdio stream to which the assembler code should be output. The label for the beginning of the function need not be output by this macro. That has already been done when the macro is run. To determine which registers to save, the macro can refer to the array `regs_ever_live`: element r is nonzero if hard register r is used anywhere within the function. This implies the function prologue should save register r, provided it is not one of the call-used registers. (`FUNCTION_EPILOGUE` must likewise use `regs_ever_live`.) On machines that have "register windows", the function entry code does not save on the stack the registers that are in the windows, even if they are supposed to be preserved by function calls; instead it takes appropriate steps to "push" the register stack, if any non-call-used registers are used in the function. On machines where functions may or may not have frame-pointers, the function entry code must vary accordingly; it must set up the frame pointer if one is wanted, and not otherwise. To determine whether a frame pointer is in wanted, the macro can refer to the variable `frame_pointer_needed`. The variable's value will be 1 at run time in a function that needs a frame pointer. See section 17.7.4 Eliminating Frame Pointer and Arg Pointer. The function entry code is responsible for allocating any stack space required for the function. This stack space consists of the regions listed below. In most cases, these regions are allocated in the order listed, with the last listed region closest to the top of the stack (the lowest address if `STACK_GROWS_DOWNWARD` is defined, and the highest address if it is not defined). You can use a different order for a machine if doing so is more convenient or required for compatibility reasons. Except in cases where required by standard or by a debugger, there is no reason why the stack layout used by GCC need agree with that used by other compilers for a machine.
  - A region of `current_function_pretend_args_size` bytes of uninitialized space just underneath the first argument arriving on the stack. (This may not be at the very start of the allocated stack region if the calling sequence has pushed anything else since pushing the stack arguments. But usually, on such machines, nothing else has been pushed yet, because the function prologue itself does all the pushing.) This region is used on machines where an argument may be passed partly in registers and partly in memory, and, in some cases to support the features in ``varargs.h'` and ``stdargs.h'`.
  - An area of memory used to save certain registers used by the function. The size of this area, which may also include space for such things as the return address and pointers to previous stack frames, is machine-specific and usually depends on which registers have been used in the function. Machines with register windows often do not require a save area.
  - A region of at least size bytes, possibly rounded up to an allocation boundary, to contain the local variables of the function. On some machines, this region and the save area may occur in the opposite order, with the save area closer to the top of the stack.
  - Optionally, when `ACCUMULATE_OUTGOING_ARGS` is defined, a region of `current_function_outgoing_args_size` bytes to be used for outgoing argument lists of the function. See section 17.7.5 Passing Function Arguments on the Stack.
  Normally, it is necessary for the macros `FUNCTION_PROLOGUE` and `FUNCTION_EPILOGUE` to treat leaf functions specially. The C variable `leaf_function` is nonzero for such a function.
`EXIT_IGNORE_STACK`
  Define this macro as a C expression that is nonzero if the return instruction or the function epilogue ignores the value of the stack pointer; in other words, if it is safe to delete an instruction to adjust the stack pointer before a return from the function. Note that this macro's value is relevant only for functions for which frame pointers are maintained. It is never safe to delete a final stack adjustment in a function that has no frame pointer, and the compiler knows this regardless of `EXIT_IGNORE_STACK`.
`EPILOGUE_USES (regno)`
  Define this macro as a C expression that is nonzero for registers are used by the epilogue or the ``return'` pattern. The stack and frame pointer registers are already be assumed to be used as needed.
`FUNCTION_EPILOGUE (file, size)`
  A C compound statement that outputs the assembler code for exit from a function. The epilogue is responsible for restoring the saved registers and stack pointer to their values when the function was called, and returning control to the caller. This macro takes the same arguments as the macro `FUNCTION_PROLOGUE`, and the registers to restore are determined from `regs_ever_live` and `CALL_USED_REGISTERS` in the same way. On some machines, there is a single instruction that does all the work of returning from the function. On these machines, give that instruction the name ``return'` and do not define the macro `FUNCTION_EPILOGUE` at all. Do not define a pattern named ``return'` if you want the `FUNCTION_EPILOGUE` to be used. If you want the target switches to control whether return instructions or epilogues are used, define a ``return'` pattern with a validity condition that tests the target switches appropriately. If the ``return'` pattern's validity condition is false, epilogues will be used. On machines where functions may or may not have frame-pointers, the function exit code must vary accordingly. Sometimes the code for these two cases is completely different. To determine whether a frame pointer is wanted, the macro can refer to the variable `frame_pointer_needed`. The variable's value will be 1 when compiling a function that needs a frame pointer. Normally, `FUNCTION_PROLOGUE` and `FUNCTION_EPILOGUE` must treat leaf functions specially. The C variable `leaf_function` is nonzero for such a function. See section 17.5.4 Handling Leaf Functions. On some machines, some functions pop their arguments on exit while others leave that for the caller to do. For example, the 68020 when given ``-mrtd'` pops arguments in functions that take a fixed number of arguments. Your definition of the macro `RETURN_POPS_ARGS` decides which functions pop their own arguments. `FUNCTION_EPILOGUE` needs to know what was decided. The variable that is called `current_function_pops_args` is the number of bytes of its arguments that a function should pop. See section 17.7.7 How Scalar Function Values Are Returned.
`DELAY_SLOTS_FOR_EPILOGUE`
  Define this macro if the function epilogue contains delay slots to which instructions from the rest of the function can be "moved". The definition should be a C expression whose value is an integer representing the number of delay slots there.
`ELIGIBLE_FOR_EPILOGUE_DELAY (insn, n)`
  A C expression that returns 1 if insn can be placed in delay slot number n of the epilogue. The argument n is an integer which identifies the delay slot now being considered (since different slots may have different rules of eligibility). It is never negative and is always less than the number of epilogue delay slots (what `DELAY_SLOTS_FOR_EPILOGUE` returns). If you reject a particular insn for a given delay slot, in principle, it may be reconsidered for a subsequent delay slot. Also, other insns may (at least in principle) be considered for the so far unfilled delay slot. The insns accepted to fill the epilogue delay slots are put in an RTL list made with `insn_list` objects, stored in the variable `current_function_epilogue_delay_list`. The insn for the first delay slot comes first in the list. Your definition of the macro `FUNCTION_EPILOGUE` should fill the delay slots by outputting the insns in this list, usually by calling `final_scan_insn`. You need not define this macro if you did not define `DELAY_SLOTS_FOR_EPILOGUE`.
`ASM_OUTPUT_MI_THUNK (file, thunk_fndecl, delta, function)`
  A C compound statement that outputs the assembler code for a thunk function, used to implement C++ virtual function calls with multiple inheritance. The thunk acts as a wrapper around a virtual function, adjusting the implicit object parameter before handing control off to the real function. First, emit code to add the integer delta to the location that contains the incoming first argument. Assume that this argument contains a pointer, and is the one used to pass the `this` pointer in C++. This is the incoming argument before the function prologue, e.g. ``%o0'` on a sparc. The addition must preserve the values of all other incoming arguments. After the addition, emit code to jump to function, which is a `FUNCTION_DECL`. This is a direct pure jump, not a call, and does not touch the return address. Hence returning from FUNCTION will return to whoever called the current ``thunk'`. The effect must be as if function had been called directly with the adjusted first argument. This macro is responsible for emitting all of the code for a thunk function; `FUNCTION_PROLOGUE` and `FUNCTION_EPILOGUE` are not invoked. The thunk_fndecl is redundant. (delta and function have already been extracted from it.) It might possibly be useful on some targets, but probably not. If you do not define this macro, the target-independent code in the C++ frontend will generate a less efficient heavyweight thunk that calls function instead of jumping to it. The generic approach does not support varargs.

### 17.7.11 Generating Code for Profiling

These macros will help you generate code for profiling.

`FUNCTION_PROFILER (file, labelno)`
  A C statement or compound statement to output to file some assembler code to call the profiling subroutine `mcount`. Before calling, the assembler code must load the address of a counter variable into a register where `mcount` expects to find the address. The name of this variable is ``LP'` followed by the number labelno, so you would generate the name using ``LP%d'` in a `fprintf`. The details of how the address should be passed to `mcount` are determined by your operating system environment, not by GNU CC. To figure them out, compile a small program for profiling using the system's installed C compiler and look at the assembler code that results.
`PROFILE_BEFORE_PROLOGUE`
  Define this macro if the code for function profiling should come before the function prologue. Normally, the profiling code comes after.
`FUNCTION_BLOCK_PROFILER (file, labelno)`
  A C statement or compound statement to output to file some assembler code to initialize basic-block profiling for the current object module. The global compile flag `profile_block_flag` distinguishes two profile modes.
  `profile_block_flag != 2`
    Output code to call the subroutine `__bb_init_func` once per object module, passing it as its sole argument the address of a block allocated in the object module. The name of the block is a local symbol made with this statement:

        ASM_GENERATE_INTERNAL_LABEL (buffer, "LPBX", 0);

    Of course, since you are writing the definition of `ASM_GENERATE_INTERNAL_LABEL` as well as that of this macro, you can take a short cut in the definition of this macro and use the name that you know will result. The first word of this block is a flag which will be nonzero if the object module has already been initialized. So test this word first, and do not call `__bb_init_func` if the flag is nonzero. BLOCK_OR_LABEL contains a unique number which may be used to generate a label as a branch destination when `__bb_init_func` will not be called. Described in assembler language, the code to be output looks like:

          cmp (LPBX0),0
          bne local_label
          parameter1 <- LPBX0
          call __bb_init_func
        local_label:

  `profile_block_flag == 2`
    Output code to call the subroutine `__bb_init_trace_func` and pass two parameters to it. The first parameter is the same as for `__bb_init_func`. The second parameter is the number of the first basic block of the function as given by BLOCK_OR_LABEL. Note that `__bb_init_trace_func` has to be called, even if the object module has been initialized already. Described in assembler language, the code to be output looks like:

        parameter1 <- LPBX0
        parameter2 <- BLOCK_OR_LABEL
        call __bb_init_trace_func

`BLOCK_PROFILER (file, blockno)`
  A C statement or compound statement to output to file some assembler code to increment the count associated with the basic block number blockno. The global compile flag `profile_block_flag` distinguishes two profile modes.
  `profile_block_flag != 2`
    Output code to increment the counter directly. Basic blocks are numbered separately from zero within each compilation. The count associated with block number blockno is at index blockno in a vector of words; the name of this array is a local symbol made with this statement:

        ASM_GENERATE_INTERNAL_LABEL (buffer, "LPBX", 2);

    Of course, since you are writing the definition of `ASM_GENERATE_INTERNAL_LABEL` as well as that of this macro, you can take a short cut in the definition of this macro and use the name that you know will result. Described in assembler language, the code to be output looks like:

        inc (LPBX2+4*BLOCKNO)

  `profile_block_flag == 2`
    Output code to initialize the global structure `__bb` and call the function `__bb_trace_func`, which will increment the counter. `__bb` consists of two words. In the first word, the current basic block number, as given by BLOCKNO, has to be stored. In the second word, the address of a block allocated in the object module has to be stored. The address is given by the label created with this statement:

        ASM_GENERATE_INTERNAL_LABEL (buffer, "LPBX", 0);

    Described in assembler language, the code to be output looks like:

        move BLOCKNO -> (__bb)
        move LPBX0 -> (__bb+4)
        call __bb_trace_func

`FUNCTION_BLOCK_PROFILER_EXIT (file)`
  A C statement or compound statement to output to file assembler code to call function `__bb_trace_ret`. The assembler code should only be output if the global compile flag `profile_block_flag` == 2. This macro has to be used at every place where code for returning from a function is generated (e.g. `FUNCTION_EPILOGUE`). Although you have to write the definition of `FUNCTION_EPILOGUE` as well, you have to define this macro to tell the compiler, that the proper call to `__bb_trace_ret` is produced.
`MACHINE_STATE_SAVE (id)`
  A C statement or compound statement to save all registers, which may be clobbered by a function call, including condition codes. The `asm` statement will be mostly likely needed to handle this task. Local labels in the assembler code can be concatenated with the string id, to obtain a unique lable name. Registers or condition codes clobbered by `FUNCTION_PROLOGUE` or `FUNCTION_EPILOGUE` must be saved in the macros `FUNCTION_BLOCK_PROFILER`, `FUNCTION_BLOCK_PROFILER_EXIT` and `BLOCK_PROFILER` prior calling `__bb_init_trace_func`, `__bb_trace_ret` and `__bb_trace_func` respectively.
`MACHINE_STATE_RESTORE (id)`
  A C statement or compound statement to restore all registers, including condition codes, saved by `MACHINE_STATE_SAVE`. Registers or condition codes clobbered by `FUNCTION_PROLOGUE` or `FUNCTION_EPILOGUE` must be restored in the macros `FUNCTION_BLOCK_PROFILER`, `FUNCTION_BLOCK_PROFILER_EXIT` and `BLOCK_PROFILER` after calling `__bb_init_trace_func`, `__bb_trace_ret` and `__bb_trace_func` respectively.
`BLOCK_PROFILER_CODE`
  A C function or functions which are needed in the library to support block profiling.

## 17.8 Implementing the Varargs Macros

GNU CC comes with an implementation of ``varargs.h'` and ``stdarg.h'` that work without change on machines that pass arguments on the stack. Other machines require their own implementations of varargs, and the two machine independent header files must have conditionals to include it.

ANSI ``stdarg.h'` differs from traditional ``varargs.h'` mainly in the calling convention for `va_start`. The traditional implementation takes just one argument, which is the variable in which to store the argument pointer. The ANSI implementation of `va_start` takes an additional second argument. The user is supposed to write the last named argument of the function here.

However, `va_start` should not use this argument. The way to find the end of the named arguments is with the built-in functions described below.

`__builtin_saveregs ()`
  Use this built-in function to save the argument registers in memory so that the varargs mechanism can access them. Both ANSI and traditional versions of `va_start` must use `__builtin_saveregs`, unless you use `SETUP_INCOMING_VARARGS` (see below) instead. On some machines, `__builtin_saveregs` is open-coded under the control of the macro `EXPAND_BUILTIN_SAVEREGS`. On other machines, it calls a routine written in assembler language, found in ``libgcc2.c'`. Code generated for the call to `__builtin_saveregs` appears at the beginning of the function, as opposed to where the call to `__builtin_saveregs` is written, regardless of what the code is. This is because the registers must be saved before the function starts to use them for its own purposes.
`__builtin_args_info (category)`
  Use this built-in function to find the first anonymous arguments in registers. In general, a machine may have several categories of registers used for arguments, each for a particular category of data types. (For example, on some machines, floating-point registers are used for floating-point arguments while other arguments are passed in the general registers.) To make non-varargs functions use the proper calling convention, you have defined the `CUMULATIVE_ARGS` data type to record how many registers in each category have been used so far `__builtin_args_info` accesses the same data structure of type `CUMULATIVE_ARGS` after the ordinary argument layout is finished with it, with category specifying which word to access. Thus, the value indicates the first unused register in a given category. Normally, you would use `__builtin_args_info` in the implementation of `va_start`, accessing each category just once and storing the value in the `va_list` object. This is because `va_list` will have to update the values, and there is no way to alter the values accessed by `__builtin_args_info`.
`__builtin_next_arg (lastarg)`
  This is the equivalent of `__builtin_args_info`, for stack arguments. It returns the address of the first anonymous stack argument, as type `void *`. If `ARGS_GROW_DOWNWARD`, it returns the address of the location above the first anonymous stack argument. Use it in `va_start` to initialize the pointer for fetching arguments from the stack. Also use it in `va_start` to verify that the second parameter lastarg is the last named argument of the current function.
`__builtin_classify_type (object)`
  Since each machine has its own conventions for which data types are passed in which kind of register, your implementation of `va_arg` has to embody these conventions. The easiest way to categorize the specified data type is to use `__builtin_classify_type` together with `sizeof` and `__alignof__`. `__builtin_classify_type` ignores the value of object, considering only its data type. It returns an integer describing what kind of type that is--integer, floating, pointer, structure, and so on. The file ``typeclass.h'` defines an enumeration that you can use to interpret the values of `__builtin_classify_type`.

These machine description macros help implement varargs:

`EXPAND_BUILTIN_SAVEREGS (args)`
  If defined, is a C expression that produces the machine-specific code for a call to `__builtin_saveregs`. This code will be moved to the very beginning of the function, before any parameter access are made. The return value of this function should be an RTX that contains the value to use as the return of `__builtin_saveregs`. The argument args is a `tree_list` containing the arguments that were passed to `__builtin_saveregs`. If this macro is not defined, the compiler will output an ordinary call to the library function ``__builtin_saveregs'`.
`SETUP_INCOMING_VARARGS (args_so_far, mode, type,`
  pretend_args_size, second_time) This macro offers an alternative to using `__builtin_saveregs` and defining the macro `EXPAND_BUILTIN_SAVEREGS`. Use it to store the anonymous register arguments into the stack so that all the arguments appear to have been passed consecutively on the stack. Once this is done, you can use the standard implementation of varargs that works for machines that pass all their arguments on the stack. The argument args_so_far is the `CUMULATIVE_ARGS` data structure, containing the values that obtain after processing of the named arguments. The arguments mode and type describe the last named argument--its machine mode and its data type as a tree node. The macro implementation should do two things: first, push onto the stack all the argument registers not used for the named arguments, and second, store the size of the data thus pushed into the `int`-valued variable whose name is supplied as the argument pretend_args_size. The value that you store here will serve as additional offset for setting up the stack frame. Because you must generate code to push the anonymous arguments at compile time without knowing their data types, `SETUP_INCOMING_VARARGS` is only useful on machines that have just a single category of argument register and use it uniformly for all data types. If the argument second_time is nonzero, it means that the arguments of the function are being analyzed for the second time. This happens for an inline function, which is not actually compiled until the end of the source file. The macro `SETUP_INCOMING_VARARGS` should not generate any instructions in this case.
`STRICT_ARGUMENT_NAMING`
  Define this macro to be a nonzero value if the location where a function argument is passed depends on whether or not it is a named argument. This macro controls how the named argument to `FUNCTION_ARG` is set for varargs and stdarg functions. If this macro returns a nonzero value, the named argument is always true for named arguments, and false for unnamed arguments. If it returns a value of zero, but `SETUP_INCOMING_VARARGS` is defined, then all arguments are treated as named. Otherwise, all named arguments except the last are treated as named. You need not define this macro if it always returns zero.

## 17.9 Trampolines for Nested Functions

A trampoline is a small piece of code that is created at run time when the address of a nested function is taken. It normally resides on the stack, in the stack frame of the containing function. These macros tell GNU CC how to generate code to allocate and initialize a trampoline.

The instructions in the trampoline must do two things: load a constant address into the static chain register, and jump to the real address of the nested function. On CISC machines such as the m68k, this requires two instructions, a move immediate and a jump. Then the two addresses exist in the trampoline as word-long immediate operands. On RISC machines, it is often necessary to load each address into a register in two parts. Then pieces of each address form separate immediate operands.

The code generated to initialize the trampoline must store the variable parts--the static chain value and the function address--into the immediate operands of the instructions. On a CISC machine, this is simply a matter of copying each address to a memory reference at the proper offset from the start of the trampoline. On a RISC machine, it may be necessary to take out pieces of the address and store them separately.

`TRAMPOLINE_TEMPLATE (file)`
  A C statement to output, on the stream file, assembler code for a block of data that contains the constant parts of a trampoline. This code should not include a label--the label is taken care of automatically. If you do not define this macro, it means no template is needed for the target. Do not define this macro on systems where the block move code to copy the trampoline into place would be larger than the code to generate it on the spot.
`TRAMPOLINE_SECTION`
  The name of a subroutine to switch to the section in which the trampoline template is to be placed (see section 17.14 Dividing the Output into Sections (Texts, Data, ...)). The default is a value of ``readonly_data_section'`, which places the trampoline in the section containing read-only data.
`TRAMPOLINE_SIZE`
  A C expression for the size in bytes of the trampoline, as an integer.
`TRAMPOLINE_ALIGNMENT`
  Alignment required for trampolines, in bits. If you don't define this macro, the value of `BIGGEST_ALIGNMENT` is used for aligning trampolines.
`INITIALIZE_TRAMPOLINE (addr, fnaddr, static_chain)`
  A C statement to initialize the variable parts of a trampoline. addr is an RTX for the address of the trampoline; fnaddr is an RTX for the address of the nested function; static_chain is an RTX for the static chain value that should be passed to the function when it is called.
`ALLOCATE_TRAMPOLINE (fp)`
  A C expression to allocate run-time space for a trampoline. The expression value should be an RTX representing a memory reference to the space for the trampoline. If this macro is not defined, by default the trampoline is allocated as a stack slot. This default is right for most machines. The exceptions are machines where it is impossible to execute instructions in the stack area. On such machines, you may have to implement a separate stack, using this macro in conjunction with `FUNCTION_PROLOGUE` and `FUNCTION_EPILOGUE`. fp points to a data structure, a `struct function`, which describes the compilation status of the immediate containing function of the function which the trampoline is for. Normally (when `ALLOCATE_TRAMPOLINE` is not defined), the stack slot for the trampoline is in the stack frame of this containing function. Other allocation strategies probably must do something analogous with this information.

Implementing trampolines is difficult on many machines because they have separate instruction and data caches. Writing into a stack location fails to clear the memory in the instruction cache, so when the program jumps to that location, it executes the old contents.

Here are two possible solutions. One is to clear the relevant parts of the instruction cache whenever a trampoline is set up. The other is to make all trampolines identical, by having them jump to a standard subroutine. The former technique makes trampoline execution faster; the latter makes initialization faster.

To clear the instruction cache when a trampoline is initialized, define the following macros which describe the shape of the cache.

`INSN_CACHE_SIZE`
  The total size in bytes of the cache.
`INSN_CACHE_LINE_WIDTH`
  The length in bytes of each cache line. The cache is divided into cache lines which are disjoint slots, each holding a contiguous chunk of data fetched from memory. Each time data is brought into the cache, an entire line is read at once. The data loaded into a cache line is always aligned on a boundary equal to the line size.
`INSN_CACHE_DEPTH`
  The number of alternative cache lines that can hold any particular memory location.

Alternatively, if the machine has system calls or instructions to clear the instruction cache directly, you can define the following macro.

`CLEAR_INSN_CACHE (BEG, END)`
  If defined, expands to a C expression clearing the instruction cache in the specified interval. If it is not defined, and the macro INSN_CACHE_SIZE is defined, some generic code is generated to clear the cache. The definition of this macro would typically be a series of `asm` statements. Both BEG and END are both pointer expressions.

To use a standard subroutine, define the following macro. In addition, you must make sure that the instructions in a trampoline fill an entire cache line with identical instructions, or else ensure that the beginning of the trampoline code is always aligned at the same point in its cache line. Look in ``m68k.h'` as a guide.

`TRANSFER_FROM_TRAMPOLINE`
  Define this macro if trampolines need a special subroutine to do their work. The macro should expand to a series of `asm` statements which will be compiled with GNU CC. They go in a library function named `__transfer_from_trampoline`. If you need to avoid executing the ordinary prologue code of a compiled C function when you jump to the subroutine, you can do so by placing a special label of your own in the assembler code. Use one `asm` statement to generate an assembler label, and another to make the label global. Then trampolines can use that label to jump directly to your special assembler code.

## 17.10 Implicit Calls to Library Routines

Here is an explanation of implicit calls to library routines.

`MULSI3_LIBCALL`
  A C string constant giving the name of the function to call for multiplication of one signed full-word by another. If you do not define this macro, the default name is used, which is `__mulsi3`, a function defined in ``libgcc.a'`.
`DIVSI3_LIBCALL`
  A C string constant giving the name of the function to call for division of one signed full-word by another. If you do not define this macro, the default name is used, which is `__divsi3`, a function defined in ``libgcc.a'`.
`UDIVSI3_LIBCALL`
  A C string constant giving the name of the function to call for division of one unsigned full-word by another. If you do not define this macro, the default name is used, which is `__udivsi3`, a function defined in ``libgcc.a'`.
`MODSI3_LIBCALL`
  A C string constant giving the name of the function to call for the remainder in division of one signed full-word by another. If you do not define this macro, the default name is used, which is `__modsi3`, a function defined in ``libgcc.a'`.
`UMODSI3_LIBCALL`
  A C string constant giving the name of the function to call for the remainder in division of one unsigned full-word by another. If you do not define this macro, the default name is used, which is `__umodsi3`, a function defined in ``libgcc.a'`.
`MULDI3_LIBCALL`
  A C string constant giving the name of the function to call for multiplication of one signed double-word by another. If you do not define this macro, the default name is used, which is `__muldi3`, a function defined in ``libgcc.a'`.
`DIVDI3_LIBCALL`
  A C string constant giving the name of the function to call for division of one signed double-word by another. If you do not define this macro, the default name is used, which is `__divdi3`, a function defined in ``libgcc.a'`.
`UDIVDI3_LIBCALL`
  A C string constant giving the name of the function to call for division of one unsigned full-word by another. If you do not define this macro, the default name is used, which is `__udivdi3`, a function defined in ``libgcc.a'`.
`MODDI3_LIBCALL`
  A C string constant giving the name of the function to call for the remainder in division of one signed double-word by another. If you do not define this macro, the default name is used, which is `__moddi3`, a function defined in ``libgcc.a'`.
`UMODDI3_LIBCALL`
  A C string constant giving the name of the function to call for the remainder in division of one unsigned full-word by another. If you do not define this macro, the default name is used, which is `__umoddi3`, a function defined in ``libgcc.a'`.
`INIT_TARGET_OPTABS`
  Define this macro as a C statement that declares additional library routines renames existing ones. `init_optabs` calls this macro after initializing all the normal library routines.
`TARGET_EDOM`
  The value of `EDOM` on the target machine, as a C integer constant expression. If you don't define this macro, GNU CC does not attempt to deposit the value of `EDOM` into `errno` directly. Look in ``/usr/include/errno.h'` to find the value of `EDOM` on your system. If you do not define `TARGET_EDOM`, then compiled code reports domain errors by calling the library function and letting it report the error. If mathematical functions on your system use `matherr` when there is an error, then you should leave `TARGET_EDOM` undefined so that `matherr` is used normally.
`GEN_ERRNO_RTX`
  Define this macro as a C expression to create an rtl expression that refers to the global "variable" `errno`. (On certain systems, `errno` may not actually be a variable.) If you don't define this macro, a reasonable default is used.
`TARGET_MEM_FUNCTIONS`
  Define this macro if GNU CC should generate calls to the System V (and ANSI C) library functions `memcpy` and `memset` rather than the BSD functions `bcopy` and `bzero`.
`LIBGCC_NEEDS_DOUBLE`
  Define this macro if only `float` arguments cannot be passed to library routines (so they must be converted to `double`). This macro affects both how library calls are generated and how the library routines in ``libgcc1.c'` accept their arguments. It is useful on machines where floating and fixed point arguments are passed differently, such as the i860.
`FLOAT_ARG_TYPE`
  Define this macro to override the type used by the library routines to pick up arguments of type `float`. (By default, they use a union of `float` and `int`.) The obvious choice would be `float`---but that won't work with traditional C compilers that expect all arguments declared as `float` to arrive as `double`. To avoid this conversion, the library routines ask for the value as some other type and then treat it as a `float`. On some systems, no other type will work for this. For these systems, you must use `LIBGCC_NEEDS_DOUBLE` instead, to force conversion of the values `double` before they are passed.
`FLOATIFY (passed-value)`
  Define this macro to override the way library routines redesignate a `float` argument as a `float` instead of the type it was passed as. The default is an expression which takes the `float` field of the union.
`FLOAT_VALUE_TYPE`
  Define this macro to override the type used by the library routines to return values that ought to have type `float`. (By default, they use `int`.) The obvious choice would be `float`---but that won't work with traditional C compilers gratuitously convert values declared as `float` into `double`.
`INTIFY (float-value)`
  Define this macro to override the way the value of a `float`-returning library routine should be packaged in order to return it. These functions are actually declared to return type `FLOAT_VALUE_TYPE` (normally `int`). These values can't be returned as type `float` because traditional C compilers would gratuitously convert the value to a `double`. A local variable named `intify` is always available when the macro `INTIFY` is used. It is a union of a `float` field named `f` and a field named `i` whose type is `FLOAT_VALUE_TYPE` or `int`. If you don't define this macro, the default definition works by copying the value through that union.
`nongcc_SI_type`
  Define this macro as the name of the data type corresponding to `SImode` in the system's own C compiler. You need not define this macro if that type is `long int`, as it usually is.
`nongcc_word_type`
  Define this macro as the name of the data type corresponding to the word_mode in the system's own C compiler. You need not define this macro if that type is `long int`, as it usually is.
`perform_...`
  Define these macros to supply explicit C statements to carry out various arithmetic operations on types `float` and `double` in the library routines in ``libgcc1.c'`. See that file for a full list of these macros and their arguments. On most machines, you don't need to define any of these macros, because the C compiler that comes with the system takes care of doing them.
`NEXT_OBJC_RUNTIME`
  Define this macro to generate code for Objective C message sending using the calling convention of the NeXT system. This calling convention involves passing the object, the selector and the method arguments all at once to the method-lookup library function. The default calling convention passes just the object and the selector to the lookup function, which returns a pointer to the method.

## 17.11 Addressing Modes

This is about addressing modes.

`HAVE_POST_INCREMENT`
  A C expression that is nonzero the machine supports post-increment addressing.
`HAVE_PRE_INCREMENT`
`HAVE_POST_DECREMENT`
`HAVE_PRE_DECREMENT`
  Similar for other kinds of addressing.
`CONSTANT_ADDRESS_P (x)`
  A C expression that is 1 if the RTX x is a constant which is a valid address. On most machines, this can be defined as `CONSTANT_P (x)`, but a few machines are more restrictive in which constant addresses are supported. `CONSTANT_P` accepts integer-values expressions whose values are not explicitly known, such as `symbol_ref`, `label_ref`, and `high` expressions and `const` arithmetic expressions, in addition to `const_int` and `const_double` expressions.
`MAX_REGS_PER_ADDRESS`
  A number, the maximum number of registers that can appear in a valid memory address. Note that it is up to you to specify a value equal to the maximum number that `GO_IF_LEGITIMATE_ADDRESS` would ever accept.
`GO_IF_LEGITIMATE_ADDRESS (mode, x, label)`
  A C compound statement with a conditional `goto label;` executed if x (an RTX) is a legitimate memory address on the target machine for a memory operand of mode mode. It usually pays to define several simpler macros to serve as subroutines for this one. Otherwise it may be too complicated to understand. This macro must exist in two variants: a strict variant and a non-strict one. The strict variant is used in the reload pass. It must be defined so that any pseudo-register that has not been allocated a hard register is considered a memory reference. In contexts where some kind of register is required, a pseudo-register with no hard register must be rejected. The non-strict variant is used in other passes. It must be defined to accept all pseudo-registers in every context where some kind of register is required. Compiler source files that want to use the strict variant of this macro define the macro `REG_OK_STRICT`. You should use an `#ifdef REG_OK_STRICT` conditional to define the strict variant in that case and the non-strict variant otherwise. Subroutines to check for acceptable registers for various purposes (one for base registers, one for index registers, and so on) are typically among the subroutines used to define `GO_IF_LEGITIMATE_ADDRESS`. Then only these subroutine macros need have two variants; the higher levels of macros may be the same whether strict or not. Normally, constant addresses which are the sum of a `symbol_ref` and an integer are stored inside a `const` RTX to mark them as constant. Therefore, there is no need to recognize such sums specifically as legitimate addresses. Normally you would simply recognize any `const` as legitimate. Usually `PRINT_OPERAND_ADDRESS` is not prepared to handle constant sums that are not marked with `const`. It assumes that a naked `plus` indicates indexing. If so, then you must reject such naked constant sums as illegitimate addresses, so that none of them will be given to `PRINT_OPERAND_ADDRESS`. On some machines, whether a symbolic address is legitimate depends on the section that the address refers to. On these machines, define the macro `ENCODE_SECTION_INFO` to store the information into the `symbol_ref`, and then check for it here. When you see a `const`, you will have to look inside it to find the `symbol_ref` in order to determine the section. See section 17.16 Defining the Output Assembler Language. The best way to modify the name string is by adding text to the beginning, with suitable punctuation to prevent any ambiguity. Allocate the new name in `saveable_obstack`. You will have to modify `ASM_OUTPUT_LABELREF` to remove and decode the added text and output the name accordingly, and define `STRIP_NAME_ENCODING` to access the original name string. You can check the information stored here into the `symbol_ref` in the definitions of the macros `GO_IF_LEGITIMATE_ADDRESS` and `PRINT_OPERAND_ADDRESS`.
`REG_OK_FOR_BASE_P (x)`
  A C expression that is nonzero if x (assumed to be a `reg` RTX) is valid for use as a base register. For hard registers, it should always accept those which the hardware permits and reject the others. Whether the macro accepts or rejects pseudo registers must be controlled by `REG_OK_STRICT` as described above. This usually requires two variant definitions, of which `REG_OK_STRICT` controls the one actually used.
`REG_MODE_OK_FOR_BASE_P (x, mode)`
  A C expression that is just like `REG_OK_FOR_BASE_P`, except that that expression may examine the mode of the memory reference in mode. You should define this macro if the mode of the memory reference affects whether a register may be used as a base register. If you define this macro, the compiler will use it instead of `REG_OK_FOR_BASE_P`.
`REG_OK_FOR_INDEX_P (x)`
  A C expression that is nonzero if x (assumed to be a `reg` RTX) is valid for use as an index register. The difference between an index register and a base register is that the index register may be scaled. If an address involves the sum of two registers, neither one of them scaled, then either one may be labeled the "base" and the other the "index"; but whichever labeling is used must fit the machine's constraints of which registers may serve in each capacity. The compiler will try both labelings, looking for one that is valid, and will reload one or both registers only if neither labeling works.
`LEGITIMIZE_ADDRESS (x, oldx, mode, win)`
  A C compound statement that attempts to replace x with a valid memory address for an operand of mode mode. win will be a C statement label elsewhere in the code; the macro definition may use

      GO_IF_LEGITIMATE_ADDRESS (mode, x, win);

  to avoid further processing if the address has become legitimate. x will always be the result of a call to `break_out_memory_refs`, and oldx will be the operand that was given to that function to produce x. The code generated by this macro should not alter the substructure of x. If it transforms x into a more legitimate form, it should assign x (which will always be a C variable) a new value. It is not necessary for this macro to come up with a legitimate address. The compiler has standard ways of doing so in all cases. In fact, it is safe for this macro to do nothing. But often a machine-dependent strategy can generate better code.
`LEGITIMIZE_RELOAD_ADDRESS (x, mode, opnum, type, ind_levels, win)`
  A C compound statement that attempts to replace x, which is an address that needs reloading, with a valid memory address for an operand of mode mode. win will be a C statement label elsewhere in the code. It is not necessary to define this macro, but it might be useful for performance reasons. For example, on the i386, it is sometimes possible to use a single reload register instead of two by reloading a sum of two pseudo registers into a register. On the other hand, for number of RISC processors offsets are limited so that often an intermediate address needs to be generated in order to address a stack slot. By defining LEGITIMIZE_RELOAD_ADDRESS appropriately, the intermediate addresses generated for adjacent some stack slots can be made identical, and thus be shared. Note: This macro should be used with caution. It is necessary to know something of how reload works in order to effectively use this, and it is quite easy to produce macros that build in too much knowledge of reload internals. Note: This macro must be able to reload an address created by a previous invocation of this macro. If it fails to handle such addresses then the compiler may generate incorrect code or abort. The macro definition should use `push_reload` to indicate parts that need reloading; opnum, type and ind_levels are usually suitable to be passed unaltered to `push_reload`. The code generated by this macro must not alter the substructure of x. If it transforms x into a more legitimate form, it should assign x (which will always be a C variable) a new value. This also applies to parts that you change indirectly by calling `push_reload`. The macro definition may use `strict_memory_address_p` to test if the address has become legitimate. If you want to change only a part of x, one standard way of doing this is to use `copy_rtx`. Note, however, that is unshares only a single level of rtl. Thus, if the part to be changed is not at the top level, you'll need to replace first the top leve It is not necessary for this macro to come up with a legitimate address; but often a machine-dependent strategy can generate better code.
`GO_IF_MODE_DEPENDENT_ADDRESS (addr, label)`
  A C statement or compound statement with a conditional `goto label;` executed if memory address x (an RTX) can have different meanings depending on the machine mode of the memory reference it is used for or if the address is valid for some modes but not others. Autoincrement and autodecrement addresses typically have mode-dependent effects because the amount of the increment or decrement is the size of the operand being addressed. Some machines have other mode-dependent addresses. Many RISC machines have no mode-dependent addresses. You may assume that addr is a valid address for the machine.
`LEGITIMATE_CONSTANT_P (x)`
  A C expression that is nonzero if x is a legitimate constant for an immediate operand on the target machine. You can assume that x satisfies `CONSTANT_P`, so you need not check this. In fact, ``1'` is a suitable definition for this macro on machines where anything `CONSTANT_P` is valid.

## 17.12 Condition Code Status

This describes the condition code status.

The file ``conditions.h'` defines a variable `cc_status` to describe how the condition code was computed (in case the interpretation of the condition code depends on the instruction that it was set by). This variable contains the RTL expressions on which the condition code is currently based, and several standard flags.

Sometimes additional machine-specific flags must be defined in the machine description header file. It can also add additional machine-specific information by defining `CC_STATUS_MDEP`.

`CC_STATUS_MDEP`
  C code for a data type which is used for declaring the `mdep` component of `cc_status`. It defaults to `int`. This macro is not used on machines that do not use `cc0`.
`CC_STATUS_MDEP_INIT`
  A C expression to initialize the `mdep` field to "empty". The default definition does nothing, since most machines don't use the field anyway. If you want to use the field, you should probably define this macro to initialize it. This macro is not used on machines that do not use `cc0`.
`NOTICE_UPDATE_CC (exp, insn)`
  A C compound statement to set the components of `cc_status` appropriately for an insn insn whose body is exp. It is this macro's responsibility to recognize insns that set the condition code as a byproduct of other activity as well as those that explicitly set `(cc0)`. This macro is not used on machines that do not use `cc0`. If there are insns that do not set the condition code but do alter other machine registers, this macro must check to see whether they invalidate the expressions that the condition code is recorded as reflecting. For example, on the 68000, insns that store in address registers do not set the condition code, which means that usually `NOTICE_UPDATE_CC` can leave `cc_status` unaltered for such insns. But suppose that the previous insn set the condition code based on location ``a4@(102)'` and the current insn stores a new value in ``a4'`. Although the condition code is not changed by this, it will no longer be true that it reflects the contents of ``a4@(102)'`. Therefore, `NOTICE_UPDATE_CC` must alter `cc_status` in this case to say that nothing is known about the condition code value. The definition of `NOTICE_UPDATE_CC` must be prepared to deal with the results of peephole optimization: insns whose patterns are `parallel` RTXs containing various `reg`, `mem` or constants which are just the operands. The RTL structure of these insns is not sufficient to indicate what the insns actually do. What `NOTICE_UPDATE_CC` should do when it sees one is just to run `CC_STATUS_INIT`. A possible definition of `NOTICE_UPDATE_CC` is to call a function that looks at an attribute (see section 16.15 Instruction Attributes) named, for example, ``cc'`. This avoids having detailed information about patterns in two places, the ``md'` file and in `NOTICE_UPDATE_CC`.
`EXTRA_CC_MODES`
  A list of names to be used for additional modes for condition code values in registers (see section 16.10 Defining Jump Instruction Patterns). These names are added to `enum machine_mode` and all have class `MODE_CC`. By convention, they should start with ``CC'` and end with ``mode'`. You should only define this macro if your machine does not use `cc0` and only if additional modes are required.
`EXTRA_CC_NAMES`
  A list of C strings giving the names for the modes listed in `EXTRA_CC_MODES`. For example, the Sparc defines this macro and `EXTRA_CC_MODES` as

      #define EXTRA_CC_MODES CC_NOOVmode, CCFPmode, CCFPEmode
      #define EXTRA_CC_NAMES "CC_NOOV", "CCFP", "CCFPE"

  This macro is not required if `EXTRA_CC_MODES` is not defined.
`SELECT_CC_MODE (op, x, y)`
  Returns a mode from class `MODE_CC` to be used when comparison operation code op is applied to rtx x and y. For example, on the Sparc, `SELECT_CC_MODE` is defined as (see see section 16.10 Defining Jump Instruction Patterns for a description of the reason for this definition)

      #define SELECT_CC_MODE(OP,X,Y) \
        (GET_MODE_CLASS (GET_MODE (X)) == MODE_FLOAT          \
         ? ((OP == EQ || OP == NE) ? CCFPmode : CCFPEmode)    \
         : ((GET_CODE (X) == PLUS || GET_CODE (X) == MINUS    \
             || GET_CODE (X) == NEG) \
            ? CC_NOOVmode : CCmode))

  You need not define this macro if `EXTRA_CC_MODES` is not defined.
`CANONICALIZE_COMPARISON (code, op0, op1)`
  One some machines not all possible comparisons are defined, but you can convert an invalid comparison into a valid one. For example, the Alpha does not have a `GT` comparison, but you can use an `LT` comparison instead and swap the order of the operands. On such machines, define this macro to be a C statement to do any required conversions. code is the initial comparison code and op0 and op1 are the left and right operands of the comparison, respectively. You should modify code, op0, and op1 as required. GNU CC will not assume that the comparison resulting from this macro is valid but will see if the resulting insn matches a pattern in the ``md'` file. You need not define this macro if it would never change the comparison code or operands.
`REVERSIBLE_CC_MODE (mode)`
  A C expression whose value is one if it is always safe to reverse a comparison whose mode is mode. If `SELECT_CC_MODE` can ever return mode for a floating-point inequality comparison, then `REVERSIBLE_CC_MODE (mode)` must be zero. You need not define this macro if it would always returns zero or if the floating-point format is anything other than `IEEE_FLOAT_FORMAT`. For example, here is the definition used on the Sparc, where floating-point inequality comparisons are always given `CCFPEmode`:

      #define REVERSIBLE_CC_MODE(MODE)  ((MODE) != CCFPEmode)

## 17.13 Describing Relative Costs of Operations

These macros let you describe the relative speed of various operations on the target machine.

`CONST_COSTS (x, code, outer_code)`
  A part of a C `switch` statement that describes the relative costs of constant RTL expressions. It must contain `case` labels for expression codes `const_int`, `const`, `symbol_ref`, `label_ref` and `const_double`. Each case must ultimately reach a `return` statement to return the relative cost of the use of that kind of constant value in an expression. The cost may depend on the precise value of the constant, which is available for examination in x, and the rtx code of the expression in which it is contained, found in outer_code. code is the expression code--redundant, since it can be obtained with `GET_CODE (x)`.
`RTX_COSTS (x, code, outer_code)`
  Like `CONST_COSTS` but applies to nonconstant RTL expressions. This can be used, for example, to indicate how costly a multiply instruction is. In writing this macro, you can use the construct `COSTS_N_INSNS (n)` to specify a cost equal to n fast instructions. outer_code is the code of the expression in which x is contained. This macro is optional; do not define it if the default cost assumptions are adequate for the target machine.
`DEFAULT_RTX_COSTS (x, code, outer_code)`
  This macro, if defined, is called for any case not handled by the `RTX_COSTS` or `CONST_COSTS` macros. This eliminates the need to put case labels into the macro, but the code, or any functions it calls, must assume that the RTL in x could be of any type that has not already been handled. The arguments are the same as for `RTX_COSTS`, and the macro should execute a return statement giving the cost of any RTL expressions that it can handle. The default cost calculation is used for any RTL for which this macro does not return a value. This macro is optional; do not define it if the default cost assumptions are adequate for the target machine.
`ADDRESS_COST (address)`
  An expression giving the cost of an addressing mode that contains address. If not defined, the cost is computed from the address expression and the `CONST_COSTS` values. For most CISC machines, the default cost is a good approximation of the true cost of the addressing mode. However, on RISC machines, all instructions normally have the same length and execution time. Hence all addresses will have equal costs. In cases where more than one form of an address is known, the form with the lowest cost will be used. If multiple forms have the same, lowest, cost, the one that is the most complex will be used. For example, suppose an address that is equal to the sum of a register and a constant is used twice in the same basic block. When this macro is not defined, the address will be computed in a register and memory references will be indirect through that register. On machines where the cost of the addressing mode containing the sum is no higher than that of a simple indirect reference, this will produce an additional instruction and possibly require an additional register. Proper specification of this macro eliminates this overhead for such machines. Similar use of this macro is made in strength reduction of loops. address need not be valid as an address. In such a case, the cost is not relevant and can be any value; invalid addresses need not be assigned a different cost. On machines where an address involving more than one register is as cheap as an address computation involving only one register, defining `ADDRESS_COST` to reflect this can cause two registers to be live over a region of code where only one would have been if `ADDRESS_COST` were not defined in that manner. This effect should be considered in the definition of this macro. Equivalent costs should probably only be given to addresses with different numbers of registers on machines with lots of registers. This macro will normally either not be defined or be defined as a constant.
`REGISTER_MOVE_COST (from, to)`
  A C expression for the cost of moving data from a register in class from to one in class to. The classes are expressed using the enumeration values such as `GENERAL_REGS`. A value of 2 is the default; other values are interpreted relative to that. It is not required that the cost always equal 2 when from is the same as to; on some machines it is expensive to move between registers if they are not general registers. If reload sees an insn consisting of a single `set` between two hard registers, and if `REGISTER_MOVE_COST` applied to their classes returns a value of 2, reload does not check to ensure that the constraints of the insn are met. Setting a cost of other than 2 will allow reload to verify that the constraints are met. You should do this if the ``movm'` pattern's constraints do not allow such copying.
`MEMORY_MOVE_COST (mode, class, in)`
  A C expression for the cost of moving data of mode mode between a register of class class and memory; in is zero if the value is to be written to memory, non-zero if it is to be read in. This cost is relative to those in `REGISTER_MOVE_COST`. If moving between registers and memory is more expensive than between two registers, you should define this macro to express the relative cost. If you do not define this macro, GNU CC uses a default cost of 4 plus the cost of copying via a secondary reload register, if one is needed. If your machine requires a secondary reload register to copy between memory and a register of class but the reload mechanism is more complex than copying via an intermediate, define this macro to reflect the actual cost of the move. GNU CC defines the function `memory_move_secondary_cost` if secondary reloads are needed. It computes the costs due to copying via a secondary register. If your machine copies from memory using a secondary register in the conventional way but the default base value of 4 is not correct for your machine, define this macro to add some other value to the result of that function. The arguments to that function are the same as to this macro.
`BRANCH_COST`
  A C expression for the cost of a branch instruction. A value of 1 is the default; other values are interpreted relative to that.

Here are additional macros which do not specify precise relative costs, but only that certain actions are more expensive than GNU CC would ordinarily expect.

`SLOW_BYTE_ACCESS`
  Define this macro as a C expression which is nonzero if accessing less than a word of memory (i.e. a `char` or a `short`) is no faster than accessing a word of memory, i.e., if such access require more than one instruction or if there is no difference in cost between byte and (aligned) word loads. When this macro is not defined, the compiler will access a field by finding the smallest containing object; when it is defined, a fullword load will be used if alignment permits. Unless bytes accesses are faster than word accesses, using word accesses is preferable since it may eliminate subsequent memory access if subsequent accesses occur to other fields in the same word of the structure, but to different bytes.
`SLOW_ZERO_EXTEND`
  Define this macro if zero-extension (of a `char` or `short` to an `int`) can be done faster if the destination is a register that is known to be zero. If you define this macro, you must have instruction patterns that recognize RTL structures like this:

      (set (strict_low_part (subreg:QI (reg:SI ...) 0)) ...)

  and likewise for `HImode`.
`SLOW_UNALIGNED_ACCESS`
  Define this macro to be the value 1 if unaligned accesses have a cost many times greater than aligned accesses, for example if they are emulated in a trap handler. When this macro is non-zero, the compiler will act as if `STRICT_ALIGNMENT` were non-zero when generating code for block moves. This can cause significantly more instructions to be produced. Therefore, do not set this macro non-zero if unaligned accesses only add a cycle or two to the time for a memory access. If the value of this macro is always zero, it need not be defined.
`DONT_REDUCE_ADDR`
  Define this macro to inhibit strength reduction of memory addresses. (On some machines, such strength reduction seems to do harm rather than good.)
`MOVE_RATIO`
  The threshold of number of scalar memory-to-memory move insns, below which a sequence of insns should be generated instead of a string move insn or a library call. Increasing the value will always make code faster, but eventually incurs high cost in increased code size. Note that on machines with no memory-to-memory move insns, this macro denotes the corresponding number of memory-to-memory sequences. If you don't define this, a reasonable default is used.
`MOVE_BY_PIECES_P (size, alignment)`
  A C expression used to determine whether `move_by_pieces` will be used to copy a chunk of memory, or whether some other block move mechanism will be used. Defaults to 1 if `move_by_pieces_ninsns` returns less than `MOVE_RATIO`.
`MOVE_MAX_PIECES`
  A C expression used by `move_by_pieces` to determine the largest unit a load or store used to copy memory is. Defaults to `MOVE_MAX`.
`USE_LOAD_POST_INCREMENT (mode)`
  A C expression used to determine whether a load postincrement is a good thing for `move_by_pieces` to use for a given mode. Defaults to the value of `HAVE_POST_INCREMENT`.
`USE_LOAD_PRE_INCREMENT (mode)`
  A C expression used to determine whether a load preincrement is a good thing for `move_by_pieces` to use for a given mode. Defaults to the value of `HAVE_PRE_INCREMENT`.
`USE_STORE_POST_INCREMENT (mode)`
  A C expression used to determine whether a store postincrement is a good thing for `move_by_pieces` to use for a given mode. Defaults to the value of `HAVE_POST_INCREMENT`.
`USE_STORE_PRE_INCREMENT (mode)`
  This macro is used to determine whether a store preincrement is a good thing for `move_by_pieces` to use for a given mode. Defaults to the value of `HAVE_PRE_INCREMENT`.
`NO_FUNCTION_CSE`
  Define this macro if it is as good or better to call a constant function address than to call an address kept in a register.
`NO_RECURSIVE_FUNCTION_CSE`
  Define this macro if it is as good or better for a function to call itself with an explicit address than to call an address kept in a register.
`ADJUST_COST (insn, link, dep_insn, cost)`
  A C statement (sans semicolon) to update the integer variable cost based on the relationship between insn that is dependent on dep_insn through the dependence link. The default is to make no adjustment to cost. This can be used for example to specify to the scheduler that an output- or anti-dependence does not incur the same cost as a data-dependence.
`ADJUST_PRIORITY (insn)`
  A C statement (sans semicolon) to update the integer scheduling priority `INSN_PRIORITY(insn)`. Reduce the priority to execute the insn earlier, increase the priority to execute insn later. Do not define this macro if you do not need to adjust the scheduling priorities of insns.

## 17.14 Dividing the Output into Sections (Texts, Data, ...)

An object file is divided into sections containing different types of data. In the most common case, there are three sections: the text section, which holds instructions and read-only data; the data section, which holds initialized writable data; and the bss section, which holds uninitialized data. Some systems have other kinds of sections.

The compiler must tell the assembler when to switch sections. These macros control what commands to output to tell the assembler this. You can also define additional sections.

`TEXT_SECTION_ASM_OP`
  A C expression whose value is a string containing the assembler operation that should precede instructions and read-only data. Normally `".text"` is right.
`DATA_SECTION_ASM_OP`
  A C expression whose value is a string containing the assembler operation to identify the following data as writable initialized data. Normally `".data"` is right.
`SHARED_SECTION_ASM_OP`
  If defined, a C expression whose value is a string containing the assembler operation to identify the following data as shared data. If not defined, `DATA_SECTION_ASM_OP` will be used.
`BSS_SECTION_ASM_OP`
  If defined, a C expression whose value is a string containing the assembler operation to identify the following data as uninitialized global data. If not defined, and neither `ASM_OUTPUT_BSS` nor `ASM_OUTPUT_ALIGNED_BSS` are defined, uninitialized global data will be output in the data section if ``-fno-common'` is passed, otherwise `ASM_OUTPUT_COMMON` will be used.
`SHARED_BSS_SECTION_ASM_OP`
  If defined, a C expression whose value is a string containing the assembler operation to identify the following data as uninitialized global shared data. If not defined, and `BSS_SECTION_ASM_OP` is, the latter will be used.
`INIT_SECTION_ASM_OP`
  If defined, a C expression whose value is a string containing the assembler operation to identify the following data as initialization code. If not defined, GNU CC will assume such a section does not exist.
`EXTRA_SECTIONS`
  A list of names for sections other than the standard two, which are `in_text` and `in_data`. You need not define this macro on a system with no other sections (that GCC needs to use).
`EXTRA_SECTION_FUNCTIONS`
  One or more functions to be defined in ``varasm.c'`. These functions should do jobs analogous to those of `text_section` and `data_section`, for your additional sections. Do not define this macro if you do not define `EXTRA_SECTIONS`.
`READONLY_DATA_SECTION`
  On most machines, read-only variables, constants, and jump tables are placed in the text section. If this is not the case on your machine, this macro should be defined to be the name of a function (either `data_section` or a function defined in `EXTRA_SECTIONS`) that switches to the section to be used for read-only items. If these items should be placed in the text section, this macro should not be defined.
`SELECT_SECTION (exp, reloc)`
  A C statement or statements to switch to the appropriate section for output of exp. You can assume that exp is either a `VAR_DECL` node or a constant of some sort. reloc indicates whether the initial value of exp requires link-time relocations. Select the section by calling `text_section` or one of the alternatives for other sections. Do not define this macro if you put all read-only variables and constants in the read-only data section (usually the text section).
`SELECT_RTX_SECTION (mode, rtx)`
  A C statement or statements to switch to the appropriate section for output of rtx in mode mode. You can assume that rtx is some kind of constant in RTL. The argument mode is redundant except in the case of a `const_int` rtx. Select the section by calling `text_section` or one of the alternatives for other sections. Do not define this macro if you put all constants in the read-only data section.
`JUMP_TABLES_IN_TEXT_SECTION`
  Define this macro to be an expression with a non-zero value if jump tables (for `tablejump` insns) should be output in the text section, along with the assembler instructions. Otherwise, the readonly data section is used. This macro is irrelevant if there is no separate readonly data section.
`ENCODE_SECTION_INFO (decl)`
  Define this macro if references to a symbol must be treated differently depending on something about the variable or function named by the symbol (such as what section it is in). The macro definition, if any, is executed immediately after the rtl for decl has been created and stored in `DECL_RTL (decl)`. The value of the rtl will be a `mem` whose address is a `symbol_ref`. The usual thing for this macro to do is to record a flag in the `symbol_ref` (such as `SYMBOL_REF_FLAG`) or to store a modified name string in the `symbol_ref` (if one bit is not enough information).
`STRIP_NAME_ENCODING (var, sym_name)`
  Decode sym_name and store the real name part in var, sans the characters that encode section info. Define this macro if `ENCODE_SECTION_INFO` alters the symbol's name string.
`UNIQUE_SECTION_P (decl)`
  A C expression which evaluates to true if decl should be placed into a unique section for some target-specific reason. If you do not define this macro, the default is ``0'`. Note that the flag ``-ffunction-sections'` will also cause functions to be placed into unique sections.
`UNIQUE_SECTION (decl, reloc)`
  A C statement to build up a unique section name, expressed as a STRING_CST node, and assign it to ``DECL_SECTION_NAME (decl)'`. reloc indicates whether the initial value of exp requires link-time relocations. If you do not define this macro, GNU CC will use the symbol name prefixed by ``.'` as the section name.

## 17.15 Position Independent Code

This section describes macros that help implement generation of position independent code. Simply defining these macros is not enough to generate valid PIC; you must also add support to the macros `GO_IF_LEGITIMATE_ADDRESS` and `PRINT_OPERAND_ADDRESS`, as well as `LEGITIMIZE_ADDRESS`. You must modify the definition of ``movsi'` to do something appropriate when the source operand contains a symbolic address. You may also need to alter the handling of switch statements so that they use relative addresses.

`PIC_OFFSET_TABLE_REGNUM`
  The register number of the register used to address a table of static data addresses in memory. In some cases this register is defined by a processor's "application binary interface" (ABI). When this macro is defined, RTL is generated for this register once, as with the stack pointer and frame pointer registers. If this macro is not defined, it is up to the machine-dependent files to allocate such a register (if necessary).
`PIC_OFFSET_TABLE_REG_CALL_CLOBBERED`
  Define this macro if the register defined by `PIC_OFFSET_TABLE_REGNUM` is clobbered by calls. Do not define this macro if `PIC_OFFSET_TABLE_REGNUM` is not defined.
`FINALIZE_PIC`
  By generating position-independent code, when two different programs (A and B) share a common library (libC.a), the text of the library can be shared whether or not the library is linked at the same address for both programs. In some of these environments, position-independent code requires not only the use of different addressing modes, but also special code to enable the use of these addressing modes. The `FINALIZE_PIC` macro serves as a hook to emit these special codes once the function is being compiled into assembly code, but not before. (It is not done before, because in the case of compiling an inline function, it would lead to multiple PIC prologues being included in functions which used inline functions and were compiled to assembly language.)
`LEGITIMATE_PIC_OPERAND_P (x)`
  A C expression that is nonzero if x is a legitimate immediate operand on the target machine when generating position independent code. You can assume that x satisfies `CONSTANT_P`, so you need not check this. You can also assume flag_pic is true, so you need not check it either. You need not define this macro if all constants (including `SYMBOL_REF`) can be immediate operands when generating position independent code.

## 17.16 Defining the Output Assembler Language

This section describes macros whose principal purpose is to describe how to write instructions in assembler language--rather than what the instructions do.

### 17.16.1 The Overall Framework of an Assembler File

This describes the overall framework of an assembler file.

`ASM_FILE_START (stream)`
  A C expression which outputs to the stdio stream stream some appropriate text to go at the start of an assembler file. Normally this macro is defined to output a line containing ``#NO_APP'`, which is a comment that has no effect on most assemblers but tells the GNU assembler that it can save time by not checking for certain assembler constructs. On systems that use SDB, it is necessary to output certain commands; see ``attasm.h'`.
`ASM_FILE_END (stream)`
  A C expression which outputs to the stdio stream stream some appropriate text to go at the end of an assembler file. If this macro is not defined, the default is to output nothing special at the end of the file. Most systems don't require any definition. On systems that use SDB, it is necessary to output certain commands; see ``attasm.h'`.
`ASM_IDENTIFY_GCC (file)`
  A C statement to output assembler commands which will identify the object file as having been compiled with GNU CC (or another GNU compiler). If you don't define this macro, the string ``gcc_compiled.:'` is output. This string is calculated to define a symbol which, on BSD systems, will never be defined for any other reason. GDB checks for the presence of this symbol when reading the symbol table of an executable. On non-BSD systems, you must arrange communication with GDB in some other fashion. If GDB is not used on your system, you can define this macro with an empty body.
`ASM_COMMENT_START`
  A C string constant describing how to begin a comment in the target assembler language. The compiler assumes that the comment will end at the end of the line.
`ASM_APP_ON`
  A C string constant for text to be output before each `asm` statement or group of consecutive ones. Normally this is `"#APP"`, which is a comment that has no effect on most assemblers but tells the GNU assembler that it must check the lines that follow for all valid assembler constructs.
`ASM_APP_OFF`
  A C string constant for text to be output after each `asm` statement or group of consecutive ones. Normally this is `"#NO_APP"`, which tells the GNU assembler to resume making the time-saving assumptions that are valid for ordinary compiler output.
`ASM_OUTPUT_SOURCE_FILENAME (stream, name)`
  A C statement to output COFF information or DWARF debugging information which indicates that filename name is the current source file to the stdio stream stream. This macro need not be defined if the standard form of output for the file format in use is appropriate.
`OUTPUT_QUOTED_STRING (stream, name)`
  A C statement to output the string string to the stdio stream stream. If you do not call the function `output_quoted_string` in your config files, GNU CC will only call it to output filenames to the assembler source. So you can use it to canonicalize the format of the filename using this macro.
`ASM_OUTPUT_SOURCE_LINE (stream, line)`
  A C statement to output DBX or SDB debugging information before code for line number line of the current source file to the stdio stream stream. This macro need not be defined if the standard form of debugging information for the debugger in use is appropriate.
`ASM_OUTPUT_IDENT (stream, string)`
  A C statement to output something to the assembler file to handle a ``#ident'` directive containing the text string. If this macro is not defined, nothing is output for a ``#ident'` directive.
`ASM_OUTPUT_SECTION_NAME (stream, decl, name, reloc)`
  A C statement to output something to the assembler file to switch to section name for object decl which is either a `FUNCTION_DECL`, a `VAR_DECL` or `NULL_TREE`. reloc indicates whether the initial value of exp requires link-time relocations. Some target formats do not support arbitrary sections. Do not define this macro in such cases. At present this macro is only used to support section attributes. When this macro is undefined, section attributes are disabled.
`OBJC_PROLOGUE`
  A C statement to output any assembler statements which are required to precede any Objective C object definitions or message sending. The statement is executed only when compiling an Objective C program.

### 17.16.2 Output of Data

This describes data output.

`ASM_OUTPUT_LONG_DOUBLE (stream, value)`
`ASM_OUTPUT_DOUBLE (stream, value)`
`ASM_OUTPUT_FLOAT (stream, value)`
`ASM_OUTPUT_THREE_QUARTER_FLOAT (stream, value)`
`ASM_OUTPUT_SHORT_FLOAT (stream, value)`
`ASM_OUTPUT_BYTE_FLOAT (stream, value)`
  A C statement to output to the stdio stream stream an assembler instruction to assemble a floating-point constant of `TFmode`, `DFmode`, `SFmode`, `TQFmode`, `HFmode`, or `QFmode`, respectively, whose value is value. value will be a C expression of type `REAL_VALUE_TYPE`. Macros such as `REAL_VALUE_TO_TARGET_DOUBLE` are useful for writing these definitions.
`ASM_OUTPUT_QUADRUPLE_INT (stream, exp)`
`ASM_OUTPUT_DOUBLE_INT (stream, exp)`
`ASM_OUTPUT_INT (stream, exp)`
`ASM_OUTPUT_SHORT (stream, exp)`
`ASM_OUTPUT_CHAR (stream, exp)`
  A C statement to output to the stdio stream stream an assembler instruction to assemble an integer of 16, 8, 4, 2 or 1 bytes, respectively, whose value is value. The argument exp will be an RTL expression which represents a constant value. Use ``output_addr_const (stream, exp)'` to output this value as an assembler expression. For sizes larger than `UNITS_PER_WORD`, if the action of a macro would be identical to repeatedly calling the macro corresponding to a size of `UNITS_PER_WORD`, once for each word, you need not define the macro.
`ASM_OUTPUT_BYTE (stream, value)`
  A C statement to output to the stdio stream stream an assembler instruction to assemble a single byte containing the number value.
`ASM_BYTE_OP`
  A C string constant giving the pseudo-op to use for a sequence of single-byte constants. If this macro is not defined, the default is `"byte"`.
`ASM_OUTPUT_ASCII (stream, ptr, len)`
  A C statement to output to the stdio stream stream an assembler instruction to assemble a string constant containing the len bytes at ptr. ptr will be a C expression of type `char *` and len a C expression of type `int`. If the assembler has a `.ascii` pseudo-op as found in the Berkeley Unix assembler, do not define the macro `ASM_OUTPUT_ASCII`.
`CONSTANT_POOL_BEFORE_FUNCTION`
  You may define this macro as a C expression. You should define the expression to have a non-zero value if GNU CC should output the constant pool for a function before the code for the function, or a zero value if GNU CC should output the constant pool after the function. If you do not define this macro, the usual case, GNU CC will output the constant pool before the function.
`ASM_OUTPUT_POOL_PROLOGUE (file funname fundecl size)`
  A C statement to output assembler commands to define the start of the constant pool for a function. funname is a string giving the name of the function. Should the return type of the function be required, it can be obtained via fundecl. size is the size, in bytes, of the constant pool that will be written immediately after this call. If no constant-pool prefix is required, the usual case, this macro need not be defined.
`ASM_OUTPUT_SPECIAL_POOL_ENTRY (file, x, mode, align, labelno, jumpto)`
  A C statement (with or without semicolon) to output a constant in the constant pool, if it needs special treatment. (This macro need not do anything for RTL expressions that can be output normally.) The argument file is the standard I/O stream to output the assembler code on. x is the RTL expression for the constant to output, and mode is the machine mode (in case x is a ``const_int'`). align is the required alignment for the value x; you should output an assembler directive to force this much alignment. The argument labelno is a number to use in an internal label for the address of this pool entry. The definition of this macro is responsible for outputting the label definition at the proper place. Here is how to do this:

      ASM_OUTPUT_INTERNAL_LABEL (file, "LC", labelno);

  When you output a pool entry specially, you should end with a `goto` to the label jumpto. This will prevent the same pool entry from being output a second time in the usual manner. You need not define this macro if it would do nothing.
`CONSTANT_AFTER_FUNCTION_P (exp)`
  Define this macro as a C expression which is nonzero if the constant exp, of type `tree`, should be output after the code for a function. The compiler will normally output all constants before the function; you need not define this macro if this is OK.
`ASM_OUTPUT_POOL_EPILOGUE (file funname fundecl size)`
  A C statement to output assembler commands to at the end of the constant pool for a function. funname is a string giving the name of the function. Should the return type of the function be required, you can obtain it via fundecl. size is the size, in bytes, of the constant pool that GNU CC wrote immediately before this call. If no constant-pool epilogue is required, the usual case, you need not define this macro.
`IS_ASM_LOGICAL_LINE_SEPARATOR (C)`
  Define this macro as a C expression which is nonzero if C is used as a logical line separator by the assembler. If you do not define this macro, the default is that only the character ``;'` is treated as a logical line separator.
`ASM_OPEN_PAREN`
`ASM_CLOSE_PAREN`
  These macros are defined as C string constant, describing the syntax in the assembler for grouping arithmetic expressions. The following definitions are correct for most assemblers:

      #define ASM_OPEN_PAREN "("
      #define ASM_CLOSE_PAREN ")"

These macros are provided by ``real.h'` for writing the definitions of `ASM_OUTPUT_DOUBLE` and the like:

`REAL_VALUE_TO_TARGET_SINGLE (x, l)`
`REAL_VALUE_TO_TARGET_DOUBLE (x, l)`
`REAL_VALUE_TO_TARGET_LONG_DOUBLE (x, l)`
  These translate x, of type `REAL_VALUE_TYPE`, to the target's floating point representation, and store its bit pattern in the array of `long int` whose address is l. The number of elements in the output array is determined by the size of the desired target floating point data type: 32 bits of it go in each `long int` array element. Each array element holds 32 bits of the result, even if `long int` is wider than 32 bits on the host machine. The array element values are designed so that you can print them out using `fprintf` in the order they should appear in the target machine's memory.
`REAL_VALUE_TO_DECIMAL (x, format, string)`
  This macro converts x, of type `REAL_VALUE_TYPE`, to a decimal number and stores it as a string into string. You must pass, as string, the address of a long enough block of space to hold the result. The argument format is a `printf`-specification that serves as a suggestion for how to format the output string.

### 17.16.3 Output of Uninitialized Variables

Each of the macros in this section is used to do the whole job of outputting a single uninitialized variable.

`ASM_OUTPUT_COMMON (stream, name, size, rounded)`
  A C statement (sans semicolon) to output to the stdio stream stream the assembler definition of a common-label named name whose size is size bytes. The variable rounded is the size rounded up to whatever alignment the caller wants. Use the expression `assemble_name (stream, name)` to output the name itself; before and after that, output the additional assembler syntax for defining the name, and a newline. This macro controls how the assembler definitions of uninitialized common global variables are output.
`ASM_OUTPUT_ALIGNED_COMMON (stream, name, size, alignment)`
  Like `ASM_OUTPUT_COMMON` except takes the required alignment as a separate, explicit argument. If you define this macro, it is used in place of `ASM_OUTPUT_COMMON`, and gives you more flexibility in handling the required alignment of the variable. The alignment is specified as the number of bits.
`ASM_OUTPUT_DECL_COMMON (stream, decl, name, size, alignment)`
  Like `ASM_OUTPUT_ALIGNED_COMMON` except that it takes an additional argument - the decl of the variable to be output, if there is one. This macro can be called with decl == NULL_TREE. If you define this macro, it is used in place of both `ASM_OUTPUT_COMMON` and `ASM_OUTPUT_ALIGNED_COMMON`, and gives you more flexibility in handling the destination of the variable.
`ASM_OUTPUT_SHARED_COMMON (stream, name, size, rounded)`
  If defined, it is similar to `ASM_OUTPUT_COMMON`, except that it is used when name is shared. If not defined, `ASM_OUTPUT_COMMON` will be used.
`ASM_OUTPUT_BSS (stream, decl, name, size, rounded)`
  A C statement (sans semicolon) to output to the stdio stream stream the assembler definition of uninitialized global decl named name whose size is size bytes. The variable rounded is the size rounded up to whatever alignment the caller wants. Try to use function `asm_output_bss` defined in ``varasm.c'` when defining this macro. If unable, use the expression `assemble_name (stream, name)` to output the name itself; before and after that, output the additional assembler syntax for defining the name, and a newline. This macro controls how the assembler definitions of uninitialized global variables are output. This macro exists to properly support languages like `c++` which do not have `common` data. However, this macro currently is not defined for all targets. If this macro and `ASM_OUTPUT_ALIGNED_BSS` are not defined then `ASM_OUTPUT_COMMON` or `ASM_OUTPUT_ALIGNED_COMMON` or `ASM_OUTPUT_DECL_COMMON` is used.
`ASM_OUTPUT_ALIGNED_BSS (stream, decl, name, size, alignment)`
  Like `ASM_OUTPUT_BSS` except takes the required alignment as a separate, explicit argument. If you define this macro, it is used in place of `ASM_OUTPUT_BSS`, and gives you more flexibility in handling the required alignment of the variable. The alignment is specified as the number of bits. Try to use function `asm_output_aligned_bss` defined in file ``varasm.c'` when defining this macro.
`ASM_OUTPUT_SHARED_BSS (stream, decl, name, size, rounded)`
  If defined, it is similar to `ASM_OUTPUT_BSS`, except that it is used when name is shared. If not defined, `ASM_OUTPUT_BSS` will be used.
`ASM_OUTPUT_LOCAL (stream, name, size, rounded)`
  A C statement (sans semicolon) to output to the stdio stream stream the assembler definition of a local-common-label named name whose size is size bytes. The variable rounded is the size rounded up to whatever alignment the caller wants. Use the expression `assemble_name (stream, name)` to output the name itself; before and after that, output the additional assembler syntax for defining the name, and a newline. This macro controls how the assembler definitions of uninitialized static variables are output.
`ASM_OUTPUT_ALIGNED_LOCAL (stream, name, size, alignment)`
  Like `ASM_OUTPUT_LOCAL` except takes the required alignment as a separate, explicit argument. If you define this macro, it is used in place of `ASM_OUTPUT_LOCAL`, and gives you more flexibility in handling the required alignment of the variable. The alignment is specified as the number of bits.
`ASM_OUTPUT_DECL_LOCAL (stream, decl, name, size, alignment)`
  Like `ASM_OUTPUT_ALIGNED_LOCAL` except that it takes an additional parameter - the decl of variable to be output, if there is one. This macro can be called with decl == NULL_TREE. If you define this macro, it is used in place of `ASM_OUTPUT_LOCAL` and `ASM_OUTPUT_ALIGNED_LOCAL`, and gives you more flexibility in handling the destination of the variable.
`ASM_OUTPUT_SHARED_LOCAL (stream, name, size, rounded)`
  If defined, it is similar to `ASM_OUTPUT_LOCAL`, except that it is used when name is shared. If not defined, `ASM_OUTPUT_LOCAL` will be used.

### 17.16.4 Output and Generation of Labels

This is about outputting labels.

`ASM_OUTPUT_LABEL (stream, name)`
  A C statement (sans semicolon) to output to the stdio stream stream the assembler definition of a label named name. Use the expression `assemble_name (stream, name)` to output the name itself; before and after that, output the additional assembler syntax for defining the name, and a newline.
`ASM_DECLARE_FUNCTION_NAME (stream, name, decl)`
  A C statement (sans semicolon) to output to the stdio stream stream any text necessary for declaring the name name of a function which is being defined. This macro is responsible for outputting the label definition (perhaps using `ASM_OUTPUT_LABEL`). The argument decl is the `FUNCTION_DECL` tree node representing the function. If this macro is not defined, then the function name is defined in the usual manner as a label (by means of `ASM_OUTPUT_LABEL`).
`ASM_DECLARE_FUNCTION_SIZE (stream, name, decl)`
  A C statement (sans semicolon) to output to the stdio stream stream any text necessary for declaring the size of a function which is being defined. The argument name is the name of the function. The argument decl is the `FUNCTION_DECL` tree node representing the function. If this macro is not defined, then the function size is not defined.
`ASM_DECLARE_OBJECT_NAME (stream, name, decl)`
  A C statement (sans semicolon) to output to the stdio stream stream any text necessary for declaring the name name of an initialized variable which is being defined. This macro must output the label definition (perhaps using `ASM_OUTPUT_LABEL`). The argument decl is the `VAR_DECL` tree node representing the variable. If this macro is not defined, then the variable name is defined in the usual manner as a label (by means of `ASM_OUTPUT_LABEL`).
`ASM_FINISH_DECLARE_OBJECT (stream, decl, toplevel, atend)`
  A C statement (sans semicolon) to finish up declaring a variable name once the compiler has processed its initializer fully and thus has had a chance to determine the size of an array when controlled by an initializer. This is used on systems where it's necessary to declare something about the size of the object. If you don't define this macro, that is equivalent to defining it to do nothing.
`ASM_GLOBALIZE_LABEL (stream, name)`
  A C statement (sans semicolon) to output to the stdio stream stream some commands that will make the label name global; that is, available for reference from other files. Use the expression `assemble_name (stream, name)` to output the name itself; before and after that, output the additional assembler syntax for making that name global, and a newline.
`ASM_WEAKEN_LABEL`
  A C statement (sans semicolon) to output to the stdio stream stream some commands that will make the label name weak; that is, available for reference from other files but only used if no other definition is available. Use the expression `assemble_name (stream, name)` to output the name itself; before and after that, output the additional assembler syntax for making that name weak, and a newline. If you don't define this macro, GNU CC will not support weak symbols and you should not define the `SUPPORTS_WEAK` macro.
`SUPPORTS_WEAK`
  A C expression which evaluates to true if the target supports weak symbols. If you don't define this macro, ``defaults.h'` provides a default definition. If `ASM_WEAKEN_LABEL` is defined, the default definition is ``1'`; otherwise, it is ``0'`. Define this macro if you want to control weak symbol support with a compiler flag such as ``-melf'`.
`MAKE_DECL_ONE_ONLY`
  A C statement (sans semicolon) to mark decl to be emitted as a public symbol such that extra copies in multiple translation units will be discarded by the linker. Define this macro if your object file format provides support for this concept, such as the ``COMDAT'` section flags in the Microsoft Windows PE/COFF format, and this support requires changes to decl, such as putting it in a separate section.
`SUPPORTS_ONE_ONLY`
  A C expression which evaluates to true if the target supports one-only semantics. If you don't define this macro, ``varasm.c'` provides a default definition. If `MAKE_DECL_ONE_ONLY` is defined, the default definition is ``1'`; otherwise, it is ``0'`. Define this macro if you want to control one-only symbol support with a compiler flag, or if setting the `DECL_ONE_ONLY` flag is enough to mark a declaration to be emitted as one-only.
`ASM_OUTPUT_EXTERNAL (stream, decl, name)`
  A C statement (sans semicolon) to output to the stdio stream stream any text necessary for declaring the name of an external symbol named name which is referenced in this compilation but not defined. The value of decl is the tree node for the declaration. This macro need not be defined if it does not need to output anything. The GNU assembler and most Unix assemblers don't require anything.
`ASM_OUTPUT_EXTERNAL_LIBCALL (stream, symref)`
  A C statement (sans semicolon) to output on stream an assembler pseudo-op to declare a library function name external. The name of the library function is given by symref, which has type `rtx` and is a `symbol_ref`. This macro need not be defined if it does not need to output anything. The GNU assembler and most Unix assemblers don't require anything.
`ASM_OUTPUT_LABELREF (stream, name)`
  A C statement (sans semicolon) to output to the stdio stream stream a reference in assembler syntax to a label named name. This should add ``_'` to the front of the name, if that is customary on your operating system, as it is in most Berkeley Unix systems. This macro is used in `assemble_name`.
`ASM_OUTPUT_INTERNAL_LABEL (stream, prefix, num)`
  A C statement to output to the stdio stream stream a label whose name is made from the string prefix and the number num. It is absolutely essential that these labels be distinct from the labels used for user-level functions and variables. Otherwise, certain programs will have name conflicts with internal labels. It is desirable to exclude internal labels from the symbol table of the object file. Most assemblers have a naming convention for labels that should be excluded; on many systems, the letter ``L'` at the beginning of a label has this effect. You should find out what convention your system uses, and follow it. The usual definition of this macro is as follows:

      fprintf (stream, "L%s%d:\n", prefix, num)

`ASM_GENERATE_INTERNAL_LABEL (string, prefix, num)`
  A C statement to store into the string string a label whose name is made from the string prefix and the number num. This string, when output subsequently by `assemble_name`, should produce the output that `ASM_OUTPUT_INTERNAL_LABEL` would produce with the same prefix and num. If the string begins with ``*'`, then `assemble_name` will output the rest of the string unchanged. It is often convenient for `ASM_GENERATE_INTERNAL_LABEL` to use ``*'` in this way. If the string doesn't start with ``*'`, then `ASM_OUTPUT_LABELREF` gets to output the string, and may change it. (Of course, `ASM_OUTPUT_LABELREF` is also part of your machine description, so you should know what it does on your machine.)
`ASM_FORMAT_PRIVATE_NAME (outvar, name, number)`
  A C expression to assign to outvar (which is a variable of type `char *`) a newly allocated string made from the string name and the number number, with some suitable punctuation added. Use `alloca` to get space for the string. The string will be used as an argument to `ASM_OUTPUT_LABELREF` to produce an assembler label for an internal static variable whose name is name. Therefore, the string must be such as to result in valid assembler code. The argument number is different each time this macro is executed; it prevents conflicts between similarly-named internal static variables in different scopes. Ideally this string should not be a valid C identifier, to prevent any conflict with the user's own symbols. Most assemblers allow periods or percent signs in assembler symbols; putting at least one of these between the name and the number will suffice.
`ASM_OUTPUT_DEF (stream, name, value)`
  A C statement to output to the stdio stream stream assembler code which defines (equates) the symbol name to have the value value. If SET_ASM_OP is defined, a default definition is provided which is correct for most systems.
`ASM_OUTPUT_DEFINE_LABEL_DIFFERENCE_SYMBOL (stream, symbol, high, low)`
  A C statement to output to the stdio stream stream assembler code which defines (equates) the symbol symbol to have a value equal to the difference of the two symbols high and low, i.e. high minus low. GNU CC guarantees that the symbols high and low are already known by the assembler so that the difference resolves into a constant. If SET_ASM_OP is defined, a default definition is provided which is correct for most systems.
`ASM_OUTPUT_WEAK_ALIAS (stream, name, value)`
  A C statement to output to the stdio stream stream assembler code which defines (equates) the weak symbol name to have the value value. Define this macro if the target only supports weak aliases; define ASM_OUTPUT_DEF instead if possible.
`OBJC_GEN_METHOD_LABEL (buf, is_inst, class_name, cat_name, sel_name)`
  Define this macro to override the default assembler names used for Objective C methods. The default name is a unique method number followed by the name of the class (e.g. ``_1_Foo'`). For methods in categories, the name of the category is also included in the assembler name (e.g. ``_1_Foo_Bar'`). These names are safe on most systems, but make debugging difficult since the method's selector is not present in the name. Therefore, particular systems define other ways of computing names. buf is an expression of type `char *` which gives you a buffer in which to store the name; its length is as long as class_name, cat_name and sel_name put together, plus 50 characters extra. The argument is_inst specifies whether the method is an instance method or a class method; class_name is the name of the class; cat_name is the name of the category (or NULL if the method is not in a category); and sel_name is the name of the selector. On systems where the assembler can handle quoted names, you can use this macro to provide more human-readable names.

### 17.16.5 How Initialization Functions Are Handled

The compiled code for certain languages includes constructors (also called initialization routines)---functions to initialize data in the program when the program is started. These functions need to be called before the program is "started"---that is to say, before `main` is called.

Compiling some languages generates destructors (also called termination routines) that should be called when the program terminates.

To make the initialization and termination functions work, the compiler must output something in the assembler code to cause those functions to be called at the appropriate time. When you port the compiler to a new system, you need to specify how to do this.

There are two major ways that GCC currently supports the execution of initialization and termination functions. Each way has two variants. Much of the structure is common to all four variations.

The linker must build two lists of these functions--a list of initialization functions, called `__CTOR_LIST__`, and a list of termination functions, called `__DTOR_LIST__`.

Each list always begins with an ignored function pointer (which may hold 0, -1, or a count of the function pointers after it, depending on the environment). This is followed by a series of zero or more function pointers to constructors (or destructors), followed by a function pointer containing zero.

Depending on the operating system and its executable file format, either ``crtstuff.c'` or ``libgcc2.c'` traverses these lists at startup time and exit time. Constructors are called in reverse order of the list; destructors in forward order.

The best way to handle static constructors works only for object file formats which provide arbitrarily-named sections. A section is set aside for a list of constructors, and another for a list of destructors. Traditionally these are called ``.ctors'` and ``.dtors'`. Each object file that defines an initialization function also puts a word in the constructor section to point to that function. The linker accumulates all these words into one contiguous ``.ctors'` section. Termination functions are handled similarly.

To use this method, you need appropriate definitions of the macros `ASM_OUTPUT_CONSTRUCTOR` and `ASM_OUTPUT_DESTRUCTOR`. Usually you can get them by including ``svr4.h'`.

When arbitrary sections are available, there are two variants, depending upon how the code in ``crtstuff.c'` is called. On systems that support an init section which is executed at program startup, parts of ``crtstuff.c'` are compiled into that section. The program is linked by the `gcc` driver like this:

    ld -o output_file crtbegin.o ... crtend.o -lgcc

The head of a function (`__do_global_ctors`) appears in the init section of ``crtbegin.o'`; the remainder of the function appears in the init section of ``crtend.o'`. The linker will pull these two parts of the section together, making a whole function. If any of the user's object files linked into the middle of it contribute code, then that code will be executed as part of the body of `__do_global_ctors`.

To use this variant, you must define the `INIT_SECTION_ASM_OP` macro properly.

If no init section is available, do not define `INIT_SECTION_ASM_OP`. Then `__do_global_ctors` is built into the text section like all other functions, and resides in ``libgcc.a'`. When GCC compiles any function called `main`, it inserts a procedure call to `__main` as the first executable code after the function prologue. The `__main` function, also defined in ``libgcc2.c'`, simply calls ``__do_global_ctors'`.

In file formats that don't support arbitrary sections, there are again two variants. In the simplest variant, the GNU linker (GNU `ld`) and an `a.out' format must be used. In this case, `ASM_OUTPUT_CONSTRUCTOR` is defined to produce a `.stabs` entry of type ``N_SETT'`, referencing the name `__CTOR_LIST__`, and with the address of the void function containing the initialization code as its value. The GNU linker recognizes this as a request to add the value to a "set"; the values are accumulated, and are eventually placed in the executable as a vector in the format described above, with a leading (ignored) count and a trailing zero element. `ASM_OUTPUT_DESTRUCTOR` is handled similarly. Since no init section is available, the absence of `INIT_SECTION_ASM_OP` causes the compilation of `main` to call `__main` as above, starting the initialization process.

The last variant uses neither arbitrary sections nor the GNU linker. This is preferable when you want to do dynamic linking and when using file formats which the GNU linker does not support, such as `ECOFF'. In this case, `ASM_OUTPUT_CONSTRUCTOR` does not produce an `N_SETT` symbol; initialization and termination functions are recognized simply by their names. This requires an extra program in the linkage step, called `collect2`. This program pretends to be the linker, for use with GNU CC; it does its job by running the ordinary linker, but also arranges to include the vectors of initialization and termination functions. These functions are called via `__main` as described above.

Choosing among these configuration options has been simplified by a set of operating-system-dependent files in the ``config'` subdirectory. These files define all of the relevant parameters. Usually it is sufficient to include one into your specific machine-dependent configuration file. These files are:

``aoutos.h'`
  For operating systems using the `a.out' format.
``next.h'`
  For operating systems using the `MachO' format.
``svr3.h'`
  For System V Release 3 and similar systems using `COFF' format.
``svr4.h'`
  For System V Release 4 and similar systems using `ELF' format.
``vms.h'`
  For the VMS operating system.

### 17.16.6 Macros Controlling Initialization Routines

Here are the macros that control how the compiler handles initialization and termination functions:

`INIT_SECTION_ASM_OP`
  If defined, a C string constant for the assembler operation to identify the following data as initialization code. If not defined, GNU CC will assume such a section does not exist. When you are using special sections for initialization and termination functions, this macro also controls how ``crtstuff.c'` and ``libgcc2.c'` arrange to run the initialization functions.
`HAS_INIT_SECTION`
  If defined, `main` will not call `__main` as described above. This macro should be defined for systems that control the contents of the init section on a symbol-by-symbol basis, such as OSF/1, and should not be defined explicitly for systems that support `INIT_SECTION_ASM_OP`.
`LD_INIT_SWITCH`
  If defined, a C string constant for a switch that tells the linker that the following symbol is an initialization routine.
`LD_FINI_SWITCH`
  If defined, a C string constant for a switch that tells the linker that the following symbol is a finalization routine.
`INVOKE__main`
  If defined, `main` will call `__main` despite the presence of `INIT_SECTION_ASM_OP`. This macro should be defined for systems where the init section is not actually run automatically, but is still useful for collecting the lists of constructors and destructors.
`ASM_OUTPUT_CONSTRUCTOR (stream, name)`
  Define this macro as a C statement to output on the stream stream the assembler code to arrange to call the function named name at initialization time. Assume that name is the name of a C function generated automatically by the compiler. This function takes no arguments. Use the function `assemble_name` to output the name name; this performs any system-specific syntactic transformations such as adding an underscore. If you don't define this macro, nothing special is output to arrange to call the function. This is correct when the function will be called in some other manner--for example, by means of the `collect2` program, which looks through the symbol table to find these functions by their names.
`ASM_OUTPUT_DESTRUCTOR (stream, name)`
  This is like `ASM_OUTPUT_CONSTRUCTOR` but used for termination functions rather than initialization functions.

If your system uses `collect2` as the means of processing constructors, then that program normally uses `nm` to scan an object file for constructor functions to be called. On certain kinds of systems, you can define these macros to make `collect2` work faster (and, in some cases, make it work at all):

`OBJECT_FORMAT_COFF`
  Define this macro if the system uses COFF (Common Object File Format) object files, so that `collect2` can assume this format and scan object files directly for dynamic constructor/destructor functions.
`OBJECT_FORMAT_ROSE`
  Define this macro if the system uses ROSE format object files, so that `collect2` can assume this format and scan object files directly for dynamic constructor/destructor functions. These macros are effective only in a native compiler; `collect2` as part of a cross compiler always uses `nm` for the target machine.
`REAL_NM_FILE_NAME`
  Define this macro as a C string constant containing the file name to use to execute `nm`. The default is to search the path normally for `nm`. If your system supports shared libraries and has a program to list the dynamic dependencies of a given library or executable, you can define these macros to enable support for running initialization and termination functions in shared libraries:
`LDD_SUFFIX`
  Define this macro to a C string constant containing the name of the program which lists dynamic dependencies, like `"ldd"` under SunOS 4.
`PARSE_LDD_OUTPUT (PTR)`
  Define this macro to be C code that extracts filenames from the output of the program denoted by `LDD_SUFFIX`. PTR is a variable of type `char *` that points to the beginning of a line of output from `LDD_SUFFIX`. If the line lists a dynamic dependency, the code must advance PTR to the beginning of the filename on that line. Otherwise, it must set PTR to `NULL`.

### 17.16.7 Output of Assembler Instructions

This describes assembler instruction output.

`REGISTER_NAMES`
  A C initializer containing the assembler's names for the machine registers, each one as a C string constant. This is what translates register numbers in the compiler into assembler language.
`ADDITIONAL_REGISTER_NAMES`
  If defined, a C initializer for an array of structures containing a name and a register number. This macro defines additional names for hard registers, thus allowing the `asm` option in declarations to refer to registers using alternate names.
`ASM_OUTPUT_OPCODE (stream, ptr)`
  Define this macro if you are using an unusual assembler that requires different names for the machine instructions. The definition is a C statement or statements which output an assembler instruction opcode to the stdio stream stream. The macro-operand ptr is a variable of type `char *` which points to the opcode name in its "internal" form--the form that is written in the machine description. The definition should output the opcode name to stream, performing any translation you desire, and increment the variable ptr to point at the end of the opcode so that it will not be output twice. In fact, your macro definition may process less than the entire opcode name, or more than the opcode name; but if you want to process text that includes ``%'`-sequences to substitute operands, you must take care of the substitution yourself. Just be sure to increment ptr over whatever text should not be output normally. If you need to look at the operand values, they can be found as the elements of `recog_operand`. If the macro definition does nothing, the instruction is output in the usual way.
`FINAL_PRESCAN_INSN (insn, opvec, noperands)`
  If defined, a C statement to be executed just prior to the output of assembler code for insn, to modify the extracted operands so they will be output differently. Here the argument opvec is the vector containing the operands extracted from insn, and noperands is the number of elements of the vector which contain meaningful data for this insn. The contents of this vector are what will be used to convert the insn template into assembler code, so you can change the assembler output by changing the contents of the vector. This macro is useful when various assembler syntaxes share a single file of instruction patterns; by defining this macro differently, you can cause a large class of instructions to be output differently (such as with rearranged operands). Naturally, variations in assembler syntax affecting individual insn patterns ought to be handled by writing conditional output routines in those patterns. If this macro is not defined, it is equivalent to a null statement.
`FINAL_PRESCAN_LABEL`
  If defined, `FINAL_PRESCAN_INSN` will be called on each `CODE_LABEL`. In that case, opvec will be a null pointer and noperands will be zero.
`PRINT_OPERAND (stream, x, code)`
  A C compound statement to output to stdio stream stream the assembler syntax for an instruction operand x. x is an RTL expression. code is a value that can be used to specify one of several ways of printing the operand. It is used when identical operands must be printed differently depending on the context. code comes from the ``%'` specification that was used to request printing of the operand. If the specification was just ``%digit'` then code is 0; if the specification was ``%ltr digit'` then code is the ASCII code for ltr. If x is a register, this macro should print the register's name. The names can be found in an array `reg_names` whose type is `char *[]`. `reg_names` is initialized from `REGISTER_NAMES`. When the machine description has a specification ``%punct'` (a ``%'` followed by a punctuation character), this macro is called with a null pointer for x and the punctuation character for code.
`PRINT_OPERAND_PUNCT_VALID_P (code)`
  A C expression which evaluates to true if code is a valid punctuation character for use in the `PRINT_OPERAND` macro. If `PRINT_OPERAND_PUNCT_VALID_P` is not defined, it means that no punctuation characters (except for the standard one, ``%'`) are used in this way.
`PRINT_OPERAND_ADDRESS (stream, x)`
  A C compound statement to output to stdio stream stream the assembler syntax for an instruction operand that is a memory reference whose address is x. x is an RTL expression. On some machines, the syntax for a symbolic address depends on the section that the address refers to. On these machines, define the macro `ENCODE_SECTION_INFO` to store the information into the `symbol_ref`, and then check for it here. See section 17.16 Defining the Output Assembler Language.
`DBR_OUTPUT_SEQEND(file)`
  A C statement, to be executed after all slot-filler instructions have been output. If necessary, call `dbr_sequence_length` to determine the number of slots filled in a sequence (zero if not currently outputting a sequence), to decide how many no-ops to output, or whatever. Don't define this macro if it has nothing to do, but it is helpful in reading assembly output if the extent of the delay sequence is made explicit (e.g. with white space). Note that output routines for instructions with delay slots must be prepared to deal with not being output as part of a sequence (i.e. when the scheduling pass is not run, or when no slot fillers could be found.) The variable `final_sequence` is null when not processing a sequence, otherwise it contains the `sequence` rtx being output.
`REGISTER_PREFIX`
`LOCAL_LABEL_PREFIX`
`USER_LABEL_PREFIX`
`IMMEDIATE_PREFIX`
  If defined, C string expressions to be used for the ``%R'`, ``%L'`, ``%U'`, and ``%I'` options of `asm_fprintf` (see ``final.c'`). These are useful when a single ``md'` file must support multiple assembler formats. In that case, the various ``tm.h'` files can define these macros differently.
`ASSEMBLER_DIALECT`
  If your target supports multiple dialects of assembler language (such as different opcodes), define this macro as a C expression that gives the numeric index of the assembler language dialect to use, with zero as the first variant. If this macro is defined, you may use constructs of the form ``{option0|option1|option2...}'` in the output templates of patterns (see section 16.4 Output Templates and Operand Substitution) or in the first argument of `asm_fprintf`. This construct outputs ``option0'`, ``option1'` or ``option2'`, etc., if the value of `ASSEMBLER_DIALECT` is zero, one or two, etc. Any special characters within these strings retain their usual meaning. If you do not define this macro, the characters ``{'`, ``|'` and ``}'` do not have any special meaning when used in templates or operands to `asm_fprintf`. Define the macros `REGISTER_PREFIX`, `LOCAL_LABEL_PREFIX`, `USER_LABEL_PREFIX` and `IMMEDIATE_PREFIX` if you can express the variations in assembler language syntax with that mechanism. Define `ASSEMBLER_DIALECT` and use the ``{option0|option1}'` syntax if the syntax variant are larger and involve such things as different opcodes or operand order.
`ASM_OUTPUT_REG_PUSH (stream, regno)`
  A C expression to output to stream some assembler code which will push hard register number regno onto the stack. The code need not be optimal, since this macro is used only when profiling.
`ASM_OUTPUT_REG_POP (stream, regno)`
  A C expression to output to stream some assembler code which will pop hard register number regno off of the stack. The code need not be optimal, since this macro is used only when profiling.

### 17.16.8 Output of Dispatch Tables

This concerns dispatch tables.

`ASM_OUTPUT_ADDR_DIFF_ELT (stream, body, value, rel)`
  A C statement to output to the stdio stream stream an assembler pseudo-instruction to generate a difference between two labels. value and rel are the numbers of two internal labels. The definitions of these labels are output using `ASM_OUTPUT_INTERNAL_LABEL`, and they must be printed in the same way here. For example,

      fprintf (stream, "\t.word L%d-L%d\n",
               value, rel)

  You must provide this macro on machines where the addresses in a dispatch table are relative to the table's own address. If defined, GNU CC will also use this macro on all machines when producing PIC. body is the body of the ADDR_DIFF_VEC; it is provided so that the mode and flags can be read.
`ASM_OUTPUT_ADDR_VEC_ELT (stream, value)`
  This macro should be provided on machines where the addresses in a dispatch table are absolute. The definition should be a C statement to output to the stdio stream stream an assembler pseudo-instruction to generate a reference to a label. value is the number of an internal label whose definition is output using `ASM_OUTPUT_INTERNAL_LABEL`. For example,

      fprintf (stream, "\t.word L%d\n", value)

`ASM_OUTPUT_CASE_LABEL (stream, prefix, num, table)`
  Define this if the label before a jump-table needs to be output specially. The first three arguments are the same as for `ASM_OUTPUT_INTERNAL_LABEL`; the fourth argument is the jump-table which follows (a `jump_insn` containing an `addr_vec` or `addr_diff_vec`). This feature is used on system V to output a `swbeg` statement for the table. If this macro is not defined, these labels are output with `ASM_OUTPUT_INTERNAL_LABEL`.
`ASM_OUTPUT_CASE_END (stream, num, table)`
  Define this if something special must be output at the end of a jump-table. The definition should be a C statement to be executed after the assembler code for the table is written. It should write the appropriate code to stdio stream stream. The argument table is the jump-table insn, and num is the label-number of the preceding label. If this macro is not defined, nothing special is output at the end of the jump-table.

### 17.16.9 Assembler Commands for Exception Regions

This describes commands marking the start and the end of an exception region.

`ASM_OUTPUT_EH_REGION_BEG ()`
  A C expression to output text to mark the start of an exception region. This macro need not be defined on most platforms.
`ASM_OUTPUT_EH_REGION_END ()`
  A C expression to output text to mark the end of an exception region. This macro need not be defined on most platforms.
`EXCEPTION_SECTION ()`
  A C expression to switch to the section in which the main exception table is to be placed (see section 17.14 Dividing the Output into Sections (Texts, Data, ...)). The default is a section named `.gcc_except_table` on machines that support named sections via `ASM_OUTPUT_SECTION_NAME`, otherwise if ``-fpic'` or ``-fPIC'` is in effect, the `data_section`, otherwise the `readonly_data_section`.
`EH_FRAME_SECTION_ASM_OP`
  If defined, a C string constant for the assembler operation to switch to the section for exception handling frame unwind information. If not defined, GNU CC will provide a default definition if the target supports named sections. ``crtstuff.c'` uses this macro to switch to the appropriate section. You should define this symbol if your target supports DWARF 2 frame unwind information and the default definition does not work.
`OMIT_EH_TABLE ()`
  A C expression that is nonzero if the normal exception table output should be omitted. This macro need not be defined on most platforms.
`EH_TABLE_LOOKUP ()`
  Alternate runtime support for looking up an exception at runtime and finding the associated handler, if the default method won't work. This macro need not be defined on most platforms.
`DOESNT_NEED_UNWINDER`
  A C expression that decides whether or not the current function needs to have a function unwinder generated for it. See the file `except.c` for details on when to define this, and how.
`MASK_RETURN_ADDR`
  An rtx used to mask the return address found via RETURN_ADDR_RTX, so that it does not contain any extraneous set bits in it.
`DWARF2_UNWIND_INFO`
  Define this macro to 0 if your target supports DWARF 2 frame unwind information, but it does not yet work with exception handling. Otherwise, if your target supports this information (if it defines ``INCOMING_RETURN_ADDR_RTX'` and either ``UNALIGNED_INT_ASM_OP'` or ``OBJECT_FORMAT_ELF'`), GCC will provide a default definition of 1. If this macro is defined to 1, the DWARF 2 unwinder will be the default exception handling mechanism; otherwise, setjmp/longjmp will be used by default. If this macro is defined to anything, the DWARF 2 unwinder will be used instead of inline unwinders and __unwind_function in the non-setjmp case.

### 17.16.10 Assembler Commands for Alignment

This describes commands for alignment.

`LABEL_ALIGN_AFTER_BARRIER (label)`
  The alignment (log base 2) to put in front of label, which follows a BARRIER. This macro need not be defined if you don't want any special alignment to be done at such a time. Most machine descriptions do not currently define the macro.
`LOOP_ALIGN (label)`
  The alignment (log base 2) to put in front of label, which follows a NOTE_INSN_LOOP_BEG note. This macro need not be defined if you don't want any special alignment to be done at such a time. Most machine descriptions do not currently define the macro.
`LABEL_ALIGN (label)`
  The alignment (log base 2) to put in front of label. If LABEL_ALIGN_AFTER_BARRIER / LOOP_ALIGN specify a different alignment, the maximum of the specified values is used.
`ASM_OUTPUT_SKIP (stream, nbytes)`
  A C statement to output to the stdio stream stream an assembler instruction to advance the location counter by nbytes bytes. Those bytes should be zero when loaded. nbytes will be a C expression of type `int`.
`ASM_NO_SKIP_IN_TEXT`
  Define this macro if `ASM_OUTPUT_SKIP` should not be used in the text section because it fails to put zeros in the bytes that are skipped. This is true on many Unix systems, where the pseudo--op to skip bytes produces no-op instructions rather than zeros when used in the text section.
`ASM_OUTPUT_ALIGN (stream, power)`
  A C statement to output to the stdio stream stream an assembler command to advance the location counter to a multiple of 2 to the power bytes. power will be a C expression of type `int`.
`ASM_OUTPUT_MAX_SKIP_ALIGN (stream, power, max_skip)`
  A C statement to output to the stdio stream stream an assembler command to advance the location counter to a multiple of 2 to the power bytes, but only if max_skip or fewer bytes are needed to satisfy the alignment request. power and max_skip will be a C expression of type `int`.

## 17.17 Controlling Debugging Information Format

This describes how to specify debugging information.

### 17.17.1 Macros Affecting All Debugging Formats

These macros affect all debugging formats.

`DBX_REGISTER_NUMBER (regno)`
  A C expression that returns the DBX register number for the compiler register number regno. In simple cases, the value of this expression may be regno itself. But sometimes there are some registers that the compiler knows about and DBX does not, or vice versa. In such cases, some register may need to have one number in the compiler and another for DBX. If two registers have consecutive numbers inside GNU CC, and they can be used as a pair to hold a multiword value, then they must have consecutive numbers after renumbering with `DBX_REGISTER_NUMBER`. Otherwise, debuggers will be unable to access such a pair, because they expect register pairs to be consecutive in their own numbering scheme. If you find yourself defining `DBX_REGISTER_NUMBER` in way that does not preserve register pairs, then what you must do instead is redefine the actual register numbering scheme.
`DEBUGGER_AUTO_OFFSET (x)`
  A C expression that returns the integer offset value for an automatic variable having address x (an RTL expression). The default computation assumes that x is based on the frame-pointer and gives the offset from the frame-pointer. This is required for targets that produce debugging output for DBX or COFF-style debugging output for SDB and allow the frame-pointer to be eliminated when the ``-g'` options is used.
`DEBUGGER_ARG_OFFSET (offset, x)`
  A C expression that returns the integer offset value for an argument having address x (an RTL expression). The nominal offset is offset.
`PREFERRED_DEBUGGING_TYPE`
  A C expression that returns the type of debugging output GNU CC should produce when the user specifies just ``-g'`. Define this if you have arranged for GNU CC to support more than one format of debugging output. Currently, the allowable values are `DBX_DEBUG`, `SDB_DEBUG`, `DWARF_DEBUG`, `DWARF2_DEBUG`, and `XCOFF_DEBUG`. When the user specifies ``-ggdb'`, GNU CC normally also uses the value of this macro to select the debugging output format, but with two exceptions. If `DWARF2_DEBUGGING_INFO` is defined and `LINKER_DOES_NOT_WORK_WITH_DWARF2` is not defined, GNU CC uses the value `DWARF2_DEBUG`. Otherwise, if `DBX_DEBUGGING_INFO` is defined, GNU CC uses `DBX_DEBUG`. The value of this macro only affects the default debugging output; the user can always get a specific type of output by using ``-gstabs'`, ``-gcoff'`, ``-gdwarf-1'`, ``-gdwarf-2'`, or ``-gxcoff'`.

### 17.17.2 Specific Options for DBX Output

These are specific options for DBX output.

`DBX_DEBUGGING_INFO`
  Define this macro if GNU CC should produce debugging output for DBX in response to the ``-g'` option.
`XCOFF_DEBUGGING_INFO`
  Define this macro if GNU CC should produce XCOFF format debugging output in response to the ``-g'` option. This is a variant of DBX format.
`DEFAULT_GDB_EXTENSIONS`
  Define this macro to control whether GNU CC should by default generate GDB's extended version of DBX debugging information (assuming DBX-format debugging information is enabled at all). If you don't define the macro, the default is 1: always generate the extended information if there is any occasion to.
`DEBUG_SYMS_TEXT`
  Define this macro if all `.stabs` commands should be output while in the text section.
`ASM_STABS_OP`
  A C string constant naming the assembler pseudo op to use instead of `.stabs` to define an ordinary debugging symbol. If you don't define this macro, `.stabs` is used. This macro applies only to DBX debugging information format.
`ASM_STABD_OP`
  A C string constant naming the assembler pseudo op to use instead of `.stabd` to define a debugging symbol whose value is the current location. If you don't define this macro, `.stabd` is used. This macro applies only to DBX debugging information format.
`ASM_STABN_OP`
  A C string constant naming the assembler pseudo op to use instead of `.stabn` to define a debugging symbol with no name. If you don't define this macro, `.stabn` is used. This macro applies only to DBX debugging information format.
`DBX_NO_XREFS`
  Define this macro if DBX on your system does not support the construct ``xstagname'`. On some systems, this construct is used to describe a forward reference to a structure named tagname. On other systems, this construct is not supported at all.
`DBX_CONTIN_LENGTH`
  A symbol name in DBX-format debugging information is normally continued (split into two separate `.stabs` directives) when it exceeds a certain length (by default, 80 characters). On some operating systems, DBX requires this splitting; on others, splitting must not be done. You can inhibit splitting by defining this macro with the value zero. You can override the default splitting-length by defining this macro as an expression for the length you desire.
`DBX_CONTIN_CHAR`
  Normally continuation is indicated by adding a ``\'` character to the end of a `.stabs` string when a continuation follows. To use a different character instead, define this macro as a character constant for the character you want to use. Do not define this macro if backslash is correct for your system.
`DBX_STATIC_STAB_DATA_SECTION`
  Define this macro if it is necessary to go to the data section before outputting the ``.stabs'` pseudo-op for a non-global static variable.
`DBX_TYPE_DECL_STABS_CODE`
  The value to use in the "code" field of the `.stabs` directive for a typedef. The default is `N_LSYM`.
`DBX_STATIC_CONST_VAR_CODE`
  The value to use in the "code" field of the `.stabs` directive for a static variable located in the text section. DBX format does not provide any "right" way to do this. The default is `N_FUN`.
`DBX_REGPARM_STABS_CODE`
  The value to use in the "code" field of the `.stabs` directive for a parameter passed in registers. DBX format does not provide any "right" way to do this. The default is `N_RSYM`.
`DBX_REGPARM_STABS_LETTER`
  The letter to use in DBX symbol data to identify a symbol as a parameter passed in registers. DBX format does not customarily provide any way to do this. The default is `'P'`.
`DBX_MEMPARM_STABS_LETTER`
  The letter to use in DBX symbol data to identify a symbol as a stack parameter. The default is `'p'`.
`DBX_FUNCTION_FIRST`
  Define this macro if the DBX information for a function and its arguments should precede the assembler code for the function. Normally, in DBX format, the debugging information entirely follows the assembler code.
`DBX_LBRAC_FIRST`
  Define this macro if the `N_LBRAC` symbol for a block should precede the debugging information for variables and functions defined in that block. Normally, in DBX format, the `N_LBRAC` symbol comes first.
`DBX_BLOCKS_FUNCTION_RELATIVE`
  Define this macro if the value of a symbol describing the scope of a block (`N_LBRAC` or `N_RBRAC`) should be relative to the start of the enclosing function. Normally, GNU C uses an absolute address.
`DBX_USE_BINCL`
  Define this macro if GNU C should generate `N_BINCL` and `N_EINCL` stabs for included header files, as on Sun systems. This macro also directs GNU C to output a type number as a pair of a file number and a type number within the file. Normally, GNU C does not generate `N_BINCL` or `N_EINCL` stabs, and it outputs a single number for a type number.

### 17.17.3 Open-Ended Hooks for DBX Format

These are hooks for DBX format.

`DBX_OUTPUT_LBRAC (stream, name)`
  Define this macro to say how to output to stream the debugging information for the start of a scope level for variable names. The argument name is the name of an assembler symbol (for use with `assemble_name`) whose value is the address where the scope begins.
`DBX_OUTPUT_RBRAC (stream, name)`
  Like `DBX_OUTPUT_LBRAC`, but for the end of a scope level.
`DBX_OUTPUT_ENUM (stream, type)`
  Define this macro if the target machine requires special handling to output an enumeration type. The definition should be a C statement (sans semicolon) to output the appropriate information to stream for the type type.
`DBX_OUTPUT_FUNCTION_END (stream, function)`
  Define this macro if the target machine requires special output at the end of the debugging information for a function. The definition should be a C statement (sans semicolon) to output the appropriate information to stream. function is the `FUNCTION_DECL` node for the function.
`DBX_OUTPUT_STANDARD_TYPES (syms)`
  Define this macro if you need to control the order of output of the standard data types at the beginning of compilation. The argument syms is a `tree` which is a chain of all the predefined global symbols, including names of data types. Normally, DBX output starts with definitions of the types for integers and characters, followed by all the other predefined types of the particular language in no particular order. On some machines, it is necessary to output different particular types first. To do this, define `DBX_OUTPUT_STANDARD_TYPES` to output those symbols in the necessary order. Any predefined types that you don't explicitly output will be output afterward in no particular order. Be careful not to define this macro so that it works only for C. There are no global variables to access most of the built-in types, because another language may have another set of types. The way to output a particular type is to look through syms to see if you can find it. Here is an example:

      {
        tree decl;
        for (decl = syms; decl; decl = TREE_CHAIN (decl))
          if (!strcmp (IDENTIFIER_POINTER (DECL_NAME (decl)),
                       "long int"))
            dbxout_symbol (decl);
        ...
      }

  This does nothing if the expected type does not exist. See the function `init_decl_processing` in ``c-decl.c'` to find the names to use for all the built-in C types. Here is another way of finding a particular type:

      {
        tree decl;
        for (decl = syms; decl; decl = TREE_CHAIN (decl))
          if (TREE_CODE (decl) == TYPE_DECL
              && (TREE_CODE (TREE_TYPE (decl))
                  == INTEGER_CST)
              && TYPE_PRECISION (TREE_TYPE (decl)) == 16
              && TYPE_UNSIGNED (TREE_TYPE (decl)))
            /* This must be unsigned short.  */
            dbxout_symbol (decl);
        ...
      }

`NO_DBX_FUNCTION_END`
  Some stabs encapsulation formats (in particular ECOFF), cannot handle the `.stabs "",N_FUN,,0,0,Lscope-function-1` gdb dbx extention construct. On those machines, define this macro to turn this feature off without disturbing the rest of the gdb extensions.

### 17.17.4 File Names in DBX Format

This describes file names in DBX format.

`DBX_WORKING_DIRECTORY`
  Define this if DBX wants to have the current directory recorded in each object file. Note that the working directory is always recorded if GDB extensions are enabled.
`DBX_OUTPUT_MAIN_SOURCE_FILENAME (stream, name)`
  A C statement to output DBX debugging information to the stdio stream stream which indicates that file name is the main source file--the file specified as the input file for compilation. This macro is called only once, at the beginning of compilation. This macro need not be defined if the standard form of output for DBX debugging information is appropriate.
`DBX_OUTPUT_MAIN_SOURCE_DIRECTORY (stream, name)`
  A C statement to output DBX debugging information to the stdio stream stream which indicates that the current directory during compilation is named name. This macro need not be defined if the standard form of output for DBX debugging information is appropriate.
`DBX_OUTPUT_MAIN_SOURCE_FILE_END (stream, name)`
  A C statement to output DBX debugging information at the end of compilation of the main source file name. If you don't define this macro, nothing special is output at the end of compilation, which is correct for most machines.
`DBX_OUTPUT_SOURCE_FILENAME (stream, name)`
  A C statement to output DBX debugging information to the stdio stream stream which indicates that file name is the current source file. This output is generated each time input shifts to a different source file as a result of ``#include'`, the end of an included file, or a ``#line'` command. This macro need not be defined if the standard form of output for DBX debugging information is appropriate.

### 17.17.5 Macros for SDB and DWARF Output

Here are macros for SDB and DWARF output.

`SDB_DEBUGGING_INFO`
  Define this macro if GNU CC should produce COFF-style debugging output for SDB in response to the ``-g'` option.
`DWARF_DEBUGGING_INFO`
  Define this macro if GNU CC should produce dwarf format debugging output in response to the ``-g'` option.
`DWARF2_DEBUGGING_INFO`
  Define this macro if GNU CC should produce dwarf version 2 format debugging output in response to the ``-g'` option. To support optional call frame debugging information, you must also define `INCOMING_RETURN_ADDR_RTX` and either set `RTX_FRAME_RELATED_P` on the prologue insns if you use RTL for the prologue, or call `dwarf2out_def_cfa` and `dwarf2out_reg_save` as appropriate from `FUNCTION_PROLOGUE` if you don't.
`DWARF2_FRAME_INFO`
  Define this macro to a nonzero value if GNU CC should always output Dwarf 2 frame information. If `DWARF2_UNWIND_INFO` (see section 17.16.9 Assembler Commands for Exception Regions is nonzero, GNU CC will output this information not matter how you define `DWARF2_FRAME_INFO`.
`LINKER_DOES_NOT_WORK_WITH_DWARF2`
  Define this macro if the linker does not work with Dwarf version 2. Normally, if the user specifies only ``-ggdb'` GNU CC will use Dwarf version 2 if available; this macro disables this. See the description of the `PREFERRED_DEBUGGING_TYPE` macro for more details.
`PUT_SDB_...`
  Define these macros to override the assembler syntax for the special SDB assembler directives. See ``sdbout.c'` for a list of these macros and their arguments. If the standard syntax is used, you need not define them yourself.
`SDB_DELIM`
  Some assemblers do not support a semicolon as a delimiter, even between SDB assembler directives. In that case, define this macro to be the delimiter to use (usually ``\n'`). It is not necessary to define a new set of `PUT_SDB_op` macros if this is the only change required.
`SDB_GENERATE_FAKE`
  Define this macro to override the usual method of constructing a dummy name for anonymous structure and union types. See ``sdbout.c'` for more information.
`SDB_ALLOW_UNKNOWN_REFERENCES`
  Define this macro to allow references to unknown structure, union, or enumeration tags to be emitted. Standard COFF does not allow handling of unknown references, MIPS ECOFF has support for it.
`SDB_ALLOW_FORWARD_REFERENCES`
  Define this macro to allow references to structure, union, or enumeration tags that have not yet been seen to be handled. Some assemblers choke if forward tags are used, while some require it.

## 17.18 Cross Compilation and Floating Point

While all modern machines use 2's complement representation for integers, there are a variety of representations for floating point numbers. This means that in a cross-compiler the representation of floating point numbers in the compiled program may be different from that used in the machine doing the compilation.

Because different representation systems may offer different amounts of range and precision, the cross compiler cannot safely use the host machine's floating point arithmetic. Therefore, floating point constants must be represented in the target machine's format. This means that the cross compiler cannot use `atof` to parse a floating point constant; it must have its own special routine to use instead. Also, constant folding must emulate the target machine's arithmetic (or must not be done at all).

The macros in the following table should be defined only if you are cross compiling between different floating point formats.

Otherwise, don't define them. Then default definitions will be set up which use `double` as the data type, `==` to test for equality, etc.

You don't need to worry about how many times you use an operand of any of these macros. The compiler never uses operands which have side effects.

`REAL_VALUE_TYPE`
  A macro for the C data type to be used to hold a floating point value in the target machine's format. Typically this would be a `struct` containing an array of `int`.
`REAL_VALUES_EQUAL (x, y)`
  A macro for a C expression which compares for equality the two values, x and y, both of type `REAL_VALUE_TYPE`.
`REAL_VALUES_LESS (x, y)`
  A macro for a C expression which tests whether x is less than y, both values being of type `REAL_VALUE_TYPE` and interpreted as floating point numbers in the target machine's representation.
`REAL_VALUE_LDEXP (x, scale)`
  A macro for a C expression which performs the standard library function `ldexp`, but using the target machine's floating point representation. Both x and the value of the expression have type `REAL_VALUE_TYPE`. The second argument, scale, is an integer.
`REAL_VALUE_FIX (x)`
  A macro whose definition is a C expression to convert the target-machine floating point value x to a signed integer. x has type `REAL_VALUE_TYPE`.
`REAL_VALUE_UNSIGNED_FIX (x)`
  A macro whose definition is a C expression to convert the target-machine floating point value x to an unsigned integer. x has type `REAL_VALUE_TYPE`.
`REAL_VALUE_RNDZINT (x)`
  A macro whose definition is a C expression to round the target-machine floating point value x towards zero to an integer value (but still as a floating point number). x has type `REAL_VALUE_TYPE`, and so does the value.
`REAL_VALUE_UNSIGNED_RNDZINT (x)`
  A macro whose definition is a C expression to round the target-machine floating point value x towards zero to an unsigned integer value (but still represented as a floating point number). x has type `REAL_VALUE_TYPE`, and so does the value.
`REAL_VALUE_ATOF (string, mode)`
  A macro for a C expression which converts string, an expression of type `char *`, into a floating point number in the target machine's representation for mode mode. The value has type `REAL_VALUE_TYPE`.
`REAL_INFINITY`
  Define this macro if infinity is a possible floating point value, and therefore division by 0 is legitimate.
`REAL_VALUE_ISINF (x)`
  A macro for a C expression which determines whether x, a floating point value, is infinity. The value has type `int`. By default, this is defined to call `isinf`.
`REAL_VALUE_ISNAN (x)`
  A macro for a C expression which determines whether x, a floating point value, is a "nan" (not-a-number). The value has type `int`. By default, this is defined to call `isnan`.

Define the following additional macros if you want to make floating point constant folding work while cross compiling. If you don't define them, cross compilation is still possible, but constant folding will not happen for floating point values.

`REAL_ARITHMETIC (output, code, x, y)`
  A macro for a C statement which calculates an arithmetic operation of the two floating point values x and y, both of type `REAL_VALUE_TYPE` in the target machine's representation, to produce a result of the same type and representation which is stored in output (which will be a variable). The operation to be performed is specified by code, a tree code which will always be one of the following: `PLUS_EXPR`, `MINUS_EXPR`, `MULT_EXPR`, `RDIV_EXPR`, `MAX_EXPR`, `MIN_EXPR`. The expansion of this macro is responsible for checking for overflow. If overflow happens, the macro expansion should execute the statement `return 0;`, which indicates the inability to perform the arithmetic operation requested.
`REAL_VALUE_NEGATE (x)`
  A macro for a C expression which returns the negative of the floating point value x. Both x and the value of the expression have type `REAL_VALUE_TYPE` and are in the target machine's floating point representation. There is no way for this macro to report overflow, since overflow can't happen in the negation operation.
`REAL_VALUE_TRUNCATE (mode, x)`
  A macro for a C expression which converts the floating point value x to mode mode. Both x and the value of the expression are in the target machine's floating point representation and have type `REAL_VALUE_TYPE`. However, the value should have an appropriate bit pattern to be output properly as a floating constant whose precision accords with mode mode. There is no way for this macro to report overflow.
`REAL_VALUE_TO_INT (low, high, x)`
  A macro for a C expression which converts a floating point value x into a double-precision integer which is then stored into low and high, two variables of type int.
`REAL_VALUE_FROM_INT (x, low, high, mode)`
  A macro for a C expression which converts a double-precision integer found in low and high, two variables of type int, into a floating point value which is then stored into x. The value is in the target machine's representation for mode mode and has the type `REAL_VALUE_TYPE`.

## 17.19 Miscellaneous Parameters

Here are several miscellaneous parameters.

`PREDICATE_CODES`
  Define this if you have defined special-purpose predicates in the file ``machine.c'`. This macro is called within an initializer of an array of structures. The first field in the structure is the name of a predicate and the second field is an array of rtl codes. For each predicate, list all rtl codes that can be in expressions matched by the predicate. The list should have a trailing comma. Here is an example of two entries in the list for a typical RISC machine:

      #define PREDICATE_CODES \
        {"gen_reg_rtx_operand", {SUBREG, REG}},  \
        {"reg_or_short_cint_operand", {SUBREG, REG, CONST_INT}},

  Defining this macro does not affect the generated code (however, incorrect definitions that omit an rtl code that may be matched by the predicate can cause the compiler to malfunction). Instead, it allows the table built by ``genrecog'` to be more compact and efficient, thus speeding up the compiler. The most important predicates to include in the list specified by this macro are those used in the most insn patterns.
`CASE_VECTOR_MODE`
  An alias for a machine mode name. This is the machine mode that elements of a jump-table should have.
`CASE_VECTOR_SHORTEN_MODE (min_offset, max_offset, body)`
  Optional: return the preferred mode for an `addr_diff_vec` when the minimum and maximum offset are known. If you define this, it enables extra code in branch shortening to deal with `addr_diff_vec`. To make this work, you also have to define INSN_ALIGN and make the alignment for `addr_diff_vec` explicit. The body argument is provided so that the offset_unsigned and scale flags can be updated.
`CASE_VECTOR_PC_RELATIVE`
  Define this macro to be a C expression to indicate when jump-tables should contain relative addresses. If jump-tables never contain relative addresses, then you need not define this macro.
`CASE_DROPS_THROUGH`
  Define this if control falls through a `case` insn when the index value is out of range. This means the specified default-label is actually ignored by the `case` insn proper.
`CASE_VALUES_THRESHOLD`
  Define this to be the smallest number of different values for which it is best to use a jump-table instead of a tree of conditional branches. The default is four for machines with a `casesi` instruction and five otherwise. This is best for most machines.
`LOOP_TEST_THRESHOLD`
  Define this to be the maximum number of insns to move around when moving a loop test from the top of a loop to the bottom and seeing whether to duplicate it. The default is thirty.
`WORD_REGISTER_OPERATIONS`
  Define this macro if operations between registers with integral mode smaller than a word are always performed on the entire register. Most RISC machines have this property and most CISC machines do not.
`LOAD_EXTEND_OP (mode)`
  Define this macro to be a C expression indicating when insns that read memory in mode, an integral mode narrower than a word, set the bits outside of mode to be either the sign-extension or the zero-extension of the data read. Return `SIGN_EXTEND` for values of mode for which the insn sign-extends, `ZERO_EXTEND` for which it zero-extends, and `NIL` for other modes. This macro is not called with mode non-integral or with a width greater than or equal to `BITS_PER_WORD`, so you may return any value in this case. Do not define this macro if it would always return `NIL`. On machines where this macro is defined, you will normally define it as the constant `SIGN_EXTEND` or `ZERO_EXTEND`.
`SHORT_IMMEDIATES_SIGN_EXTEND`
  Define this macro if loading short immediate values into registers sign extends.
`IMPLICIT_FIX_EXPR`
  An alias for a tree code that should be used by default for conversion of floating point values to fixed point. Normally, `FIX_ROUND_EXPR` is used.
`FIXUNS_TRUNC_LIKE_FIX_TRUNC`
  Define this macro if the same instructions that convert a floating point number to a signed fixed point number also convert validly to an unsigned one.
`EASY_DIV_EXPR`
  An alias for a tree code that is the easiest kind of division to compile code for in the general case. It may be `TRUNC_DIV_EXPR`, `FLOOR_DIV_EXPR`, `CEIL_DIV_EXPR` or `ROUND_DIV_EXPR`. These four division operators differ in how they round the result to an integer. `EASY_DIV_EXPR` is used when it is permissible to use any of those kinds of division and the choice should be made on the basis of efficiency.
`MOVE_MAX`
  The maximum number of bytes that a single instruction can move quickly between memory and registers or between two memory locations.
`MAX_MOVE_MAX`
  The maximum number of bytes that a single instruction can move quickly between memory and registers or between two memory locations. If this is undefined, the default is `MOVE_MAX`. Otherwise, it is the constant value that is the largest value that `MOVE_MAX` can have at run-time.
`SHIFT_COUNT_TRUNCATED`
  A C expression that is nonzero if on this machine the number of bits actually used for the count of a shift operation is equal to the number of bits needed to represent the size of the object being shifted. When this macro is non-zero, the compiler will assume that it is safe to omit a sign-extend, zero-extend, and certain bitwise `and' instructions that truncates the count of a shift operation. On machines that have instructions that act on bitfields at variable positions, which may include `bit test' instructions, a nonzero `SHIFT_COUNT_TRUNCATED` also enables deletion of truncations of the values that serve as arguments to bitfield instructions. If both types of instructions truncate the count (for shifts) and position (for bitfield operations), or if no variable-position bitfield instructions exist, you should define this macro. However, on some machines, such as the 80386 and the 680x0, truncation only applies to shift operations and not the (real or pretended) bitfield operations. Define `SHIFT_COUNT_TRUNCATED` to be zero on such machines. Instead, add patterns to the ``md'` file that include the implied truncation of the shift instructions. You need not define this macro if it would always have the value of zero.
`TRULY_NOOP_TRUNCATION (outprec, inprec)`
  A C expression which is nonzero if on this machine it is safe to "convert" an integer of inprec bits to one of outprec bits (where outprec is smaller than inprec) by merely operating on it as if it had only outprec bits. On many machines, this expression can be 1. When `TRULY_NOOP_TRUNCATION` returns 1 for a pair of sizes for modes for which `MODES_TIEABLE_P` is 0, suboptimal code can result. If this is the case, making `TRULY_NOOP_TRUNCATION` return 0 in such cases may improve things.
`STORE_FLAG_VALUE`
  A C expression describing the value returned by a comparison operator with an integral mode and stored by a store-flag instruction (``scond'`) when the condition is true. This description must apply to all the ``scond'` patterns and all the comparison operators whose results have a `MODE_INT` mode. A value of 1 or -1 means that the instruction implementing the comparison operator returns exactly 1 or -1 when the comparison is true and 0 when the comparison is false. Otherwise, the value indicates which bits of the result are guaranteed to be 1 when the comparison is true. This value is interpreted in the mode of the comparison operation, which is given by the mode of the first operand in the ``scond'` pattern. Either the low bit or the sign bit of `STORE_FLAG_VALUE` be on. Presently, only those bits are used by the compiler. If `STORE_FLAG_VALUE` is neither 1 or -1, the compiler will generate code that depends only on the specified bits. It can also replace comparison operators with equivalent operations if they cause the required bits to be set, even if the remaining bits are undefined. For example, on a machine whose comparison operators return an `SImode` value and where `STORE_FLAG_VALUE` is defined as ``0x80000000'`, saying that just the sign bit is relevant, the expression

      (ne:SI (and:SI x (const_int power-of-2)) (const_int 0))

  can be converted to

      (ashift:SI x (const_int n))

  where n is the appropriate shift count to move the bit being tested into the sign bit. There is no way to describe a machine that always sets the low-order bit for a true value, but does not guarantee the value of any other bits, but we do not know of any machine that has such an instruction. If you are trying to port GNU CC to such a machine, include an instruction to perform a logical-and of the result with 1 in the pattern for the comparison operators and let us know (see section 8.3 How to Report Bugs). Often, a machine will have multiple instructions that obtain a value from a comparison (or the condition codes). Here are rules to guide the choice of value for `STORE_FLAG_VALUE`, and hence the instructions to be used:
  - Use the shortest sequence that yields a valid definition for `STORE_FLAG_VALUE`. It is more efficient for the compiler to "normalize" the value (convert it to, e.g., 1 or 0) than for the comparison operators to do so because there may be opportunities to combine the normalization with other operations.
  - For equal-length sequences, use a value of 1 or -1, with -1 being slightly preferred on machines with expensive jumps and 1 preferred on other machines.
  - As a second choice, choose a value of ``0x80000001'` if instructions exist that set both the sign and low-order bits but do not define the others.
  - Otherwise, use a value of ``0x80000000'`.
  Many machines can produce both the value chosen for `STORE_FLAG_VALUE` and its negation in the same number of instructions. On those machines, you should also define a pattern for those cases, e.g., one matching

      (set A (neg:m (ne:m B C)))

  Some machines can also perform `and` or `plus` operations on condition code values with less instructions than the corresponding ``scond'` insn followed by `and` or `plus`. On those machines, define the appropriate patterns. Use the names `incscc` and `decscc`, respectively, for the patterns which perform `plus` or `minus` operations on condition code values. See ``rs6000.md'` for some examples. The GNU Superoptizer can be used to find such instruction sequences on other machines. You need not define `STORE_FLAG_VALUE` if the machine has no store-flag instructions.
`FLOAT_STORE_FLAG_VALUE`
  A C expression that gives a non-zero floating point value that is returned when comparison operators with floating-point results are true. Define this macro on machine that have comparison operations that return floating-point values. If there are no such operations, do not define this macro.
`Pmode`
  An alias for the machine mode for pointers. On most machines, define this to be the integer mode corresponding to the width of a hardware pointer; `SImode` on 32-bit machine or `DImode` on 64-bit machines. On some machines you must define this to be one of the partial integer modes, such as `PSImode`. The width of `Pmode` must be at least as large as the value of `POINTER_SIZE`. If it is not equal, you must define the macro `POINTERS_EXTEND_UNSIGNED` to specify how pointers are extended to `Pmode`.
`FUNCTION_MODE`
  An alias for the machine mode used for memory references to functions being called, in `call` RTL expressions. On most machines this should be `QImode`.
`INTEGRATE_THRESHOLD (decl)`
  A C expression for the maximum number of instructions above which the function decl should not be inlined. decl is a `FUNCTION_DECL` node. The default definition of this macro is 64 plus 8 times the number of arguments that the function accepts. Some people think a larger threshold should be used on RISC machines.
`SCCS_DIRECTIVE`
  Define this if the preprocessor should ignore `#sccs` directives and print no error message.
`NO_IMPLICIT_EXTERN_C`
  Define this macro if the system header files support C++ as well as C. This macro inhibits the usual method of using system header files in C++, which is to pretend that the file's contents are enclosed in ``extern "C" {...}'`.
`HANDLE_PRAGMA (getc, ungetc, name)`
  Define this macro if you want to implement any pragmas. If defined, it is a C expression whose value is 1 if the pragma was handled by the macro, zero otherwise. The argument getc is a function of type ``int (*)(void)'` which will return the next character in the input stream, or EOF if no characters are left. The argument ungetc is a function of type ``void (*)(int)'` which will push a character back into the input stream. The argument name is the word following #pragma in the input stream. The input stream pointer will be pointing just beyond the end of this word. The input stream should be left undistrubed if the expression returns zero, otherwise it should be pointing at the next character after the end of the pragma. Any characters remaining on the line will be ignored. It is generally a bad idea to implement new uses of `#pragma`. The only reason to define this macro is for compatibility with other compilers that do support `#pragma` for the sake of any user programs which already use it. If the pragma can be implemented by atttributes then the macro ``INSERT_ATTRIBUTES'` might be a useful one to define as well. Note: older versions of this macro only had two arguments: stream and token. The macro was changed in order to allow it to work when gcc is built both with and without a cpp library.
`HANDLE_SYSV_PRAGMA`
  Define this macro (to a value of 1) if you want the System V style pragmas ``#pragma pack(<n>)'` and ``#pragma weak <name> [=<value>]'` to be supported by gcc. The pack pragma specifies the maximum alignment (in bytes) of fields within a structure, in much the same way as the ``__aligned__'` and ``__packed__'` `__attribute__`s do. A pack value of zero resets the behaviour to the default. The weak pragma only works if `SUPPORTS_WEAK` and `ASM_WEAKEN_LABEL` are defined. If enabled it allows the creation of specifically named weak labels, optionally with a value.
`HANDLE_PRAGMA_PACK_PUSH_POP`
  Define this macro (to a value of 1) if you want to support the Win32 style pragmas ``#pragma pack(push,<n>)'` and ``#pragma pack(pop)'`. The pack(push,<n>) pragma specifies the maximum alignment (in bytes) of fields within a structure, in much the same way as the ``__aligned__'` and ``__packed__'` `__attribute__`s do. A pack value of zero resets the behaviour to the default. Successive invocations of this pragma cause the previous values to be stacked, so that invocations of ``#pragma pack(pop)'` will return to the previous value.
`VALID_MACHINE_DECL_ATTRIBUTE (decl, attributes, identifier, args)`
  If defined, a C expression whose value is nonzero if identifier with arguments args is a valid machine specific attribute for decl. The attributes in attributes have previously been assigned to decl.
`VALID_MACHINE_TYPE_ATTRIBUTE (type, attributes, identifier, args)`
  If defined, a C expression whose value is nonzero if identifier with arguments args is a valid machine specific attribute for type. The attributes in attributes have previously been assigned to type.
`COMP_TYPE_ATTRIBUTES (type1, type2)`
  If defined, a C expression whose value is zero if the attributes on type1 and type2 are incompatible, one if they are compatible, and two if they are nearly compatible (which causes a warning to be generated).
`SET_DEFAULT_TYPE_ATTRIBUTES (type)`
  If defined, a C statement that assigns default attributes to newly defined type.
`MERGE_MACHINE_TYPE_ATTRIBUTES (type1, type2)`
  Define this macro if the merging of type attributes needs special handling. If defined, the result is a list of the combined TYPE_ATTRIBUTES of type1 and type2. It is assumed that comptypes has already been called and returned 1.
`MERGE_MACHINE_DECL_ATTRIBUTES (olddecl, newdecl)`
  Define this macro if the merging of decl attributes needs special handling. If defined, the result is a list of the combined DECL_MACHINE_ATTRIBUTES of olddecl and newdecl. newdecl is a duplicate declaration of olddecl. Examples of when this is needed are when one attribute overrides another, or when an attribute is nullified by a subsequent definition.
`INSERT_ATTRIBUTES (node, attr_ptr, prefix_ptr)`
  Define this macro if you want to be able to add attributes to a decl when it is being created. This is normally useful for backends which wish to implement a pragma by using the attributes which correspond to the pragma's effect. The node argument is the decl which is being created. The attr_ptr argument is a pointer to the attribute list for this decl. The prefix_ptr is a pointer to the list of attributes that have appeared after the specifiers and modifiers of the declaration, but before the declaration proper.
`SET_DEFAULT_DECL_ATTRIBUTES (decl, attributes)`
  If defined, a C statement that assigns default attributes to newly defined decl.
`DOLLARS_IN_IDENTIFIERS`
  Define this macro to control use of the character ``$'` in identifier names. 0 means ``$'` is not allowed by default; 1 means it is allowed. 1 is the default; there is no need to define this macro in that case. This macro controls the compiler proper; it does not affect the preprocessor.
`NO_DOLLAR_IN_LABEL`
  Define this macro if the assembler does not accept the character ``$'` in label names. By default constructors and destructors in G++ have ``$'` in the identifiers. If this macro is defined, ``.'` is used instead.
`NO_DOT_IN_LABEL`
  Define this macro if the assembler does not accept the character ``.'` in label names. By default constructors and destructors in G++ have names that use ``.'`. If this macro is defined, these names are rewritten to avoid ``.'`.
`DEFAULT_MAIN_RETURN`
  Define this macro if the target system expects every program's `main` function to return a standard "success" value by default (if no other value is explicitly returned). The definition should be a C statement (sans semicolon) to generate the appropriate rtl instructions. It is used only when compiling the end of `main`.
`HAVE_ATEXIT`
  Define this if the target system supports the function `atexit` from the ANSI C standard. If this is not defined, and `INIT_SECTION_ASM_OP` is not defined, a default `exit` function will be provided to support C++.
`EXIT_BODY`
  Define this if your `exit` function needs to do something besides calling an external function `_cleanup` before terminating with `_exit`. The `EXIT_BODY` macro is only needed if neither `HAVE_ATEXIT` nor `INIT_SECTION_ASM_OP` are defined.
`INSN_SETS_ARE_DELAYED (insn)`
  Define this macro as a C expression that is nonzero if it is safe for the delay slot scheduler to place instructions in the delay slot of insn, even if they appear to use a resource set or clobbered in insn. insn is always a `jump_insn` or an `insn`; GNU CC knows that every `call_insn` has this behavior. On machines where some `insn` or `jump_insn` is really a function call and hence has this behavior, you should define this macro. You need not define this macro if it would always return zero.
`INSN_REFERENCES_ARE_DELAYED (insn)`
  Define this macro as a C expression that is nonzero if it is safe for the delay slot scheduler to place instructions in the delay slot of insn, even if they appear to set or clobber a resource referenced in insn. insn is always a `jump_insn` or an `insn`. On machines where some `insn` or `jump_insn` is really a function call and its operands are registers whose use is actually in the subroutine it calls, you should define this macro. Doing so allows the delay slot scheduler to move instructions which copy arguments into the argument registers into the delay slot of insn. You need not define this macro if it would always return zero.
`MACHINE_DEPENDENT_REORG (insn)`
  In rare cases, correct code generation requires extra machine dependent processing between the second jump optimization pass and delayed branch scheduling. On those machines, define this macro as a C statement to act on the code starting at insn.
`MULTIPLE_SYMBOL_SPACES`
  Define this macro if in some cases global symbols from one translation unit may not be bound to undefined symbols in another translation unit without user intervention. For instance, under Microsoft Windows symbols must be explicitly imported from shared libraries (DLLs).
`MAX_CONDITIONAL_EXECUTE`
  A C expression for the maximum number of instructions to execute via conditional execution instructions instead of a branch. A value of `BRANCH_COST`+1 is the default if the machine does not use `cc0`, and 1 if it does use `cc0`.
`ISSUE_RATE`
  A C expression that returns how many instructions can be issued at the same time if the machine is a superscalar machine. This is only used by the ``Haifa'` scheduler, and not the traditional scheduler.
`MD_SCHED_INIT (file, verbose`
  A C statement which is executed by the ``Haifa'` scheduler at the beginning of each block of instructions that are to be scheduled. file is either a null pointer, or a stdio stream to write any debug output to. verbose is the verbose level provided by ``-fsched-verbose-'`n.
`MD_SCHED_REORDER (file, verbose, ready, n_ready)`
  A C statement which is executed by the ``Haifa'` scheduler after it has scheduled the ready list to allow the machine description to reorder it (for example to combine two small instructions together on ``VLIW'` machines). file is either a null pointer, or a stdio stream to write any debug output to. verbose is the verbose level provided by ``-fsched-verbose-'`n. ready is a pointer to the ready list of instructions that are ready to be scheduled. n_ready is the number of elements in the ready list. The scheduler reads the ready list in reverse order, starting with ready[n_ready-1] and going to ready[0].
`MD_SCHED_VARIABLE_ISSUE (file, verbose, insn, more)`
  A C statement which is executed by the ``Haifa'` scheduler after it has scheduled an insn from the ready list. file is either a null pointer, or a stdio stream to write any debug output to. verbose is the verbose level provided by ``-fsched-verbose-'`n. insn is the instruction that was scheduled. more is the number of instructions that can be issued in the current cycle. The ``MD_SCHED_VARIABLE_ISSUE'` macro is responsible for updating the value of more (typically by more--).
`MAX_INTEGER_COMPUTATION_MODE`
  Define this to the largest integer machine mode which can be used for operations other than load, store and copy operations. You need only define this macro if the target holds values larger than `word_mode` in general purpose registers. Most targets should not define this macro.
`MATH_LIBRARY`
  Define this macro as a C string constant for the linker argument to link in the system math library, or ``""'` if the target does not have a separate math library. You need only define this macro if the default of ``"-lm"'` is wrong.

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