path: root/gcc/gcc.info-20

This is Info file gcc.info, produced by Makeinfo version 1.68 from the
input file gcc.texi.

   This file documents the use and the internals of the GNU compiler.

   Published by the Free Software Foundation 59 Temple Place - Suite 330
Boston, MA 02111-1307 USA

   Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997 Free
Software Foundation, Inc.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the sections entitled "GNU General Public License," "Funding for
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resulting derived work is distributed under the terms of a permission
notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the sections entitled "GNU General Public
License," "Funding for Free Software," and "Protect Your Freedom--Fight
`Look And Feel'", and this permission notice, may be included in
translations approved by the Free Software Foundation instead of in the
original English.


File: gcc.info,  Node: Function Units,  Prev: Delay Slots,  Up: Insn Attributes

Specifying Function Units
-------------------------

   On most RISC machines, there are instructions whose results are not
available for a specific number of cycles.  Common cases are
instructions that load data from memory.  On many machines, a pipeline
stall will result if the data is referenced too soon after the load
instruction.

   In addition, many newer microprocessors have multiple function
units, usually one for integer and one for floating point, and often
will incur pipeline stalls when a result that is needed is not yet
ready.

   The descriptions in this section allow the specification of how much
time must elapse between the execution of an instruction and the time
when its result is used.  It also allows specification of when the
execution of an instruction will delay execution of similar instructions
due to function unit conflicts.

   For the purposes of the specifications in this section, a machine is
divided into "function units", each of which execute a specific class
of instructions in first-in-first-out order.  Function units that
accept one instruction each cycle and allow a result to be used in the
succeeding instruction (usually via forwarding) need not be specified.
Classic RISC microprocessors will normally have a single function unit,
which we can call `memory'.  The newer "superscalar" processors will
often have function units for floating point operations, usually at
least a floating point adder and multiplier.

   Each usage of a function units by a class of insns is specified with
a `define_function_unit' expression, which looks like this:

     (define_function_unit NAME MULTIPLICITY SIMULTANEITY
                           TEST READY-DELAY ISSUE-DELAY
                          [CONFLICT-LIST])

   NAME is a string giving the name of the function unit.

   MULTIPLICITY is an integer specifying the number of identical units
in the processor.  If more than one unit is specified, they will be
scheduled independently.  Only truly independent units should be
counted; a pipelined unit should be specified as a single unit.  (The
only common example of a machine that has multiple function units for a
single instruction class that are truly independent and not pipelined
are the two multiply and two increment units of the CDC 6600.)

   SIMULTANEITY specifies the maximum number of insns that can be
executing in each instance of the function unit simultaneously or zero
if the unit is pipelined and has no limit.

   All `define_function_unit' definitions referring to function unit
NAME must have the same name and values for MULTIPLICITY and
SIMULTANEITY.

   TEST is an attribute test that selects the insns we are describing
in this definition.  Note that an insn may use more than one function
unit and a function unit may be specified in more than one
`define_function_unit'.

   READY-DELAY is an integer that specifies the number of cycles after
which the result of the instruction can be used without introducing any
stalls.

   ISSUE-DELAY is an integer that specifies the number of cycles after
the instruction matching the TEST expression begins using this unit
until a subsequent instruction can begin.  A cost of N indicates an N-1
cycle delay.  A subsequent instruction may also be delayed if an
earlier instruction has a longer READY-DELAY value.  This blocking
effect is computed using the SIMULTANEITY, READY-DELAY, ISSUE-DELAY,
and CONFLICT-LIST terms.  For a normal non-pipelined function unit,
SIMULTANEITY is one, the unit is taken to block for the READY-DELAY
cycles of the executing insn, and smaller values of ISSUE-DELAY are
ignored.

   CONFLICT-LIST is an optional list giving detailed conflict costs for
this unit.  If specified, it is a list of condition test expressions to
be applied to insns chosen to execute in NAME following the particular
insn matching TEST that is already executing in NAME.  For each insn in
the list, ISSUE-DELAY specifies the conflict cost; for insns not in the
list, the cost is zero.  If not specified, CONFLICT-LIST defaults to
all instructions that use the function unit.

   Typical uses of this vector are where a floating point function unit
can pipeline either single- or double-precision operations, but not
both, or where a memory unit can pipeline loads, but not stores, etc.

   As an example, consider a classic RISC machine where the result of a
load instruction is not available for two cycles (a single "delay"
instruction is required) and where only one load instruction can be
executed simultaneously.  This would be specified as:

     (define_function_unit "memory" 1 1 (eq_attr "type" "load") 2 0)

   For the case of a floating point function unit that can pipeline
either single or double precision, but not both, the following could be
specified:

     (define_function_unit
        "fp" 1 0 (eq_attr "type" "sp_fp") 4 4 [(eq_attr "type" "dp_fp")])
     (define_function_unit
        "fp" 1 0 (eq_attr "type" "dp_fp") 4 4 [(eq_attr "type" "sp_fp")])

   *Note:* The scheduler attempts to avoid function unit conflicts and
uses all the specifications in the `define_function_unit' expression.
It has recently come to our attention that these specifications may not
allow modeling of some of the newer "superscalar" processors that have
insns using multiple pipelined units.  These insns will cause a
potential conflict for the second unit used during their execution and
there is no way of representing that conflict.  We welcome any examples
of how function unit conflicts work in such processors and suggestions
for their representation.


File: gcc.info,  Node: Target Macros,  Next: Config,  Prev: Machine Desc,  Up: Top

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'.

* Menu:

* Driver::              Controlling how the driver runs the compilation passes.
* Run-time Target::     Defining `-m' options like `-m68000' and `-m68020'.
* Storage Layout::      Defining sizes and alignments of data.
* Type Layout::         Defining sizes and properties of basic user data types.
* Registers::           Naming and describing the hardware registers.
* Register Classes::    Defining the classes of hardware registers.
* Stack and Calling::   Defining which way the stack grows and by how much.
* Varargs::		Defining the varargs macros.
* Trampolines::         Code set up at run time to enter a nested function.
* Library Calls::       Controlling how library routines are implicitly called.
* Addressing Modes::    Defining addressing modes valid for memory operands.
* Condition Code::      Defining how insns update the condition code.
* Costs::               Defining relative costs of different operations.
* Sections::            Dividing storage into text, data, and other sections.
* PIC::			Macros for position independent code.
* Assembler Format::    Defining how to write insns and pseudo-ops to output.
* Debugging Info::      Defining the format of debugging output.
* Cross-compilation::   Handling floating point for cross-compilers.
* Misc::                Everything else.


File: gcc.info,  Node: Driver,  Next: Run-time Target,  Up: Target Macros

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.

`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'.

     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.

`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.  *Note Target 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/'.


File: gcc.info,  Node: Run-time Target,  Next: Storage Layout,  Prev: Driver,  Up: Target Macros

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, and a number, which contains the bits to set in
     `target_flags'.  A negative number says to clear bits instead; the
     negative of the number is which bits to clear.  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},     \
              { "", 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, and the address of a variable.  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 } }

`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)'
     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.

     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.


File: gcc.info,  Node: Storage Layout,  Next: Type Layout,  Prev: Run-time Target,  Up: Target Macros

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'.  *Note Run-time Target::.

`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
     sign-extended and zero if they are zero-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 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 `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 `STACK_BOUNDARY', the stack
     may be momentarily unaligned while pushing arguments.

`FUNCTION_BOUNDARY'
     Alignment required for a function entry point, in bits.

`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 static
     variable.  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 (STRUCT, SIZE, ALIGN)'
     Define this macro as an expression for the overall size of a
     structure (given by STRUCT as a tree node) when the size computed
     from the fields is SIZE and the alignment is ALIGN.

     The default is to round SIZE up to a multiple of ALIGN.

`ROUND_TYPE_ALIGN (STRUCT, COMPUTED, SPECIFIED)'
     Define this macro as an expression for the alignment of a structure
     (given by STRUCT 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.

`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'
     (*note Config::.) 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 *Note Function
     Entry::), 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.


File: gcc.info,  Node: Type Layout,  Next: Registers,  Prev: Storage Layout,  Up: Target Macros

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'.