gcc/tm.texi

@c Copyright (C) 1988,89,92,93,94,96 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.

@node Target Macros
@chapter Target Description Macros
@cindex machine description macros
@cindex target description macros
@cindex macros, target description
@cindex @file{tm.h} macros

In addition to the file @file{@var{machine}.md}, a machine description
includes a C header file conventionally given the name
@file{@var{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 @file{.md} file.  The file @file{tm.h} should be
a link to @file{@var{machine}.h}.  The header file @file{config.h}
includes @file{tm.h} and most compiler source files include
@file{config.h}.

@menu
* Driver::              Controlling how the driver runs the compilation passes.
* Run-time Target::     Defining @samp{-m} options like @samp{-m68000} and @samp{-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.
@end menu

@node Driver
@section Controlling the Compilation Driver, @file{gcc}
@cindex driver
@cindex controlling the compilation driver

@c prevent bad page break with this line
You can control the compilation driver.

@table @code
@findex SWITCH_TAKES_ARG
@item SWITCH_TAKES_ARG (@var{char})
A C expression which determines whether the option @samp{-@var{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
@code{DEFAULT_SWITCH_TAKES_ARG}, which handles the standard options
properly.  You need not define @code{SWITCH_TAKES_ARG} unless you
wish to add additional options which take arguments.  Any redefinition
should call @code{DEFAULT_SWITCH_TAKES_ARG} and then check for
additional options.

@findex WORD_SWITCH_TAKES_ARG
@item WORD_SWITCH_TAKES_ARG (@var{name})
A C expression which determines whether the option @samp{-@var{name}}
takes arguments.  The value should be the number of arguments that
option takes--zero, for many options.  This macro rather than
@code{SWITCH_TAKES_ARG} is used for multi-character option names.

By default, this macro is defined as
@code{DEFAULT_WORD_SWITCH_TAKES_ARG}, which handles the standard options
properly.  You need not define @code{WORD_SWITCH_TAKES_ARG} unless you
wish to add additional options which take arguments.  Any redefinition
should call @code{DEFAULT_WORD_SWITCH_TAKES_ARG} and then check for
additional options.

@findex SWITCHES_NEED_SPACES
@item 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 @code{""}.

@findex CPP_SPEC
@item 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.

@findex NO_BUILTIN_SIZE_TYPE
@item NO_BUILTIN_SIZE_TYPE
If this macro is defined, the preprocessor will not define the builtin macro
@code{__SIZE_TYPE__}.  The macro @code{__SIZE_TYPE__} must then be defined
by @code{CPP_SPEC} instead.

This should be defined if @code{SIZE_TYPE} depends on target dependent flags
which are not accessible to the preprocessor.  Otherwise, it should not
be defined.

@findex NO_BUILTIN_PTRDIFF_TYPE
@item NO_BUILTIN_PTRDIFF_TYPE
If this macro is defined, the preprocessor will not define the builtin macro
@code{__PTRDIFF_TYPE__}.  The macro @code{__PTRDIFF_TYPE__} must then be
defined by @code{CPP_SPEC} instead.

This should be defined if @code{PTRDIFF_TYPE} depends on target dependent flags
which are not accessible to the preprocessor.  Otherwise, it should not
be defined.

@findex SIGNED_CHAR_SPEC
@item 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
@samp{-D__CHAR_UNSIGNED__} to CPP if @code{char} will be treated as
@code{unsigned char} by @code{cc1}.

Do not define this macro unless you need to override the default
definition.

@findex CC1_SPEC
@item CC1_SPEC
A C string constant that tells the GNU CC driver program options to
pass to @code{cc1}.  It can also specify how to translate options you
give to GNU CC into options for GNU CC to pass to the @code{cc1}.

Do not define this macro if it does not need to do anything.

@findex CC1PLUS_SPEC
@item CC1PLUS_SPEC
A C string constant that tells the GNU CC driver program options to
pass to @code{cc1plus}.  It can also specify how to translate options you
give to GNU CC into options for GNU CC to pass to the @code{cc1plus}.

Do not define this macro if it does not need to do anything.

@findex ASM_SPEC
@item 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 @file{sun3.h} for an example of this.

Do not define this macro if it does not need to do anything.

@findex ASM_FINAL_SPEC
@item 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 @file{mips.h} for
an example of this.

Do not define this macro if it does not need to do anything.

@findex LINK_SPEC
@item 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.

@findex LIB_SPEC
@item LIB_SPEC
Another C string constant used much like @code{LINK_SPEC}.  The difference
between the two is that @code{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 @file{gcc.c}.

@findex LIBGCC_SPEC
@item LIBGCC_SPEC
Another C string constant that tells the GNU CC driver program
how and when to place a reference to @file{libgcc.a} into the
linker command line.  This constant is placed both before and after
the value of @code{LIB_SPEC}.

If this macro is not defined, the GNU CC driver provides a default that
passes the string @samp{-lgcc} to the linker unless the @samp{-shared}
option is specified.

@findex STARTFILE_SPEC
@item STARTFILE_SPEC
Another C string constant used much like @code{LINK_SPEC}.  The
difference between the two is that @code{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 @file{gcc.c}.

@findex ENDFILE_SPEC
@item ENDFILE_SPEC
Another C string constant used much like @code{LINK_SPEC}.  The
difference between the two is that @code{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.

@findex EXTRA_SPECS
@item EXTRA_SPECS
Define this macro to provide additional specifications to put in the
@file{specs} file that can be used in various specifications like
@code{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.

@code{EXTRA_SPECS} is useful when an architecture contains several
related targets, which have various @code{..._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 @code{EXTRA_SPECS} to
define either @code{_CALL_SYSV} when the System V calling sequence is
used or @code{_CALL_AIX} when the older AIX-based calling sequence is
used.

The @file{config/rs6000/rs6000.h} target file defines:

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

#define CPP_SYS_DEFAULT ""
@end example

The @file{config/rs6000/sysv.h} target file defines:
@smallexample
#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"
@end smallexample

while the @file{config/rs6000/eabiaix.h} target file defines
@code{CPP_SYSV_DEFAULT} as:

@smallexample
#undef CPP_SYSV_DEFAULT
#define CPP_SYSV_DEFAULT "-D_CALL_AIX"
@end smallexample

@findex LINK_LIBGCC_SPECIAL
@item LINK_LIBGCC_SPECIAL
Define this macro if the driver program should find the library
@file{libgcc.a} itself and should not pass @samp{-L} options to the
linker.  If you do not define this macro, the driver program will pass
the argument @samp{-lgcc} to tell the linker to do the search and will
pass @samp{-L} options to it.

@findex LINK_LIBGCC_SPECIAL_1
@item LINK_LIBGCC_SPECIAL_1
Define this macro if the driver program should find the library
@file{libgcc.a}.  If you do not define this macro, the driver program will pass
the argument @samp{-lgcc} to tell the linker to do the search.
This macro is similar to @code{LINK_LIBGCC_SPECIAL}, except that it does
not affect @samp{-L} options.

@findex MULTILIB_DEFAULTS
@item 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
@code{MULTILIB_OPTIONS}.

Do not define this macro if @code{MULTILIB_OPTIONS} is not defined in
the target makefile fragment or if none of the options listed in
@code{MULTILIB_OPTIONS} are set by default.
@xref{Target Fragment}.

@findex RELATIVE_PREFIX_NOT_LINKDIR
@item RELATIVE_PREFIX_NOT_LINKDIR
Define this macro to tell @code{gcc} that it should only translate
a @samp{-B} prefix into a @samp{-L} linker option if the prefix
indicates an absolute file name.

@findex STANDARD_EXEC_PREFIX
@item STANDARD_EXEC_PREFIX
Define this macro as a C string constant if you wish to override the
standard choice of @file{/usr/local/lib/gcc-lib/} as the default prefix to
try when searching for the executable files of the compiler.

@findex MD_EXEC_PREFIX
@item MD_EXEC_PREFIX
If defined, this macro is an additional prefix to try after
@code{STANDARD_EXEC_PREFIX}.  @code{MD_EXEC_PREFIX} is not searched
when the @samp{-b} option is used, or the compiler is built as a cross
compiler.

@findex STANDARD_STARTFILE_PREFIX
@item STANDARD_STARTFILE_PREFIX
Define this macro as a C string constant if you wish to override the
standard choice of @file{/usr/local/lib/} as the default prefix to
try when searching for startup files such as @file{crt0.o}.

@findex MD_STARTFILE_PREFIX
@item MD_STARTFILE_PREFIX
If defined, this macro supplies an additional prefix to try after the
standard prefixes.  @code{MD_EXEC_PREFIX} is not searched when the
@samp{-b} option is used, or when the compiler is built as a cross
compiler.

@findex MD_STARTFILE_PREFIX_1
@item MD_STARTFILE_PREFIX_1
If defined, this macro supplies yet another prefix to try after the
standard prefixes.  It is not searched when the @samp{-b} option is
used, or when the compiler is built as a cross compiler.

@findex INIT_ENVIRONMENT
@item INIT_ENVIRONMENT
Define this macro as a C string constant if you with 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 @code{putenv} to
initialize the necessary environment variables.

@findex LOCAL_INCLUDE_DIR
@item LOCAL_INCLUDE_DIR
Define this macro as a C string constant if you wish to override the
standard choice of @file{/usr/local/include} as the default prefix to
try when searching for local header files.  @code{LOCAL_INCLUDE_DIR}
comes before @code{SYSTEM_INCLUDE_DIR} in the search order.

Cross compilers do not use this macro and do not search either
@file{/usr/local/include} or its replacement.

@findex SYSTEM_INCLUDE_DIR
@item 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.  @code{SYSTEM_INCLUDE_DIR} comes before
@code{STANDARD_INCLUDE_DIR} in the search order.

Cross compilers do not use this macro and do not search the directory
specified.

@findex STANDARD_INCLUDE_DIR
@item STANDARD_INCLUDE_DIR
Define this macro as a C string constant if you wish to override the
standard choice of @file{/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
@file{/usr/include} or its replacement.

@findex INCLUDE_DEFAULTS
@item INCLUDE_DEFAULTS
Define this macro if you wish to override the entire default search path
for include files.  The default search path includes
@code{GCC_INCLUDE_DIR}, @code{LOCAL_INCLUDE_DIR},
@code{SYSTEM_INCLUDE_DIR}, @code{GPLUSPLUS_INCLUDE_DIR}, and
@code{STANDARD_INCLUDE_DIR}.  In addition, @code{GPLUSPLUS_INCLUDE_DIR}
and @code{GCC_INCLUDE_DIR} are defined automatically by @file{Makefile},
and specify private search areas for GCC.  The directory
@code{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 two elements: the directory name (a
string constant) and a flag for C++-only directories.  Mark the end of
the array with a null element.  For example, here is the definition used
for VMS:

@example
#define INCLUDE_DEFAULTS \
@{                                       \
  @{ "GNU_GXX_INCLUDE:", 1@},             \
  @{ "GNU_CC_INCLUDE:", 0@},              \
  @{ "SYS$SYSROOT:[SYSLIB.]", 0@},        \
  @{ ".", 0@},                            \
  @{ 0, 0@}                               \
@}
@end example
@end table

Here is the order of prefixes tried for exec files:

@enumerate
@item
Any prefixes specified by the user with @samp{-B}.

@item
The environment variable @code{GCC_EXEC_PREFIX}, if any.

@item
The directories specified by the environment variable @code{COMPILER_PATH}.

@item
The macro @code{STANDARD_EXEC_PREFIX}.

@item
@file{/usr/lib/gcc/}.

@item
The macro @code{MD_EXEC_PREFIX}, if any.
@end enumerate

Here is the order of prefixes tried for startfiles:

@enumerate
@item
Any prefixes specified by the user with @samp{-B}.

@item
The environment variable @code{GCC_EXEC_PREFIX}, if any.

@item
The directories specified by the environment variable @code{LIBRARY_PATH}
(native only, cross compilers do not use this).

@item
The macro @code{STANDARD_EXEC_PREFIX}.

@item
@file{/usr/lib/gcc/}.

@item
The macro @code{MD_EXEC_PREFIX}, if any.

@item
The macro @code{MD_STARTFILE_PREFIX}, if any.

@item
The macro @code{STANDARD_STARTFILE_PREFIX}.

@item
@file{/lib/}.

@item
@file{/usr/lib/}.
@end enumerate

@node Run-time Target
@section Run-time Target Specification
@cindex run-time target specification
@cindex predefined macros
@cindex target specifications

@c prevent bad page break with this line
Here are run-time target specifications.

@table @code
@findex CPP_PREDEFINES
@item CPP_PREDEFINES
Define this to be a string constant containing @samp{-D} options to
define the predefined macros that identify this machine and system.
These macros will be predefined unless the @samp{-ansi} option is
specified.

In addition, a parallel set of macros are predefined, whose names are
made by appending @samp{__} at the beginning and at the end.  These
@samp{__} macros are permitted by the ANSI standard, so they are
predefined regardless of whether @samp{-ansi} is specified.

For example, on the Sun, one can use the following value:

@smallexample
"-Dmc68000 -Dsun -Dunix"
@end smallexample

The result is to define the macros @code{__mc68000__}, @code{__sun__}
and @code{__unix__} unconditionally, and the macros @code{mc68000},
@code{sun} and @code{unix} provided @samp{-ansi} is not specified.

@findex extern int target_flags
@item extern int target_flags;
This declaration should be present.

@cindex optional hardware or system features
@cindex features, optional, in system conventions
@item TARGET_@dots{}
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 @code{TARGET_68020} that tests a bit in
@code{target_flags}.

Define a macro @code{TARGET_@var{featurename}} for each such option.
Its definition should test a bit in @code{target_flags}; for example:

@smallexample
#define TARGET_68020 (target_flags & 1)
@end smallexample

One place where these macros are used is in the condition-expressions
of instruction patterns.  Note how @code{TARGET_68020} appears
frequently in the 68000 machine description file, @file{m68k.md}.
Another place they are used is in the definitions of the other
macros in the @file{@var{machine}.h} file.

@findex TARGET_SWITCHES
@item TARGET_SWITCHES
This macro defines names of command options to set and clear
bits in @code{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
@code{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 @samp{-m} to the specified name.

One of the subgroupings should have a null string.  The number in
this grouping is the default value for @code{target_flags}.  Any
target options act starting with that value.

Here is an example which defines @samp{-m68000} and @samp{-m68020}
with opposite meanings, and picks the latter as the default:

@smallexample
#define TARGET_SWITCHES \
  @{ @{ "68020", 1@},      \
    @{ "68000", -1@},     \
    @{ "", 1@}@}
@end smallexample

@findex TARGET_OPTIONS
@item TARGET_OPTIONS
This macro is similar to @code{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
@code{char *}, is set to the variable part of the given option if the fixed
part matches.  The actual option name is made by appending @samp{-m} to the
specified name.

Here is an example which defines @samp{-mshort-data-@var{number}}.  If the
given option is @samp{-mshort-data-512}, the variable @code{m88k_short_data}
will be set to the string @code{"512"}.

@smallexample
extern char *m88k_short_data;
#define TARGET_OPTIONS \
 @{ @{ "short-data-", &m88k_short_data @} @}
@end smallexample

@findex TARGET_VERSION
@item TARGET_VERSION
This macro is a C statement to print on @code{stderr} a string
describing the particular machine description choice.  Every machine
description should define @code{TARGET_VERSION}.  For example:

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

@findex OVERRIDE_OPTIONS
@item OVERRIDE_OPTIONS
Sometimes certain combinations of command options do not make sense on
a particular target machine.  You can define a macro
@code{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
@samp{-O}.  That is what @code{OPTIMIZATION_OPTIONS} is for.

@findex OPTIMIZATION_OPTIONS
@item OPTIMIZATION_OPTIONS (@var{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.

@var{level} is the optimization level specified; 2 if @samp{-O2} is
specified, 1 if @samp{-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.

@strong{Do not examine @code{write_symbols} in
this macro!} The debugging options are not supposed to alter the
generated code.

@findex CAN_DEBUG_WITHOUT_FP
@item 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
@samp{-fomit-frame-pointer} option whenever @samp{-O} is specified.
@end table

@node Storage Layout
@section Storage Layout
@cindex 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 @code{target_flags}.
@xref{Run-time Target}.

@table @code
@findex BITS_BIG_ENDIAN
@item 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 @code{BYTES_BIG_ENDIAN}.

@findex BYTES_BIG_ENDIAN
@item 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.

@findex WORDS_BIG_ENDIAN
@item 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.

@findex LIBGCC2_WORDS_BIG_ENDIAN
@item 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.

@findex FLOAT_WORDS_BIG_ENDIAN
@item FLOAT_WORDS_BIG_ENDIAN
Define this macro to have the value 1 if @code{DFmode}, @code{XFmode} or
@code{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.

@findex BITS_PER_UNIT
@item BITS_PER_UNIT
Define this macro to be the number of bits in an addressable storage
unit (byte); normally 8.

@findex BITS_PER_WORD
@item BITS_PER_WORD
Number of bits in a word; normally 32.

@findex MAX_BITS_PER_WORD
@item MAX_BITS_PER_WORD
Maximum number of bits in a word.  If this is undefined, the default is
@code{BITS_PER_WORD}.  Otherwise, it is the constant value that is the
largest value that @code{BITS_PER_WORD} can have at run-time.

@findex UNITS_PER_WORD
@item UNITS_PER_WORD
Number of storage units in a word; normally 4.

@findex MIN_UNITS_PER_WORD
@item MIN_UNITS_PER_WORD
Minimum number of units in a word.  If this is undefined, the default is
@code{UNITS_PER_WORD}.  Otherwise, it is the constant value that is the
smallest value that @code{UNITS_PER_WORD} can have at run-time.

@findex POINTER_SIZE
@item POINTER_SIZE
Width of a pointer, in bits.  You must specify a value no wider than the
width of @code{Pmode}.  If it is not equal to the width of @code{Pmode},
you must define @code{POINTERS_EXTEND_UNSIGNED}.

@findex POINTERS_EXTEND_UNSIGNED
@item POINTERS_EXTEND_UNSIGNED
A C expression whose value is nonzero if pointers that need to be
extended from being @code{POINTER_SIZE} bits wide to @code{Pmode}
are sign-extended and zero if they are zero-extended.

You need not define this macro if the @code{POINTER_SIZE} is equal
to the width of @code{Pmode}.

@findex PROMOTE_MODE
@item PROMOTE_MODE (@var{m}, @var{unsignedp}, @var{type})
A macro to update @var{m} and @var{unsignedp} when an object whose type
is @var{type} and which has the specified mode and signedness is to be
stored in a register.  This macro is only called when @var{type} is a
scalar type.

On most RISC machines, which only have operations that operate on a full
register, define this macro to set @var{m} to @code{word_mode} if
@var{m} is an integer mode narrower than @code{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 @var{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
@var{unsignedp} according to which kind of extension is more efficient.

Do not define this macro if it would never modify @var{m}.

@findex PROMOTE_FUNCTION_ARGS
@item PROMOTE_FUNCTION_ARGS
Define this macro if the promotion described by @code{PROMOTE_MODE}
should also be done for outgoing function arguments.

@findex PROMOTE_FUNCTION_RETURN
@item PROMOTE_FUNCTION_RETURN
Define this macro if the promotion described by @code{PROMOTE_MODE}
should also be done for the return value of functions.

If this macro is defined, @code{FUNCTION_VALUE} must perform the same
promotions done by @code{PROMOTE_MODE}.

@findex PROMOTE_FOR_CALL_ONLY
@item PROMOTE_FOR_CALL_ONLY
Define this macro if the promotion described by @code{PROMOTE_MODE}
should @emph{only} be performed for outgoing function arguments or
function return values, as specified by @code{PROMOTE_FUNCTION_ARGS}
and @code{PROMOTE_FUNCTION_RETURN}, respectively.

@findex PARM_BOUNDARY
@item 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.

@findex STACK_BOUNDARY
@item 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).

@cindex @code{PUSH_ROUNDING}, interaction with @code{STACK_BOUNDARY}
If @code{PUSH_ROUNDING} is not defined, the stack will always be aligned
to the specified boundary.  If @code{PUSH_ROUNDING} is defined and specifies a
less strict alignment than @code{STACK_BOUNDARY}, the stack may be
momentarily unaligned while pushing arguments.

@findex FUNCTION_BOUNDARY
@item FUNCTION_BOUNDARY
Alignment required for a function entry point, in bits.

@findex BIGGEST_ALIGNMENT
@item BIGGEST_ALIGNMENT
Biggest alignment that any data type can require on this machine, in bits.

@findex BIGGEST_FIELD_ALIGNMENT
@item BIGGEST_FIELD_ALIGNMENT
Biggest alignment that any structure field can require on this machine,
in bits.  If defined, this overrides @code{BIGGEST_ALIGNMENT} for
structure fields only.

@findex ADJUST_FIELD_ALIGN
@item ADJUST_FIELD_ALIGN (@var{field}, @var{computed})
An expression for the alignment of a structure field @var{field} if the
alignment computed in the usual way is @var{computed}.  GNU CC uses
this value instead of the value in @code{BIGGEST_ALIGNMENT} or
@code{BIGGEST_FIELD_ALIGNMENT}, if defined, for structure fields only.

@findex MAX_OFILE_ALIGNMENT
@item 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
@code{__attribute__ ((aligned (@var{n})))} construct.  If not defined,
the default value is @code{BIGGEST_ALIGNMENT}.

@findex DATA_ALIGNMENT
@item DATA_ALIGNMENT (@var{type}, @var{basic-align})
If defined, a C expression to compute the alignment for a static
variable.  @var{type} is the data type, and @var{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 @var{basic-align} is used.

@findex strcpy
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 @code{strcpy} calls that copy
constants to character arrays can be done inline.

@findex CONSTANT_ALIGNMENT
@item CONSTANT_ALIGNMENT (@var{constant}, @var{basic-align})
If defined, a C expression to compute the alignment given to a constant
that is being placed in memory.  @var{constant} is the constant and
@var{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 @var{basic-align} is used.

The typical use of this macro is to increase alignment for string
constants to be word aligned so that @code{strcpy} calls that copy
constants can be done inline.

@findex EMPTY_FIELD_BOUNDARY
@item EMPTY_FIELD_BOUNDARY
Alignment in bits to be given to a structure bit field that follows an
empty field such as @code{int : 0;}.

Note that @code{PCC_BITFIELD_TYPE_MATTERS} also affects the alignment
that results from an empty field.

@findex STRUCTURE_SIZE_BOUNDARY
@item 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
@code{BITS_PER_UNIT}.

@findex STRICT_ALIGNMENT
@item 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.

@findex PCC_BITFIELD_TYPE_MATTERS
@item 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 (@code{int},
@code{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 @code{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 @samp{insv}, @samp{extv}, and
@samp{extzv} insns that can directly reference memory.

The other known way of making bitfields work is to define
@code{STRUCTURE_SIZE_BOUNDARY} as large as @code{BIGGEST_ALIGNMENT}.
Then every structure can be accessed with fullwords.

Unless the machine has bitfield instructions or you define
@code{STRUCTURE_SIZE_BOUNDARY} that way, you must define
@code{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:

@example
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);
@}
@end example

If this prints 2 and 5, then the compiler's behavior is what you would
get from @code{PCC_BITFIELD_TYPE_MATTERS}.

@findex BITFIELD_NBYTES_LIMITED
@item BITFIELD_NBYTES_LIMITED
Like PCC_BITFIELD_TYPE_MATTERS except that its effect is limited to
aligning a bitfield within the structure.

@findex ROUND_TYPE_SIZE
@item ROUND_TYPE_SIZE (@var{struct}, @var{size}, @var{align})
Define this macro as an expression for the overall size of a structure
(given by @var{struct} as a tree node) when the size computed from the
fields is @var{size} and the alignment is @var{align}.

The default is to round @var{size} up to a multiple of @var{align}.

@findex ROUND_TYPE_ALIGN
@item ROUND_TYPE_ALIGN (@var{struct}, @var{computed}, @var{specified})
Define this macro as an expression for the alignment of a structure
(given by @var{struct} as a tree node) if the alignment computed in the
usual way is @var{computed} and the alignment explicitly specified was
@var{specified}.

The default is to use @var{specified} if it is larger; otherwise, use
the smaller of @var{computed} and @code{BIGGEST_ALIGNMENT}

@findex MAX_FIXED_MODE_SIZE
@item 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, @code{GET_MODE_BITSIZE
(DImode)} is assumed.

@findex CHECK_FLOAT_VALUE
@item CHECK_FLOAT_VALUE (@var{mode}, @var{value}, @var{overflow})
A C statement to validate the value @var{value} (of type
@code{double}) for mode @var{mode}.  This means that you check whether
@var{value} fits within the possible range of values for mode
@var{mode} on this target machine.  The mode @var{mode} is always
a mode of class @code{MODE_FLOAT}.  @var{overflow} is nonzero if
the value is already known to be out of range.

If @var{value} is not valid or if @var{overflow} is nonzero, you should
set @var{overflow} to 1 and then assign some valid value to @var{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.

@findex TARGET_FLOAT_FORMAT
@item TARGET_FLOAT_FORMAT
A code distinguishing the floating point format of the target machine.
There are three defined values:

@table @code
@findex IEEE_FLOAT_FORMAT
@item 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.

@findex VAX_FLOAT_FORMAT
@item VAX_FLOAT_FORMAT
This code indicates the peculiar format used on the Vax.

@findex UNKNOWN_FLOAT_FORMAT
@item UNKNOWN_FLOAT_FORMAT
This code indicates any other format.
@end table

The value of this macro is compared with @code{HOST_FLOAT_FORMAT}
(@pxref{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 @code{FLOAT_WORDS_BIG_ENDIAN} for the target
machine and @code{HOST_FLOAT_WORDS_BIG_ENDIAN} for the host.
@end table

@node Type Layout
@section 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.

@table @code
@findex INT_TYPE_SIZE
@item INT_TYPE_SIZE
A C expression for the size in bits of the type @code{int} on the
target machine.  If you don't define this, the default is one word.

@findex MAX_INT_TYPE_SIZE
@item MAX_INT_TYPE_SIZE
Maximum number for the size in bits of the type @code{int} on the target
machine.  If this is undefined, the default is @code{INT_TYPE_SIZE}.
Otherwise, it is the constant value that is the largest value that
@code{INT_TYPE_SIZE} can have at run-time.  This is used in @code{cpp}.

@findex SHORT_TYPE_SIZE
@item SHORT_TYPE_SIZE
A C expression for the size in bits of the type @code{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.)

@findex LONG_TYPE_SIZE
@item LONG_TYPE_SIZE
A C expression for the size in bits of the type @code{long} on the
target machine.  If you don't define this, the default is one word.

@findex MAX_LONG_TYPE_SIZE
@item MAX_LONG_TYPE_SIZE
Maximum number for the size in bits of the type @code{long} on the
target machine.  If this is undefined, the default is
@code{LONG_TYPE_SIZE}.  Otherwise, it is the constant value that is the
largest value that @code{LONG_TYPE_SIZE} can have at run-time.  This is
used in @code{cpp}.

@findex LONG_LONG_TYPE_SIZE
@item LONG_LONG_TYPE_SIZE
A C expression for the size in bits of the type @code{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.

@findex CHAR_TYPE_SIZE
@item CHAR_TYPE_SIZE
A C expression for the size in bits of the type @code{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.)

@findex MAX_CHAR_TYPE_SIZE
@item MAX_CHAR_TYPE_SIZE
Maximum number for the size in bits of the type @code{char} on the
target machine.  If this is undefined, the default is
@code{CHAR_TYPE_SIZE}.  Otherwise, it is the constant value that is the
largest value that @code{CHAR_TYPE_SIZE} can have at run-time.  This is
used in @code{cpp}.

@findex FLOAT_TYPE_SIZE
@item FLOAT_TYPE_SIZE
A C expression for the size in bits of the type @code{float} on the
target machine.  If you don't define this, the default is one word.

@findex DOUBLE_TYPE_SIZE
@item DOUBLE_TYPE_SIZE
A C expression for the size in bits of the type @code{double} on the
target machine.  If you don't define this, the default is two
words.

@findex LONG_DOUBLE_TYPE_SIZE
@item LONG_DOUBLE_TYPE_SIZE
A C expression for the size in bits of the type @code{long double} on
the target machine.  If you don't define this, the default is two
words.

@findex DEFAULT_SIGNED_CHAR
@item DEFAULT_SIGNED_CHAR
An expression whose value is 1 or 0, according to whether the type
@code{char} should be signed or unsigned by default.  The user can
always override this default with the options @samp{-fsigned-char}
and @samp{-funsigned-char}.

@findex DEFAULT_SHORT_ENUMS
@item DEFAULT_SHORT_ENUMS
A C expression to determine whether to give an @code{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
@code{enum} types should be allocated like @code{int}.

If you don't define the macro, the default is 0.

@findex SIZE_TYPE
@item SIZE_TYPE
A C expression for a string describing the name of the data type to use
for size values.  The typedef name @code{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 @code{unsigned} if
appropriate, and finally @code{int}.  The string must exactly match one
of the data type names defined in the function
@code{init_decl_processing} in the file @file{c-decl.c}.  You may not
omit @code{int} or change the order---that would cause the compiler to
crash on startup.

If you don't define this macro, the default is @code{"long unsigned
int"}.

@findex PTRDIFF_TYPE
@item 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
@code{ptrdiff_t} is defined using the contents of the string.  See
@code{SIZE_TYPE} above for more information.

If you don't define this macro, the default is @code{"long int"}.

@findex WCHAR_TYPE
@item WCHAR_TYPE
A C expression for a string describing the name of the data type to use
for wide characters.  The typedef name @code{wchar_t} is defined using
the contents of the string.  See @code{SIZE_TYPE} above for more
information.

If you don't define this macro, the default is @code{"int"}.

@findex WCHAR_TYPE_SIZE
@item WCHAR_TYPE_SIZE
A C expression for the size in bits of the data type for wide
characters.  This is used in @code{cpp}, which cannot make use of
@code{WCHAR_TYPE}.

@findex MAX_WCHAR_TYPE_SIZE
@item 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
@code{WCHAR_TYPE_SIZE}.  Otherwise, it is the constant value that is the
largest value that @code{WCHAR_TYPE_SIZE} can have at run-time.  This is
used in @code{cpp}.

@findex OBJC_INT_SELECTORS
@item OBJC_INT_SELECTORS
Define this macro if the type of Objective C selectors should be
@code{int}.

If this macro is not defined, then selectors should have the type
@code{struct objc_selector *}.

@findex OBJC_SELECTORS_WITHOUT_LABELS
@item 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.

@findex TARGET_BELL
@item TARGET_BELL
A C constant expression for the integer value for escape sequence
@samp{\a}.

@findex TARGET_TAB
@findex TARGET_BS
@findex TARGET_NEWLINE
@item TARGET_BS
@itemx TARGET_TAB
@itemx TARGET_NEWLINE
C constant expressions for the integer values for escape sequences
@samp{\b}, @samp{\t} and @samp{\n}.

@findex TARGET_VT
@findex TARGET_FF
@findex TARGET_CR
@item TARGET_VT
@itemx TARGET_FF
@itemx TARGET_CR
C constant expressions for the integer values for escape sequences
@samp{\v}, @samp{\f} and @samp{\r}.
@end table

@node Registers
@section Register Usage
@cindex 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 @ref{Register Classes}.  For information
on using registers to access a stack frame, see @ref{Frame Registers}.
For passing values in registers, see @ref{Register Arguments}.
For returning values in registers, see @ref{Scalar Return}.

@menu
* Register Basics::		Number and kinds of registers.
* Allocation Order::		Order in which registers are allocated.
* Values in Registers::		What kinds of values each reg can hold.
* Leaf Functions::		Renumbering registers for leaf functions.
* Stack Registers::		Handling a register stack such as 80387.
* Obsolete Register Macros::	Macros formerly used for the 80387.
@end menu

@node Register Basics
@subsection Basic Characteristics of Registers

@c prevent bad page break with this line
Registers have various characteristics.

@table @code
@findex FIRST_PSEUDO_REGISTER
@item FIRST_PSEUDO_REGISTER
Number of hardware registers known to the compiler.  They receive
numbers 0 through @code{FIRST_PSEUDO_REGISTER-1}; thus, the first
pseudo register's number really is assigned the number
@code{FIRST_PSEUDO_REGISTER}.

@item FIXED_REGISTERS
@findex FIXED_REGISTERS
@cindex fixed register
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 @var{n}th number is 1 if
register @var{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 @code{CONDITIONAL_REGISTER_USAGE}, or by
the user with the command options @samp{-ffixed-@var{reg}},
@samp{-fcall-used-@var{reg}} and @samp{-fcall-saved-@var{reg}}.

@findex CALL_USED_REGISTERS
@item CALL_USED_REGISTERS
@cindex call-used register
@cindex call-clobbered register
@cindex call-saved register
Like @code{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 @code{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.

@findex CONDITIONAL_REGISTER_USAGE
@findex fixed_regs
@findex call_used_regs
@item CONDITIONAL_REGISTER_USAGE
Zero or more C statements that may conditionally modify two variables
@code{fixed_regs} and @code{call_used_regs} (both of type @code{char
[]}) after they have been initialized from the two preceding macros.

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.

@cindex disabling certain registers
@cindex controlling register usage
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
@code{fixed_regs} and @code{call_used_regs} to 1 for each of the
registers in the classes which should not be used by GCC.  Also define
the macro @code{REG_CLASS_FROM_LETTER} to return @code{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 @code{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.)

@findex NON_SAVING_SETJMP
@item NON_SAVING_SETJMP
If this macro is defined and has a nonzero value, it means that
@code{setjmp} and related functions fail to save the registers, or that
@code{longjmp} fails to restore them.  To compensate, the compiler
avoids putting variables in registers in functions that use
@code{setjmp}.

@findex INCOMING_REGNO
@item INCOMING_REGNO (@var{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 @var{out} as seen by the calling
function.  Return @var{out} if register number @var{out} is not an
outbound register.

@findex OUTGOING_REGNO
@item OUTGOING_REGNO (@var{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 @var{in} as seen by the called
function.  Return @var{in} if register number @var{in} is not an inbound
register.

@ignore
@findex PC_REGNUM
@item PC_REGNUM
If the program counter has a register number, define this as that
register number.  Otherwise, do not define it.
@end ignore
@end table

@node Allocation Order
@subsection Order of Allocation of Registers
@cindex order of register allocation
@cindex register allocation order

@c prevent bad page break with this line
Registers are allocated in order.

@table @code
@findex REG_ALLOC_ORDER
@item 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 @code{REG_ALLOC_ORDER} to be an initializer that lists
the highest numbered allocatable register first.

@findex ORDER_REGS_FOR_LOCAL_ALLOC
@item 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 @code{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
@code{reg_alloc_order} before execution of the macro.

On most machines, it is not necessary to define this macro.
@end table

@node Values in Registers
@subsection 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.

@table @code
@findex HARD_REGNO_NREGS
@item HARD_REGNO_NREGS (@var{regno}, @var{mode})
A C expression for the number of consecutive hard registers, starting
at register number @var{regno}, required to hold a value of mode
@var{mode}.

On a machine where all registers are exactly one word, a suitable
definition of this macro is

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

@findex HARD_REGNO_MODE_OK
@item HARD_REGNO_MODE_OK (@var{regno}, @var{mode})
A C expression that is nonzero if it is permissible to store a value
of mode @var{mode} in hard register number @var{regno} (or in several
registers starting with that one).  For a machine where all registers
are equivalent, a suitable definition is

@smallexample
#define HARD_REGNO_MODE_OK(REGNO, MODE) 1
@end smallexample

It is not necessary for this macro to check for the numbers of fixed
registers, because the allocation mechanism considers them to be always
occupied.

@cindex register pairs
On some machines, double-precision values must be kept in even/odd
register pairs.  The way to implement that is to define this macro
to reject odd register numbers for such modes.

@ignore
@c I think this is not true now
GNU CC assumes that it can always move values between registers and
(suitably addressed) memory locations.  If it is impossible to move a
value of a certain mode between memory and certain registers, then
@code{HARD_REGNO_MODE_OK} must not allow this mode in those registers.
@end ignore

The minimum requirement for a mode to be OK in a register is that the
@samp{mov@var{mode}} instruction pattern support moves between the
register and any other hard register for which the mode is OK; and that
moving a value into the register and back out not alter it.

Since the same instruction used to move @code{SImode} will work for all
narrower integer modes, it is not necessary on any machine for
@code{HARD_REGNO_MODE_OK} to distinguish between these modes, provided
you define patterns @samp{movhi}, etc., to take advantage of this.  This
is useful because of the interaction between @code{HARD_REGNO_MODE_OK}
and @code{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 @emph{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,
@code{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
@code{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 @code{GENERAL_REGS}, they will not
be used unless some pattern's constraint asks for one.

@findex MODES_TIEABLE_P
@item MODES_TIEABLE_P (@var{mode1}, @var{mode2})
A C expression that is nonzero if it is desirable to choose register
allocation so as to avoid move instructions between a value of mode
@var{mode1} and a value of mode @var{mode2}.

If @code{HARD_REGNO_MODE_OK (@var{r}, @var{mode1})} and
@code{HARD_REGNO_MODE_OK (@var{r}, @var{mode2})} are ever different
for any @var{r}, then @code{MODES_TIEABLE_P (@var{mode1},
@var{mode2})} must be zero.
@end table

@node Leaf Functions
@subsection Handling Leaf Functions

@cindex leaf functions
@cindex functions, leaf
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.

@table @code
@findex LEAF_REGISTERS
@item 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.

@findex LEAF_REG_REMAP
@item LEAF_REG_REMAP (@var{regno})
A C expression whose value is the register number to which @var{regno}
should be renumbered, when a function is treated as a leaf function.

If @var{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.
@end table

@findex leaf_function
Normally, @code{FUNCTION_PROLOGUE} and @code{FUNCTION_EPILOGUE} must
treat leaf functions specially.  It can test the C variable
@code{leaf_function} which is nonzero for leaf functions.  (The variable
@code{leaf_function} is defined only if @code{LEAF_REGISTERS} is
defined.)
@c changed this to fix overfull.  ALSO:  why the "it" at the beginning
@c of the next paragraph?!  --mew 2feb93

@node Stack Registers
@subsection 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.

@table @code
@findex STACK_REGS
@item STACK_REGS
Define this if the machine has any stack-like registers.

@findex FIRST_STACK_REG
@item FIRST_STACK_REG
The number of the first stack-like register.  This one is the top
of the stack.

@findex LAST_STACK_REG
@item LAST_STACK_REG
The number of the last stack-like register.  This one is the bottom of
the stack.
@end table

@node Obsolete Register Macros
@subsection 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!

@table @code
@findex OVERLAPPING_REGNO_P
@item OVERLAPPING_REGNO_P (@var{regno})
If defined, this is a C expression whose value is nonzero if hard
register number @var{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 @emph{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.

@findex INSN_CLOBBERS_REGNO_P
@item INSN_CLOBBERS_REGNO_P (@var{insn}, @var{regno})
If defined, this is a C expression whose value should be nonzero if
the insn @var{insn} has the effect of mysteriously clobbering the
contents of hard register number @var{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.

@cindex death notes
@findex PRESERVE_DEATH_INFO_REGNO_P
@item PRESERVE_DEATH_INFO_REGNO_P (@var{regno})
If defined, this is a C expression whose value is nonzero if correct
@code{REG_DEAD} notes are needed for hard register number @var{regno}
after reload.

You would arrange to preserve death info for a register when some of the
code in the machine description which is executed to write the assembler
code looks at the death notes.  This is necessary only when the actual
hardware feature which GNU CC thinks of as a register is not actually a
register of the usual sort.  (It might, for example, be a hardware
stack.)

It is also useful for peepholes and linker relaxation.

If this macro is not defined, it means that no death notes need to be
preserved, and some may even be incorrect.  This is the usual situation.
@end table

@node Register Classes
@section Register Classes
@cindex register class definitions
@cindex class definitions, register

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 @dfn{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.

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

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

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

The way classes other than @code{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 @code{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 @code{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 (@code{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
@code{PREFERRED_RELOAD_CLASS} can always have a possible value to return.

@table @code
@findex enum reg_class
@item enum reg_class
An enumeral type that must be defined with all the register class names
as enumeral values.  @code{NO_REGS} must be first.  @code{ALL_REGS}
must be the last register class, followed by one more enumeral value,
@code{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 @code{int}.  The number serves as an index
in many of the tables described below.

@findex N_REG_CLASSES
@item N_REG_CLASSES
The number of distinct register classes, defined as follows:

@example
#define N_REG_CLASSES (int) LIM_REG_CLASSES
@end example

@findex REG_CLASS_NAMES
@item 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.

@findex REG_CLASS_CONTENTS
@item REG_CLASS_CONTENTS
An initializer containing the contents of the register classes, as integers
which are bit masks.  The @var{n}th integer specifies the contents of class
@var{n}.  The way the integer @var{mask} is interpreted is that
register @var{r} is in the class if @code{@var{mask} & (1 << @var{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 @code{HARD_REG_SET} which is defined in @file{hard-reg-set.h}.

@findex REGNO_REG_CLASS
@item REGNO_REG_CLASS (@var{regno})
A C expression whose value is a register class containing hard register
@var{regno}.  In general there is more than one such class; choose a class
which is @dfn{minimal}, meaning that no smaller class also contains the
register.

@findex BASE_REG_CLASS
@item 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.

@findex INDEX_REG_CLASS
@item 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).

@findex REG_CLASS_FROM_LETTER
@item REG_CLASS_FROM_LETTER (@var{char})
A C expression which defines the machine-dependent operand constraint
letters for register classes.  If @var{char} is such a letter, the
value should be the register class corresponding to it.  Otherwise,
the value should be @code{NO_REGS}.  The register letter @samp{r},
corresponding to class @code{GENERAL_REGS}, will not be passed
to this macro; you do not need to handle it.

@findex REGNO_OK_FOR_BASE_P
@item REGNO_OK_FOR_BASE_P (@var{num})
A C expression which is nonzero if register number @var{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.

@findex REGNO_OK_FOR_INDEX_P
@item REGNO_OK_FOR_INDEX_P (@var{num})
A C expression which is nonzero if register number @var{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.

@findex PREFERRED_RELOAD_CLASS
@item PREFERRED_RELOAD_CLASS (@var{x}, @var{class})
A C expression that places additional restrictions on the register class
to use when it is necessary to copy value @var{x} into a register in class
@var{class}.  The value is a register class; perhaps @var{class}, or perhaps
another, smaller class.  On many machines, the following definition is
safe:

@example
#define PREFERRED_RELOAD_CLASS(X,CLASS) CLASS
@end example

Sometimes returning a more restrictive class makes better code.  For
example, on the 68000, when @var{x} is an integer constant that is in range
for a @samp{moveq} instruction, the value of this macro is always
@code{DATA_REGS} as long as @var{class} includes the data registers.
Requiring a data register guarantees that a @samp{moveq} will be used.

If @var{x} is a @code{const_double}, by returning @code{NO_REGS}
you can force @var{x} into a memory constant.  This is useful on
certain machines where immediate floating values cannot be loaded into
certain kinds of registers.

@findex PREFERRED_OUTPUT_RELOAD_CLASS
@item PREFERRED_OUTPUT_RELOAD_CLASS (@var{x}, @var{class})
Like @code{PREFERRED_RELOAD_CLASS}, but for output reloads instead of
input reloads.  If you don't define this macro, the default is to use
@var{class}, unchanged.

@findex LIMIT_RELOAD_CLASS
@item LIMIT_RELOAD_CLASS (@var{mode}, @var{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
@var{mode} in a reload register for which class @var{class} would
ordinarily be used.

Unlike @code{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 @var{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.

@findex SECONDARY_RELOAD_CLASS
@findex SECONDARY_INPUT_RELOAD_CLASS
@findex SECONDARY_OUTPUT_RELOAD_CLASS
@item SECONDARY_RELOAD_CLASS (@var{class}, @var{mode}, @var{x})
@itemx SECONDARY_INPUT_RELOAD_CLASS (@var{class}, @var{mode}, @var{x})
@itemx SECONDARY_OUTPUT_RELOAD_CLASS (@var{class}, @var{mode}, @var{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
@samp{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 @var{x} to a
register @var{class} in @var{mode} requires an intermediate register,
you should define @code{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 @var{class} in @var{mode} to @var{x} requires an
intermediate or scratch register, @code{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
@code{SECONDARY_RELOAD_CLASS} should be used instead of defining both
macros identically.

The values returned by these macros are often @code{GENERAL_REGS}.
Return @code{NO_REGS} if no spare register is needed; i.e., if @var{x}
can be directly copied to or from a register of @var{class} in
@var{mode} without requiring a scratch register.  Do not define this
macro if it would always return @code{NO_REGS}.

If a scratch register is required (either with or without an
intermediate register), you should define patterns for
@samp{reload_in@var{m}} or @samp{reload_out@var{m}}, as required
(@pxref{Standard Names}.  These patterns, which will normally be
implemented with a @code{define_expand}, should be similar to the
@samp{mov@var{m}} 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 @var{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.

@var{x} might be a pseudo-register or a @code{subreg} of a
pseudo-register, which could either be in a hard register or in memory.
Use @code{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 @code{mov@var{m}} pattern should use memory as a
intermediate storage.  This case often occurs between floating-point and
general registers.

@findex SECONDARY_MEMORY_NEEDED
@item SECONDARY_MEMORY_NEEDED (@var{class1}, @var{class2}, @var{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
@var{m} in registers of @var{class1} can only be copied to registers of
class @var{class2} by storing a register of @var{class1} into memory
and loading that memory location into a register of @var{class2}.

Do not define this macro if its value would always be zero.

@findex SECONDARY_MEMORY_NEEDED_RTX
@item SECONDARY_MEMORY_NEEDED_RTX (@var{mode})
Normally when @code{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
@code{SECONDARY_MEMORY_NEEDED}.

@findex SECONDARY_MEMORY_NEEDED_MODE
@item SECONDARY_MEMORY_NEEDED_MODE (@var{mode})
When the compiler needs a secondary memory location to copy between two
registers of mode @var{mode}, it normally allocates sufficient memory to
hold a quantity of @code{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 @var{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 @file{alpha.h} for
details.

Do not define this macro if you do not define
@code{SECONDARY_MEMORY_NEEDED} or if widening @var{mode} to a mode that
is @code{BITS_PER_WORD} bits wide is correct for your machine.

@findex SMALL_REGISTER_CLASSES
@item SMALL_REGISTER_CLASSES
Normally the compiler avoids choosing registers that have been
explicitly mentioned in the rtl as spill registers (these registers are
normally those used to pass parameters and return values).  However,
some machines have so few registers of certain classes that there
would not be enough registers to use as spill registers if this were
done.

Define @code{SMALL_REGISTER_CLASSES} on these machines.  When it is
defined, the compiler allows registers explicitly used in the rtl to be
used as spill registers but avoids extending the lifetime of these
registers.

It is always safe to define this macro, 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 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.

@findex CLASS_LIKELY_SPILLED_P
@item CLASS_LIKELY_SPILLED_P (@var{class})
A C expression whose value is nonzero if pseudos that have been assigned
to registers of class @var{class} would likely be spilled because
registers of @var{class} are needed for spill registers.

The default value of this macro returns 1 if @var{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 pseudo
allocated by @file{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
@file{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.

@findex CLASS_MAX_NREGS
@item CLASS_MAX_NREGS (@var{class}, @var{mode})
A C expression for the maximum number of consecutive registers
of class @var{class} needed to hold a value of mode @var{mode}.

This is closely related to the macro @code{HARD_REGNO_NREGS}.  In fact,
the value of the macro @code{CLASS_MAX_NREGS (@var{class}, @var{mode})}
should be the maximum value of @code{HARD_REGNO_NREGS (@var{regno},
@var{mode})} for all @var{regno} values in the class @var{class}.

This macro helps control the handling of multiple-word values
in the reload pass.

@item 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, @file{alpha.h} defines this macro as
@code{FLOAT_REGS}.
@end table

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

@table @code
@findex CONST_OK_FOR_LETTER_P
@item CONST_OK_FOR_LETTER_P (@var{value}, @var{c})
A C expression that defines the machine-dependent operand constraint letters
that specify particular ranges of integer values.  If @var{c} is one
of those letters, the expression should check that @var{value}, an integer,
is in the appropriate range and return 1 if so, 0 otherwise.  If @var{c} is
not one of those letters, the value should be 0 regardless of @var{value}.

@findex CONST_DOUBLE_OK_FOR_LETTER_P
@item CONST_DOUBLE_OK_FOR_LETTER_P (@var{value}, @var{c})
A C expression that defines the machine-dependent operand constraint
letters that specify particular ranges of @code{const_double} values.

If @var{c} is one of those letters, the expression should check that
@var{value}, an RTX of code @code{const_double}, is in the appropriate
range and return 1 if so, 0 otherwise.  If @var{c} is not one of those
letters, the value should be 0 regardless of @var{value}.

@code{const_double} is used for all floating-point constants and for
@code{DImode} fixed-point constants.  A given letter can accept either
or both kinds of values.  It can use @code{GET_MODE} to distinguish
between these kinds.

@findex EXTRA_CONSTRAINT
@item EXTRA_CONSTRAINT (@var{value}, @var{c})
A C expression that defines the optional machine-dependent constraint
letters 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 @var{value} corresponds to the operand type
represented by the constraint letter @var{c}.  If @var{c} is not defined
as an extra constraint, the value returned should be 0 regardless of
@var{value}.

For example, on the ROMP, load instructions cannot have their output in r0 if
the memory reference contains a symbolic address.  Constraint letter
@samp{Q} is defined as representing a memory address that does
@emph{not} contain a symbolic address.  An alternative is specified with
a @samp{Q} constraint on the input and @samp{r} on the output.  The next
alternative specifies @samp{m} on the input and a register class that
does not include r0 on the output.
@end table

@node Stack and Calling
@section Stack Layout and Calling Conventions
@cindex calling conventions

@c prevent bad page break with this line
This describes the stack layout and calling conventions.

@menu
* Frame Layout::
* Frame Registers::
* Elimination::
* Stack Arguments::
* Register Arguments::
* Scalar Return::
* Aggregate Return::
* Caller Saves::
* Function Entry::
* Profiling::
@end menu

@node Frame Layout
@subsection Basic Stack Layout
@cindex stack frame layout
@cindex frame layout

@c prevent bad page break with this line
Here is the basic stack layout.

@table @code
@findex STACK_GROWS_DOWNWARD
@item 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 @dots{},'' it means that the
compiler checks this macro only with @code{#ifdef} so the precise
definition used does not matter.

@findex FRAME_GROWS_DOWNWARD
@item FRAME_GROWS_DOWNWARD
Define this macro if the addresses of local variable slots are at negative
offsets from the frame pointer.

@findex ARGS_GROW_DOWNWARD
@item ARGS_GROW_DOWNWARD
Define this macro if successive arguments to a function occupy decreasing
addresses on the stack.

@findex STARTING_FRAME_OFFSET
@item STARTING_FRAME_OFFSET
Offset from the frame pointer to the first local variable slot to be allocated.

If @code{FRAME_GROWS_DOWNWARD}, find the next slot's offset by
subtracting the first slot's length from @code{STARTING_FRAME_OFFSET}.
Otherwise, it is found by adding the length of the first slot to the
value @code{STARTING_FRAME_OFFSET}.
@c i'm not sure if the above is still correct.. had to change it to get
@c rid of an overfull.  --mew 2feb93

@findex STACK_POINTER_OFFSET
@item 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 @code{ARGS_GROW_DOWNWARD}, this is the offset to the location above
the first location at which outgoing arguments are placed.

@findex FIRST_PARM_OFFSET
@item FIRST_PARM_OFFSET (@var{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 @code{ARGS_GROW_DOWNWARD}, this is the offset to the location above
the first argument's address.

@findex STACK_DYNAMIC_OFFSET
@item STACK_DYNAMIC_OFFSET (@var{fundecl})
Offset from the stack pointer register to an item dynamically allocated
on the stack, e.g., by @code{alloca}.

The default value for this macro is @code{STACK_POINTER_OFFSET} plus the
length of the outgoing arguments.  The default is correct for most
machines.  See @file{function.c} for details.

@findex DYNAMIC_CHAIN_ADDRESS
@item DYNAMIC_CHAIN_ADDRESS (@var{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
@var{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 @var{frameaddr}---that is, the stack frame address is also the
address of the stack word that points to the previous frame.

@findex SETUP_FRAME_ADDRESSES
@item 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.
This macro will seldom need to be defined.

@findex RETURN_ADDR_RTX
@item RETURN_ADDR_RTX (@var{count}, @var{frameaddr})
A C expression whose value is RTL representing the value of the return
address for the frame @var{count} steps up from the current frame.
@var{frameaddr} is the frame pointer of the @var{count} frame, or
the frame pointer of the @var{count} @minus{} 1 frame if
@code{RETURN_ADDR_IN_PREVIOUS_FRAME} is defined.

@findex RETURN_ADDR_IN_PREVIOUS_FRAME
@item 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.
@end table

@need 2000
@node Frame Registers
@subsection Registers That Address the Stack Frame

@c prevent bad page break with this line
This discusses registers that address the stack frame.

@table @code
@findex STACK_POINTER_REGNUM
@item STACK_POINTER_REGNUM
The register number of the stack pointer register, which must also be a
fixed register according to @code{FIXED_REGISTERS}.  On most machines,
the hardware determines which register this is.

@findex FRAME_POINTER_REGNUM
@item 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.

@findex HARD_FRAME_POINTER_REGNUM
@item 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
@code{FRAME_POINTER_REGNUM} the number of a special, fixed register to
be used internally until the offset is known, and define
@code{HARD_FRAME_POINTER_REGNUM} to be actual the 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 @code{ELIMINABLE_REGS} how to eliminate
@code{FRAME_POINTER_REGNUM} into either @code{HARD_FRAME_POINTER_REGNUM}
or @code{STACK_POINTER_REGNUM}.

Do not define this macro if it would be the same as
@code{FRAME_POINTER_REGNUM}.

@findex ARG_POINTER_REGNUM
@item 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
@code{FIXED_REGISTERS}, or arrange to be able to eliminate it
(@pxref{Elimination}).

@findex RETURN_ADDRESS_POINTER_REGNUM
@item 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
@code{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.

@findex STATIC_CHAIN_REGNUM
@findex STATIC_CHAIN_INCOMING_REGNUM
@item STATIC_CHAIN_REGNUM
@itemx 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 @code{STATIC_CHAIN_INCOMING_REGNUM}, while the register
number as seen by the calling function is @code{STATIC_CHAIN_REGNUM}.  If
these registers are the same, @code{STATIC_CHAIN_INCOMING_REGNUM} need
not be defined.@refill

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.

@findex STATIC_CHAIN
@findex STATIC_CHAIN_INCOMING
@item STATIC_CHAIN
@itemx STATIC_CHAIN_INCOMING
If the static chain is passed in memory, these macros provide rtx giving
@code{mem} expressions that denote where they are stored.
@code{STATIC_CHAIN} and @code{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.@refill

@findex stack_pointer_rtx
@findex frame_pointer_rtx
@findex arg_pointer_rtx
The variables @code{stack_pointer_rtx}, @code{frame_pointer_rtx}, and
@code{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.
@end table

@node Elimination
@subsection Eliminating Frame Pointer and Arg Pointer

@c prevent bad page break with this line
This is about eliminating the frame pointer and arg pointer.

@table @code
@findex FRAME_POINTER_REQUIRED
@item 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
@code{FRAME_POINTER_REQUIRED} says.  You don't need to worry about
them.@refill

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 @code{FIXED_REGISTERS} for more information.

@findex INITIAL_FRAME_POINTER_OFFSET
@findex get_frame_size
@item INITIAL_FRAME_POINTER_OFFSET (@var{depth-var})
A C statement to store in the variable @var{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 @code{get_frame_size ()} and the tables of
registers @code{regs_ever_live} and @code{call_used_regs}.

If @code{ELIMINABLE_REGS} is defined, this macro will be not be used and
need not be defined.  Otherwise, it must be defined even if
@code{FRAME_POINTER_REQUIRED} is defined to always be true; in that
case, you may set @var{depth-var} to anything.

@findex ELIMINABLE_REGS
@item 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:
@example
#define ELIMINABLE_REGS  \
@{@{ARG_POINTER_REGNUM, STACK_POINTER_REGNUM@}, \
 @{ARG_POINTER_REGNUM, FRAME_POINTER_REGNUM@}, \
 @{FRAME_POINTER_REGNUM, STACK_POINTER_REGNUM@}@}
@end example

Note that the elimination of the argument pointer with the stack pointer is
specified first since that is the preferred elimination.

@findex CAN_ELIMINATE
@item CAN_ELIMINATE (@var{from-reg}, @var{to-reg})
A C expression that returns non-zero if the compiler is allowed to try
to replace register number @var{from-reg} with register number
@var{to-reg}.  This macro need only be defined if @code{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.

@findex INITIAL_ELIMINATION_OFFSET
@item INITIAL_ELIMINATION_OFFSET (@var{from-reg}, @var{to-reg}, @var{offset-var})
This macro is similar to @code{INITIAL_FRAME_POINTER_OFFSET}.  It
specifies the initial difference between the specified pair of
registers.  This macro must be defined if @code{ELIMINABLE_REGS} is
defined.

@findex LONGJMP_RESTORE_FROM_STACK
@item LONGJMP_RESTORE_FROM_STACK
Define this macro if the @code{longjmp} function restores registers from
the stack frames, rather than from those saved specifically by
@code{setjmp}.  Certain quantities must not be kept in registers across
a call to @code{setjmp} on such machines.
@end table

@node Stack Arguments
@subsection Passing Function Arguments on the Stack
@cindex arguments on stack
@cindex stack arguments

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.

@table @code
@findex PROMOTE_PROTOTYPES
@item PROMOTE_PROTOTYPES
Define this macro if an argument declared in a prototype as an
integral type smaller than @code{int} should actually be passed as an
@code{int}.  In addition to avoiding errors in certain cases of
mismatch, it also makes for better code on certain machines.

@findex PUSH_ROUNDING
@item PUSH_ROUNDING (@var{npushed})
A C expression that is the number of bytes actually pushed onto the
stack when an instruction attempts to push @var{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

@example
#define PUSH_ROUNDING(BYTES) (BYTES)
@end example

@noindent
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

@example
#define PUSH_ROUNDING(BYTES) (((BYTES) + 1) & ~1)
@end example

@findex ACCUMULATE_OUTGOING_ARGS
@findex current_function_outgoing_args_size
@item ACCUMULATE_OUTGOING_ARGS
If defined, the maximum amount of space required for outgoing arguments
will be computed and placed into the variable
@code{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 @code{PUSH_ROUNDING} and @code{ACCUMULATE_OUTGOING_ARGS}
is not proper.

@findex REG_PARM_STACK_SPACE
@item REG_PARM_STACK_SPACE (@var{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 @var{fndecl}.

This space can be allocated by the caller, or be a part of the
machine-dependent stack frame: @code{OUTGOING_REG_PARM_STACK_SPACE} says
which.
@c above is overfull.  not sure what to do.  --mew 5feb93  did
@c something, not sure if it looks good.  --mew 10feb93

@findex MAYBE_REG_PARM_STACK_SPACE
@findex FINAL_REG_PARM_STACK_SPACE
@item MAYBE_REG_PARM_STACK_SPACE
@itemx FINAL_REG_PARM_STACK_SPACE (@var{const_size}, @var{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, @code{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
@code{REG_PARM_STACK_SPACE}.

@findex OUTGOING_REG_PARM_STACK_SPACE
@item 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 @code{ACCUMULATE_OUTGOING_ARGS} is defined, this macro controls
whether the space for these arguments counts in the value of
@code{current_function_outgoing_args_size}.

@findex STACK_PARMS_IN_REG_PARM_AREA
@item STACK_PARMS_IN_REG_PARM_AREA
Define this macro if @code{REG_PARM_STACK_SPACE} is defined, but the
stack parameters don't skip the area specified by it.
@c i changed this, makes more sens and it should have taken care of the
@c overfull.. not as specific, tho.  --mew 5feb93

Normally, when a parameter is not passed in registers, it is placed on the
stack beyond the @code{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.

@findex RETURN_POPS_ARGS
@item RETURN_POPS_ARGS (@var{fundecl}, @var{funtype}, @var{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.

@var{fundecl} is a C variable whose value is a tree node that describes
the function in question.  Normally it is a node of type
@code{FUNCTION_DECL} that describes the declaration of the function.
From this you can obtain the DECL_MACHINE_ATTRIBUTES of the function.

@var{funtype} is a C variable whose value is a tree node that
describes the function in question.  Normally it is a node of type
@code{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, @var{funtype}
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.

@var{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 @var{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 @code{printf}) pop
nothing (the caller pops all).  When this convention is in use,
@var{funtype} is examined to determine whether a function takes a fixed
number of arguments.
@end table

@node Register Arguments
@subsection Passing Arguments in Registers
@cindex arguments in registers
@cindex registers arguments

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.

@table @code
@findex FUNCTION_ARG
@item FUNCTION_ARG (@var{cum}, @var{mode}, @var{type}, @var{named})
A C expression that controls whether a function argument is passed
in a register, and which register.

The arguments are @var{cum}, which summarizes all the previous
arguments; @var{mode}, the machine mode of the argument; @var{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 @var{named},
which is 1 for an ordinary argument and 0 for nameless arguments that
correspond to @samp{@dots{}} in the called function's prototype.

The value of the expression is usually either a @code{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 @code{parallel} RTX.  This is
used when an argument is passed in multiple locations.  The mode of the
of the @code{parallel} should be the mode of the entire argument.  The
@code{parallel} holds any number of @code{expr_list} pairs; each one
describes where part of the argument is passed.  In each @code{expr_list},
the first operand can be either a @code{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 @code{reg}, then the mode indicates
how large this part of the argument is.  The second operand of the
@code{expr_list} is a @code{const_int} which gives the offset in bytes
into the entire argument where this part starts.

@cindex @file{stdarg.h} and register arguments
The usual way to make the ANSI library @file{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 @code{FUNCTION_ARG} return 0 whenever @var{named} is 0.

@cindex @code{MUST_PASS_IN_STACK}, and @code{FUNCTION_ARG}
@cindex @code{REG_PARM_STACK_SPACE}, and @code{FUNCTION_ARG}
You may use the macro @code{MUST_PASS_IN_STACK (@var{mode}, @var{type})}
in the definition of this macro to determine if this argument is of a
type that must be passed in the stack.  If @code{REG_PARM_STACK_SPACE}
is not defined and @code{FUNCTION_ARG} returns non-zero for such an
argument, the compiler will abort.  If @code{REG_PARM_STACK_SPACE} is
defined, the argument will be computed in the stack and then loaded into
a register.

@findex FUNCTION_INCOMING_ARG
@item FUNCTION_INCOMING_ARG (@var{cum}, @var{mode}, @var{type}, @var{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, @code{FUNCTION_ARG} computes the register in which
the caller passes the value, and @code{FUNCTION_INCOMING_ARG} should
be defined in a similar fashion to tell the function being called
where the arguments will arrive.

If @code{FUNCTION_INCOMING_ARG} is not defined, @code{FUNCTION_ARG}
serves both purposes.@refill

@findex FUNCTION_ARG_PARTIAL_NREGS
@item FUNCTION_ARG_PARTIAL_NREGS (@var{cum}, @var{mode}, @var{type}, @var{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 @var{n} words of arguments are passed in registers, and the rest
on the stack.  If a multi-word argument (a @code{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.

@code{FUNCTION_ARG} for these arguments should return the first
register to be used by the caller for this argument; likewise
@code{FUNCTION_INCOMING_ARG}, for the called function.

@findex FUNCTION_ARG_PASS_BY_REFERENCE
@item FUNCTION_ARG_PASS_BY_REFERENCE (@var{cum}, @var{mode}, @var{type}, @var{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 @code{REG_PARM_STACK_SPACE} is not defined, a suitable
definition of this macro might be
@smallexample
#define FUNCTION_ARG_PASS_BY_REFERENCE\
(CUM, MODE, TYPE, NAMED)  \
  MUST_PASS_IN_STACK (MODE, TYPE)
@end smallexample
@c this is *still* too long.  --mew 5feb93

@findex FUNCTION_ARG_CALLEE_COPIES
@item FUNCTION_ARG_CALLEE_COPIES (@var{cum}, @var{mode}, @var{type}, @var{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.

@findex CUMULATIVE_ARGS
@item CUMULATIVE_ARGS
A C type for declaring a variable that is used as the first argument of
@code{FUNCTION_ARG} and other related values.  For some target machines,
the type @code{int} suffices and can hold the number of bytes of
argument so far.

There is no need to record in @code{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
@code{CUMULATIVE_ARGS}; however, the data structure must exist and
should not be empty, so use @code{int}.

@findex INIT_CUMULATIVE_ARGS
@item INIT_CUMULATIVE_ARGS (@var{cum}, @var{fntype}, @var{libname}, @var{indirect})
A C statement (sans semicolon) for initializing the variable @var{cum}
for the state at the beginning of the argument list.  The variable has
type @code{CUMULATIVE_ARGS}.  The value of @var{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
@var{indirect} is nonzero when processing an indirect call, for example
a call through a function pointer.  The value of @var{indirect} is zero
for a call to an explicitly named function, a library function call, or when
@code{INIT_CUMULATIVE_ARGS} is used to find arguments for the function
being compiled.

When processing a call to a compiler support library function,
@var{libname} identifies which one.  It is a @code{symbol_ref} rtx which
contains the name of the function, as a string.  @var{libname} is 0 when
an ordinary C function call is being processed.  Thus, each time this
macro is called, either @var{libname} or @var{fntype} is nonzero, but
never both of them at once.

@findex INIT_CUMULATIVE_INCOMING_ARGS
@item INIT_CUMULATIVE_INCOMING_ARGS (@var{cum}, @var{fntype}, @var{libname})
Like @code{INIT_CUMULATIVE_ARGS} but overrides it for the purposes of
finding the arguments for the function being compiled.  If this macro is
undefined, @code{INIT_CUMULATIVE_ARGS} is used instead.

The value passed for @var{libname} is always 0, since library routines
with special calling conventions are never compiled with GNU CC.  The
argument @var{libname} exists for symmetry with
@code{INIT_CUMULATIVE_ARGS}.
@c could use "this macro" in place of @code{INIT_CUMULATIVE_ARGS}, maybe.
@c --mew 5feb93   i switched the order of the sentences.  --mew 10feb93

@findex FUNCTION_ARG_ADVANCE
@item FUNCTION_ARG_ADVANCE (@var{cum}, @var{mode}, @var{type}, @var{named})
A C statement (sans semicolon) to update the summarizer variable
@var{cum} to advance past an argument in the argument list.  The
values @var{mode}, @var{type} and @var{named} describe that argument.
Once this is done, the variable @var{cum} is suitable for analyzing
the @emph{following} argument with @code{FUNCTION_ARG}, etc.@refill

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.

@findex FUNCTION_ARG_PADDING
@item FUNCTION_ARG_PADDING (@var{mode}, @var{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
@code{enum direction}: either @code{upward} to pad above the argument,
@code{downward} to pad below, or @code{none} to inhibit padding.

The @emph{amount} of padding is always just enough to reach the next
multiple of @code{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 @code{int}, and upward otherwise.

@findex FUNCTION_ARG_BOUNDARY
@item FUNCTION_ARG_BOUNDARY (@var{mode}, @var{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,
@code{PARM_BOUNDARY} is used for all arguments.

@findex FUNCTION_ARG_REGNO_P
@item FUNCTION_ARG_REGNO_P (@var{regno})
A C expression that is nonzero if @var{regno} is the number of a hard
register in which function arguments are sometimes passed.  This does
@emph{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.
@end table

@node Scalar Return
@subsection How Scalar Function Values Are Returned
@cindex return values in registers
@cindex values, returned by functions
@cindex scalars, returned as values

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

@table @code
@findex TRADITIONAL_RETURN_FLOAT
@item TRADITIONAL_RETURN_FLOAT
Define this macro if @samp{-traditional} should not cause functions
declared to return @code{float} to convert the value to @code{double}.

@findex FUNCTION_VALUE
@item FUNCTION_VALUE (@var{valtype}, @var{func})
A C expression to create an RTX representing the place where a
function returns a value of data type @var{valtype}.  @var{valtype} is
a tree node representing a data type.  Write @code{TYPE_MODE
(@var{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).@refill

The value of the expression is usually a @code{reg} RTX for the hard
register where the return value is stored.  The value can also be a
@code{parallel} RTX, if the return value is in multiple places.  See
@code{FUNCTION_ARG} for an explanation of the @code{parallel} form.

If @code{PROMOTE_FUNCTION_RETURN} is defined, you must apply the same
promotion rules specified in @code{PROMOTE_MODE} if @var{valtype} is a
scalar type.

If the precise function being called is known, @var{func} is a tree
node (@code{FUNCTION_DECL}) for it; otherwise, @var{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.@refill

@code{FUNCTION_VALUE} is not used for return vales with aggregate data
types, because these are returned in another way.  See
@code{STRUCT_VALUE_REGNUM} and related macros, below.

@findex FUNCTION_OUTGOING_VALUE
@item FUNCTION_OUTGOING_VALUE (@var{valtype}, @var{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, @code{FUNCTION_VALUE} computes the register in which
the caller will see the value.  @code{FUNCTION_OUTGOING_VALUE} should be
defined in a similar fashion to tell the function where to put the
value.@refill

If @code{FUNCTION_OUTGOING_VALUE} is not defined,
@code{FUNCTION_VALUE} serves both purposes.@refill

@code{FUNCTION_OUTGOING_VALUE} is not used for return vales with
aggregate data types, because these are returned in another way.  See
@code{STRUCT_VALUE_REGNUM} and related macros, below.

@findex LIBCALL_VALUE
@item LIBCALL_VALUE (@var{mode})
A C expression to create an RTX representing the place where a library
function returns a value of mode @var{mode}.  If the precise function
being called is known, @var{func} is a tree node
(@code{FUNCTION_DECL}) for it; otherwise, @var{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.@refill

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 @code{LIBRARY_VALUE} need not be concerned aggregate
data types, because none of the library functions returns such types.

@findex FUNCTION_VALUE_REGNO_P
@item FUNCTION_VALUE_REGNO_P (@var{regno})
A C expression that is nonzero if @var{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 @code{double}, say) need not be
recognized by this macro.  So for most machines, this definition
suffices:

@example
#define FUNCTION_VALUE_REGNO_P(N) ((N) == 0)
@end example

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.

@findex APPLY_RESULT_SIZE
@item APPLY_RESULT_SIZE
Define this macro if @samp{untyped_call} and @samp{untyped_return}
need more space than is implied by @code{FUNCTION_VALUE_REGNO_P} for
saving and restoring an arbitrary return value.
@end table

@node Aggregate Return
@subsection How Large Values Are Returned
@cindex aggregates as return values
@cindex large return values
@cindex returning aggregate values
@cindex structure value address

When a function value's mode is @code{BLKmode} (and in some other
cases), the value is not returned according to @code{FUNCTION_VALUE}
(@pxref{Scalar Return}).  Instead, the caller passes the address of a
block of memory in which the value should be stored.  This address
is called the @dfn{structure value address}.

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

@table @code
@findex RETURN_IN_MEMORY
@item RETURN_IN_MEMORY (@var{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 @var{type} will be a C expression of type
@code{tree}, representing the data type of the value.

Note that values of mode @code{BLKmode} must be explicitly handled
by this macro.  Also, the option @samp{-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 @code{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 @code{DEFAULT_PCC_STRUCT_RETURN}
to indicate this.

@findex DEFAULT_PCC_STRUCT_RETURN
@item 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 @code{RETURN_IN_MEMORY} macro.

If not defined, this defaults to the value 1.

@findex STRUCT_VALUE_REGNUM
@item STRUCT_VALUE_REGNUM
If the structure value address is passed in a register, then
@code{STRUCT_VALUE_REGNUM} should be the number of that register.

@findex STRUCT_VALUE
@item STRUCT_VALUE
If the structure value address is not passed in a register, define
@code{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.

@findex STRUCT_VALUE_INCOMING_REGNUM
@item 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.

@findex STRUCT_VALUE_INCOMING
@item STRUCT_VALUE_INCOMING
If the incoming location is not a register, then you should define
@code{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 @code{mem} which refers to the frame
pointer.  A definition of 0 means that the address is passed as an
``invisible'' first argument.

@findex PCC_STATIC_STRUCT_RETURN
@item 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 @samp{-fpcc-struct-return} mode, but it does
nothing when you use @samp{-freg-struct-return} mode.
@end table

@node Caller Saves
@subsection 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.

@table @code
@findex DEFAULT_CALLER_SAVES
@item DEFAULT_CALLER_SAVES
Define this macro if function calls on the target machine do not preserve
any registers; in other words, if @code{CALL_USED_REGISTERS} has 1
for all registers.  This macro enables @samp{-fcaller-saves} by default.
Eventually that option will be enabled by default on all machines and both
the option and this macro will be eliminated.

@findex CALLER_SAVE_PROFITABLE
@item CALLER_SAVE_PROFITABLE (@var{refs}, @var{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: @code{4 * @var{calls} < @var{refs}}.
@end table

@node Function Entry
@subsection Function Entry and Exit
@cindex function entry and exit
@cindex prologue
@cindex epilogue

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

@table @code
@findex FUNCTION_PROLOGUE
@item FUNCTION_PROLOGUE (@var{file}, @var{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 @var{size} additional bytes of storage for the
local variables.  @var{size} is an integer.  @var{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.

@findex regs_ever_live
To determine which registers to save, the macro can refer to the array
@code{regs_ever_live}: element @var{r} is nonzero if hard register
@var{r} is used anywhere within the function.  This implies the function
prologue should save register @var{r}, provided it is not one of the
call-used registers.  (@code{FUNCTION_EPILOGUE} must likewise use
@code{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.

@findex frame_pointer_needed
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
@code{frame_pointer_needed}.  The variable's value will be 1 at run
time in a function that needs a frame pointer.  @xref{Elimination}.

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 @code{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.

@itemize @bullet
@item
@findex current_function_pretend_args_size
A region of @code{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 @file{varargs.h} and @file{stdargs.h}.

@item
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.

@item
A region of at least @var{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.

@item
@cindex @code{ACCUMULATE_OUTGOING_ARGS} and stack frames
Optionally, when @code{ACCUMULATE_OUTGOING_ARGS} is defined, a region of
@code{current_function_outgoing_args_size} bytes to be used for outgoing
argument lists of the function.  @xref{Stack Arguments}.
@end itemize

Normally, it is necessary for the macros @code{FUNCTION_PROLOGUE} and
@code{FUNCTION_EPILOGUE} to treat leaf functions specially.  The C
variable @code{leaf_function} is nonzero for such a function.

@findex EXIT_IGNORE_STACK
@item 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 @code{EXIT_IGNORE_STACK}.

@findex EPILOGUE_USES
@item EPILOGUE_USES (@var{regno})
Define this macro as a C expression that is nonzero for registers are
used by the epilogue or the @samp{return} pattern.  The stack and frame
pointer registers are already be assumed to be used as needed.

@findex FUNCTION_EPILOGUE
@item FUNCTION_EPILOGUE (@var{file}, @var{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 @code{FUNCTION_PROLOGUE}, and the
registers to restore are determined from @code{regs_ever_live} and
@code{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 @samp{return} and do not define the macro
@code{FUNCTION_EPILOGUE} at all.

Do not define a pattern named @samp{return} if you want the
@code{FUNCTION_EPILOGUE} to be used.  If you want the target switches
to control whether return instructions or epilogues are used, define a
@samp{return} pattern with a validity condition that tests the target
switches appropriately.  If the @samp{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
@code{frame_pointer_needed}.  The variable's value will be 1 when compiling
a function that needs a frame pointer.

Normally, @code{FUNCTION_PROLOGUE} and @code{FUNCTION_EPILOGUE} must
treat leaf functions specially.  The C variable @code{leaf_function} is
nonzero for such a function.  @xref{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 @samp{-mrtd} pops arguments in functions that take a fixed
number of arguments.

@findex current_function_pops_args
Your definition of the macro @code{RETURN_POPS_ARGS} decides which
functions pop their own arguments.  @code{FUNCTION_EPILOGUE} needs to
know what was decided.  The variable that is called
@code{current_function_pops_args} is the number of bytes of its
arguments that a function should pop.  @xref{Scalar Return}.
@c what is the "its arguments" in the above sentence referring to, pray
@c tell?  --mew 5feb93

@findex DELAY_SLOTS_FOR_EPILOGUE
@item 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.

@findex ELIGIBLE_FOR_EPILOGUE_DELAY
@item ELIGIBLE_FOR_EPILOGUE_DELAY (@var{insn}, @var{n})
A C expression that returns 1 if @var{insn} can be placed in delay
slot number @var{n} of the epilogue.

The argument @var{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 @code{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.

@findex current_function_epilogue_delay_list
@findex final_scan_insn
The insns accepted to fill the epilogue delay slots are put in an RTL
list made with @code{insn_list} objects, stored in the variable
@code{current_function_epilogue_delay_list}.  The insn for the first
delay slot comes first in the list.  Your definition of the macro
@code{FUNCTION_EPILOGUE} should fill the delay slots by outputting the
insns in this list, usually by calling @code{final_scan_insn}.

You need not define this macro if you did not define
@code{DELAY_SLOTS_FOR_EPILOGUE}.

@findex ASM_OUTPUT_MI_THUNK
@item ASM_OUTPUT_MI_THUNK (@var{file}, @var{thunk_fndecl}, @var{delta}, @var{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 @var{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 @code{this} pointer
in C++.  This is the incoming argument @emph{before} the function prologue,
e.g. @samp{%o0} on a sparc.  The addition must preserve the values of
all other incoming arguments.

After the addition, emit code to jump to @var{function}, which is a
@code{FUNCTION_DECL}.  This is a direct pure jump, not a call, and does
not touch the return address.  Hence returning from @var{FUNCTION} will
return to whoever called the current @samp{thunk}.

The effect must be as if @var{function} had been called directly with
the adjusted first argument.  This macro is responsible for emitting all
of the code for a thunk function; @code{FUNCTION_PROLOGUE} and
@code{FUNCTION_EPILOGUE} are not invoked.

The @var{thunk_fndecl} is redundant.  (@var{delta} and @var{function}
have already been extracted from it.)  It might possibly be useful on
some targets, but probably not.

For many targets, the target-independent code in the C++ frontend will
be sufficient and you can leave this macro undefined.  You need to
define this macro if the code generated by default would clobber any of
the incoming arguments; this is only likely if parameters can be passed
in registers.  You should also define this macro if the default code is
sub-optimal.
@end table

@node Profiling
@subsection Generating Code for Profiling
@cindex profiling, code generation

These macros will help you generate code for profiling.

@table @code
@findex FUNCTION_PROFILER
@item FUNCTION_PROFILER (@var{file}, @var{labelno})
A C statement or compound statement to output to @var{file} some
assembler code to call the profiling subroutine @code{mcount}.
Before calling, the assembler code must load the address of a
counter variable into a register where @code{mcount} expects to
find the address.  The name of this variable is @samp{LP} followed
by the number @var{labelno}, so you would generate the name using
@samp{LP%d} in a @code{fprintf}.

@findex mcount
The details of how the address should be passed to @code{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.

@findex PROFILE_BEFORE_PROLOGUE
@item PROFILE_BEFORE_PROLOGUE
Define this macro if the code for function profiling should come before
the function prologue.  Normally, the profiling code comes after.

@findex FUNCTION_BLOCK_PROFILER
@vindex profile_block_flag
@item FUNCTION_BLOCK_PROFILER (@var{file}, @var{labelno})
A C statement or compound statement to output to @var{file} some
assembler code to initialize basic-block profiling for the current
object module.  The global compile flag @code{profile_block_flag}
distingishes two profile modes.

@table @code
@findex __bb_init_func
@item profile_block_flag != 2
Output code to call the subroutine @code{__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:

@smallexample
ASM_GENERATE_INTERNAL_LABEL (@var{buffer}, "LPBX", 0);
@end smallexample

Of course, since you are writing the definition of
@code{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 @code{__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 @code{__bb_init_func}
will not be called.

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

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

@findex __bb_init_trace_func
@item profile_block_flag == 2
Output code to call the subroutine @code{__bb_init_trace_func}
and pass two parameters to it.  The first parameter is the same as
for @code{__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 @code{__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:
@example
parameter1 <- LPBX0
parameter2 <- BLOCK_OR_LABEL
call __bb_init_trace_func
@end example
@end table

@findex BLOCK_PROFILER
@vindex profile_block_flag
@item BLOCK_PROFILER (@var{file}, @var{blockno})
A C statement or compound statement to output to @var{file} some
assembler code to increment the count associated with the basic
block number @var{blockno}.  The global compile flag
@code{profile_block_flag} distingishes two profile modes.

@table @code
@item 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 @var{blockno} is at index
@var{blockno} in a vector of words; the name of this array is a local
symbol made with this statement:

@smallexample
ASM_GENERATE_INTERNAL_LABEL (@var{buffer}, "LPBX", 2);
@end smallexample

@c This paragraph is the same as one a few paragraphs up.
@c That is not an error.
Of course, since you are writing the definition of
@code{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:

@smallexample
inc (LPBX2+4*BLOCKNO)
@end smallexample

@vindex __bb
@findex __bb_trace_func
@item profile_block_flag == 2
Output code to initialize the global structure @code{__bb} and
call the function @code{__bb_trace_func}, which will increment the
counter.

@code{__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:

@smallexample
ASM_GENERATE_INTERNAL_LABEL (@var{buffer}, "LPBX", 0);
@end smallexample

Described in assembler language, the code to be output looks like:
@example
move BLOCKNO -> (__bb)
move LPBX0 -> (__bb+4)
call __bb_trace_func
@end example
@end table

@findex FUNCTION_BLOCK_PROFILER_EXIT
@findex __bb_trace_ret
@vindex profile_block_flag
@item FUNCTION_BLOCK_PROFILER_EXIT (@var{file})
A C statement or compound statement to output to @var{file}
assembler code to call function @code{__bb_trace_ret}.  The
assembler code should only be output
if the global compile flag @code{profile_block_flag} == 2.  This
macro has to be used at every place where code for returning from
a function is generated (e.g. @code{FUNCTION_EPILOGUE}).  Although
you have to write the definition of @code{FUNCTION_EPILOGUE}
as well, you have to define this macro to tell the compiler, that
the proper call to @code{__bb_trace_ret} is produced.

@findex MACHINE_STATE_SAVE
@findex __bb_init_trace_func
@findex __bb_trace_func
@findex __bb_trace_ret
@item MACHINE_STATE_SAVE (@var{id})
A C statement or compound statement to save all registers, which may
be clobbered by a function call, including condition codes.  The
@code{asm} statement will be mostly likely needed to handle this
task.  Local labels in the assembler code can be concatenated with the
string @var{id}, to obtain a unique lable name.

Registers or condition codes clobbered by @code{FUNCTION_PROLOGUE} or
@code{FUNCTION_EPILOGUE} must be saved in the macros
@code{FUNCTION_BLOCK_PROFILER}, @code{FUNCTION_BLOCK_PROFILER_EXIT} and
@code{BLOCK_PROFILER} prior calling @code{__bb_init_trace_func},
@code{__bb_trace_ret} and @code{__bb_trace_func} respectively.

@findex MACHINE_STATE_RESTORE
@findex __bb_init_trace_func
@findex __bb_trace_func
@findex __bb_trace_ret
@item MACHINE_STATE_RESTORE (@var{id})
A C statement or compound statement to restore all registers, including
condition codes, saved by @code{MACHINE_STATE_SAVE}.

Registers or condition codes clobbered by @code{FUNCTION_PROLOGUE} or
@code{FUNCTION_EPILOGUE} must be restored in the macros
@code{FUNCTION_BLOCK_PROFILER}, @code{FUNCTION_BLOCK_PROFILER_EXIT} and
@code{BLOCK_PROFILER} after calling @code{__bb_init_trace_func},
@code{__bb_trace_ret} and @code{__bb_trace_func} respectively.

@findex BLOCK_PROFILER_CODE
@item BLOCK_PROFILER_CODE
A C function or functions which are needed in the library to
support block profiling.
@end table

@node Varargs
@section Implementing the Varargs Macros
@cindex varargs implementation

GNU CC comes with an implementation of @file{varargs.h} and
@file{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 @file{stdarg.h} differs from traditional @file{varargs.h} mainly in
the calling convention for @code{va_start}.  The traditional
implementation takes just one argument, which is the variable in which
to store the argument pointer.  The ANSI implementation of
@code{va_start} takes an additional second argument.  The user is
supposed to write the last named argument of the function here.

However, @code{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.

@table @code
@findex __builtin_saveregs
@item __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 @code{va_start} must use @code{__builtin_saveregs}, unless
you use @code{SETUP_INCOMING_VARARGS} (see below) instead.

On some machines, @code{__builtin_saveregs} is open-coded under the
control of the macro @code{EXPAND_BUILTIN_SAVEREGS}.  On other machines,
it calls a routine written in assembler language, found in
@file{libgcc2.c}.

Code generated for the call to @code{__builtin_saveregs} appears at the
beginning of the function, as opposed to where the call to
@code{__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.
@c i rewrote the first sentence above to fix an overfull hbox. --mew
@c 10feb93

@findex __builtin_args_info
@item __builtin_args_info (@var{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 @code{CUMULATIVE_ARGS} data type to record how many
registers in each category have been used so far

@code{__builtin_args_info} accesses the same data structure of type
@code{CUMULATIVE_ARGS} after the ordinary argument layout is finished
with it, with @var{category} specifying which word to access.  Thus, the
value indicates the first unused register in a given category.

Normally, you would use @code{__builtin_args_info} in the implementation
of @code{va_start}, accessing each category just once and storing the
value in the @code{va_list} object.  This is because @code{va_list} will
have to update the values, and there is no way to alter the
values accessed by @code{__builtin_args_info}.

@findex __builtin_next_arg
@item __builtin_next_arg (@var{lastarg})
This is the equivalent of @code{__builtin_args_info}, for stack
arguments.  It returns the address of the first anonymous stack
argument, as type @code{void *}. If @code{ARGS_GROW_DOWNWARD}, it
returns the address of the location above the first anonymous stack
argument.  Use it in @code{va_start} to initialize the pointer for
fetching arguments from the stack.  Also use it in @code{va_start} to
verify that the second parameter @var{lastarg} is the last named argument
of the current function.

@findex __builtin_classify_type
@item __builtin_classify_type (@var{object})
Since each machine has its own conventions for which data types are
passed in which kind of register, your implementation of @code{va_arg}
has to embody these conventions.  The easiest way to categorize the
specified data type is to use @code{__builtin_classify_type} together
with @code{sizeof} and @code{__alignof__}.

@code{__builtin_classify_type} ignores the value of @var{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 @file{typeclass.h} defines an enumeration that you can use to
interpret the values of @code{__builtin_classify_type}.
@end table

These machine description macros help implement varargs:

@table @code
@findex EXPAND_BUILTIN_SAVEREGS
@item EXPAND_BUILTIN_SAVEREGS (@var{args})
If defined, is a C expression that produces the machine-specific code
for a call to @code{__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 @code{__builtin_saveregs}.

The argument @var{args} is a @code{tree_list} containing the arguments
that were passed to @code{__builtin_saveregs}.

If this macro is not defined, the compiler will output an ordinary
call to the library function @samp{__builtin_saveregs}.

@c !!! a bug in texinfo; how to make the entry on the @item line allow
@c more than one line of text... help...  --mew 10feb93
@findex SETUP_INCOMING_VARARGS
@item SETUP_INCOMING_VARARGS (@var{args_so_far}, @var{mode}, @var{type},
@var{pretend_args_size}, @var{second_time})
This macro offers an alternative to using @code{__builtin_saveregs} and
defining the macro @code{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 @var{args_so_far} is the @code{CUMULATIVE_ARGS} data
structure, containing the values that obtain after processing of the
named arguments.  The arguments @var{mode} and @var{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 @emph{not} used for the named
arguments, and second, store the size of the data thus pushed into the
@code{int}-valued variable whose name is supplied as the argument
@var{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,
@code{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 @var{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 @code{SETUP_INCOMING_VARARGS} should
not generate any instructions in this case.

@findex STRICT_ARGUMENT_NAMING
@item STRICT_ARGUMENT_NAMING
Define this macro if the location where a function argument is passed
depends on whether or not it is a named argument.

This macro controls how the @var{named} argument to @code{FUNCTION_ARG}
is set for varargs and stdarg functions.  With this macro defined,
the @var{named} argument is always true for named arguments, and false for
unnamed arguments.  If this is not defined, but @code{SETUP_INCOMING_VARARGS}
is defined, then all arguments are treated as named.  Otherwise, all named
arguments except the last are treated as named.
@end table

@node Trampolines
@section Trampolines for Nested Functions
@cindex trampolines for nested functions
@cindex nested functions, trampolines for

A @dfn{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.

@table @code
@findex TRAMPOLINE_TEMPLATE
@item TRAMPOLINE_TEMPLATE (@var{file})
A C statement to output, on the stream @var{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.

@findex TRAMPOLINE_SECTION
@item TRAMPOLINE_SECTION
The name of a subroutine to switch to the section in which the
trampoline template is to be placed (@pxref{Sections}).  The default is
a value of @samp{readonly_data_section}, which places the trampoline in
the section containing read-only data.

@findex TRAMPOLINE_SIZE
@item TRAMPOLINE_SIZE
A C expression for the size in bytes of the trampoline, as an integer.

@findex TRAMPOLINE_ALIGNMENT
@item TRAMPOLINE_ALIGNMENT
Alignment required for trampolines, in bits.

If you don't define this macro, the value of @code{BIGGEST_ALIGNMENT}
is used for aligning trampolines.

@findex INITIALIZE_TRAMPOLINE
@item INITIALIZE_TRAMPOLINE (@var{addr}, @var{fnaddr}, @var{static_chain})
A C statement to initialize the variable parts of a trampoline.
@var{addr} is an RTX for the address of the trampoline; @var{fnaddr} is
an RTX for the address of the nested function; @var{static_chain} is an
RTX for the static chain value that should be passed to the function
when it is called.

@findex ALLOCATE_TRAMPOLINE
@item ALLOCATE_TRAMPOLINE (@var{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.

@cindex @code{FUNCTION_EPILOGUE} and trampolines
@cindex @code{FUNCTION_PROLOGUE} and trampolines
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 @code{FUNCTION_PROLOGUE} and
@code{FUNCTION_EPILOGUE}.

@var{fp} points to a data structure, a @code{struct function}, which
describes the compilation status of the immediate containing function of
the function which the trampoline is for.  Normally (when
@code{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.
@end table

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.

@table @code
@findex INSN_CACHE_SIZE
@item INSN_CACHE_SIZE
The total size in bytes of the cache.

@findex INSN_CACHE_LINE_WIDTH
@item 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.

@findex INSN_CACHE_DEPTH
@item INSN_CACHE_DEPTH
The number of alternative cache lines that can hold any particular memory
location.
@end table

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

@table @code
@findex CLEAR_INSN_CACHE
@item CLEAR_INSN_CACHE (@var{BEG}, @var{END})
If defined, expands to a C expression clearing the @emph{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
@code{asm} statements.  Both @var{BEG} and @var{END} are both pointer
expressions.
@end table

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 @file{m68k.h} as a guide.

@table @code
@findex TRANSFER_FROM_TRAMPOLINE
@item TRANSFER_FROM_TRAMPOLINE
Define this macro if trampolines need a special subroutine to do their
work.  The macro should expand to a series of @code{asm} statements
which will be compiled with GNU CC.  They go in a library function named
@code{__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 @code{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.
@end table

@node Library Calls
@section Implicit Calls to Library Routines
@cindex library subroutine names
@cindex @file{libgcc.a}

@c prevent bad page break with this line
Here is an explanation of implicit calls to library routines.

@table @code
@findex MULSI3_LIBCALL
@item 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 @code{__mulsi3},
a function defined in @file{libgcc.a}.

@findex DIVSI3_LIBCALL
@item 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 @code{__divsi3}, a
function defined in @file{libgcc.a}.

@findex UDIVSI3_LIBCALL
@item 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 @code{__udivsi3}, a
function defined in @file{libgcc.a}.

@findex MODSI3_LIBCALL
@item 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
@code{__modsi3}, a function defined in @file{libgcc.a}.

@findex UMODSI3_LIBCALL
@item 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
@code{__umodsi3}, a function defined in @file{libgcc.a}.

@findex MULDI3_LIBCALL
@item 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 @code{__muldi3},
a function defined in @file{libgcc.a}.

@findex DIVDI3_LIBCALL
@item 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 @code{__divdi3}, a
function defined in @file{libgcc.a}.

@findex UDIVDI3_LIBCALL
@item 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 @code{__udivdi3}, a
function defined in @file{libgcc.a}.

@findex MODDI3_LIBCALL
@item 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
@code{__moddi3}, a function defined in @file{libgcc.a}.

@findex UMODDI3_LIBCALL
@item 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
@code{__umoddi3}, a function defined in @file{libgcc.a}.

@findex INIT_TARGET_OPTABS
@item INIT_TARGET_OPTABS
Define this macro as a C statement that declares additional library
routines renames existing ones. @code{init_optabs} calls this macro after
initializing all the normal library routines.

@findex TARGET_EDOM
@cindex @code{EDOM}, implicit usage
@item TARGET_EDOM
The value of @code{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 @code{EDOM} into @code{errno} directly.  Look in
@file{/usr/include/errno.h} to find the value of @code{EDOM} on your
system.

If you do not define @code{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 @code{matherr} when
there is an error, then you should leave @code{TARGET_EDOM} undefined so
that @code{matherr} is used normally.

@findex GEN_ERRNO_RTX
@cindex @code{errno}, implicit usage
@item GEN_ERRNO_RTX
Define this macro as a C expression to create an rtl expression that
refers to the global ``variable'' @code{errno}.  (On certain systems,
@code{errno} may not actually be a variable.)  If you don't define this
macro, a reasonable default is used.

@findex TARGET_MEM_FUNCTIONS
@cindex @code{bcopy}, implicit usage
@cindex @code{memcpy}, implicit usage
@cindex @code{bzero}, implicit usage
@cindex @code{memset}, implicit usage
@item TARGET_MEM_FUNCTIONS
Define this macro if GNU CC should generate calls to the System V
(and ANSI C) library functions @code{memcpy} and @code{memset}
rather than the BSD functions @code{bcopy} and @code{bzero}.

@findex LIBGCC_NEEDS_DOUBLE
@item LIBGCC_NEEDS_DOUBLE
Define this macro if only @code{float} arguments cannot be passed to
library routines (so they must be converted to @code{double}).  This
macro affects both how library calls are generated and how the library
routines in @file{libgcc1.c} accept their arguments.  It is useful on
machines where floating and fixed point arguments are passed
differently, such as the i860.

@findex FLOAT_ARG_TYPE
@item FLOAT_ARG_TYPE
Define this macro to override the type used by the library routines to
pick up arguments of type @code{float}.  (By default, they use a union
of @code{float} and @code{int}.)

The obvious choice would be @code{float}---but that won't work with
traditional C compilers that expect all arguments declared as @code{float}
to arrive as @code{double}.  To avoid this conversion, the library routines
ask for the value as some other type and then treat it as a @code{float}.

On some systems, no other type will work for this.  For these systems,
you must use @code{LIBGCC_NEEDS_DOUBLE} instead, to force conversion of
the values @code{double} before they are passed.

@findex FLOATIFY
@item FLOATIFY (@var{passed-value})
Define this macro to override the way library routines redesignate a
@code{float} argument as a @code{float} instead of the type it was
passed as.  The default is an expression which takes the @code{float}
field of the union.

@findex FLOAT_VALUE_TYPE
@item FLOAT_VALUE_TYPE
Define this macro to override the type used by the library routines to
return values that ought to have type @code{float}.  (By default, they
use @code{int}.)

The obvious choice would be @code{float}---but that won't work with
traditional C compilers gratuitously convert values declared as
@code{float} into @code{double}.

@findex INTIFY
@item INTIFY (@var{float-value})
Define this macro to override the way the value of a
@code{float}-returning library routine should be packaged in order to
return it.  These functions are actually declared to return type
@code{FLOAT_VALUE_TYPE} (normally @code{int}).

These values can't be returned as type @code{float} because traditional
C compilers would gratuitously convert the value to a @code{double}.

A local variable named @code{intify} is always available when the macro
@code{INTIFY} is used.  It is a union of a @code{float} field named
@code{f} and a field named @code{i} whose type is
@code{FLOAT_VALUE_TYPE} or @code{int}.

If you don't define this macro, the default definition works by copying
the value through that union.

@findex nongcc_SI_type
@item nongcc_SI_type
Define this macro as the name of the data type corresponding to
@code{SImode} in the system's own C compiler.

You need not define this macro if that type is @code{long int}, as it usually
is.

@findex nongcc_word_type
@item 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 @code{long int}, as it usually
is.

@findex perform_@dots{}
@item perform_@dots{}
Define these macros to supply explicit C statements to carry out various
arithmetic operations on types @code{float} and @code{double} in the
library routines in @file{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.

@findex NEXT_OBJC_RUNTIME
@item 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.
@end table

@node Addressing Modes
@section Addressing Modes
@cindex addressing modes

@c prevent bad page break with this line
This is about addressing modes.

@table @code
@findex HAVE_POST_INCREMENT
@item HAVE_POST_INCREMENT
Define this macro if the machine supports post-increment addressing.

@findex HAVE_PRE_INCREMENT
@findex HAVE_POST_DECREMENT
@findex HAVE_PRE_DECREMENT
@item HAVE_PRE_INCREMENT
@itemx HAVE_POST_DECREMENT
@itemx HAVE_PRE_DECREMENT
Similar for other kinds of addressing.

@findex CONSTANT_ADDRESS_P
@item CONSTANT_ADDRESS_P (@var{x})
A C expression that is 1 if the RTX @var{x} is a constant which
is a valid address.  On most machines, this can be defined as
@code{CONSTANT_P (@var{x})}, but a few machines are more restrictive
in which constant addresses are supported.

@findex CONSTANT_P
@code{CONSTANT_P} accepts integer-values expressions whose values are
not explicitly known, such as @code{symbol_ref}, @code{label_ref}, and
@code{high} expressions and @code{const} arithmetic expressions, in
addition to @code{const_int} and @code{const_double} expressions.

@findex MAX_REGS_PER_ADDRESS
@item 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 @code{GO_IF_LEGITIMATE_ADDRESS} would ever
accept.

@findex GO_IF_LEGITIMATE_ADDRESS
@item GO_IF_LEGITIMATE_ADDRESS (@var{mode}, @var{x}, @var{label})
A C compound statement with a conditional @code{goto @var{label};}
executed if @var{x} (an RTX) is a legitimate memory address on the
target machine for a memory operand of mode @var{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.

@findex REG_OK_STRICT
Compiler source files that want to use the strict variant of this
macro define the macro @code{REG_OK_STRICT}.  You should use an
@code{#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 @code{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.@refill

Normally, constant addresses which are the sum of a @code{symbol_ref}
and an integer are stored inside a @code{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 @code{const} as legitimate.

Usually @code{PRINT_OPERAND_ADDRESS} is not prepared to handle constant
sums that are not marked with  @code{const}.  It assumes that a naked
@code{plus} indicates indexing.  If so, then you @emph{must} reject such
naked constant sums as illegitimate addresses, so that none of them will
be given to @code{PRINT_OPERAND_ADDRESS}.

@cindex @code{ENCODE_SECTION_INFO} and address validation
On some machines, whether a symbolic address is legitimate depends on
the section that the address refers to.  On these machines, define the
macro @code{ENCODE_SECTION_INFO} to store the information into the
@code{symbol_ref}, and then check for it here.  When you see a
@code{const}, you will have to look inside it to find the
@code{symbol_ref} in order to determine the section.  @xref{Assembler
Format}.

@findex saveable_obstack
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 @code{saveable_obstack}.  You will have to modify
@code{ASM_OUTPUT_LABELREF} to remove and decode the added text and
output the name accordingly, and define @code{STRIP_NAME_ENCODING} to
access the original name string.

You can check the information stored here into the @code{symbol_ref} in
the definitions of the macros @code{GO_IF_LEGITIMATE_ADDRESS} and
@code{PRINT_OPERAND_ADDRESS}.

@findex REG_OK_FOR_BASE_P
@item REG_OK_FOR_BASE_P (@var{x})
A C expression that is nonzero if @var{x} (assumed to be a @code{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 @code{REG_OK_STRICT} as described above.  This usually
requires two variant definitions, of which @code{REG_OK_STRICT}
controls the one actually used.

@findex REG_OK_FOR_INDEX_P
@item REG_OK_FOR_INDEX_P (@var{x})
A C expression that is nonzero if @var{x} (assumed to be a @code{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.

@findex LEGITIMIZE_ADDRESS
@item LEGITIMIZE_ADDRESS (@var{x}, @var{oldx}, @var{mode}, @var{win})
A C compound statement that attempts to replace @var{x} with a valid
memory address for an operand of mode @var{mode}.  @var{win} will be a
C statement label elsewhere in the code; the macro definition may use

@example
GO_IF_LEGITIMATE_ADDRESS (@var{mode}, @var{x}, @var{win});
@end example

@noindent
to avoid further processing if the address has become legitimate.

@findex break_out_memory_refs
@var{x} will always be the result of a call to @code{break_out_memory_refs},
and @var{oldx} will be the operand that was given to that function to produce
@var{x}.

The code generated by this macro should not alter the substructure of
@var{x}.  If it transforms @var{x} into a more legitimate form, it
should assign @var{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.

@findex GO_IF_MODE_DEPENDENT_ADDRESS
@item GO_IF_MODE_DEPENDENT_ADDRESS (@var{addr}, @var{label})
A C statement or compound statement with a conditional @code{goto
@var{label};} executed if memory address @var{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 @var{addr} is a valid address for the machine.

@findex LEGITIMATE_CONSTANT_P
@item LEGITIMATE_CONSTANT_P (@var{x})
A C expression that is nonzero if @var{x} is a legitimate constant for
an immediate operand on the target machine.  You can assume that
@var{x} satisfies @code{CONSTANT_P}, so you need not check this.  In fact,
@samp{1} is a suitable definition for this macro on machines where
anything @code{CONSTANT_P} is valid.@refill
@end table

@node Condition Code
@section Condition Code Status
@cindex condition code status

@c prevent bad page break with this line
This describes the condition code status.

@findex cc_status
The file @file{conditions.h} defines a variable @code{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 @code{CC_STATUS_MDEP}.

@table @code
@findex CC_STATUS_MDEP
@item CC_STATUS_MDEP
C code for a data type which is used for declaring the @code{mdep}
component of @code{cc_status}.  It defaults to @code{int}.

This macro is not used on machines that do not use @code{cc0}.

@findex CC_STATUS_MDEP_INIT
@item CC_STATUS_MDEP_INIT
A C expression to initialize the @code{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 @code{cc0}.

@findex NOTICE_UPDATE_CC
@item NOTICE_UPDATE_CC (@var{exp}, @var{insn})
A C compound statement to set the components of @code{cc_status}
appropriately for an insn @var{insn} whose body is @var{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 @code{(cc0)}.

This macro is not used on machines that do not use @code{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
@code{NOTICE_UPDATE_CC} can leave @code{cc_status} unaltered for such
insns.  But suppose that the previous insn set the condition code
based on location @samp{a4@@(102)} and the current insn stores a new
value in @samp{a4}.  Although the condition code is not changed by
this, it will no longer be true that it reflects the contents of
@samp{a4@@(102)}.  Therefore, @code{NOTICE_UPDATE_CC} must alter
@code{cc_status} in this case to say that nothing is known about the
condition code value.

The definition of @code{NOTICE_UPDATE_CC} must be prepared to deal
with the results of peephole optimization: insns whose patterns are
@code{parallel} RTXs containing various @code{reg}, @code{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
@code{NOTICE_UPDATE_CC} should do when it sees one is just to run
@code{CC_STATUS_INIT}.

A possible definition of @code{NOTICE_UPDATE_CC} is to call a function
that looks at an attribute (@pxref{Insn Attributes}) named, for example,
@samp{cc}.  This avoids having detailed information about patterns in
two places, the @file{md} file and in @code{NOTICE_UPDATE_CC}.

@findex EXTRA_CC_MODES
@item EXTRA_CC_MODES
A list of names to be used for additional modes for condition code
values in registers (@pxref{Jump Patterns}).  These names are added
to @code{enum machine_mode} and all have class @code{MODE_CC}.  By
convention, they should start with @samp{CC} and end with @samp{mode}.

You should only define this macro if your machine does not use @code{cc0}
and only if additional modes are required.

@findex EXTRA_CC_NAMES
@item EXTRA_CC_NAMES
A list of C strings giving the names for the modes listed in
@code{EXTRA_CC_MODES}.  For example, the Sparc defines this macro and
@code{EXTRA_CC_MODES} as

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

This macro is not required if @code{EXTRA_CC_MODES} is not defined.

@findex SELECT_CC_MODE
@item SELECT_CC_MODE (@var{op}, @var{x}, @var{y})
Returns a mode from class @code{MODE_CC} to be used when comparison
operation code @var{op} is applied to rtx @var{x} and @var{y}.  For
example, on the Sparc, @code{SELECT_CC_MODE} is defined as (see
@pxref{Jump Patterns} for a description of the reason for this
definition)

@smallexample
#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))
@end smallexample

You need not define this macro if @code{EXTRA_CC_MODES} is not defined.

@findex CANONICALIZE_COMPARISON
@item CANONICALIZE_COMPARISON (@var{code}, @var{op0}, @var{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 @code{GT} comparison, but you can use an @code{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.  @var{code} is the initial comparison code