2002-04-24 Matt Hiller <hiller@redhat.com>

[pf3gnuchains/gcc-fork.git] / gcc / doc / tm.texi

@c Copyright (C) 1988,1989,1992,1993,1994,1995,1996,1997,1998,1999,2000,2001,2002
@c 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 and Functions
@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} and a C source file named @file{@var{machine}.c}.
The 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}.  The
source file defines a variable @code{targetm}, which is a structure
containing pointers to functions and data relating to the target
machine.  @file{@var{machine}.c} should also contain their definitions,
if they are not defined elsewhere in GCC, and other functions called
through the macros defined in the @file{.h} file.

@menu
* Target Structure::    The @code{targetm} variable.
* Driver::              Controlling how the driver runs the compilation passes.
* Run-time Target::     Defining @samp{-m} options like @option{-m68000} and @option{-m68020}.
* Per-Function Data::   Defining data structures for per-function information.
* Storage Layout::      Defining sizes and alignments of data.
* Type Layout::         Defining sizes and properties of basic user data types.
* Escape Sequences::    Defining the value of target character escape sequences
* 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.
* Scheduling::          Adjusting the behavior of the instruction scheduler.
* 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.
* Floating Point::      Handling floating point for cross-compilers.
* Mode Switching::      Insertion of mode-switching instructions.
* Target Attributes::   Defining target-specific uses of @code{__attribute__}.
* MIPS Coprocessors::   MIPS coprocessor support and how to customize it.
* Misc::                Everything else.
@end menu

@node Target Structure
@section The Global @code{targetm} Variable
@cindex target hooks
@cindex target functions

@deftypevar {struct gcc_target} targetm
The target @file{.c} file must define the global @code{targetm} variable
which contains pointers to functions and data relating to the target
machine.  The variable is declared in @file{target.h};
@file{target-def.h} defines the macro @code{TARGET_INITIALIZER} which is
used to initialize the variable, and macros for the default initializers
for elements of the structure.  The @file{.c} file should override those
macros for which the default definition is inappropriate.  For example:
@smallexample
#include "target.h"
#include "target-def.h"

/* @r{Initialize the GCC target structure.}  */

#undef TARGET_COMP_TYPE_ATTRIBUTES
#define TARGET_COMP_TYPE_ATTRIBUTES @var{machine}_comp_type_attributes

struct gcc_target targetm = TARGET_INITIALIZER;
@end smallexample
@end deftypevar

Where a macro should be defined in the @file{.c} file in this manner to
form part of the @code{targetm} structure, it is documented below as a
``Target Hook'' with a prototype.  Many macros will change in future
from being defined in the @file{.h} file to being part of the
@code{targetm} structure.

@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 @option{-@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 @option{-@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 SWITCH_CURTAILS_COMPILATION
@item SWITCH_CURTAILS_COMPILATION (@var{char})
A C expression which determines whether the option @option{-@var{char}}
stops compilation before the generation of an executable.  The value is
boolean, nonzero if the option does stop an executable from being
generated, zero otherwise.

By default, this macro is defined as
@code{DEFAULT_SWITCH_CURTAILS_COMPILATION}, which handles the standard
options properly.  You need not define
@code{SWITCH_CURTAILS_COMPILATION} unless you wish to add additional
options which affect the generation of an executable.  Any redefinition
should call @code{DEFAULT_SWITCH_CURTAILS_COMPILATION} 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 TARGET_OPTION_TRANSLATE_TABLE
@item TARGET_OPTION_TRANSLATE_TABLE
If defined, a list of pairs of strings, the first of which is a
potential command line target to the @file{gcc} driver program, and the
second of which is a space-separated (tabs and other whitespace are not
supported) list of options with which to replace the first option.  The
target defining this list is responsible for assuring that the results
are valid.  Replacement options may not be the @code{--opt} style, they
must be the @code{-opt} style.  It is the intention of this macro to
provide a mechanism for substitution that affects the multilibs chosen,
such as one option that enables many options, some of which select
multilibs.  Example nonsensical definition, where @code{-malt-abi},
@code{-EB}, and @code{-mspoo} cause different multilibs to be chosen:

@example
#define TARGET_OPTION_TRANSLATE_TABLE \
@{ "-fast",   "-march=fast-foo -malt-abi -I/usr/fast-foo" @}, \
@{ "-compat", "-EB -malign=4 -mspoo" @}
@end example

@findex CPP_SPEC
@item CPP_SPEC
A C string constant that tells the GCC driver program options to
pass to CPP@.  It can also specify how to translate options you
give to GCC into options for GCC to pass to the CPP@.

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

@findex CPLUSPLUS_CPP_SPEC
@item CPLUSPLUS_CPP_SPEC
This macro is just like @code{CPP_SPEC}, but is used for C++, rather
than C@.  If you do not define this macro, then the value of
@code{CPP_SPEC} (if any) will be used instead.

@findex NO_BUILTIN_SIZE_TYPE
@item NO_BUILTIN_SIZE_TYPE
If this macro is defined, the preprocessor will not define the built-in 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 built-in 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 NO_BUILTIN_WCHAR_TYPE
@item NO_BUILTIN_WCHAR_TYPE
If this macro is defined, the preprocessor will not define the built-in macro
@code{__WCHAR_TYPE__}.  The macro @code{__WCHAR_TYPE__} must then be
defined by @code{CPP_SPEC} instead.

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

@findex NO_BUILTIN_WINT_TYPE
@item NO_BUILTIN_WINT_TYPE
If this macro is defined, the preprocessor will not define the built-in macro
@code{__WINT_TYPE__}.  The macro @code{__WINT_TYPE__} must then be
defined by @code{CPP_SPEC} instead.

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

@findex CC1_SPEC
@item CC1_SPEC
A C string constant that tells the GCC driver program options to
pass to @code{cc1}, @code{cc1plus}, @code{f771}, and the other language
front ends.
It can also specify how to translate options you give to GCC into options
for GCC to pass to front ends.

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 GCC driver program options to
pass to @code{cc1plus}.  It can also specify how to translate options you
give to GCC into options for GCC to pass to the @code{cc1plus}.

Do not define this macro if it does not need to do anything.
Note that everything defined in CC1_SPEC is already passed to
@code{cc1plus} so there is no need to duplicate the contents of
CC1_SPEC in CC1PLUS_SPEC@.

@findex ASM_SPEC
@item ASM_SPEC
A C string constant that tells the GCC driver program options to
pass to the assembler.  It can also specify how to translate options
you give to GCC into options for GCC 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 GCC 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 GCC driver program options to
pass to the linker.  It can also specify how to translate options you
give to GCC into options for GCC 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 GCC 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 GCC driver provides a default that
passes the string @option{-lgcc} to the linker.

@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 THREAD_MODEL_SPEC
@item THREAD_MODEL_SPEC
GCC @code{-v} will print the thread model GCC was configured to use.
However, this doesn't work on platforms that are multilibbed on thread
models, such as AIX 4.3.  On such platforms, define
@code{THREAD_MODEL_SPEC} such that it evaluates to a string without
blanks that names one of the recognized thread models.  @code{%*}, the
default value of this macro, will expand to the value of
@code{thread_file} set in @file{config.gcc}.

@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{@dots{}_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 @option{-L} options to the
linker.  If you do not define this macro, the driver program will pass
the argument @option{-lgcc} to tell the linker to do the search and will
pass @option{-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 @option{-lgcc} to tell the linker to do the search.
This macro is similar to @code{LINK_LIBGCC_SPECIAL}, except that it does
not affect @option{-L} options.

@findex LINK_GCC_C_SEQUENCE_SPEC
@item LINK_GCC_C_SEQUENCE_SPEC
The sequence in which libgcc and libc are specified to the linker.
By default this is @code{%G %L %G}.

@findex LINK_COMMAND_SPEC
@item LINK_COMMAND_SPEC
A C string constant giving the complete command line need to execute the
linker.  When you do this, you will need to update your port each time a
change is made to the link command line within @file{gcc.c}.  Therefore,
define this macro only if you need to completely redefine the command
line for invoking the linker and there is no other way to accomplish
the effect you need.  Overriding this macro may be avoidable by overriding
@code{LINK_GCC_C_SEQUENCE_SPEC} instead.

@findex LINK_ELIMINATE_DUPLICATE_LDIRECTORIES
@item LINK_ELIMINATE_DUPLICATE_LDIRECTORIES
A nonzero value causes @command{collect2} to remove duplicate @option{-L@var{directory}} search
directories from linking commands.  Do not give it a nonzero value if
removing duplicate search directories changes the linker's semantics.

@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 @option{-B} prefix into a @option{-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 @option{-b} option is used, or the compiler is built as a cross
compiler.  If you define @code{MD_EXEC_PREFIX}, then be sure to add it
to the list of directories used to find the assembler in @file{configure.in}.

@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
@option{-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 @option{-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 wish to set environment
variables for programs called by the driver, such as the assembler and
loader.  The driver passes the value of this macro to @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 search either @file{/usr/local/include} or its
replacement.

@findex MODIFY_TARGET_NAME
@item MODIFY_TARGET_NAME
Define this macro if you with to define command-line switches that modify the
default target name

For each switch, you can include a string to be appended to the first
part of the configuration name or a string to be deleted from the
configuration name, if present.  The definition should be an initializer
for an array of structures.  Each array element should have three
elements: the switch name (a string constant, including the initial
dash), one of the enumeration codes @code{ADD} or @code{DELETE} to
indicate whether the string should be inserted or deleted, and the string
to be inserted or deleted (a string constant).

For example, on a machine where @samp{64} at the end of the
configuration name denotes a 64-bit target and you want the @option{-32}
and @option{-64} switches to select between 32- and 64-bit targets, you would
code

@smallexample
#define MODIFY_TARGET_NAME \
  @{ @{ "-32", DELETE, "64"@}, \
     @{"-64", ADD, "64"@}@}
@end smallexample


@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 STANDARD_INCLUDE_COMPONENT
@item STANDARD_INCLUDE_COMPONENT
The ``component'' corresponding to @code{STANDARD_INCLUDE_DIR}.
See @code{INCLUDE_DEFAULTS}, below, for the description of components.
If you do not define this macro, no component is used.

@findex INCLUDE_DEFAULTS
@item INCLUDE_DEFAULTS
Define this macro if you wish to override the entire default search path
for include files.  For a native compiler, the default search path
usually consists of @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 four elements: the directory name (a
string constant), the component name (also a string constant), a flag
for C++-only directories,
and a flag showing that the includes in the directory don't need to be
wrapped in @code{extern @samp{C}} when compiling C++.  Mark the end of
the array with a null element.

The component name denotes what GNU package the include file is part of,
if any, in all upper-case letters.  For example, it might be @samp{GCC}
or @samp{BINUTILS}.  If the package is part of a vendor-supplied
operating system, code the component name as @samp{0}.

For example, here is the definition used for VAX/VMS:

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

Here is the order of prefixes tried for exec files:

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

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

@item
The directories specified by the environment variable @code{LIBRARY_PATH}
(or port-specific name; 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 @option{-D} options to
define the predefined macros that identify this machine and system.
These macros will be predefined unless the @option{-ansi} option (or a
@option{-std} option for strict ISO C conformance) 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 ISO standard, so they are
predefined regardless of whether @option{-ansi} or a @option{-std} option
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 @option{-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}.  It is
recommended that a helper macro @code{TARGET_MASK_@var{featurename}}
is defined for each bit-value to test, and used in
@code{TARGET_@var{featurename}} and @code{TARGET_SWITCHES}.  For
example:

@smallexample
#define TARGET_MASK_68020 1
#define TARGET_68020 (target_flags & TARGET_MASK_68020)
@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, a number, which contains the bits to set in
@code{target_flags}, and a second string which is the description
displayed by @option{--help}.  If the number is negative then the bits specified
by the number are cleared instead of being set.  If the description
string is present but empty, then no help information will be displayed
for that option, but it will not count as an undocumented option.  The
actual option name is made by appending @samp{-m} to the specified name.
Non-empty description strings should be marked with @code{N_(@dots{})} for
@command{xgettext}.  Please do not mark empty strings because the empty
string is reserved by GNU gettext. @code{gettext("")} returns the header entry
of the message catalog with meta information, not the empty string.

In addition to the description for @option{--help},
more detailed documentation for each option should be added to
@file{invoke.texi}.

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 @option{-m68000} and @option{-m68020}
with opposite meanings, and picks the latter as the default:

@smallexample
#define TARGET_SWITCHES \
  @{ @{ "68020", TARGET_MASK_68020, "" @},      \
    @{ "68000", -TARGET_MASK_68020, \
      N_("Compile for the 68000") @}, \
    @{ "", TARGET_MASK_68020, "" @}@}
@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, the address of a variable, and a description string.
Non-empty description strings should be marked with @code{N_(@dots{})} for
@command{xgettext}.  Please do not mark empty strings because the empty
string is reserved by GNU gettext. @code{gettext("")} returns the header entry
of the message catalog with meta information, not the empty string.

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.  Again, each option should
also be documented in @file{invoke.texi}.

Here is an example which defines @option{-mshort-data-@var{number}}.  If the
given option is @option{-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, \
     N_("Specify the size of the short data section") @} @}
@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
@option{-O}.  That is what @code{OPTIMIZATION_OPTIONS} is for.

@findex OPTIMIZATION_OPTIONS
@item OPTIMIZATION_OPTIONS (@var{level}, @var{size})
Some machines may desire to change what optimizations are performed for
various optimization levels.   This macro, if defined, is executed once
just after the optimization level is determined and before the remainder
of the command options have been parsed.  Values set in this macro are
used as the default values for the other command line options.

@var{level} is the optimization level specified; 2 if @option{-O2} is
specified, 1 if @option{-O} is specified, and 0 if neither is specified.

@var{size} is nonzero if @option{-Os} is specified and zero otherwise.

You should not use this macro to change options that are not
machine-specific.  These should uniformly selected by the same
optimization level on all supported machines.  Use this macro to enable
machine-specific optimizations.

@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, GCC will turn on the
@option{-fomit-frame-pointer} option whenever @option{-O} is specified.
@end table

@node Per-Function Data
@section Defining data structures for per-function information.
@cindex per-function data
@cindex data structures

If the target needs to store information on a per-function basis, GCC
provides a macro and a couple of variables to allow this.  Note, just
using statics to store the information is a bad idea, since GCC supports
nested functions, so you can be halfway through encoding one function
when another one comes along.

GCC defines a data structure called @code{struct function} which
contains all of the data specific to an individual function.  This
structure contains a field called @code{machine} whose type is
@code{struct machine_function *}, which can be used by targets to point
to their own specific data.

If a target needs per-function specific data it should define the type
@code{struct machine_function} and also the macro
@code{INIT_EXPANDERS}.  This macro should be used to initialize some or
all of the function pointers @code{init_machine_status},
@code{free_machine_status} and @code{mark_machine_status}.  These
pointers are explained below.

One typical use of per-function, target specific data is to create an
RTX to hold the register containing the function's return address.  This
RTX can then be used to implement the @code{__builtin_return_address}
function, for level 0.

Note---earlier implementations of GCC used a single data area to hold
all of the per-function information.  Thus when processing of a nested
function began the old per-function data had to be pushed onto a
stack, and when the processing was finished, it had to be popped off the
stack.  GCC used to provide function pointers called
@code{save_machine_status} and @code{restore_machine_status} to handle
the saving and restoring of the target specific information.  Since the
single data area approach is no longer used, these pointers are no
longer supported.

The macro and function pointers are described below.

@table @code
@findex INIT_EXPANDERS
@item   INIT_EXPANDERS
Macro called to initialize any target specific information.  This macro
is called once per function, before generation of any RTL has begun.
The intention of this macro is to allow the initialization of the
function pointers below.

@findex init_machine_status
@item   init_machine_status
This is a @code{void (*)(struct function *)} function pointer.  If this
pointer is non-@code{NULL} it will be called once per function, before function
compilation starts, in order to allow the target to perform any target
specific initialization of the @code{struct function} structure.  It is
intended that this would be used to initialize the @code{machine} of
that structure.

@findex free_machine_status
@item   free_machine_status
This is a @code{void (*)(struct function *)} function pointer.  If this
pointer is non-@code{NULL} it will be called once per function, after the
function has been compiled, in order to allow any memory allocated
during the @code{init_machine_status} function call to be freed.

@findex mark_machine_status
@item   mark_machine_status
This is a @code{void (*)(struct function *)} function pointer.  If this
pointer is non-@code{NULL} it will be called once per function in order to mark
any data items in the @code{struct machine_function} structure which
need garbage collection.

@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; GCC 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 @code{WORDS_BIG_ENDIAN} is not constant.  This must be a
constant value with the same meaning as @code{WORDS_BIG_ENDIAN}, which will be
used only when compiling @file{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).  If you do not define this macro the default is 8.

@findex BITS_PER_WORD
@item BITS_PER_WORD
Number of bits in a word.  If you do not define this macro, the default
is @code{BITS_PER_UNIT * UNITS_PER_WORD}.

@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}.  If you do not specify
a value the default is @code{BITS_PER_WORD}.

@findex POINTERS_EXTEND_UNSIGNED
@item POINTERS_EXTEND_UNSIGNED
A C expression whose value is greater than zero if pointers that need to be
extended from being @code{POINTER_SIZE} bits wide to @code{Pmode} are to
be zero-extended and zero if they are to be sign-extended.  If the value
is less then zero then there must be an "ptr_extend" instruction that
extends a pointer from @code{POINTER_SIZE} to @code{Pmode}.

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 to the minimum alignment enforced by hardware for the
stack pointer on this machine.  The definition is a C expression for the
desired alignment (measured in bits).  This value is used as a default
if @code{PREFERRED_STACK_BOUNDARY} is not defined.  On most machines,
this should be the same as @code{PARM_BOUNDARY}.

@findex PREFERRED_STACK_BOUNDARY
@item PREFERRED_STACK_BOUNDARY
Define this macro if you wish to preserve a certain alignment for the
stack pointer, greater than what the hardware enforces.  The definition
is a C expression for the desired alignment (measured in bits).  This
macro must evaluate to a value equal to or larger than
@code{STACK_BOUNDARY}.

@findex FORCE_PREFERRED_STACK_BOUNDARY_IN_MAIN
@item FORCE_PREFERRED_STACK_BOUNDARY_IN_MAIN
A C expression that evaluates true if @code{PREFERRED_STACK_BOUNDARY} is
not guaranteed by the runtime and we should emit code to align the stack
at the beginning of @code{main}.

@cindex @code{PUSH_ROUNDING}, interaction with @code{PREFERRED_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{PREFERRED_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 MINIMUM_ATOMIC_ALIGNMENT
@item MINIMUM_ATOMIC_ALIGNMENT
If defined, the smallest alignment, in bits, that can be given to an
object that can be referenced in one operation, without disturbing any
nearby object.  Normally, this is @code{BITS_PER_UNIT}, but may be larger
on machines that don't have byte or half-word store operations.

@findex BIGGEST_FIELD_ALIGNMENT
@item BIGGEST_FIELD_ALIGNMENT
Biggest alignment that any structure or union field can require on this
machine, in bits.  If defined, this overrides @code{BIGGEST_ALIGNMENT} for
structure and union fields only, unless the field alignment has been set
by the @code{__attribute__ ((aligned (@var{n})))} construct.

@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}.  GCC uses
this value instead of the value in @code{BIGGEST_ALIGNMENT} or
@code{BIGGEST_FIELD_ALIGNMENT}, if defined.

@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 variable in
the static store.  @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 LOCAL_ALIGNMENT
@item LOCAL_ALIGNMENT (@var{type}, @var{basic-align})
If defined, a C expression to compute the alignment for a variable in
the local store.  @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.

One use of this macro is to increase alignment of medium-size data to
make it all fit in fewer cache lines.

@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 bit-fields and the structures that contain them.

The behavior is that the type written for a bit-field (@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 bit-field 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 bit-field 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
bit-fields 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 bit-fields 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 bit-field 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 GCC use the same conventions for laying out
bit-fields 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 @code{PCC_BITFIELD_TYPE_MATTERS} except that its effect is limited
to aligning a bit-field within the structure.

@findex MEMBER_TYPE_FORCES_BLK
@item MEMBER_TYPE_FORCES_BLK (@var{field})
Return 1 if a structure or array containing @var{field} should be accessed using
@code{BLKMODE}.

Normally, this is not needed.  See the file @file{c4x.h} for an example
of how to use this macro to prevent a structure having a floating point
field from being accessed in an integer mode.

@findex ROUND_TYPE_SIZE
@item ROUND_TYPE_SIZE (@var{type}, @var{computed}, @var{specified})
Define this macro as an expression for the overall size of a type
(given by @var{type} as a tree node) when the size computed in the
usual way is @var{computed} and the alignment is @var{specified}.

The default is to round @var{computed} up to a multiple of @var{specified}.

@findex ROUND_TYPE_SIZE_UNIT
@item ROUND_TYPE_SIZE_UNIT (@var{type}, @var{computed}, @var{specified})
Similar to @code{ROUND_TYPE_SIZE}, but sizes and alignments are
specified in units (bytes).  If you define @code{ROUND_TYPE_SIZE},
you must also define this macro and they must be defined consistently
with each other.

@findex ROUND_TYPE_ALIGN
@item ROUND_TYPE_ALIGN (@var{type}, @var{computed}, @var{specified})
Define this macro as an expression for the alignment of a type (given
by @var{type} 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 VECTOR_MODE_SUPPORTED_P
@item VECTOR_MODE_SUPPORTED_P(@var{mode})
Define this macro to be nonzero if the port is prepared to handle insns
involving vector mode @var{mode}.  At the very least, it must have move
patterns for this mode.

@findex STACK_SAVEAREA_MODE
@item STACK_SAVEAREA_MODE (@var{save_level})
If defined, an expression of type @code{enum machine_mode} that
specifies the mode of the save area operand of a
@code{save_stack_@var{level}} named pattern (@pxref{Standard Names}).
@var{save_level} is one of @code{SAVE_BLOCK}, @code{SAVE_FUNCTION}, or
@code{SAVE_NONLOCAL} and selects which of the three named patterns is
having its mode specified.

You need not define this macro if it always returns @code{Pmode}.  You
would most commonly define this macro if the
@code{save_stack_@var{level}} patterns need to support both a 32- and a
64-bit mode.

@findex STACK_SIZE_MODE
@item STACK_SIZE_MODE
If defined, an expression of type @code{enum machine_mode} that
specifies the mode of the size increment operand of an
@code{allocate_stack} named pattern (@pxref{Standard Names}).

You need not define this macro if it always returns @code{word_mode}.
You would most commonly define this macro if the @code{allocate_stack}
pattern needs to support both a 32- and a 64-bit mode.

@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 five 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 ``D float'' format used on the VAX@.

@findex IBM_FLOAT_FORMAT
@item IBM_FLOAT_FORMAT
This code indicates the format used on the IBM System/370.

@findex C4X_FLOAT_FORMAT
@item C4X_FLOAT_FORMAT
This code indicates the format used on the TMS320C3x/C4x.

@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}, which
is defined by the @command{configure} script, 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}.

@findex MODE_HAS_NANS
@item MODE_HAS_NANS (@var{mode})
When defined, this macro should be true if @var{mode} has a NaN
representation.  The compiler assumes that NaNs are not equal to
anything (including themselves) and that addition, subtraction,
multiplication and division all return NaNs when one operand is
NaN@.

By default, this macro is true if @var{mode} is a floating-point
mode and the target floating-point format is IEEE@.

@findex MODE_HAS_INFINITIES
@item MODE_HAS_INFINITIES (@var{mode})
This macro should be true if @var{mode} can represent infinity.  At
present, the compiler uses this macro to decide whether @samp{x - x}
is always defined.  By default, the macro is true when @var{mode}
is a floating-point mode and the target format is IEEE@.

@findex MODE_HAS_SIGNED_ZEROS
@item MODE_HAS_SIGNED_ZEROS (@var{mode})
True if @var{mode} distinguishes between positive and negative zero.
The rules are expected to follow the IEEE standard:

@itemize @bullet
@item
@samp{x + x} has the same sign as @samp{x}.

@item
If the sum of two values with opposite sign is zero, the result is
positive for all rounding modes expect towards @minus{}infinity, for
which it is negative.

@item
The sign of a product or quotient is negative when exactly one
of the operands is negative.
@end itemize

The default definition is true if @var{mode} is a floating-point
mode and the target format is IEEE@.

@findex MODE_HAS_SIGN_DEPENDENT_ROUNDING
@item MODE_HAS_SIGN_DEPENDENT_ROUNDING (@var{mode})
If defined, this macro should be true for @var{mode} if it has at
least one rounding mode in which @samp{x} and @samp{-x} can be
rounded to numbers of different magnitude.  Two such modes are
towards @minus{}infinity and towards +infinity.

The default definition of this macro is true if @var{mode} is
a floating-point mode and the target format is IEEE@.

@findex ROUND_TOWARDS_ZERO
@item ROUND_TOWARDS_ZERO
If defined, this macro should be true if the prevailing rounding
mode is towards zero.  A true value has the following effects:

@itemize @bullet
@item
@code{MODE_HAS_SIGN_DEPENDENT_ROUNDING} will be false for all modes.

@item
@file{libgcc.a}'s floating-point emulator will round towards zero
rather than towards nearest.

@item
The compiler's floating-point emulator will round towards zero after
doing arithmetic, and when converting from the internal float format to
the target format.
@end itemize

The macro does not affect the parsing of string literals.  When the
primary rounding mode is towards zero, library functions like
@code{strtod} might still round towards nearest, and the compiler's
parser should behave like the target's @code{strtod} where possible.

Not defining this macro is equivalent to returning zero.

@findex LARGEST_EXPONENT_IS_NORMAL
@item LARGEST_EXPONENT_IS_NORMAL (@var{size})
This macro should only be defined when the target float format is
described as IEEE@.  It should return true if floats with @var{size}
bits do not have a NaN or infinity representation, but use the largest
exponent for normal numbers instead.

Defining this macro to true for @var{size} causes @code{MODE_HAS_NANS}
and @code{MODE_HAS_INFINITIES} to be false for @var{size}-bit modes.
It also affects the way @file{libgcc.a} and @file{real.c} emulate
floating-point arithmetic.

The default definition of this macro returns false for all sizes.
@end table

@deftypefn {Target Hook} bool TARGET_MS_BITFIELD_LAYOUT_P (tree @var{record_type})
This target hook returns @code{true} if bit-fields in the given
@var{record_type} are to be laid out following the rules of Microsoft
Visual C/C++, namely: (i) a bit-field won't share the same storage
unit with the previous bit-field if their underlying types have
different sizes, and the bit-field will be aligned to the highest
alignment of the underlying types of itself and of the previous
bit-field; (ii) a zero-sized bit-field will affect the alignment of
the whole enclosing structure, even if it is unnamed; except that
(iii) a zero-sized bit-field will be disregarded unless it follows
another bit-field of non-zero size.  If this hook returns @code{true},
other macros that control bit-field layout are ignored.
@end deftypefn

@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 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 ADA_LONG_TYPE_SIZE
@item ADA_LONG_TYPE_SIZE
On some machines, the size used for the Ada equivalent of the type
@code{long} by a native Ada compiler differs from that used by C.  In
that situation, define this macro to be a C expression to be used for
the size of that type.  If you don't define this, the default is the
value of @code{LONG_TYPE_SIZE}.

@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 this
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
@code{BITS_PER_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 BOOL_TYPE_SIZE
@item BOOL_TYPE_SIZE
A C expression for the size in bits of the C++ type @code{bool} on the
target machine.  If you don't define this, the default is
@code{CHAR_TYPE_SIZE}.

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

@findex INTEL_EXTENDED_IEEE_FORMAT
Define this macro to be 1 if the target machine uses 80-bit floating-point
values with 128-bit size and alignment.  This is used in @file{real.c}.

@findex WIDEST_HARDWARE_FP_SIZE
@item WIDEST_HARDWARE_FP_SIZE
A C expression for the size in bits of the widest floating-point format
supported by the hardware.  If you define this macro, you must specify a
value less than or equal to the value of @code{LONG_DOUBLE_TYPE_SIZE}.
If you do not define this macro, the value of @code{LONG_DOUBLE_TYPE_SIZE}
is the default.

@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 @option{-fsigned-char}
and @option{-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 GCOV_TYPE_SIZE
@item GCOV_TYPE_SIZE
A C expression for the size in bits of the type used for gcov counters on the
target machine.  If you don't define this, the default is one
@code{LONG_TYPE_SIZE} in case it is greater or equal to 64-bit and
@code{LONG_LONG_TYPE_SIZE} otherwise.  You may want to re-define the type to
ensure atomicity for counters in multithreaded programs.

@findex WINT_TYPE
@item WINT_TYPE
A C expression for a string describing the name of the data type to
use for wide characters passed to @code{printf} and returned from
@code{getwc}.  The typedef name @code{wint_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{"unsigned int"}.

@findex INTMAX_TYPE
@item INTMAX_TYPE
A C expression for a string describing the name of the data type that
can represent any value of any standard or extended signed integer type.
The typedef name @code{intmax_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 the first of
@code{"int"}, @code{"long int"}, or @code{"long long int"} that has as
much precision as @code{long long int}.

@findex UINTMAX_TYPE
@item UINTMAX_TYPE
A C expression for a string describing the name of the data type that
can represent any value of any standard or extended unsigned integer
type.  The typedef name @code{uintmax_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 the first of
@code{"unsigned int"}, @code{"long unsigned int"}, or @code{"long long
unsigned int"} that has as much precision as @code{long long unsigned
int}.

@findex TARGET_PTRMEMFUNC_VBIT_LOCATION
@item TARGET_PTRMEMFUNC_VBIT_LOCATION
The C++ compiler represents a pointer-to-member-function with a struct
that looks like:

@example
  struct @{
    union @{
      void (*fn)();
      ptrdiff_t vtable_index;
    @};
    ptrdiff_t delta;
  @};
@end example

@noindent
The C++ compiler must use one bit to indicate whether the function that
will be called through a pointer-to-member-function is virtual.
Normally, we assume that the low-order bit of a function pointer must
always be zero.  Then, by ensuring that the vtable_index is odd, we can
distinguish which variant of the union is in use.  But, on some
platforms function pointers can be odd, and so this doesn't work.  In
that case, we use the low-order bit of the @code{delta} field, and shift
the remainder of the @code{delta} field to the left.

GCC will automatically make the right selection about where to store
this bit using the @code{FUNCTION_BOUNDARY} setting for your platform.
However, some platforms such as ARM/Thumb have @code{FUNCTION_BOUNDARY}
set such that functions always start at even addresses, but the lowest
bit of pointers to functions indicate whether the function at that
address is in ARM or Thumb mode.  If this is the case of your
architecture, you should define this macro to
@code{ptrmemfunc_vbit_in_delta}.

In general, you should not have to define this macro.  On architectures
in which function addresses are always even, according to
@code{FUNCTION_BOUNDARY}, GCC will automatically define this macro to
@code{ptrmemfunc_vbit_in_pfn}.

@findex TARGET_VTABLE_USES_DESCRIPTORS
@item TARGET_VTABLE_USES_DESCRIPTORS
Normally, the C++ compiler uses function pointers in vtables.  This
macro allows the target to change to use ``function descriptors''
instead.  Function descriptors are found on targets for whom a
function pointer is actually a small data structure.  Normally the
data structure consists of the actual code address plus a data
pointer to which the function's data is relative.

If vtables are used, the value of this macro should be the number
of words that the function descriptor occupies.
@end table

@node Escape Sequences
@section Target Character Escape Sequences
@cindex escape sequences

By default, GCC assumes that the C character escape sequences take on
their ASCII values for the target.  If this is not correct, you must
explicitly define all of the macros below.

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

@findex TARGET_ESC
@item TARGET_ESC
A C constant expression for the integer value of the target escape
character.  As an extension, GCC evaluates the escape sequences
@samp{\e} and @samp{\E} to this.

@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.
@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 @option{-ffixed-@var{reg}},
@option{-fcall-used-@var{reg}} and @option{-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 CALL_REALLY_USED_REGISTERS
@item CALL_REALLY_USED_REGISTERS
@cindex call-used register
@cindex call-clobbered register
@cindex call-saved register
Like @code{CALL_USED_REGISTERS} except this macro doesn't require
that the entire set of @code{FIXED_REGISTERS} be included.
(@code{CALL_USED_REGISTERS} must be a superset of @code{FIXED_REGISTERS}).
This macro is optional.  If not specified, it defaults to the value
of @code{CALL_USED_REGISTERS}.

@findex HARD_REGNO_CALL_PART_CLOBBERED
@item HARD_REGNO_CALL_PART_CLOBBERED (@var{regno}, @var{mode})
@cindex call-used register
@cindex call-clobbered register
@cindex call-saved register
A C expression that is nonzero if it is not permissible to store a
value of mode @var{mode} in hard register number @var{regno} across a
call without some part of it being clobbered.  For most machines this
macro need not be defined.  It is only required for machines that do not
preserve the entire contents of a register across a call.

@findex CONDITIONAL_REGISTER_USAGE
@findex fixed_regs
@findex call_used_regs
@item CONDITIONAL_REGISTER_USAGE
Zero or more C statements that may conditionally modify five variables
@code{fixed_regs}, @code{call_used_regs}, @code{global_regs},
@code{reg_names}, and @code{reg_class_contents}, to take into account
any dependence of these register sets on target flags.  The first three
of these are of type @code{char []} (interpreted as Boolean vectors).
@code{global_regs} is a @code{const char *[]}, and
@code{reg_class_contents} is a @code{HARD_REG_SET}.  Before the macro is
called, @code{fixed_regs}, @code{call_used_regs},
@code{reg_class_contents}, and @code{reg_names} have been initialized
from @code{FIXED_REGISTERS}, @code{CALL_USED_REGISTERS},
@code{REG_CLASS_CONTENTS}, and @code{REGISTER_NAMES}, respectively.
@code{global_regs} has been cleared, and any @option{-ffixed-@var{reg}},
@option{-fcall-used-@var{reg}} and @option{-fcall-saved-@var{reg}}
command options have been applied.

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.

@findex LOCAL_REGNO
@item LOCAL_REGNO (@var{regno})
Define this macro if the target machine has register windows.  This C
expression returns true if the register is call-saved but is in the
register window.  Unlike most call-saved registers, such registers
need not be explicitly restored on function exit or during non-local
gotos.

@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 GCC 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 allocable 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

You need not include code 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.  You can implement that by defining this macro to reject
odd register numbers for such modes.

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

Since the same instruction used to move @code{word_mode} 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 a value of mode
@var{mode1} is accessible in mode @var{mode2} without copying.

If @code{HARD_REGNO_MODE_OK (@var{r}, @var{mode1})} and
@code{HARD_REGNO_MODE_OK (@var{r}, @var{mode2})} are always the same for
any @var{r}, then @code{MODES_TIEABLE_P (@var{mode1}, @var{mode2})}
should be nonzero.  If they differ for any @var{r}, you should define
this macro to return zero unless some other mechanism ensures the
accessibility of the value in a narrower mode.

You should define this macro to return nonzero in as many cases as
possible since doing so will allow GCC to perform better register
allocation.

@findex AVOID_CCMODE_COPIES
@item AVOID_CCMODE_COPIES
Define this macro if the compiler should avoid copies to/from @code{CCmode}
registers.  You should only define this macro if support for copying to/from
@code{CCmode} is incomplete.
@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''.

GCC 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
Name of a char 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
GCC 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 @minus{}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 current_function_is_leaf
@findex current_function_uses_only_leaf_regs
@code{TARGET_ASM_FUNCTION_PROLOGUE} and
@code{TARGET_ASM_FUNCTION_EPILOGUE} must usually treat leaf functions
specially.  They can test the C variable @code{current_function_is_leaf}
which is nonzero for leaf functions.  @code{current_function_is_leaf} is
set prior to local register allocation and is valid for the remaining
compiler passes.  They can also test the C variable
@code{current_function_uses_only_leaf_regs} which is nonzero for leaf
functions which only use leaf registers.
@code{current_function_uses_only_leaf_regs} is valid after reload and is
only useful 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, GCC 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 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}.
In this situation, the first integer in each sub-initializer corresponds to
registers 0 through 31, the second integer to registers 32 through 63, and
so on.

@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 MODE_BASE_REG_CLASS
@item MODE_BASE_REG_CLASS (@var{mode})
This is a variation of the @code{BASE_REG_CLASS} macro which allows
the selection of a base register in a mode depenedent manner.  If
@var{mode} is VOIDmode then it should return the same value as
@code{BASE_REG_CLASS}.

@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_MODE_OK_FOR_BASE_P
@item REGNO_MODE_OK_FOR_BASE_P (@var{num}, @var{mode})
A C expression that is just like @code{REGNO_OK_FOR_BASE_P}, except that
that expression may examine the mode of the memory reference in
@var{mode}.  You should define this macro if the mode of the memory
reference affects whether a register may be used as a base register.  If
you define this macro, the compiler will use it instead of
@code{REGNO_OK_FOR_BASE_P}.

@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 @minus{}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 an
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 nonzero 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
On some machines, it is risky to let hard registers live across arbitrary
insns.  Typically, these machines have instructions that require values
to be in specific registers (like an accumulator), and reload will fail
if the required hard register is used for another purpose across such an
insn.

Define @code{SMALL_REGISTER_CLASSES} to be an expression with a nonzero
value on these machines.  When this macro has a nonzero value, the
compiler will try to minimize the lifetime of hard registers.

It is always safe to define this macro with a nonzero value, but if you
unnecessarily define it, you will reduce the amount of optimizations
that can be performed in some cases.  If you do not define this macro
with a nonzero value when it is required, the compiler will run out of
spill registers and print a fatal error message.  For most machines, you
should not define this macro at all.

@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 pseudos
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_MODE
If defined, a C expression for a class that contains registers for
which the compiler may not change modes arbitrarily.

@item CLASS_CANNOT_CHANGE_MODE_P(@var{from}, @var{to})
A C expression that is true if, for a register in
@code{CLASS_CANNOT_CHANGE_MODE}, the requested mode punning is invalid.

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 @code{CLASS_CANNOT_CHANGE_MODE}
as @code{FLOAT_REGS} and @code{CLASS_CANNOT_CHANGE_MODE_P} restricts
mode changes to same-size modes.

Compare this to IA-64, which extends floating-point values to 82-bits,
and stores 64-bit integers in a different format than 64-bit doubles.
Therefore @code{CLASS_CANNOT_CHANGE_MODE_P} is always true.
@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 (@samp{I}, @samp{J}, @samp{K}, @dots{} @samp{P}) 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
(@samp{G} or @samp{H}).

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.  Any letter that is not
elsewhere defined and not matched by @code{REG_CLASS_FROM_LETTER}
may be used.  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::
* Exception Handling::
* Stack Checking::
* Frame Registers::
* Elimination::
* Stack Arguments::
* Register Arguments::
* Scalar Return::
* Aggregate Return::
* Caller Saves::
* Function Entry::
* Profiling::
* Tail Calls::
@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 STACK_PUSH_CODE
@item STACK_PUSH_CODE

This macro defines the operation used when something is pushed
on the stack.  In RTL, a push operation will be
@code{(set (mem (STACK_PUSH_CODE (reg sp))) ...)}

The choices are @code{PRE_DEC}, @code{POST_DEC}, @code{PRE_INC},
and @code{POST_INC}.  Which of these is correct depends on
the stack direction and on whether the stack pointer points
to the last item on the stack or whether it points to the
space for the next item on the stack.

The default is @code{PRE_DEC} when @code{STACK_GROWS_DOWNWARD} is
defined, which is almost always right, and @code{PRE_INC} otherwise,
which is often wrong.

@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.  You will seldom need to
define this macro.

@findex BUILTIN_SETJMP_FRAME_VALUE
@item BUILTIN_SETJMP_FRAME_VALUE
If defined, a C expression that contains an rtx that is used to store
the address of the current frame into the built in @code{setjmp} buffer.
The default value, @code{virtual_stack_vars_rtx}, is correct for most
machines.  One reason you may need to define this macro is if
@code{hard_frame_pointer_rtx} is the appropriate value on your machine.

@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, after
the prologue.  @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.

The value of the expression must always be the correct address when
@var{count} is zero, but may be @code{NULL_RTX} if there is not way to
determine the return address of other frames.

@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.

@findex INCOMING_RETURN_ADDR_RTX
@item INCOMING_RETURN_ADDR_RTX
A C expression whose value is RTL representing the location of the
incoming return address at the beginning of any function, before the
prologue.  This RTL is either a @code{REG}, indicating that the return
value is saved in @samp{REG}, or a @code{MEM} representing a location in
the stack.

You only need to define this macro if you want to support call frame
debugging information like that provided by DWARF 2.

If this RTL is a @code{REG}, you should also define
@code{DWARF_FRAME_RETURN_COLUMN} to @code{DWARF_FRAME_REGNUM (REGNO)}.

@findex INCOMING_FRAME_SP_OFFSET
@item INCOMING_FRAME_SP_OFFSET
A C expression whose value is an integer giving the offset, in bytes,
from the value of the stack pointer register to the top of the stack
frame at the beginning of any function, before the prologue.  The top of
the frame is defined to be the value of the stack pointer in the
previous frame, just before the call instruction.

You only need to define this macro if you want to support call frame
debugging information like that provided by DWARF 2.

@findex ARG_POINTER_CFA_OFFSET
@item ARG_POINTER_CFA_OFFSET (@var{fundecl})
A C expression whose value is an integer giving the offset, in bytes,
from the argument pointer to the canonical frame address (cfa).  The
final value should coincide with that calculated by
@code{INCOMING_FRAME_SP_OFFSET}.  Which is unfortunately not usable
during virtual register instantiation.

The default value for this macro is @code{FIRST_PARM_OFFSET (fundecl)},
which is correct for most machines; in general, the arguments are found
immediately before the stack frame.  Note that this is not the case on
some targets that save registers into the caller's frame, such as SPARC
and rs6000, and so such targets need to define this macro.

You only need to define this macro if the default is incorrect, and you
want to support call frame debugging information like that provided by
DWARF 2.

@findex SMALL_STACK
@item SMALL_STACK
Define this macro if the stack size for the target is very small.  This
has the effect of disabling gcc's built-in @samp{alloca}, though
@samp{__builtin_alloca} is not affected.
@end table

@node Exception Handling
@subsection Exception Handling Support
@cindex exception handling

@table @code
@findex EH_RETURN_DATA_REGNO
@item EH_RETURN_DATA_REGNO (@var{N})
A C expression whose value is the @var{N}th register number used for
data by exception handlers, or @code{INVALID_REGNUM} if fewer than
@var{N} registers are usable.

The exception handling library routines communicate with the exception
handlers via a set of agreed upon registers.  Ideally these registers
should be call-clobbered; it is possible to use call-saved registers,
but may negatively impact code size.  The target must support at least
2 data registers, but should define 4 if there are enough free registers.

You must define this macro if you want to support call frame exception
handling like that provided by DWARF 2.

@findex EH_RETURN_STACKADJ_RTX
@item EH_RETURN_STACKADJ_RTX
A C expression whose value is RTL representing a location in which
to store a stack adjustment to be applied before function return.
This is used to unwind the stack to an exception handler's call frame.
It will be assigned zero on code paths that return normally.

Typically this is a call-clobbered hard register that is otherwise
untouched by the epilogue, but could also be a stack slot.

You must define this macro if you want to support call frame exception
handling like that provided by DWARF 2.

@findex EH_RETURN_HANDLER_RTX
@item EH_RETURN_HANDLER_RTX
A C expression whose value is RTL representing a location in which
to store the address of an exception handler to which we should
return.  It will not be assigned on code paths that return normally.

Typically this is the location in the call frame at which the normal
return address is stored.  For targets that return by popping an
address off the stack, this might be a memory address just below
the @emph{target} call frame rather than inside the current call
frame.  @code{EH_RETURN_STACKADJ_RTX} will have already been assigned,
so it may be used to calculate the location of the target call frame.

Some targets have more complex requirements than storing to an
address calculable during initial code generation.  In that case
the @code{eh_return} instruction pattern should be used instead.

If you want to support call frame exception handling, you must
define either this macro or the @code{eh_return} instruction pattern.

@findex ASM_PREFERRED_EH_DATA_FORMAT
@item ASM_PREFERRED_EH_DATA_FORMAT(@var{code}, @var{global})
This macro chooses the encoding of pointers embedded in the exception
handling sections.  If at all possible, this should be defined such
that the exception handling section will not require dynamic relocations,
and so may be read-only.

@var{code} is 0 for data, 1 for code labels, 2 for function pointers.
@var{global} is true if the symbol may be affected by dynamic relocations.
The macro should return a combination of the @code{DW_EH_PE_*} defines
as found in @file{dwarf2.h}.

If this macro is not defined, pointers will not be encoded but
represented directly.

@findex ASM_MAYBE_OUTPUT_ENCODED_ADDR_RTX
@item ASM_MAYBE_OUTPUT_ENCODED_ADDR_RTX(@var{file}, @var{encoding}, @var{size}, @var{addr}, @var{done})
This macro allows the target to emit whatever special magic is required
to represent the encoding chosen by @code{ASM_PREFERRED_EH_DATA_FORMAT}.
Generic code takes care of pc-relative and indirect encodings; this must
be defined if the target uses text-relative or data-relative encodings.

This is a C statement that branches to @var{done} if the format was
handled.  @var{encoding} is the format chosen, @var{size} is the number
of bytes that the format occupies, @var{addr} is the @code{SYMBOL_REF}
to be emitted.

@findex MD_FALLBACK_FRAME_STATE_FOR
@item MD_FALLBACK_FRAME_STATE_FOR(@var{context}, @var{fs}, @var{success})
This macro allows the target to add cpu and operating system specific
code to the call-frame unwinder for use when there is no unwind data
available.  The most common reason to implement this macro is to unwind
through signal frames.

This macro is called from @code{uw_frame_state_for} in @file{unwind-dw2.c}
and @file{unwind-ia64.c}.  @var{context} is an @code{_Unwind_Context};
@var{fs} is an @code{_Unwind_FrameState}.  Examine @code{context->ra}
for the address of the code being executed and @code{context->cfa} for
the stack pointer value.  If the frame can be decoded, the register save
addresses should be updated in @var{fs} and the macro should branch to
@var{success}.  If the frame cannot be decoded, the macro should do
nothing.
@end table

@node Stack Checking
@subsection Specifying How Stack Checking is Done

GCC will check that stack references are within the boundaries of
the stack, if the @option{-fstack-check} is specified, in one of three ways:

@enumerate
@item
If the value of the @code{STACK_CHECK_BUILTIN} macro is nonzero, GCC
will assume that you have arranged for stack checking to be done at
appropriate places in the configuration files, e.g., in
@code{TARGET_ASM_FUNCTION_PROLOGUE}.  GCC will do not other special
processing.

@item
If @code{STACK_CHECK_BUILTIN} is zero and you defined a named pattern
called @code{check_stack} in your @file{md} file, GCC will call that
pattern with one argument which is the address to compare the stack
value against.  You must arrange for this pattern to report an error if
the stack pointer is out of range.

@item
If neither of the above are true, GCC will generate code to periodically
``probe'' the stack pointer using the values of the macros defined below.
@end enumerate

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

@table @code
@findex STACK_CHECK_BUILTIN
@item STACK_CHECK_BUILTIN
A nonzero value if stack checking is done by the configuration files in a
machine-dependent manner.  You should define this macro if stack checking
is require by the ABI of your machine or if you would like to have to stack
checking in some more efficient way than GCC's portable approach.
The default value of this macro is zero.

@findex STACK_CHECK_PROBE_INTERVAL
@item STACK_CHECK_PROBE_INTERVAL
An integer representing the interval at which GCC must generate stack
probe instructions.  You will normally define this macro to be no larger
than the size of the ``guard pages'' at the end of a stack area.  The
default value of 4096 is suitable for most systems.

@findex STACK_CHECK_PROBE_LOAD
@item STACK_CHECK_PROBE_LOAD
A integer which is nonzero if GCC should perform the stack probe
as a load instruction and zero if GCC should use a store instruction.
The default is zero, which is the most efficient choice on most systems.

@findex STACK_CHECK_PROTECT
@item STACK_CHECK_PROTECT
The number of bytes of stack needed to recover from a stack overflow,
for languages where such a recovery is supported.  The default value of
75 words should be adequate for most machines.

@findex STACK_CHECK_MAX_FRAME_SIZE
@item STACK_CHECK_MAX_FRAME_SIZE
The maximum size of a stack frame, in bytes.  GCC will generate probe
instructions in non-leaf functions to ensure at least this many bytes of
stack are available.  If a stack frame is larger than this size, stack
checking will not be reliable and GCC will issue a warning.  The
default is chosen so that GCC only generates one instruction on most
systems.  You should normally not change the default value of this macro.

@findex STACK_CHECK_FIXED_FRAME_SIZE
@item STACK_CHECK_FIXED_FRAME_SIZE
GCC uses this value to generate the above warning message.  It
represents the amount of fixed frame used by a function, not including
space for any callee-saved registers, temporaries and user variables.
You need only specify an upper bound for this amount and will normally
use the default of four words.

@findex STACK_CHECK_MAX_VAR_SIZE
@item STACK_CHECK_MAX_VAR_SIZE
The maximum size, in bytes, of an object that GCC will place in the
fixed area of the stack frame when the user specifies
@option{-fstack-check}.
GCC computed the default from the values of the above macros and you will
normally not need to override that default.
@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 the actual hard register number
used for the frame pointer.

You should define this macro only in the very rare circumstances when it
is not possible to calculate the offset between the frame pointer and
the automatic variables until after register allocation has been
completed.  When this macro is defined, you must also indicate in your
definition of @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.

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.

@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.

@findex DWARF_FRAME_REGISTERS
@item DWARF_FRAME_REGISTERS
This macro specifies the maximum number of hard registers that can be
saved in a call frame.  This is used to size data structures used in
DWARF2 exception handling.

Prior to GCC 3.0, this macro was needed in order to establish a stable
exception handling ABI in the face of adding new hard registers for ISA
extensions.  In GCC 3.0 and later, the EH ABI is insulated from changes
in the number of hard registers.  Nevertheless, this macro can still be
used to reduce the runtime memory requirements of the exception handling
routines, which can be substantial if the ISA contains a lot of
registers that are not call-saved.

If this macro is not defined, it defaults to
@code{FIRST_PSEUDO_REGISTER}.

@findex PRE_GCC3_DWARF_FRAME_REGISTERS
@item PRE_GCC3_DWARF_FRAME_REGISTERS

This macro is similar to @code{DWARF_FRAME_REGISTERS}, but is provided
for backward compatibility in pre GCC 3.0 compiled code.

If this macro is not defined, it defaults to
@code{DWARF_FRAME_REGISTERS}.

@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.

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 nonzero 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.
@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
A C expression whose value is nonzero 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.  If the macro is not defined in target
header files, it defaults to 0.

@findex PUSH_ARGS
@item PUSH_ARGS
A C expression.  If nonzero, push insns will be used to pass
outgoing arguments.
If the target machine does not have a push instruction, set it to zero.
That directs GCC to use an alternate strategy: to
allocate the entire argument block and then store the arguments into
it.  When @code{PUSH_ARGS} is nonzero, @code{PUSH_ROUNDING} must be defined too.
On some machines, the definition

@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.

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
A C expression.  If nonzero, 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.

Setting both @code{PUSH_ARGS} 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},
which can be zero if GCC is calling a library function.

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, GCC 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 @code{DECL_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{fundecl}
will contain an identifier node for the library function.  Thus, if
you need to distinguish among various library functions, you can do so
by their names.  Note that ``library function'' in this context means
a function used to perform arithmetic, whose name is known specially
in the compiler and was not mentioned in the C code being compiled.

@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.

@findex CALL_POPS_ARGS
@item   CALL_POPS_ARGS (@var{cum})
A C expression that should indicate the number of bytes a call sequence
pops off the stack.  It is added to the value of @code{RETURN_POPS_ARGS}
when compiling a function call.

@var{cum} is the variable in which all arguments to the called function
have been accumulated.

On certain architectures, such as the SH5, a call trampoline is used
that pops certain registers off the stack, depending on the arguments
that have been passed to the function.  Since this is a property of the
call site, not of the called function, @code{RETURN_POPS_ARGS} is not
appropriate.

@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.
@var{type} can be an incomplete type if a syntax error has previously
occurred.

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 must be a @code{reg} RTX for the hard
register in which to pass this part of the argument, and the mode of the
register RTX 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 of where this part starts.
As a special exception the first @code{expr_list} in the @code{parallel}
RTX may have a first operand of zero.  This indicates that the entire
argument is also stored on the stack.

The last time this macro is called, it is called with @code{MODE ==
VOIDmode}, and its result is passed to the @code{call} or @code{call_value}
pattern as operands 2 and 3 respectively.

@cindex @file{stdarg.h} and register arguments
The usual way to make the ISO 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 nonzero 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 MUST_PASS_IN_STACK
@item MUST_PASS_IN_STACK (@var{mode}, @var{type})
Define as a C expression that evaluates to nonzero if we do not know how
to pass TYPE solely in registers.  The file @file{expr.h} defines a
definition that is usually appropriate, refer to @file{expr.h} for additional
documentation.

@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.

@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, that 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 @code{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 FUNCTION_ARG_REG_LITTLE_ENDIAN
@item FUNCTION_ARG_REG_LITTLE_ENDIAN
If defined TRUE on a big-endian system then structure arguments passed
(and returned) in registers are passed in a little-endian manner instead of
the big-endian manner.  On the HP-UX IA64 and PA64 platforms structures are
aligned differently then integral values and setting this value to true will
allow for the special handling of structure arguments and return values.

@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_LIBCALL_ARGS
@item INIT_CUMULATIVE_LIBCALL_ARGS (@var{cum}, @var{mode}, @var{libname})
Like @code{INIT_CUMULATIVE_ARGS} but only used for outgoing libcalls,
it gets a @code{MODE} argument instead of @var{fntype}, that would be
@code{NULL}.  @var{indirect} would always be zero, too.  If this macro
is not defined, @code{INIT_CUMULATIVE_ARGS (cum, NULL_RTX, libname,
0)} is used instead.

@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 GCC@.  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.

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 PAD_VARARGS_DOWN
@item PAD_VARARGS_DOWN
If defined, a C expression which determines whether the default
implementation of va_arg will attempt to pad down before reading the
next argument, if that argument is smaller than its aligned space as
controlled by @code{PARM_BOUNDARY}.  If this macro is not defined, all such
arguments are padded down if @code{BYTES_BIG_ENDIAN} is true.

@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.

@findex LOAD_ARGS_REVERSED
@item LOAD_ARGS_REVERSED
If defined, the order in which arguments are loaded into their
respective argument registers is reversed so that the last
argument is loaded first.  This macro only affects arguments
passed in registers.

@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 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).

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.

@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.

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

@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.

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 @option{-fpcc-struct-return}
takes effect regardless of this macro.  On most systems, it is
possible to leave the macro undefined; this causes a default
definition to be used, whose value is the constant 1 for @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 @option{-fpcc-struct-return} mode, but it does
nothing when you use @option{-freg-struct-return} mode.
@end table

@node Caller Saves
@subsection Caller-Saves Register Allocation

If you enable it, GCC 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.  When defined, this macro enables @option{-fcaller-saves}
by default for all optimization levels.  It has no effect for optimization
levels 2 and higher, where @option{-fcaller-saves} is the default.

@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}}.

@findex HARD_REGNO_CALLER_SAVE_MODE
@item HARD_REGNO_CALLER_SAVE_MODE (@var{regno}, @var{nregs})
A C expression specifying which mode is required for saving @var{nregs}
of a pseudo-register in call-clobbered hard register @var{regno}.  If
@var{regno} is unsuitable for caller save, @code{VOIDmode} should be
returned.  For most machines this macro need not be defined since GCC
will select the smallest suitable mode.
@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.

@deftypefn {Target Hook} void TARGET_ASM_FUNCTION_PROLOGUE (FILE *@var{file}, HOST_WIDE_INT @var{size})
If defined, a function 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{TARGET_ASM_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.
@end deftypefn

@deftypefn {Target Hook} void TARGET_ASM_FUNCTION_END_PROLOGUE (FILE *@var{file})
If defined, a function that outputs assembler code at the end of a
prologue.  This should be used when the function prologue is being
emitted as RTL, and you have some extra assembler that needs to be
emitted.  @xref{prologue instruction pattern}.
@end deftypefn

@deftypefn {Target Hook} void TARGET_ASM_FUNCTION_BEGIN_EPILOGUE (FILE *@var{file})
If defined, a function that outputs assembler code at the start of an
epilogue.  This should be used when the function epilogue is being
emitted as RTL, and you have some extra assembler that needs to be
emitted.  @xref{epilogue instruction pattern}.
@end deftypefn

@deftypefn {Target Hook} void TARGET_ASM_FUNCTION_EPILOGUE (FILE *@var{file}, HOST_WIDE_INT @var{size})
If defined, a function 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{TARGET_ASM_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{TARGET_ASM_FUNCTION_EPILOGUE} at all.

Do not define a pattern named @samp{return} if you want the
@code{TARGET_ASM_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{TARGET_ASM_FUNCTION_PROLOGUE} and
@code{TARGET_ASM_FUNCTION_EPILOGUE} must treat leaf functions specially.
The C variable @code{current_function_is_leaf} 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 @option{-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{TARGET_ASM_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
@end deftypefn

@table @code

@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 @code{<varargs.h>} and @code{<stdarg.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{TARGET_ASM_FUNCTION_PROLOGUE} and
@code{TARGET_ASM_FUNCTION_EPILOGUE} to treat leaf functions specially.
The C variable @code{current_function_is_leaf} 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 that 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 EH_USES
@item EH_USES (@var{regno})
Define this macro as a C expression that is nonzero for registers that are
used by the exception handling mechanism, and so should be considered live
on entry to an exception edge.

@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{TARGET_ASM_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{TARGET_ASM_FUNCTION_PROLOGUE}
and @code{TARGET_ASM_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.

If you do not define this macro, the target-independent code in the C++
front end will generate a less efficient heavyweight thunk that calls
@var{function} instead of jumping to it.  The generic approach does
not support varargs.
@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}.

@findex mcount
The details of how @code{mcount} expects to be called are determined by
your operating system environment, not by GCC@.  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.

Older implementations of @code{mcount} expect the address of a counter
variable to be loaded into some register.  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 PROFILE_HOOK
@item PROFILE_HOOK
A C statement or compound statement to output to @var{file} some assembly
code to call the profiling subroutine @code{mcount} even the target does
not support profiling.

@findex NO_PROFILE_COUNTERS
@item NO_PROFILE_COUNTERS
Define this macro if the @code{mcount} subroutine on your system does
not need a counter variable allocated for each function.  This is true
for almost all modern implementations.  If you define this macro, you
must not use the @var{labelno} argument to @code{FUNCTION_PROFILER}.

@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 TARGET_ALLOWS_PROFILING_WITHOUT_FRAME_POINTER
@item TARGET_ALLOWS_PROFILING_WITHOUT_FRAME_POINTER
On some targets, it is impossible to use profiling when the frame
pointer has been omitted.  For example, on x86 GNU/Linux systems,
the @code{mcount} routine provided by the GNU C Library finds the
address of the routine that called the routine that called @code{mcount}
by looking in the immediate caller's stack frame.  If the immediate
caller has no frame pointer, this lookup will fail.

By default, GCC assumes that the target does allow profiling when the
frame pointer is omitted.  This macro should be defined to a C
expression that evaluates to @code{false} if the target does not allow
profiling when the frame pointer is omitted.

@end table

@node Tail Calls
@subsection Permitting tail calls
@cindex tail calls

@table @code
@findex FUNCTION_OK_FOR_SIBCALL
@item FUNCTION_OK_FOR_SIBCALL (@var{decl})
A C expression that evaluates to true if it is ok to perform a sibling
call to @var{decl} from the current function.

It is not uncommon for limitations of calling conventions to prevent
tail calls to functions outside the current unit of translation, or
during PIC compilation.  Use this macro to enforce these restrictions,
as the @code{sibcall} md pattern can not fail, or fall over to a
``normal'' call.
@end table

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

GCC comes with an implementation of @code{<varargs.h>} and
@code{<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.

ISO @code{<stdarg.h>} differs from traditional @code{<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 ISO 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 ISO 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 ()
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}.

@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 are obtained after processing 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 to be a nonzero value if the location where a function
argument is passed depends on whether or not it is a named argument.

This macro controls how the @var{named} argument to @code{FUNCTION_ARG}
is set for varargs and stdarg functions.  If this macro returns a
nonzero value, the @var{named} argument is always true for named
arguments, and false for unnamed arguments.  If it returns a value of
zero, 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.

You need not define this macro if it always returns zero.

@findex PRETEND_OUTGOING_VARARGS_NAMED
@item PRETEND_OUTGOING_VARARGS_NAMED
If you need to conditionally change ABIs so that one works with
@code{SETUP_INCOMING_VARARGS}, but the other works like neither
@code{SETUP_INCOMING_VARARGS} nor @code{STRICT_ARGUMENT_NAMING} was
defined, then define this macro to return nonzero if
@code{SETUP_INCOMING_VARARGS} is used, zero otherwise.
Otherwise, you should not define this macro.
@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 GCC 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 TRAMPOLINE_ADJUST_ADDRESS
@item TRAMPOLINE_ADJUST_ADDRESS (@var{addr})
A C statement that should perform any machine-specific adjustment in
the address of the trampoline.  Its argument contains the address that
was passed to @code{INITIALIZE_TRAMPOLINE}.  In case the address to be
used for a function call should be different from the address in which
the template was stored, the different address should be assigned to
@var{addr}.  If this macro is not defined, @var{addr} will be used for
function calls.

@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{TARGET_ASM_FUNCTION_EPILOGUE} and trampolines
@cindex @code{TARGET_ASM_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{TARGET_ASM_FUNCTION_PROLOGUE}
and @code{TARGET_ASM_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
@code{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 GCC@.  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 FLOAT_LIB_COMPARE_RETURNS_BOOL (@var{mode}, @var{comparison})
@item FLOAT_LIB_COMPARE_RETURNS_BOOL
Define this macro as a C statement that returns nonzero if a call to
the floating point comparison library function will return a boolean
value that indicates the result of the comparison.  It should return
zero if one of gcc's own libgcc functions is called.

Most ports don't need to define this macro.

@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, GCC 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{memmove}, implicit usage
@cindex @code{bzero}, implicit usage
@cindex @code{memset}, implicit usage
@item TARGET_MEM_FUNCTIONS
Define this macro if GCC should generate calls to the ISO C
(and System V) library functions @code{memcpy}, @code{memmove} 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 @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{libgcc.a} accept their arguments.  It is useful on
machines where floating and fixed point arguments are passed
differently, such as the i860.

@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_PRE_INCREMENT
@findex HAVE_PRE_DECREMENT
@findex HAVE_POST_INCREMENT
@findex HAVE_POST_DECREMENT
@item HAVE_PRE_INCREMENT
@itemx HAVE_PRE_DECREMENT
@itemx HAVE_POST_INCREMENT
@itemx HAVE_POST_DECREMENT
A C expression that is nonzero if the machine supports pre-increment,
pre-decrement, post-increment, or post-decrement addressing respectively.

@findex HAVE_POST_MODIFY_DISP
@findex HAVE_PRE_MODIFY_DISP
@item HAVE_PRE_MODIFY_DISP
@itemx HAVE_POST_MODIFY_DISP
A C expression that is nonzero if the machine supports pre- or
post-address side-effect generation involving constants other than
the size of the memory operand.

@findex HAVE_POST_MODIFY_REG
@findex HAVE_PRE_MODIFY_REG
@item HAVE_PRE_MODIFY_REG
@itemx HAVE_POST_MODIFY_REG
A C expression that is nonzero if the machine supports pre- or
post-address side-effect generation involving a register displacement.

@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.

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_MODE_OK_FOR_BASE_P
@item REG_MODE_OK_FOR_BASE_P (@var{x}, @var{mode})
A C expression that is just like @code{REG_OK_FOR_BASE_P}, except that
that expression may examine the mode of the memory reference in
@var{mode}.  You should define this macro if the mode of the memory
reference affects whether a register may be used as a base register.  If
you define this macro, the compiler will use it instead of
@code{REG_OK_FOR_BASE_P}.

@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 FIND_BASE_TERM
@item FIND_BASE_TERM (@var{x})
A C expression to determine the base term of address @var{x}.
This macro is used in only one place: `find_base_term' in alias.c.

It is always safe for this macro to not be defined.  It exists so
that alias analysis can understand machine-dependent addresses.

The typical use of this macro is to handle addresses containing
a label_ref or symbol_ref within an UNSPEC@.

@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 LEGITIMIZE_RELOAD_ADDRESS
@item LEGITIMIZE_RELOAD_ADDRESS (@var{x}, @var{mode}, @var{opnum}, @var{type}, @var{ind_levels}, @var{win})
A C compound statement that attempts to replace @var{x}, which is an address
that needs reloading, with a valid memory address for an operand of mode
@var{mode}.  @var{win} will be a C statement label elsewhere in the code.
It is not necessary to define this macro, but it might be useful for
performance reasons.

For example, on the i386, it is sometimes possible to use a single
reload register instead of two by reloading a sum of two pseudo
registers into a register.  On the other hand, for number of RISC
processors offsets are limited so that often an intermediate address
needs to be generated in order to address a stack slot.  By defining
@code{LEGITIMIZE_RELOAD_ADDRESS} appropriately, the intermediate addresses
generated for adjacent some stack slots can be made identical, and thus
be shared.

@emph{Note}: This macro should be used with caution.  It is necessary
to know something of how reload works in order to effectively use this,
and it is quite easy to produce macros that build in too much knowledge
of reload internals.

@emph{Note}: This macro must be able to reload an address created by a
previous invocation of this macro.  If it fails to handle such addresses
then the compiler may generate incorrect code or abort.

@findex push_reload
The macro definition should use @code{push_reload} to indicate parts that
need reloading; @var{opnum}, @var{type} and @var{ind_levels} are usually
suitable to be passed unaltered to @code{push_reload}.

The code generated by this macro must 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.
This also applies to parts that you change indirectly by calling
@code{push_reload}.

@findex strict_memory_address_p
The macro definition may use @code{strict_memory_address_p} to test if
the address has become legitimate.

@findex copy_rtx
If you want to change only a part of @var{x}, one standard way of doing
this is to use @code{copy_rtx}.  Note, however, that is unshares only a
single level of rtl.  Thus, if the part to be changed is not at the
top level, you'll need to replace first the top level.
It is not necessary for this macro to come up with a legitimate
address;  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.
@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 additional modes for condition code values in registers
(@pxref{Jump Patterns}).  This macro should expand to a sequence of
calls of the macro @code{CC} separated by white space.  @code{CC} takes
two arguments.  The first is the enumeration name of the mode, which
should begin with @samp{CC} and end with @samp{mode}.  The second is a C
string giving the printable name of the mode; it should be the same as
the first argument, but with the trailing @samp{mode} removed.

You should only define this macro if additional modes are required.

A sample definition of @code{EXTRA_CC_MODES} is:
@smallexample
#define EXTRA_CC_MODES            \
    CC(CC_NOOVmode, "CC_NOOV")    \
    CC(CCFPmode, "CCFP")          \
    CC(CCFPEmode, "CCFPE")
@end smallexample

@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})
On 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
and @var{op0} and @var{op1} are the left and right operands of the
comparison, respectively.  You should modify @var{code}, @var{op0}, and
@var{op1} as required.

GCC will not assume that the comparison resulting from this macro is
valid but will see if the resulting insn matches a pattern in the
@file{md} file.

You need not define this macro if it would never change the comparison
code or operands.

@findex REVERSIBLE_CC_MODE
@item REVERSIBLE_CC_MODE (@var{mode})
A C expression whose value is one if it is always safe to reverse a
comparison whose mode is @var{mode}.  If @code{SELECT_CC_MODE}
can ever return @var{mode} for a floating-point inequality comparison,
then @code{REVERSIBLE_CC_MODE (@var{mode})} must be zero.

You need not define this macro if it would always returns zero or if the
floating-point format is anything other than @code{IEEE_FLOAT_FORMAT}.
For example, here is the definition used on the Sparc, where floating-point
inequality comparisons are always given @code{CCFPEmode}:

@smallexample
#define REVERSIBLE_CC_MODE(MODE)  ((MODE) != CCFPEmode)
@end smallexample

@findex REVERSE_CONDITION (@var{code}, @var{mode})
A C expression whose value is reversed condition code of the @var{code} for
comparison done in CC_MODE @var{mode}.  The macro is used only in case
@code{REVERSIBLE_CC_MODE (@var{mode})} is nonzero.  Define this macro in case
machine has some non-standard way how to reverse certain conditionals.  For
instance in case all floating point conditions are non-trapping, compiler may
freely convert unordered compares to ordered one.  Then definition may look
like:

@smallexample
#define REVERSE_CONDITION(CODE, MODE) \
   ((MODE) != CCFPmode ? reverse_condition (CODE) \
    : reverse_condition_maybe_unordered (CODE))
@end smallexample

@findex REVERSE_CONDEXEC_PREDICATES_P
@item REVERSE_CONDEXEC_PREDICATES_P (@var{code1}, @var{code2})
A C expression that returns true if the conditional execution predicate
@var{code1} is the inverse of @var{code2} and vice versa.  Define this to
return 0 if the target has conditional execution predicates that cannot be
reversed safely.  If no expansion is specified, this macro is defined as
follows:

@smallexample
#define REVERSE_CONDEXEC_PREDICATES_P (x, y) \
   ((x) == reverse_condition (y))
@end smallexample

@end table

@node Costs
@section Describing Relative Costs of Operations
@cindex costs of instructions
@cindex relative costs
@cindex speed of instructions

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

@table @code
@findex CONST_COSTS
@item CONST_COSTS (@var{x}, @var{code}, @var{outer_code})
A part of a C @code{switch} statement that describes the relative costs
of constant RTL expressions.  It must contain @code{case} labels for
expression codes @code{const_int}, @code{const}, @code{symbol_ref},
@code{label_ref} and @code{const_double}.  Each case must ultimately
reach a @code{return} statement to return the relative cost of the use
of that kind of constant value in an expression.  The cost may depend on
the precise value of the constant, which is available for examination in
@var{x}, and the rtx code of the expression in which it is contained,
found in @var{outer_code}.

@var{code} is the expression code---redundant, since it can be
obtained with @code{GET_CODE (@var{x})}.

@findex RTX_COSTS
@findex COSTS_N_INSNS
@item RTX_COSTS (@var{x}, @var{code}, @var{outer_code})
Like @code{CONST_COSTS} but applies to nonconstant RTL expressions.
This can be used, for example, to indicate how costly a multiply
instruction is.  In writing this macro, you can use the construct
@code{COSTS_N_INSNS (@var{n})} to specify a cost equal to @var{n} fast
instructions.  @var{outer_code} is the code of the expression in which
@var{x} is contained.

This macro is optional; do not define it if the default cost assumptions
are adequate for the target machine.

@findex DEFAULT_RTX_COSTS
@item DEFAULT_RTX_COSTS (@var{x}, @var{code}, @var{outer_code})
This macro, if defined, is called for any case not handled by the
@code{RTX_COSTS} or @code{CONST_COSTS} macros.  This eliminates the need
to put case labels into the macro, but the code, or any functions it
calls, must assume that the RTL in @var{x} could be of any type that has
not already been handled.  The arguments are the same as for
@code{RTX_COSTS}, and the macro should execute a return statement giving
the cost of any RTL expressions that it can handle.  The default cost
calculation is used for any RTL for which this macro does not return a
value.

This macro is optional; do not define it if the default cost assumptions
are adequate for the target machine.

@findex ADDRESS_COST
@item ADDRESS_COST (@var{address})
An expression giving the cost of an addressing mode that contains
@var{address}.  If not defined, the cost is computed from
the @var{address} expression and the @code{CONST_COSTS} values.

For most CISC machines, the default cost is a good approximation of the
true cost of the addressing mode.  However, on RISC machines, all
instructions normally have the same length and execution time.  Hence
all addresses will have equal costs.

In cases where more than one form of an address is known, the form with
the lowest cost will be used.  If multiple forms have the same, lowest,
cost, the one that is the most complex will be used.

For example, suppose an address that is equal to the sum of a register
and a constant is used twice in the same basic block.  When this macro
is not defined, the address will be computed in a register and memory
references will be indirect through that register.  On machines where
the cost of the addressing mode containing the sum is no higher than
that of a simple indirect reference, this will produce an additional
instruction and possibly require an additional register.  Proper
specification of this macro eliminates this overhead for such machines.

Similar use of this macro is made in strength reduction of loops.

@var{address} need not be valid as an address.  In such a case, the cost
is not relevant and can be any value; invalid addresses need not be
assigned a different cost.

On machines where an address involving more than one register is as
cheap as an address computation involving only one register, defining
@code{ADDRESS_COST} to reflect this can cause two registers to be live
over a region of code where only one would have been if
@code{ADDRESS_COST} were not defined in that manner.  This effect should
be considered in the definition of this macro.  Equivalent costs should
probably only be given to addresses with different numbers of registers
on machines with lots of registers.

This macro will normally either not be defined or be defined as a
constant.

@findex REGISTER_MOVE_COST
@item REGISTER_MOVE_COST (@var{mode}, @var{from}, @var{to})
A C expression for the cost of moving data of mode @var{mode} from a
register in class @var{from} to one in class @var{to}.  The classes are
expressed using the enumeration values such as @code{GENERAL_REGS}.  A
value of 2 is the default; other values are interpreted relative to
that.

It is not required that the cost always equal 2 when @var{from} is the
same as @var{to}; on some machines it is expensive to move between
registers if they are not general registers.

If reload sees an insn consisting of a single @code{set} between two
hard registers, and if @code{REGISTER_MOVE_COST} applied to their
classes returns a value of 2, reload does not check to ensure that the
constraints of the insn are met.  Setting a cost of other than 2 will
allow reload to verify that the constraints are met.  You should do this
if the @samp{mov@var{m}} pattern's constraints do not allow such copying.

@findex MEMORY_MOVE_COST
@item MEMORY_MOVE_COST (@var{mode}, @var{class}, @var{in})
A C expression for the cost of moving data of mode @var{mode} between a
register of class @var{class} and memory; @var{in} is zero if the value
is to be written to memory, nonzero if it is to be read in.  This cost
is relative to those in @code{REGISTER_MOVE_COST}.  If moving between
registers and memory is more expensive than between two registers, you
should define this macro to express the relative cost.

If you do not define this macro, GCC uses a default cost of 4 plus
the cost of copying via a secondary reload register, if one is
needed.  If your machine requires a secondary reload register to copy
between memory and a register of @var{class} but the reload mechanism is
more complex than copying via an intermediate, define this macro to
reflect the actual cost of the move.

GCC defines the function @code{memory_move_secondary_cost} if
secondary reloads are needed.  It computes the costs due to copying via
a secondary register.  If your machine copies from memory using a
secondary register in the conventional way but the default base value of
4 is not correct for your machine, define this macro to add some other
value to the result of that function.  The arguments to that function
are the same as to this macro.

@findex BRANCH_COST
@item BRANCH_COST
A C expression for the cost of a branch instruction.  A value of 1 is
the default; other values are interpreted relative to that.
@end table

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

@table @code
@findex SLOW_BYTE_ACCESS
@item SLOW_BYTE_ACCESS
Define this macro as a C expression which is nonzero if accessing less
than a word of memory (i.e.@: a @code{char} or a @code{short}) is no
faster than accessing a word of memory, i.e., if such access
require more than one instruction or if there is no difference in cost
between byte and (aligned) word loads.

When this macro is not defined, the compiler will access a field by
finding the smallest containing object; when it is defined, a fullword
load will be used if alignment permits.  Unless bytes accesses are
faster than word accesses, using word accesses is preferable since it
may eliminate subsequent memory access if subsequent accesses occur to
other fields in the same word of the structure, but to different bytes.

@findex SLOW_UNALIGNED_ACCESS
@item SLOW_UNALIGNED_ACCESS (@var{mode}, @var{alignment})
Define this macro to be the value 1 if memory accesses described by the
@var{mode} and @var{alignment} parameters have a cost many times greater
than aligned accesses, for example if they are emulated in a trap
handler.

When this macro is nonzero, the compiler will act as if
@code{STRICT_ALIGNMENT} were nonzero when generating code for block
moves.  This can cause significantly more instructions to be produced.
Therefore, do not set this macro nonzero if unaligned accesses only add a
cycle or two to the time for a memory access.

If the value of this macro is always zero, it need not be defined.  If
this macro is defined, it should produce a nonzero value when
@code{STRICT_ALIGNMENT} is nonzero.

@findex DONT_REDUCE_ADDR
@item DONT_REDUCE_ADDR
Define this macro to inhibit strength reduction of memory addresses.
(On some machines, such strength reduction seems to do harm rather
than good.)

@findex MOVE_RATIO
@item MOVE_RATIO
The threshold of number of scalar memory-to-memory move insns, @emph{below}
which a sequence of insns should be generated instead of a
string move insn or a library call.  Increasing the value will always
make code faster, but eventually incurs high cost in increased code size.

Note that on machines where the corresponding move insn is a
@code{define_expand} that emits a sequence of insns, this macro counts
the number of such sequences.

If you don't define this, a reasonable default is used.

@findex MOVE_BY_PIECES_P
@item MOVE_BY_PIECES_P (@var{size}, @var{alignment})
A C expression used to determine whether @code{move_by_pieces} will be used to
copy a chunk of memory, or whether some other block move mechanism
will be used.  Defaults to 1 if @code{move_by_pieces_ninsns} returns less
than @code{MOVE_RATIO}.

@findex MOVE_MAX_PIECES
@item MOVE_MAX_PIECES
A C expression used by @code{move_by_pieces} to determine the largest unit
a load or store used to copy memory is.  Defaults to @code{MOVE_MAX}.

@findex USE_LOAD_POST_INCREMENT
@item USE_LOAD_POST_INCREMENT (@var{mode})
A C expression used to determine whether a load postincrement is a good
thing to use for a given mode.  Defaults to the value of
@code{HAVE_POST_INCREMENT}.

@findex USE_LOAD_POST_DECREMENT
@item USE_LOAD_POST_DECREMENT (@var{mode})
A C expression used to determine whether a load postdecrement is a good
thing to use for a given mode.  Defaults to the value of
@code{HAVE_POST_DECREMENT}.

@findex USE_LOAD_PRE_INCREMENT
@item USE_LOAD_PRE_INCREMENT (@var{mode})
A C expression used to determine whether a load preincrement is a good
thing to use for a given mode.  Defaults to the value of
@code{HAVE_PRE_INCREMENT}.

@findex USE_LOAD_PRE_DECREMENT
@item USE_LOAD_PRE_DECREMENT (@var{mode})
A C expression used to determine whether a load predecrement is a good
thing to use for a given mode.  Defaults to the value of
@code{HAVE_PRE_DECREMENT}.

@findex USE_STORE_POST_INCREMENT
@item USE_STORE_POST_INCREMENT (@var{mode})
A C expression used to determine whether a store postincrement is a good
thing to use for a given mode.  Defaults to the value of
@code{HAVE_POST_INCREMENT}.

@findex USE_STORE_POST_DECREMENT
@item USE_STORE_POST_DECREMENT (@var{mode})
A C expression used to determine whether a store postdecrement is a good
thing to use for a given mode.  Defaults to the value of
@code{HAVE_POST_DECREMENT}.

@findex USE_STORE_PRE_INCREMENT
@item USE_STORE_PRE_INCREMENT (@var{mode})
This macro is used to determine whether a store preincrement is a good
thing to use for a given mode.  Defaults to the value of
@code{HAVE_PRE_INCREMENT}.

@findex USE_STORE_PRE_DECREMENT
@item USE_STORE_PRE_DECREMENT (@var{mode})
This macro is used to determine whether a store predecrement is a good
thing to use for a given mode.  Defaults to the value of
@code{HAVE_PRE_DECREMENT}.

@findex NO_FUNCTION_CSE
@item NO_FUNCTION_CSE
Define this macro if it is as good or better to call a constant
function address than to call an address kept in a register.

@findex NO_RECURSIVE_FUNCTION_CSE
@item NO_RECURSIVE_FUNCTION_CSE
Define this macro if it is as good or better for a function to call
itself with an explicit address than to call an address kept in a
register.
@end table

@node Scheduling
@section Adjusting the Instruction Scheduler

The instruction scheduler may need a fair amount of machine-specific
adjustment in order to produce good code.  GCC provides several target
hooks for this purpose.  It is usually enough to define just a few of
them: try the first ones in this list first.

@deftypefn {Target Hook} int TARGET_SCHED_ISSUE_RATE (void)
This hook returns the maximum number of instructions that can ever issue
at the same time on the target machine.  The default is one.  This value
must be constant over the entire compilation.  If you need it to vary
depending on what the instructions are, you must use
@samp{TARGET_SCHED_VARIABLE_ISSUE}.
@end deftypefn

@deftypefn {Target Hook} int TARGET_SCHED_VARIABLE_ISSUE (FILE *@var{file}, int @var{verbose}, rtx @var{insn}, int @var{more})
This hook is executed by the scheduler after it has scheduled an insn
from the ready list.  It should return the number of insns which can
still be issued in the current cycle.  Normally this is
@samp{@w{@var{more} - 1}}.  You should define this hook if some insns
take more machine resources than others, so that fewer insns can follow
them in the same cycle.  @var{file} is either a null pointer, or a stdio
stream to write any debug output to.  @var{verbose} is the verbose level
provided by @option{-fsched-verbose-@var{n}}.  @var{insn} is the
instruction that was scheduled.
@end deftypefn

@deftypefn {Target Hook} int TARGET_SCHED_ADJUST_COST (rtx @var{insn}, rtx @var{link}, rtx @var{dep_insn}, int @var{cost})
This function corrects the value of @var{cost} based on the relationship
between @var{insn} and @var{dep_insn} through the dependence @var{link}.
It should return the new value.  The default is to make no adjustment to
@var{cost}.  This can be used for example to specify to the scheduler
that an output- or anti-dependence does not incur the same cost as a
data-dependence.
@end deftypefn

@deftypefn {Target Hook} int TARGET_SCHED_ADJUST_PRIORITY (rtx @var{insn}, int @var{priority})
This hook adjusts the integer scheduling priority @var{priority} of
@var{insn}.  It should return the new priority.  Reduce the priority to
execute @var{insn} earlier, increase the priority to execute @var{insn}
later.  Do not define this hook if you do not need to adjust the
scheduling priorities of insns.
@end deftypefn

@deftypefn {Target Hook} int TARGET_SCHED_REORDER (FILE *@var{file}, int @var{verbose}, rtx *@var{ready}, int *@var{n_readyp}, int @var{clock})
This hook is executed by the scheduler after it has scheduled the ready
list, to allow the machine description to reorder it (for example to
combine two small instructions together on @samp{VLIW} machines).
@var{file} is either a null pointer, or a stdio stream to write any
debug output to.  @var{verbose} is the verbose level provided by
@option{-fsched-verbose-@var{n}}.  @var{ready} is a pointer to the ready
list of instructions that are ready to be scheduled.  @var{n_readyp} is
a pointer to the number of elements in the ready list.  The scheduler
reads the ready list in reverse order, starting with
@var{ready}[@var{*n_readyp}-1] and going to @var{ready}[0].  @var{clock}
is the timer tick of the scheduler.  You may modify the ready list and
the number of ready insns.  The return value is the number of insns that
can issue this cycle; normally this is just @code{issue_rate}.  See also
@samp{TARGET_SCHED_REORDER2}.
@end deftypefn

@deftypefn {Target Hook} int TARGET_SCHED_REORDER2 (FILE *@var{file}, int @var{verbose}, rtx *@var{ready}, int *@var{n_ready}, @var{clock})
Like @samp{TARGET_SCHED_REORDER}, but called at a different time.  That
function is called whenever the scheduler starts a new cycle.  This one
is called once per iteration over a cycle, immediately after
@samp{TARGET_SCHED_VARIABLE_ISSUE}; it can reorder the ready list and
return the number of insns to be scheduled in the same cycle.  Defining
this hook can be useful if there are frequent situations where
scheduling one insn causes other insns to become ready in the same
cycle.  These other insns can then be taken into account properly.
@end deftypefn

@deftypefn {Target Hook} void TARGET_SCHED_INIT (FILE *@var{file}, int @var{verbose}, int @var{max_ready})
This hook is executed by the scheduler at the beginning of each block of
instructions that are to be scheduled.  @var{file} is either a null
pointer, or a stdio stream to write any debug output to.  @var{verbose}
is the verbose level provided by @option{-fsched-verbose-@var{n}}.
@var{max_ready} is the maximum number of insns in the current scheduling
region that can be live at the same time.  This can be used to allocate
scratch space if it is needed, e.g. by @samp{TARGET_SCHED_REORDER}.
@end deftypefn

@deftypefn {Target Hook} void TARGET_SCHED_FINISH (FILE *@var{file}, int @var{verbose})
This hook is executed by the scheduler at the end of each block of
instructions that are to be scheduled.  It can be used to perform
cleanup of any actions done by the other scheduling hooks.  @var{file}
is either a null pointer, or a stdio stream to write any debug output
to.  @var{verbose} is the verbose level provided by
@option{-fsched-verbose-@var{n}}.
@end deftypefn

@deftypefn {Target Hook} rtx TARGET_SCHED_CYCLE_DISPLAY (int @var{clock}, rtx @var{last})
This hook is called in verbose mode only, at the beginning of each pass
over a basic block.  It should insert an insn into the chain after
@var{last}, which has no effect, but records the value @var{clock} in
RTL dumps and assembly output.  Define this hook only if you need this
level of detail about what the scheduler is doing.
@end deftypefn

@node Sections
@section Dividing the Output into Sections (Texts, Data, @dots{})
@c the above section title is WAY too long.  maybe cut the part between
@c the (...)?  --mew 10feb93

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

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

@table @code
@findex TEXT_SECTION_ASM_OP
@item TEXT_SECTION_ASM_OP
A C expression whose value is a string, including spacing, containing the
assembler operation that should precede instructions and read-only data.
Normally @code{"\t.text"} is right.

@findex TEXT_SECTION
@item TEXT_SECTION
A C statement that switches to the default section containing instructions.
Normally this is not needed, as simply defining @code{TEXT_SECTION_ASM_OP}
is enough.  The MIPS port uses this to sort all functions after all data
declarations.

@findex DATA_SECTION_ASM_OP
@item DATA_SECTION_ASM_OP
A C expression whose value is a string, including spacing, containing the
assembler operation to identify the following data as writable initialized
data.  Normally @code{"\t.data"} is right.

@findex SHARED_SECTION_ASM_OP
@item SHARED_SECTION_ASM_OP
If defined, a C expression whose value is a string, including spacing,
containing the assembler operation to identify the following data as
shared data.  If not defined, @code{DATA_SECTION_ASM_OP} will be used.

@findex BSS_SECTION_ASM_OP
@item BSS_SECTION_ASM_OP
If defined, a C expression whose value is a string, including spacing,
containing the assembler operation to identify the following data as
uninitialized global data.  If not defined, and neither
@code{ASM_OUTPUT_BSS} nor @code{ASM_OUTPUT_ALIGNED_BSS} are defined,
uninitialized global data will be output in the data section if
@option{-fno-common} is passed, otherwise @code{ASM_OUTPUT_COMMON} will be
used.

@findex SHARED_BSS_SECTION_ASM_OP
@item SHARED_BSS_SECTION_ASM_OP
If defined, a C expression whose value is a string, including spacing,
containing the assembler operation to identify the following data as
uninitialized global shared data.  If not defined, and
@code{BSS_SECTION_ASM_OP} is, the latter will be used.

@findex INIT_SECTION_ASM_OP
@item INIT_SECTION_ASM_OP
If defined, a C expression whose value is a string, including spacing,
containing the assembler operation to identify the following data as
initialization code.  If not defined, GCC will assume such a section does
not exist.

@findex FINI_SECTION_ASM_OP
@item FINI_SECTION_ASM_OP
If defined, a C expression whose value is a string, including spacing,
containing the assembler operation to identify the following data as
finalization code.  If not defined, GCC will assume such a section does
not exist.

@findex CRT_CALL_STATIC_FUNCTION
@item CRT_CALL_STATIC_FUNCTION (@var{section_op}, @var{function})
If defined, an ASM statement that switches to a different section
via @var{section_op}, calls @var{function}, and switches back to
the text section.  This is used in @file{crtstuff.c} if
@code{INIT_SECTION_ASM_OP} or @code{FINI_SECTION_ASM_OP} to calls
to initialization and finalization functions from the init and fini
sections.  By default, this macro uses a simple function call.  Some
ports need hand-crafted assembly code to avoid dependencies on
registers initialized in the function prologue or to ensure that
constant pools don't end up too far way in the text section.

@findex FORCE_CODE_SECTION_ALIGN
@item FORCE_CODE_SECTION_ALIGN
If defined, an ASM statement that aligns a code section to some
arbitrary boundary.  This is used to force all fragments of the
@code{.init} and @code{.fini} sections to have to same alignment
and thus prevent the linker from having to add any padding.

@findex EXTRA_SECTIONS
@findex in_text
@findex in_data
@item EXTRA_SECTIONS
A list of names for sections other than the standard two, which are
@code{in_text} and @code{in_data}.  You need not define this macro
on a system with no other sections (that GCC needs to use).

@findex EXTRA_SECTION_FUNCTIONS
@findex text_section
@findex data_section
@item EXTRA_SECTION_FUNCTIONS
One or more functions to be defined in @file{varasm.c}.  These
functions should do jobs analogous to those of @code{text_section} and
@code{data_section}, for your additional sections.  Do not define this
macro if you do not define @code{EXTRA_SECTIONS}.

@findex READONLY_DATA_SECTION
@item READONLY_DATA_SECTION
On most machines, read-only variables, constants, and jump tables are
placed in the text section.  If this is not the case on your machine,
this macro should be defined to be the name of a function (either
@code{data_section} or a function defined in @code{EXTRA_SECTIONS}) that
switches to the section to be used for read-only items.

If these items should be placed in the text section, this macro should
not be defined.

@findex SELECT_SECTION
@item SELECT_SECTION (@var{exp}, @var{reloc}, @var{align})
A C statement or statements to switch to the appropriate section for
output of @var{exp}.  You can assume that @var{exp} is either a
@code{VAR_DECL} node or a constant of some sort.  @var{reloc}
indicates whether the initial value of @var{exp} requires link-time
relocations.  Bit 1 is set when variable contains local relocations
only, while bit 2 is set for global relocations.
Select the section by calling @code{text_section} or one
of the alternatives for other sections.  @var{align} is the constant
alignment in bits.

Do not define this macro if you put all read-only variables and
constants in the read-only data section (usually the text section).

@findex SELECT_RTX_SECTION
@item SELECT_RTX_SECTION (@var{mode}, @var{rtx}, @var{align})
A C statement or statements to switch to the appropriate section for
output of @var{rtx} in mode @var{mode}.  You can assume that @var{rtx}
is some kind of constant in RTL@.  The argument @var{mode} is redundant
except in the case of a @code{const_int} rtx.  Select the section by
calling @code{text_section} or one of the alternatives for other
sections.  @var{align} is the constant alignment in bits.

Do not define this macro if you put all constants in the read-only
data section.

@findex JUMP_TABLES_IN_TEXT_SECTION
@item JUMP_TABLES_IN_TEXT_SECTION
Define this macro to be an expression with a nonzero value if jump
tables (for @code{tablejump} insns) should be output in the text
section, along with the assembler instructions.  Otherwise, the
readonly data section is used.

This macro is irrelevant if there is no separate readonly data section.

@findex ENCODE_SECTION_INFO
@item ENCODE_SECTION_INFO (@var{decl}, @var{new_decl_p})
Define this macro if references to a symbol or a constant must be
treated differently depending on something about the variable or
function named by the symbol (such as what section it is in).

The macro definition, if any, is executed under two circumstances.  One
is immediately after the rtl for @var{decl} that represents a variable
or a function has been created and stored in @code{DECL_RTL(@var{decl})}.
The value of the rtl will be a @code{mem} whose address is a @code{symbol_ref}.
The other is immediately after the rtl for @var{decl} that represents a
constant has been created and stored in @code{TREE_CST_RTL (@var{decl})}.
The macro is called once for each distinct constant in a source file.

The @var{new_decl_p} argument will be true if this is the first time that
@code{ENCODE_SECTION_INFO} has been invoked on this decl.  It will
be false for subsequent invocations, which will happen for duplicate
declarations.  Whether or not anything must be done for the duplicate
declaration depends on whether @code{ENCODE_SECTION_INFO} examines
@code{DECL_ATTRIBUTES}.

@cindex @code{SYMBOL_REF_FLAG}, in @code{ENCODE_SECTION_INFO}
The usual thing for this macro to do is to record a flag in the
@code{symbol_ref} (such as @code{SYMBOL_REF_FLAG}) or to store a
modified name string in the @code{symbol_ref} (if one bit is not
enough information).

@findex STRIP_NAME_ENCODING
@item STRIP_NAME_ENCODING (@var{var}, @var{sym_name})
Decode @var{sym_name} and store the real name part in @var{var}, sans
the characters that encode section info.  Define this macro if
@code{ENCODE_SECTION_INFO} alters the symbol's name string.

@findex UNIQUE_SECTION
@item UNIQUE_SECTION (@var{decl}, @var{reloc})
A C statement to build up a unique section name, expressed as a
@code{STRING_CST} node, and assign it to @samp{DECL_SECTION_NAME (@var{decl})}.
@var{reloc} indicates whether the initial value of @var{exp} requires
link-time relocations.  If you do not define this macro, GCC will use
the symbol name prefixed by @samp{.} as the section name.  Note - this
macro can now be called for uninitialized data items as well as
initialized data and functions.
@end table

@node PIC
@section Position Independent Code
@cindex position independent code
@cindex PIC

This section describes macros that help implement generation of position
independent code.  Simply defining these macros is not enough to
generate valid PIC; you must also add support to the macros
@code{GO_IF_LEGITIMATE_ADDRESS} and @code{PRINT_OPERAND_ADDRESS}, as
well as @code{LEGITIMIZE_ADDRESS}.  You must modify the definition of
@samp{movsi} to do something appropriate when the source operand
contains a symbolic address.  You may also need to alter the handling of
switch statements so that they use relative addresses.
@c i rearranged the order of the macros above to try to force one of
@c them to the next line, to eliminate an overfull hbox. --mew 10feb93

@table @code
@findex PIC_OFFSET_TABLE_REGNUM
@item PIC_OFFSET_TABLE_REGNUM
The register number of the register used to address a table of static
data addresses in memory.  In some cases this register is defined by a
processor's ``application binary interface'' (ABI)@.  When this macro
is defined, RTL is generated for this register once, as with the stack
pointer and frame pointer registers.  If this macro is not defined, it
is up to the machine-dependent files to allocate such a register (if
necessary).  Note that this register must be fixed when in use (e.g.@:
when @code{flag_pic} is true).

@findex PIC_OFFSET_TABLE_REG_CALL_CLOBBERED
@item PIC_OFFSET_TABLE_REG_CALL_CLOBBERED
Define this macro if the register defined by
@code{PIC_OFFSET_TABLE_REGNUM} is clobbered by calls.  Do not define
this macro if @code{PIC_OFFSET_TABLE_REGNUM} is not defined.

@findex FINALIZE_PIC
@item FINALIZE_PIC
By generating position-independent code, when two different programs (A
and B) share a common library (libC.a), the text of the library can be
shared whether or not the library is linked at the same address for both
programs.  In some of these environments, position-independent code
requires not only the use of different addressing modes, but also
special code to enable the use of these addressing modes.

The @code{FINALIZE_PIC} macro serves as a hook to emit these special
codes once the function is being compiled into assembly code, but not
before.  (It is not done before, because in the case of compiling an
inline function, it would lead to multiple PIC prologues being
included in functions which used inline functions and were compiled to
assembly language.)

@findex LEGITIMATE_PIC_OPERAND_P
@item LEGITIMATE_PIC_OPERAND_P (@var{x})
A C expression that is nonzero if @var{x} is a legitimate immediate
operand on the target machine when generating position independent code.
You can assume that @var{x} satisfies @code{CONSTANT_P}, so you need not
check this.  You can also assume @var{flag_pic} is true, so you need not
check it either.  You need not define this macro if all constants
(including @code{SYMBOL_REF}) can be immediate operands when generating
position independent code.
@end table

@node Assembler Format
@section Defining the Output Assembler Language

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

@menu
* File Framework::       Structural information for the assembler file.
* Data Output::          Output of constants (numbers, strings, addresses).
* Uninitialized Data::   Output of uninitialized variables.
* Label Output::         Output and generation of labels.
* Initialization::       General principles of initialization
                           and termination routines.
* Macros for Initialization::
                         Specific macros that control the handling of
                           initialization and termination routines.
* Instruction Output::   Output of actual instructions.
* Dispatch Tables::      Output of jump tables.
* Exception Region Output:: Output of exception region code.
* Alignment Output::     Pseudo ops for alignment and skipping data.
@end menu

@node File Framework
@subsection The Overall Framework of an Assembler File
@cindex assembler format
@cindex output of assembler code

@c prevent bad page break with this line
This describes the overall framework of an assembler file.

@table @code
@findex ASM_FILE_START
@item ASM_FILE_START (@var{stream})
A C expression which outputs to the stdio stream @var{stream}
some appropriate text to go at the start of an assembler file.

Normally this macro is defined to output a line containing
@samp{#NO_APP}, which is a comment that has no effect on most
assemblers but tells the GNU assembler that it can save time by not
checking for certain assembler constructs.

On systems that use SDB, it is necessary to output certain commands;
see @file{attasm.h}.

@findex ASM_FILE_END
@item ASM_FILE_END (@var{stream})
A C expression which outputs to the stdio stream @var{stream}
some appropriate text to go at the end of an assembler file.

If this macro is not defined, the default is to output nothing
special at the end of the file.  Most systems don't require any
definition.

On systems that use SDB, it is necessary to output certain commands;
see @file{attasm.h}.

@findex ASM_COMMENT_START
@item ASM_COMMENT_START
A C string constant describing how to begin a comment in the target
assembler language.  The compiler assumes that the comment will end at
the end of the line.

@findex ASM_APP_ON
@item ASM_APP_ON
A C string constant for text to be output before each @code{asm}
statement or group of consecutive ones.  Normally this is
@code{"#APP"}, which is a comment that has no effect on most
assemblers but tells the GNU assembler that it must check the lines
that follow for all valid assembler constructs.

@findex ASM_APP_OFF
@item ASM_APP_OFF
A C string constant for text to be output after each @code{asm}
statement or group of consecutive ones.  Normally this is
@code{"#NO_APP"}, which tells the GNU assembler to resume making the
time-saving assumptions that are valid for ordinary compiler output.

@findex ASM_OUTPUT_SOURCE_FILENAME
@item ASM_OUTPUT_SOURCE_FILENAME (@var{stream}, @var{name})
A C statement to output COFF information or DWARF debugging information
which indicates that filename @var{name} is the current source file to
the stdio stream @var{stream}.

This macro need not be defined if the standard form of output
for the file format in use is appropriate.

@findex OUTPUT_QUOTED_STRING
@item OUTPUT_QUOTED_STRING (@var{stream}, @var{string})
A C statement to output the string @var{string} to the stdio stream
@var{stream}.  If you do not call the function @code{output_quoted_string}
in your config files, GCC will only call it to output filenames to
the assembler source.  So you can use it to canonicalize the format
of the filename using this macro.

@findex ASM_OUTPUT_SOURCE_LINE
@item ASM_OUTPUT_SOURCE_LINE (@var{stream}, @var{line})
A C statement to output DBX or SDB debugging information before code
for line number @var{line} of the current source file to the
stdio stream @var{stream}.

This macro need not be defined if the standard form of debugging
information for the debugger in use is appropriate.

@findex ASM_OUTPUT_IDENT
@item ASM_OUTPUT_IDENT (@var{stream}, @var{string})
A C statement to output something to the assembler file to handle a
@samp{#ident} directive containing the text @var{string}.  If this
macro is not defined, nothing is output for a @samp{#ident} directive.

@findex OBJC_PROLOGUE
@item OBJC_PROLOGUE
A C statement to output any assembler statements which are required to
precede any Objective-C object definitions or message sending.  The
statement is executed only when compiling an Objective-C program.
@end table

@deftypefn {Target Hook} void TARGET_ASM_NAMED_SECTION (const char *@var{name}, unsigned int @var{flags}, unsigned int @var{align})
Output assembly directives to switch to section @var{name}.  The section
should have attributes as specified by @var{flags}, which is a bit mask
of the @code{SECTION_*} flags defined in @file{output.h}.  If @var{align}
is nonzero, it contains an alignment in bytes to be used for the section,
otherwise some target default should be used.  Only targets that must
specify an alignment within the section directive need pay attention to
@var{align} -- we will still use @code{ASM_OUTPUT_ALIGN}.
@end deftypefn

@deftypefn {Target Hook} bool TARGET_HAVE_NAMED_SECTIONS
This flag is true if the target supports @code{TARGET_ASM_NAMED_SECTION}.
@end deftypefn

@deftypefn {Target Hook} {unsigned int} TARGET_SECTION_TYPE_FLAGS (tree @var{decl}, const char *@var{name}, int @var{reloc})
Choose a set of section attributes for use by @code{TARGET_ASM_NAMED_SECTION}
based on a variable or function decl, a section name, and whether or not the
declaration's initializer may contain runtime relocations.  @var{decl} may be
 null, in which case read-write data should be assumed.

The default version if this function handles choosing code vs data,
read-only vs read-write data, and @code{flag_pic}.  You should only
need to override this if your target has special flags that might be
set via @code{__attribute__}.
@end deftypefn

@need 2000
@node Data Output
@subsection Output of Data


@deftypevr {Target Hook} {const char *} TARGET_ASM_BYTE_OP
@deftypevrx {Target Hook} {const char *} TARGET_ASM_ALIGNED_HI_OP
@deftypevrx {Target Hook} {const char *} TARGET_ASM_ALIGNED_SI_OP
@deftypevrx {Target Hook} {const char *} TARGET_ASM_ALIGNED_DI_OP
@deftypevrx {Target Hook} {const char *} TARGET_ASM_ALIGNED_TI_OP
@deftypevrx {Target Hook} {const char *} TARGET_ASM_UNALIGNED_HI_OP
@deftypevrx {Target Hook} {const char *} TARGET_ASM_UNALIGNED_SI_OP
@deftypevrx {Target Hook} {const char *} TARGET_ASM_UNALIGNED_DI_OP
@deftypevrx {Target Hook} {const char *} TARGET_ASM_UNALIGNED_TI_OP
These hooks specify assembly directives for creating certain kinds
of integer object.  The @code{TARGET_ASM_BYTE_OP} directive creates a
byte-sized object, the @code{TARGET_ASM_ALIGNED_HI_OP} one creates an
aligned two-byte object, and so on.  Any of the hooks may be
@code{NULL}, indicating that no suitable directive is available.

The compiler will print these strings at the start of a new line,
followed immediately by the object's initial value.  In most cases,
the string should contain a tab, a pseudo-op, and then another tab.
@end deftypevr

@deftypefn {Target Hook} bool TARGET_ASM_INTEGER (rtx @var{x}, unsigned int @var{size}, int @var{aligned_p})
The @code{assemble_integer} function uses this hook to output an
integer object.  @var{x} is the object's value, @var{size} is its size
in bytes and @var{aligned_p} indicates whether it is aligned.  The
function should return @code{true} if it was able to output the
object.  If it returns false, @code{assemble_integer} will try to
split the object into smaller parts.

The default implementation of this hook will use the
@code{TARGET_ASM_BYTE_OP} family of strings, returning @code{false}
when the relevant string is @code{NULL}.
@end deftypefn

@table @code
@findex OUTPUT_ADDR_CONST_EXTRA
@item OUTPUT_ADDR_CONST_EXTRA (@var{stream}, @var{x}, @var{fail})
A C statement to recognize @var{rtx} patterns that
@code{output_addr_const} can't deal with, and output assembly code to
@var{stream} corresponding to the pattern @var{x}.  This may be used to
allow machine-dependent @code{UNSPEC}s to appear within constants.

If @code{OUTPUT_ADDR_CONST_EXTRA} fails to recognize a pattern, it must
@code{goto fail}, so that a standard error message is printed.  If it
prints an error message itself, by calling, for example,
@code{output_operand_lossage}, it may just complete normally.

@findex ASM_OUTPUT_ASCII
@item ASM_OUTPUT_ASCII (@var{stream}, @var{ptr}, @var{len})
A C statement to output to the stdio stream @var{stream} an assembler
instruction to assemble a string constant containing the @var{len}
bytes at @var{ptr}.  @var{ptr} will be a C expression of type
@code{char *} and @var{len} a C expression of type @code{int}.

If the assembler has a @code{.ascii} pseudo-op as found in the
Berkeley Unix assembler, do not define the macro
@code{ASM_OUTPUT_ASCII}.

@findex ASM_OUTPUT_FDESC
@item ASM_OUTPUT_FDESC (@var{stream}, @var{decl}, @var{n})
A C statement to output word @var{n} of a function descriptor for
@var{decl}.  This must be defined if @code{TARGET_VTABLE_USES_DESCRIPTORS}
is defined, and is otherwise unused.

@findex CONSTANT_POOL_BEFORE_FUNCTION
@item CONSTANT_POOL_BEFORE_FUNCTION
You may define this macro as a C expression.  You should define the
expression to have a nonzero value if GCC should output the constant
pool for a function before the code for the function, or a zero value if
GCC should output the constant pool after the function.  If you do
not define this macro, the usual case, GCC will output the constant
pool before the function.

@findex ASM_OUTPUT_POOL_PROLOGUE
@item ASM_OUTPUT_POOL_PROLOGUE (@var{file}, @var{funname}, @var{fundecl}, @var{size})
A C statement to output assembler commands to define the start of the
constant pool for a function.  @var{funname} is a string giving
the name of the function.  Should the return type of the function
be required, it can be obtained via @var{fundecl}.  @var{size}
is the size, in bytes, of the constant pool that will be written
immediately after this call.

If no constant-pool prefix is required, the usual case, this macro need
not be defined.

@findex ASM_OUTPUT_SPECIAL_POOL_ENTRY
@item ASM_OUTPUT_SPECIAL_POOL_ENTRY (@var{file}, @var{x}, @var{mode}, @var{align}, @var{labelno}, @var{jumpto})
A C statement (with or without semicolon) to output a constant in the
constant pool, if it needs special treatment.  (This macro need not do
anything for RTL expressions that can be output normally.)

The argument @var{file} is the standard I/O stream to output the
assembler code on.  @var{x} is the RTL expression for the constant to
output, and @var{mode} is the machine mode (in case @var{x} is a
@samp{const_int}).  @var{align} is the required alignment for the value
@var{x}; you should output an assembler directive to force this much
alignment.

The argument @var{labelno} is a number to use in an internal label for
the address of this pool entry.  The definition of this macro is
responsible for outputting the label definition at the proper place.
Here is how to do this:

@example
ASM_OUTPUT_INTERNAL_LABEL (@var{file}, "LC", @var{labelno});
@end example

When you output a pool entry specially, you should end with a
@code{goto} to the label @var{jumpto}.  This will prevent the same pool
entry from being output a second time in the usual manner.

You need not define this macro if it would do nothing.

@findex CONSTANT_AFTER_FUNCTION_P
@item CONSTANT_AFTER_FUNCTION_P (@var{exp})
Define this macro as a C expression which is nonzero if the constant
@var{exp}, of type @code{tree}, should be output after the code for a
function.  The compiler will normally output all constants before the
function; you need not define this macro if this is OK@.

@findex ASM_OUTPUT_POOL_EPILOGUE
@item ASM_OUTPUT_POOL_EPILOGUE (@var{file} @var{funname} @var{fundecl} @var{size})
A C statement to output assembler commands to at the end of the constant
pool for a function.  @var{funname} is a string giving the name of the
function.  Should the return type of the function be required, you can
obtain it via @var{fundecl}.  @var{size} is the size, in bytes, of the
constant pool that GCC wrote immediately before this call.

If no constant-pool epilogue is required, the usual case, you need not
define this macro.

@findex IS_ASM_LOGICAL_LINE_SEPARATOR
@item IS_ASM_LOGICAL_LINE_SEPARATOR (@var{C})
Define this macro as a C expression which is nonzero if @var{C} is
used as a logical line separator by the assembler.

If you do not define this macro, the default is that only
the character @samp{;} is treated as a logical line separator.
@end table

@deftypevr {Target Hook} {const char *} TARGET_ASM_OPEN_PAREN
@deftypevrx {Target Hook} {const char *} TARGET_ASM_CLOSE_PAREN
These target hooks are C string constants, describing the syntax in the
assembler for grouping arithmetic expressions.  If not overridden, they
default to normal parentheses, which is correct for most assemblers.
@end deftypevr

  These macros are provided by @file{real.h} for writing the definitions
of @code{ASM_OUTPUT_DOUBLE} and the like:

@table @code
@item REAL_VALUE_TO_TARGET_SINGLE (@var{x}, @var{l})
@itemx REAL_VALUE_TO_TARGET_DOUBLE (@var{x}, @var{l})
@itemx REAL_VALUE_TO_TARGET_LONG_DOUBLE (@var{x}, @var{l})
@findex REAL_VALUE_TO_TARGET_SINGLE
@findex REAL_VALUE_TO_TARGET_DOUBLE
@findex REAL_VALUE_TO_TARGET_LONG_DOUBLE
These translate @var{x}, of type @code{REAL_VALUE_TYPE}, to the target's
floating point representation, and store its bit pattern in the variable
@var{l}.  For @code{REAL_VALUE_TO_TARGET_SINGLE}, this variable should
be a simple @code{long int}.  For the others, it should be an array of
@code{long int}.  The number of elements in this array is determined by
the size of the desired target floating point data type: 32 bits of it
go in each @code{long int} array element.  Each array element holds 32
bits of the result, even if @code{long int} is wider than 32 bits on the
host machine.

The array element values are designed so that you can print them out
using @code{fprintf} in the order they should appear in the target
machine's memory.

@item REAL_VALUE_TO_DECIMAL (@var{x}, @var{format}, @var{string})
@findex REAL_VALUE_TO_DECIMAL
This macro converts @var{x}, of type @code{REAL_VALUE_TYPE}, to a
decimal number and stores it as a string into @var{string}.
You must pass, as @var{string}, the address of a long enough block
of space to hold the result.

The argument @var{format} is a @code{printf}-specification that serves
as a suggestion for how to format the output string.
@end table

@node Uninitialized Data
@subsection Output of Uninitialized Variables

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

@table @code
@findex ASM_OUTPUT_COMMON
@item ASM_OUTPUT_COMMON (@var{stream}, @var{name}, @var{size}, @var{rounded})
A C statement (sans semicolon) to output to the stdio stream
@var{stream} the assembler definition of a common-label named
@var{name} whose size is @var{size} bytes.  The variable @var{rounded}
is the size rounded up to whatever alignment the caller wants.

Use the expression @code{assemble_name (@var{stream}, @var{name})} to
output the name itself; before and after that, output the additional
assembler syntax for defining the name, and a newline.

This macro controls how the assembler definitions of uninitialized
common global variables are output.

@findex ASM_OUTPUT_ALIGNED_COMMON
@item ASM_OUTPUT_ALIGNED_COMMON (@var{stream}, @var{name}, @var{size}, @var{alignment})
Like @code{ASM_OUTPUT_COMMON} except takes the required alignment as a
separate, explicit argument.  If you define this macro, it is used in
place of @code{ASM_OUTPUT_COMMON}, and gives you more flexibility in
handling the required alignment of the variable.  The alignment is specified
as the number of bits.

@findex ASM_OUTPUT_ALIGNED_DECL_COMMON
@item ASM_OUTPUT_ALIGNED_DECL_COMMON (@var{stream}, @var{decl}, @var{name}, @var{size}, @var{alignment})
Like @code{ASM_OUTPUT_ALIGNED_COMMON} except that @var{decl} of the
variable to be output, if there is one, or @code{NULL_TREE} if there
is no corresponding variable.  If you define this macro, GCC will use it
in place of both @code{ASM_OUTPUT_COMMON} and
@code{ASM_OUTPUT_ALIGNED_COMMON}.  Define this macro when you need to see
the variable's decl in order to chose what to output.

@findex ASM_OUTPUT_SHARED_COMMON
@item ASM_OUTPUT_SHARED_COMMON (@var{stream}, @var{name}, @var{size}, @var{rounded})
If defined, it is similar to @code{ASM_OUTPUT_COMMON}, except that it
is used when @var{name} is shared.  If not defined, @code{ASM_OUTPUT_COMMON}
will be used.

@findex ASM_OUTPUT_BSS
@item ASM_OUTPUT_BSS (@var{stream}, @var{decl}, @var{name}, @var{size}, @var{rounded})
A C statement (sans semicolon) to output to the stdio stream
@var{stream} the assembler definition of uninitialized global @var{decl} named
@var{name} whose size is @var{size} bytes.  The variable @var{rounded}
is the size rounded up to whatever alignment the caller wants.

Try to use function @code{asm_output_bss} defined in @file{varasm.c} when
defining this macro.  If unable, use the expression
@code{assemble_name (@var{stream}, @var{name})} to output the name itself;
before and after that, output the additional assembler syntax for defining
the name, and a newline.

This macro controls how the assembler definitions of uninitialized global
variables are output.  This macro exists to properly support languages like
C++ which do not have @code{common} data.  However, this macro currently
is not defined for all targets.  If this macro and
@code{ASM_OUTPUT_ALIGNED_BSS} are not defined then @code{ASM_OUTPUT_COMMON}
or @code{ASM_OUTPUT_ALIGNED_COMMON} or
@code{ASM_OUTPUT_ALIGNED_DECL_COMMON} is used.

@findex ASM_OUTPUT_ALIGNED_BSS
@item ASM_OUTPUT_ALIGNED_BSS (@var{stream}, @var{decl}, @var{name}, @var{size}, @var{alignment})
Like @code{ASM_OUTPUT_BSS} except takes the required alignment as a
separate, explicit argument.  If you define this macro, it is used in
place of @code{ASM_OUTPUT_BSS}, and gives you more flexibility in
handling the required alignment of the variable.  The alignment is specified
as the number of bits.

Try to use function @code{asm_output_aligned_bss} defined in file
@file{varasm.c} when defining this macro.

@findex ASM_OUTPUT_SHARED_BSS
@item ASM_OUTPUT_SHARED_BSS (@var{stream}, @var{decl}, @var{name}, @var{size}, @var{rounded})
If defined, it is similar to @code{ASM_OUTPUT_BSS}, except that it
is used when @var{name} is shared.  If not defined, @code{ASM_OUTPUT_BSS}
will be used.

@findex ASM_OUTPUT_LOCAL
@item ASM_OUTPUT_LOCAL (@var{stream}, @var{name}, @var{size}, @var{rounded})
A C statement (sans semicolon) to output to the stdio stream
@var{stream} the assembler definition of a local-common-label named
@var{name} whose size is @var{size} bytes.  The variable @var{rounded}
is the size rounded up to whatever alignment the caller wants.

Use the expression @code{assemble_name (@var{stream}, @var{name})} to
output the name itself; before and after that, output the additional
assembler syntax for defining the name, and a newline.

This macro controls how the assembler definitions of uninitialized
static variables are output.

@findex ASM_OUTPUT_ALIGNED_LOCAL
@item ASM_OUTPUT_ALIGNED_LOCAL (@var{stream}, @var{name}, @var{size}, @var{alignment})
Like @code{ASM_OUTPUT_LOCAL} except takes the required alignment as a
separate, explicit argument.  If you define this macro, it is used in
place of @code{ASM_OUTPUT_LOCAL}, and gives you more flexibility in
handling the required alignment of the variable.  The alignment is specified
as the number of bits.

@findex ASM_OUTPUT_ALIGNED_DECL_LOCAL
@item ASM_OUTPUT_ALIGNED_DECL_LOCAL (@var{stream}, @var{decl}, @var{name}, @var{size}, @var{alignment})
Like @code{ASM_OUTPUT_ALIGNED_DECL} except that @var{decl} of the
variable to be output, if there is one, or @code{NULL_TREE} if there
is no corresponding variable.  If you define this macro, GCC will use it
in place of both @code{ASM_OUTPUT_DECL} and
@code{ASM_OUTPUT_ALIGNED_DECL}.  Define this macro when you need to see
the variable's decl in order to chose what to output.

@findex ASM_OUTPUT_SHARED_LOCAL
@item ASM_OUTPUT_SHARED_LOCAL (@var{stream}, @var{name}, @var{size}, @var{rounded})
If defined, it is similar to @code{ASM_OUTPUT_LOCAL}, except that it
is used when @var{name} is shared.  If not defined, @code{ASM_OUTPUT_LOCAL}
will be used.
@end table

@node Label Output
@subsection Output and Generation of Labels

@c prevent bad page break with this line
This is about outputting labels.

@table @code
@findex ASM_OUTPUT_LABEL
@findex assemble_name
@item ASM_OUTPUT_LABEL (@var{stream}, @var{name})
A C statement (sans semicolon) to output to the stdio stream
@var{stream} the assembler definition of a label named @var{name}.
Use the expression @code{assemble_name (@var{stream}, @var{name})} to
output the name itself; before and after that, output the additional
assembler syntax for defining the name, and a newline.

@findex ASM_DECLARE_FUNCTION_NAME
@item ASM_DECLARE_FUNCTION_NAME (@var{stream}, @var{name}, @var{decl})
A C statement (sans semicolon) to output to the stdio stream
@var{stream} any text necessary for declaring the name @var{name} of a
function which is being defined.  This macro is responsible for
outputting the label definition (perhaps using
@code{ASM_OUTPUT_LABEL}).  The argument @var{decl} is the
@code{FUNCTION_DECL} tree node representing the function.

If this macro is not defined, then the function name is defined in the
usual manner as a label (by means of @code{ASM_OUTPUT_LABEL}).

@findex ASM_DECLARE_FUNCTION_SIZE
@item ASM_DECLARE_FUNCTION_SIZE (@var{stream}, @var{name}, @var{decl})
A C statement (sans semicolon) to output to the stdio stream
@var{stream} any text necessary for declaring the size of a function
which is being defined.  The argument @var{name} is the name of the
function.  The argument @var{decl} is the @code{FUNCTION_DECL} tree node
representing the function.

If this macro is not defined, then the function size is not defined.

@findex ASM_DECLARE_OBJECT_NAME
@item ASM_DECLARE_OBJECT_NAME (@var{stream}, @var{name}, @var{decl})
A C statement (sans semicolon) to output to the stdio stream
@var{stream} any text necessary for declaring the name @var{name} of an
initialized variable which is being defined.  This macro must output the
label definition (perhaps using @code{ASM_OUTPUT_LABEL}).  The argument
@var{decl} is the @code{VAR_DECL} tree node representing the variable.

If this macro is not defined, then the variable name is defined in the
usual manner as a label (by means of @code{ASM_OUTPUT_LABEL}).

@findex ASM_DECLARE_REGISTER_GLOBAL
@item ASM_DECLARE_REGISTER_GLOBAL (@var{stream}, @var{decl}, @var{regno}, @var{name})
A C statement (sans semicolon) to output to the stdio stream
@var{stream} any text necessary for claiming a register @var{regno}
for a global variable @var{decl} with name @var{name}.

If you don't define this macro, that is equivalent to defining it to do
nothing.

@findex  ASM_FINISH_DECLARE_OBJECT
@item ASM_FINISH_DECLARE_OBJECT (@var{stream}, @var{decl}, @var{toplevel}, @var{atend})
A C statement (sans semicolon) to finish up declaring a variable name
once the compiler has processed its initializer fully and thus has had a
chance to determine the size of an array when controlled by an
initializer.  This is used on systems where it's necessary to declare
something about the size of the object.

If you don't define this macro, that is equivalent to defining it to do
nothing.

@findex ASM_GLOBALIZE_LABEL
@item ASM_GLOBALIZE_LABEL (@var{stream}, @var{name})
A C statement (sans semicolon) to output to the stdio stream
@var{stream} some commands that will make the label @var{name} global;
that is, available for reference from other files.  Use the expression
@code{assemble_name (@var{stream}, @var{name})} to output the name
itself; before and after that, output the additional assembler syntax
for making that name global, and a newline.

@findex ASM_WEAKEN_LABEL
@item ASM_WEAKEN_LABEL (@var{stream}, @var{name})
A C statement (sans semicolon) to output to the stdio stream
@var{stream} some commands that will make the label @var{name} weak;
that is, available for reference from other files but only used if
no other definition is available.  Use the expression
@code{assemble_name (@var{stream}, @var{name})} to output the name
itself; before and after that, output the additional assembler syntax
for making that name weak, and a newline.

If you don't define this macro or @code{ASM_WEAKEN_DECL}, GCC will not
support weak symbols and you should not define the @code{SUPPORTS_WEAK}
macro.

@findex ASM_WEAKEN_DECL
@item ASM_WEAKEN_DECL (@var{stream}, @var{decl}, @var{name}, @var{value})
Combines (and replaces) the function of @code{ASM_WEAKEN_LABEL} and
@code{ASM_OUTPUT_WEAK_ALIAS}, allowing access to the associated function
or variable decl.  If @var{value} is not @code{NULL}, this C statement
should output to the stdio stream @var{stream} assembler code which
defines (equates) the weak symbol @var{name} to have the value
@var{value}.  If @var{value} is @code{NULL}, it should output commands
to make @var{name} weak.

@findex SUPPORTS_WEAK
@item SUPPORTS_WEAK
A C expression which evaluates to true if the target supports weak symbols.

If you don't define this macro, @file{defaults.h} provides a default
definition.  If either @code{ASM_WEAKEN_LABEL} or @code{ASM_WEAKEN_DECL}
is defined, the default definition is @samp{1}; otherwise, it is
@samp{0}.  Define this macro if you want to control weak symbol support
with a compiler flag such as @option{-melf}.

@findex MAKE_DECL_ONE_ONLY (@var{decl})
@item MAKE_DECL_ONE_ONLY
A C statement (sans semicolon) to mark @var{decl} to be emitted as a
public symbol such that extra copies in multiple translation units will
be discarded by the linker.  Define this macro if your object file
format provides support for this concept, such as the @samp{COMDAT}
section flags in the Microsoft Windows PE/COFF format, and this support
requires changes to @var{decl}, such as putting it in a separate section.

@findex SUPPORTS_ONE_ONLY
@item SUPPORTS_ONE_ONLY
A C expression which evaluates to true if the target supports one-only
semantics.

If you don't define this macro, @file{varasm.c} provides a default
definition.  If @code{MAKE_DECL_ONE_ONLY} is defined, the default
definition is @samp{1}; otherwise, it is @samp{0}.  Define this macro if
you want to control one-only symbol support with a compiler flag, or if
setting the @code{DECL_ONE_ONLY} flag is enough to mark a declaration to
be emitted as one-only.

@findex ASM_OUTPUT_EXTERNAL
@item ASM_OUTPUT_EXTERNAL (@var{stream}, @var{decl}, @var{name})
A C statement (sans semicolon) to output to the stdio stream
@var{stream} any text necessary for declaring the name of an external
symbol named @var{name} which is referenced in this compilation but
not defined.  The value of @var{decl} is the tree node for the
declaration.

This macro need not be defined if it does not need to output anything.
The GNU assembler and most Unix assemblers don't require anything.

@findex ASM_OUTPUT_EXTERNAL_LIBCALL
@item ASM_OUTPUT_EXTERNAL_LIBCALL (@var{stream}, @var{symref})
A C statement (sans semicolon) to output on @var{stream} an assembler
pseudo-op to declare a library function name external.  The name of the
library function is given by @var{symref}, which has type @code{rtx} and
is a @code{symbol_ref}.

This macro need not be defined if it does not need to output anything.
The GNU assembler and most Unix assemblers don't require anything.

@findex ASM_OUTPUT_LABELREF
@item ASM_OUTPUT_LABELREF (@var{stream}, @var{name})
A C statement (sans semicolon) to output to the stdio stream
@var{stream} a reference in assembler syntax to a label named
@var{name}.  This should add @samp{_} to the front of the name, if that
is customary on your operating system, as it is in most Berkeley Unix
systems.  This macro is used in @code{assemble_name}.

@findex ASM_OUTPUT_SYMBOL_REF
@item ASM_OUTPUT_SYMBOL_REF (@var{stream}, @var{sym})
A C statement (sans semicolon) to output a reference to
@code{SYMBOL_REF} @var{sym}.  If not defined, @code{assemble_name}
will be used to output the name of the symbol.  This macro may be used
to modify the way a symbol is referenced depending on information
encoded by @code{ENCODE_SECTION_INFO}.

@findex ASM_OUTPUT_LABEL_REF
@item ASM_OUTPUT_LABEL_REF (@var{stream}, @var{buf})
A C statement (sans semicolon) to output a reference to @var{buf}, the
result of ASM_GENERATE_INTERNAL_LABEL.  If not defined,
@code{assemble_name} will be used to output the name of the symbol.
This macro is not used by @code{output_asm_label}, or the @code{%l}
specifier that calls it; the intention is that this macro should be set
when it is necessary to output a label differently when its address
is being taken.

@findex ASM_OUTPUT_INTERNAL_LABEL
@item ASM_OUTPUT_INTERNAL_LABEL (@var{stream}, @var{prefix}, @var{num})
A C statement to output to the stdio stream @var{stream} a label whose
name is made from the string @var{prefix} and the number @var{num}.

It is absolutely essential that these labels be distinct from the labels
used for user-level functions and variables.  Otherwise, certain programs
will have name conflicts with internal labels.

It is desirable to exclude internal labels from the symbol table of the
object file.  Most assemblers have a naming convention for labels that
should be excluded; on many systems, the letter @samp{L} at the
beginning of a label has this effect.  You should find out what
convention your system uses, and follow it.

The usual definition of this macro is as follows:

@example
fprintf (@var{stream}, "L%s%d:\n", @var{prefix}, @var{num})
@end example

@findex ASM_OUTPUT_DEBUG_LABEL
@item ASM_OUTPUT_DEBUG_LABEL (@var{stream}, @var{prefix}, @var{num})
A C statement to output to the stdio stream @var{stream} a debug info
label whose name is made from the string @var{prefix} and the number
@var{num}.  This is useful for VLIW targets, where debug info labels
may need to be treated differently than branch target labels.  On some
systems, branch target labels must be at the beginning of instruction
bundles, but debug info labels can occur in the middle of instruction
bundles.

If this macro is not defined, then @code{ASM_OUTPUT_INTERNAL_LABEL} will be
used.

@findex ASM_OUTPUT_ALTERNATE_LABEL_NAME
@item ASM_OUTPUT_ALTERNATE_LABEL_NAME (@var{stream}, @var{string})
A C statement to output to the stdio stream @var{stream} the string
@var{string}.

The default definition of this macro is as follows:

@example
fprintf (@var{stream}, "%s:\n", LABEL_ALTERNATE_NAME (INSN))
@end example

@findex ASM_GENERATE_INTERNAL_LABEL
@item ASM_GENERATE_INTERNAL_LABEL (@var{string}, @var{prefix}, @var{num})
A C statement to store into the string @var{string} a label whose name
is made from the string @var{prefix} and the number @var{num}.

This string, when output subsequently by @code{assemble_name}, should
produce the output that @code{ASM_OUTPUT_INTERNAL_LABEL} would produce
with the same @var{prefix} and @var{num}.

If the string begins with @samp{*}, then @code{assemble_name} will
output the rest of the string unchanged.  It is often convenient for
@code{ASM_GENERATE_INTERNAL_LABEL} to use @samp{*} in this way.  If the
string doesn't start with @samp{*}, then @code{ASM_OUTPUT_LABELREF} gets
to output the string, and may change it.  (Of course,
@code{ASM_OUTPUT_LABELREF} is also part of your machine description, so
you should know what it does on your machine.)

@findex ASM_FORMAT_PRIVATE_NAME
@item ASM_FORMAT_PRIVATE_NAME (@var{outvar}, @var{name}, @var{number})
A C expression to assign to @var{outvar} (which is a variable of type
@code{char *}) a newly allocated string made from the string
@var{name} and the number @var{number}, with some suitable punctuation
added.  Use @code{alloca} to get space for the string.

The string will be used as an argument to @code{ASM_OUTPUT_LABELREF} to
produce an assembler label for an internal static variable whose name is
@var{name}.  Therefore, the string must be such as to result in valid
assembler code.  The argument @var{number} is different each time this
macro is executed; it prevents conflicts between similarly-named
internal static variables in different scopes.

Ideally this string should not be a valid C identifier, to prevent any
conflict with the user's own symbols.  Most assemblers allow periods
or percent signs in assembler symbols; putting at least one of these
between the name and the number will suffice.

@findex ASM_OUTPUT_DEF
@item ASM_OUTPUT_DEF (@var{stream}, @var{name}, @var{value})
A C statement to output to the stdio stream @var{stream} assembler code
which defines (equates) the symbol @var{name} to have the value @var{value}.

@findex SET_ASM_OP
If @code{SET_ASM_OP} is defined, a default definition is provided which is
correct for most systems.

@findex ASM_OUTPUT_DEF_FROM_DECLS
@item ASM_OUTPUT_DEF_FROM_DECLS (@var{stream}, @var{decl_of_name}, @var{decl_of_value})
A C statement to output to the stdio stream @var{stream} assembler code
which defines (equates) the symbol whose tree node is @var{decl_of_name}
to have the value of the tree node @var{decl_of_value}.  This macro will
be used in preference to @samp{ASM_OUTPUT_DEF} if it is defined and if
the tree nodes are available.

@findex ASM_OUTPUT_DEFINE_LABEL_DIFFERENCE_SYMBOL
@item ASM_OUTPUT_DEFINE_LABEL_DIFFERENCE_SYMBOL (@var{stream}, @var{symbol}, @var{high}, @var{low})
A C statement to output to the stdio stream @var{stream} assembler code
which defines (equates) the symbol @var{symbol} to have a value equal to
the difference of the two symbols @var{high} and @var{low},
i.e.@: @var{high} minus @var{low}.  GCC guarantees that the symbols @var{high}
and @var{low} are already known by the assembler so that the difference
resolves into a constant.

@findex SET_ASM_OP
If @code{SET_ASM_OP} is defined, a default definition is provided which is
correct for most systems.

@findex ASM_OUTPUT_WEAK_ALIAS
@item ASM_OUTPUT_WEAK_ALIAS (@var{stream}, @var{name}, @var{value})
A C statement to output to the stdio stream @var{stream} assembler code
which defines (equates) the weak symbol @var{name} to have the value
@var{value}.  If @var{value} is @code{NULL}, it defines @var{name} as
an undefined weak symbol.

Define this macro if the target only supports weak aliases; define
@code{ASM_OUTPUT_DEF} instead if possible.

@findex OBJC_GEN_METHOD_LABEL
@item OBJC_GEN_METHOD_LABEL (@var{buf}, @var{is_inst}, @var{class_name}, @var{cat_name}, @var{sel_name})
Define this macro to override the default assembler names used for
Objective-C methods.

The default name is a unique method number followed by the name of the
class (e.g.@: @samp{_1_Foo}).  For methods in categories, the name of
the category is also included in the assembler name (e.g.@:
@samp{_1_Foo_Bar}).

These names are safe on most systems, but make debugging difficult since
the method's selector is not present in the name.  Therefore, particular
systems define other ways of computing names.

@var{buf} is an expression of type @code{char *} which gives you a
buffer in which to store the name; its length is as long as
@var{class_name}, @var{cat_name} and @var{sel_name} put together, plus
50 characters extra.

The argument @var{is_inst} specifies whether the method is an instance
method or a class method; @var{class_name} is the name of the class;
@var{cat_name} is the name of the category (or @code{NULL} if the method is not
in a category); and @var{sel_name} is the name of the selector.

On systems where the assembler can handle quoted names, you can use this
macro to provide more human-readable names.

@findex ASM_DECLARE_CLASS_REFERENCE
@item ASM_DECLARE_CLASS_REFERENCE (@var{stream}, @var{name})
A C statement (sans semicolon) to output to the stdio stream
@var{stream} commands to declare that the label @var{name} is an
Objective-C class reference.  This is only needed for targets whose
linkers have special support for NeXT-style runtimes.

@findex ASM_DECLARE_UNRESOLVED_REFERENCE
@item ASM_DECLARE_UNRESOLVED_REFERENCE (@var{stream}, @var{name})
A C statement (sans semicolon) to output to the stdio stream
@var{stream} commands to declare that the label @var{name} is an
unresolved Objective-C class reference.  This is only needed for targets
whose linkers have special support for NeXT-style runtimes.
@end table

@node Initialization
@subsection How Initialization Functions Are Handled
@cindex initialization routines
@cindex termination routines
@cindex constructors, output of
@cindex destructors, output of

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

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

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

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

@findex __CTOR_LIST__
@findex __DTOR_LIST__
The linker must build two lists of these functions---a list of
initialization functions, called @code{__CTOR_LIST__}, and a list of
termination functions, called @code{__DTOR_LIST__}.

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

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

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

This method will be chosen as the default by @file{target-def.h} if
@code{TARGET_ASM_NAMED_SECTION} is defined.  A target that does not
support arbitrary sections, but does support special designated
constructor and destructor sections may define @code{CTORS_SECTION_ASM_OP}
and @code{DTORS_SECTION_ASM_OP} to achieve the same effect.

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

@example
ld -o @var{output_file} crti.o crtbegin.o @dots{} -lgcc crtend.o crtn.o
@end example

The prologue of a function (@code{__init}) appears in the @code{.init}
section of @file{crti.o}; the epilogue appears in @file{crtn.o}.  Likewise
for the function @code{__fini} in the @dfn{.fini} section.  Normally these
files are provided by the operating system or by the GNU C library, but
are provided by GCC for a few targets.

The objects @file{crtbegin.o} and @file{crtend.o} are (for most targets)
compiled from @file{crtstuff.c}.  They contain, among other things, code
fragments within the @code{.init} and @code{.fini} sections that branch
to routines in the @code{.text} section.  The linker will pull all parts
of a section together, which results in a complete @code{__init} function
that invokes the routines we need at startup.

To use this variant, you must define the @code{INIT_SECTION_ASM_OP}
macro properly.

If no init section is available, when GCC compiles any function called
@code{main} (or more accurately, any function designated as a program
entry point by the language front end calling @code{expand_main_function}),
it inserts a procedure call to @code{__main} as the first executable code
after the function prologue.  The @code{__main} function is defined
in @file{libgcc2.c} and runs the global constructors.

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

The last variant uses neither arbitrary sections nor the GNU linker.
This is preferable when you want to do dynamic linking and when using
file formats which the GNU linker does not support, such as `ECOFF'@.  In
this case, @code{TARGET_HAVE_CTORS_DTORS} is false, initialization and
termination functions are recognized simply by their names.  This requires
an extra program in the linkage step, called @command{collect2}.  This program
pretends to be the linker, for use with GCC; it does its job by running
the ordinary linker, but also arranges to include the vectors of
initialization and termination functions.  These functions are called
via @code{__main} as described above.  In order to use this method,
@code{use_collect2} must be defined in the target in @file{config.gcc}.

@ifinfo
The following section describes the specific macros that control and
customize the handling of initialization and termination functions.
@end ifinfo

@node Macros for Initialization
@subsection Macros Controlling Initialization Routines

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

@table @code
@findex INIT_SECTION_ASM_OP
@item INIT_SECTION_ASM_OP
If defined, a C string constant, including spacing, for the assembler
operation to identify the following data as initialization code.  If not
defined, GCC will assume such a section does not exist.  When you are
using special sections for initialization and termination functions, this
macro also controls how @file{crtstuff.c} and @file{libgcc2.c} arrange to
run the initialization functions.

@item HAS_INIT_SECTION
@findex HAS_INIT_SECTION
If defined, @code{main} will not call @code{__main} as described above.
This macro should be defined for systems that control start-up code
on a symbol-by-symbol basis, such as OSF/1, and should not
be defined explicitly for systems that support @code{INIT_SECTION_ASM_OP}.

@item LD_INIT_SWITCH
@findex LD_INIT_SWITCH
If defined, a C string constant for a switch that tells the linker that
the following symbol is an initialization routine.

@item LD_FINI_SWITCH
@findex LD_FINI_SWITCH
If defined, a C string constant for a switch that tells the linker that
the following symbol is a finalization routine.

@item COLLECT_SHARED_INIT_FUNC (@var{stream}, @var{func})
If defined, a C statement that will write a function that can be
automatically called when a shared library is loaded.  The function
should call @var{func}, which takes no arguments.  If not defined, and
the object format requires an explicit initialization function, then a
function called @code{_GLOBAL__DI} will be generated.

This function and the following one are used by collect2 when linking a
shared library that needs constructors or destructors, or has DWARF2
exception tables embedded in the code.

@item COLLECT_SHARED_FINI_FUNC (@var{stream}, @var{func})
If defined, a C statement that will write a function that can be
automatically called when a shared library is unloaded.  The function
should call @var{func}, which takes no arguments.  If not defined, and
the object format requires an explicit finalization function, then a
function called @code{_GLOBAL__DD} will be generated.

@item INVOKE__main
@findex INVOKE__main
If defined, @code{main} will call @code{__main} despite the presence of
@code{INIT_SECTION_ASM_OP}.  This macro should be defined for systems
where the init section is not actually run automatically, but is still
useful for collecting the lists of constructors and destructors.

@item SUPPORTS_INIT_PRIORITY
@findex SUPPORTS_INIT_PRIORITY
If nonzero, the C++ @code{init_priority} attribute is supported and the
compiler should emit instructions to control the order of initialization
of objects.  If zero, the compiler will issue an error message upon
encountering an @code{init_priority} attribute.
@end table

@deftypefn {Target Hook} bool TARGET_HAVE_CTORS_DTORS
This value is true if the target supports some ``native'' method of
collecting constructors and destructors to be run at startup and exit.
It is false if we must use @command{collect2}.
@end deftypefn

@deftypefn {Target Hook} void TARGET_ASM_CONSTRUCTOR (rtx @var{symbol}, int @var{priority})
If defined, a function that outputs assembler code to arrange to call
the function referenced by @var{symbol} at initialization time.

Assume that @var{symbol} is a @code{SYMBOL_REF} for a function taking
no arguments and with no return value.  If the target supports initialization
priorities, @var{priority} is a value between 0 and @code{MAX_INIT_PRIORITY};
otherwise it must be @code{DEFAULT_INIT_PRIORITY}.

If this macro is not defined by the target, a suitable default will
be chosen if (1) the target supports arbitrary section names, (2) the
target defines @code{CTORS_SECTION_ASM_OP}, or (3) @code{USE_COLLECT2}
is not defined.
@end deftypefn

@deftypefn {Target Hook} void TARGET_ASM_DESTRUCTOR (rtx @var{symbol}, int @var{priority})
This is like @code{TARGET_ASM_CONSTRUCTOR} but used for termination
functions rather than initialization functions.
@end deftypefn

If @code{TARGET_HAVE_CTORS_DTORS} is true, the initialization routine
generated for the generated object file will have static linkage.

If your system uses @command{collect2} as the means of processing
constructors, then that program normally uses @command{nm} to scan
an object file for constructor functions to be called.

On certain kinds of systems, you can define these macros to make
@command{collect2} work faster (and, in some cases, make it work at all):

@table @code
@findex OBJECT_FORMAT_COFF
@item OBJECT_FORMAT_COFF
Define this macro if the system uses COFF (Common Object File Format)
object files, so that @command{collect2} can assume this format and scan
object files directly for dynamic constructor/destructor functions.

@findex OBJECT_FORMAT_ROSE
@item OBJECT_FORMAT_ROSE
Define this macro if the system uses ROSE format object files, so that
@command{collect2} can assume this format and scan object files directly
for dynamic constructor/destructor functions.

These macros are effective only in a native compiler; @command{collect2} as
part of a cross compiler always uses @command{nm} for the target machine.

@findex REAL_NM_FILE_NAME
@item REAL_NM_FILE_NAME
Define this macro as a C string constant containing the file name to use
to execute @command{nm}.  The default is to search the path normally for
@command{nm}.

If your system supports shared libraries and has a program to list the
dynamic dependencies of a given library or executable, you can define
these macros to enable support for running initialization and
termination functions in shared libraries:

@findex LDD_SUFFIX
@item LDD_SUFFIX
Define this macro to a C string constant containing the name of the program
which lists dynamic dependencies, like @command{"ldd"} under SunOS 4.

@findex PARSE_LDD_OUTPUT
@item PARSE_LDD_OUTPUT (@var{ptr})
Define this macro to be C code that extracts filenames from the output
of the program denoted by @code{LDD_SUFFIX}.  @var{ptr} is a variable
of type @code{char *} that points to the beginning of a line of output
from @code{LDD_SUFFIX}.  If the line lists a dynamic dependency, the
code must advance @var{ptr} to the beginning of the filename on that
line.  Otherwise, it must set @var{ptr} to @code{NULL}.
@end table

@node Instruction Output
@subsection Output of Assembler Instructions

@c prevent bad page break with this line
This describes assembler instruction output.

@table @code
@findex REGISTER_NAMES
@item REGISTER_NAMES
A C initializer containing the assembler's names for the machine
registers, each one as a C string constant.  This is what translates
register numbers in the compiler into assembler language.

@findex ADDITIONAL_REGISTER_NAMES
@item ADDITIONAL_REGISTER_NAMES
If defined, a C initializer for an array of structures containing a name
and a register number.  This macro defines additional names for hard
registers, thus allowing the @code{asm} option in declarations to refer
to registers using alternate names.

@findex ASM_OUTPUT_OPCODE
@item ASM_OUTPUT_OPCODE (@var{stream}, @var{ptr})
Define this macro if you are using an unusual assembler that
requires different names for the machine instructions.

The definition is a C statement or statements which output an
assembler instruction opcode to the stdio stream @var{stream}.  The
macro-operand @var{ptr} is a variable of type @code{char *} which
points to the opcode name in its ``internal'' form---the form that is
written in the machine description.  The definition should output the
opcode name to @var{stream}, performing any translation you desire, and
increment the variable @var{ptr} to point at the end of the opcode
so that it will not be output twice.

In fact, your macro definition may process less than the entire opcode
name, or more than the opcode name; but if you want to process text
that includes @samp{%}-sequences to substitute operands, you must take
care of the substitution yourself.  Just be sure to increment
@var{ptr} over whatever text should not be output normally.

@findex recog_data.operand
If you need to look at the operand values, they can be found as the
elements of @code{recog_data.operand}.

If the macro definition does nothing, the instruction is output
in the usual way.

@findex FINAL_PRESCAN_INSN
@item FINAL_PRESCAN_INSN (@var{insn}, @var{opvec}, @var{noperands})
If defined, a C statement to be executed just prior to the output of
assembler code for @var{insn}, to modify the extracted operands so
they will be output differently.

Here the argument @var{opvec} is the vector containing the operands
extracted from @var{insn}, and @var{noperands} is the number of
elements of the vector which contain meaningful data for this insn.
The contents of this vector are what will be used to convert the insn
template into assembler code, so you can change the assembler output
by changing the contents of the vector.

This macro is useful when various assembler syntaxes share a single
file of instruction patterns; by defining this macro differently, you
can cause a large class of instructions to be output differently (such
as with rearranged operands).  Naturally, variations in assembler
syntax affecting individual insn patterns ought to be handled by
writing conditional output routines in those patterns.

If this macro is not defined, it is equivalent to a null statement.

@findex FINAL_PRESCAN_LABEL
@item FINAL_PRESCAN_LABEL
If defined, @code{FINAL_PRESCAN_INSN} will be called on each
@code{CODE_LABEL}.  In that case, @var{opvec} will be a null pointer and
@var{noperands} will be zero.

@findex PRINT_OPERAND
@item PRINT_OPERAND (@var{stream}, @var{x}, @var{code})
A C compound statement to output to stdio stream @var{stream} the
assembler syntax for an instruction operand @var{x}.  @var{x} is an
RTL expression.

@var{code} is a value that can be used to specify one of several ways
of printing the operand.  It is used when identical operands must be
printed differently depending on the context.  @var{code} comes from
the @samp{%} specification that was used to request printing of the
operand.  If the specification was just @samp{%@var{digit}} then
@var{code} is 0; if the specification was @samp{%@var{ltr}
@var{digit}} then @var{code} is the ASCII code for @var{ltr}.

@findex reg_names
If @var{x} is a register, this macro should print the register's name.
The names can be found in an array @code{reg_names} whose type is
@code{char *[]}.  @code{reg_names} is initialized from
@code{REGISTER_NAMES}.

When the machine description has a specification @samp{%@var{punct}}
(a @samp{%} followed by a punctuation character), this macro is called
with a null pointer for @var{x} and the punctuation character for
@var{code}.

@findex PRINT_OPERAND_PUNCT_VALID_P
@item PRINT_OPERAND_PUNCT_VALID_P (@var{code})
A C expression which evaluates to true if @var{code} is a valid
punctuation character for use in the @code{PRINT_OPERAND} macro.  If
@code{PRINT_OPERAND_PUNCT_VALID_P} is not defined, it means that no
punctuation characters (except for the standard one, @samp{%}) are used
in this way.

@findex PRINT_OPERAND_ADDRESS
@item PRINT_OPERAND_ADDRESS (@var{stream}, @var{x})
A C compound statement to output to stdio stream @var{stream} the
assembler syntax for an instruction operand that is a memory reference
whose address is @var{x}.  @var{x} is an RTL expression.

@cindex @code{ENCODE_SECTION_INFO} usage
On some machines, the syntax for a symbolic address 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.  @xref{Assembler Format}.

@findex DBR_OUTPUT_SEQEND
@findex dbr_sequence_length
@item DBR_OUTPUT_SEQEND(@var{file})
A C statement, to be executed after all slot-filler instructions have
been output.  If necessary, call @code{dbr_sequence_length} to
determine the number of slots filled in a sequence (zero if not
currently outputting a sequence), to decide how many no-ops to output,
or whatever.

Don't define this macro if it has nothing to do, but it is helpful in
reading assembly output if the extent of the delay sequence is made
explicit (e.g.@: with white space).

@findex final_sequence
Note that output routines for instructions with delay slots must be
prepared to deal with not being output as part of a sequence
(i.e.@: when the scheduling pass is not run, or when no slot fillers could be
found.)  The variable @code{final_sequence} is null when not
processing a sequence, otherwise it contains the @code{sequence} rtx
being output.

@findex REGISTER_PREFIX
@findex LOCAL_LABEL_PREFIX
@findex USER_LABEL_PREFIX
@findex IMMEDIATE_PREFIX
@findex asm_fprintf
@item REGISTER_PREFIX
@itemx LOCAL_LABEL_PREFIX
@itemx USER_LABEL_PREFIX
@itemx IMMEDIATE_PREFIX
If defined, C string expressions to be used for the @samp{%R}, @samp{%L},
@samp{%U}, and @samp{%I} options of @code{asm_fprintf} (see
@file{final.c}).  These are useful when a single @file{md} file must
support multiple assembler formats.  In that case, the various @file{tm.h}
files can define these macros differently.

@item ASM_FPRINTF_EXTENSIONS(@var{file}, @var{argptr}, @var{format})
@findex ASM_FPRINTF_EXTENSIONS
If defined this macro should expand to a series of @code{case}
statements which will be parsed inside the @code{switch} statement of
the @code{asm_fprintf} function.  This allows targets to define extra
printf formats which may useful when generating their assembler
statements.  Note that upper case letters are reserved for future
generic extensions to asm_fprintf, and so are not available to target
specific code.  The output file is given by the parameter @var{file}.
The varargs input pointer is @var{argptr} and the rest of the format
string, starting the character after the one that is being switched
upon, is pointed to by @var{format}.

@findex ASSEMBLER_DIALECT
@item ASSEMBLER_DIALECT
If your target supports multiple dialects of assembler language (such as
different opcodes), define this macro as a C expression that gives the
numeric index of the assembler language dialect to use, with zero as the
first variant.

If this macro is defined, you may use constructs of the form
@smallexample
@samp{@{option0|option1|option2@dots{}@}}
@end smallexample
@noindent
in the output templates of patterns (@pxref{Output Template}) or in the
first argument of @code{asm_fprintf}.  This construct outputs
@samp{option0}, @samp{option1}, @samp{option2}, etc., if the value of
@code{ASSEMBLER_DIALECT} is zero, one, two, etc.  Any special characters
within these strings retain their usual meaning.  If there are fewer
alternatives within the braces than the value of
@code{ASSEMBLER_DIALECT}, the construct outputs nothing.

If you do not define this macro, the characters @samp{@{}, @samp{|} and
@samp{@}} do not have any special meaning when used in templates or
operands to @code{asm_fprintf}.

Define the macros @code{REGISTER_PREFIX}, @code{LOCAL_LABEL_PREFIX},
@code{USER_LABEL_PREFIX} and @code{IMMEDIATE_PREFIX} if you can express
the variations in assembler language syntax with that mechanism.  Define
@code{ASSEMBLER_DIALECT} and use the @samp{@{option0|option1@}} syntax
if the syntax variant are larger and involve such things as different
opcodes or operand order.

@findex ASM_OUTPUT_REG_PUSH
@item ASM_OUTPUT_REG_PUSH (@var{stream}, @var{regno})
A C expression to output to @var{stream} some assembler code
which will push hard register number @var{regno} onto the stack.
The code need not be optimal, since this macro is used only when
profiling.

@findex ASM_OUTPUT_REG_POP
@item ASM_OUTPUT_REG_POP (@var{stream}, @var{regno})
A C expression to output to @var{stream} some assembler code
which will pop hard register number @var{regno} off of the stack.
The code need not be optimal, since this macro is used only when
profiling.
@end table

@node Dispatch Tables
@subsection Output of Dispatch Tables

@c prevent bad page break with this line
This concerns dispatch tables.

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

@example
fprintf (@var{stream}, "\t.word L%d-L%d\n",
         @var{value}, @var{rel})
@end example

You must provide this macro on machines where the addresses in a
dispatch table are relative to the table's own address.  If defined, GCC
will also use this macro on all machines when producing PIC@.
@var{body} is the body of the @code{ADDR_DIFF_VEC}; it is provided so that the
mode and flags can be read.

@findex ASM_OUTPUT_ADDR_VEC_ELT
@item ASM_OUTPUT_ADDR_VEC_ELT (@var{stream}, @var{value})
This macro should be provided on machines where the addresses
in a dispatch table are absolute.

The definition should be a C statement to output to the stdio stream
@var{stream} an assembler pseudo-instruction to generate a reference to
a label.  @var{value} is the number of an internal label whose
definition is output using @code{ASM_OUTPUT_INTERNAL_LABEL}.
For example,

@example
fprintf (@var{stream}, "\t.word L%d\n", @var{value})
@end example

@findex ASM_OUTPUT_CASE_LABEL
@item ASM_OUTPUT_CASE_LABEL (@var{stream}, @var{prefix}, @var{num}, @var{table})
Define this if the label before a jump-table needs to be output
specially.  The first three arguments are the same as for
@code{ASM_OUTPUT_INTERNAL_LABEL}; the fourth argument is the
jump-table which follows (a @code{jump_insn} containing an
@code{addr_vec} or @code{addr_diff_vec}).

This feature is used on system V to output a @code{swbeg} statement
for the table.

If this macro is not defined, these labels are output with
@code{ASM_OUTPUT_INTERNAL_LABEL}.

@findex ASM_OUTPUT_CASE_END
@item ASM_OUTPUT_CASE_END (@var{stream}, @var{num}, @var{table})
Define this if something special must be output at the end of a
jump-table.  The definition should be a C statement to be executed
after the assembler code for the table is written.  It should write
the appropriate code to stdio stream @var{stream}.  The argument
@var{table} is the jump-table insn, and @var{num} is the label-number
of the preceding label.

If this macro is not defined, nothing special is output at the end of
the jump-table.
@end table

@node Exception Region Output
@subsection Assembler Commands for Exception Regions

@c prevent bad page break with this line

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

@table @code
@findex EH_FRAME_SECTION_NAME
@item EH_FRAME_SECTION_NAME
If defined, a C string constant for the name of the section containing
exception handling frame unwind information.  If not defined, GCC will
provide a default definition if the target supports named sections.
@file{crtstuff.c} uses this macro to switch to the appropriate section.

You should define this symbol if your target supports DWARF 2 frame
unwind information and the default definition does not work.

@findex EH_FRAME_IN_DATA_SECTION
@item EH_FRAME_IN_DATA_SECTION
If defined, DWARF 2 frame unwind information will be placed in the
data section even though the target supports named sections.  This
might be necessary, for instance, if the system linker does garbage
collection and sections cannot be marked as not to be collected.

Do not define this macro unless @code{TARGET_ASM_NAMED_SECTION} is
also defined.

@findex MASK_RETURN_ADDR
@item MASK_RETURN_ADDR
An rtx used to mask the return address found via @code{RETURN_ADDR_RTX}, so
that it does not contain any extraneous set bits in it.

@findex DWARF2_UNWIND_INFO
@item DWARF2_UNWIND_INFO
Define this macro to 0 if your target supports DWARF 2 frame unwind
information, but it does not yet work with exception handling.
Otherwise, if your target supports this information (if it defines
@samp{INCOMING_RETURN_ADDR_RTX} and either @samp{UNALIGNED_INT_ASM_OP}
or @samp{OBJECT_FORMAT_ELF}), GCC will provide a default definition of
1.

If this macro is defined to 1, the DWARF 2 unwinder will be the default
exception handling mechanism; otherwise, @code{setjmp}/@code{longjmp} will be used by
default.

If this macro is defined to anything, the DWARF 2 unwinder will be used
instead of inline unwinders and @code{__unwind_function} in the non-@code{setjmp} case.

@findex DWARF_CIE_DATA_ALIGNMENT
@item DWARF_CIE_DATA_ALIGNMENT
This macro need only be defined if the target might save registers in the
function prologue at an offset to the stack pointer that is not aligned to
@code{UNITS_PER_WORD}.  The definition should be the negative minimum
alignment if @code{STACK_GROWS_DOWNWARD} is defined, and the positive
minimum alignment otherwise.  @xref{SDB and DWARF}.  Only applicable if
the target supports DWARF 2 frame unwind information.

@end table

@deftypefn {Target Hook} void TARGET_ASM_EXCEPTION_SECTION ()
If defined, a function that switches to the section in which the main
exception table is to be placed (@pxref{Sections}).  The default is a
function that switches to a section named @code{.gcc_except_table} on
machines that support named sections via
@code{TARGET_ASM_NAMED_SECTION}, otherwise if @option{-fpic} or
@option{-fPIC} is in effect, the @code{data_section}, otherwise the
@code{readonly_data_section}.
@end deftypefn

@deftypefn {Target Hook} void TARGET_ASM_EH_FRAME_SECTION ()
If defined, a function that switches to the section in which the DWARF 2
frame unwind information to be placed (@pxref{Sections}).  The default
is a function that outputs a standard GAS section directive, if
@code{EH_FRAME_SECTION_NAME} is defined, or else a data section
directive followed by a synthetic label.
@end deftypefn

@node Alignment Output
@subsection Assembler Commands for Alignment

@c prevent bad page break with this line
This describes commands for alignment.

@table @code
@findex JUMP_ALIGN
@item JUMP_ALIGN (@var{label})
The alignment (log base 2) to put in front of @var{label}, which is
a common destination of jumps and has no fallthru incoming edge.

This macro need not be defined if you don't want any special alignment
to be done at such a time.  Most machine descriptions do not currently
define the macro.

Unless it's necessary to inspect the @var{label} parameter, it is better
to set the variable @var{align_jumps} in the target's
@code{OVERRIDE_OPTIONS}.  Otherwise, you should try to honor the user's
selection in @var{align_jumps} in a @code{JUMP_ALIGN} implementation.

@findex LABEL_ALIGN_AFTER_BARRIER
@item LABEL_ALIGN_AFTER_BARRIER (@var{label})
The alignment (log base 2) to put in front of @var{label}, which follows
a @code{BARRIER}.

This macro need not be defined if you don't want any special alignment
to be done at such a time.  Most machine descriptions do not currently
define the macro.

@findex LABEL_ALIGN_AFTER_BARRIER_MAX_SKIP
@item LABEL_ALIGN_AFTER_BARRIER_MAX_SKIP
The maximum number of bytes to skip when applying
@code{LABEL_ALIGN_AFTER_BARRIER}.  This works only if
@code{ASM_OUTPUT_MAX_SKIP_ALIGN} is defined.

@findex LOOP_ALIGN
@item LOOP_ALIGN (@var{label})
The alignment (log base 2) to put in front of @var{label}, which follows
a @code{NOTE_INSN_LOOP_BEG} note.

This macro need not be defined if you don't want any special alignment
to be done at such a time.  Most machine descriptions do not currently
define the macro.

Unless it's necessary to inspect the @var{label} parameter, it is better
to set the variable @code{align_loops} in the target's
@code{OVERRIDE_OPTIONS}.  Otherwise, you should try to honor the user's
selection in @code{align_loops} in a @code{LOOP_ALIGN} implementation.

@findex LOOP_ALIGN_MAX_SKIP
@item LOOP_ALIGN_MAX_SKIP
The maximum number of bytes to skip when applying @code{LOOP_ALIGN}.
This works only if @code{ASM_OUTPUT_MAX_SKIP_ALIGN} is defined.

@findex LABEL_ALIGN
@item LABEL_ALIGN (@var{label})
The alignment (log base 2) to put in front of @var{label}.
If @code{LABEL_ALIGN_AFTER_BARRIER} / @code{LOOP_ALIGN} specify a different alignment,
the maximum of the specified values is used.

Unless it's necessary to inspect the @var{label} parameter, it is better
to set the variable @code{align_labels} in the target's
@code{OVERRIDE_OPTIONS}.  Otherwise, you should try to honor the user's
selection in @code{align_labels} in a @code{LABEL_ALIGN} implementation.

@findex LABEL_ALIGN_MAX_SKIP
@item LABEL_ALIGN_MAX_SKIP
The maximum number of bytes to skip when applying @code{LABEL_ALIGN}.
This works only if @code{ASM_OUTPUT_MAX_SKIP_ALIGN} is defined.

@findex ASM_OUTPUT_SKIP
@item ASM_OUTPUT_SKIP (@var{stream}, @var{nbytes})
A C statement to output to the stdio stream @var{stream} an assembler
instruction to advance the location counter by @var{nbytes} bytes.
Those bytes should be zero when loaded.  @var{nbytes} will be a C
expression of type @code{int}.

@findex ASM_NO_SKIP_IN_TEXT
@item ASM_NO_SKIP_IN_TEXT
Define this macro if @code{ASM_OUTPUT_SKIP} should not be used in the
text section because it fails to put zeros in the bytes that are skipped.
This is true on many Unix systems, where the pseudo--op to skip bytes
produces no-op instructions rather than zeros when used in the text
section.

@findex ASM_OUTPUT_ALIGN
@item ASM_OUTPUT_ALIGN (@var{stream}, @var{power})
A C statement to output to the stdio stream @var{stream} an assembler
command to advance the location counter to a multiple of 2 to the
@var{power} bytes.  @var{power} will be a C expression of type @code{int}.

@findex ASM_OUTPUT_MAX_SKIP_ALIGN
@item ASM_OUTPUT_MAX_SKIP_ALIGN (@var{stream}, @var{power}, @var{max_skip})
A C statement to output to the stdio stream @var{stream} an assembler
command to advance the location counter to a multiple of 2 to the
@var{power} bytes, but only if @var{max_skip} or fewer bytes are needed to
satisfy the alignment request.  @var{power} and @var{max_skip} will be
a C expression of type @code{int}.
@end table

@need 3000
@node Debugging Info
@section Controlling Debugging Information Format

@c prevent bad page break with this line
This describes how to specify debugging information.

@menu
* All Debuggers::      Macros that affect all debugging formats uniformly.
* DBX Options::        Macros enabling specific options in DBX format.
* DBX Hooks::          Hook macros for varying DBX format.
* File Names and DBX:: Macros controlling output of file names in DBX format.
* SDB and DWARF::      Macros for SDB (COFF) and DWARF formats.
* VMS Debug::          Macros for VMS debug format.
@end menu

@node All Debuggers
@subsection Macros Affecting All Debugging Formats

@c prevent bad page break with this line
These macros affect all debugging formats.

@table @code
@findex DBX_REGISTER_NUMBER
@item DBX_REGISTER_NUMBER (@var{regno})
A C expression that returns the DBX register number for the compiler
register number @var{regno}.  In the default macro provided, the value
of this expression will be @var{regno} itself.  But sometimes there are
some registers that the compiler knows about and DBX does not, or vice
versa.  In such cases, some register may need to have one number in the
compiler and another for DBX@.

If two registers have consecutive numbers inside GCC, and they can be
used as a pair to hold a multiword value, then they @emph{must} have
consecutive numbers after renumbering with @code{DBX_REGISTER_NUMBER}.
Otherwise, debuggers will be unable to access such a pair, because they
expect register pairs to be consecutive in their own numbering scheme.

If you find yourself defining @code{DBX_REGISTER_NUMBER} in way that
does not preserve register pairs, then what you must do instead is
redefine the actual register numbering scheme.

@findex DEBUGGER_AUTO_OFFSET
@item DEBUGGER_AUTO_OFFSET (@var{x})
A C expression that returns the integer offset value for an automatic
variable having address @var{x} (an RTL expression).  The default
computation assumes that @var{x} is based on the frame-pointer and
gives the offset from the frame-pointer.  This is required for targets
that produce debugging output for DBX or COFF-style debugging output
for SDB and allow the frame-pointer to be eliminated when the
@option{-g} options is used.

@findex DEBUGGER_ARG_OFFSET
@item DEBUGGER_ARG_OFFSET (@var{offset}, @var{x})
A C expression that returns the integer offset value for an argument
having address @var{x} (an RTL expression).  The nominal offset is
@var{offset}.

@findex PREFERRED_DEBUGGING_TYPE
@item PREFERRED_DEBUGGING_TYPE
A C expression that returns the type of debugging output GCC should
produce when the user specifies just @option{-g}.  Define
this if you have arranged for GCC to support more than one format of
debugging output.  Currently, the allowable values are @code{DBX_DEBUG},
@code{SDB_DEBUG}, @code{DWARF_DEBUG}, @code{DWARF2_DEBUG},
@code{XCOFF_DEBUG}, @code{VMS_DEBUG}, and @code{VMS_AND_DWARF2_DEBUG}.

When the user specifies @option{-ggdb}, GCC normally also uses the
value of this macro to select the debugging output format, but with two
exceptions.  If @code{DWARF2_DEBUGGING_INFO} is defined and
@code{LINKER_DOES_NOT_WORK_WITH_DWARF2} is not defined, GCC uses the
value @code{DWARF2_DEBUG}.  Otherwise, if @code{DBX_DEBUGGING_INFO} is
defined, GCC uses @code{DBX_DEBUG}.

The value of this macro only affects the default debugging output; the
user can always get a specific type of output by using @option{-gstabs},
@option{-gcoff}, @option{-gdwarf-1}, @option{-gdwarf-2}, @option{-gxcoff},
or @option{-gvms}.
@end table

@node DBX Options
@subsection Specific Options for DBX Output

@c prevent bad page break with this line
These are specific options for DBX output.

@table @code
@findex DBX_DEBUGGING_INFO
@item DBX_DEBUGGING_INFO
Define this macro if GCC should produce debugging output for DBX
in response to the @option{-g} option.

@findex XCOFF_DEBUGGING_INFO
@item XCOFF_DEBUGGING_INFO
Define this macro if GCC should produce XCOFF format debugging output
in response to the @option{-g} option.  This is a variant of DBX format.

@findex DEFAULT_GDB_EXTENSIONS
@item DEFAULT_GDB_EXTENSIONS
Define this macro to control whether GCC should by default generate
GDB's extended version of DBX debugging information (assuming DBX-format
debugging information is enabled at all).  If you don't define the
macro, the default is 1: always generate the extended information
if there is any occasion to.

@findex DEBUG_SYMS_TEXT
@item DEBUG_SYMS_TEXT
Define this macro if all @code{.stabs} commands should be output while
in the text section.

@findex ASM_STABS_OP
@item ASM_STABS_OP
A C string constant, including spacing, naming the assembler pseudo op to
use instead of @code{"\t.stabs\t"} to define an ordinary debugging symbol.
If you don't define this macro, @code{"\t.stabs\t"} is used.  This macro
applies only to DBX debugging information format.

@findex ASM_STABD_OP
@item ASM_STABD_OP
A C string constant, including spacing, naming the assembler pseudo op to
use instead of @code{"\t.stabd\t"} to define a debugging symbol whose
value is the current location.  If you don't define this macro,
@code{"\t.stabd\t"} is used.  This macro applies only to DBX debugging
information format.

@findex ASM_STABN_OP
@item ASM_STABN_OP
A C string constant, including spacing, naming the assembler pseudo op to
use instead of @code{"\t.stabn\t"} to define a debugging symbol with no
name.  If you don't define this macro, @code{"\t.stabn\t"} is used.  This
macro applies only to DBX debugging information format.

@findex DBX_NO_XREFS
@item DBX_NO_XREFS
Define this macro if DBX on your system does not support the construct
@samp{xs@var{tagname}}.  On some systems, this construct is used to
describe a forward reference to a structure named @var{tagname}.
On other systems, this construct is not supported at all.

@findex DBX_CONTIN_LENGTH
@item DBX_CONTIN_LENGTH
A symbol name in DBX-format debugging information is normally
continued (split into two separate @code{.stabs} directives) when it
exceeds a certain length (by default, 80 characters).  On some
operating systems, DBX requires this splitting; on others, splitting
must not be done.  You can inhibit splitting by defining this macro
with the value zero.  You can override the default splitting-length by
defining this macro as an expression for the length you desire.

@findex DBX_CONTIN_CHAR
@item DBX_CONTIN_CHAR
Normally continuation is indicated by adding a @samp{\} character to
the end of a @code{.stabs} string when a continuation follows.  To use
a different character instead, define this macro as a character
constant for the character you want to use.  Do not define this macro
if backslash is correct for your system.

@findex DBX_STATIC_STAB_DATA_SECTION
@item DBX_STATIC_STAB_DATA_SECTION
Define this macro if it is necessary to go to the data section before
outputting the @samp{.stabs} pseudo-op for a non-global static
variable.

@findex DBX_TYPE_DECL_STABS_CODE
@item DBX_TYPE_DECL_STABS_CODE
The value to use in the ``code'' field of the @code{.stabs} directive
for a typedef.  The default is @code{N_LSYM}.

@findex DBX_STATIC_CONST_VAR_CODE
@item DBX_STATIC_CONST_VAR_CODE
The value to use in the ``code'' field of the @code{.stabs} directive
for a static variable located in the text section.  DBX format does not
provide any ``right'' way to do this.  The default is @code{N_FUN}.

@findex DBX_REGPARM_STABS_CODE
@item DBX_REGPARM_STABS_CODE
The value to use in the ``code'' field of the @code{.stabs} directive
for a parameter passed in registers.  DBX format does not provide any
``right'' way to do this.  The default is @code{N_RSYM}.

@findex DBX_REGPARM_STABS_LETTER
@item DBX_REGPARM_STABS_LETTER
The letter to use in DBX symbol data to identify a symbol as a parameter
passed in registers.  DBX format does not customarily provide any way to
do this.  The default is @code{'P'}.

@findex DBX_MEMPARM_STABS_LETTER
@item DBX_MEMPARM_STABS_LETTER
The letter to use in DBX symbol data to identify a symbol as a stack
parameter.  The default is @code{'p'}.

@findex DBX_FUNCTION_FIRST
@item DBX_FUNCTION_FIRST
Define this macro if the DBX information for a function and its
arguments should precede the assembler code for the function.  Normally,
in DBX format, the debugging information entirely follows the assembler
code.

@findex DBX_LBRAC_FIRST
@item DBX_LBRAC_FIRST
Define this macro if the @code{N_LBRAC} symbol for a block should
precede the debugging information for variables and functions defined in
that block.  Normally, in DBX format, the @code{N_LBRAC} symbol comes
first.

@findex DBX_BLOCKS_FUNCTION_RELATIVE
@item DBX_BLOCKS_FUNCTION_RELATIVE
Define this macro if the value of a symbol describing the scope of a
block (@code{N_LBRAC} or @code{N_RBRAC}) should be relative to the start
of the enclosing function.  Normally, GCC uses an absolute address.

@findex DBX_USE_BINCL
@item DBX_USE_BINCL
Define this macro if GCC should generate @code{N_BINCL} and
@code{N_EINCL} stabs for included header files, as on Sun systems.  This
macro also directs GCC to output a type number as a pair of a file
number and a type number within the file.  Normally, GCC does not
generate @code{N_BINCL} or @code{N_EINCL} stabs, and it outputs a single
number for a type number.
@end table

@node DBX Hooks
@subsection Open-Ended Hooks for DBX Format

@c prevent bad page break with this line
These are hooks for DBX format.

@table @code
@findex DBX_OUTPUT_LBRAC
@item DBX_OUTPUT_LBRAC (@var{stream}, @var{name})
Define this macro to say how to output to @var{stream} the debugging
information for the start of a scope level for variable names.  The
argument @var{name} is the name of an assembler symbol (for use with
@code{assemble_name}) whose value is the address where the scope begins.

@findex DBX_OUTPUT_RBRAC
@item DBX_OUTPUT_RBRAC (@var{stream}, @var{name})
Like @code{DBX_OUTPUT_LBRAC}, but for the end of a scope level.

@findex DBX_OUTPUT_ENUM
@item DBX_OUTPUT_ENUM (@var{stream}, @var{type})
Define this macro if the target machine requires special handling to
output an enumeration type.  The definition should be a C statement
(sans semicolon) to output the appropriate information to @var{stream}
for the type @var{type}.

@findex DBX_OUTPUT_FUNCTION_END
@item DBX_OUTPUT_FUNCTION_END (@var{stream}, @var{function})
Define this macro if the target machine requires special output at the
end of the debugging information for a function.  The definition should
be a C statement (sans semicolon) to output the appropriate information
to @var{stream}.  @var{function} is the @code{FUNCTION_DECL} node for
the function.

@findex DBX_OUTPUT_STANDARD_TYPES
@item DBX_OUTPUT_STANDARD_TYPES (@var{syms})
Define this macro if you need to control the order of output of the
standard data types at the beginning of compilation.  The argument
@var{syms} is a @code{tree} which is a chain of all the predefined
global symbols, including names of data types.

Normally, DBX output starts with definitions of the types for integers
and characters, followed by all the other predefined types of the
particular language in no particular order.

On some machines, it is necessary to output different particular types
first.  To do this, define @code{DBX_OUTPUT_STANDARD_TYPES} to output
those symbols in the necessary order.  Any predefined types that you
don't explicitly output will be output afterward in no particular order.

Be careful not to define this macro so that it works only for C@.  There
are no global variables to access most of the built-in types, because
another language may have another set of types.  The way to output a
particular type is to look through @var{syms} to see if you can find it.
Here is an example:

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

@noindent
This does nothing if the expected type does not exist.

See the function @code{init_decl_processing} in @file{c-decl.c} to find
the names to use for all the built-in C types.

Here is another way of finding a particular type:

@c this is still overfull.  --mew 10feb93
@smallexample
@{
  tree decl;
  for (decl = syms; decl; decl = TREE_CHAIN (decl))
    if (TREE_CODE (decl) == TYPE_DECL
        && (TREE_CODE (TREE_TYPE (decl))
            == INTEGER_CST)
        && TYPE_PRECISION (TREE_TYPE (decl)) == 16
        && TYPE_UNSIGNED (TREE_TYPE (decl)))
@group
      /* @r{This must be @code{unsigned short}.}  */
      dbxout_symbol (decl);
  @dots{}
@}
@end group
@end smallexample

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

@end table

@node File Names and DBX
@subsection File Names in DBX Format

@c prevent bad page break with this line
This describes file names in DBX format.

@table @code
@findex DBX_WORKING_DIRECTORY
@item DBX_WORKING_DIRECTORY
Define this if DBX wants to have the current directory recorded in each
object file.

Note that the working directory is always recorded if GDB extensions are
enabled.

@findex DBX_OUTPUT_MAIN_SOURCE_FILENAME
@item DBX_OUTPUT_MAIN_SOURCE_FILENAME (@var{stream}, @var{name})
A C statement to output DBX debugging information to the stdio stream
@var{stream} which indicates that file @var{name} is the main source
file---the file specified as the input file for compilation.
This macro is called only once, at the beginning of compilation.

This macro need not be defined if the standard form of output
for DBX debugging information is appropriate.

@findex DBX_OUTPUT_MAIN_SOURCE_DIRECTORY
@item DBX_OUTPUT_MAIN_SOURCE_DIRECTORY (@var{stream}, @var{name})
A C statement to output DBX debugging information to the stdio stream
@var{stream} which indicates that the current directory during
compilation is named @var{name}.

This macro need not be defined if the standard form of output
for DBX debugging information is appropriate.

@findex DBX_OUTPUT_MAIN_SOURCE_FILE_END
@item DBX_OUTPUT_MAIN_SOURCE_FILE_END (@var{stream}, @var{name})
A C statement to output DBX debugging information at the end of
compilation of the main source file @var{name}.

If you don't define this macro, nothing special is output at the end
of compilation, which is correct for most machines.

@findex DBX_OUTPUT_SOURCE_FILENAME
@item DBX_OUTPUT_SOURCE_FILENAME (@var{stream}, @var{name})
A C statement to output DBX debugging information to the stdio stream
@var{stream} which indicates that file @var{name} is the current source
file.  This output is generated each time input shifts to a different
source file as a result of @samp{#include}, the end of an included file,
or a @samp{#line} command.

This macro need not be defined if the standard form of output
for DBX debugging information is appropriate.
@end table

@need 2000
@node SDB and DWARF
@subsection Macros for SDB and DWARF Output

@c prevent bad page break with this line
Here are macros for SDB and DWARF output.

@table @code
@findex SDB_DEBUGGING_INFO
@item SDB_DEBUGGING_INFO
Define this macro if GCC should produce COFF-style debugging output
for SDB in response to the @option{-g} option.

@findex DWARF_DEBUGGING_INFO
@item DWARF_DEBUGGING_INFO
Define this macro if GCC should produce dwarf format debugging output
in response to the @option{-g} option.

@findex DWARF2_DEBUGGING_INFO
@item DWARF2_DEBUGGING_INFO
Define this macro if GCC should produce dwarf version 2 format
debugging output in response to the @option{-g} option.

To support optional call frame debugging information, you must also
define @code{INCOMING_RETURN_ADDR_RTX} and either set
@code{RTX_FRAME_RELATED_P} on the prologue insns if you use RTL for the
prologue, or call @code{dwarf2out_def_cfa} and @code{dwarf2out_reg_save}
as appropriate from @code{TARGET_ASM_FUNCTION_PROLOGUE} if you don't.

@findex DWARF2_FRAME_INFO
@item DWARF2_FRAME_INFO
Define this macro to a nonzero value if GCC should always output
Dwarf 2 frame information.  If @code{DWARF2_UNWIND_INFO}
(@pxref{Exception Region Output} is nonzero, GCC will output this
information not matter how you define @code{DWARF2_FRAME_INFO}.

@findex LINKER_DOES_NOT_WORK_WITH_DWARF2
@item LINKER_DOES_NOT_WORK_WITH_DWARF2
Define this macro if the linker does not work with Dwarf version 2.
Normally, if the user specifies only @option{-ggdb} GCC will use Dwarf
version 2 if available; this macro disables this.  See the description
of the @code{PREFERRED_DEBUGGING_TYPE} macro for more details.

@findex DWARF2_GENERATE_TEXT_SECTION_LABEL
@item DWARF2_GENERATE_TEXT_SECTION_LABEL
By default, the Dwarf 2 debugging information generator will generate a
label to mark the beginning of the text section.  If it is better simply
to use the name of the text section itself, rather than an explicit label,
to indicate the beginning of the text section, define this macro to zero.

@findex DWARF2_ASM_LINE_DEBUG_INFO
@item DWARF2_ASM_LINE_DEBUG_INFO
Define this macro to be a nonzero value if the assembler can generate Dwarf 2
line debug info sections.  This will result in much more compact line number
tables, and hence is desirable if it works.

@findex PUT_SDB_@dots{}
@item PUT_SDB_@dots{}
Define these macros to override the assembler syntax for the special
SDB assembler directives.  See @file{sdbout.c} for a list of these
macros and their arguments.  If the standard syntax is used, you need
not define them yourself.

@findex SDB_DELIM
@item SDB_DELIM
Some assemblers do not support a semicolon as a delimiter, even between
SDB assembler directives.  In that case, define this macro to be the
delimiter to use (usually @samp{\n}).  It is not necessary to define
a new set of @code{PUT_SDB_@var{op}} macros if this is the only change
required.

@findex SDB_GENERATE_FAKE
@item SDB_GENERATE_FAKE
Define this macro to override the usual method of constructing a dummy
name for anonymous structure and union types.  See @file{sdbout.c} for
more information.

@findex SDB_ALLOW_UNKNOWN_REFERENCES
@item SDB_ALLOW_UNKNOWN_REFERENCES
Define this macro to allow references to unknown structure,
union, or enumeration tags to be emitted.  Standard COFF does not
allow handling of unknown references, MIPS ECOFF has support for
it.

@findex SDB_ALLOW_FORWARD_REFERENCES
@item SDB_ALLOW_FORWARD_REFERENCES
Define this macro to allow references to structure, union, or
enumeration tags that have not yet been seen to be handled.  Some
assemblers choke if forward tags are used, while some require it.
@end table

@need 2000
@node VMS Debug
@subsection Macros for VMS Debug Format

@c prevent bad page break with this line
Here are macros for VMS debug format.

@table @code
@findex VMS_DEBUGGING_INFO
@item VMS_DEBUGGING_INFO
Define this macro if GCC should produce debugging output for VMS
in response to the @option{-g} option.  The default behavior for VMS
is to generate minimal debug info for a traceback in the absence of
@option{-g} unless explicitly overridden with @option{-g0}.  This
behavior is controlled by @code{OPTIMIZATION_OPTIONS} and
@code{OVERRIDE_OPTIONS}.
@end table

@node Floating Point
@section Cross Compilation and Floating Point
@cindex cross compilation and floating point
@cindex floating point and cross compilation

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

Because different representation systems may offer different amounts of
range and precision, all floating point constants must be represented in
the target machine's format.  Therefore, the cross compiler cannot
safely use the host machine's floating point arithmetic; it must emulate
the target's arithmetic.  To ensure consistency, GCC always uses
emulation to work with floating point values, even when the host and
target floating point formats are identical.

The following macros are provided by @file{real.h} for the compiler to
use.  All parts of the compiler which generate or optimize
floating-point calculations must use these macros.  They may evaluate
their operands more than once, so operands must not have side effects.

@defmac REAL_VALUE_TYPE
The C data type to be used to hold a floating point value in the target
machine's format.  Typically this is a @code{struct} containing an
array of @code{HOST_WIDE_INT}, but all code should treat it as an opaque
quantity.
@end defmac

@deftypefn Macro int REAL_VALUES_EQUAL (REAL_VALUE_TYPE @var{x}, REAL_VALUE_TYPE @var{y})
Compares for equality the two values, @var{x} and @var{y}.  If the target
floating point format supports negative zeroes and/or NaNs,
@samp{REAL_VALUES_EQUAL (-0.0, 0.0)} is true, and
@samp{REAL_VALUES_EQUAL (NaN, NaN)} is false.
@end deftypefn

@deftypefn Macro int REAL_VALUES_LESS (REAL_VALUE_TYPE @var{x}, REAL_VALUE_TYPE @var{y})
Tests whether @var{x} is less than @var{y}.
@end deftypefn

@findex ldexp
@deftypefn Macro REAL_VALUE_TYPE REAL_VALUE_LDEXP (REAL_VALUE_TYPE @var{x}, int @var{scale})
Multiplies @var{x} by 2 raised to the power @var{scale}.
@end deftypefn

@deftypefn Macro HOST_WIDE_INT REAL_VALUE_FIX (REAL_VALUE_TYPE @var{x})
Truncates @var{x} to a signed integer, rounding toward zero.
@end deftypefn

@deftypefn Macro {unsigned HOST_WIDE_INT} REAL_VALUE_UNSIGNED_FIX (REAL_VALUE_TYPE @var{x})
Truncates @var{x} to an unsigned integer, rounding toward zero.  If
@var{x} is negative, returns zero.
@end deftypefn

@deftypefn Macro REAL_VALUE_TYPE REAL_VALUE_RNDZINT (REAL_VALUE_TYPE @var{x})
Rounds the target-machine floating point value @var{x} towards zero to an
integer value, but leaves it represented as a floating point number.
@end deftypefn

@deftypefn Macro REAL_VALUE_TYPE REAL_VALUE_UNSIGNED_RNDZINT (REAL_VALUE_TYPE @var{x})
Rounds the target-machine floating point value @var{x} towards zero to an
unsigned integer value, but leaves it represented as a floating point
number.  If @var{x} is negative, returns (positive) zero.
@end deftypefn

@deftypefn Macro REAL_VALUE_TYPE REAL_VALUE_ATOF (const char *@var{string}, enum machine_mode @var{mode})
Converts @var{string} into a floating point number in the target machine's
representation for mode @var{mode}.  This routine can handle both
decimal and hexadecimal floating point constants, using the syntax
defined by the C language for both.
@end deftypefn

@deftypefn Macro int REAL_VALUE_NEGATIVE (REAL_VALUE_TYPE @var{x})
Returns 1 if @var{x} is negative (including negative zero), 0 otherwise.  
@end deftypefn

@deftypefn Macro int REAL_VALUE_ISINF (REAL_VALUE_TYPE @var{x})
Determines whether @var{x} represents infinity (positive or negative).
@end deftypefn

@deftypefn Macro int REAL_VALUE_ISNAN (REAL_VALUE_TYPE @var{x})
Determines whether @var{x} represents a ``NaN'' (not-a-number).
@end deftypefn

@deftypefn Macro void REAL_ARITHMETIC (REAL_VALUE_TYPE @var{output}, enum tree_code @var{code}, REAL_VALUE_TYPE @var{x}, REAL_VALUE_TYPE @var{y})
Calculates an arithmetic operation on the two floating point values
@var{x} and @var{y}, storing the result in @var{output} (which must be a
variable).

The operation to be performed is specified by @var{code}.  Only the
following codes are supported: @code{PLUS_EXPR}, @code{MINUS_EXPR},
@code{MULT_EXPR}, @code{RDIV_EXPR}, @code{MAX_EXPR}, @code{MIN_EXPR}.

If @code{REAL_ARITHMETIC} is asked to evaluate division by zero and the
target's floating point format cannot represent infinity, it will call
@code{abort}.  Callers should check for this situation first, using
@code{MODE_HAS_INFINITIES}.  @xref{Storage Layout}.
@end deftypefn

@deftypefn Macro REAL_VALUE_TYPE REAL_VALUE_NEGATE (REAL_VALUE_TYPE @var{x})
Returns the negative of the floating point value @var{x}.
@end deftypefn

@deftypefn Macro REAL_VALUE_TYPE REAL_VALUE_ABS (REAL_VALUE_TYPE @var{x})
Returns the absolute value of @var{x}.
@end deftypefn

@deftypefn Macro REAL_VALUE_TYPE REAL_VALUE_TRUNCATE (REAL_VALUE_TYPE @var{mode}, enum machine_mode @var{x})
Truncates the floating point value @var{x} to fit in @var{mode}.  The
return value is still a full-size @code{REAL_VALUE_TYPE}, but it has an
appropriate bit pattern to be output asa floating constant whose
precision accords with mode @var{mode}.
@end deftypefn

@deftypefn Macro void REAL_VALUE_TO_INT (HOST_WIDE_INT @var{low}, HOST_WIDE_INT @var{high}, REAL_VALUE_TYPE @var{x})
Converts a floating point value @var{x} into a double-precision integer
which is then stored into @var{low} and @var{high}.  If the value is not
integral, it is truncated.
@end deftypefn

@deftypefn Macro void REAL_VALUE_FROM_INT (REAL_VALUE_TYPE @var{x}, HOST_WIDE_INT @var{low}, HOST_WIDE_INT @var{high}, enum machine_mode @var{mode})
@findex REAL_VALUE_FROM_INT
Converts a double-precision integer found in @var{low} and @var{high},
into a floating point value which is then stored into @var{x}.  The
value is truncated to fit in mode @var{mode}.
@end deftypefn

@node Mode Switching
@section Mode Switching Instructions
@cindex mode switching
The following macros control mode switching optimizations:

@table @code
@findex OPTIMIZE_MODE_SWITCHING
@item OPTIMIZE_MODE_SWITCHING (@var{entity})
Define this macro if the port needs extra instructions inserted for mode
switching in an optimizing compilation.

For an example, the SH4 can perform both single and double precision
floating point operations, but to perform a single precision operation,
the FPSCR PR bit has to be cleared, while for a double precision
operation, this bit has to be set.  Changing the PR bit requires a general
purpose register as a scratch register, hence these FPSCR sets have to
be inserted before reload, i.e.@: you can't put this into instruction emitting
or @code{MACHINE_DEPENDENT_REORG}.

You can have multiple entities that are mode-switched, and select at run time
which entities actually need it.  @code{OPTIMIZE_MODE_SWITCHING} should
return nonzero for any @var{entity} that needs mode-switching.
If you define this macro, you also have to define
@code{NUM_MODES_FOR_MODE_SWITCHING}, @code{MODE_NEEDED},
@code{MODE_PRIORITY_TO_MODE} and @code{EMIT_MODE_SET}.
@code{NORMAL_MODE} is optional.

@findex NUM_MODES_FOR_MODE_SWITCHING
@item NUM_MODES_FOR_MODE_SWITCHING
If you define @code{OPTIMIZE_MODE_SWITCHING}, you have to define this as
initializer for an array of integers.  Each initializer element
N refers to an entity that needs mode switching, and specifies the number
of different modes that might need to be set for this entity.
The position of the initializer in the initializer - starting counting at
zero - determines the integer that is used to refer to the mode-switched
entity in question.
In macros that take mode arguments / yield a mode result, modes are
represented as numbers 0 @dots{} N @minus{} 1.  N is used to specify that no mode
switch is needed / supplied.

@findex MODE_NEEDED
@item MODE_NEEDED (@var{entity}, @var{insn})
@var{entity} is an integer specifying a mode-switched entity.  If
@code{OPTIMIZE_MODE_SWITCHING} is defined, you must define this macro to
return an integer value not larger than the corresponding element in
@code{NUM_MODES_FOR_MODE_SWITCHING}, to denote the mode that @var{entity} must
be switched into prior to the execution of @var{insn}.

@findex NORMAL_MODE
@item NORMAL_MODE (@var{entity})
If this macro is defined, it is evaluated for every @var{entity} that needs
mode switching.  It should evaluate to an integer, which is a mode that
@var{entity} is assumed to be switched to at function entry and exit.

@findex MODE_PRIORITY_TO_MODE
@item MODE_PRIORITY_TO_MODE (@var{entity}, @var{n})
This macro specifies the order in which modes for @var{entity} are processed.
0 is the highest priority, @code{NUM_MODES_FOR_MODE_SWITCHING[@var{entity}] - 1} the
lowest.  The value of the macro should be an integer designating a mode
for @var{entity}.  For any fixed @var{entity}, @code{mode_priority_to_mode}
(@var{entity}, @var{n}) shall be a bijection in 0 @dots{}
@code{num_modes_for_mode_switching[@var{entity}] - 1}.

@findex EMIT_MODE_SET
@item EMIT_MODE_SET (@var{entity}, @var{mode}, @var{hard_regs_live})
Generate one or more insns to set @var{entity} to @var{mode}.
@var{hard_reg_live} is the set of hard registers live at the point where
the insn(s) are to be inserted.
@end table

@node Target Attributes
@section Defining target-specific uses of @code{__attribute__}
@cindex target attributes
@cindex machine attributes
@cindex attributes, target-specific

Target-specific attributes may be defined for functions, data and types.
These are described using the following target hooks; they also need to
be documented in @file{extend.texi}.

@deftypevr {Target Hook} {const struct attribute_spec *} TARGET_ATTRIBUTE_TABLE
If defined, this target hook points to an array of @samp{struct
attribute_spec} (defined in @file{tree.h}) specifying the machine
specific attributes for this target and some of the restrictions on the
entities to which these attributes are applied and the arguments they
take.
@end deftypevr

@deftypefn {Target Hook} int TARGET_COMP_TYPE_ATTRIBUTES (tree @var{type1}, tree @var{type2})
If defined, this target hook is a function which returns zero if the attributes on
@var{type1} and @var{type2} are incompatible, one if they are compatible,
and two if they are nearly compatible (which causes a warning to be
generated).  If this is not defined, machine-specific attributes are
supposed always to be compatible.
@end deftypefn

@deftypefn {Target Hook} void TARGET_SET_DEFAULT_TYPE_ATTRIBUTES (tree @var{type})
If defined, this target hook is a function which assigns default attributes to
newly defined @var{type}.
@end deftypefn

@deftypefn {Target Hook} tree TARGET_MERGE_TYPE_ATTRIBUTES (tree @var{type1}, tree @var{type2})
Define this target hook if the merging of type attributes needs special
handling.  If defined, the result is a list of the combined
@code{TYPE_ATTRIBUTES} of @var{type1} and @var{type2}.  It is assumed
that @code{comptypes} has already been called and returned 1.  This
function may call @code{merge_attributes} to handle machine-independent
merging.
@end deftypefn

@deftypefn {Target Hook} tree TARGET_MERGE_DECL_ATTRIBUTES (tree @var{olddecl}, tree @var{newdecl})
Define this target hook if the merging of decl attributes needs special
handling.  If defined, the result is a list of the combined
@code{DECL_ATTRIBUTES} of @var{olddecl} and @var{newdecl}.
@var{newdecl} is a duplicate declaration of @var{olddecl}.  Examples of
when this is needed are when one attribute overrides another, or when an
attribute is nullified by a subsequent definition.  This function may
call @code{merge_attributes} to handle machine-independent merging.

@findex TARGET_DLLIMPORT_DECL_ATTRIBUTES
If the only target-specific handling you require is @samp{dllimport} for
Windows targets, you should define the macro
@code{TARGET_DLLIMPORT_DECL_ATTRIBUTES}.  This links in a function
called @code{merge_dllimport_decl_attributes} which can then be defined
as the expansion of @code{TARGET_MERGE_DECL_ATTRIBUTES}.  This is done
in @file{i386/cygwin.h} and @file{i386/i386.c}, for example.
@end deftypefn

@deftypefn {Target Hook} void TARGET_INSERT_ATTRIBUTES (tree @var{node}, tree *@var{attr_ptr})
Define this target hook if you want to be able to add attributes to a decl
when it is being created.  This is normally useful for back ends which
wish to implement a pragma by using the attributes which correspond to
the pragma's effect.  The @var{node} argument is the decl which is being
created.  The @var{attr_ptr} argument is a pointer to the attribute list
for this decl.  The list itself should not be modified, since it may be
shared with other decls, but attributes may be chained on the head of
the list and @code{*@var{attr_ptr}} modified to point to the new
attributes, or a copy of the list may be made if further changes are
needed.
@end deftypefn

@deftypefn {Target Hook} bool TARGET_FUNCTION_ATTRIBUTE_INLINABLE_P (tree @var{fndecl})
@cindex inlining
This target hook returns @code{true} if it is ok to inline @var{fndecl}
into the current function, despite its having target-specific
attributes, @code{false} otherwise.  By default, if a function has a
target specific attribute attached to it, it will not be inlined.
@end deftypefn

@node MIPS Coprocessors
@section Defining coprocessor specifics for MIPS targets.
@cindex MIPS coprocessor-definition macros

The MIPS specification allows MIPS implementations to have as many as 4
coprocessors, each with as many as 32 private registers.  gcc supports
accessing these registers and transferring values between the registers
and memory using asm-ized variables.  For example:

@smallexample
  register unsigned int cp0count asm ("c0r1");
  unsigned int d;

  d = cp0count + 3;
@end smallexample

(``c0r1'' is the default name of register 1 in coprocessor 0; alternate
names may be added as described below, or the default names may be
overridden entirely in @code{SUBTARGET_CONDITIONAL_REGISTER_USAGE}.)

Coprocessor registers are assumed to be epilogue-used; sets to them will
be preserved even if it does not appear that the register is used again
later in the function.

Another note: according to the MIPS spec, coprocessor 1 (if present) is
the FPU.  One accesses COP1 registers through standard mips
floating-point support; they are not included in this mechanism.

There is one macro used in defining the MIPS coprocessor interface which
you may want to override in subtargets; it is described below.

@table @code

@item ALL_COP_ADDITIONAL_REGISTER_NAMES
@findex ALL_COP_ADDITIONAL_REGISTER_NAMES
A comma-separated list (with leading comma) of pairs describing the
alternate names of coprocessor registers.  The format of each entry should be
@smallexample
@{ @var{alternatename}, @var{register_number}@}
@end smallexample
Default: empty.

@end table

@node Misc
@section Miscellaneous Parameters
@cindex parameters, miscellaneous

@c prevent bad page break with this line
Here are several miscellaneous parameters.

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

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

Defining this macro does not affect the generated code (however,
incorrect definitions that omit an rtl code that may be matched by the
predicate can cause the compiler to malfunction).  Instead, it allows
the table built by @file{genrecog} to be more compact and efficient,
thus speeding up the compiler.  The most important predicates to include
in the list specified by this macro are those used in the most insn
patterns.

For each predicate function named in @code{PREDICATE_CODES}, a
declaration will be generated in @file{insn-codes.h}.

@item SPECIAL_MODE_PREDICATES
@findex SPECIAL_MODE_PREDICATES
Define this if you have special predicates that know special things
about modes.  Genrecog will warn about certain forms of
@code{match_operand} without a mode; if the operand predicate is
listed in @code{SPECIAL_MODE_PREDICATES}, the warning will be
suppressed.

Here is an example from the IA-32 port (@code{ext_register_operand}
specially checks for @code{HImode} or @code{SImode} in preparation
for a byte extraction from @code{%ah} etc.).

@smallexample
#define SPECIAL_MODE_PREDICATES \
  "ext_register_operand",
@end smallexample

@findex CASE_VECTOR_MODE
@item CASE_VECTOR_MODE
An alias for a machine mode name.  This is the machine mode that
elements of a jump-table should have.

@findex CASE_VECTOR_SHORTEN_MODE
@item CASE_VECTOR_SHORTEN_MODE (@var{min_offset}, @var{max_offset}, @var{body})
Optional: return the preferred mode for an @code{addr_diff_vec}
when the minimum and maximum offset are known.  If you define this,
it enables extra code in branch shortening to deal with @code{addr_diff_vec}.
To make this work, you also have to define INSN_ALIGN and
make the alignment for @code{addr_diff_vec} explicit.
The @var{body} argument is provided so that the offset_unsigned and scale
flags can be updated.

@findex CASE_VECTOR_PC_RELATIVE
@item CASE_VECTOR_PC_RELATIVE
Define this macro to be a C expression to indicate when jump-tables
should contain relative addresses.  If jump-tables never contain
relative addresses, then you need not define this macro.

@findex CASE_DROPS_THROUGH
@item CASE_DROPS_THROUGH
Define this if control falls through a @code{case} insn when the index
value is out of range.  This means the specified default-label is
actually ignored by the @code{case} insn proper.

@findex CASE_VALUES_THRESHOLD
@item CASE_VALUES_THRESHOLD
Define this to be the smallest number of different values for which it
is best to use a jump-table instead of a tree of conditional branches.
The default is four for machines with a @code{casesi} instruction and
five otherwise.  This is best for most machines.

@findex WORD_REGISTER_OPERATIONS
@item WORD_REGISTER_OPERATIONS
Define this macro if operations between registers with integral mode
smaller than a word are always performed on the entire register.
Most RISC machines have this property and most CISC machines do not.

@findex LOAD_EXTEND_OP
@item LOAD_EXTEND_OP (@var{mode})
Define this macro to be a C expression indicating when insns that read
memory in @var{mode}, an integral mode narrower than a word, set the
bits outside of @var{mode} to be either the sign-extension or the
zero-extension of the data read.  Return @code{SIGN_EXTEND} for values
of @var{mode} for which the
insn sign-extends, @code{ZERO_EXTEND} for which it zero-extends, and
@code{NIL} for other modes.

This macro is not called with @var{mode} non-integral or with a width
greater than or equal to @code{BITS_PER_WORD}, so you may return any
value in this case.  Do not define this macro if it would always return
@code{NIL}.  On machines where this macro is defined, you will normally
define it as the constant @code{SIGN_EXTEND} or @code{ZERO_EXTEND}.

@findex SHORT_IMMEDIATES_SIGN_EXTEND
@item SHORT_IMMEDIATES_SIGN_EXTEND
Define this macro if loading short immediate values into registers sign
extends.

@findex FIXUNS_TRUNC_LIKE_FIX_TRUNC
@item FIXUNS_TRUNC_LIKE_FIX_TRUNC
Define this macro if the same instructions that convert a floating
point number to a signed fixed point number also convert validly to an
unsigned one.

@findex MOVE_MAX
@item MOVE_MAX
The maximum number of bytes that a single instruction can move quickly
between memory and registers or between two memory locations.

@findex MAX_MOVE_MAX
@item MAX_MOVE_MAX
The maximum number of bytes that a single instruction can move quickly
between memory and registers or between two memory locations.  If this
is undefined, the default is @code{MOVE_MAX}.  Otherwise, it is the
constant value that is the largest value that @code{MOVE_MAX} can have
at run-time.

@findex SHIFT_COUNT_TRUNCATED
@item SHIFT_COUNT_TRUNCATED
A C expression that is nonzero if on this machine the number of bits
actually used for the count of a shift operation is equal to the number
of bits needed to represent the size of the object being shifted.  When
this macro is nonzero, the compiler will assume that it is safe to omit
a sign-extend, zero-extend, and certain bitwise `and' instructions that
truncates the count of a shift operation.  On machines that have
instructions that act on bit-fields at variable positions, which may
include `bit test' instructions, a nonzero @code{SHIFT_COUNT_TRUNCATED}
also enables deletion of truncations of the values that serve as
arguments to bit-field instructions.

If both types of instructions truncate the count (for shifts) and
position (for bit-field operations), or if no variable-position bit-field
instructions exist, you should define this macro.

However, on some machines, such as the 80386 and the 680x0, truncation
only applies to shift operations and not the (real or pretended)
bit-field operations.  Define @code{SHIFT_COUNT_TRUNCATED} to be zero on
such machines.  Instead, add patterns to the @file{md} file that include
the implied truncation of the shift instructions.

You need not define this macro if it would always have the value of zero.

@findex TRULY_NOOP_TRUNCATION
@item TRULY_NOOP_TRUNCATION (@var{outprec}, @var{inprec})
A C expression which is nonzero if on this machine it is safe to
``convert'' an integer of @var{inprec} bits to one of @var{outprec}
bits (where @var{outprec} is smaller than @var{inprec}) by merely
operating on it as if it had only @var{outprec} bits.

On many machines, this expression can be 1.

@c rearranged this, removed the phrase "it is reported that".  this was
@c to fix an overfull hbox.  --mew 10feb93
When @code{TRULY_NOOP_TRUNCATION} returns 1 for a pair of sizes for
modes for which @code{MODES_TIEABLE_P} is 0, suboptimal code can result.
If this is the case, making @code{TRULY_NOOP_TRUNCATION} return 0 in
such cases may improve things.

@findex STORE_FLAG_VALUE
@item STORE_FLAG_VALUE
A C expression describing the value returned by a comparison operator
with an integral mode and stored by a store-flag instruction
(@samp{s@var{cond}}) when the condition is true.  This description must
apply to @emph{all} the @samp{s@var{cond}} patterns and all the
comparison operators whose results have a @code{MODE_INT} mode.

A value of 1 or @minus{}1 means that the instruction implementing the
comparison operator returns exactly 1 or @minus{}1 when the comparison is true
and 0 when the comparison is false.  Otherwise, the value indicates
which bits of the result are guaranteed to be 1 when the comparison is
true.  This value is interpreted in the mode of the comparison
operation, which is given by the mode of the first operand in the
@samp{s@var{cond}} pattern.  Either the low bit or the sign bit of
@code{STORE_FLAG_VALUE} be on.  Presently, only those bits are used by
the compiler.

If @code{STORE_FLAG_VALUE} is neither 1 or @minus{}1, the compiler will
generate code that depends only on the specified bits.  It can also
replace comparison operators with equivalent operations if they cause
the required bits to be set, even if the remaining bits are undefined.
For example, on a machine whose comparison operators return an
@code{SImode} value and where @code{STORE_FLAG_VALUE} is defined as
@samp{0x80000000}, saying that just the sign bit is relevant, the
expression

@smallexample
(ne:SI (and:SI @var{x} (const_int @var{power-of-2})) (const_int 0))
@end smallexample

@noindent
can be converted to

@smallexample
(ashift:SI @var{x} (const_int @var{n}))
@end smallexample

@noindent
where @var{n} is the appropriate shift count to move the bit being
tested into the sign bit.

There is no way to describe a machine that always sets the low-order bit
for a true value, but does not guarantee the value of any other bits,
but we do not know of any machine that has such an instruction.  If you
are trying to port GCC to such a machine, include an instruction to
perform a logical-and of the result with 1 in the pattern for the
comparison operators and let us know at @email{gcc@@gcc.gnu.org}.

Often, a machine will have multiple instructions that obtain a value
from a comparison (or the condition codes).  Here are rules to guide the
choice of value for @code{STORE_FLAG_VALUE}, and hence the instructions
to be used:

@itemize @bullet
@item
Use the shortest sequence that yields a valid definition for
@code{STORE_FLAG_VALUE}.  It is more efficient for the compiler to
``normalize'' the value (convert it to, e.g., 1 or 0) than for the
comparison operators to do so because there may be opportunities to
combine the normalization with other operations.

@item
For equal-length sequences, use a value of 1 or @minus{}1, with @minus{}1 being
slightly preferred on machines with expensive jumps and 1 preferred on
other machines.

@item
As a second choice, choose a value of @samp{0x80000001} if instructions
exist that set both the sign and low-order bits but do not define the
others.

@item
Otherwise, use a value of @samp{0x80000000}.
@end itemize

Many machines can produce both the value chosen for
@code{STORE_FLAG_VALUE} and its negation in the same number of
instructions.  On those machines, you should also define a pattern for
those cases, e.g., one matching

@smallexample
(set @var{A} (neg:@var{m} (ne:@var{m} @var{B} @var{C})))
@end smallexample

Some machines can also perform @code{and} or @code{plus} operations on
condition code values with less instructions than the corresponding
@samp{s@var{cond}} insn followed by @code{and} or @code{plus}.  On those
machines, define the appropriate patterns.  Use the names @code{incscc}
and @code{decscc}, respectively, for the patterns which perform
@code{plus} or @code{minus} operations on condition code values.  See
@file{rs6000.md} for some examples.  The GNU Superoptizer can be used to
find such instruction sequences on other machines.

You need not define @code{STORE_FLAG_VALUE} if the machine has no store-flag
instructions.

@findex FLOAT_STORE_FLAG_VALUE
@item FLOAT_STORE_FLAG_VALUE (@var{mode})
A C expression that gives a nonzero @code{REAL_VALUE_TYPE} value that is
returned when comparison operators with floating-point results are true.
Define this macro on machine that have comparison operations that return
floating-point values.  If there are no such operations, do not define
this macro.

@findex Pmode
@item Pmode
An alias for the machine mode for pointers.  On most machines, define
this to be the integer mode corresponding to the width of a hardware
pointer; @code{SImode} on 32-bit machine or @code{DImode} on 64-bit machines.
On some machines you must define this to be one of the partial integer
modes, such as @code{PSImode}.

The width of @code{Pmode} must be at least as large as the value of
@code{POINTER_SIZE}.  If it is not equal, you must define the macro
@code{POINTERS_EXTEND_UNSIGNED} to specify how pointers are extended
to @code{Pmode}.

@findex FUNCTION_MODE
@item FUNCTION_MODE
An alias for the machine mode used for memory references to functions
being called, in @code{call} RTL expressions.  On most machines this
should be @code{QImode}.

@findex INTEGRATE_THRESHOLD
@item INTEGRATE_THRESHOLD (@var{decl})
A C expression for the maximum number of instructions above which the
function @var{decl} should not be inlined.  @var{decl} is a
@code{FUNCTION_DECL} node.

The default definition of this macro is 64 plus 8 times the number of
arguments that the function accepts.  Some people think a larger
threshold should be used on RISC machines.

@findex STDC_0_IN_SYSTEM_HEADERS
@item STDC_0_IN_SYSTEM_HEADERS
In normal operation, the preprocessor expands @code{__STDC__} to the
constant 1, to signify that GCC conforms to ISO Standard C@.  On some
hosts, like Solaris, the system compiler uses a different convention,
where @code{__STDC__} is normally 0, but is 1 if the user specifies
strict conformance to the C Standard.

Defining @code{STDC_0_IN_SYSTEM_HEADERS} makes GNU CPP follows the host
convention when processing system header files, but when processing user
files @code{__STDC__} will always expand to 1.

@findex SCCS_DIRECTIVE
@item SCCS_DIRECTIVE
Define this if the preprocessor should ignore @code{#sccs} directives
and print no error message.

@findex NO_IMPLICIT_EXTERN_C
@item NO_IMPLICIT_EXTERN_C
Define this macro if the system header files support C++ as well as C@.
This macro inhibits the usual method of using system header files in
C++, which is to pretend that the file's contents are enclosed in
@samp{extern "C" @{@dots{}@}}.

@findex HANDLE_PRAGMA
@item HANDLE_PRAGMA (@var{getc}, @var{ungetc}, @var{name})
This macro is no longer supported.  You must use
@code{REGISTER_TARGET_PRAGMAS} instead.

@findex REGISTER_TARGET_PRAGMAS
@findex #pragma
@findex pragma
@item REGISTER_TARGET_PRAGMAS (@var{pfile})
Define this macro if you want to implement any target-specific pragmas.
If defined, it is a C expression which makes a series of calls to
@code{cpp_register_pragma} for each pragma, with @var{pfile} passed as
the first argument to to these functions.  The macro may also do any
setup required for the pragmas.

The primary reason to define this macro is to provide compatibility with
other compilers for the same target.  In general, we discourage
definition of target-specific pragmas for GCC@.

If the pragma can be implemented by attributes then you should consider
defining the target hook @samp{TARGET_INSERT_ATTRIBUTES} as well.

Preprocessor macros that appear on pragma lines are not expanded.  All
@samp{#pragma} directives that do not match any registered pragma are
silently ignored, unless the user specifies @option{-Wunknown-pragmas}.

@deftypefun void cpp_register_pragma (cpp_reader *@var{pfile}, const char *@var{space}, const char *@var{name}, void (*@var{callback}) (cpp_reader *))

Each call to @code{cpp_register_pragma} establishes one pragma.  The
@var{callback} routine will be called when the preprocessor encounters a
pragma of the form

@smallexample
#pragma [@var{space}] @var{name} @dots{}
@end smallexample

@var{space} is the case-sensitive namespace of the pragma, or
@code{NULL} to put the pragma in the global namespace.  The callback
routine receives @var{pfile} as its first argument, which can be passed
on to cpplib's functions if necessary.  You can lex tokens after the
@var{name} by calling @code{c_lex}.  Tokens that are not read by the
callback will be silently ignored.  The end of the line is indicated by
a token of type @code{CPP_EOF}.

For an example use of this routine, see @file{c4x.h} and the callback
routines defined in @file{c4x-c.c}.

Note that the use of @code{c_lex} is specific to the C and C++
compilers.  It will not work in the Java or Fortran compilers, or any
other language compilers for that matter.  Thus if @code{c_lex} is going
to be called from target-specific code, it must only be done so when
building the C and C++ compilers.  This can be done by defining the
variables @code{c_target_objs} and @code{cxx_target_objs} in the
target entry in the @file{config.gcc} file.  These variables should name
the target-specific, language-specific object file which contains the
code that uses @code{c_lex}.  Note it will also be necessary to add a
rule to the makefile fragment pointed to by @code{tmake_file} that shows
how to build this object file.
@end deftypefun

@findex HANDLE_SYSV_PRAGMA
@findex #pragma
@findex pragma
@item HANDLE_SYSV_PRAGMA
Define this macro (to a value of 1) if you want the System V style
pragmas @samp{#pragma pack(<n>)} and @samp{#pragma weak <name>
[=<value>]} to be supported by gcc.

The pack pragma specifies the maximum alignment (in bytes) of fields
within a structure, in much the same way as the @samp{__aligned__} and
@samp{__packed__} @code{__attribute__}s do.  A pack value of zero resets
the behavior to the default.

The weak pragma only works if @code{SUPPORTS_WEAK} and
@code{ASM_WEAKEN_LABEL} are defined.  If enabled it allows the creation
of specifically named weak labels, optionally with a value.

@findex HANDLE_PRAGMA_PACK_PUSH_POP
@findex #pragma
@findex pragma
@item HANDLE_PRAGMA_PACK_PUSH_POP
Define this macro (to a value of 1) if you want to support the Win32
style pragmas @samp{#pragma pack(push,@var{n})} and @samp{#pragma
pack(pop)}.  The @samp{pack(push,@var{n})} pragma specifies the maximum alignment
(in bytes) of fields within a structure, in much the same way as the
@samp{__aligned__} and @samp{__packed__} @code{__attribute__}s do.  A
pack value of zero resets the behavior to the default.  Successive
invocations of this pragma cause the previous values to be stacked, so
that invocations of @samp{#pragma pack(pop)} will return to the previous
value.

@findex DOLLARS_IN_IDENTIFIERS
@item DOLLARS_IN_IDENTIFIERS
Define this macro to control use of the character @samp{$} in identifier
names.  0 means @samp{$} is not allowed by default; 1 means it is allowed.
1 is the default; there is no need to define this macro in that case.
This macro controls the compiler proper; it does not affect the preprocessor.

@findex NO_DOLLAR_IN_LABEL
@item NO_DOLLAR_IN_LABEL
Define this macro if the assembler does not accept the character
@samp{$} in label names.  By default constructors and destructors in
G++ have @samp{$} in the identifiers.  If this macro is defined,
@samp{.} is used instead.

@findex NO_DOT_IN_LABEL
@item NO_DOT_IN_LABEL
Define this macro if the assembler does not accept the character
@samp{.} in label names.  By default constructors and destructors in G++
have names that use @samp{.}.  If this macro is defined, these names
are rewritten to avoid @samp{.}.

@findex DEFAULT_MAIN_RETURN
@item DEFAULT_MAIN_RETURN
Define this macro if the target system expects every program's @code{main}
function to return a standard ``success'' value by default (if no other
value is explicitly returned).

The definition should be a C statement (sans semicolon) to generate the
appropriate rtl instructions.  It is used only when compiling the end of
@code{main}.

@item NEED_ATEXIT
@findex NEED_ATEXIT
Define this if the target system lacks the function @code{atexit}
from the ISO C standard.  If this macro is defined, a default definition
will be provided to support C++.  If @code{ON_EXIT} is not defined,
a default @code{exit} function will also be provided.

@item ON_EXIT
@findex ON_EXIT
Define this macro if the target has another way to implement atexit
functionality without replacing @code{exit}.  For instance, SunOS 4 has
a similar @code{on_exit} library function.

The definition should be a functional macro which can be used just like
the @code{atexit} function.

@item EXIT_BODY
@findex EXIT_BODY
Define this if your @code{exit} function needs to do something
besides calling an external function @code{_cleanup} before
terminating with @code{_exit}.  The @code{EXIT_BODY} macro is
only needed if @code{NEED_ATEXIT} is defined and @code{ON_EXIT} is not
defined.

@findex INSN_SETS_ARE_DELAYED
@item INSN_SETS_ARE_DELAYED (@var{insn})
Define this macro as a C expression that is nonzero if it is safe for the
delay slot scheduler to place instructions in the delay slot of @var{insn},
even if they appear to use a resource set or clobbered in @var{insn}.
@var{insn} is always a @code{jump_insn} or an @code{insn}; GCC knows that
every @code{call_insn} has this behavior.  On machines where some @code{insn}
or @code{jump_insn} is really a function call and hence has this behavior,
you should define this macro.

You need not define this macro if it would always return zero.

@findex INSN_REFERENCES_ARE_DELAYED
@item INSN_REFERENCES_ARE_DELAYED (@var{insn})
Define this macro as a C expression that is nonzero if it is safe for the
delay slot scheduler to place instructions in the delay slot of @var{insn},
even if they appear to set or clobber a resource referenced in @var{insn}.
@var{insn} is always a @code{jump_insn} or an @code{insn}.  On machines where
some @code{insn} or @code{jump_insn} is really a function call and its operands
are registers whose use is actually in the subroutine it calls, you should
define this macro.  Doing so allows the delay slot scheduler to move
instructions which copy arguments into the argument registers into the delay
slot of @var{insn}.

You need not define this macro if it would always return zero.

@findex MACHINE_DEPENDENT_REORG
@item MACHINE_DEPENDENT_REORG (@var{insn})
In rare cases, correct code generation requires extra machine
dependent processing between the second jump optimization pass and
delayed branch scheduling.  On those machines, define this macro as a C
statement to act on the code starting at @var{insn}.

@findex MULTIPLE_SYMBOL_SPACES
@item MULTIPLE_SYMBOL_SPACES
Define this macro if in some cases global symbols from one translation
unit may not be bound to undefined symbols in another translation unit
without user intervention.  For instance, under Microsoft Windows
symbols must be explicitly imported from shared libraries (DLLs).

@findex MD_ASM_CLOBBERS
@item MD_ASM_CLOBBERS (@var{clobbers})
A C statement that adds to @var{clobbers} @code{STRING_CST} trees for
any hard regs the port wishes to automatically clobber for all asms.

@findex MAX_INTEGER_COMPUTATION_MODE
@item MAX_INTEGER_COMPUTATION_MODE
Define this to the largest integer machine mode which can be used for
operations other than load, store and copy operations.

You need only define this macro if the target holds values larger than
@code{word_mode} in general purpose registers.  Most targets should not define
this macro.

@findex MATH_LIBRARY
@item MATH_LIBRARY
Define this macro as a C string constant for the linker argument to link
in the system math library, or @samp{""} if the target does not have a
separate math library.

You need only define this macro if the default of @samp{"-lm"} is wrong.

@findex LIBRARY_PATH_ENV
@item LIBRARY_PATH_ENV
Define this macro as a C string constant for the environment variable that
specifies where the linker should look for libraries.

You need only define this macro if the default of @samp{"LIBRARY_PATH"}
is wrong.

@findex TARGET_HAS_F_SETLKW
@item TARGET_HAS_F_SETLKW
Define this macro if the target supports file locking with fcntl / F_SETLKW@.
Note that this functionality is part of POSIX@.
Defining @code{TARGET_HAS_F_SETLKW} will enable the test coverage code
to use file locking when exiting a program, which avoids race conditions
if the program has forked.

@findex MAX_CONDITIONAL_EXECUTE
@item MAX_CONDITIONAL_EXECUTE

A C expression for the maximum number of instructions to execute via
conditional execution instructions instead of a branch.  A value of
@code{BRANCH_COST}+1 is the default if the machine does not use cc0, and
1 if it does use cc0.

@findex IFCVT_MODIFY_TESTS
@item IFCVT_MODIFY_TESTS
A C expression to modify the tests in @code{TRUE_EXPR}, and
@code{FALSE_EXPR} for use in converting insns in @code{TEST_BB},
@code{THEN_BB}, @code{ELSE_BB}, and @code{JOIN_BB} basic blocks to
conditional execution.  Set either @code{TRUE_EXPR} or @code{FALSE_EXPR}
to a null pointer if the tests cannot be converted.

@findex IFCVT_MODIFY_INSN
@item IFCVT_MODIFY_INSN
A C expression to modify the @code{PATTERN} of an @code{INSN} that is to
be converted to conditional execution format.

@findex IFCVT_MODIFY_FINAL
@item IFCVT_MODIFY_FINAL
A C expression to perform any final machine dependent modifications in
converting code to conditional execution in the basic blocks
@code{TEST_BB}, @code{THEN_BB}, @code{ELSE_BB}, and @code{JOIN_BB}.

@findex IFCVT_MODIFY_CANCEL
@item IFCVT_MODIFY_CANCEL
A C expression to cancel any machine dependent modifications in
converting code to conditional execution in the basic blocks
@code{TEST_BB}, @code{THEN_BB}, @code{ELSE_BB}, and @code{JOIN_BB}.
@end table

@deftypefn {Target Hook} void TARGET_INIT_BUILTINS ()
Define this hook if you have any machine-specific built-in functions
that need to be defined.  It should be a function that performs the
necessary setup.

Machine specific built-in functions can be useful to expand special machine
instructions that would otherwise not normally be generated because
they have no equivalent in the source language (for example, SIMD vector
instructions or prefetch instructions).

To create a built-in function, call the function @code{builtin_function}
which is defined by the language front end.  You can use any type nodes set
up by @code{build_common_tree_nodes} and @code{build_common_tree_nodes_2};
only language front ends that use those two functions will call
@samp{TARGET_INIT_BUILTINS}.
@end deftypefn

@deftypefn {Target Hook} rtx TARGET_EXPAND_BUILTIN (tree @var{exp}, rtx @var{target}, rtx @var{subtarget}, enum machine_mode @var{mode}, int @var{ignore})

Expand a call to a machine specific built-in function that was set up by
@samp{TARGET_INIT_BUILTINS}.  @var{exp} is the expression for the
function call; the result should go to @var{target} if that is
convenient, and have mode @var{mode} if that is convenient.
@var{subtarget} may be used as the target for computing one of
@var{exp}'s operands.  @var{ignore} is nonzero if the value is to be
ignored.  This function should return the result of the call to the
built-in function.
@end deftypefn

@table @code
@findex MD_CAN_REDIRECT_BRANCH
@item MD_CAN_REDIRECT_BRANCH(@var{branch1}, @var{branch2})

Take a branch insn in @var{branch1} and another in @var{branch2}.
Return true if redirecting @var{branch1} to the destination of
@var{branch2} is possible.

On some targets, branches may have a limited range.  Optimizing the
filling of delay slots can result in branches being redirected, and this
may in turn cause a branch offset to overflow.

@findex ALLOCATE_INITIAL_VALUE
@item ALLOCATE_INITIAL_VALUE(@var{hard_reg})

When the initial value of a hard register has been copied in a pseudo
register, it is often not necessary to actually allocate another register
to this pseudo register, because the original hard register or a stack slot
it has been saved into can be used.  @code{ALLOCATE_INITIAL_VALUE}, if
defined, is called at the start of register allocation once for each
hard register that had its initial value copied by using
@code{get_func_hard_reg_initial_val} or @code{get_hard_reg_initial_val}.
Possible values are @code{NULL_RTX}, if you don't want
to do any special allocation, a @code{REG} rtx---that would typically be
the hard register itself, if it is known not to be clobbered---or a
@code{MEM}.
If you are returning a @code{MEM}, this is only a hint for the allocator;
it might decide to use another register anyways.
You may use @code{current_function_leaf_function} in the definition of the
macro, functions that use @code{REG_N_SETS}, to determine if the hard
register in question will not be clobbered.

@findex TARGET_OBJECT_SUFFIX
@item TARGET_OBJECT_SUFFIX
Define this macro to be a C string representing the suffix for object
files on your target machine.  If you do not define this macro, GCC will
use @samp{.o} as the suffix for object files.

@findex TARGET_EXECUTABLE_SUFFIX
@item TARGET_EXECUTABLE_SUFFIX
Define this macro to be a C string representing the suffix to be
automatically added to executable files on your target machine.  If you
do not define this macro, GCC will use the null string as the suffix for
executable files.

@findex COLLECT_EXPORT_LIST
@item COLLECT_EXPORT_LIST
If defined, @code{collect2} will scan the individual object files
specified on its command line and create an export list for the linker.
Define this macro for systems like AIX, where the linker discards
object files that are not referenced from @code{main} and uses export
lists.

@end table

@deftypefn {Target Hook} bool TARGET_CANNOT_MODIFY_JUMPS_P (void)
This target hook returns @code{true} past the point in which new jump
instructions could be created.  On machines that require a register for
every jump such as the SHmedia ISA of SH5, this point would typically be
reload, so this target hook should be defined to a function such as:

@smallexample
static bool
cannot_modify_jumps_past_reload_p ()
@{
  return (reload_completed || reload_in_progress);
@}
@end smallexample
@end deftypefn