* tm.texi (RETURN_ADDR_OFFSET): Document.

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

@c Copyright (C) 1988,1989,1992,1993,1994,1995,1996,1997,1998,1999,2000,2001,
@c 2002, 2003 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.

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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{""}.
@end defmac

@defmac 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:

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

@defmac DRIVER_SELF_SPECS
A list of specs for the driver itself.  It should be a suitable
initializer for an array of strings, with no surrounding braces.

The driver applies these specs to its own command line between loading
default @file{specs} files (but not command-line specified ones) and
choosing the multilib directory or running any subcommands.  It
applies them in the order given, so each spec can depend on the
options added by earlier ones.  It is also possible to remove options
using @samp{%<@var{option}} in the usual way.

This macro can be useful when a port has several interdependent target
options.  It provides a way of standardizing the command line so
that the other specs are easier to write.

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

@defmac OPTION_DEFAULT_SPECS
A list of specs used to support configure-time default options (i.e.@:
@option{--with} options) in the driver.  It should be a suitable initializer
for an array of structures, each containing two strings, without the
outermost pair of surrounding braces.

The first item in the pair is the name of the default.  This must match
the code in @file{config.gcc} for the target.  The second item is a spec
to apply if a default with this name was specified.  The string
@samp{%(VALUE)} in the spec will be replaced by the value of the default
everywhere it occurs.

The driver will apply these specs to its own command line between loading
default @file{specs} files and processing @code{DRIVER_SELF_SPECS}, using
the same mechanism as @code{DRIVER_SELF_SPECS}.

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

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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@.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac AS_NEEDS_DASH_FOR_PIPED_INPUT
Define this macro, with no value, if the driver should give the assembler
an argument consisting of a single dash, @option{-}, to instruct it to
read from its standard input (which will be a pipe connected to the
output of the compiler proper).  This argument is given after any
@option{-o} option specifying the name of the output file.

If you do not define this macro, the assembler is assumed to read its
standard input if given no non-option arguments.  If your assembler
cannot read standard input at all, use a @samp{%@{pipe:%e@}} construct;
see @file{mips.h} for instance.
@end defmac

@defmac 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.
@end defmac

@defmac 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}.
@end defmac

@defmac 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.
@end defmac

@defmac 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}.
@end defmac

@defmac 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.
@end defmac

@defmac 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}.
@end defmac

@defmac SYSROOT_SUFFIX_SPEC
Define this macro to add a suffix to the target sysroot when GCC is 
configured with a sysroot.  This will cause GCC to search for usr/lib, 
et al, within sysroot+suffix. 
@end defmac

@defmac SYSROOT_HEADERS_SUFFIX_SPEC
Define this macro to add a headers_suffix to the target sysroot when 
GCC is configured with a sysroot.  This will cause GCC to pass the 
updated sysroot+headers_suffix to CPP@, causing it to search for
usr/include, et al, within sysroot+headers_suffix.
@end defmac

@defmac 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-sysv: %(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
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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}.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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}.
@end defmac

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

@defmac 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.
@end defmac

@defmac 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}.
@end defmac

@defmac 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}.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac MODIFY_TARGET_NAME
Define this macro if you wish 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
@end defmac

@defmac 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.
@end defmac

@defmac 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 ignore this macro and do not search either
@file{/usr/include} or its replacement.
@end defmac

@defmac 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.
@end defmac

@defmac 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 defmac

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.

@defmac TARGET_CPU_CPP_BUILTINS ()
This function-like macro expands to a block of code that defines
built-in preprocessor macros and assertions for the target cpu, using
the functions @code{builtin_define}, @code{builtin_define_std} and
@code{builtin_assert}.  When the front end
calls this macro it provides a trailing semicolon, and since it has
finished command line option processing your code can use those
results freely.

@code{builtin_assert} takes a string in the form you pass to the
command-line option @option{-A}, such as @code{cpu=mips}, and creates
the assertion.  @code{builtin_define} takes a string in the form
accepted by option @option{-D} and unconditionally defines the macro.

@code{builtin_define_std} takes a string representing the name of an
object-like macro.  If it doesn't lie in the user's namespace,
@code{builtin_define_std} defines it unconditionally.  Otherwise, it
defines a version with two leading underscores, and another version
with two leading and trailing underscores, and defines the original
only if an ISO standard was not requested on the command line.  For
example, passing @code{unix} defines @code{__unix}, @code{__unix__}
and possibly @code{unix}; passing @code{_mips} defines @code{__mips},
@code{__mips__} and possibly @code{_mips}, and passing @code{_ABI64}
defines only @code{_ABI64}.

You can also test for the C dialect being compiled.  The variable
@code{c_language} is set to one of @code{clk_c}, @code{clk_cplusplus}
or @code{clk_objective_c}.  Note that if we are preprocessing
assembler, this variable will be @code{clk_c} but the function-like
macro @code{preprocessing_asm_p()} will return true, so you might want
to check for that first.  If you need to check for strict ANSI, the
variable @code{flag_iso} can be used.  The function-like macro
@code{preprocessing_trad_p()} can be used to check for traditional
preprocessing.
@end defmac

@defmac TARGET_OS_CPP_BUILTINS ()
Similarly to @code{TARGET_CPU_CPP_BUILTINS} but this macro is optional
and is used for the target operating system instead.
@end defmac

@defmac TARGET_OBJFMT_CPP_BUILTINS ()
Similarly to @code{TARGET_CPU_CPP_BUILTINS} but this macro is optional
and is used for the target object format.  @file{elfos.h} uses this
macro to define @code{__ELF__}, so you probably do not need to define
it yourself.
@end defmac

@deftypevar {extern int} target_flags
This declaration should be present.
@end deftypevar

@cindex optional hardware or system features
@cindex features, optional, in system conventions

@defmac TARGET_@var{featurename}
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{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 & 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.
@end defmac

@defmac 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", MASK_68020, "" @},     \
    @{ "68000", -MASK_68020,          \
      N_("Compile for the 68000") @}, \
    @{ "", MASK_68020, "" @},          \
  @}
@end smallexample
@end defmac

@defmac 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 option
name, the address of a variable, a description string, and a value.
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.

If the value listed in the table is @code{NULL}, then the variable, type
@code{char *}, is set to the variable part of the given option if the
fixed part matches.  In other words, if the first part of the option
matches what's in the table, the variable will be set to point to the
rest of the option.  This allows the user to specify a value for that
option.  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}.

If the value listed in the table is non-@code{NULL}, then the option
must match the option in the table exactly (with @samp{-m}), and the
variable is set to point to the value listed in the table.

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"), 0 @} @}
@end smallexample

Here is a variant of the above that allows the user to also specify
just @option{-mshort-data} where a default of @code{"64"} is used.

@smallexample
extern char *m88k_short_data;
#define TARGET_OPTIONS \
 @{ @{ "short-data-", &m88k_short_data, \
     N_("Specify the size of the short data section"), 0 @} \
    @{ "short-data", &m88k_short_data, "", "64" @},
    @}
@end smallexample

Here is an example which defines @option{-mno-alu}, @option{-malu1}, and
@option{-malu2} as a three-state switch, along with suitable macros for
checking the state of the option (documentation is elided for brevity).

@smallexample
[chip.c]
char *chip_alu = ""; /* Specify default here.  */

[chip.h]
extern char *chip_alu;
#define TARGET_OPTIONS \
  @{ @{ "no-alu", &chip_alu, "", "" @}, \
     @{ "alu1", &chip_alu, "", "1" @}, \
     @{ "alu2", &chip_alu, "", "2" @}, @}
#define TARGET_ALU (chip_alu[0] != '\0')
#define TARGET_ALU1 (chip_alu[0] == '1')
#define TARGET_ALU2 (chip_alu[0] == '2')
@end smallexample
@end defmac

@defmac 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
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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 defmac

@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 the function pointer
@code{init_machine_status}.  This pointer is 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.

@defmac 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 pointer @code{init_machine_status}.
@end defmac

@deftypevar {void (*)(struct function *)} init_machine_status
If this function 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.

@code{struct machine_function} structures are expected to be freed by GC.
Generally, any memory that they reference must be allocated by using
@code{ggc_alloc}, including the structure itself.
@end deftypevar

@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}.

@defmac 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}.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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}.
@end defmac

@defmac 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.
@end defmac

@defmac UNITS_PER_WORD
Number of storage units in a word; normally 4.
@end defmac

@defmac 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.
@end defmac

@defmac 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}.
@end defmac

@defmac 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}.
@end defmac

@defmac 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}.
@end defmac

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

@defmac 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}.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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}.
@end defmac

@defmac 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}.
@end defmac

@defmac 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.
@end defmac

@defmac FUNCTION_BOUNDARY
Alignment required for a function entry point, in bits.
@end defmac

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

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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 (including applying of
@code{BIGGEST_ALIGNMENT} and @code{BIGGEST_FIELD_ALIGNMENT} to the
alignment) is @var{computed}.  It overrides alignment only if the
field alignment has not been set by the
@code{__attribute__ ((aligned (@var{n})))} construct.
@end defmac

@defmac 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}.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

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

If @code{PCC_BITFIELD_TYPE_MATTERS} is true, it overrides this macro.
@end defmac

@defmac 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}.
@end defmac

@defmac 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.
@end defmac

@defmac 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 named 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 named 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.)

An unnamed bit-field will not affect the alignment of the containing
structure.

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}.
@end defmac

@defmac BITFIELD_NBYTES_LIMITED
Like @code{PCC_BITFIELD_TYPE_MATTERS} except that its effect is limited
to aligning a bit-field within the structure.
@end defmac

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

If @var{field} is the only field in the structure, @var{mode} is its
mode, otherwise @var{mode} is VOIDmode.  @var{mode} is provided in the
case where structures of one field would require the structure's mode to
retain the field's mode.

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.
@end defmac

@defmac 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}
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac TARGET_FLOAT_FORMAT
A code distinguishing the floating point format of the target machine.
There are four defined values:

@ftable @code
@item IEEE_FLOAT_FORMAT
This code indicates IEEE floating point.  It is the default; there is no
need to define @code{TARGET_FLOAT_FORMAT} when the format is IEEE@.

@item VAX_FLOAT_FORMAT
This code indicates the ``F float'' (for @code{float}) and ``D float''
or ``G float'' formats (for @code{double}) used on the VAX and PDP-11@.

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

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

If your target uses a floating point format other than these, you must
define a new @var{name}_FLOAT_FORMAT code for it, and add support for
it to @file{real.c}.

The ordering of the component words of floating point values stored in
memory is controlled by @code{FLOAT_WORDS_BIG_ENDIAN}.
@end defmac

@defmac 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@.
@end defmac

@defmac 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@.
@end defmac

@defmac 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@.
@end defmac

@defmac 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@.
@end defmac

@defmac 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.
@end defmac

@defmac LARGEST_EXPONENT_IS_NORMAL (@var{size})
This macro 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 defmac

@deftypefn {Target Hook} bool TARGET_VECTOR_OPAQUE_P (tree @var{type})
This target hook should return @code{true} a vector is opaque.  That
is, if no cast is needed when copying a vector value of type
@var{type} into another vector lvalue of the same size.  Vector opaque
types cannot be initialized.  The default is that there are no such
types.
@end deftypefn

@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 nonzero size.  If this hook returns @code{true},
other macros that control bit-field layout are ignored.

When a bit-field is inserted into a packed record, the whole size
of the underlying type is used by one or more same-size adjacent
bit-fields (that is, if its long:3, 32 bits is used in the record,
and any additional adjacent long bit-fields are packed into the same
chunk of 32 bits. However, if the size changes, a new field of that
size is allocated). In an unpacked record, this is the same as using
alignment, but not equivalent when packing.

If both MS bit-fields and @samp{__attribute__((packed))} are used,
the latter will take precedence. If @samp{__attribute__((packed))} is
used on a single field when MS bit-fields are in use, it will take
precedence for that field, but the alignment of the rest of the structure
may affect its placement.
@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.

@defmac 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.
@end defmac

@defmac 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.)
@end defmac

@defmac 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.
@end defmac

@defmac 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}.
@end defmac

@defmac 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}.
@end defmac

@defmac 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.
@end defmac

@defmac 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}.
@end defmac

@defmac BOOL_TYPE_SIZE
A C expression for the size in bits of the C++ type @code{bool} and
C99 type @code{_Bool} on the target machine.  If you don't define
this, and you probably shouldn't, the default is @code{CHAR_TYPE_SIZE}.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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}.
@end defmac

@defmac TARGET_FLT_EVAL_METHOD
A C expression for the value for @code{FLT_EVAL_METHOD} in @file{float.h},
assuming, if applicable, that the floating-point control word is in its
default state.  If you do not define this macro the value of
@code{FLT_EVAL_METHOD} will be zero.
@end defmac

@defmac 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.
@end defmac

@defmac 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}.
@end defmac

@defmac 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.
@end defmac

@defmac 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"}.
@end defmac

@defmac 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"}.
@end defmac

@defmac 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"}.
@end defmac

@defmac 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}.
@end defmac

@defmac 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}.
@end defmac

@defmac 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.
@end defmac

@defmac 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"}.
@end defmac

@defmac 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}.
@end defmac

@defmac 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}.
@end defmac

@defmac 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}.
@end defmac

@defmac 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 defmac

@defmac TARGET_VTABLE_ENTRY_ALIGN
By default, the vtable entries are void pointers, the so the alignment
is the same as pointer alignment.  The value of this macro specifies
the alignment of the vtable entry in bits.  It should be defined only
when special alignment is necessary. */
@end defmac

@defmac TARGET_VTABLE_DATA_ENTRY_DISTANCE
There are a few non-descriptor entries in the vtable at offsets below
zero.  If these entries must be padded (say, to preserve the alignment
specified by @code{TARGET_VTABLE_ENTRY_ALIGN}), set this to the number
of words in each data entry.
@end defmac

@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.  All of them must evaluate
to constants; they are used in @code{case} statements.

@findex TARGET_BELL
@findex TARGET_CR
@findex TARGET_ESC
@findex TARGET_FF
@findex TARGET_NEWLINE
@findex TARGET_TAB
@findex TARGET_VT
@multitable {@code{TARGET_NEWLINE}} {Escape} {ASCII character}
@item Macro                 @tab Escape             @tab ASCII character
@item @code{TARGET_BELL}    @tab @kbd{\a}           @tab @code{07}, @code{BEL}
@item @code{TARGET_CR}      @tab @kbd{\r}           @tab @code{0D}, @code{CR}
@item @code{TARGET_ESC}     @tab @kbd{\e}, @kbd{\E} @tab @code{1B}, @code{ESC}
@item @code{TARGET_FF}      @tab @kbd{\f}           @tab @code{0C}, @code{FF}
@item @code{TARGET_NEWLINE} @tab @kbd{\n}           @tab @code{0A}, @code{LF}
@item @code{TARGET_TAB}     @tab @kbd{\t}           @tab @code{09}, @code{HT}
@item @code{TARGET_VT}      @tab @kbd{\v}           @tab @code{0B}, @code{VT}
@end multitable

@noindent
Note that the @kbd{\e} and @kbd{\E} escapes are GNU extensions, not
part of the C standard.

@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.

@defmac 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}.
@end defmac

@defmac 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}}.
@end defmac

@defmac 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.
@end defmac

@defmac 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}.
@end defmac

@defmac 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.
@end defmac

@findex fixed_regs
@findex call_used_regs
@findex global_regs
@findex reg_names
@findex reg_class_contents
@defmac 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} / @code{REG_CLASS_FROM_CONSTRAINT}
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.)
@end defmac

@defmac 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}.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

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

@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.

@defmac 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.
@end defmac

@defmac 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 defmac

@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.

@defmac 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
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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 defmac

@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.

@defmac 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.
@end defmac

@defmac 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 defmac

@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.  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.  Furthermore, the existing
support for stack-like registers is specific to the 80387 floating
point coprocessor.  If you have a new architecture that uses
stack-like registers, you will need to do substantial work on
@file{reg-stack.c} and write your machine description to cooperate
with it, as well as defining these macros.

@defmac STACK_REGS
Define this if the machine has any stack-like registers.
@end defmac

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

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

@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.

@deftp {Data type} {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.
@end deftp

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

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

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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 dependent manner.  If
@var{mode} is VOIDmode then it should return the same value as
@code{BASE_REG_CLASS}.
@end defmac

@defmac 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).
@end defmac

@defmac CONSTRAINT_LEN (@var{char}, @var{str})
For the constraint at the start of @var{str}, which starts with the letter
@var{c}, return the length.  This allows you to have register class /
constant / extra constraints that are longer than a single letter;
you don't need to define this macro if you can do with single-letter
constraints only.  The definition of this macro should use
DEFAULT_CONSTRAINT_LEN for all the characters that you don't want
to handle specially.
There are some sanity checks in genoutput.c that check the constraint lengths
for the md file, so you can also use this macro to help you while you are
transitioning from a byzantine single-letter-constraint scheme: when you
return a negative length for a constraint you want to re-use, genoutput
will complain about every instance where it is used in the md file.
@end defmac

@defmac 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.
@end defmac

@defmac REG_CLASS_FROM_CONSTRAINT (@var{char}, @var{str})
Like @code{REG_CLASS_FROM_LETTER}, but you also get the constraint string
passed in @var{str}, so that you can use suffixes to distinguish between
different variants.
@end defmac

@defmac 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.
@end defmac

@defmac 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}.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac SECONDARY_RELOAD_CLASS (@var{class}, @var{mode}, @var{x})
@defmacx SECONDARY_INPUT_RELOAD_CLASS (@var{class}, @var{mode}, @var{x})
@defmacx 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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}.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac CANNOT_CHANGE_MODE_CLASS (@var{from}, @var{to}, @var{class})
If defined, a C expression that returns nonzero for a @var{class} for which
a change from mode @var{from} to mode @var{to} 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{CANNOT_CHANGE_MODE_CLASS}
as below:

@example
#define CANNOT_CHANGE_MODE_CLASS(FROM, TO, CLASS) \
  (GET_MODE_SIZE (FROM) != GET_MODE_SIZE (TO) \
   ? reg_classes_intersect_p (FLOAT_REGS, (CLASS)) : 0)
@end example
@end defmac

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

@defmac 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}.
@end defmac

@defmac CONST_OK_FOR_CONSTRAINT_P (@var{value}, @var{c}, @var{str})
Like @code{CONST_OK_FOR_LETTER_P}, but you also get the constraint
string passed in @var{str}, so that you can use suffixes to distinguish
between different variants.
@end defmac

@defmac 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.
@end defmac

@defmac CONST_DOUBLE_OK_FOR_CONSTRAINT_P (@var{value}, @var{c}, @var{str})
Like @code{CONST_DOUBLE_OK_FOR_LETTER_P}, but you also get the constraint
string passed in @var{str}, so that you can use suffixes to distinguish
between different variants.
@end defmac

@defmac 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} /
@code{REG_CLASS_FROM_CONSTRAINT}
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 defmac

@defmac EXTRA_CONSTRAINT_STR (@var{value}, @var{c}, @var{str})
Like @code{EXTRA_CONSTRAINT}, but you also get the constraint string passed
in @var{str}, so that you can use suffixes to distinguish between different
variants.
@end defmac

@defmac EXTRA_MEMORY_CONSTRAINT (@var{c}, @var{str})
A C expression that defines the optional machine-dependent constraint
letters, amongst those accepted by @code{EXTRA_CONSTRAINT}, that should
be treated like memory constraints by the reload pass.

It should return 1 if the operand type represented by the constraint 
at the start of @var{str}, the first letter of which is the letter @var{c},
 comprises a subset of all memory references including
all those whose address is simply a base register.  This allows the reload 
pass to reload an operand, if it does not directly correspond to the operand 
type of @var{c}, by copying its address into a base register.

For example, on the S/390, some instructions do not accept arbitrary
memory references, but only those that do not make use of an index
register.  The constraint letter @samp{Q} is defined via
@code{EXTRA_CONSTRAINT} as representing a memory address of this type.
If the letter @samp{Q} is marked as @code{EXTRA_MEMORY_CONSTRAINT},
a @samp{Q} constraint can handle any memory operand, because the
reload pass knows it can be reloaded by copying the memory address
into a base register if required.  This is analogous to the way
a @samp{o} constraint can handle any memory operand.
@end defmac

@defmac EXTRA_ADDRESS_CONSTRAINT (@var{c}, @var{str})
A C expression that defines the optional machine-dependent constraint
letters, amongst those accepted by @code{EXTRA_CONSTRAINT} /
@code{EXTRA_CONSTRAINT_STR}, that should
be treated like address constraints by the reload pass.

It should return 1 if the operand type represented by the constraint 
at the start of @var{str}, which starts with the letter @var{c}, comprises
a subset of all memory addresses including
all those that consist of just a base register.  This allows the reload 
pass to reload an operand, if it does not directly correspond to the operand 
type of @var{str}, by copying it into a base register.

Any constraint marked as @code{EXTRA_ADDRESS_CONSTRAINT} can only
be used with the @code{address_operand} predicate.  It is treated 
analogously to the @samp{p} constraint.
@end defmac

@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.

@defmac 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.
@end defmac

@defmac 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))) @dots{})}

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.
@end defmac

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

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

@defmac 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
@end defmac

@defmac STACK_ALIGNMENT_NEEDED
Define to zero to disable final alignment of the stack during reload.
The nonzero default for this macro is suitable for most ports.

On ports where @code{STARTING_FRAME_OFFSET} is nonzero or where there
is a register save block following the local block that doesn't require
alignment to @code{STACK_BOUNDARY}, it may be beneficial to disable
stack alignment and do it in the backend.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac RETURN_ADDR_IN_PREVIOUS_FRAME
Define this if the return address of a particular stack frame is accessed
from the frame pointer of the previous stack frame.
@end defmac

@defmac 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)}.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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 defmac

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

@defmac 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.
@end defmac

@defmac 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.

Do not define this macro if the stack pointer is saved and restored
by the regular prolog and epilog code in the call frame itself; in 
this case, the exception handling library routines will update the 
stack location to be restored in place.  Otherwise, you must define 
this macro if you want to support call frame exception handling like 
that provided by DWARF 2.
@end defmac

@defmac 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.  If defined, @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.
@end defmac

@defmac RETURN_ADDR_OFFSET
If defined, an integer-valued C expression for which rtl will be generated
to add it to the exception handler address before it is searched in the
exception handling tables, and to subtract it again from the address before
using it to return to the exception handler.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.

For proper signal handling in Java this macro is accompanied by
@code{MAKE_THROW_FRAME}, defined in @file{libjava/include/*-signal.h} headers.
@end defmac

@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.

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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 defmac

@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.

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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}.
@end defmac

@defmac 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}).
@end defmac

@defmac 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.
@end defmac

@defmac STATIC_CHAIN_REGNUM
@defmacx 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.
@end defmac

@defmac STATIC_CHAIN
@defmacx 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.
@end defmac

@defmac 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}.
@end defmac

@defmac 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 defmac

@defmac DWARF_REG_TO_UNWIND_COLUMN (@var{regno})

Define this macro if the target's representation for dwarf registers
is different than the internal representation for unwind column.
Given a dwarf register, this macro should return the internal unwind
column number to use instead.

See the PowerPC's SPE target for an example.  
@end defmac

@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.

@defmac 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.
@end defmac

@findex get_frame_size
@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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 defmac

@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.

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac PUSH_ARGS_REVERSED
A C expression.  If nonzero, function arguments will be evaluated from
last to first, rather than from first to last.  If this macro is not
defined, it defaults to @code{PUSH_ARGS} on targets where the stack
and args grow in opposite directions, and 0 otherwise.
@end defmac

@defmac 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
@end defmac

@findex current_function_outgoing_args_size
@defmac 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.
@end defmac

@defmac 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.
@end defmac
@c above is overfull.  not sure what to do.  --mew 5feb93  did
@c something, not sure if it looks good.  --mew 10feb93

@defmac MAYBE_REG_PARM_STACK_SPACE
@defmacx 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}.
@end defmac

@defmac 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}.
@end defmac

@defmac 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.
@end defmac

@defmac 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.
@end defmac

@defmac 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 defmac

@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.

@defmac 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
@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 pass