OS/2 Professional/OS2PRO194.ISO/os2/prgramer/unix/emx/gnu/doc/tm.tex

Text File | 1992-10-23 | 238.4 KB | 5,655 lines

@c Copyright (C) 1988, 1989, 1992 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.

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

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

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

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

@table @code
@findex SWITCH_TAKES_ARG
@item SWITCH_TAKES_ARG (@var{char})
A C expression which determines whether the option @samp{-@var{char}}
takes arguments.  The value should be the number of arguments that
option takes--zero, for many options.

By default, this macro is defined to handle the standard options
properly.  You need not define it unless you wish to add additional
options which take arguments.

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

By default, this macro is defined to handle the standard options
properly.  You need not define it unless you wish to add additional
options which take arguments.

@findex SWITCHES_NEED_SPACES
@item SWITCHES_NEED_SPACES
A string-valued C expression which is nonempty if the linker needs a
space between the @samp{-L} or @samp{-o} option and its argument.

If this macro is not defined, the default value is 0.

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

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

@findex SIGNED_CHAR_SPEC
@item SIGNED_CHAR_SPEC
A C string constant that tells the GNU CC driver program options to
pass to CPP.  By default, this macro is defined to pass the option
@samp{-D__CHAR_UNSIGNED__} to CPP if @code{char} will be treated as
@code{unsigned char} by @code{cc1}.

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

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

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

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

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

@findex ASM_SPEC
@item ASM_SPEC
A C string constant that tells the GNU CC driver program options to
pass to the assembler.  It can also specify how to translate options
you give to GNU CC into options for GNU CC to pass to the assembler.
See the file @file{sun3.h} for an example of this.

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

@findex ASM_FINAL_SPEC
@item ASM_FINAL_SPEC
A C string constant that tells the GNU CC driver program how to
run any programs which cleanup after the normal assembler.
Normally, this is not needed.  See the file @file{mips.h} for
an example of this.

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

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

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

@findex LIB_SPEC
@item LIB_SPEC
Another C string constant used much like @code{LINK_SPEC}.  The difference
between the two is that @code{LIB_SPEC} is used at the end of the
command given to the linker.

If this macro is not defined, a default is provided that
loads the standard C library from the usual place.  See @file{gcc.c}.

@findex STARTFILE_SPEC
@item STARTFILE_SPEC
Another C string constant used much like @code{LINK_SPEC}.  The
difference between the two is that @code{STARTFILE_SPEC} is used at
the very beginning of the command given to the linker.

If this macro is not defined, a default is provided that loads the
standard C startup file from the usual place.  See @file{gcc.c}.

@findex ENDFILE_SPEC
@item ENDFILE_SPEC
Another C string constant used much like @code{LINK_SPEC}.  The
difference between the two is that @code{ENDFILE_SPEC} is used at
the very end of the command given to the linker.

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

@findex LINK_LIBGCC_SPECIAL
@item LINK_LIBGCC_SPECIAL
Define this macro meaning that @code{gcc} should find the
library @file{libgcc.a} by hand, rather than passing the argument
@samp{-lgcc} to tell the linker to do the search.

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

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

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

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

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

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

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

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

@findex SYSTEM_INCLUDE_DIR
@item SYSTEM_INCLUDE_DIR
Define this macro as a C string constant if you wish to specify a
system-specific directory to search for header files before the standard
directory.  @code{SYSTEM_INCLUDE_DIR} comes before
@code{STANDARD_INCLUDE_DIR} in the search order.

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

@findex STANDARD_INCLUDE_DIR
@item STANDARD_INCLUDE_DIR
Define this macro as a C string constant if you wish to override the
standard choice of @file{/usr/include} as the default prefix to
try when searching for header files.

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

@findex INCLUDE_DEFAULTS
@item INCLUDE_DEFAULTS
Define this macro if you wish to override the entire default search path
for include files.  The default search path includes
@code{GPLUSPLUS_INCLUDE_DIR}, @code{GCC_INCLUDE_DIR},
@code{LOCAL_INCLUDE_DIR}, @code{SYSTEM_INCLUDE_DIR}, and
@code{STANDARD_INCLUDE_DIR}.  In addition, the macros
@code{GPLUSPLUS_INCLUDE_DIR} and @code{GCC_INCLUDE_DIR} are defined
automatically by @file{Makefile}, and specify private search areas for
GCC.  The directory @code{GPLUSPLUS_INCLUDE_DIR} is used only for C++
programs.

The definition should be an initializer for an array of structures.
Each array element should have two elements: the directory name (a
string constant) and a flag for C++-only directories.  Mark the end of
the array with a null element.  For example, here is the definition used
for VMS:

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

Here is the order of prefixes tried for exec files:

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

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

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

@item
The macro @code{STANDARD_EXEC_PREFIX}.

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

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

Here is the order of prefixes tried for startfiles:

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

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

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

@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

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

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

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

@example
"-Dmc68000 -Dsun -Dunix"
@end example

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

@findex STDC_VALUE
@item STDC_VALUE
Define the value to be assigned to the built-in macro @code{__STDC__}.
The default is the value @samp{1}.

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

@cindex optional hardware or system features
@cindex features, optional, in system conventions
@item TARGET_@dots{}
This series of macros is to allow compiler command arguments to
enable or disable the use of optional features of the target machine.
For example, one machine description serves both the 68000 and
the 68020; a command argument tells the compiler whether it should
use 68020-only instructions or not.  This command argument works
by means of a macro @code{TARGET_68020} that tests a bit in
@code{target_flags}.

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

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

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

@findex TARGET_SWITCHES
@item TARGET_SWITCHES
This macro defines names of command options to set and clear
bits in @code{target_flags}.  Its definition is an initializer
with a subgrouping for each command option.

Each subgrouping contains a string constant, that defines the option
name, and a number, which contains the bits to set in
@code{target_flags}.  A negative number says to clear bits instead;
the negative of the number is which bits to clear.  The actual option
name is made by appending @samp{-m} to the specified name.

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

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

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

@findex TARGET_OPTIONS
@item TARGET_OPTIONS
This macro is similar to @code{TARGET_SWITCHES} but defines names of command
options that have values.  Its definition is an initializer with a
subgrouping for each command option.

Each subgrouping contains a string constant, that defines the fixed part
of the option name, and the address of a variable.  The variable, type
@code{char *}, is set to the variable part of the given option if the fixed
part matches.  The actual option name is made by appending @samp{-m} to the
specified name.

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

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

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

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

@findex OVERRIDE_OPTIONS
@item OVERRIDE_OPTIONS
Sometimes certain combinations of command options do not make sense on
a particular target machine.  You can define a macro
@code{OVERRIDE_OPTIONS} to take account of this.  This macro, if
defined, is executed once just after all the command options have been
parsed.

Don't use this macro to turn on various extra optimizations for
@samp{-O}.  That is what @code{OPTIMIZATION_OPTIONS} is for.

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

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

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

@node Storage Layout
@section Storage Layout
@cindex storage layout

Note that the definitions of the macros in this table which are sizes or
alignments measured in bits do not need to be constant.  They can be C
expressions that refer to static variables, such as the @code{target_flags}.
@xref{Run-time Target}.

@table @code
@findex BITS_BIG_ENDIAN
@item BITS_BIG_ENDIAN
Define this macro to be the value 1 if the most significant bit in a
byte has the lowest number; otherwise define it to be the value zero.
This means that bit-field instructions count from the most significant
bit.  If the machine has no bit-field instructions, this macro is
irrelevant.

This macro does not affect the way structure fields are packed into
bytes or words; that is controlled by @code{BYTES_BIG_ENDIAN}.

@findex BYTES_BIG_ENDIAN
@item BYTES_BIG_ENDIAN
Define this macro to be 1 if the most significant byte in a word has the
lowest number.

@findex WORDS_BIG_ENDIAN
@item WORDS_BIG_ENDIAN
Define this macro to be 1 if, in a multiword object, the most
significant word has the lowest number.  This applies to both memory
locations and registers; GNU CC fundamentally assumes that the order of
words in memory is the same as the order in registers.

@findex BITS_PER_UNIT
@item BITS_PER_UNIT
Number of bits in an addressable storage unit (byte); normally 8.

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

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

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

@findex POINTER_SIZE
@item POINTER_SIZE
Width of a pointer, in bits.

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

On most RISC machines, which only have operations that operate on a full
register, define this macro to set @var{m} to @code{word_mode} if
@var{m} is an integer mode narrower than @code{BITS_PER_WORD}.  In most
cases, only integer modes should be widened because wider-precision
floating-point operations are usually more expensive than their narrower
counterparts.

For most machines, the macro definition does not change @var{unsignedp}.
However, some machines, have instructions that preferentially handle
either signed or unsigned quanities of certain modes.  For example, on
the DEC Alpha, 32-bit loads from memory and 32-bit add instructions
sign-extend the result to 64 bits.  On such machines, set
@var{unsignedp} according to which kind of extension is more efficient.

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

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

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

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

@findex PARM_BOUNDARY
@item PARM_BOUNDARY
Normal alignment required for function parameters on the stack, in
bits.  All stack parameters receive at least this much alignment
regardless of data type.  On most machines, this is the same as the
size of an integer.

@findex STACK_BOUNDARY
@item STACK_BOUNDARY
Define this macro if you wish to preserve a certain alignment for
the stack pointer.  The definition is a C expression
for the desired alignment (measured in bits).

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

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

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

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

@findex MAX_OFILE_ALIGNMENT
@item MAX_OFILE_ALIGNMENT
Biggest alignment supported by the object file format of this machine.
Use this macro to limit the alignment which can be specified using the
@code{__attribute__ ((aligned (@var{n})))} construct.  If not defined,
the default value is @code{BIGGEST_ALIGNMENT}.

@findex DATA_ALIGNMENT
@item DATA_ALIGNMENT (@var{type}, @var{basic-align})
If defined, a C expression to compute the alignment for a static
variable.  @var{type} is the data type, and @var{basic-align} is the
alignment that the object would ordinarily have.  The value of this
macro is used instead of that alignment to align the object.

If this macro is not defined, then @var{basic-align} is used.

@findex strcpy
One use of this macro is to increase alignment of medium-size data to
make it all fit in fewer cache lines.  Another is to cause character
arrays to be word-aligned so that @code{strcpy} calls that copy
constants to character arrays can be done inline.

@findex CONSTANT_ALIGNMENT
@item CONSTANT_ALIGNMENT (@var{constant}, @var{basic-align})
If defined, a C expression to compute the alignment given to a constant
that is being placed in memory.  @var{constant} is the constant and
@var{basic-align} is the alignment that the object would ordinarily
have.  The value of this macro is used instead of that alignment to
align the object.

If this macro is not defined, then @var{basic-align} is used.

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

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

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

@findex STRUCTURE_SIZE_BOUNDARY
@item STRUCTURE_SIZE_BOUNDARY
Number of bits which any structure or union's size must be a multiple of.
Each structure or union's size is rounded up to a multiple of this.

If you do not define this macro, the default is the same as
@code{BITS_PER_UNIT}.

@findex STRICT_ALIGNMENT
@item STRICT_ALIGNMENT
Define this macro to be the value 1 if instructions will fail to work
if given data not on the nominal alignment.  If instructions will merely
go slower in that case, define this macro as 0.

@findex PCC_BITFIELD_TYPE_MATTERS
@item PCC_BITFIELD_TYPE_MATTERS
Define this if you wish to imitate the way many other C compilers handle
alignment of bitfields and the structures that contain them.

The behavior is that the type written for a bitfield (@code{int},
@code{short}, or other integer type) imposes an alignment for the
entire structure, as if the structure really did contain an ordinary
field of that type.  In addition, the bitfield is placed within the
structure so that it would fit within such a field, not crossing a
boundary for it.

Thus, on most machines, a bitfield whose type is written as @code{int}
would not cross a four-byte boundary, and would force four-byte
alignment for the whole structure.  (The alignment used may not be four
bytes; it is controlled by the other alignment parameters.)

If the macro is defined, its definition should be a C expression;
a nonzero value for the expression enables this behavior.

Note that if this macro is not defined, or its value is zero, some
bitfields may cross more than one alignment boundary.  The compiler can
support such references if there are @samp{insv}, @samp{extv}, and
@samp{extzv} insns that can directly reference memory.

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

Unless the machine has bitfield instructions or you define
@code{STRUCTURE_SIZE_BOUNDARY} that way, you must define
@code{PCC_BITFIELD_TYPE_MATTERS} to have a nonzero value.

If your aim is to make GNU CC use the same conventions for laying out
bitfields as are used by another compiler, here is how to investigate
what the other compiler does.  Compile and run this program:

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

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

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

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

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

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

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

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

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

@findex MAX_FIXED_MODE_SIZE
@item MAX_FIXED_MODE_SIZE
An integer expression for the size in bits of the largest integer
machine mode that should actually be used.  All integer machine modes of
this size or smaller can be used for structures and unions with the
appropriate sizes.  If this macro is undefined, @code{GET_MODE_BITSIZE
(DImode)} is assumed.

@findex CHECK_FLOAT_VALUE
@item CHECK_FLOAT_VALUE (@var{mode}, @var{value})
A C statement to validate the value @var{value} (of type
@code{double}) for mode @var{mode}.  This means that you check whether
@var{value} fits within the possible range of values for mode
@var{mode} on this target machine.  The mode @var{mode} is always
@code{SFmode} or @code{DFmode}.

@findex error
If @var{value} is not valid, you should call @code{error} to print an
error message and then assign some valid value to @var{value}.
Allowing an invalid value to go through the compiler can produce
incorrect assembler code which may even cause Unix assemblers to
crash.

This macro need not be defined if there is no work for it to do.

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

@table @code
@findex IEEE_FLOAT_FORMAT
@item IEEE_FLOAT_FORMAT
This code indicates IEEE floating point.  It is the default; there is no
need to define this macro when the format is IEEE.

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

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

The value of this macro is compared with @code{HOST_FLOAT_FORMAT}
(@pxref{Config}) to determine whether the target machine has the same
format as the host machine.  If any other formats are actually in use on
supported machines, new codes should be defined for them.
@end table

@node Type Layout
@section Layout of Source Language Data Types

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

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

@findex SHORT_TYPE_SIZE
@item SHORT_TYPE_SIZE
A C expression for the size in bits of the type @code{short} on the
target machine.  If you don't define this, the default is half a word.
(If this would be less than one storage unit, it is rounded up to one
unit.)

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

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

@findex CHAR_TYPE_SIZE
@item CHAR_TYPE_SIZE
A C expression for the size in bits of the type @code{char} on the
target machine.  If you don't define this, the default is one quarter
of a word.  (If this would be less than one storage unit, it is rounded up
to one unit.)

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

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

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

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

@findex DEFAULT_SHORT_ENUMS
@item DEFAULT_SHORT_ENUMS
A C expression to determine whether to give an @code{enum} type
only as many bytes as it takes to represent the range of possible values
of that type.  A nonzero value means to do that; a zero value means all
@code{enum} types should be allocated like @code{int}.

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

@findex SIZE_TYPE
@item SIZE_TYPE
A C expression for a string describing the name of the data type to use
for size values.  The typedef name @code{size_t} is defined using the
contents of the string.

The string can contain more than one keyword.  If so, separate them with
spaces, and write first any length keyword, then @code{unsigned} if
appropriate, and finally @code{int}.  The string must exactly match one
of the data type names defined in the function
@code{init_decl_processing} in the file @file{c-decl.c}.  You may not
omit @code{int} or change the order---that would cause the compiler to
crash on startup.

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

@findex PTRDIFF_TYPE
@item PTRDIFF_TYPE
A C expression for a string describing the name of the data type to use
for the result of subtracting two pointers.  The typedef name
@code{ptrdiff_t} is defined using the contents of the string.  See
@code{SIZE_TYPE} above for more information.

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

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

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

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

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

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

@findex OBJC_SELECTORS_WITHOUT_LABELS
@item OBJC_SELECTORS_WITHOUT_LABELS
Define this macro if the compiler can group all the selectors together
into a vector and use just one label at the beginning of the vector.
Otherwise, the compiler must give each selector its own assembler
label.

On certain machines, it is important to have a separate label for each
selector because this enables the linker to eliminate duplicate selectors.

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

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

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

@node Registers
@section Register Usage
@cindex register usage

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

The description of which registers a specific instruction can use is
done with register classes; see @ref{Register Classes}.  For information
on using registers to access a stack frame, see @ref{Frame Registers}.
For passing values in registers, see @ref{Register Arguments}.
For returning values in registers, see @ref{Scalar Return}.

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

@node Register Basics
@subsection Basic Characteristics of Registers

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

@item FIXED_REGISTERS
@findex FIXED_REGISTERS
@cindex fixed register
An initializer that says which registers are used for fixed purposes
all throughout the compiled code and are therefore not available for
general allocation.  These would include the stack pointer, the frame
pointer (except on machines where that can be used as a general
register when no frame pointer is needed), the program counter on
machines where that is considered one of the addressable registers,
and any other numbered register with a standard use.

This information is expressed as a sequence of numbers, separated by
commas and surrounded by braces.  The @var{n}th number is 1 if
register @var{n} is fixed, 0 otherwise.

The table initialized from this macro, and the table initialized by
the following one, may be overridden at run time either automatically,
by the actions of the macro @code{CONDITIONAL_REGISTER_USAGE}, or by
the user with the command options @samp{-ffixed-@var{reg}},
@samp{-fcall-used-@var{reg}} and @samp{-fcall-saved-@var{reg}}.

@findex CALL_USED_REGISTERS
@item CALL_USED_REGISTERS
@cindex call-used register
@cindex call-clobbered register
@cindex call-saved register
Like @code{FIXED_REGISTERS} but has 1 for each register that is
clobbered (in general) by function calls as well as for fixed
registers.  This macro therefore identifies the registers that are not
available for general allocation of values that must live across
function calls.

If a register has 0 in @code{CALL_USED_REGISTERS}, the compiler
automatically saves it on function entry and restores it on function
exit, if the register is used within the function.

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

This is necessary in case the fixed or call-clobbered registers depend
on target flags.

You need not define this macro if it has no work to do.

@cindex disabling certain registers
@cindex controlling register usage
If the usage of an entire class of registers depends on the target
flags, you may indicate this to GCC by using this macro to modify
@code{fixed_regs} and @code{call_used_regs} to 1 for each of the
registers in the classes which should not be used by GCC.  Also define
the macro @code{REG_CLASS_FROM_LETTER} to return @code{NO_REGS} if it
is called with a letter for a class that shouldn't be used.

(However, if this class is not included in @code{GENERAL_REGS} and all
of the insn patterns whose constraints permit this class are
controlled by target switches, then GCC will automatically avoid using
these registers when the target switches are opposed to them.)

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

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

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

@table @code
@findex REG_ALLOC_ORDER
@item REG_ALLOC_ORDER
If defined, an initializer for a vector of integers, containing the
numbers of hard registers in the order in which GNU CC should prefer
to use them (from most preferred to least).

If this macro is not defined, registers are used lowest numbered first
(all else being equal).

One use of this macro is on machines where the highest numbered
registers must always be saved and the save-multiple-registers
instruction supports only sequences of consecutive registers.  On such
machines, define @code{REG_ALLOC_ORDER} to be an initializer that lists
the highest numbered allocatable register first.

@findex ORDER_REGS_FOR_LOCAL_ALLOC
@item ORDER_REGS_FOR_LOCAL_ALLOC
A C statement (sans semicolon) to choose the order in which to allocate
hard registers for pseudo-registers local to a basic block.

Store the desired order of registers in the array
@code{reg_alloc_order}.  Element 0 should be the register to allocate
first; element 1, the next register; and so on.

The macro body should not assume anything about the contents of
@code{reg_alloc_order} before execution of the macro.

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

@node Values in Registers
@subsection How Values Fit in Registers

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

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

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

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

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

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

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

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

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

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

Since the same instruction used to move @code{SImode} will work for all
narrower integer modes, it is not necessary on any machine for
@code{HARD_REGNO_MODE_OK} to distinguish between these modes, provided
you define patterns @samp{movhi}, etc., to take advantage of this.  This
is useful because of the interaction between @code{HARD_REGNO_MODE_OK}
and @code{MODES_TIEABLE_P}; it is very desirable for all integer modes
to be tieable.

Many machines have special registers for floating point arithmetic.
Often people assume that floating point machine modes are allowed only
in floating point registers.  This is not true.  Any registers that
can hold integers can safely @emph{hold} a floating point machine
mode, whether or not floating arithmetic can be done on it in those
registers.  Integer move instructions can be used to move the values.

On some machines, though, the converse is true: fixed-point machine
modes may not go in floating registers.  This is true if the floating
registers normalize any value stored in them, because storing a
non-floating value there would garble it.  In this case,
@code{HARD_REGNO_MODE_OK} should reject fixed-point machine modes in
floating registers.  But if the floating registers do not automatically
normalize, if you can store any bit pattern in one and retrieve it
unchanged without a trap, then any machine mode may go in a floating
register, so you can define this macro to say so.

On some machines, such as the Sparc and the Mips, we get better code
by defining @code{HARD_REGNO_MODE_OK} to forbid integers in floating
registers, even though the hardware is capable of handling them.  This
is because transferring values between floating registers and general
registers is so slow that it is better to keep the integer in memory.

The primary significance of special floating registers is rather that
they are the registers acceptable in floating point arithmetic
instructions.  However, this is of no concern to
@code{HARD_REGNO_MODE_OK}.  You handle it by writing the proper
constraints for those instructions.

On some machines, the floating registers are especially slow to access,
so that it is better to store a value in a stack frame than in such a
register if floating point arithmetic is not being done.  As long as the
floating registers are not in class @code{GENERAL_REGS}, they will not
be used unless some pattern's constraint asks for one.

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

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

@node Leaf Functions
@subsection Handling Leaf Functions

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

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

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

@table @code
@findex LEAF_REGISTERS
@item LEAF_REGISTERS
A C initializer for a vector, indexed by hard register number, which
contains 1 for a register that is allowable in a candidate for leaf
function treatment.

If leaf function treatment involves renumbering the registers, then the
registers marked here should be the ones before renumbering---those that
GNU CC would ordinarily allocate.  The registers which will actually be
used in the assembler code, after renumbering, should not be marked with 1
in this vector.

Define this macro only if the target machine offers a way to optimize
the treatment of leaf functions.

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

If @var{regno} is a register number which should not appear in a leaf
function before renumbering, then the expression should yield -1, which
will cause the compiler to abort.

Define this macro only if the target machine offers a way to optimize the
treatment of leaf functions, and registers need to be renumbered to do
this.

@findex REG_LEAF_ALLOC_ORDER
@item REG_LEAF_ALLOC_ORDER
If defined, an initializer for a vector of integers, containing the
numbers of hard registers in the order in which the GNU CC should prefer
to use them (from most preferred to least) in a leaf function.  If this
macro is not defined, REG_ALLOC_ORDER is used for both non-leaf and
leaf-functions.
@end table

@findex leaf_function
Normally, it is necessary for @code{FUNCTION_PROLOGUE} and
@code{FUNCTION_EPILOGUE} to treat leaf functions specially.  It can test
the C variable @code{leaf_function} which is nonzero for leaf functions.
(The variable @code{leaf_function} is defined only if
@code{LEAF_REGISTERS} is defined.)

@node Stack Registers
@subsection Registers That Form a Stack

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

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

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

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

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

@node Obsolete Register Macros
@subsection Obsolete Macros for Controlling Register Usage

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

@table @code
@findex OVERLAPPING_REGNO_P
@item OVERLAPPING_REGNO_P (@var{regno})
If defined, this is a C expression whose value is nonzero if hard
register number @var{regno} is an overlapping register.  This means a
hard register which overlaps a hard register with a different number.
(Such overlap is undesirable, but occasionally it allows a machine to
be supported which otherwise could not be.)  This macro must return
nonzero for @emph{all} the registers which overlap each other.  GNU CC
can use an overlapping register only in certain limited ways.  It can
be used for allocation within a basic block, and may be spilled for
reloading; that is all.

If this macro is not defined, it means that none of the hard registers
overlap each other.  This is the usual situation.

@findex INSN_CLOBBERS_REGNO_P
@item INSN_CLOBBERS_REGNO_P (@var{insn}, @var{regno})
If defined, this is a C expression whose value should be nonzero if
the insn @var{insn} has the effect of mysteriously clobbering the
contents of hard register number @var{regno}.  By ``mysterious'' we
mean that the insn's RTL expression doesn't describe such an effect.

If this macro is not defined, it means that no insn clobbers registers
mysteriously.  This is the usual situation; all else being equal,
it is best for the RTL expression to show all the activity.

@cindex death notes
@findex PRESERVE_DEATH_INFO_REGNO_P
@item PRESERVE_DEATH_INFO_REGNO_P (@var{regno})
If defined, this is a C expression whose value is nonzero if accurate
@code{REG_DEAD} notes are needed for hard register number @var{regno}
at the time of outputting the assembler code.  When this is so, a few
optimizations that take place after register allocation and could
invalidate the death notes are not done when this register is
involved.

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

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

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

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

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

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

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

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

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

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

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

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

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

@table @code
@findex enum reg_class
@item enum reg_class
An enumeral type that must be defined with all the register class names
as enumeral values.  @code{NO_REGS} must be first.  @code{ALL_REGS}
must be the last register class, followed by one more enumeral value,
@code{LIM_REG_CLASSES}, which is not a register class but rather
tells how many classes there are.

Each register class has a number, which is the value of casting
the class name to type @code{int}.  The number serves as an index
in many of the tables described below.

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

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

@findex REG_CLASS_NAMES
@item REG_CLASS_NAMES
An initializer containing the names of the register classes as C string
constants.  These names are used in writing some of the debugging dumps.

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

When the machine has more than 32 registers, an integer does not suffice.
Then the integers are replaced by sub-initializers, braced groupings containing
several integers.  Each sub-initializer must be suitable as an initializer
for the type @code{HARD_REG_SET} which is defined in @file{hard-reg-set.h}.

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

@findex BASE_REG_CLASS
@item BASE_REG_CLASS
A macro whose definition is the name of the class to which a valid
base register must belong.  A base register is one used in an address
which is the register value plus a displacement.

@findex INDEX_REG_CLASS
@item INDEX_REG_CLASS
A macro whose definition is the name of the class to which a valid
index register must belong.  An index register is one used in an
address where its value is either multiplied by a scale factor or
added to another register (as well as added to a displacement).

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

@findex REGNO_OK_FOR_BASE_P
@item REGNO_OK_FOR_BASE_P (@var{num})
A C expression which is nonzero if register number @var{num} is
suitable for use as a base register in operand addresses.  It may be
either a suitable hard register or a pseudo register that has been
allocated such a hard register.

@findex REGNO_OK_FOR_INDEX_P
@item REGNO_OK_FOR_INDEX_P (@var{num})
A C expression which is nonzero if register number @var{num} is
suitable for use as an index register in operand addresses.  It may be
either a suitable hard register or a pseudo register that has been
allocated such a hard register.

The difference between an index register and a base register is that
the index register may be scaled.  If an address involves the sum of
two registers, neither one of them scaled, then either one may be
labeled the ``base'' and the other the ``index''; but whichever
labeling is used must fit the machine's constraints of which registers
may serve in each capacity.  The compiler will try both labelings,
looking for one that is valid, and will reload one or both registers
only if neither labeling works.

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

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

@noindent
is safe.

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

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

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

@findex LIMIT_RELOAD_CLASS
@item LIMIT_RELOAD_CLASS (@var{mode}, @var{class})
A C expression that places additional restrictions on the register class
to use when it is necessary to be able to hold a value of mode
@var{mode} in a reload register for which class @var{class} would
ordinarily be used.

Unlike @code{PREFERRED_RELOAD_CLASS}, this macro should be used when
there are certain modes that simply can't go in certain reload classes.

The value is a register class; perhaps @var{class}, or perhaps another,
smaller class.

Don't define this macro unless the target machine has limitations which
require the macro to do something nontrivial.

@findex SECONDARY_RELOAD_CLASS
@findex SECONDARY_INPUT_RELOAD_CLASS
@findex SECONDARY_OUTPUT_RELOAD_CLASS
@item SECONDARY_RELOAD_CLASS (@var{class}, @var{mode}, @var{x})
@itemx SECONDARY_INPUT_RELOAD_CLASS (@var{class}, @var{mode}, @var{x})
@itemx SECONDARY_OUTPUT_RELOAD_CLASS (@var{class}, @var{mode}, @var{x})
Many machines have some registers that cannot be copied directly to or
from memory or even from other types of registers.  An example is the
@samp{MQ} register, which on most machines, can only be copied to or
from general registers, but not memory.  Some machines allow copying all
registers to and from memory, but require a scratch register for stores
to some memory locations (e.g., those with symbolic address on the RT,
and those with certain symbolic address on the Sparc when compiling
PIC).  In some cases, both an intermediate and a scratch register are
required.

You should define these macros to indicate to the reload phase that it may
need to allocate at least one register for a reload in addition to the
register to contain the data.  Specifically, if copying @var{x} to a
register @var{class} in @var{mode} requires an intermediate register,
you should define @code{SECONDARY_INPUT_RELOAD_CLASS} to return the
largest register class all of whose registers can be used as
intermediate registers or scratch registers.

If copying a register @var{class} in @var{mode} to @var{x} requires an
intermediate or scratch register, you should define
@code{SECONDARY_OUTPUT_RELOAD_CLASS} to return the largest register
class required.  If the requirements for input and output reloads are
the same, the macro @code{SECONDARY_RELOAD_CLASS} should be used instead
of defining both macros identically.

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

If a scratch register is required (either with or without an
intermediate register), you should define patterns for
@samp{reload_in@var{m}} or @samp{reload_out@var{m}}, as required
(@pxref{Standard Names}.  These patterns, which will normally be
implemented with a @code{define_expand}, should be similar to the
@samp{mov@var{m}} patterns, except that operand 2 is the scratch
register.

Define constraints for the reload register and scratch register that
contain a single register class.  If the original reload register (whose
class is @var{class}) can meet the constraint given in the pattern, the
value returned by these macros is used for the class of the scratch
register.  Otherwise, two additional reload registers are required.
Their classes are obtained from the constraints in the insn pattern.

@var{x} might be a pseudo-register or a @code{subreg} of a
pseudo-register, which could either be in a hard register or in memory.
Use @code{true_regnum} to find out; it will return -1 if the pseudo is
in memory and the hard register number if it is in a register.

These macros should not be used in the case where a particular class of
registers can only be copied to memory and not to another class of
registers.  In that case, secondary reload registers are not needed and
would not be helpful.  Instead, a stack location must be used to perform
the copy and the @code{mov@var{m}} pattern should use memory as a
intermediate storage.  This case often occurs between floating-point and
general registers.

@findex SECONDARY_MEMORY_NEEDED
@item SECONDARY_MEMORY_NEEDED (@var{class1}, @var{class2}, @var{m})
Certain machines have the property that some registers cannot be copied
to some other registers without using memory.  Define this macro on
those machines to be a C expression that is non-zero if objects of mode
@var{m} in registers of @var{class1} can only be copied to registers of
class @var{class2} by storing a register of @var{class1} into memory
and loading that memory location into a register of @var{class2}.

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

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

On those machines, you should define @code{SMALL_REGISTER_CLASSES}.
When it is defined, the compiler allows registers explicitly used in the
rtl to be used as spill registers but prevents the compiler from
extending the lifetime of these registers.

Defining this macro is always safe, but unnecessarily defining this macro
will reduce the amount of optimizations that can be performed in some
cases.  If this macro is not defined but needs to be, the compiler will
run out of reload registers and print a fatal error message.

For most machines, this macro should not be defined.

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

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

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

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

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

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

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

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

@findex EXTRA_CONSTRAINT
@item EXTRA_CONSTRAINT (@var{value}, @var{c})
A C expression that defines the optional machine-dependent constraint
letters that can be used to segregate specific types of operands,
usually memory references, for the target machine.  Normally this macro
will not be defined.  If it is required for a particular target machine,
it should return 1 if @var{value} corresponds to the operand type
represented by the constraint letter @var{c}.  If @var{c} is not defined
as an extra constraint, the value returned should be 0 regardless of
@var{value}.

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

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

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

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

@table @code
@findex STACK_GROWS_DOWNWARD
@item STACK_GROWS_DOWNWARD
Define this macro if pushing a word onto the stack moves the stack
pointer to a smaller address.

When we say, ``define this macro if @dots{},'' it means that the
compiler checks this macro only with @code{#ifdef} so the precise
definition used does not matter.

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

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

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

If @code{FRAME_GROWS_DOWNWARD}, the next slot's offset is found by
subtracting the length of the first slot from @code{STARTING_FRAME_OFFSET}.
Otherwise, it is found by adding the length of the first slot to
the value @code{STARTING_FRAME_OFFSET}.

@findex STACK_POINTER_OFFSET
@item STACK_POINTER_OFFSET
Offset from the stack pointer register to the first location at which
outgoing arguments are placed.  If not specified, the default value of
zero is used.  This is the proper value for most machines.

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

@findex FIRST_PARM_OFFSET
@item FIRST_PARM_OFFSET (@var{fundecl})
Offset from the argument pointer register to the first argument's
address.  On some machines it may depend on the data type of the
function.

If @code{ARGS_GROW_DOWNWARD}, this is the offset to the location above
the first argument's address.

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

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

@findex DYNAMIC_CHAIN_ADDRESS
@item DYNAMIC_CHAIN_ADDRESS (@var{frameaddr})
A C expression whose value is RTL representing the address in a stack
frame where the pointer to the caller's frame is stored.  Assume that
@var{frameaddr} is an RTL expression for the address of the stack frame
itself.

If you don't define this macro, the default is to return the value
of @var{frameaddr}---that is, the stack frame address is also the
address of the stack word that points to the previous frame.
@end table

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

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

@findex FRAME_POINTER_REGNUM
@item FRAME_POINTER_REGNUM
The register number of the frame pointer register, which is used to
access automatic variables in the stack frame.  On some machines, the
hardware determines which register this is.  On other machines, you can
choose any register you wish for this purpose.

@findex ARG_POINTER_REGNUM
@item ARG_POINTER_REGNUM
The register number of the arg pointer register, which is used to access
the function's argument list.  On some machines, this is the same as the
frame pointer register.  On some machines, the hardware determines which
register this is.  On other machines, you can choose any register you
wish for this purpose.  If this is not the same register as the frame
pointer register, then you must mark it as a fixed register according to
@code{FIXED_REGISTERS}, or arrange to be able to eliminate it
(@pxref{Elimination}).

@findex STATIC_CHAIN_REGNUM
@findex STATIC_CHAIN_INCOMING_REGNUM
@item STATIC_CHAIN_REGNUM
@itemx STATIC_CHAIN_INCOMING_REGNUM
Register numbers used for passing a function's static chain
pointer.  If register windows are used, @code{STATIC_CHAIN_INCOMING_REGNUM}
is the register number as seen by the called function, while
@code{STATIC_CHAIN_REGNUM} is the register number as seen by the calling
function.  If these registers are the same,
@code{STATIC_CHAIN_INCOMING_REGNUM} need not be defined.@refill

The static chain register need not be a fixed register.

If the static chain is passed in memory, these macros should not be
defined; instead, the next two macros should be defined.

@findex STATIC_CHAIN
@findex STATIC_CHAIN_INCOMING
@item STATIC_CHAIN
@itemx STATIC_CHAIN_INCOMING
If the static chain is passed in memory, these macros provide rtx giving
@code{mem} expressions that denote where they are stored.
@code{STATIC_CHAIN} and @code{STATIC_CHAIN_INCOMING} give the locations
as seen by the calling and called functions, respectively.  Often the former
will be at an offset from the stack pointer and the latter at an offset from
the frame pointer.@refill

@findex stack_pointer_rtx
@findex frame_pointer_rtx
@findex arg_pointer_rtx
The variables @code{stack_pointer_rtx}, @code{frame_pointer_rtx}, and
@code{arg_pointer_rtx} will have been initialized prior to the use of these
macros and should be used to refer to those items.

If the static chain is passed in a register, the two previous macros should
be defined instead.
@end table

@node Elimination
@subsection Eliminating Frame Pointer and Arg Pointer

@table @code
@findex FRAME_POINTER_REQUIRED
@item FRAME_POINTER_REQUIRED
A C expression which is nonzero if a function must have and use a frame
pointer.  This expression is evaluated  in the reload pass.  If its value is
nonzero the function will have a frame pointer.

The expression can in principle examine the current function and decide
according to the facts, but on most machines the constant 0 or the
constant 1 suffices.  Use 0 when the machine allows code to be generated
with no frame pointer, and doing so saves some time or space.  Use 1
when there is no possible advantage to avoiding a frame pointer.

In certain cases, the compiler does not know how to produce valid code
without a frame pointer.  The compiler recognizes those cases and
automatically gives the function a frame pointer regardless of what
@code{FRAME_POINTER_REQUIRED} says.  You don't need to worry about
them.@refill

In a function that does not require a frame pointer, the frame pointer
register can be allocated for ordinary usage, unless you mark it as a
fixed register.  See @code{FIXED_REGISTERS} for more information.

This macro is ignored and need not be defined if @code{ELIMINABLE_REGS}
is defined.

@findex INITIAL_FRAME_POINTER_OFFSET
@findex get_frame_size
@item INITIAL_FRAME_POINTER_OFFSET (@var{depth-var})
A C statement to store in the variable @var{depth-var} the difference
between the frame pointer and the stack pointer values immediately after
the function prologue.  The value would be computed from information
such as the result of @code{get_frame_size ()} and the tables of
registers @code{regs_ever_live} and @code{call_used_regs}.

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

@findex ELIMINABLE_REGS
@item ELIMINABLE_REGS
If defined, this macro specifies a table of register pairs used to
eliminate unneeded registers that point into the stack frame.  If it is not
defined, the only elimination attempted by the compiler is to replace
references to the frame pointer with references to the stack pointer.

The definition of this macro is a list of structure initializations, each
of which specifies an original and replacement register.

On some machines, the position of the argument pointer is not known until
the compilation is completed.  In such a case, a separate hard register
must be used for the argument pointer.  This register can be eliminated by
replacing it with either the frame pointer or the argument pointer,
depending on whether or not the frame pointer has been eliminated.

In this case, you might specify:
@example
#define ELIMINABLE_REGS  \
@{@{ARG_POINTER_REGNUM, STACK_POINTER_REGNUM@}, \
 @{ARG_POINTER_REGNUM, FRAME_POINTER_REGNUM@}, \
 @{FRAME_POINTER_REGNUM, STACK_POINTER_REGNUM@}@}
@end example

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

@findex CAN_ELIMINATE
@item CAN_ELIMINATE (@var{from-reg}, @var{to-reg})
A C expression that returns non-zero if the compiler is allowed to try
to replace register number @var{from-reg} with register number
@var{to-reg}.  This macro need only be defined if @code{ELIMINABLE_REGS}
is defined, and will usually be the constant 1, since most of the cases
preventing register elimination are things that the compiler already
knows about.

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

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

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

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

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

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

If the target machine does not have a push instruction, do not define
this macro.  That directs GNU CC to use an alternate strategy: to
allocate the entire argument block and then store the arguments into
it.

On some machines, the definition

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

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

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

@findex ACCUMULATE_OUTGOING_ARGS
@findex current_function_outgoing_args_size
@item ACCUMULATE_OUTGOING_ARGS
If defined, the maximum amount of space required for outgoing arguments
will be computed and placed into the variable
@code{current_function_outgoing_args_size}.  No space will be pushed
onto the stack for each call; instead, the function prologue should
increase the stack frame size by this amount.

It is not proper to define both @code{PUSH_ROUNDING} and
@code{ACCUMULATE_OUTGOING_ARGS}.

@findex REG_PARM_STACK_SPACE
@item REG_PARM_STACK_SPACE (@var{fndecl})
Define this macro if functions should assume that stack space has been
allocated for arguments even when their values are passed in
registers.

The value of this macro is the size, in bytes, of the area reserved for
arguments passed in registers for the function represented by @var{fndecl}.

This space can either be allocated by the caller or be a part of the
machine-dependent stack frame: @code{OUTGOING_REG_PARM_STACK_SPACE}
says which.

@findex MAYBE_REG_PARM_STACK_SPACE
@findex FINAL_REG_PARM_STACK_SPACE
@item MAYBE_REG_PARM_STACK_SPACE
@item FINAL_REG_PARM_STACK_SPACE (@var{const_size}, @var{var_size})
Define these macros in addition to the one above if functions might
allocate stack space for arguments even when their values are passed
in registers.  These should be used when the stack space allocated
for arguments in registers is not a simple constant independent of the
function declaration.

The value of the first macro is the size, in bytes, of the area that
we should initially assume would be reserved for arguments passed in registers.

The value of the second macro is the actual size, in bytes, of the area
that will be reserved for arguments passed in registers.  This takes two
arguments: an integer representing the number of bytes of fixed sized
arguments on the stack, and a tree representing the number of bytes of
variable sized arguments on the stack.

When these macros are defined, @code{REG_PARM_STACK_SPACE} will only be
called for libcall functions, the current function, or for a function
being called when it is known that such stack space must be allocated.
In each case this value can be easily computed.

When deciding whether a called function needs such stack space, and how
much space to reserve, GNU CC uses these two macros instead of
@code{REG_PARM_STACK_SPACE}.

@findex OUTGOING_REG_PARM_STACK_SPACE
@item OUTGOING_REG_PARM_STACK_SPACE
Define this if it is the responsibility of the caller to allocate the area
reserved for arguments passed in registers.

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

@findex STACK_PARMS_IN_REG_PARM_AREA
@item STACK_PARMS_IN_REG_PARM_AREA
Define this macro if @code{REG_PARM_STACK_SPACE} is defined but stack
parameters don't skip the area specified by @code{REG_PARM_STACK_SPACE}.

Normally, when a parameter is not passed in registers, it is placed on the
stack beyond the @code{REG_PARM_STACK_SPACE} area.  Defining this macro
suppresses this behavior and causes the parameter to be passed on the
stack in its natural location.

@findex RETURN_POPS_ARGS
@item RETURN_POPS_ARGS (@var{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{funtype} is a C variable whose value is a tree node that
describes the function in question.  Normally it is a node of type
@code{FUNCTION_TYPE} that describes the data type of the function.
From this it is possible to obtain the data types of the value and
arguments (if known).

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

@var{stack-size} is the number of bytes of arguments passed on the
stack.  If a variable number of bytes is passed, it is zero, and
argument popping will always be the responsibility of the calling function.

On the Vax, all functions always pop their arguments, so the definition
of this macro is @var{stack-size}.  On the 68000, using the standard
calling convention, no functions pop their arguments, so the value of
the macro is always 0 in this case.  But an alternative calling
convention is available in which functions that take a fixed number of
arguments pop them but other functions (such as @code{printf}) pop
nothing (the caller pops all).  When this convention is in use,
@var{funtype} is examined to determine whether a function takes a fixed
number of arguments.
@end table

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

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

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

The arguments are @var{cum}, which summarizes all the previous
arguments; @var{mode}, the machine mode of the argument; @var{type},
the data type of the argument as a tree node or 0 if that is not known
(which happens for C support library functions); and @var{named},
which is 1 for an ordinary argument and 0 for nameless arguments that
correspond to @samp{@dots{}} in the called function's prototype.

The value of the expression should either be 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.

@cindex @file{stdarg.h} and register arguments
The usual way to make the ANSI library @file{stdarg.h} work on a machine
where some arguments are usually passed in registers, is to cause
nameless arguments to be passed on the stack instead.  This is done
by making @code{FUNCTION_ARG} return 0 whenever @var{named} is 0.

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

@findex FUNCTION_INCOMING_ARG
@item FUNCTION_INCOMING_ARG (@var{cum}, @var{mode}, @var{type}, @var{named})
Define this macro if the target machine has ``register windows'', so
that the register in which a function sees an arguments is not
necessarily the same as the one in which the caller passed the
argument.

For such machines, @code{FUNCTION_ARG} computes the register in which
the caller passes the value, and @code{FUNCTION_INCOMING_ARG} should
be defined in a similar fashion to tell the function being called
where the arguments will arrive.

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

@findex FUNCTION_ARG_PARTIAL_NREGS
@item FUNCTION_ARG_PARTIAL_NREGS (@var{cum}, @var{mode}, @var{type}, @var{named})
A C expression for the number of words, at the beginning of an
argument, must be put in registers.  The value must be zero for
arguments that are passed entirely in registers or that are entirely
pushed on the stack.

On some machines, certain arguments must be passed partially in
registers and partially in memory.  On these machines, typically the
first @var{n} words of arguments are passed in registers, and the rest
on the stack.  If a multi-word argument (a @code{double} or a
structure) crosses that boundary, its first few words must be passed
in registers and the rest must be pushed.  This macro tells the
compiler when this occurs, and how many of the words should go in
registers.

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

@findex FUNCTION_ARG_PASS_BY_REFERENCE
@item FUNCTION_ARG_PASS_BY_REFERENCE (@var{cum}, @var{mode}, @var{type}, @var{named})
A C expression that indicates when an argument must be passed by reference.
If nonzero for an argument, a copy of that argument is made in memory and a
pointer to the argument is passed instead of the argument itself.
The pointer is passed in whatever way is appropriate for passing a pointer
to that type.

On machines where @code{REG_PARM_STACK_SPACE} is not defined, a suitable
definition of this macro might be
@example
#define FUNCTION_ARG_PASS_BY_REFERENCE(CUM, MODE, TYPE, NAMED)  \
  MUST_PASS_IN_STACK (MODE, TYPE)
@end example

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

There is no need to record in @code{CUMULATIVE_ARGS} anything about the
arguments that have been passed on the stack.  The compiler has other
variables to keep track of that.  For target machines on which all
arguments are passed on the stack, there is no need to store anything in
@code{CUMULATIVE_ARGS}; however, the data structure must exist and
should not be empty, so use @code{int}.

@findex INIT_CUMULATIVE_ARGS
@item INIT_CUMULATIVE_ARGS (@var{cum}, @var{fntype}, @var{libname})
A C statement (sans semicolon) for initializing the variable @var{cum}
for the state at the beginning of the argument list.  The variable has
type @code{CUMULATIVE_ARGS}.  The value of @var{fntype} is the tree node
for the data type of the function which will receive the args, or 0
if the args are to a compiler support library function.

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

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

The argument @var{libname} exists for symmetry with
@code{INIT_CUMULATIVE_ARGS}.  The value passed for @var{libname} is
always 0, since library routines with special calling conventions are
never compiled with GNU CC.

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

This macro need not do anything if the argument in question was passed
on the stack.  The compiler knows how to track the amount of stack space
used for arguments without any special help.

@findex FUNCTION_ARG_PADDING
@item FUNCTION_ARG_PADDING (@var{mode}, @var{type})
If defined, a C expression which determines whether, and in which direction,
to pad out an argument with extra space.  The value should be of type
@code{enum direction}: either @code{upward} to pad above the argument,
@code{downward} to pad below, or @code{none} to inhibit padding.

This macro does not control the @emph{amount} of padding; that is
always just enough to reach the next multiple of @code{FUNCTION_ARG_BOUNDARY}.

This macro has a default definition which is right for most systems.
For little-endian machines, the default is to pad upward.  For
big-endian machines, the default is to pad downward for an argument of
constant size shorter than an @code{int}, and upward otherwise.

@findex FUNCTION_ARG_BOUNDARY
@item FUNCTION_ARG_BOUNDARY (@var{mode}, @var{type})
If defined, a C expression that gives the alignment boundary, in bits,
of an argument with the specified mode and type.  If it is not defined,
@code{PARM_BOUNDARY} is used for all arguments.

@findex FUNCTION_ARG_REGNO_P
@item FUNCTION_ARG_REGNO_P (@var{regno})
A C expression that is nonzero if @var{regno} is the number of a hard
register in which function arguments are sometimes passed.  This does
@emph{not} include implicit arguments such as the static chain and
the structure-value address.  On many machines, no registers can be
used for this purpose since all function arguments are pushed on the
stack.
@end table

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

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

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

@findex FUNCTION_VALUE
@item FUNCTION_VALUE (@var{valtype}, @var{func})
A C expression to create an RTX representing the place where a
function returns a value of data type @var{valtype}.  @var{valtype} is
a tree node representing a data type.  Write @code{TYPE_MODE
(@var{valtype})} to get the machine mode used to represent that type.
On many machines, only the mode is relevant.  (Actually, on most
machines, scalar values are returned in the same place regardless of
mode).@refill

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

If the precise function being called is known, @var{func} is a tree
node (@code{FUNCTION_DECL}) for it; otherwise, @var{func} is a null
pointer.  This makes it possible to use a different value-returning
convention for specific functions when all their calls are
known.@refill

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

@findex FUNCTION_OUTGOING_VALUE
@item FUNCTION_OUTGOING_VALUE (@var{valtype}, @var{func})
Define this macro if the target machine has ``register windows''
so that the register in which a function returns its value is not
the same as the one in which the caller sees the value.

For such machines, @code{FUNCTION_VALUE} computes the register in
which the caller will see the value, and
@code{FUNCTION_OUTGOING_VALUE} should be defined in a similar fashion
to tell the function where to put the value.@refill

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

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

@findex LIBCALL_VALUE
@item LIBCALL_VALUE (@var{mode})
A C expression to create an RTX representing the place where a library
function returns a value of mode @var{mode}.  If the precise function
being called is known, @var{func} is a tree node
(@code{FUNCTION_DECL}) for it; otherwise, @var{func} is a null
pointer.  This makes it possible to use a different value-returning
convention for specific functions when all their calls are
known.@refill

Note that ``library function'' in this context means a compiler
support routine, used to perform arithmetic, whose name is known
specially by the compiler and was not mentioned in the C code being
compiled.

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

@findex FUNCTION_VALUE_REGNO_P
@item FUNCTION_VALUE_REGNO_P (@var{regno})
A C expression that is nonzero if @var{regno} is the number of a hard
register in which the values of called function may come back.

A register whose use for returning values is limited to serving as the
second of a pair (for a value of type @code{double}, say) need not be
recognized by this macro.  So for most machines, this definition
suffices:

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

If the machine has register windows, so that the caller and the called
function use different registers for the return value, this macro
should recognize only the caller's register numbers.
@end table

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

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

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

@table @code
@findex RETURN_IN_MEMORY
@item RETURN_IN_MEMORY (@var{type})
A C expression which can inhibit the returning of certain function
values in registers, based on the type of value.  A nonzero value says
to return the function value in memory, just as large structures are
always returned.  Here @var{type} will be a C expression of type
@code{tree}, representing the data type of the value.

Note that values of mode @code{BLKmode} are returned in memory
regardless of this macro.  Also, the option @samp{-fpcc-struct-return}
takes effect regardless of this macro.  On most systems, it is
possible to leave the macro undefined; this causes a default
definition to be used, whose value is the constant 0.

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

@findex STRUCT_VALUE
@item STRUCT_VALUE
If the structure value address is not passed in a register, define
@code{STRUCT_VALUE} as an expression returning an RTX for the place
where the address is passed.  If it returns 0, the address is passed as
an ``invisible'' first argument.

@findex STRUCT_VALUE_INCOMING_REGNUM
@item STRUCT_VALUE_INCOMING_REGNUM
On some architectures the place where the structure value address
is found by the called function is not the same place that the
caller put it.  This can be due to register windows, or it could
be because the function prologue moves it to a different place.

If the incoming location of the structure value address is in a
register, define this macro as the register number.

@findex STRUCT_VALUE_INCOMING
@item STRUCT_VALUE_INCOMING
If the incoming location is not a register, define
@code{STRUCT_VALUE_INCOMING} as an expression for an RTX for where the
called function should find the value.  If it should find the value on
the stack, define this to create a @code{mem} which refers to the frame
pointer.  A definition of 0 means that the address is passed as an
``invisible'' first argument.

@findex PCC_STATIC_STRUCT_RETURN
@item PCC_STATIC_STRUCT_RETURN
Define this macro if the usual system convention on the target machine
for returning structures and unions is for the called function to return
the address of a static variable containing the value.  GNU CC does not
normally use this convention, even if it is the usual one, but does use
it if @samp{-fpcc-struct-return} is specified.

Do not define this if the usual system convention is for the caller to
pass an address to the subroutine.
@end table

@node Caller Saves
@subsection Caller-Saves Register Allocation

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

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

@findex CALLER_SAVE_PROFITABLE
@item CALLER_SAVE_PROFITABLE (@var{refs}, @var{calls})
A C expression to determine whether it is worthwhile to consider placing
a pseudo-register in a call-clobbered hard register and saving and
restoring it around each function call.  The expression should be 1 when
this is worth doing, and 0 otherwise.

If you don't define this macro, a default is used which is good on most
machines: @code{4 * @var{calls} < @var{refs}}.
@end table

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

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

@table @code
@findex FUNCTION_PROLOGUE
@item FUNCTION_PROLOGUE (@var{file}, @var{size})
A C compound statement that outputs the assembler code for entry to a
function.  The prologue is responsible for setting up the stack frame,
initializing the frame pointer register, saving registers that must be
saved, and allocating @var{size} additional bytes of storage for the
local variables.  @var{size} is an integer.  @var{file} is a stdio
stream to which the assembler code should be output.

The label for the beginning of the function need not be output by this
macro.  That has already been done when the macro is run.

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

On machines that have ``register windows'', the function entry code does
not save on the stack the registers that are in the windows, even if
they are supposed to be preserved by function calls; instead it takes
appropriate steps to ``push'' the register stack, if any non-call-used
registers are used in the function.

@findex frame_pointer_needed
On machines where functions may or may not have frame-pointers, the
function entry code must vary accordingly; it must set up the frame
pointer if one is wanted, and not otherwise.  To determine whether a
frame pointer is in wanted, the macro can refer to the variable
@code{frame_pointer_needed}.  The variable's value will be 1 at run
time in a function that needs a frame pointer.  @xref{Elimination}.

The function entry code is responsible for allocating any stack space
required for the function.  This stack space consists of the regions
listed below.  In most cases, these regions are allocated in the
order listed, with the last listed region closest to the top of the
stack (the lowest address if @code{STACK_GROWS_DOWNWARD} is defined, and
the highest address if it is not defined).  You can use a different order
for a machine if doing so is more convenient or required for
compatibility reasons.  Except in cases where required by standard
or by a debugger, there is no reason why the stack layout used by GCC
need agree with that used by other compilers for a machine.

@itemize @bullet
@item
@findex current_function_pretend_args_size
A region of @code{current_function_pretend_args_size} bytes of
uninitialized space just underneath the first argument arriving on the
stack.  (This may not be at the very start of the allocated stack region
if the calling sequence has pushed anything else since pushing the stack
arguments.  But usually, on such machines, nothing else has been pushed
yet, because the function prologue itself does all the pushing.)  This
region is used on machines where an argument may be passed partly in
registers and partly in memory, and, in some cases to support the
features in @file{varargs.h} and @file{stdargs.h}.

@item
An area of memory used to save certain registers used by the function.
The size of this area, which may also include space for such things as
the return address and pointers to previous stack frames, is
machine-specific and usually depends on which registers have been used
in the function.  Machines with register windows often do not require
a save area.

@item
A region of at least @var{size} bytes, possibly rounded up to an allocation
boundary, to contain the local variables of the function.  On some machines,
this region and the save area may occur in the opposite order, with the
save area closer to the top of the stack.

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

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

@findex EXIT_IGNORE_STACK
@item EXIT_IGNORE_STACK
Define this macro as a C expression that is nonzero if the return
instruction or the function epilogue ignores the value of the stack
pointer; in other words, if it is safe to delete an instruction to
adjust the stack pointer before a return from the function.

Note that this macro's value is relevant only for functions for which
frame pointers are maintained.  It is never safe to delete a final
stack adjustment in a function that has no frame pointer, and the
compiler knows this regardless of @code{EXIT_IGNORE_STACK}.

@findex FUNCTION_EPILOGUE
@item FUNCTION_EPILOGUE (@var{file}, @var{size})
A C compound statement that outputs the assembler code for exit from a
function.  The epilogue is responsible for restoring the saved
registers and stack pointer to their values when the function was
called, and returning control to the caller.  This macro takes the
same arguments as the macro @code{FUNCTION_PROLOGUE}, and the
registers to restore are determined from @code{regs_ever_live} and
@code{CALL_USED_REGISTERS} in the same way.

On some machines, there is a single instruction that does all the work
of returning from the function.  On these machines, give that
instruction the name @samp{return} and do not define the macro
@code{FUNCTION_EPILOGUE} at all.

Do not define a pattern named @samp{return} if you want the
@code{FUNCTION_EPILOGUE} to be used.  If you want the target switches
to control whether return instructions or epilogues are used, define a
@samp{return} pattern with a validity condition that tests the target
switches appropriately.  If the @samp{return} pattern's validity
condition is false, epilogues will be used.

On machines where functions may or may not have frame-pointers, the
function exit code must vary accordingly.  Sometimes the code for these
two cases is completely different.  To determine whether a frame pointer
is wanted, the macro can refer to the variable
@code{frame_pointer_needed}.  The variable's value will be 1 at run time
in a function that needs a frame pointer.

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

On some machines, some functions pop their arguments on exit while
others leave that for the caller to do.  For example, the 68020 when
given @samp{-mrtd} pops arguments in functions that take a fixed
number of arguments.

@findex current_function_pops_args
Your definition of the macro @code{RETURN_POPS_ARGS} decides which
functions pop their own arguments.  @code{FUNCTION_EPILOGUE} needs to
know what was decided.  The variable @code{current_function_pops_args}
is the number of bytes of its arguments that a function should pop.
@xref{Scalar Return}.

@findex DELAY_SLOTS_FOR_EPILOGUE
@item DELAY_SLOTS_FOR_EPILOGUE
Define this macro if the function epilogue contains delay slots to which
instructions from the rest of the function can be ``moved''.  The
definition should be a C expression whose value is an integer
representing the number of delay slots there.

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

The argument @var{n} is an integer which identifies the delay slot now
being considered (since different slots may have different rules of
eligibility).  It is never negative and is always less than the number
of epilogue delay slots (what @code{DELAY_SLOTS_FOR_EPILOGUE} returns).
If you reject a particular insn for a given delay slot, in principle, it
may be reconsidered for a subsequent delay slot.  Also, other insns may
(at least in principle) be considered for the so far unfilled delay
slot.

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

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

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

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

@findex mcount
The details of how the address should be passed to @code{mcount} are
determined by your operating system environment, not by GNU CC.  To
figure them out, compile a small program for profiling using the
system's installed C compiler and look at the assembler code that
results.

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

@findex FUNCTION_BLOCK_PROFILER
@findex __bb_init_func
@item FUNCTION_BLOCK_PROFILER (@var{file}, @var{labelno})
A C statement or compound statement to output to @var{file} some
assembler code to initialize basic-block profiling for the current
object module.  This code should call the subroutine
@code{__bb_init_func} once per object module, passing it as its sole
argument the address of a block allocated in the object module.

The name of the block is a local symbol made with this statement:

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

Of course, since you are writing the definition of
@code{ASM_GENERATE_INTERNAL_LABEL} as well as that of this macro, you
can take a short cut in the definition of this macro and use the name
that you know will result.

The first word of this block is a flag which will be nonzero if the
object module has already been initialized.  So test this word first,
and do not call @code{__bb_init_func} if the flag is nonzero.

@findex BLOCK_PROFILER
@item BLOCK_PROFILER (@var{file}, @var{blockno})
A C statement or compound statement to increment the count associated
with the basic block number @var{blockno}.  Basic blocks are numbered
separately from zero within each compilation.  The count associated
with block number @var{blockno} is at index @var{blockno} in a vector
of words; the name of this array is a local symbol made with this
statement:

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

Of course, since you are writing the definition of
@code{ASM_GENERATE_INTERNAL_LABEL} as well as that of this macro, you
can take a short cut in the definition of this macro and use the name
that you know will result.
@end table

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

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

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

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

@table @code
@findex __builtin_saveregs
@item __builtin_saveregs ()
Use this built-in function to save the argument registers in memory so
that the varargs mechanism can access them.  Both ANSI and traditional
versions of @code{va_start} must use @code{__builtin_saveregs}, unless
you use @code{SETUP_INCOMING_VARARGS} (see below) instead.

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

Regardless of what code is generated for the call to
@code{__builtin_saveregs}, it appears at the beginning of the function,
not where the call to @code{__builtin_saveregs} is written.  This is
because the registers must be saved before the function starts to use
them for its own purposes.

@findex __builtin_args_info
@item __builtin_args_info (@var{category})
Use this built-in function to find the first anonymous arguments in
registers.

In general, a machine may have several categories of registers used for
arguments, each for a particular category of data types.  (For example,
on some machines, floating-point registers are used for floating-point
arguments while other arguments are passed in the general registers.)
To make non-varargs functions use the proper calling convention, you
have defined the @code{CUMULATIVE_ARGS} data type to record how many
registers in each category have been used so far

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

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

@findex __builtin_next_arg
@item __builtin_next_arg ()
This is the equivalent of @code{__builtin_args_info}, for stack
arguments.  It returns the address of the first anonymous stack
argument, as type @code{void *}. If @code{ARGS_GROW_DOWNWARD}, it
returns the address of the location above the first anonymous stack
argument. Use it in @code{va_start} to initialize the pointer for
fetching arguments from the stack.

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

@code{__builtin_classify_type} ignores the value of @var{object},
considering only its data type.  It returns an integer describing what
kind of type that is---integer, floating, pointer, structure, and so on.

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

These machine description macros help implement varargs:

@table @code
@findex EXPAND_BUILTIN_SAVEREGS
@item EXPAND_BUILTIN_SAVEREGS (@var{args})
If defined, is a C expression that produces the machine-specific code
for a call to @code{__builtin_saveregs}.  This code will be moved to the
very beginning of the function, before any parameter access are made.
The return value of this function should be an RTX that contains the
value to use as the return of @code{__builtin_saveregs}.

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

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

@findex SETUP_INCOMING_VARARGS
@item SETUP_INCOMING_VARARGS (@var{args_so_far}, @var{mode}, @var{type}, @var{pretend_args_size}, @var{second_time})
This macro offers an alternative to using @code{__builtin_saveregs} and
defining the macro @code{EXPAND_BUILTIN_SAVEREGS}.  Use it to store the
anonymous register arguments into the stack so that all the arguments
appear to have been passed consecutively on the stack.  Once this is
done, you can use the standard implementation of varargs that works for
machines that pass all their arguments on the stack.

The argument @var{args_so_far} is the @code{CUMULATIVE_ARGS} data
structure, containing the values that obtain after processing of the
named arguments.  The arguments @var{mode} and @var{type} describe the
last named argument---its machine mode and its data type as a tree node.

The macro implementation should do two things: first, push onto the
stack all the argument registers @emph{not} used for the named
arguments, and second, store the size of the data thus pushed into the
@code{int}-valued variable whose name is supplied as the argument
@var{pretend_args_size}.  The value that you store here will serve as
additional offset for setting up the stack frame.

Because you must generate code to push the anonymous arguments at
compile time without knowing their data types,
@code{SETUP_INCOMING_VARARGS} is only useful on machines that have just
a single category of argument register and use it uniformly for all data
types.

If the argument @var{second_time} is nonzero, it means that the
arguments of the function are being analyzed for the second time.  This
happens for an inline function, which is not actually compiled until the
end of the source file.  The macro @code{SETUP_INCOMING_VARARGS} should
not generate any instructions in this case.
@end table

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

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

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

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

@table @code
@findex TRAMPOLINE_TEMPLATE
@item TRAMPOLINE_TEMPLATE (@var{file})
A C statement to output, on the stream @var{file}, assembler code for a
block of data that contains the constant parts of a trampoline.  This
code should not include a label---the label is taken care of
automatically.

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

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

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

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

@findex ALLOCATE_TRAMPOLINE
@item ALLOCATE_TRAMPOLINE (@var{fp})
A C expression to allocate run-time space for a trampoline.  The
expression value should be an RTX representing a memory reference to the
space for the trampoline.

@cindex @code{FUNCTION_EPILOGUE} and trampolines
@cindex @code{FUNCTION_PROLOGUE} and trampolines
If this macro is not defined, by default the trampoline is allocated as
a stack slot.  This default is right for most machines.  The exceptions
are machines where it is impossible to execute instructions in the stack
area.  On such machines, you may have to implement a separate stack,
using this macro in conjunction with @code{FUNCTION_PROLOGUE} and
@code{FUNCTION_EPILOGUE}.

@var{fp} points to a data structure, a @code{struct function}, which
describes the compilation status of the immediate containing function of
the function which the trampoline is for.  Normally (when
@code{ALLOCATE_TRAMPOLINE} is not defined), the stack slot for the
trampoline is in the stack frame of this containing function.  Other
allocation strategies probably must do something analogous with this
information.
@end table

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

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

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

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

@findex INSN_CACHE_LINE_WIDTH
@item INSN_CACHE_LINE_WIDTH
The length in bytes of each cache line.  The cache is divided into cache
lines which are disjoint slots, each holding a contiguous chunk of data
fetched from memory.  Each time data is brought into the cache, an
entire line is read at once.  The data loaded into a cache line is
always aligned on a boundary equal to the line size.

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

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

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

If you need to avoid executing the ordinary prologue code of a compiled
C function when you jump to the subroutine, you can do so by placing a
special label of your own in the assembler code.  Use one @code{asm}
statement to generate an assembler label, and another to make the label
global.  Then trampolines can use that label to jump directly to your
special assembler code.
@end table

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

@table @code
@findex MULSI3_LIBCALL
@item MULSI3_LIBCALL
A C string constant giving the name of the function to call for
multiplication of one signed full-word by another.  If you do not
define this macro, the default name is used, which is @code{__mulsi3},
a function defined in @file{libgcc.a}.

@findex DIVSI3_LIBCALL
@item DIVSI3_LIBCALL
A C string constant giving the name of the function to call for
division of one signed full-word by another.  If you do not define
this macro, the default name is used, which is @code{__divsi3}, a
function defined in @file{libgcc.a}.

@findex UDIVSI3_LIBCALL
@item UDIVSI3_LIBCALL
A C string constant giving the name of the function to call for
division of one unsigned full-word by another.  If you do not define
this macro, the default name is used, which is @code{__udivsi3}, a
function defined in @file{libgcc.a}.

@findex MODSI3_LIBCALL
@item MODSI3_LIBCALL
A C string constant giving the name of the function to call for the
remainder in division of one signed full-word by another.  If you do
not define this macro, the default name is used, which is
@code{__modsi3}, a function defined in @file{libgcc.a}.

@findex UMODSI3_LIBCALL
@item UMODSI3_LIBCALL
A C string constant giving the name of the function to call for the
remainder in division of one unsigned full-word by another.  If you do
not define this macro, the default name is used, which is
@code{__umodsi3}, a function defined in @file{libgcc.a}.

@findex MULDI3_LIBCALL
@item MULDI3_LIBCALL
A C string constant giving the name of the function to call for
multiplication of one signed double-word by another.  If you do not
define this macro, the default name is used, which is @code{__muldi3},
a function defined in @file{libgcc.a}.

@findex DIVDI3_LIBCALL
@item DIVDI3_LIBCALL
A C string constant giving the name of the function to call for
division of one signed double-word by another.  If you do not define
this macro, the default name is used, which is @code{__divdi3}, a
function defined in @file{libgcc.a}.

@findex UDIVDI3_LIBCALL
@item UDIVDI3_LIBCALL
A C string constant giving the name of the function to call for
division of one unsigned full-word by another.  If you do not define
this macro, the default name is used, which is @code{__udivdi3}, a
function defined in @file{libgcc.a}.

@findex MODDI3_LIBCALL
@item MODDI3_LIBCALL
A C string constant giving the name of the function to call for the
remainder in division of one signed double-word by another.  If you do
not define this macro, the default name is used, which is
@code{__moddi3}, a function defined in @file{libgcc.a}.

@findex UMODDI3_LIBCALL
@item UMODDI3_LIBCALL
A C string constant giving the name of the function to call for the
remainder in division of one unsigned full-word by another.  If you do
not define this macro, the default name is used, which is
@code{__umoddi3}, a function defined in @file{libgcc.a}.

@findex TARGET_EDOM
@cindex @code{EDOM}, implicit usage
@item TARGET_EDOM
The value of @code{EDOM} on the target machine, as a C integer constant
expression.  If you don't define this macro, GNU CC does not attempt to
deposit the value of @code{EDOM} into @code{errno} directly.  Look in
@file{/usr/include/errno.h} to find the value of @code{EDOM} on your
system.

If you do not define @code{TARGET_EDOM}, then compiled code reports
domain errors by calling the library function and letting it report the
error.  If mathematical functions on your system use @code{matherr} when
there is an error, then you should leave @code{TARGET_EDOM} undefined so
that @code{matherr} is used normally.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

@findex perform_@dots{}
@item perform_@dots{}
Define these macros to supply explicit C statements to carry out various
arithmetic operations on types @code{float} and @code{double} in the
library routines in @file{libgcc1.c}.  See that file for a full list
of these macros and their arguments.

On most machines, you don't need to define any of these macros, because
the C compiler that comes with the system takes care of doing them.

@findex NEXT_OBJC_RUNTIME
@item NEXT_OBJC_RUNTIME
Define this macro to generate code for Objective C message sending using
the calling convention of the NeXT system.  This calling convention
involves passing the object, the selector and the method arguments all
at once to the method-lookup library function.

The default calling convention passes just the object and the selector
to the lookup function, which returns a pointer to the method.
@end table

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

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

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

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

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

@findex MAX_REGS_PER_ADDRESS
@item MAX_REGS_PER_ADDRESS
A number, the maximum number of registers that can appear in a valid
memory address.  Note that it is up to you to specify a value equal to
the maximum number that @code{GO_IF_LEGITIMATE_ADDRESS} would ever
accept.

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

It usually pays to define several simpler macros to serve as
subroutines for this one.  Otherwise it may be too complicated to
understand.

This macro must exist in two variants: a strict variant and a
non-strict one.  The strict variant is used in the reload pass.  It
must be defined so that any pseudo-register that has not been
allocated a hard register is considered a memory reference.  In
contexts where some kind of register is required, a pseudo-register
with no hard register must be rejected.

The non-strict variant is used in other passes.  It must be defined to
accept all pseudo-registers in every context where some kind of
register is required.

@findex REG_OK_STRICT
Compiler source files that want to use the strict variant of this
macro define the macro @code{REG_OK_STRICT}.  You should use an
@code{#ifdef REG_OK_STRICT} conditional to define the strict variant
in that case and the non-strict variant otherwise.

Typically among the subroutines used to define
@code{GO_IF_LEGITIMATE_ADDRESS} are subroutines to check for
acceptable registers for various purposes (one for base registers, one
for index registers, and so on).  Then only these subroutine macros
need have two variants; the higher levels of macros may be the same
whether strict or not.@refill

Normally, constant addresses which are the sum of a @code{symbol_ref}
and an integer are stored inside a @code{const} RTX to mark them as
constant.  Therefore, there is no need to recognize such sums
specifically as legitimate addresses.  Normally you would simply
recognize any @code{const} as legitimate.

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

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

@findex saveable_obstack
The best way to modify the name string is by adding text to the
beginning, with suitable punctuation to prevent any ambiguity.  Allocate
the new name in @code{saveable_obstack}.  You will have to modify
@code{ASM_OUTPUT_LABELREF} to remove and decode the added text and
output the name accordingly, and define @code{STRIP_NAME_ENCODING} to
access the original name string.

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

@findex REG_OK_FOR_BASE_P
@item REG_OK_FOR_BASE_P (@var{x})
A C expression that is nonzero if @var{x} (assumed to be a @code{reg}
RTX) is valid for use as a base register.  For hard registers, it
should always accept those which the hardware permits and reject the
others.  Whether the macro accepts or rejects pseudo registers must be
controlled by @code{REG_OK_STRICT} as described above.  This usually
requires two variant definitions, of which @code{REG_OK_STRICT}
controls the one actually used.

@findex REG_OK_FOR_INDEX_P
@item REG_OK_FOR_INDEX_P (@var{x})
A C expression that is nonzero if @var{x} (assumed to be a @code{reg}
RTX) is valid for use as an index register.

The difference between an index register and a base register is that
the index register may be scaled.  If an address involves the sum of
two registers, neither one of them scaled, then either one may be
labeled the ``base'' and the other the ``index''; but whichever
labeling is used must fit the machine's constraints of which registers
may serve in each capacity.  The compiler will try both labelings,
looking for one that is valid, and will reload one or both registers
only if neither labeling works.

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

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

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

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

The code generated by this macro should not alter the substructure of
@var{x}.  If it transforms @var{x} into a more legitimate form, it
should assign @var{x} (which will always be a C variable) a new value.

It is not necessary for this macro to come up with a legitimate
address.  The compiler has standard ways of doing so in all cases.  In
fact, it is safe for this macro to do nothing.  But often a
machine-dependent strategy can generate better code.

@findex GO_IF_MODE_DEPENDENT_ADDRESS
@item GO_IF_MODE_DEPENDENT_ADDRESS (@var{addr}, @var{label})
A C statement or compound statement with a conditional @code{goto
@var{label};} executed if memory address @var{x} (an RTX) can have
different meanings depending on the machine mode of the memory
reference it is used for or if the address is valid for some modes
but not others.

Autoincrement and autodecrement addresses typically have mode-dependent
effects because the amount of the increment or decrement is the size
of the operand being addressed.  Some machines have other mode-dependent
addresses.  Many RISC machines have no mode-dependent addresses.

You may assume that @var{addr} is a valid address for the machine.

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

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

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

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

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

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

@findex CC_STATUS_MDEP_INIT
@item CC_STATUS_MDEP_INIT
A C expression to initialize the @code{mdep} field to ``empty''.
The default definition does nothing, since most machines don't use
the field anyway.  If you want to use the field, you should probably
define this macro to initialize it.

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

@findex NOTICE_UPDATE_CC
@item NOTICE_UPDATE_CC (@var{exp}, @var{insn})
A C compound statement to set the components of @code{cc_status}
appropriately for an insn @var{insn} whose body is @var{exp}.  It is
this macro's responsibility to recognize insns that set the condition
code as a byproduct of other activity as well as those that explicitly
set @code{(cc0)}.

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

If there are insns that do not set the condition code but do alter
other machine registers, this macro must check to see whether they
invalidate the expressions that the condition code is recorded as
reflecting.  For example, on the 68000, insns that store in address
registers do not set the condition code, which means that usually
@code{NOTICE_UPDATE_CC} can leave @code{cc_status} unaltered for such
insns.  But suppose that the previous insn set the condition code
based on location @samp{a4@@(102)} and the current insn stores a new
value in @samp{a4}.  Although the condition code is not changed by
this, it will no longer be true that it reflects the contents of
@samp{a4@@(102)}.  Therefore, @code{NOTICE_UPDATE_CC} must alter
@code{cc_status} in this case to say that nothing is known about the
condition code value.

The definition of @code{NOTICE_UPDATE_CC} must be prepared to deal
with the results of peephole optimization: insns whose patterns are
@code{parallel} RTXs containing various @code{reg}, @code{mem} or
constants which are just the operands.  The RTL structure of these
insns is not sufficient to indicate what the insns actually do.  What
@code{NOTICE_UPDATE_CC} should do when it sees one is just to run
@code{CC_STATUS_INIT}.

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

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

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

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

@example
#define EXTRA_CC_MODES CC_NOOVmode, CCFPmode
#define EXTRA_CC_NAMES "CC_NOOV", "CCFP"
@end example

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

@findex MEMORY_MOVE_COST
@item MEMORY_MOVE_COST (@var{m})
A C expression for the cost of moving data of mode @var{m} between a
register and memory.  A value of 2 is the default; this cost is relative
to those in @code{REGISTER_MOVE_COST}.

If moving between registers and memory is more expensive than between
two registers, you should define this macro to express the relative cost.

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

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

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

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

@findex SLOW_ZERO_EXTEND
@item SLOW_ZERO_EXTEND
Define this macro if zero-extension (of a @code{char} or @code{short}
to an @code{int}) can be done faster if the destination is a register
that is known to be zero.

If you define this macro, you must have instruction patterns that
recognize RTL structures like this:

@example
(set (strict_low_part (subreg:QI (reg:SI @dots{}) 0)) @dots{})
@end example

@noindent
and likewise for @code{HImode}.

@findex SLOW_UNALIGNED_ACCESS
@item SLOW_UNALIGNED_ACCESS
Define this macro to be the value 1 if unaligned accesses have a cost
many times greater than aligned accesses, for example if they are
emulated in a trap handler.

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

If the value of this macro is always zero, it need not be defined.

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

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

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

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

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

@findex ADJUST_COST
@item ADJUST_COST (@var{insn}, @var{link}, @var{dep_insn}, @var{cost})
A C statement (sans semicolon) to update the integer variable @var{cost}
based on the relationship between @var{insn} that is dependent on
@var{dep_insn} through the dependence @var{link}.  The default is to
make no adjustment to @var{cost}.  This can be used for example to
specify to the scheduler that an output- or anti-dependence does not
incur the same cost as a data-dependence.
@end table

@node Sections
@section Dividing the Output into Sections (Texts, Data, @dots{})

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

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

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

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

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

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

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

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

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

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

@findex SELECT_SECTION
@item SELECT_SECTION (@var{exp}, @var{reloc})
A C statement or statements to switch to the appropriate section for
output of @var{exp}.  You can assume that @var{exp} is either a
@code{VAR_DECL} node or a constant of some sort.  @var{reloc}
indicates whether the initial value of @var{exp} requires link-time
relocations.  Select the section by calling @code{text_section} or one
of the alternatives for other sections.

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

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

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

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

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

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

The macro definition, if any, is executed immediately after the rtl for
@var{decl} has been created and stored in @code{DECL_RTL (@var{decl})}.
The value of the rtl will be a @code{mem} whose address is a
@code{symbol_ref}.

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

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

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

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

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

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

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

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

@node Assembler Format
@section Defining the Output Assembler Language

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

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

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

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

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

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

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

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

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

@findex ASM_IDENTIFY_GCC
@item ASM_IDENTIFY_GCC (@var{file})
A C statement to output assembler commands which will identify
the object file as having been compiled with GNU CC (or another
GNU compiler).

If you don't define this macro, the string @samp{gcc_compiled.:}
is output.  This string is calculated to define a symbol which,
on BSD systems, will never be defined for any other reason.
GDB checks for the presence of this symbol when reading the
symbol table of an executable.

On non-BSD systems, you must arrange communication with GDB in
some other fashion.  If GDB is not used on your system, you can
define this macro with an empty body.

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

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

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

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

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

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

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

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

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

@node Data Output
@subsection Output of Data

@table @code
@findex ASM_OUTPUT_LONG_DOUBLE
@findex ASM_OUTPUT_DOUBLE
@findex ASM_OUTPUT_FLOAT
@item ASM_OUTPUT_LONG_DOUBLE (@var{stream}, @var{value})
@item ASM_OUTPUT_DOUBLE (@var{stream}, @var{value})
@item ASM_OUTPUT_FLOAT (@var{stream}, @var{value})
A C statement to output to the stdio stream @var{stream} an assembler
instruction to assemble a floating-point constant of @code{TFmode},
@code{DFmode} or @code{SFmode}, respectively, whose value is
@var{value}.  @var{value} will be a C expression of type
@code{REAL_VALUE__TYPE}, usually @code{double}.@refill

@findex ASM_OUTPUT_QUADRUPLE_INT
@findex ASM_OUTPUT_DOUBLE_INT
@findex ASM_OUTPUT_INT
@findex ASM_OUTPUT_SHORT
@findex ASM_OUTPUT_CHAR
@findex output_addr_const
@item ASM_OUTPUT_QUADRUPLE_INT (@var{stream}, @var{exp})
@item ASM_OUTPUT_DOUBLE_INT (@var{stream}, @var{exp})
@item ASM_OUTPUT_INT (@var{stream}, @var{exp})
@itemx ASM_OUTPUT_SHORT (@var{stream}, @var{exp})
@itemx ASM_OUTPUT_CHAR (@var{stream}, @var{exp})
A C statement to output to the stdio stream @var{stream} an assembler
instruction to assemble an integer of 16, 8, 4, 2 or 1 bytes,
respectively, whose value is @var{value}.  The argument @var{exp} will
be an RTL expression which represents a constant value.  Use
@samp{output_addr_const (@var{stream}, @var{exp})} to output this value
as an assembler expression.@refill

For sizes larger than @code{UNITS_PER_WORD}, if the action of a macro
would be identical to repeatedly calling the macro corresponding to
a size of @code{UNITS_PER_WORD}, once for each word, you need not define
the macro.

@findex ASM_OUTPUT_BYTE
@item ASM_OUTPUT_BYTE (@var{stream}, @var{value})
A C statement to output to the stdio stream @var{stream} an assembler
instruction to assemble a single byte containing the number @var{value}.

@findex ASM_BYTE_OP
@item ASM_BYTE_OP
A C string constant giving the pseudo-op to use for a sequence of
single-byte constants.  If this macro is not defined, the default is
@code{"byte"}.

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

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

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

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

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

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

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

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

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

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

@findex ASM_OPEN_PAREN
@findex ASM_CLOSE_PAREN
@item ASM_OPEN_PAREN
@itemx ASM_CLOSE_PAREN
These macros are defined as C string constant, describing the syntax
in the assembler for grouping arithmetic expressions.  The following
definitions are correct for most assemblers:

@example
#define ASM_OPEN_PAREN "("
#define ASM_CLOSE_PAREN ")"
@end example
@end table

@node Uninitialized Data
@subsection Output of Uninitialized Variables

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

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

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

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

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

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

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

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

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

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

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

@node Label Output
@subsection Output and Generation of Labels

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

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

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

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

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

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

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

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

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

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

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

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

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

@findex ASM_OUTPUT_LABELREF_AS_INT
@item ASM_OUTPUT_LABELREF_AS_INT (@var{file}, @var{label})
Define this macro for systems that use the program @code{collect2}.
The definition should be a C statement to output a word containing
a reference to the label @var{label}.

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

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

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

The usual definition of this macro is as follows:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

To use this method, you need appropriate definitions of the macros
@code{ASM_OUTPUT_CONSTRUCTOR} and @code{ASM_OUTPUT_DESTRUCTOR}.  Usually
you can get them by including @file{svr4.h}.

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

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

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

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

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

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

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

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

@table @file
@item aoutos.h
For operating systems using the `a.out' format.

@item next.h
For operating systems using the `MachO' format.

@item svr3.h
For System V Release 3 and similar systems using `COFF' format.

@item svr4.h
For System V Release 4 and similar systems using `ELF' format.

@item vms.h
For the VMS operating system.
@end table

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

@node Macros for Initialization
@subsection Macros Controlling Initialization Routines

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

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

@item ASM_OUTPUT_CONSTRUCTOR (@var{stream}, @var{name})
@findex ASM_OUTPUT_CONSTRUCTOR
Define this macro as a C statement to output on the stream @var{stream}
the assembler code to arrange to call the function named @var{name} at
initialization time.

Assume that @var{name} is the name of a C function generated
automatically by the compiler.  This function takes no arguments.  Use
the function @code{assemble_name} to output the name @var{name}; this
performs any system-specific syntactic transformations such as adding an
underscore.

If you don't define this macro, nothing special is output to arrange to
call the function.  This is correct when the function will be called in
some other manner---for example, by means of the @code{collect2} program,
which looks through the symbol table to find these functions by their
names.  If you want to use @code{collect2}, then you need to arrange for
it to be built and installed and used on your system.

@item ASM_OUTPUT_DESTRUCTOR (@var{stream}, @var{name})
@findex ASM_OUTPUT_DESTRUCTOR
This is like @code{ASM_OUTPUT_CONSTRUCTOR} but used for termination
functions rather than initialization functions.
@end table

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

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

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

These macros are effective only in a native compiler; @code{collect2} as
part of a cross compiler always uses @code{nm}.

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

@node Instruction Output
@subsection Output of Assembler Instructions

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

@cindex @code{ENCODE_SECTION_INFO} usage
On some machines, the syntax for a symbolic address depends on the
section that the address refers to.  On these machines, define the macro
@code{ENCODE_SECTION_INFO} to store the information into the
@code{symbol_ref}, and then check for it here.  @xref{Assembler Format}.

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

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

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

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

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

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

@node Dispatch Tables
@subsection Output of Dispatch Tables

@table @code
@cindex dispatch table
@findex ASM_OUTPUT_ADDR_DIFF_ELT
@item ASM_OUTPUT_ADDR_DIFF_ELT (@var{stream}, @var{value}, @var{rel})
This macro should be provided on machines where the addresses
in a dispatch table are relative to the table's own address.

The definition should be a C statement to output to the stdio stream
@var{stream} an assembler pseudo-instruction to generate a difference
between two labels.  @var{value} and @var{rel} are the numbers of two
internal labels.  The definitions of these labels are output using
@code{ASM_OUTPUT_INTERNAL_LABEL}, and they must be printed in the same
way here.  For example,

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

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

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

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

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

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

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

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

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

@node Alignment Output
@subsection Assembler Commands for Alignment

@table @code
@findex ASM_OUTPUT_ALIGN_CODE
@item ASM_OUTPUT_ALIGN_CODE (@var{file})
A C expression to output text to align the location counter in the way
that is desirable at a point in the code that is reached only by
jumping.

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

@findex ASM_OUTPUT_LOOP_ALIGN
@item ASM_OUTPUT_LOOP_ALIGN (@var{file})
A C expression to output text to align the location counter in the way
that is desirable at the beginning of a loop.

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

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

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

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

@node Debugging Info
@section Controlling Debugging Information Format

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

@node All Debuggers
@subsection Macros Affecting All Debugging Formats

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

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

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

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

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

@node DBX Options
@subsection Specific Options for DBX Output

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Here is another way of finding a particular type:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

@table @code
@findex REAL_VALUE_TYPE
@item REAL_VALUE_TYPE
A macro for the C data type to be used to hold a floating point value
in the target machine's format.  Typically this would be a
@code{struct} containing an array of @code{int}.

@findex REAL_VALUES_EQUAL
@item REAL_VALUES_EQUAL (@var{x}, @var{y})
A macro for a C expression which compares for equality the two values,
@var{x} and @var{y}, both of type @code{REAL_VALUE_TYPE}.

@findex REAL_VALUES_LESS
@item REAL_VALUES_LESS (@var{x}, @var{y})
A macro for a C expression which tests whether @var{x} is less than
@var{y}, both values being of type @code{REAL_VALUE_TYPE} and
interpreted as floating point numbers in the target machine's
representation.

@findex REAL_VALUE_LDEXP
@findex ldexp
@item REAL_VALUE_LDEXP (@var{x}, @var{scale})
A macro for a C expression which performs the standard library
function @code{ldexp}, but using the target machine's floating point
representation.  Both @var{x} and the value of the expression have
type @code{REAL_VALUE_TYPE}.  The second argument, @var{scale}, is an
integer.

@findex REAL_VALUE_FIX
@item REAL_VALUE_FIX (@var{x})
A macro whose definition is a C expression to convert the target-machine
floating point value @var{x} to a signed integer.  @var{x} has type
@code{REAL_VALUE_TYPE}.

@findex REAL_VALUE_UNSIGNED_FIX
@item REAL_VALUE_UNSIGNED_FIX (@var{x})
A macro whose definition is a C expression to convert the target-machine
floating point value @var{x} to an unsigned integer.  @var{x} has type
@code{REAL_VALUE_TYPE}.

@findex REAL_VALUE_FIX_TRUNCATE
@item REAL_VALUE_FIX_TRUNCATE (@var{x})
A macro whose definition is a C expression to convert the target-machine
floating point value @var{x} to a signed integer, rounding toward 0.
@var{x} has type @code{REAL_VALUE_TYPE}.

@findex REAL_VALUE_UNSIGNED_FIX_TRUNCATE
@item REAL_VALUE_UNSIGNED_FIX_TRUNCATE (@var{x})
A macro whose definition is a C expression to convert the target-machine
floating point value @var{x} to an unsigned integer, rounding toward 0.
@var{x} has type @code{REAL_VALUE_TYPE}.

@findex REAL_VALUE_ATOF
@item REAL_VALUE_ATOF (@var{string})
A macro for a C expression which converts @var{string}, an expression
of type @code{char *}, into a floating point number in the target
machine's representation.  The value has type @code{REAL_VALUE_TYPE}.

@findex REAL_INFINITY
@item REAL_INFINITY
Define this macro if infinity is a possible floating point value, and
therefore division by 0 is legitimate.

@findex REAL_VALUE_ISINF
@findex isinf
@item REAL_VALUE_ISINF (@var{x})
A macro for a C expression which determines whether @var{x}, a floating
point value, is infinity.  The value has type @code{int}.
By default, this is defined to call @code{isinf}.

@findex REAL_VALUE_ISNAN
@findex isnan
@item REAL_VALUE_ISNAN (@var{x})
A macro for a C expression which determines whether @var{x}, a floating
point value, is a ``nan'' (not-a-number).  The value has type
@code{int}.  By default, this is defined to call @code{isnan}.
@end table

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

@table @code
@findex REAL_ARITHMETIC
@item REAL_ARITHMETIC (@var{output}, @var{code}, @var{x}, @var{y})
A macro for a C statement which calculates an arithmetic operation of
the two floating point values @var{x} and @var{y}, both of type
@code{REAL_VALUE_TYPE} in the target machine's representation, to
produce a result of the same type and representation which is stored
in @var{output} (which will be a variable).

The operation to be performed is specified by @var{code}, a tree code
which will always be one of the following: @code{PLUS_EXPR},
@code{MINUS_EXPR}, @code{MULT_EXPR}, @code{RDIV_EXPR},
@code{MAX_EXPR}, @code{MIN_EXPR}.@refill

@cindex overflow while constant folding
The expansion of this macro is responsible for checking for overflow.
If overflow happens, the macro expansion should execute the statement
@code{return 0;}, which indicates the inability to perform the
arithmetic operation requested.

@findex REAL_VALUE_NEGATE
@item REAL_VALUE_NEGATE (@var{x})
A macro for a C expression which returns the negative of the floating
point value @var{x}.  Both @var{x} and the value of the expression
have type @code{REAL_VALUE_TYPE} and are in the target machine's
floating point representation.

There is no way for this macro to report overflow, since overflow
can't happen in the negation operation.

@findex REAL_VALUE_TRUNCATE
@item REAL_VALUE_TRUNCATE (@var{mode}, @var{x})
A macro for a C expression which converts the floating point value
@var{x} to mode @var{mode}.

Both @var{x} and the value of the expression have type
@code{REAL_VALUE_TYPE} and are in the target machine's floating point
representation.  However, the value should have an appropriate bit
pattern to be output properly as a floating constant whose precision
accords with mode @var{mode}.

There is no way for this macro to report overflow.

@findex REAL_VALUE_TO_INT
@item REAL_VALUE_TO_INT (@var{low}, @var{high}, @var{x})
A macro for a C expression which converts a floating point value
@var{x} into a double-precision integer which is then stored into
@var{low} and @var{high}, two variables of type @var{int}.

@item REAL_VALUE_FROM_INT (@var{x}, @var{low}, @var{high})
@findex REAL_VALUE_FROM_INT
A macro for a C expression which converts a double-precision integer
found in @var{low} and @var{high}, two variables of type @var{int},
into a floating point value which is then stored into @var{x}.
@end table

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

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

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

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

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

@findex CASE_VECTOR_PC_RELATIVE
@item CASE_VECTOR_PC_RELATIVE
Define this macro if jump-tables should contain relative addresses.

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

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

@findex BYTE_LOADS_ZERO_EXTEND
@item BYTE_LOADS_ZERO_EXTEND
Define this macro if an instruction to load a value narrower than a
word from memory into a register also zero-extends the value to the whole
register.

@findex BYTE_LOADS_SIGN_EXTEND
@item BYTE_LOADS_SIGN_EXTEND
Define this macro if an instruction to load a value narrower than a
word from memory into a register also sign-extends the value to the whole
register.

@findex IMPLICIT_FIX_EXPR
@item IMPLICIT_FIX_EXPR
An alias for a tree code that should be used by default for conversion
of floating point values to fixed point.  Normally,
@code{FIX_ROUND_EXPR} is used.@refill

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

@findex EASY_DIV_EXPR
@item EASY_DIV_EXPR
An alias for a tree code that is the easiest kind of division to
compile code for in the general case.  It may be
@code{TRUNC_DIV_EXPR}, @code{FLOOR_DIV_EXPR}, @code{CEIL_DIV_EXPR} or
@code{ROUND_DIV_EXPR}.  These four division operators differ in how
they round the result to an integer.  @code{EASY_DIV_EXPR} is used
when it is permissible to use any of those kinds of division and the
choice should be made on the basis of efficiency.@refill

@findex MOVE_MAX
@item MOVE_MAX
The maximum number of bytes that a single instruction can move quickly
from memory to memory.

@findex SHIFT_COUNT_TRUNCATED
@item SHIFT_COUNT_TRUNCATED
Defining this macro causes the compiler to omit a sign-extend,
zero-extend, or bitwise `and' instruction that truncates the count of a
shift operation to a width equal to the number of bits needed to
represent the size of the object being shifted.  On machines that have
instructions that act on bitfields at variable positions, which may
include `bit test' instructions, defining @code{SHIFT_COUNT_TRUNCATED}
also enables deletion of truncations of the values that serve as
arguments to bitfield instructions.

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

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

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

On many machines, this expression can be 1.

It is reported that suboptimal code can result when
@code{TRULY_NOOP_TRUNCATION} returns 1 for a pair of sizes for modes for
which @code{MODES_TIEABLE_P} is 0.  If this is the case, making
@code{TRULY_NOOP_TRUNCATION} return 0 in such cases may improve things.

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

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

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

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

@noindent
can be converted to

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

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

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

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

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

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

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

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

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

@findex FLOAT_STORE_FLAG_VALUE
@item FLOAT_STORE_FLAG_VALUE
A C expression that gives a non-zero floating point value that is
returned when comparison operators with floating-point results are true.
Define this macro on machine that have comparison operations that return
floating-point values.  If there are no such operations, do not define
this macro.

@findex Pmode
@item Pmode
An alias for the machine mode for pointers.  Normally the definition
can be

@example
#define Pmode SImode
@end example

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

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

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

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

@findex HANDLE_PRAGMA
@findex #pragma
@findex pragma
@item HANDLE_PRAGMA (@var{stream})
Define this macro if you want to implement any pragmas.  If defined, it
should be a C statement to be executed when @code{#pragma} is seen.  The
argument @var{stream} is the stdio input stream from which the source
text can be read.

It is generally a bad idea to implement new uses of @code{#pragma}.  The
only reason to define this macro is for compatibility with other
compilers that do support @code{#pragma} for the sake of any user
programs which already use it.

@findex DOLLARS_IN_IDENTIFIERS
@item DOLLARS_IN_IDENTIFIERS
Define this macro to control use of the character @samp{$} in identifier
names.  The value should be 0, 1, or 2.  0 means @samp{$} is not allowed
by default; 1 means it is allowed by default if @samp{-traditional} is
used; 2 means it is allowed by default provided @samp{-ansi} is not used.
1 is the default; there is no need to define this macro in that case.

@findex NO_DOLLAR_IN_LABEL
@item NO_DOLLAR_IN_LABEL
Define this macro if the assembler does not accept the character
@samp{$} in label names.  By default constructors and destructors in
G++ have @samp{$} in the identifiers.  If this macro is defined,
@samp{.} is used instead.

@findex DEFAULT_MAIN_RETURN
@item DEFAULT_MAIN_RETURN
Define this macro if the target system expects every program's @code{main}
function to return a standard ``success'' value by default (if no other
value is explicitly returned).

The definition should be a C statement (sans semicolon) to generate the
appropriate rtl instructions.  It is used only when compiling the end of
@code{main}.

@item HAVE_ATEXIT
@findex HAVE_ATEXIT
Define this if the target system supports the function
@code{atexit} from the ANSI C standard.  If this is not defined,
and @code{INIT_SECTION_ASM_OP} is not defined, a default
@code{exit} function will be provided to support C++.

@item EXIT_BODY
@findex EXIT_BODY
Define this if your @code{exit} function needs to do something
besides calling an external function @code{_cleanup} before
terminating with @code{_exit}.  The @code{EXIT_BODY} macro is
only needed if netiher @code{HAVE_ATEXIT} nor
@code{INIT_SECTION_ASM_OP} are defined.
@end table
@end ifset