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@c Copyright (C) 1988-2021 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.
@node Target Macros
@chapter Target Description Macros and Functions
@cindex machine description macros
@cindex target description macros
@cindex macros, target description
@cindex @file{tm.h} macros
In addition to the file @file{@var{machine}.md}, a machine description
includes a C header file conventionally given the name
@file{@var{machine}.h} and a C source file named @file{@var{machine}.c}.
The header file defines numerous macros that convey the information
about the target machine that does not fit into the scheme of the
@file{.md} file. The file @file{tm.h} should be a link to
@file{@var{machine}.h}. The header file @file{config.h} includes
@file{tm.h} and most compiler source files include @file{config.h}. The
source file defines a variable @code{targetm}, which is a structure
containing pointers to functions and data relating to the target
machine. @file{@var{machine}.c} should also contain their definitions,
if they are not defined elsewhere in GCC, and other functions called
through the macros defined in the @file{.h} file.
@menu
* Target Structure:: The @code{targetm} variable.
* Driver:: Controlling how the driver runs the compilation passes.
* Run-time Target:: Defining @samp{-m} options like @option{-m68000} and @option{-m68020}.
* Per-Function Data:: Defining data structures for per-function information.
* Storage Layout:: Defining sizes and alignments of data.
* Type Layout:: Defining sizes and properties of basic user data types.
* 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.
* Anchored Addresses:: Defining how @option{-fsection-anchors} should work.
* Condition Code:: Defining how insns update the condition code.
* Costs:: Defining relative costs of different operations.
* Scheduling:: Adjusting the behavior of the instruction scheduler.
* Sections:: Dividing storage into text, data, and other sections.
* PIC:: Macros for position independent code.
* Assembler Format:: Defining how to write insns and pseudo-ops to output.
* Debugging Info:: Defining the format of debugging output.
* Floating Point:: Handling floating point for cross-compilers.
* Mode Switching:: Insertion of mode-switching instructions.
* Target Attributes:: Defining target-specific uses of @code{__attribute__}.
* Emulated TLS:: Emulated TLS support.
* MIPS Coprocessors:: MIPS coprocessor support and how to customize it.
* PCH Target:: Validity checking for precompiled headers.
* C++ ABI:: Controlling C++ ABI changes.
* D Language and ABI:: Controlling D ABI changes.
* Named Address Spaces:: Adding support for named address spaces
* Misc:: Everything else.
@end menu
@node Target Structure
@section The Global @code{targetm} Variable
@cindex target hooks
@cindex target functions
@deftypevar {struct gcc_target} targetm
The target @file{.c} file must define the global @code{targetm} variable
which contains pointers to functions and data relating to the target
machine. The variable is declared in @file{target.h};
@file{target-def.h} defines the macro @code{TARGET_INITIALIZER} which is
used to initialize the variable, and macros for the default initializers
for elements of the structure. The @file{.c} file should override those
macros for which the default definition is inappropriate. For example:
@smallexample
#include "target.h"
#include "target-def.h"
/* @r{Initialize the GCC target structure.} */
#undef TARGET_COMP_TYPE_ATTRIBUTES
#define TARGET_COMP_TYPE_ATTRIBUTES @var{machine}_comp_type_attributes
struct gcc_target targetm = TARGET_INITIALIZER;
@end smallexample
@end deftypevar
Where a macro should be defined in the @file{.c} file in this manner to
form part of the @code{targetm} structure, it is documented below as a
``Target Hook'' with a prototype. Many macros will change in future
from being defined in the @file{.h} file to being part of the
@code{targetm} structure.
Similarly, there is a @code{targetcm} variable for hooks that are
specific to front ends for C-family languages, documented as ``C
Target Hook''. This is declared in @file{c-family/c-target.h}, the
initializer @code{TARGETCM_INITIALIZER} in
@file{c-family/c-target-def.h}. If targets initialize @code{targetcm}
themselves, they should set @code{target_has_targetcm=yes} in
@file{config.gcc}; otherwise a default definition is used.
Similarly, there is a @code{targetm_common} variable for hooks that
are shared between the compiler driver and the compilers proper,
documented as ``Common Target Hook''. This is declared in
@file{common/common-target.h}, the initializer
@code{TARGETM_COMMON_INITIALIZER} in
@file{common/common-target-def.h}. If targets initialize
@code{targetm_common} themselves, they should set
@code{target_has_targetm_common=yes} in @file{config.gcc}; otherwise a
default definition is used.
Similarly, there is a @code{targetdm} variable for hooks that are
specific to the D language front end, documented as ``D Target Hook''.
This is declared in @file{d/d-target.h}, the initializer
@code{TARGETDM_INITIALIZER} in @file{d/d-target-def.h}. If targets
initialize @code{targetdm} themselves, they should set
@code{target_has_targetdm=yes} in @file{config.gcc}; otherwise a default
definition is used.
@node Driver
@section Controlling the Compilation Driver, @file{gcc}
@cindex driver
@cindex controlling the compilation driver
@c prevent bad page break with this line
You can control the compilation driver.
@defmac DRIVER_SELF_SPECS
A list of specs for the driver itself. It should be a suitable
initializer for an array of strings, with no surrounding braces.
The driver applies these specs to its own command line between loading
default @file{specs} files (but not command-line specified ones) and
choosing the multilib directory or running any subcommands. It
applies them in the order given, so each spec can depend on the
options added by earlier ones. It is also possible to remove options
using @samp{%<@var{option}} in the usual way.
This macro can be useful when a port has several interdependent target
options. It provides a way of standardizing the command line so
that the other specs are easier to write.
Do not define this macro if it does not need to do anything.
@end defmac
@defmac OPTION_DEFAULT_SPECS
A list of specs used to support configure-time default options (i.e.@:
@option{--with} options) in the driver. It should be a suitable initializer
for an array of structures, each containing two strings, without the
outermost pair of surrounding braces.
The first item in the pair is the name of the default. This must match
the code in @file{config.gcc} for the target. The second item is a spec
to apply if a default with this name was specified. The string
@samp{%(VALUE)} in the spec will be replaced by the value of the default
everywhere it occurs.
The driver will apply these specs to its own command line between loading
default @file{specs} files and processing @code{DRIVER_SELF_SPECS}, using
the same mechanism as @code{DRIVER_SELF_SPECS}.
Do not define this macro if it does not need to do anything.
@end defmac
@defmac CPP_SPEC
A C string constant that tells the GCC driver program options to
pass to CPP@. It can also specify how to translate options you
give to GCC into options for GCC to pass to the CPP@.
Do not define this macro if it does not need to do anything.
@end defmac
@defmac CPLUSPLUS_CPP_SPEC
This macro is just like @code{CPP_SPEC}, but is used for C++, rather
than C@. If you do not define this macro, then the value of
@code{CPP_SPEC} (if any) will be used instead.
@end defmac
@defmac CC1_SPEC
A C string constant that tells the GCC driver program options to
pass to @code{cc1}, @code{cc1plus}, @code{f771}, and the other language
front ends.
It can also specify how to translate options you give to GCC into options
for GCC to pass to front ends.
Do not define this macro if it does not need to do anything.
@end defmac
@defmac CC1PLUS_SPEC
A C string constant that tells the GCC driver program options to
pass to @code{cc1plus}. It can also specify how to translate options you
give to GCC into options for GCC to pass to the @code{cc1plus}.
Do not define this macro if it does not need to do anything.
Note that everything defined in CC1_SPEC is already passed to
@code{cc1plus} so there is no need to duplicate the contents of
CC1_SPEC in CC1PLUS_SPEC@.
@end defmac
@defmac ASM_SPEC
A C string constant that tells the GCC driver program options to
pass to the assembler. It can also specify how to translate options
you give to GCC into options for GCC to pass to the assembler.
See the file @file{sun3.h} for an example of this.
Do not define this macro if it does not need to do anything.
@end defmac
@defmac ASM_FINAL_SPEC
A C string constant that tells the GCC driver program how to
run any programs which cleanup after the normal assembler.
Normally, this is not needed. See the file @file{mips.h} for
an example of this.
Do not define this macro if it does not need to do anything.
@end defmac
@defmac AS_NEEDS_DASH_FOR_PIPED_INPUT
Define this macro, with no value, if the driver should give the assembler
an argument consisting of a single dash, @option{-}, to instruct it to
read from its standard input (which will be a pipe connected to the
output of the compiler proper). This argument is given after any
@option{-o} option specifying the name of the output file.
If you do not define this macro, the assembler is assumed to read its
standard input if given no non-option arguments. If your assembler
cannot read standard input at all, use a @samp{%@{pipe:%e@}} construct;
see @file{mips.h} for instance.
@end defmac
@defmac LINK_SPEC
A C string constant that tells the GCC driver program options to
pass to the linker. It can also specify how to translate options you
give to GCC into options for GCC to pass to the linker.
Do not define this macro if it does not need to do anything.
@end defmac
@defmac LIB_SPEC
Another C string constant used much like @code{LINK_SPEC}. The difference
between the two is that @code{LIB_SPEC} is used at the end of the
command given to the linker.
If this macro is not defined, a default is provided that
loads the standard C library from the usual place. See @file{gcc.c}.
@end defmac
@defmac LIBGCC_SPEC
Another C string constant that tells the GCC driver program
how and when to place a reference to @file{libgcc.a} into the
linker command line. This constant is placed both before and after
the value of @code{LIB_SPEC}.
If this macro is not defined, the GCC driver provides a default that
passes the string @option{-lgcc} to the linker.
@end defmac
@defmac REAL_LIBGCC_SPEC
By default, if @code{ENABLE_SHARED_LIBGCC} is defined, the
@code{LIBGCC_SPEC} is not directly used by the driver program but is
instead modified to refer to different versions of @file{libgcc.a}
depending on the values of the command line flags @option{-static},
@option{-shared}, @option{-static-libgcc}, and @option{-shared-libgcc}. On
targets where these modifications are inappropriate, define
@code{REAL_LIBGCC_SPEC} instead. @code{REAL_LIBGCC_SPEC} tells the
driver how to place a reference to @file{libgcc} on the link command
line, but, unlike @code{LIBGCC_SPEC}, it is used unmodified.
@end defmac
@defmac USE_LD_AS_NEEDED
A macro that controls the modifications to @code{LIBGCC_SPEC}
mentioned in @code{REAL_LIBGCC_SPEC}. If nonzero, a spec will be
generated that uses @option{--as-needed} or equivalent options and the
shared @file{libgcc} in place of the
static exception handler library, when linking without any of
@code{-static}, @code{-static-libgcc}, or @code{-shared-libgcc}.
@end defmac
@defmac LINK_EH_SPEC
If defined, this C string constant is added to @code{LINK_SPEC}.
When @code{USE_LD_AS_NEEDED} is zero or undefined, it also affects
the modifications to @code{LIBGCC_SPEC} mentioned in
@code{REAL_LIBGCC_SPEC}.
@end defmac
@defmac STARTFILE_SPEC
Another C string constant used much like @code{LINK_SPEC}. The
difference between the two is that @code{STARTFILE_SPEC} is used at
the very beginning of the command given to the linker.
If this macro is not defined, a default is provided that loads the
standard C startup file from the usual place. See @file{gcc.c}.
@end defmac
@defmac ENDFILE_SPEC
Another C string constant used much like @code{LINK_SPEC}. The
difference between the two is that @code{ENDFILE_SPEC} is used at
the very end of the command given to the linker.
Do not define this macro if it does not need to do anything.
@end defmac
@defmac THREAD_MODEL_SPEC
GCC @code{-v} will print the thread model GCC was configured to use.
However, this doesn't work on platforms that are multilibbed on thread
models, such as AIX 4.3. On such platforms, define
@code{THREAD_MODEL_SPEC} such that it evaluates to a string without
blanks that names one of the recognized thread models. @code{%*}, the
default value of this macro, will expand to the value of
@code{thread_file} set in @file{config.gcc}.
@end defmac
@defmac SYSROOT_SUFFIX_SPEC
Define this macro to add a suffix to the target sysroot when GCC is
configured with a sysroot. This will cause GCC to search for usr/lib,
et al, within sysroot+suffix.
@end defmac
@defmac SYSROOT_HEADERS_SUFFIX_SPEC
Define this macro to add a headers_suffix to the target sysroot when
GCC is configured with a sysroot. This will cause GCC to pass the
updated sysroot+headers_suffix to CPP, causing it to search for
usr/include, et al, within sysroot+headers_suffix.
@end defmac
@defmac EXTRA_SPECS
Define this macro to provide additional specifications to put in the
@file{specs} file that can be used in various specifications like
@code{CC1_SPEC}.
The definition should be an initializer for an array of structures,
containing a string constant, that defines the specification name, and a
string constant that provides the specification.
Do not define this macro if it does not need to do anything.
@code{EXTRA_SPECS} is useful when an architecture contains several
related targets, which have various @code{@dots{}_SPECS} which are similar
to each other, and the maintainer would like one central place to keep
these definitions.
For example, the PowerPC System V.4 targets use @code{EXTRA_SPECS} to
define either @code{_CALL_SYSV} when the System V calling sequence is
used or @code{_CALL_AIX} when the older AIX-based calling sequence is
used.
The @file{config/rs6000/rs6000.h} target file defines:
@smallexample
#define EXTRA_SPECS \
@{ "cpp_sysv_default", CPP_SYSV_DEFAULT @},
#define CPP_SYS_DEFAULT ""
@end smallexample
The @file{config/rs6000/sysv.h} target file defines:
@smallexample
#undef CPP_SPEC
#define CPP_SPEC \
"%@{posix: -D_POSIX_SOURCE @} \
%@{mcall-sysv: -D_CALL_SYSV @} \
%@{!mcall-sysv: %(cpp_sysv_default) @} \
%@{msoft-float: -D_SOFT_FLOAT@} %@{mcpu=403: -D_SOFT_FLOAT@}"
#undef CPP_SYSV_DEFAULT
#define CPP_SYSV_DEFAULT "-D_CALL_SYSV"
@end smallexample
while the @file{config/rs6000/eabiaix.h} target file defines
@code{CPP_SYSV_DEFAULT} as:
@smallexample
#undef CPP_SYSV_DEFAULT
#define CPP_SYSV_DEFAULT "-D_CALL_AIX"
@end smallexample
@end defmac
@defmac LINK_LIBGCC_SPECIAL_1
Define this macro if the driver program should find the library
@file{libgcc.a}. If you do not define this macro, the driver program will pass
the argument @option{-lgcc} to tell the linker to do the search.
@end defmac
@defmac LINK_GCC_C_SEQUENCE_SPEC
The sequence in which libgcc and libc are specified to the linker.
By default this is @code{%G %L %G}.
@end defmac
@defmac POST_LINK_SPEC
Define this macro to add additional steps to be executed after linker.
The default value of this macro is empty string.
@end defmac
@defmac LINK_COMMAND_SPEC
A C string constant giving the complete command line need to execute the
linker. When you do this, you will need to update your port each time a
change is made to the link command line within @file{gcc.c}. Therefore,
define this macro only if you need to completely redefine the command
line for invoking the linker and there is no other way to accomplish
the effect you need. Overriding this macro may be avoidable by overriding
@code{LINK_GCC_C_SEQUENCE_SPEC} instead.
@end defmac
@hook TARGET_ALWAYS_STRIP_DOTDOT
@defmac MULTILIB_DEFAULTS
Define this macro as a C expression for the initializer of an array of
string to tell the driver program which options are defaults for this
target and thus do not need to be handled specially when using
@code{MULTILIB_OPTIONS}.
Do not define this macro if @code{MULTILIB_OPTIONS} is not defined in
the target makefile fragment or if none of the options listed in
@code{MULTILIB_OPTIONS} are set by default.
@xref{Target Fragment}.
@end defmac
@defmac RELATIVE_PREFIX_NOT_LINKDIR
Define this macro to tell @command{gcc} that it should only translate
a @option{-B} prefix into a @option{-L} linker option if the prefix
indicates an absolute file name.
@end defmac
@defmac 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 compiler is built as a cross
compiler. If you define @code{MD_EXEC_PREFIX}, then be sure to add it
to the list of directories used to find the assembler in @file{configure.ac}.
@end defmac
@defmac STANDARD_STARTFILE_PREFIX
Define this macro as a C string constant if you wish to override the
standard choice of @code{libdir} as the default prefix to
try when searching for startup files such as @file{crt0.o}.
@code{STANDARD_STARTFILE_PREFIX} is not searched when the compiler
is built as a cross compiler.
@end defmac
@defmac STANDARD_STARTFILE_PREFIX_1
Define this macro as a C string constant if you wish to override the
standard choice of @code{/lib} as a prefix to try after the default prefix
when searching for startup files such as @file{crt0.o}.
@code{STANDARD_STARTFILE_PREFIX_1} is not searched when the compiler
is built as a cross compiler.
@end defmac
@defmac STANDARD_STARTFILE_PREFIX_2
Define this macro as a C string constant if you wish to override the
standard choice of @code{/lib} as yet another prefix to try after the
default prefix when searching for startup files such as @file{crt0.o}.
@code{STANDARD_STARTFILE_PREFIX_2} is not searched when the compiler
is built as a cross compiler.
@end defmac
@defmac MD_STARTFILE_PREFIX
If defined, this macro supplies an additional prefix to try after the
standard prefixes. @code{MD_EXEC_PREFIX} is not searched when the
compiler is built as a cross compiler.
@end defmac
@defmac MD_STARTFILE_PREFIX_1
If defined, this macro supplies yet another prefix to try after the
standard prefixes. It is not searched when the compiler is built as a
cross compiler.
@end defmac
@defmac INIT_ENVIRONMENT
Define this macro as a C string constant if you wish to set environment
variables for programs called by the driver, such as the assembler and
loader. The driver passes the value of this macro to @code{putenv} to
initialize the necessary environment variables.
@end defmac
@defmac LOCAL_INCLUDE_DIR
Define this macro as a C string constant if you wish to override the
standard choice of @file{/usr/local/include} as the default prefix to
try when searching for local header files. @code{LOCAL_INCLUDE_DIR}
comes before @code{NATIVE_SYSTEM_HEADER_DIR} (set in
@file{config.gcc}, normally @file{/usr/include}) in the search order.
Cross compilers do not search either @file{/usr/local/include} or its
replacement.
@end defmac
@defmac NATIVE_SYSTEM_HEADER_COMPONENT
The ``component'' corresponding to @code{NATIVE_SYSTEM_HEADER_DIR}.
See @code{INCLUDE_DEFAULTS}, below, for the description of components.
If you do not define this macro, no component is used.
@end defmac
@defmac INCLUDE_DEFAULTS
Define this macro if you wish to override the entire default search path
for include files. For a native compiler, the default search path
usually consists of @code{GCC_INCLUDE_DIR}, @code{LOCAL_INCLUDE_DIR},
@code{GPLUSPLUS_INCLUDE_DIR}, and
@code{NATIVE_SYSTEM_HEADER_DIR}. In addition, @code{GPLUSPLUS_INCLUDE_DIR}
and @code{GCC_INCLUDE_DIR} are defined automatically by @file{Makefile},
and specify private search areas for GCC@. The directory
@code{GPLUSPLUS_INCLUDE_DIR} is used only for C++ programs.
The definition should be an initializer for an array of structures.
Each array element should have four elements: the directory name (a
string constant), the component name (also a string constant), a flag
for C++-only directories,
and a flag showing that the includes in the directory don't need to be
wrapped in @code{extern @samp{C}} when compiling C++. Mark the end of
the array with a null element.
The component name denotes what GNU package the include file is part of,
if any, in all uppercase letters. For example, it might be @samp{GCC}
or @samp{BINUTILS}. If the package is part of a vendor-supplied
operating system, code the component name as @samp{0}.
For example, here is the definition used for VAX/VMS:
@smallexample
#define INCLUDE_DEFAULTS \
@{ \
@{ "GNU_GXX_INCLUDE:", "G++", 1, 1@}, \
@{ "GNU_CC_INCLUDE:", "GCC", 0, 0@}, \
@{ "SYS$SYSROOT:[SYSLIB.]", 0, 0, 0@}, \
@{ ".", 0, 0, 0@}, \
@{ 0, 0, 0, 0@} \
@}
@end smallexample
@end defmac
Here is the order of prefixes tried for exec files:
@enumerate
@item
Any prefixes specified by the user with @option{-B}.
@item
The environment variable @code{GCC_EXEC_PREFIX} or, if @code{GCC_EXEC_PREFIX}
is not set and the compiler has not been installed in the configure-time
@var{prefix}, the location in which the compiler has actually been installed.
@item
The directories specified by the environment variable @code{COMPILER_PATH}.
@item
The macro @code{STANDARD_EXEC_PREFIX}, if the compiler has been installed
in the configured-time @var{prefix}.
@item
The location @file{/usr/libexec/gcc/}, but only if this is a native compiler.
@item
The location @file{/usr/lib/gcc/}, but only if this is a native compiler.
@item
The macro @code{MD_EXEC_PREFIX}, if defined, but only if this is a native
compiler.
@end enumerate
Here is the order of prefixes tried for startfiles:
@enumerate
@item
Any prefixes specified by the user with @option{-B}.
@item
The environment variable @code{GCC_EXEC_PREFIX} or its automatically determined
value based on the installed toolchain location.
@item
The directories specified by the environment variable @code{LIBRARY_PATH}
(or port-specific name; native only, cross compilers do not use this).
@item
The macro @code{STANDARD_EXEC_PREFIX}, but only if the toolchain is installed
in the configured @var{prefix} or this is a native compiler.
@item
The location @file{/usr/lib/gcc/}, but only if this is a native compiler.
@item
The macro @code{MD_EXEC_PREFIX}, if defined, but only if this is a native
compiler.
@item
The macro @code{MD_STARTFILE_PREFIX}, if defined, but only if this is a
native compiler, or we have a target system root.
@item
The macro @code{MD_STARTFILE_PREFIX_1}, if defined, but only if this is a
native compiler, or we have a target system root.
@item
The macro @code{STANDARD_STARTFILE_PREFIX}, with any sysroot modifications.
If this path is relative it will be prefixed by @code{GCC_EXEC_PREFIX} and
the machine suffix or @code{STANDARD_EXEC_PREFIX} and the machine suffix.
@item
The macro @code{STANDARD_STARTFILE_PREFIX_1}, but only if this is a native
compiler, or we have a target system root. The default for this macro is
@file{/lib/}.
@item
The macro @code{STANDARD_STARTFILE_PREFIX_2}, but only if this is a native
compiler, or we have a target system root. The default for this macro is
@file{/usr/lib/}.
@end enumerate
@node Run-time Target
@section Run-time Target Specification
@cindex run-time target specification
@cindex predefined macros
@cindex target specifications
@c prevent bad page break with this line
Here are run-time target specifications.
@defmac TARGET_CPU_CPP_BUILTINS ()
This function-like macro expands to a block of code that defines
built-in preprocessor macros and assertions for the target CPU, using
the functions @code{builtin_define}, @code{builtin_define_std} and
@code{builtin_assert}. When the front end
calls this macro it provides a trailing semicolon, and since it has
finished command line option processing your code can use those
results freely.
@code{builtin_assert} takes a string in the form you pass to the
command-line option @option{-A}, such as @code{cpu=mips}, and creates
the assertion. @code{builtin_define} takes a string in the form
accepted by option @option{-D} and unconditionally defines the macro.
@code{builtin_define_std} takes a string representing the name of an
object-like macro. If it doesn't lie in the user's namespace,
@code{builtin_define_std} defines it unconditionally. Otherwise, it
defines a version with two leading underscores, and another version
with two leading and trailing underscores, and defines the original
only if an ISO standard was not requested on the command line. For
example, passing @code{unix} defines @code{__unix}, @code{__unix__}
and possibly @code{unix}; passing @code{_mips} defines @code{__mips},
@code{__mips__} and possibly @code{_mips}, and passing @code{_ABI64}
defines only @code{_ABI64}.
You can also test for the C dialect being compiled. The variable
@code{c_language} is set to one of @code{clk_c}, @code{clk_cplusplus}
or @code{clk_objective_c}. Note that if we are preprocessing
assembler, this variable will be @code{clk_c} but the function-like
macro @code{preprocessing_asm_p()} will return true, so you might want
to check for that first. If you need to check for strict ANSI, the
variable @code{flag_iso} can be used. The function-like macro
@code{preprocessing_trad_p()} can be used to check for traditional
preprocessing.
@end defmac
@defmac TARGET_OS_CPP_BUILTINS ()
Similarly to @code{TARGET_CPU_CPP_BUILTINS} but this macro is optional
and is used for the target operating system instead.
@end defmac
@defmac TARGET_OBJFMT_CPP_BUILTINS ()
Similarly to @code{TARGET_CPU_CPP_BUILTINS} but this macro is optional
and is used for the target object format. @file{elfos.h} uses this
macro to define @code{__ELF__}, so you probably do not need to define
it yourself.
@end defmac
@deftypevar {extern int} target_flags
This variable is declared in @file{options.h}, which is included before
any target-specific headers.
@end deftypevar
@hook TARGET_DEFAULT_TARGET_FLAGS
This variable specifies the initial value of @code{target_flags}.
Its default setting is 0.
@end deftypevr
@cindex optional hardware or system features
@cindex features, optional, in system conventions
@hook TARGET_HANDLE_OPTION
This hook is called whenever the user specifies one of the
target-specific options described by the @file{.opt} definition files
(@pxref{Options}). It has the opportunity to do some option-specific
processing and should return true if the option is valid. The default
definition does nothing but return true.
@var{decoded} specifies the option and its arguments. @var{opts} and
@var{opts_set} are the @code{gcc_options} structures to be used for
storing option state, and @var{loc} is the location at which the
option was passed (@code{UNKNOWN_LOCATION} except for options passed
via attributes).
@end deftypefn
@hook TARGET_HANDLE_C_OPTION
This target hook is called whenever the user specifies one of the
target-specific C language family options described by the @file{.opt}
definition files(@pxref{Options}). It has the opportunity to do some
option-specific processing and should return true if the option is
valid. The arguments are like for @code{TARGET_HANDLE_OPTION}. The
default definition does nothing but return false.
In general, you should use @code{TARGET_HANDLE_OPTION} to handle
options. However, if processing an option requires routines that are
only available in the C (and related language) front ends, then you
should use @code{TARGET_HANDLE_C_OPTION} instead.
@end deftypefn
@hook TARGET_OBJC_CONSTRUCT_STRING_OBJECT
@hook TARGET_OBJC_DECLARE_UNRESOLVED_CLASS_REFERENCE
@hook TARGET_OBJC_DECLARE_CLASS_DEFINITION
@hook TARGET_STRING_OBJECT_REF_TYPE_P
@hook TARGET_CHECK_STRING_OBJECT_FORMAT_ARG
@hook TARGET_OVERRIDE_OPTIONS_AFTER_CHANGE
@defmac C_COMMON_OVERRIDE_OPTIONS
This is similar to the @code{TARGET_OPTION_OVERRIDE} hook
but is only used in the C
language frontends (C, Objective-C, C++, Objective-C++) and so can be
used to alter option flag variables which only exist in those
frontends.
@end defmac
@hook TARGET_OPTION_OPTIMIZATION_TABLE
Some machines may desire to change what optimizations are performed for
various optimization levels. This variable, if defined, describes
options to enable at particular sets of optimization levels. These
options are processed once
just after the optimization level is determined and before the remainder
of the command options have been parsed, so may be overridden by other
options passed explicitly.
This processing is run once at program startup and when the optimization
options are changed via @code{#pragma GCC optimize} or by using the
@code{optimize} attribute.
@end deftypevr
@hook TARGET_OPTION_INIT_STRUCT
@defmac SWITCHABLE_TARGET
Some targets need to switch between substantially different subtargets
during compilation. For example, the MIPS target has one subtarget for
the traditional MIPS architecture and another for MIPS16. Source code
can switch between these two subarchitectures using the @code{mips16}
and @code{nomips16} attributes.
Such subtargets can differ in things like the set of available
registers, the set of available instructions, the costs of various
operations, and so on. GCC caches a lot of this type of information
in global variables, and recomputing them for each subtarget takes a
significant amount of time. The compiler therefore provides a facility
for maintaining several versions of the global variables and quickly
switching between them; see @file{target-globals.h} for details.
Define this macro to 1 if your target needs this facility. The default
is 0.
@end defmac
@hook TARGET_FLOAT_EXCEPTIONS_ROUNDING_SUPPORTED_P
@node Per-Function Data
@section Defining data structures for per-function information.
@cindex per-function data
@cindex data structures
If the target needs to store information on a per-function basis, GCC
provides a macro and a couple of variables to allow this. Note, just
using statics to store the information is a bad idea, since GCC supports
nested functions, so you can be halfway through encoding one function
when another one comes along.
GCC defines a data structure called @code{struct function} which
contains all of the data specific to an individual function. This
structure contains a field called @code{machine} whose type is
@code{struct machine_function *}, which can be used by targets to point
to their own specific data.
If a target needs per-function specific data it should define the type
@code{struct machine_function} and also the macro @code{INIT_EXPANDERS}.
This macro should be used to initialize the function pointer
@code{init_machine_status}. This pointer is explained below.
One typical use of per-function, target specific data is to create an
RTX to hold the register containing the function's return address. This
RTX can then be used to implement the @code{__builtin_return_address}
function, for level 0.
Note---earlier implementations of GCC used a single data area to hold
all of the per-function information. Thus when processing of a nested
function began the old per-function data had to be pushed onto a
stack, and when the processing was finished, it had to be popped off the
stack. GCC used to provide function pointers called
@code{save_machine_status} and @code{restore_machine_status} to handle
the saving and restoring of the target specific information. Since the
single data area approach is no longer used, these pointers are no
longer supported.
@defmac INIT_EXPANDERS
Macro called to initialize any target specific information. This macro
is called once per function, before generation of any RTL has begun.
The intention of this macro is to allow the initialization of the
function pointer @code{init_machine_status}.
@end defmac
@deftypevar {void (*)(struct function *)} init_machine_status
If this function pointer is non-@code{NULL} it will be called once per
function, before function compilation starts, in order to allow the
target to perform any target specific initialization of the
@code{struct function} structure. It is intended that this would be
used to initialize the @code{machine} of that structure.
@code{struct machine_function} structures are expected to be freed by GC@.
Generally, any memory that they reference must be allocated by using
GC allocation, including the structure itself.
@end deftypevar
@node Storage Layout
@section Storage Layout
@cindex storage layout
Note that the definitions of the macros in this table which are sizes or
alignments measured in bits do not need to be constant. They can be C
expressions that refer to static variables, such as the @code{target_flags}.
@xref{Run-time Target}.
@defmac BITS_BIG_ENDIAN
Define this macro to have the value 1 if the most significant bit in a
byte has the lowest number; otherwise define it to have the value zero.
This means that bit-field instructions count from the most significant
bit. If the machine has no bit-field instructions, then this must still
be defined, but it doesn't matter which value it is defined to. This
macro need not be a constant.
This macro does not affect the way structure fields are packed into
bytes or words; that is controlled by @code{BYTES_BIG_ENDIAN}.
@end defmac
@defmac BYTES_BIG_ENDIAN
Define this macro to have the value 1 if the most significant byte in a
word has the lowest number. This macro need not be a constant.
@end defmac
@defmac WORDS_BIG_ENDIAN
Define this macro to have the value 1 if, in a multiword object, the
most significant word has the lowest number. This applies to both
memory locations and registers; see @code{REG_WORDS_BIG_ENDIAN} if the
order of words in memory is not the same as the order in registers. This
macro need not be a constant.
@end defmac
@defmac REG_WORDS_BIG_ENDIAN
On some machines, the order of words in a multiword object differs between
registers in memory. In such a situation, define this macro to describe
the order of words in a register. The macro @code{WORDS_BIG_ENDIAN} controls
the order of words in memory.
@end defmac
@defmac FLOAT_WORDS_BIG_ENDIAN
Define this macro to have the value 1 if @code{DFmode}, @code{XFmode} or
@code{TFmode} floating point numbers are stored in memory with the word
containing the sign bit at the lowest address; otherwise define it to
have the value 0. This macro need not be a constant.
You need not define this macro if the ordering is the same as for
multi-word integers.
@end defmac
@defmac BITS_PER_WORD
Number of bits in a word. If you do not define this macro, the default
is @code{BITS_PER_UNIT * UNITS_PER_WORD}.
@end defmac
@defmac MAX_BITS_PER_WORD
Maximum number of bits in a word. If this is undefined, the default is
@code{BITS_PER_WORD}. Otherwise, it is the constant value that is the
largest value that @code{BITS_PER_WORD} can have at run-time.
@end defmac
@defmac UNITS_PER_WORD
Number of storage units in a word; normally the size of a general-purpose
register, a power of two from 1 or 8.
@end defmac
@defmac MIN_UNITS_PER_WORD
Minimum number of units in a word. If this is undefined, the default is
@code{UNITS_PER_WORD}. Otherwise, it is the constant value that is the
smallest value that @code{UNITS_PER_WORD} can have at run-time.
@end defmac
@defmac POINTER_SIZE
Width of a pointer, in bits. You must specify a value no wider than the
width of @code{Pmode}. If it is not equal to the width of @code{Pmode},
you must define @code{POINTERS_EXTEND_UNSIGNED}. If you do not specify
a value the default is @code{BITS_PER_WORD}.
@end defmac
@defmac POINTERS_EXTEND_UNSIGNED
A C expression that determines how pointers should be extended from
@code{ptr_mode} to either @code{Pmode} or @code{word_mode}. It is
greater than zero if pointers should be zero-extended, zero if they
should be sign-extended, and negative if some other sort of conversion
is needed. In the last case, the extension is done by the target's
@code{ptr_extend} instruction.
You need not define this macro if the @code{ptr_mode}, @code{Pmode}
and @code{word_mode} are all the same width.
@end defmac
@defmac PROMOTE_MODE (@var{m}, @var{unsignedp}, @var{type})
A macro to update @var{m} and @var{unsignedp} when an object whose type
is @var{type} and which has the specified mode and signedness is to be
stored in a register. This macro is only called when @var{type} is a
scalar type.
On most RISC machines, which only have operations that operate on a full
register, define this macro to set @var{m} to @code{word_mode} if
@var{m} is an integer mode narrower than @code{BITS_PER_WORD}. In most
cases, only integer modes should be widened because wider-precision
floating-point operations are usually more expensive than their narrower
counterparts.
For most machines, the macro definition does not change @var{unsignedp}.
However, some machines, have instructions that preferentially handle
either signed or unsigned quantities of certain modes. For example, on
the DEC Alpha, 32-bit loads from memory and 32-bit add instructions
sign-extend the result to 64 bits. On such machines, set
@var{unsignedp} according to which kind of extension is more efficient.
Do not define this macro if it would never modify @var{m}.
@end defmac
@hook TARGET_C_EXCESS_PRECISION
Return a value, with the same meaning as the C99 macro
@code{FLT_EVAL_METHOD} that describes which excess precision should be
applied.
@hook TARGET_PROMOTE_FUNCTION_MODE
@defmac PARM_BOUNDARY
Normal alignment required for function parameters on the stack, in
bits. All stack parameters receive at least this much alignment
regardless of data type. On most machines, this is the same as the
size of an integer.
@end defmac
@defmac STACK_BOUNDARY
Define this macro to the minimum alignment enforced by hardware for the
stack pointer on this machine. The definition is a C expression for the
desired alignment (measured in bits). This value is used as a default
if @code{PREFERRED_STACK_BOUNDARY} is not defined. On most machines,
this should be the same as @code{PARM_BOUNDARY}.
@end defmac
@defmac PREFERRED_STACK_BOUNDARY
Define this macro if you wish to preserve a certain alignment for the
stack pointer, greater than what the hardware enforces. The definition
is a C expression for the desired alignment (measured in bits). This
macro must evaluate to a value equal to or larger than
@code{STACK_BOUNDARY}.
@end defmac
@defmac INCOMING_STACK_BOUNDARY
Define this macro if the incoming stack boundary may be different
from @code{PREFERRED_STACK_BOUNDARY}. This macro must evaluate
to a value equal to or larger than @code{STACK_BOUNDARY}.
@end defmac
@defmac FUNCTION_BOUNDARY
Alignment required for a function entry point, in bits.
@end defmac
@defmac BIGGEST_ALIGNMENT
Biggest alignment that any data type can require on this machine, in
bits. Note that this is not the biggest alignment that is supported,
just the biggest alignment that, when violated, may cause a fault.
@end defmac
@hook TARGET_ABSOLUTE_BIGGEST_ALIGNMENT
@defmac MALLOC_ABI_ALIGNMENT
Alignment, in bits, a C conformant malloc implementation has to
provide. If not defined, the default value is @code{BITS_PER_WORD}.
@end defmac
@defmac ATTRIBUTE_ALIGNED_VALUE
Alignment used by the @code{__attribute__ ((aligned))} construct. If
not defined, the default value is @code{BIGGEST_ALIGNMENT}.
@end defmac
@defmac MINIMUM_ATOMIC_ALIGNMENT
If defined, the smallest alignment, in bits, that can be given to an
object that can be referenced in one operation, without disturbing any
nearby object. Normally, this is @code{BITS_PER_UNIT}, but may be larger
on machines that don't have byte or half-word store operations.
@end defmac
@defmac BIGGEST_FIELD_ALIGNMENT
Biggest alignment that any structure or union field can require on this
machine, in bits. If defined, this overrides @code{BIGGEST_ALIGNMENT} for
structure and union fields only, unless the field alignment has been set
by the @code{__attribute__ ((aligned (@var{n})))} construct.
@end defmac
@defmac ADJUST_FIELD_ALIGN (@var{field}, @var{type}, @var{computed})
An expression for the alignment of a structure field @var{field} of
type @var{type} if the alignment computed in the usual way (including
applying of @code{BIGGEST_ALIGNMENT} and @code{BIGGEST_FIELD_ALIGNMENT} to the
alignment) is @var{computed}. It overrides alignment only if the
field alignment has not been set by the
@code{__attribute__ ((aligned (@var{n})))} construct. Note that @var{field}
may be @code{NULL_TREE} in case we just query for the minimum alignment
of a field of type @var{type} in structure context.
@end defmac
@defmac MAX_STACK_ALIGNMENT
Biggest stack alignment guaranteed by the backend. Use this macro
to specify the maximum alignment of a variable on stack.
If not defined, the default value is @code{STACK_BOUNDARY}.
@c FIXME: The default should be @code{PREFERRED_STACK_BOUNDARY}.
@c But the fix for PR 32893 indicates that we can only guarantee
@c maximum stack alignment on stack up to @code{STACK_BOUNDARY}, not
@c @code{PREFERRED_STACK_BOUNDARY}, if stack alignment isn't supported.
@end defmac
@defmac MAX_OFILE_ALIGNMENT
Biggest alignment supported by the object file format of this machine.
Use this macro to limit the alignment which can be specified using the
@code{__attribute__ ((aligned (@var{n})))} construct for functions and
objects with static storage duration. The alignment of automatic
objects may exceed the object file format maximum up to the maximum
supported by GCC. If not defined, the default value is
@code{BIGGEST_ALIGNMENT}.
On systems that use ELF, the default (in @file{config/elfos.h}) is
the largest supported 32-bit ELF section alignment representable on
a 32-bit host e.g.@: @samp{(((uint64_t) 1 << 28) * 8)}.
On 32-bit ELF the largest supported section alignment in bits is
@samp{(0x80000000 * 8)}, but this is not representable on 32-bit hosts.
@end defmac
@hook TARGET_LOWER_LOCAL_DECL_ALIGNMENT
@hook TARGET_STATIC_RTX_ALIGNMENT
@defmac DATA_ALIGNMENT (@var{type}, @var{basic-align})
If defined, a C expression to compute the alignment for a variable in
the static store. @var{type} is the data type, and @var{basic-align} is
the alignment that the object would ordinarily have. The value of this
macro is used instead of that alignment to align the object.
If this macro is not defined, then @var{basic-align} is used.
@findex strcpy
One use of this macro is to increase alignment of medium-size data to
make it all fit in fewer cache lines. Another is to cause character
arrays to be word-aligned so that @code{strcpy} calls that copy
constants to character arrays can be done inline.
@end defmac
@defmac DATA_ABI_ALIGNMENT (@var{type}, @var{basic-align})
Similar to @code{DATA_ALIGNMENT}, but for the cases where the ABI mandates
some alignment increase, instead of optimization only purposes. E.g.@
AMD x86-64 psABI says that variables with array type larger than 15 bytes
must be aligned to 16 byte boundaries.
If this macro is not defined, then @var{basic-align} is used.
@end defmac
@hook TARGET_CONSTANT_ALIGNMENT
@defmac LOCAL_ALIGNMENT (@var{type}, @var{basic-align})
If defined, a C expression to compute the alignment for a variable in
the local store. @var{type} is the data type, and @var{basic-align} is
the alignment that the object would ordinarily have. The value of this
macro is used instead of that alignment to align the object.
If this macro is not defined, then @var{basic-align} is used.
One use of this macro is to increase alignment of medium-size data to
make it all fit in fewer cache lines.
If the value of this macro has a type, it should be an unsigned type.
@end defmac
@hook TARGET_VECTOR_ALIGNMENT
@defmac STACK_SLOT_ALIGNMENT (@var{type}, @var{mode}, @var{basic-align})
If defined, a C expression to compute the alignment for stack slot.
@var{type} is the data type, @var{mode} is the widest mode available,
and @var{basic-align} is the alignment that the slot would ordinarily
have. The value of this macro is used instead of that alignment to
align the slot.
If this macro is not defined, then @var{basic-align} is used when
@var{type} is @code{NULL}. Otherwise, @code{LOCAL_ALIGNMENT} will
be used.
This macro is to set alignment of stack slot to the maximum alignment
of all possible modes which the slot may have.
If the value of this macro has a type, it should be an unsigned type.
@end defmac
@defmac LOCAL_DECL_ALIGNMENT (@var{decl})
If defined, a C expression to compute the alignment for a local
variable @var{decl}.
If this macro is not defined, then
@code{LOCAL_ALIGNMENT (TREE_TYPE (@var{decl}), DECL_ALIGN (@var{decl}))}
is used.
One use of this macro is to increase alignment of medium-size data to
make it all fit in fewer cache lines.
If the value of this macro has a type, it should be an unsigned type.
@end defmac
@defmac MINIMUM_ALIGNMENT (@var{exp}, @var{mode}, @var{align})
If defined, a C expression to compute the minimum required alignment
for dynamic stack realignment purposes for @var{exp} (a type or decl),
@var{mode}, assuming normal alignment @var{align}.
If this macro is not defined, then @var{align} will be used.
@end defmac
@defmac EMPTY_FIELD_BOUNDARY
Alignment in bits to be given to a structure bit-field that follows an
empty field such as @code{int : 0;}.
If @code{PCC_BITFIELD_TYPE_MATTERS} is true, it overrides this macro.
@end defmac
@defmac STRUCTURE_SIZE_BOUNDARY
Number of bits which any structure or union's size must be a multiple of.
Each structure or union's size is rounded up to a multiple of this.
If you do not define this macro, the default is the same as
@code{BITS_PER_UNIT}.
@end defmac
@defmac STRICT_ALIGNMENT
Define this macro to be the value 1 if instructions will fail to work
if given data not on the nominal alignment. If instructions will merely
go slower in that case, define this macro as 0.
@end defmac
@defmac PCC_BITFIELD_TYPE_MATTERS
Define this if you wish to imitate the way many other C compilers handle
alignment of bit-fields and the structures that contain them.
The behavior is that the type written for a named bit-field (@code{int},
@code{short}, or other integer type) imposes an alignment for the entire
structure, as if the structure really did contain an ordinary field of
that type. In addition, the bit-field is placed within the structure so
that it would fit within such a field, not crossing a boundary for it.
Thus, on most machines, a named bit-field whose type is written as
@code{int} would not cross a four-byte boundary, and would force
four-byte alignment for the whole structure. (The alignment used may
not be four bytes; it is controlled by the other alignment parameters.)
An unnamed bit-field will not affect the alignment of the containing
structure.
If the macro is defined, its definition should be a C expression;
a nonzero value for the expression enables this behavior.
Note that if this macro is not defined, or its value is zero, some
bit-fields may cross more than one alignment boundary. The compiler can
support such references if there are @samp{insv}, @samp{extv}, and
@samp{extzv} insns that can directly reference memory.
The other known way of making bit-fields work is to define
@code{STRUCTURE_SIZE_BOUNDARY} as large as @code{BIGGEST_ALIGNMENT}.
Then every structure can be accessed with fullwords.
Unless the machine has bit-field instructions or you define
@code{STRUCTURE_SIZE_BOUNDARY} that way, you must define
@code{PCC_BITFIELD_TYPE_MATTERS} to have a nonzero value.
If your aim is to make GCC use the same conventions for laying out
bit-fields as are used by another compiler, here is how to investigate
what the other compiler does. Compile and run this program:
@smallexample
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 smallexample
If this prints 2 and 5, then the compiler's behavior is what you would
get from @code{PCC_BITFIELD_TYPE_MATTERS}.
@end defmac
@defmac BITFIELD_NBYTES_LIMITED
Like @code{PCC_BITFIELD_TYPE_MATTERS} except that its effect is limited
to aligning a bit-field within the structure.
@end defmac
@hook TARGET_ALIGN_ANON_BITFIELD
@hook TARGET_NARROW_VOLATILE_BITFIELD
@hook TARGET_MEMBER_TYPE_FORCES_BLK
@defmac ROUND_TYPE_ALIGN (@var{type}, @var{computed}, @var{specified})
Define this macro as an expression for the alignment of a type (given
by @var{type} as a tree node) if the alignment computed in the usual
way is @var{computed} and the alignment explicitly specified was
@var{specified}.
The default is to use @var{specified} if it is larger; otherwise, use
the smaller of @var{computed} and @code{BIGGEST_ALIGNMENT}
@end defmac
@defmac MAX_FIXED_MODE_SIZE
An integer expression for the size in bits of the largest integer
machine mode that should actually be used. All integer machine modes of
this size or smaller can be used for structures and unions with the
appropriate sizes. If this macro is undefined, @code{GET_MODE_BITSIZE
(DImode)} is assumed.
@end defmac
@defmac STACK_SAVEAREA_MODE (@var{save_level})
If defined, an expression of type @code{machine_mode} that
specifies the mode of the save area operand of a
@code{save_stack_@var{level}} named pattern (@pxref{Standard Names}).
@var{save_level} is one of @code{SAVE_BLOCK}, @code{SAVE_FUNCTION}, or
@code{SAVE_NONLOCAL} and selects which of the three named patterns is
having its mode specified.
You need not define this macro if it always returns @code{Pmode}. You
would most commonly define this macro if the
@code{save_stack_@var{level}} patterns need to support both a 32- and a
64-bit mode.
@end defmac
@defmac STACK_SIZE_MODE
If defined, an expression of type @code{machine_mode} that
specifies the mode of the size increment operand of an
@code{allocate_stack} named pattern (@pxref{Standard Names}).
You need not define this macro if it always returns @code{word_mode}.
You would most commonly define this macro if the @code{allocate_stack}
pattern needs to support both a 32- and a 64-bit mode.
@end defmac
@hook TARGET_LIBGCC_CMP_RETURN_MODE
@hook TARGET_LIBGCC_SHIFT_COUNT_MODE
@hook TARGET_UNWIND_WORD_MODE
@hook TARGET_MS_BITFIELD_LAYOUT_P
@hook TARGET_DECIMAL_FLOAT_SUPPORTED_P
@hook TARGET_FIXED_POINT_SUPPORTED_P
@hook TARGET_EXPAND_TO_RTL_HOOK
@hook TARGET_INSTANTIATE_DECLS
@hook TARGET_MANGLE_TYPE
@node Type Layout
@section Layout of Source Language Data Types
These macros define the sizes and other characteristics of the standard
basic data types used in programs being compiled. Unlike the macros in
the previous section, these apply to specific features of C and related
languages, rather than to fundamental aspects of storage layout.
@defmac INT_TYPE_SIZE
A C expression for the size in bits of the type @code{int} on the
target machine. If you don't define this, the default is one word.
@end defmac
@defmac SHORT_TYPE_SIZE
A C expression for the size in bits of the type @code{short} on the
target machine. If you don't define this, the default is half a word.
(If this would be less than one storage unit, it is rounded up to one
unit.)
@end defmac
@defmac LONG_TYPE_SIZE
A C expression for the size in bits of the type @code{long} on the
target machine. If you don't define this, the default is one word.
@end defmac
@defmac ADA_LONG_TYPE_SIZE
On some machines, the size used for the Ada equivalent of the type
@code{long} by a native Ada compiler differs from that used by C@. In
that situation, define this macro to be a C expression to be used for
the size of that type. If you don't define this, the default is the
value of @code{LONG_TYPE_SIZE}.
@end defmac
@defmac LONG_LONG_TYPE_SIZE
A C expression for the size in bits of the type @code{long long} on the
target machine. If you don't define this, the default is two
words. If you want to support GNU Ada on your machine, the value of this
macro must be at least 64.
@end defmac
@defmac CHAR_TYPE_SIZE
A C expression for the size in bits of the type @code{char} on the
target machine. If you don't define this, the default is
@code{BITS_PER_UNIT}.
@end defmac
@defmac BOOL_TYPE_SIZE
A C expression for the size in bits of the C++ type @code{bool} and
C99 type @code{_Bool} on the target machine. If you don't define
this, and you probably shouldn't, the default is @code{CHAR_TYPE_SIZE}.
@end defmac
@defmac FLOAT_TYPE_SIZE
A C expression for the size in bits of the type @code{float} on the
target machine. If you don't define this, the default is one word.
@end defmac
@defmac DOUBLE_TYPE_SIZE
A C expression for the size in bits of the type @code{double} on the
target machine. If you don't define this, the default is two
words.
@end defmac
@defmac LONG_DOUBLE_TYPE_SIZE
A C expression for the size in bits of the type @code{long double} on
the target machine. If you don't define this, the default is two
words.
@end defmac
@defmac SHORT_FRACT_TYPE_SIZE
A C expression for the size in bits of the type @code{short _Fract} on
the target machine. If you don't define this, the default is
@code{BITS_PER_UNIT}.
@end defmac
@defmac FRACT_TYPE_SIZE
A C expression for the size in bits of the type @code{_Fract} on
the target machine. If you don't define this, the default is
@code{BITS_PER_UNIT * 2}.
@end defmac
@defmac LONG_FRACT_TYPE_SIZE
A C expression for the size in bits of the type @code{long _Fract} on
the target machine. If you don't define this, the default is
@code{BITS_PER_UNIT * 4}.
@end defmac
@defmac LONG_LONG_FRACT_TYPE_SIZE
A C expression for the size in bits of the type @code{long long _Fract} on
the target machine. If you don't define this, the default is
@code{BITS_PER_UNIT * 8}.
@end defmac
@defmac SHORT_ACCUM_TYPE_SIZE
A C expression for the size in bits of the type @code{short _Accum} on
the target machine. If you don't define this, the default is
@code{BITS_PER_UNIT * 2}.
@end defmac
@defmac ACCUM_TYPE_SIZE
A C expression for the size in bits of the type @code{_Accum} on
the target machine. If you don't define this, the default is
@code{BITS_PER_UNIT * 4}.
@end defmac
@defmac LONG_ACCUM_TYPE_SIZE
A C expression for the size in bits of the type @code{long _Accum} on
the target machine. If you don't define this, the default is
@code{BITS_PER_UNIT * 8}.
@end defmac
@defmac LONG_LONG_ACCUM_TYPE_SIZE
A C expression for the size in bits of the type @code{long long _Accum} on
the target machine. If you don't define this, the default is
@code{BITS_PER_UNIT * 16}.
@end defmac
@defmac LIBGCC2_GNU_PREFIX
This macro corresponds to the @code{TARGET_LIBFUNC_GNU_PREFIX} target
hook and should be defined if that hook is overriden to be true. It
causes function names in libgcc to be changed to use a @code{__gnu_}
prefix for their name rather than the default @code{__}. A port which
uses this macro should also arrange to use @file{t-gnu-prefix} in
the libgcc @file{config.host}.
@end defmac
@defmac WIDEST_HARDWARE_FP_SIZE
A C expression for the size in bits of the widest floating-point format
supported by the hardware. If you define this macro, you must specify a
value less than or equal to the value of @code{LONG_DOUBLE_TYPE_SIZE}.
If you do not define this macro, the value of @code{LONG_DOUBLE_TYPE_SIZE}
is the default.
@end defmac
@defmac DEFAULT_SIGNED_CHAR
An expression whose value is 1 or 0, according to whether the type
@code{char} should be signed or unsigned by default. The user can
always override this default with the options @option{-fsigned-char}
and @option{-funsigned-char}.
@end defmac
@hook TARGET_DEFAULT_SHORT_ENUMS
@defmac SIZE_TYPE
A C expression for a string describing the name of the data type to use
for size values. The typedef name @code{size_t} is defined using the
contents of the string.
The string can contain more than one keyword. If so, separate them with
spaces, and write first any length keyword, then @code{unsigned} if
appropriate, and finally @code{int}. The string must exactly match one
of the data type names defined in the function
@code{c_common_nodes_and_builtins} in the file @file{c-family/c-common.c}.
You may not omit @code{int} or change the order---that would cause the
compiler to crash on startup.
If you don't define this macro, the default is @code{"long unsigned
int"}.
@end defmac
@defmac SIZETYPE
GCC defines internal types (@code{sizetype}, @code{ssizetype},
@code{bitsizetype} and @code{sbitsizetype}) for expressions
dealing with size. This macro is a C expression for a string describing
the name of the data type from which the precision of @code{sizetype}
is extracted.
The string has the same restrictions as @code{SIZE_TYPE} string.
If you don't define this macro, the default is @code{SIZE_TYPE}.
@end defmac
@defmac PTRDIFF_TYPE
A C expression for a string describing the name of the data type to use
for the result of subtracting two pointers. The typedef name
@code{ptrdiff_t} is defined using the contents of the string. See
@code{SIZE_TYPE} above for more information.
If you don't define this macro, the default is @code{"long int"}.
@end defmac
@defmac WCHAR_TYPE
A C expression for a string describing the name of the data type to use
for wide characters. The typedef name @code{wchar_t} is defined using
the contents of the string. See @code{SIZE_TYPE} above for more
information.
If you don't define this macro, the default is @code{"int"}.
@end defmac
@defmac WCHAR_TYPE_SIZE
A C expression for the size in bits of the data type for wide
characters. This is used in @code{cpp}, which cannot make use of
@code{WCHAR_TYPE}.
@end defmac
@defmac WINT_TYPE
A C expression for a string describing the name of the data type to
use for wide characters passed to @code{printf} and returned from
@code{getwc}. The typedef name @code{wint_t} is defined using the
contents of the string. See @code{SIZE_TYPE} above for more
information.
If you don't define this macro, the default is @code{"unsigned int"}.
@end defmac
@defmac INTMAX_TYPE
A C expression for a string describing the name of the data type that
can represent any value of any standard or extended signed integer type.
The typedef name @code{intmax_t} is defined using the contents of the
string. See @code{SIZE_TYPE} above for more information.
If you don't define this macro, the default is the first of
@code{"int"}, @code{"long int"}, or @code{"long long int"} that has as
much precision as @code{long long int}.
@end defmac
@defmac UINTMAX_TYPE
A C expression for a string describing the name of the data type that
can represent any value of any standard or extended unsigned integer
type. The typedef name @code{uintmax_t} is defined using the contents
of the string. See @code{SIZE_TYPE} above for more information.
If you don't define this macro, the default is the first of
@code{"unsigned int"}, @code{"long unsigned int"}, or @code{"long long
unsigned int"} that has as much precision as @code{long long unsigned
int}.
@end defmac
@defmac SIG_ATOMIC_TYPE
@defmacx INT8_TYPE
@defmacx INT16_TYPE
@defmacx INT32_TYPE
@defmacx INT64_TYPE
@defmacx UINT8_TYPE
@defmacx UINT16_TYPE
@defmacx UINT32_TYPE
@defmacx UINT64_TYPE
@defmacx INT_LEAST8_TYPE
@defmacx INT_LEAST16_TYPE
@defmacx INT_LEAST32_TYPE
@defmacx INT_LEAST64_TYPE
@defmacx UINT_LEAST8_TYPE
@defmacx UINT_LEAST16_TYPE
@defmacx UINT_LEAST32_TYPE
@defmacx UINT_LEAST64_TYPE
@defmacx INT_FAST8_TYPE
@defmacx INT_FAST16_TYPE
@defmacx INT_FAST32_TYPE
@defmacx INT_FAST64_TYPE
@defmacx UINT_FAST8_TYPE
@defmacx UINT_FAST16_TYPE
@defmacx UINT_FAST32_TYPE
@defmacx UINT_FAST64_TYPE
@defmacx INTPTR_TYPE
@defmacx UINTPTR_TYPE
C expressions for the standard types @code{sig_atomic_t},
@code{int8_t}, @code{int16_t}, @code{int32_t}, @code{int64_t},
@code{uint8_t}, @code{uint16_t}, @code{uint32_t}, @code{uint64_t},
@code{int_least8_t}, @code{int_least16_t}, @code{int_least32_t},
@code{int_least64_t}, @code{uint_least8_t}, @code{uint_least16_t},
@code{uint_least32_t}, @code{uint_least64_t}, @code{int_fast8_t},
@code{int_fast16_t}, @code{int_fast32_t}, @code{int_fast64_t},
@code{uint_fast8_t}, @code{uint_fast16_t}, @code{uint_fast32_t},
@code{uint_fast64_t}, @code{intptr_t}, and @code{uintptr_t}. See
@code{SIZE_TYPE} above for more information.
If any of these macros evaluates to a null pointer, the corresponding
type is not supported; if GCC is configured to provide
@code{<stdint.h>} in such a case, the header provided may not conform
to C99, depending on the type in question. The defaults for all of
these macros are null pointers.
@end defmac
@defmac TARGET_PTRMEMFUNC_VBIT_LOCATION
The C++ compiler represents a pointer-to-member-function with a struct
that looks like:
@smallexample
struct @{
union @{
void (*fn)();
ptrdiff_t vtable_index;
@};
ptrdiff_t delta;
@};
@end smallexample
@noindent
The C++ compiler must use one bit to indicate whether the function that
will be called through a pointer-to-member-function is virtual.
Normally, we assume that the low-order bit of a function pointer must
always be zero. Then, by ensuring that the vtable_index is odd, we can
distinguish which variant of the union is in use. But, on some
platforms function pointers can be odd, and so this doesn't work. In
that case, we use the low-order bit of the @code{delta} field, and shift
the remainder of the @code{delta} field to the left.
GCC will automatically make the right selection about where to store
this bit using the @code{FUNCTION_BOUNDARY} setting for your platform.
However, some platforms such as ARM/Thumb have @code{FUNCTION_BOUNDARY}
set such that functions always start at even addresses, but the lowest
bit of pointers to functions indicate whether the function at that
address is in ARM or Thumb mode. If this is the case of your
architecture, you should define this macro to
@code{ptrmemfunc_vbit_in_delta}.
In general, you should not have to define this macro. On architectures
in which function addresses are always even, according to
@code{FUNCTION_BOUNDARY}, GCC will automatically define this macro to
@code{ptrmemfunc_vbit_in_pfn}.
@end defmac
@defmac TARGET_VTABLE_USES_DESCRIPTORS
Normally, the C++ compiler uses function pointers in vtables. This
macro allows the target to change to use ``function descriptors''
instead. Function descriptors are found on targets for whom a
function pointer is actually a small data structure. Normally the
data structure consists of the actual code address plus a data
pointer to which the function's data is relative.
If vtables are used, the value of this macro should be the number
of words that the function descriptor occupies.
@end defmac
@defmac TARGET_VTABLE_ENTRY_ALIGN
By default, the vtable entries are void pointers, the so the alignment
is the same as pointer alignment. The value of this macro specifies
the alignment of the vtable entry in bits. It should be defined only
when special alignment is necessary. */
@end defmac
@defmac TARGET_VTABLE_DATA_ENTRY_DISTANCE
There are a few non-descriptor entries in the vtable at offsets below
zero. If these entries must be padded (say, to preserve the alignment
specified by @code{TARGET_VTABLE_ENTRY_ALIGN}), set this to the number
of words in each data entry.
@end defmac
@node Registers
@section Register Usage
@cindex register usage
This section explains how to describe what registers the target machine
has, and how (in general) they can be used.
The description of which registers a specific instruction can use is
done with register classes; see @ref{Register Classes}. For information
on using registers to access a stack frame, see @ref{Frame Registers}.
For passing values in registers, see @ref{Register Arguments}.
For returning values in registers, see @ref{Scalar Return}.
@menu
* Register Basics:: Number and kinds of registers.
* Allocation Order:: Order in which registers are allocated.
* Values in Registers:: What kinds of values each reg can hold.
* Leaf Functions:: Renumbering registers for leaf functions.
* Stack Registers:: Handling a register stack such as 80387.
@end menu
@node Register Basics
@subsection Basic Characteristics of Registers
@c prevent bad page break with this line
Registers have various characteristics.
@defmac FIRST_PSEUDO_REGISTER
Number of hardware registers known to the compiler. They receive
numbers 0 through @code{FIRST_PSEUDO_REGISTER-1}; thus, the first
pseudo register's number really is assigned the number
@code{FIRST_PSEUDO_REGISTER}.
@end defmac
@defmac FIXED_REGISTERS
@cindex fixed register
An initializer that says which registers are used for fixed purposes
all throughout the compiled code and are therefore not available for
general allocation. These would include the stack pointer, the frame
pointer (except on machines where that can be used as a general
register when no frame pointer is needed), the program counter on
machines where that is considered one of the addressable registers,
and any other numbered register with a standard use.
This information is expressed as a sequence of numbers, separated by
commas and surrounded by braces. The @var{n}th number is 1 if
register @var{n} is fixed, 0 otherwise.
The table initialized from this macro, and the table initialized by
the following one, may be overridden at run time either automatically,
by the actions of the macro @code{CONDITIONAL_REGISTER_USAGE}, or by
the user with the command options @option{-ffixed-@var{reg}},
@option{-fcall-used-@var{reg}} and @option{-fcall-saved-@var{reg}}.
@end defmac
@defmac CALL_USED_REGISTERS
@cindex call-used register
@cindex call-clobbered register
@cindex call-saved register
Like @code{FIXED_REGISTERS} but has 1 for each register that is
clobbered (in general) by function calls as well as for fixed
registers. This macro therefore identifies the registers that are not
available for general allocation of values that must live across
function calls.
If a register has 0 in @code{CALL_USED_REGISTERS}, the compiler
automatically saves it on function entry and restores it on function
exit, if the register is used within the function.
Exactly one of @code{CALL_USED_REGISTERS} and @code{CALL_REALLY_USED_REGISTERS}
must be defined. Modern ports should define @code{CALL_REALLY_USED_REGISTERS}.
@end defmac
@defmac CALL_REALLY_USED_REGISTERS
@cindex call-used register
@cindex call-clobbered register
@cindex call-saved register
Like @code{CALL_USED_REGISTERS} except this macro doesn't require
that the entire set of @code{FIXED_REGISTERS} be included.
(@code{CALL_USED_REGISTERS} must be a superset of @code{FIXED_REGISTERS}).
Exactly one of @code{CALL_USED_REGISTERS} and @code{CALL_REALLY_USED_REGISTERS}
must be defined. Modern ports should define @code{CALL_REALLY_USED_REGISTERS}.
@end defmac
@cindex call-used register
@cindex call-clobbered register
@cindex call-saved register
@hook TARGET_FNTYPE_ABI
@hook TARGET_INSN_CALLEE_ABI
@cindex call-used register
@cindex call-clobbered register
@cindex call-saved register
@hook TARGET_HARD_REGNO_CALL_PART_CLOBBERED
@hook TARGET_GET_MULTILIB_ABI_NAME
@findex fixed_regs
@findex call_used_regs
@findex global_regs
@findex reg_names
@findex reg_class_contents
@hook TARGET_CONDITIONAL_REGISTER_USAGE
@defmac INCOMING_REGNO (@var{out})
Define this macro if the target machine has register windows. This C
expression returns the register number as seen by the called function
corresponding to the register number @var{out} as seen by the calling
function. Return @var{out} if register number @var{out} is not an
outbound register.
@end defmac
@defmac OUTGOING_REGNO (@var{in})
Define this macro if the target machine has register windows. This C
expression returns the register number as seen by the calling function
corresponding to the register number @var{in} as seen by the called
function. Return @var{in} if register number @var{in} is not an inbound
register.
@end defmac
@defmac LOCAL_REGNO (@var{regno})
Define this macro if the target machine has register windows. This C
expression returns true if the register is call-saved but is in the
register window. Unlike most call-saved registers, such registers
need not be explicitly restored on function exit or during non-local
gotos.
@end defmac
@defmac PC_REGNUM
If the program counter has a register number, define this as that
register number. Otherwise, do not define it.
@end defmac
@node Allocation Order
@subsection Order of Allocation of Registers
@cindex order of register allocation
@cindex register allocation order
@c prevent bad page break with this line
Registers are allocated in order.
@defmac REG_ALLOC_ORDER
If defined, an initializer for a vector of integers, containing the
numbers of hard registers in the order in which GCC should prefer
to use them (from most preferred to least).
If this macro is not defined, registers are used lowest numbered first
(all else being equal).
One use of this macro is on machines where the highest numbered
registers must always be saved and the save-multiple-registers
instruction supports only sequences of consecutive registers. On such
machines, define @code{REG_ALLOC_ORDER} to be an initializer that lists
the highest numbered allocable register first.
@end defmac
@defmac ADJUST_REG_ALLOC_ORDER
A C statement (sans semicolon) to choose the order in which to allocate
hard registers for pseudo-registers local to a basic block.
Store the desired register order in the array @code{reg_alloc_order}.
Element 0 should be the register to allocate first; element 1, the next
register; and so on.
The macro body should not assume anything about the contents of
@code{reg_alloc_order} before execution of the macro.
On most machines, it is not necessary to define this macro.
@end defmac
@defmac HONOR_REG_ALLOC_ORDER
Normally, IRA tries to estimate the costs for saving a register in the
prologue and restoring it in the epilogue. This discourages it from
using call-saved registers. If a machine wants to ensure that IRA
allocates registers in the order given by REG_ALLOC_ORDER even if some
call-saved registers appear earlier than call-used ones, then define this
macro as a C expression to nonzero. Default is 0.
@end defmac
@defmac IRA_HARD_REGNO_ADD_COST_MULTIPLIER (@var{regno})
In some case register allocation order is not enough for the
Integrated Register Allocator (@acronym{IRA}) to generate a good code.
If this macro is defined, it should return a floating point value
based on @var{regno}. The cost of using @var{regno} for a pseudo will
be increased by approximately the pseudo's usage frequency times the
value returned by this macro. Not defining this macro is equivalent
to having it always return @code{0.0}.
On most machines, it is not necessary to define this macro.
@end defmac
@node Values in Registers
@subsection How Values Fit in Registers
This section discusses the macros that describe which kinds of values
(specifically, which machine modes) each register can hold, and how many
consecutive registers are needed for a given mode.
@hook TARGET_HARD_REGNO_NREGS
@defmac HARD_REGNO_NREGS_HAS_PADDING (@var{regno}, @var{mode})
A C expression that is nonzero if a value of mode @var{mode}, stored
in memory, ends with padding that causes it to take up more space than
in registers starting at register number @var{regno} (as determined by
multiplying GCC's notion of the size of the register when containing
this mode by the number of registers returned by
@code{TARGET_HARD_REGNO_NREGS}). By default this is zero.
For example, if a floating-point value is stored in three 32-bit
registers but takes up 128 bits in memory, then this would be
nonzero.
This macros only needs to be defined if there are cases where
@code{subreg_get_info}
would otherwise wrongly determine that a @code{subreg} can be
represented by an offset to the register number, when in fact such a
@code{subreg} would contain some of the padding not stored in
registers and so not be representable.
@end defmac
@defmac HARD_REGNO_NREGS_WITH_PADDING (@var{regno}, @var{mode})
For values of @var{regno} and @var{mode} for which
@code{HARD_REGNO_NREGS_HAS_PADDING} returns nonzero, a C expression
returning the greater number of registers required to hold the value
including any padding. In the example above, the value would be four.
@end defmac
@defmac REGMODE_NATURAL_SIZE (@var{mode})
Define this macro if the natural size of registers that hold values
of mode @var{mode} is not the word size. It is a C expression that
should give the natural size in bytes for the specified mode. It is
used by the register allocator to try to optimize its results. This
happens for example on SPARC 64-bit where the natural size of
floating-point registers is still 32-bit.
@end defmac
@hook TARGET_HARD_REGNO_MODE_OK
@defmac HARD_REGNO_RENAME_OK (@var{from}, @var{to})
A C expression that is nonzero if it is OK to rename a hard register
@var{from} to another hard register @var{to}.
One common use of this macro is to prevent renaming of a register to
another register that is not saved by a prologue in an interrupt
handler.
The default is always nonzero.
@end defmac
@hook TARGET_MODES_TIEABLE_P
@hook TARGET_HARD_REGNO_SCRATCH_OK
@defmac AVOID_CCMODE_COPIES
Define this macro if the compiler should avoid copies to/from @code{CCmode}
registers. You should only define this macro if support for copying to/from
@code{CCmode} is incomplete.
@end defmac
@node Leaf Functions
@subsection Handling Leaf Functions
@cindex leaf functions
@cindex functions, leaf
On some machines, a leaf function (i.e., one which makes no calls) can run
more efficiently if it does not make its own register window. Often this
means it is required to receive its arguments in the registers where they
are passed by the caller, instead of the registers where they would
normally arrive.
The special treatment for leaf functions generally applies only when
other conditions are met; for example, often they may use only those
registers for its own variables and temporaries. We use the term ``leaf
function'' to mean a function that is suitable for this special
handling, so that functions with no calls are not necessarily ``leaf
functions''.
GCC assigns register numbers before it knows whether the function is
suitable for leaf function treatment. So it needs to renumber the
registers in order to output a leaf function. The following macros
accomplish this.
@defmac LEAF_REGISTERS
Name of a char vector, indexed by hard register number, which
contains 1 for a register that is allowable in a candidate for leaf
function treatment.
If leaf function treatment involves renumbering the registers, then the
registers marked here should be the ones before renumbering---those that
GCC would ordinarily allocate. The registers which will actually be
used in the assembler code, after renumbering, should not be marked with 1
in this vector.
Define this macro only if the target machine offers a way to optimize
the treatment of leaf functions.
@end defmac
@defmac LEAF_REG_REMAP (@var{regno})
A C expression whose value is the register number to which @var{regno}
should be renumbered, when a function is treated as a leaf function.
If @var{regno} is a register number which should not appear in a leaf
function before renumbering, then the expression should yield @minus{}1, which
will cause the compiler to abort.
Define this macro only if the target machine offers a way to optimize the
treatment of leaf functions, and registers need to be renumbered to do
this.
@end defmac
@findex current_function_is_leaf
@findex current_function_uses_only_leaf_regs
@code{TARGET_ASM_FUNCTION_PROLOGUE} and
@code{TARGET_ASM_FUNCTION_EPILOGUE} must usually treat leaf functions
specially. They can test the C variable @code{current_function_is_leaf}
which is nonzero for leaf functions. @code{current_function_is_leaf} is
set prior to local register allocation and is valid for the remaining
compiler passes. They can also test the C variable
@code{current_function_uses_only_leaf_regs} which is nonzero for leaf
functions which only use leaf registers.
@code{current_function_uses_only_leaf_regs} is valid after all passes
that modify the instructions have been run and is only useful if
@code{LEAF_REGISTERS} is defined.
@c changed this to fix overfull. ALSO: why the "it" at the beginning
@c of the next paragraph?! --mew 2feb93
@node Stack Registers
@subsection Registers That Form a Stack
There are special features to handle computers where some of the
``registers'' form a stack. Stack registers are normally written by
pushing onto the stack, and are numbered relative to the top of the
stack.
Currently, GCC can only handle one group of stack-like registers, and
they must be consecutively numbered. Furthermore, the existing
support for stack-like registers is specific to the 80387 floating
point coprocessor. If you have a new architecture that uses
stack-like registers, you will need to do substantial work on
@file{reg-stack.c} and write your machine description to cooperate
with it, as well as defining these macros.
@defmac STACK_REGS
Define this if the machine has any stack-like registers.
@end defmac
@defmac STACK_REG_COVER_CLASS
This is a cover class containing the stack registers. Define this if
the machine has any stack-like registers.
@end defmac
@defmac FIRST_STACK_REG
The number of the first stack-like register. This one is the top
of the stack.
@end defmac
@defmac LAST_STACK_REG
The number of the last stack-like register. This one is the bottom of
the stack.
@end defmac
@node Register Classes
@section Register Classes
@cindex register class definitions
@cindex class definitions, register
On many machines, the numbered registers are not all equivalent.
For example, certain registers may not be allowed for indexed addressing;
certain registers may not be allowed in some instructions. These machine
restrictions are described to the compiler using @dfn{register classes}.
You define a number of register classes, giving each one a name and saying
which of the registers belong to it. Then you can specify register classes
that are allowed as operands to particular instruction patterns.
@findex ALL_REGS
@findex NO_REGS
In general, each register will belong to several classes. In fact, one
class must be named @code{ALL_REGS} and contain all the registers. Another
class must be named @code{NO_REGS} and contain no registers. Often the
union of two classes will be another class; however, this is not required.
@findex GENERAL_REGS
One of the classes must be named @code{GENERAL_REGS}. There is nothing
terribly special about the name, but the operand constraint letters
@samp{r} and @samp{g} specify this class. If @code{GENERAL_REGS} is
the same as @code{ALL_REGS}, just define it as a macro which expands
to @code{ALL_REGS}.
Order the classes so that if class @var{x} is contained in class @var{y}
then @var{x} has a lower class number than @var{y}.
The way classes other than @code{GENERAL_REGS} are specified in operand
constraints is through machine-dependent operand constraint letters.
You can define such letters to correspond to various classes, then use
them in operand constraints.
You must define the narrowest register classes for allocatable
registers, so that each class either has no subclasses, or that for
some mode, the move cost between registers within the class is
cheaper than moving a register in the class to or from memory
(@pxref{Costs}).
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,
or even internal compiler errors when reload cannot find a register in the
class computed via @code{reg_class_subunion}.
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{TARGET_HARD_REGNO_MODE_OK}.
Register classes used for input-operands of bitwise-and or shift
instructions have a special requirement: each such class must have, for
each fixed-point machine mode, a subclass whose registers can transfer that
mode to or from memory. For example, on some machines, the operations for
single-byte values (@code{QImode}) are limited to certain registers. When
this is so, each register class that is used in a bitwise-and or shift
instruction must have a subclass consisting of registers from which
single-byte values can be loaded or stored. This is so that
@code{PREFERRED_RELOAD_CLASS} can always have a possible value to return.
@deftp {Data type} {enum reg_class}
An enumerated type that must be defined with all the register class names
as enumerated values. @code{NO_REGS} must be first. @code{ALL_REGS}
must be the last register class, followed by one more enumerated value,
@code{LIM_REG_CLASSES}, which is not a register class but rather
tells how many classes there are.
Each register class has a number, which is the value of casting
the class name to type @code{int}. The number serves as an index
in many of the tables described below.
@end deftp
@defmac N_REG_CLASSES
The number of distinct register classes, defined as follows:
@smallexample
#define N_REG_CLASSES (int) LIM_REG_CLASSES
@end smallexample
@end defmac
@defmac REG_CLASS_NAMES
An initializer containing the names of the register classes as C string
constants. These names are used in writing some of the debugging dumps.
@end defmac
@defmac REG_CLASS_CONTENTS
An initializer containing the contents of the register classes, as integers
which are bit masks. The @var{n}th integer specifies the contents of class
@var{n}. The way the integer @var{mask} is interpreted is that
register @var{r} is in the class if @code{@var{mask} & (1 << @var{r})} is 1.
When the machine has more than 32 registers, an integer does not suffice.
Then the integers are replaced by sub-initializers, braced groupings containing
several integers. Each sub-initializer must be suitable as an initializer
for the type @code{HARD_REG_SET} which is defined in @file{hard-reg-set.h}.
In this situation, the first integer in each sub-initializer corresponds to
registers 0 through 31, the second integer to registers 32 through 63, and
so on.
@end defmac
@defmac REGNO_REG_CLASS (@var{regno})
A C expression whose value is a register class containing hard register
@var{regno}. In general there is more than one such class; choose a class
which is @dfn{minimal}, meaning that no smaller class also contains the
register.
@end defmac
@defmac BASE_REG_CLASS
A macro whose definition is the name of the class to which a valid
base register must belong. A base register is one used in an address
which is the register value plus a displacement.
@end defmac
@defmac MODE_BASE_REG_CLASS (@var{mode})
This is a variation of the @code{BASE_REG_CLASS} macro which allows
the selection of a base register in a mode dependent manner. If
@var{mode} is VOIDmode then it should return the same value as
@code{BASE_REG_CLASS}.
@end defmac
@defmac MODE_BASE_REG_REG_CLASS (@var{mode})
A C expression whose value is the register class to which a valid
base register must belong in order to be used in a base plus index
register address. You should define this macro if base plus index
addresses have different requirements than other base register uses.
@end defmac
@defmac MODE_CODE_BASE_REG_CLASS (@var{mode}, @var{address_space}, @var{outer_code}, @var{index_code})
A C expression whose value is the register class to which a valid
base register for a memory reference in mode @var{mode} to address
space @var{address_space} must belong. @var{outer_code} and @var{index_code}
define the context in which the base register occurs. @var{outer_code} is
the code of the immediately enclosing expression (@code{MEM} for the top level
of an address, @code{ADDRESS} for something that occurs in an
@code{address_operand}). @var{index_code} is the code of the corresponding
index expression if @var{outer_code} is @code{PLUS}; @code{SCRATCH} otherwise.
@end defmac
@defmac INDEX_REG_CLASS
A macro whose definition is the name of the class to which a valid
index register must belong. An index register is one used in an
address where its value is either multiplied by a scale factor or
added to another register (as well as added to a displacement).
@end defmac
@defmac 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.
@end defmac
@defmac REGNO_MODE_OK_FOR_BASE_P (@var{num}, @var{mode})
A C expression that is just like @code{REGNO_OK_FOR_BASE_P}, except that
that expression may examine the mode of the memory reference in
@var{mode}. You should define this macro if the mode of the memory
reference affects whether a register may be used as a base register. If
you define this macro, the compiler will use it instead of
@code{REGNO_OK_FOR_BASE_P}. The mode may be @code{VOIDmode} for
addresses that appear outside a @code{MEM}, i.e., as an
@code{address_operand}.
@end defmac
@defmac REGNO_MODE_OK_FOR_REG_BASE_P (@var{num}, @var{mode})
A C expression which is nonzero if register number @var{num} is suitable for
use as a base register in base plus index operand addresses, accessing
memory in mode @var{mode}. It may be either a suitable hard register or a
pseudo register that has been allocated such a hard register. You should
define this macro if base plus index addresses have different requirements
than other base register uses.
Use of this macro is deprecated; please use the more general
@code{REGNO_MODE_CODE_OK_FOR_BASE_P}.
@end defmac
@defmac REGNO_MODE_CODE_OK_FOR_BASE_P (@var{num}, @var{mode}, @var{address_space}, @var{outer_code}, @var{index_code})
A C expression which is nonzero if register number @var{num} is
suitable for use as a base register in operand addresses, accessing
memory in mode @var{mode} in address space @var{address_space}.
This is similar to @code{REGNO_MODE_OK_FOR_BASE_P}, except
that that expression may examine the context in which the register
appears in the memory reference. @var{outer_code} is the code of the
immediately enclosing expression (@code{MEM} if at the top level of the
address, @code{ADDRESS} for something that occurs in an
@code{address_operand}). @var{index_code} is the code of the
corresponding index expression if @var{outer_code} is @code{PLUS};
@code{SCRATCH} otherwise. The mode may be @code{VOIDmode} for addresses
that appear outside a @code{MEM}, i.e., as an @code{address_operand}.
@end defmac
@defmac REGNO_OK_FOR_INDEX_P (@var{num})
A C expression which is nonzero if register number @var{num} is
suitable for use as an index register in operand addresses. It may be
either a suitable hard register or a pseudo register that has been
allocated such a hard register.
The difference between an index register and a base register is that
the index register may be scaled. If an address involves the sum of
two registers, neither one of them scaled, then either one may be
labeled the ``base'' and the other the ``index''; but whichever
labeling is used must fit the machine's constraints of which registers
may serve in each capacity. The compiler will try both labelings,
looking for one that is valid, and will reload one or both registers
only if neither labeling works.
@end defmac
@hook TARGET_PREFERRED_RENAME_CLASS
@hook TARGET_PREFERRED_RELOAD_CLASS
@defmac PREFERRED_RELOAD_CLASS (@var{x}, @var{class})
A C expression that places additional restrictions on the register class
to use when it is necessary to copy value @var{x} into a register in class
@var{class}. The value is a register class; perhaps @var{class}, or perhaps
another, smaller class. On many machines, the following definition is
safe:
@smallexample
#define PREFERRED_RELOAD_CLASS(X,CLASS) CLASS
@end smallexample
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.
One case where @code{PREFERRED_RELOAD_CLASS} must not return
@var{class} is if @var{x} is a legitimate constant which cannot be
loaded into some register class. By returning @code{NO_REGS} you can
force @var{x} into a memory location. For example, rs6000 can load
immediate values into general-purpose registers, but does not have an
instruction for loading an immediate value into a floating-point
register, so @code{PREFERRED_RELOAD_CLASS} returns @code{NO_REGS} when
@var{x} is a floating-point constant. If the constant cannot be loaded
into any kind of register, code generation will be better if
@code{TARGET_LEGITIMATE_CONSTANT_P} makes the constant illegitimate instead
of using @code{TARGET_PREFERRED_RELOAD_CLASS}.
If an insn has pseudos in it after register allocation, reload will go
through the alternatives and call repeatedly @code{PREFERRED_RELOAD_CLASS}
to find the best one. Returning @code{NO_REGS}, in this case, makes
reload add a @code{!} in front of the constraint: the x86 back-end uses
this feature to discourage usage of 387 registers when math is done in
the SSE registers (and vice versa).
@end defmac
@hook TARGET_PREFERRED_OUTPUT_RELOAD_CLASS
@defmac LIMIT_RELOAD_CLASS (@var{mode}, @var{class})
A C expression that places additional restrictions on the register class
to use when it is necessary to be able to hold a value of mode
@var{mode} in a reload register for which class @var{class} would
ordinarily be used.
Unlike @code{PREFERRED_RELOAD_CLASS}, this macro should be used when
there are certain modes that simply cannot go in certain reload classes.
The value is a register class; perhaps @var{class}, or perhaps another,
smaller class.
Don't define this macro unless the target machine has limitations which
require the macro to do something nontrivial.
@end defmac
@hook TARGET_SECONDARY_RELOAD
@defmac SECONDARY_RELOAD_CLASS (@var{class}, @var{mode}, @var{x})
@defmacx SECONDARY_INPUT_RELOAD_CLASS (@var{class}, @var{mode}, @var{x})
@defmacx SECONDARY_OUTPUT_RELOAD_CLASS (@var{class}, @var{mode}, @var{x})
These macros are obsolete, new ports should use the target hook
@code{TARGET_SECONDARY_RELOAD} instead.
These are obsolete macros, replaced by the @code{TARGET_SECONDARY_RELOAD}
target hook. Older ports still 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 were supposed to define @code{SECONDARY_INPUT_RELOAD_CLASS} to return the
largest register class all of whose registers can be used as
intermediate registers or scratch registers.
If copying a register @var{class} in @var{mode} to @var{x} requires an
intermediate or scratch register, @code{SECONDARY_OUTPUT_RELOAD_CLASS}
was supposed to be defined be defined to return the largest register
class required. If the
requirements for input and output reloads were the same, the macro
@code{SECONDARY_RELOAD_CLASS} should have been 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 were supposed to define patterns for
@samp{reload_in@var{m}} or @samp{reload_out@var{m}}, as required
(@pxref{Standard Names}. These patterns, which were normally
implemented with a @code{define_expand}, should be similar to the
@samp{mov@var{m}} patterns, except that operand 2 is the scratch
register.
These patterns need constraints for the reload register and scratch
register that
contain a single register class. If the original reload register (whose
class is @var{class}) can meet the constraint given in the pattern, the
value returned by these macros is used for the class of the scratch
register. Otherwise, two additional reload registers are required.
Their classes are obtained from the constraints in the insn pattern.
@var{x} might be a pseudo-register or a @code{subreg} of a
pseudo-register, which could either be in a hard register or in memory.
Use @code{true_regnum} to find out; it will return @minus{}1 if the pseudo is
in memory and the hard register number if it is in a register.
These macros should not be used in the case where a particular class of
registers can only be copied to memory and not to another class of
registers. In that case, secondary reload registers are not needed and
would not be helpful. Instead, a stack location must be used to perform
the copy and the @code{mov@var{m}} pattern should use memory as an
intermediate storage. This case often occurs between floating-point and
general registers.
@end defmac
@hook TARGET_SECONDARY_MEMORY_NEEDED
@defmac SECONDARY_MEMORY_NEEDED_RTX (@var{mode})
Normally when @code{TARGET_SECONDARY_MEMORY_NEEDED} is defined, the compiler
allocates a stack slot for a memory location needed for register copies.
If this macro is defined, the compiler instead uses the memory location
defined by this macro.
Do not define this macro if you do not define
@code{TARGET_SECONDARY_MEMORY_NEEDED}.
@end defmac
@hook TARGET_SECONDARY_MEMORY_NEEDED_MODE
@hook TARGET_SELECT_EARLY_REMAT_MODES
@hook TARGET_CLASS_LIKELY_SPILLED_P
@hook TARGET_CLASS_MAX_NREGS
@defmac CLASS_MAX_NREGS (@var{class}, @var{mode})
A C expression for the maximum number of consecutive registers
of class @var{class} needed to hold a value of mode @var{mode}.
This is closely related to the macro @code{TARGET_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{TARGET_HARD_REGNO_NREGS (@var{regno},
@var{mode})} for all @var{regno} values in the class @var{class}.
This macro helps control the handling of multiple-word values
in the reload pass.
@end defmac
@hook TARGET_CAN_CHANGE_MODE_CLASS
@hook TARGET_IRA_CHANGE_PSEUDO_ALLOCNO_CLASS
@hook TARGET_LRA_P
@hook TARGET_REGISTER_PRIORITY
@hook TARGET_REGISTER_USAGE_LEVELING_P
@hook TARGET_DIFFERENT_ADDR_DISPLACEMENT_P
@hook TARGET_CANNOT_SUBSTITUTE_MEM_EQUIV_P
@hook TARGET_LEGITIMIZE_ADDRESS_DISPLACEMENT
@hook TARGET_SPILL_CLASS
@hook TARGET_ADDITIONAL_ALLOCNO_CLASS_P
@hook TARGET_CSTORE_MODE
@hook TARGET_COMPUTE_PRESSURE_CLASSES
@node Stack and Calling
@section Stack Layout and Calling Conventions
@cindex calling conventions
@c prevent bad page break with this line
This describes the stack layout and calling conventions.
@menu
* Frame Layout::
* Exception Handling::
* Stack Checking::
* Frame Registers::
* Elimination::
* Stack Arguments::
* Register Arguments::
* Scalar Return::
* Aggregate Return::
* Caller Saves::
* Function Entry::
* Profiling::
* Tail Calls::
* Shrink-wrapping separate components::
* Stack Smashing Protection::
* Miscellaneous Register Hooks::
@end menu
@node Frame Layout
@subsection Basic Stack Layout
@cindex stack frame layout
@cindex frame layout
@c prevent bad page break with this line
Here is the basic stack layout.
@defmac STACK_GROWS_DOWNWARD
Define this macro to be true if pushing a word onto the stack moves the stack
pointer to a smaller address, and false otherwise.
@end defmac
@defmac STACK_PUSH_CODE
This macro defines the operation used when something is pushed
on the stack. In RTL, a push operation will be
@code{(set (mem (STACK_PUSH_CODE (reg sp))) @dots{})}
The choices are @code{PRE_DEC}, @code{POST_DEC}, @code{PRE_INC},
and @code{POST_INC}. Which of these is correct depends on
the stack direction and on whether the stack pointer points
to the last item on the stack or whether it points to the
space for the next item on the stack.
The default is @code{PRE_DEC} when @code{STACK_GROWS_DOWNWARD} is
true, which is almost always right, and @code{PRE_INC} otherwise,
which is often wrong.
@end defmac
@defmac FRAME_GROWS_DOWNWARD
Define this macro to nonzero value if the addresses of local variable slots
are at negative offsets from the frame pointer.
@end defmac
@defmac ARGS_GROW_DOWNWARD
Define this macro if successive arguments to a function occupy decreasing
addresses on the stack.
@end defmac
@hook TARGET_STARTING_FRAME_OFFSET
@defmac STACK_ALIGNMENT_NEEDED
Define to zero to disable final alignment of the stack during reload.
The nonzero default for this macro is suitable for most ports.
On ports where @code{TARGET_STARTING_FRAME_OFFSET} is nonzero or where there
is a register save block following the local block that doesn't require
alignment to @code{STACK_BOUNDARY}, it may be beneficial to disable
stack alignment and do it in the backend.
@end defmac
@defmac STACK_POINTER_OFFSET
Offset from the stack pointer register to the first location at which
outgoing arguments are placed. If not specified, the default value of
zero is used. This is the proper value for most machines.
If @code{ARGS_GROW_DOWNWARD}, this is the offset to the location above
the first location at which outgoing arguments are placed.
@end defmac
@defmac FIRST_PARM_OFFSET (@var{fundecl})
Offset from the argument pointer register to the first argument's
address. On some machines it may depend on the data type of the
function.
If @code{ARGS_GROW_DOWNWARD}, this is the offset to the location above
the first argument's address.
@end defmac
@defmac STACK_DYNAMIC_OFFSET (@var{fundecl})
Offset from the stack pointer register to an item dynamically allocated
on the stack, e.g., by @code{alloca}.
The default value for this macro is @code{STACK_POINTER_OFFSET} plus the
length of the outgoing arguments. The default is correct for most
machines. See @file{function.c} for details.
@end defmac
@defmac INITIAL_FRAME_ADDRESS_RTX
A C expression whose value is RTL representing the address of the initial
stack frame. This address is passed to @code{RETURN_ADDR_RTX} and
@code{DYNAMIC_CHAIN_ADDRESS}. If you don't define this macro, a reasonable
default value will be used. Define this macro in order to make frame pointer
elimination work in the presence of @code{__builtin_frame_address (count)} and
@code{__builtin_return_address (count)} for @code{count} not equal to zero.
@end defmac
@defmac DYNAMIC_CHAIN_ADDRESS (@var{frameaddr})
A C expression whose value is RTL representing the address in a stack
frame where the pointer to the caller's frame is stored. Assume that
@var{frameaddr} is an RTL expression for the address of the stack frame
itself.
If you don't define this macro, the default is to return the value
of @var{frameaddr}---that is, the stack frame address is also the
address of the stack word that points to the previous frame.
@end defmac
@defmac SETUP_FRAME_ADDRESSES
A C expression that produces the machine-specific code to
setup the stack so that arbitrary frames can be accessed. For example,
on the SPARC, we must flush all of the register windows to the stack
before we can access arbitrary stack frames. You will seldom need to
define this macro. The default is to do nothing.
@end defmac
@hook TARGET_BUILTIN_SETJMP_FRAME_VALUE
@defmac FRAME_ADDR_RTX (@var{frameaddr})
A C expression whose value is RTL representing the value of the frame
address for the current frame. @var{frameaddr} is the frame pointer
of the current frame. This is used for __builtin_frame_address.
You need only define this macro if the frame address is not the same
as the frame pointer. Most machines do not need to define it.
@end defmac
@defmac RETURN_ADDR_RTX (@var{count}, @var{frameaddr})
A C expression whose value is RTL representing the value of the return
address for the frame @var{count} steps up from the current frame, after
the prologue. @var{frameaddr} is the frame pointer of the @var{count}
frame, or the frame pointer of the @var{count} @minus{} 1 frame if
@code{RETURN_ADDR_IN_PREVIOUS_FRAME} is nonzero.
The value of the expression must always be the correct address when
@var{count} is zero, but may be @code{NULL_RTX} if there is no way to
determine the return address of other frames.
@end defmac
@defmac RETURN_ADDR_IN_PREVIOUS_FRAME
Define this macro to nonzero value if the return address of a particular
stack frame is accessed from the frame pointer of the previous stack
frame. The zero default for this macro is suitable for most ports.
@end defmac
@defmac INCOMING_RETURN_ADDR_RTX
A C expression whose value is RTL representing the location of the
incoming return address at the beginning of any function, before the
prologue. This RTL is either a @code{REG}, indicating that the return
value is saved in @samp{REG}, or a @code{MEM} representing a location in
the stack.
You only need to define this macro if you want to support call frame
debugging information like that provided by DWARF 2.
If this RTL is a @code{REG}, you should also define
@code{DWARF_FRAME_RETURN_COLUMN} to @code{DWARF_FRAME_REGNUM (REGNO)}.
@end defmac
@defmac DWARF_ALT_FRAME_RETURN_COLUMN
A C expression whose value is an integer giving a DWARF 2 column
number that may be used as an alternative return column. The column
must not correspond to any gcc hard register (that is, it must not
be in the range of @code{DWARF_FRAME_REGNUM}).
This macro can be useful if @code{DWARF_FRAME_RETURN_COLUMN} is set to a
general register, but an alternative column needs to be used for signal
frames. Some targets have also used different frame return columns
over time.
@end defmac
@defmac DWARF_ZERO_REG
A C expression whose value is an integer giving a DWARF 2 register
number that is considered to always have the value zero. This should
only be defined if the target has an architected zero register, and
someone decided it was a good idea to use that register number to
terminate the stack backtrace. New ports should avoid this.
@end defmac
@hook TARGET_DWARF_HANDLE_FRAME_UNSPEC
@hook TARGET_DWARF_POLY_INDETERMINATE_VALUE
@defmac INCOMING_FRAME_SP_OFFSET
A C expression whose value is an integer giving the offset, in bytes,
from the value of the stack pointer register to the top of the stack
frame at the beginning of any function, before the prologue. The top of
the frame is defined to be the value of the stack pointer in the
previous frame, just before the call instruction.
You only need to define this macro if you want to support call frame
debugging information like that provided by DWARF 2.
@end defmac
@defmac DEFAULT_INCOMING_FRAME_SP_OFFSET
Like @code{INCOMING_FRAME_SP_OFFSET}, but must be the same for all
functions of the same ABI, and when using GAS @code{.cfi_*} directives
must also agree with the default CFI GAS emits. Define this macro
only if @code{INCOMING_FRAME_SP_OFFSET} can have different values
between different functions of the same ABI or when
@code{INCOMING_FRAME_SP_OFFSET} does not agree with GAS default CFI.
@end defmac
@defmac ARG_POINTER_CFA_OFFSET (@var{fundecl})
A C expression whose value is an integer giving the offset, in bytes,
from the argument pointer to the canonical frame address (cfa). The
final value should coincide with that calculated by
@code{INCOMING_FRAME_SP_OFFSET}. Which is unfortunately not usable
during virtual register instantiation.
The default value for this macro is
@code{FIRST_PARM_OFFSET (fundecl) + crtl->args.pretend_args_size},
which is correct for most machines; in general, the arguments are found
immediately before the stack frame. Note that this is not the case on
some targets that save registers into the caller's frame, such as SPARC
and rs6000, and so such targets need to define this macro.
You only need to define this macro if the default is incorrect, and you
want to support call frame debugging information like that provided by
DWARF 2.
@end defmac
@defmac FRAME_POINTER_CFA_OFFSET (@var{fundecl})
If defined, a C expression whose value is an integer giving the offset
in bytes from the frame pointer to the canonical frame address (cfa).
The final value should coincide with that calculated by
@code{INCOMING_FRAME_SP_OFFSET}.
Normally the CFA is calculated as an offset from the argument pointer,
via @code{ARG_POINTER_CFA_OFFSET}, but if the argument pointer is
variable due to the ABI, this may not be possible. If this macro is
defined, it implies that the virtual register instantiation should be
based on the frame pointer instead of the argument pointer. Only one
of @code{FRAME_POINTER_CFA_OFFSET} and @code{ARG_POINTER_CFA_OFFSET}
should be defined.
@end defmac
@defmac CFA_FRAME_BASE_OFFSET (@var{fundecl})
If defined, a C expression whose value is an integer giving the offset
in bytes from the canonical frame address (cfa) to the frame base used
in DWARF 2 debug information. The default is zero. A different value
may reduce the size of debug information on some ports.
@end defmac
@node Exception Handling
@subsection Exception Handling Support
@cindex exception handling
@defmac EH_RETURN_DATA_REGNO (@var{N})
A C expression whose value is the @var{N}th register number used for
data by exception handlers, or @code{INVALID_REGNUM} if fewer than
@var{N} registers are usable.
The exception handling library routines communicate with the exception
handlers via a set of agreed upon registers. Ideally these registers
should be call-clobbered; it is possible to use call-saved registers,
but may negatively impact code size. The target must support at least
2 data registers, but should define 4 if there are enough free registers.
You must define this macro if you want to support call frame exception
handling like that provided by DWARF 2.
@end defmac
@defmac EH_RETURN_STACKADJ_RTX
A C expression whose value is RTL representing a location in which
to store a stack adjustment to be applied before function return.
This is used to unwind the stack to an exception handler's call frame.
It will be assigned zero on code paths that return normally.
Typically this is a call-clobbered hard register that is otherwise
untouched by the epilogue, but could also be a stack slot.
Do not define this macro if the stack pointer is saved and restored
by the regular prolog and epilog code in the call frame itself; in
this case, the exception handling library routines will update the
stack location to be restored in place. Otherwise, you must define
this macro if you want to support call frame exception handling like
that provided by DWARF 2.
@end defmac
@defmac EH_RETURN_HANDLER_RTX
A C expression whose value is RTL representing a location in which
to store the address of an exception handler to which we should
return. It will not be assigned on code paths that return normally.
Typically this is the location in the call frame at which the normal
return address is stored. For targets that return by popping an
address off the stack, this might be a memory address just below
the @emph{target} call frame rather than inside the current call
frame. If defined, @code{EH_RETURN_STACKADJ_RTX} will have already
been assigned, so it may be used to calculate the location of the
target call frame.
Some targets have more complex requirements than storing to an
address calculable during initial code generation. In that case
the @code{eh_return} instruction pattern should be used instead.
If you want to support call frame exception handling, you must
define either this macro or the @code{eh_return} instruction pattern.
@end defmac
@defmac RETURN_ADDR_OFFSET
If defined, an integer-valued C expression for which rtl will be generated
to add it to the exception handler address before it is searched in the
exception handling tables, and to subtract it again from the address before
using it to return to the exception handler.
@end defmac
@defmac ASM_PREFERRED_EH_DATA_FORMAT (@var{code}, @var{global})
This macro chooses the encoding of pointers embedded in the exception
handling sections. If at all possible, this should be defined such
that the exception handling section will not require dynamic relocations,
and so may be read-only.
@var{code} is 0 for data, 1 for code labels, 2 for function pointers.
@var{global} is true if the symbol may be affected by dynamic relocations.
The macro should return a combination of the @code{DW_EH_PE_*} defines
as found in @file{dwarf2.h}.
If this macro is not defined, pointers will not be encoded but
represented directly.
@end defmac
@defmac ASM_MAYBE_OUTPUT_ENCODED_ADDR_RTX (@var{file}, @var{encoding}, @var{size}, @var{addr}, @var{done})
This macro allows the target to emit whatever special magic is required
to represent the encoding chosen by @code{ASM_PREFERRED_EH_DATA_FORMAT}.
Generic code takes care of pc-relative and indirect encodings; this must
be defined if the target uses text-relative or data-relative encodings.
This is a C statement that branches to @var{done} if the format was
handled. @var{encoding} is the format chosen, @var{size} is the number
of bytes that the format occupies, @var{addr} is the @code{SYMBOL_REF}
to be emitted.
@end defmac
@defmac MD_FALLBACK_FRAME_STATE_FOR (@var{context}, @var{fs})
This macro allows the target to add CPU and operating system specific
code to the call-frame unwinder for use when there is no unwind data
available. The most common reason to implement this macro is to unwind
through signal frames.
This macro is called from @code{uw_frame_state_for} in
@file{unwind-dw2.c}, @file{unwind-dw2-xtensa.c} and
@file{unwind-ia64.c}. @var{context} is an @code{_Unwind_Context};
@var{fs} is an @code{_Unwind_FrameState}. Examine @code{context->ra}
for the address of the code being executed and @code{context->cfa} for
the stack pointer value. If the frame can be decoded, the register
save addresses should be updated in @var{fs} and the macro should
evaluate to @code{_URC_NO_REASON}. If the frame cannot be decoded,
the macro should evaluate to @code{_URC_END_OF_STACK}.
For proper signal handling in Java this macro is accompanied by
@code{MAKE_THROW_FRAME}, defined in @file{libjava/include/*-signal.h} headers.
@end defmac
@defmac MD_HANDLE_UNWABI (@var{context}, @var{fs})
This macro allows the target to add operating system specific code to the
call-frame unwinder to handle the IA-64 @code{.unwabi} unwinding directive,
usually used for signal or interrupt frames.
This macro is called from @code{uw_update_context} in libgcc's
@file{unwind-ia64.c}. @var{context} is an @code{_Unwind_Context};
@var{fs} is an @code{_Unwind_FrameState}. Examine @code{fs->unwabi}
for the abi and context in the @code{.unwabi} directive. If the
@code{.unwabi} directive can be handled, the register save addresses should
be updated in @var{fs}.
@end defmac
@defmac TARGET_USES_WEAK_UNWIND_INFO
A C expression that evaluates to true if the target requires unwind
info to be given comdat linkage. Define it to be @code{1} if comdat
linkage is necessary. The default is @code{0}.
@end defmac
@node Stack Checking
@subsection Specifying How Stack Checking is Done
GCC will check that stack references are within the boundaries of the
stack, if the option @option{-fstack-check} is specified, in one of
three ways:
@enumerate
@item
If the value of the @code{STACK_CHECK_BUILTIN} macro is nonzero, GCC
will assume that you have arranged for full stack checking to be done
at appropriate places in the configuration files. GCC will not do
other special processing.
@item
If @code{STACK_CHECK_BUILTIN} is zero and the value of the
@code{STACK_CHECK_STATIC_BUILTIN} macro is nonzero, GCC will assume
that you have arranged for static stack checking (checking of the
static stack frame of functions) to be done at appropriate places
in the configuration files. GCC will only emit code to do dynamic
stack checking (checking on dynamic stack allocations) using the third
approach below.
@item
If neither of the above are true, GCC will generate code to periodically
``probe'' the stack pointer using the values of the macros defined below.
@end enumerate
If neither STACK_CHECK_BUILTIN nor STACK_CHECK_STATIC_BUILTIN is defined,
GCC will change its allocation strategy for large objects if the option
@option{-fstack-check} is specified: they will always be allocated
dynamically if their size exceeds @code{STACK_CHECK_MAX_VAR_SIZE} bytes.
@defmac STACK_CHECK_BUILTIN
A nonzero value if stack checking is done by the configuration files in a
machine-dependent manner. You should define this macro if stack checking
is required by the ABI of your machine or if you would like to do stack
checking in some more efficient way than the generic approach. The default
value of this macro is zero.
@end defmac
@defmac STACK_CHECK_STATIC_BUILTIN
A nonzero value if static stack checking is done by the configuration files
in a machine-dependent manner. You should define this macro if you would
like to do static stack checking in some more efficient way than the generic
approach. The default value of this macro is zero.
@end defmac
@defmac STACK_CHECK_PROBE_INTERVAL_EXP
An integer specifying the interval at which GCC must generate stack probe
instructions, defined as 2 raised to this integer. You will normally
define this macro so that the interval be no larger than the size of
the ``guard pages'' at the end of a stack area. The default value
of 12 (4096-byte interval) is suitable for most systems.
@end defmac
@defmac STACK_CHECK_MOVING_SP
An integer which is nonzero if GCC should move the stack pointer page by page
when doing probes. This can be necessary on systems where the stack pointer
contains the bottom address of the memory area accessible to the executing
thread at any point in time. In this situation an alternate signal stack
is required in order to be able to recover from a stack overflow. The
default value of this macro is zero.
@end defmac
@defmac STACK_CHECK_PROTECT
The number of bytes of stack needed to recover from a stack overflow, for
languages where such a recovery is supported. The default value of 4KB/8KB
with the @code{setjmp}/@code{longjmp}-based exception handling mechanism and
8KB/12KB with other exception handling mechanisms should be adequate for most
architectures and operating systems.
@end defmac
The following macros are relevant only if neither STACK_CHECK_BUILTIN
nor STACK_CHECK_STATIC_BUILTIN is defined; you can omit them altogether
in the opposite case.
@defmac STACK_CHECK_MAX_FRAME_SIZE
The maximum size of a stack frame, in bytes. GCC will generate probe
instructions in non-leaf functions to ensure at least this many bytes of
stack are available. If a stack frame is larger than this size, stack
checking will not be reliable and GCC will issue a warning. The
default is chosen so that GCC only generates one instruction on most
systems. You should normally not change the default value of this macro.
@end defmac
@defmac STACK_CHECK_FIXED_FRAME_SIZE
GCC uses this value to generate the above warning message. It
represents the amount of fixed frame used by a function, not including
space for any callee-saved registers, temporaries and user variables.
You need only specify an upper bound for this amount and will normally
use the default of four words.
@end defmac
@defmac STACK_CHECK_MAX_VAR_SIZE
The maximum size, in bytes, of an object that GCC will place in the
fixed area of the stack frame when the user specifies
@option{-fstack-check}.
GCC computed the default from the values of the above macros and you will
normally not need to override that default.
@end defmac
@hook TARGET_STACK_CLASH_PROTECTION_ALLOCA_PROBE_RANGE
@need 2000
@node Frame Registers
@subsection Registers That Address the Stack Frame
@c prevent bad page break with this line
This discusses registers that address the stack frame.
@defmac STACK_POINTER_REGNUM
The register number of the stack pointer register, which must also be a
fixed register according to @code{FIXED_REGISTERS}. On most machines,
the hardware determines which register this is.
@end defmac
@defmac FRAME_POINTER_REGNUM
The register number of the frame pointer register, which is used to
access automatic variables in the stack frame. On some machines, the
hardware determines which register this is. On other machines, you can
choose any register you wish for this purpose.
@end defmac
@defmac HARD_FRAME_POINTER_REGNUM
On some machines the offset between the frame pointer and starting
offset of the automatic variables is not known until after register
allocation has been done (for example, because the saved registers are
between these two locations). On those machines, define
@code{FRAME_POINTER_REGNUM} the number of a special, fixed register to
be used internally until the offset is known, and define
@code{HARD_FRAME_POINTER_REGNUM} to be the actual hard register number
used for the frame pointer.
You should define this macro only in the very rare circumstances when it
is not possible to calculate the offset between the frame pointer and
the automatic variables until after register allocation has been
completed. When this macro is defined, you must also indicate in your
definition of @code{ELIMINABLE_REGS} how to eliminate
@code{FRAME_POINTER_REGNUM} into either @code{HARD_FRAME_POINTER_REGNUM}
or @code{STACK_POINTER_REGNUM}.
Do not define this macro if it would be the same as
@code{FRAME_POINTER_REGNUM}.
@end defmac
@defmac ARG_POINTER_REGNUM
The register number of the arg pointer register, which is used to access
the function's argument list. On some machines, this is the same as the
frame pointer register. On some machines, the hardware determines which
register this is. On other machines, you can choose any register you
wish for this purpose. If this is not the same register as the frame
pointer register, then you must mark it as a fixed register according to
@code{FIXED_REGISTERS}, or arrange to be able to eliminate it
(@pxref{Elimination}).
@end defmac
@defmac HARD_FRAME_POINTER_IS_FRAME_POINTER
Define this to a preprocessor constant that is nonzero if
@code{hard_frame_pointer_rtx} and @code{frame_pointer_rtx} should be
the same. The default definition is @samp{(HARD_FRAME_POINTER_REGNUM
== FRAME_POINTER_REGNUM)}; you only need to define this macro if that
definition is not suitable for use in preprocessor conditionals.
@end defmac
@defmac HARD_FRAME_POINTER_IS_ARG_POINTER
Define this to a preprocessor constant that is nonzero if
@code{hard_frame_pointer_rtx} and @code{arg_pointer_rtx} should be the
same. The default definition is @samp{(HARD_FRAME_POINTER_REGNUM ==
ARG_POINTER_REGNUM)}; you only need to define this macro if that
definition is not suitable for use in preprocessor conditionals.
@end defmac
@defmac RETURN_ADDRESS_POINTER_REGNUM
The register number of the return address pointer register, which is used to
access the current function's return address from the stack. On some
machines, the return address is not at a fixed offset from the frame
pointer or stack pointer or argument pointer. This register can be defined
to point to the return address on the stack, and then be converted by
@code{ELIMINABLE_REGS} into either the frame pointer or stack pointer.
Do not define this macro unless there is no other way to get the return
address from the stack.
@end defmac
@defmac STATIC_CHAIN_REGNUM
@defmacx STATIC_CHAIN_INCOMING_REGNUM
Register numbers used for passing a function's static chain pointer. If
register windows are used, the register number as seen by the called
function is @code{STATIC_CHAIN_INCOMING_REGNUM}, while the register
number as seen by the calling function is @code{STATIC_CHAIN_REGNUM}. If
these registers are the same, @code{STATIC_CHAIN_INCOMING_REGNUM} need
not be defined.
The static chain register need not be a fixed register.
If the static chain is passed in memory, these macros should not be
defined; instead, the @code{TARGET_STATIC_CHAIN} hook should be used.
@end defmac
@hook TARGET_STATIC_CHAIN
@defmac DWARF_FRAME_REGISTERS
This macro specifies the maximum number of hard registers that can be
saved in a call frame. This is used to size data structures used in
DWARF2 exception handling.
Prior to GCC 3.0, this macro was needed in order to establish a stable
exception handling ABI in the face of adding new hard registers for ISA
extensions. In GCC 3.0 and later, the EH ABI is insulated from changes
in the number of hard registers. Nevertheless, this macro can still be
used to reduce the runtime memory requirements of the exception handling
routines, which can be substantial if the ISA contains a lot of
registers that are not call-saved.
If this macro is not defined, it defaults to
@code{FIRST_PSEUDO_REGISTER}.
@end defmac
@defmac PRE_GCC3_DWARF_FRAME_REGISTERS
This macro is similar to @code{DWARF_FRAME_REGISTERS}, but is provided
for backward compatibility in pre GCC 3.0 compiled code.
If this macro is not defined, it defaults to
@code{DWARF_FRAME_REGISTERS}.
@end defmac
@defmac DWARF_REG_TO_UNWIND_COLUMN (@var{regno})
Define this macro if the target's representation for dwarf registers
is different than the internal representation for unwind column.
Given a dwarf register, this macro should return the internal unwind
column number to use instead.
@end defmac
@defmac DWARF_FRAME_REGNUM (@var{regno})
Define this macro if the target's representation for dwarf registers
used in .eh_frame or .debug_frame is different from that used in other
debug info sections. Given a GCC hard register number, this macro
should return the .eh_frame register number. The default is
@code{DBX_REGISTER_NUMBER (@var{regno})}.
@end defmac
@defmac DWARF2_FRAME_REG_OUT (@var{regno}, @var{for_eh})
Define this macro to map register numbers held in the call frame info
that GCC has collected using @code{DWARF_FRAME_REGNUM} to those that
should be output in .debug_frame (@code{@var{for_eh}} is zero) and
.eh_frame (@code{@var{for_eh}} is nonzero). The default is to
return @code{@var{regno}}.
@end defmac
@defmac REG_VALUE_IN_UNWIND_CONTEXT
Define this macro if the target stores register values as
@code{_Unwind_Word} type in unwind context. It should be defined if
target register size is larger than the size of @code{void *}. The
default is to store register values as @code{void *} type.
@end defmac
@defmac ASSUME_EXTENDED_UNWIND_CONTEXT
Define this macro to be 1 if the target always uses extended unwind
context with version, args_size and by_value fields. If it is undefined,
it will be defined to 1 when @code{REG_VALUE_IN_UNWIND_CONTEXT} is
defined and 0 otherwise.
@end defmac
@defmac DWARF_LAZY_REGISTER_VALUE (@var{regno}, @var{value})
Define this macro if the target has pseudo DWARF registers whose
values need to be computed lazily on demand by the unwinder (such as when
referenced in a CFA expression). The macro returns true if @var{regno}
is such a register and stores its value in @samp{*@var{value}} if so.
@end defmac
@node Elimination
@subsection Eliminating Frame Pointer and Arg Pointer
@c prevent bad page break with this line
This is about eliminating the frame pointer and arg pointer.
@hook TARGET_FRAME_POINTER_REQUIRED
@defmac ELIMINABLE_REGS
This macro specifies a table of register pairs used to eliminate
unneeded registers that point into the stack frame.
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:
@smallexample
#define ELIMINABLE_REGS \
@{@{ARG_POINTER_REGNUM, STACK_POINTER_REGNUM@}, \
@{ARG_POINTER_REGNUM, FRAME_POINTER_REGNUM@}, \
@{FRAME_POINTER_REGNUM, STACK_POINTER_REGNUM@}@}
@end smallexample
Note that the elimination of the argument pointer with the stack pointer is
specified first since that is the preferred elimination.
@end defmac
@hook TARGET_CAN_ELIMINATE
@defmac INITIAL_ELIMINATION_OFFSET (@var{from-reg}, @var{to-reg}, @var{offset-var})
This macro returns the initial difference between the specified pair
of registers. The value would be computed from information
such as the result of @code{get_frame_size ()} and the tables of
registers @code{df_regs_ever_live_p} and @code{call_used_regs}.
@end defmac
@hook TARGET_COMPUTE_FRAME_LAYOUT
@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.
@hook TARGET_PROMOTE_PROTOTYPES
@hook TARGET_PUSH_ARGUMENT
@defmac PUSH_ARGS_REVERSED
A C expression. If nonzero, function arguments will be evaluated from
last to first, rather than from first to last. If this macro is not
defined, it defaults to @code{PUSH_ARGS} on targets where the stack
and args grow in opposite directions, and 0 otherwise.
@end defmac
@defmac PUSH_ROUNDING (@var{npushed})
A C expression that is the number of bytes actually pushed onto the
stack when an instruction attempts to push @var{npushed} bytes.
On some machines, the definition
@smallexample
#define PUSH_ROUNDING(BYTES) (BYTES)
@end smallexample
@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
@smallexample
#define PUSH_ROUNDING(BYTES) (((BYTES) + 1) & ~1)
@end smallexample
If the value of this macro has a type, it should be an unsigned type.
@end defmac
@findex outgoing_args_size
@findex crtl->outgoing_args_size
@defmac ACCUMULATE_OUTGOING_ARGS
A C expression. If nonzero, the maximum amount of space required for outgoing arguments
will be computed and placed into
@code{crtl->outgoing_args_size}. No space will be pushed
onto the stack for each call; instead, the function prologue should
increase the stack frame size by this amount.
Setting both @code{PUSH_ARGS} and @code{ACCUMULATE_OUTGOING_ARGS}
is not proper.
@end defmac
@defmac REG_PARM_STACK_SPACE (@var{fndecl})
Define this macro if functions should assume that stack space has been
allocated for arguments even when their values are passed in
registers.
The value of this macro is the size, in bytes, of the area reserved for
arguments passed in registers for the function represented by @var{fndecl},
which can be zero if GCC is calling a library function.
The argument @var{fndecl} can be the FUNCTION_DECL, or the type itself
of the function.
This space can be allocated by the caller, or be a part of the
machine-dependent stack frame: @code{OUTGOING_REG_PARM_STACK_SPACE} says
which.
@end defmac
@c above is overfull. not sure what to do. --mew 5feb93 did
@c something, not sure if it looks good. --mew 10feb93
@defmac INCOMING_REG_PARM_STACK_SPACE (@var{fndecl})
Like @code{REG_PARM_STACK_SPACE}, but for incoming register arguments.
Define this macro if space guaranteed when compiling a function body
is different to space required when making a call, a situation that
can arise with K&R style function definitions.
@end defmac
@defmac OUTGOING_REG_PARM_STACK_SPACE (@var{fntype})
Define this to a nonzero value if it is the responsibility of the
caller to allocate the area reserved for arguments passed in registers
when calling a function of @var{fntype}. @var{fntype} may be NULL
if the function called is a library function.
If @code{ACCUMULATE_OUTGOING_ARGS} is defined, this macro controls
whether the space for these arguments counts in the value of
@code{crtl->outgoing_args_size}.
@end defmac
@defmac STACK_PARMS_IN_REG_PARM_AREA
Define this macro if @code{REG_PARM_STACK_SPACE} is defined, but the
stack parameters don't skip the area specified by it.
@c i changed this, makes more sens and it should have taken care of the
@c overfull.. not as specific, tho. --mew 5feb93
Normally, when a parameter is not passed in registers, it is placed on the
stack beyond the @code{REG_PARM_STACK_SPACE} area. Defining this macro
suppresses this behavior and causes the parameter to be passed on the
stack in its natural location.
@end defmac
@hook TARGET_RETURN_POPS_ARGS
@defmac CALL_POPS_ARGS (@var{cum})
A C expression that should indicate the number of bytes a call sequence
pops off the stack. It is added to the value of @code{RETURN_POPS_ARGS}
when compiling a function call.
@var{cum} is the variable in which all arguments to the called function
have been accumulated.
On certain architectures, such as the SH5, a call trampoline is used
that pops certain registers off the stack, depending on the arguments
that have been passed to the function. Since this is a property of the
call site, not of the called function, @code{RETURN_POPS_ARGS} is not
appropriate.
@end defmac
@node Register Arguments
@subsection Passing Arguments in Registers
@cindex arguments in registers
@cindex registers arguments
This section describes the macros which let you control how various
types of arguments are passed in registers or how they are arranged in
the stack.
@hook TARGET_FUNCTION_ARG
@hook TARGET_MUST_PASS_IN_STACK
@hook TARGET_FUNCTION_INCOMING_ARG
@hook TARGET_USE_PSEUDO_PIC_REG
@hook TARGET_INIT_PIC_REG
@hook TARGET_ARG_PARTIAL_BYTES
@hook TARGET_PASS_BY_REFERENCE
@hook TARGET_CALLEE_COPIES
@defmac CUMULATIVE_ARGS
A C type for declaring a variable that is used as the first argument
of @code{TARGET_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}.
@end defmac
@defmac OVERRIDE_ABI_FORMAT (@var{fndecl})
If defined, this macro is called before generating any code for a
function, but after the @var{cfun} descriptor for the function has been
created. The back end may use this macro to update @var{cfun} to
reflect an ABI other than that which would normally be used by default.
If the compiler is generating code for a compiler-generated function,
@var{fndecl} may be @code{NULL}.
@end defmac
@defmac INIT_CUMULATIVE_ARGS (@var{cum}, @var{fntype}, @var{libname}, @var{fndecl}, @var{n_named_args})
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.
For direct calls that are not libcalls, @var{fndecl} contain the
declaration node of the function. @var{fndecl} is also set when
@code{INIT_CUMULATIVE_ARGS} is used to find arguments for the function
being compiled. @var{n_named_args} is set to the number of named
arguments, including a structure return address if it is passed as a
parameter, when making a call. When processing incoming arguments,
@var{n_named_args} is set to @minus{}1.
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.
@end defmac
@defmac INIT_CUMULATIVE_LIBCALL_ARGS (@var{cum}, @var{mode}, @var{libname})
Like @code{INIT_CUMULATIVE_ARGS} but only used for outgoing libcalls,
it gets a @code{MODE} argument instead of @var{fntype}, that would be
@code{NULL}. @var{indirect} would always be zero, too. If this macro
is not defined, @code{INIT_CUMULATIVE_ARGS (cum, NULL_RTX, libname,
0)} is used instead.
@end defmac
@defmac INIT_CUMULATIVE_INCOMING_ARGS (@var{cum}, @var{fntype}, @var{libname})
Like @code{INIT_CUMULATIVE_ARGS} but overrides it for the purposes of
finding the arguments for the function being compiled. If this macro is
undefined, @code{INIT_CUMULATIVE_ARGS} is used instead.
The value passed for @var{libname} is always 0, since library routines
with special calling conventions are never compiled with GCC@. The
argument @var{libname} exists for symmetry with
@code{INIT_CUMULATIVE_ARGS}.
@c could use "this macro" in place of @code{INIT_CUMULATIVE_ARGS}, maybe.
@c --mew 5feb93 i switched the order of the sentences. --mew 10feb93
@end defmac
@hook TARGET_FUNCTION_ARG_ADVANCE
@hook TARGET_FUNCTION_ARG_OFFSET
@hook TARGET_FUNCTION_ARG_PADDING
@defmac PAD_VARARGS_DOWN
If defined, a C expression which determines whether the default
implementation of va_arg will attempt to pad down before reading the
next argument, if that argument is smaller than its aligned space as
controlled by @code{PARM_BOUNDARY}. If this macro is not defined, all such
arguments are padded down if @code{BYTES_BIG_ENDIAN} is true.
@end defmac
@defmac BLOCK_REG_PADDING (@var{mode}, @var{type}, @var{first})
Specify padding for the last element of a block move between registers and
memory. @var{first} is nonzero if this is the only element. Defining this
macro allows better control of register function parameters on big-endian
machines, without using @code{PARALLEL} rtl. In particular,
@code{MUST_PASS_IN_STACK} need not test padding and mode of types in
registers, as there is no longer a "wrong" part of a register; For example,
a three byte aggregate may be passed in the high part of a register if so
required.
@end defmac
@hook TARGET_FUNCTION_ARG_BOUNDARY
@hook TARGET_FUNCTION_ARG_ROUND_BOUNDARY
@defmac 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 defmac
@hook TARGET_SPLIT_COMPLEX_ARG
@hook TARGET_BUILD_BUILTIN_VA_LIST
@hook TARGET_ENUM_VA_LIST_P
@hook TARGET_FN_ABI_VA_LIST
@hook TARGET_CANONICAL_VA_LIST_TYPE
@hook TARGET_GIMPLIFY_VA_ARG_EXPR
@hook TARGET_VALID_POINTER_MODE
@hook TARGET_REF_MAY_ALIAS_ERRNO
@hook TARGET_TRANSLATE_MODE_ATTRIBUTE
@hook TARGET_SCALAR_MODE_SUPPORTED_P
@hook TARGET_VECTOR_MODE_SUPPORTED_P
@hook TARGET_COMPATIBLE_VECTOR_TYPES_P
@hook TARGET_ARRAY_MODE
@hook TARGET_ARRAY_MODE_SUPPORTED_P
@hook TARGET_LIBGCC_FLOATING_MODE_SUPPORTED_P
@hook TARGET_FLOATN_MODE
@hook TARGET_FLOATN_BUILTIN_P
@hook TARGET_SMALL_REGISTER_CLASSES_FOR_MODE_P
@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.
@hook TARGET_FUNCTION_VALUE
@defmac FUNCTION_VALUE (@var{valtype}, @var{func})
This macro has been deprecated. Use @code{TARGET_FUNCTION_VALUE} for
a new target instead.
@end defmac
@defmac 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}.
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.
@end defmac
@hook TARGET_LIBCALL_VALUE
@defmac 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:
@smallexample
#define FUNCTION_VALUE_REGNO_P(N) ((N) == 0)
@end smallexample
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.
This macro has been deprecated. Use @code{TARGET_FUNCTION_VALUE_REGNO_P}
for a new target instead.
@end defmac
@hook TARGET_FUNCTION_VALUE_REGNO_P
@defmac APPLY_RESULT_SIZE
Define this macro if @samp{untyped_call} and @samp{untyped_return}
need more space than is implied by @code{FUNCTION_VALUE_REGNO_P} for
saving and restoring an arbitrary return value.
@end defmac
@hook TARGET_OMIT_STRUCT_RETURN_REG
@hook TARGET_RETURN_IN_MSB
@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{TARGET_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.
@hook TARGET_RETURN_IN_MEMORY
@defmac DEFAULT_PCC_STRUCT_RETURN
Define this macro to be 1 if all structure and union return values must be
in memory. Since this results in slower code, this should be defined
only if needed for compatibility with other compilers or with an ABI@.
If you define this macro to be 0, then the conventions used for structure
and union return values are decided by the @code{TARGET_RETURN_IN_MEMORY}
target hook.
If not defined, this defaults to the value 1.
@end defmac
@hook TARGET_STRUCT_VALUE_RTX
@defmac PCC_STATIC_STRUCT_RETURN
Define this macro if the usual system convention on the target machine
for returning structures and unions is for the called function to return
the address of a static variable containing the value.
Do not define this if the usual system convention is for the caller to
pass an address to the subroutine.
This macro has effect in @option{-fpcc-struct-return} mode, but it does
nothing when you use @option{-freg-struct-return} mode.
@end defmac
@hook TARGET_GET_RAW_RESULT_MODE
@hook TARGET_GET_RAW_ARG_MODE
@hook TARGET_EMPTY_RECORD_P
@hook TARGET_WARN_PARAMETER_PASSING_ABI
@node Caller Saves
@subsection Caller-Saves Register Allocation
If you enable it, GCC can save registers around function calls. This
makes it possible to use call-clobbered registers to hold variables that
must live across calls.
@defmac HARD_REGNO_CALLER_SAVE_MODE (@var{regno}, @var{nregs})
A C expression specifying which mode is required for saving @var{nregs}
of a pseudo-register in call-clobbered hard register @var{regno}. If
@var{regno} is unsuitable for caller save, @code{VOIDmode} should be
returned. For most machines this macro need not be defined since GCC
will select the smallest suitable mode.
@end defmac
@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.
@hook TARGET_ASM_PRINT_PATCHABLE_FUNCTION_ENTRY
@hook TARGET_ASM_FUNCTION_PROLOGUE
@hook TARGET_ASM_FUNCTION_END_PROLOGUE
@hook TARGET_ASM_FUNCTION_BEGIN_EPILOGUE
@hook TARGET_ASM_FUNCTION_EPILOGUE
@itemize @bullet
@item
@findex pretend_args_size
@findex crtl->args.pretend_args_size
A region of @code{crtl->args.pretend_args_size} bytes of
uninitialized space just underneath the first argument arriving on the
stack. (This may not be at the very start of the allocated stack region
if the calling sequence has pushed anything else since pushing the stack
arguments. But usually, on such machines, nothing else has been pushed
yet, because the function prologue itself does all the pushing.) This
region is used on machines where an argument may be passed partly in
registers and partly in memory, and, in some cases to support the
features in @code{<stdarg.h>}.
@item
An area of memory used to save certain registers used by the function.
The size of this area, which may also include space for such things as
the return address and pointers to previous stack frames, is
machine-specific and usually depends on which registers have been used
in the function. Machines with register windows often do not require
a save area.
@item
A region of at least @var{size} bytes, possibly rounded up to an allocation
boundary, to contain the local variables of the function. On some machines,
this region and the save area may occur in the opposite order, with the
save area closer to the top of the stack.
@item
@cindex @code{ACCUMULATE_OUTGOING_ARGS} and stack frames
Optionally, when @code{ACCUMULATE_OUTGOING_ARGS} is defined, a region of
@code{crtl->outgoing_args_size} bytes to be used for outgoing
argument lists of the function. @xref{Stack Arguments}.
@end itemize
@defmac 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. The
default is 0.
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}.
@end defmac
@defmac EPILOGUE_USES (@var{regno})
Define this macro as a C expression that is nonzero for registers that are
used by the epilogue or the @samp{return} pattern. The stack and frame
pointer registers are already assumed to be used as needed.
@end defmac
@defmac EH_USES (@var{regno})
Define this macro as a C expression that is nonzero for registers that are
used by the exception handling mechanism, and so should be considered live
on entry to an exception edge.
@end defmac
@hook TARGET_ASM_OUTPUT_MI_THUNK
@hook TARGET_ASM_CAN_OUTPUT_MI_THUNK
@node Profiling
@subsection Generating Code for Profiling
@cindex profiling, code generation
These macros will help you generate code for profiling.
@defmac FUNCTION_PROFILER (@var{file}, @var{labelno})
A C statement or compound statement to output to @var{file} some
assembler code to call the profiling subroutine @code{mcount}.
@findex mcount
The details of how @code{mcount} expects to be called are determined by
your operating system environment, not by GCC@. To figure them out,
compile a small program for profiling using the system's installed C
compiler and look at the assembler code that results.
Older implementations of @code{mcount} expect the address of a counter
variable to be loaded into some register. The name of this variable is
@samp{LP} followed by the number @var{labelno}, so you would generate
the name using @samp{LP%d} in a @code{fprintf}.
@end defmac
@defmac PROFILE_HOOK
A C statement or compound statement to output to @var{file} some assembly
code to call the profiling subroutine @code{mcount} even the target does
not support profiling.
@end defmac
@defmac NO_PROFILE_COUNTERS
Define this macro to be an expression with a nonzero value if the
@code{mcount} subroutine on your system does not need a counter variable
allocated for each function. This is true for almost all modern
implementations. If you define this macro, you must not use the
@var{labelno} argument to @code{FUNCTION_PROFILER}.
@end defmac
@defmac PROFILE_BEFORE_PROLOGUE
Define this macro if the code for function profiling should come before
the function prologue. Normally, the profiling code comes after.
@end defmac
@hook TARGET_KEEP_LEAF_WHEN_PROFILED
@node Tail Calls
@subsection Permitting tail calls
@cindex tail calls
@hook TARGET_FUNCTION_OK_FOR_SIBCALL
@hook TARGET_EXTRA_LIVE_ON_ENTRY
@hook TARGET_SET_UP_BY_PROLOGUE
@hook TARGET_WARN_FUNC_RETURN
@node Shrink-wrapping separate components
@subsection Shrink-wrapping separate components
@cindex shrink-wrapping separate components
The prologue may perf