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This is Info file gcc.info, produced by Makeinfo version 1.67 from the
input file gcc.texi.
This file documents the use and the internals of the GNU compiler.
Published by the Free Software Foundation 59 Temple Place - Suite 330
Boston, MA 02111-1307 USA
Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998
Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the sections entitled "GNU General Public License," "Funding for
Free Software," and "Protect Your Freedom--Fight `Look And Feel'" are
included exactly as in the original, and provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.
Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the sections entitled "GNU General Public
License," "Funding for Free Software," and "Protect Your Freedom--Fight
`Look And Feel'", and this permission notice, may be included in
translations approved by the Free Software Foundation instead of in the
original English.

File: gcc.info, Node: Frame Registers, Next: Elimination, Prev: Stack Checking, Up: Stack and Calling
Registers That Address the Stack Frame
--------------------------------------
This discusses registers that address the stack frame.
`STACK_POINTER_REGNUM'
The register number of the stack pointer register, which must also
be a fixed register according to `FIXED_REGISTERS'. On most
machines, the hardware determines which register this is.
`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.
`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
`FRAME_POINTER_REGNUM' the number of a special, fixed register to
be used internally until the offset is known, and define
`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 `ELIMINABLE_REGS' how to
eliminate `FRAME_POINTER_REGNUM' into either
`HARD_FRAME_POINTER_REGNUM' or `STACK_POINTER_REGNUM'.
Do not define this macro if it would be the same as
`FRAME_POINTER_REGNUM'.
`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 `FIXED_REGISTERS', or
arrange to be able to eliminate it (*note Elimination::.).
`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 `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.
`STATIC_CHAIN_REGNUM'
`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 `STATIC_CHAIN_INCOMING_REGNUM',
while the register number as seen by the calling function is
`STATIC_CHAIN_REGNUM'. If these registers are the same,
`STATIC_CHAIN_INCOMING_REGNUM' need not be defined.
The static chain register need not be a fixed register.
If the static chain is passed in memory, these macros should not be
defined; instead, the next two macros should be defined.
`STATIC_CHAIN'
`STATIC_CHAIN_INCOMING'
If the static chain is passed in memory, these macros provide rtx
giving `mem' expressions that denote where they are stored.
`STATIC_CHAIN' and `STATIC_CHAIN_INCOMING' give the locations as
seen by the calling and called functions, respectively. Often the
former will be at an offset from the stack pointer and the latter
at an offset from the frame pointer.
The variables `stack_pointer_rtx', `frame_pointer_rtx', and
`arg_pointer_rtx' will have been initialized prior to the use of
these macros and should be used to refer to those items.
If the static chain is passed in a register, the two previous
macros should be defined instead.

File: gcc.info, Node: Elimination, Next: Stack Arguments, Prev: Frame Registers, Up: Stack and Calling
Eliminating Frame Pointer and Arg Pointer
-----------------------------------------
This is about eliminating the frame pointer and arg pointer.
`FRAME_POINTER_REQUIRED'
A C expression which is nonzero if a function must have and use a
frame pointer. This expression is evaluated in the reload pass.
If its value is nonzero the function will have a frame pointer.
The expression can in principle examine the current function and
decide according to the facts, but on most machines the constant 0
or the constant 1 suffices. Use 0 when the machine allows code to
be generated with no frame pointer, and doing so saves some time
or space. Use 1 when there is no possible advantage to avoiding a
frame pointer.
In certain cases, the compiler does not know how to produce valid
code without a frame pointer. The compiler recognizes those cases
and automatically gives the function a frame pointer regardless of
what `FRAME_POINTER_REQUIRED' says. You don't need to worry about
them.
In a function that does not require a frame pointer, the frame
pointer register can be allocated for ordinary usage, unless you
mark it as a fixed register. See `FIXED_REGISTERS' for more
information.
`INITIAL_FRAME_POINTER_OFFSET (DEPTH-VAR)'
A C statement to store in the variable DEPTH-VAR the difference
between the frame pointer and the stack pointer values immediately
after the function prologue. The value would be computed from
information such as the result of `get_frame_size ()' and the
tables of registers `regs_ever_live' and `call_used_regs'.
If `ELIMINABLE_REGS' is defined, this macro will be not be used and
need not be defined. Otherwise, it must be defined even if
`FRAME_POINTER_REQUIRED' is defined to always be true; in that
case, you may set DEPTH-VAR to anything.
`ELIMINABLE_REGS'
If defined, this macro specifies a table of register pairs used to
eliminate unneeded registers that point into the stack frame. If
it is not defined, the only elimination attempted by the compiler
is to replace references to the frame pointer with references to
the stack pointer.
The definition of this macro is a list of structure
initializations, each of which specifies an original and
replacement register.
On some machines, the position of the argument pointer is not
known until the compilation is completed. In such a case, a
separate hard register must be used for the argument pointer.
This register can be eliminated by replacing it with either the
frame pointer or the argument pointer, depending on whether or not
the frame pointer has been eliminated.
In this case, you might specify:
#define ELIMINABLE_REGS \
{{ARG_POINTER_REGNUM, STACK_POINTER_REGNUM}, \
{ARG_POINTER_REGNUM, FRAME_POINTER_REGNUM}, \
{FRAME_POINTER_REGNUM, STACK_POINTER_REGNUM}}
Note that the elimination of the argument pointer with the stack
pointer is specified first since that is the preferred elimination.
`CAN_ELIMINATE (FROM-REG, TO-REG)'
A C expression that returns non-zero if the compiler is allowed to
try to replace register number FROM-REG with register number
TO-REG. This macro need only be defined if `ELIMINABLE_REGS' is
defined, and will usually be the constant 1, since most of the
cases preventing register elimination are things that the compiler
already knows about.
`INITIAL_ELIMINATION_OFFSET (FROM-REG, TO-REG, OFFSET-VAR)'
This macro is similar to `INITIAL_FRAME_POINTER_OFFSET'. It
specifies the initial difference between the specified pair of
registers. This macro must be defined if `ELIMINABLE_REGS' is
defined.
`LONGJMP_RESTORE_FROM_STACK'
Define this macro if the `longjmp' function restores registers from
the stack frames, rather than from those saved specifically by
`setjmp'. Certain quantities must not be kept in registers across
a call to `setjmp' on such machines.

File: gcc.info, Node: Stack Arguments, Next: Register Arguments, Prev: Elimination, Up: Stack and Calling
Passing Function Arguments on the Stack
---------------------------------------
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.
`PROMOTE_PROTOTYPES'
Define this macro if an argument declared in a prototype as an
integral type smaller than `int' should actually be passed as an
`int'. In addition to avoiding errors in certain cases of
mismatch, it also makes for better code on certain machines.
`PUSH_ROUNDING (NPUSHED)'
A C expression that is the number of bytes actually pushed onto the
stack when an instruction attempts to push NPUSHED bytes.
If the target machine does not have a push instruction, do not
define this macro. That directs GNU CC to use an alternate
strategy: to allocate the entire argument block and then store the
arguments into it.
On some machines, the definition
#define PUSH_ROUNDING(BYTES) (BYTES)
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
#define PUSH_ROUNDING(BYTES) (((BYTES) + 1) & ~1)
`ACCUMULATE_OUTGOING_ARGS'
If defined, the maximum amount of space required for outgoing
arguments will be computed and placed into the variable
`current_function_outgoing_args_size'. No space will be pushed
onto the stack for each call; instead, the function prologue should
increase the stack frame size by this amount.
Defining both `PUSH_ROUNDING' and `ACCUMULATE_OUTGOING_ARGS' is
not proper.
`REG_PARM_STACK_SPACE (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 FNDECL.
This space can be allocated by the caller, or be a part of the
machine-dependent stack frame: `OUTGOING_REG_PARM_STACK_SPACE' says
which.
`MAYBE_REG_PARM_STACK_SPACE'
`FINAL_REG_PARM_STACK_SPACE (CONST_SIZE, VAR_SIZE)'
Define these macros in addition to the one above if functions might
allocate stack space for arguments even when their values are
passed in registers. These should be used when the stack space
allocated for arguments in registers is not a simple constant
independent of the function declaration.
The value of the first macro is the size, in bytes, of the area
that we should initially assume would be reserved for arguments
passed in registers.
The value of the second macro is the actual size, in bytes, of the
area that will be reserved for arguments passed in registers.
This takes two arguments: an integer representing the number of
bytes of fixed sized arguments on the stack, and a tree
representing the number of bytes of variable sized arguments on
the stack.
When these macros are defined, `REG_PARM_STACK_SPACE' will only be
called for libcall functions, the current function, or for a
function being called when it is known that such stack space must
be allocated. In each case this value can be easily computed.
When deciding whether a called function needs such stack space,
and how much space to reserve, GNU CC uses these two macros
instead of `REG_PARM_STACK_SPACE'.
`OUTGOING_REG_PARM_STACK_SPACE'
Define this if it is the responsibility of the caller to allocate
the area reserved for arguments passed in registers.
If `ACCUMULATE_OUTGOING_ARGS' is defined, this macro controls
whether the space for these arguments counts in the value of
`current_function_outgoing_args_size'.
`STACK_PARMS_IN_REG_PARM_AREA'
Define this macro if `REG_PARM_STACK_SPACE' is defined, but the
stack parameters don't skip the area specified by it.
Normally, when a parameter is not passed in registers, it is
placed on the stack beyond the `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.
`RETURN_POPS_ARGS (FUNDECL, FUNTYPE, STACK-SIZE)'
A C expression that should indicate the number of bytes of its own
arguments that a function pops on returning, or 0 if the function
pops no arguments and the caller must therefore pop them all after
the function returns.
FUNDECL is a C variable whose value is a tree node that describes
the function in question. Normally it is a node of type
`FUNCTION_DECL' that describes the declaration of the function.
From this you can obtain the DECL_MACHINE_ATTRIBUTES of the
function.
FUNTYPE is a C variable whose value is a tree node that describes
the function in question. Normally it is a node of type
`FUNCTION_TYPE' that describes the data type of the function.
From this it is possible to obtain the data types of the value and
arguments (if known).
When a call to a library function is being considered, FUNDECL
will contain an identifier node for the library function. Thus, if
you need to distinguish among various library functions, you can
do so by their names. Note that "library function" in this
context means a function used to perform arithmetic, whose name is
known specially in the compiler and was not mentioned in the C
code being compiled.
STACK-SIZE is the number of bytes of arguments passed on the
stack. If a variable number of bytes is passed, it is zero, and
argument popping will always be the responsibility of the calling
function.
On the Vax, all functions always pop their arguments, so the
definition of this macro is STACK-SIZE. On the 68000, using the
standard calling convention, no functions pop their arguments, so
the value of the macro is always 0 in this case. But an
alternative calling convention is available in which functions
that take a fixed number of arguments pop them but other functions
(such as `printf') pop nothing (the caller pops all). When this
convention is in use, FUNTYPE is examined to determine whether a
function takes a fixed number of arguments.

File: gcc.info, Node: Register Arguments, Next: Scalar Return, Prev: Stack Arguments, Up: Stack and Calling
Passing Arguments in Registers
------------------------------
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.
`FUNCTION_ARG (CUM, MODE, TYPE, NAMED)'
A C expression that controls whether a function argument is passed
in a register, and which register.
The arguments are CUM, which summarizes all the previous
arguments; MODE, the machine mode of the argument; TYPE, the data
type of the argument as a tree node or 0 if that is not known
(which happens for C support library functions); and NAMED, which
is 1 for an ordinary argument and 0 for nameless arguments that
correspond to `...' in the called function's prototype.
The value of the expression is usually either a `reg' RTX for the
hard register in which to pass the argument, or zero to pass the
argument on the stack.
For machines like the Vax and 68000, where normally all arguments
are pushed, zero suffices as a definition.
The value of the expression can also be a `parallel' RTX. This is
used when an argument is passed in multiple locations. The mode
of the of the `parallel' should be the mode of the entire
argument. The `parallel' holds any number of `expr_list' pairs;
each one describes where part of the argument is passed. In each
`expr_list', the first operand can be either a `reg' RTX for the
hard register in which to pass this part of the argument, or zero
to pass the argument on the stack. If this operand is a `reg',
then the mode indicates how large this part of the argument is.
The second operand of the `expr_list' is a `const_int' which gives
the offset in bytes into the entire argument where this part
starts.
The usual way to make the ANSI library `stdarg.h' work on a machine
where some arguments are usually passed in registers, is to cause
nameless arguments to be passed on the stack instead. This is done
by making `FUNCTION_ARG' return 0 whenever NAMED is 0.
You may use the macro `MUST_PASS_IN_STACK (MODE, TYPE)' in the
definition of this macro to determine if this argument is of a
type that must be passed in the stack. If `REG_PARM_STACK_SPACE'
is not defined and `FUNCTION_ARG' returns non-zero for such an
argument, the compiler will abort. If `REG_PARM_STACK_SPACE' is
defined, the argument will be computed in the stack and then
loaded into a register.
`FUNCTION_INCOMING_ARG (CUM, MODE, TYPE, NAMED)'
Define this macro if the target machine has "register windows", so
that the register in which a function sees an arguments is not
necessarily the same as the one in which the caller passed the
argument.
For such machines, `FUNCTION_ARG' computes the register in which
the caller passes the value, and `FUNCTION_INCOMING_ARG' should be
defined in a similar fashion to tell the function being called
where the arguments will arrive.
If `FUNCTION_INCOMING_ARG' is not defined, `FUNCTION_ARG' serves
both purposes.
`FUNCTION_ARG_PARTIAL_NREGS (CUM, MODE, TYPE, NAMED)'
A C expression for the number of words, at the beginning of an
argument, must be put in registers. The value must be zero for
arguments that are passed entirely in registers or that are
entirely pushed on the stack.
On some machines, certain arguments must be passed partially in
registers and partially in memory. On these machines, typically
the first N words of arguments are passed in registers, and the
rest on the stack. If a multi-word argument (a `double' or a
structure) crosses that boundary, its first few words must be
passed in registers and the rest must be pushed. This macro tells
the compiler when this occurs, and how many of the words should go
in registers.
`FUNCTION_ARG' for these arguments should return the first
register to be used by the caller for this argument; likewise
`FUNCTION_INCOMING_ARG', for the called function.
`FUNCTION_ARG_PASS_BY_REFERENCE (CUM, MODE, TYPE, NAMED)'
A C expression that indicates when an argument must be passed by
reference. If nonzero for an argument, a copy of that argument is
made in memory and a pointer to the argument is passed instead of
the argument itself. The pointer is passed in whatever way is
appropriate for passing a pointer to that type.
On machines where `REG_PARM_STACK_SPACE' is not defined, a suitable
definition of this macro might be
#define FUNCTION_ARG_PASS_BY_REFERENCE\
(CUM, MODE, TYPE, NAMED) \
MUST_PASS_IN_STACK (MODE, TYPE)
`FUNCTION_ARG_CALLEE_COPIES (CUM, MODE, TYPE, NAMED)'
If defined, a C expression that indicates when it is the called
function's responsibility to make a copy of arguments passed by
invisible reference. Normally, the caller makes a copy and passes
the address of the copy to the routine being called. When
FUNCTION_ARG_CALLEE_COPIES is defined and is nonzero, the caller
does not make a copy. Instead, it passes a pointer to the "live"
value. The called function must not modify this value. If it can
be determined that the value won't be modified, it need not make a
copy; otherwise a copy must be made.
`CUMULATIVE_ARGS'
A C type for declaring a variable that is used as the first
argument of `FUNCTION_ARG' and other related values. For some
target machines, the type `int' suffices and can hold the number
of bytes of argument so far.
There is no need to record in `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 `CUMULATIVE_ARGS'; however, the data structure
must exist and should not be empty, so use `int'.
`INIT_CUMULATIVE_ARGS (CUM, FNTYPE, LIBNAME, INDIRECT)'
A C statement (sans semicolon) for initializing the variable CUM
for the state at the beginning of the argument list. The variable
has type `CUMULATIVE_ARGS'. The value of FNTYPE is the tree node
for the data type of the function which will receive the args, or 0
if the args are to a compiler support library function. The value
of INDIRECT is nonzero when processing an indirect call, for
example a call through a function pointer. The value of INDIRECT
is zero for a call to an explicitly named function, a library
function call, or when `INIT_CUMULATIVE_ARGS' is used to find
arguments for the function being compiled.
When processing a call to a compiler support library function,
LIBNAME identifies which one. It is a `symbol_ref' rtx which
contains the name of the function, as a string. LIBNAME is 0 when
an ordinary C function call is being processed. Thus, each time
this macro is called, either LIBNAME or FNTYPE is nonzero, but
never both of them at once.
`INIT_CUMULATIVE_INCOMING_ARGS (CUM, FNTYPE, LIBNAME)'
Like `INIT_CUMULATIVE_ARGS' but overrides it for the purposes of
finding the arguments for the function being compiled. If this
macro is undefined, `INIT_CUMULATIVE_ARGS' is used instead.
The value passed for LIBNAME is always 0, since library routines
with special calling conventions are never compiled with GNU CC.
The argument LIBNAME exists for symmetry with
`INIT_CUMULATIVE_ARGS'.
`FUNCTION_ARG_ADVANCE (CUM, MODE, TYPE, NAMED)'
A C statement (sans semicolon) to update the summarizer variable
CUM to advance past an argument in the argument list. The values
MODE, TYPE and NAMED describe that argument. Once this is done,
the variable CUM is suitable for analyzing the *following*
argument with `FUNCTION_ARG', etc.
This macro need not do anything if the argument in question was
passed on the stack. The compiler knows how to track the amount
of stack space used for arguments without any special help.
`FUNCTION_ARG_PADDING (MODE, TYPE)'
If defined, a C expression which determines whether, and in which
direction, to pad out an argument with extra space. The value
should be of type `enum direction': either `upward' to pad above
the argument, `downward' to pad below, or `none' to inhibit
padding.
The *amount* of padding is always just enough to reach the next
multiple of `FUNCTION_ARG_BOUNDARY'; this macro does not control
it.
This macro has a default definition which is right for most
systems. For little-endian machines, the default is to pad
upward. For big-endian machines, the default is to pad downward
for an argument of constant size shorter than an `int', and upward
otherwise.
`FUNCTION_ARG_BOUNDARY (MODE, TYPE)'
If defined, a C expression that gives the alignment boundary, in
bits, of an argument with the specified mode and type. If it is
not defined, `PARM_BOUNDARY' is used for all arguments.
`FUNCTION_ARG_REGNO_P (REGNO)'
A C expression that is nonzero if REGNO is the number of a hard
register in which function arguments are sometimes passed. This
does *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.

File: gcc.info, Node: Scalar Return, Next: Aggregate Return, Prev: Register Arguments, Up: Stack and Calling
How Scalar Function Values Are Returned
---------------------------------------
This section discusses the macros that control returning scalars as
values--values that can fit in registers.
`TRADITIONAL_RETURN_FLOAT'
Define this macro if `-traditional' should not cause functions
declared to return `float' to convert the value to `double'.
`FUNCTION_VALUE (VALTYPE, FUNC)'
A C expression to create an RTX representing the place where a
function returns a value of data type VALTYPE. VALTYPE is a tree
node representing a data type. Write `TYPE_MODE (VALTYPE)' to get
the machine mode used to represent that type. On many machines,
only the mode is relevant. (Actually, on most machines, scalar
values are returned in the same place regardless of mode).
The value of the expression is usually a `reg' RTX for the hard
register where the return value is stored. The value can also be a
`parallel' RTX, if the return value is in multiple places. See
`FUNCTION_ARG' for an explanation of the `parallel' form.
If `PROMOTE_FUNCTION_RETURN' is defined, you must apply the same
promotion rules specified in `PROMOTE_MODE' if VALTYPE is a scalar
type.
If the precise function being called is known, FUNC is a tree node
(`FUNCTION_DECL') for it; otherwise, FUNC is a null pointer. This
makes it possible to use a different value-returning convention
for specific functions when all their calls are known.
`FUNCTION_VALUE' is not used for return vales with aggregate data
types, because these are returned in another way. See
`STRUCT_VALUE_REGNUM' and related macros, below.
`FUNCTION_OUTGOING_VALUE (VALTYPE, FUNC)'
Define this macro if the target machine has "register windows" so
that the register in which a function returns its value is not the
same as the one in which the caller sees the value.
For such machines, `FUNCTION_VALUE' computes the register in which
the caller will see the value. `FUNCTION_OUTGOING_VALUE' should be
defined in a similar fashion to tell the function where to put the
value.
If `FUNCTION_OUTGOING_VALUE' is not defined, `FUNCTION_VALUE'
serves both purposes.
`FUNCTION_OUTGOING_VALUE' is not used for return vales with
aggregate data types, because these are returned in another way.
See `STRUCT_VALUE_REGNUM' and related macros, below.
`LIBCALL_VALUE (MODE)'
A C expression to create an RTX representing the place where a
library function returns a value of mode MODE. If the precise
function being called is known, FUNC is a tree node
(`FUNCTION_DECL') for it; otherwise, FUNC is a null pointer. This
makes it possible to use a different value-returning convention
for specific functions when all their calls are known.
Note that "library function" in this context means a compiler
support routine, used to perform arithmetic, whose name is known
specially by the compiler and was not mentioned in the C code being
compiled.
The definition of `LIBRARY_VALUE' need not be concerned aggregate
data types, because none of the library functions returns such
types.
`FUNCTION_VALUE_REGNO_P (REGNO)'
A C expression that is nonzero if 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 `double', say) need not
be recognized by this macro. So for most machines, this definition
suffices:
#define FUNCTION_VALUE_REGNO_P(N) ((N) == 0)
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.
`APPLY_RESULT_SIZE'
Define this macro if `untyped_call' and `untyped_return' need more
space than is implied by `FUNCTION_VALUE_REGNO_P' for saving and
restoring an arbitrary return value.

File: gcc.info, Node: Aggregate Return, Next: Caller Saves, Prev: Scalar Return, Up: Stack and Calling
How Large Values Are Returned
-----------------------------
When a function value's mode is `BLKmode' (and in some other cases),
the value is not returned according to `FUNCTION_VALUE' (*note 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
"structure value address".
This section describes how to control returning structure values in
memory.
`RETURN_IN_MEMORY (TYPE)'
A C expression which can inhibit the returning of certain function
values in registers, based on the type of value. A nonzero value
says to return the function value in memory, just as large
structures are always returned. Here TYPE will be a C expression
of type `tree', representing the data type of the value.
Note that values of mode `BLKmode' must be explicitly handled by
this macro. Also, the option `-fpcc-struct-return' takes effect
regardless of this macro. On most systems, it is possible to
leave the macro undefined; this causes a default definition to be
used, whose value is the constant 1 for `BLKmode' values, and 0
otherwise.
Do not use this macro to indicate that structures and unions
should always be returned in memory. You should instead use
`DEFAULT_PCC_STRUCT_RETURN' to indicate this.
`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 `RETURN_IN_MEMORY' macro.
If not defined, this defaults to the value 1.
`STRUCT_VALUE_REGNUM'
If the structure value address is passed in a register, then
`STRUCT_VALUE_REGNUM' should be the number of that register.
`STRUCT_VALUE'
If the structure value address is not passed in a register, define
`STRUCT_VALUE' as an expression returning an RTX for the place
where the address is passed. If it returns 0, the address is
passed as an "invisible" first argument.
`STRUCT_VALUE_INCOMING_REGNUM'
On some architectures the place where the structure value address
is found by the called function is not the same place that the
caller put it. This can be due to register windows, or it could
be because the function prologue moves it to a different place.
If the incoming location of the structure value address is in a
register, define this macro as the register number.
`STRUCT_VALUE_INCOMING'
If the incoming location is not a register, then you should define
`STRUCT_VALUE_INCOMING' as an expression for an RTX for where the
called function should find the value. If it should find the
value on the stack, define this to create a `mem' which refers to
the frame pointer. A definition of 0 means that the address is
passed as an "invisible" first argument.
`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 `-fpcc-struct-return' mode, but it does
nothing when you use `-freg-struct-return' mode.

File: gcc.info, Node: Caller Saves, Next: Function Entry, Prev: Aggregate Return, Up: Stack and Calling
Caller-Saves Register Allocation
--------------------------------
If you enable it, GNU CC can save registers around function calls.
This makes it possible to use call-clobbered registers to hold
variables that must live across calls.
`DEFAULT_CALLER_SAVES'
Define this macro if function calls on the target machine do not
preserve any registers; in other words, if `CALL_USED_REGISTERS'
has 1 for all registers. This macro enables `-fcaller-saves' by
default. Eventually that option will be enabled by default on all
machines and both the option and this macro will be eliminated.
`CALLER_SAVE_PROFITABLE (REFS, CALLS)'
A C expression to determine whether it is worthwhile to consider
placing a pseudo-register in a call-clobbered hard register and
saving and restoring it around each function call. The expression
should be 1 when this is worth doing, and 0 otherwise.
If you don't define this macro, a default is used which is good on
most machines: `4 * CALLS < REFS'.

File: gcc.info, Node: Function Entry, Next: Profiling, Prev: Caller Saves, Up: Stack and Calling
Function Entry and Exit
-----------------------
This section describes the macros that output function entry
("prologue") and exit ("epilogue") code.
`FUNCTION_PROLOGUE (FILE, SIZE)'
A C compound statement that outputs the assembler code for entry
to a function. The prologue is responsible for setting up the
stack frame, initializing the frame pointer register, saving
registers that must be saved, and allocating SIZE additional bytes
of storage for the local variables. SIZE is an integer. FILE is
a stdio stream to which the assembler code should be output.
The label for the beginning of the function need not be output by
this macro. That has already been done when the macro is run.
To determine which registers to save, the macro can refer to the
array `regs_ever_live': element R is nonzero if hard register R is
used anywhere within the function. This implies the function
prologue should save register R, provided it is not one of the
call-used registers. (`FUNCTION_EPILOGUE' must likewise use
`regs_ever_live'.)
On machines that have "register windows", the function entry code
does not save on the stack the registers that are in the windows,
even if they are supposed to be preserved by function calls;
instead it takes appropriate steps to "push" the register stack,
if any non-call-used registers are used in the function.
On machines where functions may or may not have frame-pointers, the
function entry code must vary accordingly; it must set up the frame
pointer if one is wanted, and not otherwise. To determine whether
a frame pointer is in wanted, the macro can refer to the variable
`frame_pointer_needed'. The variable's value will be 1 at run
time in a function that needs a frame pointer. *Note
Elimination::.
The function entry code is responsible for allocating any stack
space required for the function. This stack space consists of the
regions listed below. In most cases, these regions are allocated
in the order listed, with the last listed region closest to the
top of the stack (the lowest address if `STACK_GROWS_DOWNWARD' is
defined, and the highest address if it is not defined). You can
use a different order for a machine if doing so is more convenient
or required for compatibility reasons. Except in cases where
required by standard or by a debugger, there is no reason why the
stack layout used by GCC need agree with that used by other
compilers for a machine.
* A region of `current_function_pretend_args_size' bytes of
uninitialized space just underneath the first argument
arriving on the stack. (This may not be at the very start of
the allocated stack region if the calling sequence has pushed
anything else since pushing the stack arguments. But
usually, on such machines, nothing else has been pushed yet,
because the function prologue itself does all the pushing.)
This region is used on machines where an argument may be
passed partly in registers and partly in memory, and, in some
cases to support the features in `varargs.h' and `stdargs.h'.
* 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.
* A region of at least 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.
* Optionally, when `ACCUMULATE_OUTGOING_ARGS' is defined, a
region of `current_function_outgoing_args_size' bytes to be
used for outgoing argument lists of the function. *Note
Stack Arguments::.
Normally, it is necessary for the macros `FUNCTION_PROLOGUE' and
`FUNCTION_EPILOGUE' to treat leaf functions specially. The C
variable `leaf_function' is nonzero for such a function.
`EXIT_IGNORE_STACK'
Define this macro as a C expression that is nonzero if the return
instruction or the function epilogue ignores the value of the stack
pointer; in other words, if it is safe to delete an instruction to
adjust the stack pointer before a return from the function.
Note that this macro's value is relevant only for functions for
which frame pointers are maintained. It is never safe to delete a
final stack adjustment in a function that has no frame pointer,
and the compiler knows this regardless of `EXIT_IGNORE_STACK'.
`EPILOGUE_USES (REGNO)'
Define this macro as a C expression that is nonzero for registers
are used by the epilogue or the `return' pattern. The stack and
frame pointer registers are already be assumed to be used as
needed.
`FUNCTION_EPILOGUE (FILE, SIZE)'
A C compound statement that outputs the assembler code for exit
from a function. The epilogue is responsible for restoring the
saved registers and stack pointer to their values when the
function was called, and returning control to the caller. This
macro takes the same arguments as the macro `FUNCTION_PROLOGUE',
and the registers to restore are determined from `regs_ever_live'
and `CALL_USED_REGISTERS' in the same way.
On some machines, there is a single instruction that does all the
work of returning from the function. On these machines, give that
instruction the name `return' and do not define the macro
`FUNCTION_EPILOGUE' at all.
Do not define a pattern named `return' if you want the
`FUNCTION_EPILOGUE' to be used. If you want the target switches
to control whether return instructions or epilogues are used,
define a `return' pattern with a validity condition that tests the
target switches appropriately. If the `return' pattern's validity
condition is false, epilogues will be used.
On machines where functions may or may not have frame-pointers, the
function exit code must vary accordingly. Sometimes the code for
these two cases is completely different. To determine whether a
frame pointer is wanted, the macro can refer to the variable
`frame_pointer_needed'. The variable's value will be 1 when
compiling a function that needs a frame pointer.
Normally, `FUNCTION_PROLOGUE' and `FUNCTION_EPILOGUE' must treat
leaf functions specially. The C variable `leaf_function' is
nonzero for such a function. *Note Leaf Functions::.
On some machines, some functions pop their arguments on exit while
others leave that for the caller to do. For example, the 68020
when given `-mrtd' pops arguments in functions that take a fixed
number of arguments.
Your definition of the macro `RETURN_POPS_ARGS' decides which
functions pop their own arguments. `FUNCTION_EPILOGUE' needs to
know what was decided. The variable that is called
`current_function_pops_args' is the number of bytes of its
arguments that a function should pop. *Note Scalar Return::.
`DELAY_SLOTS_FOR_EPILOGUE'
Define this macro if the function epilogue contains delay slots to
which instructions from the rest of the function can be "moved".
The definition should be a C expression whose value is an integer
representing the number of delay slots there.
`ELIGIBLE_FOR_EPILOGUE_DELAY (INSN, N)'
A C expression that returns 1 if INSN can be placed in delay slot
number N of the epilogue.
The argument N is an integer which identifies the delay slot now
being considered (since different slots may have different rules of
eligibility). It is never negative and is always less than the
number of epilogue delay slots (what `DELAY_SLOTS_FOR_EPILOGUE'
returns). If you reject a particular insn for a given delay slot,
in principle, it may be reconsidered for a subsequent delay slot.
Also, other insns may (at least in principle) be considered for
the so far unfilled delay slot.
The insns accepted to fill the epilogue delay slots are put in an
RTL list made with `insn_list' objects, stored in the variable
`current_function_epilogue_delay_list'. The insn for the first
delay slot comes first in the list. Your definition of the macro
`FUNCTION_EPILOGUE' should fill the delay slots by outputting the
insns in this list, usually by calling `final_scan_insn'.
You need not define this macro if you did not define
`DELAY_SLOTS_FOR_EPILOGUE'.
`ASM_OUTPUT_MI_THUNK (FILE, THUNK_FNDECL, DELTA, FUNCTION)'
A C compound statement that outputs the assembler code for a thunk
function, used to implement C++ virtual function calls with
multiple inheritance. The thunk acts as a wrapper around a
virtual function, adjusting the implicit object parameter before
handing control off to the real function.
First, emit code to add the integer DELTA to the location that
contains the incoming first argument. Assume that this argument
contains a pointer, and is the one used to pass the `this' pointer
in C++. This is the incoming argument *before* the function
prologue, e.g. `%o0' on a sparc. The addition must preserve the
values of all other incoming arguments.
After the addition, emit code to jump to FUNCTION, which is a
`FUNCTION_DECL'. This is a direct pure jump, not a call, and does
not touch the return address. Hence returning from FUNCTION will
return to whoever called the current `thunk'.
The effect must be as if FUNCTION had been called directly with
the adjusted first argument. This macro is responsible for
emitting all of the code for a thunk function; `FUNCTION_PROLOGUE'
and `FUNCTION_EPILOGUE' are not invoked.
The THUNK_FNDECL is redundant. (DELTA and FUNCTION have already
been extracted from it.) It might possibly be useful on some
targets, but probably not.
If you do not define this macro, the target-independent code in
the C++ frontend will generate a less efficient heavyweight thunk
that calls FUNCTION instead of jumping to it. The generic
approach does not support varargs.
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