4.4BSD/usr/src/contrib/gcc-2.3.3/gcc.info-15
This is Info file gcc.info, produced by Makeinfo-1.49 from the input
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File: gcc.info, Node: Register Classes, Next: Stack and Calling, Prev: Registers, Up: Target Macros
Register Classes
================
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 "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.
In general, each register will belong to several classes. In fact,
one class must be named `ALL_REGS' and contain all the registers.
Another class must be named `NO_REGS' and contain no registers. Often
the union of two classes will be another class; however, this is not
required.
One of the classes must be named `GENERAL_REGS'. There is nothing
terribly special about the name, but the operand constraint letters `r'
and `g' specify this class. If `GENERAL_REGS' is the same as
`ALL_REGS', just define it as a macro which expands to `ALL_REGS'.
Order the classes so that if class X is contained in class Y then X
has a lower class number than Y.
The way classes other than `GENERAL_REGS' are specified in operand
constraints is through machine-dependent operand constraint letters.
You can define such letters to correspond to various classes, then use
them in operand constraints.
You should define a class for the union of two classes whenever some
instruction allows both classes. For example, if an instruction allows
either a floating point (coprocessor) register or a general register
for a certain operand, you should define a class `FLOAT_OR_GENERAL_REGS'
which includes both of them. Otherwise you will get suboptimal code.
You must also specify certain redundant information about the
register classes: for each class, which classes contain it and which
ones are contained in it; for each pair of classes, the largest class
contained in their union.
When a value occupying several consecutive registers is expected in a
certain class, all the registers used must belong to that class.
Therefore, register classes cannot be used to enforce a requirement for
a register pair to start with an even-numbered register. The way to
specify this requirement is with `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 (`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 `PREFERRED_RELOAD_CLASS' can always have a possible value to
return.
`enum reg_class'
An enumeral type that must be defined with all the register class
names as enumeral values. `NO_REGS' must be first. `ALL_REGS'
must be the last register class, followed by one more enumeral
value, `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 `int'. The number serves as an index in
many of the tables described below.
`N_REG_CLASSES'
The number of distinct register classes, defined as follows:
#define N_REG_CLASSES (int) LIM_REG_CLASSES
`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.
`REG_CLASS_CONTENTS'
An initializer containing the contents of the register classes, as
integers which are bit masks. The Nth integer specifies the
contents of class N. The way the integer MASK is interpreted is
that register R is in the class if `MASK & (1 << 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
`HARD_REG_SET' which is defined in `hard-reg-set.h'.
`REGNO_REG_CLASS (REGNO)'
A C expression whose value is a register class containing hard
register REGNO. In general there is more than one such class;
choose a class which is "minimal", meaning that no smaller class
also contains the register.
`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.
`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).
`REG_CLASS_FROM_LETTER (CHAR)'
A C expression which defines the machine-dependent operand
constraint letters for register classes. If CHAR is such a
letter, the value should be the register class corresponding to
it. Otherwise, the value should be `NO_REGS'. The register
letter `r', corresponding to class `GENERAL_REGS', will not be
passed to this macro; you do not need to handle it.
`REGNO_OK_FOR_BASE_P (NUM)'
A C expression which is nonzero if register number NUM is suitable
for use as a base register in operand addresses. It may be either
a suitable hard register or a pseudo register that has been
allocated such a hard register.
`REGNO_OK_FOR_INDEX_P (NUM)'
A C expression which is nonzero if register number 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.
`PREFERRED_RELOAD_CLASS (X, CLASS)'
A C expression that places additional restrictions on the register
class to use when it is necessary to copy value X into a register
in class CLASS. The value is a register class; perhaps CLASS, or
perhaps another, smaller class. On many machines, the definition
#define PREFERRED_RELOAD_CLASS(X,CLASS) CLASS
is safe.
Sometimes returning a more restrictive class makes better code.
For example, on the 68000, when X is an integer constant that is
in range for a `moveq' instruction, the value of this macro is
always `DATA_REGS' as long as CLASS includes the data registers.
Requiring a data register guarantees that a `moveq' will be used.
If X is a `const_double', by returning `NO_REGS' you can force X
into a memory constant. This is useful on certain machines where
immediate floating values cannot be loaded into certain kinds of
registers.
`PREFERRED_OUTPUT_RELOAD_CLASS (X, CLASS)'
Like `PREFERRED_RELOAD_CLASS', but for output reloads instead of
input reloads. If you don't define this macro, the default is to
use CLASS, unchanged.
`LIMIT_RELOAD_CLASS (MODE, 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 MODE in a reload register for which class CLASS would
ordinarily be used.
Unlike `PREFERRED_RELOAD_CLASS', this macro should be used when
there are certain modes that simply can't go in certain reload
classes.
The value is a register class; perhaps 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.
`SECONDARY_RELOAD_CLASS (CLASS, MODE, X)'
`SECONDARY_INPUT_RELOAD_CLASS (CLASS, MODE, X)'
`SECONDARY_OUTPUT_RELOAD_CLASS (CLASS, MODE, X)'
Many machines have some registers that cannot be copied directly
to or from memory or even from other types of registers. An
example is the `MQ' register, which on most machines, can only be
copied to or from general registers, but not memory. Some
machines allow copying all registers to and from memory, but
require a scratch register for stores to some memory locations
(e.g., those with symbolic address on the RT, and those with
certain symbolic address on the Sparc when compiling PIC). In
some cases, both an intermediate and a scratch register are
required.
You should define these macros to indicate to the reload phase
that it may need to allocate at least one register for a reload in
addition to the register to contain the data. Specifically, if
copying X to a register CLASS in MODE requires an intermediate
register, you should define `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 CLASS in MODE to X requires an intermediate
or scratch register, you should define
`SECONDARY_OUTPUT_RELOAD_CLASS' to return the largest register
class required. If the requirements for input and output reloads
are the same, the macro `SECONDARY_RELOAD_CLASS' should be used
instead of defining both macros identically.
The values returned by these macros are often `GENERAL_REGS'.
Return `NO_REGS' if no spare register is needed; i.e., if X can be
directly copied to or from a register of CLASS in MODE without
requiring a scratch register. Do not define this macro if it
would always return `NO_REGS'.
If a scratch register is required (either with or without an
intermediate register), you should define patterns for
`reload_inM' or `reload_outM', as required (*note Standard
Names::.. These patterns, which will normally be implemented with
a `define_expand', should be similar to the `movM' patterns,
except that operand 2 is the scratch register.
Define constraints for the reload register and scratch register
that contain a single register class. If the original reload
register (whose class is 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.
X might be a pseudo-register or a `subreg' of a pseudo-register,
which could either be in a hard register or in memory. Use
`true_regnum' to find out; it will return -1 if the pseudo is in
memory and the hard register number if it is in a register.
These macros should not be used in the case where a particular
class of registers can only be copied to memory and not to another
class of registers. In that case, secondary reload registers are
not needed and would not be helpful. Instead, a stack location
must be used to perform the copy and the `movM' pattern should use
memory as a intermediate storage. This case often occurs between
floating-point and general registers.
`SECONDARY_MEMORY_NEEDED (CLASS1, CLASS2, M)'
Certain machines have the property that some registers cannot be
copied to some other registers without using memory. Define this
macro on those machines to be a C expression that is non-zero if
objects of mode M in registers of CLASS1 can only be copied to
registers of class CLASS2 by storing a register of CLASS1 into
memory and loading that memory location into a register of CLASS2.
Do not define this macro if its value would always be zero.
`SMALL_REGISTER_CLASSES'
Normally the compiler will avoid choosing spill registers from
registers that have been explicitly mentioned in the rtl (these
registers are normally those used to pass parameters and return
values). However, some machines have so few registers of certain
classes that there would not be enough registers to use as spill
registers if this were done.
On those machines, you should define `SMALL_REGISTER_CLASSES'.
When it is defined, the compiler allows registers explicitly used
in the rtl to be used as spill registers but prevents the compiler
from extending the lifetime of these registers.
Defining this macro is always safe, but unnecessarily defining
this macro will reduce the amount of optimizations that can be
performed in some cases. If this macro is not defined but needs
to be, the compiler will run out of reload registers and print a
fatal error message.
For most machines, this macro should not be defined.
`CLASS_MAX_NREGS (CLASS, MODE)'
A C expression for the maximum number of consecutive registers of
class CLASS needed to hold a value of mode MODE.
This is closely related to the macro `HARD_REGNO_NREGS'. In fact,
the value of the macro `CLASS_MAX_NREGS (CLASS, MODE)' should be
the maximum value of `HARD_REGNO_NREGS (REGNO, MODE)' for all
REGNO values in the class CLASS.
This macro helps control the handling of multiple-word values in
the reload pass.
Three other special macros describe which operands fit which
constraint letters.
`CONST_OK_FOR_LETTER_P (VALUE, C)'
A C expression that defines the machine-dependent operand
constraint letters that specify particular ranges of integer
values. If C is one of those letters, the expression should check
that VALUE, an integer, is in the appropriate range and return 1
if so, 0 otherwise. If C is not one of those letters, the value
should be 0 regardless of VALUE.
`CONST_DOUBLE_OK_FOR_LETTER_P (VALUE, C)'
A C expression that defines the machine-dependent operand
constraint letters that specify particular ranges of
`const_double' values.
If C is one of those letters, the expression should check that
VALUE, an RTX of code `const_double', is in the appropriate range
and return 1 if so, 0 otherwise. If C is not one of those
letters, the value should be 0 regardless of VALUE.
`const_double' is used for all floating-point constants and for
`DImode' fixed-point constants. A given letter can accept either
or both kinds of values. It can use `GET_MODE' to distinguish
between these kinds.
`EXTRA_CONSTRAINT (VALUE, C)'
A C expression that defines the optional machine-dependent
constraint letters that can be used to segregate specific types of
operands, usually memory references, for the target machine.
Normally this macro will not be defined. If it is required for a
particular target machine, it should return 1 if VALUE corresponds
to the operand type represented by the constraint letter C. If C
is not defined as an extra constraint, the value returned should
be 0 regardless of VALUE.
For example, on the ROMP, load instructions cannot have their
output in r0 if the memory reference contains a symbolic address.
Constraint letter `Q' is defined as representing a memory address
that does *not* contain a symbolic address. An alternative is
specified with a `Q' constraint on the input and `r' on the
output. The next alternative specifies `m' on the input and a
register class that does not include r0 on the output.
File: gcc.info, Node: Stack and Calling, Next: Varargs, Prev: Register Classes, Up: Target Macros
Describing Stack Layout and Calling Conventions
===============================================
* Menu:
* Frame Layout::
* Frame Registers::
* Elimination::
* Stack Arguments::
* Register Arguments::
* Scalar Return::
* Aggregate Return::
* Caller Saves::
* Function Entry::
* Profiling::
File: gcc.info, Node: Frame Layout, Next: Frame Registers, Up: Stack and Calling
Basic Stack Layout
------------------
`STACK_GROWS_DOWNWARD'
Define this macro if pushing a word onto the stack moves the stack
pointer to a smaller address.
When we say, "define this macro if ...," it means that the
compiler checks this macro only with `#ifdef' so the precise
definition used does not matter.
`FRAME_GROWS_DOWNWARD'
Define this macro if the addresses of local variable slots are at
negative offsets from the frame pointer.
`ARGS_GROW_DOWNWARD'
Define this macro if successive arguments to a function occupy
decreasing addresses on the stack.
`STARTING_FRAME_OFFSET'
Offset from the frame pointer to the first local variable slot to
be allocated.
If `FRAME_GROWS_DOWNWARD', the next slot's offset is found by
subtracting the length of the first slot from
`STARTING_FRAME_OFFSET'. Otherwise, it is found by adding the
length of the first slot to the value `STARTING_FRAME_OFFSET'.
`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 `ARGS_GROW_DOWNWARD', this is the offset to the location above
the first location at which outgoing arguments are placed.
`FIRST_PARM_OFFSET (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 `ARGS_GROW_DOWNWARD', this is the offset to the location above
the first argument's address.
`STACK_DYNAMIC_OFFSET (FUNDECL)'
Offset from the stack pointer register to an item dynamically
allocated on the stack, e.g., by `alloca'.
The default value for this macro is `STACK_POINTER_OFFSET' plus the
length of the outgoing arguments. The default is correct for most
machines. See `function.c' for details.
`DYNAMIC_CHAIN_ADDRESS (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 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 FRAMEADDR--that is, the stack frame address is also the address
of the stack word that points to the previous frame.
File: gcc.info, Node: Frame Registers, Next: Elimination, Prev: Frame Layout, Up: Stack and Calling
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.
`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::.).
`STATIC_CHAIN_REGNUM'
`STATIC_CHAIN_INCOMING_REGNUM'
Register numbers used for passing a function's static chain
pointer. If register windows are used,
`STATIC_CHAIN_INCOMING_REGNUM' is the register number as seen by
the called function, while `STATIC_CHAIN_REGNUM' is the register
number as seen by the calling function. 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
-----------------------------------------
`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.
This macro is ignored and need not be defined if `ELIMINABLE_REGS'
is defined.
`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 as `char' or `short' in
a prototype 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.
It is not proper to define both `PUSH_ROUNDING' and
`ACCUMULATE_OUTGOING_ARGS'.
`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 either 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 stack
parameters don't skip the area specified by `REG_PARM_STACK_SPACE'.
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 (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.
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, FUNTYPE
will contain an identifier node for the library function. Thus, if
you need to distinguish among various library functions, you can
do so by their names. Note that "library function" in this
context means a function used to perform arithmetic, whose name is
known specially in the compiler and was not mentioned in the C
code being compiled.
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 should either be 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 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)
`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)'
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.
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 argument LIBNAME exists for symmetry with
`INIT_CUMULATIVE_ARGS'. The value passed for LIBNAME is always 0,
since library routines with special calling conventions are never
compiled with GNU CC.
`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.
This macro does not control the *amount* of padding; that is
always just enough to reach the next multiple of
`FUNCTION_ARG_BOUNDARY'.
This macro has a default definition which is right for most
systems. For little-endian machines, the default is to pad upward.
For big-endian machines, the default is to pad downward for an
argument of constant size shorter than an `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).
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, and `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.
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' are returned in memory
regardless of 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 0.
`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, 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. GNU CC does not normally use this convention, even if it
is the usual one, but does use it if `-fpcc-struct-return' is
specified.
Do not define this if the usual system convention is for the
caller to pass an address to the subroutine.
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'.