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INFO-DIR-SECTION Programming
START-INFO-DIR-ENTRY
* gccint: (gccint).            Internals of the GNU Compiler Collection.
END-INFO-DIR-ENTRY
   This file documents the internals of the GNU compilers.

   Published by the Free Software Foundation
59 Temple Place - Suite 330
Boston, MA 02111-1307 USA

File: gccint.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.

 - Macro: 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.

 - Macro: 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.

 - Macro: 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'.

 - Macro: 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::).

 - Macro: 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.

 - Macro: STATIC_CHAIN_REGNUM
 - Macro: 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.

 - Macro: STATIC_CHAIN
 - Macro: 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.

 - Macro: 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
     `FIRST_PSEUDO_REGISTER'.

 - Macro: PRE_GCC3_DWARF_FRAME_REGISTERS
     This macro is similar to `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
     `DWARF_FRAME_REGISTERS'.

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

     See the PowerPC's SPE target for an example.


File: gccint.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.

 - Macro: 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.

 - Macro: 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.

 - Macro: 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.

 - Macro: CAN_ELIMINATE (FROM-REG, TO-REG)
     A C expression that returns nonzero 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.

 - Macro: 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.


File: gccint.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.

 - Target Hook: bool TARGET_PROMOTE_PROTOTYPES (tree FNTYPE)
     This target hook returns `true' 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.  The default is to not promote prototypes.

 - Macro: PUSH_ARGS
     A C expression.  If nonzero, push insns will be used to pass
     outgoing arguments.  If the target machine does not have a push
     instruction, set it to zero.  That directs GCC to use an alternate
     strategy: to allocate the entire argument block and then store the
     arguments into it.  When `PUSH_ARGS' is nonzero, `PUSH_ROUNDING'
     must be defined too.

 - Macro: 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 `PUSH_ARGS' on targets where the
     stack and args grow in opposite directions, and 0 otherwise.

 - Macro: 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.

     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)

 - Macro: ACCUMULATE_OUTGOING_ARGS
     A C expression.  If nonzero, the maximum amount of space required
     for outgoing arguments will be computed and placed into the
     variable `current_function_outgoing_args_size'.  No space will be
     pushed onto the stack for each call; instead, the function
     prologue should increase the stack frame size by this amount.

     Setting both `PUSH_ARGS' and `ACCUMULATE_OUTGOING_ARGS' is not
     proper.

 - Macro: 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, which can be zero if GCC is calling a
     library function.

     This space can be allocated by the caller, or be a part of the
     machine-dependent stack frame: `OUTGOING_REG_PARM_STACK_SPACE' says
     which.

 - Macro: MAYBE_REG_PARM_STACK_SPACE
 - Macro: 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, GCC uses these two macros instead of
     `REG_PARM_STACK_SPACE'.

 - Macro: 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'.

 - Macro: 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.

 - Macro: 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_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.

 - Macro: CALL_POPS_ARGS (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
     `RETURN_POPS_ARGS' when compiling a function call.

     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,
     `RETURN_POPS_ARGS' is not appropriate.


File: gccint.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.

 - Macro: 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.  TYPE can
     be an incomplete type if a syntax error has previously occurred.

     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 `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 must be a `reg' RTX for the hard
     register in which to pass this part of the argument, and the mode
     of the register RTX indicates how large this part of the argument
     is.  The second operand of the `expr_list' is a `const_int' which
     gives the offset in bytes into the entire argument of where this
     part starts.  As a special exception the first `expr_list' in the
     `parallel' RTX may have a first operand of zero.  This indicates
     that the entire argument is also stored on the stack.

     The last time this macro is called, it is called with `MODE ==
     VOIDmode', and its result is passed to the `call' or `call_value'
     pattern as operands 2 and 3 respectively.

     The usual way to make the ISO 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 nonzero 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.

 - Macro: MUST_PASS_IN_STACK (MODE, TYPE)
     Define as a C expression that evaluates to nonzero if we do not
     know how to pass TYPE solely in registers.  The file `expr.h'
     defines a definition that is usually appropriate, refer to
     `expr.h' for additional documentation.

 - Macro: 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.

 - Macro: FUNCTION_ARG_PARTIAL_NREGS (CUM, MODE, TYPE, NAMED)
     A C expression for the number of words, at the beginning of an
     argument, that 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.

 - Macro: 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)

 - Macro: 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.

 - Macro: FUNCTION_ARG_KEEP_AS_REFERENCE (CUM, MODE, TYPE, NAMED)
     If defined, a C expression that indicates when it is more
     desirable to keep an argument passed by invisible reference as a
     reference, rather than copying it to a pseudo register.

 - Macro: 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'.

 - Macro: INIT_CUMULATIVE_ARGS (CUM, FNTYPE, LIBNAME, FNDECL)
     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.  For
     direct calls that are not libcalls, FNDECL contain the declaration
     node of the function.  FNDECL is also set 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.

 - Macro: INIT_CUMULATIVE_LIBCALL_ARGS (CUM, MODE, LIBNAME)
     Like `INIT_CUMULATIVE_ARGS' but only used for outgoing libcalls,
     it gets a `MODE' argument instead of FNTYPE, that would be `NULL'.
     INDIRECT would always be zero, too.  If this macro is not
     defined, `INIT_CUMULATIVE_ARGS (cum, NULL_RTX, libname, 0)' is
     used instead.

 - Macro: 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 GCC.  The
     argument LIBNAME exists for symmetry with `INIT_CUMULATIVE_ARGS'.

 - Macro: 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.

 - Macro: 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.

 - Macro: 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 `PARM_BOUNDARY'.  If this macro is not
     defined, all such arguments are padded down if `BYTES_BIG_ENDIAN'
     is true.

 - Macro: BLOCK_REG_PADDING (MODE, TYPE, FIRST)
     Specify padding for the last element of a block move between
     registers and memory.  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
     `PARALLEL' rtl.  In particular, `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.

 - Macro: 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.

 - Macro: 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.

 - Macro: SPLIT_COMPLEX_ARGS
     Define this macro to a nonzero value if complex function arguments
     should be split into their corresponding components.  By default,
     GCC will attempt to pack complex arguments into the target's word
     size.  Some ABIs require complex arguments to be split and treated
     as their individual components.  For example, on AIX64, complex
     floats should be passed in a pair of floating point registers,
     even though a complex float would fit in one 64-bit floating point
     register.

 - Macro: LOAD_ARGS_REVERSED
     If defined, the order in which arguments are loaded into their
     respective argument registers is reversed so that the last
     argument is loaded first.  This macro only affects arguments
     passed in registers.


File: gccint.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.

 - Macro: 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 `TARGET_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.

 - Macro: 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.

 - Macro: 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.

 - Macro: 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.

 - Macro: 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: gccint.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.

 - Target Hook: bool RETURN_IN_MEMORY (tree TYPE, tree FNTYPE)
     This target hook should return a nonzero value to say to return the
     function value in memory, just as large structures are always
     returned.  Here TYPE will be the data type of the value, and FNTYPE
     will be the type of the function doing the returning, or `NULL' for
     libcalls.

     Note that values of mode `BLKmode' must be explicitly handled by
     this function.  Also, the option `-fpcc-struct-return' takes
     effect regardless of this macro.  On most systems, it is possible
     to leave the hook 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 hook to indicate that structures and unions should
     always be returned in memory.  You should instead use
     `DEFAULT_PCC_STRUCT_RETURN' to indicate this.

 - Macro: 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.

 - Target Hook: rtx TARGET_STRUCT_VALUE_RTX (tree FNDECL, int INCOMING)
     This target hook should return the location of the structure value
     address (normally a `mem' or `reg'), or 0 if the address is passed
     as an "invisible" first argument.  Note that FNDECL may be `NULL',
     for libcalls.

     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.
     INCOMING is `true' when the location is needed in the context of
     the called function, and `false' in the context of the caller.

     If INCOMING is `true' and the address is to be found on the stack,
     return a `mem' which refers to the frame pointer.

 - Macro: 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: gccint.info,  Node: Caller Saves,  Next: Function Entry,  Prev: Aggregate Return,  Up: Stack and Calling

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.

 - Macro: 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.  When defined, this macro enables
     `-fcaller-saves' by default for all optimization levels.  It has
     no effect for optimization levels 2 and higher, where
     `-fcaller-saves' is the default.

 - Macro: 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'.

 - Macro: HARD_REGNO_CALLER_SAVE_MODE (REGNO, NREGS)
     A C expression specifying which mode is required for saving NREGS
     of a pseudo-register in call-clobbered hard register REGNO.  If
     REGNO is unsuitable for caller save, `VOIDmode' should be
     returned.  For most machines this macro need not be defined since
     GCC will select the smallest suitable mode.

