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INFO-DIR-SECTION Programming
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* gcc: (gcc).                  The GNU Compiler Collection.
END-INFO-DIR-ENTRY
   This file documents the use and the internals of the GNU compiler.


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File: gcc.info,  Node: Frame Layout,  Next: Stack Checking,  Up: Stack and Calling

Basic Stack Layout
------------------

   Here is the 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', find the next slot's offset by
     subtracting the first slot's length 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.

`SETUP_FRAME_ADDRESSES'
     If defined, a C expression that produces the machine-specific code
     to setup the stack so that arbitrary frames can be accessed.  For
     example, on the Sparc, we must flush all of the register windows
     to the stack before we can access arbitrary stack frames.  You
     will seldom need to define this macro.

`BUILTIN_SETJMP_FRAME_VALUE'
     If defined, a C expression that contains an rtx that is used to
     store the address of the current frame into the built in `setjmp'
     buffer.  The default value, `virtual_stack_vars_rtx', is correct
     for most machines.  One reason you may need to define this macro
     is if `hard_frame_pointer_rtx' is the appropriate value on your
     machine.

`RETURN_ADDR_RTX (COUNT, FRAMEADDR)'
     A C expression whose value is RTL representing the value of the
     return address for the frame COUNT steps up from the current
     frame, after the prologue.  FRAMEADDR is the frame pointer of the
     COUNT frame, or the frame pointer of the COUNT - 1 frame if
     `RETURN_ADDR_IN_PREVIOUS_FRAME' is defined.

     The value of the expression must always be the correct address when
     COUNT is zero, but may be `NULL_RTX' if there is not way to
     determine the return address of other frames.

`RETURN_ADDR_IN_PREVIOUS_FRAME'
     Define this if the return address of a particular stack frame is
     accessed from the frame pointer of the previous stack frame.

`INCOMING_RETURN_ADDR_RTX'
     A C expression whose value is RTL representing the location of the
     incoming return address at the beginning of any function, before
     the prologue.  This RTL is either a `REG', indicating that the
     return value is saved in `REG', or a `MEM' representing a location
     in the stack.

     You only need to define this macro if you want to support call
     frame debugging information like that provided by DWARF 2.

     If this RTL is a `REG', you should also define
     `DWARF_FRAME_RETURN_COLUMN' to `DWARF_FRAME_REGNUM (REGNO)'.

`INCOMING_FRAME_SP_OFFSET'
     A C expression whose value is an integer giving the offset, in
     bytes, from the value of the stack pointer register to the top of
     the stack frame at the beginning of any function, before the
     prologue.  The top of the frame is defined to be the value of the
     stack pointer in the previous frame, just before the call
     instruction.

     You only need to define this macro if you want to support call
     frame debugging information like that provided by DWARF 2.

`ARG_POINTER_CFA_OFFSET (FUNDECL)'
     A C expression whose value is an integer giving the offset, in
     bytes, from the argument pointer to the canonical frame address
     (cfa).  The final value should coincide with that calculated by
     `INCOMING_FRAME_SP_OFFSET'.  Which is unfortunately not usable
     during virtual register instantiation.

     The default value for this macro is `FIRST_PARM_OFFSET (fundecl)',
     which is correct for most machines; in general, the arguments are
     found immediately before the stack frame.  Note that this is not
     the case on some targets that save registers into the caller's
     frame, such as SPARC and rs6000, and so such targets need to
     define this macro.

     You only need to define this macro if the default is incorrect,
     and you want to support call frame debugging information like that
     provided by DWARF 2.

`EH_RETURN_DATA_REGNO (N)'
     A C expression whose value is the Nth register number used for
     data by exception handlers, or `INVALID_REGNUM' if fewer than N
     registers are usable.

     The exception handling library routines communicate with the
     exception handlers via a set of agreed upon registers.  Ideally
     these registers should be call-clobbered; it is possible to use
     call-saved registers, but may negatively impact code size.  The
     target must support at least 2 data registers, but should define 4
     if there are enough free registers.

     You must define this macro if you want to support call frame
     exception handling like that provided by DWARF 2.

`EH_RETURN_STACKADJ_RTX'
     A C expression whose value is RTL representing a location in which
     to store a stack adjustment to be applied before function return.
     This is used to unwind the stack to an exception handler's call
     frame.  It will be assigned zero on code paths that return
     normally.

     Typically this is a call-clobbered hard register that is otherwise
     untouched by the epilogue, but could also be a stack slot.

     You must define this macro if you want to support call frame
     exception handling like that provided by DWARF 2.

`EH_RETURN_HANDLER_RTX'
     A C expression whose value is RTL representing a location in which
     to store the address of an exception handler to which we should
     return.  It will not be assigned on code paths that return
     normally.

     Typically this is the location in the call frame at which the
     normal return address is stored.  For targets that return by
     popping an address off the stack, this might be a memory address
     just below the _target_ call frame rather than inside the current
     call frame.  `EH_RETURN_STACKADJ_RTX' will have already been
     assigned, so it may be used to calculate the location of the
     target call frame.

     Some targets have more complex requirements than storing to an
     address calculable during initial code generation.  In that case
     the `eh_return' instruction pattern should be used instead.

     If you want to support call frame exception handling, you must
     define either this macro or the `eh_return' instruction pattern.

`ASM_PREFERRED_EH_DATA_FORMAT(CODE, GLOBAL)'
     This macro chooses the encoding of pointers embedded in the
     exception handling sections.  If at all possible, this should be
     defined such that the exception handling section will not require
     dynamic relocations, and so may be read-only.

     CODE is 0 for data, 1 for code labels, 2 for function pointers.
     GLOBAL is true if the symbol may be affected by dynamic
     relocations.  The macro should return a combination of the
     `DW_EH_PE_*' defines as found in `dwarf2.h'.

     If this macro is not defined, pointers will not be encoded but
     represented directly.

`ASM_MAYBE_OUTPUT_ENCODED_ADDR_RTX(FILE, ENCODING, SIZE, ADDR, DONE)'
     This macro allows the target to emit whatever special magic is
     required to represent the encoding chosen by
     `ASM_PREFERRED_EH_DATA_FORMAT'.  Generic code takes care of
     pc-relative and indirect encodings; this must be defined if the
     target uses text-relative or data-relative encodings.

     This is a C statement that branches to DONE if the format was
     handled.  ENCODING is the format chosen, SIZE is the number of
     bytes that the format occupies, ADDR is the `SYMBOL_REF' to be
     emitted.

`SMALL_STACK'
     Define this macro if the stack size for the target is very small.
     This has the effect of disabling gcc's built-in `alloca', though
     `__builtin_alloca' is not affected.


File: gcc.info,  Node: Stack Checking,  Next: Frame Registers,  Prev: Frame Layout,  Up: Stack and Calling

Specifying How Stack Checking is Done
-------------------------------------

   GCC will check that stack references are within the boundaries of
the stack, if the `-fstack-check' is specified, in one of three ways:

  1. If the value of the `STACK_CHECK_BUILTIN' macro is nonzero, GCC
     will assume that you have arranged for stack checking to be done at
     appropriate places in the configuration files, e.g., in
     `FUNCTION_PROLOGUE'.  GCC will do not other special processing.

  2. If `STACK_CHECK_BUILTIN' is zero and you defined a named pattern
     called `check_stack' in your `md' file, GCC will call that pattern
     with one argument which is the address to compare the stack value
     against.  You must arrange for this pattern to report an error if
     the stack pointer is out of range.

  3. If neither of the above are true, GCC will generate code to
     periodically "probe" the stack pointer using the values of the
     macros defined below.

   Normally, you will use the default values of these macros, so GCC
will use the third approach.

`STACK_CHECK_BUILTIN'
     A nonzero value if stack checking is done by the configuration
     files in a machine-dependent manner.  You should define this macro
     if stack checking is require by the ABI of your machine or if you
     would like to have to stack checking in some more efficient way
     than GCC's portable approach.  The default value of this macro is
     zero.

`STACK_CHECK_PROBE_INTERVAL'
     An integer representing the interval at which GCC must generate
     stack probe instructions.  You will normally define this macro to
     be no larger than the size of the "guard pages" at the end of a
     stack area.  The default value of 4096 is suitable for most
     systems.

`STACK_CHECK_PROBE_LOAD'
     A integer which is nonzero if GCC should perform the stack probe
     as a load instruction and zero if GCC should use a store
     instruction.  The default is zero, which is the most efficient
     choice on most systems.

`STACK_CHECK_PROTECT'
     The number of bytes of stack needed to recover from a stack
     overflow, for languages where such a recovery is supported.  The
     default value of 75 words should be adequate for most machines.

`STACK_CHECK_MAX_FRAME_SIZE'
     The maximum size of a stack frame, in bytes.  GCC will generate
     probe instructions in non-leaf functions to ensure at least this
     many bytes of stack are available.  If a stack frame is larger
     than this size, stack checking will not be reliable and GCC will
     issue a warning.  The default is chosen so that GCC only generates
     one instruction on most systems.  You should normally not change
     the default value of this macro.

`STACK_CHECK_FIXED_FRAME_SIZE'
     GCC uses this value to generate the above warning message.  It
     represents the amount of fixed frame used by a function, not
     including space for any callee-saved registers, temporaries and
     user variables.  You need only specify an upper bound for this
     amount and will normally use the default of four words.

`STACK_CHECK_MAX_VAR_SIZE'
     The maximum size, in bytes, of an object that GCC will place in the
     fixed area of the stack frame when the user specifies
     `-fstack-check'.  GCC computed the default from the values of the
     above macros and you will normally not need to override that
     default.


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 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.

`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'
     A C expression whose value is nonzero 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.  If the macro is not defined in target header files, it
     defaults to 0.

`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.  On some machines, the definition

`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)

`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.

`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.

`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, GCC 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.  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 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 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.

`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.

`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, 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.

`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_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.

`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'.

`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.

`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.

`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.

`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: 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, GCC 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.  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.

`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'.

`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.

