21.8.5 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.