i.e., as an `address_operand'. -- Macro: REGNO_MODE_OK_FOR_REG_BASE_P (NUM, MODE) A C expression which is nonzero if register number NUM is suitable for use as a base register in base plus index operand addresses, accessing memory in mode MODE. It may be either a suitable hard register or a pseudo register that has been allocated such a hard register. You should define this macro if base plus index addresses have different requirements than other base register uses. Use of this macro is deprecated; please use the more general `REGNO_MODE_CODE_OK_FOR_BASE_P'. -- Macro: REGNO_MODE_CODE_OK_FOR_BASE_P (NUM, MODE, OUTER_CODE, INDEX_CODE) A C expression that is just like `REGNO_MODE_OK_FOR_BASE_P', except that that expression may examine the context in which the register appears in the memory reference. OUTER_CODE is the code of the immediately enclosing expression (`MEM' if at the top level of the address, `ADDRESS' for something that occurs in an `address_operand'). INDEX_CODE is the code of the corresponding index expression if OUTER_CODE is `PLUS'; `SCRATCH' otherwise. The mode may be `VOIDmode' for addresses that appear outside a `MEM', i.e., as an `address_operand'. -- Macro: REGNO_OK_FOR_INDEX_P (NUM) A C expression which is nonzero if register number NUM is suitable for use as an index register in operand addresses. It may be either a suitable hard register or a pseudo register that has been allocated such a hard register. The difference between an index register and a base register is that the index register may be scaled. If an address involves the sum of two registers, neither one of them scaled, then either one may be labeled the "base" and the other the "index"; but whichever labeling is used must fit the machine's constraints of which registers may serve in each capacity. The compiler will try both labelings, looking for one that is valid, and will reload one or both registers only if neither labeling works. -- Macro: PREFERRED_RELOAD_CLASS (X, CLASS) A C expression that places additional restrictions on the register class to use when it is necessary to copy value X into a register in class CLASS. The value is a register class; perhaps CLASS, or perhaps another, smaller class. On many machines, the following definition is safe: #define PREFERRED_RELOAD_CLASS(X,CLASS) CLASS Sometimes returning a more restrictive class makes better code. For example, on the 68000, when X is an integer constant that is in range for a `moveq' instruction, the value of this macro is always `DATA_REGS' as long as CLASS includes the data registers. Requiring a data register guarantees that a `moveq' will be used. One case where `PREFERRED_RELOAD_CLASS' must not return CLASS is if X is a legitimate constant which cannot be loaded into some register class. By returning `NO_REGS' you can force X into a memory location. For example, rs6000 can load immediate values into general-purpose registers, but does not have an instruction for loading an immediate value into a floating-point register, so `PREFERRED_RELOAD_CLASS' returns `NO_REGS' when X is a floating-point constant. If the constant can't be loaded into any kind of register, code generation will be better if `LEGITIMATE_CONSTANT_P' makes the constant illegitimate instead of using `PREFERRED_RELOAD_CLASS'. If an insn has pseudos in it after register allocation, reload will go through the alternatives and call repeatedly `PREFERRED_RELOAD_CLASS' to find the best one. Returning `NO_REGS', in this case, makes reload add a `!' in front of the constraint: the x86 back-end uses this feature to discourage usage of 387 registers when math is done in the SSE registers (and vice versa). -- Macro: PREFERRED_OUTPUT_RELOAD_CLASS (X, CLASS) Like `PREFERRED_RELOAD_CLASS', but for output reloads instead of input reloads. If you don't define this macro, the default is to use CLASS, unchanged. You can also use `PREFERRED_OUTPUT_RELOAD_CLASS' to discourage reload from using some alternatives, like `PREFERRED_RELOAD_CLASS'. -- Macro: LIMIT_RELOAD_CLASS (MODE, CLASS) A C expression that places additional restrictions on the register class to use when it is necessary to be able to hold a value of mode MODE in a reload register for which class CLASS would ordinarily be used. Unlike `PREFERRED_RELOAD_CLASS', this macro should be used when there are certain modes that simply can't go in certain reload classes. The value is a register class; perhaps CLASS, or perhaps another, smaller class. Don't define this macro unless the target machine has limitations which require the macro to do something nontrivial. -- Target Hook: enum reg_class TARGET_SECONDARY_RELOAD (bool IN_P, rtx X, enum reg_class RELOAD_CLASS, enum machine_mode RELOAD_MODE, secondary_reload_info *SRI) Many machines have some registers that cannot be copied directly to or from memory or even from other types of registers. An example is the `MQ' register, which on most machines, can only be copied to or from general registers, but not memory. Below, we shall be using the term 'intermediate register' when a move operation cannot be performed directly, but has to be done by copying the source into the intermediate register first, and then copying the intermediate register to the destination. An intermediate register always has the same mode as source and destination. Since it holds the actual value being copied, reload might apply optimizations to re-use an intermediate register and eliding the copy from the source when it can determine that the intermediate register still holds the required value. Another kind of secondary reload is required on some machines which allow copying all registers to and from memory, but require a scratch register for stores to some memory locations (e.g., those with symbolic address on the RT, and those with certain symbolic address on the SPARC when compiling PIC). Scratch registers need not have the same mode as the value being copied, and usually hold a different value that that being copied. Special patterns in the md file are needed to describe how the copy is performed with the help of the scratch register; these patterns also describe the number, register class(es) and mode(s) of the scratch register(s). In some cases, both an intermediate and a scratch register are required. For input reloads, this target hook is called with nonzero IN_P, and X is an rtx that needs to be copied to a register of class RELOAD_CLASS in RELOAD_MODE. For output reloads, this target hook is called with zero IN_P, and a register of class RELOAD_CLASS needs to be copied to rtx X in RELOAD_MODE. If copying a register of RELOAD_CLASS from/to X requires an intermediate register, the hook `secondary_reload' should return the register class required for this intermediate register. If no intermediate register is required, it should return NO_REGS. If more than one intermediate register is required, describe the one that is closest in the copy chain to the reload register. If scratch registers are needed, you also have to describe how to perform the copy from/to the reload register to/from this closest intermediate register. Or if no intermediate register is required, but still a scratch register is needed, describe the copy from/to the reload register to/from the reload operand X. You do this by setting `sri->icode' to the instruction code of a pattern in the md file which performs the move. Operands 0 and 1 are the output and input of this copy, respectively. Operands from operand 2 onward are for scratch operands. These scratch operands must have a mode, and a single-register-class output constraint. When an intermediate register is used, the `secondary_reload' hook will be called again to determine how to copy the intermediate register to/from the reload operand X, so your hook must also have code to handle the register class of the intermediate operand. X might be a pseudo-register or a `subreg' of a pseudo-register, which could either be in a hard register or in memory. Use `true_regnum' to find out; it will return -1 if the pseudo is in memory and the hard register number if it is in a register. Scratch operands in memory (constraint `"=m"' / `"=&m"') are currently not supported. For the time being, you will have to continue to use `SECONDARY_MEMORY_NEEDED' for that purpose. `copy_cost' also uses this target hook to find out how values are copied. If you want it to include some extra cost for the need to allocate (a) scratch register(s), set `sri->extra_cost' to the additional cost. Or if two dependent moves are supposed to have a lower cost than the sum of the individual moves due to expected fortuitous scheduling and/or special forwarding logic, you can set `sri->extra_cost' to a negative amount. -- Macro: SECONDARY_RELOAD_CLASS (CLASS, MODE, X) -- Macro: SECONDARY_INPUT_RELOAD_CLASS (CLASS, MODE, X) -- Macro: SECONDARY_OUTPUT_RELOAD_CLASS (CLASS, MODE, X) These macros are obsolete, new ports should use the target hook `TARGET_SECONDARY_RELOAD' instead. These are obsolete macros, replaced by the `TARGET_SECONDARY_RELOAD' target hook. Older ports still define these macros to indicate to the reload phase that it may need to allocate at least one register for a reload in addition to the register to contain the data. Specifically, if copying X to a register CLASS in MODE requires an intermediate register, you were supposed to define `SECONDARY_INPUT_RELOAD_CLASS' to return the largest register class all of whose registers can be used as intermediate registers or scratch registers. If copying a register CLASS in MODE to X requires an intermediate or scratch register, `SECONDARY_OUTPUT_RELOAD_CLASS' was supposed to be defined be defined to return the largest register class required. If the requirements for input and output reloads were the same, the macro `SECONDARY_RELOAD_CLASS' should have been used instead of defining both macros identically. The values returned by these macros are often `GENERAL_REGS'. Return `NO_REGS' if no spare register is needed; i.e., if X can be directly copied to or from a register of CLASS in MODE without requiring a scratch register. Do not define this macro if it would always return `NO_REGS'. If a scratch register is required (either with or without an intermediate register), you were supposed to define patterns for `reload_inM' or `reload_outM', as required (*note Standard Names::. These patterns, which were normally implemented with a `define_expand', should be similar to the `movM' patterns, except that operand 2 is the scratch register. These patterns need constraints for the reload register and scratch register that contain a single register class. If the original reload register (whose class is CLASS) can meet the constraint given in the pattern, the value returned by these macros is used for the class of the scratch register. Otherwise, two additional reload registers are required. Their classes are obtained from the constraints in the insn pattern. X might be a pseudo-register or a `subreg' of a pseudo-register, which could either be in a hard register or in memory. Use `true_regnum' to find out; it will return -1 if the pseudo is in memory and the hard register number if it is in a register. These macros should not be used in the case where a particular class of registers can only be copied to memory and not to another class of registers. In that case, secondary reload registers are not needed and would not be helpful. Instead, a stack location must be used to perform the copy and the `movM' pattern should use memory as an intermediate storage. This case often occurs between floating-point and general registers. -- Macro: SECONDARY_MEMORY_NEEDED (CLASS1, CLASS2, M) Certain machines have the property that some registers cannot be copied to some other registers without using memory. Define this macro on those machines to be a C expression that is nonzero if objects of mode M in registers of CLASS1 can only be copied to registers of class CLASS2 by storing a register of CLASS1 into memory and loading that memory location into a register of CLASS2. Do not define this macro if its value would always be zero. -- Macro: SECONDARY_MEMORY_NEEDED_RTX (MODE) Normally when `SECONDARY_MEMORY_NEEDED' is defined, the compiler allocates a stack slot for a memory location needed for register copies. If this macro is defined, the compiler instead uses the memory location defined by this macro. Do not define this macro if you do not define `SECONDARY_MEMORY_NEEDED'. -- Macro: SECONDARY_MEMORY_NEEDED_MODE (MODE) When the compiler needs a secondary memory location to copy between two registers of mode MODE, it normally allocates sufficient memory to hold a quantity of `BITS_PER_WORD' bits and performs the store and load operations in a mode that many bits wide and whose class is the same as that of MODE. This is right thing to do on most machines because it ensures that all bits of the register are copied and prevents accesses to the registers in a narrower mode, which some machines prohibit for floating-point registers. However, this default behavior is not correct on some machines, such as the DEC Alpha, that store short integers in floating-point registers differently than in integer registers. On those machines, the default widening will not work correctly and you must define this macro to suppress that widening in some cases. See the file `alpha.h' for details. Do not define this macro if you do not define `SECONDARY_MEMORY_NEEDED' or if widening MODE to a mode that is `BITS_PER_WORD' bits wide is correct for your machine. -- Macro: SMALL_REGISTER_CLASSES On some machines, it is risky to let hard registers live across arbitrary insns. Typically, these machines have instructions that require values to be in specific registers (like an accumulator), and reload will fail if the required hard register is used for another purpose across such an insn. Define `SMALL_REGISTER_CLASSES' to be an expression with a nonzero value on these machines. When this macro has a nonzero value, the compiler will try to minimize the lifetime of hard registers. It is always safe to define this macro with a nonzero value, but if you unnecessarily define it, you will reduce the amount of optimizations that can be performed in some cases. If you do not define this macro with a nonzero value when it is required, the compiler will run out of spill registers and print a fatal error message. For most machines, you should not define this macro at all. -- Macro: CLASS_LIKELY_SPILLED_P (CLASS) A C expression whose value is nonzero if pseudos that have been assigned to registers of class CLASS would likely be spilled because registers of CLASS are needed for spill registers. The default value of this macro returns 1 if CLASS has exactly one register and zero otherwise. On most machines, this default should be used. Only define this macro to some other expression if pseudos allocated by `local-alloc.c' end up in memory because their hard registers were needed for spill registers. If this macro returns nonzero for those classes, those pseudos will only be allocated by `global.c', which knows how to reallocate the pseudo to another register. If there would not be another register available for reallocation, you should not change the definition of this macro since the only effect of such a definition would be to slow down register allocation. -- Macro: CLASS_MAX_NREGS (CLASS, MODE) A C expression for the maximum number of consecutive registers of class CLASS needed to hold a value of mode MODE. This is closely related to the macro `HARD_REGNO_NREGS'. In fact, the value of the macro `CLASS_MAX_NREGS (CLASS, MODE)' should be the maximum value of `HARD_REGNO_NREGS (REGNO, MODE)' for all REGNO values in the class CLASS. This macro helps control the handling of multiple-word values in the reload pass. -- Macro: CANNOT_CHANGE_MODE_CLASS (FROM, TO, CLASS) If defined, a C expression that returns nonzero for a CLASS for which a change from mode FROM to mode TO is invalid. For the example, loading 32-bit integer or floating-point objects into floating-point registers on the Alpha extends them to 64 bits. Therefore loading a 64-bit object and then storing it as a 32-bit object does not store the low-order 32 bits, as would be the case for a normal register. Therefore, `alpha.h' defines `CANNOT_CHANGE_MODE_CLASS' as below: #define CANNOT_CHANGE_MODE_CLASS(FROM, TO, CLASS) \ (GET_MODE_SIZE (FROM) != GET_MODE_SIZE (TO) \ ? reg_classes_intersect_p (FLOAT_REGS, (CLASS)) : 0)  File: gccint.info, Node: Old Constraints, Next: Stack and Calling, Prev: Register Classes, Up: Target Macros 15.9 Obsolete Macros for Defining Constraints ============================================= Machine-specific constraints can be defined with these macros instead of the machine description constructs described in *note Define Constraints::. This mechanism is obsolete. New ports should not use it; old ports should convert to the new mechanism. -- Macro: CONSTRAINT_LEN (CHAR, STR) For the constraint at the start of STR, which starts with the letter C, return the length. This allows you to have register class / constant / extra constraints that are longer than a single letter; you don't need to define this macro if you can do with single-letter constraints only. The definition of this macro should use DEFAULT_CONSTRAINT_LEN for all the characters that you don't want to handle specially. There are some sanity checks in genoutput.c that check the constraint lengths for the md file, so you can also use this macro to help you while you are transitioning from a byzantine single-letter-constraint scheme: when you return a negative length for a constraint you want to re-use, genoutput will complain about every instance where it is used in the md file. -- Macro: REG_CLASS_FROM_LETTER (CHAR) A C expression which defines the machine-dependent operand constraint letters for register classes. If CHAR is such a letter, the value should be the register class corresponding to it. Otherwise, the value should be `NO_REGS'. The register letter `r', corresponding to class `GENERAL_REGS', will not be passed to this macro; you do not need to handle it. -- Macro: REG_CLASS_FROM_CONSTRAINT (CHAR, STR) Like `REG_CLASS_FROM_LETTER', but you also get the constraint string passed in STR, so that you can use suffixes to distinguish between different variants. -- Macro: CONST_OK_FOR_LETTER_P (VALUE, C) A C expression that defines the machine-dependent operand constraint letters (`I', `J', `K', ... `P') that specify particular ranges of integer values. If C is one of those letters, the expression should check that VALUE, an integer, is in the appropriate range and return 1 if so, 0 otherwise. If C is not one of those letters, the value should be 0 regardless of VALUE. -- Macro: CONST_OK_FOR_CONSTRAINT_P (VALUE, C, STR) Like `CONST_OK_FOR_LETTER_P', but you also get the constraint string passed in STR, so that you can use suffixes to distinguish between different variants. -- Macro: CONST_DOUBLE_OK_FOR_LETTER_P (VALUE, C) A C expression that defines the machine-dependent operand constraint letters that specify particular ranges of `const_double' values (`G' or `H'). If C is one of those letters, the expression should check that VALUE, an RTX of code `const_double', is in the appropriate range and return 1 if so, 0 otherwise. If C is not one of those letters, the value should be 0 regardless of VALUE. `const_double' is used for all floating-point constants and for `DImode' fixed-point constants. A given letter can accept either or both kinds of values. It can use `GET_MODE' to distinguish between these kinds. -- Macro: CONST_DOUBLE_OK_FOR_CONSTRAINT_P (VALUE, C, STR) Like `CONST_DOUBLE_OK_FOR_LETTER_P', but you also get the constraint string passed in STR, so that you can use suffixes to distinguish between different variants. -- Macro: EXTRA_CONSTRAINT (VALUE, C) A C expression that defines the optional machine-dependent constraint letters that can be used to segregate specific types of operands, usually memory references, for the target machine. Any letter that is not elsewhere defined and not matched by `REG_CLASS_FROM_LETTER' / `REG_CLASS_FROM_CONSTRAINT' may be used. Normally this macro will not be defined. If it is required for a particular target machine, it should return 1 if VALUE corresponds to the operand type represented by the constraint letter C. If C is not defined as an extra constraint, the value returned should be 0 regardless of VALUE. For example, on the ROMP, load instructions cannot have their output in r0 if the memory reference contains a symbolic address. Cons !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~§çħŧƧǧȧɧʧ˧̧ͧΧϧЧѧҧӧԧէ֧קا٧ڧۧܧݧާߧ traint letter `Q' is defined as representing a memory address that does _not_ contain a symbolic address. An alternative is specified with a `Q' constraint on the input and `r' on the output. The next alternative specifies `m' on the input and a register class that does not include r0 on the output. -- Macro: EXTRA_CONSTRAINT_STR (VALUE, C, STR) Like `EXTRA_CONSTRAINT', but you also get the constraint string passed in STR, so that you can use suffixes to distinguish between different variants. -- Macro: EXTRA_MEMORY_CONSTRAINT (C, STR) A C expression that defines the optional machine-dependent constraint letters, amongst those accepted by `EXTRA_CONSTRAINT', that should be treated like memory constraints by the reload pass. It should return 1 if the operand type represented by the constraint at the start of STR, the first letter of which is the letter C, comprises a subset of all memory references including all those whose address is simply a base register. This allows the reload pass to reload an operand, if it does not directly correspond to the operand type of C, by copying its address into a base register. For example, on the S/390, some instructions do not accept arbitrary memory references, but only those that do not make use of an index register. The constraint letter `Q' is defined via `EXTRA_CONSTRAINT' as representing a memory address of this type. If the letter `Q' is marked as `EXTRA_MEMORY_CONSTRAINT', a `Q' constraint can handle any memory operand, because the reload pass knows it can be reloaded by copying the memory address into a base register if required. This is analogous to the way a `o' constraint can handle any memory operand. -- Macro: EXTRA_ADDRESS_CONSTRAINT (C, STR) A C expression that defines the optional machine-dependent constraint letters, amongst those accepted by `EXTRA_CONSTRAINT' / `EXTRA_CONSTRAINT_STR', that should be treated like address constraints by the reload pass. It should return 1 if the operand type represented by the constraint at the start of STR, which starts with the letter C, comprises a subset of all memory addresses including all those that consist of just a base register. This allows the reload pass to reload an operand, if it does not directly correspond to the operand type of STR, by copying it into a base register. Any constraint marked as `EXTRA_ADDRESS_CONSTRAINT' can only be used with the `address_operand' predicate. It is treated analogously to the `p' constraint.  File: gccint.info, Node: Stack and Calling, Next: Varargs, Prev: Old Constraints, Up: Target Macros 15.10 Stack Layout and Calling Conventions ========================================== This describes the stack layout and calling conventions. * Menu: * Frame Layout:: * Exception Handling:: * Stack Checking:: * Frame Registers:: * Elimination:: * Stack Arguments:: * Register Arguments:: * Scalar Return:: * Aggregate Return:: * Caller Saves:: * Function Entry:: * Profiling:: * Tail Calls:: * Stack Smashing Protection::  File: gccint.info, Node: Frame Layout, Next: Exception Handling, Up: Stack and Calling 15.10.1 Basic Stack Layout -------------------------- Here is the basic stack layout. -- Macro: 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. -- Macro: STACK_PUSH_CODE This macro defines the operation used when something is pushed on the stack. In RTL, a push operation will be `(set (mem (STACK_PUSH_CODE (reg sp))) ...)' The choices are `PRE_DEC', `POST_DEC', `PRE_INC', and `POST_INC'. Which of these is correct depends on the stack direction and on whether the stack pointer points to the last item on the stack or whether it points to the space for the next item on the stack. The default is `PRE_DEC' when `STACK_GROWS_DOWNWARD' is defined, which is almost always right, and `PRE_INC' otherwise, which is often wrong. -- Macro: FRAME_GROWS_DOWNWARD Define this macro to nonzero value if the addresses of local variable slots are at negative offsets from the frame pointer. -- Macro: ARGS_GROW_DOWNWARD Define this macro if successive arguments to a function occupy decreasing addresses on the stack. -- Macro: 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'. -- Macro: STACK_ALIGNMENT_NEEDED Define to zero to disable final alignment of the stack during reload. The nonzero default for this macro is suitable for most ports. On ports where `STARTING_FRAME_OFFSET' is nonzero or where there is a register save block following the local block that doesn't require alignment to `STACK_BOUNDARY', it may be beneficial to disable stack alignment and do it in the backend. -- Macro: 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. -- Macro: 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. -- Macro: 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. -- Macro: INITIAL_FRAME_ADDRESS_RTX A C expression whose value is RTL representing the address of the initial stack frame. This address is passed to `RETURN_ADDR_RTX' and `DYNAMIC_CHAIN_ADDRESS'. If you don't define this macro, a reasonable default value will be used. Define this macro in order to make frame pointer elimination work in the presence of `__builtin_frame_address (count)' and `__builtin_return_address (count)' for `count' not equal to zero. -- Macro: 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. -- Macro: 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. -- Target Hook: bool TARGET_BUILTIN_SETJMP_FRAME_VALUE () This target hook should return 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 target hook is if `hard_frame_pointer_rtx' is the appropriate value on your machine. -- Macro: FRAME_ADDR_RTX (FRAMEADDR) A C expression whose value is RTL representing the value of the frame address for the current frame. FRAMEADDR is the frame pointer of the current frame. This is used for __builtin_frame_address. You need only define this macro if the frame address is not the same as the frame pointer. Most machines do not need to define it. -- Macro: 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. -- Macro: 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. -- Macro: 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)'. -- Macro: DWARF_ALT_FRAME_RETURN_COLUMN A C expression whose value is an integer giving a DWARF 2 column number that may be used as an alternative return column. The column must not correspond to any gcc hard register (that is, it must not be in the range of `DWARF_FRAME_REGNUM'). This macro can be useful if `DWARF_FRAME_RETURN_COLUMN' is set to a general register, but an alternative column needs to be used for signal frames. Some targets have also used different frame return columns over time. -- Macro: DWARF_ZERO_REG A C expression whose value is an integer giving a DWARF 2 register number that is considered to always have the value zero. This should only be defined if the target has an architected zero register, and someone decided it was a good idea to use that register number to terminate the stack backtrace. New ports should avoid this. -- Target Hook: void TARGET_DWARF_HANDLE_FRAME_UNSPEC (const char *LABEL, rtx PATTERN, int INDEX) This target hook allows the backend to emit frame-related insns that contain UNSPECs or UNSPEC_VOLATILEs. The DWARF 2 call frame debugging info engine will invoke it on insns of the form (set (reg) (unspec [...] UNSPEC_INDEX)) and (set (reg) (unspec_volatile [...] UNSPECV_INDEX)). to let the backend emit the call frame instructions. LABEL is the CFI label attached to the insn, PATTERN is the pattern of the insn and INDEX is `UNSPEC_INDEX' or `UNSPECV_INDEX'. -- Macro: 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. -- Macro: 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. -- Macro: FRAME_POINTER_CFA_OFFSET (FUNDECL) If defined, a C expression whose value is an integer giving the offset in bytes from the frame pointer to the canonical frame address (cfa). The final value should coincide with that calculated by `INCOMING_FRAME_SP_OFFSET'. Normally the CFA is calculated as an offset from the argument pointer, via `ARG_POINTER_CFA_OFFSET', but if the argument pointer is variable due to the ABI, this may not be possible. If this macro is defined, it implies that the virtual register instantiation should be based on the frame pointer instead of the argument pointer. Only one of `FRAME_POINTER_CFA_OFFSET' and `ARG_POINTER_CFA_OFFSET' should be defined. -- Macro: CFA_FRAME_BASE_OFFSET (FUNDECL) If defined, a C expression whose value is an integer giving the offset in bytes from the canonical frame address (cfa) to the frame base used in DWARF 2 debug information. The default is zero. A different value may reduce the size of debug information on some ports.  File: gccint.info, Node: Exception Handling, Next: Stack Checking, Prev: Frame Layout, Up: Stack and Calling 15.10.2 Exception Handling Support ---------------------------------- -- Macro: 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. -- Macro: 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. Do not define this macro if the stack pointer is saved and restored by the regular prolog and epilog code in the call frame itself; in this case, the exception handling library routines will update the stack location to be restored in place. Otherwise, you must define this macro if you want to support call frame exception handling like that provided by DWARF 2. -- Macro: 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. If defined, `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. -- Macro: RETURN_ADDR_OFFSET If defined, an integer-valued C expression for which rtl will be generated to add it to the exception handler address before it is searched in the exception handling tables, and to subtract it again from the address before using it to return to the exception handler. -- Macro: 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. -- Macro: 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. -- Macro: MD_UNWIND_SUPPORT A string specifying a file to be #include'd in unwind-dw2.c. The file so included typically defines `MD_FALLBACK_FRAME_STATE_FOR'. -- Macro: MD_FALLBACK_FRAME_STATE_FOR (CONTEXT, FS) This macro allows the target to add CPU and operating system specific code to the call-frame unwinder for use when there is no unwind data available. The most common reason to implement this macro is to unwind through signal frames. This macro is called from `uw_frame_state_for' in `unwind-dw2.c', `unwind-dw2-xtensa.c' and `unwind-ia64.c'. CONTEXT is an `_Unwind_Context'; FS is an `_Unwind_FrameState'. Examine `context->ra' for the address of the code being executed and `context->cfa' for the stack pointer value. If the frame can be decoded, the register save addresses should be updated in FS and the macro should evaluate to `_URC_NO_REASON'. If the frame cannot be decoded, the macro should evaluate to `_URC_END_OF_STACK'. For proper signal handling in Java this macro is accompanied by `MAKE_THROW_FRAME', defined in `libjava/include/*-signal.h' headers. -- Macro: MD_HANDLE_UNWABI (CONTEXT, FS) This macro allows the target to add operating system specific code to the call-frame unwinder to handle the IA-64 `.unwabi' unwinding directive, usually used for signal or interrupt frames. This macro is called from `uw_update_context' in `unwind-ia64.c'. CONTEXT is an `_Unwind_Context'; FS is an `_Unwind_FrameState'. Examine `fs->unwabi' for the abi and context in the `.unwabi' directive. If the `.unwabi' directive can be handled, the register save addresses should be updated in FS. -- Macro: TARGET_USES_WEAK_UNWIND_INFO A C expression that evaluates to true if the target requires unwind info to be given comdat linkage. Define it to be `1' if comdat linkage is necessary. The default is `0'.  File: gccint.info, Node: Stack Checking, Next: Frame Registers, Prev: Exception Handling, Up: Stack and Calling 15.10.3 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 `TARGET_ASM_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. -- Macro: 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. -- Macro: 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. -- Macro: 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. -- Macro: 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. -- Macro: 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. -- 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. -- Macro: 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: gccint.info, Node: Frame Registers, Next: Elimination, Prev: Stack Checking, Up: Stack and Calling 15.10.4 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. -- Macro: DWARF_FRAME_REGNUM (REGNO) Define this macro if the target's representation for dwarf registers used in .eh_frame or .debug_frame is different from that used in other debug info sections. Given a GCC hard register number, this macro should return the .eh_frame register number. The default is `DBX_REGISTER_NUMBER (REGNO)'. -- Macro: DWARF2_FRAME_REG_OUT (REGNO, FOR_EH) Define this macro to map register numbers held in the call frame info that GCC has collected using `DWARF_FRAME_REGNUM' to those that should be output in .debug_frame (`FOR_EH' is zero) and .eh_frame (`FOR_EH' is nonzero). The default is to return `REGNO'.  File: gccint.info, Node: Elimination, Next: Stack Arguments, Prev: Frame Registers, Up: Stack and Calling 15.10.5 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 15.10.6 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: OUTGOING_REG_PARM_STACK_SPACE Define this to a nonzero value 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 15.10.7 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 t