This is doc/gccint.info, produced by makeinfo version 4.2 from doc/gccint.texi. INFO-DIR-SECTION Programming START-INFO-DIR-ENTRY * gccint: (gccint). Internals of the GNU Compiler Collection. END-INFO-DIR-ENTRY This file documents the internals of the GNU compilers. Published by the Free Software Foundation 59 Temple Place - Suite 330 Boston, MA 02111-1307 USA Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled "GNU Free Documentation License". (a) The FSF's Front-Cover Text is: A GNU Manual (b) The FSF's Back-Cover Text is: You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.  File: gccint.info, Node: Frame Layout, Next: Exception Handling, 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. `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. `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. `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: gccint.info, Node: Exception Handling, Next: Stack Checking, Prev: Frame Layout, Up: Stack and Calling Exception Handling Support -------------------------- `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. `MD_FALLBACK_FRAME_STATE_FOR(CONTEXT, FS, SUCCESS)' 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' 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 branch to SUCCESS. If the frame cannot be decoded, the macro should do nothing.  File: gccint.info, Node: Stack Checking, Next: Frame Registers, Prev: Exception Handling, 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 `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. `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: gccint.info, Node: Frame Registers, Next: Elimination, Prev: Stack Checking, Up: Stack and Calling Registers That Address the Stack Frame -------------------------------------- This discusses registers that address the stack frame. `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. `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'. `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'.  File: gccint.info, Node: Elimination, Next: Stack Arguments, Prev: Frame Registers, Up: Stack and Calling Eliminating Frame Pointer and Arg Pointer ----------------------------------------- This is about eliminating the frame pointer and arg pointer. `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.  File: gccint.info, Node: Stack Arguments, Next: Register Arguments, Prev: Elimination, Up: Stack and Calling Passing Function Arguments on the Stack --------------------------------------- The macros in this section control how arguments are passed on the stack. See the following section for other macros that control passing certain arguments in registers. `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_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. `CALL_POPS_ARGS (CUM)' A C expression that should indicate the number of bytes a call sequence pops off the stack. It is added to the value of `RETURN_POPS_ARGS' when compiling a function call. CUM is the variable in which all arguments to the called function have been accumulated. On certain architectures, such as the SH5, a call trampoline is used that pops certain registers off the stack, depending on the arguments that have been passed to the function. Since this is a property of the call site, not of the called function, `RETURN_POPS_ARGS' is not appropriate.  File: gccint.info, Node: Register Arguments, Next: Scalar Return, Prev: Stack Arguments, Up: Stack and Calling Passing Arguments in Registers ------------------------------ This section describes the macros which let you control how various types of arguments are passed in registers or how they are arranged in the stack. `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 last time this macro is called, it is called with `MODE == VOIDmode', and its result is passed to the `call' or `call_value' pattern as operands 2 and 3 respectively. The usual way to make the ISO library `stdarg.h' work on a machine where some arguments are usually passed in registers, is to cause nameless arguments to be passed on the stack instead. This is done by making `FUNCTION_ARG' return 0 whenever NAMED is 0. You may use the macro `MUST_PASS_IN_STACK (MODE, TYPE)' in the definition of this macro to determine if this argument is of a type that must be passed in the stack. If `REG_PARM_STACK_SPACE' is not defined and `FUNCTION_ARG' returns nonzero for such an argument, the compiler will abort. If `REG_PARM_STACK_SPACE' is defined, the argument will be computed in the stack and then loaded into a register. `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. `FUNCTION_ARG_REG_LITTLE_ENDIAN' If defined TRUE on a big-endian system then structure arguments passed (and returned) in registers are passed in a little-endian manner instead of the big-endian manner. On the HP-UX IA64 and PA64 platforms structures are aligned differently then integral values and setting this value to true will allow for the special handling of structure arguments and return values. `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: gccint.info, Node: Scalar Return, Next: Aggregate Return, Prev: Register Arguments, Up: Stack and Calling How Scalar Function Values Are Returned --------------------------------------- This section discusses the macros that control returning scalars as values--values that can fit in registers. `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.