This is Info file gcc.info, produced by Makeinfo version 1.67 from the input file gcc.texi. This file documents the use and the internals of the GNU compiler. 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 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the sections entitled "GNU General Public License," "Funding for Free Software," and "Protect Your Freedom--Fight `Look And Feel'" are included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the sections entitled "GNU General Public License," "Funding for Free Software," and "Protect Your Freedom--Fight `Look And Feel'", and this permission notice, may be included in translations approved by the Free Software Foundation instead of in the original English.  File: gcc.info, Node: Frame Registers, Next: Elimination, Prev: Stack Checking, Up: Stack and Calling Registers That Address the Stack Frame -------------------------------------- This discusses registers that address the stack frame. `STACK_POINTER_REGNUM' The register number of the stack pointer register, which must also be a fixed register according to `FIXED_REGISTERS'. On most machines, the hardware determines which register this is. `FRAME_POINTER_REGNUM' The register number of the frame pointer register, which is used to access automatic variables in the stack frame. On some machines, the hardware determines which register this is. On other machines, you can choose any register you wish for this purpose. `HARD_FRAME_POINTER_REGNUM' On some machines the offset between the frame pointer and starting offset of the automatic variables is not known until after register allocation has been done (for example, because the saved registers are between these two locations). On those machines, define `FRAME_POINTER_REGNUM' the number of a special, fixed register to be used internally until the offset is known, and define `HARD_FRAME_POINTER_REGNUM' to be the actual hard register number used for the frame pointer. You should define this macro only in the very rare circumstances when it is not possible to calculate the offset between the frame pointer and the automatic variables until after register allocation has been completed. When this macro is defined, you must also indicate in your definition of `ELIMINABLE_REGS' how to eliminate `FRAME_POINTER_REGNUM' into either `HARD_FRAME_POINTER_REGNUM' or `STACK_POINTER_REGNUM'. Do not define this macro if it would be the same as `FRAME_POINTER_REGNUM'. `ARG_POINTER_REGNUM' The register number of the arg pointer register, which is used to access the function's argument list. On some machines, this is the same as the frame pointer register. On some machines, the hardware determines which register this is. On other machines, you can choose any register you wish for this purpose. If this is not the same register as the frame pointer register, then you must mark it as a fixed register according to `FIXED_REGISTERS', or arrange to be able to eliminate it (*note Elimination::.). `RETURN_ADDRESS_POINTER_REGNUM' The register number of the return address pointer register, which is used to access the current function's return address from the stack. On some machines, the return address is not at a fixed offset from the frame pointer or stack pointer or argument pointer. This register can be defined to point to the return address on the stack, and then be converted by `ELIMINABLE_REGS' into either the frame pointer or stack pointer. Do not define this macro unless there is no other way to get the return address from the stack. `STATIC_CHAIN_REGNUM' `STATIC_CHAIN_INCOMING_REGNUM' Register numbers used for passing a function's static chain pointer. If register windows are used, the register number as seen by the called function is `STATIC_CHAIN_INCOMING_REGNUM', while the register number as seen by the calling function is `STATIC_CHAIN_REGNUM'. If these registers are the same, `STATIC_CHAIN_INCOMING_REGNUM' need not be defined. The static chain register need not be a fixed register. If the static chain is passed in memory, these macros should not be defined; instead, the next two macros should be defined. `STATIC_CHAIN' `STATIC_CHAIN_INCOMING' If the static chain is passed in memory, these macros provide rtx giving `mem' expressions that denote where they are stored. `STATIC_CHAIN' and `STATIC_CHAIN_INCOMING' give the locations as seen by the calling and called functions, respectively. Often the former will be at an offset from the stack pointer and the latter at an offset from the frame pointer. The variables `stack_pointer_rtx', `frame_pointer_rtx', and `arg_pointer_rtx' will have been initialized prior to the use of these macros and should be used to refer to those items. If the static chain is passed in a register, the two previous macros should be defined instead.  File: gcc.info, Node: Elimination, Next: Stack Arguments, Prev: Frame Registers, Up: Stack and Calling Eliminating Frame Pointer and Arg Pointer ----------------------------------------- This is about eliminating the frame pointer and arg pointer. `FRAME_POINTER_REQUIRED' A C expression which is nonzero if a function must have and use a frame pointer. This expression is evaluated in the reload pass. If its value is nonzero the function will have a frame pointer. The expression can in principle examine the current function and decide according to the facts, but on most machines the constant 0 or the constant 1 suffices. Use 0 when the machine allows code to be generated with no frame pointer, and doing so saves some time or space. Use 1 when there is no possible advantage to avoiding a frame pointer. In certain cases, the compiler does not know how to produce valid code without a frame pointer. The compiler recognizes those cases and automatically gives the function a frame pointer regardless of what `FRAME_POINTER_REQUIRED' says. You don't need to worry about them. In a function that does not require a frame pointer, the frame pointer register can be allocated for ordinary usage, unless you mark it as a fixed register. See `FIXED_REGISTERS' for more information. `INITIAL_FRAME_POINTER_OFFSET (DEPTH-VAR)' A C statement to store in the variable DEPTH-VAR the difference between the frame pointer and the stack pointer values immediately after the function prologue. The value would be computed from information such as the result of `get_frame_size ()' and the tables of registers `regs_ever_live' and `call_used_regs'. If `ELIMINABLE_REGS' is defined, this macro will be not be used and need not be defined. Otherwise, it must be defined even if `FRAME_POINTER_REQUIRED' is defined to always be true; in that case, you may set DEPTH-VAR to anything. `ELIMINABLE_REGS' If defined, this macro specifies a table of register pairs used to eliminate unneeded registers that point into the stack frame. If it is not defined, the only elimination attempted by the compiler is to replace references to the frame pointer with references to the stack pointer. The definition of this macro is a list of structure initializations, each of which specifies an original and replacement register. On some machines, the position of the argument pointer is not known until the compilation is completed. In such a case, a separate hard register must be used for the argument pointer. This register can be eliminated by replacing it with either the frame pointer or the argument pointer, depending on whether or not the frame pointer has been eliminated. In this case, you might specify: #define ELIMINABLE_REGS \ {{ARG_POINTER_REGNUM, STACK_POINTER_REGNUM}, \ {ARG_POINTER_REGNUM, FRAME_POINTER_REGNUM}, \ {FRAME_POINTER_REGNUM, STACK_POINTER_REGNUM}} Note that the elimination of the argument pointer with the stack pointer is specified first since that is the preferred elimination. `CAN_ELIMINATE (FROM-REG, TO-REG)' A C expression that returns non-zero if the compiler is allowed to try to replace register number FROM-REG with register number TO-REG. This macro need only be defined if `ELIMINABLE_REGS' is defined, and will usually be the constant 1, since most of the cases preventing register elimination are things that the compiler already knows about. `INITIAL_ELIMINATION_OFFSET (FROM-REG, TO-REG, OFFSET-VAR)' This macro is similar to `INITIAL_FRAME_POINTER_OFFSET'. It specifies the initial difference between the specified pair of registers. This macro must be defined if `ELIMINABLE_REGS' is defined. `LONGJMP_RESTORE_FROM_STACK' Define this macro if the `longjmp' function restores registers from the stack frames, rather than from those saved specifically by `setjmp'. Certain quantities must not be kept in registers across a call to `setjmp' on such machines.  File: gcc.info, Node: Stack Arguments, Next: Register Arguments, Prev: Elimination, Up: Stack and Calling Passing Function Arguments on the Stack --------------------------------------- The macros in this section control how arguments are passed on the stack. See the following section for other macros that control passing certain arguments in registers. `PROMOTE_PROTOTYPES' Define this macro 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. `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. If the target machine does not have a push instruction, do not define this macro. That directs GNU CC to use an alternate strategy: to allocate the entire argument block and then store the arguments into it. 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' If defined, 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. Defining both `PUSH_ROUNDING' 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. 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, GNU CC uses these two macros instead of `REG_PARM_STACK_SPACE'. `OUTGOING_REG_PARM_STACK_SPACE' Define this if it is the responsibility of the caller to allocate the area reserved for arguments passed in registers. If `ACCUMULATE_OUTGOING_ARGS' is defined, this macro controls whether the space for these arguments counts in the value of `current_function_outgoing_args_size'. `STACK_PARMS_IN_REG_PARM_AREA' Define this macro if `REG_PARM_STACK_SPACE' is defined, but the stack parameters don't skip the area specified by it. Normally, when a parameter is not passed in registers, it is placed on the stack beyond the `REG_PARM_STACK_SPACE' area. Defining this macro suppresses this behavior and causes the parameter to be passed on the stack in its natural location. `RETURN_POPS_ARGS (FUNDECL, FUNTYPE, STACK-SIZE)' A C expression that should indicate the number of bytes of its own arguments that a function pops on returning, or 0 if the function pops no arguments and the caller must therefore pop them all after the function returns. FUNDECL is a C variable whose value is a tree node that describes the function in question. Normally it is a node of type `FUNCTION_DECL' that describes the declaration of the function. From this you can obtain the DECL_MACHINE_ATTRIBUTES of the function. FUNTYPE is a C variable whose value is a tree node that describes the function in question. Normally it is a node of type `FUNCTION_TYPE' that describes the data type of the function. From this it is possible to obtain the data types of the value and arguments (if known). When a call to a library function is being considered, FUNDECL will contain an identifier node for the library function. Thus, if you need to distinguish among various library functions, you can do so by their names. Note that "library function" in this context means a function used to perform arithmetic, whose name is known specially in the compiler and was not mentioned in the C code being compiled. STACK-SIZE is the number of bytes of arguments passed on the stack. If a variable number of bytes is passed, it is zero, and argument popping will always be the responsibility of the calling function. On the Vax, all functions always pop their arguments, so the definition of this macro is STACK-SIZE. On the 68000, using the standard calling convention, no functions pop their arguments, so the value of the macro is always 0 in this case. But an alternative calling convention is available in which functions that take a fixed number of arguments pop them but other functions (such as `printf') pop nothing (the caller pops all). When this convention is in use, FUNTYPE is examined to determine whether a function takes a fixed number of arguments.  File: gcc.info, Node: Register Arguments, Next: Scalar Return, Prev: Stack Arguments, Up: Stack and Calling Passing Arguments in Registers ------------------------------ This section describes the macros which let you control how various types of arguments are passed in registers or how they are arranged in the stack. `FUNCTION_ARG (CUM, MODE, TYPE, NAMED)' A C expression that controls whether a function argument is passed in a register, and which register. The arguments are CUM, which summarizes all the previous arguments; MODE, the machine mode of the argument; TYPE, the data type of the argument as a tree node or 0 if that is not known (which happens for C support library functions); and NAMED, which is 1 for an ordinary argument and 0 for nameless arguments that correspond to `...' in the called function's prototype. 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 can be either a `reg' RTX for the hard register in which to pass this part of the argument, or zero to pass the argument on the stack. If this operand is a `reg', then the mode 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 where this part starts. The usual way to make the ANSI 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 non-zero 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. `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, must be put in registers. The value must be zero for arguments that are passed entirely in registers or that are entirely pushed on the stack. On some machines, certain arguments must be passed partially in registers and partially in memory. On these machines, typically the first N words of arguments are passed in registers, and the rest on the stack. If a multi-word argument (a `double' or a structure) crosses that boundary, its first few words must be passed in registers and the rest must be pushed. This macro tells the compiler when this occurs, and how many of the words should go in registers. `FUNCTION_ARG' for these arguments should return the first register to be used by the caller for this argument; likewise `FUNCTION_INCOMING_ARG', for the called function. `FUNCTION_ARG_PASS_BY_REFERENCE (CUM, MODE, TYPE, NAMED)' A C expression that indicates when an argument must be passed by reference. If nonzero for an argument, a copy of that argument is made in memory and a pointer to the argument is passed instead of the argument itself. The pointer is passed in whatever way is appropriate for passing a pointer to that type. On machines where `REG_PARM_STACK_SPACE' is not defined, a suitable definition of this macro might be #define FUNCTION_ARG_PASS_BY_REFERENCE\ (CUM, MODE, TYPE, NAMED) \ MUST_PASS_IN_STACK (MODE, TYPE) `FUNCTION_ARG_CALLEE_COPIES (CUM, MODE, TYPE, NAMED)' If defined, a C expression that indicates when it is the called function's responsibility to make a copy of arguments passed by invisible reference. Normally, the caller makes a copy and passes the address of the copy to the routine being called. When FUNCTION_ARG_CALLEE_COPIES is defined and is nonzero, the caller does not make a copy. Instead, it passes a pointer to the "live" value. The called function must not modify this value. If it can be determined that the value won't be modified, it need not make a copy; otherwise a copy must be made. `CUMULATIVE_ARGS' A C type for declaring a variable that is used as the first argument of `FUNCTION_ARG' and other related values. For some target machines, the type `int' suffices and can hold the number of bytes of argument so far. There is no need to record in `CUMULATIVE_ARGS' anything about the arguments that have been passed on the stack. The compiler has other variables to keep track of that. For target machines on which all arguments are passed on the stack, there is no need to store anything in `CUMULATIVE_ARGS'; however, the data structure must exist and should not be empty, so use `int'. `INIT_CUMULATIVE_ARGS (CUM, FNTYPE, LIBNAME, INDIRECT)' A C statement (sans semicolon) for initializing the variable CUM for the state at the beginning of the argument list. The variable has type `CUMULATIVE_ARGS'. The value of FNTYPE is the tree node for the data type of the function which will receive the args, or 0 if the args are to a compiler support library function. The value of INDIRECT is nonzero when processing an indirect call, for example a call through a function pointer. The value of INDIRECT is zero for a call to an explicitly named function, a library function call, or when `INIT_CUMULATIVE_ARGS' is used to find arguments for the function being compiled. When processing a call to a compiler support library function, LIBNAME identifies which one. It is a `symbol_ref' rtx which contains the name of the function, as a string. LIBNAME is 0 when an ordinary C function call is being processed. Thus, each time this macro is called, either LIBNAME or FNTYPE is nonzero, but never both of them at once. `INIT_CUMULATIVE_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 GNU CC. 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. `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.  File: gcc.info, Node: Scalar Return, Next: Aggregate Return, Prev: Register Arguments, Up: Stack and Calling How Scalar Function Values Are Returned --------------------------------------- This section discusses the macros that control returning scalars as values--values that can fit in registers. `TRADITIONAL_RETURN_FLOAT' Define this macro if `-traditional' should not cause functions declared to return `float' to convert the value to `double'. `FUNCTION_VALUE (VALTYPE, FUNC)' A C expression to create an RTX representing the place where a function returns a value of data type VALTYPE. VALTYPE is a tree node representing a data type. Write `TYPE_MODE (VALTYPE)' to get the machine mode used to represent that type. On many machines, only the mode is relevant. (Actually, on most machines, scalar values are returned in the same place regardless of mode). The value of the expression is usually a `reg' RTX for the hard register where the return value is stored. The value can also be a `parallel' RTX, if the return value is in multiple places. See `FUNCTION_ARG' for an explanation of the `parallel' form. If `PROMOTE_FUNCTION_RETURN' is defined, you must apply the same promotion rules specified in `PROMOTE_MODE' if VALTYPE is a scalar type. If the precise function being called is known, FUNC is a tree node (`FUNCTION_DECL') for it; otherwise, FUNC is a null pointer. This makes it possible to use a different value-returning convention for specific functions when all their calls are known. `FUNCTION_VALUE' is not used for return vales with aggregate data types, because these are returned in another way. See `STRUCT_VALUE_REGNUM' and related macros, below. `FUNCTION_OUTGOING_VALUE (VALTYPE, FUNC)' Define this macro if the target machine has "register windows" so that the register in which a function returns its value is not the same as the one in which the caller sees the value. For such machines, `FUNCTION_VALUE' computes the register in which the caller will see the value. `FUNCTION_OUTGOING_VALUE' should be defined in a similar fashion to tell the function where to put the value. If `FUNCTION_OUTGOING_VALUE' is not defined, `FUNCTION_VALUE' serves both purposes. `FUNCTION_OUTGOING_VALUE' is not used for return vales with aggregate data types, because these are returned in another way. See `STRUCT_VALUE_REGNUM' and related macros, below. `LIBCALL_VALUE (MODE)' A C expression to create an RTX representing the place where a library function returns a value of mode MODE. If the precise function being called is known, FUNC is a tree node (`FUNCTION_DECL') for it; otherwise, FUNC is a null pointer. This makes it possible to use a different value-returning convention for specific functions when all their calls are known. Note that "library function" in this context means a compiler support routine, used to perform arithmetic, whose name is known specially by the compiler and was not mentioned in the C code being compiled. The definition of `LIBRARY_VALUE' need not be concerned aggregate data types, because none of the library functions returns such types. `FUNCTION_VALUE_REGNO_P (REGNO)' A C expression that is nonzero if REGNO is the number of a hard register in which the values of called function may come back. A register whose use for returning values is limited to serving as the second of a pair (for a value of type `double', say) need not be recognized by this macro. So for most machines, this definition suffices: #define FUNCTION_VALUE_REGNO_P(N) ((N) == 0) If the machine has register windows, so that the caller and the called function use different registers for the return value, this macro should recognize only the caller's register numbers. `APPLY_RESULT_SIZE' Define this macro if `untyped_call' and `untyped_return' need more space than is implied by `FUNCTION_VALUE_REGNO_P' for saving and restoring an arbitrary return value.  File: gcc.info, Node: Aggregate Return, Next: Caller Saves, Prev: Scalar Return, Up: Stack and Calling How Large Values Are Returned ----------------------------- When a function value's mode is `BLKmode' (and in some other cases), the value is not returned according to `FUNCTION_VALUE' (*note Scalar Return::.). Instead, the caller passes the address of a block of memory in which the value should be stored. This address is called the "structure value address". This section describes how to control returning structure values in memory. `RETURN_IN_MEMORY (TYPE)' A C expression which can inhibit the returning of certain function values in registers, based on the type of value. A nonzero value says to return the function value in memory, just as large structures are always returned. Here TYPE will be a C expression of type `tree', representing the data type of the value. Note that values of mode `BLKmode' must be explicitly handled by this macro. Also, the option `-fpcc-struct-return' takes effect regardless of this macro. On most systems, it is possible to leave the macro undefined; this causes a default definition to be used, whose value is the constant 1 for `BLKmode' values, and 0 otherwise. Do not use this macro to indicate that structures and unions should always be returned in memory. You should instead use `DEFAULT_PCC_STRUCT_RETURN' to indicate this. `DEFAULT_PCC_STRUCT_RETURN' Define this macro to be 1 if all structure and union return values must be in memory. Since this results in slower code, this should be defined only if needed for compatibility with other compilers or with an ABI. If you define this macro to be 0, then the conventions used for structure and union return values are decided by the `RETURN_IN_MEMORY' macro. If not defined, this defaults to the value 1. `STRUCT_VALUE_REGNUM' If the structure value address is passed in a register, then `STRUCT_VALUE_REGNUM' should be the number of that register. `STRUCT_VALUE' If the structure value address is not passed in a register, define `STRUCT_VALUE' as an expression returning an RTX for the place where the address is passed. If it returns 0, the address is passed as an "invisible" first argument. `STRUCT_VALUE_INCOMING_REGNUM' On some architectures the place where the structure value address is found by the called function is not the same place that the caller put it. This can be due to register windows, or it could be because the function prologue moves it to a different place. If the incoming location of the structure value address is in a register, define this macro as the register number. `STRUCT_VALUE_INCOMING' If the incoming location is not a register, then you should define `STRUCT_VALUE_INCOMING' as an expression for an RTX for where the called function should find the value. If it should find the value on the stack, define this to create a `mem' which refers to the frame pointer. A definition of 0 means that the address is passed as an "invisible" first argument. `PCC_STATIC_STRUCT_RETURN' Define this macro if the usual system convention on the target machine for returning structures and unions is for the called function to return the address of a static variable containing the value. Do not define this if the usual system convention is for the caller to pass an address to the subroutine. This macro has effect in `-fpcc-struct-return' mode, but it does nothing when you use `-freg-struct-return' mode.  File: gcc.info, Node: Caller Saves, Next: Function Entry, Prev: Aggregate Return, Up: Stack and Calling Caller-Saves Register Allocation -------------------------------- If you enable it, GNU CC can save registers around function calls. This makes it possible to use call-clobbered registers to hold variables that must live across calls. `DEFAULT_CALLER_SAVES' Define this macro if function calls on the target machine do not preserve any registers; in other words, if `CALL_USED_REGISTERS' has 1 for all registers. This macro enables `-fcaller-saves' by default. Eventually that option will be enabled by default on all machines and both the option and this macro will be eliminated. `CALLER_SAVE_PROFITABLE (REFS, CALLS)' A C expression to determine whether it is worthwhile to consider placing a pseudo-register in a call-clobbered hard register and saving and restoring it around each function call. The expression should be 1 when this is worth doing, and 0 otherwise. If you don't define this macro, a default is used which is good on most machines: `4 * CALLS < REFS'.  File: gcc.info, Node: Function Entry, Next: Profiling, Prev: Caller Saves, Up: Stack and Calling Function Entry and Exit ----------------------- This section describes the macros that output function entry ("prologue") and exit ("epilogue") code. `FUNCTION_PROLOGUE (FILE, SIZE)' A C compound statement that outputs the assembler code for entry to a function. The prologue is responsible for setting up the stack frame, initializing the frame pointer register, saving registers that must be saved, and allocating SIZE additional bytes of storage for the local variables. SIZE is an integer. FILE is a stdio stream to which the assembler code should be output. The label for the beginning of the function need not be output by this macro. That has already been done when the macro is run. To determine which registers to save, the macro can refer to the array `regs_ever_live': element R is nonzero if hard register R is used anywhere within the function. This implies the function prologue should save register R, provided it is not one of the call-used registers. (`FUNCTION_EPILOGUE' must likewise use `regs_ever_live'.) On machines that have "register windows", the function entry code does not save on the stack the registers that are in the windows, even if they are supposed to be preserved by function calls; instead it takes appropriate steps to "push" the register stack, if any non-call-used registers are used in the function. On machines where functions may or may not have frame-pointers, the function entry code must vary accordingly; it must set up the frame pointer if one is wanted, and not otherwise. To determine whether a frame pointer is in wanted, the macro can refer to the variable `frame_pointer_needed'. The variable's value will be 1 at run time in a function that needs a frame pointer. *Note Elimination::. The function entry code is responsible for allocating any stack space required for the function. This stack space consists of the regions listed below. In most cases, these regions are allocated in the order listed, with the last listed region closest to the top of the stack (the lowest address if `STACK_GROWS_DOWNWARD' is defined, and the highest address if it is not defined). You can use a different order for a machine if doing so is more convenient or required for compatibility reasons. Except in cases where required by standard or by a debugger, there is no reason why the stack layout used by GCC need agree with that used by other compilers for a machine. * A region of `current_function_pretend_args_size' bytes of uninitialized space just underneath the first argument arriving on the stack. (This may not be at the very start of the allocated stack region if the calling sequence has pushed anything else since pushing the stack arguments. But usually, on such machines, nothing else has been pushed yet, because the function prologue itself does all the pushing.) This region is used on machines where an argument may be passed partly in registers and partly in memory, and, in some cases to support the features in `varargs.h' and `stdargs.h'. * An area of memory used to save certain registers used by the function. The size of this area, which may also include space for such things as the return address and pointers to previous stack frames, is machine-specific and usually depends on which registers have been used in the function. Machines with register windows often do not require a save area. * A region of at least SIZE bytes, possibly rounded up to an allocation boundary, to contain the local variables of the function. On some machines, this region and the save area may occur in the opposite order, with the save area closer to the top of the stack. * Optionally, when `ACCUMULATE_OUTGOING_ARGS' is defined, a region of `current_function_outgoing_args_size' bytes to be used for outgoing argument lists of the function. *Note Stack Arguments::. Normally, it is necessary for the macros `FUNCTION_PROLOGUE' and `FUNCTION_EPILOGUE' to treat leaf functions specially. The C variable `leaf_function' is nonzero for such a function. `EXIT_IGNORE_STACK' Define this macro as a C expression that is nonzero if the return instruction or the function epilogue ignores the value of the stack pointer; in other words, if it is safe to delete an instruction to adjust the stack pointer before a return from the function. Note that this macro's value is relevant only for functions for which frame pointers are maintained. It is never safe to delete a final stack adjustment in a function that has no frame pointer, and the compiler knows this regardless of `EXIT_IGNORE_STACK'. `EPILOGUE_USES (REGNO)' Define this macro as a C expression that is nonzero for registers are used by the epilogue or the `return' pattern. The stack and frame pointer registers are already be assumed to be used as needed. `FUNCTION_EPILOGUE (FILE, SIZE)' A C compound statement that outputs the assembler code for exit from a function. The epilogue is responsible for restoring the saved registers and stack pointer to their values when the function was called, and returning control to the caller. This macro takes the same arguments as the macro `FUNCTION_PROLOGUE', and the registers to restore are determined from `regs_ever_live' and `CALL_USED_REGISTERS' in the same way. On some machines, there is a single instruction that does all the work of returning from the function. On these machines, give that instruction the name `return' and do not define the macro `FUNCTION_EPILOGUE' at all. Do not define a pattern named `return' if you want the `FUNCTION_EPILOGUE' to be used. If you want the target switches to control whether return instructions or epilogues are used, define a `return' pattern with a validity condition that tests the target switches appropriately. If the `return' pattern's validity condition is false, epilogues will be used. On machines where functions may or may not have frame-pointers, the function exit code must vary accordingly. Sometimes the code for these two cases is completely different. To determine whether a frame pointer is wanted, the macro can refer to the variable `frame_pointer_needed'. The variable's value will be 1 when compiling a function that needs a frame pointer. Normally, `FUNCTION_PROLOGUE' and `FUNCTION_EPILOGUE' must treat leaf functions specially. The C variable `leaf_function' is nonzero for such a function. *Note Leaf Functions::. On some machines, some functions pop their arguments on exit while others leave that for the caller to do. For example, the 68020 when given `-mrtd' pops arguments in functions that take a fixed number of arguments. Your definition of the macro `RETURN_POPS_ARGS' decides which functions pop their own arguments. `FUNCTION_EPILOGUE' needs to know what was decided. The variable that is called `current_function_pops_args' is the number of bytes of its arguments that a function should pop. *Note Scalar Return::. `DELAY_SLOTS_FOR_EPILOGUE' Define this macro if the function epilogue contains delay slots to which instructions from the rest of the function can be "moved". The definition should be a C expression whose value is an integer representing the number of delay slots there. `ELIGIBLE_FOR_EPILOGUE_DELAY (INSN, N)' A C expression that returns 1 if INSN can be placed in delay slot number N of the epilogue. The argument N is an integer which identifies the delay slot now being considered (since different slots may have different rules of eligibility). It is never negative and is always less than the number of epilogue delay slots (what `DELAY_SLOTS_FOR_EPILOGUE' returns). If you reject a particular insn for a given delay slot, in principle, it may be reconsidered for a subsequent delay slot. Also, other insns may (at least in principle) be considered for the so far unfilled delay slot. The insns accepted to fill the epilogue delay slots are put in an RTL list made with `insn_list' objects, stored in the variable `current_function_epilogue_delay_list'. The insn for the first delay slot comes first in the list. Your definition of the macro `FUNCTION_EPILOGUE' should fill the delay slots by outputting the insns in this list, usually by calling `final_scan_insn'. You need not define this macro if you did not define `DELAY_SLOTS_FOR_EPILOGUE'. `ASM_OUTPUT_MI_THUNK (FILE, THUNK_FNDECL, DELTA, FUNCTION)' A C compound statement that outputs the assembler code for a thunk function, used to implement C++ virtual function calls with multiple inheritance. The thunk acts as a wrapper around a virtual function, adjusting the implicit object parameter before handing control off to the real function. First, emit code to add the integer DELTA to the location that contains the incoming first argument. Assume that this argument contains a pointer, and is the one used to pass the `this' pointer in C++. This is the incoming argument *before* the function prologue, e.g. `%o0' on a sparc. The addition must preserve the values of all other incoming arguments. After the addition, emit code to jump to FUNCTION, which is a `FUNCTION_DECL'. This is a direct pure jump, not a call, and does not touch the return address. Hence returning from FUNCTION will return to whoever called the current `thunk'. The effect must be as if FUNCTION had been called directly with the adjusted first argument. This macro is responsible for emitting all of the code for a thunk function; `FUNCTION_PROLOGUE' and `FUNCTION_EPILOGUE' are not invoked. The THUNK_FNDECL is redundant. (DELTA and FUNCTION have already been extracted from it.) It might possibly be useful on some targets, but probably not. If you do not define this macro, the target-independent code in the C++ frontend will generate a less efficient heavyweight thunk that calls FUNCTION instead of jumping to it. The generic approach does not support varargs.