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GNU Info File | 1995-06-16 | 43.5 KB | 928 lines

This is Info file gcc.info, produced by Makeinfo-1.55 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 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: 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 should either be 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 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)' 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. 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). 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'. `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'. File: gcc.info, Node: Profiling, Prev: Function Entry, Up: Stack and Calling Generating Code for Profiling ----------------------------- These macros will help you generate code for profiling. `FUNCTION_PROFILER (FILE, LABELNO)' A C statement or compound statement to output to FILE some assembler code to call the profiling subroutine `mcount'. Before calling, the assembler code must load the address of a counter variable into a register where `mcount' expects to find the address. The name of this variable is `LP' followed by the number LABELNO, so you would generate the name using `LP%d' in a `fprintf'. The details of how the address should be passed to `mcount' are determined by your operating system environment, not by GNU CC. To figure them out, compile a small program for profiling using the system's installed C compiler and look at the assembler code that results. `PROFILE_BEFORE_PROLOGUE' Define this macro if the code for function profiling should come before the function prologue. Normally, the profiling code comes after. `FUNCTION_BLOCK_PROFILER (FILE, LABELNO)' A C statement or compound statement to output to FILE some assembler code to initialize basic-block profiling for the current object module. This code should call the subroutine `__bb_init_func' once per object module, passing it as its sole argument the address of a block allocated in the object module. The name of the block is a local symbol made with this statement: ASM_GENERATE_INTERNAL_LABEL (BUFFER, "LPBX", 0); Of course, since you are writing the definition of `ASM_GENERATE_INTERNAL_LABEL' as well as that of this macro, you can take a short cut in the definition of this macro and use the name that you know will result. The first word of this block is a flag which will be nonzero if the object module has already been initialized. So test this word first, and do not call `__bb_init_func' if the flag is nonzero. `BLOCK_PROFILER (FILE, BLOCKNO)' A C statement or compound statement to increment the count associated with the basic block number BLOCKNO. Basic blocks are numbered separately from zero within each compilation. The count associated with block number BLOCKNO is at index BLOCKNO in a vector of words; the name of this array is a local symbol made with this statement: ASM_GENERATE_INTERNAL_LABEL (BUFFER, "LPBX", 2); Of course, since you are writing the definition of `ASM_GENERATE_INTERNAL_LABEL' as well as that of this macro, you can take a short cut in the definition of this macro and use the name that you know will result. `BLOCK_PROFILER_CODE' A C function or functions which are needed in the library to support block profiling. File: gcc.info, Node: Varargs, Next: Trampolines, Prev: Stack and Calling, Up: Target Macros Implementing the Varargs Macros =============================== GNU CC comes with an implementation of `varargs.h' and `stdarg.h' that work without change on machines that pass arguments on the stack. Other machines require their own implementations of varargs, and the two machine independent header files must have conditionals to include it. ANSI `stdarg.h' differs from traditional `varargs.h' mainly in the calling convention for `va_start'. The traditional implementation takes just one argument, which is the variable in which to store the argument pointer. The ANSI implementation of `va_start' takes an additional second argument. The user is supposed to write the last named argument of the function here. However, `va_start' should not use this argument. The way to find the end of the named arguments is with the built-in functions described below. `__builtin_saveregs ()' Use this built-in function to save the argument registers in memory so that the varargs mechanism can access them. Both ANSI and traditional versions of `va_start' must use `__builtin_saveregs', unless you use `SETUP_INCOMING_VARARGS' (see below) instead. On some machines, `__builtin_saveregs' is open-coded under the control of the macro `EXPAND_BUILTIN_SAVEREGS'. On other machines, it calls a routine written in assembler language, found in `libgcc2.c'. Code generated for the call to `__builtin_saveregs' appears at the beginning of the function, as opposed to where the call to `__builtin_saveregs' is written, regardless of what the code is. This is because the registers must be saved before the function starts to use them for its own purposes. `__builtin_args_info (CATEGORY)' Use this built-in function to find the first anonymous arguments in registers. In general, a machine may have several categories of registers used for arguments, each for a particular category of data types. (For example, on some machines, floating-point registers are used for floating-point arguments while other arguments are passed in the general registers.) To make non-varargs functions use the proper calling convention, you have defined the `CUMULATIVE_ARGS' data type to record how many registers in each category have been used so far `__builtin_args_info' accesses the same data structure of type `CUMULATIVE_ARGS' after the ordinary argument layout is finished with it, with CATEGORY specifying which word to access. Thus, the value indicates the first unused register in a given category. Normally, you would use `__builtin_args_info' in the implementation of `va_start', accessing each category just once and storing the value in the `va_list' object. This is because `va_list' will have to update the values, and there is no way to alter the values accessed by `__builtin_args_info'. `__builtin_next_arg (LASTARG)' This is the equivalent of `__builtin_args_info', for stack arguments. It returns the address of the first anonymous stack argument, as type `void *'. If `ARGS_GROW_DOWNWARD', it returns the address of the location above the first anonymous stack argument. Use it in `va_start' to initialize the pointer for fetching arguments from the stack. Also use it in `va_start' to verify that the second parameter LASTARG is the last named argument of the current function. `__builtin_classify_type (OBJECT)' Since each machine has its own conventions for which data types are passed in which kind of register, your implementation of `va_arg' has to embody these conventions. The easiest way to categorize the specified data type is to use `__builtin_classify_type' together with `sizeof' and `__alignof__'. `__builtin_classify_type' ignores the value of OBJECT, considering only its data type. It returns an integer describing what kind of type that is--integer, floating, pointer, structure, and so on. The file `typeclass.h' defines an enumeration that you can use to interpret the values of `__builtin_classify_type'. These machine description macros help implement varargs: `EXPAND_BUILTIN_SAVEREGS (ARGS)' If defined, is a C expression that produces the machine-specific code for a call to `__builtin_saveregs'. This code will be moved to the very beginning of the function, before any parameter access are made. The return value of this function should be an RTX that contains the value to use as the return of `__builtin_saveregs'. The argument ARGS is a `tree_list' containing the arguments that were passed to `__builtin_saveregs'. If this macro is not defined, the compiler will output an ordinary call to the library function `__builtin_saveregs'. `SETUP_INCOMING_VARARGS (ARGS_SO_FAR, MODE, TYPE,' PRETEND_ARGS_SIZE, SECOND_TIME) This macro offers an alternative to using `__builtin_saveregs' and defining the macro `EXPAND_BUILTIN_SAVEREGS'. Use it to store the anonymous register arguments into the stack so that all the arguments appear to have been passed consecutively on the stack. Once this is done, you can use the standard implementation of varargs that works for machines that pass all their arguments on the stack. The argument ARGS_SO_FAR is the `CUMULATIVE_ARGS' data structure, containing the values that obtain after processing of the named arguments. The arguments MODE and TYPE describe the last named argument--its machine mode and its data type as a tree node. The macro implementation should do two things: first, push onto the stack all the argument registers *not* used for the named arguments, and second, store the size of the data thus pushed into the `int'-valued variable whose name is supplied as the argument PRETEND_ARGS_SIZE. The value that you store here will serve as additional offset for setting up the stack frame. Because you must generate code to push the anonymous arguments at compile time without knowing their data types, `SETUP_INCOMING_VARARGS' is only useful on machines that have just a single category of argument register and use it uniformly for all data types. If the argument SECOND_TIME is nonzero, it means that the arguments of the function are being analyzed for the second time. This happens for an inline function, which is not actually compiled until the end of the source file. The macro `SETUP_INCOMING_VARARGS' should not generate any instructions in this case. `STRICT_ARGUMENT_NAMING' Define this macro if the location where a function argument is passed depends on whether or not it is a named argument. This macro controls how the NAMED argument to `FUNCTION_ARG' is set for varargs and stdarg functions. With this macro defined, the NAMED argument is always true for named arguments, and false for unnamed arguments. If this is not defined, but `SETUP_INCOMING_VARARGS' is defined, then all arguments are treated as named. Otherwise, all named arguments except the last are treated as named. File: gcc.info, Node: Trampolines, Next: Library Calls, Prev: Varargs, Up: Target Macros Trampolines for Nested Functions ================================ A "trampoline" is a small piece of code that is created at run time when the address of a nested function is taken. It normally resides on the stack, in the stack frame of the containing function. These macros tell GNU CC how to generate code to allocate and initialize a trampoline. The instructions in the trampoline must do two things: load a constant address into the static chain register, and jump to the real address of the nested function. On CISC machines such as the m68k, this requires two instructions, a move immediate and a jump. Then the two addresses exist in the trampoline as word-long immediate operands. On RISC machines, it is often necessary to load each address into a register in two parts. Then pieces of each address form separate immediate operands. The code generated to initialize the trampoline must store the variable parts--the static chain value and the function address--into the immediate operands of the instructions. On a CISC machine, this is simply a matter of copying each address to a memory reference at the proper offset from the start of the trampoline. On a RISC machine, it may be necessary to take out pieces of the address and store them separately. `TRAMPOLINE_TEMPLATE (FILE)' A C statement to output, on the stream FILE, assembler code for a block of data that contains the constant parts of a trampoline. This code should not include a label--the label is taken care of automatically. `TRAMPOLINE_SECTION' The name of a subroutine to switch to the section in which the trampoline template is to be placed (*note Sections::.). The default is a value of `readonly_data_section', which places the trampoline in the section containing read-only data. `TRAMPOLINE_SIZE' A C expression for the size in bytes of the trampoline, as an integer. `TRAMPOLINE_ALIGNMENT' Alignment required for trampolines, in bits. If you don't define this macro, the value of `BIGGEST_ALIGNMENT' is used for aligning trampolines. `INITIALIZE_TRAMPOLINE (ADDR, FNADDR, STATIC_CHAIN)' A C statement to initialize the variable parts of a trampoline. aDDR is an RTX for the address of the trampoline; FNADDR is an RTX for the address of the nested function; STATIC_CHAIN is an RTX for the static chain value that should be passed to the function when it is called. `ALLOCATE_TRAMPOLINE (FP)' A C expression to allocate run-time space for a trampoline. The expression value should be an RTX representing a memory reference to the space for the trampoline. If this macro is not defined, by default the trampoline is allocated as a stack slot. This default is right for most machines. The exceptions are machines where it is impossible to execute instructions in the stack area. On such machines, you may have to implement a separate stack, using this macro in conjunction with `FUNCTION_PROLOGUE' and `FUNCTION_EPILOGUE'. FP points to a data structure, a `struct function', which describes the compilation status of the immediate containing function of the function which the trampoline is for. Normally (when `ALLOCATE_TRAMPOLINE' is not defined), the stack slot for the trampoline is in the stack frame of this containing function. Other allocation strategies probably must do something analogous with this information. Implementing trampolines is difficult on many machines because they have separate instruction and data caches. Writing into a stack location fails to clear the memory in the instruction cache, so when the program jumps to that location, it executes the old contents. Here are two possible solutions. One is to clear the relevant parts of the instruction cache whenever a trampoline is set up. The other is to make all trampolines identical, by having them jump to a standard subroutine. The former technique makes trampoline execution faster; the latter makes initialization faster. To clear the instruction cache when a trampoline is initialized, define the following macros which describe the shape of the cache. `INSN_CACHE_SIZE' The total size in bytes of the cache. `INSN_CACHE_LINE_WIDTH' The length in bytes of each cache line. The cache is divided into cache lines which are disjoint slots, each holding a contiguous chunk of data fetched from memory. Each time data is brought into the cache, an entire line is read at once. The data loaded into a cache line is always aligned on a boundary equal to the line size. `INSN_CACHE_DEPTH' The number of alternative cache lines that can hold any particular memory location. Alternatively, if the machine has system calls or instructions to clear the instruction cache directly, you can define the following macro. `CLEAR_INSN_CACHE (BEG, END)' If defined, expands to a C expression clearing the *instruction cache* in the specified interval. If it is not defined, and the macro INSN_CACHE_SIZE is defined, some generic code is generated to clear the cache. The definition of this macro would typically be a series of `asm' statements. Both BEG and END are both pointer expressions. To use a standard subroutine, define the following macro. In addition, you must make sure that the instructions in a trampoline fill an entire cache line with identical instructions, or else ensure that the beginning of the trampoline code is always aligned at the same point in its cache line. Look in `m68k.h' as a guide. `TRANSFER_FROM_TRAMPOLINE' Define this macro if trampolines need a special subroutine to do their work. The macro should expand to a series of `asm' statements which will be compiled with GNU CC. They go in a library function named `__transfer_from_trampoline'. If you need to avoid executing the ordinary prologue code of a compiled C function when you jump to the subroutine, you can do so by placing a special label of your own in the assembler code. Use one `asm' statement to generate an assembler label, and another to make the label global. Then trampolines can use that label to jump directly to your special assembler code.