This is doc/gccint.info, produced by makeinfo version 4.5 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: 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: gccint.info, Node: Caller Saves, Next: Function Entry, Prev: Aggregate Return, Up: Stack and Calling Caller-Saves Register Allocation -------------------------------- If you enable it, GCC 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. When defined, this macro enables `-fcaller-saves' by default for all optimization levels. It has no effect for optimization levels 2 and higher, where `-fcaller-saves' is the default. `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'. `HARD_REGNO_CALLER_SAVE_MODE (REGNO, NREGS)' A C expression specifying which mode is required for saving NREGS of a pseudo-register in call-clobbered hard register REGNO. If REGNO is unsuitable for caller save, `VOIDmode' should be returned. For most machines this macro need not be defined since GCC will select the smallest suitable mode.  File: gccint.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. - Target Hook: void TARGET_ASM_FUNCTION_PROLOGUE (FILE *FILE, HOST_WIDE_INT SIZE) If defined, a function 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. (`TARGET_ASM_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. - Target Hook: void TARGET_ASM_FUNCTION_END_PROLOGUE (FILE *FILE) If defined, a function that outputs assembler code at the end of a prologue. This should be used when the function prologue is being emitted as RTL, and you have some extra assembler that needs to be emitted. *Note prologue instruction pattern::. - Target Hook: void TARGET_ASM_FUNCTION_BEGIN_EPILOGUE (FILE *FILE) If defined, a function that outputs assembler code at the start of an epilogue. This should be used when the function epilogue is being emitted as RTL, and you have some extra assembler that needs to be emitted. *Note epilogue instruction pattern::. - Target Hook: void TARGET_ASM_FUNCTION_EPILOGUE (FILE *FILE, HOST_WIDE_INT SIZE) If defined, a function 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 `TARGET_ASM_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 `TARGET_ASM_FUNCTION_EPILOGUE' at all. Do not define a pattern named `return' if you want the `TARGET_ASM_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, `TARGET_ASM_FUNCTION_PROLOGUE' and `TARGET_ASM_FUNCTION_EPILOGUE' must treat leaf functions specially. The C variable `current_function_is_leaf' 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. `TARGET_ASM_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::. * 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 `' and `'. * 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 `TARGET_ASM_FUNCTION_PROLOGUE' and `TARGET_ASM_FUNCTION_EPILOGUE' to treat leaf functions specially. The C variable `current_function_is_leaf' 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 that are used by the epilogue or the `return' pattern. The stack and frame pointer registers are already be assumed to be used as needed. `EH_USES (REGNO)' Define this macro as a C expression that is nonzero for registers that are used by the exception handling mechanism, and so should be considered live on entry to an exception edge. `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 `TARGET_ASM_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; `TARGET_ASM_FUNCTION_PROLOGUE' and `TARGET_ASM_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++ front end will generate a less efficient heavyweight thunk that calls FUNCTION instead of jumping to it. The generic approach does not support varargs.  File: gccint.info, Node: Profiling, Next: Tail Calls, 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'. The details of how `mcount' expects to be called are determined by your operating system environment, not by GCC. 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. Older implementations of `mcount' expect the address of a counter variable to be loaded into some register. The name of this variable is `LP' followed by the number LABELNO, so you would generate the name using `LP%d' in a `fprintf'. `PROFILE_HOOK' A C statement or compound statement to output to FILE some assembly code to call the profiling subroutine `mcount' even the target does not support profiling. `NO_PROFILE_COUNTERS' Define this macro if the `mcount' subroutine on your system does not need a counter variable allocated for each function. This is true for almost all modern implementations. If you define this macro, you must not use the LABELNO argument to `FUNCTION_PROFILER'. `PROFILE_BEFORE_PROLOGUE' Define this macro if the code for function profiling should come before the function prologue. Normally, the profiling code comes after. `TARGET_ALLOWS_PROFILING_WITHOUT_FRAME_POINTER' On some targets, it is impossible to use profiling when the frame pointer has been omitted. For example, on x86 GNU/Linux systems, the `mcount' routine provided by the GNU C Library finds the address of the routine that called the routine that called `mcount' by looking in the immediate caller's stack frame. If the immediate caller has no frame pointer, this lookup will fail. By default, GCC assumes that the target does allow profiling when the frame pointer is omitted. This macro should be defined to a C expression that evaluates to `false' if the target does not allow profiling when the frame pointer is omitted.  File: gccint.info, Node: Tail Calls, Prev: Profiling, Up: Stack and Calling Permitting tail calls --------------------- `FUNCTION_OK_FOR_SIBCALL (DECL)' A C expression that evaluates to true if it is ok to perform a sibling call to DECL from the current function. It is not uncommon for limitations of calling conventions to prevent tail calls to functions outside the current unit of translation, or during PIC compilation. Use this macro to enforce these restrictions, as the `sibcall' md pattern can not fail, or fall over to a "normal" call.  File: gccint.info, Node: Varargs, Next: Trampolines, Prev: Stack and Calling, Up: Target Macros Implementing the Varargs Macros =============================== GCC comes with an implementation of `' and `' 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. ISO `' differs from traditional `' 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 ISO 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 ISO 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 ()' 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'. `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 are obtained after processing 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 to be a nonzero value 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. If this macro returns a nonzero value, the NAMED argument is always true for named arguments, and false for unnamed arguments. If it returns a value of zero, but `SETUP_INCOMING_VARARGS' is defined, then all arguments are treated as named. Otherwise, all named arguments except the last are treated as named. You need not define this macro if it always returns zero. `PRETEND_OUTGOING_VARARGS_NAMED' If you need to conditionally change ABIs so that one works with `SETUP_INCOMING_VARARGS', but the other works like neither `SETUP_INCOMING_VARARGS' nor `STRICT_ARGUMENT_NAMING' was defined, then define this macro to return nonzero if `SETUP_INCOMING_VARARGS' is used, zero otherwise. Otherwise, you should not define this macro.  File: gccint.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 GCC 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. If you do not define this macro, it means no template is needed for the target. Do not define this macro on systems where the block move code to copy the trampoline into place would be larger than the code to generate it on the spot. `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. `TRAMPOLINE_ADJUST_ADDRESS (ADDR)' A C statement that should perform any machine-specific adjustment in the address of the trampoline. Its argument contains the address that was passed to `INITIALIZE_TRAMPOLINE'. In case the address to be used for a function call should be different from the address in which the template was stored, the different address should be assigned to ADDR. If this macro is not defined, ADDR will be used for function calls. `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 `TARGET_ASM_FUNCTION_PROLOGUE' and `TARGET_ASM_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 GCC. 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.  File: gccint.info, Node: Library Calls, Next: Addressing Modes, Prev: Trampolines, Up: Target Macros Implicit Calls to Library Routines ================================== Here is an explanation of implicit calls to library routines. `MULSI3_LIBCALL' A C string constant giving the name of the function to call for multiplication of one signed full-word by another. If you do not define this macro, the default name is used, which is `__mulsi3', a function defined in `libgcc.a'. `DIVSI3_LIBCALL' A C string constant giving the name of the function to call for division of one signed full-word by another. If you do not define this macro, the default name is used, which is `__divsi3', a function defined in `libgcc.a'. `UDIVSI3_LIBCALL' A C string constant giving the name of the function to call for division of one unsigned full-word by another. If you do not define this macro, the default name is used, which is `__udivsi3', a function defined in `libgcc.a'. `MODSI3_LIBCALL' A C string constant giving the name of the function to call for the remainder in division of one signed full-word by another. If you do not define this macro, the default name is used, which is `__modsi3', a function defined in `libgcc.a'. `UMODSI3_LIBCALL' A C string constant giving the name of the function to call for the remainder in division of one unsigned full-word by another. If you do not define this macro, the default name is used, which is `__umodsi3', a function defined in `libgcc.a'. `MULDI3_LIBCALL' A C string constant giving the name of the function to call for multiplication of one signed double-word by another. If you do not define this macro, the default name is used, which is `__muldi3', a function defined in `libgcc.a'. `DIVDI3_LIBCALL' A C string constant giving the name of the function to call for division of one signed double-word by another. If you do not define this macro, the default name is used, which is `__divdi3', a function defined in `libgcc.a'. `UDIVDI3_LIBCALL' A C string constant giving the name of the function to call for division of one unsigned full-word by another. If you do not define this macro, the default name is used, which is `__udivdi3', a function defined in `libgcc.a'. `MODDI3_LIBCALL' A C string constant giving the name of the function to call for the remainder in division of one signed double-word by another. If you do not define this macro, the default name is used, which is `__moddi3', a function defined in `libgcc.a'. `UMODDI3_LIBCALL' A C string constant giving the name of the function to call for the remainder in division of one unsigned full-word by another. If you do not define this macro, the default name is used, which is `__umoddi3', a function defined in `libgcc.a'. `INIT_TARGET_OPTABS' Define this macro as a C statement that declares additional library routines renames existing ones. `init_optabs' calls this macro after initializing all the normal library routines. `FLOAT_LIB_COMPARE_RETURNS_BOOL' Define this macro as a C statement that returns nonzero if a call to the floating point comparison library function will return a boolean value that indicates the result of the comparison. It should return zero if one of gcc's own libgcc functions is called. Most ports don't need to define this macro. `TARGET_EDOM' The value of `EDOM' on the target machine, as a C integer constant expression. If you don't define this macro, GCC does not attempt to deposit the value of `EDOM' into `errno' directly. Look in `/usr/include/errno.h' to find the value of `EDOM' on your system. If you do not define `TARGET_EDOM', then compiled code reports domain errors by calling the library function and letting it report the error. If mathematical functions on your system use `matherr' when there is an error, then you should leave `TARGET_EDOM' undefined so that `matherr' is used normally. `GEN_ERRNO_RTX' Define this macro as a C expression to create an rtl expression that refers to the global "variable" `errno'. (On certain systems, `errno' may not actually be a variable.) If you don't define this macro, a reasonable default is used. `TARGET_MEM_FUNCTIONS' Define this macro if GCC should generate calls to the ISO C (and System V) library functions `memcpy', `memmove' and `memset' rather than the BSD functions `bcopy' and `bzero'. `LIBGCC_NEEDS_DOUBLE' Define this macro if `float' arguments cannot be passed to library routines (so they must be converted to `double'). This macro affects both how library calls are generated and how the library routines in `libgcc.a' accept their arguments. It is useful on machines where floating and fixed point arguments are passed differently, such as the i860. `NEXT_OBJC_RUNTIME' Define this macro to generate code for Objective-C message sending using the calling convention of the NeXT system. This calling convention involves passing the object, the selector and the method arguments all at once to the method-lookup library function. The default calling convention passes just the object and the selector to the lookup function, which returns a pointer to the method.