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GNU Info File | 1994-02-07 | 44.1 KB | 949 lines

This is Info file gcc.info, produced by Makeinfo-1.49 from the input file gcc.texi. This file documents the use and the internals of the GNU compiler. Copyright (C) 1988, 1989, 1992 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" 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" 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: 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, in the case that `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 `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 at run time in a function that needs a frame pointer. Normally, it is necessary for `FUNCTION_PROLOGUE' and `FUNCTION_EPILOGUE' to 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 `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 ----------------------------- `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. 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'. Regardless of what code is generated for the call to `__builtin_saveregs', it appears at the beginning of the function, not where the call to `__builtin_saveregs' is written. 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 ()' 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. `__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. 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. 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. File: gcc.info, Node: Library Calls, Next: Addressing Modes, Prev: Trampolines, Up: Target Macros 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'. `TARGET_EDOM' The value of `EDOM' on the target machine, as a C integer constant expression. If you don't define this macro, GNU CC 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 GNU CC should generate calls to the System V (and ANSI C) library functions `memcpy' and `memset' rather than the BSD functions `bcopy' and `bzero'. `LIBGCC_NEEDS_DOUBLE' Define this macro if only `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 `libgcc1.c' accept their arguments. It is useful on machines where floating and fixed point arguments are passed differently, such as the i860. `FLOAT_ARG_TYPE' Define this macro to override the type used by the library routines to pick up arguments of type `float'. (By default, they use a union of `float' and `int'.) The obvious choice would be `float'--but that won't work with traditional C compilers that expect all arguments declared as `float' to arrive as `double'. To avoid this conversion, the library routines ask for the value as some other type and then treat it as a `float'. On some systems, no other type will work for this. For these systems, you must use `LIBGCC_NEEDS_DOUBLE' instead, to force conversion of the values `double' before they are passed. `FLOATIFY (PASSED-VALUE)' Define this macro to override the way library routines redesignate a `float' argument as a `float' instead of the type it was passed as. The default is an expression which takes the `float' field of the union. `FLOAT_VALUE_TYPE' Define this macro to override the type used by the library routines to return values that ought to have type `float'. (By default, they use `int'.) The obvious choice would be `float'--but that won't work with traditional C compilers gratuitously convert values declared as `float' into `double'. `INTIFY (FLOAT-VALUE)' Define this macro to override the way the value of a `float'-returning library routine should be packaged in order to return it. These functions are actually declared to return type `FLOAT_VALUE_TYPE' (normally `int'). These values can't be returned as type `float' because traditional C compilers would gratuitously convert the value to a `double'. A local variable named `intify' is always available when the macro `INTIFY' is used. It is a union of a `float' field named `f' and a field named `i' whose type is `FLOAT_VALUE_TYPE' or `int'. If you don't define this macro, the default definition works by copying the value through that union. `nongcc_SI_type' Define this macro as the name of the data type corresponding to `SImode' in the system's own C compiler. You need not define this macro if that type is `int', as it usually is. `perform_...' Define these macros to supply explicit C statements to carry out various arithmetic operations on types `float' and `double' in the library routines in `libgcc1.c'. See that file for a full list of these macros and their arguments. On most machines, you don't need to define any of these macros, because the C compiler that comes with the system takes care of doing them. `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. File: gcc.info, Node: Addressing Modes, Next: Condition Code, Prev: Library Calls, Up: Target Macros Addressing Modes ================ `HAVE_POST_INCREMENT' Define this macro if the machine supports post-increment addressing. `HAVE_PRE_INCREMENT' `HAVE_POST_DECREMENT' `HAVE_PRE_DECREMENT' Similar for other kinds of addressing. `CONSTANT_ADDRESS_P (X)' A C expression that is 1 if the RTX X is a constant which is a valid address. On most machines, this can be defined as `CONSTANT_P (X)', but a few machines are more restrictive in which constant addresses are supported. `CONSTANT_P' accepts integer-values expressions whose values are not explicitly known, such as `symbol_ref', `label_ref', and `high' expressions and `const' arithmetic expressions, in addition to `const_int' and `const_double' expressions. `MAX_REGS_PER_ADDRESS' A number, the maximum number of registers that can appear in a valid memory address. Note that it is up to you to specify a value equal to the maximum number that `GO_IF_LEGITIMATE_ADDRESS' would ever accept. `GO_IF_LEGITIMATE_ADDRESS (MODE, X, LABEL)' A C compound statement with a conditional `goto LABEL;' executed if X (an RTX) is a legitimate memory address on the target machine for a memory operand of mode MODE. It usually pays to define several simpler macros to serve as subroutines for this one. Otherwise it may be too complicated to understand. This macro must exist in two variants: a strict variant and a non-strict one. The strict variant is used in the reload pass. It must be defined so that any pseudo-register that has not been allocated a hard register is considered a memory reference. In contexts where some kind of register is required, a pseudo-register with no hard register must be rejected. The non-strict variant is used in other passes. It must be defined to accept all pseudo-registers in every context where some kind of register is required. Compiler source files that want to use the strict variant of this macro define the macro `REG_OK_STRICT'. You should use an `#ifdef REG_OK_STRICT' conditional to define the strict variant in that case and the non-strict variant otherwise. Typically among the subroutines used to define `GO_IF_LEGITIMATE_ADDRESS' are subroutines to check for acceptable registers for various purposes (one for base registers, one for index registers, and so on). Then only these subroutine macros need have two variants; the higher levels of macros may be the same whether strict or not. Normally, constant addresses which are the sum of a `symbol_ref' and an integer are stored inside a `const' RTX to mark them as constant. Therefore, there is no need to recognize such sums specifically as legitimate addresses. Normally you would simply recognize any `const' as legitimate. Usually `PRINT_OPERAND_ADDRESS' is not prepared to handle constant sums that are not marked with `const'. It assumes that a naked `plus' indicates indexing. If so, then you *must* reject such naked constant sums as illegitimate addresses, so that none of them will be given to `PRINT_OPERAND_ADDRESS'. On some machines, whether a symbolic address is legitimate depends on the section that the address refers to. On these machines, define the macro `ENCODE_SECTION_INFO' to store the information into the `symbol_ref', and then check for it here. When you see a `const', you will have to look inside it to find the `symbol_ref' in order to determine the section. *Note Assembler Format::. The best way to modify the name string is by adding text to the beginning, with suitable punctuation to prevent any ambiguity. Allocate the new name in `saveable_obstack'. You will have to modify `ASM_OUTPUT_LABELREF' to remove and decode the added text and output the name accordingly, and define `STRIP_NAME_ENCODING' to access the original name string. You can check the information stored here into the `symbol_ref' in the definitions of `GO_IF_LEGITIMATE_ADDRESS' and `PRINT_OPERAND_ADDRESS'. `REG_OK_FOR_BASE_P (X)' A C expression that is nonzero if X (assumed to be a `reg' RTX) is valid for use as a base register. For hard registers, it should always accept those which the hardware permits and reject the others. Whether the macro accepts or rejects pseudo registers must be controlled by `REG_OK_STRICT' as described above. This usually requires two variant definitions, of which `REG_OK_STRICT' controls the one actually used. `REG_OK_FOR_INDEX_P (X)' A C expression that is nonzero if X (assumed to be a `reg' RTX) is valid for use as an index register. The difference between an index register and a base register is that the index register may be scaled. If an address involves the sum of two registers, neither one of them scaled, then either one may be labeled the "base" and the other the "index"; but whichever labeling is used must fit the machine's constraints of which registers may serve in each capacity. The compiler will try both labelings, looking for one that is valid, and will reload one or both registers only if neither labeling works. `LEGITIMIZE_ADDRESS (X, OLDX, MODE, WIN)' A C compound statement that attempts to replace X with a valid memory address for an operand of mode MODE. WIN will be a C statement label elsewhere in the code; the macro definition may use GO_IF_LEGITIMATE_ADDRESS (MODE, X, WIN); to avoid further processing if the address has become legitimate. X will always be the result of a call to `break_out_memory_refs', and OLDX will be the operand that was given to that function to produce X. The code generated by this macro should not alter the substructure of X. If it transforms X into a more legitimate form, it should assign X (which will always be a C variable) a new value. It is not necessary for this macro to come up with a legitimate address. The compiler has standard ways of doing so in all cases. In fact, it is safe for this macro to do nothing. But often a machine-dependent strategy can generate better code. `GO_IF_MODE_DEPENDENT_ADDRESS (ADDR, LABEL)' A C statement or compound statement with a conditional `goto LABEL;' executed if memory address X (an RTX) can have different meanings depending on the machine mode of the memory reference it is used for or if the address is valid for some modes but not others. Autoincrement and autodecrement addresses typically have mode-dependent effects because the amount of the increment or decrement is the size of the operand being addressed. Some machines have other mode-dependent addresses. Many RISC machines have no mode-dependent addresses. You may assume that ADDR is a valid address for the machine. `LEGITIMATE_CONSTANT_P (X)' A C expression that is nonzero if X is a legitimate constant for an immediate operand on the target machine. You can assume that X satisfies `CONSTANT_P', so you need not check this. In fact, `1' is a suitable definition for this macro on machines where anything `CONSTANT_P' is valid. File: gcc.info, Node: Condition Code, Next: Costs, Prev: Addressing Modes, Up: Target Macros Condition Code Status ===================== The file `conditions.h' defines a variable `cc_status' to describe how the condition code was computed (in case the interpretation of the condition code depends on the instruction that it was set by). This variable contains the RTL expressions on which the condition code is currently based, and several standard flags. Sometimes additional machine-specific flags must be defined in the machine description header file. It can also add additional machine-specific information by defining `CC_STATUS_MDEP'. `CC_STATUS_MDEP' C code for a data type which is used for declaring the `mdep' component of `cc_status'. It defaults to `int'. This macro is not used on machines that do not use `cc0'. `CC_STATUS_MDEP_INIT' A C expression to initialize the `mdep' field to "empty". The default definition does nothing, since most machines don't use the field anyway. If you want to use the field, you should probably define this macro to initialize it. This macro is not used on machines that do not use `cc0'. `NOTICE_UPDATE_CC (EXP, INSN)' A C compound statement to set the components of `cc_status' appropriately for an insn INSN whose body is EXP. It is this macro's responsibility to recognize insns that set the condition code as a byproduct of other activity as well as those that explicitly set `(cc0)'. This macro is not used on machines that do not use `cc0'. If there are insns that do not set the condition code but do alter other machine registers, this macro must check to see whether they invalidate the expressions that the condition code is recorded as reflecting. For example, on the 68000, insns that store in address registers do not set the condition code, which means that usually `NOTICE_UPDATE_CC' can leave `cc_status' unaltered for such insns. But suppose that the previous insn set the condition code based on location `a4@(102)' and the current insn stores a new value in `a4'. Although the condition code is not changed by this, it will no longer be true that it reflects the contents of `a4@(102)'. Therefore, `NOTICE_UPDATE_CC' must alter `cc_status' in this case to say that nothing is known about the condition code value. The definition of `NOTICE_UPDATE_CC' must be prepared to deal with the results of peephole optimization: insns whose patterns are `parallel' RTXs containing various `reg', `mem' or constants which are just the operands. The RTL structure of these insns is not sufficient to indicate what the insns actually do. What `NOTICE_UPDATE_CC' should do when it sees one is just to run `CC_STATUS_INIT'. A possible definition of `NOTICE_UPDATE_CC' is to call a function that looks at an attribute (*note Insn Attributes::.) named, for example, `cc'. This avoids having detailed information about patterns in two places, the `md' file and in `NOTICE_UPDATE_CC'. `EXTRA_CC_MODES' A list of names to be used for additional modes for condition code values in registers (*note Jump Patterns::.). These names are added to `enum machine_mode' and all have class `MODE_CC'. By convention, they should start with `CC' and end with `mode'. You should only define this macro if your machine does not use `cc0' and only if additional modes are required. `EXTRA_CC_NAMES' A list of C strings giving the names for the modes listed in `EXTRA_CC_MODES'. For example, the Sparc defines this macro and `EXTRA_CC_MODES' as #define EXTRA_CC_MODES CC_NOOVmode, CCFPmode #define EXTRA_CC_NAMES "CC_NOOV", "CCFP" This macro is not required if `EXTRA_CC_MODES' is not defined. `SELECT_CC_MODE (OP, X, Y)' Returns a mode from class `MODE_CC' to be used when comparison operation code OP is applied to rtx X and Y. For example, on the Sparc, `SELECT_CC_MODE' is defined as (see *note Jump Patterns::. for a description of the reason for this definition) #define SELECT_CC_MODE(OP,X,Y) \ (GET_MODE_CLASS (GET_MODE (X)) == MODE_FLOAT \ ? ((OP == EQ || OP == NE) ? CCFPmode : CCFPEmode) \ : ((GET_CODE (X) == PLUS || GET_CODE (X) == MINUS || GET_CODE (X) == NEG) \ ? CC_NOOVmode : CCmode)) This macro is not required if `EXTRA_CC_MODES' is not defined.