This is Info file gcc.info, produced by Makeinfo version 1.68 from the input file gcc.texi. This file documents the use and the internals of the GNU compiler. Published by the Free Software Foundation 59 Temple Place - Suite 330 Boston, MA 02111-1307 USA Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997 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: Registers, Next: Register Classes, Prev: Type Layout, Up: Target Macros Register Usage ============== This section explains how to describe what registers the target machine has, and how (in general) they can be used. The description of which registers a specific instruction can use is done with register classes; see *Note Register Classes::. For information on using registers to access a stack frame, see *Note Frame Registers::. For passing values in registers, see *Note Register Arguments::. For returning values in registers, see *Note Scalar Return::. * Menu: * Register Basics:: Number and kinds of registers. * Allocation Order:: Order in which registers are allocated. * Values in Registers:: What kinds of values each reg can hold. * Leaf Functions:: Renumbering registers for leaf functions. * Stack Registers:: Handling a register stack such as 80387. * Obsolete Register Macros:: Macros formerly used for the 80387.  File: gcc.info, Node: Register Basics, Next: Allocation Order, Up: Registers Basic Characteristics of Registers ---------------------------------- Registers have various characteristics. `FIRST_PSEUDO_REGISTER' Number of hardware registers known to the compiler. They receive numbers 0 through `FIRST_PSEUDO_REGISTER-1'; thus, the first pseudo register's number really is assigned the number `FIRST_PSEUDO_REGISTER'. `FIXED_REGISTERS' An initializer that says which registers are used for fixed purposes all throughout the compiled code and are therefore not available for general allocation. These would include the stack pointer, the frame pointer (except on machines where that can be used as a general register when no frame pointer is needed), the program counter on machines where that is considered one of the addressable registers, and any other numbered register with a standard use. This information is expressed as a sequence of numbers, separated by commas and surrounded by braces. The Nth number is 1 if register N is fixed, 0 otherwise. The table initialized from this macro, and the table initialized by the following one, may be overridden at run time either automatically, by the actions of the macro `CONDITIONAL_REGISTER_USAGE', or by the user with the command options `-ffixed-REG', `-fcall-used-REG' and `-fcall-saved-REG'. `CALL_USED_REGISTERS' Like `FIXED_REGISTERS' but has 1 for each register that is clobbered (in general) by function calls as well as for fixed registers. This macro therefore identifies the registers that are not available for general allocation of values that must live across function calls. If a register has 0 in `CALL_USED_REGISTERS', the compiler automatically saves it on function entry and restores it on function exit, if the register is used within the function. `CONDITIONAL_REGISTER_USAGE' Zero or more C statements that may conditionally modify two variables `fixed_regs' and `call_used_regs' (both of type `char []') after they have been initialized from the two preceding macros. This is necessary in case the fixed or call-clobbered registers depend on target flags. You need not define this macro if it has no work to do. If the usage of an entire class of registers depends on the target flags, you may indicate this to GCC by using this macro to modify `fixed_regs' and `call_used_regs' to 1 for each of the registers in the classes which should not be used by GCC. Also define the macro `REG_CLASS_FROM_LETTER' to return `NO_REGS' if it is called with a letter for a class that shouldn't be used. (However, if this class is not included in `GENERAL_REGS' and all of the insn patterns whose constraints permit this class are controlled by target switches, then GCC will automatically avoid using these registers when the target switches are opposed to them.) `NON_SAVING_SETJMP' If this macro is defined and has a nonzero value, it means that `setjmp' and related functions fail to save the registers, or that `longjmp' fails to restore them. To compensate, the compiler avoids putting variables in registers in functions that use `setjmp'. `INCOMING_REGNO (OUT)' Define this macro if the target machine has register windows. This C expression returns the register number as seen by the called function corresponding to the register number OUT as seen by the calling function. Return OUT if register number OUT is not an outbound register. `OUTGOING_REGNO (IN)' Define this macro if the target machine has register windows. This C expression returns the register number as seen by the calling function corresponding to the register number IN as seen by the called function. Return IN if register number IN is not an inbound register.  File: gcc.info, Node: Allocation Order, Next: Values in Registers, Prev: Register Basics, Up: Registers Order of Allocation of Registers -------------------------------- Registers are allocated in order. `REG_ALLOC_ORDER' If defined, an initializer for a vector of integers, containing the numbers of hard registers in the order in which GNU CC should prefer to use them (from most preferred to least). If this macro is not defined, registers are used lowest numbered first (all else being equal). One use of this macro is on machines where the highest numbered registers must always be saved and the save-multiple-registers instruction supports only sequences of consecutive registers. On such machines, define `REG_ALLOC_ORDER' to be an initializer that lists the highest numbered allocable register first. `ORDER_REGS_FOR_LOCAL_ALLOC' A C statement (sans semicolon) to choose the order in which to allocate hard registers for pseudo-registers local to a basic block. Store the desired register order in the array `reg_alloc_order'. Element 0 should be the register to allocate first; element 1, the next register; and so on. The macro body should not assume anything about the contents of `reg_alloc_order' before execution of the macro. On most machines, it is not necessary to define this macro.  File: gcc.info, Node: Values in Registers, Next: Leaf Functions, Prev: Allocation Order, Up: Registers How Values Fit in Registers --------------------------- This section discusses the macros that describe which kinds of values (specifically, which machine modes) each register can hold, and how many consecutive registers are needed for a given mode. `HARD_REGNO_NREGS (REGNO, MODE)' A C expression for the number of consecutive hard registers, starting at register number REGNO, required to hold a value of mode MODE. On a machine where all registers are exactly one word, a suitable definition of this macro is #define HARD_REGNO_NREGS(REGNO, MODE) \ ((GET_MODE_SIZE (MODE) + UNITS_PER_WORD - 1) \ / UNITS_PER_WORD)) `HARD_REGNO_MODE_OK (REGNO, MODE)' A C expression that is nonzero if it is permissible to store a value of mode MODE in hard register number REGNO (or in several registers starting with that one). For a machine where all registers are equivalent, a suitable definition is #define HARD_REGNO_MODE_OK(REGNO, MODE) 1 You need not include code to check for the numbers of fixed registers, because the allocation mechanism considers them to be always occupied. On some machines, double-precision values must be kept in even/odd register pairs. You can implement that by defining this macro to reject odd register numbers for such modes. The minimum requirement for a mode to be OK in a register is that the `movMODE' instruction pattern support moves between the register and other hard register in the same class and that moving a value into the register and back out not alter it. Since the same instruction used to move `word_mode' will work for all narrower integer modes, it is not necessary on any machine for `HARD_REGNO_MODE_OK' to distinguish between these modes, provided you define patterns `movhi', etc., to take advantage of this. This is useful because of the interaction between `HARD_REGNO_MODE_OK' and `MODES_TIEABLE_P'; it is very desirable for all integer modes to be tieable. Many machines have special registers for floating point arithmetic. Often people assume that floating point machine modes are allowed only in floating point registers. This is not true. Any registers that can hold integers can safely *hold* a floating point machine mode, whether or not floating arithmetic can be done on it in those registers. Integer move instructions can be used to move the values. On some machines, though, the converse is true: fixed-point machine modes may not go in floating registers. This is true if the floating registers normalize any value stored in them, because storing a non-floating value there would garble it. In this case, `HARD_REGNO_MODE_OK' should reject fixed-point machine modes in floating registers. But if the floating registers do not automatically normalize, if you can store any bit pattern in one and retrieve it unchanged without a trap, then any machine mode may go in a floating register, so you can define this macro to say so. The primary significance of special floating registers is rather that they are the registers acceptable in floating point arithmetic instructions. However, this is of no concern to `HARD_REGNO_MODE_OK'. You handle it by writing the proper constraints for those instructions. On some machines, the floating registers are especially slow to access, so that it is better to store a value in a stack frame than in such a register if floating point arithmetic is not being done. As long as the floating registers are not in class `GENERAL_REGS', they will not be used unless some pattern's constraint asks for one. `MODES_TIEABLE_P (MODE1, MODE2)' A C expression that is nonzero if a value of mode MODE1 is accessible in mode MODE2 without copying. If `HARD_REGNO_MODE_OK (R, MODE1)' and `HARD_REGNO_MODE_OK (R, MODE2)' are always the same for any R, then `MODES_TIEABLE_P (MODE1, MODE2)' should be nonzero. If they differ for any R, you should define this macro to return zero unless some other mechanism ensures the accessibility of the value in a narrower mode. You should define this macro to return nonzero in as many cases as possible since doing so will allow GNU CC to perform better register allocation.  File: gcc.info, Node: Leaf Functions, Next: Stack Registers, Prev: Values in Registers, Up: Registers Handling Leaf Functions ----------------------- On some machines, a leaf function (i.e., one which makes no calls) can run more efficiently if it does not make its own register window. Often this means it is required to receive its arguments in the registers where they are passed by the caller, instead of the registers where they would normally arrive. The special treatment for leaf functions generally applies only when other conditions are met; for example, often they may use only those registers for its own variables and temporaries. We use the term "leaf function" to mean a function that is suitable for this special handling, so that functions with no calls are not necessarily "leaf functions". GNU CC assigns register numbers before it knows whether the function is suitable for leaf function treatment. So it needs to renumber the registers in order to output a leaf function. The following macros accomplish this. `LEAF_REGISTERS' A C initializer for a vector, indexed by hard register number, which contains 1 for a register that is allowable in a candidate for leaf function treatment. If leaf function treatment involves renumbering the registers, then the registers marked here should be the ones before renumbering--those that GNU CC would ordinarily allocate. The registers which will actually be used in the assembler code, after renumbering, should not be marked with 1 in this vector. Define this macro only if the target machine offers a way to optimize the treatment of leaf functions. `LEAF_REG_REMAP (REGNO)' A C expression whose value is the register number to which REGNO should be renumbered, when a function is treated as a leaf function. If REGNO is a register number which should not appear in a leaf function before renumbering, then the expression should yield -1, which will cause the compiler to abort. Define this macro only if the target machine offers a way to optimize the treatment of leaf functions, and registers need to be renumbered to do this. Normally, `FUNCTION_PROLOGUE' and `FUNCTION_EPILOGUE' must treat leaf functions specially. It can test the C variable `leaf_function' which is nonzero for leaf functions. (The variable `leaf_function' is defined only if `LEAF_REGISTERS' is defined.)  File: gcc.info, Node: Stack Registers, Next: Obsolete Register Macros, Prev: Leaf Functions, Up: Registers Registers That Form a Stack --------------------------- There are special features to handle computers where some of the "registers" form a stack, as in the 80387 coprocessor for the 80386. Stack registers are normally written by pushing onto the stack, and are numbered relative to the top of the stack. Currently, GNU CC can only handle one group of stack-like registers, and they must be consecutively numbered. `STACK_REGS' Define this if the machine has any stack-like registers. `FIRST_STACK_REG' The number of the first stack-like register. This one is the top of the stack. `LAST_STACK_REG' The number of the last stack-like register. This one is the bottom of the stack.  File: gcc.info, Node: Obsolete Register Macros, Prev: Stack Registers, Up: Registers Obsolete Macros for Controlling Register Usage ---------------------------------------------- These features do not work very well. They exist because they used to be required to generate correct code for the 80387 coprocessor of the 80386. They are no longer used by that machine description and may be removed in a later version of the compiler. Don't use them! `OVERLAPPING_REGNO_P (REGNO)' If defined, this is a C expression whose value is nonzero if hard register number REGNO is an overlapping register. This means a hard register which overlaps a hard register with a different number. (Such overlap is undesirable, but occasionally it allows a machine to be supported which otherwise could not be.) This macro must return nonzero for *all* the registers which overlap each other. GNU CC can use an overlapping register only in certain limited ways. It can be used for allocation within a basic block, and may be spilled for reloading; that is all. If this macro is not defined, it means that none of the hard registers overlap each other. This is the usual situation. `INSN_CLOBBERS_REGNO_P (INSN, REGNO)' If defined, this is a C expression whose value should be nonzero if the insn INSN has the effect of mysteriously clobbering the contents of hard register number REGNO. By "mysterious" we mean that the insn's RTL expression doesn't describe such an effect. If this macro is not defined, it means that no insn clobbers registers mysteriously. This is the usual situation; all else being equal, it is best for the RTL expression to show all the activity. `PRESERVE_DEATH_INFO_REGNO_P (REGNO)' If defined, this is a C expression whose value is nonzero if correct `REG_DEAD' notes are needed for hard register number REGNO after reload. You would arrange to preserve death info for a register when some of the code in the machine description which is executed to write the assembler code looks at the death notes. This is necessary only when the actual hardware feature which GNU CC thinks of as a register is not actually a register of the usual sort. (It might, for example, be a hardware stack.) It is also useful for peepholes and linker relaxation. If this macro is not defined, it means that no death notes need to be preserved, and some may even be incorrect. This is the usual situation.  File: gcc.info, Node: Register Classes, Next: Stack and Calling, Prev: Registers, Up: Target Macros Register Classes ================ On many machines, the numbered registers are not all equivalent. For example, certain registers may not be allowed for indexed addressing; certain registers may not be allowed in some instructions. These machine restrictions are described to the compiler using "register classes". You define a number of register classes, giving each one a name and saying which of the registers belong to it. Then you can specify register classes that are allowed as operands to particular instruction patterns. In general, each register will belong to several classes. In fact, one class must be named `ALL_REGS' and contain all the registers. Another class must be named `NO_REGS' and contain no registers. Often the union of two classes will be another class; however, this is not required. One of the classes must be named `GENERAL_REGS'. There is nothing terribly special about the name, but the operand constraint letters `r' and `g' specify this class. If `GENERAL_REGS' is the same as `ALL_REGS', just define it as a macro which expands to `ALL_REGS'. Order the classes so that if class X is contained in class Y then X has a lower class number than Y. The way classes other than `GENERAL_REGS' are specified in operand constraints is through machine-dependent operand constraint letters. You can define such letters to correspond to various classes, then use them in operand constraints. You should define a class for the union of two classes whenever some instruction allows both classes. For example, if an instruction allows either a floating point (coprocessor) register or a general register for a certain operand, you should define a class `FLOAT_OR_GENERAL_REGS' which includes both of them. Otherwise you will get suboptimal code. You must also specify certain redundant information about the register classes: for each class, which classes contain it and which ones are contained in it; for each pair of classes, the largest class contained in their union. When a value occupying several consecutive registers is expected in a certain class, all the registers used must belong to that class. Therefore, register classes cannot be used to enforce a requirement for a register pair to start with an even-numbered register. The way to specify this requirement is with `HARD_REGNO_MODE_OK'. Register classes used for input-operands of bitwise-and or shift instructions have a special requirement: each such class must have, for each fixed-point machine mode, a subclass whose registers can transfer that mode to or from memory. For example, on some machines, the operations for single-byte values (`QImode') are limited to certain registers. When this is so, each register class that is used in a bitwise-and or shift instruction must have a subclass consisting of registers from which single-byte values can be loaded or stored. This is so that `PREFERRED_RELOAD_CLASS' can always have a possible value to return. `enum reg_class' An enumeral type that must be defined with all the register class names as enumeral values. `NO_REGS' must be first. `ALL_REGS' must be the last register class, followed by one more enumeral value, `LIM_REG_CLASSES', which is not a register class but rather tells how many classes there are. Each register class has a number, which is the value of casting the class name to type `int'. The number serves as an index in many of the tables described below. `N_REG_CLASSES' The number of distinct register classes, defined as follows: #define N_REG_CLASSES (int) LIM_REG_CLASSES `REG_CLASS_NAMES' An initializer containing the names of the register classes as C string constants. These names are used in writing some of the debugging dumps. `REG_CLASS_CONTENTS' An initializer containing the contents of the register classes, as integers which are bit masks. The Nth integer specifies the contents of class N. The way the integer MASK is interpreted is that register R is in the class if `MASK & (1 << R)' is 1. When the machine has more than 32 registers, an integer does not suffice. Then the integers are replaced by sub-initializers, braced groupings containing several integers. Each sub-initializer must be suitable as an initializer for the type `HARD_REG_SET' which is defined in `hard-reg-set.h'. `REGNO_REG_CLASS (REGNO)' A C expression whose value is a register class containing hard register REGNO. In general there is more than one such class; choose a class which is "minimal", meaning that no smaller class also contains the register. `BASE_REG_CLASS' A macro whose definition is the name of the class to which a valid base register must belong. A base register is one used in an address which is the register value plus a displacement. `INDEX_REG_CLASS' A macro whose definition is the name of the class to which a valid index register must belong. An index register is one used in an address where its value is either multiplied by a scale factor or added to another register (as well as added to a displacement). `REG_CLASS_FROM_LETTER (CHAR)' A C expression which defines the machine-dependent operand constraint letters for register classes. If CHAR is such a letter, the value should be the register class corresponding to it. Otherwise, the value should be `NO_REGS'. The register letter `r', corresponding to class `GENERAL_REGS', will not be passed to this macro; you do not need to handle it. `REGNO_OK_FOR_BASE_P (NUM)' A C expression which is nonzero if register number NUM is suitable for use as a base register in operand addresses. It may be either a suitable hard register or a pseudo register that has been allocated such a hard register. `REGNO_MODE_OK_FOR_BASE_P (NUM, MODE)' A C expression that is just like `REGNO_OK_FOR_BASE_P', except that that expression may examine the mode of the memory reference in MODE. You should define this macro if the mode of the memory reference affects whether a register may be used as a base register. If you define this macro, the compiler will use it instead of `REGNO_OK_FOR_BASE_P'. `REGNO_OK_FOR_INDEX_P (NUM)' A C expression which is nonzero if register number NUM is suitable for use as an index register in operand addresses. It may be either a suitable hard register or a pseudo register that has been allocated such a hard 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. `PREFERRED_RELOAD_CLASS (X, CLASS)' A C expression that places additional restrictions on the register class to use when it is necessary to copy value X into a register in class CLASS. The value is a register class; perhaps CLASS, or perhaps another, smaller class. On many machines, the following definition is safe: #define PREFERRED_RELOAD_CLASS(X,CLASS) CLASS Sometimes returning a more restrictive class makes better code. For example, on the 68000, when X is an integer constant that is in range for a `moveq' instruction, the value of this macro is always `DATA_REGS' as long as CLASS includes the data registers. Requiring a data register guarantees that a `moveq' will be used. If X is a `const_double', by returning `NO_REGS' you can force X into a memory constant. This is useful on certain machines where immediate floating values cannot be loaded into certain kinds of registers. `PREFERRED_OUTPUT_RELOAD_CLASS (X, CLASS)' Like `PREFERRED_RELOAD_CLASS', but for output reloads instead of input reloads. If you don't define this macro, the default is to use CLASS, unchanged. `LIMIT_RELOAD_CLASS (MODE, CLASS)' A C expression that places additional restrictions on the register class to use when it is necessary to be able to hold a value of mode MODE in a reload register for which class CLASS would ordinarily be used. Unlike `PREFERRED_RELOAD_CLASS', this macro should be used when there are certain modes that simply can't go in certain reload classes. The value is a register class; perhaps CLASS, or perhaps another, smaller class. Don't define this macro unless the target machine has limitations which require the macro to do something nontrivial. `SECONDARY_RELOAD_CLASS (CLASS, MODE, X)' `SECONDARY_INPUT_RELOAD_CLASS (CLASS, MODE, X)' `SECONDARY_OUTPUT_RELOAD_CLASS (CLASS, MODE, X)' Many machines have some registers that cannot be copied directly to or from memory or even from other types of registers. An example is the `MQ' register, which on most machines, can only be copied to or from general registers, but not memory. Some machines allow copying all registers to and from memory, but require a scratch register for stores to some memory locations (e.g., those with symbolic address on the RT, and those with certain symbolic address on the Sparc when compiling PIC). In some cases, both an intermediate and a scratch register are required. You should define these macros to indicate to the reload phase that it may need to allocate at least one register for a reload in addition to the register to contain the data. Specifically, if copying X to a register CLASS in MODE requires an intermediate register, you should define `SECONDARY_INPUT_RELOAD_CLASS' to return the largest register class all of whose registers can be used as intermediate registers or scratch registers. If copying a register CLASS in MODE to X requires an intermediate or scratch register, `SECONDARY_OUTPUT_RELOAD_CLASS' should be defined to return the largest register class required. If the requirements for input and output reloads are the same, the macro `SECONDARY_RELOAD_CLASS' should be used instead of defining both macros identically. The values returned by these macros are often `GENERAL_REGS'. Return `NO_REGS' if no spare register is needed; i.e., if X can be directly copied to or from a register of CLASS in MODE without requiring a scratch register. Do not define this macro if it would always return `NO_REGS'. If a scratch register is required (either with or without an intermediate register), you should define patterns for `reload_inM' or `reload_outM', as required (*note Standard Names::.. These patterns, which will normally be implemented with a `define_expand', should be similar to the `movM' patterns, except that operand 2 is the scratch register. Define constraints for the reload register and scratch register that contain a single register class. If the original reload register (whose class is CLASS) can meet the constraint given in the pattern, the value returned by these macros is used for the class of the scratch register. Otherwise, two additional reload registers are required. Their classes are obtained from the constraints in the insn pattern. X might be a pseudo-register or a `subreg' of a pseudo-register, which could either be in a hard register or in memory. Use `true_regnum' to find out; it will return -1 if the pseudo is in memory and the hard register number if it is in a register. These macros should not be used in the case where a particular class of registers can only be copied to memory and not to another class of registers. In that case, secondary reload registers are not needed and would not be helpful. Instead, a stack location must be used to perform the copy and the `movM' pattern should use memory as a intermediate storage. This case often occurs between floating-point and general registers. `SECONDARY_MEMORY_NEEDED (CLASS1, CLASS2, M)' Certain machines have the property that some registers cannot be copied to some other registers without using memory. Define this macro on those machines to be a C expression that is non-zero if objects of mode M in registers of CLASS1 can only be copied to registers of class CLASS2 by storing a register of CLASS1 into memory and loading that memory location into a register of CLASS2. Do not define this macro if its value would always be zero. `SECONDARY_MEMORY_NEEDED_RTX (MODE)' Normally when `SECONDARY_MEMORY_NEEDED' is defined, the compiler allocates a stack slot for a memory location needed for register copies. If this macro is defined, the compiler instead uses the memory location defined by this macro. Do not define this macro if you do not define `SECONDARY_MEMORY_NEEDED'. `SECONDARY_MEMORY_NEEDED_MODE (MODE)' When the compiler needs a secondary memory location to copy between two registers of mode MODE, it normally allocates sufficient memory to hold a quantity of `BITS_PER_WORD' bits and performs the store and load operations in a mode that many bits wide and whose class is the same as that of MODE. This is right thing to do on most machines because it ensures that all bits of the register are copied and prevents accesses to the registers in a narrower mode, which some machines prohibit for floating-point registers. However, this default behavior is not correct on some machines, such as the DEC Alpha, that store short integers in floating-point registers differently than in integer registers. On those machines, the default widening will not work correctly and you must define this macro to suppress that widening in some cases. See the file `alpha.h' for details. Do not define this macro if you do not define `SECONDARY_MEMORY_NEEDED' or if widening MODE to a mode that is `BITS_PER_WORD' bits wide is correct for your machine. `SMALL_REGISTER_CLASSES' Normally the compiler avoids choosing registers that have been explicitly mentioned in the rtl as spill registers (these registers are normally those used to pass parameters and return values). However, some machines have so few registers of certain classes that there would not be enough registers to use as spill registers if this were done. Define `SMALL_REGISTER_CLASSES' to be an expression with a non-zero value on these machines. When this macro has a non-zero value, the compiler allows registers explicitly used in the rtl to be used as spill registers but avoids extending the lifetime of these registers. It is always safe to define this macro with a non-zero value, but if you unnecessarily define it, you will reduce the amount of optimizations that can be performed in some cases. If you do not define this macro with a non-zero value when it is required, the compiler will run out of spill registers and print a fatal error message. For most machines, you should not define this macro at all. `CLASS_LIKELY_SPILLED_P (CLASS)' A C expression whose value is nonzero if pseudos that have been assigned to registers of class CLASS would likely be spilled because registers of CLASS are needed for spill registers. The default value of this macro returns 1 if CLASS has exactly one register and zero otherwise. On most machines, this default should be used. Only define this macro to some other expression if pseudo allocated by `local-alloc.c' end up in memory because their hard registers were needed for spill registers. If this macro returns nonzero for those classes, those pseudos will only be allocated by `global.c', which knows how to reallocate the pseudo to another register. If there would not be another register available for reallocation, you should not change the definition of this macro since the only effect of such a definition would be to slow down register allocation. `CLASS_MAX_NREGS (CLASS, MODE)' A C expression for the maximum number of consecutive registers of class CLASS needed to hold a value of mode MODE. This is closely related to the macro `HARD_REGNO_NREGS'. In fact, the value of the macro `CLASS_MAX_NREGS (CLASS, MODE)' should be the maximum value of `HARD_REGNO_NREGS (REGNO, MODE)' for all REGNO values in the class CLASS. This macro helps control the handling of multiple-word values in the reload pass. `CLASS_CANNOT_CHANGE_SIZE' If defined, a C expression for a class that contains registers which the compiler must always access in a mode that is the same size as the mode in which it loaded the register. For the example, loading 32-bit integer or floating-point objects into floating-point registers on the Alpha extends them to 64-bits. Therefore loading a 64-bit object and then storing it as a 32-bit object does not store the low-order 32-bits, as would be the case for a normal register. Therefore, `alpha.h' defines this macro as `FLOAT_REGS'. Three other special macros describe which operands fit which constraint letters. `CONST_OK_FOR_LETTER_P (VALUE, C)' A C expression that defines the machine-dependent operand constraint letters (`I', `J', `K', ... `P') that specify particular ranges of integer values. If C is one of those letters, the expression should check that VALUE, an integer, is in the appropriate range and return 1 if so, 0 otherwise. If C is not one of those letters, the value should be 0 regardless of VALUE. `CONST_DOUBLE_OK_FOR_LETTER_P (VALUE, C)' A C expression that defines the machine-dependent operand constraint letters that specify particular ranges of `const_double' values (`G' or `H'). If C is one of those letters, the expression should check that VALUE, an RTX of code `const_double', is in the appropriate range and return 1 if so, 0 otherwise. If C is not one of those letters, the value should be 0 regardless of VALUE. `const_double' is used for all floating-point constants and for `DImode' fixed-point constants. A given letter can accept either or both kinds of values. It can use `GET_MODE' to distinguish between these kinds. `EXTRA_CONSTRAINT (VALUE, C)' A C expression that defines the optional machine-dependent constraint letters ( ``Q', `R', `S', `T', `U') that can' be used to segregate specific types of operands, usually memory references, for the target machine. Normally this macro will not be defined. If it is required for a particular target machine, it should return 1 if VALUE corresponds to the operand type represented by the constraint letter C. If C is not defined as an extra constraint, the value returned should be 0 regardless of VALUE. For example, on the ROMP, load instructions cannot have their output in r0 if the memory reference contains a symbolic address. Constraint letter `Q' is defined as representing a memory address that does *not* contain a symbolic address. An alternative is specified with a `Q' constraint on the input and `r' on the output. The next alternative specifies `m' on the input and a register class that does not include r0 on the output.  File: gcc.info, Node: Stack and Calling, Next: Varargs, Prev: Register Classes, Up: Target Macros Stack Layout and Calling Conventions ==================================== This describes the stack layout and calling conventions. * Menu: * Frame Layout:: * Stack Checking:: * Frame Registers:: * Elimination:: * Stack Arguments:: * Register Arguments:: * Scalar Return:: * Aggregate Return:: * Caller Saves:: * Function Entry:: * Profiling::  File: gcc.info, Node: Frame Layout, Next: Stack Checking, Up: Stack and Calling Basic Stack Layout ------------------ Here is the basic stack layout. `STACK_GROWS_DOWNWARD' Define this macro if pushing a word onto the stack moves the stack pointer to a smaller address. When we say, "define this macro if ...," it means that the compiler checks this macro only with `#ifdef' so the precise definition used does not matter. `FRAME_GROWS_DOWNWARD' Define this macro if the addresses of local variable slots are at negative offsets from the frame pointer. `ARGS_GROW_DOWNWARD' Define this macro if successive arguments to a function occupy decreasing addresses on the stack. `STARTING_FRAME_OFFSET' Offset from the frame pointer to the first local variable slot to be allocated. If `FRAME_GROWS_DOWNWARD', find the next slot's offset by subtracting the first slot's length from `STARTING_FRAME_OFFSET'. Otherwise, it is found by adding the length of the first slot to the value `STARTING_FRAME_OFFSET'. `STACK_POINTER_OFFSET' Offset from the stack pointer register to the first location at which outgoing arguments are placed. If not specified, the default value of zero is used. This is the proper value for most machines. If `ARGS_GROW_DOWNWARD', this is the offset to the location above the first location at which outgoing arguments are placed. `FIRST_PARM_OFFSET (FUNDECL)' Offset from the argument pointer register to the first argument's address. On some machines it may depend on the data type of the function. If `ARGS_GROW_DOWNWARD', this is the offset to the location above the first argument's address. `STACK_DYNAMIC_OFFSET (FUNDECL)' Offset from the stack pointer register to an item dynamically allocated on the stack, e.g., by `alloca'. The default value for this macro is `STACK_POINTER_OFFSET' plus the length of the outgoing arguments. The default is correct for most machines. See `function.c' for details. `DYNAMIC_CHAIN_ADDRESS (FRAMEADDR)' A C expression whose value is RTL representing the address in a stack frame where the pointer to the caller's frame is stored. Assume that FRAMEADDR is an RTL expression for the address of the stack frame itself. If you don't define this macro, the default is to return the value of FRAMEADDR--that is, the stack frame address is also the address of the stack word that points to the previous frame. `SETUP_FRAME_ADDRESSES ()' If defined, a C expression that produces the machine-specific code to setup the stack so that arbitrary frames can be accessed. For example, on the Sparc, we must flush all of the register windows to the stack before we can access arbitrary stack frames. This macro will seldom need to be defined. `RETURN_ADDR_RTX (COUNT, FRAMEADDR)' A C expression whose value is RTL representing the value of the return address for the frame COUNT steps up from the current frame, after the prologue. FRAMEADDR is the frame pointer of the COUNT frame, or the frame pointer of the COUNT - 1 frame if `RETURN_ADDR_IN_PREVIOUS_FRAME' is defined. The value of the expression must always be the correct address when COUNT is zero, but may be `NULL_RTX' if there is not way to determine the return address of other frames. `RETURN_ADDR_IN_PREVIOUS_FRAME' Define this if the return address of a particular stack frame is accessed from the frame pointer of the previous stack frame. `INCOMING_RETURN_ADDR_RTX' A C expression whose value is RTL representing the location of the incoming return address at the beginning of any function, before the prologue. This RTL is either a `REG', indicating that the return value is saved in `REG', or a `MEM' representing a location in the stack. You only need to define this macro if you want to support call frame debugging information like that provided by DWARF 2. `INCOMING_FRAME_SP_OFFSET' A C expression whose value is an integer giving the offset, in bytes, from the value of the stack pointer register to the top of the stack frame at the beginning of any function, before the prologue. The top of the frame is defined to be the value of the stack pointer in the previous frame, just before the call instruction. You only need to define this macro if you want to support call frame debugging information like that provided by DWARF 2.  File: gcc.info, Node: Stack Checking, Next: Frame Registers, Prev: Frame Layout, Up: Stack and Calling Specifying How Stack Checking is Done ------------------------------------- GNU CC will check that stack references are within the boundaries of the stack, if the `-fstack-check' is specified, in one of three ways: 1. If the value of the `STACK_CHECK_BUILTIN' macro is nonzero, GNU CC will assume that you have arranged for stack checking to be done at appropriate places in the configuration files, e.g., in `FUNCTION_PROLOGUE'. GNU CC will do not other special processing. 2. If `STACK_CHECK_BUILTIN' is zero and you defined a named pattern called `check_stack' in your `md' file, GNU CC will call that pattern with one argument which is the address to compare the stack value against. You must arrange for this pattern to report an error if the stack pointer is out of range. 3. If neither of the above are true, GNU CC will generate code to periodically "probe" the stack pointer using the values of the macros defined below. Normally, you will use the default values of these macros, so GNU CC will use the third approach. `STACK_CHECK_BUILTIN' A nonzero value if stack checking is done by the configuration files in a machine-dependent manner. You should define this macro if stack checking is require by the ABI of your machine or if you would like to have to stack checking in some more efficient way than GNU CC's portable approach. The default value of this macro is zero. `STACK_CHECK_PROBE_INTERVAL' An integer representing the interval at which GNU CC must generate stack probe instructions. You will normally define this macro to be no larger than the size of the "guard pages" at the end of a stack area. The default value of 4096 is suitable for most systems. `STACK_CHECK_PROBE_LOAD' A integer which is nonzero if GNU CC should perform the stack probe as a load instruction and zero if GNU CC should use a store instruction. The default is zero, which is the most efficient choice on most systems. `STACK_CHECK_PROTECT' The number of bytes of stack needed to recover from a stack overflow, for languages where such a recovery is supported. The default value of 75 words should be adequate for most machines. `STACK_CHECK_MAX_FRAME_SIZE' The maximum size of a stack frame, in bytes. GNU CC will generate probe instructions in non-leaf functions to ensure at least this many bytes of stack are available. If a stack frame is larger than this size, stack checking will not be reliable and GNU CC will issue a warning. The default is chosen so that GNU CC only generates one instruction on most systems. You should normally not change the default value of this macro. `STACK_CHECK_FIXED_FRAME_SIZE' GNU CC uses this value to generate the above warning message. It represents the amount of fixed frame used by a function, not including space for any callee-saved registers, temporaries and user variables. You need only specify an upper bound for this amount and will normally use the default of four words. `STACK_CHECK_MAX_VAR_SIZE' The maximum size, in bytes, of an object that GNU CC will place in the fixed area of the stack frame when the user specifies `-fstack-check'. GNU CC computed the default from the values of the above macros and you will normally not need to override that default.