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GNU Info File | 1992-02-16 | 48.8 KB | 1,150 lines

This is Info file gcc.info, produced by Makeinfo-1.43 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 section entitled "GNU General Public License" is 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 section entitled "GNU General Public License" 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: Storage Layout, Next: Type Layout, Prev: Run-time Target, Up: Machine Macros Storage Layout ============== Note that the definitions of the macros in this table which are sizes or alignments measured in bits do not need to be constant. They can be C expressions that refer to static variables, such as the `target_flags'. *Note Run-time Target::. `BITS_BIG_ENDIAN' Define this macro to be the value 1 if the most significant bit in a byte has the lowest number; otherwise define it to be the value zero. This means that bit-field instructions count from the most significant bit. If the machine has no bit-field instructions, this macro is irrelevant. This macro does not affect the way structure fields are packed into bytes or words; that is controlled by `BYTES_BIG_ENDIAN'. `BYTES_BIG_ENDIAN' Define this macro to be 1 if the most significant byte in a word has the lowest number. `WORDS_BIG_ENDIAN' Define this macro to be 1 if, in a multiword object, the most significant word has the lowest number. `BITS_PER_UNIT' Number of bits in an addressable storage unit (byte); normally 8. `BITS_PER_WORD' Number of bits in a word; normally 32. `MAX_BITS_PER_WORD' Maximum number of bits in a word. If this is undefined, the default is `BITS_PER_WORD'. Otherwise, it is the constant value that is the largest value that `BITS_PER_WORD' can have at run-time. `UNITS_PER_WORD' Number of storage units in a word; normally 4. `POINTER_SIZE' Width of a pointer, in bits. `PARM_BOUNDARY' Normal alignment required for function parameters on the stack, in bits. All stack parameters receive least this much alignment regardless of data type. On most machines, this is the same as the size of an integer. `STACK_BOUNDARY' Define this macro if you wish to preserve a certain alignment for the stack pointer. The definition is a C expression for the desired alignment (measured in bits). If `PUSH_ROUNDING' is not defined, the stack will always be aligned to the specified boundary. If `PUSH_ROUNDING' is defined and specifies a less strict alignment than `STACK_BOUNDARY', the stack may be momentarily unaligned while pushing arguments. `FUNCTION_BOUNDARY' Alignment required for a function entry point, in bits. `BIGGEST_ALIGNMENT' Biggest alignment that any data type can require on this machine, in bits. `BIGGEST_FIELD_ALIGNMENT' Biggest alignment that any structure field can require on this machine, in bits. `MAX_OFILE_ALIGNMENT' Biggest alignment supported by the object file format of this machine. Use this macro to limit the alignment which can be specified using the `__attribute__ ((aligned (N)))' construct. If not defined, the default value is `BIGGEST_ALIGNMENT'. `DATA_ALIGNMENT (TYPE, BASIC-ALIGN)' If defined, a C expression to compute the alignment for a static variable. TYPE is the data type, and BASIC-ALIGN is the alignment that the object would ordinarily have. The value of this macro is used instead of that alignment to align the object. If this macro is not defined, then BASIC-ALIGN is used. One use of this macro is to increase alignment of medium-size data to make it all fit in fewer cache lines. Another is to cause character arrays to be word-aligned so that `strcpy' calls that copy constants to character arrays can be done inline. `CONSTANT_ALIGNMENT (CONSTANT, BASIC-ALIGN)' If defined, a C expression to compute the alignment given to a constant that is being placed in memory. CONSTANT is the constant and BASIC-ALIGN is the alignment that the object would ordinarily have. The value of this macro is used instead of that alignment to align the object. If this macro is not defined, then BASIC-ALIGN is used. The typical use of this macro is to increase alignment for string constants to be word aligned so that `strcpy' calls that copy constants can be done inline. `EMPTY_FIELD_BOUNDARY' Alignment in bits to be given to a structure bit field that follows an empty field such as `int : 0;'. `STRUCTURE_SIZE_BOUNDARY' Number of bits which any structure or union's size must be a multiple of. Each structure or union's size is rounded up to a multiple of this. If you do not define this macro, the default is the same as `BITS_PER_UNIT'. `STRICT_ALIGNMENT' Define this if instructions will fail to work if given data not on the nominal alignment. If instructions will merely go slower in that case, do not define this macro. `PCC_BITFIELD_TYPE_MATTERS' Define this if you wish to imitate the way many other C compilers handle alignment of bitfields and the structures that contain them. The behavior is that the type written for a bitfield (`int', `short', or other integer type) imposes an alignment for the entire structure, as if the structure really did contain an ordinary field of that type. In addition, the bitfield is placed within the structure so that it would fit within such a field, not crossing a boundary for it. Thus, on most machines, a bitfield whose type is written as `int' would not cross a four-byte boundary, and would force four-byte alignment for the whole structure. (The alignment used may not be four bytes; it is controlled by the other alignment parameters.) If the macro is defined, its definition should be a C expression; a nonzero value for the expression enables this behavior. Note that if this macro is not defined, or its value is zero, some bitfields may cross more than one alignment boundary. The compiler can support such references if there are `insv', `extv', and `extzv' insns that can directly reference memory. The other known way of making bitfields work is to define `STRUCTURE_SIZE_BOUNDARY' as large as `BIGGEST_ALIGNMENT'. Then every structure can be accessed with fullwords. Unless the machine has bitfield instructions or you define `STRUCTURE_SIZE_BOUNDARY' that way, you must define `PCC_BITFIELD_TYPE_MATTERS' to have a nonzero value. `BITFIELD_NBYTES_LIMITED' Like PCC_BITFIELD_TYPE_MATTERS except that its effect is limited to aligning a bitfield within the structure. `ROUND_TYPE_SIZE (STRUCT, SIZE, ALIGN)' Define this macro as an expression for the overall size of a structure (given by STRUCT as a tree node) when the size computed from the fields is SIZE and the alignment is ALIGN. The default is to round SIZE up to a multiple of ALIGN. `ROUND_TYPE_ALIGN (STRUCT, COMPUTED, SPECIFIED)' Define this macro as an expression for the alignment of a structure (given by STRUCT as a tree node) if the alignment computed in the usual way is COMPUTED and the alignment explicitly specified was SPECIFIED. The default is to use SPECIFIED if it is larger; otherwise, use the smaller of COMPUTED and `BIGGEST_ALIGNMENT' `MAX_FIXED_MODE_SIZE' An integer expression for the size in bits of the largest integer machine mode that should actually be used. All integer machine modes of this size or smaller can be used for structures and unions with the appropriate sizes. If this macro is undefined, `GET_MODE_BITSIZE (DImode)' is assumed. `CHECK_FLOAT_VALUE (MODE, VALUE)' A C statement to validate the value VALUE (of type `double') for mode MODE. This means that you check whether VALUE fits within the possible range of values for mode MODE on this target machine. The mode MODE is always `SFmode' or `DFmode'. If VALUE is not valid, you should call `error' to print an error message and then assign some valid value to VALUE. Allowing an invalid value to go through the compiler can produce incorrect assembler code which may even cause Unix assemblers to crash. This macro need not be defined if there is no work for it to do. `TARGET_FLOAT_FORMAT' A code distinguishing the floating point format of the target machine. There are three defined values: `IEEE_FLOAT_FORMAT' This code indicates IEEE floating point. It is the default; there is no need to define this macro when the format is IEEE. `VAX_FLOAT_FORMAT' This code indicates the peculiar format used on the Vax. `UNKNOWN_FLOAT_FORMAT' This code indicates any other format. The value of this macro is compared with `HOST_FLOAT_FORMAT' (*note Config::.) to determine whether the target machine has the same format as the host machine. If any other formats are actually in use on supported machines, new codes should be defined for them. File: gcc.info, Node: Type Layout, Next: Registers, Prev: Storage Layout, Up: Machine Macros Layout of Source Language Data Types ==================================== These macros define the sizes and other characteristics of the standard basic data types used in programs being compiled. Unlike the macros in the previous section, these apply to specific features of C and related languages, rather than to fundamental aspects of storage layout. `INT_TYPE_SIZE' A C expression for the size in bits of the type `int' on the target machine. If you don't define this, the default is one word. `SHORT_TYPE_SIZE' A C expression for the size in bits of the type `short' on the target machine. If you don't define this, the default is half a word. (If this would be less than one storage unit, it is rounded up to one unit.) `LONG_TYPE_SIZE' A C expression for the size in bits of the type `long' on the target machine. If you don't define this, the default is one word. `LONG_LONG_TYPE_SIZE' A C expression for the size in bits of the type `long long' on the target machine. If you don't define this, the default is two words. `CHAR_TYPE_SIZE' A C expression for the size in bits of the type `char' on the target machine. If you don't define this, the default is one quarter of a word. (If this would be less than one storage unit, it is rounded up to one unit.) `FLOAT_TYPE_SIZE' A C expression for the size in bits of the type `float' on the target machine. If you don't define this, the default is one word. `DOUBLE_TYPE_SIZE' A C expression for the size in bits of the type `double' on the target machine. If you don't define this, the default is two words. `LONG_DOUBLE_TYPE_SIZE' A C expression for the size in bits of the type `long double' on the target machine. If you don't define this, the default is two words. `DEFAULT_SIGNED_CHAR' An expression whose value is 1 or 0, according to whether the type `char' should be signed or unsigned by default. The user can always override this default with the options `-fsigned-char' and `-funsigned-char'. `DEFAULT_SHORT_ENUMS' A C expression to determine whether to give an `enum' type only as many bytes as it takes to represent the range of possible values of that type. A nonzero value means to do that; a zero value means all `enum' types should be allocated like `int'. If you don't define the macro, the default is 0. `SIZE_TYPE' A C expression for a string describing the name of the data type to use for size values. The typedef name `size_t' is defined using the contents of the string. The string can contain more than one keyword. If so, separate them with spaces, and write first any length keyword, then `unsigned' if appropriate, and finally `int'. The string must exactly match one of the data type names defined in the function `init_decl_processing' in the file `c-decl.c'. You may not omit `int' or change the order--that would cause the compiler to crash on startup. If you don't define this macro, the default is `"long unsigned int"'. `PTRDIFF_TYPE' A C expression for a string describing the name of the data type to use for the result of subtracting two pointers. The typedef name `ptrdiff_t' is defined using the contents of the string. See `SIZE_TYPE' above for more information. If you don't define this macro, the default is `"long int"'. `WCHAR_TYPE' A C expression for a string describing the name of the data type to use for wide characters. The typedef name `wchar_t' is defined using the contents of the string. See `SIZE_TYPE' above for more information. If you don't define this macro, the default is `"int"'. `WCHAR_TYPE_SIZE' A C expression for the size in bits of the data type for wide characters. This is used in `cpp', which cannot make use of `WCHAR_TYPE'. `OBJC_INT_SELECTORS' Define this macro if the type of Objective C selectors should be `int'. If this macro is not defined, then selectors should have the type `struct objc_selector *'. `OBJC_NONUNIQUE_SELECTORS' Define this macro if Objective C selector-references will be made unique by the linker (this is the default). In this case, each selector-reference will be given a separate assembler label. Otherwise, the selector-references will be gathered into an array with a single assembler label. `MULTIBYTE_CHARS' Define this macro to enable support for multibyte characters in the input to GNU CC. This requires that the host system support the ANSI C library functions for converting multibyte characters to wide characters. `TARGET_BELL' A C constant expression for the integer value for escape sequence `\a'. `TARGET_BS' `TARGET_TAB' `TARGET_NEWLINE' C constant expressions for the integer values for escape sequences `\b', `\t' and `\n'. `TARGET_VT' `TARGET_FF' `TARGET_CR' C constant expressions for the integer values for escape sequences `\v', `\f' and `\r'. File: gcc.info, Node: Registers, Next: Register Classes, Prev: Type Layout, Up: Machine 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 ---------------------------------- `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'. File: gcc.info, Node: Allocation Order, Next: Values in Registers, Prev: Register Basics, Up: Registers Order of Allocation of Registers -------------------------------- `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 allocatable 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 order of registers 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 It is not necessary for this macro 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. The way to implement that is to define 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 any other hard register for which the mode is OK; and that moving a value into the register and back out not alter it. Since the same instruction used to move `SImode' 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 and this macro should 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 it is desirable to choose register allocation so as to avoid move instructions between a value of mode MODE1 and a value of mode MODE2. If `HARD_REGNO_MODE_OK (R, MODE1)' and `HARD_REGNO_MODE_OK (R, MODE2)' are ever different for any R, then `MODES_TIEABLE_P (MODE1, MODE2)' must be zero. 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 make 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. Also, the leaf function may use only those registers for its own variables and temporaries. 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. `REG_LEAF_ALLOC_ORDER' If defined, an initializer for a vector of integers, containing the numbers of hard registers in the order in which the GNU CC should prefer to use them (from most preferred to least) in a leaf function. If this macro is not defined, REG_ALLOC_ORDER is used for both non-leaf and leaf-functions. 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. 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 accurate `REG_DEAD' notes are needed for hard register number REGNO at the time of outputting the assembler code. When this is so, a few optimizations that take place after register allocation and could invalidate the death notes are not done when this register is involved. 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.) If this macro is not defined, it means that no death notes need to be preserved. This is the usual situation. File: gcc.info, Node: Register Classes, Next: Stack and Calling, Prev: Registers, Up: Machine 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 that 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'. `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_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 definition #define PREFERRED_RELOAD_CLASS(X,CLASS) CLASS is safe. 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. `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, you should define `SECONDARY_OUTPUT_RELOAD_CLASS' 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. `SMALL_REGISTER_CLASSES' Normally the compiler will avoid choosing spill registers from registers that have been explicitly mentioned in the rtl (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. On those machines, you should define `SMALL_REGISTER_CLASSES'. When it is defined, the compiler allows registers explicitly used in the rtl to be used as spill registers but prevents the compiler from extending the lifetime of these registers. Defining this macro is always safe, but unnecessarily defining this macro will reduce the amount of optimizations that can be performed in some cases. If this macro is not defined but needs to be, the compiler will run out of reload registers and print a fatal error message. For most machines, this macro should not be defined. `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. 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 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. 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 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: Machine Macros Describing Stack Layout and Calling Conventions =============================================== * Menu: * Frame Layout:: * Frame Registers:: * Elimination:: * Stack Arguments:: * Register Arguments:: * Scalar Return:: * Aggregate Return:: * Caller Saves:: * Function Entry:: * Profiling:: File: gcc.info, Node: Frame Layout, Next: Frame Registers, Up: Stack and Calling 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', the next slot's offset is found by subtracting the length of the first slot 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.