This is doc/gccint.info, produced by makeinfo version 4.1 from doc/gccint.texi. INFO-DIR-SECTION Programming START-INFO-DIR-ENTRY * gccint: (gccint). Internals of the GNU Compiler Collection. END-INFO-DIR-ENTRY This file documents the internals of the GNU compilers. Published by the Free Software Foundation 59 Temple Place - Suite 330 Boston, MA 02111-1307 USA Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled "GNU Free Documentation License". (a) The FSF's Front-Cover Text is: A GNU Manual (b) The FSF's Back-Cover Text is: You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.  File: gccint.info, Node: Type Layout, Next: Escape Sequences, Prev: Storage Layout, Up: Target 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. `ADA_LONG_TYPE_SIZE' On some machines, the size used for the Ada equivalent of the type `long' by a native Ada compiler differs from that used by C. In that situation, define this macro to be a C expression to be used for the size of that type. If you don't define this, the default is the value of `LONG_TYPE_SIZE'. `MAX_LONG_TYPE_SIZE' Maximum number for the size in bits of the type `long' on the target machine. If this is undefined, the default is `LONG_TYPE_SIZE'. Otherwise, it is the constant value that is the largest value that `LONG_TYPE_SIZE' can have at run-time. This is used in `cpp'. `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. If you want to support GNU Ada on your machine, the value of this macro must be at least 64. `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 `BITS_PER_UNIT'. `MAX_CHAR_TYPE_SIZE' Maximum number for the size in bits of the type `char' on the target machine. If this is undefined, the default is `CHAR_TYPE_SIZE'. Otherwise, it is the constant value that is the largest value that `CHAR_TYPE_SIZE' can have at run-time. This is used in `cpp'. `BOOL_TYPE_SIZE' A C expression for the size in bits of the C++ type `bool' and C99 type `_Bool' on the target machine. If you don't define this, and you probably shouldn't, the default is `CHAR_TYPE_SIZE'. `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. Maximum number for the size in bits of the type `long double' on the target machine. If this is undefined, the default is `LONG_DOUBLE_TYPE_SIZE'. Otherwise, it is the constant value that is the largest value that `LONG_DOUBLE_TYPE_SIZE' can have at run-time. This is used in `cpp'. Define this macro to be 1 if the target machine uses 80-bit floating-point values with 128-bit size and alignment. This is used in `real.c'. `WIDEST_HARDWARE_FP_SIZE' A C expression for the size in bits of the widest floating-point format supported by the hardware. If you define this macro, you must specify a value less than or equal to the value of `LONG_DOUBLE_TYPE_SIZE'. If you do not define this macro, the value of `LONG_DOUBLE_TYPE_SIZE' is the default. `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'. `MAX_WCHAR_TYPE_SIZE' Maximum number for the size in bits of the data type for wide characters. If this is undefined, the default is `WCHAR_TYPE_SIZE'. Otherwise, it is the constant value that is the largest value that `WCHAR_TYPE_SIZE' can have at run-time. This is used in `cpp'. `GCOV_TYPE_SIZE' A C expression for the size in bits of the type used for gcov counters on the target machine. If you don't define this, the default is one `LONG_TYPE_SIZE' in case it is greater or equal to 64-bit and `LONG_LONG_TYPE_SIZE' otherwise. You may want to re-define the type to ensure atomicity for counters in multithreaded programs. `WINT_TYPE' A C expression for a string describing the name of the data type to use for wide characters passed to `printf' and returned from `getwc'. The typedef name `wint_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 `"unsigned int"'. `INTMAX_TYPE' A C expression for a string describing the name of the data type that can represent any value of any standard or extended signed integer type. The typedef name `intmax_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 the first of `"int"', `"long int"', or `"long long int"' that has as much precision as `long long int'. `UINTMAX_TYPE' A C expression for a string describing the name of the data type that can represent any value of any standard or extended unsigned integer type. The typedef name `uintmax_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 the first of `"unsigned int"', `"long unsigned int"', or `"long long unsigned int"' that has as much precision as `long long unsigned int'. `TARGET_PTRMEMFUNC_VBIT_LOCATION' The C++ compiler represents a pointer-to-member-function with a struct that looks like: struct { union { void (*fn)(); ptrdiff_t vtable_index; }; ptrdiff_t delta; }; The C++ compiler must use one bit to indicate whether the function that will be called through a pointer-to-member-function is virtual. Normally, we assume that the low-order bit of a function pointer must always be zero. Then, by ensuring that the vtable_index is odd, we can distinguish which variant of the union is in use. But, on some platforms function pointers can be odd, and so this doesn't work. In that case, we use the low-order bit of the `delta' field, and shift the remainder of the `delta' field to the left. GCC will automatically make the right selection about where to store this bit using the `FUNCTION_BOUNDARY' setting for your platform. However, some platforms such as ARM/Thumb have `FUNCTION_BOUNDARY' set such that functions always start at even addresses, but the lowest bit of pointers to functions indicate whether the function at that address is in ARM or Thumb mode. If this is the case of your architecture, you should define this macro to `ptrmemfunc_vbit_in_delta'. In general, you should not have to define this macro. On architectures in which function addresses are always even, according to `FUNCTION_BOUNDARY', GCC will automatically define this macro to `ptrmemfunc_vbit_in_pfn'. `TARGET_VTABLE_USES_DESCRIPTORS' Normally, the C++ compiler uses function pointers in vtables. This macro allows the target to change to use "function descriptors" instead. Function descriptors are found on targets for whom a function pointer is actually a small data structure. Normally the data structure consists of the actual code address plus a data pointer to which the function's data is relative. If vtables are used, the value of this macro should be the number of words that the function descriptor occupies.  File: gccint.info, Node: Escape Sequences, Next: Registers, Prev: Type Layout, Up: Target Macros Target Character Escape Sequences ================================= By default, GCC assumes that the C character escape sequences take on their ASCII values for the target. If this is not correct, you must explicitly define all of the macros below. `TARGET_BELL' A C constant expression for the integer value for escape sequence `\a'. `TARGET_ESC' A C constant expression for the integer value of the target escape character. As an extension, GCC evaluates the escape sequences `\e' and `\E' to this. `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: gccint.info, Node: Registers, Next: Register Classes, Prev: Escape Sequences, 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.  File: gccint.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. `CALL_REALLY_USED_REGISTERS' Like `CALL_USED_REGISTERS' except this macro doesn't require that the entire set of `FIXED_REGISTERS' be included. (`CALL_USED_REGISTERS' must be a superset of `FIXED_REGISTERS'). This macro is optional. If not specified, it defaults to the value of `CALL_USED_REGISTERS'. `HARD_REGNO_CALL_PART_CLOBBERED (REGNO, MODE)' A C expression that is nonzero if it is not permissible to store a value of mode MODE in hard register number REGNO across a call without some part of it being clobbered. For most machines this macro need not be defined. It is only required for machines that do not preserve the entire contents of a register across a call. `CONDITIONAL_REGISTER_USAGE' Zero or more C statements that may conditionally modify five variables `fixed_regs', `call_used_regs', `global_regs', `reg_names', and `reg_class_contents', to take into account any dependence of these register sets on target flags. The first three of these are of type `char []' (interpreted as Boolean vectors). `global_regs' is a `const char *[]', and `reg_class_contents' is a `HARD_REG_SET'. Before the macro is called, `fixed_regs', `call_used_regs', `reg_class_contents', and `reg_names' have been initialized from `FIXED_REGISTERS', `CALL_USED_REGISTERS', `REG_CLASS_CONTENTS', and `REGISTER_NAMES', respectively. `global_regs' has been cleared, and any `-ffixed-REG', `-fcall-used-REG' and `-fcall-saved-REG' command options have been applied. 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. `LOCAL_REGNO (REGNO)' Define this macro if the target machine has register windows. This C expression returns true if the register is call-saved but is in the register window. Unlike most call-saved registers, such registers need not be explicitly restored on function exit or during non-local gotos.  File: gccint.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 GCC 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: gccint.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 GCC to perform better register allocation. `AVOID_CCMODE_COPIES' Define this macro if the compiler should avoid copies to/from `CCmode' registers. You should only define this macro if support for copying to/from `CCmode' is incomplete.  File: gccint.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". GCC 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' Name of a char 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 GCC 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. `TARGET_ASM_FUNCTION_PROLOGUE' and `TARGET_ASM_FUNCTION_EPILOGUE' must usually treat leaf functions specially. They can test the C variable `current_function_is_leaf' which is nonzero for leaf functions. `current_function_is_leaf' is set prior to local register allocation and is valid for the remaining compiler passes. They can also test the C variable `current_function_uses_only_leaf_regs' which is nonzero for leaf functions which only use leaf registers. `current_function_uses_only_leaf_regs' is valid after reload and is only useful if `LEAF_REGISTERS' is defined.  File: gccint.info, Node: Stack Registers, 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, GCC 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: gccint.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'. In this situation, the first integer in each sub-initializer corresponds to registers 0 through 31, the second integer to registers 32 through 63, and so on. `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. `MODE_BASE_REG_CLASS (MODE)' This is a variation of the `BASE_REG_CLASS' macro which allows the selection of a base register in a mode depenedent manner. If MODE is VOIDmode then it should return the same value as `BASE_REG_CLASS'. `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 an 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 nonzero 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' On some machines, it is risky to let hard registers live across arbitrary insns. Typically, these machines have instructions that require values to be in specific registers (like an accumulator), and reload will fail if the required hard register is used for another purpose across such an insn. Define `SMALL_REGISTER_CLASSES' to be an expression with a nonzero value on these machines. When this macro has a nonzero value, the compiler will try to minimize the lifetime of hard registers. It is always safe to define this macro with a nonzero 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 nonzero 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 pseudos 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_MODE' If defined, a C expression for a class that contains registers for which the compiler may not change modes arbitrarily. `CLASS_CANNOT_CHANGE_MODE_P(FROM, TO)' A C expression that is true if, for a register in `CLASS_CANNOT_CHANGE_MODE', the requested mode punning is invalid. 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 `CLASS_CANNOT_CHANGE_MODE' as `FLOAT_REGS' and `CLASS_CANNOT_CHANGE_MODE_P' restricts mode changes to same-size modes. Compare this to IA-64, which extends floating-point values to 82 bits, and stores 64-bit integers in a different format than 64-bit doubles. Therefore `CLASS_CANNOT_CHANGE_MODE_P' is always true. 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 that can be used to segregate specific types of operands, usually memory references, for the target machine. Any letter that is not elsewhere defined and not matched by `REG_CLASS_FROM_LETTER' may be used. 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: gccint.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:: * Exception Handling:: * Stack Checking:: * Frame Registers:: * Elimination:: * Stack Arguments:: * Register Arguments:: * Scalar Return:: * Aggregate Return:: * Caller Saves:: * Function Entry:: * Profiling:: * Tail Calls::