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This is Info file gcc.info, produced by Makeinfo-1.54 from the input file gcc.texi. This file documents the use and the internals of the GNU compiler. Published by the Free Software Foundation 675 Massachusetts Avenue Cambridge, MA 02139 USA Copyright (C) 1988, 1989, 1992, 1993 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the sections entitled "GNU General Public License" and "Protect Your Freedom--Fight `Look And Feel'" are included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the sections entitled "GNU General Public License" and "Protect Your Freedom--Fight `Look And Feel'", and this permission notice, may be included in translations approved by the Free Software Foundation instead of in the original English. File: gcc.info, Node: Driver, Next: Run-time Target, Up: Target Macros Controlling the Compilation Driver, `gcc' ========================================= `SWITCH_TAKES_ARG (CHAR)' A C expression which determines whether the option `-CHAR' takes arguments. The value should be the number of arguments that option takes-zero, for many options. By default, this macro is defined to handle the standard options properly. You need not define it unless you wish to add additional options which take arguments. `WORD_SWITCH_TAKES_ARG (NAME)' A C expression which determines whether the option `-NAME' takes arguments. The value should be the number of arguments that option takes-zero, for many options. This macro rather than `SWITCH_TAKES_ARG' is used for multi-character option names. By default, this macro is defined as `DEFAULT_WORD_SWITCH_TAKES_ARG', which handles the standard options properly. You need not define `WORD_SWITCH_TAKES_ARG' unless you wish to add additional options which take arguments. Any redefinition should call `DEFAULT_WORD_SWITCH_TAKES_ARG' and then check for additional options. `SWITCHES_NEED_SPACES' A string-valued C expression which is nonempty if the linker needs a space between the `-L' or `-o' option and its argument. If this macro is not defined, the default value is 0. `CPP_SPEC' A C string constant that tells the GNU CC driver program options to pass to CPP. It can also specify how to translate options you give to GNU CC into options for GNU CC to pass to the CPP. Do not define this macro if it does not need to do anything. `NO_BUILTIN_SIZE_TYPE' If this macro is defined, the preprocessor will not define the builtin macro `__SIZE_TYPE__'. The macro `__SIZE_TYPE__' must then be defined by `CPP_SPEC' instead. This should be defined if `SIZE_TYPE' depends on target dependent flags which are not accessible to the preprocessor. Otherwise, it should not be defined. `NO_BUILTIN_PTRDIFF_TYPE' If this macro is defined, the preprocessor will not define the builtin macro `__PTRDIFF_TYPE__'. The macro `__PTRDIFF_TYPE__' must then be defined by `CPP_SPEC' instead. This should be defined if `PTRDIFF_TYPE' depends on target dependent flags which are not accessible to the preprocessor. Otherwise, it should not be defined. `SIGNED_CHAR_SPEC' A C string constant that tells the GNU CC driver program options to pass to CPP. By default, this macro is defined to pass the option `-D__CHAR_UNSIGNED__' to CPP if `char' will be treated as `unsigned char' by `cc1'. Do not define this macro unless you need to override the default definition. `CC1_SPEC' A C string constant that tells the GNU CC driver program options to pass to `cc1'. It can also specify how to translate options you give to GNU CC into options for GNU CC to pass to the `cc1'. Do not define this macro if it does not need to do anything. `CC1PLUS_SPEC' A C string constant that tells the GNU CC driver program options to pass to `cc1plus'. It can also specify how to translate options you give to GNU CC into options for GNU CC to pass to the `cc1plus'. Do not define this macro if it does not need to do anything. `ASM_SPEC' A C string constant that tells the GNU CC driver program options to pass to the assembler. It can also specify how to translate options you give to GNU CC into options for GNU CC to pass to the assembler. See the file `sun3.h' for an example of this. Do not define this macro if it does not need to do anything. `ASM_FINAL_SPEC' A C string constant that tells the GNU CC driver program how to run any programs which cleanup after the normal assembler. Normally, this is not needed. See the file `mips.h' for an example of this. Do not define this macro if it does not need to do anything. `LINK_SPEC' A C string constant that tells the GNU CC driver program options to pass to the linker. It can also specify how to translate options you give to GNU CC into options for GNU CC to pass to the linker. Do not define this macro if it does not need to do anything. `LIB_SPEC' Another C string constant used much like `LINK_SPEC'. The difference between the two is that `LIB_SPEC' is used at the end of the command given to the linker. If this macro is not defined, a default is provided that loads the standard C library from the usual place. See `gcc.c'. `STARTFILE_SPEC' Another C string constant used much like `LINK_SPEC'. The difference between the two is that `STARTFILE_SPEC' is used at the very beginning of the command given to the linker. If this macro is not defined, a default is provided that loads the standard C startup file from the usual place. See `gcc.c'. `ENDFILE_SPEC' Another C string constant used much like `LINK_SPEC'. The difference between the two is that `ENDFILE_SPEC' is used at the very end of the command given to the linker. Do not define this macro if it does not need to do anything. `LINK_LIBGCC_SPECIAL' Define this macro meaning that `gcc' should find the library `libgcc.a' by hand, rather than passing the argument `-lgcc' to tell the linker to do the search; also, `gcc' should not generate `-L' options to pass to the linker (as it normally does). `LINK_LIBGCC_SPECIAL_1' Define this macro meaning that `gcc' should find the library `libgcc.a' by hand, rather than passing the argument `-lgcc' to tell the linker to do the search. `RELATIVE_PREFIX_NOT_LINKDIR' Define this macro to tell `gcc' that it should only translate a `-B' prefix into a `-L' linker option if the prefix indicates an absolute file name. `STANDARD_EXEC_PREFIX' Define this macro as a C string constant if you wish to override the standard choice of `/usr/local/lib/gcc-lib/' as the default prefix to try when searching for the executable files of the compiler. `MD_EXEC_PREFIX' If defined, this macro is an additional prefix to try after `STANDARD_EXEC_PREFIX'. `MD_EXEC_PREFIX' is not searched when the `-b' option is used, or the compiler is built as a cross compiler. `STANDARD_STARTFILE_PREFIX' Define this macro as a C string constant if you wish to override the standard choice of `/usr/local/lib/' as the default prefix to try when searching for startup files such as `crt0.o'. `MD_STARTFILE_PREFIX' If defined, this macro supplies an additional prefix to try after the standard prefixes. `MD_EXEC_PREFIX' is not searched when the `-b' option is used, or when the compiler is built as a cross compiler. `MD_STARTFILE_PREFIX_1' If defined, this macro supplies yet another prefix to try after the standard prefixes. It is not searched when the `-b' option is used, or when the compiler is built as a cross compiler. `LOCAL_INCLUDE_DIR' Define this macro as a C string constant if you wish to override the standard choice of `/usr/local/include' as the default prefix to try when searching for local header files. `LOCAL_INCLUDE_DIR' comes before `SYSTEM_INCLUDE_DIR' in the search order. Cross compilers do not use this macro and do not search either `/usr/local/include' or its replacement. `SYSTEM_INCLUDE_DIR' Define this macro as a C string constant if you wish to specify a system-specific directory to search for header files before the standard directory. `SYSTEM_INCLUDE_DIR' comes before `STANDARD_INCLUDE_DIR' in the search order. Cross compilers do not use this macro and do not search the directory specified. `STANDARD_INCLUDE_DIR' Define this macro as a C string constant if you wish to override the standard choice of `/usr/include' as the default prefix to try when searching for header files. Cross compilers do not use this macro and do not search either `/usr/include' or its replacement. `INCLUDE_DEFAULTS' Define this macro if you wish to override the entire default search path for include files. The default search path includes `GCC_INCLUDE_DIR', `LOCAL_INCLUDE_DIR', `SYSTEM_INCLUDE_DIR', `GPLUSPLUS_INCLUDE_DIR', and `STANDARD_INCLUDE_DIR'. In addition, `GPLUSPLUS_INCLUDE_DIR' and `GCC_INCLUDE_DIR' are defined automatically by `Makefile', and specify private search areas for GCC. The directory `GPLUSPLUS_INCLUDE_DIR' is used only for C++ programs. The definition should be an initializer for an array of structures. Each array element should have two elements: the directory name (a string constant) and a flag for C++-only directories. Mark the end of the array with a null element. For example, here is the definition used for VMS: #define INCLUDE_DEFAULTS \ { \ { "GNU_GXX_INCLUDE:", 1}, \ { "GNU_CC_INCLUDE:", 0}, \ { "SYS$SYSROOT:[SYSLIB.]", 0}, \ { ".", 0}, \ { 0, 0} \ } Here is the order of prefixes tried for exec files: 1. Any prefixes specified by the user with `-B'. 2. The environment variable `GCC_EXEC_PREFIX', if any. 3. The directories specified by the environment variable `COMPILER_PATH'. 4. The macro `STANDARD_EXEC_PREFIX'. 5. `/usr/lib/gcc/'. 6. The macro `MD_EXEC_PREFIX', if any. Here is the order of prefixes tried for startfiles: 1. Any prefixes specified by the user with `-B'. 2. The environment variable `GCC_EXEC_PREFIX', if any. 3. The directories specified by the environment variable `LIBRARY_PATH'. 4. The macro `STANDARD_EXEC_PREFIX'. 5. `/usr/lib/gcc/'. 6. The macro `MD_EXEC_PREFIX', if any. 7. The macro `MD_STARTFILE_PREFIX', if any. 8. The macro `STANDARD_STARTFILE_PREFIX'. 9. `/lib/'. 10. `/usr/lib/'. File: gcc.info, Node: Run-time Target, Next: Storage Layout, Prev: Driver, Up: Target Macros Run-time Target Specification ============================= `CPP_PREDEFINES' Define this to be a string constant containing `-D' options to define the predefined macros that identify this machine and system. These macros will be predefined unless the `-ansi' option is specified. In addition, a parallel set of macros are predefined, whose names are made by appending `__' at the beginning and at the end. These `__' macros are permitted by the ANSI standard, so they are predefined regardless of whether `-ansi' is specified. For example, on the Sun, one can use the following value: "-Dmc68000 -Dsun -Dunix" The result is to define the macros `__mc68000__', `__sun__' and `__unix__' unconditionally, and the macros `mc68000', `sun' and `unix' provided `-ansi' is not specified. `STDC_VALUE' Define the value to be assigned to the built-in macro `__STDC__'. The default is the value `1'. `extern int target_flags;' This declaration should be present. `TARGET_...' This series of macros is to allow compiler command arguments to enable or disable the use of optional features of the target machine. For example, one machine description serves both the 68000 and the 68020; a command argument tells the compiler whether it should use 68020-only instructions or not. This command argument works by means of a macro `TARGET_68020' that tests a bit in `target_flags'. Define a macro `TARGET_FEATURENAME' for each such option. Its definition should test a bit in `target_flags'; for example: #define TARGET_68020 (target_flags & 1) One place where these macros are used is in the condition-expressions of instruction patterns. Note how `TARGET_68020' appears frequently in the 68000 machine description file, `m68k.md'. Another place they are used is in the definitions of the other macros in the `MACHINE.h' file. `TARGET_SWITCHES' This macro defines names of command options to set and clear bits in `target_flags'. Its definition is an initializer with a subgrouping for each command option. Each subgrouping contains a string constant, that defines the option name, and a number, which contains the bits to set in `target_flags'. A negative number says to clear bits instead; the negative of the number is which bits to clear. The actual option name is made by appending `-m' to the specified name. One of the subgroupings should have a null string. The number in this grouping is the default value for `target_flags'. Any target options act starting with that value. Here is an example which defines `-m68000' and `-m68020' with opposite meanings, and picks the latter as the default: #define TARGET_SWITCHES \ { { "68020", 1}, \ { "68000", -1}, \ { "", 1}} `TARGET_OPTIONS' This macro is similar to `TARGET_SWITCHES' but defines names of command options that have values. Its definition is an initializer with a subgrouping for each command option. Each subgrouping contains a string constant, that defines the fixed part of the option name, and the address of a variable. The variable, type `char *', is set to the variable part of the given option if the fixed part matches. The actual option name is made by appending `-m' to the specified name. Here is an example which defines `-mshort-data-NUMBER'. If the given option is `-mshort-data-512', the variable `m88k_short_data' will be set to the string `"512"'. extern char *m88k_short_data; #define TARGET_OPTIONS \ { { "short-data-", &m88k_short_data } } `TARGET_VERSION' This macro is a C statement to print on `stderr' a string describing the particular machine description choice. Every machine description should define `TARGET_VERSION'. For example: #ifdef MOTOROLA #define TARGET_VERSION \ fprintf (stderr, " (68k, Motorola syntax)"); #else #define TARGET_VERSION \ fprintf (stderr, " (68k, MIT syntax)"); #endif `OVERRIDE_OPTIONS' Sometimes certain combinations of command options do not make sense on a particular target machine. You can define a macro `OVERRIDE_OPTIONS' to take account of this. This macro, if defined, is executed once just after all the command options have been parsed. Don't use this macro to turn on various extra optimizations for `-O'. That is what `OPTIMIZATION_OPTIONS' is for. `OPTIMIZATION_OPTIONS (LEVEL)' Some machines may desire to change what optimizations are performed for various optimization levels. This macro, if defined, is executed once just after the optimization level is determined and before the remainder of the command options have been parsed. Values set in this macro are used as the default values for the other command line options. LEVEL is the optimization level specified; 2 if -O2 is specified, 1 if -O is specified, and 0 if neither is specified. *Do not examine `write_symbols' in this macro!* The debugging options are not supposed to alter the generated code. File: gcc.info, Node: Storage Layout, Next: Type Layout, Prev: Run-time Target, Up: Target 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, then this must still be defined, but it doesn't matter which value it is defined to. 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. This applies to both memory locations and registers; GNU CC fundamentally assumes that the order of words in memory is the same as the order in registers. `BITS_PER_UNIT' Define this macro to be the 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. `PROMOTE_MODE (M, UNSIGNEDP, TYPE)' A macro to update M and UNSIGNEDP when an object whose type is TYPE and which has the specified mode and signedness is to be stored in a register. This macro is only called when TYPE is a scalar type. On most RISC machines, which only have operations that operate on a full register, define this macro to set M to `word_mode' if M is an integer mode narrower than `BITS_PER_WORD'. In most cases, only integer modes should be widened because wider-precision floating-point operations are usually more expensive than their narrower counterparts. For most machines, the macro definition does not change UNSIGNEDP. However, some machines, have instructions that preferentially handle either signed or unsigned quantities of certain modes. For example, on the DEC Alpha, 32-bit loads from memory and 32-bit add instructions sign-extend the result to 64 bits. On such machines, set UNSIGNEDP according to which kind of extension is more efficient. Do not define this macro if it would never modify M. `PROMOTE_FUNCTION_ARGS' Define this macro if the promotion described by `PROMOTE_MODE' should also be done for outgoing function arguments. `PROMOTE_FUNCTION_RETURN' Define this macro if the promotion described by `PROMOTE_MODE' should also be done for the return value of functions. If this macro is defined, `FUNCTION_VALUE' must perform the same promotions done by `PROMOTE_MODE'. `PARM_BOUNDARY' Normal alignment required for function parameters on the stack, in bits. All stack parameters receive at 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. If defined, this overrides `BIGGEST_ALIGNMENT' for structure fields only. `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;'. Note that `PCC_BITFIELD_TYPE_MATTERS' also affects the alignment that results from an empty field. `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 macro to be the value 1 if instructions will fail to work if given data not on the nominal alignment. If instructions will merely go slower in that case, define this macro as 0. `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. If your aim is to make GNU CC use the same conventions for laying out bitfields as are used by another compiler, here is how to investigate what the other compiler does. Compile and run this program: struct foo1 { char x; char :0; char y; }; struct foo2 { char x; int :0; char y; }; main () { printf ("Size of foo1 is %d\n", sizeof (struct foo1)); printf ("Size of foo2 is %d\n", sizeof (struct foo2)); exit (0); } If this prints 2 and 5, then the compiler's behavior is what you would get from `PCC_BITFIELD_TYPE_MATTERS'. `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: 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. `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_SELECTORS_WITHOUT_LABELS' Define this macro if the compiler can group all the selectors together into a vector and use just one label at the beginning of the vector. Otherwise, the compiler must give each selector its own assembler label. On certain machines, it is important to have a separate label for each selector because this enables the linker to eliminate duplicate selectors. `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: Target Macros Register Usage ============== This section explains how to describe what registers the target machine has, and how (in general) they can be used. The description of which registers a specific instruction can use is done with register classes; see *Note Register Classes::. For information on using registers to access a stack frame, see *Note Frame Registers::. For passing values in registers, see *Note Register Arguments::. For returning values in registers, see *Note Scalar Return::. * Menu: * Register Basics:: Number and kinds of registers. * Allocation Order:: Order in which registers are allocated. * Values in Registers:: What kinds of values each reg can hold. * Leaf Functions:: Renumbering registers for leaf functions. * Stack Registers:: Handling a register stack such as 80387. * Obsolete Register Macros:: Macros formerly used for the 80387. File: gcc.info, Node: Register Basics, Next: Allocation Order, Up: Registers Basic Characteristics of Registers ---------------------------------- `FIRST_PSEUDO_REGISTER' Number of hardware registers known to the compiler. They receive numbers 0 through `FIRST_PSEUDO_REGISTER-1'; thus, the first pseudo register's number really is assigned the number `FIRST_PSEUDO_REGISTER'. `FIXED_REGISTERS' An initializer that says which registers are used for fixed purposes all throughout the compiled code and are therefore not available for general allocation. These would include the stack pointer, the frame pointer (except on machines where that can be used as a general register when no frame pointer is needed), the program counter on machines where that is considered one of the addressable registers, and any other numbered register with a standard use. This information is expressed as a sequence of numbers, separated by commas and surrounded by braces. The Nth number is 1 if register N is fixed, 0 otherwise. The table initialized from this macro, and the table initialized by the following one, may be overridden at run time either automatically, by the actions of the macro `CONDITIONAL_REGISTER_USAGE', or by the user with the command options `-ffixed-REG', `-fcall-used-REG' and `-fcall-saved-REG'. `CALL_USED_REGISTERS' Like `FIXED_REGISTERS' but has 1 for each register that is clobbered (in general) by function calls as well as for fixed registers. This macro therefore identifies the registers that are not available for general allocation of values that must live across function calls. If a register has 0 in `CALL_USED_REGISTERS', the compiler automatically saves it on function entry and restores it on function exit, if the register is used within the function. `CONDITIONAL_REGISTER_USAGE' Zero or more C statements that may conditionally modify two variables `fixed_regs' and `call_used_regs' (both of type `char []') after they have been initialized from the two preceding macros. This is necessary in case the fixed or call-clobbered registers depend on target flags. You need not define this macro if it has no work to do. If the usage of an entire class of registers depends on the target flags, you may indicate this to GCC by using this macro to modify `fixed_regs' and `call_used_regs' to 1 for each of the registers in the classes which should not be used by GCC. Also define the macro `REG_CLASS_FROM_LETTER' to return `NO_REGS' if it is called with a letter for a class that shouldn't be used. (However, if this class is not included in `GENERAL_REGS' and all of the insn patterns whose constraints permit this class are controlled by target switches, then GCC will automatically avoid using these registers when the target switches are opposed to them.) `NON_SAVING_SETJMP' If this macro is defined and has a nonzero value, it means that `setjmp' and related functions fail to save the registers, or that `longjmp' fails to restore them. To compensate, the compiler avoids putting variables in registers in functions that use `setjmp'. `INCOMING_REGNO (OUT)' Define this macro if the target machine has register windows. This C expression returns the register number as seen by the called function corresponding to the register number OUT as seen by the calling function. Return OUT if register number OUT is not an outbound register. `OUTGOING_REGNO (IN)' Define this macro if the target machine has register windows. This C expression returns the register number as seen by the calling function corresponding to the register number IN as seen by the called function. Return IN if register number IN is not an inbound register. File: gcc.info, Node: Allocation Order, Next: Values in Registers, Prev: Register Basics, Up: Registers Order of Allocation of Registers -------------------------------- `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 register order in the array `reg_alloc_order'. Element 0 should be the register to allocate first; element 1, the next register; and so on. The macro body should not assume anything about the contents of `reg_alloc_order' before execution of the macro. On most machines, it is not necessary to define this macro. File: gcc.info, Node: Values in Registers, Next: Leaf Functions, Prev: Allocation Order, Up: Registers How Values Fit in Registers --------------------------- This section discusses the macros that describe which kinds of values (specifically, which machine modes) each register can hold, and how many consecutive registers are needed for a given mode. `HARD_REGNO_NREGS (REGNO, MODE)' A C expression for the number of consecutive hard registers, starting at register number REGNO, required to hold a value of mode MODE. On a machine where all registers are exactly one word, a suitable definition of this macro is #define HARD_REGNO_NREGS(REGNO, MODE) \ ((GET_MODE_SIZE (MODE) + UNITS_PER_WORD - 1) \ / UNITS_PER_WORD)) `HARD_REGNO_MODE_OK (REGNO, MODE)' A C expression that is nonzero if it is permissible to store a value of mode MODE in hard register number REGNO (or in several registers starting with that one). For a machine where all registers are equivalent, a suitable definition is #define HARD_REGNO_MODE_OK(REGNO, MODE) 1 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, so you can define this macro to say so. On some machines, such as the Sparc and the Mips, we get better code by defining `HARD_REGNO_MODE_OK' to forbid integers in floating registers, even though the hardware is capable of handling them. This is because transferring values between floating registers and general registers is so slow that it is better to keep the integer in memory. 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 makes no calls) can run more efficiently if it does not make its own register window. Often this means it is required to receive its arguments in the registers where they are passed by the caller, instead of the registers where they would normally arrive. The special treatment for leaf functions generally applies only when other conditions are met; for example, often they may use only those registers for its own variables and temporaries. We use the term "leaf function" to mean a function that is suitable for this special handling, so that functions with no calls are not necessarily "leaf functions". GNU CC assigns register numbers before it knows whether the function is suitable for leaf function treatment. So it needs to renumber the registers in order to output a leaf function. The following macros accomplish this. `LEAF_REGISTERS' A C initializer for a vector, indexed by hard register number, which contains 1 for a register that is allowable in a candidate for leaf function treatment. If leaf function treatment involves renumbering the registers, then the registers marked here should be the ones before renumbering--those that GNU CC would ordinarily allocate. The registers which will actually be used in the assembler code, after renumbering, should not be marked with 1 in this vector. Define this macro only if the target machine offers a way to optimize the treatment of leaf functions. `LEAF_REG_REMAP (REGNO)' A C expression whose value is the register number to which REGNO should be renumbered, when a function is treated as a leaf function. If REGNO is a register number which should not appear in a leaf function before renumbering, then the expression should yield -1, which will cause the compiler to abort. Define this macro only if the target machine offers a way to optimize the treatment of leaf functions, and registers need to be renumbered to do this. `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, `FUNCTION_PROLOGUE' and `FUNCTION_EPILOGUE' must treat leaf functions specially. It can test the C variable `leaf_function' which is nonzero for leaf functions. (The variable `leaf_function' is defined only if `LEAF_REGISTERS' is defined.) File: gcc.info, Node: Stack Registers, Next: Obsolete Register Macros, Prev: Leaf Functions, Up: Registers Registers That Form a Stack --------------------------- There are special features to handle computers where some of the "registers" form a stack, as in the 80387 coprocessor for the 80386. Stack registers are normally written by pushing onto the stack, and are numbered relative to the top of the stack. Currently, GNU CC can only handle one group of stack-like registers, and they must be consecutively numbered. `STACK_REGS' Define this if the machine has any stack-like registers. `FIRST_STACK_REG' The number of the first stack-like register. This one is the top of the stack. `LAST_STACK_REG' The number of the last stack-like register. This one is the bottom of the stack.