--- gcc/gcc.info-14 2018/04/24 18:00:43 1.1.1.4 +++ gcc/gcc.info-14 2018/04/24 18:24:08 1.1.1.8 @@ -1,9 +1,13 @@ -This is Info file gcc.info, produced by Makeinfo-1.49 from the input +This is Info file gcc.info, produced by Makeinfo-1.55 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. + Published by the Free Software Foundation 59 Temple Place - Suite 330 +Boston, MA 02111-1307 USA + + Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995 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 @@ -11,1173 +15,956 @@ 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. +that the sections entitled "GNU General Public License," "Funding for +Free Software," and "Protect Your Freedom--Fight `Look And Feel'" are +included exactly as in the original, and provided that the entire +resulting derived work is distributed under the terms of a permission +notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the sections entitled "GNU General Public -License" 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 to handle the standard options - properly. You need not define it unless you wish to add additional - options which take arguments. - -`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. - -`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. - -`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 - `GPLUSPLUS_INCLUDE_DIR', `GCC_INCLUDE_DIR', `LOCAL_INCLUDE_DIR', - `SYSTEM_INCLUDE_DIR', and `STANDARD_INCLUDE_DIR'. In addition, - the macros `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, - 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. 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' - 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 quanities 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'. - - -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, 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, it is necessary for `FUNCTION_PROLOGUE' and -`FUNCTION_EPILOGUE' to treat leaf functions specially. It can test the -C variable `leaf_function' which is nonzero for leaf functions. (The -variable `leaf_function' is defined only if `LEAF_REGISTERS' is -defined.) - - -File: gcc.info, Node: Stack Registers, Next: Obsolete Register Macros, Prev: Leaf Functions, Up: Registers - -Registers That Form a Stack ---------------------------- - - There are special features to handle computers where some of the -"registers" form a stack, as in the 80387 coprocessor for the 80386. -Stack registers are normally written by pushing onto the stack, and are -numbered relative to the top of the stack. - - Currently, GNU CC can only handle one group of stack-like registers, -and they must be consecutively numbered. - -`STACK_REGS' - Define this if the machine has any stack-like registers. - -`FIRST_STACK_REG' - The number of the first stack-like register. This one is the top - of the stack. - -`LAST_STACK_REG' - The number of the last stack-like register. This one is the - bottom of the stack. - - -File: gcc.info, Node: Obsolete Register Macros, Prev: Stack Registers, Up: Registers - -Obsolete Macros for Controlling Register Usage ----------------------------------------------- - - These features do not work very well. They exist because they used -to be required to generate correct code for the 80387 coprocessor of the -80386. They are no longer used by that machine description and may be -removed in a later version of the compiler. Don't use them! - -`OVERLAPPING_REGNO_P (REGNO)' - If defined, this is a C expression whose value is nonzero if hard - register number REGNO is an overlapping register. This means a - hard register which overlaps a hard register with a different - number. (Such overlap is undesirable, but occasionally it allows a - machine to be supported which otherwise could not be.) This macro - must return nonzero for *all* the registers which overlap each - other. GNU CC can use an overlapping register only in certain - limited ways. It can be used for allocation within a basic block, - and may be spilled for reloading; that is all. - - If this macro is not defined, it means that none of the hard - registers overlap each other. This is the usual situation. - -`INSN_CLOBBERS_REGNO_P (INSN, REGNO)' - If defined, this is a C expression whose value should be nonzero if - the insn INSN has the effect of mysteriously clobbering the - contents of hard register number REGNO. By "mysterious" we mean - that the insn's RTL expression doesn't describe such an effect. - - If this macro is not defined, it means that no insn clobbers - registers mysteriously. This is the usual situation; all else - being equal, it is best for the RTL expression to show all the - activity. - -`PRESERVE_DEATH_INFO_REGNO_P (REGNO)' - If defined, this is a C expression whose value is nonzero if - 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. +License," "Funding for Free Software," and "Protect Your Freedom--Fight +`Look And Feel'", and this permission notice, may be included in +translations approved by the Free Software Foundation instead of in the +original English. + + +File: gcc.info, Node: Machine Modes, Next: Constants, Prev: Flags, Up: RTL + +Machine Modes +============= + + A machine mode describes a size of data object and the +representation used for it. In the C code, machine modes are +represented by an enumeration type, `enum machine_mode', defined in +`machmode.def'. Each RTL expression has room for a machine mode and so +do certain kinds of tree expressions (declarations and types, to be +precise). + + In debugging dumps and machine descriptions, the machine mode of an +RTL expression is written after the expression code with a colon to +separate them. The letters `mode' which appear at the end of each +machine mode name are omitted. For example, `(reg:SI 38)' is a `reg' +expression with machine mode `SImode'. If the mode is `VOIDmode', it +is not written at all. + + Here is a table of machine modes. The term "byte" below refers to an +object of `BITS_PER_UNIT' bits (*note Storage Layout::.). + +`QImode' + "Quarter-Integer" mode represents a single byte treated as an + integer. + +`HImode' + "Half-Integer" mode represents a two-byte integer. + +`PSImode' + "Partial Single Integer" mode represents an integer which occupies + four bytes but which doesn't really use all four. On some + machines, this is the right mode to use for pointers. + +`SImode' + "Single Integer" mode represents a four-byte integer. + +`PDImode' + "Partial Double Integer" mode represents an integer which occupies + eight bytes but which doesn't really use all eight. On some + machines, this is the right mode to use for certain pointers. + +`DImode' + "Double Integer" mode represents an eight-byte integer. + +`TImode' + "Tetra Integer" (?) mode represents a sixteen-byte integer. + +`SFmode' + "Single Floating" mode represents a single-precision (four byte) + floating point number. + +`DFmode' + "Double Floating" mode represents a double-precision (eight byte) + floating point number. + +`XFmode' + "Extended Floating" mode represents a triple-precision (twelve + byte) floating point number. This mode is used for IEEE extended + floating point. On some systems not all bits within these bytes + will actually be used. + +`TFmode' + "Tetra Floating" mode represents a quadruple-precision (sixteen + byte) floating point number. + +`CCmode' + "Condition Code" mode represents the value of a condition code, + which is a machine-specific set of bits used to represent the + result of a comparison operation. Other machine-specific modes + may also be used for the condition code. These modes are not used + on machines that use `cc0' (see *note Condition Code::.). + +`BLKmode' + "Block" mode represents values that are aggregates to which none of + the other modes apply. In RTL, only memory references can have + this mode, and only if they appear in string-move or vector + instructions. On machines which have no such instructions, + `BLKmode' will not appear in RTL. + +`VOIDmode' + Void mode means the absence of a mode or an unspecified mode. For + example, RTL expressions of code `const_int' have mode `VOIDmode' + because they can be taken to have whatever mode the context + requires. In debugging dumps of RTL, `VOIDmode' is expressed by + the absence of any mode. + +`SCmode, DCmode, XCmode, TCmode' + These modes stand for a complex number represented as a pair of + floating point values. The floating point values are in `SFmode', + `DFmode', `XFmode', and `TFmode', respectively. + +`CQImode, CHImode, CSImode, CDImode, CTImode, COImode' + These modes stand for a complex number represented as a pair of + integer values. The integer values are in `QImode', `HImode', + `SImode', `DImode', `TImode', and `OImode', respectively. + + The machine description defines `Pmode' as a C macro which expands +into the machine mode used for addresses. Normally this is the mode +whose size is `BITS_PER_WORD', `SImode' on 32-bit machines. + + The only modes which a machine description must support are +`QImode', and the modes corresponding to `BITS_PER_WORD', +`FLOAT_TYPE_SIZE' and `DOUBLE_TYPE_SIZE'. The compiler will attempt to +use `DImode' for 8-byte structures and unions, but this can be +prevented by overriding the definition of `MAX_FIXED_MODE_SIZE'. +Alternatively, you can have the compiler use `TImode' for 16-byte +structures and unions. Likewise, you can arrange for the C type `short +int' to avoid using `HImode'. + + Very few explicit references to machine modes remain in the compiler +and these few references will soon be removed. Instead, the machine +modes are divided into mode classes. These are represented by the +enumeration type `enum mode_class' defined in `machmode.h'. The +possible mode classes are: + +`MODE_INT' + Integer modes. By default these are `QImode', `HImode', `SImode', + `DImode', and `TImode'. + +`MODE_PARTIAL_INT' + The "partial integer" modes, `PSImode' and `PDImode'. + +`MODE_FLOAT' + floating point modes. By default these are `SFmode', `DFmode', + `XFmode' and `TFmode'. + +`MODE_COMPLEX_INT' + Complex integer modes. (These are not currently implemented). + +`MODE_COMPLEX_FLOAT' + Complex floating point modes. By default these are `SCmode', + `DCmode', `XCmode', and `TCmode'. + +`MODE_FUNCTION' + Algol or Pascal function variables including a static chain. + (These are not currently implemented). + +`MODE_CC' + Modes representing condition code values. These are `CCmode' plus + any modes listed in the `EXTRA_CC_MODES' macro. *Note Jump + Patterns::, also see *Note Condition Code::. + +`MODE_RANDOM' + This is a catchall mode class for modes which don't fit into the + above classes. Currently `VOIDmode' and `BLKmode' are in + `MODE_RANDOM'. + + Here are some C macros that relate to machine modes: + +`GET_MODE (X)' + Returns the machine mode of the RTX X. + +`PUT_MODE (X, NEWMODE)' + Alters the machine mode of the RTX X to be NEWMODE. + +`NUM_MACHINE_MODES' + Stands for the number of machine modes available on the target + machine. This is one greater than the largest numeric value of any + machine mode. + +`GET_MODE_NAME (M)' + Returns the name of mode M as a string. + +`GET_MODE_CLASS (M)' + Returns the mode class of mode M. + +`GET_MODE_WIDER_MODE (M)' + Returns the next wider natural mode. For example, the expression + `GET_MODE_WIDER_MODE (QImode)' returns `HImode'. + +`GET_MODE_SIZE (M)' + Returns the size in bytes of a datum of mode M. + +`GET_MODE_BITSIZE (M)' + Returns the size in bits of a datum of mode M. + +`GET_MODE_MASK (M)' + Returns a bitmask containing 1 for all bits in a word that fit + within mode M. This macro can only be used for modes whose + bitsize is less than or equal to `HOST_BITS_PER_INT'. + +`GET_MODE_ALIGNMENT (M))' + Return the required alignment, in bits, for an object of mode M. + +`GET_MODE_UNIT_SIZE (M)' + Returns the size in bytes of the subunits of a datum of mode M. + This is the same as `GET_MODE_SIZE' except in the case of complex + modes. For them, the unit size is the size of the real or + imaginary part. + +`GET_MODE_NUNITS (M)' + Returns the number of units contained in a mode, i.e., + `GET_MODE_SIZE' divided by `GET_MODE_UNIT_SIZE'. + +`GET_CLASS_NARROWEST_MODE (C)' + Returns the narrowest mode in mode class C. + + The global variables `byte_mode' and `word_mode' contain modes whose +classes are `MODE_INT' and whose bitsizes are either `BITS_PER_UNIT' or +`BITS_PER_WORD', respectively. On 32-bit machines, these are `QImode' +and `SImode', respectively. + + +File: gcc.info, Node: Constants, Next: Regs and Memory, Prev: Machine Modes, Up: RTL + +Constant Expression Types +========================= + + The simplest RTL expressions are those that represent constant +values. + +`(const_int I)' + This type of expression represents the integer value I. I is + customarily accessed with the macro `INTVAL' as in `INTVAL (EXP)', + which is equivalent to `XWINT (EXP, 0)'. + + There is only one expression object for the integer value zero; it + is the value of the variable `const0_rtx'. Likewise, the only + expression for integer value one is found in `const1_rtx', the only + expression for integer value two is found in `const2_rtx', and the + only expression for integer value negative one is found in + `constm1_rtx'. Any attempt to create an expression of code + `const_int' and value zero, one, two or negative one will return + `const0_rtx', `const1_rtx', `const2_rtx' or `constm1_rtx' as + appropriate. + + Similarly, there is only one object for the integer whose value is + `STORE_FLAG_VALUE'. It is found in `const_true_rtx'. If + `STORE_FLAG_VALUE' is one, `const_true_rtx' and `const1_rtx' will + point to the same object. If `STORE_FLAG_VALUE' is -1, + `const_true_rtx' and `constm1_rtx' will point to the same object. + +`(const_double:M ADDR I0 I1 ...)' + Represents either a floating-point constant of mode M or an + integer constant too large to fit into `HOST_BITS_PER_WIDE_INT' + bits but small enough to fit within twice that number of bits (GNU + CC does not provide a mechanism to represent even larger + constants). In the latter case, M will be `VOIDmode'. + + ADDR is used to contain the `mem' expression that corresponds to + the location in memory that at which the constant can be found. If + it has not been allocated a memory location, but is on the chain + of all `const_double' expressions in this compilation (maintained + using an undisplayed field), ADDR contains `const0_rtx'. If it is + not on the chain, ADDR contains `cc0_rtx'. ADDR is customarily + accessed with the macro `CONST_DOUBLE_MEM' and the chain field via + `CONST_DOUBLE_CHAIN'. + + If M is `VOIDmode', the bits of the value are stored in I0 and I1. + I0 is customarily accessed with the macro `CONST_DOUBLE_LOW' and + I1 with `CONST_DOUBLE_HIGH'. + + If the constant is floating point (regardless of its precision), + then the number of integers used to store the value depends on the + size of `REAL_VALUE_TYPE' (*note Cross-compilation::.). The + integers represent a floating point number, but not precisely in + the target machine's or host machine's floating point format. To + convert them to the precise bit pattern used by the target + machine, use the macro `REAL_VALUE_TO_TARGET_DOUBLE' and friends + (*note Data Output::.). + + The macro `CONST0_RTX (MODE)' refers to an expression with value 0 + in mode MODE. If mode MODE is of mode class `MODE_INT', it + returns `const0_rtx'. Otherwise, it returns a `CONST_DOUBLE' + expression in mode MODE. Similarly, the macro `CONST1_RTX (MODE)' + refers to an expression with value 1 in mode MODE and similarly + for `CONST2_RTX'. + +`(const_string STR)' + Represents a constant string with value STR. Currently this is + used only for insn attributes (*note Insn Attributes::.) since + constant strings in C are placed in memory. + +`(symbol_ref:MODE SYMBOL)' + Represents the value of an assembler label for data. SYMBOL is a + string that describes the name of the assembler label. If it + starts with a `*', the label is the rest of SYMBOL not including + the `*'. Otherwise, the label is SYMBOL, usually prefixed with + `_'. + + The `symbol_ref' contains a mode, which is usually `Pmode'. + Usually that is the only mode for which a symbol is directly valid. + +`(label_ref LABEL)' + Represents the value of an assembler label for code. It contains + one operand, an expression, which must be a `code_label' that + appears in the instruction sequence to identify the place where + the label should go. + + The reason for using a distinct expression type for code label + references is so that jump optimization can distinguish them. + +`(const:M EXP)' + Represents a constant that is the result of an assembly-time + arithmetic computation. The operand, EXP, is an expression that + contains only constants (`const_int', `symbol_ref' and `label_ref' + expressions) combined with `plus' and `minus'. However, not all + combinations are valid, since the assembler cannot do arbitrary + arithmetic on relocatable symbols. + + M should be `Pmode'. + +`(high:M EXP)' + Represents the high-order bits of EXP, usually a `symbol_ref'. + The number of bits is machine-dependent and is normally the number + of bits specified in an instruction that initializes the high + order bits of a register. It is used with `lo_sum' to represent + the typical two-instruction sequence used in RISC machines to + reference a global memory location. + + M should be `Pmode'. + + +File: gcc.info, Node: Regs and Memory, Next: Arithmetic, Prev: Constants, Up: RTL + +Registers and Memory +==================== + + Here are the RTL expression types for describing access to machine +registers and to main memory. + +`(reg:M N)' + For small values of the integer N (those that are less than + `FIRST_PSEUDO_REGISTER'), this stands for a reference to machine + register number N: a "hard register". For larger values of N, it + stands for a temporary value or "pseudo register". The compiler's + strategy is to generate code assuming an unlimited number of such + pseudo registers, and later convert them into hard registers or + into memory references. + + M is the machine mode of the reference. It is necessary because + machines can generally refer to each register in more than one + mode. For example, a register may contain a full word but there + may be instructions to refer to it as a half word or as a single + byte, as well as instructions to refer to it as a floating point + number of various precisions. + + Even for a register that the machine can access in only one mode, + the mode must always be specified. + + The symbol `FIRST_PSEUDO_REGISTER' is defined by the machine + description, since the number of hard registers on the machine is + an invariant characteristic of the machine. Note, however, that + not all of the machine registers must be general registers. All + the machine registers that can be used for storage of data are + given hard register numbers, even those that can be used only in + certain instructions or can hold only certain types of data. + + A hard register may be accessed in various modes throughout one + function, but each pseudo register is given a natural mode and is + accessed only in that mode. When it is necessary to describe an + access to a pseudo register using a nonnatural mode, a `subreg' + expression is used. + + A `reg' expression with a machine mode that specifies more than + one word of data may actually stand for several consecutive + registers. If in addition the register number specifies a + hardware register, then it actually represents several consecutive + hardware registers starting with the specified one. + + Each pseudo register number used in a function's RTL code is + represented by a unique `reg' expression. + + Some pseudo register numbers, those within the range of + `FIRST_VIRTUAL_REGISTER' to `LAST_VIRTUAL_REGISTER' only appear + during the RTL generation phase and are eliminated before the + optimization phases. These represent locations in the stack frame + that cannot be determined until RTL generation for the function + has been completed. The following virtual register numbers are + defined: + + `VIRTUAL_INCOMING_ARGS_REGNUM' + This points to the first word of the incoming arguments + passed on the stack. Normally these arguments are placed + there by the caller, but the callee may have pushed some + arguments that were previously passed in registers. + + When RTL generation is complete, this virtual register is + replaced by the sum of the register given by + `ARG_POINTER_REGNUM' and the value of `FIRST_PARM_OFFSET'. + + `VIRTUAL_STACK_VARS_REGNUM' + If `FRAME_GROWS_DOWNWARD' is defined, this points to + immediately above the first variable on the stack. + Otherwise, it points to the first variable on the stack. + + `VIRTUAL_STACK_VARS_REGNUM' is replaced with the sum of the + register given by `FRAME_POINTER_REGNUM' and the value + `STARTING_FRAME_OFFSET'. + + `VIRTUAL_STACK_DYNAMIC_REGNUM' + This points to the location of dynamically allocated memory + on the stack immediately after the stack pointer has been + adjusted by the amount of memory desired. + + This virtual register is replaced by the sum of the register + given by `STACK_POINTER_REGNUM' and the value + `STACK_DYNAMIC_OFFSET'. + + `VIRTUAL_OUTGOING_ARGS_REGNUM' + This points to the location in the stack at which outgoing + arguments should be written when the stack is pre-pushed + (arguments pushed using push insns should always use + `STACK_POINTER_REGNUM'). + + This virtual register is replaced by the sum of the register + given by `STACK_POINTER_REGNUM' and the value + `STACK_POINTER_OFFSET'. + +`(subreg:M REG WORDNUM)' + `subreg' expressions are used to refer to a register in a machine + mode other than its natural one, or to refer to one register of a + multi-word `reg' that actually refers to several registers. + + Each pseudo-register has a natural mode. If it is necessary to + operate on it in a different mode--for example, to perform a + fullword move instruction on a pseudo-register that contains a + single byte--the pseudo-register must be enclosed in a `subreg'. + In such a case, WORDNUM is zero. + + Usually M is at least as narrow as the mode of REG, in which case + it is restricting consideration to only the bits of REG that are + in M. + + Sometimes M is wider than the mode of REG. These `subreg' + expressions are often called "paradoxical". They are used in + cases where we want to refer to an object in a wider mode but do + not care what value the additional bits have. The reload pass + ensures that paradoxical references are only made to hard + registers. + + The other use of `subreg' is to extract the individual registers of + a multi-register value. Machine modes such as `DImode' and + `TImode' can indicate values longer than a word, values which + usually require two or more consecutive registers. To access one + of the registers, use a `subreg' with mode `SImode' and a WORDNUM + that says which register. + + Storing in a non-paradoxical `subreg' has undefined results for + bits belonging to the same word as the `subreg'. This laxity makes + it easier to generate efficient code for such instructions. To + represent an instruction that preserves all the bits outside of + those in the `subreg', use `strict_low_part' around the `subreg'. + + The compilation parameter `WORDS_BIG_ENDIAN', if set to 1, says + that word number zero is the most significant part; otherwise, it + is the least significant part. + + Between the combiner pass and the reload pass, it is possible to + have a paradoxical `subreg' which contains a `mem' instead of a + `reg' as its first operand. After the reload pass, it is also + possible to have a non-paradoxical `subreg' which contains a + `mem'; this usually occurs when the `mem' is a stack slot which + replaced a pseudo register. + + Note that it is not valid to access a `DFmode' value in `SFmode' + using a `subreg'. On some machines the most significant part of a + `DFmode' value does not have the same format as a single-precision + floating value. + + It is also not valid to access a single word of a multi-word value + in a hard register when less registers can hold the value than + would be expected from its size. For example, some 32-bit + machines have floating-point registers that can hold an entire + `DFmode' value. If register 10 were such a register `(subreg:SI + (reg:DF 10) 1)' would be invalid because there is no way to + convert that reference to a single machine register. The reload + pass prevents `subreg' expressions such as these from being formed. + + The first operand of a `subreg' expression is customarily accessed + with the `SUBREG_REG' macro and the second operand is customarily + accessed with the `SUBREG_WORD' macro. + +`(scratch:M)' + This represents a scratch register that will be required for the + execution of a single instruction and not used subsequently. It is + converted into a `reg' by either the local register allocator or + the reload pass. + + `scratch' is usually present inside a `clobber' operation (*note + Side Effects::.). + +`(cc0)' + This refers to the machine's condition code register. It has no + operands and may not have a machine mode. There are two ways to + use it: + + * To stand for a complete set of condition code flags. This is + best on most machines, where each comparison sets the entire + series of flags. + + With this technique, `(cc0)' may be validly used in only two + contexts: as the destination of an assignment (in test and + compare instructions) and in comparison operators comparing + against zero (`const_int' with value zero; that is to say, + `const0_rtx'). + + * To stand for a single flag that is the result of a single + condition. This is useful on machines that have only a + single flag bit, and in which comparison instructions must + specify the condition to test. + + With this technique, `(cc0)' may be validly used in only two + contexts: as the destination of an assignment (in test and + compare instructions) where the source is a comparison + operator, and as the first operand of `if_then_else' (in a + conditional branch). + + There is only one expression object of code `cc0'; it is the value + of the variable `cc0_rtx'. Any attempt to create an expression of + code `cc0' will return `cc0_rtx'. + + Instructions can set the condition code implicitly. On many + machines, nearly all instructions set the condition code based on + the value that they compute or store. It is not necessary to + record these actions explicitly in the RTL because the machine + description includes a prescription for recognizing the + instructions that do so (by means of the macro + `NOTICE_UPDATE_CC'). *Note Condition Code::. Only instructions + whose sole purpose is to set the condition code, and instructions + that use the condition code, need mention `(cc0)'. + + On some machines, the condition code register is given a register + number and a `reg' is used instead of `(cc0)'. This is usually the + preferable approach if only a small subset of instructions modify + the condition code. Other machines store condition codes in + general registers; in such cases a pseudo register should be used. + + Some machines, such as the Sparc and RS/6000, have two sets of + arithmetic instructions, one that sets and one that does not set + the condition code. This is best handled by normally generating + the instruction that does not set the condition code, and making a + pattern that both performs the arithmetic and sets the condition + code register (which would not be `(cc0)' in this case). For + examples, search for `addcc' and `andcc' in `sparc.md'. + +`(pc)' + This represents the machine's program counter. It has no operands + and may not have a machine mode. `(pc)' may be validly used only + in certain specific contexts in jump instructions. + + There is only one expression object of code `pc'; it is the value + of the variable `pc_rtx'. Any attempt to create an expression of + code `pc' will return `pc_rtx'. + + All instructions that do not jump alter the program counter + implicitly by incrementing it, but there is no need to mention + this in the RTL. + +`(mem:M ADDR)' + This RTX represents a reference to main memory at an address + represented by the expression ADDR. M specifies how large a unit + of memory is accessed. + + +File: gcc.info, Node: Arithmetic, Next: Comparisons, Prev: Regs and Memory, Up: RTL + +RTL Expressions for Arithmetic +============================== + + Unless otherwise specified, all the operands of arithmetic +expressions must be valid for mode M. An operand is valid for mode M +if it has mode M, or if it is a `const_int' or `const_double' and M is +a mode of class `MODE_INT'. + + For commutative binary operations, constants should be placed in the +second operand. + +`(plus:M X Y)' + Represents the sum of the values represented by X and Y carried + out in machine mode M. + +`(lo_sum:M X Y)' + Like `plus', except that it represents that sum of X and the + low-order bits of Y. The number of low order bits is + machine-dependent but is normally the number of bits in a `Pmode' + item minus the number of bits set by the `high' code (*note + Constants::.). + + M should be `Pmode'. + +`(minus:M X Y)' + Like `plus' but represents subtraction. + +`(compare:M X Y)' + Represents the result of subtracting Y from X for purposes of + comparison. The result is computed without overflow, as if with + infinite precision. + + Of course, machines can't really subtract with infinite precision. + However, they can pretend to do so when only the sign of the + result will be used, which is the case when the result is stored + in the condition code. And that is the only way this kind of + expression may validly be used: as a value to be stored in the + condition codes. + + The mode M is not related to the modes of X and Y, but instead is + the mode of the condition code value. If `(cc0)' is used, it is + `VOIDmode'. Otherwise it is some mode in class `MODE_CC', often + `CCmode'. *Note Condition Code::. + + Normally, X and Y must have the same mode. Otherwise, `compare' + is valid only if the mode of X is in class `MODE_INT' and Y is a + `const_int' or `const_double' with mode `VOIDmode'. The mode of X + determines what mode the comparison is to be done in; thus it must + not be `VOIDmode'. + + If one of the operands is a constant, it should be placed in the + second operand and the comparison code adjusted as appropriate. + + A `compare' specifying two `VOIDmode' constants is not valid since + there is no way to know in what mode the comparison is to be + performed; the comparison must either be folded during the + compilation or the first operand must be loaded into a register + while its mode is still known. + +`(neg:M X)' + Represents the negation (subtraction from zero) of the value + represented by X, carried out in mode M. + +`(mult:M X Y)' + Represents the signed product of the values represented by X and Y + carried out in machine mode M. + + Some machines support a multiplication that generates a product + wider than the operands. Write the pattern for this as + + (mult:M (sign_extend:M X) (sign_extend:M Y)) + + where M is wider than the modes of X and Y, which need not be the + same. + + Write patterns for unsigned widening multiplication similarly using + `zero_extend'. + +`(div:M X Y)' + Represents the quotient in signed division of X by Y, carried out + in machine mode M. If M is a floating point mode, it represents + the exact quotient; otherwise, the integerized quotient. + + Some machines have division instructions in which the operands and + quotient widths are not all the same; you should represent such + instructions using `truncate' and `sign_extend' as in, + + (truncate:M1 (div:M2 X (sign_extend:M2 Y))) + +`(udiv:M X Y)' + Like `div' but represents unsigned division. + +`(mod:M X Y)' +`(umod:M X Y)' + Like `div' and `udiv' but represent the remainder instead of the + quotient. + +`(smin:M X Y)' +`(smax:M X Y)' + Represents the smaller (for `smin') or larger (for `smax') of X + and Y, interpreted as signed integers in mode M. + +`(umin:M X Y)' +`(umax:M X Y)' + Like `smin' and `smax', but the values are interpreted as unsigned + integers. + +`(not:M X)' + Represents the bitwise complement of the value represented by X, + carried out in mode M, which must be a fixed-point machine mode. + +`(and:M X Y)' + Represents the bitwise logical-and of the values represented by X + and Y, carried out in machine mode M, which must be a fixed-point + machine mode. + +`(ior:M X Y)' + Represents the bitwise inclusive-or of the values represented by X + and Y, carried out in machine mode M, which must be a fixed-point + mode. + +`(xor:M X Y)' + Represents the bitwise exclusive-or of the values represented by X + and Y, carried out in machine mode M, which must be a fixed-point + mode. + +`(ashift:M X C)' + Represents the result of arithmetically shifting X left by C + places. X have mode M, a fixed-point machine mode. C be a + fixed-point mode or be a constant with mode `VOIDmode'; which mode + is determined by the mode called for in the machine description + entry for the left-shift instruction. For example, on the Vax, + the mode of C is `QImode' regardless of M. + +`(lshiftrt:M X C)' +`(ashiftrt:M X C)' + Like `ashift' but for right shift. Unlike the case for left shift, + these two operations are distinct. + +`(rotate:M X C)' +`(rotatert:M X C)' + Similar but represent left and right rotate. If C is a constant, + use `rotate'. + +`(abs:M X)' + Represents the absolute value of X, computed in mode M. + +`(sqrt:M X)' + Represents the square root of X, computed in mode M. Most often M + will be a floating point mode. + +`(ffs:M X)' + Represents one plus the index of the least significant 1-bit in X, + represented as an integer of mode M. (The value is zero if X is + zero.) The mode of X need not be M; depending on the target + machine, various mode combinations may be valid. + + +File: gcc.info, Node: Comparisons, Next: Bit Fields, Prev: Arithmetic, Up: RTL + +Comparison Operations +===================== + + Comparison operators test a relation on two operands and are +considered to represent a machine-dependent nonzero value described by, +but not necessarily equal to, `STORE_FLAG_VALUE' (*note Misc::.) if the +relation holds, or zero if it does not. The mode of the comparison +operation is independent of the mode of the data being compared. If +the comparison operation is being tested (e.g., the first operand of an +`if_then_else'), the mode must be `VOIDmode'. If the comparison +operation is producing data to be stored in some variable, the mode +must be in class `MODE_INT'. All comparison operations producing data +must use the same mode, which is machine-specific. + + There are two ways that comparison operations may be used. The +comparison operators may be used to compare the condition codes `(cc0)' +against zero, as in `(eq (cc0) (const_int 0))'. Such a construct +actually refers to the result of the preceding instruction in which the +condition codes were set. The instructing setting the condition code +must be adjacent to the instruction using the condition code; only +`note' insns may separate them. + + Alternatively, a comparison operation may directly compare two data +objects. The mode of the comparison is determined by the operands; they +must both be valid for a common machine mode. A comparison with both +operands constant would be invalid as the machine mode could not be +deduced from it, but such a comparison should never exist in RTL due to +constant folding. + + In the example above, if `(cc0)' were last set to `(compare X Y)', +the comparison operation is identical to `(eq X Y)'. Usually only one +style of comparisons is supported on a particular machine, but the +combine pass will try to merge the operations to produce the `eq' shown +in case it exists in the context of the particular insn involved. + + Inequality comparisons come in two flavors, signed and unsigned. +Thus, there are distinct expression codes `gt' and `gtu' for signed and +unsigned greater-than. These can produce different results for the same +pair of integer values: for example, 1 is signed greater-than -1 but not +unsigned greater-than, because -1 when regarded as unsigned is actually +`0xffffffff' which is greater than 1. + + The signed comparisons are also used for floating point values. +Floating point comparisons are distinguished by the machine modes of +the operands. + +`(eq:M X Y)' + 1 if the values represented by X and Y are equal, otherwise 0. + +`(ne:M X Y)' + 1 if the values represented by X and Y are not equal, otherwise 0. + +`(gt:M X Y)' + 1 if the X is greater than Y. If they are fixed-point, the + comparison is done in a signed sense. + +`(gtu:M X Y)' + Like `gt' but does unsigned comparison, on fixed-point numbers + only. + +`(lt:M X Y)' +`(ltu:M X Y)' + Like `gt' and `gtu' but test for "less than". + +`(ge:M X Y)' +`(geu:M X Y)' + Like `gt' and `gtu' but test for "greater than or equal". + +`(le:M X Y)' +`(leu:M X Y)' + Like `gt' and `gtu' but test for "less than or equal". + +`(if_then_else COND THEN ELSE)' + This is not a comparison operation but is listed here because it is + always used in conjunction with a comparison operation. To be + precise, COND is a comparison expression. This expression + represents a choice, according to COND, between the value + represented by THEN and the one represented by ELSE. + + On most machines, `if_then_else' expressions are valid only to + express conditional jumps. + +`(cond [TEST1 VALUE1 TEST2 VALUE2 ...] DEFAULT)' + Similar to `if_then_else', but more general. Each of TEST1, + TEST2, ... is performed in turn. The result of this expression is + the VALUE corresponding to the first non-zero test, or DEFAULT if + none of the tests are non-zero expressions. + + This is currently not valid for instruction patterns and is + supported only for insn attributes. *Note Insn Attributes::. + + +File: gcc.info, Node: Bit Fields, Next: Conversions, Prev: Comparisons, Up: RTL + +Bit Fields +========== + + Special expression codes exist to represent bitfield instructions. +These types of expressions are lvalues in RTL; they may appear on the +left side of an assignment, indicating insertion of a value into the +specified bit field. + +`(sign_extract:M LOC SIZE POS)' + This represents a reference to a sign-extended bit field contained + or starting in LOC (a memory or register reference). The bit field + is SIZE bits wide and starts at bit POS. The compilation option + `BITS_BIG_ENDIAN' says which end of the memory unit POS counts + from. + + If LOC is in memory, its mode must be a single-byte integer mode. + If LOC is in a register, the mode to use is specified by the + operand of the `insv' or `extv' pattern (*note Standard Names::.) + and is usually a full-word integer mode. + + The mode of POS is machine-specific and is also specified in the + `insv' or `extv' pattern. + + The mode M is the same as the mode that would be used for LOC if + it were a register. + +`(zero_extract:M LOC SIZE POS)' + Like `sign_extract' but refers to an unsigned or zero-extended bit + field. The same sequence of bits are extracted, but they are + filled to an entire word with zeros instead of by sign-extension. + + +File: gcc.info, Node: Conversions, Next: RTL Declarations, Prev: Bit Fields, Up: RTL + +Conversions +=========== + + All conversions between machine modes must be represented by +explicit conversion operations. For example, an expression which is +the sum of a byte and a full word cannot be written as `(plus:SI +(reg:QI 34) (reg:SI 80))' because the `plus' operation requires two +operands of the same machine mode. Therefore, the byte-sized operand +is enclosed in a conversion operation, as in + + (plus:SI (sign_extend:SI (reg:QI 34)) (reg:SI 80)) + + The conversion operation is not a mere placeholder, because there +may be more than one way of converting from a given starting mode to +the desired final mode. The conversion operation code says how to do +it. + + For all conversion operations, X must not be `VOIDmode' because the +mode in which to do the conversion would not be known. The conversion +must either be done at compile-time or X must be placed into a register. + +`(sign_extend:M X)' + Represents the result of sign-extending the value X to machine + mode M. M must be a fixed-point mode and X a fixed-point value of + a mode narrower than M. + +`(zero_extend:M X)' + Represents the result of zero-extending the value X to machine + mode M. M must be a fixed-point mode and X a fixed-point value of + a mode narrower than M. + +`(float_extend:M X)' + Represents the result of extending the value X to machine mode M. + m must be a floating point mode and X a floating point value of a + mode narrower than M. + +`(truncate:M X)' + Represents the result of truncating the value X to machine mode M. + M must be a fixed-point mode and X a fixed-point value of a mode + wider than M. + +`(float_truncate:M X)' + Represents the result of truncating the value X to machine mode M. + M must be a floating point mode and X a floating point value of a + mode wider than M. + +`(float:M X)' + Represents the result of converting fixed point value X, regarded + as signed, to floating point mode M. + +`(unsigned_float:M X)' + Represents the result of converting fixed point value X, regarded + as unsigned, to floating point mode M. + +`(fix:M X)' + When M is a fixed point mode, represents the result of converting + floating point value X to mode M, regarded as signed. How + rounding is done is not specified, so this operation may be used + validly in compiling C code only for integer-valued operands. + +`(unsigned_fix:M X)' + Represents the result of converting floating point value X to + fixed point mode M, regarded as unsigned. How rounding is done is + not specified. + +`(fix:M X)' + When M is a floating point mode, represents the result of + converting floating point value X (valid for mode M) to an + integer, still represented in floating point mode M, by rounding + towards zero. + + +File: gcc.info, Node: RTL Declarations, Next: Side Effects, Prev: Conversions, Up: RTL + +Declarations +============ + + Declaration expression codes do not represent arithmetic operations +but rather state assertions about their operands. + +`(strict_low_part (subreg:M (reg:N R) 0))' + This expression code is used in only one context: as the + destination operand of a `set' expression. In addition, the + operand of this expression must be a non-paradoxical `subreg' + expression. + + The presence of `strict_low_part' says that the part of the + register which is meaningful in mode N, but is not part of mode M, + is not to be altered. Normally, an assignment to such a subreg is + allowed to have undefined effects on the rest of the register when + M is less than a word. - \ No newline at end of file