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* Gcc: (gcc).                           Using and Porting the GNU C compiler.
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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 have the value 1 if the most significant bit
     in a byte has the lowest number; otherwise define it to have 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 need not be a
     constant.

     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 have the value 1 if the most significant byte
     in a word has the lowest number.  This macro need not be a
     constant.

`WORDS_BIG_ENDIAN'
     Define this macro to have the value 1 if, in a multiword object,
     the most significant word has the lowest number.  This applies to
     both memory locations and registers; GCC fundamentally assumes
     that the order of words in memory is the same as the order in
     registers.  This macro need not be a constant.

`LIBGCC2_WORDS_BIG_ENDIAN'
     Define this macro if WORDS_BIG_ENDIAN is not constant.  This must
     be a constant value with the same meaning as WORDS_BIG_ENDIAN,
     which will be used only when compiling libgcc2.c.  Typically the
     value will be set based on preprocessor defines.

`FLOAT_WORDS_BIG_ENDIAN'
     Define this macro to have the value 1 if `DFmode', `XFmode' or
     `TFmode' floating point numbers are stored in memory with the word
     containing the sign bit at the lowest address; otherwise define it
     to have the value 0.  This macro need not be a constant.

     You need not define this macro if the ordering is the same as for
     multi-word integers.

`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.

`MIN_UNITS_PER_WORD'
     Minimum number of units in a word.  If this is undefined, the
     default is `UNITS_PER_WORD'.  Otherwise, it is the constant value
     that is the smallest value that `UNITS_PER_WORD' can have at
     run-time.

`POINTER_SIZE'
     Width of a pointer, in bits.  You must specify a value no wider
     than the width of `Pmode'.  If it is not equal to the width of
     `Pmode', you must define `POINTERS_EXTEND_UNSIGNED'.

`POINTERS_EXTEND_UNSIGNED'
     A C expression whose value is nonzero if pointers that need to be
     extended from being `POINTER_SIZE' bits wide to `Pmode' are to be
     zero-extended and zero if they are to be sign-extended.

     You need not define this macro if the `POINTER_SIZE' is equal to
     the width of `Pmode'.

`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'.

`PROMOTE_FOR_CALL_ONLY'
     Define this macro if the promotion described by `PROMOTE_MODE'
     should *only* be performed for outgoing function arguments or
     function return values, as specified by `PROMOTE_FUNCTION_ARGS'
     and `PROMOTE_FUNCTION_RETURN', respectively.

`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 there is a guaranteed alignment for the stack
     pointer on this machine.  The definition is a C expression for the
     desired alignment (measured in bits).  This value is used as a
     default if PREFERRED_STACK_BOUNDARY is not defined.

`PREFERRED_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 STACK_BOUNDARY is also
     defined, this macro must evaluate to a value equal to or larger
     than STACK_BOUNDARY.

     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 `PREFERRED_STACK_BOUNDARY',
     the stack may be momentarily unaligned while pushing arguments.

`FUNCTION_BOUNDARY'
     Alignment required for a function entry point, in bits.

`FUNCTION_BOUNDARY_MAX_SKIP (MAX_SKIP)'
     If defined, the maximum number of padding bytes to insert to force
     the start a function to a particular alignment.  If MAX_SKIP is
     smaller than the number of bytes needed to satisfy an alignment
     request, then no alignment is made.  The compiler will choose a
     reasonable default value if you do not define this macro.  This
     macro has no effct if `ASM_OUTPUT_MAX_SKIP_ALIGN' is not defined.

`BIGGEST_ALIGNMENT'
     Biggest alignment that any data type can require on this machine,
     in bits.

`MINIMUM_ATOMIC_ALIGNMENT'
     If defined, the smallest alignment, in bits, that can be given to
     an object that can be referenced in one operation, without
     disturbing any nearby object.  Normally, this is `BITS_PER_UNIT',
     but may be larger on machines that don't have byte or half-word
     store operations.

`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.

`ADJUST_FIELD_ALIGN (FIELD, COMPUTED)'
     An expression for the alignment of a structure field FIELD if the
     alignment computed in the usual way is COMPUTED.  GCC uses this
     value instead of the value in `BIGGEST_ALIGNMENT' or
     `BIGGEST_FIELD_ALIGNMENT', if defined, 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
     variables in the static store.  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.

`LOCAL_ALIGNMENT (TYPE, BASIC-ALIGN)'
     If defined, a C expression to compute the alignment for a
     variables in the local store.  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.

`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 GCC 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 (TYPE, COMPUTED, SPECIFIED)'
     Define this macro as an expression for the overall size of a type
     (given by TYPE as a tree node) when the size computed in the usual
     way is COMPUTED and the alignment is SPECIFIED.

     The default is to round COMPUTED up to a multiple of SPECIFIED.

`ROUND_TYPE_ALIGN (TYPE, COMPUTED, SPECIFIED)'
     Define this macro as an expression for the alignment of a type
     (given by TYPE 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.

`VECTOR_MODE_SUPPORTED_P(MODE)'
     Define this macro to be nonzero if the port is prepared to handle
     insns involving vector mode MODE.  At the very least, it must have
     move patterns for this mode.

`STACK_SAVEAREA_MODE (SAVE_LEVEL)'
     If defined, an expression of type `enum machine_mode' that
     specifies the mode of the save area operand of a
     `save_stack_LEVEL' named pattern (*note Standard Names::.).
     SAVE_LEVEL is one of `SAVE_BLOCK', `SAVE_FUNCTION', or
     `SAVE_NONLOCAL' and selects which of the three named patterns is
     having its mode specified.

     You need not define this macro if it always returns `Pmode'.  You
     would most commonly define this macro if the `save_stack_LEVEL'
     patterns need to support both a 32- and a 64-bit mode.

`STACK_SIZE_MODE'
     If defined, an expression of type `enum machine_mode' that
     specifies the mode of the size increment operand of an
     `allocate_stack' named pattern (*note Standard Names::.).

     You need not define this macro if it always returns `word_mode'.
     You would most commonly define this macro if the `allocate_stack'
     pattern needs to support both a 32- and a 64-bit mode.

`CHECK_FLOAT_VALUE (MODE, VALUE, OVERFLOW)'
     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 a mode of class `MODE_FLOAT'.  OVERFLOW
     is nonzero if the value is already known to be out of range.

     If VALUE is not valid or if OVERFLOW is nonzero, you should set
     OVERFLOW to 1 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.

     The ordering of the component words of floating point values
     stored in memory is controlled by `FLOAT_WORDS_BIG_ENDIAN' for the
     target machine and `HOST_FLOAT_WORDS_BIG_ENDIAN' for the host.

`DEFAULT_VTABLE_THUNKS'
     GCC supports two ways of implementing C++ vtables:  traditional or
     with so-called "thunks".  The flag `-fvtable-thunk' chooses
     between them.  Define this macro to be a C expression for the
     default value of that flag.  If `DEFAULT_VTABLE_THUNKS' is 0, GCC
     uses the traditional implementation by default.  The "thunk"
     implementation is more efficient (especially if you have provided
     an implementation of `ASM_OUTPUT_MI_THUNK', see *Note Function
     Entry::), but is not binary compatible with code compiled using
     the traditional implementation.  If you are writing a new port,
     define `DEFAULT_VTABLE_THUNKS' to 1.

     If you do not define this macro, the default for `-fvtable-thunk'
     is 0.


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.

`MAX_INT_TYPE_SIZE'
     Maximum number for the size in bits of the type `int' on the target
     machine.  If this is undefined, the default is `INT_TYPE_SIZE'.
     Otherwise, it is the constant value that is the largest value that
     `INT_TYPE_SIZE' can have at run-time.  This is used in `cpp'.

`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.

`MAX_LONG_TYPE_SIZE'
     Maximum number for the size in bits of the type `long' on the
     target machine.  If this is undefined, the default is
     `LONG_TYPE_SIZE'.  Otherwise, it is the constant value that is the
     largest value that `LONG_TYPE_SIZE' can have at run-time.  This is
     used in `cpp'.

`LONG_LONG_TYPE_SIZE'
     A C expression for the size in bits of the type `long long' on the
     target machine.  If you don't define this, the default is two
     words.  If you want to support GNU Ada on your machine, the value
     of macro must be at least 64.

`CHAR_TYPE_SIZE'
     A C expression for the size in bits of the type `char' on the
     target machine.  If you don't define this, the default is one
     quarter of a word.  (If this would be less than one storage unit,
     it is rounded up to one unit.)

`MAX_CHAR_TYPE_SIZE'
     Maximum number for the size in bits of the type `char' on the
     target machine.  If this is undefined, the default is
     `CHAR_TYPE_SIZE'.  Otherwise, it is the constant value that is the
     largest value that `CHAR_TYPE_SIZE' can have at run-time.  This is
     used in `cpp'.

`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.

`WIDEST_HARDWARE_FP_SIZE'
     A C expression for the size in bits of the widest floating-point
     format supported by the hardware.  If you define this macro, you
     must specify a value less than or equal to the value of
     `LONG_DOUBLE_TYPE_SIZE'.  If you do not define this macro, the
     value of `LONG_DOUBLE_TYPE_SIZE' is the default.

`DEFAULT_SIGNED_CHAR'
     An expression whose value is 1 or 0, according to whether the type
     `char' should be signed or unsigned by default.  The user can
     always override this default with the options `-fsigned-char' and
     `-funsigned-char'.

`DEFAULT_SHORT_ENUMS'
     A C expression to determine whether to give an `enum' type only as
     many bytes as it takes to represent the range of possible values
     of that type.  A nonzero value means to do that; a zero value
     means all `enum' types should be allocated like `int'.

     If you don't define the macro, the default is 0.

`SIZE_TYPE'
     A C expression for a string describing the name of the data type
     to use for size values.  The typedef name `size_t' is defined
     using the contents of the string.

     The string can contain more than one keyword.  If so, separate
     them with spaces, and write first any length keyword, then
     `unsigned' if appropriate, and finally `int'.  The string must
     exactly match one of the data type names defined in the function
     `init_decl_processing' in the file `c-decl.c'.  You may not omit
     `int' or change the order--that would cause the compiler to crash
     on startup.

     If you don't define this macro, the default is `"long unsigned
     int"'.

`PTRDIFF_TYPE'
     A C expression for a string describing the name of the data type
     to use for the result of subtracting two pointers.  The typedef
     name `ptrdiff_t' is defined using the contents of the string.  See
     `SIZE_TYPE' above for more information.

     If you don't define this macro, the default is `"long int"'.

`WCHAR_TYPE'
     A C expression for a string describing the name of the data type
     to use for wide characters.  The typedef name `wchar_t' is defined
     using the contents of the string.  See `SIZE_TYPE' above for more
     information.

     If you don't define this macro, the default is `"int"'.

`WCHAR_TYPE_SIZE'
     A C expression for the size in bits of the data type for wide
     characters.  This is used in `cpp', which cannot make use of
     `WCHAR_TYPE'.

`MAX_WCHAR_TYPE_SIZE'
     Maximum number for the size in bits of the data type for wide
     characters.  If this is undefined, the default is
     `WCHAR_TYPE_SIZE'.  Otherwise, it is the constant value that is the
     largest value that `WCHAR_TYPE_SIZE' can have at run-time.  This is
     used in `cpp'.

`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.


File: gcc.info,  Node: Register Basics,  Next: Allocation Order,  Up: Registers

Basic Characteristics of Registers
----------------------------------

   Registers have various characteristics.

`FIRST_PSEUDO_REGISTER'
     Number of hardware registers known to the compiler.  They receive
     numbers 0 through `FIRST_PSEUDO_REGISTER-1'; thus, the first
     pseudo register's number really is assigned the number
     `FIRST_PSEUDO_REGISTER'.

`FIXED_REGISTERS'
     An initializer that says which registers are used for fixed
     purposes all throughout the compiled code and are therefore not
     available for general allocation.  These would include the stack
     pointer, the frame pointer (except on machines where that can be
     used as a general register when no frame pointer is needed), the
     program counter on machines where that is considered one of the
     addressable registers, and any other numbered register with a
     standard use.

     This information is expressed as a sequence of numbers, separated
     by commas and surrounded by braces.  The Nth number is 1 if
     register N is fixed, 0 otherwise.

     The table initialized from this macro, and the table initialized by
     the following one, may be overridden at run time either
     automatically, by the actions of the macro
     `CONDITIONAL_REGISTER_USAGE', or by the user with the command
     options `-ffixed-REG', `-fcall-used-REG' and `-fcall-saved-REG'.

`CALL_USED_REGISTERS'
     Like `FIXED_REGISTERS' but has 1 for each register that is
     clobbered (in general) by function calls as well as for fixed
     registers.  This macro therefore identifies the registers that are
     not available for general allocation of values that must live
     across function calls.

     If a register has 0 in `CALL_USED_REGISTERS', the compiler
     automatically saves it on function entry and restores it on
     function exit, if the register is used within the function.

`HARD_REGNO_CALL_PART_CLOBBERED (REGNO, MODE)'
     A C expression that is non-zero if it is not permissible to store a
     value of mode MODE in hard register number REGNO across a call
     without some part of it being clobbered.  For most machines this
     macro need not be defined.  It is only required for machines that
     do not preserve the entire contents of a register across a call.

`CONDITIONAL_REGISTER_USAGE'
     Zero or more C statements that may conditionally modify four
     variables `fixed_regs', `call_used_regs', `global_regs' (these
     three are of type `char []') and `reg_class_contents' (of type
     `HARD_REG_SET').  Before the macro is called `fixed_regs',
     `call_used_regs' and `reg_class_contents' have been initialized
     from `FIXED_REGISTERS', `CALL_USED_REGISTERS' and
     `REG_CLASS_CONTENTS', respectively, `global_regs' has been
     cleared, and any `-ffixed-REG', `-fcall-used-REG' and
     `-fcall-saved-REG' command options have been applied.

     This is necessary in case the fixed or call-clobbered registers
     depend on target flags.

     You need not define this macro if it has no work to do.

     If the usage of an entire class of registers depends on the target
     flags, you may indicate this to GCC by using this macro to modify
     `fixed_regs' and `call_used_regs' to 1 for each of the registers
     in the classes which should not be used by GCC.  Also define the
     macro `REG_CLASS_FROM_LETTER' to return `NO_REGS' if it is called
     with a letter for a class that shouldn't be used.

     (However, if this class is not included in `GENERAL_REGS' and all
     of the insn patterns whose constraints permit this class are
     controlled by target switches, then GCC will automatically avoid
     using these registers when the target switches are opposed to
     them.)

`NON_SAVING_SETJMP'
     If this macro is defined and has a nonzero value, it means that
     `setjmp' and related functions fail to save the registers, or that
     `longjmp' fails to restore them.  To compensate, the compiler
     avoids putting variables in registers in functions that use
     `setjmp'.

`INCOMING_REGNO (OUT)'
     Define this macro if the target machine has register windows.
     This C expression returns the register number as seen by the
     called function corresponding to the register number OUT as seen
     by the calling function.  Return OUT if register number OUT is not
     an outbound register.

`OUTGOING_REGNO (IN)'
     Define this macro if the target machine has register windows.
     This C expression returns the register number as seen by the
     calling function corresponding to the register number IN as seen
     by the called function.  Return IN if register number IN is not an
     inbound register.


File: gcc.info,  Node: Allocation Order,  Next: Values in Registers,  Prev: Register Basics,  Up: Registers

Order of Allocation of Registers
--------------------------------

   Registers are allocated in order.

`REG_ALLOC_ORDER'
     If defined, an initializer for a vector of integers, containing the
     numbers of hard registers in the order in which GCC should prefer
     to use them (from most preferred to least).

     If this macro is not defined, registers are used lowest numbered
     first (all else being equal).

     One use of this macro is on machines where the highest numbered
     registers must always be saved and the save-multiple-registers
     instruction supports only sequences of consecutive registers.  On
     such machines, define `REG_ALLOC_ORDER' to be an initializer that
     lists the highest numbered allocable register first.

`ORDER_REGS_FOR_LOCAL_ALLOC'
     A C statement (sans semicolon) to choose the order in which to
     allocate hard registers for pseudo-registers local to a basic
     block.

     Store the desired register order in the array `reg_alloc_order'.
     Element 0 should be the register to allocate first; element 1, the
     next register; and so on.

     The macro body should not assume anything about the contents of
     `reg_alloc_order' before execution of the macro.

     On most machines, it is not necessary to define this macro.


File: 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)

`ALTER_HARD_SUBREG (TGT_MODE, WORD, SRC_MODE, REGNO)'
     A C expression that returns an adjusted hard register number for

          (subreg:TGT_MODE (reg:SRC_MODE REGNO) WORD)

     This may be needed if the target machine has mixed sized big-endian
     registers, like Sparc v9.

`HARD_REGNO_MODE_OK (REGNO, MODE)'
     A C expression that is nonzero if it is permissible to store a
     value of mode MODE in hard register number REGNO (or in several
     registers starting with that one).  For a machine where all
     registers are equivalent, a suitable definition is

          #define HARD_REGNO_MODE_OK(REGNO, MODE) 1

     You need not include code to check for the numbers of fixed
     registers, because the allocation mechanism considers them to be
     always occupied.

     On some machines, double-precision values must be kept in even/odd
     register pairs.  You can implement that by defining this macro to
     reject odd register numbers for such modes.

     The minimum requirement for a mode to be OK in a register is that
     the `movMODE' instruction pattern support moves between the
     register and other hard register in the same class and that moving
     a value into the register and back out not alter it.

     Since the same instruction used to move `word_mode' will work for
     all narrower integer modes, it is not necessary on any machine for
     `HARD_REGNO_MODE_OK' to distinguish between these modes, provided
     you define patterns `movhi', etc., to take advantage of this.  This
     is useful because of the interaction between `HARD_REGNO_MODE_OK'
     and `MODES_TIEABLE_P'; it is very desirable for all integer modes
     to be tieable.

     Many machines have special registers for floating point arithmetic.
     Often people assume that floating point machine modes are allowed
     only in floating point registers.  This is not true.  Any
     registers that can hold integers can safely *hold* a floating
     point machine mode, whether or not floating arithmetic can be done
     on it in those registers.  Integer move instructions can be used
     to move the values.

     On some machines, though, the converse is true: fixed-point machine
     modes may not go in floating registers.  This is true if the
     floating registers normalize any value stored in them, because
     storing a non-floating value there would garble it.  In this case,
     `HARD_REGNO_MODE_OK' should reject fixed-point machine modes in
     floating registers.  But if the floating registers do not
     automatically normalize, if you can store any bit pattern in one
     and retrieve it unchanged without a trap, then any machine mode
     may go in a floating register, so you can define this macro to say
     so.

     The primary significance of special floating registers is rather
     that they are the registers acceptable in floating point arithmetic
     instructions.  However, this is of no concern to
     `HARD_REGNO_MODE_OK'.  You handle it by writing the proper
     constraints for those instructions.

     On some machines, the floating registers are especially slow to
     access, so that it is better to store a value in a stack frame
     than in such a register if floating point arithmetic is not being
     done.  As long as the floating registers are not in class
     `GENERAL_REGS', they will not be used unless some pattern's
     constraint asks for one.

`MODES_TIEABLE_P (MODE1, MODE2)'
     A C expression that is nonzero if a value of mode MODE1 is
     accessible in mode MODE2 without copying.

     If `HARD_REGNO_MODE_OK (R, MODE1)' and `HARD_REGNO_MODE_OK (R,
     MODE2)' are always the same for any R, then `MODES_TIEABLE_P
     (MODE1, MODE2)' should be nonzero.  If they differ for any R, you
     should define this macro to return zero unless some other
     mechanism ensures the accessibility of the value in a narrower
     mode.

     You should define this macro to return nonzero in as many cases as
     possible since doing so will allow GCC to perform better register
     allocation.

`AVOID_CCMODE_COPIES'
     Define this macro if the compiler should avoid copies to/from
     `CCmode' registers.  You should only define this macro if support
     for copying to/from `CCmode' is incomplete.


File: 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".

   GCC assigns register numbers before it knows whether the function is
suitable for leaf function treatment.  So it needs to renumber the
registers in order to output a leaf function.  The following macros
accomplish this.

`LEAF_REGISTERS'
     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 GCC would ordinarily allocate.  The
     registers which will actually be used in the assembler code, after
     renumbering, should not be marked with 1 in this vector.

     Define this macro only if the target machine offers a way to
     optimize the treatment of leaf functions.

`LEAF_REG_REMAP (REGNO)'
     A C expression whose value is the register number to which REGNO
     should be renumbered, when a function is treated as a leaf
     function.

     If REGNO is a register number which should not appear in a leaf
     function before renumbering, then the expression should yield -1,
     which will cause the compiler to abort.

     Define this macro only if the target machine offers a way to
     optimize the treatment of leaf functions, and registers need to be
     renumbered to do this.

   Normally, `FUNCTION_PROLOGUE' and `FUNCTION_EPILOGUE' must treat
leaf functions specially.  They can test the C variable
`current_function_is_leaf' which is nonzero for leaf functions.
`current_function_is_leaf' is set prior to local register allocation
and is valid for the remaining compiler passes.  They can also test the
C variable `current_function_uses_only_leaf_regs' which is nonzero for
leaf functions which only use leaf registers.
`current_function_uses_only_leaf_regs' is valid after reload and is
only useful if `LEAF_REGISTERS' is defined.


File: gcc.info,  Node: Stack Registers,  Prev: Leaf Functions,  Up: Registers

Registers That Form a Stack
---------------------------

   There are special features to handle computers where some of the
"registers" form a stack, as in the 80387 coprocessor for the 80386.
Stack registers are normally written by pushing onto the stack, and are
numbered relative to the top of the stack.

   Currently, GCC can only handle one group of stack-like registers, and
they must be consecutively numbered.

`STACK_REGS'
     Define this if the machine has any stack-like registers.

`FIRST_STACK_REG'
     The number of the first stack-like register.  This one is the top
     of the stack.

`LAST_STACK_REG'
     The number of the last stack-like register.  This one is the
     bottom of the stack.

