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(gcc-295.info)Machine Constraints

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Constraints for Particular Machines
-----------------------------------

   Whenever possible, you should use the general-purpose constraint
letters in `asm' arguments, since they will convey meaning more readily
to people reading your code.  Failing that, use the constraint letters
that usually have very similar meanings across architectures.  The most
commonly used constraints are `m' and `r' (for memory and
general-purpose registers respectively; Note: Simple Constraints.),
and `I', usually the letter indicating the most common
immediate-constant format.

   For each machine architecture, the `config/MACHINE.h' file defines
additional constraints.  These constraints are used by the compiler
itself for instruction generation, as well as for `asm' statements;
therefore, some of the constraints are not particularly interesting for
`asm'.  The constraints are defined through these macros:

`REG_CLASS_FROM_LETTER'
     Register class constraints (usually lower case).

`CONST_OK_FOR_LETTER_P'
     Immediate constant constraints, for non-floating point constants of
     word size or smaller precision (usually upper case).

`CONST_DOUBLE_OK_FOR_LETTER_P'
     Immediate constant constraints, for all floating point constants
     and for constants of greater than word size precision (usually
     upper case).

`EXTRA_CONSTRAINT'
     Special cases of registers or memory.  This macro is not required,
     and is only defined for some machines.

   Inspecting these macro definitions in the compiler source for your
machine is the best way to be certain you have the right constraints.
However, here is a summary of the machine-dependent constraints
available on some particular machines.

*ARM family--`arm.h'*

    `f'
          Floating-point register

    `F'
          One of the floating-point constants 0.0, 0.5, 1.0, 2.0, 3.0,
          4.0, 5.0 or 10.0

    `G'
          Floating-point constant that would satisfy the constraint `F'
          if it were negated

    `I'
          Integer that is valid as an immediate operand in a data
          processing instruction.  That is, an integer in the range 0
          to 255 rotated by a multiple of 2

    `J'
          Integer in the range -4095 to 4095

    `K'
          Integer that satisfies constraint `I' when inverted (ones
          complement)

    `L'
          Integer that satisfies constraint `I' when negated (twos
          complement)

    `M'
          Integer in the range 0 to 32

    `Q'
          A memory reference where the exact address is in a single
          register (``m'' is preferable for `asm' statements)

    `R'
          An item in the constant pool

    `S'
          A symbol in the text segment of the current file

*AMD 29000 family--`a29k.h'*

    `l'
          Local register 0

    `b'
          Byte Pointer (`BP') register

    `q'
          `Q' register

    `h'
          Special purpose register

    `A'
          First accumulator register

    `a'
          Other accumulator register

    `f'
          Floating point register

    `I'
          Constant greater than 0, less than 0x100

    `J'
          Constant greater than 0, less than 0x10000

    `K'
          Constant whose high 24 bits are on (1)

    `L'
          16 bit constant whose high 8 bits are on (1)

    `M'
          32 bit constant whose high 16 bits are on (1)

    `N'
          32 bit negative constant that fits in 8 bits

    `O'
          The constant 0x80000000 or, on the 29050, any 32 bit constant
          whose low 16 bits are 0.

    `P'
          16 bit negative constant that fits in 8 bits

    `G'
    `H'
          A floating point constant (in `asm' statements, use the
          machine independent `E' or `F' instead)

*IBM RS6000--`rs6000.h'*

    `b'
          Address base register

    `f'
          Floating point register

    `h'
          `MQ', `CTR', or `LINK' register

    `q'
          `MQ' register

    `c'
          `CTR' register

    `l'
          `LINK' register

    `x'
          `CR' register (condition register) number 0

    `y'
          `CR' register (condition register)

    `z'
          `FPMEM' stack memory for FPR-GPR transfers

    `I'
          Signed 16 bit constant

    `J'
          Constant whose low 16 bits are 0

    `K'
          Constant whose high 16 bits are 0

    `L'
          Constant suitable as a mask operand

    `M'
          Constant larger than 31

    `N'
          Exact power of 2

    `O'
          Zero

    `P'
          Constant whose negation is a signed 16 bit constant

    `G'
          Floating point constant that can be loaded into a register
          with one instruction per word

    `Q'
          Memory operand that is an offset from a register (`m' is
          preferable for `asm' statements)

    `R'
          AIX TOC entry

    `S'
          Constant suitable as a 64-bit mask operand

    `U'
          System V Release 4 small data area reference

*Intel 386--`i386.h'*

    `q'
          `a', `b', `c', or `d' register

    `A'
          `a', or `d' register (for 64-bit ints)

    `f'
          Floating point register

    `t'
          First (top of stack) floating point register

    `u'
          Second floating point register

    `a'
          `a' register

    `b'
          `b' register

    `c'
          `c' register

    `d'
          `d' register

    `D'
          `di' register

    `S'
          `si' register

    `I'
          Constant in range 0 to 31 (for 32 bit shifts)

    `J'
          Constant in range 0 to 63 (for 64 bit shifts)

    `K'
          `0xff'

    `L'
          `0xffff'

    `M'
          0, 1, 2, or 3 (shifts for `lea' instruction)

    `N'
          Constant in range 0 to 255 (for `out' instruction)

    `G'
          Standard 80387 floating point constant

*Intel 960--`i960.h'*

    `f'
          Floating point register (`fp0' to `fp3')

    `l'
          Local register (`r0' to `r15')

    `b'
          Global register (`g0' to `g15')

    `d'
          Any local or global register

    `I'
          Integers from 0 to 31

    `J'
          0

    `K'
          Integers from -31 to 0

    `G'
          Floating point 0

    `H'
          Floating point 1

*MIPS--`mips.h'*

    `d'
          General-purpose integer register

    `f'
          Floating-point register (if available)

    `h'
          `Hi' register

    `l'
          `Lo' register

    `x'
          `Hi' or `Lo' register

    `y'
          General-purpose integer register

    `z'
          Floating-point status register

    `I'
          Signed 16 bit constant (for arithmetic instructions)

    `J'
          Zero

    `K'
          Zero-extended 16-bit constant (for logic instructions)

    `L'
          Constant with low 16 bits zero (can be loaded with `lui')

    `M'
          32 bit constant which requires two instructions to load (a
          constant which is not `I', `K', or `L')

    `N'
          Negative 16 bit constant

    `O'
          Exact power of two

    `P'
          Positive 16 bit constant

    `G'
          Floating point zero

    `Q'
          Memory reference that can be loaded with more than one
          instruction (`m' is preferable for `asm' statements)

    `R'
          Memory reference that can be loaded with one instruction (`m'
          is preferable for `asm' statements)

    `S'
          Memory reference in external OSF/rose PIC format (`m' is
          preferable for `asm' statements)

*Motorola 680x0--`m68k.h'*

    `a'
          Address register

    `d'
          Data register

    `f'
          68881 floating-point register, if available

    `x'
          Sun FPA (floating-point) register, if available

    `y'
          First 16 Sun FPA registers, if available

    `I'
          Integer in the range 1 to 8

    `J'
          16 bit signed number

    `K'
          Signed number whose magnitude is greater than 0x80

    `L'
          Integer in the range -8 to -1

    `M'
          Signed number whose magnitude is greater than 0x100

    `G'
          Floating point constant that is not a 68881 constant

    `H'
          Floating point constant that can be used by Sun FPA

*SPARC--`sparc.h'*

    `f'
          Floating-point register that can hold 32 or 64 bit values.

    `e'
          Floating-point register that can hold 64 or 128 bit values.

    `I'
          Signed 13 bit constant

    `J'
          Zero

    `K'
          32 bit constant with the low 12 bits clear (a constant that
          can be loaded with the `sethi' instruction)

    `G'
          Floating-point zero

    `H'
          Signed 13 bit constant, sign-extended to 32 or 64 bits

    `Q'
          Memory reference that can be loaded with one instruction
          (`m' is more appropriate for `asm' statements)

    `S'
          Constant, or memory address

    `T'
          Memory address aligned to an 8-byte boundary

    `U'
          Even register