GNU Info
(gcc-295.info)Modifiers
Constraint Modifier Characters
------------------------------
Here are constraint modifier characters.
`='
Means that this operand is write-only for this instruction: the
previous value is discarded and replaced by output data.
`+'
Means that this operand is both read and written by the
instruction.
When the compiler fixes up the operands to satisfy the constraints,
it needs to know which operands are inputs to the instruction and
which are outputs from it. `=' identifies an output; `+'
identifies an operand that is both input and output; all other
operands are assumed to be input only.
`&'
Means (in a particular alternative) that this operand is an
"earlyclobber" operand, which is modified before the instruction is
finished using the input operands. Therefore, this operand may
not lie in a register that is used as an input operand or as part
of any memory address.
`&' applies only to the alternative in which it is written. In
constraints with multiple alternatives, sometimes one alternative
requires `&' while others do not. See, for example, the `movdf'
insn of the 68000.
An input operand can be tied to an earlyclobber operand if its only
use as an input occurs before the early result is written. Adding
alternatives of this form often allows GCC to produce better code
when only some of the inputs can be affected by the earlyclobber.
See, for example, the `mulsi3' insn of the ARM.
`&' does not obviate the need to write `='.
`%'
Declares the instruction to be commutative for this operand and the
following operand. This means that the compiler may interchange
the two operands if that is the cheapest way to make all operands
fit the constraints. This is often used in patterns for addition
instructions that really have only two operands: the result must
go in one of the arguments. Here for example, is how the 68000
halfword-add instruction is defined:
(define_insn "addhi3"
[(set (match_operand:HI 0 "general_operand" "=m,r")
(plus:HI (match_operand:HI 1 "general_operand" "%0,0")
(match_operand:HI 2 "general_operand" "di,g")))]
...)
`#'
Says that all following characters, up to the next comma, are to be
ignored as a constraint. They are significant only for choosing
register preferences.
`*'
Says that the following character should be ignored when choosing
register preferences. `*' has no effect on the meaning of the
constraint as a constraint, and no effect on reloading.
Here is an example: the 68000 has an instruction to sign-extend a
halfword in a data register, and can also sign-extend a value by
copying it into an address register. While either kind of
register is acceptable, the constraints on an address-register
destination are less strict, so it is best if register allocation
makes an address register its goal. Therefore, `*' is used so
that the `d' constraint letter (for data register) is ignored when
computing register preferences.
(define_insn "extendhisi2"
[(set (match_operand:SI 0 "general_operand" "=*d,a")
(sign_extend:SI
(match_operand:HI 1 "general_operand" "0,g")))]
...)
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