Here are constraint modifier characters.
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.
If you specify
+ in a constraint, you put it in the
first character of the constraint string.
& applies only to the alternative in which it is written. In
constraints with multiple alternatives, sometimes one alternative
& 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
(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")))] ...)
*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
constraint letter (for data register) is ignored when computing
(define_insn "extendhisi2" [(set (match_operand:SI 0 "general_operand" "=*d,a") (sign_extend:SI (match_operand:HI 1 "general_operand" "0,g")))] ...)