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Re: combination of read/write and earlyclobber constraint modifier

On 01-07-14 21:58, Marc Glisse wrote:
So my question is: is the combination of '&' and '+' supported ? If so,
what is the exact semantics ? If not, should we warn or give an error ?
I don't think we can define any reasonable semantics for &+.  My
recommendation would be for this to be considered a hard error.

Uh? The doc explicitly says "An input operand can be tied to an earlyclobber
operand" and goes on to explain why that is useful. It avoids using the same
register for other input when they are identical.

Hi Marc,

That part of the doc refers to the mulsi3 insn for ARM as example:
;; Use `&' and then `0' to prevent the operands 0 and 1 being the same
(define_insn "*arm_mulsi3"
  [(set (match_operand:SI          0 "s_register_operand" "=&r,&r")
        (mult:SI (match_operand:SI 2 "s_register_operand" "r,r")
                 (match_operand:SI 1 "s_register_operand" "%0,r")))]
  "TARGET_32BIT && !arm_arch6"
  "mul%?\\t%0, %2, %1"
  [(set_attr "type" "mul")
   (set_attr "predicable" "yes")]

Note that there's no combination of & and + here.

AFAIU, the 'tie' established here is from input operand 1 to an earlyclobber output operand 0 using the '0' matching constraint.

Having said that, I don't understand the comment, AFAIU it should be: 'Use '0' to make sure operands 0 and 1 are the same, and use '&' to make sure operands 0 and 2 are not the same.

- Tom

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