My question is: where and how would you suggest we do this
optimization. With peephole2? Or in combine? In i386.md, I see pattern
*subsi_2 looks like what I'd like to combine these two insn into:
(define_insn "*subsi_2"
[(set (reg FLAGS_REG)
(compare
(minus:SI (match_operand:SI 1 "nonimmediate_operand" "0,0")
(match_operand:SI 2 "general_operand" "ri,rm"))
(const_int 0)))
(set (match_operand:SI 0 "nonimmediate_operand" "=rm,r")
(minus:SI (match_dup 1) (match_dup 2)))]
"ix86_match_ccmode (insn, CCGOCmode)
&& ix86_binary_operator_ok (MINUS, SImode, operands)"
"sub{l}\t{%2, %0|%0, %2}"
[(set_attr "type" "alu")
(set_attr "mode" "SI")])
That's quite similar to several PPC patterns for andi. , and they work.
If you've got two other insns that look like the set's I'd expect
combine
to merge them, and would look to see why it doesn't.
