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Re: Init df for split pass since some target use REG_NOTE in split pattern

On 07/25/2016 08:31 PM, Kito Cheng wrote:
Hi Jeff:

Oop, patch in attachment, and I hit this bug in gcc.dg/torture/vshuf-v2di.c
with our nds32 internal branch.

Hi Richard:
I think we really need reg dead note for some optimization, and btw,
here is our split pattern:

  [(set (match_operand:DI 0 "nds32_general_register_operand" "")
        (match_operand:DI 1 "nds32_general_register_operand" ""))]
  "find_regno_note (insn, REG_UNUSED, REGNO (operands[0])) != NULL
   || find_regno_note (insn, REG_UNUSED, REGNO (operands[0]) + 1) != NULL"
  [(set (match_dup 0) (match_dup 1))]
  rtx dead_note = find_regno_note (curr_insn, REG_UNUSED, REGNO (operands[0]));
  HOST_WIDE_INT offset;
  if (dead_note == NULL_RTX)
    offset = 0;
    offset = 4;
  operands[0] = simplify_gen_subreg (
                  SImode, operands[0],
                  DImode, offset);
  operands[1] = simplify_gen_subreg (
                  SImode, operands[1],
                  DImode, offset);
This seems better suited as a generic optimization than hidden away in a backend.

AFAICT you're just noticing a word of the output operand is dead and eliding the load/store for that word.

In fact, I'm a bit surprised nothing has optimized this away by the time reload/LRA is done. You might spend some time walking through lower-subreg to see if it can be easily extended to handle your case.


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