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

[Jeff Law <law at redhat dot com>]

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:
>
> (define_split
>   [(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;
>   else
>     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.

Jeff