On Fri, Aug 26, 2022 at 8:55 AM Aldy Hernandez <al...@redhat.com> wrote:
>
> [pinskia: I'm CCing you as the author of the match.pd pattern.]
>
> So, as I wrap up the work here (latest patch attached), I see there's
> another phiopt regression (not spaceship related). I was hoping
> someone could either give me a hand, or offer some guidance.
>
> The failure is in gcc.dg/tree-ssa/phi-opt-24.c.
>
> We fail to transform the following into -A:
>
> /* { dg-options "-O2 -fno-signed-zeros -fdump-tree-phiopt" } */
>
> float f0(float A)
> {
> // A == 0? A : -A same as -A
> if (A == 0) return A;
> return -A;
> }
>
> This is because the abs/negative match.pd pattern here:
>
> /* abs/negative simplifications moved from fold_cond_expr_with_comparison,
> Need to handle (A - B) case as fold_cond_expr_with_comparison does.
> Need to handle UN* comparisons.
> ...
> ...
>
> Sees IL that has the 0.0 propagated.
>
> Instead of:
>
> <bb 2> [local count: 1073741824]:
> if (A_2(D) == 0.0)
> goto <bb 4>; [34.00%]
> else
> goto <bb 3>; [66.00%]
>
> <bb 3> [local count: 708669601]:
> _3 = -A_2(D);
>
> <bb 4> [local count: 1073741824]:
> # _1 = PHI <A_2(D)(2), _3(3)>
>
> It now sees:
>
> <bb 4> [local count: 1073741824]:
> # _1 = PHI <0.0(2), _3(3)>
>
> which it leaves untouched, causing the if conditional to survive.
>
> Is this something that can be done by improving the match.pd pattern,
> or should be done elsewhere?
Oh the pattern which is supposed to catch this does:
(simplify
(cnd (cmp @0 zerop) integer_zerop (negate@1 @0))
(if (!HONOR_SIGNED_ZEROS (type))
@1))
Notice the integer_zerop here.
fold_cond_expr_with_comparison has integer_zerop there too.
I am not 100% sure you can replace A_2 with 0.0 where you are doing it
as mentioned in another thread.
Thanks,
Andrew Pinski
>
> Thanks.
> Aldy
