On Mon, Aug 29, 2022 at 10:45 PM Aldy Hernandez via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> The upcoming work for frange triggers a regression in
> gcc.dg/tree-ssa/phi-opt-24.c.
>
> For -O2 -fno-signed-zeros, we fail to transform the following into -A:
>
> 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.
>
> Adding a variant to the pattern that for real_zerop fixes the problem.
>
> I am a match.pd weenie, and am avoiding touching more patterns that
> may also benefit from handling real constants. So please use simple
> words if what I'm doing isn't correct ;-).
>
> I did not include a testcase, as it's just phi-opt-24.c which will get
> triggered when I commit the frange with endpoints work.
>
> Tested on x86-64 & ppc64le Linux.
>
> OK?
>
> gcc/ChangeLog:
>
> * match.pd ((cmp @0 zerop) real_zerop (negate@1 @0)): Add variant
> for real zero.
> ---
> gcc/match.pd | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/gcc/match.pd b/gcc/match.pd
> index 1bb936fc401..5bdea300f94 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -4803,6 +4803,10 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
> (cnd (cmp @0 zerop) @0 (negate@1 @0))
> (if (!HONOR_SIGNED_ZEROS (type))
> @1))
> + (simplify
> + (cnd (cmp @0 zerop) real_zerop (negate@1 @0))
> + (if (!HONOR_SIGNED_ZEROS (type))
> + @1))
> (simplify
> (cnd (cmp @0 zerop) integer_zerop (negate@1 @0))
> (if (!HONOR_SIGNED_ZEROS (type))
Looking at zerop:
bool
zerop (const_tree expr)
{
return (integer_zerop (expr)
|| real_zerop (expr)
|| fixed_zerop (expr));
}
your patch is equivalent to changing the integer_zerop one to use zerop?
(well, with the exception of also covering fixed_zerop).
OK if doing it that way.
Thanks,
Richard.
> --
> 2.37.1
>
