Re: [match.pd PATCH] PR tree-optimization/126467: 0.0-x -> -x vs. signed zeros.

On Wed, Aug 5, 2026 at 1:06 AM Roger Sayle <[email protected]> wrote:
>
>
> This is my proposed solution to PR tree-optimization/126467, where we're
> inappropriately converting 0.0 - x to -x when we honor IEEE signed zeros.
> This transformation is valid with -Ofast, but by default +0.0 - +0.0
> should return +0.0, but -(+0.0) is -0.0.  Likewise when x is NaN, 0.0 - x
> may change the payload, but -x is guaranteed not to.  My fix is to
> separate the logic for this transformation from that for FP addition.
>
> Technically, we could do slightly better by introducing a
> tree_expr_negative_p (complementing and mutually recursive with the
> existing tree_expr_nonnegative_p), but that's a bigger change and
> less suitable for backporting to release branches, i.e. a follow-up.
>
> I agree with Alexander Monakov that an alternate fix might be to
> correctly reuse the existing fold_real_zero_addition_p functionality
> by constructing and garbage collecting a NEGATE_EXPR tree on each call,
> but this seems a little less efficient.
>
> This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> and make -k check, both with and without --target_board=unix{-m32}
> with no new failures.  Ok for mainline?

`!HONOR_NANS (type) || !tree_expr_maybe_nan_p (@1)`
You don't need the HONOR_NANS part here since tree_expr_maybe_nan_p
already handles that.
Otherwise ok.

Thanks,
Andrea

>
>
> 2026-08-05  Roger Sayle  <[email protected]>
>
> gcc/ChangeLog
>         PR tree-optimization/126467
>         * match.pd (0.0 - x -> -x): Update the conditions under which
>         the transformation is performed, disallowing x = +0.0 when we
>         honor signed zeros.
>
> gcc/testsuite/ChangeLog
>         PR tree-optimization/126467
>         * gcc.dg/pr126467-1.c: New test case.
>         * gcc.dg/pr126467-2.c: Likewise.
>         * gcc.dg/pr96392.c: Fix incorrect test case.
>