Re: [PATCH] PR middle-end/98420: Don't fold x - x to 0.0 with -frounding-math

[Richard Biener <richard.guenther@gmail.com>]

On Fri, Mar 11, 2022 at 12:31 AM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> This patch addresses PR middle-end/98420, which is inappropriate constant
> folding of x - x to 0.0 (in match.pd) when -frounding-math is specified.
> Specifically, x - x may be -0.0 with FE_DOWNWARD as the rounding mode.
>
> To summarize, the desired IEEE behaviour, x - x for floating point x,
> (1) can't be folded to 0.0 by default, due to the possibility of NaN or Inf
> (2) can be folded to 0.0 with -ffinite-math-only
> (3) can't be folded to 0.0 with -ffinite-math-only -frounding-math
> (4) can be folded with -ffinite-math-only -frounding-math -fno-signed-zeros
>
> Technically, this is a regression from GCC 4.1 (according to godbolt.org)
> so hopefully this patch is suitable during stage4.
>
> This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> and make -k check with no new failures.  Ok for mainline?

+         && !tree_expr_maybe_infinite_p (@0)
+         && (!flag_rounding_math || !HONOR_SIGNED_ZEROS (type))))
   { build_zero_cst (type); }))

HONOR_SIGN_DEPENDENT_ROUNDING (type) instead of flag_rounding_math?

OK with that change.

Richard.

>
> 2022-03-10  Roger Sayle  <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
>         PR middle-end/98420
>         * match.pd (minus @0 @0): Additional checks for -fno-rounding-math
>         (the defaut) or -fno-signed-zeros.
>
> gcc/testsuite/ChangeLog
>         PR middle-end/98420
>         * gcc.dg/pr98420.c: New test case.
>
>
> Thanks in advance,
> Roger
> --
>