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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/96793] __builtin_floor produces wrong result when
 rounding direction is FE_DOWNWARD
Date: Fri, 28 Aug 2020 09:40:07 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96793

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |glisse at gcc dot gnu.org

--- Comment #10 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Uro=C5=A1 Bizjak from comment #7)
> Created attachment 49144 [details]
> Proposed patch
>=20
> Patch in testing.

OTOH we _do_ try to compensate for HONOR_SIGNED_ZEROS so I wonder whether
we really need to punt for -frounding-math?  I also doubt glibc gets it
correct, does it?

I guess

  /* Compensate: xa =3D xa - (xa > operand1 ? 1 : 0) */
  tmp =3D ix86_expand_sse_compare_mask (UNGT, xa, res, !do_floor);
  emit_insn (gen_rtx_SET (tmp, gen_rtx_AND (mode, one, tmp)));
  tmp =3D expand_simple_binop (mode, do_floor ? MINUS : PLUS,
                             xa, tmp, NULL_RTX, 0, OPTAB_DIRECT);

might be in error for FE_DOWNWARD if 0 - 0 behaves differently depending
on rounding mode...  or the compensation is off

  if (HONOR_SIGNED_ZEROS (mode))
    ix86_sse_copysign_to_positive (res, res, force_reg (mode, operand1), ma=
sk);=