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From: "ubizjak at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/98420] New: Invalid simplification of x - x with
 -frounding-math
Date: Tue, 22 Dec 2020 14:56:35 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98420

            Bug ID: 98420
           Summary: Invalid simplification of x - x with -frounding-math
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ubizjak at gmail dot com
  Target Milestone: ---

Split out from PR96793, where Marc says in c13:

-q-
x-x does depend on the rounding mode (the transformation in match.pd gets it
wrong, by the way).
-/q-

The testcase:

--cut here--
#include <fenv.h>

double
__attribute__((noinline))
foo (double a)
{
  return a - a;
}

int
main ()
{
  double res;

  fesetround (FE_DOWNWARD);

  res =3D foo (1.11);

  if (__builtin_signbit (res) =3D=3D 0)
    __builtin_abort ();

  return 0;
}
--cut here--

aborts with -O2 -ffinite-math-only -frounding-math

Please note that x - x with FE_DOWNWARD should result in -0.0.

The problem is in match.pd, where:

(simplify
 (minus @0 @0)
 (if (!FLOAT_TYPE_P (type) || !HONOR_NANS (type))
  { build_zero_cst (type); }))

should involve flag_rounding_math in some way.=