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From: "roger at nextmovesoftware dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/103406] gcc -O0 behaves differently on "DBL_MAX
 related operations" than gcc -O1 and above
Date: Thu, 25 Nov 2021 19:40:16 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103406

Roger Sayle <roger at nextmovesoftware dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |NEW
           Assignee|roger at nextmovesoftware dot com  |unassigned at gcc d=
ot gnu.org
            Summary|[12 Regression] gcc -O0     |gcc -O0 behaves differently
                   |behaves differently on      |on "DBL_MAX related
                   |"DBL_MAX related            |operations" than gcc -O1
                   |operations" than gcc -O1    |and above
                   |and above                   |
             Target|                            |x86_64

--- Comment #13 from Roger Sayle <roger at nextmovesoftware dot com> ---
The Inf - Inf =3D> 0.0 regression should now be fixed on mainline.

Hmm.  As hinted by Richard Beiner's investigation, the underlying problem is
even more pervasive.  It turns out that on x86/IA64 chips, floating point
addition is not commutative, i.e. x+y is not the same as y+x, as demonstrat=
ed
by the test program below:

#include <stdio.h>

const double pn =3D __builtin_nan("");
const double mn =3D -__builtin_nan("");

__attribute__ ((noinline, noclone))
double plus(double x, double y)
{
  return x + y;
}

int main()
{
  printf("%lf\n",plus(pn,mn));
  printf("%lf\n",plus(mn,pn));
  return 0;
}

Output:
nan
-nan

Unfortunately, GCC assumes almost everywhere the FP addition is commutative
and (as per comments #8 and #9) associative with negation/minus.  This appe=
ars
to be target property, c.f. libgcc's _FP_CHOOSENAN, but could in theory be
resolved by a -fstrict-math mode (that implies -ftrapping-math) that disabl=
es
commutativity (swapping of operands) throughout the compiler, including
reload/fold-const etc., on affected Intel-like targets.
Perhaps this PR is a duplicate now that the regression has been fixed?=