[Bug c/56020] New: FE_INVALID flag not set on comparison with NAN (unordered)

[Bug c/56020] New: FE_INVALID flag not set on comparison with NAN (unordered)

[vincent-gcc at vinc17 dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56020

             Bug #: 56020
           Summary: FE_INVALID flag not set on comparison with NAN
                    (unordered)
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: vincent-gcc@vinc17.net


GCC doesn't set the FE_INVALID flag on comparison with NAN (<=, >=, <, >), e.g.
with:

#include <stdio.h>
#include <math.h>
#include <fenv.h>

#pragma STDC FENV_ACCESS ON

int main (void)
{
  double d = NAN;
  int err;

  feclearexcept (FE_INVALID);
  d <= 0.0;
  err = ! fetestexcept(FE_INVALID);
  if (err)
    printf ("The FE_INVALID flag is not set\n");
  return err;
}

$ gcc-snapshot --version
gcc (Debian 20130113-1) 4.8.0 20130113 (experimental) [trunk revision 195136]
[...]
$ gcc-snapshot -std=c99 -Wall nancmp.c -o nancmp -lm
nancmp.c:5:0: warning: ignoring #pragma STDC FENV_ACCESS [-Wunknown-pragmas]
 #pragma STDC FENV_ACCESS ON
 ^
nancmp.c: In function 'main':
nancmp.c:13:3: warning: statement with no effect [-Wunused-value]
   d <= 0.0;
   ^
$ ./nancmp
The FE_INVALID flag is not set

The "statement with no effect" warning would also be incorrect once this bug is
fixed.

[Bug c/56020] FE_INVALID flag not set on comparison with NAN (unordered)

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56020

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-01-17
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> 2013-01-17 13:21:26 UTC ---
The comparison is dead.  FENV_ACCESS is not implemented (side-effects on
FENV are not preserved).  Use

  if (d <= 0.0)
    __asm("");

GCC uses ucomisd for the comparison, even with -fsignalling-nans.

[Bug c/56020] FE_INVALID flag not set on comparison with NAN (unordered)

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56020

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|NEW                         |RESOLVED
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=34678

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 34678, FENV_ACCESS is not implemented ...

*** This bug has been marked as a duplicate of bug 34678 ***