[Bug math/9783] New: fetestexcept(FE_INEXACT) does not return expected result when inexact trapping is disabled

["abramo dot bagnara at gmail dot com" <sourceware-bugzilla at sourceware dot org>]

Running the program below, it seems that if feenableexcept(FE_INEXACT) is not
called fetestexcept(FE_INEXACT) doesn't work as expected (and as C99 standard
provides).

Please note that according to documentation GNU extension
feenableexcept/fedisablexcept does not enable/disable inexact detection, but
does enable/disable inexact *trapping* via SIGFPE signal.

Also please ignore the incorrect return from the overly simplified signal
handler (it would generate an infinite loop on some architectures).

The bug was previously believed a gcc one
(http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37581),
but recently we got new info that shows this is not true.

$ cat sf2.c 
#define _GNU_SOURCE
#include <fenv.h>
#include <signal.h>
#include <stdio.h>

static void foo (int sig)
{
  printf("inexact\n");
}


float __attribute__((noinline)) test (float x) {
        printf("%f / %f\n", 2.0f, x);
        return 2.0f / x;
}

void t()
{
  volatile float x;
  feclearexcept (FE_ALL_EXCEPT);
  x = test (3.0f);
  printf("fetestexcept(FE_INEXACT) = %d\n", fetestexcept(FE_INEXACT));
  feclearexcept (FE_ALL_EXCEPT);
  x = test (2.0f);
  printf("fetestexcept(FE_INEXACT) = %d\n", fetestexcept(FE_INEXACT));
}

int main() {
  printf("\nWith FE_INEXACT SIGFPE disabled\n");
  t();

  printf("\nWith FE_INEXACT SIGFPE enabled\n");
  signal (SIGFPE, foo);
  feenableexcept (FE_INEXACT);
  t();
}
$ gcc -O2 -lm -mieee-with-inexact sf2.c
$ ./a.out

With FE_INEXACT SIGFPE disabled
2.000000 / 3.000000
fetestexcept(FE_INEXACT) = 0
2.000000 / 2.000000
fetestexcept(FE_INEXACT) = 0

With FE_INEXACT SIGFPE enabled
2.000000 / 3.000000
inexact
fetestexcept(FE_INEXACT) = 2097152
2.000000 / 2.000000
fetestexcept(FE_INEXACT) = 0
$

-- 
           Summary: fetestexcept(FE_INEXACT) does not return expected result
                    when inexact trapping is disabled
           Product: glibc
           Version: 2.8
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
        AssignedTo: aj at suse dot de
        ReportedBy: abramo dot bagnara at gmail dot com
                CC: bagnara at cs dot unipr dot it,glibc-bugs at sources dot
                    redhat dot com
 GCC build triplet: alphaev56-unknown-linux-gnu
  GCC host triplet: alphaev56-unknown-linux-gnu
GCC target triplet: alphaev56-unknown-linux-gnu


http://sourceware.org/bugzilla/show_bug.cgi?id=9783

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