[Bug c/57029] New: GCC doesn't set the inexact flag on inexact compile-time int-to-float conversion

[vincent-gcc at vinc17 dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57029



             Bug #: 57029

           Summary: GCC doesn't set the inexact flag on inexact

                    compile-time int-to-float conversion

    Classification: Unclassified

           Product: gcc

           Version: 4.9.0

            Status: UNCONFIRMED

          Severity: normal

          Priority: P3

         Component: c

        AssignedTo: unassig...@gcc.gnu.org

        ReportedBy: vincent-...@vinc17.net





GCC doesn't set the inexact flag on inexact compile-time int-to-float

conversion:



#include <stdio.h>

#include <fenv.h>



#pragma STDC FENV_ACCESS ON



void test1 (void)

{

  volatile float c;



  c = 0x7fffffbf;

  printf ("c = %a, inexact = %d\n", c, fetestexcept (FE_INEXACT));

}



void test2 (void)

{

  volatile float c;

  volatile int i = 0x7fffffbf;



  c = i;

  printf ("c = %a, inexact = %d\n", c, fetestexcept (FE_INEXACT));

}



int main (void)

{

  test1 ();

  test2 ();

  return 0;

}



Under Linux/x86_64:



$ gcc -std=c99 -O3 conv-int-flt-inex.c -o conv-int-flt-inex -lm

$ ./conv-int-flt-inex

c = 0x1.fffffep+30, inexact = 0

c = 0x1.fffffep+30, inexact = 32



Ditto without optimizations.



Note: the STDC FENV_ACCESS pragma is currently not supported (PR 34678), but I

don't think it is directly related (this is not an instruction ordering

problem...).



This bug has been found from:



  http://gcc.gnu.org/ml/gcc-help/2013-04/msg00164.html