gcc currently ignores FENV_ACCESS pragma set to ON, and generate incorrect code
in such cases. Before gcc recognizes this pragma, the -frounding-math option
should probably be on by default. For instance, consider the following code.
#include <stdio.h>
#include <float.h>
#include <math.h>
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
static void tstall (void)
{
volatile double x = DBL_MIN;
printf ("%.20g = %.20g\n", 1.0 + DBL_MIN, 1.0 + x);
printf ("%.20g = %.20g\n", 1.0 - DBL_MIN, 1.0 - x);
}
int main (void)
{
#ifdef FE_TONEAREST
printf ("Rounding to nearest\n");
if (fesetround (FE_TONEAREST))
printf ("Error\n");
else
tstall ();
#endif
#ifdef FE_TOWARDZERO
printf ("Rounding toward 0\n");
if (fesetround (FE_TOWARDZERO))
printf ("Error\n");
else
tstall ();
#endif
#ifdef FE_DOWNWARD
printf ("Rounding toward -inf\n");
if (fesetround (FE_DOWNWARD))
printf ("Error\n");
else
tstall ();
#endif
#ifdef FE_UPWARD
printf ("Rounding toward +inf\n");
if (fesetround (FE_UPWARD))
printf ("Error\n");
else
tstall ();
#endif
return 0;
}
By default, I get incorrect results:
Rounding to nearest
1 = 1
1 = 1
Rounding toward 0
1 = 1
1 = 0.99999999999999988898
Rounding toward -inf
1 = 1
1 = 0.99999999999999988898
Rounding toward +inf
1 = 1.000000000000000222
1 = 1
If I add the -frounding-math option, I get correct results:
Rounding to nearest
1 = 1
1 = 1
Rounding toward 0
1 = 1
0.99999999999999988898 = 0.99999999999999988898
Rounding toward -inf
1 = 1
0.99999999999999988898 = 0.99999999999999988898
Rounding toward +inf
1.000000000000000222 = 1.000000000000000222
1 = 1
--
Summary: gcc ignores FENV_ACCESS pragma set to ON
Product: gcc
Version: 4.3.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: vincent at vinc17 dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37838
