https://gcc.gnu.org/bugzilla/show_bug.cgi?id=34678
--- Comment #43 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
Note that the effect of changing the rounding mode after a computation, whether
-frounding-math is used or not, is not just that the change of rounding mode
may not be honored. If can yield inconsistencies in a block where the rounding
mode is not changed.
#include <stdio.h>
#include <stdlib.h>
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
#define CST 0x1p-200
int main (void)
{
volatile double a = CST;
double b = a, c = a, d;
printf ("%a\n", 1.0 - b);
fesetround (FE_DOWNWARD);
printf ("%a\n", 1.0 - c);
if (b == c && b == CST && c == CST)
{
printf ("%d\n", 1.0 - b == 1.0 - c);
printf ("1: %a\n", 1.0 - b);
printf ("2: %a\n", 1.0 - c);
d = b == CST ? b : (abort (), 1.0);
printf ("3: %a\n", 1.0 - d);
d = b == CST ? b : 1.0;
printf ("4: %a\n", 1.0 - d);
}
return 0;
}
With -std=c17 -frounding-math -O3 -lm, I get:
0x1p+0
0x1.fffffffffffffp-1
0
1: 0x1p+0
2: 0x1.fffffffffffffp-1
3: 0x1p+0
4: 0x1.fffffffffffffp-1
