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.
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.
Dup of bug 34678, FENV_ACCESS is not implemented ... *** This bug has been marked as a duplicate of bug 34678 ***