The program below show that gcc reorder floating point instructions in such a way to make inexact checking fruitless.
Reading generated assembler I see two problems: 1) the cast to float in x assignment is executed *after* fetestexcept and not before as it's written (and needed to get the correct result). This infringes C99 standard sequence point rules. 2) the second division is not recomputed (because CSE), then inexact flag is not changed after feclearexcept I guess that the latter is due to missing #pragma STDC FENV_ACCESS implementation, but the former undermine the whole fetestexcept usability. $ cat bug.c #include <fenv.h> #include <stdio.h> double vf = 0x0fffffff; double vg = 0x10000000; /* vf/vg is exactly representable as IEC559 64 bit floating point, while it's not representable exactly as a 32 bit one */ int main() { double a = vf; double b = vg; feclearexcept(FE_INEXACT); float x; x = a / b; printf("%i %.1000g\n", fetestexcept(FE_INEXACT), x); feclearexcept(FE_INEXACT); double y; y = a / b; printf("%i %.1000g\n", fetestexcept(FE_INEXACT), y); return 0; } $ gcc -O2 bug.c -lm $ ./a.out 0 1 0 0.9999999962747097015380859375 $ -- Summary: Wrong floating point reorder Product: gcc Version: 4.3.2 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: abramobagnara at tin dot it GCC target triplet: i486-linux-gnu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38960