https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109008
--- Comment #25 from Jakub Jelinek <jakub at gcc dot gnu.org> --- I guess more debugging tomorrow. Because with the frange_nextafter it comes up for the f1 to range p[frange] double [-2.22044604925031283432823045461545143383482334912930322712e-16 (-0x0.fffffffffffff8p-52), 4.44089209850062566865646090923090286766964669825860645425e-16 (0x0.fffffffffffff8p-51)] But as can be seen on double f1 (double eps) { double d = 1. + eps; if (d == 1.) return eps; return 0.0; } int main () { __builtin_printf ("%.32a\n", f1 (-0x0.fffffffffffff8p-52)); __builtin_printf ("%.32a\n", f1 (__builtin_nextafter (-0x0.fffffffffffff8p-52, -42.0))); __builtin_printf ("%.32a\n", f1 (0x0.fffffffffffff8p-51)); __builtin_printf ("%.32a\n", f1 (__builtin_nextafter (0x0.fffffffffffff8p-51, 42.0))); __builtin_printf ("%.32a\n", f1 (-0x1.0p-54)); __builtin_printf ("%.32a\n", f1 (__builtin_nextafter (-0x1.0p-54, -42.0))); __builtin_printf ("%.32a\n", f1 (0x1.0p-53)); __builtin_printf ("%.32a\n", f1 (__builtin_nextafter (0x1.0p-53, 42.0))); __builtin_printf ("%.32a\n", f1 (-0x0.8p-53)); __builtin_printf ("%.32a\n", f1 (__builtin_nextafter (-0x0.8p-53, -42.0))); __builtin_printf ("%.32a\n", f1 (0x0.8p-52)); __builtin_printf ("%.32a\n", f1 (__builtin_nextafter (0x0.8p-52, 42.0))); } the correct exact range is what is written in the comment, i.e. [-0x1.0p-54, 0x1.0p-53] aka [-0x0.8p-53, 0x0.8p-52].