https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103406
Roger Sayle <roger at nextmovesoftware dot com> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|ASSIGNED |NEW Assignee|roger at nextmovesoftware dot com |unassigned at gcc dot gnu.org Summary|[12 Regression] gcc -O0 |gcc -O0 behaves differently |behaves differently on |on "DBL_MAX related |"DBL_MAX related |operations" than gcc -O1 |operations" than gcc -O1 |and above |and above | Target| |x86_64 --- Comment #13 from Roger Sayle <roger at nextmovesoftware dot com> --- The Inf - Inf => 0.0 regression should now be fixed on mainline. Hmm. As hinted by Richard Beiner's investigation, the underlying problem is even more pervasive. It turns out that on x86/IA64 chips, floating point addition is not commutative, i.e. x+y is not the same as y+x, as demonstrated by the test program below: #include <stdio.h> const double pn = __builtin_nan(""); const double mn = -__builtin_nan(""); __attribute__ ((noinline, noclone)) double plus(double x, double y) { return x + y; } int main() { printf("%lf\n",plus(pn,mn)); printf("%lf\n",plus(mn,pn)); return 0; } Output: nan -nan Unfortunately, GCC assumes almost everywhere the FP addition is commutative and (as per comments #8 and #9) associative with negation/minus. This appears to be target property, c.f. libgcc's _FP_CHOOSENAN, but could in theory be resolved by a -fstrict-math mode (that implies -ftrapping-math) that disables commutativity (swapping of operands) throughout the compiler, including reload/fold-const etc., on affected Intel-like targets. Perhaps this PR is a duplicate now that the regression has been fixed?