https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91554
--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> --- It's obviously 8462 /* If this expression has side effects, show we don't know it to be a 8463 constant. Likewise if it's a pointer or aggregate type since in 8464 those case we only want literals, since those are only optimized 8465 when generating RTL, not later. 8466 And finally, if we are compiling an initializer, not code, we 8467 need to return a definite result now; there's not going to be any 8468 more optimization done. */ 8469 if (TREE_SIDE_EFFECTS (arg) 8470 || AGGREGATE_TYPE_P (TREE_TYPE (arg)) 8471 || POINTER_TYPE_P (TREE_TYPE (arg)) 8472 || cfun == 0 8473 || folding_initializer 8474 || force_folding_builtin_constant_p) 8475 return integer_zero_node; that causes this, I guess you want to use __builtin_constant_p (b != 0) instead. The docs don't explain what a "constant at compile time" is so whether for example the address of a global or the address of an automatic var would be "constant". But I'd say the above incorrectly disregards the NULL-pointer case.
