https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77964
--- Comment #15 from Jakub Jelinek <jakub at gcc dot gnu.org> --- (In reply to Jiri Slaby from comment #14) > (In reply to Andrew Pinski from comment #10) > > (In reply to Markus Trippelsdorf from comment #9) > > > Is subtracting undefined, too? > > Yes. Comparing two unrelated arrays or subtracting them is undefined. > > But they are not unrelated arrays. So what from the C standard actually > makes (and allows) gcc think they are unrelated? C doesn't have any notion of "related" declarations. > And given gcc 7 is to be released yet, can we have a switch to disable this > optimization? This is nothing new in GCC 7, you've most likely just been extremely lucky in the past that it happened to work as you expected. Other projects had to change similar UB code years ago. It isn't just a single optimization, but lots of them that rely on pointer arithmetics being defined only within the same object.
