https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79546
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> --- >-Werror=unsafe-loop-optimizations That is not enabled by default even with -Werror -W -Wall. But "missed loop optimization, the loop counter may overflow" is basically saying if i was an signed integer, the loop counter would overflow in some cases. The case depends the value of len. The only way to fix this would be rewrite the code so that case would not happen. The cases where it would happen is len = SIZET_MIN ... SIZET_MIN-0x0FFF (if I did my math correctly).
