https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113981
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> --- With -O0 or -fsanitize-trap=return you get a trap. With -fsanitize=return you get a UBsan error. With -O3 you asked the compiler to optimize the heck out of a function with undefined behaviour, what do you expect? Either that function is never called (in which case why would you want a wasted ret instruction increasing the binary size?) or it's called and has undefined behaviour, which is not going to end well with -O3. Why would you not just fix it when you see the warning?
