http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53265
--- Comment #28 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-04-30 06:07:23 UTC --- The warning is only printed if the loop has a single exit and known constant iteration count without the undefined behavior analysis, and when the warning is printed, we don't apply the aggressive analysis anyway. It is hard to warn in all cases, but what exactly would be the cases anyway, the amount of surprise on undefined behavior varies a lot. The point of the warning was to warn about the easy cases, for the rest applies what we write in bugs.html - if a suspicious program behaviour goes away with -fno-aggressive-loop-optimizations, most likely it is a fault of the compiled code, not the compiler.
