So, in a narrow, simplistic case signed integer overflow undef behaviour made it possible to replace a loop with a constant. Hooooray!

Pretending that integer overflows don't happen because of undef behaviour is... Funny. Integer overflow do happen a lot, both with signed and unsigned ints. Compilers should only apply radical optimisations in this situation if they can prove an overflow will never happen. Otherwise, they should not touch the loop.

EDIT: typos, rephrasing for clarity