g++ optimization breaks for loops

Question

A few days ago, I encountered what I believe to be a bug in g++ 5.3 concerning the nesting of for loops at higher -OX optimization levels. (Been experiencing it

Answer 1

Your code invokes undefined behaviour, since the int sum overflows. You say "this shouldn't in any way affect the other variables". Wrong. Once you have undefined behaviour, all odds are off. Anything can happen.

gcc is (in)famous for optimisations that assume there is no undefined behaviour and do let's say interesting things if undefined behaviour happens.

Solution: Don't do it.