Isn't it often the case that the optimizations throw out sequential consistency guarantees? That would introduce non-determinism in concurrent executions that

Isn't it often the case that the optimizations throw out sequential consistency guarantees? That would introduce non-determinism in concurrent executions that might have nothing to do with the semantics of the program. For example, I'm under the impression that the most recent C++ standard basically says "if you have no data races then all will be well" even though they may be benign data races like dual assignments of the same value.