One thing that bothers me is, can I rely on the compiler making those optimisations? Which ones are guaranteed for me? Those are sort of important consideration

One thing that bothers me is, can I rely on the compiler making those optimisations? Which ones are guaranteed for me? Those are sort of important considerations, because it can mean the difference between a linear fibs and a quadratic fibs, or an O(nlogn) sort and a O(n²) sort.