There can be no answer. Research is ongoing, smart people are actively trying to make optimizer better, so even if I gave a 100% correct answer now (which would be pages long), a new commit 1 minute latter will change the rules. Sometimes someone discovers what we thought was safe isn't safe in some obscure case and so we are forced to no longer apply some optimization. sometimes optimization is a compromise and we decide that the using a couple extra CPU cycles is worth it because of some other gain (a CPU cycle is often impossible to measure in the real world as things like caches tend to dominate benchmarks, so you can make this comprise many times when suddenly the total adds up to something you can measure.).

The short answer for those who don't want details: it is unlikely you can measure a difference in real world code assuming good clean code with the right algorithm.