It’s worth noting that good compilers can actually do a version of this: it’s called profile-guided optimisation <https://en.wikipedia.org/wiki/Profile-guided_o

It’s worth noting that good compilers can actually do a version of this: it’s called profile-guided optimisation <https://en.wikipedia.org/wiki/Profile-guided_optimization https://en.wikipedia.org/wiki/Profile-guided_optimization>. Not many projects use it, because it’s generally a bit of a bother (build a special version, run your profiling, then build everything all over again), but Firefox gets good mileage out of it. My vague recollection is a typical sort of speedup of 5%.