Confusion with how V8 turns function into hot function? [duplicate]

Question

I was learning the inner workings of V8 and found out that there is JIT compiler which, on the fly, optimizes the hot functions with inline caching technique. I have only two questions, firstly, is function considered as hot function as long as it is executed repeatedly one after another several times? Secondly, after what exact number of repeated execution function gets hot in V8?

Answer 1

V8 developer here. Function "hotness" is not simply determined by the number of calls to it. Instead, V8 tries to predict how useful it would be to optimize a given function by estimating the amount of time spent executing the unoptimized version of that function. The exact heuristics of how this works, which other factors are taken into account (e.g. completeness/stability of type feedback), and the threshold when optimized compilation is triggered can and do change over time.

The reason is that optimized compilation is fairly expensive, so you'd only want to do it when it's likely to pay off. ("likely" because it depends in particular on how much work the function will do in the future, and predicting the future accurately is of course impossible, so there's always some amount of guesswork and heuristics involved.)