> To find hot spots, YJIT counts how many times your program calls each function or block

At first glance this seems too simple. Compare it to JavaScript JITs, which IIRC can compile hot spots even in functions that are only called a few times (e.g. those that contain heavy loops) via on-stack replacement. (Although I’ve also heard on-stack replacement called a “party trick” - more useful for optimising benchmark scores than for real code.)

But on the other hand, Ruby’s language design might help here. Idiomatic Ruby uses blocks for loop bodies - so can Ruby JITs optimise long-running loops by treating the loop body as just another function?

> Idiomatic Ruby uses blocks for loop bodies

Yes that's something I want to dig into and explore in this chapter... when exactly does Ruby's JIT compiler activate and optimize our code? And you're right: since Ruby will JIT blocks as if they were separate function many loops will be optimized using this simple heuristic.