I haven't had a look at the YJIT design, but your reasoning: > detecting parts of code [...] and then apply a fast compiler to that part applies to the previ

I haven't had a look at the YJIT design, but your reasoning: > detecting parts of code [...] and then apply a fast compiler to that part applies to the previous JIT (MJIT) design, which created C snippets and compiled them on the fly. I don't think this is the case for YJIT, although I'm not 100% sure. Surely this isn't the case, for example, for TenderJIT, which, for the bytecode instructions, builds assembly code and (optionally) invokes the original interpreter functions.