Comment by pat_shaughnessy

That's the magic of YJIT, and what I'll describe in the rest of Chapter 4. YJIT uses a "wait-and-see" approach, and often defers compilation of the block/function until the actual types are provided by your program. And YJIT then keeps track of separate block versions based on the operand types, and can call the appropriate block version as needed.

This basic algorithm is called "Basic Block Versioning." Maxime Chevalier-Boisvert from Shopify has some great presentations online about this; for example [1].

I believe ZJIT, the newer JIT engine, uses a different approach. I'm exploring that now.

[1] https://www.youtube.com/watch?v=zO9_uTaELCw — RubyConf 2021 - YJIT - Building a new JIT Compiler inside CRuby by Maxime Chevalier Boisvert.