The escape analysis piece is what makes this particularly interesting. ZJIT can prove an object doesn't escape a method's scope and then eliminate the heap allocation entirely — the object lives on the stack (or in registers) and the load/store optimizations follow naturally once there's no indirection.

YJIT's profile-guided approach is powerful but pays a cost every time a hot path diverges from the expected type. The BBV approach in ZJIT bakes type assumptions directly into the compiled code, so you get the same specialization without the deopt overhead on the happy path. The tradeoff is code size — more type combinations means more compiled variants — but for server-side Rails apps where the method profile is fairly stable, that's usually fine.

Curious whether they're planning to share any of the escape analysis machinery upstream to YJIT, or if the JIT designs are diverging permanently.