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Why is a traditional method based JIT better than an LBBV JIT? I thought YJIT is LBBV because it's a better fit for Ruby, whereas traditional method based JIT is more suitable for static languages like Java.

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One reason is that we think we can make better use of registers. Since LBBV doesn't "see" all blocks in a particular method all at once, it's much more challenging to optimize register use across basic blocks. We've added type profiling, so ZJIT can "learn" types from the runtime.

Are you piggybacking on LLVM for registerallocation and things like that or building from scracth?

I can imagine combining BBV type branching info with register tracing would add a lot of complicated structures, but I thought the BB's of BBV was more or less analogous to SSA blocks so no middle ground to be found? (consdering if there are many megamorphic sites in the standard library?)

Usual caveat that while Java is mostly static, is has dynamic runtime semantics inherited from Smalltalk and Objective-C, with dynamic class loading, bytecode generation, proxy classes, reflection, hence why the research work on Smalltalk and Strongtalk ended up being so useful for Hotspot.

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