Support Local Variables

Discovering and supporting all of the wild semantics for local variables in Ruby was so interesting that the ZJIT team decided to write a paper about it: Support Local Variables (PDF). It’s also the first bit of academic literature that documents and explains ZJIT. It’s published at VMIL 2026, where someone on our team will be presenting it. Abstract Ruby is a dynamically typed and object-oriented programming language. Its primary implementation, CRuby, contains a bytecode virtual machine and a mature lazy basic block versioning (LBBV) just-in-time (JIT) compiler called YJIT. In order to both implement more advanced optimizations than YJIT supports and also encourage more outside contributions, we present a new method-based JIT called ZJIT. Like YJIT, ZJIT compiles from bytecode to machine code. Unlike YJIT, ZJIT has multiple global and local optimization passes. ZJIT’s high-level intermediate representation is in static single assignment (SSA) form. In order to optimize Ruby’s local…