September 4, 2026
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 variables, ZJIT lifts local variables into SSA values. This is a departure from how other Ruby compilers handle locals: other JIT compilers either leave local variables as memory loads and stores or do advanced partial evaluation to recover SSA values from memory.
While implementing locals, we (re-)discovered what features make local variables in Ruby especially challenging to compile correctly and efficiently. We demonstrate these features and illustrate how we solved these problems in ZJIT.