Deep dive into Ruby 3 YIT (Just-In-Time) Compiler architectures: MJIT vs YJIT.

YJIT (Yet Another JIT) is a lazy Basic Block Versioning JIT compiler built directly inside CRuby. It optimizes native performance significantly by compiling hot Ruby code blocks straight into machine code at runtime.

Ruby 3 introduced advanced JIT frameworks to move beyond standard virtual machine bytecode execution. The initial JIT compiler, MJIT, transformed bytecode into external C code files and invoked a system compiler (like GCC or Clang) to produce a shared library, which introduced high processing latency. Ruby 3.1 introduced YJIT, developed by Shopify using Lazy Basic Block Versioning (LBBV). YJIT compiles machine instructions directly into memory lazily as blocks execute, drastically decreasing overhead and providing massive real-world performance gains for complex web frameworks like Rails.