TruffleRuby

Shopify · Mar 31, 2020

Optimizing Ruby Lazy Initialization in TruffleRuby with Deoptimization

The post examines how TruffleRuby can optimize Ruby’s ||= operator when it is used for lazy initialization rather than repeated assignment. Static profiling of 20 popular open-source projects found 2,082 uses, with 64% meeting conservative criteria based on constant values or naming patterns for parameterless methods assigning instance or class variables. The implementation replaces the usual OrNode with an OrLazyValueDefinedNode, which counts executions of the right-hand side and deoptimizes when it is executed fewer than twice, allowing uncommon assignment paths to remain in the interpreter. In a benchmark, the change compiled code about 6% faster and produced about 63% less machine code by memory, although the post notes that benchmarking larger projects is noisy and the runtime impact is difficult to prove.