Parallel Type-checking with Saturating LVars

Given the sophistication of recent type systems, unification-based type-checking and inference can be a time-consuming phase of compilation—especially when union types are combined with subtyping. It is natural to consider improving performance through parallelism, but these algorithms are challenging to parallelize due to complicated control structure and difficulties representing data in a way that is both efficient and supports concurrency. We provide a solution to these problems based on the LVish approach to deterministic-by-default parallel programming. We extend LVish with a novel class of concurrent data structures: Saturating LVars, which are the first LVars to safely release memory during the object’s lifetime. Our new design allows us to achieve a parallel speedup on worst-case (exponential) inputs of traditional HindleyMilner inference, and on the Typed-Racket type-checking algorithm, which yields up an 8.46× parallel speedup on type-checking examples drawn from the Racket repository.