Parameterization as Abstraction: A Tractable Approach to the Dataflow Analysis of Concurrent Programs

Abstract

Dataflow analysis for concurrent programs is a problem of critical importance but, unfortunately, also an undecidable one. A key obstacle is to determine precisely how dataflow facts at a location in a given thread could be affected by operations of other threads.This problem, in turn, boils down to pairwise reachability, i.e., given program locations c₁ and c₂ in two threads T₁ and T₂, respectively, determining whether c1 and c2 are simultaneously reachable in the presence of constraints imposed by synchronization primitives. Unfortunately, pairwise reachability is undecidable for most synchronization primitives. However, we leverage the surprising result that the closely related problem of parameterized pairwise reachability of c₁ and c₂, i.e., whether for some n₁ and n₂, c₁ and c₂ are simultaneously reachable in the program T₁ n_1||T₂ n₂ comprised of n₁ copies of T₁ and n₂ copies of T₂,is not only decidable for many primitives, but efficiently so. Although parameterization makes pairwise reachability tractable it may over-approximate the set of pairwise reachable locations and can, therefore, be looked upon as an abstraction technique.Where as abstract interpretation is used for control and data abstractions, we propose the use of parameterization as an abstraction for concurrency. Leveraging abstract interpretation in conjunction with parameterization allows us to lift two desirable properties of sequential dataflow analysis to the concurrent domain, i.e., precision and scalability.