Understanding Method Level Speculation

Method level speculation (MLS) is an optimistic technique for parallelizing imperative programs, for which a variety of MLS systems and optimizations have been proposed. However, runtime performance strongly depends on the interaction between program structure and MLS system design choices, making it difficult to compare approaches or understand in a general way how programs behave under MLS. Here we develop an abstract list-based model of speculative execution that encompasses several MLS designs, and a concrete stack-based model that is suitable for implementations. Using our abstract model, we show equivalence and correctness for a variety of MLS designs, unifying in-order and out-of-order execution models. Using our concrete model, we directly explore the execution behaviour of simple imperative programs, and show how specific parallelization patterns are induced by combining common programming idioms with precise speculation decisions. This basic groundwork establishes a common basis for understanding MLS designs, and suggests more formal directions for optimizing MLS behaviour and application.