Stuttering for Free

One of the most common tools for proving behavioral refinements between transition systems is the method of simulation proofs, which has been explored extensively over the past several decades. Stuttering simulations are an extension of traditional simulations-used, for example, in CompCert-in which either the source or target of the simulation is permitted to "stutter" (stay in place) while the other side steps forward. In the interest of ensuring soundness, however, existing stuttering simulations restrict proofs to only perform a finite number of stuttering steps before making synchronous progress-a step of reasoning in which both sides of the simulation progress forward together. This restriction guarantees that a terminating program cannot be proven to simulate a non-terminating one. In this paper, we observe that the requirement to eventually achieve synchronous progress is burdensome and, what's more, unnecessary: it is possible to ensure soundness of stuttering simulations while only requiring asynchronous progress (progress on both sides of the simulation that may be achieved with only stuttering steps). Building on this observation, we develop a new simulation technique we call FreeSim (short for "freely-stuttering simulations"), mechanized in Coq, and we demonstrate its effectiveness on a range of interesting case studies. These include a simplification of the meta-theory of CompCert, as well as the DTrees library, which enriches the ITrees (Interaction Trees) library with dual non-determinism.