Exploration algorithms for explicit-state transition systems are a core back-end technology in program verification. They can be applied to programs by generating the transition system on the fly, avoiding an expensive up-front translation. An on-the-fly strategy requires significant modifications to the implementation, into a form that stores states directly as valuations of program variables. Performed manually on a per-algorithm basis, such modifications are laborious and error-prone. In this paper we present the Ijit Application Programming Interface (API), which allows users to automatically transform a given transition system exploration algorithm to one that operates on Boolean programs. The API converts system states temporarily to program states just in time for expansion via image computations, forward or backward. Using our API, we have effortlessly extended various non-trivial (e.g. infinitestate) model checking algorithms to operate on multi-threaded Boolean programs. We demonstrate the ease of use of the API, and present a case study on the impact of the just-in-time translation on these algorithms.

We present a method for accelerating explicit-state backward search algorithms for systems of arbitrarily many finite-state threads. Our method statically analyzes the program executed by the threads for the existence of simple loops. We show how such loops can be collapsed without approximation into Presburger arithmetic constraints that symbolically summarize the effect of executing the backward search algorithm along the loop in the multi-threaded program. As a result, the subsequent explicit-state search does not need to explore the summarized part of the state space. The combination of concrete and symbolic exploration gives our algorithm a concolic flavor. We demonstrate the power of this method for proving and refuting safety properties of unboundedthread programs.