SPORE: Combining Symmetry and Partial Order Reduction

Symmetry reduction (SR) and partial order reduction (POR) aim to scale up model checking by exploiting the underlying program structure: SR avoids exploring executions equivalent up to some permutation of symmetric threads, while POR avoids exploring executions equivalent up to reordering of independent instructions. While both SR and POR have been well studied individually, their combination in the context of stateless model checking has remained an open problem. In this paper, we present SPORE, the first stateless model checker that combines SR and POR in a sound, complete and optimal manner. SPORE can leverage both symmetries in the client program itself, but also internal symmetries in the underlying implementation (i.e., idempotent operations), a novel symmetry notion we introduce in this paper. Our experiments confirm that SPORE explores drastically fewer executions than tools that solely employ SR/POR, thereby greatly advancing the state-of-the-art.