Combining PDR and reverse PDR for hardware model checking

In the last few years IC3 resp. PDR attracted a lot of attention as a SAT-based hardware verification approach without needing to unroll the transition relation as in Bounded Model Checking (BMC). Motivated by different strengths of forward and backward traversal already observed in BDD based model checking and by an exponential complexity gap between original PDR and its reverted counterpart `Reverse PDR' (which starts its analysis with the initial states instead of the unsafe states as in the original PDR), we take a closer look at Reverse PDR and we present a combined forward/backward version of PDR that inherits the advantages of both original and Reverse PDR. Our experimental results on benchmarks from the Hardware Model Checking Competition demonstrate clear benefits of the combined approach.