Preprocessing Techniques for QBFs

In this paper we present sQueezeBF, an effective preprocessor for QBFs that combines various techniques for eliminating variables and/or clauses. In particular sQueezeBF combines (i) variable elimination by Q-resolution and equality reduction, and (ii) clause simplification via subsumption and self-subsumption resolution. The experimental analysis shows that sQueezeBF can produce significant reductions in the number of clauses and/or variables — up to the point that some instances are solved directly by sQueezeBF — and that it can significantly improve the efficiency of a range of state-of-the-art QBF solvers — up to the point that some instances cannot be solved without sQueezeBF preprocessing.