Reducing the Complexity of Quantified Formulas via Variable Elimination

Abstract: We present a general simplification of quantified SMT formulas using variable elimination. The simplification is based on an analysis of the ground terms occurring as arguments in function applications. We use this information to generate a system of set constraints, which is then solved to compute a set of sufficient ground terms for each variable. Universally quantified variables with a finite set of sufficient ground terms can be eliminated by instantiating them with the computed ground terms. The resulting SMT formula contains potentially fewer quantifiers and thus is potentially easier to solve. We describe how a satisfying model of the resulting formula can be modified to satisfy the original formula. Our experiments show that in many cases, this simplification considerably improves the solving time, and our evaluations using Z3 and CVC4 indicate that the idea is not specific to a particular solver, but can be applied in general.