The use of interpolants in verification is gaining more and more importance. Since theories used in applications are usually obtained as (disjoint) combinations of simpler theories, it is important to modularly reuse interpolation algorithms for the component theories. We show that a sufficient and necessary condition to do this for quantifier-free interpolation is that the component theories have the strong ( sub -) amalgamation property. Then, we provide an equivalent syntactic characterization and show that such characterization covers most theories commonly employed in verification. Finally, we design a combined quantifier-free interpolation algorithm capable of handling both convex and nonconvex theories; this algorithm subsumes and extends most existing work on combined interpolation.

The use of interpolants in model checking is becoming an enabling technology to allow fast and robust verification of hardware and software. The application of encodings based on the theory of arrays, however, is limited by the impossibility of deriving quantifier-free interpolants in general. In this paper, we show that, with a minor extension to the theory of arrays, it is possible to obtain quantifier-free interpolants. We prove this by designing an interpolating procedure, based on solving equations between array updates. Rewriting techniques are used in the key steps of the solver and its proof of correctness. To the best of our knowledge, this is the first successful attempt of computing quantifier-free interpolants for a theory of arrays.

Model checking based on Craig's interpolants ultimately relies on efficient engines, such as SMT-Solvers, to log proofs of unsatisfiability and to derive the desired interpolant by means of a set of algorithms known in literature. These algorithms, however, are designed for proofs that do not contain mixed predicates. In this paper we present a technique for transforming the propositional proof produced by an SMT-Solver in such a way that mixed predicates are eliminated. We show a number of cases in which mixed predicates arise as a consequence of state-of-the-art solving procedures (e.g. lemma on demand, theory combination, etc.). In such cases our technique can be applied to allow the reuse of known interpolation algorithms. We demonstrate with a set of experiments that our approach is viable.

Theory propagation in Satisfiability Modulo Theories is crucial for the solver's performance. It is important, however, to pay particular care to the amount of deductions to perform. The risk is in fact to clog the SAT-Solver with too many (and potentially useless clauses). In this paper we review some techniques for generating and communicating clauses to the SAT-Solver. In addition we propose a generic and flexible schema for theory propagation in which explanations for entailed facts are generated by re-using the consistency check procedure that is normally available in a theory solver. We argue that our schema can simplify the design of a theory solver, and allow a flexible form of theory propagation even for inherently hard theories (such as bit-vectors).

Efficient decision procedures for bit-vectors are essential for modern verification frameworks. This paper describes a new decision procedure for the core theory of bit-vectors that exploits a reduction to equality reasoning. The procedure is embedded in a congruence closure algorithm, whose data structures are extended in order to efficiently manage the relations between bit-vector slicings, modulo equivalence classes. The resulting procedure is incremental, backtrackable, and proof producing: it can be used as a theory-solver for a lazy SMT schema. Experiments show that our approach is comparable and often superior to bit-blasting on the core fragment, and that it also helps as a theory layer when applied over the full bit-vector theory.