2.00 pm, Seminar Hall Counterexample guided Skolem Function Synthesis from Factored Specifications Supratik Chakraborty IIT Bombay. 020916 Abstract Given a propositional formula F(x,y), a Skolem function for x is a function \psi(y), such that substituting \psi(y) for x in F gives a formula semantically equivalent to \exists x F(x, y). Automatically generating Skolem functions is of significant interest in several applications including certified QBF solving, finding strategies of players in games, synthesizing circuits and bitvector programs from specifications, disjunctive decomposition of sequential circuits, etc. In many such applications, F is given as a conjunction of factors, each of which depends on a small subset of variables. Existing algorithms for Skolem function generation ignore any such factored form and treat F as a monolithic function. This presents scalability hurdles in medium to large problem instances. In this work, we argue that exploiting the factored form of F can give significant performance improvements in practice when computing Skolem functions. We present a new CEGAR style algorithm for generating Skolem functions from factored propositional formulas. In contrast to earlier work, our algorithm neither requires a proof of QBF satisfiability nor uses composition of monolithic conjunctions of factors. We show experimentally that our algorithm generates smaller Skolem functions and outperforms stateoftheart approaches on several large benchmarks. (Joint work with S. Akshay, Shetal Shah, Ajith John and Ashutosh Trivedi)
