Solving Constrained Horn Clauses Using Dependence-Disjoint Expansions

Recursion-free Constrained Horn Clauses (CHCs) are logicprogramming problems that formulate verifying safety of programs with bounded iteration and recursion. They both formulate bounded verification problems and are generated by solvers that attempt to verify safety of unbounded iterative or recursive systems. Efficient solvers of recursion-free systems reduce the problem to solving a series of systems that can each be solved tractably. In this paper, we define a novel tractable class of recursion-free systems, named Clause-Dependence Disjoint (CDD) systems, that strictly generalizes tractable classes defined in previous work. We implemented a novel solver for recursion-free CHCs, named Shara, that reduces the problem of solving a given recursion-free system to solving an equivalent CDD system. Our evaluation of Shara on standard benchmarks indicates that, in many practical cases, it performs significantly better than a state-of-the-art implementation.