OASIS: ILP-Guided Synthesis of Loop Invariants

Finding appropriate inductive loop invariants for a program is a key challenge in verifying its functional properties. Although the problem is undecidable in general, several heuristics have been proposed to handle practical programs that tend to have simple control-flow structures. However, these heuristics only work well when the space of invariants is small. On the other hand, machine-learned techniques that use continuous optimization have a high sample complexity, i.e., the number of invariant guesses and the associated counterexamples, since the invariant is required to exactly satisfy a specification. We propose a novel technique that is able to solve complex verification problems involving programs with larger number of variables and non-linear specifications. We formulate an invariant as a piecewise low-degree polynomial, and reduce the problem of synthesizing it to a set of integer linear programming (ILP) problems. This enables the use of state-of-the-art ILP techniques that combine enumerative search with continuous optimization; thus ensuring fast convergence for a large class of verification tasks while still ensuring low sample complexity. We instantiate our technique as the open-source oasis tool using an off-the-shelf ILP solver, and evaluate it on more than 300 benchmark tasks collected from the annual SyGuS competition and recent prior work. Our experiments show that oasis outperforms the state-of-the-art tools, including the winner of last year's SyGuS competition, and is able to solve 9 challenging tasks that existing tools fail on.