Small inductive safe invariants

Abstract

Computing minimal (or even just small) certificates is a central problem in automated reasoning and, in particular, in automated formal verification. For example, Minimal Unsatisfiable Subsets (MUSes) have a wide range of applications in verification ranging from abstraction and generalization to vacuity detection and more. In this paper, we study the problem of computing minimal certificates for safety properties. In this setting, a certificate is a set of clauses Inυ such that each clause contains initial states, and their conjunction is safe (no bad states) and inductive. A certificate is minimal, if no subset of Inυ is safe and inductive. We propose a two-tiered approach for computing a Minimal Safe Inductive Subset (MSIS) of Inv. The first tier is two efficient approximation algorithms that under-and over-approximate MSIS, respectively. The second tier is an optimized reduction from MSIS to a sequence of computations of Maximal Inductive Subsets (MIS). We evaluate our approach on the HWMCC benchmarks and certificates produced by our variant of IC3. We show that our approach is several orders of magnitude more effective than the naive reduction of MSIS to MIS.