Analyzing multiple conflicts in SAT: an experimental evaluation

Unit propagation and conflict analysis are two essential ingredients of CDCL SAT Solving. The order in which unit propagation is computed does not matter when no conflict is found, because it is well known that there exists a unique unit-propagation fixpoint. However, when a conflict is found, current CDCL implementations stop and analyze that concrete conflict, even though other conflicts may exist in the unit-propagation closure. In this experimental evaluation, we report on our experience in modifying this concrete aspect in the CaDiCaL SAT Solver and try to answer the question of whether we can improve the performance of SAT Solvers by the analysis of multiple conflicts.All authors are supported by grant PID2021-122830OB-C43, funded by MCIN/AEI/ 10.13039/501100011033 and by “ERDF: A way of making Europe”Peer ReviewedPostprint (published version

Proving termination through conditional termination

We present a constraint-based method for proving conditional termination of integer programs. Building on this, we construct a framework to prove (unconditional) program termination using a powerful mechanism to combine conditional termination proofs. Our key insight is that a conditional termination proof shows termination for a subset of program execution states which do not need to be considered in the remaining analysis. This facilitates more effective termination as well as non-termination analyses, and allows handling loops with different execution phases naturally. Moreover, our method can deal with sequences of loops compositionally. In an empirical evaluation, we show that our implementation VeryMax outperforms state-of-the-art tools on a range of standard benchmarks.Peer ReviewedPostprint (author's final draft

A parametric approach for smaller and better encodings of cardinality constraints

Adequate encodings for high-level constraints are a key ingredient for the application of SAT technology. In particular, cardinality constraints state that at most (at least, or exactly) k out of n propositional variables can be true. They are crucial in many applications. Although sophisticated encodings for cardinality constraints exist, it is well known that for small n and k straightforward encodings without auxiliary variables sometimes behave better, and that the choice of the right trade-off between minimizing either the number of variables or the number of clauses is highly application-dependent. Here we build upon previous work on Cardinality Networks to get the best of several worlds: we develop an arc-consistent encoding that, by recursively decomposing the constraint into smaller ones, allows one to decide which encoding to apply to each sub-constraint. This process minimizes a function λ·num- vars + num-clauses, where λ is a parameter that can be tuned by the user. Our careful experimental evaluation shows that (e.g., for λ = 5) this new technique produces much smaller encodings in variables and clauses, and indeed strongly improves SAT solvers' performance.Postprint (author’s final draft

Incomplete SMT techniques for solving non-linear formulas over the integers

We present new methods for solving the Satisfiability Modulo Theories problem over the theory of QuantifierFree Non-linear Integer Arithmetic, SMT(QF-NIA), which consists of deciding the satisfiability of ground formulas with integer polynomial constraints. Following previous work, we propose to solve SMT(QF-NIA) instances by reducing them to linear arithmetic: non-linear monomials are linearized by abstracting them with fresh variables and by performing case splitting on integer variables with finite domain. For variables that do not have a finite domain, we can artificially introduce one by imposing a lower and an upper bound and iteratively enlarge it until a solution is found (or the procedure times out). The key for the success of the approach is to determine, at each iteration, which domains have to be enlarged. Previously, unsatisfiable cores were used to identify the domains to be changed, but no clue was obtained as to how large the new domains should be. Here, we explain two novel ways to guide this process by analyzing solutions to optimization problems: (i) to minimize the number of violated artificial domain bounds, solved via a Max-SMT solver, and (ii) to minimize the distance with respect to the artificial domains, solved via an Optimization Modulo Theories (OMT) solver. Using this SMT-based optimization technology allows smoothly extending the method to also solve Max-SMT problems over non-linear integer arithmetic. Finally, we leverage the resulting Max-SMT(QF-NIA) techniques to solve ∃∀ formulas in a fragment of quantified non-linear arithmetic that appears commonly in verification and synthesis applications.Peer ReviewedPostprint (author's final draft

Compositional safety verification with Max-SMT

We present an automated compositional program verification technique for safety properties based on conditional inductive invariants. For a given program part (e.g., a single loop) and a postcondition, we show how to, using a Max-SMT solver, an inductive invariant together with a precondition can be synthesized so that the precondition ensures the validity of the invariant and that the invariant implies the postcondition. From this, we build a bottom-up program verification framework that propagates preconditions of small program parts as postconditions for preceding program parts. The method recovers from failures to prove the validity of a precondition, using the obtained intermediate results to restrict the search space for further proof attempts. As only small program parts need to be handled at a time, our method is scalable and distributable. The derived conditions can be viewed as implicit contracts between different parts of the program, and thus enable an incremental program analysis.Peer ReviewedPostprint (published version

A heuristic approach to the design of optimal cross-docking boxes

Multinational companies frequently work with manufacturers that receive large orders for different products (or product varieties: size, shape, color, texture, material), to serve thousands of different final destinations (e.g., shops) requesting a combination of different quantities of each product. It is not the manufacturers’ task to create the individual shipments for each final destination. But manufacturers can deliver part of their production in so-called cross-docking boxes (or other containers) of a few+ (say, three) types, each type containing a given assortment, i.e., different quantities of different products. At a logistics center, the shipments for each destination are then made of cross-docking boxes plus additional “picking” units. The expensive part is the picking, since cross-docking boxes require little or no manipulation. The problem we solve in this paper is, given a large set of orders for each destination, to design the cross-docking box types in order to minimize picking. We formally define a variant of this problem and develop a heuristic method to solve it. Finally, we present extensive experimental results on a large set of real-world benchmarks proving that our approach gives high-quality solutions (optimal or near optimal) in a very limited amount of time.This work was supported in part by the Spanish Ministerio de Ciencia e Innovación (MICINN) Project under Grant RTI2018-094403-B-C33.Peer ReviewedPostprint (published version

A Write-Based Solver for SAT Modulo the Theory of Arrays

The extensional theory of arrays is one of the most important ones for applications of SAT Modulo Theories (SMT) to hardware and software verification. Here we present a new T-solver for arrays in the context of the DPLL(T) approach to SMT. The main characteristics of our solver are: (i) no translation of writes into reads is needed, (ii) there is no axiom instantiation, and (iii) the T-solver interacts with the Boolean engine by asking to split on equality literals between indices. As far as we know, this is the first accurate description of an array solver integrated in a state-of-the-art SMT solver and, unlike most state-of-the-art solvers, it is not based on a lazy instantiation of the array axioms. Moreover, it is very competitive in practice, specially on problems that require heavy reasoning on array literal

Proving termination of imperative programs using Max-SMT

We show how Max-SMT can be exploited in constraint-based program termination proving. Thanks to expressing the generation of a ranking function as a Max-SMT optimization problem where constraints are assigned different weights, quasi-ranking functions --functions that almost satisfy all conditions for ensuring well-foundedness-- are produced in a lack of ranking functions. By means of trace partitioning, this allows our method to progress in the termination analysis where other approaches would get stuck. Moreover, Max-SMT makes it easy to combine the process of building the termination argument with the usually necessary task of generating supporting invariants. The method has been implemented in a prototype that has successfully been tested on a wide set of programs.Peer Reviewe

Minimal-model-guided approaches to solving polynomial constraints and extensions

In this paper we present new methods for deciding the satisfiability of formulas involving integer polynomial constraints. In previous work we proposed to solve SMT(NIA) problems by reducing them to SMT(LIA): non-linear monomials are linearized by abstracting them with fresh variables and by performing case splitting on integer variables with finite domain. When variables do not have finite domains, artificial ones can be introduced by imposing a lower and an upper bound, and made iteratively larger until a solution is found (or the procedure times out). For the approach to be practical, unsatisfiable cores are used to guide which domains have to be relaxed (i.e., enlarged) from one iteration to the following one. However, it is not clear then how large they have to be made, which is critical. Here we propose to guide the domain relaxation step by analyzing minimal models produced by the SMT(LIA) solver. Namely, we consider two different cost functions: the number of violated artificial domain bounds, and the distance with respect to the artificial domains. We compare these approaches with other techniques on benchmarks coming from constraint-based program analysis and show the potential of the method. Finally, we describe how one of these minimal-model-guided techniques can be smoothly adapted to deal with the extension Max-SMT of SMT(NIA) and then applied to program termination proving.Peer Reviewe

Decision levels are stable: towards better SAT heuristics

We shed new light on the Literal Block Distance (LBD) and glue-based heuristics used in current SAT solvers. For this, we first introduce the concept of stickiness: given a run of a CDCL SAT solver, for each pair of literals we define, by a real value between 0 and 1, how sticky they are, basically, how frequently they are set at the same decision level. By means of a careful and detailed experimental setup and analysis, we confirm the following quite surprising fact: given a SAT instance, when running different CDCL SAT solvers on it, no matter their settings or random seeds, the stickiness relation between literals is always very similar, in a precisely defined sense. We analyze how quickly stickiness stabilizes in a run (quite quickly), and show that it is stable even under different encodings of cardinality constraints. We then describe how and why these solid new insights lead to heuristics refinements for SAT (and extensions, such as SMT) and improved information sharing in parallel solvers.Projects TIN2015-69175-C4-3-R (funded by FEDER/MINECO) and RTI2018-094403-B-C33 (funded by FEDER/Ministerio de Ciencia e Innovacion, Agencia Estatal de Investigacion, Spain)Peer ReviewedPostprint (published version