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09:00-10:30 Session 46: FLoC Plenary Lecture: Peter O'Hearn
Location: Maths LT1
Continuous Reasoning: Scaling the Impact of Formal Methods

ABSTRACT. Formal reasoning about programs is one of the oldest and most fundamental research directions in computer science. It has also been one of the most elusive. There has been a tremendous amount of valuable research in formal  methods, but rarely have formal reasoning techniques been deployed as part of the development process of large industrial codebases.


This talk describes work in continuous reasoning, where formal reasoning about a (changing) codebase is done in a fashion which mirrors the iterative, continuous model of software development that is increasingly practiced in industry. We suggest that advances in continuous reasoning will allow formal reasoning to scale to more programs, and more programmers. We describe our experience using continuous reasoning with large, rapidly changing codebases at Facebook, and we describe open problems and directions for research for the scientific community.


This a paper with the same title accompanying this talk appears in the LICS’18 proceedings.

10:30-11:00Coffee Break
11:00-12:15 Session 47F: SAT Invited Talk: Christoph Scholl
Welcome to SAT 2018
Dependency Quantified Boolean Formulas: An Overview of Solution Methods and Applications

ABSTRACT. Dependency quantified Boolean formulas (DQBFs) as a generalization of quantified Boolean formulas (QBFs) have received considerable attention in research during the last years. Here we give an overview of the solution methods developed for DQBF so far. The exposition is complemented with the discussion of various applications that can be handled with DQBF solving.

12:30-14:00Lunch Break
14:00-15:30 Session 49F: MaxSAT
Approximately Propagation Complete and Conflict Propagating Constraint Encodings

ABSTRACT. The effective use of satisfiability (SAT) solvers requires problem encodings that make good use of the reasoning techniques employed in such solvers, such as unit propagation and clause learning. Propagation completeness has been proposed as a useful property for constraint encodings as it maximizes the utility of unit propagation. Experimental results on using encodings with this property in the context of satisfiability modulo theory (SMT) solving have however remained inconclusive, as such encodings are typically very large, which increases the bookkeeping work of solvers.

In this paper, we introduce approximate propagation completeness and approximate conflict propagation as novel SAT encoding property notions. While approximate propagation completeness is a generalization of classical propagation completeness, (approximate) conflict propagation is a new concept for reasoning about how early conflicts can be detected by a SAT solver. Both notions together span a hierarchy of encoding quality choices, with classical propagation completeness as a special case. We show how to compute approximately propagation complete and conflict propagating constraint encodings with a minimal number of clauses using a reduction to MaxSAT. To evaluate the effect of such encodings, we give results on applying them in a case study.

Dynamic Polynomial Watchdog Encoding for Solving Weighted MaxSAT
SPEAKER: Tobias Paxian

ABSTRACT. In this paper we present a novel cardinality constraint encoding for solving the weighted MaxSAT problem with iterative SAT-based methods based on the Polynomial Watchdog (PW) CNF encoding for Pseudo-Boolean (PB) constraints. The watchdog of the PW encoding indicates whether the bound of the PB constraint holds. In our approach, we lift this static watchdog concept to a dynamic one allowing an incremental convergence to the optimal result. Consequently, we formulate and implement a SAT-based algorithm for our new Dynamic Polynomial Watchdog (DPW) encoding which can be applied for solving the MaxSAT problem. Furthermore, we introduce three fundamental optimizations of the PW encoding also suited for the original version leading to a significantly less encoding size.

Our experimental results show that our encoding and algorithm is competitive with state-of-the-art encodings as utilized in QMaxSAT (3rd place in last MaxSAT Evaluation 2017). Our encoding dominates two of the QMaxSAT encodings, and at the same time is able to solve unique instances. We integrated our new encoding into QMaxSAT and adapt the heuristic to choose between the only remaining encoding of QMaxSAT and our approach. This combined version solves 19 (4%) more instances in overall 30% less run time on the benchmark set of the MaxSAT Evaluation 2017. However, for the instances solved by both solvers our encoding is 2X faster than all employed encodings of QMaxSAT used in the evaluation.

Solving MaxSAT with Bit-Vector Optimization

ABSTRACT. We explore the relationships between two closely related optimization problems: MaxSAT and Optimization Modulo Bit-Vectors (OBV). Given a bit-vector or a propositional formula F and a target bit-vector T, Unweighted Partial MaxSAT maximizes the number of satisfied bits in T, while OBV maximizes the value of T. 

We propose a new OBV-based Unweighted Partial MaxSAT algorithm. Our resulting solver–Mrs. Beaver–outscores the state-of-the-art solvers when run with the settings of the Incomplete-60-Second-Timeout Track of MaxSAT Evaluation 2017. 

Mrs. Beaver is the first MaxSAT algorithm designed to be incremental in the following sense: it can be re-used across multiple invocations with different hard assumptions and target bit-vectors. We provide experimental evidence showing that enabling incrementality in MaxSAT significantly improves the performance of a MaxSAT-based Boolean Multilevel Optimization (BMO) algorithm when solving a new, critical industrial BMO application: cleaning-up weak design rule violations during the Physical Design stage of Computer-Aided-Design.

15:30-16:00Coffee Break
16:00-18:00 Session 51F: CDCL
Using Combinatorial Benchmarks to Probe the Reasoning Power of Pseudo-Boolean Solvers
SPEAKER: Jan Elffers

ABSTRACT. We study cdcl-cuttingplanes, Open-WBO, and Sat4j, three successful solvers from the Pseudo-Boolean Competition 2016, and evaluate them by performing experiments on crafted benchmarks designed to be trivial for the cutting planes (CP) proof system underlying pseudo-Boolean (PB) proof search, but yet potentially tricky for PB solvers. Our results demonstrate severe shortcomings in state-of-the-art PB solving techniques. Despite the fact that our benchmarks have linear-size tree-like CP proofs, the solvers often perform quite badly even for very small instances. Our analysis is that this shows that solvers need to explore stronger methods of pseudo-Boolean reasoning within cutting planes. We make an empirical observation from the competition data that many of the easy crafted instances are also infeasible over the rational numbers, or have small strong backdoors to PB instances without rational solutions. This raises the intriguing question whether the existence of such backdoors can be correlated with easiness/hardness. However, for some of our constructed benchmark families even rationally infeasible instances are completely beyond reach. This indicates that PB solvers need to get better not only at Boolean reasoning but even at linear programming. Finally, we compare CP-based solvers with CDCL and MIP solvers. For those of our benchmarks where the natural CNF encodings admit efficient resolution proofs, we see that the CDCL-based solver Open-WBO is orders of magnitude faster than the CP-based solvers cdcl-cuttingplanes and Sat4j (though it seems very sensitive to the ordering of the input). And the MIP solver Gurobi beats all of these solvers across the board. These experimental results point to several crucial challenges in the quest for more efficient pseudo-Boolean solvers, and we also believe that a further study of our benchmarks could shed important light on the potential and limitations of current state-of-the-art PB solving.

Machine Learning-based Restart Policy for CDCL SAT Solvers
SPEAKER: Jia Liang

ABSTRACT. Restarts are a critically important heuristic in most modern conflict-driven clause-learning (CDCL) SAT solvers. The precise reason as to why and how restarts enable CDCL solvers to scale efficiently remains obscure. In this paper we address this question, and provide some answers that enabled us to design a new effective machine learning-based restart policy. Specifically, we provide evidence that restarts improve the quality of learnt clauses as measured by one of best known clause quality metrics, namely, literal block distance (LBD). More precisely, we show that more frequent restarts decrease the LBD of learnt clauses, which in turn improves solver performance. We also note that too many restarts can be harmful because of the computational overhead of rebuilding the search tree from scratch too frequently. With this tradeoff in mind, between that of learning better clauses vs. the computational overhead of rebuilding the search tree, we introduce a new machine learning-based restart policy that predicts the quality of the next learnt clause based on the history of previously learnt clauses. The restart policy erases the solver’s search tree during its run, if it predicts that the quality of the next learnt clause is below some dynamic threshold that is determined by the solver’s history on the given input. Our machine learning-based restart policy is based on two observations gleaned from our study of LBDs of learned clauses. First, we discover that high LBD percentiles can be approximated with z-scores of the normal distribution. Second, we find that LBDs, viewed as a sequence, are correlated and hence the LBDs of past learned clauses can be used to predict the LBD of future ones. With these observations in place, and techniques to exploit them, our new restart policy is shown to effective over a large benchmark from the SAT Competition 2014 to 2017.

Chronological Backtracking
SPEAKER: Vadim Ryvchin

ABSTRACT. Non-Chronological Backtracking (NCB) has been implemented in every modern CDCL SAT solver since the original CDCL solver GRASP. NCB’s importance has never been questioned. This paper argues that NCB is not always helpful. We show how one can implement the alternative to NCB–Chronological Backtracking (CB)–in a modern SAT solver. We demonstrate that CB improves the performance of the winner of the latest SAT Competition, Maple-LCM-Dist, and the winner of the latest MaxSAT Evaluation, Open-WBO.

Centrality-Based Improvements to CDCL Heuristics
SPEAKER: Sima Jamali

ABSTRACT. There are many reasons to think that SAT solvers should be able to exploit formula structure, but no standard techniques in modern CDCL solvers make explicit use of structure. We describe modifications to modern decision and clause-deletion heuristics that exploit formula structure by using variable centrality. We show that these improve the performance of Maple LCM Dist, the winning solver from Main Track of the 2017 SAT Solver competition. In particular, using centrality in clause deletion results in solving 9 more formulas from the 2017 Main Track. We also look at a number of measures of solver performance and learned clause quality, to see how the changes affect solver execution.

19:00-21:30 FLoC reception at Ashmolean Museum

FLoC reception at Ashmolean Museum. Drinks and canapés available from 7pm (pre-booking via FLoC registration system required; guests welcome).