CAV2024: 36TH INTERNATIONAL CONFERENCE ON COMPUTER AIDED VERIFICATION
PROGRAM FOR THURSDAY, JULY 25TH
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09:00-10:10 Session 5: Decision Procedures 2
Location: MB 1.210
09:00
Scalable Bit-Blasting with Abstractions

ABSTRACT. The dominant state-of-the-art approach for solving bit-vector formulas in Satisfiability Modulo Theories (SMT) is bit-blasting, an eager reduction to propositional logic. Bit-blasting is surprisingly efficient in practice, but does not generally scale well with increasing bit-widths, especially when bit-vector arithmetic is present. In this paper, we present a novel CEGAR-style abstraction-refinement procedure for the theory of fixed-size bit-vectors that significantly improves the scalability of bit-blasting. We provide lemma schemes for various arithmetic bit-vector operators and an abduction-based framework for synthesizing refinement lemmas. We extended the state-of-the-art SMT solver Bitwuzla with our abstraction-refinement approach and show that it significantly improves solver performance on a variety of benchmark sets, including industrial benchmarks that arise from smart contract verification.

09:20
From Clauses to Klauses

ABSTRACT. Satisfiability (SAT) solvers have used the the same input format for decades: a formula in conjunctive normal form (CNF). However, cardinality constraints appear frequently in problem descriptions, with over 64% of the SAT Competition formulas containing at least one cardinality constraint and over 17% containing many large cardinality constraints. Allowing general cardinality constraints as input would simplify encodings and enable the solver to handle constraints natively or to encode them using different (and possibly dynamically changing) clausal forms. We modify the modern SAT solver cadical to handle cardinality constraints natively. Unlike the stronger cardinality reasoning in pseudo-Boolean or other systems, our incremental approach with cardinality-based propagation requires only moderate changes to a SAT solver, preserves the ability to run important inprocessing techniques, and is easily combined with existing proof-producing and validation tools. Our experimental evaluation on SAT Competition formulas shows our solver configurations with cardinality support consistently outperform other SAT and PB solvers.

09:40
Formally Certified Approximate Model Counting

ABSTRACT. Approximate model counting is the task of approximating the number of solutions to an input formula. The state-of-the-art approximate model counter for formulas in conjunctive normal form (CNF), ApproxMC, provides a scalable means of obtaining model counts with probably approximately correct (PAC)-style guarantees. Nevertheless, the validity of ApproxMC's approximation relies on a careful theoretical analysis of its randomized algorithm and the correctness of its highly optimized implementation, especially the latter's stateful interactions with an incremental CNF-XOR satisfiability solver.

We present the first certification framework for approximate model counting with formally verified guarantees on the quality of its output approximation. Our approach combines: (i) a static, once-off, formal proof of the algorithm's PAC guarantee in the Isabelle/HOL proof assistant; and (ii) dynamic, per-run, verification of ApproxMC's calls to an external CNF-XOR solver using proof certificates. We detail our general approach to establish a rigorous connection between these two parts of the verification, including our blueprint for turning the formalized, randomized algorithm into a verified proof checker, and our design of proof certificates for both ApproxMC and its internal CNF-XOR solving steps. Experimentally, we show that certificate generation adds little overhead to an approximate counter implementation, and that our certificate checker is able to fully certify 84.7% of instances with generated certificates when given the same time and memory limits as the counter.

10:00
CaDiCaL 2.0

ABSTRACT. The SAT solver CaDiCaL provides a rich feature set with a clean library interface. It has been adopted by many users, is well documented and easy to extend due to its effective testing and debugging infrastructure. In this tool paper we give a high-level introduction into the solver architecture and then go briefly over implemented techniques. We describe basic features and novel advanced usage scenarios. Experiments confirm that CaDiCaL despite this flexibility has state-of-the-art performance both in a stand-alone as well as incremental setting.

10:10-10:30 Session 6: Machine Learning and Neural Networks 1
Location: MB 1.210
10:10
Monitizer: Automating Design and Evaluation of Neural Network Monitors

ABSTRACT. The behavior of neural networks (NN) on previously unseen types of data (out-of-distribution or OOD) is typically unpredictable. Hence, detecting that an input is OOD is important for the safe operation of the NN. This can be achieved by monitoring the NN at runtime.

While various monitors have been suggested recently, their optimization for a given problem as well as comparison and reproduction remain very demanding. We present a tool for users and developers of such monitors of NNs. It allows for automatic (i) application of various types of monitors from literature to a given input NN, (ii) optimization of the monitor's hyperparameters, and (iii) experimental evaluation and comparison to other approaches. Besides, it facilitates the development of new monitoring approaches. We demonstrate the usability of the tool on several use cases of different types of users as well as on a case study comparing different approaches from recent literature.

10:20
Marabou 2.0: A Versatile Formal Analyzer of Neural Networks

ABSTRACT. This paper serves as a comprehensive system description of version 2.0 of the Marabou framework for formal analysis of neural networks. We discuss the tool's architectural design and highlight the major features and components introduced since its initial release.

10:30-11:00Coffee Break
11:00-12:00 Session 7: CAV Keynote: Lean 4: Bridging Formal Mathematics and Software Verification
Location: MB 1.210
11:00
Lean 4: Bridging Formal Mathematics and Software Verification

ABSTRACT. This talk will explore the dual applications of Lean 4, the latest iteration of the Lean proof assistant and programming language, in advancing formal mathematics and software verification. We begin with an overview of its design and implementation. We will detail how Lean 4 enables the formalization of complex mathematical theories and proofs, thereby enhancing collaboration and reliability in mathematical research. This endeavor is supported by a philosophy that promotes decentralized innovation, empowering a diverse community of researchers, developers, and enthusiasts to collaboratively push the boundaries of mathematical practice. Simultaneously, we will discuss software verification applications using Lean 4 at AWS. By leveraging Lean’s dual capabilities as both a proof assistant and a functional programming language, we achieve a cohesive approach to software development and verification. Additionally, the talk will outline future directions for Lean 4, including efforts to expand its user community, enhance user experience, and further integrate formal methods into both academic research and industrial applications.

12:00-14:00Lunch
14:00-15:30 Session 8: Software Verification 1
Location: MB 1.210
14:00
Strided Difference Bound Matrices

ABSTRACT. A wide range of symbolic analysis and optimization problems can be formalized using polyhedra. Sub-classes of polyhedra, also known as sub-polyhedral domains, are sought for their lower space and time complexity. We introduce the Strided Difference Bound Matrix (SDBM) domain, which represents a sweet spot in the context of optimizing compilers. Its expressiveness and efficient algorithms are particularly well suited to the construction of machine learning compilers. We present decision algorithms, abstract domain operators and computational complexity proofs for SDBM. We also conduct an empirical study with the MLIR compiler framework to validate the domain's practical applicability. We characterize a sub-class of SDBMs that frequently occurs in practice, and demonstrate even faster algorithms on this sub-class.

14:20
On Polynomial Expressions with C-Finite Recurrences in Loops with Nested Nondeterministic Branches

ABSTRACT. Loops are inductive constructs, which make them difficult to analyze and verify in general. One approach is to represent the inductive behaviors of the program variables in a loop by recurrences and try to solve them for closed-form solutions. These solutions can then be used to generate invariants or directly fed into an SMT-based verifier. One problem with this approach is that if a loop contains nondeterministic choices or complex operations such as non-linear assignments, then recurrences for program variables may not exist or have no closed-form solutions. In such cases, an alternative is to generate recurrences for expressions, and there has been recent work along this line. In this paper, we further work in this direction and propose a template-based method for extracting polynomial expressions that satisfy some c-finite recurrences. We show that computationally, this amounts to solving a system of quadratic equations. While in general these quadratic equations may have infinite solutions, we show that unless the individual program variables all have closed-form solutions, the desired polynomials form a finite union of vector spaces. We propose an algorithm for computing the bases of the vector spaces, and identify two cases where the bases can be computed efficiently. To demonstrate the usefulness of our results, we implemented a prototype system based on one of the special cases, and integrated it into a SMT-based verifier. Our experimental results show that the new verifier can now verify programs with non-linear properties.

14:40
Verification Algorithms for Automated Separation Logic Verifiers

ABSTRACT. Most automated program verifiers for separation logic use either symbolic execution or verification condition generation to extract proof obligations, which are then handed over to an SMT solver. Existing verification algorithms are designed to be sound, but differ in performance and completeness. These characteristics may also depend on the programs and properties to be verified. Consequently, developers and users of program verifiers have to select a verification algorithm carefully for their application domain. Taking an informed decision requires a systematic comparison of the performance and completeness characteristics of the verification algorithms used by modern separation logic verifiers, but such a comparison does not exist.

This paper describes five verification algorithms for separation logic, three that are used in existing tools and two novel algorithms that combine characteristics of existing symbolic execution and verification condition generation algorithms. A detailed evaluation of implementations of these five algorithms in the Viper infrastructure assesses their performance and completeness for different classes of input programs. Based on the experimental results, we identify candidate portfolios of algorithms that maximize completeness and performance.

15:00
SMT-based Symbolic Model-Checking for Operator Precedence Languages

ABSTRACT. Operator Precedence Languages (OPL) have been recently identified as a suitable formalism for model checking recursive procedural programs, thanks to their ability of modeling the program stack. OPL requirements can be expressed in the Precedence Oriented Temporal Logic (POTL), which features modalities to reason on the natural matching between function calls and returns, exceptions, and other advanced programming constructs that previous approaches, such as Visibly Pushdown Languages, cannot model effectively. Existing approaches for model checking of POTL have been designed following the explicit-state, automata-based approach, a feature that severely limits their scalability. In this paper, we give the first symbolic, SMT-based approach for model checking POTL properties. While previous approaches construct the automaton for both the POTL formula and the model of the program, we encode them into a (sequence of) SMT formulas. The search of a trace of the model witnessing a violation of the formula is then carried out by an SMT-solver, in a Bounded Model Checking fashion. We carried out an experimental evaluation, which shows the effectiveness of the proposed solution.

15:20
A Framework for Debugging Automated Program Verification Proofs via Proof Actions

ABSTRACT. Many program verification tools provide automation via SMT solvers, allowing them to automatically discharge many proofs. However, when a proof fails, it can be hard to understand why it failed or how to fix it. The main feedback the developer receives is simply the verification result (i.e., success or failure), with no visibility into the solver’s internal state. To assist developers using such tools, we introduce ProofPlumber, a novel and extensible proof-action framework for understanding and debugging proof failures. Proof actions act on the developer’s source-level proofs (e.g., assertions and lemmas) to determine why they failed and potentially suggest remedies. We evaluate ProofPlumber by writing a collection of proof actions that capture common proof debugging practices. We produce 17 proof actions, each only 29–176 lines of code.

15:30-16:00Coffee Break
16:00-18:10 Session 9: Synthesis and Repair
Location: MB 1.210
16:00
Relational Synthesis of Recursive Programs via Constrained Tree Automata

ABSTRACT. In this paper, we present a new synthesis method based on the novel concept of a constrained tree automaton (CTA). A CTA is a variant of a finite tree automaton (FTA) where the acceptance of a term by the automaton is conditioned upon the logical satisfiability of a formula. In the context of program synthesis, CTAs allow the construction of a more precise version space than FTAs by ruling out programs that make inconsistent assumptions about the unknown semantics of functions under synthesis. We apply our proposed algorithm to synthesizing recursive (or mutually recursive) procedures from relational specifications and demonstrate that our method allows solving synthesis problems that are beyond the scope of existing approaches.

16:20
Synthesis of Temporal Causality

ABSTRACT. We present an automata-based algorithm to synthesize omega-regular causes for omega-regular effects on executions of a reactive system, such as counterexamples uncovered by a model checker. Our theory is a generalization of temporal causality, which has recently been proposed as a framework for drawing causal relationships between trace properties on a given trace. So far, algorithms exist only for verifying a single causal relationship and, as an extension, cause synthesis through enumeration, which is complete only for a small fragment of effect properties. This work presents the first complete cause-synthesis algorithm for the class of omega-regular effects. We demonstrate the practical feasibility of our algorithm with the prototype tool CORP and evaluate its performance for cause synthesis and cause checking.

16:40
Localized Attractor Computations for Infinite-State Games

ABSTRACT. Infinite-state games are a commonly used model for the synthesis of reactive systems with unbounded data domains. Symbolic methods for solving such games need to be able to construct intricate arguments to establish the existence of winning strategies. Often, large problem instances require prohibitively complex arguments. Therefore, techniques that identify smaller and simpler sub-problems and exploit the respective results for the given game-solving task are highly desirable.

In this paper, we propose the first such technique for infinite-state games. The main idea is to enhance symbolic game-solving with the results of localized attractor computations performed in sub-games. The crux of our approach lies in identifying useful sub-games by computing permissive winning strategy templates in finite abstractions of the infinite-state game. The experimental evaluation of our method demonstrates that it outperforms existing techniques and is applicable to infinite-state games beyond the state of the art.

17:00
Syntax-Guided Automated Program Repair For Hyperproperties

ABSTRACT. We study the problem of automatically repairing infinite-state software programs w.r.t. temporal hyperproperties. As a first step, we present a repair approach for the temporal logic HyperLTL based on symbolic execution, constraint generation, and syntax-guided synthesis of repair expression (SyGuS). To improve the repair quality, we introduce the notation of a transparent repair that aims to find a patch that is as close as possible to the original program. As a practical realization, we develop an iterative repair approach. Here, we search for a sequence of repairs that are closer and closer to the original program's behavior. We implement our method in a prototype and report on encouraging experimental results using off-the-shelf SyGuS solvers.

17:20
Dynamic Programming for Symbolic Boolean Realizability and Synthesis

ABSTRACT. Inspired by recent progress in dynamic programming approaches for weighted model counting, we investigate a dynamic-programming approach in the context of boolean realizability and synthesis, which takes a conjunctive-normal-form boolean formula over input and output variables, and aims at synthesizing witness functions for the output variables in terms of the inputs. We show how graded project-join trees, obtained via tree decomposition, can be used to compute a BDD representing the realizability set for the input formulas in a bottom-up order. We then show how the intermediate BDDs generated during realizability checking phase can be applied to synthesizing the witness functions in a top-down manner. An experimental evaluation of a solver -- DPSynth -- based on these ideas demonstrates that our approach for Boolean realizabilty and synthesis has superior time and space performance over a heuristics-based approach using same symbolic representations. We discuss the advantage on scalability of the new approach, and also investigate our findings on the performance of the DP framework.

17:40
Information Flow guided Synthesis with Unbounded Communication

ABSTRACT. Information flow guided synthesis is a compositional approach to the automated construction of distributed systems where the assumptions between the components are captured as information-flow requirements. Information-flow requirements are hyperproperties that ensure that if a component needs to act on certain information that is only available in other components, then this information will be passed to the component. We present a new method for the automatic construction of information flow assumptions from specifications given as temporal safety properties. The new method is the first approach to handle situations where the required amount of information is unbounded. For example, we can analyze communication protocols that transmit messages in a potentially infinite loop. We show that component implementations can then, in principle, be constructed from the information flow requirements using a synthesis tool for hyperproperties. We additionally present a more practical synthesis technique that constructs the components using efficient methods for standard synthesis from trace properties. We have implemented the technique in the tool FlowSy, which outperforms previous approaches to distributed synthesis on several benchmarks from the literature.

18:00
The SemGuS Toolkit

ABSTRACT. Semantics-Guided Synthesis (SemGuS) is a programmable framework for defining synthesis problems in a domain- and solver-agnostic way. This paper presents the standardized SemGuS format, together with an open-source toolkit that provides a parser, a verifier, and enumerative SemGuS solvers. The paper also describes an initial set of SemGuS benchmarks, which form the basis for comparing SemGuS solvers, and presents an evaluation of the baseline enumerative solvers.

18:30-19:30 Session 10: Logic Lounge
Location: MB 1.210
18:30
Realizing Leibniz’s Dream

ABSTRACT. In the 17th Century, the philosopher, mathematician and lawyer Gottlieb Leibniz envisioned the creation of a characteristica universalis and calculus ratiocinator that would enable reasoning in law and morals as systematically as in geometry and analysis. His goal was to resolve legal disputes with the precision and clarity with which accountants settle financial discrepancies. “To take pen in hand, sit down at the abacus and, having called in a friend if they want, say to each other: Let us calculate!”

We are now, for the first time in history, positioned to realize Leibniz’s dream of automating legal reasoning. The crucial step in this process, I will argue, is the alignment of sophisticated computer science techniques with appropriate types of legal problems. Automating code-based legal reasoning, which relies on explicit statutes and regulations, differs fundamentally from automating case-based reasoning, which depends on precedents and interpretations. I will explore how formal methods and Large Language Models (LLMs) can be utilized to achieve what Leibniz envisioned three centuries ago, effectively transforming the landscape of legal reasoning through the power of modern computational technology.