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10:30-11:00Coffee Break
11:00-12:30 Session 95A: Model Checking
Location: Maths LT1
Propositional Dynamic Logic for Higher-Order Functional Programs
SPEAKER: Yuki Satake

ABSTRACT. We present an extension of propositional dynamic logic called HOT-PDL for specifying temporal properties of higher-order functional programs. The semantics of HOT-PDL is defined over Higher-Order Traces (HOTs) that model execution traces of higher-order programs. A HOT is a sequence of events such as function calls and returns, equipped with two kinds of pointers inspired by the notion of justification pointers from game semantics: one for capturing the correspondence between call and return events, and the other for capturing higher-order control flow involving a function that is passed to or returned by a higher-order function. To allow traversal of the new kinds of pointers, HOT-PDL extends PDL with new path expressions. The extension enables HOT-PDL to specify interesting properties of higher-order programs, including stack-based access control properties and those definable using dependent refinement types. We show that HOT-PDL model checking of higher-order functional programs over bounded integers is decidable via a reduction to modal $\mu$-calculus model checking of higher-order recursion schemes.

Syntax-Guided Termination Analysis

ABSTRACT. We present new algorithms for proving program termination and non-termination using syntax-guided synthesis. They exploit the symbolic encoding of program and automatically construct a formal grammar for symbolic constraints that are used to synthesize either a termination argument or a never-terminating program refinement. The constraints are then added back to the program encoding, and an off-the-shelf constraint solver decides on their fitness and on the progress of the algorithms. The evaluation of our implementation, called FreqTerm, shows that although the number of constraints is always finite and the formal grammar is limited to the syntax of the program, in the majority of cases our algorithms are effective and fast. Importantly, FreqTerm is competitive with the state-of-the-art on a wide range of terminating and non-terminating benchmarks, and it significantly outperforms the state-of-the-art on proving non-termination of a class of programs arising from large-scale Event-Condition-Action systems.

Model Checking Quantitative Hyperproperties
SPEAKER: Hazem Torfah

ABSTRACT. Hyperproperties are properties of sets of computation traces. In this paper, we study quantitative hyperproperties, which we define as hyperproperties that express a bound on the number of traces that may appear in a certain relation. For example, quantitative non-interference limits the amount of information about certain secret inputs that is leaked through the observable outputs of a system. Quantitative non-interference thus bounds the number of traces that have the same observable input but different observable output. We study quantitative hyperproperties in the setting of HyperLTL, a temporal logic for hyperproperties. We show that, while quantitative hyperproperties can be expressed in HyperLTL, the running time of the HyperLTL model checking algorithm is, depending on the type of property, exponential or even doubly exponential in the quantitative bound. We improve this complexity with a new model checking algorithm based on model-counting. The new algorithm needs only logarithmic space in the bound and therefore improves, depending on the property, exponentially or even doubly exponentially over the model checking algorithm of HyperLTL. In the worst case, the running time of the new algorithm is exponential in the size of the system. Our SAT-based prototype implementation demonstrates, however, that the counting approach is viable on systems with nontrivial quantitative information flow requirements such as a passcode checker.

Exploiting Synchrony and Symmetry in Relational Verification
SPEAKER: Lauren Pick

ABSTRACT. Relational safety specifications describe multiple runs of the same program or relate the behaviors of multiple programs. Approaches to automatic relational verification often compose the programs and analyze the result for safety, but a naively composed program can lead to difficult verification problems. We propose to exploit relational specifications for simplifying the generated verification subtasks. First, we maximize opportunities for synchronizing code fragments. Second, we compute symmetries in the specifications to reveal and avoid redundant subtasks. We have implemented these enhancements in a prototype for verifying k-safety properties on Java programs. Our evaluation confirms that our approach leads to a consistent performance speedup on a range of benchmarks.

JBMC: A Bounded Model Checking Tool for Verifying Java Bytecode

ABSTRACT. We present a bounded model checking tool for verifying Java bytecode, which is built on top of the CProver framework, named Java Bounded Model Checker (JBMC). In summary, JBMC processes Java bytecode together with a model of the standard Java libraries, with the goal of checking a set of desired properties. Experimental results show that JBMC can correctly verify a set of Java benchmarks extracted from the literature and it also performs competitively with JayHorn and JPF, which are state-of-art Java veriers based on constrained Horn clauses and path-based symbolic execution, respectively.

Eager Abstraction for Symbolic Model Checking

ABSTRACT. We present a method of abstracting parameterized or infinite-state symbolic model checking problems to finite-state problems, based on propositional skeletons and eager theory explication. The method is extensible in the sense that users can add abstractions (or refine existing abstractions) by providing axiom schemata. We evaluate the method on a collection of parameterized and infinite-state case studies, including FLASH cache coherence protocol and the Virtually Synchronous Paxos distributed consensus protocol. The approach is seen to substantially reduce the complexity of the required auxiliary invariants in comparison to invariant checking using an SMT solver.

12:30-14:00Lunch Break
14:00-15:00 Session 96A: CAV Invited Talk: Eran Yahav
Location: Maths LT1
From Programs to interpretable Deep Models, and Back

ABSTRACT. Deep learning has revolutionized many research areas. In this talk, we demonstrate how deep learning over programs is used to provide (preliminary) augmented programmer intelligence. In the first part of the talk, we show how deep learning over programs is used to tackle tasks like code completion, code summarization, and captioning. We describe a general path-based representation of source code that can be used across programming languages and learning tasks, and discuss how this representation enables different learning algorithms. In the second part, we describe techniques for extracting interpretable representations from deep models, shedding light on what has actually been learned in various tasks.

15:00-15:30 Session 98A: Polyhedra
Location: Maths LT1
Fast Numerical Program Analysis with Reinforcement Learning

ABSTRACT. We leverage reinforcement learning (RL) to speed up numerical program analysis. The key insight is to establish a correspondence between concepts in RL and program analysis. For instance, a state in RL maps to an abstract program state in the analysis, an action maps to an abstract transformer, and at every state, we have a set of sound transformers (actions) that represent different trade-offs between precision and performance. At each iteration, the agent (analysis in our case) uses a policy that is learned offline by RL to decide on the transformer which minimizes the loss of precision at fixpoint while increasing analysis performance. Our approach leverages the idea of online decomposition (applicable to popular numerical abstract domains) to define a space of approximate transformers with varying degrees of precision and performance. Using a suitably designed set of features that capture key properties of both, abstract program states and available actions, we then apply Q-learning with linear function approximation to compute an optimized context-sensitive policy that chooses transformers during analysis. We implemented our approach for the notoriously expensive Polyhedra domain and evaluated it on a set of Linux device drivers that are expensive to analyze. The results show that our approach can yield massive speedups (up to $515$x) while maintaining precision at fixpoint.

A Direct Encoding for NNC Polyhedra
SPEAKER: Anna Becchi

ABSTRACT. We present an alternative Double Description representation for the domain of NNC (not necessarily closed) polyhedra, together with the corresponding Chernikova-like conversion procedure. The representation uses no slack variable at all and provides a solution to a few technical issues caused by the encoding of an NNC polyhedron as a closed polyhedron in a higher dimension space. A preliminary experimental evaluation shows that the new conversion algorithm is able to achieve significant efficiency improvements.

15:30-16:00Coffee Break
16:00-18:00 Session 99A: Synthesis
Location: Maths LT1
What’s hard about Boolean Functional Synthesis?

ABSTRACT. Given a relational specification between Boolean inputs and outputs, the goal of Boolean functional synthesis is to synthesize each output as a function of the inputs such that the specification is met. In this paper, we first show that unless some hard conjectures in complexity theory are falsified, Boolean functional synthesis must necessarily generate exponential-sized Skolem functions, thereby requiring exponential time, in the worst-case. Given this inherent hardness, what does one do to solve the problem? We present a two-phase algorithm for Boolean functional synthesis, where the first phase is efficient both in terms of time and sizes of synthesized functions, and solves an overwhelming majority of benchmarks. To explain this surprisingly good performance, we provide a sufficient condition under which the first phase must produce exact correct answers. When this condition fails, the second phase builds upon the result of the first phase, possibly requiring exponential time and generating exponential-sized functions in the worst-case. Detailed experimental evaluation shows our algorithm to perform better than state-of-the-art techniques for the vast majority of benchmarks.

Synthesis Modulo Theories

ABSTRACT. Abstract. Program synthesis is the mechanized construction of soft- ware. One of the main difficulties is the efficient exploration of the very large solution space, and tools often require a user-provided syntactic restriction of the search space. We propose a new approach to program synthesis that combines the strengths of a counterexample-guided in- ductive synthesiser with those of a theory solver, exploring the solution space more efficiently without relying on user guidance. We call this approach CEGIS(T ), where T is a first-order theory. In this paper, we focus on one particular application of CEGIS(T ), namely the synthesis of programs that require non-trivial constants, which is a fundamentally difficult task for state-of-the-art synthesisers. We present two exemplars, one based on Fourier-Motzkin (FM) variable elimination and one based on first-order satisfiability. We demonstrate the practical value of CEGIS(T ) by automatically synthesizing programs for a set of intricate benchmarks.

Synthesizing Reactive Systems from Hyperproperties
SPEAKER: Philip Lukert

ABSTRACT. We study the reactive synthesis problem for hyperproperties given as formulas of the temporal logic HyperLTL. Hyperproperties generalize trace properties, i.e., sets of traces, to sets of sets of traces. Typical examples are information-flow policies like noninterference, which stipulate that no sensitive data must leak into the public domain. Such properties cannot be expressed in standard linear or branching-time temporal logics like LTL, CTL, or CTL*. We show that, while the synthesis problem is undecidable for full HyperLTL, it remains decidable for the exists*, exists*forall1, and the linear forall* fragments. Beyond these fragments, the synthesis problem immediately becomes undecidable. For universal HyperLTL, we present a semi-decisionprocedure that constructs implementations and counterexamples up to a given bound. We report encouraging experimental results obtained with a prototype implementation on example specifications with hyperproperties like symmetric responses, secrecy, and information flow.

Reactive Control Improvisation

ABSTRACT. Reactive synthesis is a paradigm for automatically building correct-by-construction systems that interact with an unknown or adversarial environment. We study how to do reactive synthesis when part of the specification of the system is that its behavior should be random. Randomness can be useful, for example, in a network protocol fuzz tester whose output should be varied, or a planner for a surveillance robot whose route should be unpredictable. However, existing reactive synthesis techniques do not provide a way to ensure random behavior while maintaining functional correctness. Towards this end, we generalize the recently-proposed framework of control improvisation (CI) to add reactivity. The resulting framework of reactive control improvisation provides a natural way to integrate a randomness requirement with the usual functional specifications of reactive synthesis over a finite window. We theoretically characterize when such problems are realizable, and give a general method for solving them. For specifications given by reachability or safety games or by deterministic finite automata, our method yields a polynomial-time synthesis algorithm. For various other types of specifications including temporal logic formulas, we obtain a polynomial-space algorithm and prove matching PSPACE-hardness results. We show that all of these randomized variants of reactive synthesis are no harder in a complexity-theoretic sense than their non-randomized counterparts.

Constraint-Based Synthesis of Coupling Proofs

ABSTRACT. Proof by coupling is a classical technique for proving properties about pairs of randomized algorithms by carefully relating (or coupling) two probabilistic executions. In this paper, our goal is to automatically construct such proofs for programs. First, we present f-coupled postconditions, an abstraction describing two coupled programs. Second, we show how specific forms of f-coupled postconditions imply probabilistic properties like uniformity and independence of program variables. Third, we demonstrate how to automate construction of coupling proofs by reducing the problem to solving a purely logical synthesis problem of the form ∃f. ∀X. φ, thus doing away with probabilistic reasoning. Finally, we evaluate our technique in a prototype implementation, and demonstrate its ability to construct a range of coupling proofs for various properties and randomized algorithms.

Controller Synthesis Made Real: Reach-avoid Specifications and Linear Dynamics
SPEAKER: Umang Mathur

ABSTRACT. We address the problem of synthesizing provably correct controllers for linear systems with reach-avoid specifications. Our solution uses a combination of an open-loop controller and a tracking controller, thereby reducing the problem to smaller tractable problems. We show that, once a tracking controller is fixed, the reachable states from an initial neighborhood, subject to any disturbance, can be over-approximated by a sequence of ellipsoids, with sizes that are independent of the open-loop controller. Hence, the open-loop controller can be synthesized independently to meet the reach-avoid specification for an initial neighborhood. Exploiting several techniques for tightening the over-approximations, we reduce the open-loop controller synthesis problem to satisfiability over quantifier-free linear real arithmetic. The overall synthesis algorithm, computes a tracking controller, and then iteratively covers the entire initial set to find open-loop controllers for initial neighborhoods. The algorithm is sound and, for a class of robust systems, is also complete. We present RealSyn, a tool implementing this synthesis algorithm, and we show that it scales to several high-dimensional systems with complex reach-avoid specifications.

Synthesis Of Asynchronous Reactive Programs From Temporal Specifications

ABSTRACT. Asynchronous interactions are ubiquitous in computing systems and complicate design and programming. Automatic construction (``synthesis'') of asynchronous programs from specifications could ease the difficulty, but known methods are intractable in practice. This work develops substantially simpler methods for asynchronous synthesis. An exponentially more compact automaton construction is developed for the reduction of asynchronous to synchronous synthesis. Experiments with a prototype implementation of the new method demonstrate practical feasibility. Furthermore, it is shown that for several useful classes of temporal properties, automaton-based methods can be avoided altogether and replaced with simpler Boolean constraint solving.

Syntax Guided Synthesis with Quantitative Syntactic Objectives
SPEAKER: Qinheping Hu

ABSTRACT. Automatic program synthesis promises to increase the productivity of programmers and end-users of computing devices by automating tedious and error-prone tasks. Despite the practical successes of program synthesis, we still do not have systematic frameworks to synthesize programs that are ``good'' according to certain metrics---e.g., produce programs of reasonable size or with good runtime---and to understand when synthesis can result in such good programs. In this paper, we propose QSyGuS, a unifying framework for describing syntax-guided synthesis problems with quantitative objectives over the syntax of the synthesized programs. QSyGuSbuilds on weighted (tree) grammars, a clean and foundational formalism that provides flexible support for different quantitative objectives, useful closure properties, and practical decision procedures. We then present an algorithm for solving QSyGuS. Our algorithm leverages closure properties of weighted grammars to generate intermediate problems that can be solved using non-quantitative \sygus solvers. Finally, we implement our algorithm in a tool, QuaSi, and evaluate it on 26 quantitative extensions of existing SyGuS benchmarks. QuaSi can synthesize optimal solutions in 15/26 benchmarks with times comparable to those needed to not find an arbitrary solution.

19:00-21:30 FLoC reception at Oxford Town Hall

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