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09:00-10:30 Session 125K
Location: Maths LT3
Synthesizing safe AI-based controllers for cyber-physical systems

ABSTRACT. In this talk, we will look at the problem of reasoning about safety for cyber-physical systems using semi-automated methods to learn safety invariants. We will leverage this basic technique to reason about AI-based controllers, for example, neural network based controllers trained using reinforcement learning. We show a technique that can potentially scale to controllers using 1000s of neurons, while able to prove safety of the synthesized controller. Finally, we chart a roadmap for synthesizing correct-by-design controllers that use AI-based algorithms.

Meta-Interpretive Learning of Logic Programs

ABSTRACT. Meta-Interpretive Learning (MIL) is a recent Inductive Logic Programming technique aimed at supporting learning of recursive definitions. A powerful and novel aspect of MIL is that when learning a predicate definition it automatically introduces sub-definitions, allowing decomposition into a hierarchy of reusable parts.  MIL is based on an adapted version of a Prolog meta-interpreter. Normally such a meta-interpreter derives a proof by repeatedly fetching first-order Prolog clauses whose heads unify with a given goal. By contrast, a meta-interpretive learner additionally fetches higher-order meta-rules whose heads unify with the goal, and saves the resulting meta-substitutions to form a program. This talk will overview theoretical and implementational advances in this new area including the ability to learn Turing computable functions within a constrained subset of logic programs, the use of probabilistic representations within Bayesian meta-interpretive and techniques for minimising the number of meta-rules employed. The talk will also summarise applications of MIL including the learning of regular and context-free grammars, learning from visual representations with repeated patterns, learning string transformations for spreadsheet applications, learning and optimising recursive robot strategies and learning tactics for proving correctness of programs.  The talk will conclude by pointing to the many challenges which remain to be addressed within this new area.

10:30-11:00Coffee Break
11:00-12:30 Session 127K
Location: Maths LT3
Reactive Synthesis from LTL Specification with Spot

ABSTRACT. We present ltlsynt, a new tool for reactive synthesis from LTL specifications. It relies on the efficiency of Spot to translate the input LTL specification to a deterministic parity automaton. The latter yields a parity game, which we solve with Zielonka's recursive algorithm.

The approach taken in ltlsynt was widely believed to be impractical, due to the double-exponential size of the parity game, and to the open status of the complexity of parity games resolution. ltlsynt ranked second of its track in the 2017 edition of the SYNTCOMP competition. This demonstrates the practical applicability of the parity game approach, when backed by efficient manipulations of omega-automata such as the ones provided by Spot. We present our approach and report on our experimental evaluation of ltlsynt to better understand its strengths and weaknesses.

On the Expressiveness of HyperLTL Synthesis
SPEAKER: Philip Lukert

ABSTRACT. Hyperproperties generalize trace properties, i.e., sets of traces to sets of sets of traces. Prominent examples are information-flow policies like observational determinism and noninterference. In this paper, we give an overview on the problem of automatically synthesizing implementations from hyperproperties given in the temporal logic HyperLTL. We show that HyperLTL synthesis subsumes various synthesis approaches studied in the literature: synthesis under incomplete information, distributed synthesis, symmetric synthesis, fault-tolerance synthesis, and synthesis with dynamic hierarchical information. As a case study, where the control of the flow of information and the distributivity of the system are equally important, we consider the dining cryptographers problem: neither well-studied LTL synthesis nor its distributed variant is sufficient for this problem. Our implementation of the (bounded) synthesis procedure for HyperLTL, called BoSyHyper, is able to automatically construct a solution to this problem. We present the decidability results for HyperLTL synthesis: we identify, based on the quantifier structure of the HyperLTL formula, for which fragments the synthesis problem remains decidable and for which fragments we have to rely on a bounded variation of the decision procedure.

Maximum Realizability for Linear Temporal Logic Specifications

ABSTRACT. Automatic synthesis from linear temporal logic (LTL) specifications is widely used in robotic motion planning and control of autonomous systems. A common specification pattern in such applications consists of an LTL formula describing the requirements on the behaviour of the system, together with a set of additional desirable properties. We study the synthesis problem in settings where the overall specification is unrealizable, more precisely, when some of the desirable properties have to be (temporarily) violated in order to satisfy the system's objective. We provide a quantitative semantics of sets of safety specifications, and use it to formalize the ``best-effort'' satisfaction of such soft specifications while satisfying the hard LTL specification. We propose an algorithm for synthesizing implementations that are optimal with respect to this quantitative semantics. Our method builds upon the idea of the bounded synthesis approach, and we develop a MaxSAT encoding which allows for maximizing the quantitative satisfaction of the safety specifications. We evaluate our algorithm on scenarios from robotics and power distribution networks.

Warm-Starting Fixed-Point Based Control Synthesis
SPEAKER: Zexiang Liu

ABSTRACT. In this work we propose a patching algorithm to incrementally modify controllers, synthesized to satisfy a temporal logic formula, when some of the control actions become unavailable. The main idea of the proposed algorithm is to ``warm-start" the synthesis process with an existing fixed-point based controller that has a larger action set. By exploiting the structure of the fixed-point based controllers, our algorithm avoids repeated computations while synthesizing a controller with restricted action set. Moreover, we show that the algorithm is sound and complete, that is, it provides the same guarantees as synthesizing a controller from scratch with the new action set. An example on synthesizing controllers for a simplified walking robot model under ground constraints is used to illustrate the approach. In this application, the ground constraints determine the action set and they might not be known a priori. Therefore it is of interest to quickly modify a controller synthesized for an unconstrained surface, when new constraints are encountered. Our simulations indicate that the proposed approach provides at least an order of magnitude speed-up compared to synthesizing a controller from scratch.

Safe, Automated and Formal Synthesis of Digital Controllers for Continuous Plants

ABSTRACT. We present a sound and automated approach to synthesizing safe, digital controllers for physical plants represented as linear, time-invariant models. The synthesis accounts for errors caused by the digitization effects introduced by digital controllers operating in either fixed- or floating-point arithmetic. Our approach uses counterexample-guided inductive synthesis (CEGIS): in the first phase an inductive generalisation engine produces a possible solution that is safe for some possible initial conditions but may not be safe for all initial conditions. Safety for all initial conditions is then verified in a second phase either via bounded model checking or abstract acceleration; if the verification step fails, a counterexample is provided to the inductive generalisation and the process iterates until a safe controller is obtained. We implemented our approach in a tool named DSSynth (Digital-System Synthesizer) and demonstrate its practical value by automatically synthesizing safe controllers for physical plant models from the digital control literature.

12:30-14:00Lunch Break
14:00-15:30 Session 128K
Location: Maths LT3
Scalable Synthesis with Symbolic Syntax Graphs
SPEAKER: Sumith Kulal

ABSTRACT. General-purpose program synthesizers face a tradeoff between having a rich vocabulary for output programs and the time taken to discover a solution. One performance bottleneck is the construction of a space of possible output programs that is both expressive and easy to search. In this paper we achieve both richness and scalability using a new algorithm for constructing symbolic syntax graphs out of easily specified components to represent the space of output programs. Our algorithm ensures that any program in the space is type-safe and only mutates values that are explicitly marked as mutable. It also shares structure where possible and encodes programs in a straight-line format instead of the typical bushy-tree format, which gives an inductive bias towards realistic programs. These optimizations shrink the size of the space of programs, leading to more efficient synthesis, without sacrificing expressiveness. We demonstrate the utility of our algorithm by implementing \syncro, a system for synthesis of incremental operations. We evaluate our algorithm on a suite of benchmarks and show that it performs significantly better than prior work.

On Inductive Verification and Synthesis
SPEAKER: Dennis Peuter

ABSTRACT. We study possibilities of using symbol elimination in program verification and synthesis. We consider programs in which some instructions or subprograms are not fully specified; the task is to derive conditions on the parameters (or subprograms) which imply inductivity of certain properties. We then propose a method for property-directed invariant generation and analyze its properties.

Programming by example: efficient, but not “helpful”

ABSTRACT. Programming by example (PBE) is a powerful programming paradigm based on example driven synthesis. Users can provide examples, and a tool automatically constructs a program that satisfies the examples. To investigate the impact of PBE on real-world users, we built a study around StriSynth, a tool for shell scripting by example, and recruited 27 working IT professionals to participate. In our study, we asked the users to complete three tasks with StriSynth, and the same three tasks with PowerShell, a traditional scripting language. We found that, although our participants completed the tasks more quickly with StriSynth, they reported that they believed PowerShell to be a more helpful tool.

Minimal Synthesis of String To String Functions From Examples

ABSTRACT. We study the problem of synthesizing string to string transformations from a set of input/output examples. The transformations we consider are expressed using deterministic finite automata (DFA) that read pairs of letters, one letter from the input and one from the output. The DFA corresponding to these transformations have additional constraints, ensuring that each input string is mapped to exactly one output string.

We suggest that, given a set of input/output examples, the smallest DFA consistent with the examples is a good candidate for the transformation the user was expecting. We therefore study the problem of, given a set of examples, finding a minimal DFA consistent with the examples and satisfying the functionality and totality constraints mentioned above.

We prove that, in general, this problem (the corresponding decision problem) is NP-complete. This is unlike the standard DFA minimization problem which can be solved in polynomial time. We provide several NP-hardness proofs that show the hardness of multiple (independent) variants of the problem.

Finally, we propose an algorithm for finding the minimal DFA consistent with input/output examples, that uses a reduction to SMT solvers. We implemented the algorithm, and used it to evaluate the likelihood that the minimal DFA indeed corresponds to the DFA expected by the user.

15:30-16:00Coffee Break
16:00-18:00 Session 130J
Location: Maths LT3
Towards correct-by-construction controller synthesis for self-driving cars

ABSTRACT. Can we design a provably-safe self-driving car? As the automobile evolves toward full autonomy, many driver convenience and safety automation systems are being introduced into production vehicles. Assuring the seamless and safe integration of each new automation function with existing control functions is a major challenge. The challenge gets exacerbated when considering the number of control and decision-making functionality required for a self-driving car. This talk presents recent results towards addressing this problem through the lens of formal methods and correct-by-construction controller synthesis. I will discuss three uses of controller synthesis: for composition, for supervision, and for falsification. Composition refers to the task of designing individual controllers for interacting subsystems and providing guarantees on their integrated operation. Supervision refers to the task of providing a safety envelope for a legacy controller or human drivers. Falsification refers to the task of algorithmically finding violations of specifications by given designs. I will use adaptive cruise control and lane keeping systems to demonstrate some of these ideas both in simulation, and also on a real vehicle in Mcity, the autonomous vehicle testing facility at the University of Michigan.

Competition Report: SYNTCOMP
Competition Report: SyGuS-COMP
Probabilistic and Interpretable Models of Code

ABSTRACT. I will present a method for building probabilistic models of code which combines techniques from synthesis, sampling and decision tree learning. The method scales to large datasets and is more precise and faster to train than state-of-the-art neural networks. The resulting models can be used for various tasks including statistical code synthesis, repair, debugging and translation. In the talk I will discuss what theoretical guarantees can be stated about these models, appropriate experimental results showing the models are applicable to various programming languages (e.g., Java, JavaScript, Python), and also discuss several open problems that span the intersection of program analysis, synthesis and machine learning.

19:15-21:30 Workshops dinner at Magdalen College

Workshops dinner at Magdalen College. Drinks reception from 7.15pm, to be seated by 7:45 (pre-booking via FLoC registration system required; guests welcome).