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08:30-09:00Coffee & Refreshments
09:00-10:30 Session 94E
Refinement of Parallel Algorithms down to LLVM

ABSTRACT. We present a stepwise refinement approach to develop verified parallel algorithms, down to efficient LLVM code. The resulting algorithms' performance is competitive with their counterparts implemented in C/C++.

Our approach is backwards compatible with the Isabelle Refinement Framework, such that existing sequential formalizations can easily be adapted or re-used.

As case study, we verify a simple parallel sorting algorithm, and show that it performs on par with its C++ implementation, and is competitive to state-of-the-art parallel sorting algorithms.

Taming an Authoritative Armv8 ISA Specification: L3 Validation and CakeML Compiler Verification
PRESENTER: Hrutvik Kanabar

ABSTRACT. Machine-readable specifications for the Armv8 instruction set architecture have become publicly available as part of Arm's release processes, providing an official and unambiguous source of truth for the semantics of Arm instructions. To date, compiler and machine code verification efforts have made use of unofficial theorem proving friendly specifications of Armv8, e.g. CakeML uses an L3-based specification. The validity of these verification efforts hinges upon their unofficial ISA specifications being valid with respect to the official Arm specification.

Leveraging the Sail language ecosystem, we bridge this validation gap by formally verifying that an L3-based specification simulates the official Arm specification using the HOL4 interactive theorem prover. We exercise this simulation by proving a novel compiler correctness result for CakeML with respect to Arm's official specification of the Armv8.6 A-class instruction set.

Dandelion: Certified Approximations of Elementary Functions
PRESENTER: Heiko Becker

ABSTRACT. Elementary function operations such as sin and exp cannot in general be computed exactly on today's digital computers, and thus have to be approximated. The standard approximations in library functions typically provide only a limited set of precisions, and are too inefficient for many applications. Polynomial approximations that are customized to a limited input domain and output accuracy can provide superior performance. In fact, the Remez algorithm computes the best possible approximation for a given polynomial degree, but has so far not been formally verified.

This paper presents Dandelion, an automated certificate checker for polynomial approximations of elementary functions computed with Remez-like algorithms that is fully verified in the HOL4 theorem prover. Dandelion checks whether the difference between a polynomial approximation and its target reference elementary function remains below a given error bound for all inputs in a given constraint. By extracting a verified binary with the CakeML compiler, Dandelion can validate certificates within a reasonable time, fully automating previous manually verified approximations.

10:30-11:00Coffee Break
11:00-12:30 Session 96E
Synthetic Kolmogorov Complexity in Coq
PRESENTER: Yannick Forster

ABSTRACT. We present a generalised, constructive, and machine-checked approach to Kolmogorov complexity in the constructive type theory underlying the Coq proof assistant. By proving that nonrandom numbers form a simple predicate, we obtain elegant proofs of undecidability for random and nonrandom numbers and a proof of uncomputability of Kolmogorov complexity.

We use a general and abstract definition of Kolmogorov complexity and subsequently instantiate it to several definitions frequently found in the literature.

Whereas textbook treatments of Kolmogorov complexity usually rely heavily on classical logic and the axiom of choice, we put emphasise on the constructiveness of all our arguments, however without blurring their essence. We first give a high-level proof idea using classical logic, which can be formalised using Markov's principle using folklore techniques we subsequently explain.Lastly, we show a strategy how to eliminate Markov's principle from a certain class of computability proofs, rendering all our results fully constructive.

All our results are machine-checked by the Coq proof assistant, which is enabled by using a synthetic approach to computability: Rather than formalising a model of computation, which is well-known to introduce a considerable overhead, we abstractly assume a universal function, allowing the proofs to focus on the mathematical essence.

Formalizing Szemerédi's Regularity Lemma in Lean
PRESENTER: Yaël Dillies

ABSTRACT. Szemerédi's Regularity Lemma is a fundamental result in graph theory with extensive applications to combinatorics and number theory. In essence, it says that all graphs can be approximated by well-behaved unions of random bipartite graphs. We present a formalisation in the Lean theorem prover of a strong version of this lemma in which each part of the union must be approximately the same size. This stronger version has not been formalised previously in any theorem prover. Our proof closely follows the pen-and-paper method, allowing our formalisation to provide an explicit upper bound on the number of parts. An application of this lemma is also formalised, namely Roth's theorem on arithmetic progressions in qualitative form via the triangle removal lemma.

Formalizing the divergence theorem and the Cauchy integral formula in Lean

ABSTRACT. I formalize a version of the divergence theorem for a function on a rectangular box that does not assume regularity of individual partial derivatives, only Fréchet differentiability of the vector field and integrability of its divergence. Then I use this theorem to prove Cauchy-Goursat theorem (for some simple domains) and bootstrap complex analysis in the Lean mathematical library. The main tool is the GP-integral, a version of the Henstock-Kurzweil integral suggested by J. Mawhin in 1981. The divergence theorem for this integral does not require any regularity assumptions on the derivative of a vector field.

12:30-14:00Lunch Break

Lunches will be held in Taub lobby (CAV, CSF) and in The Grand Water Research Institute (DL, NMR, IJCAR, ITP).

14:00-15:30 Session 97E
Deeper Shallow Embeddings
PRESENTER: Jacob Prinz

ABSTRACT. Deep and shallow embeddings are two popular techniques for embedding a language in a host language with complementary strengths and weaknesses. In a deep embedding, embedded constructs are defined as data in the host: this allows for syntax manipulation and facilitates metatheoretic reasoning, but is challenging to implement---especially in the case of dependently typed embedded languages. In a shallow embedding, by contrast, constructs are encoded using features of the host: this makes them quite straightforward to implement, but limits their use in practice.

In this paper, we attempt to bridge the gap between the two, by presenting a general technique for extending a shallow embedding of a type theory with a deep embedding of its typing derivations. Such embeddings are almost as straightforward to implement as shallow ones, but come with capabilities traditionally associated with deep ones. We demonstrate these increased capabilities in a number of case studies; including a DSL that only holds affine terms, and a dependently typed core language with computational beta reduction that leverages function extensionality.

Compositional Verification of Interacting Systems using Event Monads

ABSTRACT. Large software systems are usually divided into multiple components that interact with each other. How to verify interacting components in a modular way is one of the major problems in formal verification. In many cases, interaction between components can be modeled asynchronously, where events are sent without requiring a response in order to continue with execution of the component. In this paper, we propose a lightweight, event-based framework for verification of components with asynchronous interaction. We define event monads and event systems, and a Hoare logic-style calculi for reasoning about them. The framework is implemented in Isabelle and applied to several case studies, including models for distributed computing, cache-coherence protocols, and verification of partition scheduling in a real-time operating system.

Reflexive tactics for algebra, revisited

ABSTRACT. Computational reflection allows us to turn verified decision procedures into efficient automated reasoning tools in proof assistants. The typical applications of such methodology include mathematical structures that have decidable theory fragments, e.g., equational theories of commutative rings and lattices. However, such existing tools are known not to cooperate with packed classes, a methodology to define mathematical structures in dependent type theory, that allows for the sharing of vocabulary across the inheritance hierarchy. Additionally, such tools do not support homomorphisms whose domain and codomain types may differ. This paper demonstrates how to implement reflexive tactics that support packed classes and homomorphisms. As applications of our methodology, we adapt the ring and field tactics of Coq to the commutative ring and field structures of the Mathematical Components library, and apply the resulting tactics to the formal proof of the irrationality of ζ(3) by Chyzak, Mahboubi, and Sibut-Pinote, to bring more proof automation.

15:30-16:00Coffee Break
18:30-20:30 Walking tour (at Haifa)

pickup at 18:00 from the Technion (Tour at Haifa, no food will be provided)