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## Über dieses Buch

This book constitutes the proceedings of the 6th International Conference on Interactive Theorem Proving, ITP 2015, held in Nanjing, China, in August 2015. The 27 papers presented in this volume were carefully reviewed and selected from 54 submissions. The topics range from theoretical foundations to implementation aspects and applications in program verification, security and formalization of mathematics.

## Inhaltsverzeichnis

### Verified Over-Approximation of the Diameter of Propositionally Factored Transition Systems

To guarantee the completeness of bounded model checking (BMC) we require a completeness threshold. The diameter of the Kripke model of the transition system is a valid completeness threshold for BMC of safety properties. The recurrence diameter gives us an upper bound on the diameter for use in practice. Transition systems are usually described using (propositionally) factored representations. Bounds for such lifted representations are calculated in a compositional way, by first identifying and bounding atomic subsystems, and then composing those results according to subsystem dependencies to arrive at a bound for the concrete system. Compositional approaches are invalid when using the diameter to bound atomic subsystems, and valid when using the recurrence diameter. We provide a novel overapproximation of the diameter, called the sublist diameter, that is tighter than the recurrence diameter. We prove that compositional approaches are valid using it to bound atomic subsystems. Those proofs are mechanised in HOL4. We also describe a novel verified compositional bounding technique which provides tighter overall bounds compared to existing bottom-up approaches.
Mohammad Abdulaziz, Charles Gretton, Michael Norrish

### Formalization of Error-Correcting Codes: From Hamming to Modern Coding Theory

By adding redundancy to transmitted data, error-correcting codes (ECCs) make it possible to communicate reliably over noisy channels. Minimizing redundancy and (de)coding time has driven much research, culminating with Low-Density Parity-Check (LDPC) codes. At first sight, ECCs may be considered as a trustful piece of computer systems because classical results are well-understood. But ECCs are also performance-critical so that new hardware calls for new implementations whose testing is always an issue. Moreover, research about ECCs is still flourishing with papers of ever-growing complexity. In order to provide means for implementers to perform verification and for researchers to firmly assess recent advances, we have been developing a formalization of ECCs using the SSReflect extension of the Coq proof-assistant. We report on the formalization of linear ECCs, duly illustrated with a theory about the celebrated Hamming codes and the verification of the sum-product algorithm for decoding LDPC codes.
Reynald Affeldt, Jacques Garrigue

We present ROSCoq, a framework for developing certified Coq programs for robots. ROSCoq subsystems communicate using messages, as they do in the Robot Operating System (ROS). We extend the logic of events to enable holistic reasoning about the cyber-physical behavior of robotic systems. The behavior of the physical world (e.g. Newton’s laws) and associated devices (e.g. sensors, actuators) are specified axiomatically. For reasoning about physics we use and extend CoRN’s theory of constructive real analysis. Instead of floating points, our Coq programs use CoRN’s exact, yet fast computations on reals, thus enabling accurate reasoning about such computations.
As an application, we specify the behavior of an iRobot Create. Our specification captures many real world imperfections. We write a Coq program which receives requests to navigate to specific positions and computes appropriate commands for the robot. We prove correctness properties about this system. Using the ROSCoq shim, we ran the program on the robot and provide even experimental evidence of correctness.
Abhishek Anand, Ross Knepper

### Asynchronous Processing of Coq Documents: From the Kernel up to the User Interface

The work described in this paper improves the reactivity of the Coq system by completely redesigning the way it processes a formal document. By subdividing such work into independent tasks the system can give precedence to the ones of immediate interest for the user and postpone the others. On the user side, a modern interface based on the PIDE middleware aggregates and presents in a consistent way the output of the prover. Finally postponed tasks are processed exploiting modern, parallel, hardware to offer better scalability.
Bruno Barras, Carst Tankink, Enrico Tassi

### A Concrete Memory Model for CompCert

Semantics preserving compilation of low-level C programs is challenging because their semantics is implementation defined according to the C standard. This paper presents the proof of an enhanced and more concrete memory model for the CompCert C compiler which assigns a definite meaning to more C programs. In our new formally verified memory model, pointers are still abstract but are nonetheless mapped to concrete 32-bit integers. Hence, the memory is finite and it is possible to reason about the binary encoding of pointers. We prove that the existing memory model is an abstraction of our more concrete model thus validating formally the soundness of CompCert’s abstract semantics of pointers. We also show how to adapt the front-end of CompCert thus demonstrating that it should be feasible to port the whole compiler to our novel memory model.
Frédéric Besson, Sandrine Blazy, Pierre Wilke

### Validating Dominator Trees for a Fast, Verified Dominance Test

The problem of computing dominators in a control flow graph is central to numerous modern compiler optimizations. Many efficient algorithms have been proposed in the literature, but mechanizing the correctness of the most sophisticated algorithms is still considered as too hard problems, and to this date, verified compilers use less optimized implementations. In contrast, production compilers, like GCC or LLVM, implement the classic, efficient Lengauer-Tarjan algorithm [12], to compute dominator trees. And subsequent optimization phases can then determine whether a CFG node dominates another node in constant time by using their respective depth-first search numbers in the dominator tree. In this work, we aim at integrating such techniques in verified compilers. We present a formally verified validator of untrusted dominator trees, on top of which we implement and prove correct a fast dominance test following these principles. We conduct our formal development in the Coq proof assistant, and integrate it in the middle-end of the CompCertSSA verified compiler. We also provide experimental results showing performance improvement over previous formalizations.
Sandrine Blazy, Delphine Demange, David Pichardie

### Refinement to Certify Abstract Interpretations, Illustrated on Linearization for Polyhedra

Our concern is the modular development of a certified static analyzer in Coq: we extend a certified abstract domain of convex polyhedra with a linearization procedure approximating polynomial expressions. In order to help such a development, we propose a proof framework, embedded in Coq, that implements a refinement calculus.
Sylvain Boulmé, Alexandre Maréchal

### Mechanisation of AKS Algorithm: Part 1 – The Main Theorem

The AKS algorithm (by Agrawal, Kayal and Saxena) is a significant theoretical result proving “PRIMES in P”, as well as a brilliant application of ideas from finite fields. This paper describes the first step towards the goal of a full mechanisation of this result: a mechanisation of the AKS Main Theorem, which justifies the correctness (but not the complexity) of the AKS algorithm.
Hing-Lun Chan, Michael Norrish

### Machine-Checked Verification of the Correctness and Amortized Complexity of an Efficient Union-Find Implementation

Union-Find is a famous example of a simple data structure whose amortized asymptotic time complexity analysis is non-trivial. We present a Coq formalization of this analysis. Moreover, we implement Union-Find as an OCaml library and formally endow it with a modular specification that offers a full functional correctness guarantee as well as an amortized complexity bound. Reasoning in Coq about imperative OCaml code relies on the CFML tool, which is based on characteristic formulae and Separation Logic, and which we extend with time credits. Although it was known in principle that amortized analysis can be explained in terms of time credits and that time credits can be viewed as resources in Separation Logic, we believe our work is the first practical demonstration of this approach.
Arthur Charguéraud, François Pottier

### Formalizing Size-Optimal Sorting Networks: Extracting a Certified Proof Checker

Since the proof of the four color theorem in 1976, computer-generated proofs have become a reality in mathematics and computer science. During the last decade, we have seen formal proofs using verified proof assistants being used to verify the validity of such proofs.
In this paper, we describe a formalized theory of size-optimal sorting networks. From this formalization we extract a certified checker that successfully verifies computer-generated proofs of optimality on up to 8 inputs. The checker relies on an untrusted oracle to shortcut the search for witnesses on more than 1.6 million NP-complete subproblems.
Luís Cruz-Filipe, Peter Schneider-Kamp

### Proof-Producing Reflection for HOL

With an Application to Model Polymorphism
We present a reflection principle of the form “If $$\ulcorner \varphi \urcorner$$ is provable, then $$\varphi$$” implemented in the HOL4 theorem prover, assuming the existence of a large cardinal. We use the large-cardinal assumption to construct a model of HOL within HOL, and show how to ensure $$\varphi$$ has the same meaning both inside and outside of this model. Soundness of HOL implies that if $$\ulcorner \varphi \urcorner$$ is provable, then it is true in this model, and hence $$\varphi$$ holds. We additionally show how this reflection principle can be extended, assuming an infinite hierarchy of large cardinals, to implement model polymorphism, a technique designed for verifying systems with self-replacement functionality.
Benja Fallenstein, Ramana Kumar

### Improved Tool Support for Machine-Code Decompilation in HOL4

The HOL4 interactive theorem prover provides a sound logical environment for reasoning about machine-code programs. The rigour of HOL’s LCF-style kernel naturally guarantees very high levels of assurance, but it does present challenges when it comes implementing efficient proof tools. This paper presents improvements that have been made to our methodology for soundly decompiling machine-code programs to functions expressed in HOL logic. These advancements have been facilitated by the development of a domain specific language, called L3, for the specification of Instruction Set Architectures (ISAs). As a result of these improvements, decompilation is faster (on average by one to two orders of magnitude), the instruction set specifications are easier to write, and the proof tools are easier to maintain.
Anthony Fox

### A Formalized Hierarchy of Probabilistic System Types

Proof Pearl
Numerous models of probabilistic systems are studied in the literature. Coalgebra has been used to classify them into system types and compare their expressiveness. In this work, we formalize the resulting hierarchy of probabilistic system types in Isabelle/HOL by modeling the semantics of the different systems as codatatypes. This approach yields simple and concise proofs, as bisimilarity coincides with equality for codatatypes. On the way, we develop libraries of bounded sets and discrete probability distributions and integrate them with the facility for (co)datatype definitions.
Johannes Hölzl, Andreas Lochbihler, Dmitriy Traytel

### A Verified Enclosure for the Lorenz Attractor (Rough Diamond)

A rigorous numerical algorithm, formally verified with Isabelle/HOL, is used to compute an accurate enclosure for the Lorenz attractor.
Accurately enclosing the attractor is highly relevant: a similar non verified computation is part of Tucker’s proof that the Lorenz attractor is chaotic in a rigorous mathematical sense. This proof settled a conjecture that Fields medalist Stephen Smale has put on his list of eighteen important mathematical problems for the twenty-first century.
Fabian Immler

### Learning to Parse on Aligned Corpora (Rough Diamond)

One of the first big hurdles that mathematicians encounter when considering writing formal proofs is the necessity to get acquainted with the formal terminology and the parsing mechanisms used in the large ITP libraries. This includes the large number of formal symbols, the grammar of the formal languages and the advanced mechanisms instrumenting the proof assistants to correctly understand the formal expressions in the presence of ubiquitous overloading.
In this work we start to address this problem by developing approximate probabilistic parsing techniques that autonomously train disambiguation on large corpora. Unlike in standard natural language processing, we can filter the resulting parse trees by strong ITP and AR semantic methods such as typechecking and automated theorem proving, and even let the probabilistic methods self-improve based on such semantic feedback. We describe the general motivation and our first experiments, and build an online system for parsing ambiguous formulas over the Flyspeck library.
Cezary Kaliszyk, Josef Urban, Jiří Vyskočil

### A Consistent Foundation for Isabelle/HOL

The interactive theorem prover Isabelle/HOL is based on the well understood Higher-Order Logic (HOL), which is widely believed to be consistent (and provably consistent in set theory by a standard semantic argument). However, Isabelle/HOL brings its own personal touch to HOL: overloaded constant definitions, used to achieve Haskell-like type classes in the user space. These features are a delight for the users, but unfortunately are not easy to get right as an extension of HOL—they have a history of inconsistent behavior. It has been an open question under which criteria overloaded constant definitions and type definitions can be combined together while still guaranteeing consistency. This paper presents a solution to this problem: non-overlapping definitions and termination of the definition-dependency relation (tracked not only through constants but also through types) ensures relative consistency of Isabelle/HOL.
Ondřej Kunčar, Andrei Popescu

### Refinement to Imperative/HOL

Many algorithms can be implemented most efficiently with imperative data structures that support destructive update. In this paper we present an approach to automatically generate verified imperative implementations from abstract specifications in Isabelle/HOL. It is based on the Isabelle Refinement Framework, for which a lot of abstract algorithms are already formalized.
Based on Imperative/HOL, which allows to generate verified imperative code, we develop a separation logic framework with automation to make it conveniently usable. On top of this, we develop an imperative collection framework, which provides standard implementations for sets and maps like hash tables and array lists. Finally, we define a refinement calculus to refine abstract (functional) algorithms to imperative ones.
Moreover, we have implemented a tool to automate the refinement process, replacing abstract data types by efficient imperative implementations from our collection framework. As a case study, we apply our tool to automatically generate verified imperative implementations of nested depth-first search and Dijkstra’s shortest paths algorithm, which are considerably faster than the corresponding functional implementations. The nested DFS implementation is almost as fast as a C++ implementation of the same algorithm.
Peter Lammich

### Stream Fusion for Isabelle’s Code Generator

Rough Diamond
Stream fusion eliminates intermediate lists in functional code. We formalise stream fusion for finite and coinductive lists in Isabelle/HOL and implement the transformation in the code preprocessor. Our initial results show that optimisations during code extraction can boost the performance of the generated code, but the transformation requires further engineering to be usable in practice.
Andreas Lochbihler, Alexandra Maximova

### HOCore in Coq

We consider a recent publication on higher-order process calculi [12] and describe how its main results have been formalized in the Coq proof assistant. We highlight a number of important technical issues that we have uncovered in the original publication. We believe that these issues are not unique to the paper under consideration and require particular care to be avoided.
Petar Maksimović, Alan Schmitt

### Affine Arithmetic and Applications to Real-Number Proving

Accuracy and correctness are central issues in numerical analysis. To address these issues, several self-validated computation methods have been proposed in the last fifty years. Their common goal is to provide rigorously correct enclosures for calculated values, sacrificing a measure of precision for correctness. Perhaps the most widely adopted enclosure method is interval arithmetic. Interval arithmetic performs well in a wide range of cases, but often produces excessively large overestimations, unless the domain is reduced in size, e.g., by subdivision. Many extensions of interval arithmetic have been developed in order to cope with this problem. Among them, affine arithmetic provides tighter estimations by taking into account linear correlations between operands. This paper presents a formalization of affine arithmetic, written in the Prototype Verification System (PVS), along with a formally verified branch-and-bound procedure implementing that model. This procedure and its correctness property enables the implementation of a PVS strategy for automatically computing upper and lower bounds of real-valued expressions that are provably correct up to a user-specified precision.
Mariano M. Moscato, César A. Muñoz, Andrew P. Smith

### Amortized Complexity Verified

A framework for the analysis of the amortized complexity of (functional) data structures is formalized in Isabelle/HOL and applied to a number of standard examples and to three famous non-trivial ones: skew heaps, splay trees and splay heaps.
Tobias Nipkow

### Foundational Property-Based Testing

Integrating property-based testing with a proof assistant creates an interesting opportunity: reusable or tricky testing code can be formally verified using the proof assistant itself. In this work we introduce a novel methodology for formally verified property-based testing and implement it as a foundational verification framework for QuickChick, a port of QuickCheck to Coq. Our framework enables one to verify that the executable testing code is testing the right Coq property. To make verification tractable, we provide a systematic way for reasoning about the set of outcomes a random data generator can produce with non-zero probability, while abstracting away from the actual probabilities. Our framework is firmly grounded in a fully verified implementation of QuickChick itself, using the same underlying verification methodology. We also apply this methodology to a complex case study on testing an information-flow control abstract machine, demonstrating that our verification methodology is modular and scalable and that it requires minimal changes to existing code.
Zoe Paraskevopoulou, Cătălin Hriţcu, Maxime Dénès, Leonidas Lampropoulos, Benjamin C. Pierce

### A Linear First-Order Functional Intermediate Language for Verified Compilers

We present the linear first-order intermediate language IL for verified compilers. IL is a functional language with calls to a nondeterministic environment. We give IL terms a second, imperative semantic interpretation and obtain a register transfer language. For the imperative interpretation we establish a notion of live variables. Based on live variables, we formulate a decidable property called coherence ensuring that the functional and the imperative interpretation of a term coincide. We formulate a register assignment algorithm for IL and prove its correctness. The algorithm translates a functional IL program into an equivalent imperative IL program. Correctness follows from the fact that the algorithm reaches a coherent program after consistently renaming local variables. We prove that the maximal number of live variables in the initial program bounds the number of different variables in the final coherent program. The entire development is formalized in Coq.
Sigurd Schneider, Gert Smolka, Sebastian Hack

### Autosubst: Reasoning with de Bruijn Terms and Parallel Substitutions

Reasoning about syntax with binders plays an essential role in the formalization of the metatheory of programming languages. While the intricacies of binders can be ignored in paper proofs, formalizations involving binders tend to be heavyweight. We present a discipline for syntax with binders based on de Bruijn terms and parallel substitutions, with a decision procedure covering all assumption-free equational substitution lemmas. The approach is implemented in the Coq library Autosubst, which additionally derives substitution operations and proofs of substitution lemmas for custom term types. We demonstrate the effectiveness of the approach with several case studies, including part A of the POPLmark challenge.
Steven Schäfer, Tobias Tebbi, Gert Smolka

### ModuRes: A Coq Library for Modular Reasoning About Concurrent Higher-Order Imperative Programming Languages

It is well-known that it is challenging to build semantic models of type systems or logics for reasoning about concurrent higher-order imperative programming languages. One of the key challenges is that such semantic models often involve constructing solutions to certain kinds of recursive domain equations, which in practice has been a barrier to formalization efforts. Here we present the ModuRes Coq library, which provides an easy way to solve such equations. We show how the library can be used to construct models of type systems and logics for reasoning about concurrent higher-order imperative programming languages.
Filip Sieczkowski, Aleš Bizjak, Lars Birkedal

### Transfinite Constructions in Classical Type Theory

We study a transfinite construction we call tower construction in classical type theory. The construction is inductive and applies to partially ordered types. It yields the set of all points reachable from a starting point with an increasing successor function and a family of admissible suprema. Based on the construction, we obtain type-theoretic versions of the theorems of Zermelo (well-orderings), Hausdorff (maximal chains), and Bourbaki and Witt (fixed points). The development is formalized in Coq assuming excluded middle.
Gert Smolka, Steven Schäfer, Christian Doczkal

### A Mechanized Theory of Regular Trees in Dependent Type Theory

Regular trees are potentially infinite trees such that the set of distinct subtrees is finite. This paper describes an implementation of regular trees in dependent type theory. Regular trees are semantically characterised as a coinductive type and syntactically as a nested datatype. We show how tree transducers can be used to define regular tree homomorphisms by guarded corecursion. Finally, we give examples on how transducers are used to obtain decidability of properties over regular trees. All results have been mechanized in the proof assistant Coq.

### Deriving Comparators and Show Functions in Isabelle/HOL

We present an Isabelle/HOL development that allows for the automatic generation of certain operations for user-defined datatypes. Since the operations are defined within the logic, they are applicable for code generation. Triggered by the demand to provide readable error messages as well as to access efficient data structures like sorted trees in generated code, we provide show functions that compute the string representation of a given value, comparators that yield linear orders, and hash functions. Moreover, large parts of the employed machinery should be reusable for other operations like read functions, etc.
In contrast to similar mechanisms, like Haskell’s “deriving,” we do not only generate definitions, but also prove some desired properties, e.g., that a comparator indeed orders values linearly. This is achieved by a collection of tactics that discharge the corresponding proof obligations automatically.
Christian Sternagel, René Thiemann

### Formalising Knot Theory in Isabelle/HOL

This paper describes a formalization of some topics in knot theory. The formalization was carried out in the interactive proof assistant, Isabelle. The concepts that were formalized include definitions of tangles, links, framed links and various forms of equivalences between them. The formalization is based on a formulation of links in terms of tangles. We further construct and prove the invariance of the Bracket polynomial. Bracket polynomial is an invariant of framed links closely linked to the Jones polynomial. This is perhaps the first attempt to formalize any aspect of knot theory in an interactive proof assistant.
T. V. H. Prathamesh

### Pattern Matches in HOL:

A New Representation and Improved Code Generation
Pattern matching is ubiquitous in functional programming and also very useful for definitions in higher-order logic. However, it is not directly supported by higher-order logic. Therefore, the parsers of theorem provers like HOL4 and Isabelle/HOL contain a pattern-compilation algorithm. Internally, decision trees based on case constants are used. For non-trivial case expressions, there is a big discrepancy between the user’s view and the internal representation.
This paper presents a new general-purpose representation for case expressions that mirrors the input syntax in the internal representation closely. Because of this close connection, the new representation is more intuitive and often much more compact. Complicated parsers and pretty printers are no longer required. Proofs can more closely follow the user’s intentions, and code generators can produce better code. Moreover, the new representation is more general than the currently used representation, supporting guards, patterns with multiple occurrences of the same bound variable, unbound variables, arithmetic expressions in patterns, and more. This work has been implemented in the HOL4 theorem prover and integrated into CakeML’s proof-producing code generator.
Thomas Tuerk, Magnus O. Myreen, Ramana Kumar

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