ing WS1S Systems to Verify Parameterized Networks . . . . . . . . . . . . 188 Kai Baukus, Saddek Bensalem, Yassine Lakhnech and Karsten Stahl FMona: A Tool for Expressing Validation Techniques over Infinite State Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 J.-P. Bodeveix and M. Filali Transitive Closures of Regular Relations for Verifying Infinite-State Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 Bengt Jonsson and Marcus Nilsson Diagnostic and Test Generation Using Static Analysis to Improve Automatic Test Generation . . . . . . . . . . . . . 235 Marius Bozga, Jean-Claude Fernandez and Lucian Ghirvu Efficient Diagnostic Generation for Boolean Equation Systems . . . . . . . . . . . . 251 Radu Mateescu Efficient Model-Checking Compositional State Space Generation with Partial Order Reductions for Asynchronous Communicating Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 Jean-Pierre Krimm and Laurent Mounier Checking for CFFD-Preorder with Tester Processes . . . . . . . . . . . . . . . . . . . . . . . 283 Juhana Helovuo and Antti Valmari Fair Bisimulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 Thomas A. Henzinger and Sriram K. Rajamani Integrating Low Level Symmetries into Reachability Analysis . . . . . . . . . . . . . 315 Karsten Schmidt Model-Checking Tools Model Checking Support for the ASM High-Level Language . . . . . . . . . . . . . . 331 Giuseppe Del Castillo and Kirsten Winter Table of

Tools and Algorithms for the Construction and Analysis of Systems. Proc. TACAS 2009

Journal of the ACM

This article describes the development and formal verification (proof of semantic preservation) of a compiler back-end from Cminor (a simple imperative intermediate language) to PowerPC assembly code, using the Coq proof assistant both for programming the compiler and for proving its soundness. Such a verified compiler is useful in the context of formal methods applied to the certification of critical software: the verification of the compiler guarantees that the safety properties proved on the source code hold for the executable compiled code as well.

A Formally Verified Compiler Back-end

Many modern computer systems and most multicore chips (chip multiprocessors) support shared memory in hardware. In a shared memory system, each of the processor cores may read and write to a single shared address space. For a shared memory machine, the memory consistency model defines the architecturally visible behavior of its memory system. Consistency definitions provide rules about loads and stores (or memory reads and writes) and how they act upon memory. As part of supporting a memory consistency model, many machines also provide cache coherence protocols that ensure that multiple cached copies of data are kept up-to-date. The goal of this primer is to provide readers with a basic understanding of consistency and coherence. This understanding includes both the issues that must be solved as well as a variety of solutions. We present both highlevel concepts as well as specific, concrete examples from real-world systems. Table of Contents: Preface / Introduction to Consistency and Coherence / Coherence Basics / Memory Consistency Motivation and Sequential Consistency / Total Store Order and the x86 Memory Model / Relaxed Memory Consistency / Coherence Protocols / Snooping Coherence Protocols / Directory Coherence Protocols / Advanced Topics in Coherence / Author Biographies

/pdf/a-primer-on-memory-consistency-and-cache-coherence-4cn8rqoyia.pdf

A Primer on Memory Consistency and Cache Coherence

Exploiting the multiprocessors that have recently become ubiquitous requires high-performance and reliable concurrent systems code, for concurrent data structures, operating system kernels, synchronization libraries, compilers, and so on. However, concurrent programming, which is always challenging, is made much more so by two problems. First, real multiprocessors typically do not provide the sequentially consistent memory that is assumed by most work on semantics and verification. Instead, they have relaxed memory models, varying in subtle ways between processor families, in which different hardware threads may have only loosely consistent views of a shared memory. Second, the public vendor architectures, supposedly specifying what programmers can rely on, are often in ambiguous informal prose (a particularly poor medium for loose specifications), leading to widespread confusion.In this paper we focus on x86 processors. We review several recent Intel and AMD specifications, showing that all contain serious ambiguities, some are arguably too weak to program above, and some are simply unsound with respect to actual hardware. We present a new x86-TSO programmer's model that, to the best of our knowledge, suffers from none of these problems. It is mathematically precise (rigorously defined in HOL4) but can be presented as an intuitive abstract machine which should be widely accessible to working programmers. We illustrate how this can be used to reason about the correctness of a Linux spinlock implementation and describe a general theory of data-race freedom for x86-TSO. This should put x86 multiprocessor system building on a more solid foundation; it should also provide a basis for future work on verification of such systems.

/pdf/x86-tso-a-rigorous-and-usable-programmer-s-model-for-x86-52wtp003a1.pdf

x86-TSO: a rigorous and usable programmer's model for x86 multiprocessors

In this article, we consider the semantic design and verified compilation of a C-like programming language for concurrent shared-memory computation on x86 multiprocessors. The design of such a language is made surprisingly subtle by several factors: the relaxed-memory behavior of the hardware, the effects of compiler optimization on concurrent code, the need to support high-performance concurrent algorithms, and the desire for a reasonably simple programming model. In turn, this complexity makes verified compilation both essential and challenging.We describe ClightTSO, a concurrent extension of CompCert’s Clight in which the TSO-based memory model of x86 multiprocessors is exposed for high-performance code, and CompCertTSO, a formally verified compiler from ClightTSO to x86 assembly language, building on CompCert. CompCertTSO is verified in Coq: for any well-behaved and successfully compiled ClightTSO source program, any permitted observable behavior of the generated assembly code (if it does not run out of memory) is also possible in the source semantics. We also describe some verified fence-elimination optimizations, integrated into CompCertTSO.

/pdf/compcerttso-a-verified-compiler-for-relaxed-memory-42f3hbf8xf.pdf

CompCertTSO: A Verified Compiler for Relaxed-Memory Concurrency

It is rare to give a semantic definition of a full-scale programming language, despite the many potential benefits. Partly this is because the available metalanguages for expressing semantics - usually either LaTEX for informal mathematics, or the formal mathematics of a proof assistant - make it much harder than necessary to work with large definitions.We present a metalanguage specifically designed for this problem, and a tool, ott, that sanity-checks such definitions and compiles them into proof assistant code for Coq, HOL, Isabelle, and (in progress) Twelf, together with LaTEX code for production-quality typesetting, and OCaml boilerplate. The main innovations are:(1) metalanguage design to make definitions concise, and easy to read and edit;(2) an expressive but intuitive metalanguage for specifying binding structures; and (3) compilation to proof assistant code.This has been tested in substantial case studies, including modular specifications of calculi from the TAPL text, a Lightweight Java with Java JSR 277/294 module system proposals, and a large fragment of OCaml (around 306 rules), with machine proofs of various soundness results. Our aim with this work is to enable a phase change: making it feasible to work routinely, without heroic effort, with rigorous semantic definitions of realistic languages.

/pdf/ott-effective-tool-support-for-the-working-semanticist-32vssnszer.pdf

Ott: effective tool support for the working semanticist

Semantic definitions of full-scale programming languages are rarely given, despite the many potential benefits. Partly this is because the available metalanguages for expressing semantics – usually either for informal mathematics or the formal mathematics of a proof assistant – make it much harder than necessary to work with large definitions. We present a metalanguage specifically designed for this problem, and a tool, Ott, that sanity-checks such definitions and compiles them into proof assistant code for Coq, HOL, and Isabelle/HOL, together with code for production-quality typesetting, and OCaml boilerplate. The main innovations are (1) metalanguage design to make definitions concise, and easy to read and edit; (2) an expressive but intuitive metalanguage for specifying binding structures; and (3) compilation to proof assistant code. This has been tested in substantial case studies, including modular specifications of calculi from the TAPL text, a Lightweight Java with Java JSR 277/294 module system proposals, and a large fragment of OCaml (OCamllight, 310 rules), with mechanised proofs of various soundness results. Our aim with this work is to enable a phase change: making it feasible to work routinely, without heroic effort, with rigorous semantic definitions of realistic languages.

/pdf/ott-effective-tool-support-for-the-working-semanticist-1knfr4hn1b.pdf

Ott: Effective tool support for the working semanticist

We define (with machine-checked proofs in Coq) a modular operational semantics for Concurrent C minor--a language with shared memory, spawnable threads, and first-class locks. By modular we mean that one can reason about sequential control and data-flow knowing almost nothing about concurrency, and one can reason about concurrency knowing almost nothing about sequential control and data-flow constructs. We present a Concurrent Separation Logic with first-class locks and threads, and prove its soundness with respect to the operational semantics. Using our modularity principle, we proved the sequential C.S.L. rules (those inherited from sequential Separation Logic) simply by adapting Appel & Blazy's machine-checked soundness proofs. Our Concurrent C minor operational semantics is designed to connect to Leroy's optimizing (sequential) C minor compiler; we propose our modular semantics as a way to adapt Leroy's compiler-correctness proofs to the concurrent setting. Thus we will obtain end-to-end proofs: the properties you prove in Concurrent Separation Logic will be true of the program that actually executes on the machine.

/pdf/oracle-semantics-for-concurrent-separation-logic-52dbglu9ok.pdf

Francesco Zappa Nardelli

Papers

x86-TSO: a rigorous and usable programmer's model for x86 multiprocessors

CompCertTSO: A Verified Compiler for Relaxed-Memory Concurrency

Ott: effective tool support for the working semanticist

Ott: Effective tool support for the working semanticist

Oracle semantics for concurrent separation logic