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Clément Pit-Claudel

List of publications

ASPLOS 2021Effective Simulation and Debugging for a High-Level Hardware Language using Software Compilerspdf, bibClément Pit-Claudel, Thomas Bourgeat, Stella Lau, Arvind, Adam Chlipala.
Proceedings of the 26th International Conference on Architectural Support for Programming Languages and Operating Systems (ASPLOS 2021).
Rule-based hardware design languages (RHDLs) promise to enhance developer productivity by offering convenient abstractions. Advanced compiler technology keeps the cost of these abstractions low, generating circuits with excellent area and timing properties.
Unfortunately, comparatively little effort has been spent on building simulators and debuggers for these languages, so users often simulate and debug their designs at the RTL level. This is problematic because generated circuits typically suffer from poor readability, as compiler optimizations can break high-level abstractions. Worse, optimizations that operate under the assumption that concurrency is essentially free yield faster circuits but often actively hurt simulation performance on platforms with limited concurrency, like desktop computers or servers.
This paper demonstrates the benefits of completely separating the simulation and synthesis pipelines. We propose a new approach, yielding the first compiler designed for effective simulation and debugging of a language in the Bluespec family. We generate cycle-accurate C++ models that are readable, compatible with a wide range of traditional software-debugging tools, and fast (often 2 to 3x faster than circuit-level simulation). We achieve these results by optimizing for sequential performance and using static analysis to minimize redundant work. The result is a vastly-improved hardware-design experience, which we demonstrate on embedded processor designs and DSP building blocks using performance benchmarks and debugging case studies.

CoqPL 2021Automated Synthesis of Verified Firewallspdf, bibShardul Chiplunkar, Clément Pit-Claudel, Adam Chlipala.
The Seventh International Workshop on Coq for PL (CoqPL 2021).
Workshop paper.
We demonstrate correct-by-construction firewalls—stateful packet filters for TCP/IP packets—using the Fiat synthesis library. We present a general DSL for specifying their behavior independent of algorithmic implementation. We outline the design of a verified compiler in Coq, detail a few verified efficiency optimizations, and show how the compiler can easily be extended to support custom optimizations for user-defined policies.

CoqPL 2021An experience report on writing usable DSLs in Coqpdf, bibClément Pit-Claudel, Thomas Bourgeat.
The Seventh International Workshop on Coq for PL (CoqPL 2021).
Workshop paper.
Features added to Coq over the last 5 years have made it possible to create drastically more usable domain-specific languages (DSLs). We report on our experience building and using a hardware-description language embedded within Coq, highlighting how recent Coq improvements make it possible to solve longstanding pain points with Coq DSLs.

SLE 2020Untangling Mechanized Proofspdf, bibClément Pit-Claudel.
Proceedings of the 13th ACM SIGPLAN International Conference on Software Language Engineering (SLE 2020).
Distinguished artifact.
Proof assistants like Coq, Lean, or HOL4 rely heavily on stateful meta-programs called scripts to assemble proofs. Unlike pen-and-paper proofs, proof scripts only describe the steps to take (induct on x, apply a theorem, …), not the states that these steps lead to; as a result, plain proof scripts are essentially incomprehensible without the assistance of an interactive user interface able to run the script and show the corresponding proof states.
Until now, the standard process to communicate a proof without forcing readers to execute its script was to manually copy-paste intermediate proof states into the script, as source code comments — a tedious and error-prone exercise. Additional prose (such as for a book or tutorial) was likewise embedded in comments, preserving executability at the cost of a mediocre text-editing experience.
This paper describes a new approach to the development and dissemination of literate proof scripts, with a focus on the Coq proof assistant. Specifically, we describe two contributions: a compiler that interleaves Coq's output with the original proof script to produce interactive webpages that are complete, self-contained presentations of Coq proofs; and a new literate programming toolkit that allows authors to switch seamlessly between prose- and code-oriented views of the same sources, by translating back and forth between reStructuredText documents and literate Coq source files. In combination, these tools offer a new way to write, communicate, and preserve proofs, combining the flexibility of procedural proof scripts and the intelligibility of declarative proofs.

IJCAR 2020Extensible Extraction of Efficient Imperative Programs with Foreign Functions, Manually Managed Memory, and Proofspdf, bibClément Pit-Claudel, Peng Wang, Benjamin Delaware, Jason Gross, Adam Chlipala.
International Joint Conference on Automated Reasoning (IJCAR 2020).
We present an original approach to sound program extraction in a proof assistant, using syntax-driven automation to derive correct-by-construction imperative programs from nondeterministic functional source code. Our approach does not require committing to a single inflexible compilation strategy and instead makes it straightforward to create domain-specific code translators. In addition to a small set of core definitions, our framework is a large, user-extensible collection of compilation rules each phrased to handle specific language constructs, code patterns, or data manipulations. By mixing and matching these pieces of logic, users can easily tailor extraction to their own domains and programs, getting maximum performance and ensuring correctness of the resulting assembly code.
Using this approach, we complete the first proof-generating pipeline that goes automatically from high-level specifications to assembly code. In our main case study, the original specifications are phrased to resemble SQL-style queries, while the final assembly code does manual memory management, calls out to foreign data structures and functions, and is suitable to deploy on resource-constrained platforms. The pipeline runs entirely within the Coq proof assistant, leading to final, linked assembly code with overall full-functional-correctness proofs in separation logic.

PLDI 2020The Essence of Bluespec: A Core Language for Rule-Based Hardware Designpdf, bibThomas Bourgeat, Clément Pit-Claudel, Adam Chlipala, Arvind.
Proceedings of the ACM on Programming Languages (PLDI 2020).
The Bluespec hardware-description language presents a significantly higher-level view than hardware engineers are used to, exposing a simpler concurrency model that promotes formal proof, without compromising on performance of compiled circuits. Unfortunately, the cost model of Bluespec has been unclear, with performance details depending on a mix of user hints and opaque static analysis of potential concurrency conflicts within a design. In this paper we present Kôika, a derivative of Bluespec that preserves its desirable properties and yet gives direct control over the scheduling decisions that determine performance. Kôika has a novel and deterministic operational semantics that uses dynamic analysis to avoid concurrency anomalies. Our implementation includes Coq definitions of syntax, semantics, key metatheorems, and a verified compiler to circuits. We argue that most of the extra circuitry required for dynamic analysis can be eliminated by compile-time BSV-style static analysis.

ICFP 2019Narcissus: Correct-by-Construction Derivation of Decoders and Encoders from Binary Formatspdf, bibBenjamin Delaware, Sorawit Suriyakarn, Clément Pit-Claudel, Qianchuan Ye, Adam Chlipala.
Proceedings of the ACM on Programming Languages (ICFP 2019).
It is a neat result from functional programming that libraries of parser combinators can support rapid construction of decoders for quite a range of formats. With a little more work, the same combinator program can denote both a decoder and an encoder. Unfortunately, the real world is full of gnarly formats, as with the packet formats that make up the standard Internet protocol stack. Most past parser-combinator approaches cannot handle these formats, and the few exceptions require redundancy – one part of the natural grammar needs to be hand-translated into hints in multiple parts of a parser program. We show how to recover very natural and nonredundant format specifications, covering all popular network packet formats and generating both decoders and encoders automatically. The catch is that we use the Coq proof assistant to derive both kinds of artifacts using tactics, automatically, in a way that guarantees that they form inverses of each other. We used our approach to reimplement packet processing for a full Internet protocol stack, inserting our replacement into the OCaml-based MirageOS unikernel, resulting in minimal performance degradation.

ESOP 2019Meta-F⋆ : Proof Automation with SMT, Tactics, and Metaprogramspdf, bibGuido Martínez, Danel Ahman, Victor Dumitrescu, Nick Giannarakis, Chris Hawblitzel, Cătălin Hriţcu, Monal Narasimhamurthy, Zoe Paraskevopoulou, Clément Pit-Claudel, Jonathan Protzenko, Tahina Ramananandro, Aseem Rastogi, Nikhil Swamy.
Proceedings of the 28th European Symposium on Programming (ESOP 2019).
We introduce Meta-F⋆, a tactics and metaprogramming framework for the F⋆ program verifier. The main novelty of Meta-F⋆ is allowing the use of tactics and metaprogramming to discharge assertions not solvable by SMT, or to just simplify them into well-behaved SMT fragments. Plus, Meta-F⋆ can be used to generate verified code automatically.
Meta-F⋆ is implemented as an F⋆ effect, which, given the powerful effect system of F⋆, heavily increases code reuse and even enables the lightweight verification of metaprograms. Metaprograms can be either interpreted, or compiled to efficient native code that can be dynamically loaded into the F⋆ type-checker and can interoperate with interpreted code.
Evaluation on realistic case studies shows that Meta-F⋆ provides substantial gains in proof development, efficiency, and robustness.

SETTA 2018Correct-by-Construction Implementation of Runtime Monitors Using Stepwise Refinementpdf, bibTeng Zhang, John Wiegley, Theophilos Giannakopoulos, Gregory Eakman, Clément Pit-Claudel, Insup Lee, Oleg Sokolsky.
Proceedings of the 4th International Symposium on Dependable Software Engineering: Theories, Tools, and Applications (SETTA 2018).
Runtime verification (RV) is a lightweight technique for verifying traces of computer systems. One challenge in applying RV is to guarantee that the implementation of a runtime monitor correctly detects and signals unexpected events. In this paper, we present a method for deriving correct-by-construction implementations of runtime monitors from high-level specifications using Fiat, a Coq library for stepwise refinement. SMEDL (Scenario-based Meta-Event Definition Language), a domain specific language for event-driven RV, is chosen as the specification language. We propose an operational semantics for SMEDL suitable to be used in Fiat to describe the behavior of a monitor in a relational way. Then, by utilizing Fiat’s refinement calculus, we transform a declarative monitor specification into an executable runtime monitor with a proof that the behavior of the implementation is strictly a subset of that provided by the specification. Moreover, we define a predicate on the syntax structure of a monitor definition to ensure termination and determinism. Most of the proof work required to generate monitor code has been automated.

ML 2018ML as a Tactic Language, Againpdf, bibGuido Martínez, Danel Ahman, Victor Dumitrescu, Nick Giannarakis, Chris Hawblitzel, Cătălin Hriţcu, Monal Narasimhamurthy, Zoe Paraskevopoulou, Clément Pit-Claudel, Jonathan Protzenko, Tahina Ramananandro, Aseem Rastogi, Nikhil Swamy.
ML family workshop at ICFP 2018 (ML 2018).
Workshop paper.
ML originated as the meta language for constructing proofs in the LCF theorem prover. Due to its many virtues, it became a role model for object-level languages as well, inspiring an entire family of strict functional programming languages.
A member of this family is F⋆, a verification-oriented dialect of ML including dependent types and a monadic effect system, the combination of which allows to express arbitrarily complex properties of (effectful!) programs within its type system itself. To prove these properties, F⋆ relies on querying an SMT solver (usually Z3). For properties in well-supported SMT theories (e.g. linear arithmetic or uninterpreted functions) this approach works very well, allowing programmers to verify programs with relative ease. However, for poorly-supported theories (e.g. non-linear arithmetic), the SMT solver might require a prohibitive amount of resources or be wildly unpredictable. Furthermore, these rotten apples spoil the barrel, since combining even a basic provable fact from a poorly-supported theory with a well-supported one might stump the solver.
Motivated by these issues, we have recently extended F⋆ with a tactics and metaprogramming engine, which allows to attack proof obligations with user-defined procedures. Tactics are written in Meta-F⋆, which is simply the set of those F⋆ programs of a particular Tac effect, defined within the existing type-and-effect system. As such, they readily interoperate with the rest of language and can themselves be verified (to a degree). Using tactics allows the programmer to massage proof obligations before querying the SMT solver (possibly simplifying them into the well-supported fragments) or even solving them completely.
Here, we present an overview of Meta-F⋆, focusing on its design and its execution models. In addition, we discuss the benefits of tactics and metaprogramming in F⋆ through some idiomatic case studies.

SNAPL 2017The End of History? Using a Proof Assistant to Replace Language Design with Library Designpdf, bibAdam Chlipala, Benjamin Delaware, Samuel Duchovni, Jason Gross, Clément Pit-Claudel, Sorawit Suriyakarn, Peng Wang, Katherine Ye.
2nd Summit on Advances in Programming Languages (SNAPL 2017).
Functionality of software systems has exploded in part because of advances in programming-language support for packaging reusable functionality as libraries. Developers benefit from the uniformity that comes of exposing many interfaces in the same language, as opposed to stringing together hodgepodges of command-line tools. Domain-specific languages may be viewed as an evolution of the power of reusable interfaces, when those interfaces become so flexible as to deserve to be called programming languages. However, common approaches to domain-specific languages give up many of the hard-won advantages of library-building in a rich common language, and even the traditional approach poses significant challenges in learning new APIs. We suggest that instead of continuing to develop new domain-specific languages, our community should embrace library-based ecosystems within very expressive languages that mix programming and theorem proving. Our prototype framework Fiat, a library for the Coq proof assistant, turns languages into easily comprehensible libraries via the key idea of modularizing functionality and performance away from each other, the former via macros that desugar into higher-order logic and the latter via optimization scripts that derive efficient code from logical programs.

CAV 2016Trigger Selection Strategies to Stabilize Program Verifierspdf, bibK. Rustan M. Leino and Clément Pit-Claudel.
Computer Aided Verification: 28th International Conference (CAV 2016).
SMT-based program verifiers often suffer from the so-called butterfly effect, in which minor modifications to the program source cause significant instabilities in verification times, which in turn may lead to spurious verification failures and a degraded user experience. This paper identifies matching loops (ill-behaved quantifiers causing an SMT solver to repeatedly instantiate a small set of quantified formulas) as a significant contributor to these instabilities, and describes some techniques to detect and prevent them. At their core, the contributed techniques move the trigger selection logic away from the SMT solver and into the high-level verifier: this move allows authors of verifiers to annotate, rewrite, and analyze user-written quantifiers to improve the solver’s performance, using information that is easily available at the source level but would be hard to extract from the heavily encoded terms that the solver works with. The paper demonstrates three core techniques (quantifier splitting, trigger sharing, and matching loop detection) by extending the Dafny verifier with its own trigger selection routine, and demonstrates significant predictability and performance gains on both Dafny’s test suite and large verification efforts using Dafny.

CoqPL 2016Company-Coq: Taking Proof General one step closer to a real IDEpdf, bibClément Pit-Claudel, Pierre Courtieu.
The Second International Workshop on Coq for PL (CoqPL 2016).
Workshop paper.
Company-Coq is a new Emacs package that extends Proof General with a contextual auto-completion engine for Coq proofs and many additional facilities to make writing proofs easier and more efficient. Beyond fuzzy auto-completion of tactics, options, module names, and local definitions, company-coq offers offline in-editor documentation, convenient snippets, and multiple other Coq-specific IDE features.

MS thesisCompilation Using Correct-by-Construction Program Synthesishtml, pdf, bibClément Pit-Claudel.
Master's Thesis at MIT.
William A. Martin Memorial Thesis Award for Outstanding Thesis in CS.
Extracting and compiling certified programs may introduce bugs in otherwise proven-correct code, reducing the extent of the guarantees that proof assistants and correct-by-construction program-derivation frameworks provide. We explore a novel approach to extracting and compiling embedded domain-specific languages developed in a proof assistant (Coq), showing how it allows us to extend correctness guarantees all the way down to a verification-aware assembly language. Our core idea is to phrase compilation of shallowly embedded programs to a lower-level deeply embedded language as a synthesis problem, solved using simple proof-search techniques. This technique is extensible (support for individual language constructs is provided by a user-extensible database of compilation tactics and lemmas) and allows the source programs to depend on axiomatically specified methods of externally implemented data structures, delaying linking to the assembly stage. Composed with the Fiat and Bedrock frameworks, our new method provides the first proof-generating automatic translation from SQL-style relational programs into executable assembly code.

POPL 2015Fiat: Deductive Synthesis of Abstract Data Types in a Proof Assistantpdf, bibBenjamin Delaware, Clément Pit-Claudel, Jason Gross, Adam Chlipala.
Proceedings of the 42nd ACM SIGPLAN-SIGACT Symposium on Principles of Programming Languages (POPL 2015).
We present Fiat, a library for the Coq proof assistant supporting refinement of declarative specifications into efficient functional programs with a high degree of automation. Each refinement process leaves a proof trail, checkable by the normal Coq kernel, justifying its soundness. We focus on the synthesis of abstract data types that package methods with private data. We demonstrate the utility of our framework by applying it to the synthesis of query structures — abstract data types with SQL-like query and insert operations. Fiat includes a library for writing specifications of query structures in SQL-inspired notation, expressing operations over relations (tables) in terms of mathematical sets. This library includes a suite of tactics for automating the refinement of specifications into efficient, correct-by-construction OCaml code. Using these tactics, a programmer can generate such an implementation completely automatically by only specifying the equivalent of SQL indexes, data structures capturing useful views of the abstract data. Throughout we speculate on the new programming modularity possibilities enabled by an automated refinement system with proved-correct rules.

Tech reportOutlier Detection in Heterogeneous Datasets using Automatic Tuple Expansionpdf, bibClément Pit-Claudel, Zelda Mariet, Rachael Harding, Sam Madden.
Technical report.
Rapidly developing areas of information technology are generating massive amounts of data. Human errors, sensor failures, and other unforeseen circumstances unfortunately tend to undermine the quality and consistency of these datasets by introducing outliers -- data points that exhibit surprising behavior when compared to the rest of the data. Characterizing, locating, and in some cases eliminating these outliers offers interesting insight about the data under scrutiny and reinforces the confidence that one may have in conclusions drawn from otherwise noisy datasets. In this paper, we describe a tuple expansion procedure which reconstructs rich information from semantically poor SQL data types such as strings, integers, and floating point numbers. We then use this procedure as the foundation of a new user-guided outlier detection framework, dBoost, which relies on inference and statistical modeling of heterogeneous data to flag suspicious fields in database tuples. We show that this novel approach achieves good classification performance, both in traditional numerical datasets and in highly non-numerical contexts such as mostly textual datasets.