[publications]

• PLDI'24 Decidable Subtyping of Existential Types for Julia

  
  Authors: Julia Belyakova, Ben Chung, Ross Tate, Jan Vitek
  Abstract: Julia is a modern scientific-computing language that relies on multiple dispatch to implement generic libraries. While the language does not have a static type system, method declarations are decorated with expressive type annotations to determine when they are applicable. To find applicable methods, the implementation uses subtyping at run-time. We show that Julia’s subtyping is undecidable, and we propose a restriction on types to recover decidability by stratifying types into method signatures over value types—where the former can freely use bounded existential types but the latter are restricted to use-site variance. A corpus analysis suggests that nearly all Julia programs written in practice already conform to this restriction.
  Venue: PACM PL: PLDI
  DOI: 10.1145/3656421 PDF
• ECOOP'24 The Fault in Our Stars: Designing Reproducible Large-scale Code Analysis Experiments

  
  Authors: Petr Maj, Stefanie Muroya, Konrad Siek, Luca Di Grazia, Jan Vitek
  Abstract: Large-scale software repositories are a source of insights for software engineering. They offer an unmatched window into the software development process at scale. Their sheer number and size holds the promise of broadly applicable results. At the same time, that very size presents practical challenges for scaling tools and algorithms to millions of projects. A reasonable approach is to limit studies to representative samples of the population of interest. Broadly applicable conclusions can then be obtained by generalizing to the entire population. The contribution of this paper is a standardized experimental design methodology for choosing the inputs of studies working with large-scale repositories. We advocate for a methodology that clearly lays out what the population of interest is, how to sample it, and that fosters reproducibility. Along the way, we discourage researchers from using extrinsic attributes of projects such as stars, that measure some unclear notion of popularity.
  Venue: European Conference on Object-Oriented Programming (ECOOP)
  DOI: TBD PDF
• OOPSLA'23 Reusing Just-in-Time Compiled Code

  
  Authors: Meetesh Kalpesh Mehta, Sebastián Krynski, Hugo Musso Gualandi, Manas Thakur, Jan Vitek
  Abstract: Most code is executed more than once. If not entire programs then libraries remain unchanged from one run to the next. Just-in-time compilers expend considerable effort gathering insights about code they compiled many times, and often end up generating the same binary over and over again. We explore how to reuse compiled code across runs of different programs to reduce warm-up costs of dynamic languages. We propose to use speculative contextual dispatch to select versions of functions from an off-line curated code repository. That repository is a persistent database of previously compiled functions indexed by the context under which they were compiled. The repository is curated to remove redundant code and to optimize dispatch. We assess practicality by extending Ř, a compiler for the R language, and evaluating its performance. Our results suggest that the approach improves warmup times while preserving peak performance.
  Venue: PACM PL: OOPSLA
  DOI: 10.1145/3622839 PDF Artifact: 10.5281/zenodo.8330884
• PLDI'22 Deoptless: Speculation with Dispatched On-Stack Replacement and Specialized Continuations

  
  Authors: Olivier Flückiger, Jan Ječmen, Sebastián Krynski and Jan Vitek
  Abstract: Just-in-time compilation provides significant performance improvements for programs written in dynamic languages. These benefits come from the ability of the compiler to speculate about likely cases and generate optimized code for these. Unavoidably, speculations sometimes fail and the optimizations must be reverted. In some pathological cases, this can leave the program stuck with suboptimal code. In this paper we propose deoptless, a technique that replaces deoptimization points with dispatched specialized continuations. The goal of deoptless is to take a step towards providing users with a more transparent performance model in which mysterious slowdowns are less frequent and grave.
  Venue: Programming Language Design and Implementation Conference (PLDI)
  DOI: 10.1145/3519939.3523729 PDF Artifact: 10.5281/zenodo.6394618
• SLE'22 signatr: A Data-Driven Fuzzing Tool for R

  
  Authors: Alexi Turcotte, Pierre Donat-Bouillud, Filip Krikava, and Jan Vitek
  Abstract: The fast-and-loose, permissive semantics of dynamic programming languages limit the power of static analyses. For that reason, soundness is often traded for precision through dynamic program analysis. Dynamic analysis is only as good as the available runnable code, and relying solely on test suites is fraught as they do not cover the full gamut of possible behaviors. Fuzzing is an approach for automatically exercising code, and could be used to obtain more runnable code. However, the shape of user-defined data in dynamic languages is difficult to intuit, limiting a fuzzer's reach. We propose a feedback-driven blackbox fuzzing approach which draws inputs from a database of values recorded from existing code. We implement this approach in a tool called signatr for the R language. We present the insights of its design and implementation, and assess signatr's ability to uncover new behaviors by fuzzing 4,829 R functions from 100 R packages, revealing 1,195,184 new signatures.
  Venue: International Conference on Software Language Engineering (SLE)
  DOI: 10.1145/3567512.3567530 PDF Artifact: 10.5281/zenodo.7342082
• OOPSLA'21 Promises Are Made to Be Broken: Migrating R to Strict Semantics}

  
  Authors: Aviral Goel, Jan Ječmen, Sebastián Krynski, Olivier Flückiger Jan Vitek.
  Abstract: Function calls in the R language do not evaluate their arguments, these are passed to the callee as suspended computations and evaluated if needed. After 25 years of experience with the language, there are very few cases where programmers leverage delayed evaluation intentionally and laziness comes at a price in performance and complexity. This paper explores how to evolve the semantics of a lazy language towards strictness-by-default and laziness-on-demand. To provide a migration path, it is necessary to provide tooling for developers to migrate libraries without introducing errors. This paper reports on a dynamic analysis that infers strictness signatures for functions to capture both intentional and accidental laziness. Over 99% of the inferred signatures were correct when tested against clients of the libraries.
  Venue: PACM PL: OOPSLA
  PDF DOI: 10.1145/3485478 Artifact: 10.5281/zenodo.5394235
• OOPSLA'21 What We Eval in the Shadows: A Large-scale Study of Eval in R programs

  
  Authors: Aviral Goel, Pierre Donat-Bouillud, Filip Křikava, Christoph Kirsch and Jan Vitek.
  Abstract: Most dynamic languages allow users to turn text into code using various functions, often named eval, with language-dependent semantics. The widespread use of these reflective functions hinders static analysis and prevents compilers from performing optimizations. This paper aims to provide a better sense of why programmers use eval. Understanding why eval is used in practice is key to finding ways to mitigate its negative impact. We have reasons to believe that reflective feature usage is language and application domain-specific; we focus on data science code written in R and compare our results to previous work that analyzed web programming in JavaScript. We analyze 49,296,059 calls to eval from 240,327 scripts extracted from 15,401 R packages. We find that eval is indeed in widespread use; R’s eval is more pervasive and arguably dangerous than what was previously reported for JavaScript.
  Venue: PACM PL: OOPSLA
  PDF DOI: 10.1145/3485502 Artifact: 10.5281/zenodo.5415230
• OOPSLA'21 Type Stability in Julia: Avoiding Performance Pathologies in JIT Compilation

  
  Authors: Artem Pelenitsyn, Julia Belyakova, Ben Chung, Ross Tate and Jan Vitek.
  Abstract: As a scientific programming language, Julia strives for performance but also provides high-level productivity features. To avoid performance pathologies, Julia users are expected to adhere to a coding discipline that enables so-called type stability. Informally, a function is type stable if the type of the output depends only on the types of the inputs, not their values. This paper provides a formal definition of type stability as well as a stronger property of type groundedness, shows that groundedness enables compiler optimizations, and proves the compiler correct. We also perform a corpus analysis to uncover how these type-related properties manifest in practice.
  Venue: PACM PL: OOPSLA
  PDF DOI: 10.1145/3485527 Artifact: 10.5281/zenodo.5500548
• DSL'21 First-class environments in R

  
  Authors: Aviral Goel and Jan Vitek
  Abstract: The R programming language is widely used for statistical computing. To enable interactive data exploration and rapid prototyping, R encourages a dynamic programming style. This programming style is supported by features such as first-class environments. Amongst widely used languages, R has the richest interface for programmatically manipulating environments. With the flexibility afforded by reflective operations on first-class environments, come significant challenges for reasoning and optimizing user-defined code. This paper documents the reflective interface used to operate over first-class environment. We explain the rationale behind its design and conduct a large-scale study of how the interface is used in popular libraries.
  Venue: Symposium on Dynamic Languages (DLS)
  DOI: 10.1145/3486602.3486768 PDF
• ECOOP'21 CodeDJ: Reproducible Queries over Large-Scale Software Repositories

  
  Authors: Petr Maj, Konrad Siek, Alexander Kovalenko, Jan Vitek
  Abstract: Analyzing massive code bases is a staple of modern software engineering research -- a welcome side-effect of the advent of large-scale software repositories such as GitHub. Selecting which projects one should analyze is a labor-intensive process, and a process that can lead to biased results if the selection is not representative of the population of interest. One issue faced by researchers is that the interface exposed by software repositories only allows the most basic of queries. CodeDJ is an infrastructure for querying repositories composed of a persistent datastore, constantly updated with data acquired from GitHub, and an in-memory database with a Rust query interface. CodeDJ supports reproducibility, historical queries are answered deterministically using past states of the datastore; thus researchers can reproduce published results. To illustrate the benefits of CodeDJ, we identify biases in the data of a published study and, by repeating the analysis with new data, we demonstrate that the study’s conclusions were sensitive to the choice of projects.
  Venue: European Conference on Object-Oriented Programming (ECOOP)
  DOI: 10.4230/LIPIcs.ECOOP.2021.6 PDF Artifact: 10.4230/DARTS.7.2.13
• POPL'21Formally Verified Speculation and Deoptimization in a JIT Compiler

  
  Authors: Aurele Barriere, Olivier Flückiger, Sandrine Blazy, David Pichardie, Jan Vitek.
  Abstract: Just-in-time compilers for dynamic languages routinely generate code under assumptions that may be invalidated at run-time, this allows for specialization of program code to the common case in order to avoid unnecessary overheads due to uncommon cases. This form of software speculation requires support for deoptimization when some of the assumptions fail to hold. This paper presents a model just-in-time compiler with an intermediate representation that explicits the synchronization points used for deoptimization and the assumptions made by the compiler's speculation. We also present several common compiler optimizations that can leverage speculation to generate improved code. The optimizations are proved correct with the help of a proof assistant. While our work stops short of proving native code generation, we demonstrate how one could use the verified optimization to obtain significant speed ups in an end-to-end setting.
  Venue: PACM PL: Conference on Principles of Programming Languages (POPL)
  PDF DOI: 10.1145/3434327 Artifact: 10.1145/3410263
• OOPSLA'20World Age in Julia: Optimizing Method Dispatch in the Presence of Eval

  
  Authors: Julia Belyakova, Benjamin Chung, Jack Gelinas, Jameson Nash, Ross Tate, Jan Vitek.
  Abstract: Dynamic programming languages face semantic and performance challenges in the presence of features, such as eval, that can inject new code into a running program. The Julia programming language introduces the novel concept of world age to insulate optimized code from one of the most disruptive side-effects of eval: changes to the definition of an existing function. This paper provides the first formal semantics of world age in a core calculus named Juliette, and shows how world age enables compiler optimizations, such as inlining, in the presence of eval. While Julia also provides programmers with the means to bypass world age, we found that this mechanism is not used extensively: a static analysis of over 4,000 registered Julia packages shows that only 4--7% of packages are impacted by world age. This suggests that Julia's semantics aligns with programmer expectations.
  Venue: PACM PL: Conference on Object-Oriented Programming Languages, Systems and Applications (OOPSLA)
  w PDF DOI: 10.1145/3428275
  Artifact: github.com/julbinb/juliette-wa
• OOPSLA'20Designing Types for R, Empirically

  
  Authors: Alexi Turcotte, Aviral Goel, Filip Krikava, Jan Vitek.
  Abstract: The R programming language is widely used in a variety of domains. It was designed to favor an interactive style of programming with minimal syntactic and conceptual overhead. This design is well suited to interactive data analysis, but a bad fit for tools such as compilers or program analyzers which must generate native code or catch programming errors. In particular, R has no type annotations, and all operations are dynamically checked at run-time. The starting point for our work are the twin questions: what expressive power is needed to accurately type R code? and which type system is the R community willing to adopt? Both questions are difficult to answer without actually experimenting with a type system. The goal of this paper is to provide data that can feed into that design process. To this end, we perform a large corpus analysis to gain insights in the degree of polymorphism exhibited by idiomatic R code and explore potential benefits that the R community could accrue from a simple type system. As a starting point, we infer type signatures for 25,215 functions from 412 packages among the most widely used open source R libraries. We then conduct an evaluation on 8,694 clients of these packages, as well as on end-user code found on the Kaggle competition website.
  Venue: PACM PL: Conference on Object-Oriented Programming Languages, Systems and Applications (OOPSLA)
  PDF DOI: 10.1145/3428249
  Artifact: zenodo.org/record/4037278#.X6vnnfmvD3B
• OOPSLA'20Contextual Dispatch for Function Specialization

  
  Authors: Olivier Flückiger, Guido Chair, Ming-Ho Yee, Jan Jecmen, Jakob Hain, Jan Vitek.
  Abstract: In order to generate efficient code, dynamic language compilers often need information, such as dynamic types, not readily available in the program source. Leveraging a mixture of static and dynamic information, these compilers speculate on the missing information. Within one compilation unit, they specialize the generated code to the previously observed behaviors, betting that past is prologue. When speculation fails, the execution must jump back to unoptimized code. In this paper, we propose an approach to further the specialization, by disentangling classes of behaviors into separate optimization units. With contextual dispatch, functions are versioned and each version is compiled under different assumptions. When a function is invoked, the implementation dispatches to a version optimized under assumptions matching the dynamic context of the call. As a proof-of-concept, we describe a compiler for the R language which uses this approach. We evaluate contextual dispatch on a set of benchmarks and compare it to traditional speculation with deoptimization techniques. Our implementation is, on average, 1.7× faster than the GNU R reference implementation, and contextual dispatch improves the performance of 18 out of 46 programs in our benchmark suite.
  Venue: PACM PL: Conference on Object-Oriented Programming Languages, Systems and Applications (OOPSLA)
  PDF DOI: 10.1145/3428288
  Artifact: zenodo.org/record/3973073#.X6wCQy9h124
• DLS'20Sampling Optimized Code for Type Feedback

  
  Authors: Olivier Flückiger, Sebastián Krynski, Andreas Wälchli, Jan Vitek.
  Abstract: To efficiently execute dynamically typed languages, many language implementations have adopted a two-tier architec- ture. The first tier aims for low-latency startup times and collects dynamic profiles, such as the types of every program variable. The second tier provides high-throughput using an optimizing compiler that specializes code to the recorded type information. If the program behavior changes to the point that not previously seen types occur in specialized code, that specialized code becomes invalid, it is deoptimized, and control is transferred back to the first tier execution engine which will start specializing anew. However, if the program behavior becomes more specific, for instance, if a variable that was recorded as holding values of many types becomes monomorphic, no deoptimization will be triggered. Once the program is running optimized code, there are no means to notice that an opportunity for optimization has been missed or to restart specialization. We propose to employ a sampling-based profiler to moni- tor native code without any instrumentation. The absence of instrumentation means that when the profiler is not active, no overhead is incurred. When the profiler is active, the over- head can be controlled by limiting the sampling rate. Our implementation is in the context of the Ř just-in-time, opti- mizing, compiler for the R language. Based on the sampled profiles, we are able to detect when the native code produced by Ř is specialized for stale type feedback and recompile it to more type-specific code. We show that recording in our pro- filer adds an overhead of less than 3% in most cases and up to 9% in few cases when engaged and that it reliably detects stale type feedback within milliseconds.
  Venue: Dynamic Language Symposium (DLS)
  PDF DOI: 10.1145/3426422.3426984
  Artifact: zenodo.org/record/4061655#.X6wBpS9h124
• OOPSLA'19Scala Implicits are Everywhere: A large-scale study of the use of Implicits in the wild

  
  Authors: Filip Krikava, Heather Muller, Jan Vitek.
  Abstract: The Scala programming language offers two distinctive language features implicit parameters and implicit conversions, often referred together as implicits. Announced without fanfare in 2004, implicits have quickly grown to become a widely and pervasively used feature of the language. They provide a way to reduce the boilerplate code in Scala programs. They are also used to implement certain language features without having to modify the compiler. We report on a large-scale study of the use of implicits in the wild. For this, we analyzed 7,280 Scala projects hosted on GitHub, spanning over 8.1M call sites involving implicits and 370.7K implicit declarations across 18.7M lines of Scala code.
  Venue: PACM PL: Conference on Object-Oriented Programming Languages, Systems and Applications (OOPSLA)
  PDF DOI: 10.1145/3360589 Artifact: 10.5281/zenodo.3369436 Video: YouTube
  
  
• OOPSLA'19On the Design, Implementation and Use of Laziness in R

  
  Authors: Aviral Goel, Jan Vitek.
  Abstract: The R programming language has been lazy for over twenty five years. This paper presents a review of the design and implementation of call-by-need in R, and a data-driven study of how generations of programmers have put laziness to use in their code. We analyze 16,707 libraries and observe the creation of 270.9 B promises. Our data suggests that there is little supporting evidence to assert that programmers use laziness to avoid unnecessary computation or to operate over infinite data structures. For the most part R code appears to have been written without reliance on, and in many cases even knowledge of, delayed argument evaluation. The only significant exception is a small number of packages and core libraries which leverage call-by-need for meta-programming.
  Venue: PACM PL: Conference on Object-Oriented Programming Languages, Systems and Applications (OOPSLA)
  PDF DOI: 10.1145/3360579 Artifact: 10.5281/zenodo.3369573
  Video: YouTube
  
  
• DLS'19 R Melts Brains: An IR for First-Class Environments and Lazy Effectful Arguments

  
  Authors: Olivier Flückige, Ming-Ho Yee, Guido Chari, Jakob Hain, Jan Ječmen, Jan Vitek
  Abstract: The R programming language combines a number of features considered hard to analyze and implement efficiently: dynamic typing, reflection, lazy evaluation, vectorized primitive types, first-class closures, and extensive use of native code. Additionally, variable scopes are reified at runtime as first-class environments. The combination of these features renders most static program analysis techniques impractical, and thus, compiler optimizations based on them ineffective. We present our work on PIR, an intermediate representation with explicit support for first-class environments and effectful lazy evaluation. We describe two dataflow analyses on PIR: the first enables reasoning about variables and their environments, and the second infers where arguments are evaluated. Leveraging their results, we show how to elide environment creation and inline functions.
  Venue: Dynamic Language Symposium (DLS)
  DOI: 10.1145/3276945.3276946 PDF Artifact: github.com/reactorlabs/rir Video: Youtube
  
• ECOOP'19 Julia's efficient algorithm for subtyping unions and covariant tuples

  
  Authors: Benjamin Chung, Francesco Zappa Nardelli, Jan Vitek
  Abstract: The Julia programming language supports multiple dispatch and provides a rich type annotation language to specify method applicability. When multiple methods are applicable for a given call, Julia relies on subtyping between method signatures to pick the correct method to invoke. Julia’s subtyping algorithm is surprisingly complex, and determining whether it is correct remains an open question. In this paper, we focus on one piece of this problem: the interaction between union types and covariant tuples. Previous work normalized unions inside tuples to disjunctive normal form. However, this strategy has two drawbacks: complex type signatures induce space explosion, and interference between normalization and other features of Julia’s type system. In this paper, we describe the algorithm that Julia uses to compute subtyping between tuples and unions—an algorithm that is immune to space explosion and plays well with other features of the language. We prove this algorithm correct and complete against a semantic-subtyping denotational model in Coq.
  Venue: European Conference on Object-Oriented Programming (ECOOP)
  DOI: 10.4230/LIPIcs.ECOOP.2019.24 PDF Artifact: dx.doi.org/10.4230/DARTS.5.2.8 Video: Youtube
  
• TOPLAS'19On the Impact of Programming Languages on Code Quality

  
  Authors: Emery D. Berger, Celeste Hollenbeck, Petr Maj, Olga Vitek, Jan Vitek.
  Abstract: In a 2014 paper, Ray, Posnett, Devanbu, and Filkov claimed to have uncovered a statistically significant association between eleven programming languages and software defects in 729 projects hosted on GitHub. Specifically, their work answered four research questions relating to software defects and programming languages. With data and code provided by the authors, the present paper first attempts to conduct an experimental repetition of the original study. The repetition is only partially successful, due to missing code and issues with the classification of languages. The second part of this work focuses on their main claim, the association between bugs and languages, and performs a complete, independent reanalysis of the data and of the statistical modeling steps undertaken by Ray et al. in 2014. This reanalysis uncovers a number of serious flaws which reduce the number of languages with an association with defects down from eleven to only four. Moreover, the practical effect size is exceedingly small. These results thus undermine the conclusions of the original study. Correcting the record is important, as many subsequent works have cited the 2014 paper and have asserted, without evidence, a causal link between the choice of programming language for a given task and the number of software defects. Causation is not supported by the data at hand; and, in our opinion, even after fixing the methodological flaws we uncovered, too many unaccounted sources of bias remain to hope for a meaningful comparison of bug rates across languages.
  Venue: ACM Tansactions on Programming Languages (TOPLAS)
  DOI: 10.1145/3340571 PDF Artifact: github.com/PRL-PRG/TOPLAS19_Artifact Video: CurryOn version OOPSLA version (short)
  
• TECS'19Can Android Run on Time? Extending and Measuring the Android Platform's Timeliness

  
  Authors: Yin Yan, Girish Gokul, Karthik Dantu, Steven Y. Ko, Lukasz Ziarek, and Jan Vitek.
  Abstract: Time predictability is difficult to achieve in the complex, layered execution environments that are common in modern embedded devices such as smartphones. We explore adopting the Android programming model for a range of embedded applications that extends beyond mobile devices, under the constraint that changes to widely used libraries should be minimized. The challenges we explore include: the interplay between real-time activities and the rest of the system, how to express the timeliness requirements of components, and how well those requirements can be met on stock embedded platforms. We detail the design and implementation of our modifications to the Android framework along with a real-time VM and OS, and provide experimental data validating feasibility over five applications.
  Venue: Transactions on Embedded Computing Systems (TECS)
  DOI: 10.1145/3289257 PDF
  
• JFP'19 How to Evaluate the Performance of Gradual Type Systems

  
  Authors: Ben Greenman, Asumu Takikawa, Max S. New, Daniel Felty, Robert Bruce Findler, Jan Vitek and Matthias Felleisen.
  Abstract: A sound gradual type system ensures that untyped components of a program can never break the guarantees of statically typed components. This assurance relies on runtime checks, which in turn impose performance overhead in proportion to the frequency and nature of interaction between typed and untyped components. The literature on gradual typing lacks rigorous descriptions of methods for measuring the performance of gradual type systems. This gap has consequences for the implementors of gradual type systems and developers who use such systems. Without systematic evaluation of mixed-typed programs, implementors cannot precisely determine how improvements to a gradual type system affect performance. Developers cannot predict whether adding types to part of a program will significantly degrade (or improve) its performance. This paper presents the first method for evaluating the performance of sound gradual type systems. The method quantifies both the absolute performance of a gradual type system and the relative performance of two implementations of the same gradual type system. To validate the method, the paper reports on its application to twenty programs and three implementations of Typed Racket.
  Venue: Journal of Functional Programs (JFP)
  DOI: 10.1017/S0956796818000217 PDF
  
• TOPLAS'19Feature-Specific Profiling

  
  Authors: Leif Andersen, Vincent St-Amour, Jan Vitek, Matthias Felleisen.
  Abstract: While high-level languages come with significant readability and maintainability benefits, their performance remains difficult to predict. For example, programmers may unknowingly use language features inappropriately, which cause their programs to run slower than expected. To address this issue, we introduce feature-specific profiling, a technique that reports performance costs in terms of linguistic constructs. Festure-specific profilers help programmers find expensive uses of specific features of their language. We describe the architecture of a profiler that implements our approach, explain prototypes of the profiler for two languages with different characteristics and implementation strategies, and provide empirical evidence for the approach’s general usefulness as a performance debugging tool.
  Venue: ACM Tansactions on Programming Languages (TOPLAS)
  DOI: 10.1145/3275519 PDF
  
• JAR'18Verifying a Concurrent Garbage Collector with a Rely-Guarantee Methodology

  
  Authors: Yannick Zakowski, David Cachera, Delphine Demange, Gustavo Petri, David Pichardie, Suresh Jagannathan and Jan Vitek.
  Abstract: Concurrent garbage collection algorithms are a challenge for program verification. In this paper, we address this problem by proposing a mechanized proof methodology based on the Rely-Guarantee proof technique. We design a compiler intermediate representation with strong type guarantees, dedicated support for abstract concurrent data structures, and high-level iterators on runtime internals. In addition, we define an Rely-Guarantee program logic supporting an incremental proof methodology where annotations and invariants can be progressively enriched. We formalize the intermediate representation, the proof system, and prove the soundness of the methodology in the Coq proof assistant. Equipped with this, we prove a fully concurrent garbage collector where mutators never have to wait for the collector.
  Venue: Journal of Automated Reasoning (JAR)
  DOI: https://doi.org/10.1007/s10817-018-9489-x PDF
  
• OOPSLA'18 Julia: Dynamism and Performance Reconciled by Design

  
  Authors: Benjamin Chung, Lionel Zoubritzky, Jeff Bezanson, Stefang Karpinski, Viral Shah, Jan Vitek
  Abstract: Julia is a programming language for the scientific community that combines features of productivity languages, such as Python or MATLAB, with characteristics of performance-oriented languages, such as C++ or Fortran. Julia has many productivity features: dynamic typing, automatic memory management, rich type annotations, and multiple dispatch. At the same time, it lets programmers control memory layout and uses a specializing just-in-time compiler that eliminates some of the overhead of those features. This paper details these choices, and reflects on their implications for performance and usability.
  Venue: PACM PL: Conference on Object-Oriented Programming Languages, Systems and Applications (OOPSLA)
  DOI: 10.1145/3276490 PDF
• OOPSLA'18 Julia Subtyping: A Rational Reconstruction

  
  Authors: Francesco Zappa Nardelli, Artem Pelenitsyn, Julia Belyakova, Benjamin Chung, Jeff Bezanson, Jan Vitek
  Abstract: Programming languages that support multiple dispatch rely on an expressive notion of subtyping to specify method applicability. In these languages, type annotations on method declarations are used to select, out of a potentially large set of methods, the one that is most appropriate for a particular tuple of arguments. Julia is a language for scientific computing built around multiple dispatch and an expressive subtyping relation. This paper provides the first formal definition of Julia’s subtype relation and motivates its design. We validate our specification empirically with an implementation of our definition that we compare against the existing Julia implementation on a collection of real-world programs. Our subtype implementation differs on 122 subtype tests out of 6,014.476. The first 120 differences are due to a bug in Julia that was fixed once reported; the remaining 2 are under discussion.
  Venue: PACM PL: Conference on Object-Oriented Programming Languages, Systems and Applications (OOPSLA)
  DOI: 10.1145/3276483 PDF
• DLS'18Self-Contained Development Environments

  
  Authors: Guido Chari, Javeier Pimas, Olivier Flückiger, Jan Vitek
  Abstract: Operating systems are traditionally implemented in low-level, performance-oriented programming languages. These languages typically rely on minimal runtime support and provide unfettered access to the underlying hardware. Tradition has benefits: developers control the resources that the operating system manages and few performance bottle-necks cannot be overcome with clever feats of programming. On the other hand, this makes operating systems harder to understand and maintain. Furthermore, those languages have few built-in barriers against bugs. This paper is an experiment in side-stepping operating systems, and pushing functionality into the runtime of high-level programming languages. The question we try to answer is how much support is needed to run an application written in, say, Smalltalk or Python on bare metal, that is, with no underlying operating system. We present a framework named NopSys that allows this, and we validate it with the implementation of CogNos a Smalltalk virtual machine running on bare x86 hardware. Experimental results suggest that this approach is promising.
  Venue: Dynamic Language Symposium (DLS)
  DOI: 10.1145/3276945.3276948 PDF
  
• ISSTA'18Tests from Traces: Automated Unit Test Extraction for R

  
  Authors: Filip Krikava, Jan Vitek
  Abstract: Unit tests are labor-intensive to write and maintain. This paper looks into the possibility of automatically generating unit tests for a target software package from the execution traces of client code and examples. Our objective is to reduce the effort involved in creating test suites while minimizing the number and size of individual tests, and maximizing coverage. To evaluate the viability of our approach, we select a challenging target for automated test generation, namely R, a programming language that is popular for data science applications. The challenges presented by R are its extreme dynamism and lack of types. This combination decrease the efficacy of traditional test generation techniques. We present Genthat, a tool developed over the last couple of years to non-invasively record execution traces of R programs and extract unit tests from those traces. We have carried out an evaluation on 1,547 packages comprising 1.7M lines of R code. The tests generated by Genthat improved code coverage from the original rather low value of 267,113 lines to 704,450 lines.
  Venue: International Symposium on Software Testing and Analysis (ISSTA)
  DOI: 10.1145/3213846.3213863 PDF
  
• ECOOP'18 KafKa: Gradual Typing for Objects

  
  Authors: Benjamin Chung, Paley Li, Francesco Zappa Nardelli, Jan Vitek
  Abstract: The enduring popularity of dynamically typed languages has motivated research on gradual type systems to allow developers to annotate legacy dynamic code piecemeal. Type soundness for a program which contains a mixture of typed and untyped code cannot mean the traditional absence of errors. While some errors will be caught at type checking time, other errors will only be caught as the program executes. After a decade of research it there are still a number of competing approaches to providing gradual type support for object-oriented languages. We introduce a framework for comparing gradual type systems, combining a common source languages with KafKa, a core calculus for object-oriented gradual typing. KafKa decouples the semantics of gradual typing from those of the source language. KafKa is strongly typed in order to highlight where dynamic operations are required. We illustrate our approach by translating idealizations of four different gradually typed semantics into the core calculus and discuss the implications of their respective designs.
  Venue: European Conference on Object-Oriented Programming (ECOOP)
  DOI: 10.4230/LIPIcs.ECOOP.2018.12 PDF
  
• ESOP'18Soundness of a Concurrent Collector for Actors

  
  Authors: Juliana Franco, Sylvan Clebsch, Sophia Drossopoulou, Jan Vitek, Tobias Wrigstad
  Abstract: ORCA is a garbage collection protocol for actor-based programs. Multiple actors may mutate the heap while the collector is running without any dedicated synchronisation. ORCA is applicable to any actor language whose type system prevents data races and which supports causal message delivery. We present a model of ORCA which is parametric to the host language and its type system. We describe the interplay between the host language and the collector. We give invariants preserved by ORCA, and prove its soundness and completeness.
  Venue: European Symposium on Programming (ESOP)
  DOI: 10.1007/978-3-319-89884-1_31 PDF
  
• POPL'18 Correctness of Speculative Optimizations with Dynamic Deoptimization

  
  Authors: Olivier Flückiger, Gabriel Scherer, Ming-Ho Yee, Aviral Goel, Amal Ahmed and Jan Vitek
  Abstract: High-performance dynamic language implementations make heavy use of speculative optimizations to achieve speeds close to statically compiled languages. These optimizations are typically performed by a just-in-time compiler that generates code under a set of assumptions about the state of the program and its environment. In certain cases, a program may execute code compiled under assumptions that are no longer valid. The implementation must then deoptimize the program on-the-fly; this entails finding semantically equivalent code that does not rely on invalid assumptions, translating program state to that expected by the target code, and transferring control. This paper looks at the interaction between optimization and deoptimization, and shows that reasoning about speculation is surprisingly easy when assumptions are made explicit in the program representation. This insight is demonstrated on a compiler intermediate representation, named sourir, modeled after the high-level representation for a dynamic language. Traditional compiler optimizations such as constant folding, unreachable code elimination, and function inlining are shown to be correct in the presence of assumptions. Furthermore, the paper establishes the correctness of compiler transformations specific to deoptimization: namely unrestricted deoptimization, predicate hoisting, and assume composition.
  Venue: PACM PL: Symposium on Principles of Programming Languages (POPL)
  DOI: 10.1145/3158137 PDF
• OOPSLA'17Dej́à Vu: A Map of Code Duplicates on GitHub

  
  Authors: Crista Lopes, Petr Maj, Pedro Martins, Di Yang, Jakub Zitny, Hitesh Sajnani, Jan Vitek
  Abstract: Previous studies have shown that there is a non-trivial amount of duplication in source code. This paper analyzes a corpus of 4.5 million non-fork projects hosted on GitHub representing over 428 million les written in Java, C++, Python, and JavaScript. We found that this corpus has a mere 85 million unique les. In other words, 70% of the code on GitHub consists of clones of previously created les. There is considerable variation between language ecosystems. JavaScript has the highest rate of le duplication, only 6% of the les are distinct. Java, on the other hand, has the least duplication, 60% of les are distinct. Lastly, a project-level analysis shows that between 9% and 31% of the projects contain at least 80% of les that can be found elsewhere. These rates of duplication have implications for systems built on open source software as well as for researchers interested in analyzing large code bases. As a concrete artifact of this study, we have created DéjàVu, a publicly available map of code duplicates in GitHub repositories.
  Venue: PACM PL: Conference on Object-Oriented Programming Languages, Systems and Applications (OOPSLA)
  DOI: 10.1145/3133908 PDF
• OOPSLA'17 Orca: GC and Type System Co-Design for Actor Languages

  
  Authors: Sylvain Clebsch, Juliana Franco, Sophia Drossopoulou, Albert Mingkun Yang, Tobias Wrigstad, Jan Vitek
  Abstract: Orca is a concurrent and parallel garbage collector for actor programs, which does not require any stop-the-world steps, or synchronisation mechanisms, and which has been designed to support zero-copy message passing and sharing of mutable data. Orca is part of the runtime of the actor-based language Pony. Pony’s runtime was co-designed with the Pony language. This co-design allowed to exploit certain language properties in order to optimise performance of garbage collection. Namely, Orca relies on the absence of race conditions in order to avoid read/write barriers, and it leverages the actor message passing for synchronisation among actors. This paper describes Pony, its type system, and the Orca garbage collection algorithm. An evaluation of the performance of Orca suggests that is fast and scalable for idiomatic workloads.
  Venue: PACM PL: Conference on Object-Oriented Programming Languages, Systems and Applications (OOPSLA)
  DOI: 10.1145/3133896 PDF
• ECOOP'17Parallelizing Julia with a Non-invasive DSL

  
  Authors: Todd Anderson, Hai Liu, Lindsey Kuper, Ehsan Totoni, Jan Vitek, and Tatiana Shpeisman
  Abstract: Computational scientists often prototype software using productivity languages that offer high-level programming abstractions. When higher performance is needed, they are obliged to rewrite their code in a lower-level efficiency language. Different solutions have been proposed to address this tradeoff between productivity and efficiency. One promising approach is to create embedded domain-specific languages that sacrifice generality for productivity and performance, but practical experience with DSLs points to some road blocks preventing widespread adoption. This paper proposes a non-invasive domain-specific language that makes as few visible changes to the host programming model as possible. We present ParallelAccelerator, a library and compiler for high-level, high-performance scientific computing in Julia. ParallelAccelerator’s programming model is aligned with existing Julia programming idioms. Our compiler exposes the implicit parallelism in high-level array-style programs and compiles them to fast, parallel native code. Programs can also run in “library-only” mode, letting users benefit from the full Julia environment and libraries. Our results show encouraging performance improvements with very few changes to source code required. In particular, few to no additional type annotations are necessary.
  Venue: European Confence on Object-Oriented Programming (ECOOP)
  DOI: 10.4230/LIPIcs.ECOOP.2017.4 Artifact: 10.4230/DARTS.3.2.7
  PDF
• ITP'17 Verifying a Concurrent Garbage Collector using a Rely-Guarantee Methodology

  
  Authors: Yannick Zakowski, David Cachera, Delphine Demange, Gustavo Petri, David Pichardie, Suresh Jagannathan, Jan Vitek
  Abstract: Concurrent garbage collection algorithms are a challenge in the area of concurrent program verification. We proposed a mechanized proof methodology based on the popular Rely-Guarantee (RG) proof technique. We design a specific compiler intermediate representation (IR) with strong type guarantees, dedicated support for abstract concurrent data structures, and high-level iterators on runtime internals. In addition, we define an RG program logic supporting an incremental proof methodology where annotations and invariants can be progressively enriched. We formalize the IR, the proof system, and prove the soundness of the methodol- ogy in the Coq proof assistant. Equipped with this IR, we prove a fully concurrent garbage collector where mutators never have to wait for the collector.
  Venue: International Conference on Interactive Theorem Proving (ITP)
  DOI: 10.1007/978-3-319-66107-0_31 PDF
• RTAS'17 Making Android Run on Time

  Authors: Yin Yan, Karthik Dantu, Steven Y. Ko, Jan Vitek, Lukasz Ziarek
  Abstract: Time predictability is difficult to achieve in the complex, layered execution environments that are common in modern embedded devices. We consider the possibility of adopting the Android programming model for a range of embedded applications that extends beyond mobile devices, under the constraint that changes to widely used libraries should be minimized. The challenges we explore include: the interplay between real-time activities and the rest of the system, how to express the timeliness requirements of components, and how well those requirements can be met on stock embedded platforms. We report on the design and implementation of an Android virtual machine with soft-real-time support, and provides experimental data validating feasibility over three case studies.
  Venue: Real-time and Embedded Technology and Application Symposium (RTAS)
  DOI: 10.1109/RTAS.2017.38 PDF
• POPL 16Is Sound Gradual Typing Dead?

  Authors: Asumu Takikawa, Dan Feltey, Ben Greenman, Max S. New, Jan Vitek, and Matthias Felleisen
  Abstract: Programmers have come to embrace dynamically-typed languages for prototyping and delivering large and complex systems. When it comes to maintaining and evolving these systems, the lack of explicit static typing becomes a bottleneck. In response, researchers have explored the idea of gradually-typed programming languages which allow the incremental addition of type annotations to software written in one of these untyped languages. Some of these new, hybrid languages insert run-time checks at the boundary between typed and untyped code to establish type soundness for the overall system. With sound gradual typing, programmers can rely on the language implementation to provide meaningful error messages when type invariants are violated. While most research on sound gradual typing remains theoretical, the few emerging implementations suffer from performance overheads due to these checks. None of the publications on this topic comes with a comprehensive performance evaluation. Worse, a few report disastrous numbers. In response, this paper proposes a method for evaluating the performance of gradually-typed programming languages. The method hinges on exploring the space of partial conversions from untyped to typed. For each benchmark, the performance of the different versions is reported in a synthetic metric that associates runtime overhead to conversion effort. The paper reports on the results of applying the method to Typed Racket, a mature implementation of sound gradual typing, using a suite of real-world programs of various sizes and complexities. Based on these results the paper con- cludes that, given the current state of implementation technologies, sound gradual typing faces significant challenges. Conversely, it raises the question of how implementations could reduce the overheads associated with soundness and how tools could be used to steer programmers clear from pathological cases.
  Venue: Principles of Programming Languages (POPL)
  DOI: 10.1145/2914770.2837630 PDF
  Artifact: http://www.ccs.neu.edu/home/asumu/artifacts/popl-2016/
  
• ECOOP 15Concrete Types for TypeScript

  Authors: Gregor Richards, Francesco Zappa Nardelli, and Jan Vitek
  Abstract: TypeScript extends JavaScript with optional type annotations that are, by design, unsound and, that the TypeScript compiler discards as it emits code. This design point preserves programming idioms developers are familiar with, and allows them to leave their legacy code unchanged, while offering a measure of static error checking in annotated parts of the program. We present an alternative design for TypeScript that supports the same degree of dynamism but that also allows types to be strengthened to provide correctness guarantees. We report on an implementation, called StrongScript, that improves runtime performance of typed programs when run on a modified version of the V8 JavaScript engine.
  Venue: European Conference on Object Oriented Programming (ECOOP)
  DOI: 10.4230/LIPIcs.ECOOP.2015.999 PDF
  Artifact: http://plg.uwaterloo.ca/~dynjs/strongscript
  
• ECOOP'15Cooking the Books: Formalizing JMM Implementation Recipes

  Authors: Gustavo Petri, Jan Vitek and Suresh Jagannathan
  Abstract: The Java Memory Model (JMM) is intended to characterize the meaning of concurrent Java programs. Because of the model's complexity, however, its definition cannot be easily transplanted within an optimizing Java compiler, even though an important rationale for its design was to ensure Java compiler optimizations are not unduly hampered because of the language's concurrency features. In response, the JSR-133 Cookbook for Compiler Writers, an informal guide to realizing the principles underlying the JMM on different (relaxed-memory) platforms was developed. The goal of the cookbook is to give compiler writers a relatively simple, yet reasonably efficient, set of reordering-based recipes that satisfy JMM constraints. We present the first formalization of the cookbook, providing a semantic basis upon which the relationship between the recipes defined by the cookbook and the guarantees enforced by the JMM can be rigorously established. Notably, one artifact of our investigation is that the rules defined by the cookbook for compiling Java onto Power are inconsistent with the requirements of the JMM, a surprising result, and one which justifies our belief in the need for formally provable definitions to reason about sophisticated (and racy) concurrency patterns in Java, and their implementation on modern-day relaxed-memory hardware. Our formalization enables simulation arguments between an architecture-independent intermediate representation of the kind suggested by the Cookbook with machine abstractions for Power and x86. Moreover, we provide fixes for cookbook recipes that are inconsistent with the behaviors admitted by the target platform, and prove the correctness of these repairs.
  Venue: European Conference on Object-Oriented Programming, 2015
  DOI: 10.4230/LIPIcs.ECOOP.2015.999 PDF
• CACM'15The Real Software Crisis: Repeatability as a Core Value

  Authors: Shriram Krishnamurthi, Jan Vitek
  Abstract: Where is the software in programming language research? In our field, software artifacts play a central role: they are the embodiments of our ideas and contributions. Yet when we publish, we are evaluated on our ability to describe informally those artifacts in prose. Often, such prose gives only a partial, and sometimes overly rosy, view of the work. Especially so when the object of discourse is made up of tens of thousands of lines of code that interact in subtle ways with different parts of the software and hardware stack on which it is deployed. Over the years, our community's culture has evolved to value originality above everything else, and our main conferences and journals deny software its rightful place.
  Venue: Communications of the ACM, Vol. 58 No. 3, Pages 34-36
  DOI: 10.1145/2658987 PDF
• TOPLAS'14Atomicity Refinement for Verified Compilation

  Authors: Suresh Jagannathan, Vincent Laporte, Gustavo Petri, David Pichardie, Jan Vitek
  Abstract: We consider the verified compilation of high-level managed languages like Java or C# whose intermediate representations provide support for shared-memory synchronization and automatic memory management. Our development is framed in the context of the Total Store Order relaxed memory model. Ensuring complier correctness is challenging because high-level actions are translated into sequences of nonatomic actions with compiler-injected snippets of racy code; the behavior of this code depends not only on the actions of other threads but also on out-of-order executions performed by the processor. A naïve proof of correctness would require reasoning over all possible thread interleavings. In this article, we propose a refinement-based proof methodology that precisely relates concurrent code expressed at different abstraction levels, cognizant throughout of the relaxed memory semantics of the underlying processor. Our technique allows the compiler writer to reason compositionally about the atomicity of low-level concurrent code used to implement managed services. We illustrate our approach with examples taken from the verification of a concurrent garbage collector.
  Venue: ACM Transactions on Programming Languages and Systems (TOPLAS), Volume 36 Issue 2, July 2014, Article No. 6
  DOI: 10.1145/2601339 PDF
• VEE'14A Fast Abstract Syntax Tree Interpteter for R

  Authors: Tomas Kalibera Petr Maj, Floreal Morandat, Jan Vitek
  Abstract:Dynamic languages have been gaining popularity to the point that their performance is starting to matter. The effort required to develop a production-quality, high-performance runtime is, however, staggering and the expertise required to do so is often out of reach of the community maintaining a particular language. Many domain specific languages remain stuck with naive implementations, as they are easy to write and simple to maintain for domain scientists. In this paper, we try to see how far one can push a naive implementation while remaining portable and not requiring expertise in compilers and runtime systems. We choose the R language, a dynamic language used in statistics, as the target of our experiment and adopt the simplest possible implementation strategy, one based on evaluation of abstract syntax trees. We build our interpreter on top of a Java virtual machine and use only facilities available to all Java programmers. We compare our results to other implementations of R.
  Venue: Proceedings of the 10th ACM SIGPLAN/SIGOPS international conference on Virtual execution environments (VEE) Pages 89-102
  DOI: 10.1145/2674025.2576205 PDF
• ISMM'14M3: high-performance memory management from off-the-shelf components

  Authors: David Terei, Alex Aiken, Jan Vitek
  Abstract: Real-world garbage collectors in managed languages are complex. We investigate whether this complexity is really necessary and show that by having a different (but wider) interface between the collector and the developer, we can achieve high performance with off-the-shelf components for real applications. We propose to assemble a memory manager out of multiple, simple collection strategies and to expose the choice of where to use those strategies in the program to the developer. We describe and evaluate an instantiation of our design for C. Our prototype allows developers to choose on a per-type basis whether data should be reference counted or reclaimed by a tracing collector. While neither strategy is optimised, our empirical data shows that we can achieve performance that is competitive with hand-tuned C code for real-world applications.
  Venue: Proceedings of the 2014 international symposium on Memory management (ISMM) Pages 3-13
  DOI: 10.1145/2602988.2602995 PDF