SIGPLAN Research Highlights Papers
Selected September 2013
Title: A General Constraint-Centric Scheduling Framework for Spatial Architectures
Authors: Tony Nowatzki, Michael Sartin-Tarm, Lorenzo De Carli, Karthikeyan Sankaralingam, Cristian Estan, Behnam Robatmili
Venue: PLDI 2013
Scheduling of computation to hardware resources is absolutely critical to the success of spatial architectures, but it's a very tough problem. The authors present a general framework for scheduling on spatial architectures, and evaluate that framework on three diverse architectures with impressive results w.r.t. specialized schedulers. This work is an impressive study of the scheduling problem presented by these architectures and will be of interest to PL and architecture people alike.
Title: CLAP: Recording Local Executions to Reproduce Concurrency Failures
Authors: Jeff Huang, Charles Zhang, Julian Dolby
Venue: PLDI 2013
CLAP is an efficient technique for reproducing concurrency bugs that logs thread local execution and computes memory dependencies offline, allowing an observed concurrency bug to be reproduced. The principal contribution is the identification of a new approach to reproducing concurrency bugs that reduces overheads and reduces the observer effect seen in existing systems that inject synchronization points.
Title: From Relational Verification to SIMD Loop Synthesis
Authors: Gilles Barthe, Juan Manuel Crespo, Sumit Gulwani, Cesar Kunz, Mark Marron
Venue: PPoPP 2013
This paper addresses a compiler optimization problem, namely generating SSE vector code for loops with data-driven control flow and suboptimal data layout, with an interesting and unexpected combination of techniques. The initial loop restructuring step only generates pre- and post-conditions for various code fragments. A synthesis algorithm is then used to fill these fragments in. This paper won the PPoPP best paper award and was recommended by Saman Amarasinghe, one of the PC Chairs.
Title: SIMD Parallelization of Applications that Traverse Irregular Data Structures
Authors: Bin Ren, Gagan Agrawal Jim Larus, Todd Mytkowicz, Tomi Poutanen Wolfram Schulte
Venue: CGO 2013
Finding ways to parallelize important computations is now essential in creating a competitive product or service. This paper addresses the challenging problem of mapping a SIMD model of computation onto an irregular data structure and demonstrates the effectiveness of this approach for two important classes of problems: regular expression matching and evaluating forests of decision trees. The paper shows that with the proper intermediate language abstraction, such irregular computations can be mapped efficiently onto SIMD units and result in speedups of over 10 times for random forest evaluation mapping the computation onto the SSE extensions of the x86 instruction set on a single core machine. These improvements result both from parallelizing the irregular computation and carefully laying out the data being manipulated to reduce the memory latency. By showing that the approach applies to two large interesting classes of irregular computations, the authors demonstrate the generality of their approach its potential for broad impact.
Selected May 2013
Title: Set-Theoretic Foundation of Parametric Polymorphism and Subtyping
Authors: Giuseppe Castagna and Zhiwu Xu
Venue: ICFP 2011
The paper provides a semantic basis for the combination of subtyping and parametric polymorphism (generics in Java parlance) that has eluded researchers for quite some time. The paper builds on the more "intuitive" types-as-sets notion, in a principled way, and explains the issues with past attempts and give intuition for the presented models. It has immediate application in typing XML, but appears to be much broader than that. The problem of semantic definition of types is generally an important and challenging one where we expect progress to come slowly. This paper seems to represent an important next step.
Title: Safe Haskell
Authors: David Terei, David Mazières, Simon Marlow, and Simon Peyton Jones
Venue: Haskell Symposium 2012
This paper describes a language extension of Haskell that is intended to make it possible to confine and safely execute untrusted, possibly malicious code. It leverages Haskell's type system, and imposes relatively few requirements on the development of Haskell programs. There's also a "Safe Haskell" subset of the language being identified that has particularly nice properties, and the compiler is able to automatically infer whether programs are in this subset. The design and approach of Safe Haskell should be of interest to the wider CACM community, even though (or in particular, because) some of Safe Haskell's ideas are made possible because Haskell is already a relatively safe language to start with.
Work-Stealing Without the Baggage
Authors: Vivek Kumar, Daniel Frampton, Stephen M. Blackburn, David Grove, and Olivier Tardieu
Venue: OOPSLA 2012
This paper takes a well-known idea, work-stealing to exploit parallel hardware, and asks why it doesn't work as well as it should. It identifies the key sources of overhead, then conducts a substantial empirical study of 4 alternative strategies for avoiding that overhead. The result is that one of these techniques is shown to incur low-overhead (15%), while enabling just as much parallelism as the others - more bang for the buck. This is a reconsideration of a known idea that improves on it substantially.
AutoMan: a Platform for Integrating Human-Based and Digital Computation
Authors: Daniel W. Barowy, Charlie Curtsinger, Emery D. Berger, and Andrew McGregor
Venue: OOPSLA 2012
There has been a lot of buzz about mixtures of machine and human computation. For the most part there has been little programmatic support for constructing such systems. This paper makes an initial step in that direction. It defines an API that permits crowd-source computation to be invoked from a program like ordinary function calls. This is coupled with a runtime system that manages the execution of the crowd-sources computation, determines when sufficient confidence in the results is achieved, manages things like repricing and restarting crowd-source tasks, and seeks to maximize parallelism in the execution of the crowd-source tasks. This is an interesting new idea that provides a direction for people to build on.
On the Linear Ranking Problem for Integer Linear-Constraint Loops
Authors: Amir Ben-Amram and Samir Genaim
Venue: POPL 2013
The paper studies the complexity of the linear ranking problem, namely the problem that, given a loop described as a set of linear constraints over a finite set of integer variables, determines if there exists a linear ranking decreasing function for that loop. The existence of this function implies termination, although the converse may not hold. The great contribution is set by moving from rational/real valued variables, which is PTIME, to integer valued variables where the decidability of the problem was still open. The authors proved that the Linear Ranking Problem for Integer variables is decidable and it is coNP-complete. Algorithmic methods for synthesizing linear ranking functions have been introduced and restrictions have been studied in order to make the problem PTIME.
Selected October 2012
Title: And Then There Were None: A Stall-Free Real-Time Garbage Collector for Reconfigurable Hardware
Authors: David F. Bacon, Perry Cheng, and Sunil Shukla
Venue: PLDI 2012
Published as And Then There Were None: A Stall-Free Real-Time Garbage Collector for Reconfigurable Hardware in December 2013 CACM Research Highlight. Technical perspective by Eliot Moss
This is a very forward-looking paper. The authors see a future where FPGAs are an important substrate for general purpose computing, and they address the problem of garbage collection for programs realized in FPGA. The paper presents the first completely hardware implementation of a garbage collector (as opposed to hardware-assist) and the first garbage collector which completely eliminates interference to the mutator. The paper is broad in scope (hardware, software, empirical, semi-formal), and is developed from a fundamental, first-principles background. The paper is accessible yet not incremental.
SuperC: Parsing All of C by Taming the Preprocessor
Authors: Paul Gazzillo and Robert Grimm
Venue: PLDI 2012
This paper addresses the long standing problem of correctly parsing both of the two underlying languages that comprise what we know as C: C proper, and the C preprocessor. Most tools do not attempt to parse both. Most C compilers parse a single configuration of the program rather than the full set of possible configurations. The authors provide a nice overview of the challenges of parsing C and explore this concretely with the challenging example of the Linux kernel. They introduce 'Fork-Merge LR (FMLR)' parsing which they use to address the challenge of parsing across multiple configuration possibilities. The outcome is a publicly available tool, 'SuperC'. This paper is accessible to a broad audience, it has concrete applications, and has some very nice computer science behind it. The work has clear applications, addresses a very well established problem, and is bound to be a milestone publication.
Efficient Lookup-Table Protocol in Secure Multiparty Computation
Authors: John Launchbury, Iavor S. Diatchki, Thomas DuBuisson, and Andy Adams-Moran
Venue: ICFP 2012
Cloud computing provides new opportunities for individuals to cheaply take advantage of tremendous computing power for occasional expensive computations, but the capability comes with new challenges. This paper specifically focuses on privacy. In particular, you might imagine encrypting your private before sending it to the cloud computers, in order to ensure secrets are not leaked. But how can a cloud computer compute on encrypted data? The SMC (secure multi-party computation) community has studied this challenge and has come up with algorithms that can successfully operate on encrypted data, without revealing that data to the cloud computers that are actually doing the computation. Unfortunately, their algorithms are typically expressed via circuits, making an efficient implementation more challenging than it should be. This paper shows how a languages-based perspective can bring powerful tools to bear on the problem. Indeed, the paper shows how to get order-of-magnitude improvements to the implementation of AES encryption in an SMC setting. Even better: the overall structure of the AES implementation is similar to the non-SMC version. One key technical development that makes this possible is a new array lookup primitive that provides good leverage for expressing the algorithm and its efficient implementation.
Experience Report: A Do-It-Yourself High-Assurance Compiler
Authors: Lee Pike, Nis Wegmann, Sebastian Niller, and Alwyn Goodloe
Venue: ICFP 2012
This exemplary paper explains how a variety of techniques that are understood in the FP community can be applied to build domain-specific embedded languages with evidence that the compiler is correct. The techniques used include using a Turing-complete host language with a Turing-incomplete hosted language, static and dynamic type checking, the QuickCheck property-based verification tool, model checking, and aiming for a verifying rather than a verified compiler. These techniques were applied to build a language, Copilot, for verification monitors used in aviation, an application of great interest in its own right.
Selected February 2012
Title: Fast Restore of Checkpointed Memory using Working Set Estimation
Authors: Irene Zhang, Alex Garthwaite, Yuri Baskakov, Kenneth C. Barr
Venue: VEE 2011
The paper describes a clever technique for restoring checkpointed OS-level virtual machines, which significantly outperforms both eager (restore all memory pages, then start VM) and lazy (page in as needed) techniques. While the checkpoint is computed, page accesses are tracked for roughly the time required to restore the VM. Accessed pages are restored eagerly, the rest lazily. If everything were entirely deterministic, this should restore exactly the needed pages eagerly, and it seems to come close. This is a VMWare paper, and I believe the technique is used in practice. It is in many ways the ideal CACM paper: It exposes the reader to a probably new problem space, the paper is easy to read, and probably unusually easy to shorten to the CACM page limit. It is a simple but interesting and meaningful result. It also exposes the reader to the "minimum mutator utilization" metrics from the GC literature, which may themselves be interesting to some readers.
Title: SugarJ: Library-based Syntactic Language Extensibility
Authors: Sebastian Erdweg, Tillmann Rendel, Christian Kastner, Klaus Ostermann
Venue: OOPSLA 2011
This paper appeared in OOPSLA 2011, pages 391-406. It addresses the problem of adding domain-specific language extensions, including domain-specific syntax extensions, to an existing programming language through the use of library modules. The main claim is that the approach "maintains the composability and scoping properties of ordinary libraries" (abstract). The mechanism is also self-applicable, which allows one to write syntax extensions that help the process of writing syntax extensions, and it also composes at this meta-level.
The approach builds on Java, SDF, and Stratego. There is also a novel incremental parsing technique that supports changing syntax within a source file. Composition of syntax extensions works through the module import mechanism of the host language. There is, however, a caveat that grammars do not always compose, so there is a possibility that importing two syntax extensions may cause an error. In summary, the paper both combines a number of existing techniques (from SDF and Stratego) in a very elegant and well-thought out language design.
The paper has a nice demonstration of the ideas, with examples that will be well received by a CACM audience. One example shows how to write a layered XML extension, including an embedding of XML Schema. The XML Schema extension shows how context-sensitive checks (like type checking) can be accomplished in their framework.
Support for domain-specific languages is something that has been gaining steam in the PL community for years, and it seems like this paper can help bring these ideas to a wider audience. In particular, an idea from earlier work (on MetaBorg) is adapted that allows the writer of a syntax extension to use the concrete syntax of a syntax extension in the description of how to transform the extension into the underlying language (such as Java). This is itself a nice demonstration of the self-applicability of the syntax extension mechanisms and how such syntax extension mechanisms can be implemented as a library (see section 5.1). Another nice feature of the design is the well thought out way that extensions are staged during processing of a compilation unit (file). This process, described in section 4.2, doesn't seem too deep, and may seem obvious in hindsight, but it's the kind of thing that can have a long lasting impact on the field, as it just seems right. Brought together, all these mechanisms seem to make a fundamental difference, and seem to represent a leap forward in syntactic extensibility. In particular, the way that the mechanism is demonstrated to be self-applicable is extremely impressive. Furthermore, this self-applicability allows for an virtuous cycle of improvements in program libraries, as is shown to some extent in the paper with the implementation of support for concrete syntax transformations.
Selected September 2011:
Title: Understanding POWER Multiprocessors
Authors: Susmit Sarkar, University of Cambridge, Peter Sewell, University of Cambridge, Jade Alglave, Oxford University, Luc Maranget INRIA, Derek Williams, IBM
Venue: PLDI 2011
An overlapping set of authors wrote an earlier paper entitled "x86-TSO: A Rigorous and Usable Programmer's Model for x86 Multiprocessors", published in July 2010 as a Research Highlight. That paper describes the easy-to-understand, but formally describable, and empirically tested memory model for x86. This paper describes the analogous effort for IBMs Power architecture.
The paper gives a high-level description of some of the tricky behavior exhibited by Power and how to understand it. The paper builds an executable abstract machine and compares runs of the abstract machine with runs on Power hardware for a large suite of "litmus tests". Execution of the abstract machine is nontrivial and involves a search for all possible executions. The paper explains example programs and tricky behavior via simple diagrams, and it explains the abstract machine with words, while the formal definitions are all in a technical report. The paper is an impressive tour-de-force of what programming language technology can do to help understand a complicated processor. For example, the authors defined their own language for specifying abstract machines, along with a translator to executable OCaml code and other tools. Additionally, the authors did a good job of making the abstract machine as simple as possible, in the face of a massively complicated processor.
This provides an interesting contrast to the x86 paper. The x86 paper gives rise to a model (essentially TSO) simple enough to be described operationally in a way that is probably useful to a typical assembly programmer. The Power model is inherently much more complex, in that the architecture exposes more speculation, and provides a very diverse set of memory fence instructions, with very subtle semantics. The present paper derives a model that is also mathematically precise, but probably too complex for a typical programmer. Nonetheless, it fills an important void, and finally answers important questions that have been creating confusion for years. A major question throughout the development of the latest C and C++ standards was whether the provided atomic operations were implementable on PowerPC, and at what cost. This allows those questions to be precisely answered, and finally allows a compiler writer to confidently and correctly hide the architectural complexity. If the C++ standards committee had had access to such a description, we would have avoided months of debate.
Title: Data Representation Synthesis
Authors: Peter Hawkins, Stanford, Alex Aiken, Stanford, Kathleen Fisher, Tufts University, Martin Rinard, MIT, Mooly Sagiv, Tel-Aviv University
Venue: PLDI 2011
This paper describes a system that synthesizes C++ data structures and query/update algorithms from a declarative description. The motivation is that a programmer will be more likely to reason effectively about higher-level, declarative specifications and automatically-generated code. The approach is partially automated, but requires user input to select the data representation--although the paper experiments with auto-tuning to select the representation.
Title: Automated Atomicity-Violation Fixing
Authors: Guoliang Jin, University of Wisconsin-Madison, Linhai Song, University of Wisconsin-Madison, Wei Zhang, University of Wisconsin-Madison, Shan Lu, University of Wisconsin-Madison, Ben Liblit, University of Wisconsin-Madison
Venue: PLDI 2011
This paper presents AFix, a development tool that uses the results of an atomicity violation detector (in this case, CTrigger) to automatically construct patches that fix the detected violations. It does so by placing critical sections around accesses that might cause atomicity violations (using various simple static analyses), and then merging sections together to reduce overhead and the likelihood of deadlock. On several real-world bugs, AFix is able to either completely fix or partially mitigate the effect of atomicity bugs, often better than manual patches. This paper was well motivated (the example in the introduction was very nicely chosen to illustrate the pertinent issues), well written, and explained a set of intuitive techniques clearly. In addition, the results seemed very promising. The strategy presented by the paper is fairly simple, but surprisingly effective, at least for fixing bugs that fall under its failure model. While it's easy to see many sources of potential inefficiency and incompleteness in the proposed algorithm, the empirical results make the case that the technique can fix real bugs with negligible overhead. This is one of the first papers to attack the problem of automated bug fixing, so it should be of wide interest.
Title: Automating String Processing in Spreadsheets Using Input-Output Examples (POPL 2011) and Spreadsheet Table Transformations from Examples (PLDI 2011)
Authors: Sumit Gulwani (POPL 2011) and William Harris and Sumit Gulwani (PLDI 2011)
Venue: PLDI 2011
The committee selects this paper (POPL 2011) together with the follow-up paper in PLDI 2011 by William Harris and Sumit Gulwani. Each of the papers is a great paper, and we believe they would be even better together as a combined CACM article. The theme of the papers is program synthesis in service of the general population. Specifically, the papers develop synthesis techniques that make it easier to use spreadsheets. The POPL 2011 paper focuses on string manipulation, while the PLDI 2011 paper focuses on table manipulation. Both papers enable "programming by example" in the sense that a user can show, with the existing functionality of the spreadsheet, an example of what needs to be done, and the implemented tool will then generate code for doing that kind of thing and apply it to all other instances in the table. If the result isn't what the user wanted, then one can simply iterate the process. Both papers develop new program synthesis techniques and demonstrate via implementations and experiments that they work well in practice. These papers have broad appeal and show nicely that tools that are based on programming language techniques can be widely useful.
Selected February 2011:
Title: Lime: a Java-Compatible and Synthesizable Language for Heterogeneous Architectures
Authors: Joshua Auerbach, IBM Research, David F. Bacon, IBM Research, Perry Cheng, IBM Research, Rodric Rabbah, IBM Research
Venue: OOPSLA 2010
This paper describes a new language, Lime, that is designed to be compiled to and executed on a "range of architectures, from FPGAs to conventional CPUs." The language also features parallelism at many different granularities. The key to the various optimizations given is the distinction between stateful and stateless computations. The paper has a novel notion of purity which is declared and checked in a novel way that is modular: by splitting the notion of purity into the concepts of valueness and locality. Various types can be declared as value types, which must have the property of being "deeply immutable." Locality means "that no side effects can be propagated through static fields or information reachable from them." Both "local static" methods whose arguments are all value types and "local instance" methods whose arguments are all of value types are pure functions. The language also has a kind of polymorphism that allows the locality of a method to depend on the context in which it invoked (called "glocal").
Lime supports fine-grained parallelism (on value types) through collective operations, which can be user-defined. Types such as bitstrings and small arrays can be made data-parallel.
Lime also supports stream computing using tuple types and its "task" and "connect" mechanisms. The connect operator (written "=>") connects an output stream in a task to another task's input stream. Such connections are atomically buffered to help with flow control. (There is also a technique that can be used for rate matching.) To ease the connection between streams produced by tasks, Lime has a "match" operator and also statically checks connections to prevent "errors and scheduling complications". However, it does have ways to send messages upstream or downstream to aid in configuration. All these mechanisms merely describe a dataflow graph; starting the computation is done separately, allowing the compiler to output the dataflow computation to hardware and providing the opportunity for various optimizations.
Tasks can also be used for coarse-grained parallelism with its "split" and "join" primitives, that operate on streams.
Lime's support for compilation to various kinds of hardware is also aided by a mechanism that can declare when a set of types is complete. This is done by allowing classes and interfaces to name all their direct subtypes. The evaluation is impressive, with C and FPGA executions performing orders of magnitude faster compared to the baseline code, although the authors say that their FPGA backend is not completely mature.
Title: Nikola: Embedding Compiled GPU Functions in Haskell
Authors: Geoffrey Mainland, Harvard School of Engineering and Applied Sciences, Greg Morrisett, Harvard School of Engineering and Applied Sciences
Venue: Haskell Symposium 2010
This paper provides an excellent overview of the state of the art of domain-specific language (DSL) programming within a functional language such as Haskell. As running example, it introduces Nikola, a domain-specific language embedded in Haskell. Nikola is suitable for expressing first-order array computations. Nikola programs can thus be written within Haskell, having all the high-level features and the type system at the disposal. Despite being embedded, the Haskell compiler has access to a representation of the Nikola program, can analyze its sharing structure, can invoke a Nikola-specific compiler behind the scenes, generate code for the CUDA framework and have the program run on the GPU. The paper demonstrates that advantages of standalone DSLs and embedded DSLs can be combined without imposing any extra burden on the programmer. The authors show preliminary performance tests that indicate that it is possible to achieve the performance of hand-written CUDA code when starting from a Haskell program. The paper is interesting to people interested in DSLs because it provides a good overview of typical problems arising when embedding DSLs such as loss of sharing, and it gives pragmatic explanations how these problems can be avoided. It furthermore demonstrates that even embedded DSLs can generate efficient code, and therefore allow users of otherwise rather low-level languages such as CUDA to work in higher-level languages instead.
Title: The Impact of Higher-Order State and Control Effects on Local Relational Reasoning
Authors: Derek Dreyer (MPI-SWS); Georg Neis (MPI-SWS); Lars Birkedal (IT University of Copenhagen)
Venue: ICFP 2010
Understanding if two programs are equivalent is an essential part of reasoning about the correctness of program transformations. This paper applies the modern technique of step-indexed Kripke logical relations to the old problem of reasoning about program equivalence in the presence of expressive language constructs, such as recursive types, abstract types, general references and call/cc. It also discusses how reasoning ability changes under restrictions to the language, giving examples properties that are true only if the language lacks higher-order state or control effects such as call/cc. Although the mathematical machinery necessary for this work is highly technical at times, much of the presentation in the paper is given in an informal, pedagogic style based on clean "visual" proof sketches.
Title: Lightweight Modular Staging: A Pragmatic Approach to Runtime Code Generation and Compiled DSLs
Authors: Tiark Rompf, École Polytechnique Fédérale de Lausanne Martin Odersky, École Polytechnique Fédérale de Lausanne
Venue: GPCE 2010
Published as Lightweight Modular Staging: A Pragmatic Approach to Runtime Code Generation and Compiled DSLs in June 2012 CACM Research Highlight. Technical perspective by Peter Lee
The paper introduces "Lightweight Modular Staging" (LMS), a technique for expressing multi-stage programming. Staged programming allows control over the time when a piece of code is executed. In particular, code can be executed at compile time already, thereby allowing a disciplined form of program generation. Most programming languages that support staging require explicit source code annotations that delay or force the execution of a marked piece of code. LMS is different in that it does not have annotations on the term level: instead, simply changing the type signatures of a program fragment is sufficient to lift that program to a staged version. Additional optimizations can then be implemented separately (using Scala's "trait"s), without touching the original code. LMS is particularly attractive for implementing embedded domain-specific languages (DSLs), because the staging features can be used to perform code optimizations in the background and the lightweight syntax keeps the surface syntax clean. This is confirmed by the fact that the Stanford Pervasive Parallelism Lab is already using LMS successfully to build domain-specific languages. The paper is a good example how modern language technology can help to solve long-standing and difficult problems such as staging: with Scala's general-purpose features, the staging system as described in this paper can be implemented as a library, without need for further language extensions.
Selected November 2010:
Title: Complete Functional Synthesis
Authors: Viktor Kuncak, Mikaël Mayer, Ruzica Piskac, and Philippe Suter
Venue: PLDI 2010
Published as a CACM Research Highlight (February 2012).
Synthesizing program fragments from high-level specifications has the potential to make programs more readable and programmers more productive. The problem has many challenging elements, however, including how to keep specifications concise, how to convert these specifications into code, and how to integrate such synthesis into general-purpose programming languages. This paper provides a key insight: decision procedures that generate witnesses can form the basis for synthesis. As a case study, the paper develops this idea for two useful decision procedures: linear arithmetic and constraints over sets. Further, the authors show how to incorporate such synthesis into Scala as a form of pattern matching. The resulting programs are concise and easier to understand than the versions without synthesis. The decision procedures are run at compile time to synthesize the desired code, and if no solution exists, the tool reports the failure statically. In summary, this paper describes a cool idea that could substantially change programming practice.
Title: Safe to the Last Instruction: Automated Verification of a Type-Safe Operating System
Authors: Jean Yang and Chris Hawblitzel
Venue: PLDI 2010
Published as a CACM Research Highlight (December 2011).
This paper describes Verve, an operating system running on standard hardware that is verified to ensure type and memory safety. The verification is automatic, using a combination of two different layers. Due to this combination, a large part of the operating system kernel can actually be written in a high-level programming language (here: C#). The automatic nature of the verification brings verification times down to a number of minutes. This means that for the first time it is possible to develop extension of the system on the fly, and to perform quick experiments with new designs. The paper presents an entertaining and accessible introduction to the various issues that are involved in building such a system. Verve illustrates that building much more robust and secure operating systems is feasible. Given the widespread vulnerability of existing operating systems, Verve is of broad interest to the CACM community because it shows that alternatives are possible.
Selected November 2009:
Title: FastTrack: Efficient and Precise Dynamic Race Detection
Authors: Cormac Flanagan and Stephen N. Freund
Venue: PLDI 2009
Published as a CACM Research Highlight (November 2010).
Past work on dynamic data race detection had to choose between fast but imprecise race detectors that report false alarms vs. slow but precise race detectors that never report false alarms. The latter category is typically based on classical "vector clock" operations, where a single operation requires time linear in the number of program threads. The key insight behind the FastTrack dynamic data race detection algorithm introduced in this paper is that the full generality of vector clocks is not necessary in over 99% of these read and write operations in practice. Instead, a more lightweight representation of the happens-before information can be constructed using "epochs". Experimental results on Java benchmarks show that this approach is an order of magnitude faster than a traditional vector-clock race detector, thereby representing a significant advance over classical work in this area. This paper includes a comprehensive comparison with past work, including the "Goldilocks" paper. Though both papers address the same high-level problem, they take very different approaches. FastTrack has shown how past algorithms based on vector clocks can be made significantly more efficient and scalable, thereby providing a perspective on a large body of past work in this area. Another SIGPLAN Research Highlights paper, “Goldilocks”, is also highly innovative and takes a different approach that is not based on vector clocks.
Selected June 2009:
Title: The Theory of Deadlock Avoidance via Discrete Control
Authors: Yin Wang, Stephanie Lafortune, Terence Kelly, Manjunath Kudlur, and Scott Mahlke
Venue: POPL 2009
Deadlock is a difficult problem for the average programmer who must now confront multicore hardware. The Gadara project offers a very interesting possible solution. The attached POPL paper provides a theoretical foundation for dynamic deadlock avoidance in concurrent programs that employ conventional mutual exclusion and synchronization primitives. Beginning with control flow graphs extracted from program source code, it constructs a formal model of the program and then applies Discrete Control Theory to automatically synthesize deadlock avoidance control logic that is implemented by program instrumentation. At run time, the control logic avoids deadlocks by postponing lock acquisitions. Discrete Control Theory guarantees that the instrumented program cannot deadlock. The control logic is maximally permissive: it postpones lock acquisitions only when necessary to prevent deadlocks, and therefore permits maximal runtime concurrency. Additonal publications on the prototype system have appeared in top fora in DCT, systems, and programming languages: WODES '08, OSDI '08. The OSDI paper is attached as well. This research is a very nice combination of rigor and novelty. The authors have a working prototype and have applied it to real software including Apache, BIND, and OpenLDAP; it works as advertised, with very tolerable performance overhead. This approach potentially makes a significant contribution to enhancing the programmability of multicore processors for the mainstream.
Title: Generic Discrimination: Sorting and Partitioning Unshared Data in Linear Time
Author: Fritz Henglein
Venue: ICFP 2008
Everyone knows that sorting a collection of n elements requires a number of comparisons proportional to n log n. Distribution sort over certain kinds of data works in time linear in the number of elements, but is a "specialty" sort, useful only in special circumstances. After reading this paper, you may reverse this idea. Henglein shows how to generalize distribution sort to a wide range of data. Assuming a base type is suitable for distribution sort, he shows how products, sums, and lists of such data are also suitable for distribution sort, relegating n log n sorts to the "specialty" case. Henglein argues convincingly that the right interface for an abstract data type to supply is not comparison and not sorting, but a mild generalization of sorting which he calls a discriminator, in terms of which the other two may be defined. While the technique may be applied in any programming language, it fits particularly well with a functional language where it is easy to compose sort operators on component types to build sort operators on structures, and where advanced type systems allow one to ensure type safety while doing so. Henglein presents complete code in Haskell. We should expect the technique to spread to other languages; the new notion of C++ concepts, which resemble Haskell type classes, are well suited for supporting defining functions based on type structure. The paper is a joy to read. (Addendum)
Selected January 2009:
Title: Goldilocks: A Race and Transaction-Aware Java Runtime
Authors: Tayfun Elmas, Shaz Qadeer, and Serdar Tasiran
Venue: PLDI 2007
Published as a CACM Research Highlight (November 2010).
The paper describes a precise and reasonably fast data-race-detection algorithm. Their implementation takes the form of a modified JVM that detects all data races as defined by the Java memory model, and throws an exception when a race is encountered. It also supports transactional memory. As far as I know, this is the first such implementation that is fast enough to at least raise the question of whether this could be done routinely as part of production code execution. Although the performance measurements in the paper are not such that we can declare this a solved problem, I think this paper points at an interesting possible solution path to two hard (and, thanks to the widespread adoption of multicore processors) critical parallel programming problems: 1. Dealing with data races is the most difficult aspect of parallel program debugging. Since such bugs are usually not easily reproducible, they may escape testing, and may go unreported for long periods of time. Debugging may require months, since the actual data race is often no longer visible when a failure results, requiring the problem be reproduced repeatedly with increasingly instrumented programs. An exception at the time of the race avoids this problem. 2. (Not pointed out in the paper, since I think nobody appreciated it at the time.) It has become increasingly clear that we do not know how to fully specify the semantics of a multithreaded language like Java that, for security reasons, must specify some semantics for data races. The Java memory model (POPL 05) is the most successful attempt, but it has recently become clear that it too has some surprising and undesirable properties. If we could requrie that data races produce exceptions, these issues disappear. I believe there is now some ongoing work that looks at hardware support for data-race detection, that might greatly reduce the performance costs, and eventually make the "data-race as exception" model viable for a much larger class of applications. This paper is a great first step toward something that might actually result in a major advance in shared memory parallel programming, whether it uses transactional memory, or continues to be based on locks.
Title: Sound, Complete and Scalable Path-Sensitive Analysis
Author: Isil Dillig, Thomas Dillig, and Alex Aiken
Venue: PLDI 2008
Published as a CACM Research Highlight (August 2010). Retitled as: Reasoning About the Unknown in Static Analysis
This paper appeared in PLDI 2008 and was coauthored by Alex Aiken and two of his Ph.D. students, Isil Dillig and Thomas Dillig. It provides an elegant and scalable solution to a decades-old challenge in the area of program analysis viz., how to perform precise intra- and inter-procedural path-sensitive program analysis without incurring the exponential increase in space and time that arises in a naive code duplication approach? And how to perform such an analysis for recursive programs? They introduce a new approach based on constraint systems to address this problem by defining constraints that capture the exact path- and context-sensitive condition under which a program property holds. The resulting constraint system may not be solvable in general. However, to answer a "may" or a "must" query regarding a program property, they can extract necessary or sufficient conditions from the constraint system which can be solved i.e., they specialize the exact path condition to the query being asked. Further, these extracted necessary/sufficient conditions are typically small enough to make the approach scalable to large software -- they have implemented this approach in the Saturn system and used it to analyze over 6MLOC in the entire Linux kernel. They also use "unobservable variables" to model interactions with the environment (such as user input) as well as to support modular program analysis. Their first set of experimental results quantify the size of necessary and sufficient conditions for pointer operations. Their second set of results applies their approach to identifying null-dereference errors. Both experiments were performed on OpenSSH, Samba, and the Linux kernel. The results for null-dereference analysis in Figure 7 compare inter- and intra-procedural path-sensitive analysis with just intra-procedural path-sensitive analysis, and shows that the former results in order-of-magnitude reductions in false positive reports compared to the latter. I believe that the authors can do a good job of rewriting the content of this paper for the general CACM audience. Some of the formalism in the current version of the paper may not be accessible to a large audience, and will need to be toned down in the CACM version. However, the basic concepts of inter- and intra-procedural path-sensitive analysis can be conveyed by simple examples such as the queryUser() example discussed in Section 1. This paper is also a good illustration of how far approaches such as constraint systems have progressed for program analysis tools.
Selected September 2008:
Title: Formal Certification of a Compiler Back-end or: Programming a Compiler with a Proof Assistant
Author: Xavier Leroy
Venue: POPL 2006
Published as a CACM Research Highlight (July 2009). Retitled as: Formal Verification of a Realistic Compiler
In 2003, JACM published Tony Hoare's grand challenge paper on The Verifying Compiler. A verifying compiler guarantees correctness of a program before running it. Meeting such a challenge would mean that researchers have to make great progress on both how compilers are written and on verification technology. Leroy's paper presents the first *formally-verified* proof of the correctness of a *complete compilation chain*. "Formally verified" means that the entire proof is verified by the Coq proof assistant. "Complete compilation chain" means that, instead of verifying an isolated transformation or optimisation, Leroy has verified the whole chain of a lightly optimising compiler from a (deliberately modest) high level language called CMinor to PowerPC machine code, by way of four different intermediate languages. The paper shows how to certify that when the compiler generates code, then safety properties proved on the source program will hold for the executable code as well. We believe many researchers and practitioners will be interested in a glimpse of the progress on verifying software we all depend on.
Title: The Next 700 Data Description Languages
Authors: Kathleen Fisher, Yitzhak Mandelbaum, and David Walker
Venue: POPL 2006
Network administrators, financial analysts, scientists, and many others represent data in a myriad of ad hoc formats such as variable-width ASCII records and data-dependent number of fixed-width binary records. Those formats typically don't have parsing, querying, analysis, or transformation tools readily available. Thus motivated, researchers (including Fisher et al) have developed a variety of ad hoc data description languages. What is still missing, though, is a semantics and meta-theory for such languages, or anything to play the role the lambda calculus plays in underpinning programming language design and implementation. That is the gap that this paper fills, by defining a core calculus called the Data Dependency Calculus, and giving it a formal semantics and meta-theory. The paper demonstrates a formal translation from one data description language (IPADS) into DDC, and illustrates how features from other languages might similarly be translated. This is the first time anyone has tried to give a formal treatment to the wild and woolly world of data description languages. We believe many researchers and practitioners will be interested in catching up with what programming language technology can offer for solving common real-world problems related to ad hoc data.
Title: Scalable Synchronous Queues
Authors: William N. Scherer III, Doug Lea, and Michael L. Scott
VENUE: PPoPP 2006
Published as a CACM Research Highlight (May 2009)
Describes "synchronous queues", a fundamental data structure used to implement hand-offs or "rendezvous" between threads. This implementation was immediately integrated into the java.util.concurrent component of Sun's standard Java library, and has thus already had practical impact. The data structure is commonly used with the Java Executor framework and, as is shown by measurements in the paper, may rather spectacularly impact the performance of some applications using it. This is in some sense the canonical data structure that involves blocking or waiting, and for which conventional sequential performance metrics are hence inapplicable. It's a great example of how to design a data structure, for which, in a sense, only concurrency considerations matter. We believe this will expose readers to an area of increasing importance that is very different from the kind of algorithmic problems most of them will be familiar with. We have confidence that the authors will be able to retarget the paper at a much broader audience than that targeted by the current paper.
Title: Java Takes Flight: Time-portable Real-time Programming with Exotasks
Authors: Joshua Auerbach, David F. Bacon, Daniel T. Iercan, Christoph M. Kirsch, V. T. Rajan, Harald Roeck and Rainer Trummer
Venue: LCTES 2007
Java and real-time are terms that aren't often used together. This paper changes that. Real-time systems programming involves low levels of abstraction, making real-time systems difficult to test and requiring them to be re-certified every time the software or hardware environment changes. This paper introduces Exotasks to Java to raise the level of abstraction. Exotasks add deterministic timing, even in the presence of other Java threads and across changes of hardware and software platform. The authors built a quad-rotor model helicopter, the JAviator, to test the concepts of the work. They tested the system with actual flights. When the system fails, the helicopter crashes! Beyond the richness of the technical details, this paper should appeal to CACM readers. There is a real problem, clever language solutions, and an interesting application demonstration complete with photographs. The work is conducted within the broader context of IBM's effort to establish a common Java-based real time programming environment.
Title: Fault-Tolerant Typed Assembly Language
Authors: Frances Perry, Lester Mackey, George A. Reis, Jay Ligattii, David I. August, and David Walker
Venue: PLDI 2007
This paper was one of two best papers from PLDI 2007. It spans the areas of fault-tolerant computing, type systems, and production-strength compilers to provide a unique approach to formalization and detection of Single Event Upset (SEU) faults, motivated by the increasing number of transient hardware faults observed in real systems. Their approach builds on the general strategy of using redundancy for fault tolerance by executing two independent copies of the program in two separate threads. Prior to writing data out to a memory-mapped output device, the results of the two computations are checked for equivalence. (This research focuses on fault detection in processors, and assumes that other techniques such as error-correcting codes are used to achieve fault tolerance in memories.) They introduce a Fault-Tolerant Typed Assembly Language, TALFT, that ensures that all well-typed programs exhibit fail-safe behavior in the presence of transient faults i.e., a corrupt value can never be written to a memory-mapped output device. TALFT can help identify cases when conventional optimizations may not be sound from the viewpoint of fault tolerance. Finally, they implemented their reliability transformations in the Princeton VELOCITY compiler, and provided empirical results for 22 SPECint2000 and MediaBench benchmarks. These results show that the overhead of fault tolerance is about 34% on average instead of the 100% that may have been expected from dual redundancy. I especially like this paper for CACM because it spans a number of different areas, and I believe that the authors can do a good job of rewriting the content of this paper for the general CACM audience. The basic single-event-upset fault model is well motivated by current technology trends and yet simple to understand. Overall, the paper should be an eye-opener to folks who equate compilers with the Dragon book and haven't been keeping up with the latest research in compiler technology such as the fault tolerance application studied in this paper.
Selected January 2008:
Title: Dynamic Multigrained Parallelization on the Cell Broadband Engine
Authors: Filip Blagojevic, Dimitris S. Nikolopoulos, Alexandros Stamatakis, and Christos D. Antonopoulos
Venue: PPoPP 2007
With the advent of multicores, parallel programming is again one of the most important issues in computer science. This paper describes and evaluates a scheduling strategy for parallel programs executing on an IBM cell processor. It exploits coarse grain parallelism whenever there is an adequate supply of tasks. Otherwise, it attempts to exploit fine grain-parallelism. Fine grain parallelism is given lower priority because it tends to incur more overhead than coarse-grain parallelism. The target machine has an interesting, much talked about architecture. It is a heterogeneous machine, a likely characteristic of future multicores. The application is representative of an important class of applications that contain relatively simple parallelism at multiple levels. The paper describes an excellent case study in parallel programming that illustrates some of the immense difficulties that programmers are likely to face in the brave new world of multicore systems.
Note: This paper was recognized by an ad hoc SIGPLAN Committee prior to the creation of the SIGPLAN Research Highlights Committee.
SIGPLAN papers that have also appeared as CACM Research Highlights:
Wake Up and Smell the Coffee: Evaluation Methodology for the 21st Century
Authors: Stephen M. Blackburn, Kathryn S. McKinley, Robin Garner, Chris Hoffmann, Asjad M. Khan, Rotem Bentzur, Amer Diwan, Daniel Feinberg, Daniel Frampton, Samuel Z. Guyer, Martin Hirzel, Antony Hosking, Maria Jump, Han Lee, J. Eliot B. Moss, Aashish Phansalkar
Venue: OOPSLA 2006.
Published as a CACM Research Highlight (August 2008).
Composable Memory Transactions
Authors: Tim Harris, Simon Marlow, Simon Peyton-Jones, Maurice Herlihy
Venue: PPoPP 2005.
Published as a CACM Research Highlight (August 2008).
Exterminator: Automatically Correcting Memory Errors with High Probability
Authors: Gene Novark, Emery D. Berger, Benjamin G. Zorn
Venue: PLDI 2007.
Published as a CACM Research Highlight (December 2008).
Optimistic Parallelism Requires Abstractions
Authors: Milind Kulkarni, Keshav Pingali, Bruce Walter, Ganesh Ramanarayanan, Kavita Bala, L. Paul Chew
Venue: PLDI 2007.
Published as a CACM Research Highlight (September 2009).
Title: x86-TSO: A Rigorous and Usable Programmer's Model for x86 Multiprocessors
Authors: Peter Sewell, Susmit Sarkar, Scott Owens, Francesco Zappa Nardelli, Magnus O. Myreen
Venue: POPL 2009 and TPHOLs 2009.
Published as a CACM Research Highlight (July 2010).
Title: Abstracting Abstract Machines: A Systematic Approach to Higher-Order Program Analysis
Authors: David Van Horn, Matthew Might
Venue: ICFP 2010.
Published as a CACM Research Highlight (September 2011).
Title: Continuity and Robustness of Programs
Authors: Swarat Chaudhuri, Sumit Gulwani, Roberto Lublinerman
Venue: POPL 2010.
Published as a CACM Research Highlight (August 2012).