A graded Monad for deadlock-free concurrency (functional pearl)

We present a new type-oriented framework for writing shared memory multithreaded programs that the Haskell type system guarantees are deadlock-free. The implementation wraps all concurrent computation inside a graded monad and assumes a total order is defined between locks. The grades within the type of such a computation specify which locks it acquires and releases. This information is drawn from an algebra that ensures that types can, in principle, be inferred in polynomial time.Trinity College, Cambridg

Brief Announcement: Optimal Bit-Reversal Using Vector Permutations

Accepted versio

Refactoring traces to identify concurrency improvements

It is often difficult to analyse why a program executes more slowly than intended. This is particularly true for concurrent programs. We describe and evaluate a system, Rehype, which takes Java programs, performs low-overhead tracing of method calls, analyses the resulting trace-logs to detect inefficient uses of concurrency constructs, and suggests source-code-oriented improvements. Rehype deals with task-based concurrency, specifically a future-based model of tasks. Implementing the suggested improvements on an industrial API server more than doubled request-processing throughput.The first author was funded by the Engineering and Physical Sciences Research Council (EPSRC), the Cambridge Trusts, and the University of Cambridge Department of Computer Science and Technology

Source code patches from dynamic analysis

Dynamic analysis can identify improvements to programs that cannot feasibly be identified by static analysis; concurrency improvements are a motivating example. However, mapping these dynamic-analysis-based improvements back to patch-like source-code changes is non-trivial. We describe a system, Scopda, for generating source-code patches for improvements identified by execution-trace-based dynamic analysis. Scopda uses a graph-based static program representation (abstract program graph, APG), containing inter-procedural control flow and local data flow information, to analyse and transform static source-code. We demonstrate Scopda's ability to generate sensible source code patches for Java programs, though it is fundamentally language agnostic.The first author was funded by the Engineering and Physical Sciences Research Council (EPSRC), the Cambridge Trusts, and the University of Cambridge Department of Computer Science and Technology

Data-flow analyses as effects and graded monads

In static analysis, two frameworks have been studied extensively: monotone data-flow analysis and type-and-effect systems. Whilst both are seen as general analysis frameworks, their relationship has remained unclear. Here we show that monotone data-flow analyses can be encoded as effect systems in a uniform way, via algebras of transfer functions. This helps to answer questions about the most appropriate structure for general effect algebras, especially with regards capturing control-flow precisely. Via the perspective of capturing data-flow analyses, we show the recent suggestion of using effect quantales is not general enough as it excludes non-distributive analyses e.g., constant propagation. By rephrasing the McCarthy transformation, we then model monotone data-flow effects via graded monads. This provides a model of data-flow analyses that can be used to reason about analysis correctness at the semantic level, and to embed data-flow analyses into type systems.Trinity College, Cambridge (Internal Graduate Scholarship) EPSRC grant EP/T013516/

Coeffects: A calculus of context-dependent computation

The notion of context in functional languages no longer refers just to variables in scope. Context can capture additional properties of variables (usage patterns in linear logics; caching requirements in dataflow languages) as well as additional resources or properties of the execution environment (rebindable resources; platform version in a cross-platform application). The recently introduced notion of coeffects captures the latter, whole-context properties, but it failed to capture fine-grained per-variable properties. We remedy this by developing a generalized coeffect system with annotations indexed by a coeffect shape. By instantiating a concrete shape, our system captures previously studied flat (whole-context) coeffects, but also structural (per-variable) coeffects, making coeffect analyses more useful. We show that the structural system enjoys desirable syntactic properties and we give a categorical semantics using extended notions of indexed comonad. The examples presented in this paper are based on analysis of established language features (liveness, linear logics, dataflow, dynamic scoping) and we argue that such context-aware properties will also be useful for future development of languages for increasingly heterogeneous and distributed platforms

Effect systems revisited—control-flow algebra and semantics

Effect systems were originally conceived as an inference-based program analysis to capture program behaviour—as a set of (representations of) effects. Two orthogonal developments have since happened. First, motivated by static analysis, effects were generalised to values in an algebra, to better model control flow (e.g. for may/must analyses and concurrency). Second, motivated by semantic questions, the syntactic notion of set- (or semilattice-) based effect system was linked to the semantic notion of monads and more recently to graded monads which give a more precise semantic account of effects. We give a lightweight tutorial explanation of the concepts involved in these two threads and then unify them via the notion of an effect-directed semantics for a control-flow algebra of effects. For the case of effectful programming with sequencing, alternation and parallelism—illustrated with music—we identify a form of graded joinads as the appropriate structure for unifying effect analysis and semantics

An efficient and scalable platform for java source code analysis using overlaid graph representations

© 2013 IEEE. Although source code programs are commonly written as textual information, they enclose syntactic and semantic information that is usually represented as graphs. This information is used for many different purposes, such as static program analysis, advanced code search, coding guideline checking, software metrics computation, and extraction of semantic and syntactic information to create predictive models. Most of the existing systems that provide these kinds of services are designed ad hoc for the particular purpose they are aimed at. For this reason, we created ProgQuery, a platform to allow users to write their own Java program analyses in a declarative fashion, using graph representations. We modify the Java compiler to compute seven syntactic and semantic representations, and store them in a Neo4j graph database. Such representations are overlaid, meaning that syntactic and semantic nodes of the different graphs are interconnected to allow combining different kinds of information in the queries/analyses. We evaluate ProgQuery and compare it to the related systems. Our platform outperforms the other systems in analysis time, and scales better to program sizes and analysis complexity. Moreover, the queries coded show that ProgQuery is more expressive than the other approaches. The additional information stored by ProgQuery increases the database size and associated insertion time, but these increases are significantly lower than the query/analysis performance gains obtained.Spanish Department of Science, Innovation and Universities under Project RTI2018-099235-B-I00

REPAIR: Hard-error recovery via re-execution

Processor reliability at upcoming technology nodes presents significant challenges to designers from increased manufacturing variability, parametric variation and transistor wear-out leading to permanent faults. We present a design to tolerate this impact at the microarchitectural level—a chip with n cores together with one or more shared instruction re-execution units (IRUs). Instructions using a faulty component are identified and re-executed on an IRU. This design incurs no slowdown in the absence of errors and allows continued operation of all n cores after multiple hard errors on one or all cores in the structures protected by our scheme. Experiments show that a single-core chip experiences only a 23% slowdown with 1 error, rising to 43% in the presence of 5 errors. In a 4-core scenario with 4 errors on every core and a shared IRU, REPAIR enables performance of 0.68× of a fully functioning system.This work was supported by the Engineering and Physical Sciences Research Council (EPSRC) through grants EP/K026399/1 and EP/J016284/1. Experiments used the Darwin Supercomputer of the University of Cambridge High Performance Computing Service (http://www.hpc.cam.ac.uk/) funded by the Higher Education Funding Council for England and the Science and Technology Facilities Council.This is the author accepted manuscript. The final version is available from IEEE via http://dx.doi.org/10.1109/DFT.2015.731513