Graphics Processing Units (GPUs) are the accelerator of choice for performance-critical applications, yet optimizing for performance requires mastery of the complex interactions between its memory architecture and its execution model. Existing static analysis tools for GPU kernels either identify performance bugs without quantifying costs or cannot handle thread-divergent control flow, leading to significant over-approximations. We present the first static relational-cost analysis for GPU warp-level parallelism that can give exact bounds even in the presence of thread divergence. Our analysis is general and flexible, as it is parametric on the resource metric (uncoalesced accesses, bank conflicts) and on the cost relation (=, ≤, ≥). We establish a soundness theorem for our technique, provide mechanized proofs in Rocq and implement our theory in a tool called Pico . In a reproducibility experiment, Pico produced the tightest bounds in every input, outperforming the state-of-the-art tool RaCUDA in 10 kernels (1.7× better), while RaCUDA produced 4 incorrect bounds and crashed on 2 kernels. In an experiment to measure the accuracy of Pico , we studied the impact of threaddivergence in control-flow in a dataset of 226 kernels. We found that at least 75.3% of conditionals and 85.4% of loops can be captured exactly, without introducing approximation.

Go’s unique combination of structural subtyping between generics and types with non-uniform runtime representations presents significant challenges for formalising the language. We introduce WG (Welterweight Go), a core model of Go that captures key features excluded by prior work, including underlying types, type unions and type sets, and proposed new features, such as generic methods. We also develop LWG, a lower-level language that models Go’s runtime mechanisms, notably the distinction between raw struct values and interface values that carry runtime type information (RTTI). We give a type-directed compilation from WG to LWG that demonstrates how the proposed features can be implemented while observing important design and implementation goals for Go: compatibility with separate compilation, and no runtime code generation. Unlike existing approaches based on static monomorphisation, our compilation strategy uses runtime type conversions and adaptor methods to handle the complex interactions between structural subtyping, generics, and Go’s runtime infrastructure.

Session types are widely used as abstractions of asynchronous message passing systems. Refinement for such abstractions is crucial as it allows improvements of a given component without compromising its compatibility with the rest of the system. In the context of session types, the most general notion of refinement is asynchronous session subtyping, which allows message emissions to be anticipated w.r.t. a bounded amount of message consumptions. In this paper we investigate the possibility to anticipate emissions w.r.t. an unbounded amount of consumptions: to this aim we propose to consider fair compliance over asynchronous session types and fair refinement as the relation that preserves it. This allows us to propose a novel variant of session subtyping that leverages the notion of controllability from service contract theory and that is a sound characterisation of fair refinement. In addition, we show that both fair refinement and our novel subtyping are undecidable. We also present a sound algorithm which deals with examples that feature potentially unbounded buffering. Finally, we present an implementation of our algorithm and an empirical evaluation of it on synthetic benchmarks.

GPUs are progressively being integrated into modern society, playing a pivotal role in Artificial Intelligence and High-Performance Computing. Programmers need a deep understanding of the GPU programming model to avoid subtle data-races in their codes. Static verification that is sound and incomplete can guarantee data-race freedom, but the alarms it raises may be spurious and need to be validated. In this paper, we establish a True Positive Theorem for a static data-race detector for GPU programs, i.e., a result that identifies a class of programs for which our technique only raises true alarms. Our work builds on the formalism of memory access protocols , that models the concurrency operations of CUDA programs. The crux of our approach is an approximation analysis that can correctly identify true alarms, and pinpoint the conditions that make an alarm imprecise. Our approximation analysis detects when the reported locations are reachable (control independence, or CI), and when the reported locations are precise (data independence, or DI), as well identify inexact values in an alarm. In addition to a True Positive result for programs that are CI and DI, we establish the root causes of spurious alarms depending on whether CI or DI are present. We apply our theory to introduce FaialAA, the first sound and partially complete data-race detector. We evaluate FaialAA in three experiments. First, in a comparative study with the state-of-the-art tools, we show that FaialAA confirms more DRF programs than others while emitting 1.9 × fewer potential alarms. Importantly, the approximation analysis of FaialAA detects 10 undocumented data-races. Second, in an experiment studying 6 commits of data-race fixes in open source projects OpenMM and Nvidia’s MegaTron, FaialAA confirmed the buggy and fixed versions of 5 commits, while others were only able to confirm 2. Third, we show that 59.5 % of 2,770 programs are CI and DI, quantifying when the approximation analysis of FaialAA is complete. This paper is accompanied by the mechanized proofs of the theoretical results presented therein and a tool (FaialAA) implementing of our theory.

A challenge of writing concurrent message passing programs is ensuring the absence of partial deadlocks, which can cause severe memory leaks in long running systems. Several static analysis techniques have been proposed for automatically detecting partial deadlocks in Go programs. For a large enterprise code base, we found these tools too imprecise to reason about process communication that is parametric, i.e., where the number of channel communication operations or the channel capacities are determined at runtime. We present a novel approach to automatically verify the absence of partial deadlocks in Go program fragments with such parametric process communication. The key idea is to translate Go fragments to a core language that is sufficiently expressive to represent real-world parametric communication patterns and can be encoded into Dafny programs annotated with postconditions enforcing partial deadlock freedom. In situations where a fragment is partial deadlock free only when the concurrency parameters satisfy certain conditions, a suitable precondition can often be inferred. Experimental results on a real-world code base containing 583 program fragments that are beyond the reach of existing techniques have shown that the approach can verify the absence of partial deadlocks in 145 cases. For an additional 228 cases, a nontrivial precondition is inferred that the surrounding code must satisfy to ensure partial deadlock freedom.

Existing models for the analysis of concurrent processes tend to focus on fail-stop failures, where processes are either working or permanently stopped, and their state (working/stopped) is known. In fact, systems are often affected by grey failures: failures that are latent, possibly transient, and may affect the system in subtle ways that later lead to major issues (such as crashes, limited availability, overload). We introduce a model of actor-based systems with grey failures, based on two interlinked layers: an actor model, given as an asynchronous process calculus with discrete time, and a failure model that represents failure patterns to inject in the system. Our failure model captures not only fail-stop node and link failures, but also grey failures (e.g., partial, transient). We give a behavioural equivalence relation based on weak barbed bisimulation to compare systems on the basis of their ability to recover from failures, and on this basis we define some desirable properties of reliable systems. By doing so, we reduce the problem of checking reliability properties of systems to the problem of checking bisimulation.

Abstract Theories and tools based on multiparty session types offer correctness guarantees for concurrent programs that communicate using message-passing. These guarantees usually come at the cost of an intrinsically top-down approach, which requires the communication behaviour of the entire program to be specified as a global type. This paper introduces : an OCaml library that supports the development of correct message-passing programs without having to write any types. The library utilises the meta-programming facilities of OCaml to automatically infer the session types of concurrent programs and verify their compatibility (k-MC [15]). Well-typed programs, written with , do not lead to communication errors and cannot get stuck.

Abstract GPUs offer parallelism as a commodity, but they are difficult to program correctly. Static analyzers that guarantee data-race freedom (DRF) are essential to help programmers establish the correctness of their programs (kernels). However, existing approaches produce too many false alarms and struggle to handle larger programs. To address these limitations we formalize a novel compositional analysis for DRF, based on access memory protocols. These protocols are behavioral types that codify the way threads interact over shared memory. Our work includes fully mechanized proofs of our theoretical results, the first mechanized proofs in the field of DRF analysis for GPU kernels. Our theory is implemented in , a tool that outperforms the state-of-the-art. Notably, it can correctly verify at least $$1.42{\times }$$ 1.42 × more real-world kernels, and it exhibits a linear growth in 4 out of 5 experiments, while others grow exponentially in all 5 experiments.

Abstract Session types are widely used as abstractions of asynchronous message passing systems. Refinement for such abstractions is crucial as it allows improvements of a given component without compromising its compatibility with the rest of the system. In the context of session types, the most general notion of refinement is the asynchronous session subtyping, which allows to anticipate message emissions but only under certain conditions. In particular, asynchronous session subtyping rules out candidates subtypes that occur naturally in communication protocols where, e.g., two parties simultaneously send each other a finite but unspecified amount of messages before removing them from their respective buffers. To address this shortcoming, we study fair compliance over asynchronous session types and fair refinement as the relation that preserves it. This allows us to propose a novel variant of session subtyping that leverages the notion of controllability from service contract theory and that is a sound characterisation of fair refinement. In addition, we show that both fair refinement and our novel subtyping are undecidable. We also present a sound algorithm, and its implementation, which deals with examples that feature potentially unbounded buffering.

The Go programming language offers a wide range of primitives to coordinate lightweight threads, e.g., channels, waitgroups, and mutexes — all of which may cause concurrency bugs. Static checkers that guarantee the absence of bugs are essential to help programmers avoid these costly errors before their code is executed. However existing tools either miss too many bugs or cannot handle large programs. To address these limitations, we propose a static checker for Go programs which relies on performing bounded model checking of their concurrent behaviours. In contrast to previous works, our approach deals with large codebases, supports programs that have statically unknown parameters, and is extensible to additional concurrency primitives. Our work includes a detailed presentation of the extraction algorithm from Go programs to models, an algorithm to automatically check programs with statically unknown parameters, and a large scale evaluation of our approach. The latter shows that our approach outperforms the state-of-the-art.

We describe a design for generics in Go inspired by previous work on Featherweight Java by Igarashi, Pierce, and Wadler. Whereas subtyping in Java is nominal, in Go it is structural, and whereas generics in Java are defined via erasure, in Go we use monomorphisation. Although monomorphisation is widely used, we are one of the first to formalise it. Our design also supports a solution to The Expression Problem.

This paper proposes a sound procedure to verify properties of communicating session automata ( csa ), i.e., communicating automata that include multiparty session types. We introduce a new asynchronous compatibility property for csa , called k -multiparty compatibility ( k - mc ), which is a strict superset of the synchronous multiparty compatibility used in theories and tools based on session types. It is decomposed into two bounded properties: ( i ) a condition called k-safety which guarantees that, within the bound, all sent messages can be received and each automaton can make a move; and ( ii ) a condition called k-exhaustivity which guarantees that all k -reachable send actions can be fired within the bound. We show that k -exhaustivity implies existential boundedness, and soundly and completely characterises systems where each automaton behaves equivalently under bounds greater than or equal to k . We show that checking k - mc is pspace -complete, and demonstrate its scalability empirically over large systems (using partial order reduction).

Go is a popular programming language renowned for its good support for system programming and its channel-based message passing concurrency mechanism. These strengths have made it the language of choice of many platform software such as Docker and Kubernetes. In this paper, we analyse 865 Go projects from GitHub in order to understand how message passing concurrency is used in publicly available code. Our results include the following findings: (1) message passing primitives are used frequently and intensively, (2) concurrency-related features are generally clustered in specific parts of a Go project, (3) most projects use synchronous communication channels over asynchronous ones, and (4) most Go projects use simple concurrent thread topologies, which are however currently unsupported by existing static verification frameworks.

The Go programming language has been heavily adopted in industry as a language that efficiently combines systems programming with concurrency. Go's concurrency primitives, inspired by process calculi such as CCS and CSP, feature channel-based communication and lightweight threads, providing a distinct means of structuring concurrent software. Despite its popularity, the Go programming ecosystem offers little to no support for guaranteeing the correctness of message-passing concurrent programs.

Go is a production-level statically typed programming language whose design features explicit message-passing primitives and lightweight threads, enabling (and encouraging) programmers to develop concurrent systems where components interact through communication more so than by lock-based shared memory concurrency. Go can only detect global deadlocks at runtime, but provides no compile-time protection against all too common communication mismatches or partial deadlocks.

This paper studies safety, progress, and non-zeno properties of Communicating Timed Automata (CTAs), which are timed automata (TA) extended with unbounded communication channels, and presents a procedure to build timed global specifications from systems of CTAs. We define safety and progress properties for CTAs by extending properties studied in communicating finite-state machines to the timed setting. We then study non-zenoness for CTAs; our aim is to prevent scenarios in which the participants have to execute an infinite number of actions in a finite amount of time. We propose sound and decidable conditions for these properties, and demonstrate the practicality of our approach with an implementation and experimental evaluations of our theory.

Graphical choreographies, or global graphs, are general multiparty session specifications featuring expressive constructs such as forking, merging, and joining for representing application-level protocols. Global graphs can be directly translated into modelling notations such as BPMN and UML. This paper presents an algorithm whereby a global graph can be constructed from asynchronous interactions represented by communicating finite-state machines (CFSMs). Our results include: a sound and complete characterisation of a subset of safe CFSMs from which global graphs can be constructed; an algorithm to translate CFSMs to global graphs; a time complexity analysis; and an implementation of our theory, as well as an experimental evaluation.