Deoptless: Speculation with Dispatched On-Stack Replacement and Specialized Continuations
Autoři
Flückiger, O.; Ječmen, J.; Krynski, S.; Vitek, J.
Rok
2022
Publikováno
PLDI 2022: Proceedings of the 43rd ACM SIGPLAN International Conference on Programming Language Design and Implementation. New York: Association for Computing Machinery, 2022. p. 749-761. ISBN 978-1-4503-9265-5.
Typ
Stať ve sborníku
Anotace
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.
Promises Are Made To Be Broken: Migrating R to Strict Semantics
Autoři
Goel, A.; Ječmen, J.; Krynski, S.; Flückiger, O.; Vitek, J.
Rok
2021
Publikováno
Proceedings of the ACM on Programming Languages (PACMPL). 2021, 5(OOPSLA), 1-20. ISSN 2475-1421.
Typ
Článek
DOI
Anotace
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.
Sampling optimized code for type feedback
Autoři
Flückiger, O.; Wälchli, A.; Krynski, S.; Vitek, J.
Rok
2020
Publikováno
DSL_Proceedings of the 16th ACM SIGPLAN International Symposium on Dynamic Languages. New York: ACM, 2020. p. 99-111. ISBN 978-1-4503-8175-8.
Typ
Stať ve sborníku
Pracoviště
Anotace
To efficiently execute dynamically typed languages, many language implementations have adopted a two-tier architecture. The first tier aims for low-latency startup times and collects dynamic profiles, such as the dynamic types of variables. 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 polymorphic variable becomes monomorphic, nothing changes. Once the program is running optimized code, there are no means to notice that an opportunity for optimization has been missed. We propose to employ a sampling-based profiler to monitor native code without any instrumentation. The absence of instrumentation means that when the profiler is not active, no overhead is incurred. We present an implementation is in the context of the A just-in-time, optimizing compiler for the R language. Based on the sampled profiles, we are able to detect when the native code produced by A is specialized for stale type feedback and recompile it to more type-specific code. We show that sampling adds an overhead of less than 3 © 2020 ACM.