Safe optimisations for shared-memory concurrent programs

  • Authors:

  • Jaroslav Ševčík

  • Affiliations:

  • University of Cambridge, The MathWorks, Cambridge, United Kingdom

  • Venue:
  • Proceedings of the 32nd ACM SIGPLAN conference on Programming language design and implementation
  • Year:
  • 2011

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Abstract

Current proposals for concurrent shared-memory languages, including C++ and C, provide sequential consistency only for programs without data races (the DRF guarantee). While the implications of such a contract for hardware optimisations are relatively well-understood, the correctness of compiler optimisations under the DRF guarantee is less clear, and experience with Java shows that this area is error-prone. In this paper we give a rigorous study of optimisations that involve both reordering and elimination of memory reads and writes, covering many practically important optimisations. We first define powerful classes of transformations semantically, in a language-independent trace semantics. We prove that any composition of these transformations is sound with respect to the DRF guarantee, and moreover that they provide basic security guarantees (no thin-air reads) even for programs with data races. To give a concrete example, we apply our semantic results to a simple imperative language and prove that several syntactic transformations are safe for that language. We also discuss some surprising limitations of the DRF guarantee.