Clarifying and compiling C/C++ concurrency: from C++11 to POWER

  • Authors:

  • Mark Batty;Kayvan Memarian;Scott Owens;Susmit Sarkar;Peter Sewell

  • Affiliations:

  • University of Cambridge, Cambridge, United Kingdom;University of Cambridge & INRIA, Cambridge, United Kingdom;University of Cambridge, Cambridge, United Kingdom;University of Cambridge, Cambridge, United Kingdom;University of Cambridge, Cambridge, United Kingdom

  • Venue:
  • POPL '12 Proceedings of the 39th annual ACM SIGPLAN-SIGACT symposium on Principles of programming languages
  • Year:
  • 2012

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Abstract

The upcoming C and C++ revised standards add concurrency to the languages, for the first time, in the form of a subtle *relaxed memory model* (the *C++11 model*). This aims to permit compiler optimisation and to accommodate the differing relaxed-memory behaviours of mainstream multiprocessors, combining simple semantics for most code with high-performance *low-level atomics* for concurrency libraries. In this paper, we first establish two simpler but provably equivalent models for C++11, one for the full language and another for the subset without consume operations. Subsetting further to the fragment without low-level atomics, we identify a subtlety arising from atomic initialisation and prove that, under an additional condition, the model is equivalent to sequential consistency for race-free programs. We then prove our main result, the correctness of two proposed compilation schemes for the C++11 load and store concurrency primitives to Power assembly, having noted that an earlier proposal was flawed. (The main ideas apply also to ARM, which has a similar relaxed memory architecture.) This should inform the ongoing development of production compilers for C++11 and C1x, clarifies what properties of the machine architecture are required, and builds confidence in the C++11 and Power semantics.