The C11 and C++11 Concurrency Model

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of memory orders for fine-grained control over the ordering of memory accesses. The
C/C++11 memory model allows a superset of the relaxed behaviour allowed by its target
architectures. Bychoosingstrongermemoryorders, onecanforbidthisrelaxedbehaviour.
1.1 Focus of this thesis
The C and C++ memory models are defined by the International Standards Organisation
(ISO) in two lengthy standard documents [30, 8]. Prior to my work, there were
drafts describing the C/C++11 memory model, but those drafts, despite careful crafting
by experts, were not known to describe a usable language memory model. The prose
specifications were untestable, and the model was not well understood. It was not formally
established whether the design was implementable, programmable, concise, or even
internally consistent, nor had the central design tenets, laid out early in the design process
[38, 35] and reiterated by Boehm and Adve [37], been established.
In my work, I have sought to understand the C/C++11 memory model in formal
terms, to fix parts that were broken, to prove that the design is usable, and, where fixing
problems was not yet possible, to highlight outstanding issues. In this thesis I assess the
C/C++11memorymodeldesign, presentingaclear andcompletepictureofamainstream
programming-language relaxed memory model. This effort both improved the C/C++11
definition and can inform the design of future programming-language memory models.
1.2 Contributions
Chapter 3 describes a formal version of the C/C++11 memory model that was developed
in close contact with the standardisation committee. Work on this model fed corrections
back to the language specification, and as a consequence, it is very closely in tune with
the intention of the committee, and the ratified prose specification. The formal model is
written in the specification language Lem [85, 90], and is readable, precise and executable
(thefulldefinitionsareprovidedinAppendixC).Thefeaturesofthemodelareintroduced
in stages through a series of cut-down models that apply to programs that do not use all
of the language features. This chapter also presents a simplified model omits a redundant
part of the specification. This work was developed in discussion with Scott Owens, Susmit
Sarkar, and Peter Sewell, but I played the leading role. It was published in POPL in
2011 [28].
Chapter 4 describes Cppmem, a tool that takes very small programs and calculates
all of the behaviours allowed by the memory model. Cppmem is joint work with Scott
Owens, Jean Pichon, Susmit Sarkar, and Peter Sewell. I contributed to the initial design
of the tool, and the tool uses an automatic OCaml translation of my formal memory
model produced by Lem. Cppmem is invaluable for exploring the behaviour of the mem-