We show that the weak memory model introduced by the 2011 C and C++ standards does not permit many common source-tosource program transformations (such as expression linearisation and "roach motel" reorderings) that modern compilers perform and that are deemed to be correct. As such it cannot be used to define the semantics of intermediate languages of compilers, as, for instance, LLVM aimed to. We consider a number of possible local fixes, some strengthening and some weakening the model. We evaluate the proposed fixes by determining which program transformations are valid with respect to each of the patched models. We provide formal Coq proofs of their correctness or counterexamples as appropriate.

The key challenge in defining the concurrency semantics of a programming language is how to enable the most efficient compilation to existing hardware architectures, and yet forbid programs from reading thin-air values, i.e., ones that do not appear in the program. At POPL'17, Kang et al. achieved a major breakthrough by introducing the 'promising' semantics that came with results showing that it was a good candidate solution to the problem. Unfortunately, however, the promising semantics is rather complicated, and due to its complexity it contains some flaws and limitations that are very hard to address. In response, we present an alternative solution to this problem based on event structures. We show that it is indeed a solution by establishing the standard results about the semantics (DRF theorems, implementation and optimization correctness) as well as a formal connection to the semantics of Kang et al. Further, we show that it is easier to adapt, by extending the semantics to cover features (such as SC accesses) that are not supported by Kang et al. and to rule out some dubious behaviors admitted by the promising semantics.

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Abstract. Linearizability of concurrent data structures is usually proved by monolithic simulation arguments relying on the identification of the so-called linearization points. Regrettably, such proofs, whether manual or automatic, are often complicated and scale poorly to advanced non-blocking concurrency patterns, such as helping and optimistic updates.In response, we propose a more modular way of checking linearizability of concurrent queue algorithms that does not involve identifying linearization points. We reduce the task of proving linearizability with respect to the queue specification to establishing four basic properties, each of which can be proved independently by simpler arguments. As a demonstration of our approach, we verify the Herlihy and Wing queue, an algorithm that is challenging to verify by a simulation proof.

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