Verification techniques for C11 programs have advanced significantly in recent years with the development of operational semantics and associated logics for increasingly large fragments of C11. However, these semantics and logics have been developed in a restricted setting to avoid the thin-air-read problem. In this article, we propose an operational semantics that leverages an intra-thread partial order (called semantic dependencies ) induced by a recently developed denotational event-structure-based semantics. We prove that our operational semantics is sound and complete with respect to the denotational semantics. We present an associated logic that generalises a recent Owicki–Gries framework for RC11 RAR (repaired C11) with relaxed and release-acquire accesses. We describe the mechanisation of the logic in the Isabelle/HOL theorem prover, which we use to prove correctness of a number of examples.

Although early electronic computers commonly had persistent core memory that retained its contents with power off, modern computers generally do not. DRAM loses its contents when power is lost. However, DRAM has been difficult to scale to smaller feature sizes and larger capacities, making it costly to build balanced systems with sufficient amounts of directly accessible memory. Commonly proposed replacements, including Intel’s Optane product, are once again persistent. It is however unclear, and probably unlikely, that the fastest levels of the memory hierarchy will be able to adopt such technology. No such non-volatile (NVM) technology has yet taken over, but there remains a strong economic incentive to move hardware in this direction, and it would be disappointing if we continued to be constrained by the current DRAM scaling. Since current computer systems often invest great effort, in the form of software complexity, power, and computation time, to “persist” data from DRAM by rearranging and copying it to persistent storage, like magnetic disks or flash memory, it is natural and important to ask whether we can leverage persistence of part of primary memory to avoid this overhead. Such efforts are complicated by the fact that real systems are likely to remain only partially persistent; some memory components, like processor caches and device registers. may remain volatile. This seminar focused on various aspects of programming for such persistent memory systems, ranging from programming models for reasoning about and formally verifying programs that leverage persistence, to techniques for converting existing multithreaded programs (particularly, lock-free ones) to corresponding programs that also directly persist their state in NVM. We explored relationships between this problem and prior work on concurrent programming models.

Deductive verification techniques for C11 programs have advanced significantly in recent years with the development of operational semantics and associated logics for increasingly large fragments of C11. However, these semantics and logics have been developed in a restricted setting to avoid the thin-air-read problem. In this paper, we propose an operational semantics that leverages an intra-thread partial order (called semantic dependencies) induced by a recently developed denotational event-structure-based semantics. We prove that our operational semantics is sound and complete with respect to the denotational semantics. We present an associated logic that generalises a recent Owicki-Gries framework for RC11 (repaired C11), and demonstrate the use of this logic over several example proofs.

We present a denotational semantics for weak memory concurrency that avoids thin-air reads, provides data-race free programs with sequentially consistent semantics (DRF-SC), and supports a compositional refinement relation for validating optimisations. Our semantics identifies false program dependencies that might be removed by compiler optimisation, and leaves in place just the dependencies necessary to rule out thin-air reads. We show that our dependency calculation can be used to rule out thin-air reads in any axiomatic concurrency model, in particular C++. We present a tool that automatically evaluates litmus tests, show that we can augment C++ to fix the thin-air problem, and we prove that our augmentation is compatible with the previously used compilation mappings over key processor architectures. We argue that our dependency calculation offers a practical route to fixing the longstanding problem of thin-air reads in the C++ specification.