A high-level operational semantics for hardware weak memory models
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Modern processors deploy a variety of weak memory models, which for efficiency reasons may execute instructions in an order different to that specified by the program text. The consequences of instruction reordering can be complex and subtle, and can impact on ensuring correctness. In this paper we build on extensive work elucidating the semantics of assembler-level languages on hardware architectures with weak memory models (specifically TSO, ARM and POWER) and lift the principles to a straightforward operational semantics which allows reasoning at a higher level of abstraction. To this end we introduce a wide-spectrum language that encompasses operations on abstract data types as well as low-level assembler code, define its operational semantics using a novel approach to allowing reordering of instructions, and derive some refinement laws that can be used to explain behaviours of real processors. In this framework memory models are mostly distinguished via a pair-wise static ordering on instruction types that determines when later instructions may be reordered before earlier instructions. In addition, memory models may use different types of storage systems. For instance, non-multicopy atomic systems allow sibling processes to see updates to different variables in different orders. We encode the semantics in the rewriting engine Maude as a model-checking tool, and develop confidence in our framework by validating our semantics against existing sets of litmus tests -- small assembler programs -- comparing our results with those observed on hardware and in existing semantics. We also use the tool as a prototype to model check implementations of data structures from the literature against their abstract specifications.
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