Relaxed memory models

Abstract

Memory models define an interface between programs written in some language and their implementation, determining which behaviour the memory (and thus a program) is allowed to have in a given model. A minimal guarantee memory models should provide to the programmer is that well-synchronized, that is, data-race free code has a standard semantics. Traditionally, memory models are defined axiomatically, setting constraints on the order in which memory operations are allowed to occur, and the programming language semantics is implicit as determining some of these constraints. In this work we propose a new approach to formalizing a memory model in which the model itself is part of a weak operational semantics for a (possibly concurrent) programming language. We formalize in this way a model that allows write operations to the store to be buffered. This enables us to derive the ordering constraints from the weak semantics of programs, and to prove, at the programming language level, that the weak semantics implements the usual interleaving semantics for data-race free programs, hence in particular that it implements the usual semantics for sequential code.

An ACL2 Mechanization of an Axiomatic Framework for Weak Memory

“…In spite of the fact that we do not generally have sequential consistency, most weaker memory models do uphold a set of guarantees which, though they are not as strong as sequential consistency, do prohibit certain behaviors. These guarantees vary greatly from model to model [3,4,7,9,10], and the variety and abundance of these models suggests the need for a more generic framework for weak memory. Such a framework ought to be both general enough to capture the semantics of all modern architectures, and strong enough to enforce meaningful constraints that are universally upheld.…”