Putting Weak Memory in Order via a Promising Intermediate Representation

Abstract

We investigate the problem of developing an "in-order" shared-memory concurrency model for languages like C and C++, which executes instructions following their program order, and is thus more amenable to reasoning and verification compared to recent complex proposals with out-of-order execution. We demonstrate that it is possible to fully support non-atomic accesses in an in-order model in a way that validates all compiler optimizations that are performed in single-threaded code (including irrelevant load introduction). The key to doing so is to utilize the distinction between a source model (with catch-fire semantics) and an intermediate representation (IR) model (with undefined value for racy reads) and formally establish the soundness of mapping from source to IR. As for relaxed atomic accesses, an in-order model must forbid load-store reordering. We discuss the rather limited performance impact of this fact and present a pragmatic approach to this problem, which, in the long term, requires a new kind of hardware store instructions for implementing relaxed stores. The source and IR semantics proposed in this paper are based on recent versions of the promising semantics, and the correctness proofs of the mappings from the source to the IR and from the IR to Armv8 are mechanized in Coq. This work is the first to formally relate an in-order source model and an out-of-order IR model with the goal of having an in-order source semantics without any performance overhead for non-atomics.