Promising 2.0: global optimizations in relaxed memory concurrency

For more than fifteen years, researchers have tried to support global optimizations in a usable semantics for a concurrent programming language, yet this task has been proven to be very difficult because of (1) the infamous “out of thin air” problem, and (2) the subtle interaction between global and thread-local optimizations. In this paper, we present a solution to this problem by redesigning a key component of the promising semantics (PS) of Kang et al. Our updated PS 2.0 model supports all the results known about the original PS model (i.e., thread-local optimizations, hardware mappings, DRF theorems), but additionally enables transformations based on global value-range analysis as well as register promotion (i.e., making accesses to a shared location local if the location is accessed by only one thread). PS 2.0 also resolves a problem with the compilation of relaxed RMWs to ARMv8, which required an unintended extra fence.