Non-speculative load reordering in total store ordering

Abstract

Load reordering is important for performance. It allows a core to continue performing accesses to the memory system even when there are older, in-program-order, unperformed accesses (for example, due to long latency misses). The only known solution to allow such reordering in a strong consistency model such as total store ordering (TSO) has been to reorder speculatively and squash-and-re-execute if caught. We show, for the first time, that we can do the load reordering non-speculatively and leave it to the coherence protocol to handle conflicts. We can do this efficiently (without perceptible hardware or performance cost) and without deadlocks or livelocks. The important new result is that we can now irrevocably bind speculative loads. Our solution allows us to commit reordered loads out of order without having to wait (for the loads to become non-speculative) or checkpoint committed state (and rollback if needed), just to ensure correctness in the rare case of another core seeing the reordering.