Memory ordering: a value-based approach

Conventional out-of-order processors employ a multi-ported, fully-associative load queue to guarantee correct memory reference order both within a single thread of execution and across threads in a multiprocessor system. As improvements in process technology and pipelining lead to higher clock frequencies, scaling this complex structure to accommodate a larger number of in-flight loads becomes difficult if not impossible. Furthermore, each access to this complex structure consumes excessive amounts of energy. In this paper, we solve the associative load queue scalability problem by completely eliminating the associative load queue. Instead, data dependences and memory consistency constraints are enforced by simply reexecuting load instructions in program order prior to retirement. Using heuristics to filter the set of loads that must be re-executed, we show that our replay-based mechanism enables a simple, scalable, and energy-efficient FIFO load queue design with no associative lookup functionality, while sacrificing only a negligible amount of performance and cache bandwidth.