Memory Ordering: A Value-Based Approach Harold W. Cain Mikko H. Lipasti Computer Sciences Dept. Dept. of Elec. and Comp. Engr. Univ. of Wisconsin-Madison Univ. of Wisconsin-Madison
[email protected] [email protected] Abstract queues, etc.) used to find independent operations and cor- rectly execute them out of program order are often con- Conventional out-of-order processors employ a multi- strained by clock cycle time. In order to decrease clock ported, fully-associative load queue to guarantee correct cycle time, the size of these conventional structures must memory reference order both within a single thread of exe- usually decrease, also decreasing IPC. Conversely, IPC cution and across threads in a multiprocessor system. As may be increased by increasing their size, but this also improvements in process technology and pipelining lead to increases their access time and may degrade clock fre- higher clock frequencies, scaling this complex structure to quency. accommodate a larger number of in-flight loads becomes There has been much recent research on mitigating this difficult if not impossible. Furthermore, each access to this negative feedback loop by scaling structures in ways that complex structure consumes excessive amounts of energy. are amenable to high clock frequencies without negatively In this paper, we solve the associative load queue scalabil- affecting IPC. Much of this work has focused on the ity problem by completely eliminating the associative load instruction issue queue, physical register file, and bypass queue. Instead, data dependences and memory consistency paths, but very little has focused on the load queue or store constraints are enforced by simply re-executing load queue [1][18][21].