Orinoco: Ordered Issue and Unordered Commit with Non-Collapsible Queues

Modern out-of-order processors call for more aggressive scheduling techniques such as priority scheduling and out-of-order commit to make use of increasing core resources. Since these approaches prioritize the issue or commit of certain instructions, they face the conundrum of providing the capacity efficiency of scheduling structures while preserving the ideal ordering of instructions. Traditional collapsible queues are too expensive for today's processors, while state-of-the-art queue designs compromise with the pseudo-ordering of instructions, leading to performance degradation as well as other limitations. In this paper, we present Orinoco, a microarchitecture/circuit co-design that supports ordered issue and unordered commit with non-collapsible queues. We decouple the temporal ordering of instructions from their queue positions by introducing an age matrix with the bit count encoding, along with a commit dependency matrix and a memory disambiguation matrix to determine instructions to prioritize issue or commit. We leverage the Processing-in-Memory (PIM) approach and efficiently implement the matrix schedulers as 8T SRAM arrays. Orinoco achieves an average IPC improvement of 14.8% over the baseline in-order commit core with the state-of-the-art scheduler while incurring overhead equivalent to a few kilobytes of SRAM.