Direct instruction wakeup for out-of-order processors

Instruction queues consume a significant amount of power in high-performance processors, primarily due to instruction wakeup logic access to the queue structures. The wakeup logic delay is also a critical timing parameter. This paper proposes a new queue organization using a small number of successor pointers plus a small number of dynamically allocated full successor bit vectors for cases with a larger number of successors. The details of the new organization are described and it is shown to achieve the performance of CAM-based or full dependency matrix organizations using just one pointer per instruction plus eight full bit vectors. Only two full bit vectors are needed when two successor pointers are stored per instruction. Finally, a design and pre-layout of all critical structures in 70 nm technology was performed for the proposed organization as well as for a CAM-based baseline. The new design is shown to use 1/2 to 1/5th of the baseline instruction queue power, depending on queue size. It is also shown to use significantly less power than the full dependency matrix based design.Peer ReviewedPostprint (published version

A Simple Low-Energy Instruction Wakeup Mechanism

Instruction issue consumes a large amount of energy in out of order processors, largely in the wakeup logic. Proposed solutions to the problem require prediction or additional hardware complexity to reduce energy consumption and, in some cases, may have a negative impact on processor performance. This paper proposes a mechanism for instruction wakeup, which uses a multi-block instruction queue design. The blocks are turned off until the mechanism determines which blocks to access on wakeup using a simple successor tracking mechanism. The proposed approach is shown to require as little as 1.5 comparisons per committed instruction for SPEC2000 benchmarks