Stall Cycle Redistribution in a Transparent Fetch Pipeline

Power and power density are now primary design constraints for modern high performance microprocessors. Up to 70% of the dynamic power consumed can be attributed to the clocking system. A consequence of this trend is that clock gating has emerged as both a necessary and efficient method to significantly reduce dynamic power. Transparent pipelining, a recently proposed fine-grain clock gating technique, has the potential to significantly reduce clock power above and beyond conventional pipestage-level clock gating. Previous studies of transparent pipelining have focused on the circuit and implementation-related issues of this approach, while neglecting the broader microarchitectural implications. This paper aims to quantify the microarchitectural opportunities that are afforded by the use of transparent pipelining in a processor's fetch pipeline. We develop a technique, based on stall cycle redistribution, designed to improve the performance of transparent pipelining on fetch and other high utilization pipelines. We show that stall cycle redistribution can dramatically reduce the clocking overhead of an aggressively pipelined Cell-like microprocessor.