Reducing datapath energy through the isolation of short-lived operands

Abstract

We present a technique for reducing the power dissipation in the course of writebacks and commitments in a datapath that uses a dedicated architectural register file (ARF) to hold committed values. Our mechanism capitalizes on the observation that most of the produced register values are short-lived, meaning that the destination registers targeted by these values are renamed by the time the results are written back. Our technique avoids unnecessary writebacks into the result repository (a slot within the reorder buffer or a physical register) as well as writes into the ARF by caching (and isolating) short-lived operands within a small dedicated register file. Operands are cached in this manner till they can be safely discarded without jeopardizing the recovery from possible branch mispredictions or reconstruction of the precise state in case of interrupts or exceptions. The power/energy savings are validated using SPICE measurements of actual layouts in a 0.18 micron CMOS process. The energy reduction in the ROB and the ARF is in the range of 20-25% and this is achieved with no increase in the cycle time, little additional complexity and no IPC drop.