Energy Efficient Comparators for Superscalar Datapaths

Modern superscalar datapaths use aggressive execution reordering to exploit instruction-level parallelism. Comparators, either explicit or embedded into content-addressable logic, are used extensively throughout such designs to implement several key out-of-order execution mechanisms and support the memory hierarchy. The traditional comparator designs dissipate energy on a mismatch in any bit position. As mismatches occur with a much higher frequency than matches in many situations, considerable improvements in energy dissipation are to be gained by using comparators that dissipate energy predominantly on a full match and little or no energy on partial or complete mismatches. We make two contributions. First, we introduce a series of dissipate-on-match comparator designs, including designs for comparing long arguments. Second, we show how comparators, used in modern datapaths, can be chosen and organized judiciously based on the microarchitectural-level statistics to minimize the energy dissipation. We use the actual layout data and the realistic bit patterns of the comparands (obtained from the simulated execution of SPEC 2000 benchmarks) to show the energy impact from the use of the new comparator designs. For the same delay, the proposed 8-bit comparators dissipate 70 percent less energy than the traditional designs if used within issue queues and 73 percent less energy if used within load-store queues. The use of the proposed 6-bit comparators within the dependency checking logic is shown to increase the energy dissipation by 65 percent on the average compared to the traditional designs. We also find that the use of a hybrid 32-bit comparator, comprised of three traditional 8-bit blocks and one proposed 8-bit block, is the most energy-efficient solution for the use in the load-store queue, resulting in 19 percent energy reduction compared to the use of four traditional 8-bit blocks used to implement a 32-bit comparator.