Localizing the Tag Comparisons in the Wakeup Logic to Reduce Energy Consumption of the Issue Queue

There is a high demand to reduce the energy consumption of microprocessors. Among resources in a processor, the issue queue is one of the largest energy consumers, with much of the energy being consumed by the wakeup logic. The wakeup logic comprises the content-addressable memory, where tag comparisons are performed for all entries. Such global tag comparisons consume an appreciable amount of energy. This paper proposes a scheme called segmenting wakeup logic (SegWU), where the wakeup logic is segmented logically. An instruction is dispatched to an available entry in the segment selected based on the partial bits of the source tag values. At the wakeup time, a broadcast destination tag is compared only in the segment that corresponds to the partial bits of the destination tag value. Tag comparisons are not performed in the other segments. This localization of tag comparisons reduces the energy consumption. Our evaluation results for SPEC2017 benchmark programs show that SegWU reduces the number of tag comparisons by 90.3%. This lowers the IQ energy consumption, and the net reduction excluding the secondary effect (activity lowering of the front-end) and overhead is 3.0% in the base core energy consumption with no performance degradation. The overall energy reduction including the secondary effect is 6.9%.