Anthony Saporito

Two level bulk preload branch prediction

This paper describes the large capacity hierarchical branch predictor in the 5.5 GHz IBM zEnterprise EC12 microprocessor. Performance analyses in a simulation model and on zEC12 hardware demonstrate the benefit of this hierarchy compared to a smaller one level predictor. Novel structures and algorithms for two level branch prediction are presented. Prediction information about multiple branches is bulk transferred from the second level into the first upon detecting a perceived miss in the first level. The second level does not directly make branch predictions. Access to the second level is limited when it is unlikely to be productive. The second level is systematically searched in an order that is likely to provide hits as early as possible. On the workloads analyzed in the simulation model, measurements show a maximum core performance benefit of 13.8%. On the two workloads analyzed on zEC12 hardware 3.4% and 5.3% system performance improvements are achieved.