The IBM z15 High Frequency Mainframe Branch Predictor Industrial Product

Abstract

The design of the modern, enterprise-class IBM z15 branch predictor is described. Implemented as a multilevel look-ahead structure, the branch predictor is capable of predicting branch direction and target addresses, augmented with multiple auxiliary direction, target, and power predictors. Predictions are made asynchronously, and later integrated into the processor pipeline. The design is optimized for the unique workloads executed on these enterprise-class systems, including compute intensive and both large instruction and data footprint workloads. This paper highlights the major operations and functions of the IBM z15 branch predictor, including its pipeline, prediction structures and verification methodology. Explanations as to how the design matured to its current state are also provided.