Completion time multiple branch prediction for enhancing trace cache performance

The need for multiple branch prediction is inherent to wide instruction fetching. This paper presents a completion time multiple branch predictor called the Tree-based Multiple Branch Predictor (TMP) that builds on previous single branch prediction techniques. It employs a tree structure of branch predictors, or tree-node predictors, and achieves accurate multiple branch prediction by leveraging the high accuracies of the individual branch predictors. A highly-efficient TMP design uses the 2-bit saturating counters for the tree-node predictors. To achieve higher prediction rate, the TMP employs two-level schemes for the tree-node predictors resulting in a three-level TMP design. Placing the TMP at completion time reduces the critical latency in the front-end of the pipeline; the resultant longer update latency does not significantly impact the overall performance. In this paper the TMP is applied to a trace cache design and shown to be very effective in increasing its performance. Results: A realistic-size TMP (72KB) can predict 1, 2, 3, and 4 consecutive blocks with compounded prediction accuracies of 96%, 93%, 87%, and 82%, respectively. The block-based trace cache with this TMP achieves 4.75 IPC for SPECint95 on an idealized machine, which is a 20% performance improvement over the original design. This improved performance is 8% above that of a conventional I-cache design with perfect single branch prediction.