Speculative Updates of Local and Global Branch History: A Quantitative Analysis

In today's wide-issue processors, even small branch-misprediction rates introduce substantial performance penalties. Worse yet, inadequate branch prediction creates a bottleneck at the fetch stage, restricting other opportunities for improving performance. The choice of how to predict conditional-branch outcomes is the primary lever on prediction accuracy. But the choice of when to update the predictor with branch outcomes is a second powerful lever, and the subject of this paper. In history-based predictors like gshare, many mispredictions result from commit-time update of the history: typical pipelined processors predict branches in the fetch stage, but update the predictor in the commit stage, making the predictor's state temporarily outof-date. As pipelines grow longer--in particular, when branches can spend many cycles in the instruction window waiting to issue--this problem becomes worse. Prior work on this subject has discussed the need for speculative update in a glo...

• Type: Journal
• Year: 2000
• Where: JILP
• Authors: Kevin Skadron, Margaret Martonosi, Douglas W. Clark
• Keywords: Fetch Stage, JILP 2000, Prediction Accuracy, Update