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lr lhe Two-Level Adaptive branch predictor was conceived at Michigan during October, 1990. At the time, we and Mike Butler, another Michigan Ph.D. student in the HE’S research group, were collaborating extensively with Mike Shebanow, Mitch Alsup, and Hunter Scales, all of Motorola, on a paper showing that Instruction Level Parallelism was greater than two [l]. The collaboration was initiated by Mike Shebanow, a designer of Motorola’s MC88120. Shebanow was one of the original inventors of the HI’S execution model, which attempted to obtain performance by wide-issue instruction supply and multiple deep pipelines with out-of-order execution to prevent blocking. He had shown as early as 1984 that more than l/3 of the potential performance of an HI’S microengine was lost due to branch prediction misses, and had proposed [2] his Autocorrelation Predictor as a way to improve on the saturating two-bit up-down counter [3], which was the most accurate predictor at that time. As part of that collaboration, Tse-Yu Yeh and Mike Butler worked with Shebanow at Motorola the previous summer, and Yale Patt visited Motorola regularly. Our studies that summer, based on the HI’S paradigm, confirmed that the amount of work that would be thrown away due to a branch misprediction was prohibitively far too large. Thus, anything less than a very aggressive dynamic branch predictor was unacceptable. The outgrowth of that summer resulted in the Two-Level Adaptive Branch Predictor. It was first published in Micro-24, in November 1991 [4], followed by the more comprehensive study in ISCA1992 [5]. Tse-Yu Yeh presented the concept at the University of Michigan Industrial Affiliates meeting (IPoCSE) in Ann Arbor, in April, 1991, with representatives of Intel in attendance. At the time, Intel was already strongly considering a wide-issue, deeply pipelined implementation of the x86 architecture, and knew that the two-bit saturating counter mechanism would not provide sufficient prediction accuracy. Their reaction to the Two-level predictor was one of excitement. They subsequently adapted the model to their needs in what came to be the Pentium Pro microprocessor. The Two-Level predictor has continued to evolve since its beginnings in 1990, by its originators at Michigan and by other researchers at many major university and industrial research centers. Pan et. al. [6] introduced the GAS predictor, which took advantage of correlation among branches in the same equivalence class. McFarling [7] modified the use of the history register for indexing into the Pattern History Tables, reducing negative interference. He called his branch predictor gshare. Nair [8] suggested the History Register keep track of the history of the path of previous branches, rather than the history of their directions. Chang [9] augmented the set of Pattern History Tables of two-bit counters with a table of target addresses to handle indirect branches. Several authors have suggested combining compile-time information with the dynamic predictor. Chang [lo] suggested classifying branches at compile time so that the dynamic predictor would only be used on non-unidirectional branches, reducing interference. Sechrest [ll] investigated the role of adaptivity in the PAg Two-Level predictor. Young [12] proposed using profiling and code restructuring to allow static prediction while achieving prediction accuracies approaching that of a dynamic Two-Level predictor. Recently, Evers [13] has begun to study exactly how many of the branches in the History Register really contribute to predictions, and which simply get in the way.

Retrospective: alternative implementations of two-level adaptive training branch prediction