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Zen 5's 2-ahead branch predictor: how a 30 year old idea allows for new tricks

When I recently interviewed Mike Clark, he told me, “…you’ll see the actual foundational lift play out in the future on Zen 6, even though it was really Zen 5 that set the table for tha…

If the blinded truck analogy hasn’t worn too thin yet, imagine the driver keeping a small set of notes of recent turns taken or skipped and hand-drawn scribbles of how roads driven in the last few minutes were arranged and what intersections were passed. Even back then the challenge of scaling out architectural widths of 8 or more was being talked about and a 2-Ahead Branch Predictor was one of the methods that academia put forth in order to continue squeezing more and more performance out of a single core. It is perhaps merciful for Intel and AMD that typical application integer code has a substantial branch density, on the order of one every five to six instructions, which diminishes the motivation to pursue parallelized decoders much wider than that.

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