Your CPU Is Guessing the Future, and Wrong 5% of the Time

The "5%" in the title is the headline that gets people interested in branch prediction. It is also a workload-averaged number that papers over the only interesting part of the topic, which is that the misses live exactly where you can't see them. The textbook claim — pick a percentage in the 90s — is dominated by the predictable code on hot paths. The unpredictable code is where you actually pay. So I want to make a more useful claim. The interesting question about modern branch predictors isn't