alt Hacker NewsAMD CPUs have been killing it lately, but this benchmark feels quite artificial.
It's a tiny, trivial example with 1 branch that behaves in a pseudo-random way (random, but fixed seed). I'm not sure that's a really good example of real world branching.
How would the various branch predictors perform when the branch taken varies from 0% likely to 100% likely, in say, 5% increments?
How would they perform when the contents of both paths are very heavy, which involves a lot of pipeline/SE flushing?
How would they perform when many different branches all occur in sequence?
How costly are their branch mispredictions, relative to one another?
Without info like that, this feels a little pointless.
Replies
He isn't trying to determine how well it works. He's trying to determine how large it is.
It is a tiny example, but it measures something. It doesn’t handle the other performance characteristics you mention, but it has the advantage of being a basically pure measurement of the memorization ability of the branch predictors.
The blog post is not very long—not much longer than some of the comments we’ve written here about it. So, I think it is reasonable to expect the reader to be able to hold the whole thing in their head, and understand it, and understand that it is extremely targeted at a specific metric.