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AMD 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

AMD 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.

jeffbee 7mo ago
He isn't trying to determine how well it works. He's trying to determine how large it is.
- Night_Thastus 7mo ago
  Their post gives the impression that clearly AMD's branch prediction is better, because this one number is bigger. "Once more I am disappointed by Intel" While it could very well be true that the AMD branch predictor is straight-up better, the data they provided is insufficient for that conclusion.
  - vlovich123 7mo ago
    You may want to look up who Daniel Lemire is and the work he's done. What he's basically saying is "in the totality of things I've examined where Intel has come up short, this is another data point that is in line with their performance across the board". It's not "this one benchmark proves Intel sucks hurr hurr" - it's saying it's yet another data point supporting the perception that Intel is struggling against the competition.
bee_rider 7mo ago
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.

jeffbee 7mo ago

He isn't trying to determine how well it works. He's trying to determine how large it is.

Night_Thastus 7mo ago
Their post gives the impression that clearly AMD's branch prediction is better, because this one number is bigger. "Once more I am disappointed by Intel" While it could very well be true that the AMD branch predictor is straight-up better, the data they provided is insufficient for that conclusion.
- vlovich123 7mo ago
  You may want to look up who Daniel Lemire is and the work he's done. What he's basically saying is "in the totality of things I've examined where Intel has come up short, this is another data point that is in line with their performance across the board". It's not "this one benchmark proves Intel sucks hurr hurr" - it's saying it's yet another data point supporting the perception that Intel is struggling against the competition.

bee_rider 7mo ago

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.