alt Hacker NewsI guess the generate_random_value function uses the same seed every time, so the expectation is that the branch predictor should be able to memorize it with perfect accuracy.
But the memorization capacity of the branch predictor must be a trade-off, right? I guess this generate_random_value function is impossible to predict using heuristics, so I guess the question is how often we encounter 30k long branch patterns like that.
Which isn’t to say I have evidence to the contrary. I just have no idea how useful this capacity actually is, haha.
Replies
IcePic • today at 3:00 PM
https://chromium.googlesource.com/chromiumos/third_party/gcc... has some looong select/case things with lots of ifs in them, but I don't think they would hit 30k.
30k long patterns are likely rare. However in the real world there is a lot of code with 30k different branches that we use several times and so the same ability memorize/predict 30k branches is useful even though this particular example isn't realistic it still looks good.
Of course we can't generalize this to Intel bad. This pattern seems unrealistic (at least at a glance - but real experts should have real data/statistics on what real code does not just my semi-educated guess), and so perhaps Intel has better prediction algorithms for the real world that miss this example. Not being an expert in the branches real world code takes I can't comment.