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Subnormal floating-point numbers are expensive on Intel processors

69 points · 54 comments · zdw

  1. juancn · · focus · HN ↗
    Apparently it only happens on P-cores, recent E-cores have a fast path for subnormals.
    1. rustybolt · · focus · HN ↗
      That seems weird. They did throw extra hardware at it to speed it up for the efficient cores, but not for the performance cores?
      1. [deleted] · · focus · HN ↗

        [deleted]

      2. cwzwarich · · focus · HN ↗
        Probably different teams. Plus you can not underestimate the role that momentum plays in semiconductor engineering teams. If some respected person determined that subnormals are either Hard (tm) or not a Real Problem (tm), it will take a long time to correct this mistaken belief. I believe there have been some recent academic papers on FP implementation from the Intel E-Core team, which is a sign that they are a bit more with the times.
      3. gpderetta · · focus · HN ↗
        larger SIMD ALUs, which also have been around for longer?
      4. Sesse__ · · focus · HN ↗
        They are designed by different teams and have different ways of doing math. You could just as easily say “why are the efficiency cores wasting an extra cycle for each multiplication doing fixups of an uncommon case”.

        Fabian Giesen explains in more detail here: <a href="https:&#x2F;&#x2F;mastodon.gamedev.place&#x2F;@rygorous&#x2F;117277063419144390" rel="nofollow">https:&#x2F;&#x2F;mastodon.gamedev.place&#x2F;@rygorous&#x2F;117277063419144390

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