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RichFelker's profile
Rich Felker
Rich Felker
Rich Felker
@RichFelker

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Rich Felker

@RichFelker

Yeah, I do @musllibc, FOSS & infosec stuff. But now is not the time for a mostly-/only-tech Twitter feed.

musl-libc.org
Joined March 2014

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    1. Fiora Aeternal Suffering  🎃‏ @FioraAeterna 23 Oct 2017
      Replying to @FioraAeterna @hikari_no_yume and

      This GPU also stores all values as floats. Half-precision lets it use the f16 dot product unit, among other things.

      2 replies 0 retweets 3 likes
    2. Fiora Aeternal Suffering  🎃‏ @FioraAeterna 23 Oct 2017
      Replying to @FioraAeterna @hikari_no_yume and

      Suppose: half a = c * d; half b = e; So: max(a, b) -> (f32)dot.f16.max(c, d, e, 1) a -> mul.f32((f32)c, (f32)d)

      1 reply 1 retweet 4 likes
    3. Fiora Aeternal Suffering  🎃‏ @FioraAeterna 23 Oct 2017
      Replying to @FioraAeterna @hikari_no_yume and

      now when max(a, b) returns "a", it may not equal the other computation of "a"

      1 reply 1 retweet 2 likes
    4. Fiora Aeternal Suffering  🎃‏ @FioraAeterna 23 Oct 2017
      Replying to @FioraAeterna @hikari_no_yume and

      specific version of generalized problem: different computations of same value may be done different ways, e.g at higher precision than asked

      2 replies 0 retweets 1 like
    5. Det är blott stjärnorna som vet  ⭐️ other Andrea‏ @hikari_no_yume 23 Oct 2017
      Replying to @FioraAeterna @RichFelker @pikhq

      x86 has this problem when x87 and SSE are mixed

      1 reply 0 retweets 1 like
    6. Fiora Aeternal Suffering  🎃‏ @FioraAeterna 23 Oct 2017
      Replying to @hikari_no_yume @RichFelker @pikhq

      yeah, it's basically the same problem as the one that resulted in that fabulous infinite loop in PHP

      1 reply 0 retweets 2 likes
    7. Det är blott stjärnorna som vet  ⭐️ other Andrea‏ @hikari_no_yume 23 Oct 2017
      Replying to @FioraAeterna @RichFelker @pikhq

      oh man I never heard about that until now, this is amazing: http://www.exploringbinary.com/php-hangs-on-numeric-value-2-2250738585072011e-308/ …

      2 replies 1 retweet 7 likes
    8. Rich Felker‏ @RichFelker 23 Oct 2017
      Replying to @hikari_no_yume @FioraAeterna @pikhq

      This is why it's so important for compilers to comply strictly to spec & not do foolish transformations that change results.

      1 reply 0 retweets 0 likes
    9. Fiora Aeternal Suffering  🎃‏ @FioraAeterna 23 Oct 2017
      Replying to @RichFelker @hikari_no_yume @pikhq

      this one's actually worse though: the compiler didn't even do any transformations

      1 reply 0 retweets 1 like
    10. Fiora Aeternal Suffering  🎃‏ @FioraAeterna 23 Oct 2017
      Replying to @FioraAeterna @RichFelker and

      rather, x87 keeps extra internal precision between ops, and you *can't turn it off*

      5 replies 0 retweets 2 likes
      Rich Felker‏ @RichFelker 23 Oct 2017
      Replying to @FioraAeterna @hikari_no_yume @pikhq

      There's a spec on this in C around FLT_EVAL_METHOD and it's 100% deterministic if implemented correctly. GCC does it wrong in -std=gnuXX.

      10:07 AM - 23 Oct 2017
      • 2 Likes
      • Steve Canon 🤓
      1 reply 0 retweets 2 likes
        1. New conversation
        2. Fiora Aeternal Suffering  🎃‏ @FioraAeterna 23 Oct 2017
          Replying to @RichFelker @hikari_no_yume @pikhq

          yes. what i mean is that doing the naive compilation will not match the spec; you have to do extra steps to match it

          1 reply 0 retweets 0 likes
        3. Josiah Worcester‏ @pikhq 23 Oct 2017
          Replying to @FioraAeterna @RichFelker @hikari_no_yume

          Erm, if I understand it right you just implement double+double=double_t, flush those to stack, and only round when storing to double?

          1 reply 0 retweets 0 likes
        4. Rich Felker‏ @RichFelker 23 Oct 2017
          Replying to @pikhq @FioraAeterna @hikari_no_yume

          Conforming compiler has to spill doubles as double_t (long double for x86), convert to double only via explicit cast or assignment.

          3 replies 0 retweets 1 like
        5. Fiora Aeternal Suffering  🎃‏ @FioraAeterna 23 Oct 2017
          Replying to @RichFelker @pikhq @hikari_no_yume

          Eh? Opposite problem; it has to round to double after each op.

          2 replies 0 retweets 0 likes
        6. Fiora Aeternal Suffering  🎃‏ @FioraAeterna 23 Oct 2017
          Replying to @FioraAeterna @RichFelker and

          The bug can occur even if you never spill once.

          1 reply 0 retweets 0 likes
        7. Fiora Aeternal Suffering  🎃‏ @FioraAeterna 23 Oct 2017
          Replying to @FioraAeterna @RichFelker and

          ^^^^^^pic.twitter.com/iDaMG63KJ2

          2 replies 0 retweets 0 likes
        8. Rich Felker‏ @RichFelker 23 Oct 2017
          Replying to @FioraAeterna @pikhq @hikari_no_yume

          -float-store is a buggy workaround for old gcc behavior that made stuff deterministic but worse/wrong.

          0 replies 0 retweets 1 like
        9. End of conversation

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