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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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    Rich Felker‏ @RichFelker 2 Dec 2017

    Are there positive ints a,b s.t. floor((double)a/b)!=a/b ? (C expressions, 32-bit int)

    2:56 PM - 2 Dec 2017
    4 replies 0 retweets 0 likes
      1. Rich Felker‏ @RichFelker 2 Dec 2017

        Assume current rounding mode is not known.

        0 replies 0 retweets 0 likes
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      1. New conversation
      2. Steve Canon‏ @stephentyrone 2 Dec 2017
        Replying to @RichFelker

        Assuming 32b int and IEEE double, surely not, because only one rounding.

        2 replies 0 retweets 0 likes
      3. Rich Felker‏ @RichFelker 2 Dec 2017
        Replying to @stephentyrone

        There are 2 roundings: inexact / and floor.

        1 reply 0 retweets 0 likes
      4. friend void‏ @volatile_void 2 Dec 2017
        Replying to @RichFelker @stephentyrone

        The rounding taking place in (double)a/b cannot take the result on the other side of (integer truncated) a/b, though, because (integer truncated) a/b is representable as a double and would be a better result candidate than whatever candidate below it for the result of the double/

        1 reply 0 retweets 2 likes
      5. Rich Felker‏ @RichFelker 2 Dec 2017
        Replying to @volatile_void @stephentyrone

        The interesting question wasn't whether it crosses a/b but whether it crosses a/b+1. But I guess it can't do that either.

        1 reply 0 retweets 0 likes
      6. Rich Felker‏ @RichFelker 2 Dec 2017
        Replying to @RichFelker @volatile_void @stephentyrone

        BTW is that still true if a,b can range up to 53 bit? Looks borderline but probably still safe.

        1 reply 0 retweets 0 likes
      7. Steve Canon‏ @stephentyrone 2 Dec 2017
        Replying to @RichFelker @volatile_void

        Yes, as long as a and b convert exactly you’re fine.

        1 reply 0 retweets 0 likes
      8. Rich Felker‏ @RichFelker 2 Dec 2017
        Replying to @stephentyrone @volatile_void

        (2^54 - 2)/(2^54 - 1) is >1 ulp away from 1.0 so that looks ok.

        2 replies 0 retweets 0 likes
      9. Steve Canon‏ @stephentyrone 2 Dec 2017
        Replying to @RichFelker @volatile_void

        2^54 - 1 isn't representable as a double; are you sure you don't mean 2^53 - 1?

        1 reply 0 retweets 0 likes
      10. 2 more replies
      1. New conversation
      2. Josiah Worcester‏ @pikhq 2 Dec 2017
        Replying to @RichFelker

        Surely not: IEEE doubles represent 32-bit ints exactly, and the divide should be exact or have error ~= 2^-20 no matter rounding mode.

        1 reply 0 retweets 4 likes
      3. Koorogi‏ @koorogi 2 Dec 2017
        Replying to @pikhq @RichFelker

        Does the magnitude of the rounding error matter? Even a tiny error can cross an integer boundary.

        1 reply 0 retweets 0 likes
      4. Josiah Worcester‏ @pikhq 2 Dec 2017
        Replying to @koorogi @RichFelker

        I suppose, but I don't think there's any results which will actually do so? (2^32-1)/(2^32-1) has a fractional component large enough that it swamps any errors, and that seems like it's at or near the worst case.

        0 replies 0 retweets 0 likes
      5. End of conversation
      1. New conversation
      2. William D. Jones‏ @cr1901 2 Dec 2017
        Replying to @RichFelker

        z3 can probably solve this for you :D. I'd try it myself, but right now eating dinner :(.

        2 replies 0 retweets 0 likes
      3. Erika‏ @rrika9 2 Dec 2017
        Replying to @cr1901 @RichFelker

        https://rise4fun.com/Z3/8opld  but I might have encoded this wrongly

        1 reply 0 retweets 1 like
      4. William D. Jones‏ @cr1901 2 Dec 2017
        Replying to @rrika9 @RichFelker

        Still much better than I could've done, considering the circumstances (EEATINGDINNER). Tyvm :D!

        0 replies 0 retweets 0 likes
      5. End of conversation

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