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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. John Regehr‏ @johnregehr Jun 28

      I've often wondered what serious compiler support for optimizing bignum operations would look like...

      6 replies 0 retweets 11 likes
      Show this thread
    2. Steve Canon‏ @stephentyrone Jun 28
      Replying to @johnregehr

      Pretty sure that it all comes down to algebraic transforms that let you discard partial-products that do not contribute to the final result due to cancellation, which humans still seem to be better at in general.

      1 reply 1 retweet 2 likes
    3. Steve Canon‏ @stephentyrone Jun 28
      Replying to @stephentyrone @johnregehr

      (I.e. phone me when a compiler invents Montgomery multiplication.)

      2 replies 1 retweet 5 likes
    4. Steve Canon‏ @stephentyrone Jun 28
      Replying to @stephentyrone @johnregehr

      There's no real reason why a compiler *can't* invent Montgomery reduction, but so far compiler writers have been mostly uninterested in building compilers that could. This is at least partially because we don't have a good accepted idiom for compilers to save tricks for reuse, \

      1 reply 1 retweet 3 likes
    5. Steve Canon‏ @stephentyrone Jun 28
      Replying to @stephentyrone @johnregehr

      and a compiler that reinvents Montgomery reduction every time you run it would be obnoxious.

      1 reply 1 retweet 4 likes
    6. John Regehr‏ @johnregehr Jun 28
      Replying to @stephentyrone

      I happen to know about a solver-based superoptimizer that caches results

      1 reply 0 retweets 5 likes
    7. John Regehr‏ @johnregehr Jun 28
      Replying to @johnregehr @stephentyrone

      but what about super basic stuff? how many compilers have bothered to propagate constants through bignums and that sort of thing? obviously not relevant for crypto and such, but for bignum-by-default languages we'd want all this

      4 replies 0 retweets 5 likes
    8. Steve Canon‏ @stephentyrone Jun 28
      Replying to @johnregehr

      In my bignums-done-right dream, all the usual integer transforms would apply at the LLVM layer, and then we'd map down to kernels in compiler-rt.

      2 replies 0 retweets 1 like
    9. John Regehr‏ @johnregehr Jun 28
      Replying to @stephentyrone

      on one hand, bignum-by-default is clearly the right thing for non-systems languages, but otoh machine ints with always-on trapping seems fine most of the time

      1 reply 0 retweets 0 likes
    10. Steve Canon‏ @stephentyrone Jun 28
      Replying to @johnregehr

      Bignum-by-default has far better ergonomics, and actually enables some higher-level optimizations by virtue of preserving desirable algebraic properties like associativity. But it also asks a lot of the runtime and optimizer.

      3 replies 0 retweets 5 likes
      Rich Felker‏ @RichFelker Jun 28
      Replying to @stephentyrone @johnregehr

      Bignum-by-default has no operations that are fail-safe; without static bounds on value magnitudes, every operation requires allocation.

      10:53 AM - 28 Jun 2018
      3 replies 0 retweets 0 likes
        1. New conversation
        2. John Regehr‏ @johnregehr Jun 28
          Replying to @RichFelker @stephentyrone

          even so it's what I want when I'm not doing systems programming

          1 reply 0 retweets 3 likes
        3. @landley‏ @landley Jun 28
          Replying to @johnregehr @RichFelker @stephentyrone

          You want a scripting language then. That's the kind of tradeoff they make all the time.

          2 replies 0 retweets 0 likes
        4. John Regehr‏ @johnregehr Jun 28
          Replying to @landley @RichFelker @stephentyrone

          no, scripting languages are slow and are for prototyping and glue. I want this is my general-purpose language.

          1 reply 0 retweets 1 like
        5. @landley‏ @landley Jun 28
          Replying to @johnregehr @RichFelker @stephentyrone

          You want to cherry-pick _this_ decision to be "easy to program, handles general cases, but slower", but not all the other similar ones.

          1 reply 0 retweets 0 likes
        6. John Regehr‏ @johnregehr Jun 28
          Replying to @landley @RichFelker @stephentyrone

          no, this isn't an exhaustive list, it's just one thing I want in a fast AOT-compiled language like Swift or Java

          1 reply 0 retweets 1 like
        7. @landley‏ @landley Jun 28
          Replying to @johnregehr @RichFelker @stephentyrone

          Tempted to fling you at the 3-part https://landley.net/notes-2011.html#20-03-2011 … (and earlier https://landley.net/notes-2010.html#06-04-2010 …) but it's too much reading.

          0 replies 0 retweets 0 likes
        8. End of conversation
        1. New conversation
        2. Steve Canon‏ @stephentyrone Jun 28
          Replying to @RichFelker @johnregehr

          If your source values come from fixed-width integers or literals, you always have (pessimistic) bounds, but those are usually good enough.

          1 reply 0 retweets 1 like
        3. Steve Canon‏ @stephentyrone Jun 28
          Replying to @stephentyrone @RichFelker @johnregehr

          i.e. you can implement a no-allocation mode that uses pessimistic bounds and errors at compile time if it can’t prove something satisfactory. This would be usable even in most systems domains, and much safer than what we currently do.

          0 replies 0 retweets 2 likes
        4. End of conversation
        1. New conversation
        2. Ian Henderson‏ @ianh_ Jun 28
          Replying to @RichFelker @stephentyrone @johnregehr

          The situation seems similar to dynamic/fixed-size arrays—you have to either deal with overflow, deal with allocation failure, or figure out static bounds. If your language provides dynamic arrays, why not provide dynamic integers as well?

          1 reply 0 retweets 0 likes
        3. Ian Henderson‏ @ianh_ Jun 28
          Replying to @ianh_ @RichFelker and

          …and whatever optimization infrastructure you come up with for bignums could probably work on arrays too!

          0 replies 0 retweets 1 like
        4. End of conversation

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