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pcwalton's profile
Patrick Walton
Patrick Walton
Patrick Walton
@pcwalton

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Patrick Walton

@pcwalton

Research engineer at Mozilla

San Francisco, CA
pcwalton.github.io
Joined November 2009

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    1. Benjamin Brittain‏ @Brittain_Ben 5 Sep 2019
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      Replying to @RichFelker @andy_kelley @pcwalton

      Yes. You can implement naive linked lists without unsafe.

      1 reply 0 retweets 4 likes
    2. Rich Felker‏ @RichFelker 5 Sep 2019
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      Replying to @Brittain_Ben @andy_kelley @pcwalton

      OK. I meant that if the type system Rust bases its memory-safety on didn't admit *any* way to do linked lists without unsafe, the whole formalism would be a lot less impressive/valuable than I thought it was...

      2 replies 0 retweets 0 likes
    3. Andrew Kelley‏ @andy_kelley 5 Sep 2019
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      Replying to @RichFelker @Brittain_Ben @pcwalton

      I didn't even know it was possible, now I want to learn what a linked list looks like without using unsafe

      2 replies 0 retweets 2 likes
    4. Manish‏ @ManishEarth 5 Sep 2019
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      Replying to @andy_kelley @RichFelker and

      use Rc and Weak Presumably y'all are talking about doubly linked lists, singly linked lists are possible to do efficiently in safe code

      4 replies 0 retweets 6 likes
    5. Joe Groff‏ @jckarter 5 Sep 2019
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      Replying to @ManishEarth @andy_kelley and

      There's also the possibility of using non-pointer references to represent the graph, such as indices into an array or some other pool data structure that holds the nodes

      4 replies 0 retweets 8 likes
    6. Manish‏ @ManishEarth 5 Sep 2019
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      Replying to @jckarter @andy_kelley and

      Oh yeah, that works too. That's almost as efficient as the unsafe version

      2 replies 0 retweets 1 like
    7. Joe Groff‏ @jckarter 5 Sep 2019
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      Replying to @ManishEarth @andy_kelley and

      Yep, and you have the opportunity to use a type that's smaller than a pointer, or one that's stable when serialized to disk or mapped into another process or device

      1 reply 0 retweets 3 likes
    8. Benjamin Brittain‏ @Brittain_Ben 5 Sep 2019
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      Replying to @jckarter @ManishEarth and

      I think calling that a linked list is a bit of a stretch though. It certainly wouldn't satisfy most people arguing about Rust being bad.

      3 replies 0 retweets 0 likes
    9. Joe Groff‏ @jckarter 5 Sep 2019
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      Replying to @Brittain_Ben @ManishEarth and

      IMO, data structures are really about the structure, not the mechanism by which the structure is represented. C has promoted a pointer-centric view of the world, but pointers aren't the one and only way to build graphs, and they have their own tradeoffs

      1 reply 1 retweet 4 likes
    10. Patrick Walton‏ @pcwalton 5 Sep 2019
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      Replying to @jckarter @Brittain_Ben and

      Exactly. Which GPU programming makes very clear :) In OpenGL pre-4.7 or so your GPU model doesn’t even *have* the concept of pointer addresses. They’re completely abstracted away from you, so indices are the only option.

      1 reply 0 retweets 1 like
      Patrick Walton‏ @pcwalton 5 Sep 2019
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      Replying to @pcwalton @jckarter and

      It turns out that doing things with indices is often good practice even with GPU APIs that allow raw pointer address manipulation, simply so that you don’t have to keep CPU and GPU side addresses straight.

      9:12 AM - 5 Sep 2019
      • 2 Likes
      • 🏳️‍⚧️::🦀::😷::Diana says 🍾+🔥=🥓 A🐷AB Joe Groff
      3 replies 0 retweets 2 likes
        1. New conversation
        2. Patrick Walton‏ @pcwalton 5 Sep 2019
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          Replying to @pcwalton @jckarter and

          One interesting data point: In Servo Layout 2020 I suggested to @SimonSapin to try indices, but it quickly proved to be a bad idea because of concurrency concerns (we want parallel layout). We ended up just using *singly* linked trees, which worked surprisingly well!

          1 reply 0 retweets 2 likes
        3. Joe Groff‏ @jckarter 5 Sep 2019
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          Replying to @pcwalton @Brittain_Ben and

          Interesting, what were the concurrency concerns?

          2 replies 0 retweets 0 likes
        4. 2 more replies
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        2. Joe Groff‏ @jckarter 5 Sep 2019
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          Replying to @pcwalton @Brittain_Ben and

          64-bit addresses are also huge and sparse, and nobody even uses all the bits, so going with 32-bit indexes can be a good memory savings too

          1 reply 0 retweets 1 like
        3. Steve Canon‏ @stephentyrone 5 Sep 2019
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          Replying to @jckarter @pcwalton and

          And more often than not you can get away with even smaller indexes!

          1 reply 0 retweets 1 like
        4. 1 more reply
        1. your least favorite construct‏ @XMPPwocky 5 Sep 2019
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          Replying to @pcwalton @jckarter and

          sounds like a textbook use case for static types?

          0 replies 0 retweets 0 likes
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