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TimSweeneyEpic's profile
Tim Sweeney
Tim Sweeney
Tim Sweeney
@TimSweeneyEpic

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Tim Sweeney

@TimSweeneyEpic

Epic Games founder & CEO

epicgames.com
Joined August 2013

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    1. Tim Sweeney‏ @TimSweeneyEpic Feb 1
      • Report Tweet
      • Report NetzDG Violation

      Programming languages without garbage collection send us down a long path of design decisions that lead to slow compile times and fragile runtime performance cliffs.

      30 replies 41 retweets 356 likes
      Show this thread
    2. Tim Sweeney‏ @TimSweeneyEpic Feb 1
      • Report Tweet
      • Report NetzDG Violation

      First is the abandoning of covariance and contravariance, the property which guarantees sensible subtyping: that bytes are also integers, and integers are also objects, extending systematically to container types and functions.

      3 replies 3 retweets 44 likes
      Show this thread
    3. Tim Sweeney‏ @TimSweeneyEpic Feb 1
      • Report Tweet
      • Report NetzDG Violation

      Without garbage collection, offering an array of bytes where an array of integers is required requires stack-allocating an array of integers and converting each byte to an integer every time a subtype is used in place of an actual type. This is so absurd that it’s not done.

      6 replies 4 retweets 38 likes
      Show this thread
    4. Tim Sweeney‏ @TimSweeneyEpic Feb 1
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      So now when we want to recover performance, we need to write all containers and their operations using an increasingly elaborate set of templates or generic functions, which the compiler must specialize for each type at significant cost. This is what C++ and Rust do.

      4 replies 5 retweets 40 likes
      Show this thread
    5. Tim Sweeney‏ @TimSweeneyEpic Feb 1
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      Or we can create a very clunky wrapper like array_of_anything that is used wherever generic types are required, which manually casts and converts values among types dynamically each time it’s accessed. Java generics did this and they were awful.

      3 replies 4 retweets 28 likes
      Show this thread
    6. Tim Sweeney‏ @TimSweeneyEpic Feb 1
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      But if we have garbage collection, we can store our large data structures once with whatever type is required, then dynamically create wrappers that reinterpret it as any subtype that’s required. We pay the cost of GC and indirect control flow for accessors but that’s all.

      7 replies 4 retweets 40 likes
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    7. Arseny Kapoulkine‏ @zeuxcg Feb 1
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      Replying to @TimSweeneyEpic

      If you are happy with the cost of the indirect call, what stops you from creating this wrapper in a statically compiled language without GC on stack? I don't think these are related. The big issue *is* the indirect call, which can be prohibitively expensive.

      1 reply 0 retweets 2 likes
    8. Tim Sweeney‏ @TimSweeneyEpic Feb 1
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      Replying to @zeuxcg

      If a subtyping relationship is implemented by a conversion like vector<t>(const vector<u>&) that works by holding a reference to its input, then the dangling reference issues will be explosive. This led C++ down the string_view path to distinguish epemeral things.

      2 replies 0 retweets 3 likes
    9. Arseny Kapoulkine‏ @zeuxcg Feb 1
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      Replying to @TimSweeneyEpic

      It seems orthogonal to me. If you want a typed view into the original vector, you need the lifetime to match - but it doesn’t seem too different from passing a reference to a vector to some other system without any type conversions?

      2 replies 0 retweets 3 likes
      Tim Sweeney‏ @TimSweeneyEpic Feb 1
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      Replying to @zeuxcg

      What all of these subtyping-through-zero-copy-sharing-through-manual-memory-management solutions have in common is that, in a large program, the combinatorial complexity of all of these lifetime relationships is impossible for a human to comprehend. Thus libraries don't do it.

      11:13 AM - 1 Feb 2020
      • 6 Likes
      • Victor Ilyushchenko Oleksii Skidan RICHLAND GAMES Shinobi4life levitys
      2 replies 0 retweets 6 likes
        1. New conversation
        2. Jonas Bötel‏ @CodeLumpN Feb 1
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          Replying to @TimSweeneyEpic @zeuxcg

          I agree. But you are suggesting to have the computer take care of the combinatorial complexity instead. That still doesn't solve the fundamental problem that it's still impossible for a human to comprehend.

          1 reply 0 retweets 0 likes
        3. Jonas Bötel‏ @CodeLumpN Feb 1
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          Replying to @CodeLumpN @TimSweeneyEpic @zeuxcg

          Resulting in programs that are technically correct, but run out of memory. Because the actual problem the user had was not memory management but ownership. Memory management is a symptom.

          1 reply 0 retweets 0 likes
        4. 12 more replies
        1. Sebastian Sylvan‏ @ssylvan Feb 1
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          Replying to @TimSweeneyEpic @zeuxcg

          Or maybe they don't do it because it's just not that common to want something like that? 99.9% of my code is super boring and doesn't need covariant arrays and what not so the complexity really isn't warranted.

          0 replies 0 retweets 1 like
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