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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
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      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
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    2. Tim Sweeney‏ @TimSweeneyEpic Feb 1
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      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
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    3. Tim Sweeney‏ @TimSweeneyEpic Feb 1
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      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
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    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
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    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
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      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.

      10:22 AM - 1 Feb 2020
      • 4 Retweets
      • 40 Likes
      • Federico Vaggi Mr. Thee ADK Aidan david doll Prerit Singh Nader Pervez James Baicoianu 𝒜𝓁𝑒𝓎𝓈𝒾𝒾𝒶 ˳✧༚ Zanin
      7 replies 4 retweets 40 likes
        1. Tim Sweeney‏ @TimSweeneyEpic Feb 1
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          Unfortunately, most languages missed this opportunity. The C family including C# and Java are overly imperative and lost variance due to wrongly-scoped mutability. And functional languages have generally chosen type systems lacking subtypes, covariance, and contravariance.

          7 replies 5 retweets 57 likes
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        2. 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
        3. 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
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        2. Wouter‏ @wvo Feb 1
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          Replying to @TimSweeneyEpic

          How is this different from Java etc (since we have no existing language to point to as an example)? And unless you have some impressive optimization in mind, this will end up ~10x slower than the highly specialized code C++/Rust emit, which is a high cost for "proper" subtyping.

          1 reply 0 retweets 3 likes
        3. Wouter‏ @wvo Feb 1
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          Replying to @wvo @TimSweeneyEpic

          And agree with @Jonathan_Blow that specialization based typing is possible without excessive compilation cost or crazy errors (at least I am attempting so in http://strlen.com/lobster/ ). It can be super expressive without needing type classes etc.

          1 reply 0 retweets 0 likes
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        2. c0de517e/AngeloPesce‏ @kenpex Feb 1
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          Replying to @TimSweeneyEpic

          You're thinking in the context of a strict type system though. It's not entirely clear to me that we should bother, i.e. I am not persuaded (or more importantly, have seen strong empirical evidence) of the usefulness of overly strict enforcement of types in real-world code.

          1 reply 0 retweets 0 likes
        3. Layla Mah‏ @MissQuickstep Feb 1
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          Replying to @kenpex @TimSweeneyEpic

          My experience with the value of strict typing is very different from yours!

          1 reply 0 retweets 2 likes
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        2. Joe Groff‏ @jckarter Feb 1
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          Replying to @TimSweeneyEpic

          I think that’s more a property of having a JIT than a GC. C++ and Rust can instantiate wrappers just fine, but they rely on monomorphization to get good performance. C# and Java can base everything on abstractions like IEnumerable because they can devirtualize hot paths on demand

          1 reply 0 retweets 6 likes
        3. Ted van Gaalen‏ @tedvga Feb 2
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          Replying to @jckarter @TimSweeneyEpic

          it isn't a problem with 2020 Smalltalk and its VM, like in Pharo 8.0

          0 replies 0 retweets 0 likes
        4. End of conversation
        1. Ville M. Vainio‏ @vivainio Feb 1
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          Replying to @TimSweeneyEpic

          You need gc for that? As opposed to managing reference to that wrapper?

          0 replies 0 retweets 0 likes
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