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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 14 Jul 2018
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      After 3 months of coding weekends, my attempt to create a relocating nonblocking garbage collector in C++ has failed. It works in theory, but in practice the read barrier is pervasive and costly, and the threading invariants are incredibly tricky to maintain.

      11 replies 18 retweets 127 likes
      Show this thread
    2. Federico Vaggi‏ @F_Vaggi 14 Jul 2018
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      Replying to @TimSweeneyEpic

      There are a few paid versions for Java: is there something about Java semantics that makes it easier to do there ?

      1 reply 0 retweets 0 likes
    3. RJ‏ @shajorjita 14 Jul 2018
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      Replying to @F_Vaggi @TimSweeneyEpic

      JVM is implemented in c/c++. So, if it's possible in Java, it should be possible in c

      1 reply 0 retweets 1 like
    4. Tamir Bahar‏ @tmr232 14 Jul 2018
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      Replying to @shajorjita @F_Vaggi @TimSweeneyEpic

      Memory in Java is managed by JVM. In C and C++ you manage your own memory, working with raw pointers. You have no means of moving pointers around without invalidating variables and code.

      2 replies 0 retweets 1 like
    5. RJ‏ @shajorjita 14 Jul 2018
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      Replying to @tmr232 @F_Vaggi @TimSweeneyEpic

      Within c, you can ask system to move the data around and then update the pointers to point to new location. My point is, if something is possible in jvm, that should be possible in c.

      1 reply 0 retweets 1 like
    6. Tamir Bahar‏ @tmr232 14 Jul 2018
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      Replying to @shajorjita @F_Vaggi @TimSweeneyEpic

      I agree. But the semantic difference is large. JVM can track pointers and know where all the pointers in a running program are. In C you have no way of knowing what's a pointer and what isn't. That said, you *may* be able to handle well-behaved code given compiler support

      1 reply 0 retweets 2 likes
    7. Federico Vaggi‏ @F_Vaggi 14 Jul 2018
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      Replying to @tmr232 @shajorjita @TimSweeneyEpic

      I am almost certain any kind of GC system in C/C++ would be opt in for pointers that are specifically owned by the GC - ala boehm GC

      1 reply 0 retweets 0 likes
      Tim Sweeney‏ @TimSweeneyEpic 14 Jul 2018
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      Replying to @F_Vaggi @tmr232 @shajorjita

      My approach is even simpler: GC only occurs on ref<t> smart pointers and only when allocated through forwarding ref<t>(..) ctors. Thus GC is opt-in and orthogonal to explicit lifetime management of other objects.

      12:56 PM - 14 Jul 2018
      • 5 Likes
      • corvus frugilegus Brandon Paddock RJ Patrick Purcell Federico Vaggi
      2 replies 0 retweets 5 likes
        1. New conversation
        2. corvus frugilegus‏ @glaebhoerl 15 Jul 2018
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          Replying to @TimSweeneyEpic @F_Vaggi and

          How do you keep track of roots? Don't you still need to trace through arbitrary C++ structures which may be holding on to `ref<t>`s to determine if they're reachable (and where, once again, you don't know what's a pointer and what isn't)?

          1 reply 0 retweets 0 likes
        3. Tim Sweeney‏ @TimSweeneyEpic 15 Jul 2018
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          Replying to @glaebhoerl @F_Vaggi and

          I maintain a bitmask of all locations in unmanaged memory that contain ref<t>, set in the ctor and cleared in the dtor. Those bits are used to locate roots (conservatively, given races), plus there’s a dirty-flag per object to indicate recent rootedness.

          2 replies 0 retweets 3 likes
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        2. Tamir Bahar‏ @tmr232 14 Jul 2018
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          Replying to @TimSweeneyEpic @F_Vaggi @shajorjita

          So where did the cost come in, compared to other reference-based languages? I think that looking into C++/CLI might also be interesting.

          1 reply 0 retweets 0 likes
        3. Tim Sweeney‏ @TimSweeneyEpic 14 Jul 2018
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          Replying to @tmr232 @F_Vaggi @shajorjita

          When reading a garbage collector managed pointer from an object in a thread, it needs to be sanitized through a hazard pointer or similar synchronization mechanism. This is expensive, and needed for every pointer copy and operator->.

          2 replies 0 retweets 2 likes
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