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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. Joe Groff‏ @jckarter 5 Sep 2019
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      Replying to @mcclure111

      Yeah. If you call a generic function whose implementation is available to the compiler, the optimizer will generate a version where the type is substituted and the vtables become unnecessary, more like a C++ template. Sometimes we do this for non type arguments too like closures

      2 replies 1 retweet 6 likes
    2. mcc‏ @mcclure111 5 Sep 2019
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      Replying to @jckarter

      That's cool.

      1 reply 0 retweets 1 like
    3. gankra's gay‏ @Gankra_ 5 Sep 2019
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      Replying to @mcclure111 @jckarter

      to be clear: vtable pointers are never part of a struct, they are either in the ref-counting header of a box, or passed as an "argument" to polymorphic code. this must be, as e.g. Array<T> lays out struct values inline and has the same layout in polymorphic and specialized code

      1 reply 0 retweets 1 like
    4. mcc‏ @mcclure111 5 Sep 2019
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      Replying to @Gankra_ @jckarter

      that sounds smart. to be clear, the "passed as an argument" is invisible to the user?

      1 reply 0 retweets 1 like
    5. gankra's gay‏ @Gankra_ 5 Sep 2019
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      Replying to @mcclure111 @jckarter

      yep.

      1 reply 0 retweets 0 likes
    6. gankra's gay‏ @Gankra_ 5 Sep 2019
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      Replying to @Gankra_ @mcclure111 @jckarter

      you can find a bunch of notes on how generic type metadata is represented here: https://github.com/apple/swift/blob/master/docs/ABI/TypeMetadata.rst … (probably slightly out of date, they made lots of changes in the run up to ABI stability)

      1 reply 0 retweets 1 like
    7. gankra's gay‏ @Gankra_ 5 Sep 2019
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      Replying to @Gankra_ @mcclure111 @jckarter

      aw bummer, joe did y'all really rip out all the cute vtable entries for "$primitive_op, but do it for an array"? stupid well-defined is_pod query and "caring about code size" and "we have to generate this vtables at runtime oh god" ruining all the fun.

      1 reply 0 retweets 1 like
    8. Joe Groff‏ @jckarter 5 Sep 2019
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      Replying to @Gankra_ @mcclure111

      My next wild idea is to represent the type layout as a bitstring showing where the refcounted are instead of a vtable of primitive operations. Chances are this could end up being faster for runtime-generated types than going through n layers of unspecialized goo

      2 replies 0 retweets 0 likes
    9. Patrick Walton‏ @pcwalton 5 Sep 2019
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      Replying to @jckarter @Gankra_ @mcclure111

      I tried this for Rust once! It didn’t go well, but maybe Swift will do better

      1 reply 0 retweets 1 like
    10. Joe Groff‏ @jckarter 5 Sep 2019
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      Replying to @pcwalton @Gankra_ @mcclure111

      What problems did you run into?

      1 reply 0 retweets 0 likes
      Patrick Walton‏ @pcwalton 5 Sep 2019
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      Replying to @jckarter @Gankra_ @mcclure111

      I wrote an entire VM in C++ templates to interpret these layouts as quickly as possible to adjust reference counts and such. It was a large speed penalty (~30% of time profile of Rust code was spent in it or something) and didn’t even reduce code size that much.

      10:32 PM - 5 Sep 2019
      2 replies 0 retweets 0 likes
        1. New conversation
        2. Patrick Walton‏ @pcwalton 5 Sep 2019
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          Replying to @pcwalton @jckarter and

          But this was used for everything; the compiler had no specialization at all. So maybe things would be different if this was only for cold code.

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

          I was pretty burned on the whole approach after that and resolved that I’ll just use an interpreter and/or a JIT if I ever wanted to avoid template specialization again. Maybe I should have tried harder, but that was the result.

          1 reply 0 retweets 0 likes
        4. 7 more replies
        1. Ted Mielczarek‏ @TedMielczarek 6 Sep 2019
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          Replying to @pcwalton @jckarter and

          I don't know why but to me this sounds very similar to Gecko's generated DOM bindings vs. XPCOM calls through XPConnect typelibs.

          0 replies 0 retweets 0 likes
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