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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. Greg Parker‏ @gparker 4 Aug 2018
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      Replying to @jckarter @pcwalton

      Two problems: One, many call sites really want a two-entry cache (mutable and non-mutable class, for example). Two, it costs too much dirty memory to do this everywhere so you need some way to choose at compile time where to apply it.

      2 replies 0 retweets 3 likes
    2. Patrick Walton‏ @pcwalton 4 Aug 2018
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      Replying to @gparker @jckarter

      Yeah, you can chain them to make polymorphic ICs (all JS engines do this). Can you use PGO to determine which call sites are hot?

      1 reply 0 retweets 1 like
    3. Patrick Walton‏ @pcwalton 4 Aug 2018
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      Replying to @pcwalton @gparker @jckarter

      It’s hard to believe that devirtualization is never worth it. objc_msgSend is pretty much always a BTB miss and that’s gotta hurt…

      2 replies 0 retweets 1 like
    4. Joe Groff‏ @jckarter 4 Aug 2018
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      Replying to @pcwalton @gparker

      That’s not really true on newer architectures. Branch prediction with history in practice predicts msgSend fairly well

      2 replies 0 retweets 2 likes
    5. Joe Groff‏ @jckarter 4 Aug 2018
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      Replying to @jckarter @pcwalton @gparker

      Well enough that tricks that increase local code size or memory usage aren’t worth the systemwide costs, at least

      1 reply 0 retweets 0 likes
    6. Patrick Walton‏ @pcwalton 4 Aug 2018
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      Replying to @jckarter @gparker

      That would imply that JS inline caches aren't worth it. I'm skeptical, given that it's completely impossible to write a competitive JS engine without them.

      3 replies 0 retweets 1 like
    7. John McCall‏ @pathofshrines 5 Aug 2018
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      Replying to @pcwalton @jckarter @gparker

      In addition to all of Greg’s points, remember that a call cache in JS gets to skip more work; ObjC assumes you’re calling a method with the right signature.

      1 reply 0 retweets 1 like
    8. John McCall‏ @pathofshrines 5 Aug 2018
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      Replying to @pathofshrines @pcwalton and

      And the caching can do more than just devirtualization, e.g. you can devirtualize to a variant which propagates argument type information.

      2 replies 0 retweets 0 likes
    9. Patrick Walton‏ @pcwalton 5 Aug 2018
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      Replying to @pathofshrines @jckarter @gparker

      You can do that in Objective-C too! Devirtualization can result in inlining, and this unlocks a whole bunch of optimizations. I think you all are really too easily dismissing a large source of optimizations.

      3 replies 0 retweets 0 likes
    10. Joe Groff‏ @jckarter 5 Aug 2018
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      Replying to @pcwalton @pathofshrines @gparker

      These things have been tried and found to be too expensive at the system level. PGO might change the game there if the workflow issues can be smoothed over, but I think ObjC code already tends to put msgSends along cold paths like API boundaries

      1 reply 0 retweets 1 like
      Patrick Walton‏ @pcwalton 5 Aug 2018
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      Replying to @jckarter @pathofshrines @gparker

      I suspect it’d help quite a bit for the long tail of apps that aren’t carefully profiled. I see IMP caching brought up a lot, but I rarely see it actually done. objc_msgSend is a “peanut butter cost” that’s smeared over all the code. Exactly the kind of thing optzns are good at.

      10:46 AM - 5 Aug 2018 from Dogpatch, San Francisco
      1 reply 0 retweets 0 likes
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        2. Joe Groff‏ @jckarter 5 Aug 2018
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          Replying to @pcwalton @pathofshrines @gparker

          Well Swift is trying to address that by letting you use the same abstractions for cheaper when you don’t have to cross an ABI boundary. These sorts of AOT optimizations are easier to justify when they can be definitive rather than speculative thanks to stricter language semantics

          1 reply 0 retweets 0 likes
        3. Patrick Walton‏ @pcwalton 5 Aug 2018
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          Replying to @jckarter @pathofshrines @gparker

          Agreed, Swift is a much more sensible design for AOT. Obj-C was always…weird :)

          1 reply 0 retweets 0 likes
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