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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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    Tim Sweeney‏ @TimSweeneyEpic Jan 30
    • Report Tweet
    • Report NetzDG Violation

    Just eliminated C++ exceptions in a complicated multithreaded library. It’s 15% faster despite all performance sensitive functions previously being declared noexcept, with no obvious explanation.

    6:55 PM - 30 Jan 2020
    • 140 Retweets
    • 904 Likes
    • Eero Mutka Tomáš Šváb Matt Conte @ ??? Claire Wind Clout Strife Rick Hoskinson nick12 Horthrax Michael Campagnaro
    38 replies 140 retweets 904 likes
      1. New conversation
      2. Tim Sweeney‏ @TimSweeneyEpic Jan 30
        • Report Tweet
        • Report NetzDG Violation

        Exception throwing on Windows x64 is 8,000 CPU cycles in Visual C++ and 20,000 in clang, but grows to 1,800,000 when a debugger is attached to the process.

        4 replies 19 retweets 144 likes
        Show this thread
      3. Tim Sweeney‏ @TimSweeneyEpic Jan 30
        • Report Tweet
        • Report NetzDG Violation

        This cost is mind-boggling for what is essentially a goto-with-destructors. In the process of adopting the “zero cost exceptions” fallacy, modern platforms created a catastrophic performance cliff that makes the feature unusable in real software.

        5 replies 8 retweets 188 likes
        Show this thread
      4. Tim Sweeney‏ @TimSweeneyEpic Jan 30
        • Report Tweet
        • Report NetzDG Violation

        And it’s not just C++. Other languages build their exception models on top of C++ exceptions and the operating system level plumbing (why should the OS have anything to do with this?) so, for example, JavaScript exceptions are also impractically expensive.

        10 replies 6 retweets 138 likes
        Show this thread
      5. End of conversation
      1. Tim Sweeney‏ @TimSweeneyEpic Jan 31
        • Report Tweet
        • Report NetzDG Violation
        Replying to @greatcodeholio

        I remember that from win32 on x86, where exceptions seemed reasonably fast. On x64, there is no trace of exception tracking in mainline code, but some horrible handler that digs through the executable file to find tables of exception handlers.

        0 replies 0 retweets 4 likes
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      1. New conversation
      2. Massimo Tristano‏ @keebus Jan 31
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        • Report NetzDG Violation
        Replying to @TimSweeneyEpic

        I toyed with a new C-like language that supports the "try/catch" mechanism simply by passing in a second Return Address for the fail case so that when a failable function fails, it will return to the catch block of its caller without the need for the caller to check return value.

        1 reply 1 retweet 18 likes
      3. Tim Sweeney‏ @TimSweeneyEpic Jan 31
        • Report Tweet
        • Report NetzDG Violation
        Replying to @keebus

        I think that’s exactly what’s needed. Herb Sutter has been developing a similar proposal for C++. In the case of C++, the compiler has to generate a stub of code for all functions that runs local destructors and jumps to the parent handler.

        0 replies 0 retweets 12 likes
      4. End of conversation
      1. New conversation
      2. Brad Pederson‏ @PedersonDevice Jan 31
        • Report Tweet
        • Report NetzDG Violation
        Replying to @TimSweeneyEpic

        Over Christmas it was clock cycles on garbage collection, this post on exceptions. Both wastes due to non-specific intent with either memory alloc or knowing conditions of a function? Solutions to lower clock cycles is elixir to the symptom and not a solution to the root cause?

        1 reply 0 retweets 1 like
      3. Tim Sweeney‏ @TimSweeneyEpic Jan 31
        • Report Tweet
        • Report NetzDG Violation
        Replying to @PedersonDevice

        In this case I’m not writing a specific program that I can optimize, but a library framework for programming with maximum safety and programmer productivity.

        1 reply 0 retweets 14 likes
      4. 1 more reply
      1. New conversation
      2. Patrick Purcell‏ @TheAdvantag3 Jan 30
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        Replying to @TimSweeneyEpic

        Were the non performance intensive functions throwing? Ie, were the gains from avoiding the cost of throwing?

        1 reply 0 retweets 3 likes
      3. Tim Sweeney‏ @TimSweeneyEpic Jan 30
        • Report Tweet
        • Report NetzDG Violation
        Replying to @TheAdvantag3

        They weren’t throwing and a disassembly showed no visible artifacts of exceptions. But turning off the possibility of throwing exceptions gained 15% and just made the assembly code tighter with no clear pattern to the improvements.

        3 replies 2 retweets 37 likes
      4. 3 more replies

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