• pedantcoder Profile Picture

    Pierre H. 🔥🌸 @pedantcoder

    6 years ago

    About objc_direct, a thread. I should have probably anticipated that people would raise eyebrows and spent more time explaining the point in the LLVM commit, so here it is...

    steipete Profile Picture

    Peter Steinberger @steipete

    6 years ago

    About objc_direct, a thread. I should have probably anticipated that people would raise eyebrows and spent more time explaining the point in the LLVM commit, so here it is...

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  • pedantcoder Profile Picture

    Pierre H. 🔥🌸 @pedantcoder

    6 years ago

    The Obj-C dynamic dispatch comes with many costs, this is common "knowledge". However the details of it are rarely known. Beside the obvious cost of the h-lookup, it comes with 4 other kinds of costs: - codegen size - optimization barrier - static metadate - runtime metadata

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  • pedantcoder Profile Picture

    Pierre H. 🔥🌸 @pedantcoder

    6 years ago

    (1) Codegen size: In addition to `self`, `_cmd` is passed to objc_msgSend to be able to lookup your IMP. A selector is loaded from a GOT-like slot, called a selref, which in arm64 generates assembly akin to: adrp x1, mySelector@PAGE ldr x1, [x1, mySelector@PAGEOFF]

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  • pedantcoder Profile Picture

    Pierre H. 🔥🌸 @pedantcoder

    6 years ago

    This is 8 bytes that you pay at every call site. The number of calls to objc_msgSend is large enough that these 8 useless bytes add up. For example, in CloudKit, these 2 instructions represent 10.7% of __text. This is fairly typical of Objective-C heavy code.

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