Apple Silicon M1: Black Magic Fuckery

Re:Apple Silicon M1: Black Magic Fuckery by wahern on Tuesday November 24, 2020 @09:20PM (#25205304)

I didn't understand why the implementation wouldn't just do an atomic increment, but I guess Obj-C semantics provide too much magic to permit such a simple approach. The actual code, in addition to [presumably] not being inlined, does not seem easy to optimize at the hardware level: https://github.com/apple/swift-corelibs-foundation/blob/main...

The native Swift retain (swift_retain above) seems to be somewhere inside this mess: https://github.com/apple/swift/blob/main/stdlib/public/runti...

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Re:Apple Silicon M1: Black Magic Fuckery by lilyball on Wednesday November 25, 2020 @01:16AM (#25206437)

What you’ve linked is CoreFoundation’s retain, Objective-C’s can be found in https://opensource.apple.com/source/objc4/objc4-787.1/runtim... (look for objc_object::rootRetain).

The short answer for why it can’t just be an increment is because the reference count is stored in a subset of the bits of the isa pointer, and when the reference count grows too large it has to overflow into a separate sidetable. So it does separate load and CAS operations in order to implement this overflow behavior.

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