objc4

Dyld (dynamic linker) does this I think, it's effectively similar to other relocations. Each dynamic library brings its own set of selectors (I just think of them as strings in the data section of the binary, referenced indirectly via a pointer in the objc_selrefs section of the binary which can be updated as needed) and at runtime they're uniqueified. See https://www.sealiesoftware.com/blog/archive/2009/09/01/objc_... and https://www.mulle-kybernetik.com/weblog/2015/mulle_objc_sele... I actually can't find much authoritative discussion about this on the internet, just those two posts. But since the objc-runtime is open source, you can probably find it there. I think it might be this method? https://github.com/opensource-apple/objc4/blob/cd5e62a5597ea... -- But I'm not sure what GP meant by "at compilation". I suppose the compiler could

IANAL, and I don't speak for Apple, but as far as I can tell, the part about Apple is nonsense. CoreFoundation is open source: https://github.com/opensource-apple/CF libdispatch is open source: https://apple.github.io/swift-corelibs-libdispatch/post/libd... ASL is open source: https://opensource.apple.com/source/syslog/syslog-349.1.1/li... The objective C runtime is open source: https://github.com/opensource-apple/objc4/blob/master/APPLE_... icu, pthread, zlib, and libxml did not even originate at Apple.

Hmm, if I'm understanding right, rather than passively checking whether any of the other threads are in the critical section, it's actually forcing all the threads to undergo a context switch so that the PC (instruction pointer) can be read. So long as the context switch is prevented until the thread is out of the critical section, the actual value of the PC is irrelevant so long as the call succeeds: #define PC_SENTINEL 1 unsigned int count = x86_THREAD_STATE64_COUNT; kern_return_t okay = thread_get_state (thread, x86_THREAD_STATE64, (thread_state_t)&state, &count); return (okay == KERN_SUCCESS) ? state.__rip : PC_SENTINEL; https://github.com/opensource-apple/objc4/blob/master/runtim... If you have a lot of threads running, this could get really expensive. I presume it works, but this doesn't seem like an efficient approach

You can do a variant of this in userspace as well: https://github.com/opensource-apple/objc4/blob/master/runtim... Basically, issue a syscall that acquires the PC of all threads. When none of the PCs are in the critical section, then you can garbage collect old objects.

The goal of the article isn't about sheer performance — there are plenty of notes about it. If it was about pure performance, it'd be recommending moving away from objc classes and methods and using C functions or C++ classes instead, like std::atomic<>. It's meant to be a somewhat-easy-to-digest introduction to lock-free design, where applicable. What @synchronized ends up doing is far more complex — it has to be, to ensure the correctness of its purposes: https://github.com/opensource-apple/objc4/blob/master/runtim...