Tuesday, October 31, 2006


Dave MacLachlan:

By examining the code (ADC registration required) that implements objc_sync_enter and obc_sync_exit, we can see that on every @synchronized(foo) block, we are actually paying for 3 lock/unlock sequences. objc_sync_enter calls id2data, which is responsible for getting the lock associated with foo, and then locks it. objc_sync_exit also calls id2data to get the lock associated with foo, and then unlocks it. And, id2data must lock/unlock its own internal data structures so that it can safely get the lock associated with foo, so we pay for that on each call as well.

Plus the overhead of entering an exception handling block, which is currently expensive in Objective-C.

Update: two faster versions. The first takes advantage of a guarantee that (Objective-)C++ makes, and the second uses swizzling to replace the method with one that isn’t synchronized once the initial race condition is over.

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