IRGen: Make type(of:) behavior consistent in ObjC bridged contexts. #15798
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When we use type(of: x) on a class in an ObjC bridged context, the optimizer turns this into a SIL `value_metatype @objc` operation, which is supposed to get the dynamic type of the object as an ObjC class. This was previously lowered by IRGen into a `object_getClass` call, which extracts the isa pointer from the object, but is inconsistent with the `-class` method in ObjC or with the Swift-native behavior, which both look through artificial subclasses, proxies, and so on. This inconsistency led to observably different behavior between debug and release builds and between ObjC-bridged and native entry points, so provide an alternative runtime entry point that replicates the behavior of getting a native Swift class. Fixes SR-7258.
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aschwaighofer
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Apr 9, 2018
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LGTM.
I am assuming that the lookup in swift_getObjCClassFromObject of objc class objects has to be done this way based on Greg's description here: http://sealiesoftware.com/blog/archive/2009/04/14/objc_explain_Classes_and_metaclasses.html
Should there be an executable test that would have failed before this commit based on sr7258?
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When we use type(of: x) on a class in an ObjC bridged context, the optimizer turns this into a SIL
value_metatype @objcoperation, which is supposed to get the dynamic type of the object as an ObjC class. This was previously lowered by IRGen into aobject_getClasscall, which extracts the isa pointer from the object, but is inconsistent with the-classmethod in ObjC or with the Swift-native behavior, which both look through artificial subclasses, proxies, and so on. This inconsistency led to observably different behavior between debug and release builds and between ObjC-bridged and native entry points, so provide an alternative runtime entry point that replicates the behavior of getting a native Swift class. Fixes SR-7258 | rdar://problem/38797313.