[IRGen] Don't emit relative references to Objective-C class references. #21107
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Objective-C class references (which show up in the __objc_classrefs section) are always coalesced by the linker. When we relatively address them (which occurs in protocol conformance records), the linker may compute the relative offset *before* coalescing, leading to an incorrect result. The net effect is a protocol conformance record that applies to the wrong Objective-C class, causing all sorts of runtime mayhem. Switch relatively-addressed Objective-C classes over to using the Objective-C runtime name of the class. It's a less efficient encoding (since we need to go through objc_lookUpClass), but it avoids the linker bug. Fixes rdar://problem/46428085 by working around the linker bug.
Now that we're never relatively addressing an Objective-C class reference, stop emitting them as file-local (by eliminating the \01l_ prefix). This is both a minor optimization and also a way to ensure that things will break more consistently if a problem remains.
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DougGregor
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Dec 10, 2018
This test appears to have been fixed by swiftlang#21107. Fixes rdar://problem/42789939.
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Objective-C class references (which show up in the __objc_classrefs
section) are always coalesced by the linker. When we relatively
address them (which occurs in protocol conformance records), the
linker may compute the relative offset before coalescing, leading to
an incorrect result. The net effect is a protocol conformance record
that applies to the wrong Objective-C class, causing all sorts of
runtime mayhem.
Switch relatively-addressed Objective-C classes over to using the
Objective-C runtime name of the class. It's a less efficient encoding
(since we need to go through objc_lookUpClass), but it avoids the
linker bug.
Fixes rdar://problem/46428085 by working around the linker bug.