[swift-dev] Magic Metadata Symbol In Stubs

Alexis abeingessner at apple.com
Thu Nov 17 14:36:03 CST 2016

I’m currently looking at pushing the storage of the empty Dictionary singleton into rodata, using Array as a reference. Most things are pretty straightforward, except the symbol for the metadata for _EmptyArrayStorage that’s hardcoded in GlobalObjects.cpp: 

extern "C" ClassMetadata _TMCs18_EmptyArrayStorage;

How was the _TMCs18 mangle discovered? Does anything need to be done to ensure this is stable? Blindly applying that prefix to the Dictionary class sadly just leads to a linker error. For reference, the current decl for Dictionary is (after gyb expansion):

internal class _RawNativeDictionaryStorage:
  _SwiftNativeNSDictionary, _NSDictionaryCore

Related: is it completely busted that GlobalObjects.[h/cpp] is blatantly assuming the layout  of _EmptyArrayStorage? The doesn’t seem to be any @_fixed_layout annotation on it or its base classes.

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