[swift-dev] Magic Metadata Symbol In Stubs

Joe Groff jgroff at apple.com
Fri Nov 18 11:01:17 CST 2016

> On Nov 17, 2016, at 12:36 PM, Alexis via swift-dev <swift-dev at swift.org> wrote:
> 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):
> @objc_non_lazy_realization
> 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.

This is all completely unprincipled code that happens to work by accident today. The C++ code is making blind assumptions about what the Swift compiler happens to do for mangling and class layout today.


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