[PATCH] D91583: [lld] Allow --export-dynamic to override --lto-whole-program-visibility

[Fangrui Song via Phabricator via llvm-commits]

MaskRay added a comment.

>> I have a further question: is it realistic to add another bit along with VisibleToRegularObj to convey the information whether a symbol is includeInDynsym()?
>> This way virtual functions related to individual _ZTV* can be safely devirtualized, no matter how users specify --export-dynamic-symbol,--export-dynamic,--dynamic-list or add link-time shared objects to alter the includeInDynsym() state of _ZTV* symbols.

Should a new bit be introduced? If the executable defines a virtual class A which is overridden by a link-time shared object, the vtable symbol will be exported. It is a misoptimization if LTO considers devirtualizes A's member functions. How does the --lto-whole-program-visibility design deal with this pitfall?

> I looked into this some more and it turns out that vtables themselves don't actually need to be exported to be overridden, so this won't be more accurate. So I think a tri-state is the best option, with the new value tied to the --export-dynamic being a strong signal that vtables could be overridden.


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https://reviews.llvm.org/D91583