[PATCH] D91583: [LTO] Prevent devirtualization for symbols dynamically exported

[Teresa Johnson via Phabricator via llvm-commits]

tejohnson added a comment.

In D91583#2612525 <https://reviews.llvm.org/D91583#2612525>, @lanza wrote:

>> I see - so just to confirm, when compiling it isn't clear that these symbols are have internal or hidden visibility, but only during linking? Because otherwise clang should already have applied an appropriate vcall_visibility that allows WPD.
>
> Correct, when compiling we don't know what's exported and what's not, so nothing is applied. This is fundamentally similar to a problem you mentioned at a talk last year -- a source file shared between multiple apps might not be used the same everywhere, so you can only know after actually linking how it's used. So we apply stricter visibility via checking what was actually imported from the library. If `_ZTV5Thing` isn't used outside the current library we can mark it as such and then WPD operate with this extra info.

Ok thanks. Dumb question - how do you know when you are linking the shared library what will be used outside of it (because presumably you don't know this until it is linked into the consuming binaries later on in the build process)? And do you communicate this info via --export-dynamic-symbol or the like?


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