[PATCH] D91583: [LTO] Prevent devirtualization for symbols exported to dynamic linker

[Teresa Johnson via Phabricator via llvm-commits]

tejohnson added inline comments.


================
Comment at: lld/ELF/LTO.cpp:252
+    r.VisibleToDynamicLinker =
+        sym->isExportDynamic(sym->kind(), sym->visibility) ||
+        sym->inDynamicList;
----------------
tejohnson wrote:
> MaskRay wrote:
> > tejohnson wrote:
> > > MaskRay wrote:
> > > > `sym->exportDynamic || sym->inDynamicList`
> > > > 
> > > > Then `isExportDynamic` does not need to be public.
> > > sym->exportDynamic is false for linkonce_odr vtables, that was what I was referencing in this comment (otherwise I could use includeInDynsym which checks that):
> > > 
> > > > Note that I couldn't use includeInDynsym in lld because that is not set for linkonce_odr symbols that were thus have canBeOmittedFromSymbolTable set (since any referencing module must have it's own copy) - we still want to block the LTO visibility upgrade for those symbols to avoid WPD. So I am using a slightly different interface that more directly checks whether export-dynamic is in effect.
> > Sorry, I don't understand the difference. If I replace this with `sym->exportDynamic`, I don't get a test failure...
> Ah, this is a test deficiency, looks like I need to make one or more of the vtables linkonce_odr to expose it. Will address that. The reason it is an issue for linkonce_odr can be seen in createBitcodeSymbol in lld/ELF/InputFiles.cpp, where it does:
> 
>     if (canOmitFromDynSym)
>       newSym.exportDynamic = false;
> 
> The canOmitFromDynSym gets propagated via the input file but is originally set in GlobalValue::canBeOmittedFromSymbolTable() for linkonce_odr with hasAtLeastLocalUnnamedAddr().
I've improved the tests. Confirmed that the improved lld test fails if you make the change you proposed.


Repository:
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  https://reviews.llvm.org/D91583/new/

https://reviews.llvm.org/D91583