[PATCH] D41988: [ThinLTO] - Set WeakAnyLinkage for all LinkerRedefined symbols.

[Rafael Avila de Espindola via llvm-commits]

Teresa Johnson <tejohnson at google.com> writes:

> Ok I see what happens. In addRegularLTO only prevailing symbols are added
> to the Keep set used when linking the IR (and a few other cases like
> linkonce_odr). In ThinLTO, obviously we don't have the IR during the thin
> link.
>
> Here's what we currently do with the prevailing info for ThinLTO:
> - in addThinLTO we already keep track of the prevailing module for a symbol
> (PrevailingModuleForGUID)
> - this is only used right now in weak symbol resolution - for linkonce, we
> upgrade the prevailing symbol to weak, and for other WeakForLinker symbols
> we downgrade the non-prevailing to available_externally (when legal)
>
> What we probably can/should do: Use PrevailingModuleForGUID to:
> 1) prevent importing of the non-prevailing copy when invoking
> ComputeCrossModuleImport
> 2) force the non-prevailing symbols to be internalized
> via thinLTOInternalizeAndPromoteInIndex
>
> For #2, we can assert in isExported that anything in an ExportList (because
> it was imported) is the prevailing copy (after doing #1). Otherwise, if the
> symbol is not prevailing in that module then return false from isExported
> to ensure it is internalized.
>
> Does that sound right?

Part 1 sounds right. For part 2 I don't think that internalizing is
correct, we want to drop the symbol completely.

Thanks a lot for the description, it was a bit hard to get the big
picture by reading the code.

>From the description I was able to create a testcase that shows why
internalizing symbols is not sufficient, we have to drop them:

https://bugs.llvm.org/show_bug.cgi?id=35938

Thanks,
Rafael