[PATCH] D14623: [ThinLTO] Comdat importing fixes and related cleanup

[David Blaikie via llvm-commits]

On Thu, Nov 12, 2015 at 11:41 AM, Teresa Johnson via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> tejohnson created this revision.
> tejohnson added reviewers: rafael, dexonsmith.
> tejohnson added subscribers: llvm-commits, davidxl, joker.eph.
>
> A number of comdat related fixes, cleanup and test cases.
>
> I made a number of changes to get importing working when there are
> comdats, with lots of new test cases involving comdats of various
> selection types, linkage types, with and without aliases.
>
> The changes ensure we link in the full comdat group containing the
> function indicated for importing. Previously I intended to require
> that the importer specify the additional comdat group members for
> importing on successive importing requests. However, in order to get the
> comdat selection type correct it is significantly simpler to import the
> full comdat group in a single pass.
>

I thought all imported definitions were marked available_externally - in
which case I'm not sure why it would be necessary to bring in the whole
comdat group, since the comdat would never be emitted here.

I guess I'm missing something - is there a pointer to when/why this is
necessary?

(& in fact, it might be useful to have this pass make one instance of a
comdat group into an external definition (I guess that's probably not
possible unless you have true whole program knowledge - maybe there's
another linkage type which would produce the comdat group even if all the
calls are optimized away) - and mark all the other instances
available_externally? even potentially stripping the comdat groups from
modules where the call sites are really cold)


>
> Additionally, if the module linker selects the source version of a
> comdat not containing the imported function, if it is referenced
> by an imported function we also import all its comdat group member
> definitions (via the lazy linker) in the same importing pass.
> In order to support this, we are now much more aggressive about lazy
> linking global values during importing, which only need to be
> linked in (as either declarations or definitions) if referenced by an
> imported function.
>
> Also, because we may now import additional function bodies lazily, it is
> no longer possible to determine up front whether an alias's aliasee is
> going to be imported as a definition or not when we copy its prototype.
> Therefore, we now convert aliases into declarations after all lazy
> linking is complete, if the aliasee definition was not imported.
>
> Finally, since comdat members may not be declarations (including
> available_externally), after linking is complete those that were
> imported as linker declarations will be removed from comdats.
>
> http://reviews.llvm.org/D14623
>
> Files:
>   lib/Linker/LinkModules.cpp
>   test/Linker/Inputs/funcimport_comdat.ll
>   test/Linker/Inputs/funcimport_comdat2.ll
>   test/Linker/Inputs/funcimport_comdat3.ll
>   test/Linker/funcimport.ll
>   test/Linker/funcimport_comdat.ll
>   test/Linker/funcimport_comdat2.ll
>   test/Linker/funcimport_comdat3.ll
>
>
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