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

[Xinliang David Li via llvm-commits]

On Thu, Nov 12, 2015 at 1:30 PM, David Blaikie <dblaikie at gmail.com> wrote:

>
>
> On Thu, Nov 12, 2015 at 1:25 PM, Xinliang David Li <davidxl at google.com>
> wrote:
>
>>
>>
>> On Thu, Nov 12, 2015 at 12:56 PM, David Blaikie <dblaikie at gmail.com>
>> wrote:
>>
>>>
>>>
>>> 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.
>>>
>>
>> That is not the case -- there is no guarantee that the comdat defs are
>> available externally.
>>
>
> What I'm wondering is if we can make that guarantee - since we can see
> multiple instances of a comdat it seems like we might be able to benefit
> from only producing the definition of them in one place by changing the
> linkages.
>

Or waste a little compile time by always emitting comdats. Picking one file
to emit comdat is probably doable -- but may cost compile time in the
serial step of thinLTO which should be avoided.

David



>
>
> - David
>
>
>>
>> David
>>
>>
>>>
>>> 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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>>>>
>>>>
>>>
>>
>
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