[llvm] r260122 - [ThinLTO] Remove imported available externally defs from comdats.

[David Blaikie via llvm-commits]

On Mon, Feb 8, 2016 at 11:22 AM, Teresa Johnson <tejohnson at google.com>
wrote:

>
>
> On Mon, Feb 8, 2016 at 11:06 AM, David Blaikie <dblaikie at gmail.com> wrote:
>
>> I couldn't quite follow the test - but wouldn't removing things (or in
>> any way modifyidng) from a COMDAT cause problems? (the COMDAT would no
>> longer match between objects, etc?)
>>
>
> The comdat will not be emitted in the importing module if we import its
> definitions as available_externally, since any such imported defs will be
> removed from the comdat with this patch.  Any non-imported members remained
> decls and are not part of a comdat in the importing module either.
>

Ah, right - thanks! (the available_externally helps me understand)

Though perhaps we should remove the comdats, to be safe? Once we're
adding/removing things from them they're no longer accurate anyway. Is
there any benefit to keeping them? Seems like they're just likely to be a
trap/misleading thing to keep them around.


>
>
>> On Mon, Feb 8, 2016 at 10:47 AM, Teresa Johnson via llvm-commits <
>> llvm-commits at lists.llvm.org> wrote:
>>
>>> Author: tejohnson
>>> Date: Mon Feb  8 12:47:20 2016
>>> New Revision: 260122
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=260122&view=rev
>>> Log:
>>> [ThinLTO] Remove imported available externally defs from comdats.
>>>
>>> Summary:
>>> Available externally definitions are considered declarations for the
>>> linker and eventually dropped. As such they are not allowed to be
>>> in comdats. Remove any such imported functions from comdats.
>>>
>>> Reviewers: rafael
>>>
>>> Subscribers: davidxl, llvm-commits, joker.eph
>>>
>>> Differential Revision: http://reviews.llvm.org/D16120
>>>
>>> Added:
>>>     llvm/trunk/test/Linker/Inputs/funcimport_comdat.ll
>>>     llvm/trunk/test/Linker/funcimport_comdat.ll
>>> Modified:
>>>     llvm/trunk/lib/Linker/LinkModules.cpp
>>>
>>> Modified: llvm/trunk/lib/Linker/LinkModules.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Linker/LinkModules.cpp?rev=260122&r1=260121&r2=260122&view=diff
>>>
>>> ==============================================================================
>>> --- llvm/trunk/lib/Linker/LinkModules.cpp (original)
>>> +++ llvm/trunk/lib/Linker/LinkModules.cpp Mon Feb  8 12:47:20 2016
>>> @@ -722,9 +722,21 @@ void ThinLTOGlobalProcessing::processGlo
>>>        GV.setVisibility(GlobalValue::HiddenVisibility);
>>>      if (isModuleExporting())
>>>        NewExportedValues.insert(&GV);
>>> -    return;
>>> +  } else
>>> +    GV.setLinkage(getLinkage(&GV));
>>> +
>>> +  // Remove functions imported as available externally defs from
>>> comdats,
>>> +  // as this is a declaration for the linker, and will be dropped
>>> eventually.
>>> +  // It is illegal for comdats to contain declarations.
>>> +  auto *GO = dyn_cast_or_null<GlobalObject>(&GV);
>>> +  if (GO && GO->isDeclarationForLinker() && GO->hasComdat()) {
>>> +    // The IRMover should not have placed any imported declarations in
>>> +    // a comdat, so the only declaration that should be in a comdat
>>> +    // at this point would be a definition imported as
>>> available_externally.
>>> +    assert(GO->hasAvailableExternallyLinkage() &&
>>> +           "Expected comdat on definition (possibly available
>>> external)");
>>> +    GO->setComdat(nullptr);
>>>    }
>>> -  GV.setLinkage(getLinkage(&GV));
>>>  }
>>>
>>>  void ThinLTOGlobalProcessing::processGlobalsForThinLTO() {
>>>
>>> Added: llvm/trunk/test/Linker/Inputs/funcimport_comdat.ll
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Linker/Inputs/funcimport_comdat.ll?rev=260122&view=auto
>>>
>>> ==============================================================================
>>> --- llvm/trunk/test/Linker/Inputs/funcimport_comdat.ll (added)
>>> +++ llvm/trunk/test/Linker/Inputs/funcimport_comdat.ll Mon Feb  8
>>> 12:47:20 2016
>>> @@ -0,0 +1,4 @@
>>> +define i32 @main() #0 {
>>> +entry:
>>> +  ret i32 0
>>> +}
>>>
>>> Added: llvm/trunk/test/Linker/funcimport_comdat.ll
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Linker/funcimport_comdat.ll?rev=260122&view=auto
>>>
>>> ==============================================================================
>>> --- llvm/trunk/test/Linker/funcimport_comdat.ll (added)
>>> +++ llvm/trunk/test/Linker/funcimport_comdat.ll Mon Feb  8 12:47:20 2016
>>> @@ -0,0 +1,28 @@
>>> +; Do setup work for all below tests: generate bitcode and combined index
>>> +; RUN: llvm-as -function-summary %s -o %t.bc
>>> +; RUN: llvm-as -function-summary %p/Inputs/funcimport_comdat.ll -o
>>> %t2.bc
>>> +; RUN: llvm-lto -thinlto -o %t3 %t.bc %t2.bc
>>> +
>>> +; Ensure linking of comdat containing external linkage global and
>>> function
>>> +; removes the imported available_externally defs from comdat.
>>> +; RUN: llvm-link %t2.bc -functionindex=%t3.thinlto.bc
>>> -import=comdat1_func1:%t.bc -S | FileCheck %s --check-prefix=IMPORTCOMDAT
>>> +; IMPORTCOMDAT-NOT: $comdat1 = comdat any
>>> +; IMPORTCOMDAT-NOT: comdat($comdat1)
>>> +
>>> +; Ensure linking of comdat containing internal linkage function with
>>> alias
>>> +; removes the imported and promoted available_externally defs from
>>> comdat.
>>> +; RUN: llvm-link %t2.bc -functionindex=%t3.thinlto.bc
>>> -import=comdat2_func1:%t.bc -S | FileCheck %s --check-prefix=IMPORTCOMDAT2
>>> +; IMPORTCOMDAT2-NOT: $comdat2 = comdat any
>>> +; IMPORTCOMDAT2-NOT: comdat($comdat2)
>>> +
>>> +$comdat1 = comdat any
>>> + at comdat1_glob = global i32 0, comdat($comdat1)
>>> +define void @comdat1_func1() comdat($comdat1) {
>>> +  ret void
>>> +}
>>> +
>>> +$comdat2 = comdat any
>>> + at comdat2_alias = alias void (), void ()* @comdat2_func1
>>> +define internal void @comdat2_func1() comdat($comdat2) {
>>> +  ret void
>>> +}
>>>
>>>
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>>
>>
>
>
> --
> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
> 408-460-2413
>
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