r241102 - CodeGen: Assign an appropriate comdat to thunks.

[Nico Weber]

It looks like this breaks building base.dll in the Chromium project:
http://code.google.com/p/chromium/issues/detail?id=505916

On Tue, Jun 30, 2015 at 12:07 PM, Peter Collingbourne <peter at pcc.me.uk>
wrote:

> Author: pcc
> Date: Tue Jun 30 14:07:26 2015
> New Revision: 241102
>
> URL: http://llvm.org/viewvc/llvm-project?rev=241102&view=rev
> Log:
> CodeGen: Assign an appropriate comdat to thunks.
>
> Previously we were not assigning a comdat to thunks in the Microsoft ABI,
> which would have required us to emit these functions outside of a comdat.
> (Due to an inconsistency in how we were emitting objects, we were getting
> this
> right most of the time, but only when compiling with function sections.)
> This
> code generator change causes us to create a comdat for each thunk.
>
> Differential Revision: http://reviews.llvm.org/D10829
>
> Modified:
>     cfe/trunk/lib/CodeGen/CGVTables.cpp
>     cfe/trunk/test/CodeGenCXX/microsoft-abi-thunks.cpp
>
> Modified: cfe/trunk/lib/CodeGen/CGVTables.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/CGVTables.cpp?rev=241102&r1=241101&r2=241102&view=diff
>
> ==============================================================================
> --- cfe/trunk/lib/CodeGen/CGVTables.cpp (original)
> +++ cfe/trunk/lib/CodeGen/CGVTables.cpp Tue Jun 30 14:07:26 2015
> @@ -378,9 +378,6 @@ void CodeGenFunction::GenerateThunk(llvm
>    // Set the right linkage.
>    CGM.setFunctionLinkage(GD, Fn);
>
> -  if (CGM.supportsCOMDAT() && Fn->isWeakForLinker())
> -    Fn->setComdat(CGM.getModule().getOrInsertComdat(Fn->getName()));
> -
>    // Set the right visibility.
>    const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());
>    setThunkVisibility(CGM, MD, Thunk, Fn);
> @@ -450,17 +447,18 @@ void CodeGenVTables::emitThunk(GlobalDec
>      // expensive/sucky at the moment, so don't generate the thunk unless
>      // we have to.
>      // FIXME: Do something better here; GenerateVarArgsThunk is extremely
> ugly.
> -    if (!UseAvailableExternallyLinkage) {
> -      CodeGenFunction(CGM).GenerateVarArgsThunk(ThunkFn, FnInfo, GD,
> Thunk);
> -      CGM.getCXXABI().setThunkLinkage(ThunkFn, ForVTable, GD,
> -                                      !Thunk.Return.isEmpty());
> -    }
> +    if (UseAvailableExternallyLinkage)
> +      return;
> +    CodeGenFunction(CGM).GenerateVarArgsThunk(ThunkFn, FnInfo, GD, Thunk);
>    } else {
>      // Normal thunk body generation.
>      CodeGenFunction(CGM).GenerateThunk(ThunkFn, FnInfo, GD, Thunk);
> -    CGM.getCXXABI().setThunkLinkage(ThunkFn, ForVTable, GD,
> -                                    !Thunk.Return.isEmpty());
>    }
> +
> +  CGM.getCXXABI().setThunkLinkage(ThunkFn, ForVTable, GD,
> +                                  !Thunk.Return.isEmpty());
> +  if (CGM.supportsCOMDAT() && ThunkFn->isWeakForLinker())
> +
> ThunkFn->setComdat(CGM.getModule().getOrInsertComdat(ThunkFn->getName()));
>  }
>
>  void CodeGenVTables::maybeEmitThunkForVTable(GlobalDecl GD,
>
> Modified: cfe/trunk/test/CodeGenCXX/microsoft-abi-thunks.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/test/CodeGenCXX/microsoft-abi-thunks.cpp?rev=241102&r1=241101&r2=241102&view=diff
>
> ==============================================================================
> --- cfe/trunk/test/CodeGenCXX/microsoft-abi-thunks.cpp (original)
> +++ cfe/trunk/test/CodeGenCXX/microsoft-abi-thunks.cpp Tue Jun 30 14:07:26
> 2015
> @@ -91,7 +91,7 @@ struct E : D {
>
>  E::E() {}  // Emits vftable and forces thunk generation.
>
> -// CODEGEN-LABEL: define weak_odr x86_thiscallcc %struct.C* @"\01?goo at E
> @@QAEPAUB@@XZ"
> +// CODEGEN-LABEL: define weak_odr x86_thiscallcc %struct.C* @"\01?goo at E
> @@QAEPAUB@@XZ"{{.*}} comdat
>  // CODEGEN:   call x86_thiscallcc %struct.C* @"\01?goo at E@@UAEPAUC@@XZ"
>  // CODEGEN:   getelementptr inbounds i8, i8* {{.*}}, i32 4
>  // CODEGEN: ret
> @@ -124,7 +124,7 @@ struct I : D {
>
>  I::I() {}  // Emits vftable and forces thunk generation.
>
> -// CODEGEN-LABEL: define weak_odr x86_thiscallcc %struct.{{[BF]}}*
> @"\01?goo at I@@QAEPAUB@@XZ"
> +// CODEGEN-LABEL: define weak_odr x86_thiscallcc %struct.{{[BF]}}*
> @"\01?goo at I@@QAEPAUB@@XZ"{{.*}} comdat
>  // CODEGEN: %[[ORIG_RET:.*]] = call x86_thiscallcc %struct.F* @"\01?goo at I
> @@UAEPAUF@@XZ"
>  // CODEGEN: %[[ORIG_RET_i8:.*]] = bitcast %struct.F* %[[ORIG_RET]] to i8*
>  // CODEGEN: %[[VBPTR_i8:.*]] = getelementptr inbounds i8, i8*
> %[[ORIG_RET_i8]], i32 4
>
>
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