CodeGen: Don't form weak dllexport aliases

[majnemer]

Summary

The MS linker cannot do anything interesting with these, it doesn't make
sense to emit them.

This fixes PR21373.

[hans]

lgtm

test/CodeGenCXX/dllexport.cpp

608

Maybe put the PR number in the comment as well.

[majnemer]

Closed by commit rL220595 (authored by @majnemer).

lib/CodeGen/CGCXX.cpp

bool CodeGenModule::TryEmitDefinitionAsAlias(GlobalDecl AliasDecl,
   // The alias will use the linkage of the referent.  If we can't
   // support aliases with that linkage, fail.
   llvm::GlobalValue::LinkageTypes Linkage = getFunctionLinkage(AliasDecl);
 
   // We can't use an alias if the linkage is not valid for one.
   if (!llvm::GlobalAlias::isValidLinkage(Linkage))
     return true;
 
+  // Don't create a weak alias for a dllexport'd symbol.
+  if (AliasDecl.getDecl()->hasAttr<DLLExportAttr>() &&
+      llvm::GlobalValue::isWeakForLinker(Linkage))
+    return true;
+
   llvm::GlobalValue::LinkageTypes TargetLinkage =
       getFunctionLinkage(TargetDecl);
 
   // Check if we have it already.
   StringRef MangledName = getMangledName(AliasDecl);
   llvm::GlobalValue *Entry = GetGlobalValue(MangledName);
   if (Entry && !Entry->isDeclaration())
     return false;

test/CodeGenCXX/dllexport.cpp

 // RUN: %clang_cc1 -triple i686-windows-msvc   -emit-llvm -std=c++1y -O1 -mconstructor-aliases -disable-llvm-optzns -o - %s | FileCheck --check-prefix=MSC --check-prefix=M32 %s
 // RUN: %clang_cc1 -triple x86_64-windows-msvc -emit-llvm -std=c++1y -O0 -o - %s | FileCheck --check-prefix=MSC --check-prefix=M64 %s
 // RUN: %clang_cc1 -triple i686-windows-gnu    -emit-llvm -std=c++1y -O0 -o - %s | FileCheck --check-prefix=GNU --check-prefix=G32 %s
 // RUN: %clang_cc1 -triple x86_64-windows-gnu  -emit-llvm -std=c++1y -O0 -o - %s | FileCheck --check-prefix=GNU --check-prefix=G64 %s
 
-// RUN: %clang_cc1 -triple i686-pc-win32 -O1 -mconstructor-aliases -disable-llvm-optzns -std=c++1y -emit-llvm -o - %s | FileCheck %s --check-prefix=MSC --check-prefix=M32
-
 // Helper structs to make templates more expressive.
 struct ImplicitInst_Exported {};
 struct ExplicitDecl_Exported {};
 struct ExplicitInst_Exported {};
 struct ExplicitSpec_Exported {};
 struct ExplicitSpec_Def_Exported {};
 struct ExplicitSpec_InlineDef_Exported {};
 struct ExplicitSpec_NotExported {};

 // G32-DAG: declare dllimport x86_thiscallcc void @_ZN42PartiallySpecializedExportedClassTemplate2IPvE1fEv
 
 // Attributes on the instantiation take precedence over attributes on the template.
 template <typename T> struct __declspec(dllimport) ExplicitlyInstantiatedWithDifferentAttr { void f() {} };
 template struct __declspec(dllexport) ExplicitlyInstantiatedWithDifferentAttr<int>;
 USEMEMFUNC(ExplicitlyInstantiatedWithDifferentAttr<int>, f);
 // M32-DAG: define weak_odr dllexport x86_thiscallcc void @"\01?f@?$ExplicitlyInstantiatedWithDifferentAttr@H@@QAEXXZ"
 
+// Don't create weak dllexport aliases.
+struct NonExportedBaseClass {
+  virtual ~NonExportedBaseClass();
+};
+NonExportedBaseClass::~NonExportedBaseClass() {}
+
+struct __declspec(dllexport) ExportedDerivedClass : NonExportedBaseClass {};
+// M32-DAG: weak_odr dllexport x86_thiscallcc void @"\01??1ExportedDerivedClass@@UAE@XZ"
+
 
 //===----------------------------------------------------------------------===//
 // Classes with template base classes
 //===----------------------------------------------------------------------===//
 
 template <typename T> struct ClassTemplate { void func() {} };
 template <typename T> struct __declspec(dllexport) ExportedClassTemplate { void func() {} };
 template <typename T> struct __declspec(dllimport) ImportedClassTemplate { void func() {} };