[MS ABI] Never reference dllimport'd vtables

Referencing a dllimported vtable is impossible in a constexpr
constructor.  It would be friendlier to C++ programmers if we
synthesized a copy of the vftable which referenced imported virtual
functions.  This would let us initialize the object in a way which
preserves both the intent to import functionality from another DLL while
also making constexpr work.

Differential Revision: http://reviews.llvm.org/D17061

llvm-svn: 260388

3 files changed, 15 insertions, 6 deletions

diff --git a/clang/lib/AST/MicrosoftMangle.cpp b/clang/lib/AST/MicrosoftMangle.cpp
index 0634319cbd1..930cf00d9e3 100644
--- a/clang/lib/AST/MicrosoftMangle.cpp
+++ b/clang/lib/AST/MicrosoftMangle.cpp
@@ -2599,7 +2599,10 @@ void MicrosoftMangleContextImpl::mangleCXXVFTable(
   // NOTE: <cvr-qualifiers> here is always 'B' (const). <storage-class>
   // is always '6' for vftables.
   MicrosoftCXXNameMangler Mangler(*this, Out);
-  Mangler.getStream() << "\01??_7";
+  if (Derived->hasAttr<DLLImportAttr>())
+    Mangler.getStream() << "\01??_S";
+  else
+    Mangler.getStream() << "\01??_7";
   Mangler.mangleName(Derived);
   Mangler.getStream() << "6B"; // '6' for vftable, 'B' for const.
   for (const CXXRecordDecl *RD : BasePath)
diff --git a/clang/lib/AST/VTableBuilder.cpp b/clang/lib/AST/VTableBuilder.cpp
index bae018652f9..b728b279182 100644
--- a/clang/lib/AST/VTableBuilder.cpp
+++ b/clang/lib/AST/VTableBuilder.cpp
@@ -2548,7 +2548,6 @@ public:
     // Only include the RTTI component if we know that we will provide a
     // definition of the vftable.
     HasRTTIComponent = Context.getLangOpts().RTTIData &&
-                       !MostDerivedClass->hasAttr<DLLImportAttr>() &&
                        MostDerivedClass->getTemplateSpecializationKind() !=
                            TSK_ExplicitInstantiationDeclaration;
 
diff --git a/clang/lib/CodeGen/MicrosoftCXXABI.cpp b/clang/lib/CodeGen/MicrosoftCXXABI.cpp
index 0664381b44f..f47b120a90c 100644
--- a/clang/lib/CodeGen/MicrosoftCXXABI.cpp
+++ b/clang/lib/CodeGen/MicrosoftCXXABI.cpp
@@ -1673,7 +1673,16 @@ llvm::GlobalVariable *MicrosoftCXXABI::getAddrOfVTable(const CXXRecordDecl *RD,
   SmallString<256> VFTableName;
   mangleVFTableName(getMangleContext(), RD, VFPtr, VFTableName);
 
-  llvm::GlobalValue::LinkageTypes VFTableLinkage = CGM.getVTableLinkage(RD);
+  // Classes marked __declspec(dllimport) need vftables generated on the
+  // import-side in order to support features like constexpr.  No other
+  // translation unit relies on the emission of the local vftable, translation
+  // units are expected to generate them as needed.
+  //
+  // Because of this unique behavior, we maintain this logic here instead of
+  // getVTableLinkage.
+  llvm::GlobalValue::LinkageTypes VFTableLinkage =
+      RD->hasAttr<DLLImportAttr>() ? llvm::GlobalValue::LinkOnceODRLinkage
+                                   : CGM.getVTableLinkage(RD);
   bool VFTableComesFromAnotherTU =
       llvm::GlobalValue::isAvailableExternallyLinkage(VFTableLinkage) ||
       llvm::GlobalValue::isExternalLinkage(VFTableLinkage);
@@ -1746,9 +1755,7 @@ llvm::GlobalVariable *MicrosoftCXXABI::getAddrOfVTable(const CXXRecordDecl *RD,
   if (C)
     VTable->setComdat(C);
 
-  if (RD->hasAttr<DLLImportAttr>())
-    VFTable->setDLLStorageClass(llvm::GlobalValue::DLLImportStorageClass);
-  else if (RD->hasAttr<DLLExportAttr>())
+  if (RD->hasAttr<DLLExportAttr>())
     VFTable->setDLLStorageClass(llvm::GlobalValue::DLLExportStorageClass);
 
   VFTablesMap[ID] = VFTable;