[MS ABI] Never reference dllimport'd vtables

[majnemer]

Summary

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.

[rnk]

lgtm if you clarify that bit

lib/CodeGen/MicrosoftCXXABI.cpp

1676

According to you, this is a crazy special case, add comments. :)

lib/AST/MicrosoftMangle.cpp

 void MicrosoftMangleContextImpl::mangleCXXVFTable(
     const CXXRecordDecl *Derived, ArrayRef<const CXXRecordDecl *> BasePath,
     raw_ostream &Out) {
   // <mangled-name> ::= ?_7 <class-name> <storage-class>
   //                    <cvr-qualifiers> [<name>] @
   // NOTE: <cvr-qualifiers> here is always 'B' (const). <storage-class>
   // is always '6' for vftables.
   MicrosoftCXXNameMangler Mangler(*this, Out);
+  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)
     Mangler.mangleName(RD);
   Mangler.getStream() << '@';
 }
 
 void MicrosoftMangleContextImpl::mangleCXXVBTable(

lib/AST/VTableBuilder.cpp

VFTableBuilder(MicrosoftVTableContext &VTables,
         Context(MostDerivedClass->getASTContext()),
         MostDerivedClass(MostDerivedClass),
         MostDerivedClassLayout(Context.getASTRecordLayout(MostDerivedClass)),
         WhichVFPtr(*Which),
         Overriders(MostDerivedClass, CharUnits(), MostDerivedClass) {
     // 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;
 
     LayoutVFTable();
 
     if (Context.getLangOpts().DumpVTableLayouts)
       dumpLayout(llvm::outs());
   }

lib/CodeGen/MicrosoftCXXABI.cpp

if (VFPtrI == VFPtrs.end()) {
     VFTablesMap[ID] = nullptr;
     return nullptr;
   }
   VPtrInfo *VFPtr = *VFPtrI;
 
   SmallString<256> VFTableName;
   mangleVFTableName(getMangleContext(), RD, VFPtr, VFTableName);
 
-  llvm::GlobalValue::LinkageTypes VFTableLinkage = CGM.getVTableLinkage(RD);
+  llvm::GlobalValue::LinkageTypes VFTableLinkage =
+      RD->hasAttr<DLLImportAttr>() ? llvm::GlobalValue::LinkOnceODRLinkage
+                                   : CGM.getVTableLinkage(RD);
   bool VFTableComesFromAnotherTU =
       llvm::GlobalValue::isAvailableExternallyLinkage(VFTableLinkage) ||
       llvm::GlobalValue::isExternalLinkage(VFTableLinkage);
   bool VTableAliasIsRequred =
       !VFTableComesFromAnotherTU && getContext().getLangOpts().RTTIData;
 
   if (llvm::GlobalValue *VFTable =
           CGM.getModule().getNamedGlobal(VFTableName)) {
if (VTableAliasIsRequred) {
     // be referencing any RTTI data.
     // The GlobalVariable will end up being an appropriate definition of the
     // VFTable.
     VFTable = VTable;
   }
   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;
   return VTable;
 }
 
 // Compute the identity of the most derived class whose virtual table is located
 // at the given offset into RD.

test/CodeGenCXX/dllimport-rtti.cpp

 // RUN: %clang_cc1 -triple i686-windows-msvc -emit-llvm -std=c++1y -fms-extensions -O1 -disable-llvm-optzns -o - %s | FileCheck %s --check-prefix=MSVC
 // RUN: %clang_cc1 -triple i686-windows-gnu  -emit-llvm -std=c++1y -fms-extensions -O1 -disable-llvm-optzns -o - %s | FileCheck %s --check-prefix=GNU
 
 struct __declspec(dllimport) S {
   virtual void f() {}
 } s;
-// MSVC-DAG: @"\01??_7S@@6B@" = available_externally dllimport
+// MSVC: [[VF_S:.*]] = private unnamed_addr constant [2 x i8*]
+// MSVC-DAG: @"\01??_SS@@6B@" = unnamed_addr alias i8*, getelementptr inbounds ([2 x i8*], [2 x i8*]* [[VF_S]], i32 0, i32 1)
 // MSVC-DAG: @"\01??_R0?AUS@@@8" = linkonce_odr
 // MSVC-DAG: @"\01??_R1A@?0A@EA@S@@8" = linkonce_odr
 // MSVC-DAG: @"\01??_R2S@@8" = linkonce_odr
 // MSVC-DAG: @"\01??_R3S@@8" = linkonce_odr
 
 // GNU-DAG: @_ZTV1S = available_externally dllimport
 // GNU-DAG: @_ZTI1S = external dllimport
 

test/CodeGenCXX/dllimport.cpp

 template <typename T> struct __declspec(dllimport) U { void foo() {} };
 // MO1-DAG: define available_externally dllimport x86_thiscallcc void @"\01?foo@?$U@H@@QAEXXZ"
 struct __declspec(dllimport) V : public U<int> { };
 USEMEMFUNC(V, foo)
 
 struct __declspec(dllimport) W { virtual void foo() {} };
 USECLASS(W)
 // vftable:
-// MO1-DAG: @"\01??_7W@@6B@" = available_externally dllimport unnamed_addr constant [1 x i8*] [i8* bitcast (void (%struct.W*)* @"\01?foo@W@@UAEXXZ" to i8*)]
+// MO1-DAG: @"\01??_SW@@6B@" = linkonce_odr unnamed_addr constant [1 x i8*] [i8* bitcast (void (%struct.W*)* @"\01?foo@W@@UAEXXZ" to i8*)]
 // GO1-DAG: @_ZTV1W = available_externally dllimport unnamed_addr constant [3 x i8*] [i8* null, i8* null, i8* bitcast (void (%struct.W*)* @_ZN1W3fooEv to i8*)]
 
 struct __declspec(dllimport) KeyFuncClass {
   constexpr KeyFuncClass() {}
   virtual void foo();
 };
 extern constexpr KeyFuncClass keyFuncClassVar = {};
 // G32-DAG: @_ZTV12KeyFuncClass = external dllimport unnamed_addr constant [3 x i8*]

test/CodeGenCXX/microsoft-abi-vftables.cpp

 // RUN: %clang_cc1 %s -fno-rtti -triple=i386-pc-win32 -fms-extensions -emit-llvm -o - -O1 -disable-llvm-optzns | FileCheck %s -check-prefix=NO-RTTI
 // RUN: %clang_cc1 %s -triple=i386-pc-win32 -fms-extensions -emit-llvm -o - -O1 -disable-llvm-optzns | FileCheck %s -check-prefix=RTTI
 
 // RTTI-DAG: $"\01??_7S@@6B@" = comdat largest
 // RTTI-DAG: $"\01??_7V@@6B@" = comdat largest
 
-// RTTI-NOT: @"\01??_R4U@@6B@"
-
 struct S {
   virtual ~S();
 } s;
 
 // RTTI-DAG: [[VTABLE_S:@.*]] = private unnamed_addr constant [2 x i8*] [i8* bitcast ({{.*}} @"\01??_R4S@@6B@" to i8*), i8* bitcast ({{.*}} @"\01??_GS@@UAEPAXI@Z" to i8*)], comdat($"\01??_7S@@6B@")
 // RTTI-DAG: @"\01??_7S@@6B@" = unnamed_addr alias i8*, getelementptr inbounds ([2 x i8*], [2 x i8*]* [[VTABLE_S]], i32 0, i32 1)
 
 // NO-RTTI-DAG: @"\01??_7S@@6B@" = linkonce_odr unnamed_addr constant [1 x i8*] [i8* bitcast ({{.*}} @"\01??_GS@@UAEPAXI@Z" to i8*)]
 
 struct __declspec(dllimport) U {
   virtual ~U();
 } u;
 
-// RTTI-DAG: @"\01??_7U@@6B@" = available_externally dllimport unnamed_addr constant [1 x i8*] [i8* bitcast ({{.*}} @"\01??_GU@@UAEPAXI@Z" to i8*)]
+// RTTI-DAG: [[VTABLE_U:@.*]] = private unnamed_addr constant [2 x i8*] [i8* bitcast ({{.*}} @"\01??_R4U@@6B@" to i8*), i8* bitcast ({{.*}} @"\01??_GU@@UAEPAXI@Z" to i8*)]
+// RTTI-DAG: @"\01??_SU@@6B@" = unnamed_addr alias i8*, getelementptr inbounds ([2 x i8*], [2 x i8*]* [[VTABLE_U]], i32 0, i32 1)
 
-// NO-RTTI-DAG: @"\01??_7U@@6B@" = available_externally dllimport unnamed_addr constant [1 x i8*] [i8* bitcast ({{.*}} @"\01??_GU@@UAEPAXI@Z" to i8*)]
+// NO-RTTI-DAG: @"\01??_SU@@6B@" = linkonce_odr unnamed_addr constant [1 x i8*] [i8* bitcast ({{.*}} @"\01??_GU@@UAEPAXI@Z" to i8*)]
 
 struct __declspec(dllexport) V {
   virtual ~V();
 } v;
 
 // RTTI-DAG: [[VTABLE_V:@.*]] = private unnamed_addr constant [2 x i8*] [i8* bitcast ({{.*}} @"\01??_R4V@@6B@" to i8*), i8* bitcast ({{.*}} @"\01??_GV@@UAEPAXI@Z" to i8*)], comdat($"\01??_7V@@6B@")
 // RTTI-DAG: @"\01??_7V@@6B@" = dllexport unnamed_addr alias i8*, getelementptr inbounds ([2 x i8*], [2 x i8*]* [[VTABLE_V]], i32 0, i32 1)