[C++20] [Modules] [Itanium ABI] Generate the vtable in the module unit

of dynamic classes

Close llvm#70585 and reflect
itanium-cxx-abi/cxx-abi#170.

The significant change of the patch is: for dynamic classes attached to
module units, we generate the vtable to the attached module units
directly and the key functions for such classes is meaningless.

clang/lib/CodeGen/CGVTables.cpp

@@ -1046,6 +1046,11 @@ CodeGenModule::getVTableLinkage(const CXXRecordDecl *RD) {
   if (!RD->isExternallyVisible())
     return llvm::GlobalVariable::InternalLinkage;
 
+  // V-tables for non-template classes with an owning module are always
+  // uniquely emitted in that module.
+  if (Module *M = RD->getOwningModule(); M && M->isNamedModule())
+    return llvm::GlobalVariable::ExternalLinkage;
+
   // We're at the end of the translation unit, so the current key
   // function is fully correct.
   const CXXMethodDecl *keyFunction = Context.getCurrentKeyFunction(RD);
@@ -1180,6 +1185,21 @@ bool CodeGenVTables::isVTableExternal(const CXXRecordDecl *RD) {
       TSK == TSK_ExplicitInstantiationDefinition)
     return false;
 
+  // Itanium C++ ABI [5.2.3]:
+  // Virtual tables for dynamic classes are emitted as follows:
+  //
+  // - If the class is templated, the tables are emitted in every object that
+  // references any of them.
+  // - Otherwise, if the class is attached to a module, the tables are uniquely
+  // emitted in the object for the module unit in which it is defined.
+  // - Otherwise, if the class has a key function (see below), the tables are
+  // emitted in the object for the translation unit containing the definition of
+  // the key function. This is unique if the key function is not inline.
+  // - Otherwise, the tables are emitted in every object that references any of
+  // them.
+  if (Module *M = RD->getOwningModule(); M && M->isNamedModule())
+    return M != CGM.getContext().getCurrentNamedModule();
+
   // Otherwise, if the class doesn't have a key function (possibly
   // anymore), the vtable must be defined here.
   const CXXMethodDecl *keyFunction = CGM.getContext().getCurrentKeyFunction(RD);
@@ -1189,13 +1209,7 @@ bool CodeGenVTables::isVTableExternal(const CXXRecordDecl *RD) {
   const FunctionDecl *Def;
   // Otherwise, if we don't have a definition of the key function, the
   // vtable must be defined somewhere else.
-  if (!keyFunction->hasBody(Def))
-    return true;
-
-  assert(Def && "The body of the key function is not assigned to Def?");
-  // If the non-inline key function comes from another module unit, the vtable
-  // must be defined there.
-  return Def->isInAnotherModuleUnit() && !Def->isInlineSpecified();
+  return !keyFunction->hasBody(Def);
 }
 
 /// Given that we're currently at the end of the translation unit, and

clang/lib/CodeGen/CodeGenModule.cpp

@@ -6738,6 +6738,15 @@ void CodeGenModule::EmitTopLevelDecl(Decl *D) {
         if (ES->hasExternalDefinitions(D) == ExternalASTSource::EK_Never)
           DI->completeUnusedClass(*CRD);
     }
+    // If we're emitting a dynamic class from the importable module we're
+    // emitting, we always need to emit the virtual table according to the ABI
+    // requirement.
+    if (CRD->getOwningModule() &&
+        CRD->getOwningModule()->isInterfaceOrPartition() &&
+        CRD->getDefinition() && CRD->isDynamicClass() &&
+        CRD->getOwningModule() == getContext().getCurrentNamedModule())
+      EmitVTable(CRD);
+
     // Emit any static data members, they may be definitions.
     for (auto *I : CRD->decls())
       if (isa<VarDecl>(I) || isa<CXXRecordDecl>(I))

clang/lib/CodeGen/ItaniumCXXABI.cpp

@@ -1801,6 +1801,12 @@ void ItaniumCXXABI::emitVTableDefinitions(CodeGenVTables &CGVT,
   if (VTable->hasInitializer())
     return;
 
+  // If the class is attached to a C++ named module other than the one
+  // we're currently compiling, the vtable should be defined there.
+  if (Module *M = RD->getOwningModule();
+      M && M->isNamedModule() && M != CGM.getContext().getCurrentNamedModule())
+    return;
+
   ItaniumVTableContext &VTContext = CGM.getItaniumVTableContext();
   const VTableLayout &VTLayout = VTContext.getVTableLayout(RD);
   llvm::GlobalVariable::LinkageTypes Linkage = CGM.getVTableLinkage(RD);

clang/lib/Sema/SemaDecl.cpp

@@ -18127,6 +18127,15 @@ void Sema::ActOnTagFinishDefinition(Scope *S, Decl *TagD,
       if (NumInitMethods > 1 || !Def->hasInitMethod())
         Diag(RD->getLocation(), diag::err_sycl_special_type_num_init_method);
     }
+
+    // If we're defining a dynamic class in a module interface unit, we always
+    // need to produce the vtable for it even if the vtable is not used in the
+    // current TU.
+    //
+    // The case that the current class is not dynamic is handled in
+    // MarkVTableUsed.
+    if (getCurrentModule() && getCurrentModule()->isInterfaceOrPartition())
+      MarkVTableUsed(RD->getLocation(), RD, /*DefinitionRequired=*/true);
   }
 
   // Exit this scope of this tag's definition.

clang/lib/Sema/SemaDeclCXX.cpp

@@ -18697,11 +18697,16 @@ bool Sema::DefineUsedVTables() {
 
     bool DefineVTable = true;
 
-    // If this class has a key function, but that key function is
-    // defined in another translation unit, we don't need to emit the
-    // vtable even though we're using it.
     const CXXMethodDecl *KeyFunction = Context.getCurrentKeyFunction(Class);
-    if (KeyFunction && !KeyFunction->hasBody()) {
+    // V-tables for non-template classes with an owning module are always
+    // uniquely emitted in that module.
+    if (Class->getOwningModule() &&
+        Class->getOwningModule()->isInterfaceOrPartition())
+      DefineVTable = true;
+    else if (KeyFunction && !KeyFunction->hasBody()) {
+      // If this class has a key function, but that key function is
+      // defined in another translation unit, we don't need to emit the
+      // vtable even though we're using it.
       // The key function is in another translation unit.
       DefineVTable = false;
       TemplateSpecializationKind TSK =

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -2253,7 +2253,10 @@ ASTDeclReader::VisitCXXRecordDeclImpl(CXXRecordDecl *D) {
 
   // Lazily load the key function to avoid deserializing every method so we can
   // compute it.
-  if (WasDefinition) {
+  //
+  // The key function in named module is meaningless.
+  if (WasDefinition && (!D->getOwningModule() ||
+                        !D->getOwningModule()->isInterfaceOrPartition())) {
     DeclID KeyFn = readDeclID();
     if (KeyFn && D->isCompleteDefinition())
       // FIXME: This is wrong for the ARM ABI, where some other module may have
@@ -3212,6 +3215,14 @@ static bool isConsumerInterestedIn(ASTContext &Ctx, Decl *D, bool HasBody) {
     if (ES->hasExternalDefinitions(D) == ExternalASTSource::EK_Never)
       return true;
 
+  // The dynamic class defined in a named module is interesting.
+  // The code generator needs to emit its vtable there.
+  if (const auto *Class = dyn_cast<CXXRecordDecl>(D))
+    return Class->getOwningModule() &&
+           Class->getOwningModule()->isInterfaceOrPartition() &&
+           Class->getOwningModule() == Ctx.getCurrentNamedModule() &&
+           Class->getDefinition() && Class->isDynamicClass();
+
   return false;
 }
 

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -1483,10 +1483,15 @@ void ASTDeclWriter::VisitCXXRecordDecl(CXXRecordDecl *D) {
   if (D->isThisDeclarationADefinition())
     Record.AddCXXDefinitionData(D);
 
-  // Store (what we currently believe to be) the key function to avoid
-  // deserializing every method so we can compute it.
-  if (D->isCompleteDefinition())
-    Record.AddDeclRef(Context.getCurrentKeyFunction(D));
+  if (D->isCompleteDefinition()) {
+    if (D->getOwningModule() && D->getOwningModule()->isInterfaceOrPartition())
+      Writer.ModularCodegenDecls.push_back(Writer.GetDeclRef(D));
+    else {
+      // Store (what we currently believe to be) the key function to avoid
+      // deserializing every method so we can compute it.
+      Record.AddDeclRef(Context.getCurrentKeyFunction(D));
+    }
+  }
 
   Code = serialization::DECL_CXX_RECORD;
 }

clang/test/CodeGenCXX/modules-vtable.cppm

@@ -24,6 +24,8 @@
 // RUN:     %t/M-A.cppm -o %t/M-A.pcm
 // RUN: %clang_cc1 -triple %itanium_abi_triple -std=c++20 -fmodule-file=M:A=%t/M-A.pcm \
 // RUN:     %t/M-B.cppm  -emit-llvm -o - | FileCheck %t/M-B.cppm
+// RUN: %clang_cc1 -triple %itanium_abi_triple -std=c++20 \
+// RUN:     %t/M-A.pcm  -emit-llvm -o - | FileCheck %t/M-A.cppm
 
 //--- Mod.cppm
 export module Mod;
@@ -41,9 +43,10 @@ Base::~Base() {}
 // CHECK: @_ZTSW3Mod4Base = constant
 // CHECK: @_ZTIW3Mod4Base = constant
 
-// CHECK-INLINE: @_ZTVW3Mod4Base = linkonce_odr {{.*}}unnamed_addr constant
-// CHECK-INLINE: @_ZTSW3Mod4Base = linkonce_odr {{.*}}constant
-// CHECK-INLINE: @_ZTIW3Mod4Base = linkonce_odr {{.*}}constant
+// With the new Itanium C++ ABI, the linkage of vtables in modules don't need to be linkonce ODR.
+// CHECK-INLINE: @_ZTVW3Mod4Base = {{.*}}unnamed_addr constant
+// CHECK-INLINE: @_ZTSW3Mod4Base = {{.*}}constant
+// CHECK-INLINE: @_ZTIW3Mod4Base = {{.*}}constant
 
 module :private;
 int private_use() {
@@ -60,11 +63,11 @@ int use() {
 
 // CHECK-NOT: @_ZTSW3Mod4Base = constant
 // CHECK-NOT: @_ZTIW3Mod4Base = constant
-// CHECK: @_ZTVW3Mod4Base = external unnamed_addr
+// CHECK: @_ZTVW3Mod4Base = external
 
-// CHECK-INLINE: @_ZTVW3Mod4Base = linkonce_odr {{.*}}unnamed_addr constant
-// CHECK-INLINE: @_ZTSW3Mod4Base = linkonce_odr {{.*}}constant
-// CHECK-INLINE: @_ZTIW3Mod4Base = linkonce_odr {{.*}}constant
+// CHECK-INLINE-NOT: @_ZTSW3Mod4Base = constant
+// CHECK-INLINE-NOT: @_ZTIW3Mod4Base = constant
+// CHECK-INLINE: @_ZTVW3Mod4Base = external
 
 // Check the case that the declaration of the key function comes from another
 // module unit but the definition of the key function comes from the current
@@ -82,6 +85,10 @@ int a_use() {
     return 43;
 }
 
+// CHECK: @_ZTVW1M1C = unnamed_addr constant
+// CHECK: @_ZTSW1M1C = constant
+// CHECK: @_ZTIW1M1C = constant
+
 //--- M-B.cppm
 export module M:B;
 import :A;
@@ -93,6 +100,6 @@ int b_use() {
     return 43;
 }
 
-// CHECK: @_ZTVW1M1C = unnamed_addr constant
-// CHECK: @_ZTSW1M1C = constant
-// CHECK: @_ZTIW1M1C = constant
+// CHECK: @_ZTVW1M1C = external
+// CHECK-NOT: @_ZTSW1M1C = constant
+// CHECK-NOT: @_ZTIW1M1C = constant

clang/test/CodeGenCXX/pr70585.cppm

@@ -0,0 +1,47 @@
+// REQUIRES: !system-windows
+
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: cd %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/layer1.cppm -triple %itanium_abi_triple \
+// RUN:     -emit-module-interface -o %t/foo-layer1.pcm
+// RUN: %clang_cc1 -std=c++20 %t/layer2.cppm -triple %itanium_abi_triple  \
+// RUN:     -emit-module-interface -fmodule-file=foo:layer1=%t/foo-layer1.pcm \
+// RUN:     -o %t/foo-layer2.pcm
+// RUN: %clang_cc1 -std=c++20 %t/foo-layer1.pcm -S -emit-llvm -o - | FileCheck %t/layer1.cppm
+// RUN: %clang_cc1 -std=c++20 %t/foo-layer2.pcm -S -emit-llvm -o - \
+// RUN:     -fmodule-file=foo:layer1=%t/foo-layer1.pcm | FileCheck %t/layer2.cppm
+//
+// Check the case about emitting object files from sources directly.
+// RUN: %clang_cc1 -std=c++20 %t/layer1.cppm -triple %itanium_abi_triple \
+// RUN:     -S -emit-llvm -o - | FileCheck %t/layer1.cppm
+// RUN: %clang_cc1 -std=c++20 %t/layer2.cppm -triple %itanium_abi_triple -S -emit-llvm  \
+// RUN:     -fmodule-file=foo:layer1=%t/foo-layer1.pcm -o - | FileCheck %t/layer2.cppm
+
+//--- layer1.cppm
+export module foo:layer1;
+struct Fruit {
+    virtual ~Fruit() = default;
+    virtual void eval();
+};
+
+// CHECK-DAG: @_ZTVW3foo5Fruit = unnamed_addr constant
+// CHECK-DAG: @_ZTSW3foo5Fruit = constant
+// CHECK-DAG: @_ZTIW3foo5Fruit = constant
+
+// Testing that:
+// (1) The use of virtual functions won't produce the vtable.
+// (2) The definition of key functions won't produce the vtable.
+//
+//--- layer2.cppm
+export module foo:layer2;
+import :layer1;
+export void layer2_fun() {
+    Fruit *b = new Fruit();
+    b->eval();
+}
+void Fruit::eval() {}
+// CHECK: @_ZTVW3foo5Fruit = external unnamed_addr constant
+// CHECK-NOT: @_ZTSW3foo5Fruit
+// CHECK-NOT: @_ZTIW3foo5Fruit