[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 +1046,15 @@ CodeGenModule::getVTableLinkage(const CXXRecordDecl *RD) {
   if (!RD->isExternallyVisible())
     return llvm::GlobalVariable::InternalLinkage;
 
+  // Previously we'll decide the linkage of the vtable by the linkage
+  // of the key function. But within modules, the concept of key functions
+  // becomes meaningless. So the linkage of the vtable should always be
+  // external if the class is externally visible.
+  //
+  // TODO: How about the case of AppleKext, DLLExportAttr and DLLImportAttr.
+  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 +1189,21 @@ 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 +1213,7 @@ 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/ItaniumCXXABI.cpp

@@ -1801,6 +1801,12 @@ void ItaniumCXXABI::emitVTableDefinitions(CodeGenVTables &CGVT,
   if (VTable->hasInitializer())
     return;
 
+  // If the class are 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/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 +1,54 @@
+// 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
+
+//--- layer1.cppm
+export module foo:layer1;
+struct Fruit {
+    virtual ~Fruit() = default;
+    virtual void eval() = 0;
+};
+struct Banana : public Fruit {
+    Banana() {}
+    void eval() override;
+};
+
+// CHECK-DAG: @_ZTVW3foo6Banana = unnamed_addr constant
+// CHECK-DAG: @_ZTSW3foo6Banana = constant
+// CHECK-DAG: @_ZTIW3foo6Banana = constant
+//
+// 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() {
+    Banana *b = new Banana();
+    b->eval();
+}
+void Banana::eval() {
+}
+
+// CHECK-NOT: @_ZTVW3foo6Banana
+// CHECK-NOT: @_ZTSW3foo6Banana
+// CHECK-NOT: @_ZTIW3foo6Banana
+// CHECK-NOT: @_ZTVW3foo5Fruit
+// CHECK-NOT: @_ZTSW3foo5Fruit
+// CHECK-NOT: @_ZTIW3foo5Fruit