1 files changed, 43 insertions, 0 deletions
diff --git a/clang/lib/CIR/CodeGen/CIRGenVTables.cpp b/clang/lib/CIR/CodeGen/CIRGenVTables.cpp
index af8f5ae..94d856b 100644
--- a/clang/lib/CIR/CodeGen/CIRGenVTables.cpp
+++ b/clang/lib/CIR/CodeGen/CIRGenVTables.cpp
@@ -47,6 +47,49 @@ cir::RecordType CIRGenVTables::getVTableType(const VTableLayout &layout) {
   return cgm.getBuilder().getAnonRecordTy(tys, /*incomplete=*/false);
 }
 
+/// At this point in the translation unit, does it appear that can we
+/// rely on the vtable being defined elsewhere in the program?
+///
+/// The response is really only definitive when called at the end of
+/// the translation unit.
+///
+/// The only semantic restriction here is that the object file should
+/// not contain a vtable definition when that vtable is defined
+/// strongly elsewhere.  Otherwise, we'd just like to avoid emitting
+/// vtables when unnecessary.
+/// TODO(cir): this should be merged into common AST helper for codegen.
+bool CIRGenVTables::isVTableExternal(const CXXRecordDecl *rd) {
+  assert(rd->isDynamicClass() && "Non-dynamic classes have no VTable.");
+
+  // We always synthesize vtables if they are needed in the MS ABI. MSVC doesn't
+  // emit them even if there is an explicit template instantiation.
+  if (cgm.getTarget().getCXXABI().isMicrosoft())
+    return false;
+
+  // If we have an explicit instantiation declaration (and not a
+  // definition), the vtable is defined elsewhere.
+  TemplateSpecializationKind tsk = rd->getTemplateSpecializationKind();
+  if (tsk == TSK_ExplicitInstantiationDeclaration)
+    return true;
+
+  // Otherwise, if the class is an instantiated template, the
+  // vtable must be defined here.
+  if (tsk == TSK_ImplicitInstantiation ||
+      tsk == TSK_ExplicitInstantiationDefinition)
+    return false;
+
+  // Otherwise, if the class doesn't have a key function (possibly
+  // anymore), the vtable must be defined here.
+  const CXXMethodDecl *keyFunction =
+      cgm.getASTContext().getCurrentKeyFunction(rd);
+  if (!keyFunction)
+    return false;
+
+  // Otherwise, if we don't have a definition of the key function, the
+  // vtable must be defined somewhere else.
+  return !keyFunction->hasBody();
+}
+
 /// This is a callback from Sema to tell us that a particular vtable is
 /// required to be emitted in this translation unit.
 ///