Index: llvm/trunk/lib/Transforms/Utils/CloneFunction.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Utils/CloneFunction.cpp
+++ llvm/trunk/lib/Transforms/Utils/CloneFunction.cpp
@@ -210,6 +210,21 @@
       RemapInstruction(&II, VMap,
                        ModuleLevelChanges ? RF_None : RF_NoModuleLevelChanges,
                        TypeMapper, Materializer);
+
+  // Register all DICompileUnits of the old parent module in the new parent module
+  auto* OldModule = OldFunc->getParent();
+  auto* NewModule = NewFunc->getParent();
+  if (OldModule && NewModule && OldModule != NewModule && DIFinder.compile_unit_count()) {
+    auto* NMD = NewModule->getOrInsertNamedMetadata("llvm.dbg.cu");
+    // Avoid multiple insertions of the same DICompileUnit to NMD.
+    SmallPtrSet<const void*, 8> Visited;
+    for (auto* Operand : NMD->operands())
+      Visited.insert(Operand);
+    for (auto* Unit : DIFinder.compile_units())
+      // VMap.MD()[Unit] == Unit
+      if (Visited.insert(Unit).second)
+        NMD->addOperand(Unit);
+  }
 }
 
 /// Return a copy of the specified function and add it to that function's
Index: llvm/trunk/lib/Transforms/Utils/CloneModule.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Utils/CloneModule.cpp
+++ llvm/trunk/lib/Transforms/Utils/CloneModule.cpp
@@ -181,13 +181,25 @@
   }
 
   // And named metadata....
+  const auto* LLVM_DBG_CU = M.getNamedMetadata("llvm.dbg.cu");
   for (Module::const_named_metadata_iterator I = M.named_metadata_begin(),
                                              E = M.named_metadata_end();
        I != E; ++I) {
     const NamedMDNode &NMD = *I;
     NamedMDNode *NewNMD = New->getOrInsertNamedMetadata(NMD.getName());
-    for (unsigned i = 0, e = NMD.getNumOperands(); i != e; ++i)
-      NewNMD->addOperand(MapMetadata(NMD.getOperand(i), VMap));
+    if (&NMD == LLVM_DBG_CU) {
+      // Do not insert duplicate operands.
+      SmallPtrSet<const void*, 8> Visited;
+      for (const auto* Operand : NewNMD->operands())
+        Visited.insert(Operand);
+      for (const auto* Operand : NMD.operands()) {
+        auto* MappedOperand = MapMetadata(Operand, VMap);
+        if (Visited.insert(MappedOperand).second)
+          NewNMD->addOperand(MappedOperand);
+      }
+    } else
+      for (unsigned i = 0, e = NMD.getNumOperands(); i != e; ++i)
+        NewNMD->addOperand(MapMetadata(NMD.getOperand(i), VMap));
   }
 
   return New;
Index: llvm/trunk/unittests/Transforms/Utils/CloningTest.cpp
===================================================================
--- llvm/trunk/unittests/Transforms/Utils/CloningTest.cpp
+++ llvm/trunk/unittests/Transforms/Utils/CloningTest.cpp
@@ -652,6 +652,62 @@
   }
 }
 
+static int GetDICompileUnitCount(const Module& M) {
+  if (const auto* LLVM_DBG_CU = M.getNamedMetadata("llvm.dbg.cu")) {
+    return LLVM_DBG_CU->getNumOperands();
+  }
+  return 0;
+}
+
+TEST(CloneFunction, CloneFunctionToDifferentModule) {
+  StringRef ImplAssembly = R"(
+    define void @foo() {
+      ret void, !dbg !5
+    }
+
+    !llvm.module.flags = !{!0}
+    !llvm.dbg.cu = !{!2, !6}
+    !0 = !{i32 1, !"Debug Info Version", i32 3}
+    !1 = distinct !DISubprogram(unit: !2)
+    !2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3)
+    !3 = !DIFile(filename: "foo.c", directory: "/tmp")
+    !4 = distinct !DISubprogram(unit: !2)
+    !5 = !DILocation(line: 4, scope: !1)
+    !6 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3)
+  )";
+  StringRef DeclAssembly = R"(
+    declare void @foo()
+  )";
+
+  LLVMContext Context;
+  SMDiagnostic Error;
+
+  auto ImplModule = parseAssemblyString(ImplAssembly, Error, Context);
+  EXPECT_TRUE(ImplModule != nullptr);
+  // DICompileUnits: !2, !6. Only !2 is reachable from @foo().
+  EXPECT_TRUE(GetDICompileUnitCount(*ImplModule) == 2);
+  auto* ImplFunction = ImplModule->getFunction("foo");
+  EXPECT_TRUE(ImplFunction != nullptr);
+
+  auto DeclModule = parseAssemblyString(DeclAssembly, Error, Context);
+  EXPECT_TRUE(DeclModule != nullptr);
+  // No DICompileUnits defined here.
+  EXPECT_TRUE(GetDICompileUnitCount(*DeclModule) == 0);
+  auto* DeclFunction = DeclModule->getFunction("foo");
+  EXPECT_TRUE(DeclFunction != nullptr);
+
+  ValueToValueMapTy VMap;
+  VMap[ImplFunction] = DeclFunction;
+  // No args to map
+  SmallVector<ReturnInst*, 8> Returns;
+  CloneFunctionInto(DeclFunction, ImplFunction, VMap, true, Returns);
+
+  EXPECT_FALSE(verifyModule(*ImplModule, &errs()));
+  EXPECT_FALSE(verifyModule(*DeclModule, &errs()));
+  // DICompileUnit !2 shall be inserted into DeclModule.
+  EXPECT_TRUE(GetDICompileUnitCount(*DeclModule) == 1);
+}
+
 class CloneModule : public ::testing::Test {
 protected:
   void SetUp() override {