Index: include/llvm/Linker/IRMover.h
===================================================================
--- include/llvm/Linker/IRMover.h
+++ include/llvm/Linker/IRMover.h
@@ -63,7 +63,8 @@
   bool move(Module &Src, ArrayRef<GlobalValue *> ValuesToLink,
             std::function<void(GlobalValue &GV, ValueAdder Add)> AddLazyFor,
             DenseMap<unsigned, MDNode *> *ValIDToTempMDMap = nullptr,
-            bool IsMetadataLinkingPostpass = false);
+            bool IsMetadataLinkingPostpass = false,
+            bool IsFunctionImport = false);
   Module &getModule() { return Composite; }
 
 private:
Index: lib/Linker/IRMover.cpp
===================================================================
--- lib/Linker/IRMover.cpp
+++ lib/Linker/IRMover.cpp
@@ -408,6 +408,11 @@
   /// importing).
   bool IsMetadataLinkingPostpass;
 
+  /// Flag indicating that this is a module with a function being
+  /// imported for ThinLTO. Used to control the amount of metadata
+  /// linked in.
+  bool IsFunctionImport;
+
   /// Flags to pass to value mapper invocations.
   RemapFlags ValueMapperFlags = RF_MoveDistinctMDs;
 
@@ -510,11 +515,12 @@
            ArrayRef<GlobalValue *> ValuesToLink,
            std::function<void(GlobalValue &, IRMover::ValueAdder)> AddLazyFor,
            DenseMap<unsigned, MDNode *> *ValIDToTempMDMap = nullptr,
-           bool IsMetadataLinkingPostpass = false)
+           bool IsMetadataLinkingPostpass = false,
+           bool IsFunctionImport = false)
       : DstM(DstM), SrcM(SrcM), AddLazyFor(AddLazyFor), TypeMap(Set),
         GValMaterializer(this), LValMaterializer(this),
         IsMetadataLinkingPostpass(IsMetadataLinkingPostpass),
-        ValIDToTempMDMap(ValIDToTempMDMap) {
+        IsFunctionImport(IsFunctionImport), ValIDToTempMDMap(ValIDToTempMDMap) {
     for (GlobalValue *GV : ValuesToLink)
       maybeAdd(GV);
 
@@ -1271,11 +1277,45 @@
 void IRLinker::linkNamedMDNodes() {
   findNeededSubprograms();
   const NamedMDNode *SrcModFlags = SrcM.getModuleFlagsMetadata();
+  NamedMDNode *CompileUnits = SrcM.getNamedMetadata("llvm.dbg.cu");
   for (const NamedMDNode &NMD : SrcM.named_metadata()) {
     // Don't link module flags here. Do them separately.
     if (&NMD == SrcModFlags)
       continue;
     NamedMDNode *DestNMD = DstM.getOrInsertNamedMetadata(NMD.getName());
+
+    // Check if we are importing a function from the source module for
+    // ThinLTO. In that case we only need to map in metadata for imported
+    // functions, and don't need to link in the enums, types and other
+    // metadata hanging off the DICompileUnit. Create a simple DICompileUnit
+    // with just the needed DISubprograms linked, as well as the DIFile which
+    // is required.
+    if (&NMD == CompileUnits && IsFunctionImport) {
+      for (unsigned I = 0, E = CompileUnits->getNumOperands(); I != E; ++I) {
+        auto *CU = cast<DICompileUnit>(CompileUnits->getOperand(I));
+        assert(CU && "Expected valid compile unit");
+        // Map just the DIFile and DISubprogram operands.
+        auto *NewFile =
+            MapMetadata(CU->getFile(), ValueMap,
+                        ValueMapperFlags | RF_NullMapMissingGlobalValues,
+                        &TypeMap, &GValMaterializer);
+        auto *NewSubprograms =
+            MapMetadata(CU->getSubprograms().get(), ValueMap,
+                        ValueMapperFlags | RF_NullMapMissingGlobalValues,
+                        &TypeMap, &GValMaterializer);
+        // Create the simple DICompileUnit to hold the necessary
+        // mapped operands (DISubprogram and DIFile) and other simple
+        // non-metadata operands.
+        auto *NewCU = DICompileUnit::getDistinct(
+            CU->getContext(), CU->getSourceLanguage(), NewFile, nullptr,
+            CU->isOptimized(), nullptr, CU->getRuntimeVersion(), nullptr,
+            CU->getEmissionKind(), nullptr, nullptr, NewSubprograms, nullptr,
+            nullptr, nullptr, CU->getDWOId());
+        DestNMD->addOperand(NewCU);
+      }
+      continue;
+    }
+
     // Add Src elements into Dest node.
     for (const MDNode *op : NMD.operands())
       DestNMD->addOperand(MapMetadata(
@@ -1671,9 +1711,10 @@
     Module &Src, ArrayRef<GlobalValue *> ValuesToLink,
     std::function<void(GlobalValue &, ValueAdder Add)> AddLazyFor,
     DenseMap<unsigned, MDNode *> *ValIDToTempMDMap,
-    bool IsMetadataLinkingPostpass) {
+    bool IsMetadataLinkingPostpass, bool IsFunctionImport) {
   IRLinker TheIRLinker(Composite, IdentifiedStructTypes, Src, ValuesToLink,
-                       AddLazyFor, ValIDToTempMDMap, IsMetadataLinkingPostpass);
+                       AddLazyFor, ValIDToTempMDMap, IsMetadataLinkingPostpass,
+                       IsFunctionImport);
   bool RetCode = TheIRLinker.run();
   Composite.dropTriviallyDeadConstantArrays();
   return RetCode;
Index: lib/Linker/LinkModules.cpp
===================================================================
--- lib/Linker/LinkModules.cpp
+++ lib/Linker/LinkModules.cpp
@@ -806,7 +806,7 @@
                  [this](GlobalValue &GV, IRMover::ValueAdder Add) {
                    addLazyFor(GV, Add);
                  },
-                 ValIDToTempMDMap, false))
+                 ValIDToTempMDMap, false, isPerformingImport()))
     return true;
   Module &DstM = Mover.getModule();
   for (auto &P : Internalize) {
@@ -839,7 +839,7 @@
   if (Mover.move(
           Src, ValuesToLink.getArrayRef(),
           [this](GlobalValue &GV, IRMover::ValueAdder Add) { assert(false); },
-          ValIDToTempMDMap, true))
+          ValIDToTempMDMap, true, true))
     return true;
   return false;
 }
Index: test/Linker/thinlto_funcimport_debug.ll
===================================================================
--- test/Linker/thinlto_funcimport_debug.ll
+++ test/Linker/thinlto_funcimport_debug.ll
@@ -13,6 +13,7 @@
 ; that neither contains null.
 ; CHECK: !{{[0-9]+}} = distinct !DICompileUnit({{.*}} subprograms: ![[SPs1:[0-9]+]]
 ; CHECK-NOT: ![[SPs1]] = !{{{.*}}null{{.*}}}
+; CHECK-NOT: distinct !DICompileUnit({{.*}} enums:
 ; CHECK: !{{[0-9]+}} = distinct !DICompileUnit({{.*}} subprograms: ![[SPs2:[0-9]+]]
 ; CHECK-NOT: ![[SPs2]] = !{{{.*}}null{{.*}}}
 
Index: test/Transforms/FunctionImport/funcimport_debug.ll
===================================================================
--- test/Transforms/FunctionImport/funcimport_debug.ll
+++ test/Transforms/FunctionImport/funcimport_debug.ll
@@ -14,6 +14,7 @@
 ; CHECK: !{{[0-9]+}} = distinct !DICompileUnit({{.*}} subprograms: ![[SPs1:[0-9]+]]
 ; CHECK: ![[SPs1]] = !{![[MAINSP:[0-9]+]]}
 ; CHECK: ![[MAINSP]] = distinct !DISubprogram(name: "main"
+; CHECK-NOT: distinct !DICompileUnit({{.*}} enums:
 ; CHECK: !{{[0-9]+}} = distinct !DICompileUnit({{.*}} subprograms: ![[SPs2:[0-9]+]]
 ; CHECK-NOT: ![[SPs2]] = !{{{.*}}null{{.*}}}
 ; CHECK: ![[SPs2]] = !{![[FUNCSP:[0-9]+]]}