Index: llvm/lib/LTO/LTO.cpp
===================================================================
--- llvm/lib/LTO/LTO.cpp
+++ llvm/lib/LTO/LTO.cpp
@@ -1098,42 +1098,45 @@
       ThinLTO.ModuleMap.size());
   StringMap<std::map<GlobalValue::GUID, GlobalValue::LinkageTypes>> ResolvedODR;
 
-  if (Conf.OptLevel > 0) {
+  if (Conf.OptLevel > 0)
     ComputeCrossModuleImport(ThinLTO.CombinedIndex, ModuleToDefinedGVSummaries,
                              ImportLists, ExportLists);
 
-    std::set<GlobalValue::GUID> ExportedGUIDs;
-    for (auto &Res : GlobalResolutions) {
-      // First check if the symbol was flagged as having external references.
-      if (Res.second.Partition != GlobalResolution::External)
-        continue;
-      // IRName will be defined if we have seen the prevailing copy of
-      // this value. If not, no need to mark as exported from a ThinLTO
-      // partition (and we can't get the GUID).
-      if (Res.second.IRName.empty())
-        continue;
-      auto GUID = GlobalValue::getGUID(
-          GlobalValue::dropLLVMManglingEscape(Res.second.IRName));
-      // Mark exported unless index-based analysis determined it to be dead.
-      if (ThinLTO.CombinedIndex.isGUIDLive(GUID))
-        ExportedGUIDs.insert(GUID);
-    }
-
-    // Any functions referenced by the jump table in the regular LTO object must
-    // be exported.
-    for (auto &Def : ThinLTO.CombinedIndex.cfiFunctionDefs())
-      ExportedGUIDs.insert(
-          GlobalValue::getGUID(GlobalValue::dropLLVMManglingEscape(Def)));
-
-    auto isExported = [&](StringRef ModuleIdentifier, GlobalValue::GUID GUID) {
-      const auto &ExportList = ExportLists.find(ModuleIdentifier);
-      return (ExportList != ExportLists.end() &&
-              ExportList->second.count(GUID)) ||
-             ExportedGUIDs.count(GUID);
-    };
-    thinLTOInternalizeAndPromoteInIndex(ThinLTO.CombinedIndex, isExported);
+  // Figure out which symbols need to be internalized. This also needs to happen
+  // at -O0 because summary-based DCE is implemented using internalization, and
+  // we must apply DCE consistently with the full LTO module in order to avoid
+  // undefined references during the final link.
+  std::set<GlobalValue::GUID> ExportedGUIDs;
+  for (auto &Res : GlobalResolutions) {
+    // First check if the symbol was flagged as having external references.
+    if (Res.second.Partition != GlobalResolution::External)
+      continue;
+    // IRName will be defined if we have seen the prevailing copy of
+    // this value. If not, no need to mark as exported from a ThinLTO
+    // partition (and we can't get the GUID).
+    if (Res.second.IRName.empty())
+      continue;
+    auto GUID = GlobalValue::getGUID(
+        GlobalValue::dropLLVMManglingEscape(Res.second.IRName));
+    // Mark exported unless index-based analysis determined it to be dead.
+    if (ThinLTO.CombinedIndex.isGUIDLive(GUID))
+      ExportedGUIDs.insert(GUID);
   }
 
+  // Any functions referenced by the jump table in the regular LTO object must
+  // be exported.
+  for (auto &Def : ThinLTO.CombinedIndex.cfiFunctionDefs())
+    ExportedGUIDs.insert(
+        GlobalValue::getGUID(GlobalValue::dropLLVMManglingEscape(Def)));
+
+  auto isExported = [&](StringRef ModuleIdentifier, GlobalValue::GUID GUID) {
+    const auto &ExportList = ExportLists.find(ModuleIdentifier);
+    return (ExportList != ExportLists.end() &&
+            ExportList->second.count(GUID)) ||
+           ExportedGUIDs.count(GUID);
+  };
+  thinLTOInternalizeAndPromoteInIndex(ThinLTO.CombinedIndex, isExported);
+
   auto isPrevailing = [&](GlobalValue::GUID GUID,
                           const GlobalValueSummary *S) {
     return ThinLTO.PrevailingModuleForGUID[GUID] == S->modulePath();
Index: llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
===================================================================
--- llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
+++ llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -418,6 +418,14 @@
     else if (GlobalExtensionsNotEmpty() || !Extensions.empty())
       MPM.add(createBarrierNoopPass());
 
+    if (PerformThinLTO) {
+      // Drop available_externally and unreferenced globals. This is necessary
+      // with ThinLTO in order to avoid leaving undefined references to dead
+      // globals in the object file.
+      MPM.add(createEliminateAvailableExternallyPass());
+      MPM.add(createGlobalDCEPass());
+    }
+
     addExtensionsToPM(EP_EnabledOnOptLevel0, MPM);
 
     // Rename anon globals to be able to export them in the summary.
Index: llvm/test/LTO/Resolution/X86/Inputs/dead-strip-fulllto.ll
===================================================================
--- llvm/test/LTO/Resolution/X86/Inputs/dead-strip-fulllto.ll
+++ llvm/test/LTO/Resolution/X86/Inputs/dead-strip-fulllto.ll
@@ -14,3 +14,11 @@
 }
 
 declare void @dead2()
+
+define linkonce_odr i8* @odr() {
+  ret i8* bitcast (void ()* @dead3 to i8*)
+}
+
+define internal void @dead3() {
+  ret void
+}
Index: llvm/test/LTO/Resolution/X86/dead-strip-fulllto.ll
===================================================================
--- llvm/test/LTO/Resolution/X86/dead-strip-fulllto.ll
+++ llvm/test/LTO/Resolution/X86/dead-strip-fulllto.ll
@@ -1,11 +1,18 @@
 ; RUN: opt -module-summary -o %t %s
 ; RUN: opt -module-summary -o %t2 %S/Inputs/dead-strip-fulllto.ll
+
 ; RUN: llvm-lto2 run %t -r %t,main,px -r %t,live1,p -r %t,live2,p -r %t,dead2,p \
-; RUN:               %t2 -r %t2,live1,p -r %t2,live2, -r %t2,dead1,p -r %t2,dead2, \
+; RUN:               %t2 -r %t2,live1,p -r %t2,live2, -r %t2,dead1,p -r %t2,dead2, -r %t2,odr, \
 ; RUN: -save-temps -o %t3
 ; RUN: llvm-nm %t3.0 | FileCheck --check-prefix=FULL %s
 ; RUN: llvm-nm %t3.1 | FileCheck --check-prefix=THIN %s
 
+; RUN: llvm-lto2 run %t -r %t,main,px -r %t,live1,p -r %t,live2,p -r %t,dead2,p \
+; RUN:               %t2 -r %t2,live1,p -r %t2,live2, -r %t2,dead1,p -r %t2,dead2, -r %t2,odr, \
+; RUN: -save-temps -o %t3 -O0
+; RUN: llvm-nm %t3.0 | FileCheck --check-prefix=FULL %s
+; RUN: llvm-nm %t3.1 | FileCheck --check-prefix=THIN %s
+
 ; FULL-NOT: dead
 ; FULL: U live1
 ; FULL: T live2
@@ -14,6 +21,7 @@
 ; THIN-NOT: dead
 ; THIN: T live1
 ; THIN: U live2
+; THIN-NOT: odr
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"