Index: include/llvm/LTO/LTO.h
===================================================================
--- include/llvm/LTO/LTO.h
+++ include/llvm/LTO/LTO.h
@@ -59,7 +59,9 @@
 /// must apply the changes to the Module via thinLTOInternalizeModule.
 void thinLTOInternalizeAndPromoteInIndex(
     ModuleSummaryIndex &Index,
-    function_ref<bool(StringRef, GlobalValue::GUID)> isExported);
+    function_ref<bool(StringRef, GlobalValue::GUID)> isExported,
+    function_ref<bool(GlobalValue::GUID, const GlobalValueSummary *)>
+        isPrevailing);
 
 namespace lto {
 
Index: lib/LTO/LTO.cpp
===================================================================
--- lib/LTO/LTO.cpp
+++ lib/LTO/LTO.cpp
@@ -328,8 +328,12 @@
 
 static void thinLTOInternalizeAndPromoteGUID(
     GlobalValueSummaryList &GVSummaryList, GlobalValue::GUID GUID,
-    function_ref<bool(StringRef, GlobalValue::GUID)> isExported) {
+    function_ref<bool(StringRef, GlobalValue::GUID)> isExported,
+    function_ref<bool(GlobalValue::GUID, const GlobalValueSummary *)>
+        isPrevailing) {
   for (auto &S : GVSummaryList) {
+    if (!isPrevailing(GUID, S.get()))
+      continue;
     if (isExported(S->modulePath(), GUID)) {
       if (GlobalValue::isLocalLinkage(S->linkage()))
         S->setLinkage(GlobalValue::ExternalLinkage);
@@ -342,9 +346,12 @@
 // as external and non-exported values as internal.
 void llvm::thinLTOInternalizeAndPromoteInIndex(
     ModuleSummaryIndex &Index,
-    function_ref<bool(StringRef, GlobalValue::GUID)> isExported) {
+    function_ref<bool(StringRef, GlobalValue::GUID)> isExported,
+    function_ref<bool(GlobalValue::GUID, const GlobalValueSummary *)>
+        isPrevailing) {
   for (auto &I : Index)
-    thinLTOInternalizeAndPromoteGUID(I.second.SummaryList, I.first, isExported);
+    thinLTOInternalizeAndPromoteGUID(I.second.SummaryList, I.first, isExported,
+                                     isPrevailing);
 }
 
 // Requires a destructor for std::vector<InputModule>.
@@ -1139,12 +1146,12 @@
             ExportList->second.count(GUID)) ||
            ExportedGUIDs.count(GUID);
   };
-  thinLTOInternalizeAndPromoteInIndex(ThinLTO.CombinedIndex, isExported);
-
-  auto isPrevailing = [&](GlobalValue::GUID GUID,
-                          const GlobalValueSummary *S) {
+  auto isPrevailing = [&](GlobalValue::GUID GUID, const GlobalValueSummary *S) {
     return ThinLTO.PrevailingModuleForGUID[GUID] == S->modulePath();
   };
+
+  thinLTOInternalizeAndPromoteInIndex(ThinLTO.CombinedIndex, isExported,
+                                      isPrevailing);
   auto recordNewLinkage = [&](StringRef ModuleIdentifier,
                               GlobalValue::GUID GUID,
                               GlobalValue::LinkageTypes NewLinkage) {
Index: lib/LTO/ThinLTOCodeGenerator.cpp
===================================================================
--- lib/LTO/ThinLTOCodeGenerator.cpp
+++ lib/LTO/ThinLTOCodeGenerator.cpp
@@ -607,6 +607,30 @@
   return CombinedIndex;
 }
 
+static void internalizeAndPromoteInIndex(
+    const StringMap<FunctionImporter::ExportSetTy> &ExportLists,
+    const DenseSet<GlobalValue::GUID> &GUIDPreservedSymbols,
+    ModuleSummaryIndex &Index) {
+  auto isExported = [&](StringRef ModuleIdentifier, GlobalValue::GUID GUID) {
+    const auto &ExportList = ExportLists.find(ModuleIdentifier);
+    return (ExportList != ExportLists.end() &&
+            ExportList->second.count(GUID)) ||
+           GUIDPreservedSymbols.count(GUID);
+  };
+
+  DenseMap<GlobalValue::GUID, const GlobalValueSummary *> PrevailingCopy;
+  computePrevailingCopies(Index, PrevailingCopy);
+  auto isPrevailing = [&](GlobalValue::GUID GUID, const GlobalValueSummary *S) {
+    const auto &Prevailing = PrevailingCopy.find(GUID);
+    // Not in map means that there was only one copy, which must be prevailing.
+    if (Prevailing == PrevailingCopy.end())
+      return true;
+    return Prevailing->second == S;
+  };
+
+  thinLTOInternalizeAndPromoteInIndex(Index, isExported, isPrevailing);
+}
+
 /**
  * Perform promotion and renaming of exported internal functions.
  * Index is updated to reflect linkage changes from weak resolution.
@@ -642,13 +666,7 @@
 
   // Promote the exported values in the index, so that they are promoted
   // in the module.
-  auto isExported = [&](StringRef ModuleIdentifier, GlobalValue::GUID GUID) {
-    const auto &ExportList = ExportLists.find(ModuleIdentifier);
-    return (ExportList != ExportLists.end() &&
-            ExportList->second.count(GUID)) ||
-           GUIDPreservedSymbols.count(GUID);
-  };
-  thinLTOInternalizeAndPromoteInIndex(Index, isExported);
+  internalizeAndPromoteInIndex(ExportLists, GUIDPreservedSymbols, Index);
 
   promoteModule(TheModule, Index);
 }
@@ -762,13 +780,7 @@
     return;
 
   // Internalization
-  auto isExported = [&](StringRef ModuleIdentifier, GlobalValue::GUID GUID) {
-    const auto &ExportList = ExportLists.find(ModuleIdentifier);
-    return (ExportList != ExportLists.end() &&
-            ExportList->second.count(GUID)) ||
-           GUIDPreservedSymbols.count(GUID);
-  };
-  thinLTOInternalizeAndPromoteInIndex(Index, isExported);
+  internalizeAndPromoteInIndex(ExportLists, GUIDPreservedSymbols, Index);
   thinLTOInternalizeModule(TheModule,
                            ModuleToDefinedGVSummaries[ModuleIdentifier]);
 }
@@ -918,17 +930,10 @@
   // impacts the caching.
   resolveWeakForLinkerInIndex(*Index, ResolvedODR);
 
-  auto isExported = [&](StringRef ModuleIdentifier, GlobalValue::GUID GUID) {
-    const auto &ExportList = ExportLists.find(ModuleIdentifier);
-    return (ExportList != ExportLists.end() &&
-            ExportList->second.count(GUID)) ||
-           GUIDPreservedSymbols.count(GUID);
-  };
-
   // Use global summary-based analysis to identify symbols that can be
   // internalized (because they aren't exported or preserved as per callback).
   // Changes are made in the index, consumed in the ThinLTO backends.
-  thinLTOInternalizeAndPromoteInIndex(*Index, isExported);
+  internalizeAndPromoteInIndex(ExportLists, GUIDPreservedSymbols, *Index);
 
   // Make sure that every module has an entry in the ExportLists and
   // ResolvedODR maps to enable threaded access to these maps below.
Index: test/LTO/Resolution/X86/not-prevailing.ll
===================================================================
--- test/LTO/Resolution/X86/not-prevailing.ll
+++ test/LTO/Resolution/X86/not-prevailing.ll
@@ -0,0 +1,25 @@
+; RUN: opt -module-summary %s -o %t.o
+; RUN: llvm-lto2 run -o %t2.o %t.o -r %t.o,foo,x -r %t.o,zed,px -save-temps
+; RUN: llvm-objdump -d %t2.o.1 | FileCheck %s
+; RUN: llvm-readelf --symbols %t2.o.1 | FileCheck %s --check-prefix=SYMBOLS
+
+; Check that 'foo' was not inlined.
+; CHECK:      Disassembly of section .text:
+; CHECK-NEXT:  zed:
+; CHECK-NEXT:  0: {{.*}}  jmp  0 <zed+0x5>
+
+; Check that 'foo' produced as undefined.
+; SYMBOLS: NOTYPE  GLOBAL DEFAULT  UND foo
+; SYMBOLS: FUNC    GLOBAL DEFAULT    2 zed
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define weak i32 @foo() local_unnamed_addr {
+  ret i32 65
+}
+
+define i32 @zed() local_unnamed_addr {
+  %1 = tail call i32 @foo() #1
+  ret i32 %1
+}