Index: lib/LTO/ThinLTOCodeGenerator.cpp
===================================================================
--- lib/LTO/ThinLTOCodeGenerator.cpp
+++ lib/LTO/ThinLTOCodeGenerator.cpp
@@ -123,10 +123,11 @@
   return true;
 }
 
-static GlobalValue::LinkageTypes ResolveODR(const ModuleSummaryIndex &Index,
-                                            StringRef ModuleIdentifier,
-                                            GlobalValue::GUID GUID,
-                                            const GlobalValueSummary &GV) {
+static GlobalValue::LinkageTypes
+ResolveODR(const ModuleSummaryIndex &Index,
+           const FunctionImporter::ExportSetTy &ExportList,
+           StringRef ModuleIdentifier, GlobalValue::GUID GUID,
+           const GlobalValueSummary &GV) {
   auto HasMultipleCopies =
       [&](const GlobalValueInfoList &GVInfo) { return GVInfo.size() > 1; };
 
@@ -147,8 +148,13 @@
     auto &GVInfo = Index.findGlobalValueInfoList(GUID)->second;
     // We need to emit only one of these, the first module will keep
     // it, but turned into a weak while the others will drop it.
-    if (!HasMultipleCopies(GVInfo))
+    if (!HasMultipleCopies(GVInfo)) {
+      // Exported ODR needs to be promoted to not be discarded
+      if (GlobalValue::isDiscardableIfUnused(OriginalLinkage) &&
+          ExportList.count(GUID))
+        return GlobalValue::WeakODRLinkage;
       break;
+    }
     if (IsFirstDefinitionForLinker(GVInfo, Index, ModuleIdentifier))
       return GlobalValue::WeakODRLinkage;
     else
@@ -167,6 +173,7 @@
 /// one copy.
 static void ResolveODR(
     const ModuleSummaryIndex &Index,
+    const FunctionImporter::ExportSetTy &ExportList,
     const std::map<GlobalValue::GUID, GlobalValueSummary *> &DefinedGlobals,
     StringRef ModuleIdentifier,
     DenseMap<GlobalValue::GUID, GlobalValue::LinkageTypes> &ResolvedODR) {
@@ -182,7 +189,8 @@
   for (auto &GV : DefinedGlobals) {
     if (GlobalInvolvedWithAlias.count(GV.second))
       continue;
-    auto NewLinkage = ResolveODR(Index, ModuleIdentifier, GV.first, *GV.second);
+    auto NewLinkage =
+        ResolveODR(Index, ExportList, ModuleIdentifier, GV.first, *GV.second);
     if (NewLinkage != GV.second->linkage()) {
       ResolvedODR[GV.first] = NewLinkage;
     }
@@ -450,17 +458,25 @@
  */
 void ThinLTOCodeGenerator::promote(Module &TheModule,
                                    ModuleSummaryIndex &Index) {
+  auto ModuleCount = Index.modulePaths().size();
   auto ModuleIdentifier = TheModule.getModuleIdentifier();
   // Collect for each module the list of function it defines (GUID -> Summary).
   StringMap<std::map<GlobalValue::GUID, GlobalValueSummary *>>
       ModuleToDefinedGVSummaries;
   Index.collectDefinedFunctionsPerModule(ModuleToDefinedGVSummaries);
 
+  // Generate import/export list
+  StringMap<FunctionImporter::ImportMapTy> ImportLists(ModuleCount);
+  StringMap<FunctionImporter::ExportSetTy> ExportLists(ModuleCount);
+  ComputeCrossModuleImport(Index, ModuleToDefinedGVSummaries, ImportLists,
+                           ExportLists);
+  auto &ExportList = ExportLists[ModuleIdentifier];
+
   // Resolve the LinkOnceODR, trying to turn them into "available_externally"
   // where possible.
   // This is a compile-time optimization.
   DenseMap<GlobalValue::GUID, GlobalValue::LinkageTypes> ResolvedODR;
-  ResolveODR(Index, ModuleToDefinedGVSummaries[ModuleIdentifier],
+  ResolveODR(Index, ExportList, ModuleToDefinedGVSummaries[ModuleIdentifier],
              ModuleIdentifier, ResolvedODR);
   fixupODR(TheModule, ResolvedODR);
 
@@ -573,9 +589,11 @@
         LLVMContext Context;
         Context.setDiscardValueNames(LTODiscardValueNames);
         auto ModuleIdentifier = ModuleBuffer.getBufferIdentifier();
+        auto &ExportList = ExportLists[ModuleIdentifier];
 
         DenseMap<GlobalValue::GUID, GlobalValue::LinkageTypes> ResolvedODR;
-        ResolveODR(*Index, ModuleToDefinedGVSummaries[ModuleIdentifier],
+        ResolveODR(*Index, ExportList,
+                   ModuleToDefinedGVSummaries[ModuleIdentifier],
                    ModuleIdentifier, ResolvedODR);
 
         // Parse module now
Index: lib/Linker/LinkModules.cpp
===================================================================
--- lib/Linker/LinkModules.cpp
+++ lib/Linker/LinkModules.cpp
@@ -429,6 +429,10 @@
 }
 
 void ModuleLinker::addLazyFor(GlobalValue &GV, IRMover::ValueAdder Add) {
+  if (isPerformingImport())
+    // For ThinLTO we don't import more than what was required.
+    return;
+
   // Add these to the internalize list
   if (!GV.hasLinkOnceLinkage())
     return;
Index: test/Transforms/FunctionImport/Inputs/funcimport.ll
===================================================================
--- test/Transforms/FunctionImport/Inputs/funcimport.ll
+++ test/Transforms/FunctionImport/Inputs/funcimport.ll
@@ -99,4 +99,46 @@
   ret void
 }
 
+define void @referencelargelinkonce() #0 {
+entry:
+  call void @linkonceodr()
+  ret void
+}
+
+; A large enough linkonce_odr function that should never be imported
+define linkonce_odr void @linkonceodr() #0 {
+entry:
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  call void @globalfunc2()
+  ret void
+}
+
 
Index: test/Transforms/FunctionImport/funcimport.ll
===================================================================
--- test/Transforms/FunctionImport/funcimport.ll
+++ test/Transforms/FunctionImport/funcimport.ll
@@ -23,6 +23,7 @@
   call void (...) @setfuncptr()
   call void (...) @callfuncptr()
   call void (...) @weakfunc()
+  call void (...) @referencelargelinkonce()
   ret i32 0
 }
 
@@ -78,6 +79,12 @@
 ; CHECK-DAG: %0 = load void ()*, void ()** @P.llvm.
 ; CHECK-DAG: store void ()* @staticfunc2.llvm.{{.*}}, void ()** @P.llvm.
 
+; Ensure that @referencelargelinkonce definition is pulled in, but later we
+; also check that the linkonceodr function is not.
+; CHECK-DAG: define available_externally void @referencelargelinkonce()
+; INSTLIM5-DAG: declare extern_weak void @linkonceodr()
+declare void @referencelargelinkonce(...)
+
 ; Won't import weak func
 ; CHECK-DAG: declare void @weakfunc(...)
 declare void @weakfunc(...) #1