Index: include/llvm/Transforms/IPO/FunctionImport.h
===================================================================
--- include/llvm/Transforms/IPO/FunctionImport.h
+++ include/llvm/Transforms/IPO/FunctionImport.h
@@ -70,6 +70,14 @@
     const ModuleSummaryIndex &Index,
     StringMap<FunctionImporter::ImportMapTy> &ImportLists,
     StringMap<FunctionImporter::ExportSetTy> &ExportLists);
+
+/// Compute all the imports for the given module using the Index.
+///
+/// \p ImportList will be populated with a map that can be passed to
+/// FunctionImporter::importFunctions() above (see description there).
+void ComputeCrossModuleImportForModule(
+    StringRef ModulePath, const ModuleSummaryIndex &Index,
+    FunctionImporter::ImportMapTy &ImportList);
 }
 
 #endif // LLVM_FUNCTIONIMPORT_H
Index: lib/Transforms/IPO/FunctionImport.cpp
===================================================================
--- lib/Transforms/IPO/FunctionImport.cpp
+++ lib/Transforms/IPO/FunctionImport.cpp
@@ -143,7 +143,7 @@
     const std::map<GlobalValue::GUID, FunctionSummary *> &DefinedFunctions,
     SmallVectorImpl<EdgeInfo> &Worklist,
     FunctionImporter::ImportMapTy &ImportsForModule,
-    StringMap<FunctionImporter::ExportSetTy> &ExportLists) {
+    StringMap<FunctionImporter::ExportSetTy> *ExportLists = nullptr) {
   for (auto &Edge : Summary.calls()) {
     auto GUID = Edge.first;
     DEBUG(dbgs() << " edge -> " << GUID << " Threshold:" << Threshold << "\n");
@@ -176,18 +176,21 @@
 
     // Make exports in the source module.
     auto ExportModulePath = CalleeSummary->modulePath();
-    auto ExportList = ExportLists[ExportModulePath];
-    ExportList.insert(GUID);
-    // Mark all functions and globals referenced by this function as exported to
-    // the outside if they are defined in the same source module.
-    for (auto &Edge : CalleeSummary->calls()) {
-      auto CalleeGUID = Edge.first;
-      if (isGlobalExported(Index, ExportModulePath, CalleeGUID))
-        ExportList.insert(CalleeGUID);
-    }
-    for (auto &GUID : CalleeSummary->refs()) {
-      if (isGlobalExported(Index, ExportModulePath, GUID))
-        ExportList.insert(GUID);
+    if (ExportLists) {
+      auto ExportList = (*ExportLists)[ExportModulePath];
+      ExportList.insert(GUID);
+      // Mark all functions and globals referenced by this function as exported
+      // to
+      // the outside if they are defined in the same source module.
+      for (auto &Edge : CalleeSummary->calls()) {
+        auto CalleeGUID = Edge.first;
+        if (isGlobalExported(Index, ExportModulePath, CalleeGUID))
+          ExportList.insert(CalleeGUID);
+      }
+      for (auto &GUID : CalleeSummary->refs()) {
+        if (isGlobalExported(Index, ExportModulePath, GUID))
+          ExportList.insert(GUID);
+      }
     }
 
     // Insert the newly imported function to the worklist.
@@ -202,7 +205,7 @@
     const std::map<GlobalValue::GUID, FunctionSummary *> &DefinedFunctions,
     const ModuleSummaryIndex &Index,
     FunctionImporter::ImportMapTy &ImportsForModule,
-    StringMap<FunctionImporter::ExportSetTy> &ExportLists) {
+    StringMap<FunctionImporter::ExportSetTy> *ExportLists = nullptr) {
   // Worklist contains the list of function imported in this module, for which
   // we will analyse the callees and may import further down the callgraph.
   SmallVector<EdgeInfo, 128> Worklist;
@@ -233,7 +236,7 @@
 
 } // anonymous namespace
 
-/// Compute all the import and export for every module in the Index.
+/// Compute all the import and export for every module using the Index.
 void llvm::ComputeCrossModuleImport(
     const ModuleSummaryIndex &Index,
     StringMap<FunctionImporter::ImportMapTy> &ImportLists,
@@ -264,7 +267,7 @@
     DEBUG(dbgs() << "Computing import for Module '" << DefinedFunctions.first()
                  << "'\n");
     ComputeImportForModule(DefinedFunctions.second, Index, ImportsForModule,
-                           ExportLists);
+                           &ExportLists);
   }
 
 #ifndef NDEBUG
@@ -285,6 +288,45 @@
 #endif
 }
 
+/// Compute all the imports for the given module in the Index.
+void llvm::ComputeCrossModuleImportForModule(
+    StringRef ModulePath, const ModuleSummaryIndex &Index,
+    FunctionImporter::ImportMapTy &ImportList) {
+
+  // Collect the list of functions this module defines.
+  // GUID -> Summary
+  std::map<GlobalValue::GUID, FunctionSummary *> FunctionInfoMap;
+
+  for (auto &GlobalList : Index) {
+    auto GUID = GlobalList.first;
+    for (auto &GlobInfo : GlobalList.second) {
+      auto *Summary = dyn_cast_or_null<FunctionSummary>(GlobInfo->summary());
+      if (!Summary)
+        /// Ignore global variable, focus on functions
+        continue;
+      if (Summary->modulePath() != ModulePath)
+        continue;
+      DEBUG(dbgs() << "Adding definition: Module '" << Summary->modulePath()
+                   << "' defines '" << GUID << "'\n");
+      FunctionInfoMap[GUID] = Summary;
+    }
+  }
+
+  // Compute the import list for this module.
+  DEBUG(dbgs() << "Computing import for Module '" << ModulePath << "'\n");
+  ComputeImportForModule(FunctionInfoMap, Index, ImportList);
+
+#ifndef NDEBUG
+  DEBUG(dbgs() << "* Module " << ModulePath << " imports from "
+               << ImportList.size() << " modules.\n");
+  for (auto &Src : ImportList) {
+    auto SrcModName = Src.first();
+    DEBUG(dbgs() << " - " << Src.second.size() << " functions imported from "
+                 << SrcModName << "\n");
+  }
+#endif
+}
+
 // Automatically import functions in Module \p DestModule based on the summaries
 // index.
 //
@@ -462,13 +504,11 @@
       Index = IndexPtr.get();
     }
 
-    // First step is collecting the import/export lists
-    // The export list is not used yet, but could limit the amount of renaming
-    // performed in renameModuleForThinLTO()
-    StringMap<FunctionImporter::ImportMapTy> ImportLists;
+    // First step is collecting the import list.
+    FunctionImporter::ImportMapTy ImportList;
     StringMap<FunctionImporter::ExportSetTy> ExportLists;
-    ComputeCrossModuleImport(*Index, ImportLists, ExportLists);
-    auto &ImportList = ImportLists[M.getModuleIdentifier()];
+    ComputeCrossModuleImportForModule(M.getModuleIdentifier(), *Index,
+                                      ImportList);
 
     // Next we need to promote to global scope and rename any local values that
     // are potentially exported to other modules.