Index: include/llvm/Transforms/IPO/FunctionImport.h
===================================================================
--- include/llvm/Transforms/IPO/FunctionImport.h
+++ include/llvm/Transforms/IPO/FunctionImport.h
@@ -114,12 +114,13 @@
 void gatherImportedSummariesForModule(
     StringRef ModulePath,
     const StringMap<GVSummaryMapTy> &ModuleToDefinedGVSummaries,
-    const StringMap<FunctionImporter::ImportMapTy> &ImportLists,
+    const FunctionImporter::ImportMapTy &ImportListsForModule,
     std::map<std::string, GVSummaryMapTy> &ModuleToSummariesForIndex);
 
+/// Emit into \p OutputFilename the files module \p ModulePath will import from.
 std::error_code
 EmitImportsFiles(StringRef ModulePath, StringRef OutputFilename,
-                 const StringMap<FunctionImporter::ImportMapTy> &ImportLists);
+                 const FunctionImporter::ImportMapTy &ModuleImports);
 
 /// Resolve WeakForLinker values in \p TheModule based on the information
 /// recorded in the summaries during global summary-based analysis.
Index: lib/LTO/LTO.cpp
===================================================================
--- lib/LTO/LTO.cpp
+++ lib/LTO/LTO.cpp
@@ -409,7 +409,7 @@
 
   virtual ~ThinBackendProc() {}
   virtual Error start(unsigned Task, MemoryBufferRef MBRef,
-                      StringMap<FunctionImporter::ImportMapTy> &ImportLists,
+                      const FunctionImporter::ImportMapTy &ImportList,
                       MapVector<StringRef, MemoryBufferRef> &ModuleMap) = 0;
   virtual Error wait() = 0;
 };
@@ -447,7 +447,7 @@
   }
 
   Error start(unsigned Task, MemoryBufferRef MBRef,
-              StringMap<FunctionImporter::ImportMapTy> &ImportLists,
+              const FunctionImporter::ImportMapTy &ImportList,
               MapVector<StringRef, MemoryBufferRef> &ModuleMap) override {
     StringRef ModulePath = MBRef.getBufferIdentifier();
     BackendThreadPool.async(
@@ -466,7 +466,7 @@
               Err = std::move(E);
           }
         },
-        MBRef, std::ref(CombinedIndex), std::ref(ImportLists[ModulePath]),
+        MBRef, std::ref(CombinedIndex), std::ref(ImportList),
         std::ref(ModuleToDefinedGVSummaries[ModulePath]), std::ref(ModuleMap));
     return Error();
   }
@@ -530,7 +530,7 @@
   }
 
   Error start(unsigned Task, MemoryBufferRef MBRef,
-              StringMap<FunctionImporter::ImportMapTy> &ImportLists,
+              const FunctionImporter::ImportMapTy &ImportList,
               MapVector<StringRef, MemoryBufferRef> &ModuleMap) override {
     StringRef ModulePath = MBRef.getBufferIdentifier();
     std::string NewModulePath =
@@ -549,7 +549,7 @@
 
     std::map<std::string, GVSummaryMapTy> ModuleToSummariesForIndex;
     gatherImportedSummariesForModule(ModulePath, ModuleToDefinedGVSummaries,
-                                     ImportLists, ModuleToSummariesForIndex);
+                                     ImportList, ModuleToSummariesForIndex);
 
     raw_fd_ostream OS(NewModulePath + ".thinlto.bc", EC,
                       sys::fs::OpenFlags::F_None);
@@ -558,8 +558,8 @@
     WriteIndexToFile(CombinedIndex, OS, &ModuleToSummariesForIndex);
 
     if (ShouldEmitImportsFiles)
-      return errorCodeToError(EmitImportsFiles(
-          ModulePath, NewModulePath + ".imports", ImportLists));
+      return errorCodeToError(
+          EmitImportsFiles(ModulePath, NewModulePath + ".imports", ImportList));
     return Error();
   }
 
@@ -633,7 +633,7 @@
   unsigned Partition = 1;
 
   for (auto &Mod : ThinLTO.ModuleMap) {
-    if (Error E = BackendProc->start(Task, Mod.second, ImportLists,
+    if (Error E = BackendProc->start(Task, Mod.second, ImportLists[Mod.first],
                                      ThinLTO.ModuleMap))
       return E;
 
Index: lib/LTO/ThinLTOCodeGenerator.cpp
===================================================================
--- lib/LTO/ThinLTOCodeGenerator.cpp
+++ lib/LTO/ThinLTOCodeGenerator.cpp
@@ -578,7 +578,7 @@
                            ExportLists);
 
   llvm::gatherImportedSummariesForModule(ModulePath, ModuleToDefinedGVSummaries,
-                                         ImportLists,
+                                         ImportLists[ModulePath],
                                          ModuleToSummariesForIndex);
 }
 
@@ -601,7 +601,7 @@
                            ExportLists);
 
   std::error_code EC;
-  if ((EC = EmitImportsFiles(ModulePath, OutputName, ImportLists)))
+  if ((EC = EmitImportsFiles(ModulePath, OutputName, ImportLists[ModulePath])))
     report_fatal_error(Twine("Failed to open ") + OutputName +
                        " to save imports lists\n");
 }
Index: lib/Transforms/IPO/FunctionImport.cpp
===================================================================
--- lib/Transforms/IPO/FunctionImport.cpp
+++ lib/Transforms/IPO/FunctionImport.cpp
@@ -440,40 +440,35 @@
 void llvm::gatherImportedSummariesForModule(
     StringRef ModulePath,
     const StringMap<GVSummaryMapTy> &ModuleToDefinedGVSummaries,
-    const StringMap<FunctionImporter::ImportMapTy> &ImportLists,
+    const FunctionImporter::ImportMapTy &ImportListForModule,
     std::map<std::string, GVSummaryMapTy> &ModuleToSummariesForIndex) {
   // Include all summaries from the importing module.
   ModuleToSummariesForIndex[ModulePath] =
       ModuleToDefinedGVSummaries.lookup(ModulePath);
-  auto ModuleImports = ImportLists.find(ModulePath);
-  if (ModuleImports != ImportLists.end()) {
-    // Include summaries for imports.
-    for (auto &ILI : ModuleImports->second) {
-      auto &SummariesForIndex = ModuleToSummariesForIndex[ILI.first()];
-      const auto &DefinedGVSummaries =
-          ModuleToDefinedGVSummaries.lookup(ILI.first());
-      for (auto &GI : ILI.second) {
-        const auto &DS = DefinedGVSummaries.find(GI.first);
-        assert(DS != DefinedGVSummaries.end() &&
-               "Expected a defined summary for imported global value");
-        SummariesForIndex[GI.first] = DS->second;
-      }
+  // Include summaries for imports.
+  for (auto &ILI : ImportListForModule) {
+    auto &SummariesForIndex = ModuleToSummariesForIndex[ILI.first()];
+    const auto &DefinedGVSummaries =
+        ModuleToDefinedGVSummaries.lookup(ILI.first());
+    for (auto &GI : ILI.second) {
+      const auto &DS = DefinedGVSummaries.find(GI.first);
+      assert(DS != DefinedGVSummaries.end() &&
+             "Expected a defined summary for imported global value");
+      SummariesForIndex[GI.first] = DS->second;
     }
   }
 }
 
 /// Emit the files \p ModulePath will import from into \p OutputFilename.
-std::error_code llvm::EmitImportsFiles(
-    StringRef ModulePath, StringRef OutputFilename,
-    const StringMap<FunctionImporter::ImportMapTy> &ImportLists) {
-  auto ModuleImports = ImportLists.find(ModulePath);
+std::error_code
+llvm::EmitImportsFiles(StringRef ModulePath, StringRef OutputFilename,
+                       const FunctionImporter::ImportMapTy &ModuleImports) {
   std::error_code EC;
   raw_fd_ostream ImportsOS(OutputFilename, EC, sys::fs::OpenFlags::F_None);
   if (EC)
     return EC;
-  if (ModuleImports != ImportLists.end())
-    for (auto &ILI : ModuleImports->second)
-      ImportsOS << ILI.first() << "\n";
+  for (auto &ILI : ModuleImports)
+    ImportsOS << ILI.first() << "\n";
   return std::error_code();
 }