Index: lib/LTO/ThinLTOCodeGenerator.cpp
===================================================================
--- lib/LTO/ThinLTOCodeGenerator.cpp
+++ lib/LTO/ThinLTOCodeGenerator.cpp
@@ -972,6 +972,15 @@
 
   // Parallel optimizer + codegen
   {
+    // Populate the StringMaps to avoid Rehashing in threads.
+    for (auto IndexCount : ModulesOrdering) {
+      auto &ModuleBuffer = Modules[IndexCount];
+      auto ModuleIdentifier = ModuleBuffer.getBufferIdentifier();
+      ExportLists.try_emplace(ModuleIdentifier);
+      ImportLists.try_emplace(ModuleIdentifier);
+      ResolvedODR.try_emplace(ModuleIdentifier);
+      ModuleToDefinedGVSummaries.try_emplace(ModuleIdentifier);
+    }
     ThreadPool Pool(ThreadCount);
     for (auto IndexCount : ModulesOrdering) {
       auto &ModuleBuffer = Modules[IndexCount];