[ThinLTOCodeGenerator] Avoid Rehash StringMap in ThreadPool

Summary:
During threaded thinLTO, it is possible that the entry for current
module doesn't exist in StringMaps (like ExportLists, ResolvedODR,
etc.). Using operator[] might trigger a rehash for the StringMap, which
might happen on multiple threads at the same time.

rdar://problem/43846199

Reviewers: tejohnson, mehdi_amini, kromanova, pcc

Reviewed By: tejohnson

Subscribers: dang, inglorion, eraman, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D52049

llvm-svn: 342263

llvm/lib/LTO/ThinLTOCodeGenerator.cpp

@@ -950,11 +950,15 @@ void ThinLTOCodeGenerator::run() {
   // Changes are made in the index, consumed in the ThinLTO backends.
   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.
-  for (auto &DefinedGVSummaries : ModuleToDefinedGVSummaries) {
-    ExportLists[DefinedGVSummaries.first()];
-    ResolvedODR[DefinedGVSummaries.first()];
+  // Make sure that every module has an entry in the ExportLists, ImportList,
+  // GVSummary and ResolvedODR maps to enable threaded access to these maps
+  // below.
+  for (auto &Module : Modules) {
+    auto ModuleIdentifier = Module.getBufferIdentifier();
+    ExportLists[ModuleIdentifier];
+    ImportLists[ModuleIdentifier];
+    ResolvedODR[ModuleIdentifier];
+    ModuleToDefinedGVSummaries[ModuleIdentifier];
   }
 
   // Compute the ordering we will process the inputs: the rough heuristic here