Index: llvm/lib/Support/CommandLine.cpp
===================================================================
--- llvm/lib/Support/CommandLine.cpp
+++ llvm/lib/Support/CommandLine.cpp
@@ -43,6 +43,7 @@
 #include "llvm/Support/raw_ostream.h"
 #include <cstdlib>
 #include <map>
+#include <mutex>
 #include <string>
 using namespace llvm;
 using namespace cl;
@@ -207,6 +208,9 @@
   void addOption(Option *O, SubCommand *SC) {
     bool HadErrors = false;
     if (O->hasArgStr()) {
+      // Guard find/insert operations to SC->OptionsMap.
+      std::lock_guard<std::mutex> Lock(OptionsMapMutex);
+
       // If it's a DefaultOption, check to make sure it isn't already there.
       if (O->isDefaultOption() &&
           SC->OptionsMap.find(O->ArgStr) != SC->OptionsMap.end())
@@ -417,6 +421,7 @@
 
 private:
   SubCommand *ActiveSubCommand;
+  std::mutex OptionsMapMutex;
 
   Option *LookupOption(SubCommand &Sub, StringRef &Arg, StringRef &Value);
   Option *LookupLongOption(SubCommand &Sub, StringRef &Arg, StringRef &Value,