Index: llvm/lib/Support/Signposts.cpp
===================================================================
--- llvm/lib/Support/Signposts.cpp
+++ llvm/lib/Support/Signposts.cpp
@@ -14,6 +14,7 @@
 #if LLVM_SUPPORT_XCODE_SIGNPOSTS
 #include "llvm/ADT/DenseMap.h"
 #include <os/signpost.h>
+#include <mutex>
 #endif // if LLVM_SUPPORT_XCODE_SIGNPOSTS
 
 using namespace llvm;
@@ -38,9 +39,11 @@
 
   LogPtrTy SignpostLog;
   DenseMap<const void *, os_signpost_id_t> Signposts;
+  std::mutex Mutex;
 
   LogTy &getLogger() const { return *SignpostLog; }
   os_signpost_id_t getSignpostForObject(const void *O) {
+    std::lock_guard<std::mutex> Guard(Mutex);
     const auto &I = Signposts.find(O);
     if (I != Signposts.end())
       return I->second;