Index: llvm/include/llvm/Demangle/ItaniumDemangle.h
===================================================================
--- llvm/include/llvm/Demangle/ItaniumDemangle.h
+++ llvm/include/llvm/Demangle/ItaniumDemangle.h
@@ -19,6 +19,7 @@
 #include "DemangleConfig.h"
 #include "StringView.h"
 #include "Utility.h"
+#include "llvm/Support/Threading.h"
 #include <algorithm>
 #include <cassert>
 #include <cctype>
@@ -3029,12 +3030,11 @@
 #ifndef NDEBUG
   {
     // Verify table order.
-    static bool Done;
-    if (!Done) {
-      Done = true;
+    static llvm::once_flag Done;
+    llvm::call_once(Done, []() {
       for (const auto *Op = &Ops[0]; Op != &Ops[NumOps - 1]; Op++)
         assert(Op[0] < Op[1] && "Operator table is not ordered");
-    }
+    });
   }
 #endif