Index: libcxxabi/src/demangle/Utility.h
===================================================================
--- libcxxabi/src/demangle/Utility.h
+++ libcxxabi/src/demangle/Utility.h
@@ -14,6 +14,7 @@
 #define DEMANGLE_UTILITY_H
 
 #include "StringView.h"
+#include "llvm/Demangle/MicrosoftDemangleNodes.h"
 #include <cstdint>
 #include <cstdlib>
 #include <cstring>
@@ -93,6 +94,23 @@
     return *this;
   }
 
+  OutputString prepend(StringView R)
+  {
+      char *TempBuffer;
+      size_t Size = R.size();
+
+    if (Size != 0) {
+      grow(Size);
+      for (TempBuffer = (Buffer + CurrentPosition) - 1; TempBuffer >= Buffer; --TempBuffer) {
+        TempBuffer[Size] = TempBuffer[0];
+      }
+      std::memcpy(Buffer, R.begin(), Size);
+      CurrentPosition += Size;
+    }
+
+    return *this;
+  }
+
   OutputString &operator<<(StringView R) { return (*this += R); }
 
   OutputString &operator<<(char C) { return (*this += C); }
Index: llvm/include/llvm/Demangle/Utility.h
===================================================================
--- llvm/include/llvm/Demangle/Utility.h
+++ llvm/include/llvm/Demangle/Utility.h
@@ -14,6 +14,7 @@
 #define DEMANGLE_UTILITY_H
 
 #include "StringView.h"
+#include "llvm/Demangle/MicrosoftDemangleNodes.h"
 #include <cstdint>
 #include <cstdlib>
 #include <cstring>
@@ -93,6 +94,23 @@
     return *this;
   }
 
+  OutputString prepend(StringView R)
+  {
+      char *TempBuffer;
+      size_t Size = R.size();
+
+    if (Size != 0) {
+      grow(Size);
+      for (TempBuffer = (Buffer + CurrentPosition) - 1; TempBuffer >= Buffer; --TempBuffer) {
+        TempBuffer[Size] = TempBuffer[0];
+      }
+      std::memcpy(Buffer, R.begin(), Size);
+      CurrentPosition += Size;
+    }
+
+    return *this;
+  }
+
   OutputString &operator<<(StringView R) { return (*this += R); }
 
   OutputString &operator<<(char C) { return (*this += C); }
Index: llvm/unittests/Demangle/OutputStringTest.cpp
===================================================================
--- llvm/unittests/Demangle/OutputStringTest.cpp
+++ llvm/unittests/Demangle/OutputStringTest.cpp
@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "llvm/Demangle/MicrosoftDemangleNodes.h"
 #include "llvm/Demangle/Utility.h"
 #include "gtest/gtest.h"
 #include <string>
@@ -59,3 +60,12 @@
 
   std::free(OS.getBuffer());
 }
+
+TEST(OutputStringTest, Prepend)
+{
+  OutputString OS;
+
+  OS << "abc";
+  OS.prepend("def");
+  EXPECT_EQ("defabc", toString(OS));
+}