--- a/lib/Support/Twine.cpp	Fri Nov 25 19:14:25 2016 +0900
+++ b/lib/Support/Twine.cpp	Fri Oct 27 17:07:41 2017 +0900
@@ -10,6 +10,7 @@
 #include "llvm/ADT/Twine.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/Support/Debug.h"
+#include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;
 
@@ -18,6 +19,11 @@
   if (LHSKind == StdStringKind && RHSKind == EmptyKind)
     return *LHS.stdString;
 
+  // If we're storing a formatv_object, we can avoid an extra copy by formatting
+  // it immediately and returning the result.
+  if (LHSKind == FormatvObjectKind && RHSKind == EmptyKind)
+    return LHS.formatvObject->str();
+
   // Otherwise, flatten and copy the contents first.
   SmallString<256> Vec;
   return toStringRef(Vec).str();
@@ -68,6 +74,9 @@
   case Twine::SmallStringKind:
     OS << *Ptr.smallString;
     break;
+  case Twine::FormatvObjectKind:
+    OS << *Ptr.formatvObject;
+    break;
   case Twine::CharKind:
     OS << Ptr.character;
     break;
@@ -121,6 +130,9 @@
   case Twine::SmallStringKind:
     OS << "smallstring:\"" << *Ptr.smallString << "\"";
     break;
+  case Twine::FormatvObjectKind:
+    OS << "formatv:\"" << *Ptr.formatvObject << "\"";
+    break;
   case Twine::CharKind:
     OS << "char:\"" << Ptr.character << "\"";
     break;
@@ -161,10 +173,12 @@
   OS << ")";
 }
 
+#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
 LLVM_DUMP_METHOD void Twine::dump() const {
   print(dbgs());
 }
 
-void Twine::dumpRepr() const {
+LLVM_DUMP_METHOD void Twine::dumpRepr() const {
   printRepr(dbgs());
 }
+#endif