Index: llvm/include/llvm/ADT/Twine.h
===================================================================
--- llvm/include/llvm/ADT/Twine.h
+++ llvm/include/llvm/ADT/Twine.h
@@ -19,6 +19,7 @@
 
 namespace llvm {
 
+class formatv_object_base;
 class raw_ostream;
 
 /// Twine - A lightweight data structure for efficiently representing the
@@ -102,6 +103,9 @@
     /// A pointer to a SmallString instance.
     SmallStringKind,
 
+    /// A pointer to a formatv_object_base instance.
+    FormatvObjectKind,
+
     /// A char value, to render as a character.
     CharKind,
 
@@ -136,6 +140,7 @@
     const std::string *stdString;
     const StringRef *stringRef;
     const SmallVectorImpl<char> *smallString;
+    const formatv_object_base *formatvObject;
     char character;
     unsigned int decUI;
     int decI;
@@ -276,6 +281,13 @@
     assert(isValid() && "Invalid twine!");
   }
 
+  /// Construct from a formatv_object_base.
+  /*implicit*/ Twine(const formatv_object_base &Fmt)
+      : LHSKind(FormatvObjectKind), RHSKind(EmptyKind) {
+    LHS.formatvObject = &Fmt;
+    assert(isValid() && "Invalid twine!");
+  }
+
   /// Construct from a char.
   explicit Twine(char Val) : LHSKind(CharKind), RHSKind(EmptyKind) {
     LHS.character = Val;
Index: llvm/lib/Support/Twine.cpp
===================================================================
--- llvm/lib/Support/Twine.cpp
+++ llvm/lib/Support/Twine.cpp
@@ -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;
Index: llvm/unittests/ADT/TwineTest.cpp
===================================================================
--- llvm/unittests/ADT/TwineTest.cpp
+++ llvm/unittests/ADT/TwineTest.cpp
@@ -7,8 +7,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "llvm/ADT/Twine.h"
 #include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/Twine.h"
+#include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/raw_ostream.h"
 #include "gtest/gtest.h"
 using namespace llvm;
@@ -30,6 +31,7 @@
   EXPECT_EQ("hi", Twine(StringRef(std::string("hi"))).str());
   EXPECT_EQ("hi", Twine(StringRef("hithere", 2)).str());
   EXPECT_EQ("hi", Twine(SmallString<4>("hi")).str());
+  EXPECT_EQ("hi", Twine(formatv("{0}", "hi")).str());
 }
 
 TEST(TwineTest, Numbers) {
@@ -65,6 +67,10 @@
             repr(Twine().concat(Twine("hi"))));
   EXPECT_EQ("(Twine smallstring:\"hi\" empty)", 
             repr(Twine().concat(Twine(SmallString<5>("hi")))));
+  EXPECT_EQ("(Twine formatv:\"howdy\" empty)",
+            repr(Twine(formatv("howdy")).concat(Twine())));
+  EXPECT_EQ("(Twine formatv:\"howdy\" empty)",
+            repr(Twine().concat(Twine(formatv("howdy")))));
   EXPECT_EQ("(Twine smallstring:\"hey\" cstring:\"there\")", 
             repr(Twine(SmallString<7>("hey")).concat(Twine("there"))));
 
@@ -79,6 +85,10 @@
             repr(Twine("a").concat(Twine("b").concat(Twine("c")))));
   EXPECT_EQ("(Twine cstring:\"a\" rope:(Twine smallstring:\"b\" cstring:\"c\"))",
             repr(Twine("a").concat(Twine(SmallString<3>("b")).concat(Twine("c")))));
+  EXPECT_EQ("(Twine rope:(Twine cstring:\"a\" formatv:\"bc\") cstring:\"d\")",
+            repr(Twine("a")
+                     .concat(Twine(formatv("{0}{1}", "b", "c")))
+                     .concat(Twine("d"))));
 }
 
 TEST(TwineTest, toNullTerminatedStringRef) {
@@ -89,6 +99,9 @@
   EXPECT_EQ(0, *Twine(SmallString<11>("hello"))
                     .toNullTerminatedStringRef(storage)
                     .end());
+  EXPECT_EQ(0, *Twine(formatv("{0}{1}", "how", "dy"))
+                    .toNullTerminatedStringRef(storage)
+                    .end());
 }
 
   // I suppose linking in the entire code generator to add a unit test to check