Index: llvm/trunk/include/llvm/Support/YAMLTraits.h
===================================================================
--- llvm/trunk/include/llvm/Support/YAMLTraits.h
+++ llvm/trunk/include/llvm/Support/YAMLTraits.h
@@ -209,6 +209,15 @@
   // static T::value_type& element(IO &io, T &seq, size_t index);
 };
 
+/// This class should be specialized by any type that needs to be converted
+/// to/from a YAML mapping in the case where the names of the keys are not known
+/// in advance, e.g. a string map.
+template <typename T>
+struct CustomMappingTraits {
+  // static void inputOne(IO &io, StringRef key, T &elem);
+  // static void output(IO &io, T &elem);
+};
+
 // Only used for better diagnostics of missing traits
 template <typename T>
 struct MissingTrait;
@@ -358,6 +367,23 @@
   static bool const value =  (sizeof(test<SequenceTraits<T>>(nullptr)) == 1);
 };
 
+// Test if CustomMappingTraits<T> is defined on type T.
+template <class T>
+struct has_CustomMappingTraits
+{
+  typedef void (*Signature_input)(IO &io, StringRef key, T &v);
+
+  template <typename U>
+  static char test(SameType<Signature_input, &U::inputOne>*);
+
+  template <typename U>
+  static double test(...);
+
+public:
+  static bool const value =
+      (sizeof(test<CustomMappingTraits<T>>(nullptr)) == 1);
+};
+
 // has_FlowTraits<int> will cause an error with some compilers because
 // it subclasses int.  Using this wrapper only instantiates the
 // real has_FlowTraits only if the template type is a class.
@@ -493,6 +519,7 @@
                                         !has_BlockScalarTraits<T>::value &&
                                         !has_MappingTraits<T, Context>::value &&
                                         !has_SequenceTraits<T>::value &&
+                                        !has_CustomMappingTraits<T>::value &&
                                         !has_DocumentListTraits<T>::value> {};
 
 template <typename T, typename Context>
@@ -531,6 +558,7 @@
   virtual void endMapping() = 0;
   virtual bool preflightKey(const char*, bool, bool, bool &, void *&) = 0;
   virtual void postflightKey(void*) = 0;
+  virtual std::vector<StringRef> keys() = 0;
 
   virtual void beginFlowMapping() = 0;
   virtual void endFlowMapping() = 0;
@@ -819,6 +847,21 @@
 }
 
 template <typename T>
+typename std::enable_if<has_CustomMappingTraits<T>::value, void>::type
+yamlize(IO &io, T &Val, bool, EmptyContext &Ctx) {
+  if ( io.outputting() ) {
+    io.beginMapping();
+    CustomMappingTraits<T>::output(io, Val);
+    io.endMapping();
+  } else {
+    io.beginMapping();
+    for (StringRef key : io.keys())
+      CustomMappingTraits<T>::inputOne(io, key, Val);
+    io.endMapping();
+  }
+}
+
+template <typename T>
 typename std::enable_if<missingTraits<T, EmptyContext>::value, void>::type
 yamlize(IO &io, T &Val, bool, EmptyContext &Ctx) {
   char missing_yaml_trait_for_type[sizeof(MissingTrait<T>)];
@@ -1074,6 +1117,7 @@
   void endMapping() override;
   bool preflightKey(const char *, bool, bool, bool &, void *&) override;
   void postflightKey(void *) override;
+  std::vector<StringRef> keys() override;
   void beginFlowMapping() override;
   void endFlowMapping() override;
   unsigned beginSequence() override;
@@ -1157,7 +1201,7 @@
     bool isValidKey(StringRef key);
 
     NameToNode                        Mapping;
-    llvm::SmallVector<const char*, 6> ValidKeys;
+    llvm::SmallVector<std::string, 6> ValidKeys;
   };
 
   class SequenceHNode : public HNode {
@@ -1215,6 +1259,7 @@
   void endMapping() override;
   bool preflightKey(const char *key, bool, bool, bool &, void *&) override;
   void postflightKey(void *) override;
+  std::vector<StringRef> keys() override;
   void beginFlowMapping() override;
   void endFlowMapping() override;
   unsigned beginSequence() override;
@@ -1384,6 +1429,17 @@
   return In;
 }
 
+// Define non-member operator>> so that Input can stream in a string map.
+template <typename T>
+inline
+typename std::enable_if<has_CustomMappingTraits<T>::value, Input &>::type
+operator>>(Input &In, T &Val) {
+  EmptyContext Ctx;
+  if (In.setCurrentDocument())
+    yamlize(In, Val, true, Ctx);
+  return In;
+}
+
 // Provide better error message about types missing a trait specialization
 template <typename T>
 inline typename std::enable_if<missingTraits<T, EmptyContext>::value,
@@ -1457,6 +1513,21 @@
   return Out;
 }
 
+// Define non-member operator<< so that Output can stream out a string map.
+template <typename T>
+inline
+typename std::enable_if<has_CustomMappingTraits<T>::value, Output &>::type
+operator<<(Output &Out, T &Val) {
+  EmptyContext Ctx;
+  Out.beginDocuments();
+  if (Out.preflightDocument(0)) {
+    yamlize(Out, Val, true, Ctx);
+    Out.postflightDocument();
+  }
+  Out.endDocuments();
+  return Out;
+}
+
 // Provide better error message about types missing a trait specialization
 template <typename T>
 inline typename std::enable_if<missingTraits<T, EmptyContext>::value,
@@ -1476,6 +1547,18 @@
   }
 };
 
+/// Implementation of CustomMappingTraits for std::map<std::string, T>.
+template <typename T> struct StdMapStringCustomMappingTraitsImpl {
+  typedef std::map<std::string, T> map_type;
+  static void inputOne(IO &io, StringRef key, map_type &v) {
+    io.mapRequired(key.str().c_str(), v[key]);
+  }
+  static void output(IO &io, map_type &v) {
+    for (auto &p : v)
+      io.mapRequired(p.first.c_str(), p.second);
+  }
+};
+
 } // end namespace yaml
 } // end namespace llvm
 
@@ -1530,4 +1613,15 @@
   }                                                                            \
   }
 
+/// Utility for declaring that std::map<std::string, _type> should be considered
+/// a YAML map.
+#define LLVM_YAML_IS_STRING_MAP(_type)                                         \
+  namespace llvm {                                                             \
+  namespace yaml {                                                             \
+  template <>                                                                  \
+  struct CustomMappingTraits<std::map<std::string, _type>>                     \
+      : public StdMapStringCustomMappingTraitsImpl<_type> {};                  \
+  }                                                                            \
+  }
+
 #endif // LLVM_SUPPORT_YAMLTRAITS_H
Index: llvm/trunk/lib/Support/YAMLTraits.cpp
===================================================================
--- llvm/trunk/lib/Support/YAMLTraits.cpp
+++ llvm/trunk/lib/Support/YAMLTraits.cpp
@@ -118,6 +118,18 @@
   }
 }
 
+std::vector<StringRef> Input::keys() {
+  MapHNode *MN = dyn_cast<MapHNode>(CurrentNode);
+  std::vector<StringRef> Ret;
+  if (!MN) {
+    setError(CurrentNode, "not a mapping");
+    return Ret;
+  }
+  for (auto &P : MN->Mapping)
+    Ret.push_back(P.first());
+  return Ret;
+}
+
 bool Input::preflightKey(const char *Key, bool Required, bool, bool &UseDefault,
                          void *&SaveInfo) {
   UseDefault = false;
@@ -374,8 +386,8 @@
 }
 
 bool Input::MapHNode::isValidKey(StringRef Key) {
-  for (const char *K : ValidKeys) {
-    if (Key.equals(K))
+  for (std::string &K : ValidKeys) {
+    if (Key == K)
       return true;
   }
   return false;
@@ -451,6 +463,10 @@
   StateStack.pop_back();
 }
 
+std::vector<StringRef> Output::keys() {
+  report_fatal_error("invalid call");
+}
+
 bool Output::preflightKey(const char *Key, bool Required, bool SameAsDefault,
                           bool &UseDefault, void *&) {
   UseDefault = false;
Index: llvm/trunk/unittests/Support/YAMLIOTest.cpp
===================================================================
--- llvm/trunk/unittests/Support/YAMLIOTest.cpp
+++ llvm/trunk/unittests/Support/YAMLIOTest.cpp
@@ -2369,6 +2369,68 @@
   out.clear();
 }
 
+LLVM_YAML_IS_STRING_MAP(int)
+
+TEST(YAMLIO, TestCustomMapping) {
+  std::map<std::string, int> x;
+  x["foo"] = 1;
+  x["bar"] = 2;
+
+  std::string out;
+  llvm::raw_string_ostream ostr(out);
+  Output xout(ostr, nullptr, 0);
+
+  xout << x;
+  ostr.flush();
+  EXPECT_EQ("---\n"
+            "bar:             2\n"
+            "foo:             1\n"
+            "...\n",
+            out);
+
+  Input yin(out);
+  std::map<std::string, int> y;
+  yin >> y;
+  EXPECT_EQ(2ul, y.size());
+  EXPECT_EQ(1, y["foo"]);
+  EXPECT_EQ(2, y["bar"]);
+}
+
+LLVM_YAML_IS_STRING_MAP(FooBar)
+
+TEST(YAMLIO, TestCustomMappingStruct) {
+  std::map<std::string, FooBar> x;
+  x["foo"].foo = 1;
+  x["foo"].bar = 2;
+  x["bar"].foo = 3;
+  x["bar"].bar = 4;
+
+  std::string out;
+  llvm::raw_string_ostream ostr(out);
+  Output xout(ostr, nullptr, 0);
+
+  xout << x;
+  ostr.flush();
+  EXPECT_EQ("---\n"
+            "bar:             \n"
+            "  foo:             3\n"
+            "  bar:             4\n"
+            "foo:             \n"
+            "  foo:             1\n"
+            "  bar:             2\n"
+            "...\n",
+            out);
+
+  Input yin(out);
+  std::map<std::string, FooBar> y;
+  yin >> y;
+  EXPECT_EQ(2ul, y.size());
+  EXPECT_EQ(1, y["foo"].foo);
+  EXPECT_EQ(2, y["foo"].bar);
+  EXPECT_EQ(3, y["bar"].foo);
+  EXPECT_EQ(4, y["bar"].bar);
+}
+
 TEST(YAMLIO, InvalidInput) {
   // polluting 1 value in the sequence
   Input yin("---\n- foo:  3\n  bar:  5\n1\n- foo:  3\n  bar:  5\n...\n");