Index: llvm/trunk/include/llvm/Support/JSON.h
===================================================================
--- llvm/trunk/include/llvm/Support/JSON.h
+++ llvm/trunk/include/llvm/Support/JSON.h
@@ -204,7 +204,7 @@
 /// Each Value is one of the JSON kinds:
 ///   null    (nullptr_t)
 ///   boolean (bool)
-///   number  (double)
+///   number  (double or int64)
 ///   string  (StringRef)
 ///   array   (json::Array)
 ///   object  (json::Object)
@@ -226,7 +226,7 @@
 ///     fromJSON(const json::Value&, T&)->bool
 /// Deserializers are provided for:
 ///   - bool
-///   - int
+///   - int and int64_t
 ///   - double
 ///   - std::string
 ///   - vector<T>, where T is deserializable
@@ -254,6 +254,8 @@
   enum Kind {
     Null,
     Boolean,
+    /// Number values can store both int64s and doubles at full precision,
+    /// depending on what they were constructed/parsed from.
     Number,
     String,
     Array,
@@ -281,24 +283,36 @@
   Value(llvm::StringRef V) : Type(T_StringRef) { create<llvm::StringRef>(V); }
   Value(const char *V) : Type(T_StringRef) { create<llvm::StringRef>(V); }
   Value(std::nullptr_t) : Type(T_Null) {}
-  // Prevent implicit conversions to boolean.
-  template <typename T, typename = typename std::enable_if<
-                            std::is_same<T, bool>::value>::type>
+  // Boolean (disallow implicit conversions).
+  // (The last template parameter is a dummy to keep templates distinct.)
+  template <
+      typename T,
+      typename = typename std::enable_if<std::is_same<T, bool>::value>::type,
+      bool = false>
   Value(T B) : Type(T_Boolean) {
     create<bool>(B);
   }
-  // Numbers: arithmetic types that are not boolean.
+  // Integers (except boolean). Must be non-narrowing convertible to int64_t.
   template <
       typename T,
-      typename = typename std::enable_if<std::is_arithmetic<T>::value>::type,
+      typename = typename std::enable_if<std::is_integral<T>::value>::type,
       typename = typename std::enable_if<!std::is_same<T, bool>::value>::type>
-  Value(T D) : Type(T_Number) {
-    create<double>(D);
+  Value(T I) : Type(T_Integer) {
+    create<int64_t>(int64_t{I});
+  }
+  // Floating point. Must be non-narrowing convertible to double.
+  template <typename T,
+            typename =
+                typename std::enable_if<std::is_floating_point<T>::value>::type,
+            double * = nullptr>
+  Value(T D) : Type(T_Double) {
+    create<double>(double{D});
   }
   // Serializable types: with a toJSON(const T&)->Value function, found by ADL.
   template <typename T,
             typename = typename std::enable_if<std::is_same<
-                Value, decltype(toJSON(*(const T *)nullptr))>::value>>
+                Value, decltype(toJSON(*(const T *)nullptr))>::value>,
+            Value * = nullptr>
   Value(const T &V) : Value(toJSON(V)) {}
 
   Value &operator=(const Value &M) {
@@ -319,7 +333,8 @@
       return Null;
     case T_Boolean:
       return Boolean;
-    case T_Number:
+    case T_Double:
+    case T_Integer:
       return Number;
     case T_String:
     case T_StringRef:
@@ -344,12 +359,17 @@
     return llvm::None;
   }
   llvm::Optional<double> getAsNumber() const {
-    if (LLVM_LIKELY(Type == T_Number))
+    if (LLVM_LIKELY(Type == T_Double))
       return as<double>();
+    if (LLVM_LIKELY(Type == T_Integer))
+      return as<int64_t>();
     return llvm::None;
   }
+  // Succeeds if the Value is a Number, and exactly representable as int64_t.
   llvm::Optional<int64_t> getAsInteger() const {
-    if (LLVM_LIKELY(Type == T_Number)) {
+    if (LLVM_LIKELY(Type == T_Integer))
+      return as<int64_t>();
+    if (LLVM_LIKELY(Type == T_Double)) {
       double D = as<double>();
       if (LLVM_LIKELY(std::modf(D, &D) == 0.0 &&
                       D >= double(std::numeric_limits<int64_t>::min()) &&
@@ -407,9 +427,8 @@
   enum ValueType : char {
     T_Null,
     T_Boolean,
-    // FIXME: splitting Number into Double and Integer would allow us to
-    //        round-trip 64-bit integers.
-    T_Number,
+    T_Double,
+    T_Integer,
     T_StringRef,
     T_String,
     T_Object,
@@ -417,7 +436,7 @@
   };
   // All members mutable, see moveFrom().
   mutable ValueType Type;
-  mutable llvm::AlignedCharArrayUnion<bool, double, llvm::StringRef,
+  mutable llvm::AlignedCharArrayUnion<bool, double, int64_t, llvm::StringRef,
                                       std::string, json::Array, json::Object>
       Union;
 };
@@ -505,6 +524,13 @@
   }
   return false;
 }
+inline bool fromJSON(const Value &E, int64_t &Out) {
+  if (auto S = E.getAsInteger()) {
+    Out = *S;
+    return true;
+  }
+  return false;
+}
 inline bool fromJSON(const Value &E, double &Out) {
   if (auto S = E.getAsNumber()) {
     Out = *S;
Index: llvm/trunk/lib/Support/JSON.cpp
===================================================================
--- llvm/trunk/lib/Support/JSON.cpp
+++ llvm/trunk/lib/Support/JSON.cpp
@@ -104,7 +104,8 @@
   switch (Type) {
   case T_Null:
   case T_Boolean:
-  case T_Number:
+  case T_Double:
+  case T_Integer:
     memcpy(Union.buffer, M.Union.buffer, sizeof(Union.buffer));
     break;
   case T_StringRef:
@@ -127,7 +128,8 @@
   switch (Type) {
   case T_Null:
   case T_Boolean:
-  case T_Number:
+  case T_Double:
+  case T_Integer:
     memcpy(Union.buffer, M.Union.buffer, sizeof(Union.buffer));
     break;
   case T_StringRef:
@@ -152,7 +154,8 @@
   switch (Type) {
   case T_Null:
   case T_Boolean:
-  case T_Number:
+  case T_Double:
+  case T_Integer:
     break;
   case T_StringRef:
     as<StringRef>().~StringRef();
@@ -217,7 +220,7 @@
   }
 
   // On invalid syntax, parseX() functions return false and set Err.
-  bool parseNumber(char First, double &Out);
+  bool parseNumber(char First, Value &Out);
   bool parseString(std::string &Out);
   bool parseUnicode(std::string &Out);
   bool parseError(const char *Msg); // always returns false
@@ -317,25 +320,28 @@
     }
   }
   default:
-    if (isNumber(C)) {
-      double Num;
-      if (parseNumber(C, Num)) {
-        Out = Num;
-        return true;
-      } else {
-        return false;
-      }
-    }
+    if (isNumber(C))
+      return parseNumber(C, Out);
     return parseError("Invalid JSON value");
   }
 }
 
-bool Parser::parseNumber(char First, double &Out) {
+bool Parser::parseNumber(char First, Value &Out) {
+  // Read the number into a string. (Must be null-terminated for strto*).
   SmallString<24> S;
   S.push_back(First);
   while (isNumber(peek()))
     S.push_back(next());
   char *End;
+  // Try first to parse as integer, and if so preserve full 64 bits.
+  // strtoll returns long long >= 64 bits, so check it's in range too.
+  auto I = std::strtoll(S.c_str(), &End, 10);
+  if (End == S.end() && I >= std::numeric_limits<int64_t>::min() &&
+      I <= std::numeric_limits<int64_t>::max()) {
+    Out = int64_t(I);
+    return true;
+  }
+  // If it's not an integer
   Out = std::strtod(S.c_str(), &End);
   return End == S.end() || parseError("Invalid JSON value (number?)");
 }
@@ -558,8 +564,12 @@
   case T_Boolean:
     OS << (as<bool>() ? "true" : "false");
     break;
-  case T_Number:
-    OS << format("%g", as<double>());
+  case T_Double:
+    OS << format("%.*g", std::numeric_limits<double>::max_digits10,
+                 as<double>());
+    break;
+  case T_Integer:
+    OS << as<int64_t>();
     break;
   case T_StringRef:
     quote(OS, as<StringRef>());
Index: llvm/trunk/unittests/Support/JSONTest.cpp
===================================================================
--- llvm/trunk/unittests/Support/JSONTest.cpp
+++ llvm/trunk/unittests/Support/JSONTest.cpp
@@ -227,6 +227,66 @@
   }
 }
 
+// Verify special integer handling - we try to preserve exact int64 values.
+TEST(JSONTest, Integers) {
+  struct {
+    const char *Desc;
+    Value Val;
+    const char *Str;
+    llvm::Optional<int64_t> AsInt;
+    llvm::Optional<double> AsNumber;
+  } TestCases[] = {
+      {
+          "Non-integer. Stored as double, not convertible.",
+          double{1.5},
+          "1.5",
+          llvm::None,
+          1.5,
+      },
+
+      {
+          "Integer, not exact double. Stored as int64, convertible.",
+          int64_t{0x4000000000000001},
+          "4611686018427387905",
+          int64_t{0x4000000000000001},
+          double{0x4000000000000000},
+      },
+
+      {
+          "Negative integer, not exact double. Stored as int64, convertible.",
+          int64_t{-0x4000000000000001},
+          "-4611686018427387905",
+          int64_t{-0x4000000000000001},
+          double{-0x4000000000000000},
+      },
+
+      {
+          "Dynamically exact integer. Stored as double, convertible.",
+          double{0x6000000000000000},
+          "6.9175290276410819e+18",
+          int64_t{0x6000000000000000},
+          double{0x6000000000000000},
+      },
+
+      {
+          "Dynamically integer, >64 bits. Stored as double, not convertible.",
+          1.5 * double{0x8000000000000000},
+          "1.3835058055282164e+19",
+          llvm::None,
+          1.5 * double{0x8000000000000000},
+      },
+  };
+  for (const auto &T : TestCases) {
+    EXPECT_EQ(T.Str, s(T.Val)) << T.Desc;
+    llvm::Expected<Value> Doc = parse(T.Str);
+    EXPECT_TRUE(!!Doc) << T.Desc;
+    EXPECT_EQ(Doc->getAsInteger(), T.AsInt) << T.Desc;
+    EXPECT_EQ(Doc->getAsNumber(), T.AsNumber) << T.Desc;
+    EXPECT_EQ(T.Val, *Doc) << T.Desc;
+    EXPECT_EQ(T.Str, s(*Doc)) << T.Desc;
+  }
+}
+
 // Sample struct with typical JSON-mapping rules.
 struct CustomStruct {
   CustomStruct() : B(false) {}