Index: llvm/include/llvm/Support/JSON.h
===================================================================
--- llvm/include/llvm/Support/JSON.h
+++ llvm/include/llvm/Support/JSON.h
@@ -234,7 +234,7 @@
 /// Each Value is one of the JSON kinds:
 ///   null    (nullptr_t)
 ///   boolean (bool)
-///   number  (double or int64)
+///   number  (double, int64 or uint64)
 ///   string  (StringRef)
 ///   array   (json::Array)
 ///   object  (json::Object)
@@ -342,9 +342,18 @@
   Value(T B) : Type(T_Boolean) {
     create<bool>(B);
   }
-  // Integers (except boolean). Must be non-narrowing convertible to int64_t.
+
+  // Unsigned 64-bit long integers.
+  template <typename T,
+            typename = std::enable_if_t<std::is_same<T, uint64_t>::value>,
+            bool = false, bool = false>
+  Value(T V) : Type(T_UINT64) { create<uint64_t>(uint64_t{V}); }
+
+  // Integers (except boolean and uint64_t).
+  // Must be non-narrowing convertible to int64_t.
   template <typename T, typename = std::enable_if_t<std::is_integral<T>::value>,
-            typename = std::enable_if_t<!std::is_same<T, bool>::value>>
+            typename = std::enable_if_t<!std::is_same<T, bool>::value>,
+            typename = std::enable_if_t<!std::is_same<T, uint64_t>::value>>
   Value(T I) : Type(T_Integer) {
     create<int64_t>(int64_t{I});
   }
@@ -382,6 +391,7 @@
       return Boolean;
     case T_Double:
     case T_Integer:
+    case T_UINT64:
       return Number;
     case T_String:
     case T_StringRef:
@@ -410,6 +420,8 @@
       return as<double>();
     if (LLVM_LIKELY(Type == T_Integer))
       return as<int64_t>();
+    if (LLVM_LIKELY(Type == T_UINT64))
+      return as<uint64_t>();
     return llvm::None;
   }
   // Succeeds if the Value is a Number, and exactly representable as int64_t.
@@ -425,6 +437,11 @@
     }
     return llvm::None;
   }
+  llvm::Optional<uint64_t> getAsUINT64() const {
+    if (Type == T_UINT64)
+      return as<uint64_t>();
+    return llvm::None;
+  }
   llvm::Optional<llvm::StringRef> getAsString() const {
     if (Type == T_String)
       return llvm::StringRef(as<std::string>());
@@ -467,11 +484,12 @@
 
   friend class OStream;
 
-  enum ValueType : char {
+  enum ValueType : char16_t {
     T_Null,
     T_Boolean,
     T_Double,
     T_Integer,
+    T_UINT64,
     T_StringRef,
     T_String,
     T_Object,
@@ -479,8 +497,9 @@
   };
   // All members mutable, see moveFrom().
   mutable ValueType Type;
-  mutable llvm::AlignedCharArrayUnion<bool, double, int64_t, llvm::StringRef,
-                                      std::string, json::Array, json::Object>
+  mutable llvm::AlignedCharArrayUnion<bool, double, int64_t, uint64_t,
+                                      llvm::StringRef, std::string, json::Array,
+                                      json::Object>
       Union;
   friend bool operator==(const Value &, const Value &);
 };
@@ -683,6 +702,14 @@
   P.report("expected boolean");
   return false;
 }
+inline bool fromJSON(const Value &E, uint64_t &Out, Path P) {
+  if (auto S = E.getAsUINT64()) {
+    Out = *S;
+    return true;
+  }
+  P.report("expected uint64_t");
+  return false;
+}
 inline bool fromJSON(const Value &E, std::nullptr_t &Out, Path P) {
   if (auto S = E.getAsNull()) {
     Out = *S;
Index: llvm/lib/Support/JSON.cpp
===================================================================
--- llvm/lib/Support/JSON.cpp
+++ llvm/lib/Support/JSON.cpp
@@ -109,6 +109,7 @@
   case T_Boolean:
   case T_Double:
   case T_Integer:
+  case T_UINT64:
     memcpy(&Union, &M.Union, sizeof(Union));
     break;
   case T_StringRef:
@@ -133,6 +134,7 @@
   case T_Boolean:
   case T_Double:
   case T_Integer:
+  case T_UINT64:
     memcpy(&Union, &M.Union, sizeof(Union));
     break;
   case T_StringRef:
@@ -159,6 +161,7 @@
   case T_Boolean:
   case T_Double:
   case T_Integer:
+  case T_UINT64:
     break;
   case T_StringRef:
     as<StringRef>().~StringRef();
@@ -750,6 +753,8 @@
     valueBegin();
     if (V.Type == Value::T_Integer)
       OS << *V.getAsInteger();
+    else if (V.Type == Value::T_UINT64)
+      OS << *V.getAsUINT64();
     else
       OS << format("%.*g", std::numeric_limits<double>::max_digits10,
                    *V.getAsNumber());
Index: llvm/unittests/Support/JSONTest.cpp
===================================================================
--- llvm/unittests/Support/JSONTest.cpp
+++ llvm/unittests/Support/JSONTest.cpp
@@ -356,6 +356,13 @@
   }
 }
 
+// Simple uint64_t testing.
+TEST(JSONTest, U64Integers) {
+  Value Val = uint64_t{3100100100};
+  uint64_t Var =  3100100100;
+  EXPECT_EQ(Val, Var);
+}
+
 // Sample struct with typical JSON-mapping rules.
 struct CustomStruct {
   CustomStruct() : B(false) {}