1 files changed, 51 insertions, 4 deletions

diff --git a/js/public/Value.h b/js/public/Value.h
index a40e65c833..01666ed4e1 100644
--- a/js/public/Value.h
+++ b/js/public/Value.h
@@ -140,12 +140,16 @@ static_assert(sizeof(JSValueShiftedTag) == sizeof(uint64_t),
 
 #define JSVAL_TYPE_TO_TAG(type)      ((JSValueTag)(JSVAL_TAG_CLEAR | (type)))
 
+#define JSVAL_RAW64_UNDEFINED        (uint64_t(JSVAL_TAG_UNDEFINED) << 32)
+
 #define JSVAL_UPPER_EXCL_TAG_OF_PRIMITIVE_SET           JSVAL_TAG_OBJECT
 #define JSVAL_UPPER_INCL_TAG_OF_NUMBER_SET              JSVAL_TAG_INT32
 #define JSVAL_LOWER_INCL_TAG_OF_GCTHING_SET             JSVAL_TAG_STRING
 
 #elif defined(JS_PUNBOX64)
 
+#define JSVAL_RAW64_UNDEFINED        (uint64_t(JSVAL_TAG_UNDEFINED) << JSVAL_TAG_SHIFT)
+
 #define JSVAL_PAYLOAD_MASK           0x00007FFFFFFFFFFFLL
 #define JSVAL_TAG_MASK               0xFFFF800000000000LL
 #define JSVAL_TYPE_TO_TAG(type)      ((JSValueTag)(JSVAL_TAG_MAX_DOUBLE | (type)))
@@ -817,7 +821,7 @@ class MOZ_NON_PARAM alignas(8) Value
         double asDouble;
         void* asPtr;
 
-        layout() = default;
+        layout() : asBits(JSVAL_RAW64_UNDEFINED) {}
         explicit constexpr layout(uint64_t bits) : asBits(bits) {}
         explicit constexpr layout(double d) : asDouble(d) {}
     } data;
@@ -843,7 +847,7 @@ class MOZ_NON_PARAM alignas(8) Value
         size_t asWord;
         uintptr_t asUIntPtr;
 
-        layout() = default;
+        layout() : asBits(JSVAL_RAW64_UNDEFINED) {}
         explicit constexpr layout(uint64_t bits) : asBits(bits) {}
         explicit constexpr layout(double d) : asDouble(d) {}
     } data;
@@ -871,7 +875,7 @@ class MOZ_NON_PARAM alignas(8) Value
         double asDouble;
         void* asPtr;
 
-        layout() = default;
+        layout() : asBits(JSVAL_RAW64_UNDEFINED) {}
         explicit constexpr layout(uint64_t bits) : asBits(bits) {}
         explicit constexpr layout(double d) : asDouble(d) {}
     } data;
@@ -895,7 +899,7 @@ class MOZ_NON_PARAM alignas(8) Value
         size_t asWord;
         uintptr_t asUIntPtr;
 
-        layout() = default;
+        layout() : asBits(JSVAL_RAW64_UNDEFINED) {}
         explicit constexpr layout(uint64_t bits) : asBits(bits) {}
         explicit constexpr layout(double d) : asDouble(d) {}
     } data;
@@ -948,8 +952,51 @@ class MOZ_NON_PARAM alignas(8) Value
     }
 } JS_HAZ_GC_POINTER;
 
+/**
+ * This is a null-constructible structure that can convert to and from
+ * a Value, allowing UninitializedValue to be stored in unions.
+ */
+struct MOZ_NON_PARAM alignas(8) UninitializedValue
+{
+  private:
+    uint64_t bits;
+
+  public:
+    UninitializedValue() = default;
+    UninitializedValue(const UninitializedValue&) = default;
+    MOZ_IMPLICIT UninitializedValue(const Value& val) : bits(val.asRawBits()) {}
+
+    inline uint64_t asRawBits() const {
+        return bits;
+    }
+
+    inline Value& asValueRef() {
+        return *reinterpret_cast<Value*>(this);
+    }
+    inline const Value& asValueRef() const {
+        return *reinterpret_cast<const Value*>(this);
+    }
+
+    inline operator Value&() {
+        return asValueRef();
+    }
+    inline operator Value const&() const {
+        return asValueRef();
+    }
+    inline operator Value() const {
+        return asValueRef();
+    }
+
+    inline void operator=(Value const& other) {
+        asValueRef() = other;
+    }
+};
+
 static_assert(sizeof(Value) == 8, "Value size must leave three tag bits, be a binary power, and is ubiquitously depended upon everywhere");
 
+static_assert(sizeof(UninitializedValue) == sizeof(Value), "Value and UninitializedValue must be the same size");
+static_assert(alignof(UninitializedValue) == alignof(Value), "Value and UninitializedValue must have same alignment");
+
 inline bool
 IsOptimizedPlaceholderMagicValue(const Value& v)
 {