Implement ES6 symbol registry and predefined symbols

src/symbol.js

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 45 matching lines @@
 
 function SymbolValueOf() {
   if (!(IS_SYMBOL(this) || IS_SYMBOL_WRAPPER(this))) {
     throw MakeTypeError(
       'incompatible_method_receiver', ["Symbol.prototype.valueOf", this]);
   }
   return %_ValueOf(this);
 }
 
 
+function GetSymbolRegistry() {
+  var registry = %SymbolRegistry();
+  if (!('internal' in registry)) {

[arv (Not doing code reviews), 2014/03/22 17:15:49]

Maybe this could have been 

if (IS_UNDEFINED(registry.internal)) {

since the in operator is slow.

[rossberg, 2014/03/24 10:25:53]

Done.

+    registry.internal = {__proto__: null};
+    registry.for = {__proto__: null};
+    registry.keyFor = {__proto__: null};
+  }
+  return registry;
+}
+
+
+function InternalSymbol(key) {
+  var registry = GetSymbolRegistry();
+  if (!(key in registry.internal)) {
+    registry.internal[key] = %CreateSymbol(key);
+  }
+  return registry.internal[key];
+}
+
+
+function SymbolFor(key) {
+  key = TO_STRING_INLINE(key);
+  var registry = GetSymbolRegistry();
+  if (!(key in registry.for)) {
+    var symbol = %CreateSymbol(key);
+    registry.for[key] = symbol;
+    registry.keyFor[symbol] = key;
+  }
+  return registry.for[key];
+}
+
+
+function SymbolKeyFor(symbol) {
+  if (!IS_SYMBOL(symbol)) {
+    throw MakeTypeError("not_a_symbol", [symbol]);
+  }
+  return GetSymbolRegistry().keyFor[symbol];
+}
+
+
 // ES6 19.1.2.8
 function ObjectGetOwnPropertySymbols(obj) {
   if (!IS_SPEC_OBJECT(obj)) {
     throw MakeTypeError("called_on_non_object",
                         ["Object.getOwnPropertySymbols"]);
   }
 
   // TODO(arv): Proxies use a shared trap for String and Symbol keys.
 
   return ObjectGetOwnPropertyKeys(obj, true);
 }
 
 
 //-------------------------------------------------------------------
 
+var symbolCreate = InternalSymbol("@@create");

[arv (Not doing code reviews), 2014/03/22 17:15:49]

Doesn't '@' in a property name cause the object to become slow?

The [[Description]] should be "Symbol.create".

http://people.mozilla.org/~jorendorff/es6-draft.html#sec-well-known-symbols

[rossberg, 2014/03/24 10:25:53]

Objects should be completely ignorant to symbol descriptions. Am I missing
something?
Done.

[arv (Not doing code reviews), 2014/03/24 15:18:57]

You use the key on line 80. If we have a non identifier key we end up with a
slow object instead of an object with a hidden class for registry.internal.
However, I doubt this is an issue since the calls to InternalSymbol is limited.

[rossberg, 2014/03/24 15:37:56]

You're right. The dot will have the same effect, though.

But: I don't think it's an issue, quite the opposite -- this object is meant to
be a dictionary. It would actually be preferable to force this and the other
objects to be in dictionary mode from the get-go, to avoid unnecessary
transitions and polymorphism. I'll add that to the other CL.

+var symbolHasInstance = InternalSymbol("@@hasInstance");
+var symbolIsConcatSpreadable = InternalSymbol("@@isConcatSpreadable");
+var symbolIsRegExp = InternalSymbol("@@isRegExp");
+var symbolIterator = InternalSymbol("@@iterator");
+var symbolToStringTag = InternalSymbol("@@toStringTag");
+var symbolUnscopables = InternalSymbol("@@unscopables");
+
+
+//-------------------------------------------------------------------
+
 function SetUpSymbol() {
   %CheckIsBootstrapping();
 
   %SetCode($Symbol, SymbolConstructor);
   %FunctionSetPrototype($Symbol, new $Object());
 
+  %SetProperty($Symbol, "create", symbolCreate, DONT_ENUM);

[arv (Not doing code reviews), 2014/03/22 17:15:49]

Also, non writable, non configurable.

http://people.mozilla.org/~jorendorff/es6-draft.html#sec-symbol.create

[rossberg, 2014/03/24 10:25:53]

Done.

+  %SetProperty($Symbol, "hasInstance", symbolHasInstance, DONT_ENUM);
+  %SetProperty($Symbol, "isConcatSpreadable",
+      symbolIsConcatSpreadable, DONT_ENUM);
+  %SetProperty($Symbol, "isRegExp", symbolIsRegExp, DONT_ENUM);
+  %SetProperty($Symbol, "iterator", symbolIterator, DONT_ENUM);
+  %SetProperty($Symbol, "toStringTag", symbolToStringTag, DONT_ENUM);
+  %SetProperty($Symbol, "unscopables", symbolUnscopables, DONT_ENUM);
+  InstallFunctions($Symbol, DONT_ENUM, $Array(
+    "for", SymbolFor,
+    "keyFor", SymbolKeyFor
+  ));
+
   %SetProperty($Symbol.prototype, "constructor", $Symbol, DONT_ENUM);
   InstallFunctions($Symbol.prototype, DONT_ENUM, $Array(
     "toString", SymbolToString,
     "valueOf", SymbolValueOf
   ));
 }
 
 SetUpSymbol();
 
 
 function ExtendObject() {
   %CheckIsBootstrapping();
 
   InstallFunctions($Object, DONT_ENUM, $Array(
     "getOwnPropertySymbols", ObjectGetOwnPropertySymbols
   ));
 }
 
 ExtendObject();