Improve the generated code for the instanceof operator.

src/runtime.js

 // Copyright 2006-2008 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 256 matching lines @@
 
 // ECMA-262, section 11.8.7, page 54.
 function IN(x) {
   if (x == null || (!IS_OBJECT(x) && !IS_FUNCTION(x))) {
     throw %MakeTypeError('invalid_in_operator_use', [this, x]);
   }
   return %_IsNonNegativeSmi(this) ? %HasElement(x, this) : %HasProperty(x, %ToString(this));
 }
 
 
-// ECMA-262, section 11.8.6, page 54.
+// ECMA-262, section 11.8.6, page 54. To make the implementation more
+// efficient, the return value should be zero if the 'this' is an 
+// instance of F, and non-zero if not. This makes it possible to avoid
+// an expensive ToBoolean conversion in the generated code.
 function INSTANCE_OF(F) {
   var V = this;
   if (!IS_FUNCTION(F)) {
     throw %MakeTypeError('instanceof_function_expected', [V]);
   }
 
   // If V is not an object, return false.
   if (IS_NULL(V) || (!IS_OBJECT(V) && !IS_FUNCTION(V))) {
-    return false;
+    return 1;
   }
 
   // Get the prototype of F; if it is not an object, throw an error.
   var O = F.prototype;
   if (IS_NULL(O) || (!IS_OBJECT(O) && !IS_FUNCTION(O))) {
     throw %MakeTypeError('instanceof_nonobject_proto', [O]);
   }
 
   // Return whether or not O is in the prototype chain of V.
-  return %IsInPrototypeChain(O, V);
+  return %IsInPrototypeChain(O, V) ? 0 : 1;
 }
 
 
 // Get an array of property keys for the given object. Used in
 // for-in statements.
 function GET_KEYS() {
   return %GetPropertyNames(this);
 }
 
 
@@ skipping 208 matching lines @@
   throw %MakeTypeError('cannot_convert_to_primitive', []);
 }
 
 
 // NOTE: Setting the prototype for Array must take place as early as
 // possible due to code generation for array literals.  When
 // generating code for a array literal a boilerplate array is created
 // that is cloned when running the code.  It is essiential that the
 // boilerplate gets the right prototype.
 %FunctionSetPrototype($Array, new $Array(0));