Improve the performance of String.prototype.concat and the slow-case...

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 96 matching lines @@
   // If anything else gets here, we just do simple identity check.
   // Objects (including functions), null, undefined and booleans were
   // checked in the CompareStub, so there should be nothing left.
   return %_ObjectEquals(this, x) ? 0 : 1;
 }
 
 
 // ECMA-262, section 11.8.5, page 53. The 'ncr' parameter is used as
 // the result when either (or both) the operands are NaN.
 function COMPARE(x, ncr) {
-  // Fast case for numbers and strings.
-  if (IS_NUMBER(this) && IS_NUMBER(x)) {
-    return %NumberCompare(this, x, ncr);
-  }
-  if (IS_STRING(this) && IS_STRING(x)) {
-    return %StringCompare(this, x);
-  }
+  var left;
 
-  // If one of the operands is undefined, it will convert to NaN and
-  // thus the result should be as if one of the operands was NaN.
-  if (IS_UNDEFINED(this) || IS_UNDEFINED(x)) {
+  // Fast cases for string, numbers and undefined compares.
+  if (IS_STRING(this)) {
+    if (IS_STRING(x)) return %StringCompare(this, x);
+    if (IS_UNDEFINED(x)) return ncr;
+    left = this;
+  } else if (IS_NUMBER(this)) {
+    if (IS_NUMBER(x)) return %NumberCompare(this, x, ncr);
+    if (IS_UNDEFINED(x)) return ncr;
+    left = this;
+  } else if (IS_UNDEFINED(this)) {
     return ncr;
+  } else {
+    if (IS_UNDEFINED(x)) return ncr;
+    left = %ToPrimitive(this, NUMBER_HINT);
   }
 
   // Default implementation.
-  var a = %ToPrimitive(this, NUMBER_HINT);
-  var b = %ToPrimitive(x, NUMBER_HINT);
-  if (IS_STRING(a) && IS_STRING(b)) {
-    return %StringCompare(a, b);
+  var right = %ToPrimitive(x, NUMBER_HINT);
+  if (IS_STRING(left) && IS_STRING(right)) {
+    return %StringCompare(left, right);
   } else {
-    var a_number = %ToNumber(a);
-    var b_number = %ToNumber(b);
-    if (NUMBER_IS_NAN(a_number) || NUMBER_IS_NAN(b_number)) return ncr;
-    return %NumberCompare(a_number, b_number, ncr);
+    var left_number = %ToNumber(left);
+    var right_number = %ToNumber(right);
+    if (NUMBER_IS_NAN(left_number) || NUMBER_IS_NAN(right_number)) return ncr;
+    return %NumberCompare(left_number, right_number, ncr);
   }
 }
 
 
 
 /* -----------------------------------
    - - -   A r i t h m e t i c   - - -
    -----------------------------------
 */
 
@@ skipping 452 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));