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

src/runtime.js

Index: src/runtime.js
===================================================================
--- src/runtime.js	(revision 3550)
+++ src/runtime.js	(working copy)
@@ -114,30 +114,33 @@
 // 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);
   }
 }