Add a special case in Math.round for a SMI result. Also change the implementation for non-SMI case.

Review URL: http://codereview.chromium.org/981002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4146 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/runtime.cc

@@ -5325,12 +5325,21 @@ static Object* Runtime_Math_round(Arguments args) {
   NoHandleAllocation ha;
   ASSERT(args.length() == 1);
   Counters::math_round.Increment();
-
   CONVERT_DOUBLE_CHECKED(x, args[0]);
+
+  if (x > 0 && x < Smi::kMaxValue) {
+    return Smi::FromInt(static_cast<int>(x + 0.5));
+  }
+
   if (signbit(x) && x >= -0.5) return Heap::minus_zero_value();
-  double integer = ceil(x);
-  if (integer - x > 0.5) { integer -= 1.0; }
-  return Heap::NumberFromDouble(integer);
+
+  // if the magnitude is big enough, there's no place for fraction part. If we
+  // try to add 0.5 to this number, 1.0 will be added instead.
+  if (x >= 9007199254740991.0 || x <= -9007199254740991.0) {
+    return args[0];
+  }
+
+  return Heap::NumberFromDouble(floor(x + 0.5));
 }