Fix Math.round in runtime.cc and x64 optimized code.

src/runtime.cc

 // Copyright 2011 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 6579 matching lines @@
     // to make fuzz-natives test happy.
     return args[0];
   }
 
   HeapNumber* number = reinterpret_cast<HeapNumber*>(args[0]);
 
   double value = number->value();
   int exponent = number->get_exponent();
   int sign = number->get_sign();
 
-  // We compare with kSmiValueSize - 3 because (2^30 - 0.1) has exponent 29 and
-  // should be rounded to 2^30, which is not smi.
-  if (!sign && exponent <= kSmiValueSize - 3) {
+  if (exponent < -1) {
+    // Number in range ]-0.5..0.5[. These always round to +/-zero.
+    if (sign) return isolate->heap()->minus_zero_value();
+    return Smi::FromInt(0);
+  }
+
+  // We compare with kSmiValueSize - 2 because (2^30 - 0.1) has exponent 29 and
+  // should be rounded to 2^30, which is not smi (for 31-bit smis, similar
+  // agument holds for 32-bit smis).
+  if (!sign && exponent < kSmiValueSize - 2) {
     return Smi::FromInt(static_cast<int>(value + 0.5));
   }
 
   // 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 (exponent >= 52) {
     return number;
   }
 
   if (sign && value >= -0.5) return isolate->heap()->minus_zero_value();
@@ skipping 5355 matching lines @@
   } else {
     // Handle last resort GC and make sure to allow future allocations
     // to grow the heap without causing GCs (if possible).
     isolate->counters()->gc_last_resort_from_js()->Increment();
     isolate->heap()->CollectAllGarbage(false);
   }
 }
 
 
 } }  // namespace v8::internal