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

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index ceb6c106c9863dd7d58e6bf9331b70256db86c37..9d3be3361c6649fc21b68ceb7a6e752039d8dbbf 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -6597,9 +6597,16 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_RoundNumber) {
   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));
   }