Optimize JS date implementation for smi-only arrays

Review URL: http://codereview.chromium.org/8409010

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9848 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/date.js

@@ -294,8 +294,8 @@ function TimeInYear(year) {
 }
 
 
-var ymd_from_time_cache = [$NaN, $NaN, $NaN];
-var ymd_from_time_cached_time = $NaN;
+var ymd_from_time_cache = [1970, 0, 1];
+var ymd_from_time_cached_time = 0;
 
 function YearFromTime(t) {
   if (t !== ymd_from_time_cached_time) {

src/runtime.cc

@@ -7875,19 +7875,14 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_DateYMDFromTime) {
 
   FixedArrayBase* elms_base = FixedArrayBase::cast(res_array->elements());
   RUNTIME_ASSERT(elms_base->length() == 3);
-  RUNTIME_ASSERT(res_array->GetElementsKind() <= FAST_DOUBLE_ELEMENTS);
-
-  if (res_array->HasFastDoubleElements()) {
-    FixedDoubleArray* elms = FixedDoubleArray::cast(res_array->elements());
-    elms->set(0, year);
-    elms->set(1, month);
-    elms->set(2, day);
-  } else {
-    FixedArray* elms = FixedArray::cast(res_array->elements());
-    elms->set(0, Smi::FromInt(year));
-    elms->set(1, Smi::FromInt(month));
-    elms->set(2, Smi::FromInt(day));
-  }
+  RUNTIME_ASSERT(res_array->HasFastTypeElements());
+
+  MaybeObject* maybe = res_array->EnsureWritableFastElements();
+  if (maybe->IsFailure()) return maybe;
+  FixedArray* elms = FixedArray::cast(res_array->elements());
+  elms->set(0, Smi::FromInt(year));
+  elms->set(1, Smi::FromInt(month));
+  elms->set(2, Smi::FromInt(day));
 
   return isolate->heap()->undefined_value();
 }