Keep new arrays allocated with 'new Array(N)' in fast mode

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index 61e1069d0db35242be66e6f2031c9b098760b064..3d6d3e6cae19187c522a3d497ec4d33a7cf406f1 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -10576,7 +10576,30 @@ RUNTIME_FUNCTION(Runtime_GetArrayKeys) {
   ASSERT(args.length() == 2);
   CONVERT_ARG_HANDLE_CHECKED(JSObject, array, 0);
   CONVERT_NUMBER_CHECKED(uint32_t, length, Uint32, args[1]);
-  if (array->elements()->IsDictionary()) {
+
+  ElementsKind kind = array->GetElementsKind();
+  bool estimate_keys_as_length = IsFastPackedElementsKind(kind);
+  if (IsHoleyElementsKind(kind)) {
+    ElementsAccessor* accessor = array->GetElementsAccessor();
+    // If more than 10% of the array is holes, then calculate the key set, it's
+    // much faster than using the runtime to walk the prototype chain in the
+    // hole case.
+    int holes = 0;
+    int length = array->elements()->length();
+    for (int i = 0; i < length; ++i) {
+      if (!accessor->HasElement(array, array, i)) {
+        ++holes;
+      }
+    }
+    estimate_keys_as_length = holes < static_cast<int>(length / 10);
+  }
+
+  if (estimate_keys_as_length) {
+    RUNTIME_ASSERT(array->HasFastSmiOrObjectElements() ||
+                   array->HasFastDoubleElements());
+    uint32_t actual_length = static_cast<uint32_t>(array->elements()->length());
+    return *isolate->factory()->NewNumberFromUint(Min(actual_length, length));
+  } else {
     Handle<FixedArray> keys = isolate->factory()->empty_fixed_array();
     for (PrototypeIterator iter(isolate, array,
                                 PrototypeIterator::START_AT_RECEIVER);
@@ -10603,11 +10626,6 @@ RUNTIME_FUNCTION(Runtime_GetArrayKeys) {
       if (NumberToUint32(keys->get(i)) >= length) keys->set_undefined(i);
     }
     return *isolate->factory()->NewJSArrayWithElements(keys);
-  } else {
-    RUNTIME_ASSERT(array->HasFastSmiOrObjectElements() ||
-                   array->HasFastDoubleElements());
-    uint32_t actual_length = static_cast<uint32_t>(array->elements()->length());
-    return *isolate->factory()->NewNumberFromUint(Min(actual_length, length));
   }
 }
 
@@ -14931,6 +14949,15 @@ RUNTIME_FUNCTION(Runtime_InternalArrayConstructor) {
 }
 
 
+RUNTIME_FUNCTION(Runtime_NormalizeElements) {
+  HandleScope scope(isolate);
+  ASSERT(args.length() == 1);
+  CONVERT_ARG_HANDLE_CHECKED(JSObject, array, 0);
+  JSObject::NormalizeElements(array);
+  return *array;
+}
+
+
 RUNTIME_FUNCTION(Runtime_MaxSmi) {
   ASSERT(args.length() == 0);
   return Smi::FromInt(Smi::kMaxValue);