[runtime] Make elements backing store for arguments non-holey.

Change sloppy and strict arguments objects to FAST_ELEMENTS for their
backing stores, which will allow us to optimize them easier. Also fixes
a bug in the elements accessors that was triggered by this change.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/1433133003

Cr-Commit-Position: refs/heads/master@{#31942}

src/bootstrapper.cc

@@ -1354,8 +1354,8 @@ void Genesis::InitializeGlobal(Handle<JSGlobalObject> global_object,
         arguments_string, code);
     function->shared()->set_instance_class_name(*arguments_string);
 
-    Handle<Map> map =
-        factory->NewMap(JS_OBJECT_TYPE, Heap::kSloppyArgumentsObjectSize);
+    Handle<Map> map = factory->NewMap(
+        JS_OBJECT_TYPE, Heap::kSloppyArgumentsObjectSize, FAST_ELEMENTS);
     // Create the descriptor array for the arguments object.
     Map::EnsureDescriptorSlack(map, 2);
 
@@ -1414,8 +1414,8 @@ void Genesis::InitializeGlobal(Handle<JSGlobalObject> global_object,
     caller->set_setter(*poison);
 
     // Create the map. Allocate one in-object field for length.
-    Handle<Map> map = factory->NewMap(JS_OBJECT_TYPE,
-                                      Heap::kStrictArgumentsObjectSize);
+    Handle<Map> map = factory->NewMap(
+        JS_OBJECT_TYPE, Heap::kStrictArgumentsObjectSize, FAST_ELEMENTS);
     // Create the descriptor array for the arguments object.
     Map::EnsureDescriptorSlack(map, 3);
 

src/elements.cc

@@ -942,8 +942,12 @@ class ElementsAccessorBase : public ElementsAccessor {
                  ? index
                  : kMaxUInt32;
     } else {
-      Smi* smi_length = Smi::cast(JSArray::cast(holder)->length());
-      uint32_t length = static_cast<uint32_t>(smi_length->value());
+      uint32_t length =
+          holder->IsJSArray()
+              ? static_cast<uint32_t>(
+                    Smi::cast(JSArray::cast(holder)->length())->value())
+              : ElementsAccessorSubclass::GetCapacityImpl(holder,
+                                                          backing_store);
       return index < length ? index : kMaxUInt32;
     }
   }

src/objects.cc

@@ -14505,8 +14505,8 @@ int JSObject::GetFastElementsUsage() {
     case FAST_SMI_ELEMENTS:
     case FAST_DOUBLE_ELEMENTS:
     case FAST_ELEMENTS:
-      // Only JSArray have packed elements.
-      return Smi::cast(JSArray::cast(this)->length())->value();
+      return IsJSArray() ? Smi::cast(JSArray::cast(this)->length())->value()
+                         : store->length();
     case FAST_SLOPPY_ARGUMENTS_ELEMENTS:
       store = FixedArray::cast(FixedArray::cast(store)->get(1));
     // Fall through.