Fix setting array length to zero for slow elements.

src/elements.cc

Index: src/elements.cc
diff --git a/src/elements.cc b/src/elements.cc
index 4cb50a461d4a69790e70d2d5d5795e874fe7b99b..39686a2eabfa579499bdd04fd8f06cb68415e1c0 100644
--- a/src/elements.cc
+++ b/src/elements.cc
@@ -1268,7 +1268,30 @@ class DictionaryElementsAccessor
       JSArray* array,
       Object* length_object,
       uint32_t length) {
-    if (length == 0) {
+    Heap* heap = array->GetHeap();
+    int capacity = dict->Capacity();
+    uint32_t new_length = length;
+    uint32_t old_length = static_cast<uint32_t>(array->length()->Number());
+    if (new_length < old_length) {
+      // Find last non-deletable element in range of elements to be
+      // deleted and adjust range accordingly.
+      for (int i = 0; i < capacity; i++) {
+        Object* key = dict->KeyAt(i);
+        if (key->IsNumber()) {
+          uint32_t number = static_cast<uint32_t>(key->Number());
+          if (new_length <= number && number < old_length) {
+            PropertyDetails details = dict->DetailsAt(i);
+            if (details.IsDontDelete()) new_length = number + 1;
+          }
+        }
+      }
+      if (new_length != length) {
+        MaybeObject* maybe_object = heap->NumberFromUint32(new_length);
+        if (!maybe_object->To(&length_object)) return maybe_object;
+      }
+    }
+
+    if (new_length == 0) {
       // If the length of a slow array is reset to zero, we clear
       // the array and flush backing storage. This has the added
       // benefit that the array returns to fast mode.
@@ -1276,45 +1299,22 @@ class DictionaryElementsAccessor
       MaybeObject* maybe_obj = array->ResetElements();
       if (!maybe_obj->ToObject(&obj)) return maybe_obj;
     } else {
-      uint32_t new_length = length;
-      uint32_t old_length = static_cast<uint32_t>(array->length()->Number());
-      if (new_length < old_length) {
-        // Find last non-deletable element in range of elements to be
-        // deleted and adjust range accordingly.
-        Heap* heap = array->GetHeap();
-        int capacity = dict->Capacity();
-        for (int i = 0; i < capacity; i++) {
-          Object* key = dict->KeyAt(i);
-          if (key->IsNumber()) {
-            uint32_t number = static_cast<uint32_t>(key->Number());
-            if (new_length <= number && number < old_length) {
-              PropertyDetails details = dict->DetailsAt(i);
-              if (details.IsDontDelete()) new_length = number + 1;
-            }
+      // Remove elements that should be deleted.
+      int removed_entries = 0;
+      Object* the_hole_value = heap->the_hole_value();
+      for (int i = 0; i < capacity; i++) {
+        Object* key = dict->KeyAt(i);
+        if (key->IsNumber()) {
+          uint32_t number = static_cast<uint32_t>(key->Number());
+          if (new_length <= number && number < old_length) {
+            dict->SetEntry(i, the_hole_value, the_hole_value);
+            removed_entries++;
           }
         }
-        if (new_length != length) {
-          MaybeObject* maybe_object = heap->NumberFromUint32(new_length);
-          if (!maybe_object->To(&length_object)) return maybe_object;
-        }
-
-        // Remove elements that should be deleted.
-        int removed_entries = 0;
-        Object* the_hole_value = heap->the_hole_value();
-        for (int i = 0; i < capacity; i++) {
-          Object* key = dict->KeyAt(i);
-          if (key->IsNumber()) {
-            uint32_t number = static_cast<uint32_t>(key->Number());
-            if (new_length <= number && number < old_length) {
-              dict->SetEntry(i, the_hole_value, the_hole_value);
-              removed_entries++;
-            }
-          }
-        }
-
-        // Update the number of elements.
-        dict->ElementsRemoved(removed_entries);
       }
+
+      // Update the number of elements.
+      dict->ElementsRemoved(removed_entries);
     }
     return length_object;
   }