Remove over-zealous hole checking in Array.slice()

src/builtins.cc

Index: src/builtins.cc
diff --git a/src/builtins.cc b/src/builtins.cc
index d62713db4cf3ec39882ac1e717318c9253476988..97fcaebff5875553bc19e1aa33cd54bf3a61beea 100644
--- a/src/builtins.cc
+++ b/src/builtins.cc
@@ -858,23 +858,6 @@ BUILTIN(ArraySlice) {
   }
 
   JSObject* object = JSObject::cast(receiver);
-  ElementsKind kind = object->GetElementsKind();
-
-  if (IsHoleyElementsKind(kind)) {
-    bool packed = true;
-    ElementsAccessor* accessor = ElementsAccessor::ForKind(kind);
-    for (int i = 0; i < len; i++) {
-      if (!accessor->HasElement(object, object, i, elms)) {
-        packed = false;
-        break;
-      }
-    }
-    if (packed) {
-      kind = GetPackedElementsKind(kind);
-    } else if (!receiver->IsJSArray()) {
-      return CallJsBuiltin(isolate, "ArraySlice", args);
-    }
-  }
 
   ASSERT(len >= 0);
   int n_arguments = args.length() - 1;
@@ -924,6 +907,23 @@ BUILTIN(ArraySlice) {
   // Calculate the length of result array.
   int result_len = Max(final - k, 0);
 
+  ElementsKind kind = object->GetElementsKind();
+  if (IsHoleyElementsKind(kind)) {
+    bool packed = true;
+    ElementsAccessor* accessor = ElementsAccessor::ForKind(kind);
+    for (int i = k; i < final; i++) {
+      if (!accessor->HasElement(object, object, i, elms)) {
+        packed = false;
+        break;
+      }
+    }
+    if (packed) {
+      kind = GetPackedElementsKind(kind);
+    } else if (!receiver->IsJSArray()) {
+      return CallJsBuiltin(isolate, "ArraySlice", args);
+    }
+  }
+
   JSArray* result_array;
   MaybeObject* maybe_array = heap->AllocateJSArrayAndStorage(kind,
                                                              result_len,