Fix issue with Array.concat not preserving holes in the

src/runtime.cc

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 4110 matching lines @@
         IterateElements(Handle<JSObject>::cast(obj), range, visitor);
   }
 
   return nof_elements;
 }
 
 
 /**
  * A helper function of Runtime_ArrayConcat.
  *
- * The first argument is an Array of Arrays and objects. It is the same as
- * the arguments array of Array::concat JS function.
+ * The first argument is an Array of arrays and objects. It is the
+ * same as the arguments array of Array::concat JS function.
  *
- * If an argument is an Array object, the function visits array elements.
- * If an argument is not an Array object, the function visits the object
- * as if it is an one-element array.
+ * If an argument is an Array object, the function visits array
+ * elements.  If an argument is not an Array object, the function
+ * visits the object as if it is an one-element array.
  *
- * If the result array index overflows 32-bit integer, the rounded non-negative
- * number is used as new length. For example, if one array length is 2^32 - 1,
- * second array length is 1, the concatenated array length is 0.
+ * If the result array index overflows 32-bit integer, the rounded
+ * non-negative number is used as new length. For example, if one
+ * array length is 2^32 - 1, second array length is 1, the
+ * concatenated array length is 0.
  */
 static uint32_t IterateArguments(Handle<JSArray> arguments,
                                  ArrayConcatVisitor* visitor) {
   uint32_t visited_elements = 0;
   uint32_t num_of_args = static_cast<uint32_t>(arguments->length()->Number());
 
   for (uint32_t i = 0; i < num_of_args; i++) {
     Handle<Object> obj(arguments->GetElement(i));
     if (obj->IsJSArray()) {
       Handle<JSArray> array = Handle<JSArray>::cast(obj);
@@ skipping 43 matching lines @@
       } else {
         result_length++;
       }
     }
   }
 
   // Allocate an empty array, will set length and content later.
   Handle<JSArray> result = Factory::NewJSArray(0);
 
   uint32_t estimate_nof_elements = IterateArguments(arguments, NULL);
-  // If estimated number of elements is more than half of length,
-  // A fixed array (fast case) is more time & space-efficient than a dictionary.
+  // If estimated number of elements is more than half of length, a
+  // fixed array (fast case) is more time and space-efficient than a
+  // dictionary.
   bool fast_case = (estimate_nof_elements * 2) >= result_length;
 
   Handle<FixedArray> storage;
   if (fast_case) {
-    storage = Factory::NewFixedArray(result_length);
+    // The backing storage array must have non-existing elements to
+    // preserve holes across concat operations.
+    storage = Factory::NewFixedArrayWithHoles(result_length);
 
   } else {
     // TODO(126): move 25% pre-allocation logic into Dictionary::Allocate
     uint32_t at_least_space_for = estimate_nof_elements +
                                   (estimate_nof_elements >> 2);
     storage = Handle<FixedArray>::cast(
                   Factory::NewDictionary(at_least_space_for));
   }
 
   Handle<Object> len = Factory::NewNumber(static_cast<double>(result_length));
@@ skipping 1545 matching lines @@
 
 void Runtime::PerformGC(Object* result) {
   Failure* failure = Failure::cast(result);
   // Try to do a garbage collection; ignore it if it fails. The C
   // entry stub will throw an out-of-memory exception in that case.
   Heap::CollectGarbage(failure->requested(), failure->allocation_space());
 }
 
 
 } }  // namespace v8::internal