Change SmartMove no-op behavior to match SimpleMove (and ES6 spec)

The previous behavior, which caused Array.prototype.unshift() (with no args)
to have side-effects, no longer matches the spec (ES6 changed the no-arg behavior
in April 2014). The new SmartMove behavior is also compatible with current
versions of Firefox.

This is a baby step towards getting rid of SmartMove; it isolates the test
change in this patch, instead of lumping it in confusingly with all the
other test updates necessary for moving away from SmartMove.

R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/666883009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24854 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/array.js

@@ -234,6 +234,8 @@ function SmartSlice(array, start_i, del_count, len, deleted_elements) {
 // This function implements the optimized splice implementation that can use
 // special array operations to handle sparse arrays in a sensible fashion.
 function SmartMove(array, start_i, del_count, len, num_additional_args) {
+  // Bail out if no moving is necessary.
+  if (num_additional_args === del_count) return;
   // Move data to new array.
   var new_array = new InternalArray(len - del_count + num_additional_args);
   var indices = %GetArrayKeys(array, len);

test/mjsunit/array-unshift.js

@@ -37,9 +37,7 @@
 })();
 
 
-// Check that unshift with no args has a side-effect of
-// filling the holes with elements from the prototype
-// (if present, of course)
+// Check that unshift with no args has no side-effects.
 (function() {
   var len = 3;
   var array = new Array(len);
@@ -65,15 +63,15 @@
   assertTrue(delete Array.prototype[0]);
   assertTrue(delete Array.prototype[2]);
 
-  // unshift makes array own 0 and 2...
-  assertTrue(array.hasOwnProperty(0));
+  // array still owns nothing...
+  assertFalse(array.hasOwnProperty(0));
   assertFalse(array.hasOwnProperty(1));
-  assertTrue(array.hasOwnProperty(2));
+  assertFalse(array.hasOwnProperty(2));
 
   // ... so they are not affected be delete.
-  assertEquals(array[0], at0);
+  assertEquals(array[0], undefined);
   assertEquals(array[1], undefined);
-  assertEquals(array[2], at2);
+  assertEquals(array[2], undefined);
 })();
 
 
@@ -115,9 +113,7 @@
   assertTrue(delete Array.prototype[7]);
 })();
 
-// Check that unshift with no args has a side-effect of
-// filling the holes with elements from the prototype
-// (if present, of course)
+// Check that unshift with no args has no side-effects.
 (function() {
   var len = 3;
   var array = new Array(len);
@@ -142,12 +138,12 @@
 
   assertEquals(len, array.unshift());
 
-  // unshift makes array own 0 and 2...
-  assertTrue(array.hasOwnProperty(0));
+  // array still owns nothing.
+  assertFalse(array.hasOwnProperty(0));
   assertFalse(array.hasOwnProperty(1));
-  assertTrue(array.hasOwnProperty(2));
+  assertFalse(array.hasOwnProperty(2));
 
-  // ... so they are not affected be delete.
+  // ... but still sees values from array_proto.
   assertEquals(array[0], at0);
   assertEquals(array[1], undefined);
   assertEquals(array[2], at2);