Introduce builtin for Array.shift function.

src/builtins.cc

Index: src/builtins.cc
diff --git a/src/builtins.cc b/src/builtins.cc
index 18afe9119d634fe0001375f6f5dcdb4b9fd05bf3..854337784bb866494cf7aae7febe05fba88fff31 100644
--- a/src/builtins.cc
+++ b/src/builtins.cc
@@ -335,6 +335,41 @@ BUILTIN(ArrayPop) {
 }
 
 
+BUILTIN(ArrayShift) {
+  JSArray* array = JSArray::cast(*args.receiver());
+  ASSERT(array->HasFastElements());
+  Object* undefined = Heap::undefined_value();

[Mads Ager (chromium), 2010/02/12 13:27:51]

Get rid of the local variable and just put in Heap::undefined_value at the only
use site?

[antonm, 2010/02/12 14:05:55]

Done.

+
+  int len = Smi::cast(array->length())->value();
+  if (len == 0) return undefined;
+
+  // Fetch the prototype.
+  JSFunction* array_function =
+      Top::context()->global_context()->array_function();
+  JSObject* prototype = JSObject::cast(array_function->prototype());
+
+  // Get first element
+  FixedArray* elms = FixedArray::cast(array->elements());
+  Object* first = elms->get(0);
+
+  // Set the length.
+  array->set_length(Smi::FromInt(len - 1));
+
+  if (first->IsTheHole())

[Mads Ager (chromium), 2010/02/12 13:27:51]

Please use braces around the body.

[antonm, 2010/02/12 14:05:55]

Done.

+    first = prototype->GetElement(0);
+
+  for (int i = 0; i < len - 1; i++) {
+    Object* e = elms->get(i + 1);
+    if (e->IsTheHole())

[Mads Ager (chromium), 2010/02/12 13:27:51]

Please use braces around the body.

[antonm, 2010/02/12 14:05:55]

Done.

+      e = prototype->GetElement(i + 1);
+    elms->set(i, e);

[Mads Ager (chromium), 2010/02/12 13:27:51]

Will this preserve array holes?  Won't GetElement return undefined for holes in
the array?  Then set will introduce an element that wasn't there before with an
undefined value?

[antonm, 2010/02/12 14:05:55]

Nice catch, thanks a lot.  Regression test added.  Fixed in not most optimal way
for sparsed arrays (doing HasElement call).  If it'd prove to be a bottleneck,
I'd adjust.

The problem is I didn't find an API to get element which would distinguish
between hole and undefined.  It's easy to add (if indeed missing).

+  }
+  elms->set(len - 1, Heap::the_hole_value());
+
+  return first;
+}
+
+
 // -----------------------------------------------------------------------------
 //