Copy-on-write arrays.

src/parser.cc

Index: src/parser.cc
diff --git a/src/parser.cc b/src/parser.cc
index 1df7c21450253e67a1b4674da43593b5b5551a11..0fef2e2b0e9915c6cf0804ef5c133b494ab8fc6b 100644
--- a/src/parser.cc
+++ b/src/parser.cc
@@ -3461,6 +3461,12 @@ Expression* Parser::ParseArrayLiteral(bool* ok) {
     }
   }
 
+  // Simple and shallow arrays can be lazily copied, we transform the
+  // elements array to a copy-on-write array.
+  if (is_simple && depth == 1 && values.length() > 0) {
+    literals->set_map(Heap::fixed_cow_array_map());
+  }
+
   return NEW(ArrayLiteral(literals, values.elements(),
                           literal_index, is_simple, depth));
 }
@@ -3477,6 +3483,19 @@ bool CompileTimeValue::IsCompileTimeValue(Expression* expression) {
   return lit != NULL && lit->is_simple();
 }
 
+
+bool CompileTimeValue::ArrayLiteralElementNeedsInitialization(
+    Expression* value) {
+  // If value is a literal the property value is already set in the
+  // boilerplate object.
+  if (value->AsLiteral() != NULL) return false;
+  // If value is a materialized literal the property value is already set
+  // in the boilerplate object if it is simple.
+  if (CompileTimeValue::IsCompileTimeValue(value)) return false;
+  return true;
+}
+
+
 Handle<FixedArray> CompileTimeValue::GetValue(Expression* expression) {
   ASSERT(IsCompileTimeValue(expression));
   Handle<FixedArray> result = Factory::NewFixedArray(2, TENURED);