vm: Avoid list allocation for empty map literals

runtime/vm/parser.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/parser.h"
 #include "vm/flags.h"
 
 #ifndef DART_PRECOMPILED_RUNTIME
 
 #include "lib/invocation_mirror.h"
@@ skipping 13036 matching lines @@
       Type& factory_type = Type::Handle(Z, Type::New(
           factory_class, factory_type_args, type_pos, Heap::kNew));
       factory_type ^= ClassFinalizer::FinalizeType(
           current_class(), factory_type, ClassFinalizer::kFinalize);
       factory_type_args = factory_type.arguments();
       ASSERT(factory_type_args.Length() == factory_class.NumTypeArguments());
     }
     factory_type_args = factory_type_args.Canonicalize();
     ArgumentListNode* factory_param = new(Z) ArgumentListNode(literal_pos);
     // The kv_pair array is temporary and of element type dynamic. It is passed
-    // to the factory to initialize a properly typed map.
-    ArrayNode* kv_pairs = new(Z) ArrayNode(
-        TokenPos(),
-        Type::ZoneHandle(Z, Type::ArrayType()),
-        kv_pairs_list);
-    factory_param->Add(kv_pairs);
+    // to the factory to initialize a properly typed map. Pass a pre-allocated
+    // array for the common empty map literal case.
+    if (kv_pairs_list.length() == 0) {
+      LiteralNode* empty_array_literal =
+          new(Z) LiteralNode(TokenPos(), Object::empty_array());
+      factory_param->Add(empty_array_literal);
+    } else {
+      ArrayNode* kv_pairs = new(Z) ArrayNode(
+          TokenPos(),
+          Type::ZoneHandle(Z, Type::ArrayType()),
+          kv_pairs_list);
+      factory_param->Add(kv_pairs);
+    }
+
     return CreateConstructorCallNode(literal_pos,
                                      factory_type_args,
                                      factory_method,
                                      factory_param);
   }
   UNREACHABLE();
   return NULL;
 }
 
 
@@ skipping 1372 matching lines @@
     const ArgumentListNode& function_args,
     const LocalVariable* temp_for_last_arg,
     bool is_super_invocation) {
   UNREACHABLE();
   return NULL;
 }
 
 }  // namespace dart
 
 #endif  // DART_PRECOMPILED_RUNTIME