Allocate fewer empty arrays.

runtime/vm/object.h

 // 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.
 
 #ifndef VM_OBJECT_H_
 #define VM_OBJECT_H_
 
 #include <limits>
 #include "include/dart_api.h"
 #include "platform/assert.h"
@@ skipping 6010 matching lines @@
 
   // Returns true if all elements are OK for canonicalization.
   virtual bool CheckAndCanonicalizeFields(const char** error_str) const;
 
   // Make the array immutable to Dart code by switching the class pointer
   // to ImmutableArray.
   void MakeImmutable() const;
 
   static RawArray* New(intptr_t len, Heap::Space space = Heap::kNew);
 
+  // Empty array is read-only. Use only for arrays without type arguments.
+  static RawArray* NewOrEmpty(intptr_t len, Heap::Space space = Heap::kNew) {
+    return (len == 0) ? empty_array().raw() : New(len, space);
+  }

[siva, 2014/07/09 20:08:03]

may not need this if you do the explicit checks in the call locations.

+
   // Creates and returns a new array with 'new_length'. Copies all elements from
   // 'source' to the new array. 'new_length' must be greater than or equal to
   // 'source.Length()'. 'source' can be null.
   static RawArray* Grow(const Array& source,
                         intptr_t new_length,
                         Heap::Space space = Heap::kNew);
 
   // Return an Array object that contains all the elements currently present
   // in the specified Growable Object Array. This is done by first truncating
   // the Growable Object Array's backing array to the currently used size and
@@ skipping 1107 matching lines @@
 
 
 RawObject* MegamorphicCache::GetTargetFunction(const Array& array,
                                                intptr_t index) {
   return array.At((index * kEntryLength) + kTargetFunctionIndex);
 }
 
 }  // namespace dart
 
 #endif  // VM_OBJECT_H_