[turbofan] Boost performance of Array.prototype.shift by 4x.

src/objects.h

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_OBJECTS_H_
 #define V8_OBJECTS_H_
 
 #include <iosfwd>
 #include <memory>
 
@@ skipping 9580 matching lines @@
   DECLARE_PRINTER(JSArray)
   DECLARE_VERIFIER(JSArray)
 
   // Number of element slots to pre-allocate for an empty array.
   static const int kPreallocatedArrayElements = 4;
 
   // Layout description.
   static const int kLengthOffset = JSObject::kHeaderSize;
   static const int kSize = kLengthOffset + kPointerSize;
 
+  // Max. number of elements being copied in Array builtins.
+  static const int kMaxCopyElements = 16;
+
   static const int kInitialMaxFastElementArray =
       (kMaxRegularHeapObjectSize - FixedArray::kHeaderSize - kSize -
        AllocationMemento::kSize) /
       kPointerSize;
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(JSArray);
 };
 
 
@@ skipping 625 matching lines @@
   }
 };
 
 
 }  // NOLINT, false-positive due to second-order macros.
 }  // NOLINT, false-positive due to second-order macros.
 
 #include "src/objects/object-macros-undef.h"
 
 #endif  // V8_OBJECTS_H_