Create optimized versions of the Map/Set clear method

src/objects.h

 // Copyright 2012 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 "src/allocation.h"
@@ skipping 3903 matching lines @@
     return Smi::cast(get(kRemovedHolesIndex + index))->value();
   }
 
   static const int kNotFound = -1;
   static const int kMinCapacity = 4;
 
   static const int kNumberOfBucketsIndex = 0;
   static const int kNumberOfElementsIndex = kNumberOfBucketsIndex + 1;
   static const int kNumberOfDeletedElementsIndex = kNumberOfElementsIndex + 1;
   static const int kHashTableStartIndex = kNumberOfDeletedElementsIndex + 1;
+  static const int kNextTableIndex = kNumberOfElementsIndex;
 
   static const int kNumberOfBucketsOffset =
       kHeaderSize + kNumberOfBucketsIndex * kPointerSize;
   static const int kNumberOfElementsOffset =
       kHeaderSize + kNumberOfElementsIndex * kPointerSize;
   static const int kNumberOfDeletedElementsOffset =
       kHeaderSize + kNumberOfDeletedElementsIndex * kPointerSize;
   static const int kHashTableStartOffset =
       kHeaderSize + kHashTableStartIndex * kPointerSize;
+  static const int kNextTableOffset =
+      kHeaderSize + kNextTableIndex * kPointerSize;
 
   static const int kEntrySize = entrysize + 1;
   static const int kChainOffset = entrysize;
 
   static const int kLoadFactor = 2;
 
+  // NumberOfDeletedElements is set to kClearedTableSentinel when
+  // the table is cleared, which allows iterator transitions to
+  // optimize that case.
+  static const int kClearedTableSentinel = -1;
+
  private:
   static Handle<Derived> Rehash(Handle<Derived> table, int new_capacity);
 
   void SetNumberOfBuckets(int num) {
     set(kNumberOfBucketsIndex, Smi::FromInt(num));
   }
 
   void SetNumberOfElements(int num) {
     set(kNumberOfElementsIndex, Smi::FromInt(num));
   }
@@ skipping 21 matching lines @@
   }
 
   void SetNextTable(Derived* next_table) {
     set(kNextTableIndex, next_table);
   }
 
   void SetRemovedIndexAt(int index, int removed_index) {
     return set(kRemovedHolesIndex + index, Smi::FromInt(removed_index));
   }
 
-  static const int kNextTableIndex = kNumberOfElementsIndex;
   static const int kRemovedHolesIndex = kHashTableStartIndex;
 
   static const int kMaxCapacity =
       (FixedArray::kMaxLength - kHashTableStartIndex)
       / (1 + (kEntrySize * kLoadFactor));
 };
 
 
 class JSSetIterator;
 
@@ skipping 7033 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_