Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/heap.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2822 matching lines @@
     }
 
 #define ALLOCATE_VARSIZE_MAP(instance_type, field_name)                        \
     ALLOCATE_MAP(instance_type, kVariableSizeSentinel, field_name)
 
     ALLOCATE_VARSIZE_MAP(FIXED_ARRAY_TYPE, fixed_cow_array)
     ASSERT(fixed_array_map() != fixed_cow_array_map());
 
     ALLOCATE_VARSIZE_MAP(FIXED_ARRAY_TYPE, scope_info)
     ALLOCATE_MAP(HEAP_NUMBER_TYPE, HeapNumber::kSize, heap_number)
+    ALLOCATE_MAP(
+        MUTABLE_HEAP_NUMBER_TYPE, HeapNumber::kSize, mutable_heap_number)
     ALLOCATE_MAP(SYMBOL_TYPE, Symbol::kSize, symbol)
     ALLOCATE_MAP(FOREIGN_TYPE, Foreign::kSize, foreign)
 
     for (unsigned i = 0; i < ARRAY_SIZE(string_type_table); i++) {
       const StringTypeTable& entry = string_type_table[i];
       { MaybeObject* maybe_obj = AllocateMap(entry.type, entry.size);
         if (!maybe_obj->ToObject(&obj)) return false;
       }
       roots_[entry.index] = Map::cast(obj);
     }
@@ skipping 99 matching lines @@
   { MaybeObject* maybe_result = AllocateRaw(size, space, OLD_DATA_SPACE);
     if (!maybe_result->ToObject(&result)) return maybe_result;
   }
 
   HeapObject::cast(result)->set_map_no_write_barrier(heap_number_map());
   HeapNumber::cast(result)->set_value(value);
   return result;
 }
 
 
+MaybeObject* Heap::AllocateMutableHeapNumber(double value,

[Igor Sheludko, 2014/03/27 11:30:16]

This code is the same as AllocateHeapNumber() except map value. Probably it
would be better to extend AllocateHeapNumber() with a enum {IMMUTABLE,MUTABLE}
parameter. 
Same note applies to MacroAssembler and Factory.

+                                             PretenureFlag pretenure) {
+  // Statically ensure that it is safe to allocate heap numbers in paged
+  // spaces.
+  int size = HeapNumber::kSize;
+  STATIC_ASSERT(HeapNumber::kSize <= Page::kMaxRegularHeapObjectSize);
+
+  AllocationSpace space = SelectSpace(size, OLD_DATA_SPACE, pretenure);
+
+  Object* result;
+  MaybeObject* maybe_result = AllocateRaw(size, space, OLD_DATA_SPACE);
+  if (!maybe_result->ToObject(&result)) return maybe_result;
+
+  HeapObject::cast(result)->set_map_no_write_barrier(mutable_heap_number_map());
+  HeapNumber::cast(result)->set_value(value);
+  return result;
+}
+
+
 MaybeObject* Heap::AllocateCell(Object* value) {
   int size = Cell::kSize;
   STATIC_ASSERT(Cell::kSize <= Page::kMaxRegularHeapObjectSize);
 
   Object* result;
   { MaybeObject* maybe_result = AllocateRaw(size, CELL_SPACE, CELL_SPACE);
     if (!maybe_result->ToObject(&result)) return maybe_result;
   }
   HeapObject::cast(result)->set_map_no_write_barrier(cell_map());
   Cell::cast(result)->set_value(value);
@@ skipping 4839 matching lines @@
       static_cast<int>(object_sizes_last_time_[index]));
   CODE_AGE_LIST_COMPLETE(ADJUST_LAST_TIME_OBJECT_COUNT)
 #undef ADJUST_LAST_TIME_OBJECT_COUNT
 
   OS::MemCopy(object_counts_last_time_, object_counts_, sizeof(object_counts_));
   OS::MemCopy(object_sizes_last_time_, object_sizes_, sizeof(object_sizes_));
   ClearObjectStats();
 }
 
 } }  // namespace v8::internal