Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index eae7241cf1782dc59d175d36c0b35830725771b6..4bade10841c0c3d65bd0c072c0db6914b78a53c5 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -2840,6 +2840,8 @@ bool Heap::CreateInitialMaps() {
 
     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)
 
@@ -2959,6 +2961,25 @@ MaybeObject* Heap::AllocateHeapNumber(double value, PretenureFlag pretenure) {
 }
 
 
+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);