Elide write barriers and remove some heap_object->GetHeap() calls on

src/heap-inl.h

 // Copyright 2011 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 107 matching lines @@
   int size = SeqAsciiString::SizeFor(str.length());
 
   // Allocate string.
   Object* result;
   { MaybeObject* maybe_result = (size > MaxObjectSizeInPagedSpace())
                    ? lo_space_->AllocateRaw(size, NOT_EXECUTABLE)
                    : old_data_space_->AllocateRaw(size);
     if (!maybe_result->ToObject(&result)) return maybe_result;
   }
 
-  reinterpret_cast<HeapObject*>(result)->set_map(map);
+  // String maps are all immortal immovable objects.
+  reinterpret_cast<HeapObject*>(result)->set_map_unsafe(map);
   // Set length and hash fields of the allocated string.
   String* answer = String::cast(result);
   answer->set_length(str.length());
   answer->set_hash_field(hash_field);
 
   ASSERT_EQ(size, answer->Size());
 
   // Fill in the characters.
   memcpy(answer->address() + SeqAsciiString::kHeaderSize,
          str.start(), str.length());
@@ skipping 562 matching lines @@
 
 
 Heap* _inline_get_heap_() {
   return HEAP;
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_HEAP_INL_H_