Pretenure code generation corner case with new space COW arrays.

src/heap.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
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 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
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@@ skipping 5020 matching lines @@
   reinterpret_cast<FixedArray*>(result)->set_length(0);
   return result;
 }
 
 
 MaybeObject* Heap::AllocateEmptyExternalArray(ExternalArrayType array_type) {
   return AllocateExternalArray(0, array_type, NULL, TENURED);
 }
 
 
+MaybeObject* Heap::CopyAndTenureFixedCOWArray(FixedArray* src) {
+  ASSERT(InNewSpace(src));

[Hannes Payer (out of office), 2014/03/17 09:37:18]

This ASSERT may be wrong here, but it should hold in factory.cc.
This is what can happen: The factory may trigger a gc which may move src to old
space.

In that case, you could just return the old src instead of allocating a new one.
WDYT?

[mvstanton, 2014/03/17 12:25:28]

Great idea, thx, done.

+  int len = src->length();
+  Object* obj;
+  { MaybeObject* maybe_obj = AllocateRawFixedArray(len, TENURED);
+    if (!maybe_obj->ToObject(&obj)) return maybe_obj;
+  }
+  HeapObject::cast(obj)->set_map_no_write_barrier(fixed_array_map());

[Hannes Payer (out of office), 2014/03/17 09:37:18]

Can you set fixed_cow_array_map() here?

[mvstanton, 2014/03/17 12:25:28]

No because of protections in FixedArray::set() against setting elements in a COW
array. Created issue v8:3221 to track an alternate approach, that of pretenuring
all boilerplates, allowing us to remove this approach completely.

Also added a comment to that effect.

+  FixedArray* result = FixedArray::cast(obj);
+  result->set_length(len);
+
+  // Copy the content
+  DisallowHeapAllocation no_gc;
+  WriteBarrierMode mode = result->GetWriteBarrierMode(no_gc);
+  for (int i = 0; i < len; i++) result->set(i, src->get(i), mode);
+
+  HeapObject::cast(obj)->set_map_no_write_barrier(fixed_cow_array_map());
+  return result;
+}
+
+
 MaybeObject* Heap::CopyFixedArrayWithMap(FixedArray* src, Map* map) {
   int len = src->length();
   Object* obj;
   { MaybeObject* maybe_obj = AllocateRawFixedArray(len, NOT_TENURED);
     if (!maybe_obj->ToObject(&obj)) return maybe_obj;
   }
   if (InNewSpace(obj)) {
     HeapObject* dst = HeapObject::cast(obj);
     dst->set_map_no_write_barrier(map);
     CopyBlock(dst->address() + kPointerSize,
@@ skipping 2647 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