Pretenure code generation corner case with new space COW arrays.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 4e0e8a67c2217213c0c0a28939d8d147f01fec47..336608daf116c116c1f66dff6264f0671faf91ab 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -5038,6 +5038,27 @@ MaybeObject* Heap::AllocateEmptyExternalArray(ExternalArrayType array_type) {
 }
 
 
+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;