Use proper write barriers instead of RecordWrites when copying object elements.

src/elements.cc

Index: src/elements.cc
diff --git a/src/elements.cc b/src/elements.cc
index 0798918b89531226d1ca96cf29aed97b53e258de..64ac8bba809c4c75204d205a3a4a0f69920ecdd8 100644
--- a/src/elements.cc
+++ b/src/elements.cc
@@ -136,20 +136,13 @@ void CopyObjectToObjectElements(FixedArrayBase* from_base,
   FixedArray* to = FixedArray::cast(to_base);
   DCHECK(IsFastSmiOrObjectElementsKind(from_kind));
   DCHECK(IsFastSmiOrObjectElementsKind(to_kind));
-  Address to_address = to->address() + FixedArray::kHeaderSize;
-  Address from_address = from->address() + FixedArray::kHeaderSize;
-  CopyWords(reinterpret_cast<Object**>(to_address) + to_start,
-            reinterpret_cast<Object**>(from_address) + from_start,
-            static_cast<size_t>(copy_size));
-  if (IsFastObjectElementsKind(from_kind) &&
-      IsFastObjectElementsKind(to_kind)) {
-    Heap* heap = from->GetHeap();
-    if (!heap->InNewSpace(to)) {
-      heap->RecordWrites(to->address(),
-                         to->OffsetOfElementAt(to_start),
-                         copy_size);
-    }
-    heap->incremental_marking()->RecordWrites(to);
+
+  bool needs_write_barrier =
+      IsFastObjectElementsKind(from_kind) && IsFastObjectElementsKind(to_kind);

[ulan, 2015/11/17 13:51:48]

We can precompute the mode here:
WriteBarrierMode write_barrier_mode = 
   IsFastObjectElementsKind(from_kind) && IsFastObjectElementsKind(to_kind) ?
UPDATE_WRITE_BARRIER : SKIP_WRITE_BARRIER.

[Hannes Payer (out of office), 2015/11/18 09:02:17]

Done.

+  for (int i = 0; i < copy_size; i++) {
+    Object* value = from->get(from_start + i);
+    to->set(to_start + i, value,
+            needs_write_barrier ? UPDATE_WRITE_BARRIER : SKIP_WRITE_BARRIER);
   }
 }
 
@@ -160,7 +153,6 @@ static void CopyDictionaryToObjectElements(
   DisallowHeapAllocation no_allocation;
   SeededNumberDictionary* from = SeededNumberDictionary::cast(from_base);
   int copy_size = raw_copy_size;
-  Heap* heap = from->GetHeap();
   if (raw_copy_size < 0) {
     DCHECK(raw_copy_size == ElementsAccessor::kCopyToEnd ||
            raw_copy_size == ElementsAccessor::kCopyToEndAndInitializeToHole);
@@ -183,24 +175,18 @@ static void CopyDictionaryToObjectElements(
   if (to_start + copy_size > to_length) {
     copy_size = to_length - to_start;
   }
+  bool needs_write_barrier = IsFastObjectElementsKind(to_kind);
   for (int i = 0; i < copy_size; i++) {
     int entry = from->FindEntry(i + from_start);
     if (entry != SeededNumberDictionary::kNotFound) {
       Object* value = from->ValueAt(entry);
       DCHECK(!value->IsTheHole());
-      to->set(i + to_start, value, SKIP_WRITE_BARRIER);
+      to->set(i + to_start, value,
+              needs_write_barrier ? UPDATE_WRITE_BARRIER : SKIP_WRITE_BARRIER);
     } else {
       to->set_the_hole(i + to_start);
     }
   }
-  if (IsFastObjectElementsKind(to_kind)) {
-    if (!heap->InNewSpace(to)) {
-      heap->RecordWrites(to->address(),
-                         to->OffsetOfElementAt(to_start),
-                         copy_size);
-    }
-    heap->incremental_marking()->RecordWrites(to);
-  }
 }