In-object double fields unboxing (for 64-bit only).

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index f309eae9d1406150cd4dba059e238e0254d7c4ae..6eb6f0166ce5700bacbe23d7fcde1503655a7438 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -2808,12 +2808,23 @@ void MarkCompactCollector::MigrateObject(HeapObject* dst, HeapObject* src,
     Address dst_slot = dst_addr;
     DCHECK(IsAligned(size, kPointerSize));
 
+    bool may_contain_raw_values = src->MayContainRawValues();
+#if V8_DOUBLE_FIELDS_UNBOXING
+    InobjectPropertiesHelper helper(src->map());
+    bool has_only_tagged_fields = helper.all_fields_tagged();
+#endif
     for (int remaining = size / kPointerSize; remaining > 0; remaining--) {
       Object* value = Memory::Object_at(src_slot);
 
       Memory::Object_at(dst_slot) = value;
 
-      if (!src->MayContainRawValues()) {
+#if V8_DOUBLE_FIELDS_UNBOXING
+      if (!may_contain_raw_values &&
+          (has_only_tagged_fields || helper.IsTagged(src_slot - src_addr)))
+#else
+      if (!may_contain_raw_values)
+#endif
+      {
         RecordMigratedSlot(value, dst_slot);
       }