Reland "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 b929d8567f5c7463a35873db6674d0080c4d5009..57c93366b845a812a6f2193a1ea4c22a6d911b6e 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -2753,12 +2753,23 @@ 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);
       }