[heap] Do not clear mark bits of left trimmed old object start.

src/heap/heap.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/heap/heap.h"
 
 #include "src/accessors.h"
 #include "src/api.h"
 #include "src/assembler-inl.h"
 #include "src/ast/context-slot-cache.h"
@@ skipping 3182 matching lines @@
           ObjectMarking::MarkBitFrom(HeapObject::FromAddress(new_start)))) {
     IncrementalMarking::TransferMark(this, object,
                                      HeapObject::FromAddress(new_start));
   }
 
   // Technically in new space this write might be omitted (except for
   // debug mode which iterates through the heap), but to play safer
   // we still do it.
   CreateFillerObjectAt(old_start, bytes_to_trim, ClearRecordedSlots::kYes);
 
-  // Clear the mark bits of the black area that belongs now to the filler.
-  // This is an optimization. The sweeper will release black fillers anyway.
-  if (incremental_marking()->black_allocation() &&
-      Marking::IsBlackOrGrey(ObjectMarking::MarkBitFrom(object))) {
-    Page* page = Page::FromAddress(old_start);
-    page->markbits()->ClearRange(
-        page->AddressToMarkbitIndex(old_start),
-        page->AddressToMarkbitIndex(old_start + bytes_to_trim));
-  }
-
   // Initialize header of the trimmed array. Since left trimming is only
   // performed on pages which are not concurrently swept creating a filler
   // object does not require synchronization.
   Object** former_start = HeapObject::RawField(object, 0);
   int new_start_index = elements_to_trim * (element_size / kPointerSize);
   former_start[new_start_index] = map;
   former_start[new_start_index + 1] = Smi::FromInt(len - elements_to_trim);
 
   FixedArrayBase* new_object =
       FixedArrayBase::cast(HeapObject::FromAddress(new_start));
@@ skipping 3161 matching lines @@
 }
 
 
 // static
 int Heap::GetStaticVisitorIdForMap(Map* map) {
   return StaticVisitorBase::GetVisitorId(map);
 }
 
 }  // namespace internal
 }  // namespace v8