Revert of [heap] delete Heap::LeftTrimFixedAray

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index 674b03b6dafa37e8da15b8ea60691407b34b2a2d..0bd763bc763b7d22826cbd0646ac4dd69860640b 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -3082,6 +3082,24 @@ void Heap::CreateFillerObjectAt(Address addr, int size) {
 }
 
 
+bool Heap::CanMoveObjectStart(HeapObject* object) {
+  if (!FLAG_move_object_start) return false;
+
+  Address address = object->address();
+
+  if (lo_space()->Contains(object)) return false;
+
+  Page* page = Page::FromAddress(address);
+  // We can move the object start if:
+  // (1) the object is not in old space,
+  // (2) the page of the object was already swept,
+  // (3) the page was already concurrently swept. This case is an optimization
+  // for concurrent sweeping. The WasSwept predicate for concurrently swept
+  // pages is set after sweeping all pages.
+  return !InOldSpace(address) || page->WasSwept() || page->SweepingCompleted();
+}
+
+
 void Heap::AdjustLiveBytes(HeapObject* object, int by, InvocationMode mode) {
   if (incremental_marking()->IsMarking() &&
       Marking::IsBlack(Marking::MarkBitFrom(object->address()))) {
@@ -3094,6 +3112,55 @@ void Heap::AdjustLiveBytes(HeapObject* object, int by, InvocationMode mode) {
 }
 
 
+FixedArrayBase* Heap::LeftTrimFixedArray(FixedArrayBase* object,
+                                         int elements_to_trim) {
+  DCHECK(!object->IsFixedTypedArrayBase());
+  const int element_size = object->IsFixedArray() ? kPointerSize : kDoubleSize;
+  const int bytes_to_trim = elements_to_trim * element_size;
+  Map* map = object->map();
+
+  // For now this trick is only applied to objects in new and paged space.
+  // In large object space the object's start must coincide with chunk
+  // and thus the trick is just not applicable.
+  DCHECK(!lo_space()->Contains(object));
+  DCHECK(object->map() != fixed_cow_array_map());
+
+  STATIC_ASSERT(FixedArrayBase::kMapOffset == 0);
+  STATIC_ASSERT(FixedArrayBase::kLengthOffset == kPointerSize);
+  STATIC_ASSERT(FixedArrayBase::kHeaderSize == 2 * kPointerSize);
+
+  const int len = object->length();
+  DCHECK(elements_to_trim <= len);
+
+  // Calculate location of new array start.
+  Address new_start = object->address() + bytes_to_trim;
+
+  // 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(object->address(), 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.
+  DCHECK(CanMoveObjectStart(object));
+  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));
+
+  // Maintain consistency of live bytes during incremental marking
+  Marking::TransferMark(this, object->address(), new_start);
+  AdjustLiveBytes(new_object, -bytes_to_trim, Heap::CONCURRENT_TO_SWEEPER);
+
+  // Notify the heap profiler of change in object layout.
+  OnMoveEvent(new_object, object, new_object->Size());
+  return new_object;
+}
+
+
 // Force instantiation of templatized method.
 template void Heap::RightTrimFixedArray<Heap::SEQUENTIAL_TO_SWEEPER>(
     FixedArrayBase*, int);