Do not left-trim arrays when concurrent sweeping is active.

src/builtins.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 592 matching lines @@
 
   // Get first element
   ElementsAccessor* accessor = array->GetElementsAccessor();
   Object* first;
   MaybeObject* maybe_first = accessor->Get(receiver, array, 0, elms_obj);
   if (!maybe_first->To(&first)) return maybe_first;
   if (first->IsTheHole()) {
     first = heap->undefined_value();
   }
 
-  if (!heap->lo_space()->Contains(elms_obj)) {
+  if (!heap->CanMoveObjectStart(elms_obj)) {
     array->set_elements(LeftTrimFixedArray(heap, elms_obj, 1));
   } else {
     // Shift the elements.
     if (elms_obj->IsFixedArray()) {
       FixedArray* elms = FixedArray::cast(elms_obj);
       DisallowHeapAllocation no_gc;
       heap->MoveElements(elms, 0, 1, len - 1);
       elms->set(len - 1, heap->the_hole_value());
     } else {
       FixedDoubleArray* elms = FixedDoubleArray::cast(elms_obj);
@@ skipping 319 matching lines @@
 
       if (elms_obj->IsFixedDoubleArray()) {
         FixedDoubleArray* elms = FixedDoubleArray::cast(elms_obj);
         MoveDoubleElements(elms, delta, elms, 0, actual_start);
       } else {
         FixedArray* elms = FixedArray::cast(elms_obj);
         DisallowHeapAllocation no_gc;
         heap->MoveElements(elms, delta, 0, actual_start);
       }
 
-      elms_obj = LeftTrimFixedArray(heap, elms_obj, delta);
-
+      if (heap->CanMoveObjectStart(elms_obj)) {
+        // On the fast path we move the start of the object in memory.
+        elms_obj = LeftTrimFixedArray(heap, elms_obj, delta);
+      } else {
+        // This is the slow path. We are going to move the elements to the left
+        // by copying them. For trimmed values we store the hole.
+        if (elms_obj->IsFixedDoubleArray()) {
+          FixedDoubleArray* elms = FixedDoubleArray::cast(elms_obj);
+          MoveDoubleElements(elms, 0, elms, delta, len - delta);
+          FillWithHoles(elms, len - delta, len);
+        } else {
+          FixedArray* elms = FixedArray::cast(elms_obj);
+          DisallowHeapAllocation no_gc;
+          heap->MoveElements(elms, 0, delta, len - delta);
+          FillWithHoles(heap, elms, len - delta, len);
+        }
+      }
       elms_changed = true;
     } else {
       if (elms_obj->IsFixedDoubleArray()) {
         FixedDoubleArray* elms = FixedDoubleArray::cast(elms_obj);
         MoveDoubleElements(elms, actual_start + item_count,
                            elms, actual_start + actual_delete_count,
                            (len - actual_delete_count - actual_start));
         FillWithHoles(elms, new_length, len);
       } else {
         FixedArray* elms = FixedArray::cast(elms_obj);
@@ skipping 817 matching lines @@
 }
 BUILTIN_LIST_C(DEFINE_BUILTIN_ACCESSOR_C)
 BUILTIN_LIST_A(DEFINE_BUILTIN_ACCESSOR_A)
 BUILTIN_LIST_H(DEFINE_BUILTIN_ACCESSOR_H)
 BUILTIN_LIST_DEBUG_A(DEFINE_BUILTIN_ACCESSOR_A)
 #undef DEFINE_BUILTIN_ACCESSOR_C
 #undef DEFINE_BUILTIN_ACCESSOR_A
 
 
 } }  // namespace v8::internal