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

src/builtins.cc

Index: src/builtins.cc
diff --git a/src/builtins.cc b/src/builtins.cc
index b48de7f2cd52378eb5e706e1951fba1031192c4f..3e97f7648a5f4c328011d4b564e9c35bad96ff34 100644
--- a/src/builtins.cc
+++ b/src/builtins.cc
@@ -610,7 +610,7 @@ BUILTIN(ArrayShift) {
     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.
@@ -950,8 +950,23 @@ BUILTIN(ArraySplice) {
         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()) {