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

src/spaces.h

 // Copyright 2011 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 450 matching lines @@
   // are set to the value in "flags", the rest retain the current value
   // in flags_.
   void SetFlags(intptr_t flags, intptr_t mask) {
     flags_ = (flags_ & ~mask) | (flags & mask);
   }
 
   // Return all current flags.
   intptr_t GetFlags() { return flags_; }
 
 
-  // PARALLEL_SWEEPING_PENDING - This page is ready for parallel sweeping.
-  // PARALLEL_SWEEPING_IN_PROGRESS - This page is currently swept or was
-  // swept by a sweeper thread.
   // PARALLEL_SWEEPING_DONE - The page state when sweeping is complete or
   // sweeping must not be performed on that page.
+  // PARALLEL_SWEEPING_FINALIZE - A sweeper thread is done sweeping this
+  // page and will not touch the page memory anymore.
+  // PARALLEL_SWEEPING_IN_PROGRESS - This page is currently swept by a
+  // sweeper thread.
+  // PARALLEL_SWEEPING_PENDING - This page is ready for parallel sweeping.
   enum ParallelSweepingState {
     PARALLEL_SWEEPING_DONE,
+    PARALLEL_SWEEPING_FINALIZE,
     PARALLEL_SWEEPING_IN_PROGRESS,
     PARALLEL_SWEEPING_PENDING
   };
 
   ParallelSweepingState parallel_sweeping() {
     return static_cast<ParallelSweepingState>(
         NoBarrier_Load(&parallel_sweeping_));
   }
 
   void set_parallel_sweeping(ParallelSweepingState state) {
@@ skipping 2490 matching lines @@
   }
   // Must be small, since an iteration is used for lookup.
   static const int kMaxComments = 64;
 };
 #endif
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_SPACES_H_