* need compare-and-swap.
   */
  static final class LockFreeBitArray {
    private static final int LONG_ADDRESSABLE_BITS = 6;
    final AtomicLongArray data;
    private final LongAddable bitCount;

    LockFreeBitArray(long bits) {
      checkArgument(bits > 0, "data length is zero!");
      // Avoid delegating to this(long[]), since AtomicLongArray(long[]) will clone its input and