void Item_sum_xor::set_bits_from_counters()
{
  ulonglong value= 0;
  for (int i= 0; i < NUM_BIT_COUNTERS; i++)
  {
    value|= (bit_counters[i] % 2) ? (1 << i) : 0;
  }
  bits= value ^ reset_bits;
}
void Item_sum_xor::set_bits_from_counters()
{
  ulonglong value= 0;
  for (int i= 0; i < NUM_BIT_COUNTERS; i++)
  {
    value|= (bit_counters[i] % 2) ? (1 << i) : 0;
  }
  bits= value ^ reset_bits;
}