code: func (b *builtinBitCountSig) evalInt(row chunk.Row) (int64, bool, error) {
  	n, isNull, err := b.args[0].EvalInt(b.ctx, row)
  	if err != nil || isNull {
  		if err != nil && types.ErrOverflow.Equal(err) {
  			return 64, false, nil
  		}
  		return 0, true, err
  	}
  
  	var count int64
  	for ; n != 0; n = (n - 1) & n {
  		count++
  	}
  	return count, false, nil
  }
docs: // evalInt evals BIT_COUNT(N).
  // See https://dev.mysql.com/doc/refman/5.7/en/bit-functions.html#function_bit-count