@HiveDecimalWritableVersionV2
  public void mutateSetScale(int roundingPoint, int roundingMode) {
    if (!isSet) {
      return;
    }
    isSet = fastRound(roundingPoint, roundingMode, this);
    if (!isSet) {
      fastReset();
    }
  }
Allocate scratchLongs with HiveDecimal.SCRATCH_LONGS_LEN longs.
And, allocate scratch scratchBuffer with HiveDecimal.SCRATCH_BUFFER_LEN_BIG_INTEGER_BYTES bytes.
storage-api/src/java/org/apache/hadoop/hive/serde2/io/HiveDecimalWritable.java