MemReporterBase::diff_in_current_scale is defined as follows:

  inline long diff_in_current_scale(size_t s1, size_t s2) const {
    long amount = (long)(s1 - s2);
    long scale = (long)_scale;
    amount = (amount > 0) ? (amount + scale / 2) : (amount - scale / 2);
    return amount / scale;
  }

Long and size_t can have different sizes: 4 bytes and 8 bytes (LLP64). The result of 's1 - s2' might not fit into long. It might not fit into int64_t. For example: s1 is SIZE_MAX and s2 is SIZE_MAX-MAX_INT64-1.

We should calculate diff as size_t and convert a result to long. Assertions must be added to check the diff fits into long.