Fast Exact Summation Using Small and Large Superaccumulators (XSUM). XSUM is a library for performing exact summation using super-accumulators. It provides methods for exactly summing a set of floating-point numbers, where using a simple summation and the rounding which happens after each addition could be an important factor in many applications. The main algorithm is taken from the original library FUNCTIONS FOR EXACT SUMMATION described in the paper "Fast Exact Summation Using Small and Large Superaccumulators," by Radford M. Neal.