Look here: http://www.mersenne.org/various/math.php
at the Lucas-Lehmer test which uses floating point FFT's to square large numbers. I am not sure if that is what is was doing originally, but I suspect it was.
We used that prime95 program from mersenne.org quite a bit in testing because it was very close to our best max-power test for processors. It would keep both the FP and integer ALUs saturated and validated all the results so if anything was wrong it would start complaining.
