Author retrospective for optimizing matrix multiply using PHiPAC
Jeff Bilmes, Krste Asanović, CheeWhye Chin, Jim Demmel · 2014
PHiPAC was an early attempt to improve software performance by searching in a large design space of possible implementations to find the best one. At the time, in the early 1990s, the most efficient numerical linear algebra libraries were carefully hand tuned for specific microarchitectures and compilers, and were often written in assembly language. This allowed very precise tuning of an algorithm to the specifics of a current platform, and provided great opportunity for high efficiency. The prevailing thought at the time was that such an approach was necessary to produce near-peak performance. On the other hand, this approach was brittle, and required great human effort to try each code variant, and so only a tiny subset of the possible code design points could be explored. Worse, given the combined complexities of the compiler and microarchitecture, it was difficult to predict which code variants would be worth the implementation effort.