Greater performance and better efficiency: Predicated execution has shown us the way

Abstract

We have been riding a strong wave of greater and greater performance for decades, to some extent due to the combination of Moore's Law and Dennard scaling. But we are told all this is coming to an end, in part because we cannot continue to double the transistor count on the chip and we cannot run these things at higher and higher frequencies. Much of the silliness promised by multicore is just that, and not the answer. So, what are we to do? It turns out predication gave us the answer more than 30 years ago. Most of us were not paying attention. Today we have no choice. Predication happened because the compiler, the ISA, and the microarchitecture all cooperated so it could happen. That meant breaking the artificial walls in the transformation hierarchy. If we accept this as something we have to do, there are plenty of opportunities (a) for increased performance (attacking latency instead of just multicore bandwidth) and (b) for better energy efficiency. In this talk I hope to point out some of them, and then ask the obvious question: What do we need to do to make this happen?