Speculative Multithreading Does not (Necessarily) Waste Energy

While Chip Multiprocessors (CMP) with Speculative Multithreading (SM) have been gaining momentum, experienced processor designers in industry have reservations about their practical implementation. In particular, it is felt that SM is too energy-inefficient to compete against conventional superscalars. This paper challenges the commonly-held view that SM consumes excessive energy. We show a CMP with SM support that is not only faster but also more energy efficient than a state-of-the-art wide-issue superscalar. We demonstrate it with a new energy-efficient CMP micro-architecture. In addition, we identify the additional sources of energy consumption in SM, and propose energy-centric optimizations that mitigate them. Experiments with the SpecInt 2000 codes show that a CMP with 2 4-issue cores and support for SM delivers a speedup of 1.08 over a 8-issue superscalar and consumes only 54% of its power. Alternatively, for the same average power in both chips, the SM CMP is 1.6 times faster than the superscalar on average.