The Energy Impact of Aggressive Loop Fusion

Loop fusion combines corresponding iterations of different loops. As shown in previous work, it can often decrease program run time by reducing the overhead of loop control and effective address calculations, and in important cases by dramatically increasing cache or register reuse. In this paper we consider corresponding changes in program energy. By merging program phases, fusion tends to increase the uniformity, or balance of demand for system resources. On a conventional superscalar processor, increased balance tends to increase IPC, and thus dynamic power, so that fusion-induced improvements in program energy are slightly smaller than improvements in program run time. If IPC is held constant, however, by reducing frequency and voltage-particularly on a processor with multiple clock domains-then energy improvements may significantly exceed run time improvements. We demonstrate the benefits of increased program balance under a theoretical model of processor energy consumption. We then evaluate the benefits of fusion empirically on synthetic and real-world benchmarks, using our existing loop-fusing compiler, and running on a heavily modified version of the SimpleScalar/Wattch simulator. In addition to validating our theoretical model, the simulation results allow us to "tease apart" the various factors that contribute to fusion-induced time and energy savings