Is Compiling for Performance == Compiling for Power?

Energy consumption and power dissipation are increasingly becoming important design constraints in high performance microprocessors. Compilers traditionally are not exposed to the energy details of the processor. However, with the increasing power/energy problem, it is important to evaluate how the existing compiler optimizations in uence energy consumption and power dissipation in the processor. In this paper we present a quantitative study wherein we examine the e ect of the standard optimizations levels -O1 to -O4 of DEC Alpha's cc compiler on power and energy of the processor. We also evaluate the e ect of four individual optimizations on power/energy and attempt to classify them as \low energy" or \low power" optimizations. In our experiments we nd that optimizations that improve performance by reducing the number of instructions are optimized for energy. Such optimizations reduce the total amount of work done by the program. This is in contrast to optimizations that improve performance by increasing the overlap in the program during execution. The latter kind of optimizations increase the average power dissipated in the processor.