Effects of loop unrolling and use of instruction buffer on processor energy consumption

In the area of Embedded Systems, instruction memories are one of the critical components consuming significant amounts of energy. Existence of a relation between size of the compiled program, and consequently required size of the instruction memory, and the compiler optimization flags is well-known. In particular, loop transformations such as loop unrolling, while having potential to increase performance dramatically, often cause unreasonable growth in the size of the required instruction memory, causing loss of benefit of lower cycle count from overall system energy point of view. One method how to decrease energy consumption of the memories is use of instruction buffers. Often executed loops are stored in the buffer and executed from there, while main memory is not read. In this paper, we show how the compiler flag, controlling loop unrolling, influences the structure of the loops in the program. While unrolling improves performance, unrolled loops can disappear from the program completely, or grow to unreasonable size where use of instruction buffer brings no benefits from the energy point of view.