Reordering the assembly instructions in basic blocks to reduce switching activities on the instruction bus

Abstract

Execution time is no longer the only target to achieve when designing programmes for today and next-generation CMOS-based digital systems. One needs to also consider reducing power dissipation. Buses contribute to the power dissipation during the execution of a given programme since instructions and/or operands have to be fetched from the memory. Reducing power dissipation in buses has been addressed in the literature. In this study, the authors address the problem of reducing power dissipation of the instruction bus by reordering the instructions in basic blocks without increasing the executing time and the code size, and while maintaining the original functionality of the programme. The authors target embedded processors having Harvard architecture. They focus on solving this problem for programmes developed at the assembly level, since at that level the machine code can be obtained by simply running an assembler, which allows an accurate computation of switching activities on the instruction bus by considering each pair of instructions. The authors formulate this problem as an integer linear programme (ILP), and they provide two heuristics. Experimental results have shown that the proposed approach can reduce switching activities. The ILP has reduced switching activities by as high as 38%. One of the two proposed heuristics has always resulted in reducing switching activities, and its relative savings are within an average of 5% from the optimum produced using the ILP.