Current trends indicate that leakage energy consumption will be an important concern in upcoming process technologies. In this paper, we propose a compiler-based leakage energy optimization strategy. Our strategy is built upon a data-flow analysis that identifies basic blocks that do not use a given functional unit. Based on this information, the compiler then inserts activate/deactivate instructions in the code to set/reset a sleep signal which controls leakage current for functional units. Our experimental results show that the proposed compiler-based strategy is very effective in reducing leakage energy of functional units.