Loop Nest Splitting for WCET-Optimization and Predictability Improvement

This paper presents the effect of the loop nest splitting source code optimization on worst-case execution time (WCET). Loop nest splitting minimizes the number of executed if-statements in loop nests of multimedia applications. It identifies iterations where all if-statements are satisfied and splits the loop nest such that if-statements are not executed at all for large parts of the loop nest´s iteration space. Especially loops and if-statements are an inherent source of unpredictability and loss of precision for WCET analysis. This is caused by the difficulty to obtain safe and tight worst-case estimates of an application´s high-level control flow. In addition, assembly-level control flow redirections reduce predictability even more due to complex processor pipelines and branch prediction units. Loop nest splitting bases on precise mathematical models combined with genetic algorithms. On the one hand, these techniques achieve a significantly more homogeneous control flow structure. On the other hand, the precision of our analyses enables to generate very accurate high-level flow facts for loops and if-statements. The application of our implemented algorithms to three real-life benchmarks leads to average speed-ups by 25.0%-30.1%, while WCET is reduced by 34.0%-36.3%