Loop Nest Splitting for WCET-Optimization and Predictability Improvement

Abstract

This paper presents the influence of the loop nest splitting source code optimization on the worst-case execution time (WCET). Loop nest splitting minimizes the number of executed if-statements in loop nests of embedded multimedia applications. Especially loops and if-statements of high-level languages are an inherent source of unpredictability and loss of precision for WCET analysis. This is caused by the fact that it is difficult to obtain safe and tight worst-case estimates of an application's flow of control through these high-level constructs. In addition, the corresponding control flow redirections expressed at the assembly level reduce predictability even more due to the complex pipeline and branch prediction behavior of modern embedded processors.

The analysis techniques for loop nest splitting are based on precise mathematical models combined with genetic algorithms. On the one hand, these techniques achieve a significantly more homogeneous structure of the control flow. On the other hand, the precision of our analyses leads to the generation of very accurate high-level flow facts for loops and if-statements. The application of our implemented algorithms to three real-life multimedia benchmarks leads to average speed-ups by 25.0% - 30.1%, while WCET is reduced between 34.0% and 36.3%.