WCET optimization strategy based on source code refactoring

For safety-critical real-time software, if worst-case execution time (WCET) violates a time constraint, it is considered having a timeliness defect. To fix the defect early with lower cost, a WCET optimization strategy is proposed based on source code refactoring. The strategy guides programmers to search refactoring opportunities in the correct positions and perform refactorings by a reasonable sequence. To this end, the worst-case execution path (WCEP) of a target program is firstly extracted from its control flow graph. Then the WCEP is mapped onto source code by the back-annotation technique. An abstract syntax tree-based invariant path identification algorithm is developed for recognizing the invariant paths from the source-level WCEP. According to the invariant paths and loop statements, the source code is divided into four optimization regions with different priorities. Thus the searching scopes are reduced, and invalid refactorings are avoided. On the basis, the refactoring which has the lowest cost in the same region is performed first. To support the strategy, a cost model of source code refactoring is designed. It mainly considers adverse effects of refactorings on the maintainability of source code. The experimental results showed that the optimization strategy reduced WCET effectively and maximally kept the maintainability. Therefore it is more suitable for WCET optimization in an early programming phase. It is helpful to fix the defects early and then guarantee the timeliness safety of the software.