Improving WCET by applying a WC code-positioning optimization

Abstract

Applications in embedded systems often need to meet specified timing constraints. It is advantageous to not only calculate the worst-case execution time (WCET) of an application, but to also perform transformation, which reduce the WCET, since an application with a lower WCET will be less likely to violate its timing constraints. Some processors incur a pipeline delay whenever an instruction transfers control to a target that is not the next sequential instruction. Code-positioning optimizations attempt to reduce these delays by positioning the basic blocks to minimize the number of unconditional jumps and taken conditional branches that occur. Traditional code-positioning algorithms use profile data to find the frequently executed edges between basic blocks, then minimize the transfers of control along these edges to reduce the average case execution time (ACET). This paper introduces a WCET code-positioning optimization, driven by the worst-case (WC) path information from a timing analyzer, to reduce the WCET instead of ACET. This WCET optimization changes the layout of the code in memory to reduce the branch penalties along the WC paths. Unlike the frequency of edges in traditional profile-driven code positioning, the WC path may change after code-positioning decisions are made. Thus, WCET code positioning is inherently more challenging than ACET code positioning. The experimental results show that this optimization typically finds the optimal layout of the basic blocks with the minimal WCET. The results show over a 7% reduction in WCET is achieved after code positioning is performed.