Cache persistence analysis: Theory and practice

To compute a worst-case execution time (WCET) estimate for a program, the architectural effects of the underlying hardware must be modeled. For modern processors this results in the need for a cache and pipeline analysis. The timing-relevant result of the cache analysis is the categorization of the accesses to cached memory. Categorizations that are obtainable by the well-known must and may cache analysis [Ferdinand 1997] are always-hit, always-miss and not-classified. The cache persistence analysis tries to provide additional information for the not-classified case to limit the number of misses. There exists a cache persistence analysis by Ferdinand and Wilhelm based on abstract interpretation computing these classifications. In this article, we present a correctness issue with this analysis. To fix this issue, we propose two new abstract interpretation based persistence analyses and show their safety. One is based on the known may analysis and a second one on the concept of conflict counting. For fully timing compositional architectures [Wilhelm et al. 2009] the persistence information is straightforward to use. We will apply the concepts of persistence analysis for the first time to state-of-the-art architectures that exhibit both timing anomalies and domino effects. Such architectures do not allow the analyzer to quantify the costs of a single cache hit or miss in isolation. To make the usage of the persistence information feasible, we integrate the presented novel persistence analyses together with a novel path analysis approach into the industrially used WCET analyzer aiT.