Branch Target Buffers: WCET Analysis Framework and Timing Predictability

Abstract

One step in the verification of hard real-time systems is to determine upper bounds on the worst-case execution times (WCET) of tasks. To obtain tight bounds, a WCET analysis has to consider microarchitectural features like caches, branch prediction, and branch target buffers (BTB). We propose a modular WCET analysis framework for branch target buffers (BTB), which allows for easy adaptability to different BTBs. As an example, we investigate the Motorola PowerPC 56x family MPC56x, which is used in automotive and avionic systems. On a set of avionic and compiler benchmarks, our analysis improves WCET bounds on average by 13% over no BTB analysis. Capitalizing on the modularity of our framework, we explore alternative hardware designs. We propose more predictable designs, which improve obtainable WCET bounds by up to 20%, reduce analysis time considerably, and simplify the analysis. We generalize our findings and give advice concerning hardware used in real-time systems.