WCET-driven branch prediction aware code positioning

Abstract

In the past decades, embedded system designers moved from simple, predictable system designs towards complex systems equipped with caches, branch prediction units and speculative execution. This step was necessary in order to fulfill increasing requirements on computational power. Static analysis techniques considering such speculative units had to be developed to allow the estimation of an upper bound of the execution time of a program. This bound is called worst-case execution time (WCET). Its knowledge is crucial to verify whether hard real-time systems satisfy their timing constraints, and the WCET is a key parameter for the design of embedded systems.In this paper, we propose a WCET-driven branch prediction aware optimization which reorders basic blocks of a function in order to reduce the amount of jump instructions and mispredicted branches. We employed a genetic algorithm which rearranges basic blocks in order to decrease the WCET of a program. This enables a first estimation of the possible optimization potential at the cost of high optimization runtimes. To avoid time consuming repetitive WCET analyses, we developed a new algorithm employing integer-linear programming (ILP). The ILP models the worst-case execution path (WCEP) of a program and takes branch prediction effects into account. This algorithm enables short optimization runtimes at slightly decreased optimization results. In a case study, the genetic algorithm is able to reduce the benchmarks' WCET by up to 24.7% whereas our ILP-based approach is able to decrease the WCET by up to 20.0%.

A time-predictable branch predictor

“…However, for compilerdirected static prediction, the compiler has to statically determine the direction of the branch to be predicted to minimize WCET estimates (see [3,5]). In addition, the performance of static predictors like backward taken, forward not taken are highly influenced by the compiler code generation, raising the issue of code generation, in particular basic block re-ordering, for obtaining as low as possible WCET estimates [17].…”

Schedulability Aware WCET-Optimization of Periodic Preemptive Hard Real-Time Multitasking Systems

“…In [3] it was adapted to perform a WCET-oriented static code SPM optimization for the Infineon TriCore microcontroller. Later works used it to implement WCET-oriented register allocation and branch prediction aware code block positioning [5,10]. However, all these approaches have in common that they focus on single-tasking systems.…”

“…In [8] we modified it to allow for schedulability oriented code optimizations of multitasking systems. For fixed-priority systems, the WCRT of each task can be calculated using equation (10) which was proposed in [6]:…”