Improving WCET by applying worst-case path optimizations

Abstract

It is advantageous to perform compiler optimizations that attempt to lower the worst-case execution time (WCET) of an embedded application since tasks with lower WCETs are easier to schedule and more likely to meet their deadlines. Compiler writers in recent years have used profile information to detect the frequently executed paths in a program and there has been considerable effort to develop compiler optimizations to improve these paths in order to reduce the average-case execution time (ACET). In this paper, we describe an approach to reduce the WCET by adapting and applying optimizations designed for frequent paths to the worst-case (WC) paths in an application. Instead of profiling to find the frequent paths, our WCET path optimization uses feedback from a timing analyzer to detect the WC paths in a function. Since these path-based optimizations may increase code size, the subsequent effects on the WCET due to these optimizations is measured to ensure that the worst-case path optimizations actually improve the WCET before committing to a code size increase. We evaluate these WC path optimizations and present results showing the decrease in WCET versus the increase in code size.

Using Trace Scratchpads to Reduce Execution Times in Predictable Real-Time Architectures

“…Trace scratchpads are an architectural solution for WCET reduction, forming traces to speed up WC paths as a post compilation step (as opposed to a process applied during compilation, e.g. [38] Figure 14. Effects of architectural variations on the WCET reduction achieved by using both types of scratchpad in place of an instruction scratchpad alone.…”

“…How trace scratchpads are related to previous work: instruction caches [18], trace caches [25], and instruction scratchpads [20,29]. has been previously applied by Zhao [38] in a compiler optimization context. Janapsatya [14] has previously combined trace formation with instruction scratchpads in order to reduce energy consumption.…”

“…In contrast, VLIW compilers usually try to minimize ACET, and therefore aim for the most likely path. WC path-based trace formation has been previously applied by Zhao [38] within a compiler framework.…”

Worst-Case Execution Time Based Optimization of Real-Time Java Programs

“…Zhao et al present path optimizations that are applied on the worst-case path [23], such as superblock formation (i.e., merging basic blocks into a region that has one entry but multiple exits) and duplication of the worst-case path within loops. The optimizations are used to create new optimization opportunities on the worst-case path, but they also increase the code size of the functions.…”

“…For real-time systems we are interested in building new computer architectures [16] and supporting software (compilers [23], Java virtual machines [15], libraries [7]) that are optimized for the WCET. In this paper we describe the optimization of Java applications for the WCET.…”