Compiler-Directed Constant Execution Time on Flat Memory Systems

Abstract

Time predictability is a central requirement for real-time systems. The correct behavior of such a system can only be achieved if the results of programs are ready in time to affect the environment. Execution times of modern systems can vary for many reasons, meaning complex analyses must be performed to ensure that the execution time is bounded and that a task always finishes before its deadline. Care must also be taken to ensure that nefarious actors do not exploit the varying execution time to compromise the system's integrity. Avoiding variable execution times can greatly simplify systems, is inherently more secure, and eliminates the need for complex analyses. In this paper, we first argue for the value of having programs with constant execution times. We then show how the memory system around a processing core can affect execution times even on systems without intermediate storage like caches or scratch-pads. We present automatic compiler techniques for generating constant execution time programs and evaluate their implementation on the Patmos architecture. We show that combining our two compensation techniques is generally superior to either on their own. We compare the performance of our implementation to the estimates produced by the Platin worst-case execution time analyzer. While our implementation significantly impacts performance, it is generally manageable and has the potential for comparable execution times.