Static Analysis of the Worst-Case Memory Performance for Irregular Codes with Indirections

摘要

Real-time systems are subject to timing constraints, whose upper bound is given by the Worst-Case Execution Time (WCET). Cache memory behavior is difficult to predict analytically and estimating a safe and precise worst-case value is even more challenging. The worst-case memory performance (WCMP) component of the WCET can only be estimated with the precise knowledge of the stream of data addresses accessed by the code, which is determined by the access patterns and the base addresses of the data structures accessed. The regularity of strided access patterns simplifies their analysis, as they are characterized by relatively few parameters, which are often available at compile time. Unfortunately codes may exhibit irregular access patterns, which are much more difficult to statically analyze. As for the base addresses of the data structures, they are not always available at compile-time for many reasons: stack variables, dynamically allocated memory, modules compiled separately, etc. This article addresses these problems by presenting a model that predicts an upper bound of the data cache performance for codes both with regular and irregular access patterns, which is valid for any possible base addresses of the data structures. The model analyzes irregular access patterns due to the presence of indirections in the code and it can provide two kinds of predictions: a safe hard boundary that is suitable for hard real-time systems and a soft boundary whose safeness is not guaranteed but which is valid most of the times. In fact, in all our experiments the number of misses was below the soft boundary predicted by the model. This turns this soft boundary prediction into a valuable tool, particularly for non and soft real-time systems, which tolerate a percentage of the runs exceeding their deadlines.