Principles of timing anomalies in superscalar processors

“This material is presented to ensure timely dissemination of scholarly and technical work. Copyright and all rights therein are retained by authors or by other copyright holders. All persons copying this information are expected to adhere to the terms and constraints invoked by each author's copyright. In most cases, these works may not be reposted without the explicit permission of the copyright holder." “Copyright IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.”The counter-intuitive timing behavior of certain features in superscalar processors that cause severe problems for existing worst-case execution time analysis (WCET) methods is called timing anomalies. In this paper, we identify structural sources potentially causing timing anomalies in superscalar pipelines. We provide examples for cases where timing anomalies can arise in much simpler hardware architectures than commonly supposed (i.e., even in hardware containing only in-orderfunctional units). We elaborate the general principle behind timing anomalies and propose a general criterion (resource allocation criterion) that provides a necessary (but not sufficient) condition for the occurrence of timing anomalies in a processor. This principle allows to state the absence of timing anomalies for a specific combination of hardware and software and thus forms a solid theoretic foundation for the time-predictable execution of real-time software on complex processor hardware