Uppsala Dissertations from the Faculty of Science and Technology 36 __________________________________________________________________________________________ JAKOB ENGBLOM Processor Pipelines and Static Worst-Case Execution Time Analysis ACTA UNIVERSITATIS UPSALIENSIS UPPSALA 2002 Dissertation for the Degree of Doctor of Philosophy in Computer Systems pre- sented at Uppsala University, April 19, 2002. ABSTRACT Engblom, J. 2002: Processor Pipelines and Static Worst-Case Execution Time Analy- sis. Acta Universitatis Upsaliensis. Uppsala dissertations from the Faculty of Science and Technology 36. 130 pp. Uppsala. ISBN 91-554-5228-0. Worst-Case Execution Time (WCET) estimates for programs are necessary when building real-time systems. They are used to ensure timely responses from interrupts, to guarantee the throughput of cyclic tasks, as input to scheduling and schedule anal- ysis algorithms, and in many other circumstances. Traditionally, such estimates have been obtained either by measurements or labor-intensive manual analysis, which is both time consuming and error-prone. Static worst-case execution time analysis is a family of techniques that promise to quickly provide safe execution time estimates for real-time programs, simultaneously increasing system quality and decreasing the development cost. This thesis presents several contributions to the state-of-the-art in WCET analysis. We present an overall architecture for WCET analysis tools that provides a frame- work for implementing modules. Within the stable interfaces provided, modules can be independently replaced, making it easy to customize a tool for a particular target and perform performance-precision trade-offs. We have developed concrete techniques for analyzing and representing the timing behavior of programs running on pipelined processors. The representation and anal- ysis is more powerful than previous approaches in that pipeline timing effects across more than pairs of instructions can be handled, and in that no assumptions are made about the program structure. The analysis algorithm relies on a trace-driven processor simulator instead of a special-purpose processor model. This allows us to use existing simulators to adapt the analysis to a new target platform, reducing the retargeting effort. We have defined a formal mathematical model of processor pipelines, which we use to investigate the properties of pipelines and WCET analysis. We prove several interesting properties of processors with in-order issue, such as the freedom from timing anomalies and the fundamental safety of WCET analysis for certain classes of pipelines. We have also constructed a number of examples that demonstrate that tight and safe WCET analysis for pipelined processors might not be as easy as once believed. Considering the link between the analysis methods and the real world, we discuss how to build accurate software models of processor hardware, and the conditions under which accuracy is achievable. Jakob Engblom, Department of Information Technology, Uppsala University, Box 337, SE-751 05 Uppsala, Sweden, E-mail: [email protected]. Also at IAR Systems AB, Box 23051, SE-750 23, Uppsala, Sweden, E-mail: [email protected] c Jakob Engblom 2002 ISSN 1104-2516 ISBN 91-554-5228-0 Printed in Sweden by Elanders Gotab, Stockholm 2002 Distributor: Uppsala University Library, Box 510, SE-751 20 Uppsala, Sweden www.uu.se; e-mail: [email protected] Acknowledgements First of all, I would like to thank Professor Bengt Jonsson, my supervisor. We have both worked hard during the writing of this thesis, and without him and his keen eye for muddled thinking and hand-waving arguments, the quality of this work would have been much lower. This thesis work was performed as an “industry PhD Student” within the Advanced Software Technology (ASTEC) competence centre (www.astec.uu.se) at Uppsala University funded in part by NUTEK/VINNOVA. The Department of Computer Systems (DoCS, www.docs.uu.se)atUp- psala University represented the academic side and provided half of my fi- nancing. DoCS is now part of the Department of Information Technology (www.it.uu.se). The industrial partner was IAR Systems (www.iar.com). I would like to thank IAR Systems in general, and Olle Landstr¨om and Anders Berg in partic- ular, for giving me the opportunity to do an industry PhD. Taking on a PhD student is a big step for a small company like IAR was in 1997. Being an industry PhD, I have traded teaching work for development work and technical training work at IAR, which I believe has been a very succesful formula. I have felt equally at home at IAR and at the university, and hopefully I have helped increase the flow of information and ideas between industry and academia. For me personally, this duality has been very inspiring and rewarding. Despite the principle of no teaching, I have taken the chance to get a little involved in undergraduate teaching anyway, giving guest lectures for a number of real-time and computer architecture courses. Andreas “Ebbe” Ermedahl has been my team-mate in WCET analysis re- search since I started working in the project in 1997. Together, we have achieved much more than any one of us could have done by himself. I thank Ebbe for years of intense and inspiring cooperation and discussion. Friedhelm Stappert at C-Lab in Paderborn, Germany, joined our project in 1999, adding fresh perspectives and implementation manpower. Despite the i ii geographical distribution, Ebbe, Friedhelm, and I have managed to produce a prototype tool and a number of papers together. Professor Hans Hansson got me into real-time systems and WCET research via my Master’s thesis project back in 1997, and has been a great help in writing papers and discussing ideas ever since. Jan Gustafsson has provided valuable and constructive feedback on this thesis and earlier papers, and has been a valuable partner in the research for a long time. Carl von Platen at IAR has been team leader for me and the other industry PhD students at IAR (Jan Sj¨odin and Johan Runesson) for the past few years, and I thank him for many invigorating discussions. Some of our research in WCET analysis and its applications has been per- formed with the help of hard-working Master’s Thesis students: Sven Mont`an, Martin Carlsson,andMagnus Nilsson. Thanks to them, several interesting ideas have been explored that we would not have had time for otherwise. Jan Lindblad at Enea OSE Systems (www.ose.com)andJ¨orgen Hansson at CC- Systems (www.cc-systems.se) have been kind enough to help finance some of the students and have provided industrial input to our research. Over the years, I have had many interesting discussions with people in in- dustry and academia (not all on WCET analysis). I cannot list them all, but some of them are Professor David Whalley at Florida State University, Iain Bate in York, Professor Sang Lyul Min at Seoul National University, Stefan Petters in M¨unchen, Peter Altenbernd at C-Lab in Paderborn, Thomas Lundqvist at Chalmers, Professor Erik Hagersten here in Uppsala, and Raimund Kirner and Pavel Atanassov at TU Wien. Bj¨orn Victor helped me set up the LATEXdocuments, and Johan Bengtsson and I helped each other figure out how to publish our theses and how and where to print them. I would also like to thank everyone at IAR in Uppsala and DoCS for all the fun we have had together! Anna-Maria Lundins Stipendiefond at Sm˚alands Nation has provided gen- erous grants of travel money. If you are planning on getting a PhD in Uppsala, Sm˚alands is the best nation! The ARTES Network (www.artes.uu.se) has provided travel funding and some nice summer schools. Of course, life in Uppsala would have been much less fun without all of my wonderful friends. I would also like thank my parents,Lars-Ake˚ and Christina, and my brother Samuel and sister Cecilia. They have always been there for me! And a special thanks goes to that very special person who has brightened my life in the past few years and who supported me in the stressful spurt of my thesis writing, Eva. Contents 1 Introduction 1 1.1Real-TimeSystems.......................... 1 1.2EmbeddedSystems.......................... 2 1.3 Execution Time and Real-Time Systems .............. 3 1.4 Execution Time Estimates . ..................... 4 1.5UsesofWCET............................ 5 1.6 Obtaining WCET Estimates ..................... 6 1.7ProcessorPipelines.......................... 7 1.7.1 Simple Scalar Pipelines ................... 9 1.7.2 Scalar Pipelines . ..................... 10 1.7.3 Superscalar In-Order Pipelines ............... 10 1.7.4 VLIW (Very Long Instruction Word) ............ 11 1.7.5 Superscalar Out-of-Order Pipelines ............. 11 1.8PropertiesofEmbeddedHardware................. 12 1.9PropertiesofEmbeddedSoftware.................. 14 1.9.1 ProgrammingStyle...................... 14 1.9.2 Algorithmic Timing Behavior ................ 15 1.10 Industrial Practice and Attitudes .................. 16 1.11 Contributions of This Thesis .................... 17 1.12 Outline ................................ 19 2 WCET Overview and Previous Work 21 2.1ComponentsofWCETAnalysis................... 21 2.2 Flow Analysis ............................. 22 2.2.1 Flow Determination ..................... 22 2.2.2 Flow Representation ..................... 24 2.2.3 PreparationforCalculation................. 25 2.2.4 The Mapping Problem .................... 25 2.3 Low-Level Analysis .......................... 26 2.3.1 Global Low-Level Analysis ................. 26 2.3.2 Local Low-Level Analysis .................