Combining Symbolic Execution and Path Enumeration in Worst-Case Execution Time Analysis

his paper examines the problem of determining bounds on execution time of real-time programs. Execution time estimation is generally useful in real-time software verification phase, but may be used in other phases of the design and execution of real-time programs (scheduling, automatic parallelizing, etc.). This paper is devoted to the worst-case execution time (WCET) analysis. We present a static WCET analysis approach aimed to automatically extract flow information used in WCET estimate computing. The approach combines symbolic execution and path enumeration. The main idea is to avoid unfolding loops performed by symbolic execution-based approaches while providing tight and safe WCET estimate

Taming Numbers and Durations in the Model Checking Integrated Planning System

The Model Checking Integrated Planning System (MIPS) is a temporal least commitment heuristic search planner based on a flexible object-oriented workbench architecture. Its design clearly separates explicit and symbolic directed exploration algorithms from the set of on-line and off-line computed estimates and associated data structures. MIPS has shown distinguished performance in the last two international planning competitions. In the last event the description language was extended from pure propositional planning to include numerical state variables, action durations, and plan quality objective functions. Plans were no longer sequences of actions but time-stamped schedules. As a participant of the fully automated track of the competition, MIPS has proven to be a general system; in each track and every benchmark domain it efficiently computed plans of remarkable quality. This article introduces and analyzes the most important algorithmic novelties that were necessary to tackle the new layers of expressiveness in the benchmark problems and to achieve a high level of performance. The extensions include critical path analysis of sequentially generated plans to generate corresponding optimal parallel plans. The linear time algorithm to compute the parallel plan bypasses known NP hardness results for partial ordering by scheduling plans with respect to the set of actions and the imposed precedence relations. The efficiency of this algorithm also allows us to improve the exploration guidance: for each encountered planning state the corresponding approximate sequential plan is scheduled. One major strength of MIPS is its static analysis phase that grounds and simplifies parameterized predicates, functions and operators, that infers knowledge to minimize the state description length, and that detects domain object symmetries. The latter aspect is analyzed in detail. MIPS has been developed to serve as a complete and optimal state space planner, with admissible estimates, exploration engines and branching cuts. In the competition version, however, certain performance compromises had to be made, including floating point arithmetic, weighted heuristic search exploration according to an inadmissible estimate and parameterized optimization

Improving WCET Analysis Precision through Automata Product

Real-time scheduling of application requires sound estimation of the Worst-Case Execution Time (WCET) of each task. Part of the over-approximation introduced by the WCET analysis of a task comes from not taking into account the fact that the (implicit) worst-case execution path may be infeasible. This article does not address the question of finding infeasible paths but provides a new formalism of automata to describe sets of infeasible paths. This formalism combines the possibilities to express state-based path acceptance (like in regular automata), constraints on counters (in the Implicit Path Enumeration Technique fashion) and contexts of validity (like in State charts). We show the applicability of our proposal by performing infeasible paths aware WCET analyses within the OTAWA framework. We provide algorithms that transform the control flow graph and/or the constraints system supporting the WCET analysis in order to exclude the specified paths

Improving WCET Evaluation using Linear Relation Analysis

International audienceThe precision of a worst case execution time (WCET) evaluation tool on a given program is highly dependent on how the tool is able to detect and discard semantically infeasible executions of the program. In this paper, we propose to use the classical abstract interpretation-based method of linear relation analysis to discover and exploit relations between execution paths. For this purpose, we add auxiliary variables (counters) to the program to trace its execution paths. The results are easily incorporated in the classical workflow of a WCET evaluator, when the evaluator is based on the popular implicit path enumeration technique. We use existing tools-a WCET evaluator and a linear relation analyzer-to build and experiment a prototype implementation of this idea. * This work is supported by the French research fundation (ANR) as part of the W-SEPT project (ANR-12-INSE-0001

WCET Analysis: The Annotation Language Challenge

Worst-case execution time (WCET) analysis is indispensable for the successful design and development of systems, which, in addition to their functional constraints, have to satisfy hard real-time constraints. The expressiveness and usability of annotation languages, which are used by algorithms and tools for WCET analysis in order to separate feasible from infeasible program paths, have a crucial impact on the precision and performance of these algorithms and tools. In this paper, we thus propose to complement the WCET tool challenge, which has recently successfully been launched, by a second closely related challenge: the WCET annotation language challenge. We believe that contributions towards mastering this challenge will be essential for the next major step of advancing the field of WCET analysis

Worst-Case Energy Consumption Analysis for Energy-Constrained Embedded Systems

Abstract—The fact that energy is a scarce resource in many embedded real-time systems creates the need for energy-aware task schedulers, which not only guarantee timing constraints but also consider energy consumption. Unfortunately, existing approaches to analyze the worst-case execution time (WCET) of a task usually cannot be directly applied to determine its worst-case energy consumption (WCEC) due to execution time and energy consumption not being closely correlated on many state-of-the-art processors. Instead, a WCEC analyzer must take into account the particular energy characteristics of a target platform. In this paper, we present 0g, a comprehensive approach to WCEC analysis that combines different techniques to speed up the analysis and to improve results. If detailed knowledge about the energy costs of instructions on the target platform is available, our tool is able to compute upper bounds for the WCEC by statically analyzing the program code. Otherwise, a novel ap-proach allows 0g to determine the WCEC by measurement after having identified a set of suitable program inputs based on an auxiliary energy model, which specifies the energy consumption of instructions in relation to each other. Our experiments for three target platforms show that 0g provides precise WCEC estimates. I

TuBound - A Conceptually New Tool for Worst-Case Execution Time Analysis

TuBound is a conceptually new tool for the worst-case execution time (WCET) analysis of programs. A distinctive feature of TuBound is the seamless integration of a WCET analysis component and of a compiler in a uniform tool. TuBound enables the programmer to provide hints improving the precision of the WCET computation on the high-level program source code, while preserving the advantages of using an optimizing compiler and the accuracy of a WCET analysis performed on the low-level machine code. This way, TuBound ideally serves the needs of both the programmer and the WCET analysis by providing them the interface on the very abstraction level that is most appropriate and convenient to them. In this paper we present the system architecture of TuBound, discuss the internal work-flow of the tool, and report on first measurements using benchmarks from Maelardalen University. TuBound took also part in the WCET Tool Challenge 2008