Worst-case Execution Path Research Articles

WCET-Aware Control Flow Checking With Super-Nodes for Resource-Constrained Embedded Systems

Safety-critical embedded systems in application domains, such as aerospace, automotive, and industrial automation, must satisfy dual requirements of fault-tolerance and real-time predictability. Control flow checking is an effective technique for improving embedded systems’ reliability by online monitoring and checking of software control flow to detect runtime deviations from the control flow graph. However, inserting instrumentation code in every basic block incurs significant execution time overhead, which may cause the program to violate its timing constraints. In this paper, we propose to selectively instrument a subset of code regions that are larger than basic blocks, called super-nodes, in order to make the program partially resilient to control flow errors while keeping the program worst-case execution time (WCET) below a given upper bound. WCET analysis is invoked to estimate the program WCET and to identify the corresponding worst-case execution path (WCEP). An ILP formulation is used to judiciously select a subset of super-nodes on the WCEP for instrumentation, so that the best fault detection coverage is achieved without violating the given WCET upper bound. The optimization is repeated for each identified WCEP until the program WCET satisfies the WCET upper bound. Experimental results demonstrate significant improvements of fault detection coverage compared with related work.

Reducing WCET Overestimations by Correcting Errors in Loop Bound Constraints

In order to reduce overestimations of worst-case execution time (WCET), in this article, we firstly report a kind of specific WCET overestimation caused by non-orthogonal nested loops. Then, we propose a novel correction approach which has three basic steps. The first step is to locate the worst-case execution path (WCEP) in the control flow graph and then map it onto source code. The second step is to identify non-orthogonal nested loops from the WCEP by means of an abstract syntax tree. The last step is to recursively calculate the WCET errors caused by the loose loop bound constraints, and then subtract the total errors from the overestimations. The novelty lies in the fact that the WCET correction is only conducted on the non-branching part of WCEP, thus avoiding potential safety risks caused by possible WCEP switches. Experimental results show that our approach reduces the specific WCET overestimation by an average of more than 82%, and 100% of corrected WCET is no less than the actual WCET. Thus, our approach is not only effective but also safe. It will help developers to design energy-efficient and safe real-time systems.

WCET optimization strategy based on source code refactoring

For safety-critical real-time software, if worst-case execution time (WCET) violates a time constraint, it is considered having a timeliness defect. To fix the defect early with lower cost, a WCET optimization strategy is proposed based on source code refactoring. The strategy guides programmers to search refactoring opportunities in the correct positions and perform refactorings by a reasonable sequence. To this end, the worst-case execution path (WCEP) of a target program is firstly extracted from its control flow graph. Then the WCEP is mapped onto source code by the back-annotation technique. An abstract syntax tree-based invariant path identification algorithm is developed for recognizing the invariant paths from the source-level WCEP. According to the invariant paths and loop statements, the source code is divided into four optimization regions with different priorities. Thus the searching scopes are reduced, and invalid refactorings are avoided. On the basis, the refactoring which has the lowest cost in the same region is performed first. To support the strategy, a cost model of source code refactoring is designed. It mainly considers adverse effects of refactorings on the maintainability of source code. The experimental results showed that the optimization strategy reduced WCET effectively and maximally kept the maintainability. Therefore it is more suitable for WCET optimization in an early programming phase. It is helpful to fix the defects early and then guarantee the timeliness safety of the software.

Criticality: static profiling for real-time programs

With the increasing performance demand in real-time systems it becomes more and more important to provide feedback to programmers and software development tools on the performance-relevant code parts of a real-time program. So far, this information was limited to an estimation of the worst-case execution time (WCET) and its associated worst-case execution path (WCEP) only. However, both, the WCET and the WCEP, only provide partial information. Only code parts that are on one of the WCEPs are indicated to the programmer. No information is provided for all other code parts. To give a comprehensive view covering the entire code base, tools in the spirit of program profiling are required.

Threaded Prefetching: A New Instruction Memory Hierarchy for Real-TimeSystems

Cache memories have been extensively used to bridge the speed gap between high speed processors and relatively slow main memory. However, they are not widely used in real-time systems due to their unpredictable performance. This paper proposes an instruction prefetching scheme called threaded prefetching as an alternative to instruction caching in real-time systems. In the proposed threaded prefetching, an instruction block pointer called a thread is assigned to each instruction memory block and is made to point to the next block on the worst case execution path that is determined by a compile-time analysis. Also, the thread is not updated throughout the entire program execution to guarantee predictability. This paper also compares the worst case performances of various previous instruction prefetching schemes with that of the proposed threaded prefetching. By analyzing several benchmark programs, we show that the worst case performance of the proposed scheme is significantly better than those of previous instruction prefetching schemes. The results also show that when the block size is large enough the worst case performance of the proposed threaded prefetching scheme is almost as good as that of an instruction cache with 100 % hit ratio.

Evaluation of the Applicability and Profitability of Speculative Execution in Real-Time Programs

Speculative execution can enhance the average execution time by executing parts of the code which may or may not be needed. In some cases, rollback is necessary if the computation turns out to be invalid. However, speculative execution should be applied carefully to real-time systems so that the worst-case execution path will not be extended. We have developed a set of compiler transformations to detect safe opportunities for speculative execution in real-time programs and transform the code without jeopardizing the timeliness of the system. To capture various properties of real-time programs that affect applicability and profitability of speculative execution, we have conducted an experiment using randomly generated real-time programs. In this paper, we present simulation results indicating effects of speculative execution on average execution time and program timeliness. In addition, we examine impacts of various program properties, such as locality of variable references, and frequency of conditionals and while loops.