Combined WCET analysis of bitcode and machine code using control-flow relation graphs

Abstract

Static program analyses like stack usage analysis and worst-case execution time (WCET) analysis depend on the actual machine code generated by the compiler for the target system. As the analysis of binary code is costly, hard to diagnose and platform dependent, it is preferable to carry out parts of these analyses on a higher-level program representation. To this end, the higher-level code and the machine code need to be related, a difficult task due to the complexity of modern optimizing compilers. In this article, we present a novel representation called control-flow relation graphs, which provide an accurate model of the control-flow relation between machine code and the compiler's intermediate representation. In order to facilitate the integration of our approach in existing compiler frameworks, we develop a construction algorithm that builds the control-flow relation graph from partial mappings provided by the compiler. The WCET calculation method for control-flow relation graphs processes flow information from both the intermediate representation and machine code. Furthermore, we demonstrate the transformation of flow information from the IR to the machine code level, in order to use existing industrial-strength WCET analysis tools operating on machine code. We implemented the construction algorithm within the LLVM compiler framework, along with an implementation of the combined WCET calculation method. The evaluation demonstrates that the approach is able to relate bitcode (LLVM's intermediate representation) and machine code in a precise way, with a WCET increase of at most 2% when using flow facts on the bitcode level, compared to equivalent ones on the machine-code level. As the methods presented in this article provide a cost-effective way to reuse platform independent flow information, they have the potential to simplify WCET analysis, and popularize its use in the development process of real-time systems.