Discrete event systems are increasingly used as a modeling tool to assess safety and cybersecurity of complex systems. In both cases, the analysis relies on the extraction of critical sequences. This approach proves to be very powerful. It suffers, however, from the combinatorial explosion of the number of sequences to look at. To push the limits of what is feasible with reasonable computational resources, extraction algorithms use cutoffs and minimality criteria. In this article, we review the principles of extraction algorithms, and we show that there are important differences between critical sequences extracted in the context of safety analyses and those extracted in the context of cybersecurity analyses. Based on this thorough comparison, we introduce a new cutoff criterion, so-called footprint, that aims at capturing the willfulness of an intruder performing a cyberattack. We illustrate our presentation by means of three case studies, one focused on the analysis of failures and two focused on the analysis of cyberattacks and their effects on safety. We show experimentally the interest of the footprint criterion.

Array bound checking refers to determining whether all array references in a program are within their declared ranges. This checking is critical for software verification and validation because subscripting arrays beyond their declared sizes may produce unexpected results, security holes, or failures. It is available in most commercial compilers but current implementations are not as efficient and effective as one may have hoped: (1) the execution times of array bound checked programs are increased by a factor of up to 5, (2) the compilation times are increased, which is detrimental to development and debugging, (3) the related error messages do not usually carry information to locate the faulty references, and (4) the consistency between actual array sizes and formal array declarations is not often checked.This article presents two optimization techniques that deal with Points 1, 2, and 3, and a new algorithm to tackle Point 4, which is not addressed by the current literature. The first optimization technique is based on the elimination of redundant tests, to provide very accurate information about faulty references during development and testing phases. The second one is based on the insertion of unavoidable tests to provide the smallest possible slowdown during the production phase. The new algorithm ensures the absence of bound violations in every array access in the called procedure with respect to the array declarations in the calling procedure. Our experiments suggest that the optimization of array bound checking depends on several factors, not only the percentage of removed checks, usually considered as the best improvement measuring metrics. The debugging capability and compile-time and run-time performances of our techniques are better than current implementations. The execution times of SPEC95 CFP benchmarks with range checking added by PIPS, our Fortran research compiler, are slightly longer, less than 20%, than that of unchecked programs. More problems due to functional and data recursion would have to be solved to extend these results from Fortran to other languages such as C, C++, or Java, but the issues addressed in this article are nevertheless relevant.

Software debugging, maintenance and reuse have to deal with many problems from Fortran scientific codes that do not fully respect the standard specification. Our study on Linpack, PerfectClub and SPEC95 benchmarks and several industrial software reveals a large number of unprecise variable declarations that prevent program analysis, verification and parallelization. Furthermore, they decrease the readability of programs and make reverse-engineering more difficult.