How to Compute Worst-Case Execution Time by Optimization Modulo Theory and a Clever Encoding of Program Semantics

International audienceIn systems with hard real-time constraints, it is necessary to compute upper bounds on the worst-case execution time (WCET) of programs; the closer the bound to the real WCET, the better. This is especially the case of synchronous reactive control loops with a fixed clock; the WCET of the loop body must not exceed the clock period. We compute the WCET (or at least a close upper bound thereof) as the solution of an optimization modulo theory problem that takes into account the semantics of the program, in contrast to other methods that compute the longest path whether or not it is feasible according to these semantics. Optimization modulo theory extends satisfiability modulo theory (SMT) to maximization problems. Immediate encodings of WCET problems into SMT yield formulas intractable for all current production-grade solvers; this is inherent to the DPLL(T) approach to SMT implemented in these solvers. By conjoining some appropriate "cuts" to these formulas, we considerably reduce the computation time of the SMT-solver. We experimented our approach on a variety of control programs, using the OTAWA analyzer both as baseline and as underlying microarchitectural analysis for our analysis, and show notable improvement on the WCET bound on a variety of benchmarks and control programs

Physico-chemical and mechanical degradation of polyamide 11 induced by hydrolysis and thermal ageing

International audienceThis paper deals with the effect of both temperature and water activity on polyamide 11 physico-chemical and mechanical properties. The purpose of this work is to describe the ductile-brittle transition of polyamides during ageing in a wet environment using a mechanical behavior model. For that it is necessary to make physico-chemical analyzes (DSC, FTIR, GPC, viscosity) and mechanical tests (uniaxial tensile test and DMTA). These characterizations were carried out on safe and samples aged until 60 days under hydrothermal conditions in acid water, and under purely thermal conditions in a neutral environment. Changes in mechanical properties with ageing were observed in the polymer and were correlated to morphological changes deduced from the physicochemical characterizations. An increase of the second yield stress related to the recrystallization and a decrease of the strain at break due to the decrease of the molecular weight were observed during ageing