Computing time as a program variable: a way around infeasible paths

Abstract

Conditional branches connect the values of program variables with the execution paths and thus with the execution times, including the worst-case execution time (WCET). Flow analysis aims to discover this connection and represent it as loop bounds and other path constraints. Usually, a specific analysis of the dependencies between branch conditions and assign­ments to variables creates some representation of the feasible paths, for example as IPET execution-count constraints, from which a WCET bound is calculated. This paper explores another approach that uses a more direct connection between variable values and execution time. The execution time is modeled as a program variable. An analysis of the dependencies between variables, including the execution-time variable, gives a WCET bound that excludes many infeasible paths. Examples show that the approach often works, in principle. It remains to be seen if it is scalable to real programs.