Using Program Slicing to Simplify Testing

Program slicing is a technique for automatically identifying the statements of a program which a ect a selected subset of its variables. A large program can be divided into a number of smaller programs (its slices), each constructed for di erent variable subsets. The slices are typically simpler than the original program, thereby simplifying the process of testing a property of the program which only concerns the corresponding subset of its variables. However, some aspects of a program's computation are not captured by a set of variables, rendering slicing inapplicable. To overcome this di culty a program shall be rewritten in a self{checking form by the addition of assignment statements to denote these `implicit' computations. Initially this makes the program longer. However, slicing can now be applied to the introspective program, forming a slice concerned solely with the implicit computation. The simpli cation power of slicing is then improved using program transformation. To illustrate this approach, the implicit computation which dictates whether or not a program is robust shall be taken as an example. Whether or not a program is robust is not generally decidable, making the approach described here particularly appealing because the slices constructed are approximate answers to the undecidable question \Is the program p robust?".