Reasoning about Aliasing

Object-oriented systems are typically structured as complex networks of interacting mutable objects. To reason about such systems, simple and eecient techniques for coping with aliasing are needed. This paper identiies several key criteria for evaluating techniques for reasoning about aliasing, then proposes a technique which satisses these criteria. The proposed technique is a simple extension of the traditional local store technique for modelling pointers. The increasing popularity of the object-oriented style of programming has resulted in a renewed interest in reasoning about aliasing. Aliasing is common in object-oriented systems, because they are typically structured as complex networks of interacting mutable objects. Simple and eecient techniques for coping with aliasing are a prerequisite to reasoning eeectively about the behaviour of such systems. Techniques for reasoning about aliasing within data structures and aliasing between program variables were thoroughly researched during the 1970's Rey78, Hor79, Cou90]. However, the results of that research have largely been ignored in recent research on aliasing within object-oriented systems. Furthermore, some of the 1970's solutions are unnecessarily restrictive in the forms of aliasing that they allow. Object-oriented systems require more exible solutions. In the rst two sections of this paper, I reconsider the underlying problems caused by aliasing and identify the key criteria which should be used to evaluate the eeectiveness of any technique for reasoning about aliasing. Then I argue that the best technique for satisfying these criteria is one of the 1970's techniques, which I shall call the local store technique. In the remaining sections of this paper, the local store technique is gener-alised to remove the unnecessary restrictions that are usually placed upon it.