Program Slicing for Object-Oriented Programming Languages

Dipl.-Ing. Christoph Steindl

Program slicing is a program analysis technique that reduces programs to those statements that are relevant for a particular computation. A slice provides the answer to the question "What program statements potentially affect the value of variable v at statement s?" Mark Weiser introduced program slicing because he made the observation that programmers have some abstractions about the program in mind during debugging. When debugging a program one follows the dependences from the erroneous statement s back to the influencing parts of the program. These statements may influence s either because they decide whether s is executed or because they define a variable that is used by s. Program slicing computes these dependences automatically and thus assists the programmer in a lot of error prone tasks, such as debugging, program integration, software maintenance, testing, and software quality assurance.

Object-oriented programming languages have attracted more and more attention during the last years since they allow one to write programs that are more flexible, reusable and maintainable. However, the concepts of inheritance, dynamic binding and polymorphism represent new challenges for static program analysis.

The result of this thesis is the Oberon Slicing Tool, a fully operational program slicing tool for the programming language Oberon-2. It integrates state-of-the-art algorithms and applies them to a strongly-typed object-oriented programming language. It extends them to support intermodular slicing of object-oriented programs. Control and data flow analysis considers inheritance, dynamic binding and polymorphism, as well as side-effects of functions, short-circuit evaluation of Boolean expressions and aliases due to reference parameters and pointers. The algorithm for alias analysis is fast but effective by taking into account information about the type of variables and the place of their declaration. The result of static program analysis is visualized with active text elements: hypertext links connect the call sites with the possible call destinations, parameter information elements indicate the direction of data flow at calls. Since static program analysis must make conservative assumptions about actual program executions, the sets of possible aliases and call destinations due to dynamic binding are more general then necessary. We visualize these sets and allow the programmer to restrict them via user interaction. These restrictions are then used to compute more precise control and data flow information. In this way, the programmer can limit the effects of aliases and dynamic binding and bring in his knowledge about the program into the analysis.

Betreuer: Prof. Dr. H. Mössenböck