During the spring semester, I supervised three groups of students working on master’s thesis projects that I proposed. Tackling quite different problems, they all did an excellent job, and it was fun and interesting to supervise their work and to follow their results. In this post, I’m giving a brief account of their work now that they are all done. I will add links to the official thesis reports in Chalmers’s Library as soon as they become available (probably in the fall). While we’re at it: if you’re a current student looking for a master’s thesis project, make sure to check out the projects I currently offer or drop me a line.

The spec is out there — Extracting contracts from code

Pontus Doverstav and Christoffer Medin developed a tool that extracts partial JML contracts (a kind of formal specification) from Java code. Their approach differs from the most common techniques for specification inference, which ultimately reason about the semantics of code (for example, abstract interpretation techniques abstract the concrete semantics of a program on simpler domains). In contrast, Pontus and Christoffer’s work is syntax-driven, as it is based on a collection of patterns that capture how certain code implicitly suggests simple contracts. For example, a qualified call t.foo() implicitly requires that t != null; this assertion is propagated to the precondition of the method where the call appears.

The whole approach is deliberately kept simple and best effort. For instance, whenever there is a method call, we try to determine its effects based on the callee’s (partial) contract; if no callee contract is available, we just assume a worst-case behavior to preserve soundness. In the worst case, an analyzed method may be marked as “failure” by the tool, denoting that no meaningful specification could be reliably extracted. An advantage of keeping the analysis so simple is that we can apply it extensively and retain any result that is useful. When they applied their tool to thousands of methods in four open-source projects, Christoffer and Pontus found that it could extract more than one JML specification case per method on average. Many extracted contracts are simple, but a few are quite complex — such as one capturing in detail the behavior of a large switch statement covering 256 cases. The thesis also reports on some preliminary evidence that suggests the contracts extracted this way can be complementary to those a programmer would write, thus helping the analysis of programs and other applications driven by specifications.

Comparing programming languages in Google Code Jam

Alexandra Back and Emma Westman analyzed empirically the solutions submitted to Google’s Code Jam programming contest, with the overall goal of comparing features of different programming languages. The main motivation behind targeting Code Jam was finding a collection of programs that all implement the same functionality (whatever is required by each round in GCJ) in different programming languages (contestants can use any programming language of their choice), while being able to control for quality — the intuition being that a solution’s rank in the contest is strongly indicative of its quality in terms of correctness and efficiency.

The study focused on the five programming languages most widely used in the contest — C, C++, C#, Java, and Python — and analyzed over 210’000 solution programs. Since a contestant’s performance is based only on his or her ability to produce the correct output for a given, large input to the program, there is no guarantee that the program that has been submitted is the same as the one that was used to generate the correct solution. This complicated the study, and ultimately required to drop 38% of the solutions from some analyses, as they could not be compiled and executed. Nonetheless, a large number of usable solutions remained that could be fully analyzed, which gives confidence in the study’s results. Among them, Alexandra and Emma found that the highest-ranked contestants use C++ predominantly but by far not exclusively; that Python programs tend to be the most concise ones, and C# programs the most verbose; that C and C++ dominate the comparison of running time performance, even though the other languages do not fall that far behind; that C and C++ also have the smallest memory footprint, while Java has the largest (probably due to how the JVM manages memory). Check out the thesis for many more details and results.

Incremental deductive verification for a subset of the Boogie language

Leo Anttila and Mattias Åkesson developed a tool that verifies Boogie programs incrementally as they are modified in each new iteration. Boogie is a popular intermediate verification language, combining a simple imperative language and an expressive program logic. Boogie is also the name of a tool that verifies Boogie programs. Whenever a procedure changes implementation or specification, Boogie verifies it from scratch — like most other verifiers; in contrast, Leo and Mattias’s tool is able to reuse the results of previous verification runs. Suppose, for example, we have verified a procedure foo; later, we introduce a small change to foo that is still compatible with the procedure’s specification. The incremental verifier is aware of what has changed, and only verifies that, intuitively, the difference between the current version of foo and the previous one does not affect correctness.

The details of how incremental verification works are quite technical, and rely on a combination of attribute grammars and incremental parsing. My experience working with Boogie (or program verifiers that use it as back-end) suggests that such an incrementality matches well the way so-called auto-active verification tools are used in practice — namely in a stream of small changes to the source code each immediately followed by a call to the verifier, whose responsiveness is of the essence to support a smooth user experience. The thesis describes experiments simulating this mode of interaction, showing that incrementality can often reduce the running time of the verifier by 30% or more.

References

  1. Christoffer Medin and Pontus Doverstav: The spec is out there — Extracting contracts from code, Master’s thesis in computer science. Department of Computer Science and Engineering, Chalmers University of Technology, Sweden, June 2017.
  2. Alexandra Back and Emma Westman: Comparing programming languages in Google Code Jam, Master’s thesis in computer science, algorithms, languages and logic. Department of Computer Science and Engineering, Chalmers University of Technology, Sweden, June 2017.
  3. Leo Anttila and Mattias Åkesson: Incremental deductive verification for a subset of the Boogie language, Master’s thesis in computer science, algorithms, languages and logic. Department of Computer Science and Engineering, Chalmers University of Technology, Sweden, June 2017.