Theory and Practice of Language Implementation, part 3

The third and last part of the summer school started with Ondrej Lhotak talking about pointer analysis. David Bacon presented a basic introduction to garbage collection and detailed description of the real-time Metronome GC. The last lecturer for this summer school was Patrick Cousot, who gave a very basic and thus understandable introduction to abstract interpretation.

Pointer Analysis

Ondrej’s lectures were focused on pointer analysis for C and Java-like languages. He introduced it as a static analysis for several problems like for instance optimization, call-graph construction, dependency analysis and optimization, cast elimination, side effect, and escape analysis. The major problem seems to be that it is a very broad subject with many different applications, and thus, many different techniques to do the analysis.

So, before applying pointer analysis the right abstraction for a specific problem has to be chosen. Usually, this kind of analysis will result in an over approximation of the actual problem. Under approximations are unsound and the exact points-to sets are not computable. Thus, a suitable over approximation has to be found which is exact enough for a specific question to be answered.

Just to recapitulate two examples, he introduced abstractions based on type filtering, i.e, points-to sets are calculated on the bases of types of the potential object during a program execution. Furthermore, he explained field sensitive abstractions. Here the main property of interest is the field to which an object is assigned and this information is used to calculate the points-to sets. More examples and more advance techniques like refinement demand-driven analysis are shown in the slide set which will hopefully appear on the summer school webpage.

Garbage Collection and the Metronome GC

David started his lectures by showing off a bit 😉 He showed a nice demo of what the Metronome system is able to achieve in terms of real-time constraints. Definitely very interesting and it fortifies my belief that a Java-like language will be the next system programming language succeeding C/C++.

Afterwards, he gave a short introduction to mark/sweep and semi-space collectors. Useful was his explanation about the actual semantics of StopTheWorld, parallel, concurrent, and incremental GCs supported with an illustration (should remember that one). BTW: The world needs a better book on garbage collection. I don’t like the one of Jones and Lins. It does not solve my problems 🙁

Eventually, he started explaining the Metronome-2 systems. The paging approach let me wonder whether there is anything an operating system is actually doing for a modern VM. Well there is, but is it enough to justify the additional trouble? Dreaming of Singularity… The second lesson was basically dedicated to synchronization issues and parallel/concurrent data structures like lock-free stacks and which strategies are out there to implement them. This was a nice addition to what we discussed during Yannis’ lectures and gave me some basic inspiration to implement a lock-free ring buffer for multiple writers and a single reader.

His explanation of all the details of the metronome algorithm and how the different phases interact was quite difficult, and well, not to easy to follow especially since I lost the overall picture from time to time. But well, it is a hard problem and a lot of engineering without a simple solution as it seems.

Abstract Interpretation

The last topic at the summer school was abstract interpretation and the lectures were given by Mr. Abstract Interpretation Patrick Cousot himself. Even so it is a theoretical topic, I liked his introduction. Probably for some of the people it was to basic, but since I never had looked at it before, for me it was the right level of detail and complexity to get an idea what abstract interpretation is actually about.

Unfortunately, my own notes are very spares, and I was not able to retrieve the slide set up to now. However, abstract interpretation tries to provide a sound approximation of the semantics of a computer program, to enable various kinds of static analysis. The main problem here is, similar to pointer analysis, to choose the right abstraction for a given problem.

The examples I recall could have fit into Summit’s presentation about bound computation of loops as well. He introduced ideas like step-wise approximation and other techniques. But, well, unfortunately my memory is too blurry to write something down here with certainty. Beside the introduction of the formal techniques, he also spent some time on how to implement them with Ocaml examples. He also made quite some advertisement for his product ASTRÉE, which originally was developed to analyze code for avionic systems. Well, if I can get my hands on his slides, I might add some more information about the content, but for the moment that’s it.

Where to go next Year?

Need less to say, that I really enjoyed the summer school. The lectures had different levels and quality, but the overall experience was great. Hope, that there will be a VM summer school next year, somewhere on this nice planet. I would love to attend it, too. 🙂

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