Data Flow

In short

In this assignment you shall experiment with and extend the material from Chapter 2 of Nielson, Nielson and Hankin, mainly by doing a selection of the exercises similar to those in the book. The various parts are largely independent, and I have indicated when you have the necessary information to complete the assignment.

The assignment may be done alone or, preferably, in pairs. Every student should be able to explain orally and clearly every part of the assignment he hands in. The lecturer has the freedom to ask this explanation at any time, meaning that you might not be warned in advance).

Not all of the components below are worth the same number of grade point: Part 1 to 5 are weighted at 10, 20, 20, 25 and 25 percent.

The goal of the assignment

A large part of the chapter is devoted to developing a dataflow analysis framework for an imperative language. To get a thorough understanding of what is really happening underneath all that notation you shall work on the following subjects:

• How to develop the transfer functions for a given analysis
• How chaotic iteration should be performed
• How to develop transfer functions for new language constructs and instantiate these for a specific analysis
• How to prove a soundness result for an analysis
• How to deal with the extension for procedures

Sometimes I ask you to give an illustrative, but small example. The idea is that your example should be as varied and broad as possible, but on the other hand should be as concise as possible. For instance, in the case of Strongly Live Variables (see below), I expect an example where a variable is strongly live, one which is not, one which is strongly live but not live, and maybe you can think of other cases. The bottom line is that your example is an alternative for a soundness proof, so the more convincing your example, the more convinced people will be (actually, a proof also only does the job when it is convincing).

The general rule is: Motivate your answers

The various parts of the assignment

Part 1: developing a new analysis within the framework (after Week 1)

Do Exercise 2.4 for Strongly Live Variable Analysis on page 133 of NNH, formulate your answer similar to Table 2.4 on page 48.

Part 2: performing chaotic iteration (after Week 1)

Design a While program of at least five blocks which well illustrates the Strongly Live Variable Analysis (it should contain at least one while loop). Apply Chaotic Iteration to the corresponding monotone framework instance, showing also intermediate values for the iterations (similar to what I do in the slides for Available Expressions). Indicate how SLV is different for this example from the standard LV (and make sure that your example allows you to illustrate this fact).

Part 3: adding new constructs to the While language (after Week 1)

Add the following three programming constructs to the While language. Describe, in general, what has to change to accommodate each of the constructs. (Hint: sometimes it is sufficient to add new transfer functions, sometimes a change has to be made to a different parameter of the monotone framework. Deciding this is, of course, up to you.)

• A print a statement where a is an arithmetic expression. Show how to adapt the Strongly Live Analysis to accommodate programs which contain print statements. Devise a small program which shows that your extension works as expected. (You do not need to perform Chaotic Iteration. What you should do is give the equations for your program, propose a good (preferably the best) solution, indicate why you think this is a reasonable solution and verify that it fulfills the equations).
• Simultaneous assignments: a simultaneous assignment is of the form v1, ..., vn := a1, ..., an, which assigns the value of a1 to v1, a2 to v2 and so on. The semantics of such a statement is that first the sequence of expressions a1 to an is evaluated, and when this is done, the resulting values are stored in the corresponding variables in left to right order.

  Add simultaneous assignments to the While language and show how to adapt the Strongly Live Variable Analysis to accomodate it.

  Illustrate that your solution works by giving an example.

Part 4: prove a soundness result (after Week 2)

Do Mini Project 2.2 on page 131.

Part 5: procedures and context (after Week 2)

Do Exercise 2.18 for the Live Variable analysis. Work out the fibonacci example of Example 2.42 (page 101) by giving the resulting embellished monotone framework with context being call strings bounded by length k=2. (Beware: there are a few snakes in the grass here.) Remark: you do not have to work out the iteration itself. Be very precise when you give the transfer functions, especially those for call and return.

What, how and when to submit

Details can be found here. In whatever fashion you hand things in make sure things are clear and readable and on time. Make sure that you motivate your answers.

Experiences from last year

The main problems and a large part of the work lie in Part 4 and 5 of this assignment.

Part four is one many students find difficult to do. Do not be fooled to believe that the proof is a copy of that in the book: it follows the same line, but what you actually have to prove is quite different, and asks for a different point of view. In practice it turned out that most students did this well.

Part five is quite difficult, and people tend to forget quite a few details here. The main complication I guess is how to simulate scoping.

-- JurriaanHage - 15 Apr 2005