A CIL Tutorial - Department of Computer Science - ETH Zürich

Chapter 9
Type Qualier Inference
In the previous two sections, we saw how to add type-qualiers to the language, and how to do
any extra type-checking that they might require. In this section, we'll see how one can begin to do
type-qualier inference. This might be useful, for example, if we wish to make our changes to C
more unobtrusive. That is, we would like to add expressiveness to the language and type-system
without thrusting a large annotation burden onto the programmer. One way to accomplish this is
by inferring as many type qualiers as possible. Additionally, analyses like this one, which push type
qualiers around, can be used for purposes aside from type-inference. For example, type qualier
inference can be used to perform an imprecise (though still very useful) thread-escape analysis.
9.1 tut9.ml
We'll use the following method to infer type qualiers. First, we'll place a numeric IDs on each
type node in the AST. Then we'll build a graph with these numeric IDs on each node and directed
edges given by the relationships between types that we nd in the program. Next, we iterate over
the nodes until we reach a x-point that satises the relationships encoded by the graph edges.
Following that, we add the inferred type qualiers to the program. If an inferred type qualier
doesn't match the one that the programmer has given explicitly, we emit a compile-time warning.
This inference pass is intended to be followed by a type-checking phase as in one of the previous
sections. Therefore, it is important that we don't infer anything that won't type-check!
We'll need the standard library Queue module, and a few things from Chapter 7, in particular the
denition of the color type.
module Q = Queue
module T = Tut7
type colors = T.color list
Even though we are borrowing some OCaml types and functions from Tut7, the C type system for
this analysis is slightly dierent. In Tut7 for two types to be compatible, their colors had to match
exactly. In this tutorial, we use the same colors, with a dierent restriction: type A is {included
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