A CIL Tutorial - Department of Computer Science - ETH Zürich

Chapter 2
Visiting the AST
This chapter explains the use of the visitor pattern [1] for traversing and modifying the CIL AST.
The Visitor pattern dened incil.ml takes care of the boilerplate code for traversing the AST,
meaning that we don't have to write a bunch of mutually recursive functions every time we want
to walk the AST like we did in Chapter 1.
The visitor pattern in CIL uses the O(bjective) features of OCaml. We inherit from nopCilVisitor,
which simply walks the AST without doing anything. To do something other than that, we override
the base class's methods.
In this example, we want to pull out assignments to a global variable, which is given as a
parameter to tut2, from a function, which is also given as a parameter to tut2. Thus, we override
the vinst (visit instruction) method.
The result of one of the methods of the visitor class tells the visitor how to proceed next. There
are four options for the result of a visitor method:
• SkipChildren Stops the visitor from recursing into child AST nodes
• DoChildren Directs the visitor to recurse into child AST nodes
• ChangeTo x Replaces the AST node with x
• ChangeDoChildrenPost(x, f) Like ChangeTo x, but runs f on the result of rebuilding x
with the result of the visitor running on x's children.
For ChangeTo x and ChangeDoChildrenPost(x, f), check nopCilVisitor in cil.ml for the correct
types for x and f. Further, note the following important dierence between ChangeTo x and
ChangeDoChildrenPost(x, f): The visitor does not recurse into new nodes added with ChangeTo x.
The visitor does however recurse into new nodes added with ChnageDoChildrenPost(x, f), and
additionally you can give a function(f) to apply to the result.
2.1 tut2.ml
In this module, we will rst subclass CIL's default AST visitor, which simply returns DoChildren
from every method. Then, we'll run the visitor over every function denition in the Cil.file.
16