A CIL Tutorial - Department of Computer Science - ETH Zürich

CHAPTER 9. TYPE QUALIFIER INFERENCE 72
in, a subtype of, etc...} type B when the colors of type A are a subset of the colors of type B. For
example, a Red int is also a Red Blue int. Thus, the subset lattice of the colors denes the subtype
relation of our type system, and we can use it to nd the least-upper-bound of two types, which
will be important for the inference algorithm below.
9.1.1 Mark types with node IDs
First we'll introduce two visitors and two functions that run the visitors over les. The rst visitor
marks each type node in the AST with a unique numeric ID held in an attribute. The second visitor
removes these marks, which we must do before we pass the code on to gcc. The function nodeAttr
creates an attribute called "Node" parameterized by an integer node ID.
let nodeStr = "Node"
let nodeAttr (id : int) : attributes = [Attr(nodeStr, [AInt id])]
The function node of type returns the integer node ID of a type. Since, we are adding the "Node"
attributes ourselves, it is a bug if one is malformed. Hence, we use E.bug. If there is no "Node"
attribute, this implies that t is the result of calling typeOf on a constant. We group all of these
types at node 0 in the graph. Later, we'll see that this node is unreachable, and so we won't try to
infer a type qualier for it.
let node of type (t : typ) : int =
match filterAttributes nodeStr (typeAttrs t) with
| [(Attr( , [AInt id]))] → id
| [ ] → 0
| → E.s (E.bug "%a: Malformed node id on %a" d loc (!currentLoc) d type t)
The visitor typeNodeMarker visits every typ node in the AST and assigns them unique node IDs
by placing a "Node" attribute on them.
class typeNodeMarker (node count : int ref) = object(self)
inherit nopCilVisitor
method vtype (t : typ) =
let action t =
if ¬(hasAttribute nodeStr (typeAttrs t)) then begin
let attr = nodeAttr (!node count) in
incr node count;
typeAddAttributes attr t
end else t
in
ChangeDoChildrenPost(t, action)
end
The function addNodeMarks invokes the typeNodeMarker visitor on a le and returns the largest ID
assigned by the visitor. This will be the number of nodes we need in our graph.