CfgCreator

Translation of abstract syntax trees into control flow graphs

The problem of translating an abstract syntax tree into a corresponding control flow graph can be formulated as a recursive problem in which sub trees of the syntax tree are translated and their corresponding control flow graphs are connected according to the control flow semantics of the root node. For example, consider the abstract syntax tree for an if-statement:

             (  if )
            /       \
        (x < 10)  (x += 1)
          / \       / \
         x  10     x   1

This tree can be translated into a control flow graph, by translating the sub tree rooted in x < 10 and that of x+= 1 and connecting their control flow graphs according to the semantics of if:

          [x < 10]----
             |t     f|
          [x +=1 ]   |
             |

The semantics of if dictate that the first sub tree to the left is a condition, which is connected to the CFG of the second sub tree - the body of the if statement - via a control flow edge with the true label (indicated in the illustration by t), and to the CFG of any follow-up code via a false edge (indicated by f).

A problem that becomes immediately apparent in the illustration is that the result of translating a sub tree may leave us with edges for which a source node is known but the destination node depends on parents or siblings that were not considered in the translation. For example, we know that an outgoing edge from [x<10] must exist, but we do not yet know where it should lead. We refer to the set of nodes of the control flow graph with outgoing edges for which the destination node is yet to be determined as the "fringe" of the control flow graph.