  The treewidth of control flow graphs arising from structured programs is
known to be at most six. However, as a control flow graph is inherently
directed, it makes sense to consider a measure of width for digraphs instead.
We use the so-called DAG-width and show that the DAG-width of control flow
graphs arising from structured (goto-free) programs is at most three.
Additionally, we also give a linear time algorithm to compute the DAG
decomposition of these control flow graphs. One consequence of this result is
that parity games (and hence the $\mu$-calculus model checking problem), which
are known to be tractable on graphs of bounded DAG-width, can be solved
efficiently in practice on control flow graphs.
