  Dyck reachability is the standard formulation of a large domain of static
analyses, as it achieves the sweet spot between precision and efficiency, and
has thus been studied extensively. Interleaved Dyck reachability (denoted
$D_k\odot D_k$) uses two Dyck languages for increased precision (e.g., context
and field sensitivity) but is well-known to be undecidable. As many static
analyses yield a certain type of bidirected graphs, they give rise to
interleaved bidirected Dyck reachability problems. Although these problems have
seen numerous applications, their decidability and complexity has largely
remained open. In a recent work, Li et al. made the first steps in this
direction, showing that (i) $D_1\odot D_1$ reachability (i.e., when both Dyck
languages are over a single parenthesis and act as counters) is computable in
$O(n^7)$ time, while (ii) $D_k\odot D_k$ reachability is NP-hard.
  In this work we address the decidability and complexity of all variants of
interleaved bidirected Dyck reachability. First, we show that $D_1\odot D_1$
reachability can be computed in $O(n^3\cdot \alpha(n))$ time, significantly
improving over the existing $O(n^7)$ bound. Second, we show that $D_k\odot D_1$
reachability (i.e., when one language acts as a counter) is decidable, in
contrast to the non-bidirected case where decidability is open. We further
consider $D_k\odot D_1$ reachability where the counter remains linearly
bounded. Our third result shows that this bounded variant can be solved in
$O(n^2\cdot \alpha(n))$ time, while our fourth result shows that the problem
has a (conditional) quadratic lower bound, and thus our upper bound is
essentially optimal. Fifth, we show that full $D_k\odot D_k$ reachability is
undecidable. This improves the recent NP-hardness lower-bound, and shows that
the problem is equivalent to the non-bidirected case.
