+/* Not all jump threading requests are useful. In particular some
+ jump threading requests can create irreducible regions which are
+ undesirable.
+
+ This routine will examine the BB's incoming edges for jump threading
+ requests which, if acted upon, would create irreducible regions. Any
+ such jump threading requests found will be pruned away. */
+
+static void
+prune_undesirable_thread_requests (basic_block bb)
+{
+ edge e;
+ edge_iterator ei;
+ bool may_create_irreducible_region = false;
+ unsigned int num_outgoing_edges_into_loop = 0;
+
+ /* For the heuristics below, we need to know if BB has more than
+ one outgoing edge into a loop. */
+ FOR_EACH_EDGE (e, ei, bb->succs)
+ num_outgoing_edges_into_loop += ((e->flags & EDGE_LOOP_EXIT) == 0);
+
+ if (num_outgoing_edges_into_loop > 1)
+ {
+ edge backedge = NULL;
+
+ /* Consider the effect of threading the edge (0, 1) to 2 on the left
+ CFG to produce the right CFG:
+
+
+ 0 0
+ | |
+ 1<--+ 2<--------+
+ / \ | | |
+ 2 3 | 4<----+ |
+ \ / | / \ | |
+ 4---+ E 1-- | --+
+ | | |
+ E 3---+
+
+
+ Threading the (0, 1) edge to 2 effectively creates two loops
+ (2, 4, 1) and (4, 1, 3) which are neither disjoint nor nested.
+ This is not good.
+
+ However, we do need to be able to thread (0, 1) to 2 or 3
+ in the left CFG below (which creates the middle and right
+ CFGs with nested loops).
+
+ 0 0 0
+ | | |
+ 1<--+ 2<----+ 3<-+<-+
+ /| | | | | | |
+ 2 | | 3<-+ | 1--+ |
+ \| | | | | | |
+ 3---+ 1--+--+ 2-----+
+
+
+ A safe heuristic appears to be to only allow threading if BB
+ has a single incoming backedge from one of its direct successors. */
+
+ FOR_EACH_EDGE (e, ei, bb->preds)
+ {
+ if (e->flags & EDGE_DFS_BACK)
+ {
+ if (backedge)
+ {
+ backedge = NULL;
+ break;
+ }
+ else
+ {
+ backedge = e;
+ }
+ }
+ }
+
+ if (backedge && find_edge (bb, backedge->src))
+ ;
+ else
+ may_create_irreducible_region = true;
+ }
+ else
+ {
+ edge dest = NULL;
+
+ /* If we thread across the loop entry block (BB) into the
+ loop and BB is still reached from outside the loop, then
+ we would create an irreducible CFG. Consider the effect
+ of threading the edge (1, 4) to 5 on the left CFG to produce
+ the right CFG
+
+ 0 0
+ / \ / \
+ 1 2 1 2
+ \ / | |
+ 4<----+ 5<->4
+ / \ | |
+ E 5---+ E
+
+
+ Threading the (1, 4) edge to 5 creates two entry points
+ into the loop (4, 5) (one from block 1, the other from
+ block 2). A classic irreducible region.
+
+ So look at all of BB's incoming edges which are not
+ backedges and which are not threaded to the loop exit.
+ If that subset of incoming edges do not all thread
+ to the same block, then threading any of them will create
+ an irreducible region. */
+
+ FOR_EACH_EDGE (e, ei, bb->preds)
+ {
+ edge e2;
+
+ /* We ignore back edges for now. This may need refinement
+ as threading a backedge creates an inner loop which
+ we would need to verify has a single entry point.
+
+ If all backedges thread to new locations, then this
+ block will no longer have incoming backedges and we
+ need not worry about creating irreducible regions
+ by threading through BB. I don't think this happens
+ enough in practice to worry about it. */
+ if (e->flags & EDGE_DFS_BACK)
+ continue;
+
+ /* If the incoming edge threads to the loop exit, then it
+ is clearly safe. */
+ e2 = e->aux;
+ if (e2 && (e2->flags & EDGE_LOOP_EXIT))
+ continue;
+
+ /* E enters the loop header and is not threaded. We can
+ not allow any other incoming edges to thread into
+ the loop as that would create an irreducible region. */
+ if (!e2)
+ {
+ may_create_irreducible_region = true;
+ break;
+ }
+
+ /* We know that this incoming edge threads to a block inside
+ the loop. This edge must thread to the same target in
+ the loop as any previously seen threaded edges. Otherwise
+ we will create an irreducible region. */
+ if (!dest)
+ dest = e2;
+ else if (e2 != dest)
+ {
+ may_create_irreducible_region = true;
+ break;
+ }
+ }
+ }
+
+ /* If we might create an irreducible region, then cancel any of
+ the jump threading requests for incoming edges which are
+ not backedges and which do not thread to the exit block. */
+ if (may_create_irreducible_region)
+ {
+ FOR_EACH_EDGE (e, ei, bb->preds)
+ {
+ edge e2;
+
+ /* Ignore back edges. */
+ if (e->flags & EDGE_DFS_BACK)
+ continue;
+
+ e2 = e->aux;
+
+ /* If this incoming edge was not threaded, then there is
+ nothing to do. */
+ if (!e2)
+ continue;
+
+ /* If this incoming edge threaded to the loop exit,
+ then it can be ignored as it is safe. */
+ if (e2->flags & EDGE_LOOP_EXIT)
+ continue;
+
+ if (e2)
+ {
+ /* This edge threaded into the loop and the jump thread
+ request must be cancelled. */
+ if (dump_file && (dump_flags & TDF_DETAILS))
+ fprintf (dump_file, " Not threading jump %d --> %d to %d\n",
+ e->src->index, e->dest->index, e2->dest->index);
+ e->aux = NULL;
+ }
+ }
+ }
+}
+