Re: [PATCH] Fix CDDCE miscompilation (PR tree-optimization/55018)

[Steven Bosscher]

On Mon, Oct 22, 2012 at 11:09 PM, Jakub Jelinek <ja...@redhat.com> wrote:
> Wouldn't it be way cheaper to just export dfs_find_deadend from cfganal.c
> and call it in calc_dfs_tree on each unconnected bb?
> I.e. (untested with the exception of the testcase):

FWIW, dfs_find_deadend looks broken to me for this usage case. It
could return a self-loop block with more than one successor. For a
pre-order search like dominance.c needs, you'd have to look as deep as
possible, something like this:

Index: cfganal.c
===================================================================
--- cfganal.c   (revision 192696)
+++ cfganal.c   (working copy)
@@ -598,18 +598,26 @@ dfs_find_deadend (basic_block bb)
 {
   sbitmap visited = sbitmap_alloc (last_basic_block);
   sbitmap_zero (visited);
+  basic_block next_bb = NULL;
+  edge_iterator ei;
+  edge e;

   for (;;)
     {
       SET_BIT (visited, bb->index);
-      if (EDGE_COUNT (bb->succs) == 0
-          || TEST_BIT (visited, EDGE_SUCC (bb, 0)->dest->index))
+      /* Look for any not yet visited successors.
+        If all successors have been visited then
+        this is the dead end we're looking for.  */
+      FOR_EACH_EDGE (e, ei, bb->succs)
+       if (! TEST_BIT (visited, e->dest->index))
+         break;
+      if (e == NULL)
         {
           sbitmap_free (visited);
           return bb;
         }

-      bb = EDGE_SUCC (bb, 0)->dest;
+      bb = e->dest;
     }

   gcc_unreachable ();


(And the (EDGE_COUNT(bb->succs) == 0) is unnecessary for
inverted_post_order_compute because it already puts all such blocks on
the initial work list :-)

Ciao!
Steven