[Bug tree-optimization/61607] DOM missed jump threading and destroyed loops

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61607

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Like with

Index: gcc/tree-ssa-threadupdate.c
===================================================================
--- gcc/tree-ssa-threadupdate.c (revision 211969)
+++ gcc/tree-ssa-threadupdate.c (working copy)
@@ -764,6 +764,14 @@ ssa_redirect_edges (struct redirection_d
          if ((*path)[1]->type != EDGE_COPY_SRC_JOINER_BLOCK)
            EDGE_SUCC (rd->dup_blocks[0], 0)->count += e->count;

+         /* If we redirect a loop latch edge cancel its loop.  */
+         if (e->src == e->src->loop_father->latch)
+           {
+             e->src->loop_father->header = NULL;
+             e->src->loop_father->latch = NULL;
+             loops_state_set (LOOPS_NEED_FIXUP);
+           }
+
          /* Redirect the incoming edge (possibly to the joiner block) to the
             appropriate duplicate block.  */
          e2 = redirect_edge_and_branch (e, rd->dup_blocks[0]);
@@ -853,32 +861,6 @@ thread_block_1 (basic_block bb, bool nol
   redirection_data
     = new hash_table<struct redirection_data> (EDGE_COUNT (bb->succs));

-  /* If we thread the latch of the loop to its exit, the loop ceases to
-     exist.  Make sure we do not restrict ourselves in order to preserve
-     this loop.  */
-  if (loop->header == bb)
-    {
-      e = loop_latch_edge (loop);
-      vec<jump_thread_edge *> *path = THREAD_PATH (e);
-
-      if (path
-         && (((*path)[1]->type == EDGE_COPY_SRC_JOINER_BLOCK && joiners)
-             || ((*path)[1]->type == EDGE_COPY_SRC_BLOCK && !joiners)))
-       {
-         for (unsigned int i = 1; i < path->length (); i++)
-           {
-             edge e2 = (*path)[i]->e;
-
-             if (loop_exit_edge_p (loop, e2))
-               {
-                 loop->header = NULL;
-                 loop->latch = NULL;
-                 loops_state_set (LOOPS_NEED_FIXUP);
-               }
-           }
-       }
-    }
-
   /* Record each unique threaded destination into a hash table for
      efficient lookups.  */
   FOR_EACH_EDGE (e, ei, bb->preds)