[PATCH] Fix to avoid multiple latches edges due to jump threading

[Jeff Law]

If we have a joiner block and one of its outgoing edges is the latch, 
then we can end up with multiple latches for a loop when we duplicate 
the joiner.  This (conservative) patch disables threading in that case.

I don't have a test which fails without other changes, but the code is 
clearly wrong as-is.  Revisiting the loop situation is #1 on the 
priority list after all the basic work for threading through paths with 
multiple duplicated blocks is in-place.

Bootstrapped and regression tested on x86_64-unknown-linux-gnu. 
Installed on the trunk.


Jeff
        * tree-ssa-threadupdate.c (thread_through_loop_header):  Do not
        thread through a joiner which has the latch edge.

diff --git a/gcc/tree-ssa-threadupdate.c b/gcc/tree-ssa-threadupdate.c
index 97dc6cb..3fba848 100644
--- a/gcc/tree-ssa-threadupdate.c
+++ b/gcc/tree-ssa-threadupdate.c
@@ -1060,11 +1142,22 @@ thread_through_loop_header (struct loop *loop, bool 
may_peel_loop_headers)
   if (single_succ_p (header))
     goto fail;
 
+  /* If we threaded the latch using a joiner block, we cancel the
+     threading opportunity out of an abundance of caution.  However,
+     still allow threading from outside to inside the loop.  */
   if (latch->aux)
     {
       vec<jump_thread_edge *> *path = THREAD_PATH (latch);
       if ((*path)[1]->type == EDGE_COPY_SRC_JOINER_BLOCK)
-       goto fail;
+       {
+         delete_jump_thread_path (path);
+         latch->aux = NULL;
+       }
+    }
+
+  if (latch->aux)
+    {
+      vec<jump_thread_edge *> *path = THREAD_PATH (latch);
       tgt_edge = (*path)[1]->e;
       tgt_bb = tgt_edge->dest;
     }