commit 59c44a234873fc2cad816153f14444aceb01e4f8
Author: Jeff Law <law@redhat.com>
Date:   Mon Nov 2 16:23:02 2015 -0700

    [PATCH] Avoid more irreducible loops in FSM threader
    
    	* tree-ssa-threadupdate.c (valid_jump_thread_path): Also detect
    	cases where the loop latch edge is in the middle of an FSM
    	path.
    
    	* gcc.dg/tree-ssa/ssa-thread-11.c: Verify that we do not have
    	irreducible loops in the CFG.

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 3613a68..6a7d988 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,9 @@
+2015-11-02  Jeff Law <jeff@redhat.com>
+
+	* tree-ssa-threadupdate.c (valid_jump_thread_path): Also detect
+	cases where the loop latch edge is in the middle of an FSM
+	path.
+
 2015-11-03  Tom de Vries  <tom@codesourcery.com>
 
 	* tree-ssa-structalias.c (make_restrict_var_constraints): Rename to ...
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index a86920a..3dc4edc 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2015-11-02  Jeff Law  <law@redhat.com>
+
+	* gcc.dg/tree-ssa/ssa-thread-11.c: Verify that we do not have
+	irreducible loops in the CFG.
+
 2015-11-02  Alan Lawrence  <alan.lawrence@arm.com>
 
 	Revert:
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-11.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-11.c
index 6a076e1..a729f56 100644
--- a/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-11.c
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-11.c
@@ -1,5 +1,6 @@
 /* { dg-do compile { target { ! { logical_op_short_circuit || { m68k*-*-* mep*-*-* bfin*-*-* v850*-*-* moxie*-*-* m32c*-*-* fr30*-*-* mcore*-*-* frv-*-* h8300-*-* m32r-*-* mn10300-*-* msp430-*-* pdp11-*-* rl78-*-* rx-*-* vax-*-*} } } } } */
 /* { dg-options "-O2 -fdump-tree-vrp2-details" } */
+/* { dg-final { scan-tree-dump-not "IRREDUCIBLE_LOOP" "vrp2" } } */
 /* { dg-final { scan-tree-dump "FSM" "vrp2" } } */
 
 void abort (void);
diff --git a/gcc/tree-ssa-threadupdate.c b/gcc/tree-ssa-threadupdate.c
index 511433a..68650e5 100644
--- a/gcc/tree-ssa-threadupdate.c
+++ b/gcc/tree-ssa-threadupdate.c
@@ -2547,29 +2547,38 @@ valid_jump_thread_path (vec<jump_thread_edge *> *path)
 {
   unsigned len = path->length ();
   bool multiway_branch = false;
+  bool threaded_through_latch = false;
 
   /* Check that the path is connected and see if there's a multi-way
      branch on the path.  */
   for (unsigned int j = 0; j < len - 1; j++)
     {
-      if ((*path)[j]->e->dest != (*path)[j+1]->e->src)
+      edge e = (*path)[j]->e;
+      struct loop *loop = e->dest->loop_father;
+
+      if (e->dest != (*path)[j+1]->e->src)
         return false;
-      gimple *last = last_stmt ((*path)[j]->e->dest);
+
+      /* If we're threading through the loop latch back into the
+	 same loop and the destination does not dominate the loop
+	 latch, then this thread would create an irreducible loop.  */
+      if (loop->latch
+	  && loop_latch_edge (loop) == e
+	  && loop == path->last()->e->dest->loop_father
+	  && (determine_bb_domination_status (loop, path->last ()->e->dest)
+	       == DOMST_NONDOMINATING))
+	threaded_through_latch = true;
+
+      gimple *last = last_stmt (e->dest);
       multiway_branch |= (last && gimple_code (last) == GIMPLE_SWITCH);
     }
 
-  /* If we are trying to thread the loop latch to a block that does
-     not dominate the loop latch, then that will create an irreducible
-     loop.  We avoid that unless the jump thread has a multi-way
+  /* If we are trying to thread through the loop latch to a block in the
+     loop that does not dominate the loop latch, then that will create an
+     irreducible loop.  We avoid that unless the jump thread has a multi-way
      branch, in which case we have deemed it worth losing other
      loop optimizations later if we can eliminate the multi-way branch.  */
-  edge e = (*path)[0]->e;
-  struct loop *loop = e->dest->loop_father;
-  if (!multiway_branch
-      && loop->latch
-      && loop_latch_edge (loop) == e
-      && (determine_bb_domination_status (loop, path->last ()->e->dest)
-	  == DOMST_NONDOMINATING))
+  if (!multiway_branch && threaded_through_latch)
     return false;
 
   return true;