[PATCH] FIx PR81455

[Richard Biener]

Bootstrapped and tested on x86_64-unknown-linux-gnu, applied.

Richard.

2017-07-25  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/81455
	* tree-ssa-loop-unswitch.c (find_loop_guard): Make sure to
	not walk in cycles when looking for guards.

	* gcc.dg/pr81455.c: New testcase.

Index: gcc/tree-ssa-loop-unswitch.c
===================================================================
--- gcc/tree-ssa-loop-unswitch.c	(revision 250494)
+++ gcc/tree-ssa-loop-unswitch.c	(working copy)
@@ -582,8 +582,9 @@ find_loop_guard (struct loop *loop)
   gcond *cond;
   do
     {
+      basic_block next = NULL;
       if (single_succ_p (header))
-	header = single_succ (header);
+	next = single_succ (header);
       else
 	{
 	  cond = dyn_cast <gcond *> (last_stmt (header));
@@ -593,12 +594,16 @@ find_loop_guard (struct loop *loop)
 	  /* Make sure to skip earlier hoisted guards that are left
 	     in place as if (true).  */
 	  if (gimple_cond_true_p (cond))
-	    header = te->dest;
+	    next = te->dest;
 	  else if (gimple_cond_false_p (cond))
-	    header = fe->dest;
+	    next = fe->dest;
 	  else
 	    break;
 	}
+      /* Never traverse a backedge.  */
+      if (header->loop_father->header == next)
+	return NULL;
+      header = next;
     }
   while (1);
   if (!flow_bb_inside_loop_p (loop, te->dest)
Index: gcc/testsuite/gcc.dg/pr81455.c
===================================================================
--- gcc/testsuite/gcc.dg/pr81455.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/pr81455.c	(working copy)
@@ -0,0 +1,15 @@
+/* { dg-do compile } */
+/* { dg-options "-O -funswitch-loops" } */
+
+void
+jh (unsigned int aw, int sn)
+{
+  int xs;
+
+  for (xs = 0; xs < 1; ++xs)
+    aw &= 1;
+
+  while (aw < 1 || ++sn < 1)
+    {
+    }
+}