re PR tree-optimization/71661 (wrong code at -O3)

	PR tree-optimization/71661
	* tree-cfgcleanup.c (remove_forwarder_block_with_phi): Handle case when
	removal of a forwarder exposes a new natural loop.

	PR tree-optimization/71661
	* gcc.dg/tree-ssa/pr71661.c: New test.

From-SVN: r240836

gcc/ChangeLog

+2016-10-05  Jeff Law  <law@redhat.com>
+
+	PR tree-optimization/71661
+	* tree-cfgcleanup.c (remove_forwarder_block_with_phi): Handle case when
+	removal of a forwarder exposes a new natural loop.
+
 2016-10-06  Uros Bizjak  <ubizjak@gmail.com>
 
 	* config/i386/sse.md (andnot<mode>3): Add FALLTHRU comments.

gcc/testsuite/ChangeLog

+2016-10-06  Jeff Law  <law@redhat.com>
+
+	PR tree-optimization/71661
+	* gcc.dg/tree-ssa/pr71661.c: New test.
+
 2016-10-06  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/77855

gcc/testsuite/gcc.dg/tree-ssa/pr71661.c

+/* { dg-do run } */
+/* { dg-options "-O3 -fdisable-tree-ethread" } */
+
+extern void exit (int);
+
+int a, b;
+
+void
+fn1 ()
+{
+  unsigned c = 0;
+  int d;
+  b = a;
+  if (a < 0)
+    goto L1;
+  for (; a < 1; a++)
+    d = 0;
+  for (; d < 2; d++)
+    {
+      for (c = 0; c < 3; c++)
+      L1:
+        a = 2;
+    }
+}
+
+int
+main ()
+{
+  fn1 ();
+  exit (0);
+}

gcc/tree-cfgcleanup.c

@@ -840,6 +840,11 @@ remove_forwarder_block_with_phi (basic_block bb)
   if (dest == bb)
     return false;
 
+  /* Removal of forwarders may expose new natural loops and thus
+     a block may turn into a loop header.  */
+  if (current_loops && bb_loop_header_p (bb))
+    return false;
+
   /* If the destination block consists of a nonlocal label, do not
      merge it.  */
   label = first_stmt (dest);