[PR,tree-optimization/71335] Fix thread path management in backwards threader

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index af96025..bd9476e 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,14 @@ 
+2016-06-10  Jeff Law  <law@redhat.com>
+
+	PR tree-optimization/71335
+	* tree-ssa-threadbackward.c (profitable_jump_thread_path): Filter out
+	zero length paths here.
+	(convert_and_register_jump_thread_path): Remove hacks related to
+	duplicated blocks in the jump thread path.
+	(fsm_find_control_statement_thread_paths): Avoid putting the same
+	block on the thread path twice, but ensure the thread path is
+	unchanged from the caller's point of view.
+	
 2016-06-10  Jan Hubicka  <hubicka@ucw.cz>
 
 	* predict.c (predict_loops): Remove PRED_LOOP_BRANCH.
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 71a2db1..b76e477 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@ 
+2016-06-10  Jeff Law  <law@redhat.com>
+
+	PR tree-optimization/71335
+	* gcc.c-torture/execute/pr71335.c: New test.
+
 2016-06-10  David Malcolm  <dmalcolm@redhat.com>
 
 	* gcc.dg/plugin/must-tail-call-2.c: Remove all details from
diff --git a/gcc/testsuite/gcc.c-torture/execute/pr71335.c b/gcc/testsuite/gcc.c-torture/execute/pr71335.c
new file mode 100644
index 0000000..cbfd990
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/execute/pr71335.c
@@ -0,0 +1,13 @@ 
+int a;
+int
+main ()
+{
+  int b = 0;
+  while (a < 0 || b)
+    {
+      b = 0;
+      for (; b < 9; b++)
+	;
+    }
+  exit (0);
+}
diff --git a/gcc/tree-ssa-threadbackward.c b/gcc/tree-ssa-threadbackward.c
index 57f1f5c..139d376 100644
--- a/gcc/tree-ssa-threadbackward.c
+++ b/gcc/tree-ssa-threadbackward.c
@@ -109,6 +109,19 @@  profitable_jump_thread_path (vec<basic_block, va_gc> *&path,
   /* Note BBI is not in the path yet, hence the +1 in the test below
      to make sure BBI is accounted for in the path length test.  */
   int path_length = path->length ();
+
+  /* We can get a length of 0 here when the statement that
+     makes a conditional generate a compile-time constant
+     result is in the same block as the conditional.
+
+     That's not really a jump threading opportunity, but instead is
+     simple cprop & simplification.  We could handle it here if we
+     wanted by wiring up all the incoming edges.  If we run this
+     early in IPA, that might be worth doing.   For now we just
+     reject that case.  */
+  if (path_length == 0)
+      return NULL;
+
   if (path_length + 1 > PARAM_VALUE (PARAM_MAX_FSM_THREAD_LENGTH))
     {
       if (dump_file && (dump_flags & TDF_DETAILS))
@@ -376,34 +389,12 @@  convert_and_register_jump_thread_path (vec<basic_block, va_gc> *&path,
       basic_block bb1 = (*path)[path->length () - j - 1];
       basic_block bb2 = (*path)[path->length () - j - 2];
 
-      /* This can happen when we have an SSA_NAME as a PHI argument and
-	 its initialization block is the head of the PHI argument's
-	 edge.  */
-      if (bb1 == bb2)
-	continue;
-
       edge e = find_edge (bb1, bb2);
       gcc_assert (e);
       jump_thread_edge *x = new jump_thread_edge (e, EDGE_FSM_THREAD);
       jump_thread_path->safe_push (x);
     }
 
-  /* As a consequence of the test for duplicate blocks in the path
-     above, we can get a path with no blocks.  This happens if a
-     conditional can be fully evaluated at compile time using just
-     defining statements in the same block as the test.
-
-     When we no longer push the block associated with a PHI argument
-     onto the stack, then this as well as the test in the loop above
-     can be removed.  */
-  if (jump_thread_path->length () == 0)
-    {
-      jump_thread_path->release ();
-      delete jump_thread_path;
-      path->pop ();
-      return;
-    }
-
   /* Add the edge taken when the control variable has value ARG.  */
   jump_thread_edge *x
     = new jump_thread_edge (taken_edge, EDGE_NO_COPY_SRC_BLOCK);
@@ -579,10 +570,19 @@  fsm_find_control_statement_thread_paths (tree name,
 
       else
 	{
+	  /* profitable_jump_thread_path is going to push the current
+	     block onto the path.  But the path will always have the current
+	     block at this point.  So we can just pop it.  */
+	  path->pop ();
+
 	  edge taken_edge = profitable_jump_thread_path (path, var_bb,
 						     name, arg);
 	  if (taken_edge)
 	    convert_and_register_jump_thread_path (path, taken_edge);
+
+	  /* And put the current block back onto the path so that the
+	     state of the stack is unchanged when we leave.  */
+	  vec_safe_push (path, var_bb);
 	}
     }