[PATCH] Improve VRP PHI handling

[Richard Biener <rguenther at suse dot de>]

The patch for PR72851 exposes an issue with PHI handling in VRP that
is easily fixed if we allow iterating again if we are sure this is
the last iteration with the set of fixed executable edges.  It also
makes sure we always print the lattice change, even if it is to VARYING.

Bootstrap / regtest running on x86_64-unknown-linux-gnu.

Richard.

2016-08-11  Richard Biener  <rguenther@suse.de>

	* tree-vrp.c (vrp_visit_phi_node): Allow a last iteration if
	the currently executable edges have fixed ranges.  Always
	go through update_value_range.

Index: gcc/tree-vrp.c
===================================================================
--- gcc/tree-vrp.c	(revision 239361)
+++ gcc/tree-vrp.c	(working copy)
@@ -8725,6 +8703,7 @@ vrp_visit_phi_node (gphi *phi)
       print_gimple_stmt (dump_file, phi, 0, dump_flags);
     }
 
+  bool may_simulate_again = false;
   edges = 0;
   for (i = 0; i < gimple_phi_num_args (phi); i++)
     {
@@ -8747,6 +8726,12 @@ vrp_visit_phi_node (gphi *phi)
 
 	  if (TREE_CODE (arg) == SSA_NAME)
 	    {
+	      /* See if we are eventually going to change one of the args.  */
+	      gimple *def_stmt = SSA_NAME_DEF_STMT (arg);
+	      if (! gimple_nop_p (def_stmt)
+		  && prop_simulate_again_p (def_stmt))
+		may_simulate_again = true;
+
 	      vr_arg = *(get_value_range (arg));
 	      /* Do not allow equivalences or symbolic ranges to leak in from
 		 backedges.  That creates invalid equivalencies.
@@ -8822,11 +8807,14 @@ vrp_visit_phi_node (gphi *phi)
      previous one.  We don't do this if we have seen a new executable
      edge; this helps us avoid an overflow infinity for conditionals
      which are not in a loop.  If the old value-range was VR_UNDEFINED
-     use the updated range and iterate one more time.  */
+     use the updated range and iterate one more time.  If we will not
+     simulate this PHI again with the same number of edges then iterate
+     one more time.  */
   if (edges > 0
       && gimple_phi_num_args (phi) > 1
       && edges == old_edges
-      && lhs_vr->type != VR_UNDEFINED)
+      && lhs_vr->type != VR_UNDEFINED
+      && may_simulate_again)
     {
       /* Compare old and new ranges, fall back to varying if the
          values are not comparable.  */
@@ -8880,6 +8868,31 @@ vrp_visit_phi_node (gphi *phi)
       goto infinite_check;
     }
 
+  goto update_range;
+
+varying:
+  set_value_range_to_varying (&vr_result);
+
+scev_check:
+  /* If this is a loop PHI node SCEV may known more about its value-range.
+     scev_check can be reached from two paths, one is a fall through from above
+     "varying" label, the other is direct goto from code block which tries to
+     avoid infinite simulation.  */
+  if ((l = loop_containing_stmt (phi))
+      && l->header == gimple_bb (phi))
+    adjust_range_with_scev (&vr_result, l, phi, lhs);
+
+infinite_check:
+  /* If we will end up with a (-INF, +INF) range, set it to
+     VARYING.  Same if the previous max value was invalid for
+     the type and we end up with vr_result.min > vr_result.max.  */
+  if ((vr_result.type == VR_RANGE || vr_result.type == VR_ANTI_RANGE)
+      && !((vrp_val_is_max (vr_result.max) && vrp_val_is_min (vr_result.min))
+	   || compare_values (vr_result.min, vr_result.max) > 0))
+    ;
+  else
+    set_value_range_to_varying (&vr_result);
+
   /* If the new range is different than the previous value, keep
      iterating.  */
 update_range:
@@ -8902,31 +8915,6 @@ update_range:
 
   /* Nothing changed, don't add outgoing edges.  */
   return SSA_PROP_NOT_INTERESTING;
-
-varying:
-  set_value_range_to_varying (&vr_result);
-
-scev_check:
-  /* If this is a loop PHI node SCEV may known more about its value-range.
-     scev_check can be reached from two paths, one is a fall through from above
-     "varying" label, the other is direct goto from code block which tries to
-     avoid infinite simulation.  */
-  if ((l = loop_containing_stmt (phi))
-      && l->header == gimple_bb (phi))
-    adjust_range_with_scev (&vr_result, l, phi, lhs);
-
-infinite_check:
-  /* If we will end up with a (-INF, +INF) range, set it to
-     VARYING.  Same if the previous max value was invalid for
-     the type and we end up with vr_result.min > vr_result.max.  */
-  if ((vr_result.type == VR_RANGE || vr_result.type == VR_ANTI_RANGE)
-      && !((vrp_val_is_max (vr_result.max) && vrp_val_is_min (vr_result.min))
-	   || compare_values (vr_result.min, vr_result.max) > 0))
-    goto update_range;
-
-  /* No match found.  Set the LHS to VARYING.  */
-  set_value_range_to_varying (lhs_vr);
-  return SSA_PROP_VARYING;
 }
 
 /* Simplify boolean operations if the source is known