[PATCH] tree-optimization/108523 - fix endless iteration in VN

[Richard Biener <rguenther@suse.de>]

The following fixes not converging iteration in value-numbering of
PHI nodes when we use an equivalence to prove the PHI node is
degenerate.  We have to avoid the situation where we oscillate
between the two equivalent values because the result is fed back
via a backedge.

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

	PR tree-optimization/108523
	* tree-ssa-sccvn.cc (visit_phi): Avoid using the exclusive
	backedge value for the result when using predication to
	prove equivalence.
---
 gcc/tree-ssa-sccvn.cc | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/gcc/tree-ssa-sccvn.cc b/gcc/tree-ssa-sccvn.cc
index 0dba3f39aa0..edb553b07cb 100644
--- a/gcc/tree-ssa-sccvn.cc
+++ b/gcc/tree-ssa-sccvn.cc
@@ -5826,7 +5826,7 @@ visit_phi (gimple *phi, bool *inserted, bool backedges_varying_p)
   poly_int64 soff, doff;
   unsigned n_executable = 0;
   edge_iterator ei;
-  edge e;
+  edge e, sameval_e = NULL;
 
   /* TODO: We could check for this in initialization, and replace this
      with a gcc_assert.  */
@@ -5867,7 +5867,10 @@ visit_phi (gimple *phi, bool *inserted, bool backedges_varying_p)
 		 && ssa_undefined_value_p (def, false))
 	  seen_undef = def;
 	else if (sameval == VN_TOP)
-	  sameval = def;
+	  {
+	    sameval = def;
+	    sameval_e = e;
+	  }
 	else if (!expressions_equal_p (def, sameval))
 	  {
 	    /* We know we're arriving only with invariant addresses here,
@@ -5916,6 +5919,8 @@ visit_phi (gimple *phi, bool *inserted, bool backedges_varying_p)
 			    fprintf (dump_file, " are equal on edge %d -> %d\n",
 				     e->src->index, e->dest->index);
 			  }
+			if (sameval_e && (sameval_e->flags & EDGE_DFS_BACK))
+			  sameval = def;
 			continue;
 		      }
 		    /* If on all previous edges the value was equal to def
@@ -5935,7 +5940,8 @@ visit_phi (gimple *phi, bool *inserted, bool backedges_varying_p)
 				     EDGE_PRED (bb, 0)->src->index,
 				     EDGE_PRED (bb, 0)->dest->index);
 			  }
-			sameval = def;
+			if (!(e->flags & EDGE_DFS_BACK))
+			  sameval = def;
 			continue;
 		      }
 		  }
@@ -5943,6 +5949,8 @@ visit_phi (gimple *phi, bool *inserted, bool backedges_varying_p)
 	    sameval = NULL_TREE;
 	    break;
 	  }
+	else
+	  sameval_e = NULL;
       }
 
   /* If the value we want to use is flowing over the backedge and we
-- 
2.35.3