diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-97.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-97.c
new file mode 100644
index 00000000000..2f09c8baeb8
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-97.c
@@ -0,0 +1,19 @@
+/* { dg-do compile } */
+/* ethread threading does not yet catch this but it might at some point.  */
+/* { dg-options "-O -fdump-tree-fre1-details -fno-thread-jumps" } */
+
+int foo (int b, int x)
+{
+  int i, j;
+  if (b)
+    i = x;
+  if (b)
+    j = x;
+  return j == i;
+}
+
+/* Even with different undefs we should CSE a PHI node with the
+   same controlling condition.  */
+
+/* { dg-final { scan-tree-dump "Replaced redundant PHI node" "fre1" } } */
+/* { dg-final { scan-tree-dump "return 1;" "fre1" } } */
diff --git a/gcc/testsuite/gcc.dg/ubsan/pr81981.c b/gcc/testsuite/gcc.dg/ubsan/pr81981.c
index 8a6597c84c8..d201efb3f65 100644
--- a/gcc/testsuite/gcc.dg/ubsan/pr81981.c
+++ b/gcc/testsuite/gcc.dg/ubsan/pr81981.c
@@ -16,6 +16,6 @@ foo (int i)
       u[0] = i;
     }
 
-  v = u[0];		/* { dg-warning "may be used uninitialized" } */
+  v = u[0];		/* { dg-warning "may be used uninitialized" "" { xfail *-*-* } } */
   return t[0];		/* { dg-warning "may be used uninitialized" } */
 }
diff --git a/gcc/tree-ssa-sccvn.c b/gcc/tree-ssa-sccvn.c
index ae0172a143e..893b1d0ddaa 100644
--- a/gcc/tree-ssa-sccvn.c
+++ b/gcc/tree-ssa-sccvn.c
@@ -4499,7 +4499,12 @@ vn_phi_lookup (gimple *phi, bool backedges_varying_p)
       tree def = PHI_ARG_DEF_FROM_EDGE (phi, e);
       if (TREE_CODE (def) == SSA_NAME
 	  && (!backedges_varying_p || !(e->flags & EDGE_DFS_BACK)))
-	def = SSA_VAL (def);
+	{
+	  if (ssa_undefined_value_p (def, false))
+	    def = VN_TOP;
+	  else
+	    def = SSA_VAL (def);
+	}
       vp1->phiargs[e->dest_idx] = def;
     }
   vp1->type = TREE_TYPE (gimple_phi_result (phi));
@@ -4543,7 +4548,12 @@ vn_phi_insert (gimple *phi, tree result, bool backedges_varying_p)
       tree def = PHI_ARG_DEF_FROM_EDGE (phi, e);
       if (TREE_CODE (def) == SSA_NAME
 	  && (!backedges_varying_p || !(e->flags & EDGE_DFS_BACK)))
-	def = SSA_VAL (def);
+	{
+	  if (ssa_undefined_value_p (def, false))
+	    def = VN_TOP;
+	  else
+	    def = SSA_VAL (def);
+	}
       vp1->phiargs[e->dest_idx] = def;
     }
   vp1->value_id = VN_INFO (result)->value_id;