--- a/gcc/tree-ssa-loop-ivcanon.c	2018-12-06 05:05:43.841181211 +0800
+++ b/gcc/tree-ssa-loop-ivcanon.c	2018-12-06 05:03:17.545185153 +0800
@@ -726,7 +726,7 @@ try_unroll_loop_completely (struct loop
       edge_to_cancel = NULL;
     }
 
-  if (!n_unroll_found)
+  if (!n_unroll_found || SCEV_NOT_KNOWN == TREE_CODE (niter))
     return false;
 
   if (n_unroll > (unsigned) PARAM_VALUE (PARAM_MAX_COMPLETELY_PEEL_TIMES))