diff a/src/hotspot/share/opto/superword.cpp b/src/hotspot/share/opto/superword.cpp
--- a/src/hotspot/share/opto/superword.cpp
+++ b/src/hotspot/share/opto/superword.cpp
@@ -92,12 +92,15 @@
 }
 
 //------------------------------transform_loop---------------------------
 void SuperWord::transform_loop(IdealLoopTree* lpt, bool do_optimization) {
   assert(UseSuperWord, "should be");
-  // Do vectors exist on this architecture?
-  if (Matcher::vector_width_in_bytes(T_BYTE) < 2) return;
+  // SuperWord only works with power of two vector sizes.
+  int vector_width = Matcher::vector_width_in_bytes(T_BYTE);
+  if (vector_width < 2 || !is_power_of_2(vector_width)) {
+    return;
+  }
 
   assert(lpt->_head->is_CountedLoop(), "must be");
   CountedLoopNode *cl = lpt->_head->as_CountedLoop();
 
   if (!cl->is_valid_counted_loop()) return; // skip malformed counted loop