--- old/src/hotspot/share/c1/c1_LIRGenerator.cpp	2019-02-18 07:28:18.695644232 -0800
+++ new/src/hotspot/share/c1/c1_LIRGenerator.cpp	2019-02-18 07:28:18.471636278 -0800
@@ -1648,6 +1648,18 @@
   }
 }
 
+bool LIRGenerator::needs_flattened_array_check(Value array, Value value) {
+  if (array->maybe_flattened_array()) {
+    return true;
+  }
+  if (array->as_Phi() != NULL && value->type()->is_object()) {
+    // Type info gets lost during Phi merging, but we might be storing into a
+    // flattened array, so we'd better check.
+    return true;
+  }
+  return false;
+}
+
 void LIRGenerator::check_flattened_array(LIRItem& array, CodeStub* slow_path) {
   LIR_Opr array_klass_reg = new_register(T_METADATA);
 
@@ -1684,7 +1696,7 @@
   }
 
   if (needs_store_check || x->check_boolean()
-      || is_loaded_flattened_array || x->array()->maybe_flattened_array()) {
+      || is_loaded_flattened_array || needs_flattened_array_check(x->array(), x->value())) {
     value.load_item();
   } else {
     value.load_for_store(x->elt_type());
@@ -1729,7 +1741,7 @@
   } else {
     StoreFlattenedArrayStub* slow_path = NULL;
 
-    if (x->array()->maybe_flattened_array()) {
+    if (needs_flattened_array_check(x->array(), x->value())) {
       // Check if we indeed have a flattened array
       index.load_item();
       slow_path = new StoreFlattenedArrayStub(array.result(), index.result(), value.result(), state_for(x));
@@ -2046,7 +2058,7 @@
     LIR_Opr result = rlock_result(x, x->elt_type());
     LoadFlattenedArrayStub* slow_path = NULL;
 
-    if (x->array()->maybe_flattened_array()) {
+    if (needs_flattened_array_check(x->array(), x)) {
       index.load_item();
       // Check if we indeed have a flattened array
       slow_path = new LoadFlattenedArrayStub(array.result(), index.result(), result, state_for(x));
--- old/src/hotspot/share/c1/c1_LIRGenerator.hpp	2019-02-18 07:28:19.351667526 -0800
+++ new/src/hotspot/share/c1/c1_LIRGenerator.hpp	2019-02-18 07:28:19.127659572 -0800
@@ -267,6 +267,7 @@
   void do_vectorizedMismatch(Intrinsic* x);
 
   void access_flattened_array(bool is_load, LIRItem& array, LIRItem& index, LIRItem& obj_item);
+  bool needs_flattened_array_check(Value array, Value value);
   void check_flattened_array(LIRItem& array, CodeStub* slow_path);
 
  public: