--- old/src/hotspot/share/c1/c1_Instruction.cpp	2019-02-20 20:12:23.423538106 -0800
+++ new/src/hotspot/share/c1/c1_Instruction.cpp	2019-02-20 20:12:23.203530025 -0800
@@ -147,7 +147,7 @@
   return false;
 }
 
-bool Instruction::maybe_flattened_array() const {
+bool Instruction::maybe_flattened_array() {
   if (ValueArrayFlatten) {
     ciType* type = declared_type();
     if (type != NULL) {
@@ -167,6 +167,10 @@
           return true;
         }
       }
+    } else if (as_Phi() != NULL) {
+      // Type info gets lost during Phi merging, but we might be storing into a
+      // flattened array, so we should do a runtime check.
+      return true;
     }
   }
 
--- old/src/hotspot/share/c1/c1_Instruction.hpp	2019-02-20 20:12:24.043560876 -0800
+++ new/src/hotspot/share/c1/c1_Instruction.hpp	2019-02-20 20:12:23.815552503 -0800
@@ -511,7 +511,7 @@
   bool is_flattened_array() const;             // FIXME -- remove it
 
   bool is_loaded_flattened_array() const;
-  bool maybe_flattened_array() const;
+  bool maybe_flattened_array();
 
   Instruction *insert_after_same_bci(Instruction *i) {
 #ifndef PRODUCT
--- old/src/hotspot/share/c1/c1_LIRGenerator.cpp	2019-02-20 20:12:24.667583795 -0800
+++ new/src/hotspot/share/c1/c1_LIRGenerator.cpp	2019-02-20 20:12:24.443575568 -0800
@@ -1659,12 +1659,32 @@
   __ branch(lir_cond_equal, T_ILLEGAL, slow_path);
 }
 
+bool LIRGenerator::needs_flattened_array_store_check(StoreIndexed* x) {
+  if (ValueArrayFlatten && x->elt_type() == T_OBJECT && x->array()->maybe_flattened_array()) {
+    ciType* type = x->value()->declared_type();
+    if (type != NULL && type->is_klass()) {
+      ciKlass* klass = type->as_klass();
+      if (klass->is_loaded() &&
+          !(klass->is_valuetype() && klass->as_value_klass()->flatten_array()) &&
+          !klass->is_java_lang_Object() &&
+          !klass->is_interface()) {
+        // This is known to be a non-flattenable object. If the array is flattened,
+        // it will be caught by the code generated by array_store_check().
+        return false;
+      }
+    }
+    // We're not 100% sure, so let's do the flattened_array_store_check.
+    return true;
+  }
+  return false;
+}
+
 void LIRGenerator::do_StoreIndexed(StoreIndexed* x) {
   assert(x->is_pinned(),"");
   bool is_loaded_flattened_array = x->array()->is_loaded_flattened_array();
   bool needs_range_check = x->compute_needs_range_check();
   bool use_length = x->length() != NULL;
-  bool obj_store = x->elt_type() == T_ARRAY || x->elt_type() == T_OBJECT;
+  bool obj_store = x->elt_type() == T_OBJECT; assert(x->elt_type() != T_ARRAY, "never used");
   bool needs_store_check = obj_store && !is_loaded_flattened_array &&
                                         (x->value()->as_Constant() == NULL ||
                                          !get_jobject_constant(x->value())->is_null_object() ||
@@ -1684,7 +1704,7 @@
   }
 
   if (needs_store_check || x->check_boolean()
-      || is_loaded_flattened_array || x->array()->maybe_flattened_array()) {
+      || is_loaded_flattened_array || needs_flattened_array_store_check(x)) {
     value.load_item();
   } else {
     value.load_for_store(x->elt_type());
@@ -1729,7 +1749,7 @@
   } else {
     StoreFlattenedArrayStub* slow_path = NULL;
 
-    if (x->array()->maybe_flattened_array()) {
+    if (needs_flattened_array_store_check(x)) {
       // Check if we indeed have a flattened array
       index.load_item();
       slow_path = new StoreFlattenedArrayStub(array.result(), index.result(), value.result(), state_for(x));
@@ -2046,9 +2066,9 @@
     LIR_Opr result = rlock_result(x, x->elt_type());
     LoadFlattenedArrayStub* slow_path = NULL;
 
-    if (x->array()->maybe_flattened_array()) {
+    if (x->elt_type() == T_OBJECT && x->array()->maybe_flattened_array()) {
       index.load_item();
-      // Check if we indeed have a flattened array
+      // if we are loading from flattened array, load it using a runtime call
       slow_path = new LoadFlattenedArrayStub(array.result(), index.result(), result, state_for(x));
       check_flattened_array(array, slow_path);
     }
--- old/src/hotspot/share/c1/c1_LIRGenerator.hpp	2019-02-20 20:12:25.307607301 -0800
+++ new/src/hotspot/share/c1/c1_LIRGenerator.hpp	2019-02-20 20:12:25.083599074 -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_store_check(StoreIndexed* x);
   void check_flattened_array(LIRItem& array, CodeStub* slow_path);
 
  public: