Assert to protect against polymorphic string loads fires on valid stores.

src/hydrogen.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/hydrogen.h"
 
 #include <sstream>
 
 #include "src/v8.h"
 
@@ skipping 7076 matching lines @@
       return consolidated_load;
     }
   }
 
   // Elements_kind transition support.
   MapHandleList transition_target(maps->length());
   // Collect possible transition targets.
   MapHandleList possible_transitioned_maps(maps->length());
   for (int i = 0; i < maps->length(); ++i) {
     Handle<Map> map = maps->at(i);
-    DCHECK(!map->IsStringMap());
+    // Loads from strings or loads with a mix of string and non-string maps
+    // shouldn't be handled polymorphically.
+    DCHECK(access_type != LOAD || !map->IsStringMap());
     ElementsKind elements_kind = map->elements_kind();
     if (CanInlineElementAccess(map) && IsFastElementsKind(elements_kind) &&
         elements_kind != GetInitialFastElementsKind()) {
       possible_transitioned_maps.Add(map);
     }
     if (elements_kind == SLOPPY_ARGUMENTS_ELEMENTS) {
       HInstruction* result = BuildKeyedGeneric(access_type, expr, object, key,
                                                val);
       *has_side_effects = result->HasObservableSideEffects();
       return AddInstruction(result);
@@ skipping 5625 matching lines @@
   if (ShouldProduceTraceOutput()) {
     isolate()->GetHTracer()->TraceHydrogen(name(), graph_);
   }
 
 #ifdef DEBUG
   graph_->Verify(false);  // No full verify.
 #endif
 }
 
 } }  // namespace v8::internal