Let KeyedStoreIC go megamorphic (instead of generic) when the same map misses twice

src/ic/ic.cc

 // Copyright 2012 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/v8.h"
 
 #include "src/accessors.h"
 #include "src/api.h"
 #include "src/arguments.h"
 #include "src/base/bits.h"
@@ skipping 1847 matching lines @@
 
   if (IsTransitionStoreMode(store_mode)) {
     Handle<Map> transitioned_receiver_map =
         ComputeTransitionedMap(receiver_map, store_mode);
     map_added |= AddOneReceiverMapIfMissing(&target_receiver_maps,
                                             transitioned_receiver_map);
   }
 
   if (!map_added) {
     // If the miss wasn't due to an unseen map, a polymorphic stub
-    // won't help, use the generic stub.
+    // won't help. In theory we should use the generic stub, but in
+    // practice there are a number of hard-to-avoid reasons why this
+    // can happen occasionally, and where the additional logic in the
+    // megamorphic stub is beneficial because it can handle most cases
+    // without calling into the runtime.
     TRACE_GENERIC_IC(isolate(), "KeyedStoreIC", "same map added twice");
-    return generic_stub();
+    return megamorphic_stub();
   }
 
   // If the maximum number of receiver maps has been exceeded, use the
   // megamorphic version of the IC.
   if (target_receiver_maps.length() > kMaxKeyedPolymorphism) {
     return megamorphic_stub();
   }
 
   // Make sure all polymorphic handlers have the same store mode, otherwise the
   // generic stub must be used.
@@ skipping 1110 matching lines @@
 static const Address IC_utilities[] = {
 #define ADDR(name) FUNCTION_ADDR(name),
     IC_UTIL_LIST(ADDR) NULL
 #undef ADDR
 };
 
 
 Address IC::AddressFromUtilityId(IC::UtilityId id) { return IC_utilities[id]; }
 }
 }  // namespace v8::internal