add interceptors which do not mask existing properties

src/ic/handler-compiler.cc

 // Copyright 2014 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/cpu-profiler.h"
 #include "src/ic/call-optimization.h"
 #include "src/ic/handler-compiler.h"
 #include "src/ic/ic.h"
@@ skipping 293 matching lines @@
         Handle<Map> receiver_map = map();
         inline_followup = call_optimization.is_simple_api_call() &&
                           call_optimization.IsCompatibleReceiverMap(
                               receiver_map, property_holder);
       }
     }
   }
 
   Label miss;
   InterceptorVectorSlotPush(receiver());
+  auto holder_orig = holder();
+  // Temporarily swap holder to check entire chain.  What could go wrong?
+  if (holder()->GetNamedInterceptor()->non_masking()) {
+    DCHECK(!inline_followup);
+    JSObject* last = *holder();
+    PrototypeIterator iter(isolate(), last);
+    while (!iter.IsAtEnd()) {
+      last = JSObject::cast(iter.GetCurrent());
+      iter.Advance();
+    }
+    auto last_handle = handle(last);
+    set_holder(last_handle);
+  }
   Register reg = FrontendHeader(receiver(), it->name(), &miss);
+  // Now do the check again, getting the holder into the right register...
+  if (holder_orig->GetNamedInterceptor()->non_masking()) {
+    set_holder(holder_orig);
+    reg = FrontendHeader(receiver(), it->name(), &miss);
+  }
   FrontendFooter(it->name(), &miss);
   InterceptorVectorSlotPop(reg);
 
   if (inline_followup) {
     // TODO(368): Compile in the whole chain: all the interceptors in
     // prototypes and ultimate answer.
     GenerateLoadInterceptorWithFollowup(it, reg);
   } else {
+    // TODO(dcarney): add a function for calling nonmasking interceptors.
     GenerateLoadInterceptor(reg);
   }
   return GetCode(kind(), Code::FAST, it->name());
 }
 
 
 void NamedLoadHandlerCompiler::GenerateLoadPostInterceptor(
     LookupIterator* it, Register interceptor_reg) {
   Handle<JSObject> real_named_property_holder(it->GetHolder<JSObject>());
 
@@ skipping 180 matching lines @@
         DCHECK(elements_kind == DICTIONARY_ELEMENTS);
         cached_stub = LoadDictionaryElementStub(isolate()).GetCode();
       }
     }
 
     handlers->Add(cached_stub);
   }
 }
 }
 }  // namespace v8::internal