Optimize clearing of map transitions.

src/snapshot/serialize.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/snapshot/serialize.h"
 
 #include "src/accessors.h"
 #include "src/api.h"
 #include "src/base/platform/platform.h"
 #include "src/bootstrapper.h"
@@ skipping 1956 matching lines @@
 }
 
 
 // Clear and later restore the next link in the weak cell, if the object is one.
 class UnlinkWeakCellScope {
  public:
   explicit UnlinkWeakCellScope(HeapObject* object) : weak_cell_(NULL) {
     if (object->IsWeakCell()) {
       weak_cell_ = WeakCell::cast(object);
       next_ = weak_cell_->next();
-      weak_cell_->clear_next(object->GetHeap());
+      weak_cell_->clear_next(object->GetHeap()->the_hole_value());

[Yang, 2015/12/01 11:21:40]

Why does the caller need to know with what sentinel value we clear the weak
cell?

[ulan, 2015/12/01 12:49:52]

For efficiency, this function is used in a hot loop in mark-compact where we can
cache the value.

I think it is OK, since clear_next shouldn't be used by anyone except GC (and
serializer).

     }
   }
 
   ~UnlinkWeakCellScope() {
     if (weak_cell_) weak_cell_->set_next(next_, UPDATE_WEAK_WRITE_BARRIER);
   }
 
  private:
   WeakCell* weak_cell_;
   Object* next_;
@@ skipping 868 matching lines @@
   SerializedCodeData* scd = new SerializedCodeData(cached_data);
   SanityCheckResult r = scd->SanityCheck(isolate, source);
   if (r == CHECK_SUCCESS) return scd;
   cached_data->Reject();
   source->GetIsolate()->counters()->code_cache_reject_reason()->AddSample(r);
   delete scd;
   return NULL;
 }
 }  // namespace internal
 }  // namespace v8