Improve polymorphic loads on single slots.

src/small-pointer-list.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 51 matching lines @@
       list->Add(single_value());
     }
     ASSERT(IsAligned(reinterpret_cast<intptr_t>(list), kPointerAlignment));
     data_ = reinterpret_cast<intptr_t>(list) | kListTag;
   }
 
   void Clear() {
     data_ = kEmptyTag;
   }
 
+  void Sort() {
+    if ((data_ & kTagMask) == kListTag) {
+      list()->Sort(compare_value);
+    }
+  }
+
   bool is_empty() const { return length() == 0; }
 
   int length() const {
     if ((data_ & kTagMask) == kEmptyTag) return 0;
     if ((data_ & kTagMask) == kSingletonTag) return 1;
     return list()->length();
   }
 
   void Add(T* pointer) {
     ASSERT(IsAligned(reinterpret_cast<intptr_t>(pointer), kPointerAlignment));
@@ skipping 70 matching lines @@
         return (single_value() == pointer) ? 1 : 0;
       }
       return 0;
     }
     return list()->CountOccurrences(pointer, start, end);
   }
 
  private:
   typedef ZoneList<T*> PointerList;
 
+  static int compare_value(T* const* a, T* const* b) {
+    return Compare<T>(**a, **b);
+  }
+
   static const intptr_t kEmptyTag = 1;
   static const intptr_t kSingletonTag = 0;
   static const intptr_t kListTag = 2;
   static const intptr_t kTagMask = 3;
   static const intptr_t kValueMask = ~kTagMask;
 
   STATIC_ASSERT(kTagMask + 1 <= kPointerAlignment);
 
   T* single_value() const {
     ASSERT((data_ & kTagMask) == kSingletonTag);
     STATIC_ASSERT(kSingletonTag == 0);
     return reinterpret_cast<T*>(data_);
   }
 
   PointerList* list() const {
     ASSERT((data_ & kTagMask) == kListTag);
     return reinterpret_cast<PointerList*>(data_ & kValueMask);
   }
 
   intptr_t data_;
 
   DISALLOW_COPY_AND_ASSIGN(SmallPointerList);
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_SMALL_POINTER_LIST_H_