Oilpan: Reduce sizeof(Persistent) to 16 byte

Source/platform/heap/HeapTest.cpp

 /*
  * Copyright (C) 2013 Google Inc. 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
@@ skipping 6251 matching lines @@
 TEST(HeapTest, MixinConstructionNoGC)
 {
     Persistent<ClassWithMember> object = ClassWithMember::create();
     EXPECT_EQ(0, object->traceCount());
     TestMixinAllocatingObject* mixin = TestMixinAllocatingObject::create(object.get());
     EXPECT_TRUE(mixin);
     EXPECT_GT(object->traceCount(), 0);
     EXPECT_GT(mixin->traceCount(), 0);
 }
 
+TEST(HeapTest, PersistentPerfTest)

[haraken, 2015/06/29 11:48:52]

This is a micro-benchmark to measure the performance of this CL (I'm not
planning to commit the benchmark). It seems that this CL regresses the
performance by 4%. (I'd say that a 4% regression in this kind of micro benchmark
is OK, but I was expecting that this CL would improve performance.)

+{
+    const int count = 10000;
+    int sum = 0;
+    Persistent<IntWrapper>** handle = static_cast<Persistent<IntWrapper>**>(malloc(sizeof(Persistent<IntWrapper>*) * count));
+    double begin = WTF::currentTimeMS();
+    for (int i = 0; i < count; i++) {
+        handle[i] = new Persistent<IntWrapper>(IntWrapper::create(i));
+    }
+    for (int delta = 2; delta < 100; delta++) {
+        for (int i = 0; i < count; i+=delta) {
+            sum += handle[i]->get()->value();
+            delete handle[i];
+        }
+        for (int i = 0; i < count; i+=delta) {
+            handle[i] = new Persistent<IntWrapper>(IntWrapper::create(i));
+        }
+    }
+    for (int i = 0; i < count; i++) {
+        sum += handle[i]->get()->value();
+        delete handle[i];
+    }
+    double end =WTF::currentTimeMS();
+    fprintf(stderr, "%.2lf %d\n", end - begin, sum);
+}
+
 } // namespace blink