Improve efficiency of persistent sparse histograms.

base/metrics/persistent_memory_allocator.cc

 // Copyright (c) 2015 The Chromium 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 "base/metrics/persistent_memory_allocator.h"
 
 #include <assert.h>
 #include <algorithm>
 
 #include "base/files/memory_mapped_file.h"
@@ skipping 98 matching lines @@
   volatile std::atomic<uint32_t> tailptr;  // Last block of iteration queue.
   volatile BlockHeader queue;   // Empty block for linked-list head/tail.
 };
 
 // The "queue" block header is used to detect "last node" so that zero/null
 // can be used to indicate that it hasn't been added at all. It is part of
 // the SharedMetadata structure which itself is always located at offset zero.
 const PersistentMemoryAllocator::Reference
     PersistentMemoryAllocator::kReferenceQueue =
         offsetof(SharedMetadata, queue);
-const PersistentMemoryAllocator::Reference
-    PersistentMemoryAllocator::kReferenceNull = 0;
 
 PersistentMemoryAllocator::Iterator::Iterator(
     const PersistentMemoryAllocator* allocator)
     : allocator_(allocator), last_record_(kReferenceQueue), record_count_(0) {}
 
 PersistentMemoryAllocator::Iterator::Iterator(
     const PersistentMemoryAllocator* allocator,
     Reference starting_after)
     : allocator_(allocator), last_record_(starting_after), record_count_(0) {
   // Ensure that the starting point is a valid, iterable block (meaning it can
@@ skipping 630 matching lines @@
 
 FilePersistentMemoryAllocator::~FilePersistentMemoryAllocator() {}
 
 // static
 bool FilePersistentMemoryAllocator::IsFileAcceptable(
     const MemoryMappedFile& file) {
   return IsMemoryAcceptable(file.data(), file.length(), 0, true);
 }
 
 }  // namespace base