Improve efficiency of persistent sparse histograms.

base/metrics/persistent_memory_allocator.h

 // 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.
 
 #ifndef BASE_METRICS_PERSISTENT_MEMORY_ALLOCATOR_H_
 #define BASE_METRICS_PERSISTENT_MEMORY_ALLOCATOR_H_
 
 #include <stdint.h>
 
 #include <atomic>
@@ skipping 100 matching lines @@
 
     DISALLOW_COPY_AND_ASSIGN(Iterator);
   };
 
   // Returned information about the internal state of the heap.
   struct MemoryInfo {
     size_t total;
     size_t free;
   };
 
+  enum : Reference {
+    kReferenceNull = 0  // A common "null" reference value.
+  };
+
   enum : uint32_t {
     kTypeIdAny = 0  // Match any type-id inside GetAsObject().
   };
 
   // The allocator operates on any arbitrary block of memory. Creation and
   // persisting or sharing of that block with another process is the
   // responsibility of the caller. The allocator needs to know only the
   // block's |base| address, the total |size| of the block, and any internal
   // |page| size (zero if not paged) across which allocations should not span.
   // The |id| is an arbitrary value the caller can use to identify a
@@ skipping 146 matching lines @@
  protected:
   volatile char* const mem_base_;  // Memory base. (char so sizeof guaranteed 1)
   const uint32_t mem_size_;        // Size of entire memory segment.
   const uint32_t mem_page_;        // Page size allocations shouldn't cross.
 
  private:
   struct SharedMetadata;
   struct BlockHeader;
   static const uint32_t kAllocAlignment;
   static const Reference kReferenceQueue;
-  static const Reference kReferenceNull;
 
   // The shared metadata is always located at the top of the memory segment.
   // These convenience functions eliminate constant casting of the base
   // pointer within the code.
   const SharedMetadata* shared_meta() const {
     return reinterpret_cast<const SharedMetadata*>(
         const_cast<const char*>(mem_base_));
   }
   SharedMetadata* shared_meta() {
     return reinterpret_cast<SharedMetadata*>(const_cast<char*>(mem_base_));
@@ skipping 97 matching lines @@
 
  private:
   std::unique_ptr<MemoryMappedFile> mapped_file_;
 
   DISALLOW_COPY_AND_ASSIGN(FilePersistentMemoryAllocator);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_PERSISTENT_MEMORY_ALLOCATOR_H_