Support "shared" histograms between processes.

base/metrics/histogram_samples.h

 // Copyright (c) 2012 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_HISTOGRAM_SAMPLES_H_
 #define BASE_METRICS_HISTOGRAM_SAMPLES_H_
 
 #include "base/atomicops.h"
 #include "base/basictypes.h"
 #include "base/metrics/histogram_base.h"
@@ skipping 13 matching lines @@
     // by the caller. It is generally used to identify the sample-set across
     // threads and processes, though not necessarily uniquely as it is possible
     // to have multiple sample-sets representing subsets of the data.
     const uint64 id;
 
     // The sum of all the entries, effectivly the sum(sample * count) for
     // all samples. Despite being atomic, no guarantees are made on the
     // accuracy of this value; there may be races during histogram
     // accumulation and snapshotting that we choose to accept. It should
     // be treated as approximate.
-#ifdef ARCH_CPU_64_BITS
-    subtle::Atomic64 sum;
-#else  // No Atomic64 on 32-bit platforms.
+    // TODO(bcwhite): Change this to std::atomic<uint64>.
     int64 sum;
-#endif
 
     // A "redundant" count helps identify memory corruption. It redundantly
     // stores the total number of samples accumulated in the histogram. We
     // can compare this count to the sum of the counts (TotalCount() function),
     // and detect problems. Note, depending on the implementation of different
     // histogram types, there might be races during histogram accumulation
     // and snapshotting that we choose to accept. In this case, the tallies
     // might mismatch even when no memory corruption has happened.
     HistogramBase::AtomicCount redundant_count;
 
@@ skipping 60 matching lines @@
 
   // Get the index of current histogram bucket.
   // For histograms that don't use predefined buckets, it returns false.
   // Requires: !Done();
   virtual bool GetBucketIndex(size_t* index) const;
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_SAMPLES_H_