Re-land https://codereview.chromium.org/23602005: Use nobarrier atomics for lossy counters in base:…

base/metrics/sample_vector.cc

 // 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.
 
 #include "base/metrics/sample_vector.h"
 
 #include "base/logging.h"
 #include "base/metrics/bucket_ranges.h"
 
 using std::vector;
 
 namespace base {
 
 typedef HistogramBase::Count Count;
 typedef HistogramBase::Sample Sample;
 
 SampleVector::SampleVector(const BucketRanges* bucket_ranges)
     : counts_(bucket_ranges->bucket_count()),
       bucket_ranges_(bucket_ranges) {
   CHECK_GE(bucket_ranges_->bucket_count(), 1u);
 }
 
 SampleVector::~SampleVector() {}
 
 void SampleVector::Accumulate(Sample value, Count count) {
   size_t bucket_index = GetBucketIndex(value);
-  counts_[bucket_index] += count;
+  subtle::NoBarrier_Store(&counts_[bucket_index],
+      subtle::NoBarrier_Load(&counts_[bucket_index]) + count);
   IncreaseSum(count * value);
   IncreaseRedundantCount(count);
 }
 
 Count SampleVector::GetCount(Sample value) const {
   size_t bucket_index = GetBucketIndex(value);
   return counts_[bucket_index];
 }
 
 Count SampleVector::TotalCount() const {
@@ skipping 114 matching lines @@
     return;
 
   while (index_ < counts_->size()) {
     if ((*counts_)[index_] != 0)
       return;
     index_++;
   }
 }
 
 }  // namespace base