Revert of Embed a single sample in histogram metadata.

base/metrics/histogram_samples.h

Index: base/metrics/histogram_samples.h
diff --git a/base/metrics/histogram_samples.h b/base/metrics/histogram_samples.h
index 0e24c4f160e7a2a29f83eefc554efbf2dffc80f4..93f6d21c8aabd34eed61dd9c20d2e030cabada8f 100644
--- a/base/metrics/histogram_samples.h
+++ b/base/metrics/histogram_samples.h
@@ -24,64 +24,8 @@
 // elements must be of a fixed width to ensure 32/64-bit interoperability.
 // If this structure changes, bump the version number for kTypeIdHistogram
 // in persistent_histogram_allocator.cc.
-//
-// Note that though these samples are individually consistent (through the use
-// of atomic operations on the counts), there is only "eventual consistency"
-// overall when multiple threads are accessing this data. That means that the
-// sum, redundant-count, etc. could be momentarily out-of-sync with the stored
-// counts but will settle to a consistent "steady state" once all threads have
-// exited this code.
 class BASE_EXPORT HistogramSamples {
  public:
-  // A single bucket and count. To fit within a single atomic on 32-bit build
-  // architectures, both |bucket| and |count| are limited in size to 16 bits.
-  // This limits the functionality somewhat but if an entry can't fit then
-  // the full array of samples can be allocated and used.
-  struct SingleSample {
-    uint16_t bucket;
-    uint16_t count;
-  };
-
-  // A structure for managing an atomic single sample. Because this is generally
-  // used in association with other atomic values, the defined methods use
-  // acquire/release operations to guarantee ordering with outside values.
-  union AtomicSingleSample {
-    AtomicSingleSample() : as_atomic(0) {}
-    AtomicSingleSample(subtle::Atomic32 rhs) : as_atomic(rhs) {}
-
-    // Returns the single sample in an atomic manner. This in an "acquire"
-    // load. The returned sample isn't shared and thus its fields can be safely
-    // accessed.
-    SingleSample Load() const;
-
-    // Extracts the single sample in an atomic manner. If |disable| is true
-    // then this object will be set so it will never accumulate another value.
-    // This is "no barrier" so doesn't enforce ordering with other atomic ops.
-    SingleSample Extract(bool disable);
-
-    // Adds a given count to the held bucket. If not possible, it returns false
-    // and leaves the parts unchanged. Once extracted/disabled, this always
-    // returns false. This in an "acquire/release" operation.
-    bool Accumulate(size_t bucket, HistogramBase::Count count);
-
-    // Returns if the sample has been "disabled" (via Extract) and thus not
-    // allowed to accept further accumulation.
-    bool IsDisabled() const;
-
-   private:
-    // union field: The actual sample bucket and count.
-    SingleSample as_parts;
-
-    // union field: The sample as an atomic value. Atomic64 would provide
-    // more flexibility but isn't available on all builds. This can hold a
-    // special, internal "disabled" value indicating that it must not accept
-    // further accumulation.
-    subtle::Atomic32 as_atomic;
-  };
-
-  // A structure of information about the data, common to all sample containers.
-  // Because of how this is used in persistent memory, it must be a POD object
-  // that makes sense when initialized to all zeros.
   struct Metadata {
     // Expected size for 32/64-bit check.
     static constexpr size_t kExpectedInstanceSize = 24;
@@ -114,17 +58,21 @@
     // might mismatch even when no memory corruption has happened.
     HistogramBase::AtomicCount redundant_count;
 
-    // A single histogram value and associated count. This allows histograms
-    // that typically report only a single value to not require full storage
-    // to be allocated.
-    AtomicSingleSample single_sample;  // 32 bits
+    // 4 bytes of padding to explicitly extend this structure to a multiple of
+    // 64-bits. This is required to ensure the structure is the same size on
+    // both 32-bit and 64-bit builds.
+    char padding[4];
   };
 
-  // Because structures held in persistent memory must be POD, there can be no
+  // Because sturctures held in persistent memory must be POD, there can be no
   // default constructor to clear the fields. This derived class exists just
   // to clear them when being allocated on the heap.
-  struct BASE_EXPORT LocalMetadata : Metadata {
-    LocalMetadata();
+  struct LocalMetadata : Metadata {
+    LocalMetadata() {
+      id = 0;
+      sum = 0;
+      redundant_count = 0;
+    }
   };
 
   explicit HistogramSamples(uint64_t id);
@@ -164,20 +112,8 @@
   enum Operator { ADD, SUBTRACT };
   virtual bool AddSubtractImpl(SampleCountIterator* iter, Operator op) = 0;
 
-  // Accumulates to the embedded single-sample field if possible. Returns true
-  // on success, false otherwise. Sum and redundant-count are also updated in
-  // the success case.
-  bool AccumulateSingleSample(HistogramBase::Sample value,
-                              HistogramBase::Count count,
-                              size_t bucket);
-
-  // Atomically adjust the sum and redundant-count.
-  void IncreaseSumAndCount(int64_t sum, HistogramBase::Count count);
-
-  AtomicSingleSample& single_sample() { return meta_->single_sample; }
-  const AtomicSingleSample& single_sample() const {
-    return meta_->single_sample;
-  }
+  void IncreaseSum(int64_t diff);
+  void IncreaseRedundantCount(HistogramBase::Count diff);
 
  private:
   // In order to support histograms shared through an external memory segment,
@@ -209,35 +145,6 @@
   virtual bool GetBucketIndex(size_t* index) const;
 };
 
-class BASE_EXPORT SingleSampleIterator : public SampleCountIterator {
- public:
-  SingleSampleIterator(HistogramBase::Sample min,
-                       HistogramBase::Sample max,
-                       HistogramBase::Count count);
-  SingleSampleIterator(HistogramBase::Sample min,
-                       HistogramBase::Sample max,
-                       HistogramBase::Count count,
-                       size_t bucket_index);
-  ~SingleSampleIterator() override;
-
-  // SampleCountIterator:
-  bool Done() const override;
-  void Next() override;
-  void Get(HistogramBase::Sample* min,
-           HistogramBase::Sample* max,
-           HistogramBase::Count* count) const override;
-
-  // SampleVector uses predefined buckets so iterator can return bucket index.
-  bool GetBucketIndex(size_t* index) const override;
-
- private:
-  // Information about the single value to return.
-  const HistogramBase::Sample min_;
-  const HistogramBase::Sample max_;
-  const size_t bucket_index_;
-  HistogramBase::Count count_;
-};
-
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_SAMPLES_H_