stats: Remove field from GaugeImpl that was not used, by moving it to…

… the counter-specific section. (#7477)

* Remove field from GaugeImpl that was not used, by moving it to the counter-specific section. (#7477)
Signed-off-by: Joshua Marantz <jmarantz@google.com>

source/common/stats/heap_stat_data.cc

@@ -86,11 +86,9 @@ template <class BaseClass> class StatsSharedImpl : public MetricImpl<BaseClass>
 protected:
   HeapStatDataAllocator& alloc_;
 
-  // Holds backing store for both CounterImpl and GaugeImpl. This provides a level
-  // of indirection needed to enable stats created with the same name from
-  // different scopes to share the same value.
+  // Holds backing store shared by both CounterImpl and GaugeImpl. CounterImpl
+  // adds another field, pending_increment_, that is not used in Gauge.
   std::atomic<uint64_t> value_{0};
-  std::atomic<uint64_t> pending_increment_{0};
   std::atomic<uint16_t> flags_{0};
   std::atomic<uint16_t> ref_count_{0};
 };
@@ -113,8 +111,10 @@ class CounterImpl : public StatsSharedImpl<Counter> {
   void inc() override { add(1); }
   uint64_t latch() override { return pending_increment_.exchange(0); }
   void reset() override { value_ = 0; }
-  bool used() const override { return flags_ & Flags::Used; }
   uint64_t value() const override { return value_; }
+
+private:
+  std::atomic<uint64_t> pending_increment_{0};
 };
 
 class GaugeImpl : public StatsSharedImpl<Gauge> {

test/integration/stats_integration_test.cc

@@ -209,6 +209,7 @@ TEST_P(ClusterMemoryTestRunner, MemoryLargeClusterSizeWithStats) {
   // 2019/06/17  7243     49412       49700   macros for exact/upper-bound memory checks
   // 2019/06/29  7364     45685       46000   combine 2 levels of stat ref-counting into 1
   // 2019/06/30  7428     42742       43000   remove stats multiple inheritance, inline HeapStatData
+  // 2019/07/06  7477     42742       43000   fork gauge representation to drop pending_increment_
 
   // Note: when adjusting this value: EXPECT_MEMORY_EQ is active only in CI
   // 'release' builds, where we control the platform and tool-chain. So you
@@ -218,7 +219,7 @@ TEST_P(ClusterMemoryTestRunner, MemoryLargeClusterSizeWithStats) {
   // On a local clang8/libstdc++/linux flow, the memory usage was observed in
   // June 2019 to be 64 bytes higher than it is in CI/release. Your mileage may
   // vary.
-  EXPECT_MEMORY_EQ(m_per_cluster, 42742);
+  EXPECT_MEMORY_EQ(m_per_cluster, 42742); // 104 bytes higher than a debug build.
   EXPECT_MEMORY_LE(m_per_cluster, 43000);
 }