State: Initialize counters with kAvgIteration in constructor (#1652)

* State: Initialize counters with kAvgIteration in constructor

Previously, `counters` was updated in `PauseTiming()` with
`counters[name] += Counter(measurement, kAvgIteration)`.

The first `counters[name]` call inserts a counter with no flags.

There is no `operator+=` for `Counter`, so the insertion is done
by converting the `Counter` to a `double`, then constructing a
`Counter` to insert from the `double`, which drops the flags.

Pre-insert the `Counter` with the correct flags, then only
update `Counter::value`.

Introduced in 1c64a36 ([perf-counters] Fix pause/resume (#1643)).

* perf_counters_test.cc: Don't divide by iterations

Perf counters are now divided by iterations, so dividing again
in the test is wrong.

* State: Fix shadowed param error

* benchmark.cc: Fix clang-tidy error

---------

Co-authored-by: dominic <510002+dmah42@users.noreply.github.com>

2 files changed, 22 insertions, 14 deletions

diff --git a/src/benchmark.cc b/src/benchmark.cc
index 974cde6..6139e59 100644
--- a/src/benchmark.cc
+++ b/src/benchmark.cc
@@ -179,6 +179,17 @@ State::State(std::string name, IterationCount max_iters,
   BM_CHECK_LT(thread_index_, threads_)
       << "thread_index must be less than threads";
 
+  // Add counters with correct flag now.  If added with `counters[name]` in
+  // `PauseTiming`, a new `Counter` will be inserted the first time, which
+  // won't have the flag.  Inserting them now also reduces the allocations
+  // during the benchmark.
+  if (perf_counters_measurement_) {
+    for (const std::string& counter_name :
+         perf_counters_measurement_->names()) {
+      counters[counter_name] = Counter(0.0, Counter::kAvgIterations);
+    }
+  }
+
   // Note: The use of offsetof below is technically undefined until C++17
   // because State is not a standard layout type. However, all compilers
   // currently provide well-defined behavior as an extension (which is
@@ -227,9 +238,11 @@ void State::PauseTiming() {
       BM_CHECK(false) << "Perf counters read the value failed.";
     }
     for (const auto& name_and_measurement : measurements) {
-      auto name = name_and_measurement.first;
-      auto measurement = name_and_measurement.second;
-      counters[name] += Counter(measurement, Counter::kAvgIterations);
+      const std::string& name = name_and_measurement.first;
+      const double measurement = name_and_measurement.second;
+      // Counter was inserted with `kAvgIterations` flag by the constructor.
+      assert(counters.find(name) != counters.end());
+      counters[name].value += measurement;
     }
   }
 }
diff --git a/test/perf_counters_test.cc b/test/perf_counters_test.cc
index f2ef9be..b0a3ab0 100644
--- a/test/perf_counters_test.cc
+++ b/test/perf_counters_test.cc
@@ -66,22 +66,17 @@ static void CheckSimple(Results const& e) {
 double withoutPauseResumeInstrCount = 0.0;
 double withPauseResumeInstrCount = 0.0;
 
-static void CheckInstrCount(double* counter, Results const& e) {
-  BM_CHECK_GT(e.NumIterations(), 0);
-  *counter = e.GetAs<double>("INSTRUCTIONS") / e.NumIterations();
+static void SaveInstrCountWithoutResume(Results const& e) {
+  withoutPauseResumeInstrCount = e.GetAs<double>("INSTRUCTIONS");
 }
 
-static void CheckInstrCountWithoutResume(Results const& e) {
-  CheckInstrCount(&withoutPauseResumeInstrCount, e);
-}
-
-static void CheckInstrCountWithResume(Results const& e) {
-  CheckInstrCount(&withPauseResumeInstrCount, e);
+static void SaveInstrCountWithResume(Results const& e) {
+  withPauseResumeInstrCount = e.GetAs<double>("INSTRUCTIONS");
 }
 
 CHECK_BENCHMARK_RESULTS("BM_Simple", &CheckSimple);
-CHECK_BENCHMARK_RESULTS("BM_WithoutPauseResume", &CheckInstrCountWithoutResume);
-CHECK_BENCHMARK_RESULTS("BM_WithPauseResume", &CheckInstrCountWithResume);
+CHECK_BENCHMARK_RESULTS("BM_WithoutPauseResume", &SaveInstrCountWithoutResume);
+CHECK_BENCHMARK_RESULTS("BM_WithPauseResume", &SaveInstrCountWithResume);
 
 int main(int argc, char* argv[]) {
   if (!benchmark::internal::PerfCounters::kSupported) {