Rewrite complexity_test to use (hardcoded) manual time (#1757)

* Rewrite complexity_test to use (hardcoded) manual time

This test is fundamentally flaky, because it tried to read tea leafs,
and is inherently misbehaving in CI environments,
since there are unmitigated sources of noise.

That being said, the computed Big-O also depends on the `--benchmark_min_time=`

Fixes https://github.com/google/benchmark/issues/272

* Correctly compute Big-O for manual timings. Fixes #1758.

* complexity_test: do more stuff in empty loop

* Make all empty loops be a bit longer empty

Looks like on windows, some of these tests still fail,
i guess clock precision is too small.

include/benchmark/benchmark.h

@@ -1792,6 +1792,7 @@ class BENCHMARK_EXPORT BenchmarkReporter {
           real_accumulated_time(0),
           cpu_accumulated_time(0),
           max_heapbytes_used(0),
+          use_real_time_for_initial_big_o(false),
           complexity(oNone),
           complexity_lambda(),
           complexity_n(0),
@@ -1834,6 +1835,10 @@ class BENCHMARK_EXPORT BenchmarkReporter {
     // This is set to 0.0 if memory tracing is not enabled.
     double max_heapbytes_used;
 
+    // By default Big-O is computed for CPU time, but that is not what you want
+    // to happen when manual time was requested, which is stored as real time.
+    bool use_real_time_for_initial_big_o;
+
     // Keep track of arguments to compute asymptotic complexity
     BigO complexity;
     BigOFunc* complexity_lambda;

src/benchmark_runner.cc

@@ -96,6 +96,7 @@ BenchmarkReporter::Run CreateRunReport(
     } else {
       report.real_accumulated_time = results.real_time_used;
     }
+    report.use_real_time_for_initial_big_o = b.use_manual_time();
     report.cpu_accumulated_time = results.cpu_time_used;
     report.complexity_n = results.complexity_n;
     report.complexity = b.complexity();

src/complexity.cc

@@ -186,8 +186,19 @@ std::vector<BenchmarkReporter::Run> ComputeBigO(
     result_cpu = MinimalLeastSq(n, cpu_time, reports[0].complexity_lambda);
     result_real = MinimalLeastSq(n, real_time, reports[0].complexity_lambda);
   } else {
-    result_cpu = MinimalLeastSq(n, cpu_time, reports[0].complexity);
+    const BigO* InitialBigO = &reports[0].complexity;
-    result_real = MinimalLeastSq(n, real_time, result_cpu.complexity);
+    const bool use_real_time_for_initial_big_o =
+        reports[0].use_real_time_for_initial_big_o;
+    if (use_real_time_for_initial_big_o) {
+      result_real = MinimalLeastSq(n, real_time, *InitialBigO);
+      InitialBigO = &result_real.complexity;
+      // The Big-O complexity for CPU time must have the same Big-O function!
+    }
+    result_cpu = MinimalLeastSq(n, cpu_time, *InitialBigO);
+    InitialBigO = &result_cpu.complexity;
+    if (!use_real_time_for_initial_big_o) {
+      result_real = MinimalLeastSq(n, real_time, *InitialBigO);
+    }
   }
 
   // Drop the 'args' when reporting complexity.

test/BUILD

@@ -35,6 +35,7 @@ PER_SRC_TEST_ARGS = {
     "repetitions_test.cc": [" --benchmark_repetitions=3"],
     "spec_arg_test.cc": ["--benchmark_filter=BM_NotChosen"],
     "spec_arg_verbosity_test.cc": ["--v=42"],
+    "complexity_test.cc": ["--benchmark_min_time=1000000x"],
 }
 
 cc_library(

test/CMakeLists.txt

@@ -218,14 +218,8 @@ if(NOT (MSVC OR CMAKE_CXX_SIMULATE_ID STREQUAL "MSVC"))
   benchmark_add_test(NAME cxx03 COMMAND cxx03_test --benchmark_min_time=0.01s)
 endif()
 
-# Attempt to work around flaky test failures when running on Appveyor servers.
-if (DEFINED ENV{APPVEYOR})
-  set(COMPLEXITY_MIN_TIME "0.5s")
-else()
-  set(COMPLEXITY_MIN_TIME "0.01s")
-endif()
 compile_output_test(complexity_test)
-benchmark_add_test(NAME complexity_benchmark COMMAND complexity_test --benchmark_min_time=${COMPLEXITY_MIN_TIME})
+benchmark_add_test(NAME complexity_benchmark COMMAND complexity_test --benchmark_min_time=1000000x)
 
 ###############################################################################
 # GoogleTest Unit Tests

test/basic_test.cc

@@ -5,7 +5,7 @@
 
 void BM_empty(benchmark::State& state) {
   for (auto _ : state) {
-    auto iterations = state.iterations();
+    auto iterations = double(state.iterations()) * double(state.iterations());
     benchmark::DoNotOptimize(iterations);
   }
 }

test/complexity_test.cc

@@ -69,35 +69,44 @@ int AddComplexityTest(const std::string &test_name,
 
 void BM_Complexity_O1(benchmark::State &state) {
   for (auto _ : state) {
-    for (int i = 0; i < 1024; ++i) {
+    // This test requires a non-zero CPU time to avoid divide-by-zero
-      benchmark::DoNotOptimize(i);
+    benchmark::DoNotOptimize(state.iterations());
+    double tmp = state.iterations();
+    benchmark::DoNotOptimize(tmp);
+    for (benchmark::IterationCount i = 0; i < state.iterations(); ++i) {
+      benchmark::DoNotOptimize(state.iterations());
+      tmp *= state.iterations();
+      benchmark::DoNotOptimize(tmp);
     }
+
+    // always 1ns per iteration
+    state.SetIterationTime(42 * 1e-9);
   }
   state.SetComplexityN(state.range(0));
 }
-BENCHMARK(BM_Complexity_O1)->Range(1, 1 << 18)->Complexity(benchmark::o1);
-BENCHMARK(BM_Complexity_O1)->Range(1, 1 << 18)->Complexity();
 BENCHMARK(BM_Complexity_O1)
     ->Range(1, 1 << 18)
+    ->UseManualTime()
+    ->Complexity(benchmark::o1);
+BENCHMARK(BM_Complexity_O1)->Range(1, 1 << 18)->UseManualTime()->Complexity();
+BENCHMARK(BM_Complexity_O1)
+    ->Range(1, 1 << 18)
+    ->UseManualTime()
     ->Complexity([](benchmark::IterationCount) { return 1.0; });
 
-const char *one_test_name = "BM_Complexity_O1";
+const char *one_test_name = "BM_Complexity_O1/manual_time";
-const char *big_o_1_test_name = "BM_Complexity_O1_BigO";
+const char *big_o_1_test_name = "BM_Complexity_O1/manual_time_BigO";
-const char *rms_o_1_test_name = "BM_Complexity_O1_RMS";
+const char *rms_o_1_test_name = "BM_Complexity_O1/manual_time_RMS";
-const char *enum_big_o_1 = "\\([0-9]+\\)";
+const char *enum_auto_big_o_1 = "\\([0-9]+\\)";
-// FIXME: Tolerate both '(1)' and 'lgN' as output when the complexity is auto
-// deduced.
-// See https://github.com/google/benchmark/issues/272
-const char *auto_big_o_1 = "(\\([0-9]+\\))|(lgN)|(N\\^2)";
 const char *lambda_big_o_1 = "f\\(N\\)";
 
 // Add enum tests
 ADD_COMPLEXITY_CASES(one_test_name, big_o_1_test_name, rms_o_1_test_name,
-                     enum_big_o_1, /*family_index=*/0);
+                     enum_auto_big_o_1, /*family_index=*/0);
 
-// Add auto enum tests
+// Add auto tests
 ADD_COMPLEXITY_CASES(one_test_name, big_o_1_test_name, rms_o_1_test_name,
-                     auto_big_o_1, /*family_index=*/1);
+                     enum_auto_big_o_1, /*family_index=*/1);
 
 // Add lambda tests
 ADD_COMPLEXITY_CASES(one_test_name, big_o_1_test_name, rms_o_1_test_name,
@@ -107,43 +116,44 @@ ADD_COMPLEXITY_CASES(one_test_name, big_o_1_test_name, rms_o_1_test_name,
 // --------------------------- Testing BigO O(N) --------------------------- //
 // ========================================================================= //
 
-std::vector<int> ConstructRandomVector(int64_t size) {
-  std::vector<int> v;
-  v.reserve(static_cast<size_t>(size));
-  for (int i = 0; i < size; ++i) {
-    v.push_back(static_cast<int>(std::rand() % size));
-  }
-  return v;
-}
-
 void BM_Complexity_O_N(benchmark::State &state) {
-  auto v = ConstructRandomVector(state.range(0));
-  // Test worst case scenario (item not in vector)
-  const int64_t item_not_in_vector = state.range(0) * 2;
   for (auto _ : state) {
-    auto it = std::find(v.begin(), v.end(), item_not_in_vector);
+    // This test requires a non-zero CPU time to avoid divide-by-zero
-    benchmark::DoNotOptimize(it);
+    benchmark::DoNotOptimize(state.iterations());
+    double tmp = state.iterations();
+    benchmark::DoNotOptimize(tmp);
+    for (benchmark::IterationCount i = 0; i < state.iterations(); ++i) {
+      benchmark::DoNotOptimize(state.iterations());
+      tmp *= state.iterations();
+      benchmark::DoNotOptimize(tmp);
+    }
+
+    // 1ns per iteration per entry
+    state.SetIterationTime(state.range(0) * 42 * 1e-9);
   }
   state.SetComplexityN(state.range(0));
 }
 BENCHMARK(BM_Complexity_O_N)
     ->RangeMultiplier(2)
-    ->Range(1 << 10, 1 << 16)
+    ->Range(1 << 10, 1 << 20)
+    ->UseManualTime()
     ->Complexity(benchmark::oN);
 BENCHMARK(BM_Complexity_O_N)
     ->RangeMultiplier(2)
-    ->Range(1 << 10, 1 << 16)
+    ->Range(1 << 10, 1 << 20)
+    ->UseManualTime()
+    ->Complexity();
+BENCHMARK(BM_Complexity_O_N)
+    ->RangeMultiplier(2)
+    ->Range(1 << 10, 1 << 20)
+    ->UseManualTime()
     ->Complexity([](benchmark::IterationCount n) -> double {
       return static_cast<double>(n);
     });
-BENCHMARK(BM_Complexity_O_N)
-    ->RangeMultiplier(2)
-    ->Range(1 << 10, 1 << 16)
-    ->Complexity();
 
-const char *n_test_name = "BM_Complexity_O_N";
+const char *n_test_name = "BM_Complexity_O_N/manual_time";
-const char *big_o_n_test_name = "BM_Complexity_O_N_BigO";
+const char *big_o_n_test_name = "BM_Complexity_O_N/manual_time_BigO";
-const char *rms_o_n_test_name = "BM_Complexity_O_N_RMS";
+const char *rms_o_n_test_name = "BM_Complexity_O_N/manual_time_RMS";
 const char *enum_auto_big_o_n = "N";
 const char *lambda_big_o_n = "f\\(N\\)";
 
@@ -151,40 +161,57 @@ const char *lambda_big_o_n = "f\\(N\\)";
 ADD_COMPLEXITY_CASES(n_test_name, big_o_n_test_name, rms_o_n_test_name,
                      enum_auto_big_o_n, /*family_index=*/3);
 
+// Add auto tests
+ADD_COMPLEXITY_CASES(n_test_name, big_o_n_test_name, rms_o_n_test_name,
+                     enum_auto_big_o_n, /*family_index=*/4);
+
 // Add lambda tests
 ADD_COMPLEXITY_CASES(n_test_name, big_o_n_test_name, rms_o_n_test_name,
-                     lambda_big_o_n, /*family_index=*/4);
+                     lambda_big_o_n, /*family_index=*/5);
 
 // ========================================================================= //
-// ------------------------- Testing BigO O(N*lgN) ------------------------- //
+// ------------------------- Testing BigO O(NlgN) ------------------------- //
 // ========================================================================= //
 
+static const double kLog2E = 1.44269504088896340736;
 static void BM_Complexity_O_N_log_N(benchmark::State &state) {
-  auto v = ConstructRandomVector(state.range(0));
   for (auto _ : state) {
-    std::sort(v.begin(), v.end());
+    // This test requires a non-zero CPU time to avoid divide-by-zero
+    benchmark::DoNotOptimize(state.iterations());
+    double tmp = state.iterations();
+    benchmark::DoNotOptimize(tmp);
+    for (benchmark::IterationCount i = 0; i < state.iterations(); ++i) {
+      benchmark::DoNotOptimize(state.iterations());
+      tmp *= state.iterations();
+      benchmark::DoNotOptimize(tmp);
+    }
+
+    state.SetIterationTime(state.range(0) * kLog2E * std::log(state.range(0)) *
+                           42 * 1e-9);
   }
   state.SetComplexityN(state.range(0));
 }
-static const double kLog2E = 1.44269504088896340736;
 BENCHMARK(BM_Complexity_O_N_log_N)
     ->RangeMultiplier(2)
-    ->Range(1 << 10, 1 << 16)
+    ->Range(1 << 10, 1U << 24)
+    ->UseManualTime()
     ->Complexity(benchmark::oNLogN);
 BENCHMARK(BM_Complexity_O_N_log_N)
     ->RangeMultiplier(2)
-    ->Range(1 << 10, 1 << 16)
+    ->Range(1 << 10, 1U << 24)
+    ->UseManualTime()
+    ->Complexity();
+BENCHMARK(BM_Complexity_O_N_log_N)
+    ->RangeMultiplier(2)
+    ->Range(1 << 10, 1U << 24)
+    ->UseManualTime()
     ->Complexity([](benchmark::IterationCount n) {
       return kLog2E * static_cast<double>(n) * std::log(static_cast<double>(n));
     });
-BENCHMARK(BM_Complexity_O_N_log_N)
-    ->RangeMultiplier(2)
-    ->Range(1 << 10, 1 << 16)
-    ->Complexity();
 
-const char *n_lg_n_test_name = "BM_Complexity_O_N_log_N";
+const char *n_lg_n_test_name = "BM_Complexity_O_N_log_N/manual_time";
-const char *big_o_n_lg_n_test_name = "BM_Complexity_O_N_log_N_BigO";
+const char *big_o_n_lg_n_test_name = "BM_Complexity_O_N_log_N/manual_time_BigO";
-const char *rms_o_n_lg_n_test_name = "BM_Complexity_O_N_log_N_RMS";
+const char *rms_o_n_lg_n_test_name = "BM_Complexity_O_N_log_N/manual_time_RMS";
 const char *enum_auto_big_o_n_lg_n = "NlgN";
 const char *lambda_big_o_n_lg_n = "f\\(N\\)";
 
@@ -193,10 +220,15 @@ ADD_COMPLEXITY_CASES(n_lg_n_test_name, big_o_n_lg_n_test_name,
                      rms_o_n_lg_n_test_name, enum_auto_big_o_n_lg_n,
                      /*family_index=*/6);
 
-// Add lambda tests
+// NOTE: auto big-o is wron.g
+ADD_COMPLEXITY_CASES(n_lg_n_test_name, big_o_n_lg_n_test_name,
+                     rms_o_n_lg_n_test_name, enum_auto_big_o_n_lg_n,
+                     /*family_index=*/7);
+
+//// Add lambda tests
 ADD_COMPLEXITY_CASES(n_lg_n_test_name, big_o_n_lg_n_test_name,
                      rms_o_n_lg_n_test_name, lambda_big_o_n_lg_n,
-                     /*family_index=*/7);
+                     /*family_index=*/8);
 
 // ========================================================================= //
 // -------- Testing formatting of Complexity with captured args ------------ //
@@ -205,21 +237,31 @@ ADD_COMPLEXITY_CASES(n_lg_n_test_name, big_o_n_lg_n_test_name,
 void BM_ComplexityCaptureArgs(benchmark::State &state, int n) {
   for (auto _ : state) {
     // This test requires a non-zero CPU time to avoid divide-by-zero
-    auto iterations = state.iterations();
+    benchmark::DoNotOptimize(state.iterations());
-    benchmark::DoNotOptimize(iterations);
+    double tmp = state.iterations();
+    benchmark::DoNotOptimize(tmp);
+    for (benchmark::IterationCount i = 0; i < state.iterations(); ++i) {
+      benchmark::DoNotOptimize(state.iterations());
+      tmp *= state.iterations();
+      benchmark::DoNotOptimize(tmp);
+    }
+
+    state.SetIterationTime(state.range(0) * 42 * 1e-9);
   }
   state.SetComplexityN(n);
 }
 
 BENCHMARK_CAPTURE(BM_ComplexityCaptureArgs, capture_test, 100)
+    ->UseManualTime()
     ->Complexity(benchmark::oN)
     ->Ranges({{1, 2}, {3, 4}});
 
 const std::string complexity_capture_name =
-    "BM_ComplexityCaptureArgs/capture_test";
+    "BM_ComplexityCaptureArgs/capture_test/manual_time";
 
 ADD_COMPLEXITY_CASES(complexity_capture_name, complexity_capture_name + "_BigO",
-                     complexity_capture_name + "_RMS", "N", /*family_index=*/9);
+                     complexity_capture_name + "_RMS", "N",
+                     /*family_index=*/9);
 
 // ========================================================================= //
 // --------------------------- TEST CASES END ------------------------------ //

test/diagnostics_test.cc

@@ -49,7 +49,7 @@ void BM_diagnostic_test(benchmark::State& state) {
   if (called_once == false) try_invalid_pause_resume(state);
 
   for (auto _ : state) {
-    auto iterations = state.iterations();
+    auto iterations = double(state.iterations()) * double(state.iterations());
     benchmark::DoNotOptimize(iterations);
   }
 
@@ -65,7 +65,7 @@ void BM_diagnostic_test_keep_running(benchmark::State& state) {
   if (called_once == false) try_invalid_pause_resume(state);
 
   while (state.KeepRunning()) {
-    auto iterations = state.iterations();
+    auto iterations = double(state.iterations()) * double(state.iterations());
     benchmark::DoNotOptimize(iterations);
   }
 

test/link_main_test.cc

@@ -2,7 +2,7 @@
 
 void BM_empty(benchmark::State& state) {
   for (auto _ : state) {
-    auto iterations = state.iterations();
+    auto iterations = double(state.iterations()) * double(state.iterations());
     benchmark::DoNotOptimize(iterations);
   }
 }

test/memory_manager_test.cc

@@ -14,7 +14,7 @@ class TestMemoryManager : public benchmark::MemoryManager {
 
 void BM_empty(benchmark::State& state) {
   for (auto _ : state) {
-    auto iterations = state.iterations();
+    auto iterations = double(state.iterations()) * double(state.iterations());
     benchmark::DoNotOptimize(iterations);
   }
 }

test/perf_counters_test.cc

@@ -14,7 +14,7 @@ BM_DECLARE_string(benchmark_perf_counters);
 
 static void BM_Simple(benchmark::State& state) {
   for (auto _ : state) {
-    auto iterations = state.iterations();
+    auto iterations = double(state.iterations()) * double(state.iterations());
     benchmark::DoNotOptimize(iterations);
   }
 }

test/reporter_output_test.cc

@@ -96,7 +96,7 @@ ADD_CASES(TC_CSVOut, {{"^\"BM_basic\",%csv_report$"}});
 void BM_bytes_per_second(benchmark::State& state) {
   for (auto _ : state) {
     // This test requires a non-zero CPU time to avoid divide-by-zero
-    auto iterations = state.iterations();
+    auto iterations = double(state.iterations()) * double(state.iterations());
     benchmark::DoNotOptimize(iterations);
   }
   state.SetBytesProcessed(1);
@@ -128,7 +128,7 @@ ADD_CASES(TC_CSVOut, {{"^\"BM_bytes_per_second\",%csv_bytes_report$"}});
 void BM_items_per_second(benchmark::State& state) {
   for (auto _ : state) {
     // This test requires a non-zero CPU time to avoid divide-by-zero
-    auto iterations = state.iterations();
+    auto iterations = double(state.iterations()) * double(state.iterations());
     benchmark::DoNotOptimize(iterations);
   }
   state.SetItemsProcessed(1);
@@ -409,7 +409,7 @@ ADD_CASES(TC_ConsoleOut, {{"^BM_BigArgs/1073741824 %console_report$"},
 void BM_Complexity_O1(benchmark::State& state) {
   for (auto _ : state) {
     // This test requires a non-zero CPU time to avoid divide-by-zero
-    auto iterations = state.iterations();
+    auto iterations = double(state.iterations()) * double(state.iterations());
     benchmark::DoNotOptimize(iterations);
   }
   state.SetComplexityN(state.range(0));

test/skip_with_error_test.cc

@@ -143,7 +143,7 @@ ADD_CASES("BM_error_during_running_ranged_for",
 
 void BM_error_after_running(benchmark::State& state) {
   for (auto _ : state) {
-    auto iterations = state.iterations();
+    auto iterations = double(state.iterations()) * double(state.iterations());
     benchmark::DoNotOptimize(iterations);
   }
   if (state.thread_index() <= (state.threads() / 2))

test/user_counters_tabular_test.cc

@@ -64,7 +64,7 @@ ADD_CASES(TC_CSVOut, {{"%csv_header,"
 void BM_Counters_Tabular(benchmark::State& state) {
   for (auto _ : state) {
     // This test requires a non-zero CPU time to avoid divide-by-zero
-    auto iterations = state.iterations();
+    auto iterations = double(state.iterations()) * double(state.iterations());
     benchmark::DoNotOptimize(iterations);
   }
   namespace bm = benchmark;

test/user_counters_test.cc

@@ -67,7 +67,7 @@ int num_calls1 = 0;
 void BM_Counters_WithBytesAndItemsPSec(benchmark::State& state) {
   for (auto _ : state) {
     // This test requires a non-zero CPU time to avoid divide-by-zero
-    auto iterations = state.iterations();
+    auto iterations = double(state.iterations()) * double(state.iterations());
     benchmark::DoNotOptimize(iterations);
   }
   state.counters["foo"] = 1;
@@ -119,7 +119,7 @@ CHECK_BENCHMARK_RESULTS("BM_Counters_WithBytesAndItemsPSec",
 void BM_Counters_Rate(benchmark::State& state) {
   for (auto _ : state) {
     // This test requires a non-zero CPU time to avoid divide-by-zero
-    auto iterations = state.iterations();
+    auto iterations = double(state.iterations()) * double(state.iterations());
     benchmark::DoNotOptimize(iterations);
   }
   namespace bm = benchmark;
@@ -163,7 +163,7 @@ CHECK_BENCHMARK_RESULTS("BM_Counters_Rate", &CheckRate);
 void BM_Invert(benchmark::State& state) {
   for (auto _ : state) {
     // This test requires a non-zero CPU time to avoid divide-by-zero
-    auto iterations = state.iterations();
+    auto iterations = double(state.iterations()) * double(state.iterations());
     benchmark::DoNotOptimize(iterations);
   }
   namespace bm = benchmark;
@@ -204,7 +204,7 @@ CHECK_BENCHMARK_RESULTS("BM_Invert", &CheckInvert);
 void BM_Counters_InvertedRate(benchmark::State& state) {
   for (auto _ : state) {
     // This test requires a non-zero CPU time to avoid divide-by-zero
-    auto iterations = state.iterations();
+    auto iterations = double(state.iterations()) * double(state.iterations());
     benchmark::DoNotOptimize(iterations);
   }
   namespace bm = benchmark;
@@ -333,7 +333,7 @@ CHECK_BENCHMARK_RESULTS("BM_Counters_AvgThreads/threads:%int",
 void BM_Counters_AvgThreadsRate(benchmark::State& state) {
   for (auto _ : state) {
     // This test requires a non-zero CPU time to avoid divide-by-zero
-    auto iterations = state.iterations();
+    auto iterations = double(state.iterations()) * double(state.iterations());
     benchmark::DoNotOptimize(iterations);
   }
   namespace bm = benchmark;
@@ -421,7 +421,7 @@ CHECK_BENCHMARK_RESULTS("BM_Counters_IterationInvariant",
 void BM_Counters_kIsIterationInvariantRate(benchmark::State& state) {
   for (auto _ : state) {
     // This test requires a non-zero CPU time to avoid divide-by-zero
-    auto iterations = state.iterations();
+    auto iterations = double(state.iterations()) * double(state.iterations());
     benchmark::DoNotOptimize(iterations);
   }
   namespace bm = benchmark;
@@ -513,7 +513,7 @@ CHECK_BENCHMARK_RESULTS("BM_Counters_AvgIterations", &CheckAvgIterations);
 void BM_Counters_kAvgIterationsRate(benchmark::State& state) {
   for (auto _ : state) {
     // This test requires a non-zero CPU time to avoid divide-by-zero
-    auto iterations = state.iterations();
+    auto iterations = double(state.iterations()) * double(state.iterations());
     benchmark::DoNotOptimize(iterations);
   }
   namespace bm = benchmark;