Add support for other multi-threading APIs

docs/user_guide.md

@@ -830,6 +830,64 @@ BENCHMARK(BM_test)->Range(8, 8<<10)->UseRealTime();
 
 Without `UseRealTime`, CPU time is used by default.
 
+### Manual Multithreaded Benchmarks
+
+Google/benchmark uses `std::thread` as multithreading environment per default.
+If you want to use another multithreading environment (e.g. OpenMP), you can
+turn off the automatic creation of threads using the `ManualThreading` function.
+```c++
+static void BM_MultiThreaded(benchmark::State& state) {
+  // Setup code here.
+  for (auto _ : state) {
+#pragma omp parallel num_threads(state.threads)
+    // Run the multithreaded test.
+  }
+  // Teardown code here.
+}
+
+BENCHMARK(BM_MultiThreaded)->ManualThreading()->Threads(1)->Threads(2)->Threads(4);
+```
+The above example creates a parallel region in each iteration.
+This includes the setup and teardown of the parallel region in the time measurement, and it
+adds an implicit barrier at the end of each iteration.
+You can avoid these effects, if you run the whole loop in parallel.
+Then you must not use the `state` object directly, but create a `ThreadState` object in each thread.
+```c++
+static void BM_MultiThreaded(benchmark::State& state) {
+  // Setup code (shared objects) here.
+#pragma omp parallel num_threads(state.threads)
+  {
+    // Thread-local setup code here.
+    for (auto _ : benchmark::ThreadState(state)) {
+      // Run the multithreaded test.
+    }
+  }
+  // Teardown code here.
+}
+
+BENCHMARK(BM_MultiThreaded)->ManualThreading()->Threads(1)->Threads(2)->Threads(4);
+```
+If you use the `ThreadState` object and explicitly specify the number of threads, then you must
+use `ManualThreading` and the number of created `ThreadState` objects must match the number of specified threads.
+However, if you use `ThreadState` without explicitly specifying the number of threads,
+then the number of threads is determined by the number of created `ThreadState` objects.
+Specifying `ManualThreading` is optional in this case.
+```c++
+static void BM_MultiThreaded(benchmark::State& state) {
+  // Setup code (shared objects) here.
+#pragma omp parallel
+  {
+    // Thread-local setup code here.
+    for (auto _ : benchmark::ThreadState(state)) {
+      // Run the multithreaded test.
+    }
+  }
+  // Teardown code here.
+}
+
+BENCHMARK(BM_MultiThreaded);  // measures omp_get_max_threads number of threads.
+```
+
 <a name="cpu-timers" />
 
 ## CPU Timers

include/benchmark/benchmark.h

@@ -930,6 +930,9 @@ class BENCHMARK_EXPORT State {
     return max_iterations - total_iterations_ + batch_leftover_;
   }
 
+  BENCHMARK_ALWAYS_INLINE
+  int GetNumThreadStates() const { return num_thread_states_; }
+
   BENCHMARK_ALWAYS_INLINE
   std::string name() const { return name_; }
 
@@ -976,12 +979,31 @@ class BENCHMARK_EXPORT State {
   const std::string name_;
   const int thread_index_;
   const int threads_;
+  int num_thread_states_;
 
-  internal::ThreadTimer* const timer_;
   internal::ThreadManager* const manager_;
-  internal::PerfCountersMeasurement* const perf_counters_measurement_;
 
   friend class internal::BenchmarkInstance;
+  friend class ThreadState;
+
+ protected:
+  void MergeThreadStateToParent(State& parent) const;
+  bool started() const { return started_; }
+
+  internal::ThreadTimer* timer_;
+  internal::PerfCountersMeasurement* perf_counters_measurement_;
+};
+
+// ThreadState can be used in a manually multithreaded benchmark loop.
+class BENCHMARK_EXPORT ThreadState : public State {
+ public:
+  explicit ThreadState(State& s);
+  ~ThreadState();
+
+ private:
+  State* parent_;
+
+  ThreadState(const ThreadState&);
 };
 
 inline BENCHMARK_ALWAYS_INLINE bool State::KeepRunning() {
@@ -1274,6 +1296,13 @@ class BENCHMARK_EXPORT Benchmark {
   // Equivalent to ThreadRange(NumCPUs(), NumCPUs())
   Benchmark* ThreadPerCpu();
 
+  // Don't create threads. Let the user evaluate state.threads and/or use
+  // ThreadState.
+  Benchmark* ManualThreading() {
+    manual_threading_ = true;
+    return this;
+  }
+
   virtual void Run(State& state) = 0;
 
   TimeUnit GetTimeUnit() const;
@@ -1286,6 +1315,7 @@ class BENCHMARK_EXPORT Benchmark {
   const char* GetName() const;
   int ArgsCnt() const;
   const char* GetArgName(int arg) const;
+  bool GetExplicitThreading() const { return !thread_counts_.empty(); }
 
  private:
   friend class BenchmarkFamilies;
@@ -1307,6 +1337,7 @@ class BENCHMARK_EXPORT Benchmark {
   bool measure_process_cpu_time_;
   bool use_real_time_;
   bool use_manual_time_;
+  bool manual_threading_;
   BigO complexity_;
   BigOFunc* complexity_lambda_;
   std::vector<Statistics> statistics_;

src/benchmark.cc

@@ -172,8 +172,9 @@ State::State(std::string name, IterationCount max_iters,
       name_(std::move(name)),
       thread_index_(thread_i),
       threads_(n_threads),
-      timer_(timer),
+      num_thread_states_(0),
       manager_(manager),
+      timer_(timer),
       perf_counters_measurement_(perf_counters_measurement) {
   BM_CHECK(max_iterations != 0) << "At least one iteration must be run";
   BM_CHECK_LT(thread_index_, threads_)
@@ -309,6 +310,44 @@ void State::FinishKeepRunning() {
   manager_->StartStopBarrier();
 }
 
+void State::MergeThreadStateToParent(State& parent) const {
+  MutexLock l(manager_->GetBenchmarkMutex());
+  internal::MergeResults(*this, timer_, manager_);
+  assert(parent.total_iterations_ == 0 ||
+         parent.total_iterations_ == total_iterations_);
+  assert(parent.batch_leftover_ == 0 ||
+         parent.batch_leftover_ == batch_leftover_);
+  parent.total_iterations_ = total_iterations_;
+  parent.batch_leftover_ = batch_leftover_;
+  parent.started_ = parent.started_ || started_;
+  parent.finished_ = parent.finished_ || finished_;
+  parent.skipped_ =
+      (parent.error_occurred() || error_occurred())
+          ? internal::SkippedWithError
+          : (parent.skipped() || skipped() ? internal::SkippedWithMessage
+                                           : internal::NotSkipped);
+  parent.num_thread_states_++;
+}
+
+ThreadState::ThreadState(State& s)
+    : State(s.name(), s.max_iterations, s.range_, s.thread_index(), s.threads(),
+            new internal::ThreadTimer(
+                internal::ThreadTimer::CreateFromTimer(*s.timer_)),
+            s.manager_,
+            s.perf_counters_measurement_
+                ? new internal::PerfCountersMeasurement(
+                      s.perf_counters_measurement_->names())
+                : 0),
+      parent_(&s) {}
+
+ThreadState::~ThreadState() {
+  BM_CHECK(error_occurred() || iterations() >= max_iterations)
+      << "Benchmark returned before ThreadState::KeepRunning() returned false!";
+  MergeThreadStateToParent(*parent_);
+  delete timer_;
+  delete perf_counters_measurement_;
+}
+
 namespace internal {
 namespace {
 

src/benchmark_api_internal.cc

@@ -2,7 +2,10 @@
 
 #include <cinttypes>
 
+#include "counter.h"
 #include "string_util.h"
+#include "thread_manager.h"
+#include "thread_timer.h"
 
 namespace benchmark {
 namespace internal {
@@ -27,7 +30,9 @@ BenchmarkInstance::BenchmarkInstance(Benchmark* benchmark, int family_idx,
       min_time_(benchmark_.min_time_),
       min_warmup_time_(benchmark_.min_warmup_time_),
       iterations_(benchmark_.iterations_),
-      threads_(thread_count) {
+      threads_(thread_count),
+      manual_threading_(benchmark_.manual_threading_),
+      explicit_threading_(benchmark_.GetExplicitThreading()) {
   name_.function_name = benchmark_.name_;
 
   size_t arg_i = 0;
@@ -114,5 +119,16 @@ void BenchmarkInstance::Teardown() const {
     teardown_(st);
   }
 }
+
+void MergeResults(const State& st, const ThreadTimer* timer,
+                  ThreadManager* manager) NO_THREAD_SAFETY_ANALYSIS {
+  ThreadManager::Result& results = manager->results;
+  results.iterations += st.iterations();
+  results.cpu_time_used += timer->cpu_time_used();
+  results.real_time_used += timer->real_time_used();
+  results.manual_time_used += timer->manual_time_used();
+  results.complexity_n += st.complexity_length_n();
+  Increment(&results.counters, st.counters);
+}
 }  // namespace internal
 }  // namespace benchmark

src/benchmark_api_internal.h

@@ -10,6 +10,7 @@
 
 #include "benchmark/benchmark.h"
 #include "commandlineflags.h"
+#include "mutex.h"
 
 namespace benchmark {
 namespace internal {
@@ -41,6 +42,8 @@ class BenchmarkInstance {
   int threads() const { return threads_; }
   void Setup() const;
   void Teardown() const;
+  bool explicit_threading() const { return explicit_threading_; }
+  bool manual_threading() const { return manual_threading_; }
 
   State Run(IterationCount iters, int thread_id, internal::ThreadTimer* timer,
             internal::ThreadManager* manager,
@@ -66,6 +69,9 @@ class BenchmarkInstance {
   double min_warmup_time_;
   IterationCount iterations_;
   int threads_;  // Number of concurrent threads to us
+  bool manual_threading_;
+  bool explicit_threading_;  // true: Number of threads come from a Threads()
+                             // call
 
   typedef void (*callback_function)(const benchmark::State&);
   callback_function setup_ = nullptr;
@@ -78,6 +84,10 @@ bool FindBenchmarksInternal(const std::string& re,
 
 bool IsZero(double n);
 
+// only call while holding benchmark_mutex_:
+void MergeResults(const State& st, const ThreadTimer* timer,
+                  ThreadManager* manager) NO_THREAD_SAFETY_ANALYSIS;
+
 BENCHMARK_EXPORT
 ConsoleReporter::OutputOptions GetOutputOptions(bool force_no_color = false);
 

src/benchmark_register.cc

@@ -217,6 +217,7 @@ Benchmark::Benchmark(const std::string& name)
       measure_process_cpu_time_(false),
       use_real_time_(false),
       use_manual_time_(false),
+      manual_threading_(false),
       complexity_(oNone),
       complexity_lambda_(nullptr),
       setup_(nullptr),

src/benchmark_runner.cc

@@ -86,7 +86,7 @@ BenchmarkReporter::Run CreateRunReport(
   // This is the total iterations across all threads.
   report.iterations = results.iterations;
   report.time_unit = b.time_unit();
-  report.threads = b.threads();
+  report.threads = results.thread_count;
   report.repetition_index = repetition_index;
   report.repetitions = repeats;
 
@@ -130,17 +130,36 @@ void RunInThread(const BenchmarkInstance* b, IterationCount iters,
 
   State st =
       b->Run(iters, thread_id, &timer, manager, perf_counters_measurement);
-  BM_CHECK(st.skipped() || st.iterations() >= st.max_iterations)
-      << "Benchmark returned before State::KeepRunning() returned false!";
+
+  assert(b->explicit_threading() || b->threads() == 1);
+
+  if (st.GetNumThreadStates() > 0) {
+    BM_CHECK((!b->explicit_threading()) || b->manual_threading())
+        << "Benchmark " << b->name().str()
+        << " run with managed threading. It must not create ThreadStates!";
+    BM_CHECK((!b->explicit_threading()) ||
+             st.GetNumThreadStates() == b->threads())
+        << "The number of ThreadStates created by Benchmark " << b->name().str()
+        << " doesn't match the number of threads!";
+  } else {
+    BM_CHECK(st.skipped() || st.iterations() >= st.max_iterations)
+        << "Benchmark returned before State::KeepRunning() returned false!";
+  }
+
   {
     MutexLock l(manager->GetBenchmarkMutex());
     internal::ThreadManager::Result& results = manager->results;
-    results.iterations += st.iterations();
-    results.cpu_time_used += timer.cpu_time_used();
-    results.real_time_used += timer.real_time_used();
-    results.manual_time_used += timer.manual_time_used();
-    results.complexity_n += st.complexity_length_n();
-    internal::Increment(&results.counters, st.counters);
+    if (st.GetNumThreadStates() > 0) {
+      // State values as well as thread state values are summed up for
+      // complexity_n and user counters:
+      results.complexity_n += st.complexity_length_n();
+      internal::Increment(&results.counters, st.counters);
+      results.thread_count =
+          b->explicit_threading() ? b->threads() : st.GetNumThreadStates();
+    } else {
+      internal::MergeResults(st, &timer, manager);
+      results.thread_count = b->threads();
+    }
   }
   manager->NotifyThreadComplete();
 }
@@ -234,7 +253,8 @@ BenchmarkRunner::BenchmarkRunner(
       has_explicit_iteration_count(b.iterations() != 0 ||
                                    parsed_benchtime_flag.tag ==
                                        BenchTimeType::ITERS),
-      pool(b.threads() - 1),
+      num_managed_threads(b.manual_threading() ? 1 : b.threads()),
+      pool(num_managed_threads - 1),
       iters(has_explicit_iteration_count
                 ? ComputeIters(b_, parsed_benchtime_flag)
                 : 1),
@@ -260,7 +280,7 @@ BenchmarkRunner::IterationResults BenchmarkRunner::DoNIterations() {
   BM_VLOG(2) << "Running " << b.name().str() << " for " << iters << "\n";
 
   std::unique_ptr<internal::ThreadManager> manager;
-  manager.reset(new internal::ThreadManager(b.threads()));
+  manager.reset(new internal::ThreadManager(num_managed_threads));
 
   // Run all but one thread in separate threads
   for (std::size_t ti = 0; ti < pool.size(); ++ti) {
@@ -287,17 +307,18 @@ BenchmarkRunner::IterationResults BenchmarkRunner::DoNIterations() {
   manager.reset();
 
   // Adjust real/manual time stats since they were reported per thread.
-  i.results.real_time_used /= b.threads();
-  i.results.manual_time_used /= b.threads();
+  i.results.real_time_used /= i.results.thread_count;
+  i.results.manual_time_used /= i.results.thread_count;
   // If we were measuring whole-process CPU usage, adjust the CPU time too.
-  if (b.measure_process_cpu_time()) i.results.cpu_time_used /= b.threads();
+  if (b.measure_process_cpu_time())
+    i.results.cpu_time_used /= i.results.thread_count;
 
   BM_VLOG(2) << "Ran in " << i.results.cpu_time_used << "/"
              << i.results.real_time_used << "\n";
 
   // By using KeepRunningBatch a benchmark can iterate more times than
   // requested, so take the iteration count from i.results.
-  i.iters = i.results.iterations / b.threads();
+  i.iters = i.results.iterations / i.results.thread_count;
 
   // Base decisions off of real time if requested by this benchmark.
   i.seconds = i.results.cpu_time_used;

src/benchmark_runner.h

@@ -93,6 +93,7 @@ class BenchmarkRunner {
   bool warmup_done;
   const int repeats;
   const bool has_explicit_iteration_count;
+  const int num_managed_threads;  // must be before pool
 
   int num_repetitions_done = 0;
 

src/thread_manager.h

@@ -45,6 +45,7 @@ class ThreadManager {
     std::string report_label_;
     std::string skip_message_;
     internal::Skipped skipped_ = internal::NotSkipped;
+    int thread_count = 0;
     UserCounters counters;
   };
   GUARDED_BY(GetBenchmarkMutex()) Result results;