BenchmarkSuspender::dismissing.

Summary:
[Folly] BenchmarkSuspender::dismissing.

Pass a lambda to it, and the lambda will be executed while the benchmark-suspender is dismissed. Just a bit of sugar around `BenchmarkSuspender::dismiss` and `BenchmarkSuspender::rehire`.

BENCHMARK(name_void, iters) {
BenchmarkSuspender braces;
# benchmark timer is suspended
braces.dismissing([&] {
# benchmark timer is running
doSomething();
});
# benchmark timer is suspended
}

BENCHMARK(name_value, iters) {
BenchmarkSuspender braces;
# benchmark timer is suspended
auto value = braces.dismissing([&] {
# benchmark timer is running
return doSomething();
});
# benchmark timer is suspended
}

Test Plan:
Unit tests:
* `folly/test/BenchmarkTest.cpp` (actually a benchmark)

Reviewed By: njormrod@fb.com

Subscribers: net-systems@, folly-diffs@, yfeldblum, chalfant

FB internal diff: D2024166

Signature: t1:2024166:1430163281:24df0ac98cbe36372f780372ee8f7dd3722b7868

folly/Benchmark.h

@@ -17,6 +17,7 @@
 #ifndef FOLLY_BENCHMARK_H_
 #define FOLLY_BENCHMARK_H_
 
+#include <folly/ScopeGuard.h>
 #include <folly/Portability.h>
 #include <folly/Preprocessor.h> // for FB_ANONYMOUS_VARIABLE
 #include <cassert>
@@ -26,6 +27,7 @@
 #include <glog/logging.h>
 #include <gflags/gflags.h>
 #include <limits>
+#include <type_traits>
 
 DECLARE_bool(benchmark);
 
@@ -149,6 +151,13 @@ struct BenchmarkSuspender {
     CHECK_EQ(0, clock_gettime(detail::DEFAULT_CLOCK_ID, &start));
   }
 
+  template <class F>
+  auto dismissing(F f) -> typename std::result_of<F()>::type {
+    SCOPE_EXIT { rehire(); };
+    dismiss();
+    return f();
+  }
+
   /**
    * This is for use inside of if-conditions, used in BENCHMARK macros.
    * If-conditions bypass the explicit on operator bool.

folly/test/BenchmarkTest.cpp

@@ -17,7 +17,11 @@
 #include <folly/Benchmark.h>
 #include <folly/Foreach.h>
 #include <folly/String.h>
+#include <algorithm>
 #include <iostream>
+#include <numeric>
+#include <random>
+#include <vector>
 using namespace folly;
 using namespace std;
 
@@ -128,6 +132,38 @@ BENCHMARK_RELATIVE_PARAM_MULTI(paramMultiRel, 1);
 BENCHMARK_PARAM_MULTI(paramMulti, 5);
 BENCHMARK_RELATIVE_PARAM_MULTI(paramMultiRel, 5);
 
+BENCHMARK_DRAW_LINE();
+
+BENCHMARK(BenchmarkSuspender_dismissing_void, iter) {
+  BenchmarkSuspender braces;
+  mt19937_64 rng;
+  while (iter--) {
+    vector<size_t> v(1 << 12, 0);
+    iota(v.begin(), v.end(), 0);
+    shuffle(v.begin(), v.end(), rng);
+    braces.dismissing([&] {
+        sort(v.begin(), v.end());
+    });
+  }
+}
+
+BENCHMARK(BenchmarkSuspender_dismissing_value, iter) {
+  BenchmarkSuspender braces;
+  mt19937_64 rng;
+  while (iter--) {
+    vector<size_t> v(1 << 12, 0);
+    iota(v.begin(), v.end(), 0);
+    shuffle(v.begin(), v.end(), rng);
+    auto s = braces.dismissing([&] {
+        sort(v.begin(), v.end());
+        return accumulate(v.begin(), v.end(), 0, [](size_t a, size_t e) {
+            return a + e;
+        });
+    });
+    doNotOptimizeAway(s);
+  }
+}
+
 int main(int argc, char** argv) {
   gflags::ParseCommandLineFlags(&argc, &argv, true);
   runBenchmarks();