folly | benchmark | Add support to run benchmarks and get a list of results for custom printing.

Summary:
Benchmarks are great, but are missing parts that allows them to be run outside of raw binary, like in mobile context, where we generally want to showcase the list of results, instead of pretty printing them to stdout.
To allow for that - adding a new public API, and restructuring few things, to code dedupe and still maintain the same functionality.

Reviewed By: Orvid

Differential Revision: D25552087

fbshipit-source-id: 946a9fbcac3faff0a69f3d86d20f3621649c7e5a

folly/Benchmark.cpp

@@ -506,11 +506,10 @@ void checkRunMode() {
   }
 }
 
-void runBenchmarks() {
-  CHECK(!benchmarks().empty());
-
-  checkRunMode();
+namespace {
 
+std::pair<std::set<std::string>, std::vector<detail::BenchmarkResult>>
+runBenchmarksWithPrinter(BenchmarkResultsPrinter* FOLLY_NULLABLE printer) {
   vector<detail::BenchmarkResult> results;
   results.reserve(benchmarks().size() - 1);
 
@@ -526,11 +525,6 @@ void runBenchmarks() {
   auto const globalBaseline =
       runBenchmarkGetNSPerIteration(benchmarks()[baselineIndex].func, 0);
 
-  bool useCounter =
-      std::any_of(benchmarks().begin(), benchmarks().end(), [](const auto& bm) {
-        return bm.useCounter;
-      });
-  BenchmarkResultsPrinter printer;
   std::set<std::string> counterNames;
   FOR_EACH_RANGE (i, 0, benchmarks().size()) {
     if (i == baselineIndex) {
@@ -548,8 +542,8 @@ void runBenchmarks() {
     // if customized user counters is used, it cannot print the result in real
     // time as it needs to run all cases first to know the complete set of
     // counters have been used, then the header can be printed out properly
-    if (!FLAGS_json_verbose && !FLAGS_json && !useCounter) {
-      printer.print({{bm.file, bm.name, elapsed.first, elapsed.second}});
+    if (printer != nullptr) {
+      printer->print({{bm.file, bm.name, elapsed.first, elapsed.second}});
     } else {
       results.push_back({bm.file, bm.name, elapsed.first, elapsed.second});
     }
@@ -560,12 +554,43 @@ void runBenchmarks() {
     }
   }
 
+  // MEASUREMENTS DONE.
+
+  return std::make_pair(std::move(counterNames), std::move(results));
+}
+
+} // namespace
+
+namespace detail {
+
+std::vector<BenchmarkResult> runBenchmarksWithResults() {
+  return runBenchmarksWithPrinter(nullptr).second;
+}
+
+} // namespace detail
+
+void runBenchmarks() {
+  CHECK(!benchmarks().empty());
+
+  checkRunMode();
+
+  BenchmarkResultsPrinter printer;
+  bool useCounter =
+      std::any_of(benchmarks().begin(), benchmarks().end(), [](const auto& bm) {
+        return bm.useCounter;
+      });
+  // PLEASE KEEP QUIET. MEASUREMENTS IN PROGRESS.
+
+  auto benchmarkResults = runBenchmarksWithPrinter(
+      !FLAGS_json_verbose && !FLAGS_json && !useCounter ? &printer : nullptr);
+
   // PLEASE MAKE NOISE. MEASUREMENTS DONE.
+
   if (FLAGS_json_verbose || FLAGS_json) {
-    printBenchmarkResults(results);
+    printBenchmarkResults(benchmarkResults.second);
   } else {
-    printer = BenchmarkResultsPrinter{std::move(counterNames)};
-    printer.print(results);
+    printer = BenchmarkResultsPrinter{std::move(benchmarkResults.first)};
+    printer.print(benchmarkResults.second);
     printer.separator('=');
   }
 

folly/Benchmark.h

@@ -102,6 +102,12 @@ void addBenchmarkImpl(
     BenchmarkFun,
     bool useCounter);
 
+/**
+ * Runs all benchmarks defined in the program, doesn't print by default.
+ * Usually used when customized printing of results is desired.
+ */
+std::vector<BenchmarkResult> runBenchmarksWithResults();
+
 } // namespace detail
 
 /**