RFC: Add direct support for relative benchmarking to the folly/Benchmark library.

Summary:
When modifying code that lacks alternative variations, there is only a 'HEAD' benchmark - so it can be useful to keep a version of these results for latter printing in the format of a relative benchmark.

Add a new flag to simplify this use-case,
--bm_relative_to: A flag taking the path to a JSON-verbose formatted dump, which will be used when printing (in non-JSON-mode) to produce output relative to the provided prior dump.

Usage:
- generate a JSON benchmark dump, use "--bm_json_verbose"
  $ your_benchmark_binary --benchmark --bm_json_verbose  old-json
- run a benchmark printing a comparison to an old dump:
  $ your_benchmark_binary --benchmark --bm_relative_to old-json

Note: You can also use bm_json_verbose and bm_relative_to at the same time, this will print relative results and update the JSON dump

Reviewed By: luciang

Differential Revision: D26214397

fbshipit-source-id: 8134b74604b7bb5037934205e8eee5e3ffc27f15

folly/Benchmark.cpp

@@ -31,6 +31,7 @@
 
 #include <boost/regex.hpp>
 
+#include <folly/FileUtil.h>
 #include <folly/MapUtil.h>
 #include <folly/String.h>
 #include <folly/container/Foreach.h>
@@ -39,11 +40,18 @@
 using namespace std;
 
 DEFINE_bool(benchmark, false, "Run benchmarks.");
+
 DEFINE_bool(json, false, "Output in JSON format.");
-DEFINE_bool(
-    json_verbose,
-    false,
-    "Output in verbose JSON format for BenchmarkCompare");
+DEFINE_string(
+    bm_relative_to,
+    "",
+    "Print benchmark results relative to an earlier dump (via --bm_json_verbose)");
+
+DEFINE_string(
+    bm_json_verbose,
+    "",
+    "File to write verbose JSON format (for BenchmarkCompare / --bm_relative_to). "
+    "NOTE: this is written independent of the above --json / --bm_relative_to.");
 
 DEFINE_string(
     bm_regex,
@@ -368,25 +376,6 @@ static void printBenchmarkResultsAsJson(
   printf("%s\n", toPrettyJson(d).c_str());
 }
 
-static void printBenchmarkResultsAsVerboseJson(
-    const vector<detail::BenchmarkResult>& data) {
-  dynamic d;
-  benchmarkResultsToDynamic(data, d);
-  printf("%s\n", toPrettyJson(d).c_str());
-}
-
-static void printBenchmarkResults(const vector<detail::BenchmarkResult>& data) {
-  if (FLAGS_json_verbose) {
-    printBenchmarkResultsAsVerboseJson(data);
-    return;
-  } else if (FLAGS_json) {
-    printBenchmarkResultsAsJson(data);
-    return;
-  }
-
-  CHECK(FLAGS_json_verbose || FLAGS_json) << "Cannot print benchmark results";
-}
-
 void benchmarkResultsToDynamic(
     const vector<detail::BenchmarkResult>& data,
     dynamic& out) {
@@ -563,6 +552,25 @@ runBenchmarksWithPrinter(BenchmarkResultsPrinter* FOLLY_NULLABLE printer) {
   return std::make_pair(std::move(counterNames), std::move(results));
 }
 
+std::vector<detail::BenchmarkResult> resultsFromFile(
+    const std::string& filename) {
+  std::string content;
+  readFile(filename.c_str(), content);
+  std::vector<detail::BenchmarkResult> ret;
+  if (!content.empty()) {
+    benchmarkResultsFromDynamic(parseJson(content), ret);
+  }
+  return ret;
+}
+
+bool writeResultsToFile(
+    const std::vector<detail::BenchmarkResult>& results,
+    const std::string& filename) {
+  dynamic d;
+  benchmarkResultsToDynamic(results, d);
+  return writeFile(toPrettyJson(d), filename.c_str());
+}
+
 } // namespace
 
 namespace detail {
@@ -586,18 +594,26 @@ void runBenchmarks() {
   // PLEASE KEEP QUIET. MEASUREMENTS IN PROGRESS.
 
   auto benchmarkResults = runBenchmarksWithPrinter(
-      !FLAGS_json_verbose && !FLAGS_json && !useCounter ? &printer : nullptr);
+      FLAGS_bm_relative_to.empty() && !FLAGS_json && !useCounter ? &printer
+                                                                 : nullptr);
 
   // PLEASE MAKE NOISE. MEASUREMENTS DONE.
 
-  if (FLAGS_json_verbose || FLAGS_json) {
-    printBenchmarkResults(benchmarkResults.second);
+  if (FLAGS_json) {
+    printBenchmarkResultsAsJson(benchmarkResults.second);
+  } else if (!FLAGS_bm_relative_to.empty()) {
+    printResultComparison(
+        resultsFromFile(FLAGS_bm_relative_to), benchmarkResults.second);
   } else {
     printer = BenchmarkResultsPrinter{std::move(benchmarkResults.first)};
     printer.print(benchmarkResults.second);
     printer.separator('=');
   }
 
+  if (!FLAGS_bm_json_verbose.empty()) {
+    writeResultsToFile(benchmarkResults.second, FLAGS_bm_json_verbose);
+  }
+
   checkRunMode();
 }
 

folly/tools/BenchmarkCompare.cpp

@@ -28,8 +28,8 @@
  * output the relative change by your changes.
  *
  * Usage:
- * - generate a benchmark JSON dump, use //folly::benchmark's "--json_verbose"
- *      $ your_benchmark_binary --benchmark --json_verbose > old-json
+ * - generate a benchmark JSON dump, use folly::benchmark's --bm_json_verbose
+ *     $ your_benchmark_binary --benchmark --bm_json_verbose old-json
  * - compare two benchmarks & output a human-readable comparison:
  *     $ benchmark_compare old-json new-json
  */