Bug 1575453 - AUTO_PROFILER_STATS (off by default) - r=gregtatum

AUTO_PROFILER_STATS(name) can be used to time a {block}.
Statistics are gathered in a function-static variable, and printf'd when the
program ends.

Differential Revision: https://phabricator.services.mozilla.com/D42825

--HG--
extra : moz-landing-system : lando

mozglue/baseprofiler/public/BaseProfiler.h

@@ -78,6 +78,8 @@
 #  define AUTO_BASE_PROFILER_TEXT_MARKER_DOCSHELL_CAUSE( \
       markerName, text, categoryPair, docShell, cause)
 
+#  define AUTO_PROFILER_STATS(name)
+
 #else  // !MOZ_BASE_PROFILER
 
 #  include "BaseProfilingStack.h"
@@ -528,6 +530,62 @@ struct ProfilerBufferInfo {
 // buffer is being written to, and how much data is visible.
 MFBT_API Maybe<ProfilerBufferInfo> profiler_get_buffer_info();
 
+// Uncomment the following line to display profiler runtime statistics at
+// shutdown.
+// #  define PROFILER_RUNTIME_STATS
+
+#  ifdef PROFILER_RUNTIME_STATS
+struct StaticBaseProfilerStats {
+  explicit StaticBaseProfilerStats(const char* aName) : mName(aName) {}
+  ~StaticBaseProfilerStats() {
+    long long n = static_cast<long long>(mNumberDurations);
+    if (n != 0) {
+      long long sumNs = static_cast<long long>(mSumDurationsNs);
+      printf("Profiler stats `%s`: %lld ns / %lld = %lld ns\n", mName, sumNs, n,
+             sumNs / n);
+    } else {
+      printf("Profiler stats `%s`: (nothing)\n", mName);
+    }
+  }
+  Atomic<long long> mSumDurationsNs{0};
+  Atomic<long long> mNumberDurations{0};
+
+ private:
+  const char* mName;
+};
+
+class MOZ_RAII AutoProfilerStats {
+ public:
+  explicit AutoProfilerStats(
+      StaticBaseProfilerStats& aStats MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
+      : mStats(aStats), mStart(TimeStamp::NowUnfuzzed()) {
+    MOZ_GUARD_OBJECT_NOTIFIER_INIT;
+  }
+
+  ~AutoProfilerStats() {
+    mStats.mSumDurationsNs += int64_t(
+        (TimeStamp::NowUnfuzzed() - mStart).ToMicroseconds() * 1000 + 0.5);
+    ++mStats.mNumberDurations;
+  }
+
+ private:
+  MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
+
+  StaticBaseProfilerStats& mStats;
+  TimeStamp mStart;
+};
+
+#    define AUTO_PROFILER_STATS(name)                                      \
+      static ::mozilla::baseprofiler::StaticBaseProfilerStats sStat##name( \
+          #name);                                                          \
+      ::mozilla::baseprofiler::AutoProfilerStats autoStat##name(sStat##name);
+
+#  else  // PROFILER_RUNTIME_STATS
+
+#    define AUTO_PROFILER_STATS(name)
+
+#  endif  // PROFILER_RUNTIME_STATS else
+
 //---------------------------------------------------------------------------
 // Put profiling data into the profiler (labels and markers)
 //---------------------------------------------------------------------------