Support very-fast-running benchmarks

Avoid calling gettimeofday every time through the benchmarking loop, by keeping
track of how long each loop takes and doubling the number of iterations done
between time checks when they take less than 1/16'th of the total elapsed time.

3 files changed, 31 insertions, 3 deletions

diff --git a/src/bench/MilliSleep.cpp b/src/bench/Examples.cpp
index 991397a234..b6b020a971 100644
--- a/src/bench/MilliSleep.cpp
+++ b/src/bench/Examples.cpp
@@ -6,6 +6,8 @@
 #include "main.h"
 #include "utiltime.h"
 
+// Sanity test: this should loop ten times, and
+// min/max/average should be close to 100ms.
 static void Sleep100ms(benchmark::State& state)
 {
     while (state.KeepRunning()) {
@@ -14,3 +16,19 @@ static void Sleep100ms(benchmark::State& state)
 }
 
 BENCHMARK(Sleep100ms);
+
+// Extremely fast-running benchmark:
+#include <math.h>
+
+volatile double sum = 0.0; // volatile, global so not optimized away
+
+static void Trig(benchmark::State& state)
+{
+    double d = 0.01;
+    while (state.KeepRunning()) {
+        sum += sin(d);
+        d += 0.000001;
+    }
+}
+
+BENCHMARK(Trig);
diff --git a/src/bench/bench.cpp b/src/bench/bench.cpp
index 5720807609..89c3b0cc2a 100644
--- a/src/bench/bench.cpp
+++ b/src/bench/bench.cpp
@@ -36,14 +36,22 @@ BenchRunner::RunAll(double elapsedTimeForOne)
 
 bool State::KeepRunning()
 {
-    double now = gettimedouble();
+    double now;
     if (count == 0) {
-        beginTime = now;
+        beginTime = now = gettimedouble();
     }
     else {
-        double elapsedOne = now - lastTime;
+        // timeCheckCount is used to avoid calling gettime most of the time,
+        // so benchmarks that run very quickly get consistent results.
+        if ((count+1)%timeCheckCount != 0) {
+            ++count;
+            return true; // keep going
+        }
+        now = gettimedouble();
+        double elapsedOne = (now - lastTime)/timeCheckCount;
         if (elapsedOne < minTime) minTime = elapsedOne;
         if (elapsedOne > maxTime) maxTime = elapsedOne;
+        if (elapsedOne*timeCheckCount < maxElapsed/16) timeCheckCount *= 2;
     }
     lastTime = now;
     ++count;
diff --git a/src/bench/bench.h b/src/bench/bench.h
index fee9b8c38d..bf591a2be6 100644
--- a/src/bench/bench.h
+++ b/src/bench/bench.h
@@ -41,10 +41,12 @@ namespace benchmark {
         double beginTime;
         double lastTime, minTime, maxTime;
         int64_t count;
+        int64_t timeCheckCount;
     public:
         State(std::string _name, double _maxElapsed) : name(_name), maxElapsed(_maxElapsed), count(0) {
             minTime = std::numeric_limits<double>::max();
             maxTime = std::numeric_limits<double>::min();
+            timeCheckCount = 1;
         }
         bool KeepRunning();
     };