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.

src/Makefile.bench.include

@@ -7,7 +7,7 @@ bench_bench_bitcoin_SOURCES = \
   bench/bench_bitcoin.cpp \
   bench/bench.cpp \
   bench/bench.h \
-  bench/MilliSleep.cpp
+  bench/Examples.cpp
 
 bench_bench_bitcoin_CPPFLAGS = $(BITCOIN_INCLUDES) $(EVENT_CLFAGS) $(EVENT_PTHREADS_CFLAGS) -I$(builddir)/bench/
 bench_bench_bitcoin_LDADD = \

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);

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;

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();
     };