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.

1 files changed, 11 insertions, 3 deletions

diff --git a/src/bench/bench.cpp b/src/bench/bench.cpp
index 572080760..89c3b0cc2 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;