Handle the case where gettimeofday() goes backwards or returns the sa…

…me value

twice; it could cause division by zero in the unit test framework.
(We already had one fix for this in place, but it was incomplete.)

This could in theory happen on any system, since there are few guarantees
about gettimeofday(), but seems to only happen in practice on GNU/Hurd, where
gettimeofday() is cached and only updated ever so often.

R=sanjay


git-svn-id: http://snappy.googlecode.com/svn/trunk@65 03e5f5b5-db94-4691-08a0-1a8bf15f6143

snappy-test.cc

@@ -204,27 +204,32 @@ void Benchmark::Run() {
       benchmark_runs[run].cpu_time_us = benchmark_cpu_time_us;
     }
 
+    string heading = StringPrintf("%s/%d", name_.c_str(), test_case_num);
+    string human_readable_speed;
+
     nth_element(benchmark_runs,
                 benchmark_runs + kMedianPos,
                 benchmark_runs + kNumRuns,
                 BenchmarkCompareCPUTime());
     int64 real_time_us = benchmark_runs[kMedianPos].real_time_us;
     int64 cpu_time_us = benchmark_runs[kMedianPos].cpu_time_us;
-    int64 bytes_per_second = benchmark_bytes_processed * 1000000 / cpu_time_us;
-
-    string heading = StringPrintf("%s/%d", name_.c_str(), test_case_num);
-    string human_readable_speed;
-    if (bytes_per_second < 1024) {
-      human_readable_speed = StringPrintf("%dB/s", bytes_per_second);
-    } else if (bytes_per_second < 1024 * 1024) {
-      human_readable_speed = StringPrintf(
-          "%.1fkB/s", bytes_per_second / 1024.0f);
-    } else if (bytes_per_second < 1024 * 1024 * 1024) {
-      human_readable_speed = StringPrintf(
-          "%.1fMB/s", bytes_per_second / (1024.0f * 1024.0f));
+    if (cpu_time_us <= 0) {
+      human_readable_speed = "?";
     } else {
-      human_readable_speed = StringPrintf(
-          "%.1fGB/s", bytes_per_second / (1024.0f * 1024.0f * 1024.0f));
+      int64 bytes_per_second =
+          benchmark_bytes_processed * 1000000 / cpu_time_us;
+      if (bytes_per_second < 1024) {
+        human_readable_speed = StringPrintf("%dB/s", bytes_per_second);
+      } else if (bytes_per_second < 1024 * 1024) {
+        human_readable_speed = StringPrintf(
+            "%.1fkB/s", bytes_per_second / 1024.0f);
+      } else if (bytes_per_second < 1024 * 1024 * 1024) {
+        human_readable_speed = StringPrintf(
+            "%.1fMB/s", bytes_per_second / (1024.0f * 1024.0f));
+      } else {
+        human_readable_speed = StringPrintf(
+            "%.1fGB/s", bytes_per_second / (1024.0f * 1024.0f * 1024.0f));
+      }
     }
 
     fprintf(stderr,