[PATCH 2/3] perf-stat: fix +- nan% in -Aa runs

[Jim Cromie <jim.cromie@gmail.com>]

without this patch, I get computations like this:

[jimc@groucho perf]$ sudo ./perf stat -r3 -Aa  perl -e '$i++ for 1..100000'

 Performance counter stats for 'perl -e $i++ for 1..100000' (3 runs):

CPU0             12.391883 task-clock-msecs         #      0.966 CPUs    ( +-    -nan% )
CPU1             12.446571 task-clock-msecs         #      0.970 CPUs    ( +-    -nan% )
CPU2             12.408014 task-clock-msecs         #      0.967 CPUs    ( +-    -nan% )
CPU3             12.422264 task-clock-msecs         #      0.968 CPUs    ( +-    -nan% )

I couldnt see anything wrong in the caller, so fixed it in stddev_stats()

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
---
 tools/perf/builtin-stat.c |    9 +++++++--
 1 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index 71bbf72..c59d199 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -151,8 +151,13 @@ static double avg_stats(struct stats *stats)
  */
 static double stddev_stats(struct stats *stats)
 {
-	double variance = stats->M2 / (stats->n - 1);
-	double variance_mean = variance / stats->n;
+	double variance, variance_mean;
+
+	if (!stats->n)
+		return 0.0;
+
+	variance = stats->M2 / (stats->n - 1);
+	variance_mean = variance / stats->n;
 
 	return sqrt(variance_mean);
 }
-- 
1.7.4.4