1 files changed, 19 insertions, 8 deletions
diff --git a/lldb/tools/lldb-perf/lib/Metric.cpp b/lldb/tools/lldb-perf/lib/Metric.cpp
index d8e7935debf..1951cdb0250 100644
--- a/lldb/tools/lldb-perf/lib/Metric.cpp
+++ b/lldb/tools/lldb-perf/lib/Metric.cpp
@@ -57,17 +57,28 @@ Metric<T>::GetAverage () const
     return GetSum()/GetCount();
 }
 
+
+// Knuth's algorithm for stddev - massive cancellation resistant
 template <class T>
 T
-Metric<T>::GetStandardDeviation () const
+Metric<T>::GetStandardDeviation (StandardDeviationMode mode) const
 {
-    T average = GetAverage();
-    T diff_squared = 0;
-    size_t count = GetCount();
-    for (auto v : m_dataset)
-        diff_squared = diff_squared + ( (v-average)*(v-average) );
-    diff_squared = diff_squared / count;
-    return sqrt(diff_squared);
+    size_t n = 0;
+    T mean = 0;
+    T M2 = 0;
+    for (auto x : m_dataset)
+    {
+        n = n + 1;
+        T delta = x - mean;
+        mean = mean + delta/n;
+        M2 = M2+delta*(x-mean);
+    }
+    T variance;
+    if (mode == StandardDeviationMode::ePopulation || n == 1)
+        variance = M2 / n;
+    else
+        variance = M2 / (n - 1);
+    return sqrt(variance);
 }
 
 template class lldb_perf::Metric<double>;