5 files changed, 52 insertions, 19 deletions

diff --git a/lldb/tools/lldb-perf/lib/Measurement.h b/lldb/tools/lldb-perf/lib/Measurement.h
index d43a37962f3..877e618d546 100644
--- a/lldb/tools/lldb-perf/lib/Measurement.h
+++ b/lldb/tools/lldb-perf/lib/Measurement.h
@@ -113,6 +113,17 @@ public:
     }
     
     void
+    WriteAverageAndStandardDeviation (Results &results)
+    {
+        auto metric = GetMetric ();
+        auto dictionary = (Results::Dictionary*)results.GetDictionary().Add(metric.GetName(), metric.GetDescription(), lldb_perf::GetResult<typename GaugeType::ValueType> (NULL, metric.GetAverage())).get();
+        if (dictionary)
+        {
+            dictionary->Add("stddev", NULL, lldb_perf::GetResult<typename GaugeType::ValueType> (NULL, metric.GetStandardDeviation()));
+        }
+    }
+    
+    void
     WriteStandardDeviation (Results &results)
     {
         auto metric = GetMetric ();
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>;
diff --git a/lldb/tools/lldb-perf/lib/Metric.h b/lldb/tools/lldb-perf/lib/Metric.h
index 8c32019b21f..45342d25b41 100644
--- a/lldb/tools/lldb-perf/lib/Metric.h
+++ b/lldb/tools/lldb-perf/lib/Metric.h
@@ -22,6 +22,12 @@ template <class ValueType>
 class Metric
 {
 public:
+    enum class StandardDeviationMode
+    {
+        eSample,
+        ePopulation
+    };
+    
     Metric ();
     Metric (const char*, const char* = NULL);
     
@@ -38,7 +44,7 @@ public:
     GetSum () const;
     
     ValueType
-    GetStandardDeviation () const;
+    GetStandardDeviation (StandardDeviationMode mode = StandardDeviationMode::ePopulation) const;
     
     const char*
     GetName () const
diff --git a/lldb/tools/lldb-perf/lib/Results.cpp b/lldb/tools/lldb-perf/lib/Results.cpp
index 2bf74359826..6abf67e53b6 100644
--- a/lldb/tools/lldb-perf/lib/Results.cpp
+++ b/lldb/tools/lldb-perf/lib/Results.cpp
@@ -176,7 +176,7 @@ Results::Write (const char *out_path)
 #endif
 }
 
-void
+Results::ResultSP
 Results::Dictionary::AddUnsigned (const char *name, const char *description, uint64_t value)
 {
     assert (name && name[0]);
@@ -189,9 +189,10 @@ Results::Dictionary::AddUnsigned (const char *name, const char *description, uin
     }
     else
         m_dictionary[std::string(name)] = ResultSP (new Unsigned (name, description, value));
+    return m_dictionary[std::string(name)];
 }
 
-void
+Results::ResultSP
 Results::Dictionary::AddDouble (const char *name, const char *description, double value)
 {
     assert (name && name[0]);
@@ -205,8 +206,9 @@ Results::Dictionary::AddDouble (const char *name, const char *description, doubl
     }
     else
         m_dictionary[std::string(name)] = ResultSP (new Double (name, description, value));
+    return m_dictionary[std::string(name)];
 }
-void
+Results::ResultSP
 Results::Dictionary::AddString (const char *name, const char *description, const char *value)
 {
     assert (name && name[0]);
@@ -219,9 +221,10 @@ Results::Dictionary::AddString (const char *name, const char *description, const
     }
     else
         m_dictionary[std::string(name)] = ResultSP (new String (name, description, value));
+    return m_dictionary[std::string(name)];
 }
 
-void
+Results::ResultSP
 Results::Dictionary::Add (const char *name, const char *description, const ResultSP &result_sp)
 {
     assert (name && name[0]);
@@ -234,6 +237,7 @@ Results::Dictionary::Add (const char *name, const char *description, const Resul
     }
     else
         m_dictionary[std::string(name)] = result_sp;
+    return m_dictionary[std::string(name)];
 }
 
 void
@@ -249,10 +253,11 @@ Results::Dictionary::ForEach (const std::function <bool (const std::string &, co
 
 
 
-void
+Results::ResultSP
 Results::Array::Append (const ResultSP &result_sp)
 {
     m_array.push_back (result_sp);
+    return result_sp;
 }
 
 void
diff --git a/lldb/tools/lldb-perf/lib/Results.h b/lldb/tools/lldb-perf/lib/Results.h
index f15b902f911..0602138f6f0 100644
--- a/lldb/tools/lldb-perf/lib/Results.h
+++ b/lldb/tools/lldb-perf/lib/Results.h
@@ -138,7 +138,7 @@ public:
         {
         }
         
-        void
+        ResultSP
         Append (const ResultSP &result_sp);
 
         void
@@ -179,16 +179,16 @@ public:
         void
         ForEach (const std::function <bool (const std::string &, const ResultSP &)> &callback);
     
-        void
+        ResultSP
         Add (const char *name, const char *description, const ResultSP &result_sp);
         
-        void
+        ResultSP
         AddDouble (const char *name, const char *descriptiorn, double value);
         
-        void
+        ResultSP
         AddUnsigned (const char *name, const char *description, uint64_t value);
 
-        void
+        ResultSP
         AddString (const char *name, const char *description, const char *value);
 
     protected: