From d125911894a6f0426946c0fd0223a7948390ff06 Mon Sep 17 00:00:00 2001
From: Emil Sjolander <emilsj@fb.com>
Date: Fri, 12 Aug 2016 06:17:36 -0700
Subject: [PATCH] Print more detailed benchmark results including median and
 stddev

Summary: Mean was previously printed just because it was the quickest. Median is a better measurement and we need to include stddev for the sake of comparison against previous diff.

Differential Revision: D3709165

fbshipit-source-id: 67aff0877192143df82a9c24cbedb1f49616eec7
---
 benchmarks/CSSBenchmark.h | 53 +++++++++++++++++++++++++++++++--------
 1 file changed, 43 insertions(+), 10 deletions(-)

diff --git a/benchmarks/CSSBenchmark.h b/benchmarks/CSSBenchmark.h
index fdb0f5eb..cbec6008 100644
--- a/benchmarks/CSSBenchmark.h
+++ b/benchmarks/CSSBenchmark.h
@@ -9,6 +9,7 @@
 
 #pragma once
 
+#include <math.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <stdint.h>
@@ -19,7 +20,7 @@
 #define CSS_BENCHMARKS(BLOCK)                                                                      \
   int main(int argc, char const *argv[]) {                                                         \
     clock_t __start;                                                                               \
-    clock_t __end;                                                                                 \
+    clock_t __endTimes[NUM_REPETITIONS];                                                           \
     { BLOCK }                                                                                      \
     return 0;                                                                                      \
   }
@@ -27,15 +28,47 @@
 #define CSS_BENCHMARK(NAME, BLOCK)                                                                 \
   __start = clock();                                                                               \
   for (uint32_t __i = 0; __i < NUM_REPETITIONS; __i++) {                                           \
-    BLOCK                                                                                          \
+    { BLOCK }                                                                                      \
+    __endTimes[__i] = clock();                                                                     \
   }                                                                                                \
-  __end = clock();                                                                                 \
-  __printBenchmarkResult(NAME, __start, __end);
+  __printBenchmarkResult(NAME, __start, __endTimes);
 
-void __printBenchmarkResult(char *name, clock_t start, clock_t end) {
-  float total = (end - start) / (double)CLOCKS_PER_SEC * 1000;
-  float mean = total / NUM_REPETITIONS;
-  printf("%s: ", name);
-  printf("%lf ms", mean);
-  printf("\n");
+
+int __compareDoubles(const void* a, const void* b) {
+  double arg1 = *(const double*)a;
+  double arg2 = *(const double*)b;
+
+  if (arg1 < arg2) {
+    return -1;
+  }
+
+  if (arg1 > arg2) {
+    return 1;
+  }
+
+  return 0;
+}
+
+void __printBenchmarkResult(char *name, clock_t start, clock_t *endTimes) {
+  double timesInMs[NUM_REPETITIONS];
+  double mean = 0;
+  clock_t lastEnd = start;
+  for (uint32_t i = 0; i < NUM_REPETITIONS; i++) {
+    timesInMs[i] = (endTimes[i] - lastEnd) / (double)CLOCKS_PER_SEC * 1000;
+    lastEnd = endTimes[i];
+    mean += timesInMs[i];
+  }
+  mean /= NUM_REPETITIONS;
+
+  qsort(timesInMs, NUM_REPETITIONS, sizeof(double), __compareDoubles);
+  double median = timesInMs[NUM_REPETITIONS / 2];
+
+  double variance = 0;
+  for (uint32_t i = 0; i < NUM_REPETITIONS; i++) {
+    variance += pow(timesInMs[i] - mean, 2);
+  }
+  variance /= NUM_REPETITIONS;
+  double stddev = sqrt(variance);
+
+  printf("%s: median: %lf ms, stddev: %lf ms\n", name, median, stddev);
 }