math/rand: avoid use of math.Pow in tests.
The use of math.Pow for mere squaring can be extremely
slow on soft-float ARM. Even on systems with hardware
floating-point, a speedup in test duration is observed.
On amd64
Before: ok math/rand 2.009s
After: ok math/rand 0.340s
Fixes #3740.
R=dave, golang-dev, r, r
CC=golang-dev
https://golang.org/cl/6348061
diff --git a/src/pkg/math/rand/rand_test.go b/src/pkg/math/rand/rand_test.go
index bbd44e3..4d3abdb 100644
--- a/src/pkg/math/rand/rand_test.go
+++ b/src/pkg/math/rand/rand_test.go
@@ -57,16 +57,13 @@
func getStatsResults(samples []float64) *statsResults {
res := new(statsResults)
- var sum float64
- for i := range samples {
- sum += samples[i]
+ var sum, squaresum float64
+ for _, s := range samples {
+ sum += s
+ squaresum += s * s
}
res.mean = sum / float64(len(samples))
- var devsum float64
- for i := range samples {
- devsum += math.Pow(samples[i]-res.mean, 2)
- }
- res.stddev = math.Sqrt(devsum / float64(len(samples)))
+ res.stddev = math.Sqrt(squaresum/float64(len(samples)) - res.mean*res.mean)
return res
}
