Since my ASM skills are limited, for positive integers, I'm planning on
using:
// PowP computes real raised to positive integer xⁿ
func PowP(x float64, n uint32) (r float64) {
if n == 0 {
return 1.0
}
if n == 1 {
return x
}
if n == 2 {
return x * x
}
if n == 3 {
return x * x * x
}
if n == 4 {
r = x * x
return r * r
}
if n == 5 {
r = x * x
return r * r * x
}
if n == 6 {
r = x * x * x
return r * r
}
if n == 7 {
r = x * x * x
return r * r * x
}
if n == 8 {
r = x * x * x * x
return r * r
}
if n == 9 {
r = x * x * x
return r * r * r
}
if n == 10 {
r = x * x * x
return r * r * r * x
}
r = x * x * x
r = r * r * r * x
var i uint32
for i = 11; i <= 20; i++ {
r *= x
if n == i {
return
}
}
return math.Pow(x, float64(n))
}
which gives a speedup of 5-8 times for small positive integers (<20). the
case of negative integers could be handled too.
output:
BenchmarkPowP10-32 50000000 30.4 ns/op
BenchmarkPowP10std-32 5000000 247 ns/op
BenchmarkPowP20-32 20000000 98.2 ns/op
BenchmarkPowP20std-32 3000000 561 ns/op
BenchmarkPowP200-32 200000 10145 ns/op
BenchmarkPowP200std-32 200000 9231 ns/op
all files are
here: https://gist.github.com/cpmech/9f871df5b59fa8407221b0d3fb361a3d
I'm also looking at the Cephes library: http://www.netlib.org/cephes/ for
ideas
maybe we could have a similar function in Go (and many more optimised ones,
especially for complex numbers...)
cheers
d
On Friday, August 4, 2017 at 4:20:41 PM UTC+10, Dorival Pedroso wrote:
>
> I've noticed that this C code:
>
> #include "math.h"
> int main() {
> double x = 2.5;
> int Nmax = 10000000;
> for (int N=0; N<Nmax; N++) {
> for (int i=0; i<20; i++) {
> pow(x, i);
> }
> }
> }
>
> can run up to 50x faster than this Go code:
>
> package main
>
> import "math"
>
> func main() {
> x := 2.5
> Nmax := 10000000
> for N := 0; N < Nmax; N++ {
> for i := 0; i < 20; i++ {
> math.Pow(x, float64(i))
> }
> }
> }
>
> The C code was compiled with: gcc -O2 ccode.c -o ccode -lm
> then run with time ./ccode
>
> The Go code was compiled with: go build gcode.go
> then run with time ./gcode
>
> I've used the time command on Linux (Ubuntu) to get some estimate.
>
> So the question is: how can we make the Go code faster?
>
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