[go-nuts] Re: What explains this 6x slowdown ?

['Isaac Gouy' via golang-nuts]

> I believe this is simply due to the 64 bits integer division.
>
> 1. Contrary to Java or most C implementations int is 64 bits with Go on 
> AMD64, and not 32 bits. Integer division on 64 bits is more expensive than 
> for 32 bits.
> 2. When array_length is known at compile time, the compiler can replace 
> the integer division done for % with cheaper operations.
>

Well, the compile-time-constant program did not become slower with int64, 
but the run-time-value program did become faster with int32 —

package main
import "os"
import "fmt"
import "strconv"
func main() {
    var (
       element, iteration, iterations, innerloop int64
       sum float64
    )

    if len(os.Args) > 1 {
        var i,_ = strconv.Atoi(os.Args[1])
        iterations = int64(i)
    }
    fmt.Printf("iterations %d\n", iterations)
    
    var (
        array_length int64 = 100000000    
        array []float64 = make([]float64, array_length)
    )    
    
    for element = 0; element < array_length; element++ {
        array[element] = float64(element)
    }
    for iteration = 0; iteration < iterations; iteration++ {
        for innerloop = 0; innerloop < 1000000000; innerloop++ {
            sum += array[(iteration + innerloop) % array_length]
        }
    }
    fmt.Printf("sum %E\n", sum)
    array = nil
}

$ /opt/src/go1.16/go/bin/go build -o out test.go
$ time ./out 100
iterations 100
sum 5.000000E+18

real    3m3.240s

====

package main
import "os"
import "fmt"
import "strconv"
func main() {
    var (
       element, iteration, iterations, innerloop int32
       sum float64
    )

    if len(os.Args) > 1 {
        var i,_ = strconv.Atoi(os.Args[1])
        iterations = int32(i)
    }
    fmt.Printf("iterations %d\n", iterations)
    
    var (
        array_length int32 = 1000000 * iterations    
        array []float64 = make([]float64, array_length)
    )    
    
    for element = 0; element < array_length; element++ {
        array[element] = float64(element)
    }
    for iteration = 0; iteration < iterations; iteration++ {
        for innerloop = 0; innerloop < 1000000000; innerloop++ {
            sum += array[(iteration + innerloop) % array_length]
        }
    }
    fmt.Printf("sum %E\n", sum)
    array = nil
}

$ /opt/src/go1.16/go/bin/go build -o out test.go
$ time ./out 100
iterations 100
sum 5.000000E+18

real    6m57.880s

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