代码顺序和性能

Question

I wanted to find which is faster: struct vs array. So I wrote a GO code in which I write 4 int values (1,2,3 and 4) to the members of a structure and then to an array of length 4. I tried to find the time it takes to write.

Case1: First, I write values to a structure and then to an array. Here I found array to be faster than structure.

package main

import (
    "fmt"
    "time"
)

type abc struct {
    a, b, c, d int
}

func main() {

    var obj abc

    t1 := time.Now()
    obj.a = 1
    obj.b = 2
    obj.c = 3
    obj.d = 4
    t2 := time.Since(t1)

    fmt.Println("Struct access time: : ", t2)

    a := make([]int, 4)
    t3 := time.Now()
    a[0] = 1
    a[1] = 2
    a[2] = 3
    a[3] = 4
    t4 := time.Since(t3)

    fmt.Println("Array access time: : ", t4)

}

Case2: Second, I write values to an array and then a structure. Here I found structure to be faster than array.

package main

import (
    "fmt"
    "time"
)

type abc struct {
    a, b, c, d int
}

func main() {

    var obj abc

    a := make([]int, 4)
    t3 := time.Now()
    a[0] = 1
    a[1] = 2
    a[2] = 3
    a[3] = 4
    t4 := time.Since(t3)

    fmt.Println("Array access time: : ", t4)

    t1 := time.Now()
    obj.a = 1
    obj.b = 2
    obj.c = 3
    obj.d = 4
    t2 := time.Since(t1)

    fmt.Println("Struct access time: : ", t2)

}

Why the performance depends on to what I write first? The one that I write to first appears to be slower. Why is it so?

Answer 1

The other answer explained the timing difference, let's get into struct vs. slice.

If the compiler can figure out at compile time that the slice is big enough, accessing the elements of the slice and a struct will generate identical code. Of course, in reality, often the compiler won't know how big the slice is and completely different optimizations will be applied depending on if you're working with a struct or a slice, so for measuring performance you have to look at a whole program and its behavior, not just one particular operation.