Slice: append vs set performance comparison

slice_append.go

package main

import (
	"fmt"
	"reflect"
	"strings"
	"unsafe"
)

func main() {
	// var a []int
	// c := make([]int, 10)
	// d := make([]int, 0, 10)
	// fmt.Printf("normal slice \ta: %v, len %d, cap: %d\n", a, len(a), cap(a))
	// fmt.Printf("slice_len_cap same \tc: %v, len %d, cap: %d\n", c, len(c), cap(c))
	// fmt.Printf("slice_len_cap not same \td: %v, len %d, cap: %d\n", d, len(d), cap(d))

	fmt.Println(strings.Repeat("-", 25))
	fmt.Println("testAppend()...")
	testAppend()
	fmt.Println(strings.Repeat("-", 25))

	fmt.Println(strings.Repeat("-", 25))
	fmt.Println("testAppendSliceLenCapSame()...")
	testAppendSliceLenCapSame()
	fmt.Println(strings.Repeat("-", 25))

	fmt.Println(strings.Repeat("-", 25))
	fmt.Println("testAppendSliceLenCapNotSame()...")
	testAppendSliceLenCapNotSame()
	fmt.Println(strings.Repeat("-", 25))
}

func testAppend() {
	var result []int
	fmt.Printf("normal slice \ta: %v, len %d, cap: %d\n", result, len(result), cap(result))
	for i := 0; i < 5; i++ {
		fmt.Printf("appending '%d': %s\n", i, getSliceHeader(&result))
		result = append(result, i)
		fmt.Printf("appended  '%d':  %s\n", i, getSliceHeader(&result))
	}
	fmt.Println(result)
	fmt.Printf("normal slice \ta: %v, len %d, cap: %d\n", result, len(result), cap(result))
}

// len & cap same
func testAppendSliceLenCapSame() {
	var result = make([]int, 5)
	fmt.Printf("slice_len_cap same \tb: %v, len %d, cap: %d\n", result, len(result), cap(result))
	for i := 0; i < 5; i++ {
		fmt.Printf("appending '%d': %s\n", i, getSliceHeader(&result))
		result = append(result, i)
		fmt.Printf("appended  '%d':  %s\n", i, getSliceHeader(&result))
	}
	fmt.Println(result)
	fmt.Printf("slice_len_cap same \tb: %v, len %d, cap: %d\n", result, len(result), cap(result))
}

// len 0
func testAppendSliceLenCapNotSame() {
	var result = make([]int, 0, 5)
	fmt.Printf("slice_len_cap not same \td: %v, len %d, cap: %d\n", result, len(result), cap(result))
	for i := 0; i < 5; i++ {
		fmt.Printf("appending '%d': %s\n", i, getSliceHeader(&result))
		result = append(result, i)
		fmt.Printf("appended  '%d':  %s\n", i, getSliceHeader(&result))
	}
	fmt.Println(result)
	fmt.Printf("slice_len_cap not same \td: %v, len %d, cap: %d\n", result, len(result), cap(result))
}

func getSliceHeader(slice *[]int) string {
	sh := (*reflect.SliceHeader)(unsafe.Pointer(slice))
	return fmt.Sprintf("%+v", sh)
}

slice_append_test.go

package main

import "testing"

func BenchmarkTestAppend(b *testing.B) {
	var a []int
	for i := 0; i < b.N; i++ {
		a = append(a, i)
	}
}

func BenchmarkTestAppendSliceLenCapSame(b *testing.B) {
	a := make([]int, b.N)
	for i := 0; i < b.N; i++ {
		a = append(a, i)
	}
}

func BenchmarkTestAppendSliceLenCapNotSame(b *testing.B) {
	a := make([]int, 0, b.N)
	for i := 0; i < b.N; i++ {
		a = append(a, i)
	}
}

func BenchmarkSliceSet(b *testing.B) {
	a := make([]int, b.N)
	for i := 0; i < b.N; i++ {
		a[i] = i
	}
}

Benchmark: