本文是对#102 Go 官方标准编译器中实现的优化集锦汇总 内容的记录与总结.
优化1-4: 字符串和字节切片之间的转化
1.紧跟range关键字的 从字符串到字节切片的转换；
go
package main
import (
"fmt"
"strings"
"testing"
)
var cs10086 = strings.Repeat("shuang!", 10086)
func main() {
fmt.Println(testing.AllocsPerRun(1, f)) //0
fmt.Println(testing.AllocsPerRun(1, g)) //1
}
func f() {
for range []byte(cs10086) {
}
}
func g() {
bs := []byte(cs10086)
for range bs {
}
}f没有开辟内存,g开辟了一次内存.
2.映射元素读取索引语法中被用做键值的 从字节切片到字符串的转换；
go
package main
import (
"bytes"
"fmt"
"testing"
)
var name = bytes.Repeat([]byte{'x'}, 188)
var m = make(map[string]string, 10)
var s = ""
func main() {
fmt.Println(testing.AllocsPerRun(1, f2)) //0
fmt.Println(testing.AllocsPerRun(1, g2)) //1
fmt.Println(testing.AllocsPerRun(1, h2)) //1
}
func f2() {
s = m[string(name)] // 有效
}
func g2() {
key := string(name)
s = m[key] // 无效
}
func h2() {
m[string(name)] = "Golang" // 无效
}
3.字符串比较表达式中被用做比较值的 从字节切片到字符串的转换
go
package main
import (
"fmt"
"testing"
)
var x = []byte{1023: 'x'}
var y = []byte{1023: 'y'}
var b bool
func main() {
fmt.Println(testing.AllocsPerRun(1, f3)) //0
fmt.Println(testing.AllocsPerRun(1, g3)) //2
}
func f3() {
b = string(x) != string(y)
}
func g3() {
sx, sy := string(x), string(y)
b = sx == sy
}
4.含 非空字符串常量 的字符串衔接表达式中的 从字节切片到字符串的转换
go
package main
import (
"fmt"
"testing"
)
var p = []byte{1023: 'p'}
var q = []byte{1023: 'q'}
var str string
func main() {
fmt.Println(testing.AllocsPerRun(1, f4)) //1
fmt.Println(testing.AllocsPerRun(1, g4)) //3
}
func f4() {
str = ("-" + string(p) + string(q))[1:]
}
func g4() {
str = string(p) + string(q)
}
5.[]rune(aString)转换的时间和空间复杂度都是O(n),但len([]rune(aString))中的此转换 不需要开辟内存
Go 1.12引入
go
package main
import (
"fmt"
"strings"
"testing"
)
var shuang = strings.Repeat("shuang!", 10086)
func main() {
fmt.Println(testing.AllocsPerRun(1, f5)) //0
fmt.Println(testing.AllocsPerRun(1, g5)) //1
}
func f5() {
_ = len([]rune(shuang))
}
func g5() {
_ = len([]byte(shuang)) //未对len([]byte(aString))做优化
}
6.字符串衔接表达式只需开辟一次内存,无论需要衔接多少个字符串
go
package main
import (
"fmt"
"testing"
)
var h, i, j, k = "Hello", "World", "Let's", "Go"
var str6 string
func main() {
fmt.Println(testing.AllocsPerRun(1, f6)) //1
fmt.Println(testing.AllocsPerRun(1, g6)) //3
}
func f6() {
str6 = h + i + j + k
}
func g6() {
str6 = h + i
str6 += j
str6 += k
}
7.for i := range anArrayOrSlice{anArrayOrSlice[i]} = zeroElement} 形式 将被优化为一个内部的memclr操作
go
package main
const N = 1024 * 100
var arr [N]int
func clearArray() {
for i := range arr {
arr[i] = 0
}
}
func clearSlice() {
sli := arr[:]
for i := range sli {
sli[i] = 0
}
}
func clearArrayPtr() {
for i := range &arr {
arr[i] = 0
}
}
benchmark:
go
package main
import (
"testing"
)
func BenchmarkTest1(b *testing.B) {
for i := 0; i < b.N; i++ {
clearArray()
}
}
func BenchmarkTest2(b *testing.B) {
for i := 0; i < b.N; i++ {
clearSlice()
}
}
func BenchmarkTest3(b *testing.B) { //无效
for i := 0; i < b.N; i++ {
clearArrayPtr()
}
}执行结果:
go
goos: darwin
goarch: amd64
pkg: xxxx
cpu: Intel(R) Core(TM) i7-8557U CPU @ 1.70GHz
BenchmarkTest1-8 73000 15309 ns/op
BenchmarkTest2-8 76464 15167 ns/op
BenchmarkTest3-8 40194 30096 ns/op
PASS
ok xxxx 4.213s8.for k = range m {delete(m,k)}形式 将被优化为一个内部的map清空操作
9.尺寸不大于4个原生字(即int),并且字段数不超过4个的结构体值被视为是小尺寸值
go
package main
type S1 struct {
a int
}
type S2 struct {
a, b int
}
type S3 struct {
a, b, c int
}
type S4 struct {
a, b, c, d int
}
type S5 struct {
a, b, c, d, e int
}
type S6 struct {
a, b, c, d, e, f int
}
var ss1, ss2, ss3, ss4, ss5, ss6 = make([]S1, 1000), make([]S2, 1000), make([]S3, 1000), make([]S4, 1000), make([]S5, 1000), make([]S6, 1000)
var x1, x2, x3, x4, x5, x6 intbenchmark:
go
package main
import "testing"
func Benchmark_Range1(b *testing.B) {
for i := 0; i < b.N; i++ {
for _, v := range ss1 {
x1 = v.a
}
}
}
func Benchmark_Range2(b *testing.B) {
for i := 0; i < b.N; i++ {
for _, v := range ss2 {
x2 = v.a
}
}
}
func Benchmark_Range3(b *testing.B) {
for i := 0; i < b.N; i++ {
for _, v := range ss3 {
x3 = v.a
}
}
}
func Benchmark_Range4(b *testing.B) {
for i := 0; i < b.N; i++ {
for _, v := range ss4 {
x4 = v.a
}
}
}
func Benchmark_Range5(b *testing.B) {
for i := 0; i < b.N; i++ {
for _, v := range ss5 {
x5 = v.a
}
}
}
func Benchmark_Range6(b *testing.B) {
for i := 0; i < b.N; i++ {
for _, v := range ss6 {
x6 = v.a
}
}
}执行结果:
go
goos: darwin
goarch: amd64
pkg: xxxx
cpu: Intel(R) Core(TM) i7-8557U CPU @ 1.70GHz
Benchmark_Range1-8 4759434 248.4 ns/op
Benchmark_Range2-8 3910621 306.0 ns/op
Benchmark_Range3-8 3735921 328.9 ns/op
Benchmark_Range4-8 3677784 325.9 ns/op
Benchmark_Range5-8 814666 1517 ns/op
Benchmark_Range6-8 728656 1568 ns/op
PASS
ok xxxx 8.868s
因为很多一等公民,其底层结构体的元素,都没有超过4个
10.接口值包裹 指针值 比 包裹 其他类型的值 要快
go
package main
var p, p2 = new([100]int), new([100]int)
var ip interface{}
go
package main
import "testing"
func Benchmark_PointerAssign(b *testing.B) {
for i := 0; i < b.N; i++ {
p = p2
}
}
func Benchmark_BoxPointer(b *testing.B) {
for i := 0; i < b.N; i++ {
ip = p
}
}
func Benchmark_PointerAssert(b *testing.B) {
for i := 0; i < b.N; i++ {
p = ip.(*[100]int)
}
}
go
goos: darwin
goarch: amd64
pkg: xxxx
cpu: Intel(R) Core(TM) i7-8557U CPU @ 1.70GHz
Benchmark_PointerAssign-8 1000000000 0.5251 ns/op 0 B/op 0 allocs/op
Benchmark_BoxPointer-8 1000000000 0.5833 ns/op 0 B/op 0 allocs/op
Benchmark_PointerAssert-8 1000000000 0.6418 ns/op 0 B/op 0 allocs/op
PASS
ok xxxx 2.372s
11.接口值包裹 指针值 比 包裹 其他类型的值 要快
Go 1.15新增优化
go
package main
var x,y = 255,256
var ix,iy interface{}
go
package main
import "testing"
func Benchmark_x(b *testing.B) {
for i := 0; i < b.N; i++ {
ix = x
}
}
func Benchmark_y(b *testing.B) {
for i := 0; i < b.N; i++ {
iy = y
}
}
go
goos: darwin
goarch: amd64
pkg: xxxx
cpu: Intel(R) Core(TM) i7-8557U CPU @ 1.70GHz
Benchmark_x-8 565624285 2.033 ns/op 0 B/op 0 allocs/op
Benchmark_y-8 92127024 12.71 ns/op 8 B/op 1 allocs/op
PASS
ok xxxx 2.653s
12.Bounds Check Elimination
