本文是对#102 Go 官网规范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；
package mainimport ( "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没有开拓内存,g开拓了一次内存.
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；
package mainimport ( "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)] // 无效}func g2() { key := string(name) s = m[key] // 有效}func h2() { m[string(name)] = "Golang" // 有效}3.字符串比拟表达式中被用做比拟值的 从字节切片到字符串的转换
package mainimport ( "fmt" "testing")var x = []byte{1023: 'x'}var y = []byte{1023: 'y'}var b boolfunc 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 的字符串连接表达式中的 从字节切片到字符串的转换
package mainimport ( "fmt" "testing")var p = []byte{1023: 'p'}var q = []byte{1023: 'q'}var str stringfunc 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)转换的工夫和空间复杂度都是O(n),但len([]rune(aString))中的此转换 不须要开拓内存
Go 1.12引入
package mainimport ( "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.字符串连接表达式只需开拓一次内存,无论须要连接多少个字符串
package mainimport ( "fmt" "testing")var h, i, j, k = "Hello", "World", "Let's", "Go"var str6 stringfunc 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} 模式 将被优化为一个外部的memclr操作
package mainconst N = 1024 * 100var arr [N]intfunc 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:
package mainimport ( "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) { //有效 for i := 0; i < b.N; i++ { clearArrayPtr() }}执行后果:
goos: darwingoarch: amd64pkg: xxxxcpu: Intel(R) Core(TM) i7-8557U CPU @ 1.70GHzBenchmarkTest1-8 73000 15309 ns/opBenchmarkTest2-8 76464 15167 ns/opBenchmarkTest3-8 40194 30096 ns/opPASSok xxxx 4.213s8.for k = range m {delete(m,k)}模式 将被优化为一个外部的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个的构造体值被视为是小尺寸值
package maintype 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:
package mainimport "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 } }}执行后果:
goos: darwingoarch: amd64pkg: xxxxcpu: Intel(R) Core(TM) i7-8557U CPU @ 1.70GHzBenchmark_Range1-8 4759434 248.4 ns/opBenchmark_Range2-8 3910621 306.0 ns/opBenchmark_Range3-8 3735921 328.9 ns/opBenchmark_Range4-8 3677784 325.9 ns/opBenchmark_Range5-8 814666 1517 ns/opBenchmark_Range6-8 728656 1568 ns/opPASSok xxxx 8.868s因为很多一等公民,其底层构造体的元素,都没有超过4个
10.接口值包裹 指针值 比 包裹 其余类型的值 要快
package mainvar p, p2 = new([100]int), new([100]int)var ip interface{}package mainimport "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) }}goos: darwingoarch: amd64pkg: xxxxcpu: Intel(R) Core(TM) i7-8557U CPU @ 1.70GHzBenchmark_PointerAssign-8 1000000000 0.5251 ns/op 0 B/op 0 allocs/opBenchmark_BoxPointer-8 1000000000 0.5833 ns/op 0 B/op 0 allocs/opBenchmark_PointerAssert-8 1000000000 0.6418 ns/op 0 B/op 0 allocs/opPASSok xxxx 2.372s11.接口值包裹 指针值 比 包裹 其余类型的值 要快
Go 1.15新增优化
package mainvar x,y = 255,256var ix,iy interface{}package mainimport "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 }}goos: darwingoarch: amd64pkg: xxxxcpu: Intel(R) Core(TM) i7-8557U CPU @ 1.70GHzBenchmark_x-8 565624285 2.033 ns/op 0 B/op 0 allocs/opBenchmark_y-8 92127024 12.71 ns/op 8 B/op 1 allocs/opPASSok xxxx 2.653s12.Bounds Check Elimination
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