Golang unsafe包使用筆記

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Golang unsafe包使用筆記

unsafe包簡介

unsafe包提供了訪問底層記憶體的方法。是用unsafe函數可以提高訪問對象的速度。

應用情境

通常用於對大數組的遍曆。

訪問數組

package mainimport (     "fmt"     "unsafe")func main() {     array := []int{1, 2, 3}     base := uintptr(unsafe.Pointer(&array[0]))     size := unsafe.Sizeof(array[0])     ptr := unsafe.Pointer(base + 2*size)     element := *(*int)(ptr)     fmt.Println(element, array[2])}

訪問結構體

package mainimport (     "fmt"     "unsafe")type Foo struct {     A int     B int}func main() {     foo := &Foo{1, 2}     fmt.Println(foo)     base := uintptr(unsafe.Pointer(foo))     offset := unsafe.Offsetof(foo.A)     ptr := unsafe.Pointer(base + offset)     *(*int)(ptr) = 3     fmt.Println(foo)}

類型轉換

package mainimport (     "fmt"     "unsafe")func main() {     var f float64 = 0.0     var x uint64 = *(*uint64)(unsafe.Pointer(&f))     fmt.Println(f, x)}

注意事項

1. unsafe可能引起相容性問題。

效能測試

代碼：

func sum(arr []int) int {     sum := 0     for _, x := range arr {             sum += x     }     return sum}func unsafeSum(arr []int) int {     ptr := unsafe.Pointer(&arr[0])     count := len(arr)     step := unsafe.Sizeof(arr[0])     sum := 0     for i := 0; i < count; i++ {             sum += *(*int)(ptr)             ptr = unsafe.Pointer(uintptr(ptr) + step)     }     return sum}func unsafeSum2(arr []int) int {     base := unsafe.Pointer(&arr[0])     count := len(arr)     step := unsafe.Sizeof(arr[0])     sum := 0     for i := 0; i < count; i++ {             ptr := unsafe.Pointer(uintptr(base) + uintptr(i)*step)             sum += *(*int)(ptr)     }     return sum}func unsafeSum3(arr []int) int {     beg := unsafe.Pointer(&arr[0])     size := unsafe.Sizeof(arr[0])     end := unsafe.Pointer(uintptr(beg) + uintptr(len(arr))*size)     ptr := beg     sum := 0     for ptr != end {             sum += *(*int)(ptr)             ptr = unsafe.Pointer(uintptr(ptr) + size)     }     return sum}

測試代碼：

package mainimport (     "testing")var (     array = []int{             1, 2, 3, 4, 5, 6, 7, 8, 9, 10,             1, 2, 3, 4, 5, 6, 7, 8, 9, 10,             1, 2, 3, 4, 5, 6, 7, 8, 9, 10,             1, 2, 3, 4, 5, 6, 7, 8, 9, 10,             1, 2, 3, 4, 5, 6, 7, 8, 9, 10,             1, 2, 3, 4, 5, 6, 7, 8, 9, 10,             1, 2, 3, 4, 5, 6, 7, 8, 9, 10,             1, 2, 3, 4, 5, 6, 7, 8, 9, 10,             1, 2, 3, 4, 5, 6, 7, 8, 9, 10,             1, 2, 3, 4, 5, 6, 7, 8, 9, 10,     })func BenchmarkUnsafeSum(b *testing.B) {     for i := 0; i < b.N; i++ {             unsafeSum(array)     }}func BenchmarkSum(b *testing.B) {     for i := 0; i < b.N; i++ {             sum(array)     }}func TestUnsafeSum(t *testing.T) {     if result := unsafeSum(array); result != 550 {             t.Fatal("expect %v get %v", 550, result)     }}func TestUnsafeSum2(t *testing.T) {     if result := unsafeSum2(array); result != 550 {             t.Fatal("expect %v get %v", 550, result)     }}func BenchmarkUnsafeSum2(b *testing.B) {     for i := 0; i < b.N; i++ {             unsafeSum2(array)     }}func TestUnsafeSum3(t *testing.T) {     if result := unsafeSum3(array); result != 550 {             t.Fatal("expect %v get %v", 550, result)     }}func BenchmarkUnsafeSum3(b *testing.B) {     for i := 0; i < b.N; i++ {             unsafeSum3(array)     }}

結果：

BenchmarkUnsafeSum-2            20000000                94.0 ns/opBenchmarkSum-2                  10000000               129 ns/opBenchmarkUnsafeSum2-2           10000000               126 ns/opBenchmarkUnsafeSum3-2           20000000                84.0 ns/opPASSok      tq/lab/Unsafe   6.926s

參考資料

1. http://golang.org/pkg/unsafe

歷程記錄

1. 2017年06月15日 建立文檔。