Golang unsafe包使用笔记
unsafe包简介
unsafe包提供了访问底层内存的方法。是用unsafe函数可以提高访问对象的速度。
应用场景
通常用于对大数组的遍历。
访问数组
package main
import (
"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])
}
访问结构体
package main
import (
"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)
}
类型转换
package main
import (
"fmt"
"unsafe"
)
func main() {
var f float64 = 0.0
var x uint64 = *(*uint64)(unsafe.Pointer(&f))
fmt.Println(f, x)
}
注意事项
- unsafe可能引起兼容性问题。
性能测试
代码：
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
}
测试代码：
package main
import (
"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)
}
}
结果：
BenchmarkUnsafeSum-2 20000000 94.0 ns/op BenchmarkSum-2 10000000 129 ns/op BenchmarkUnsafeSum2-2 10000000 126 ns/op BenchmarkUnsafeSum3-2 20000000 84.0 ns/op PASS ok tq/lab/Unsafe 6.926s
参考资料
修订记录
- 2017年06月15日 建立文档。
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