換人！golang面試官：連怎麼避免記憶體逃逸都不知道？
阿新 • • 發佈：2021-02-25
## 問題
怎麼避免**記憶體逃逸**？
## 怎麼答
在```runtime/stubs.go:133```有個函式叫```noescape```。```noescape```可以在逃逸分析中**隱藏一個指標**。讓這個指標在逃逸分析中**不會被檢測為逃逸**。
```go
// noescape hides a pointer from escape analysis. noescape is
// the identity function but escape analysis doesn't think the
// output depends on the input. noescape is inlined and currently
// compiles down to zero instructions.
// USE CAREFULLY!
//go:nosplit
func noescape(p unsafe.Pointer) unsafe.Pointer {
x := uintptr(p)
return unsafe.Pointer(x ^ 0)
}
```
## 舉例
- 通過一個例子加深理解，接下來嘗試下怎麼通過 ```go build -gcflags=-m``` 檢視逃逸的情況。
```go
package main
import (
"unsafe"
)
type A struct {
S *string
}
func (f *A) String() string {
return *f.S
}
type ATrick struct {
S unsafe.Pointer
}
func (f *ATrick) String() string {
return *(*string)(f.S)
}
func NewA(s string) A {
return A{S: &s}
}
func NewATrick(s string) ATrick {
return ATrick{S: noescape(unsafe.Pointer(&s))}
}
func noescape(p unsafe.Pointer) unsafe.Pointer {
x := uintptr(p)
return unsafe.Pointer(x ^ 0)
}
func main() {
s := "hello"
f1 := NewA(s)
f2 := NewATrick(s)
s1 := f1.String()
s2 := f2.String()
_ = s1 + s2
}
```
執行```go build -gcflags=-m main.go```
```go
$go build -gcflags=-m main.go
# command-line-arguments
./main.go:11:6: can inline (*A).String
./main.go:19:6: can inline (*ATrick).String
./main.go:23:6: can inline NewA
./main.go:31:6: can inline noescape
./main.go:27:6: can inline NewATrick
./main.go:28:29: inlining call to noescape
./main.go:36:6: can inline main
./main.go:38:14: inlining call to NewA
./main.go:39:19: inlining call to NewATrick
./main.go:39:19: inlining call to noescape
./main.go:40:17: inlining call to (*A).String
./main.go:41:17: inlining call to (*ATrick).String
/var/folders/45/qx9lfw2s2zzgvhzg3mtzkwzc0000gn/T/go-build763863171/b001/_gomod_.go:6:6: can inline init.0
./main.go:11:7: leaking param: f to result ~r0 level=2
./main.go:19:7: leaking param: f to result ~r0 level=2
./main.go:24:16: &s escapes to heap
./main.go:23:13: moved to heap: s
./main.go:27:18: NewATrick s does not escape
./main.go:28:45: NewATrick &s does not escape
./main.go:31:15: noescape p does not escape
./main.go:38:14: main &s does not escape
./main.go:39:19: main &s does not escape
./main.go:40:10: main f1 does not escape
./main.go:41:10: main f2 does not escape
./main.go:42:9: main s1 + s2 does not escape
```
其中主要看中間一小段
```go
./main.go:24:16: &s escapes to heap //這個是NewA中的，逃逸了
./main.go:23:13: moved to heap: s
./main.go:27:18: NewATrick s does not escape // NewATrick裡的s的卻沒逃逸
./main.go:28:45: NewATrick &s does not escape
```
## 解釋
- 上段程式碼對```A```和```ATrick```同樣的功能有兩種實現：他們包含一個 ```string``` ，然後用 ```String()``` 方法返回這個字串。但是從逃逸分析看```ATrick``` 版本沒有逃逸。
- ```noescape()``` 函式的作用是遮蔽輸入和輸出的依賴關係。使編譯器不認為 ```p``` 會通過 ```x``` 逃逸， 因為 ```uintptr()``` 產生的引用是編譯器無法理解的。
- 內建的 ```uintptr``` 型別是一個真正的指標型別，但是在編譯器層面，它只是一個儲存一個 ```指標地址``` 的 ```int``` 型別。程式碼的最後一行返回 ```unsafe.Pointer``` 也是一個 ```int```。
- ```noescape()``` 在 ```runtime``` 包中使用 ```unsafe.Pointer``` 的地方被大量使用。如果作者清楚被 ```unsafe.Pointer``` 引用的資料肯定不會被逃逸，但編譯器卻不知道的情況下，這是很有用的。
- **面試中秀一秀是可以的**，如果在實際專案中如果使用這種unsafe包大概率會被同事打死。**不建議使用！** 畢竟包的名字就叫做 ```unsafe```, 而且原始碼中的註釋也寫明瞭 ```USE CAREFULLY! ```。
#### 文章推薦：
- [golang面試題：簡單聊聊記憶體逃逸？](https://mp.weixin.qq.com/s?__biz=MzAwMDAxNjU4Mg==&mid=2247483686&idx=1&sn=e48c51107191f02da5751a19a54f7d41&chksm=9aee288fad99a199c126d5ff735af7320356ce4bb5753ae59ac6231e596354499414b5705b79&token=2092782362&lang=zh_CN#rd)
- [golang面試題：字串轉成byte陣列，會發生記憶體拷貝嗎？](https://mp.weixin.qq.com/s?__biz=MzAwMDAxNjU4Mg==&mid=2247483669&idx=1&sn=88f754ddabc04eb3f66ba8ac37ee1461&chksm=9aee28bcad99a1aa1ada41cfccaffc7ef4719a9bc11c1bef45b7d1b5427c1faa12d8d0c3156f&token=2092782362&lang=zh_CN#rd)
- [golang面試題：翻轉含有`中文、數字、英文字母`的字串](https://mp.weixin.qq.com/s?__biz=MzAwMDAxNjU4Mg==&mid=2247483664&idx=1&sn=23a0cf8a78b1d9c30b2e3bc102bf421e&chksm=9aee28b9ad99a1af6c879ba4b1f6439e4c21c363f0a668f322c082ca334b62255507828f66d4&token=2092782362&lang=zh_CN#rd)
- [golang面試題：拷貝大切片一定比小切片代價大嗎？](https://mp.weixin.qq.com/s?__biz=MzAwMDAxNjU4Mg==&mid=2247483674&idx=1&sn=ce4b5fee48c54ff69127ef2bd5d91427&chksm=9aee28b3ad99a1a57eed7651a16fd4bdc35ff23937e423c5e1322a234652fd135f1a16abbece&token=2092782362&lang=zh_CN#rd)
- [golang面試題：能說說uintptr和unsafe.Pointer的區別嗎？](https://mp.weixin.qq.com/s?__biz=MzAwMDAxNjU4Mg==&mid=2247483679&idx=1&sn=7075859e59741b1d0a81dc472b8ce45f&chksm=9aee28b6ad99a1a0599416886660d9ea56bd7fec18841af0e5fe86c3daea3973732a83d7eabb&token=2092782362&lang=zh_CN#rd)
###### 如果你想每天學習一個知識點，關注我的【公】【眾】【號】【golang小白