问题
怎么避免内存逃逸？
怎么答
在runtime/stubs.go:133有个函数叫noescape。noescape可以在逃逸分析中隐藏一个指针。让这个指针在逃逸分析中不会被检测为逃逸。
// 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)
}举例
- 通过一个例子加深理解，接下来尝试下怎么通过
go build -gcflags=-m查看逃逸的情况。
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
}执行go build -gcflags=-m main.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其中主要看中间一小段
代码语言：javascript复制./main.go:24:16: &s escapes to heap //这个是NewA中的，逃逸了
./main.go:23:13: moved to heap: s
./main.go:27:18: NewATrick s does not escape // NewATrick里的s的却没逃逸
./main.go:28:45: NewATrick &s does not escape解释
- 上段代码对
A和ATrick同样的功能有两种实现：他们包含一个string，然后用String()方法返回这个字符串。但是从逃逸分析看ATrick版本没有逃逸。 noescape()函数的作用是遮蔽输入和输出的依赖关系。使编译器不认为p会通过x逃逸， 因为uintptr()产生的引用是编译器无法理解的。- 内置的
uintptr类型是一个真正的指针类型，但是在编译器层面，它只是一个存储一个指针地址的int类型。代码的最后一行返回unsafe.Pointer也是一个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引用的数据肯定不会被逃逸，但编译器却不知道的情况下，这是很有用的。- 面试中秀一秀是可以的，如果在实际项目中如果使用这种unsafe包大概率会被同事打死。不建议使用！ 毕竟包的名字就叫做
unsafe, 而且源码中的注释也写明了USE CAREFULLY!。
