golang面试题（怎么避免内存逃逸（）） Go

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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 mainimport ( "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

./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!。