为什么指向局部变量的指针转义到堆? [英] Why does a pointer to a local variable escape to the heap?

问题描述

这是我的办公环境:

[lorneli@localhost GoTest]$ go version
go version go1.9 linux/amd64

这是我的程序:

package main

type request struct {
    ID string
    size uint32
    off uint64
}

func main() {
    r := request{}
    iter := interface{}(&r) // &r escapes to heap
    iters := make([]interface{}, 0)
    iters = append(iters, iter)
}

我分配了一个request实例，并将其指针转换为interface{}. 但是当使用标志-gcflags "-m"进行分析时，我发现实例转义了 转换时堆积.为什么会这样?

I allocate a request instance and convert the pointer of it to interface{}. But when analysing with flag -gcflags "-m" , I found out the instance escapes to heap when converting. Why this happens?

这里是分析结果:

[lorneli@localhost GoTest]$ go build -gcflags "-m"
# _/mnt/hgfs/vmfolder/workspace/GoTest
./main.go:9:6: can inline main
./main.go:11:21: (interface {})(&r) escapes to heap
./main.go:11:22: &r escapes to heap
./main.go:10:15: moved to heap: r
./main.go:12:15: main make([]interface {}, 0) does not escape

我认为此案例与 逃脱分析缺陷" .

推荐答案

简化您的示例.用-gcflags='-m -m'分析.

Simplify your example. Analyze with -gcflags='-m -m'.

示例1 :

package main

func main() {
    var v int
    s := make([]*int, 0)
    s = append(s, &v) // &v escapes to heap
}

输出:

$ go version
go version devel +df8c2b905b Tue Mar 6 06:13:17 2018 +0000 linux/amd64
$ go run -gcflags='-m -m' esc.go
# command-line-arguments
./esc.go:3:6: can inline main as: func() { var v int; v = <N>; s := make([]*int, 0); s = append(s, &v) }
./esc.go:6:16: &v escapes to heap
./esc.go:6:16:  from append(s, &v) (appended to slice) at ./esc.go:6:12
./esc.go:4:6: moved to heap: v
./esc.go:5:11: main make([]*int, 0) does not escape
$ 

---

转义分析确定是否有任何对值的引用转义声明该值的函数.在函数main中声明的对变量v的引用作为函数append的参数转义:&v escapes to heap from append(s, &v)，moved to heap: v.

---

Escape analysis determines whether any references to a value escape the function in which the value is declared. A reference to the variable v, declared in function main, escapes as an argument to function append: &v escapes to heap from append(s, &v), moved to heap: v.

示例2 :

package main

func main() {
    var v int
    lc := 1
    s := make([]*int, lc)
    s[0] = &v
}

$ go run -gcflags='-m -m' esc2.go
./esc2.go:3:6: can inline main as: func() { var v int; v = <N>; lc := 1; s := make([]*int, lc); s[0] = &v }
./esc2.go:6:11: make([]*int, lc) escapes to heap
./esc2.go:6:11:     from make([]*int, lc) (too large for stack) at ./esc2.go:6:11
./esc2.go:7:9: &v escapes to heap
./esc2.go:7:9:  from s[0] (slice-element-equals) at ./esc2.go:7:7
./esc2.go:4:6: moved to heap: v
$ 

---

type slice struct {
    array unsafe.Pointer
    len   int
    cap   int
}

切片的

make返回切片描述符struct(指向基础数组，长度和容量的指针)，并分配基础切片元素数组.基础数组通常在堆上分配:make([]*int, lc) escapes to heap from make([]*int, lc).

make for a slice returns a slice descriptor struct (pointer to underlying array, length, and capacity) and allocates an underlying slice element array. The underlying array is generally allocated on the heap: make([]*int, lc) escapes to heap from make([]*int, lc).

s[0] = &v将对变量v(&v)的引用存储在堆上的基础数组中:&v escapes to heap from s[0] (slice-element-equals)，moved to heap: v.函数结束并回收其堆栈之后，引用将保留在堆上，直到对基础数组进行垃圾回收为止.

s[0] = &v stores a reference to the variable v (&v) in the underlying array on the heap: &v escapes to heap from s[0] (slice-element-equals), moved to heap: v. The reference remains on the heap, after the function ends and its stack is reclaimed, until the underlying array is garbage collected.

如果make切片容量是一个较小的(编译时)常数，在您的示例中为make([]*int, 1)，则基础数组可能会分配在堆栈上.但是，转义分析没有考虑到这一点.

If the make slice capacity is a small (compile time) constant, make([]*int, 1) in your example, the underlying array may be allocated on the stack. However, escape analysis does not take this into account.

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