0. 简介

前面我们针对Go中堆和栈的内存都做了一些分析，现在我们来分析一下Go的内存逃逸。

mallocfree
newmake
  • 指向栈对象的指针不能存在于堆中；
  • 指向栈对象的指针不能在栈对象回收后存活；
go build -gcflags="-m -m -l"-m-m-l

1. 几种逃逸分析

1.1 函数返回局部变量指针

逃逸分析结果如下：

$ go build -gcflags="-m -m -l" ./main.go
# command-line-arguments
./main.go:4:2: res escapes to heap:
./main.go:4:2:   flow: ~r2 = &res:
./main.go:4:2:     from &res (address-of) at ./main.go:6:9
./main.go:4:2:     from return &res (return) at ./main.go:6:2
./main.go:4:2: moved to heap: res
note: module requires Go 1.18

Add

1.2 interface(any)类型逃逸

逃逸分析如下：

$ go build -gcflags="-m -m -l" ./main.go
# command-line-arguments
./main.go:9:13: str escapes to heap:
./main.go:9:13:   flow: {storage for ... argument} = &{storage for str}:
./main.go:9:13:     from str (spill) at ./main.go:9:13
./main.go:9:13:     from ... argument (slice-literal-element) at ./main.go:9:12
./main.go:9:13:   flow: {heap} = {storage for ... argument}:
./main.go:9:13:     from ... argument (spill) at ./main.go:9:12
./main.go:9:13:     from fmt.Printf("%v\n", ... argument...) (call parameter) at ./main.go:9:12
./main.go:9:12: ... argument does not escape
./main.go:9:13: str escapes to heap

str escapes to heapstrmoved to heap: resstr

escapes to heap vs moved to heap

我们可以写如下代码试试：

其逃逸分析和上面的没有区别：

$ go build -gcflags="-m -m -l" ./main.go
# command-line-arguments
./main.go:8:13: str escapes to heap:
./main.go:8:13:   flow: {storage for ... argument} = &{storage for str}:
./main.go:8:13:     from str (spill) at ./main.go:8:13
./main.go:8:13:     from ... argument (slice-literal-element) at ./main.go:8:12
./main.go:8:13:   flow: {heap} = {storage for ... argument}:
./main.go:8:13:     from ... argument (spill) at ./main.go:8:12
./main.go:8:13:     from fmt.Printf("%s\n", ... argument...) (call parameter) at ./main.go:8:12
./main.go:8:12: ... argument does not escape
./main.go:8:13: str escapes to heap
note: module requires Go 1.18

str1str

$ go run main.go
hello world
0xc00009af50
0xc00009af40

fmt.Printf("%s\n", str)fmt.Printf("%p\n", &str)moved to heap: str

$ go build -gcflags="-m -m -l" ./main.go
# command-line-arguments
./main.go:6:2: str escapes to heap:
./main.go:6:2:   flow: {storage for ... argument} = &str:
./main.go:6:2:     from &str (address-of) at ./main.go:8:21
./main.go:6:2:     from &str (interface-converted) at ./main.go:8:21
./main.go:6:2:     from ... argument (slice-literal-element) at ./main.go:8:12
./main.go:6:2:   flow: {heap} = {storage for ... argument}:
./main.go:6:2:     from ... argument (spill) at ./main.go:8:12
./main.go:6:2:     from fmt.Printf("%p\n", ... argument...) (call parameter) at ./main.go:8:12
./main.go:6:2: moved to heap: str
./main.go:8:12: ... argument does not escape
note: module requires Go 1.18

strstr1

$ go run main.go
0xc00010a210
0xc00010a210
0xc000106f50

参考如下解释：

bbb

翻译过来大意是：当逃逸分析输出“b escapes to heap”时，意思是指存储在b中的值逃逸到堆上了，即任何被b引用的对象必须分配在堆上，而b自身则不需要；如果b自身也逃逸到堆上，那么逃逸分析会输出“&b escapes to heap”。

由于字符串本身是存储在只读存储区，我们使用切片更能表现以上的特性。

无逃逸

逃逸分析是：

$ go build -gcflags="-m -m -l" ./main.go
# command-line-arguments
./main.go:12:12: []int{...} does not escape
note: module requires Go 1.18

打印结果：

$ go run main.go
&i 0xc00009af20
&b 0xc00009af58
b 0xc00009af28

&i b &b

切片底层数组逃逸

fmt

逃逸分析如下：

$ go build -gcflags="-m -m -l" ./main.go
# command-line-arguments
./main.go:16:13: b escapes to heap:
./main.go:16:13:   flow: {storage for ... argument} = &{storage for b}:
./main.go:16:13:     from b (spill) at ./main.go:16:13
./main.go:16:13:     from ... argument (slice-literal-element) at ./main.go:16:13
./main.go:16:13:   flow: {heap} = {storage for ... argument}:
./main.go:16:13:     from ... argument (spill) at ./main.go:16:13
./main.go:16:13:     from fmt.Println(... argument...) (call parameter) at ./main.go:16:13
./main.go:13:12: []int{...} escapes to heap:
./main.go:13:12:   flow: b = &{storage for []int{...}}:
./main.go:13:12:     from []int{...} (spill) at ./main.go:13:12
./main.go:13:12:     from b := []int{...} (assign) at ./main.go:13:4
./main.go:13:12:   flow: {storage for b} = b:
./main.go:13:12:     from b (interface-converted) at ./main.go:16:13
./main.go:13:12: []int{...} escapes to heap
./main.go:16:13: ... argument does not escape
./main.go:16:13: b escapes to heap
note: module requires Go 1.18

b escapes to heap

$ go run main.go
&i 0xc000106f38
&b 0xc000106f58
b 0xc000120030
[1 2 3 4 5]

bb

切片对象同样发生逃逸

fmt.Println(b)fmt.Println(&b)

$ go build -gcflags="-m -m -l" ./main.go
# command-line-arguments
./main.go:13:2: b escapes to heap:
./main.go:13:2:   flow: {storage for ... argument} = &b:
./main.go:13:2:     from &b (address-of) at ./main.go:16:14
./main.go:13:2:     from &b (interface-converted) at ./main.go:16:14
./main.go:13:2:     from ... argument (slice-literal-element) at ./main.go:16:13
./main.go:13:2:   flow: {heap} = {storage for ... argument}:
./main.go:13:2:     from ... argument (spill) at ./main.go:16:13
./main.go:13:2:     from fmt.Println(... argument...) (call parameter) at ./main.go:16:13
./main.go:13:12: []int{...} escapes to heap:
./main.go:13:12:   flow: b = &{storage for []int{...}}:
./main.go:13:12:     from []int{...} (spill) at ./main.go:13:12
./main.go:13:12:     from b := []int{...} (assign) at ./main.go:13:4
./main.go:13:2: moved to heap: b
./main.go:13:12: []int{...} escapes to heap
./main.go:16:13: ... argument does not escape
note: module requires Go 1.18

moved to heap: b

$ go run main.go
&i 0xc00006af48
&b 0xc00000c030
b 0xc00001a150
&[1 2 3 4 5]

b

所以可以总结下来就是：

escapes to heapmoved to heap

1.3 申请栈空间过大

可以发现逃逸分析显示没有发生逃逸：

$ go build -gcflags="-m -m -l" ./main.go
# command-line-arguments
./main.go:13:11: make([]int, 0) does not escape
./main.go:17:12: make([]byte, 65536) does not escape
note: module requires Go 1.18

如果将切片和数组的长度都增加1，则会发生逃逸。

逃逸分析：

$ go build -gcflags="-m -m -l" ./main.go
# command-line-arguments
./main.go:21:6: a escapes to heap:
./main.go:21:6:   flow: {heap} = &a:
./main.go:21:6:     from a (too large for stack) at ./main.go:21:6
./main.go:17:12: make([]byte, 65537) escapes to heap:
./main.go:17:12:   flow: {heap} = &{storage for make([]byte, 65537)}:
./main.go:17:12:     from make([]byte, 65537) (too large for stack) at ./main.go:17:12
./main.go:21:6: moved to heap: a
./main.go:13:11: make([]int, 0) does not escape
./main.go:17:12: make([]byte, 65537) escapes to heap
note: module requires Go 1.18

65536 = 64KB1024*1024*10 = 10MB

1.4 闭包逃逸

i

$ go build -gcflags="-m -m -l" ./main.go
# command-line-arguments
./main.go:4:2: intSeq capturing by ref: i (addr=false assign=true width=8)
./main.go:5:9: func literal escapes to heap:
./main.go:5:9:   flow: ~r0 = &{storage for func literal}:
./main.go:5:9:     from func literal (spill) at ./main.go:5:9
./main.go:5:9:     from return func literal (return) at ./main.go:5:2
./main.go:4:2: i escapes to heap:
./main.go:4:2:   flow: {storage for func literal} = &i:
./main.go:4:2:     from i (captured by a closure) at ./main.go:6:3
./main.go:4:2:     from i (reference) at ./main.go:6:3
./main.go:4:2: moved to heap: i
./main.go:5:9: func literal escapes to heap
note: module requires Go 1.18

iiai

2. 总结

逃逸到堆上的内存可能会加大GC压力，所以在一些简单的场景下，我们可以避免内存逃逸，使得变量更多地分配在栈上，可以提升程序的性能。比如：

  • 不要盲目地使用指针传参，特别是参数对象很小时，虽然可以减小复制大小，但是可能会造成内存逃逸；
  • 多根据代码具体分析，根据逃逸分析结果做一些优化，提高性能。