Golang的关键字defer的使用方法

核心思想

在defer出现的地方插入了指令CALL runtime.deferproc，在函数返回的地方插入了CALL runtime.deferreturn。goroutine的控制结构中，有一张表记录defer，调用runtime.deferproc时会将需要defer的表达式记录在表中，而在调用runtime.deferreturn的时候，则会依次从defer表中“出栈”并执行

如果有多个defer，调用顺序类似栈，越后面的defer表达式越先被调用

defer链

defer信息会注册到链表，当前执行的 goroutine 持有这个链表的头指针，每个 goroutine 都有一个对应的结构体struct G，其中有一个字段指向这个defer链表头

type g struct {
// Stack parameters.
// stack describes the actual stack memory: [stack.lo, stack.hi).
// stackguard0 is the stack pointer compared in the Go stack growth prologue.
// It is stack.lo+StackGuard normally, but can be StackPreempt to trigger a preemption.
// stackguard1 is the stack pointer compared in the C stack growth prologue.
// It is stack.lo+StackGuard on g0 and gsignal stacks.
// It is ~0 on other goroutine stacks, to trigger a call to morestackc (and crash).
stack       stack   // offset known to runtime/cgo
stackguard0 uintptr // offset known to liblink
stackguard1 uintptr // offset known to liblink
_panic       *_panic // innermost panic - offset known to liblink
   // _defer 这个字段指向defer链表头
_defer       *_defer // innermost defer
   ...
}

新注册的defer会添加到链表头，所以感觉像是栈那样先进后出的调用：

源码分析

deferproc一共有两个参数，第一个是参数和返回值的大小，第二个是指向funcval的指针

// Create a new deferred function fn with siz bytes of arguments.
// The compiler turns a defer statement into a call to this.
//go:nosplit
func deferproc(siz int32, fn *funcval) { // arguments of fn follow fn
   // 获取当前goroutine
gp := getg()
if gp.m.curg != gp {
// go code on the system stack can't defer
throw("defer on system stack")
}
// the arguments of fn are in a perilous state. The stack map
// for deferproc does not describe them. So we can't let garbage
// collection or stack copying trigger until we've copied them out
// to somewhere safe. The memmove below does that.
// Until the copy completes, we can only call nosplit routines.
   // 获取调用者指针
sp := getcallersp()
   // 通过偏移获得参数
argp := uintptr(unsafe.Pointer(&fn)) + unsafe.Sizeof(fn)
callerpc := getcallerpc()
// 创建defer结构体
d := newdefer(siz)
if d._panic != nil {
throw("deferproc: d.panic != nil after newdefer")
}
   // 初始化
d.link = gp._defer
gp._defer = d
d.fn = fn
d.pc = callerpc
d.sp = sp
switch siz {
case 0:
// Do nothing.
case sys.PtrSize:
*(*uintptr)(deferArgs(d)) = *(*uintptr)(unsafe.Pointer(argp))
default:
memmove(deferArgs(d), unsafe.Pointer(argp), uintptr(siz))
}
// deferproc returns 0 normally.
// a deferred func that stops a panic
// makes the deferproc return 1.
// the code the compiler generates always
// checks the return value and jumps to the
// end of the function if deferproc returns != 0.
return0()
// No code can go here - the C return register has
// been set and must not be clobbered.
}

// 以下是_defer结构体
// A _defer holds an entry on the list of deferred calls.
// If you add a field here, add code to clear it in freedefer and deferProcStack
// This struct must match the code in cmd/compile/internal/gc/reflect.go:deferstruct
// and cmd/compile/internal/gc/ssa.go:(*state).call.
// Some defers will be allocated on the stack and some on the heap.
// All defers are logically part of the stack, so write barriers to
// initialize them are not required. All defers must be manually scanned,
// and for heap defers, marked.
type _defer struct {
   // siz 记录defer的参数和返回值共占多少字节
   // 会直接分配在_defer后面，在注册时保存参数，在执行完成时拷贝到调用者参数和返回值空间
siz     int32 // includes both arguments and results
// started 标记是否已经执行
   started bool
   // heap go1.13优化，标识是否为堆分配
heap    bool
// openDefer indicates that this _defer is for a frame with open-coded
// defers. We have only one defer record for the entire frame (which may
// currently have 0, 1, or more defers active).
   // openDefer 是否是open defer，通过这些信息可以找到未注册到链表的defer函数
openDefer bool
   // sp 记录调用者栈指针，可以通过它判断自己注册的defer是否已经执行完了
sp        uintptr  // sp at time of defer
   // pc deferproc的返回地址
pc        uintptr  // pc at time of defer
   // fn 要注册的funcval
fn        *funcval // can be nil for open-coded defers
   // _panic 指向当前的panic，表示这个defer是由这个panic触发的
_panic    *_panic  // panic that is running defer
   // link 链到前一个注册的defer结构体
link      *_defer
// If openDefer is true, the fields below record values about the stack
// frame and associated function that has the open-coded defer(s). sp
// above will be the sp for the frame, and pc will be address of the
// deferreturn call in the function.
   // 通过这些信息可以找到未注册到链表的defer函数
fd   unsafe.Pointer // funcdata for the function associated with the frame
varp uintptr        // value of varp for the stack frame
// framepc is the current pc associated with the stack frame. Together,
// with sp above (which is the sp associated with the stack frame),
// framepc/sp can be used as pc/sp pair to continue a stack trace via
// gentraceback().
framepc uintptr
}

defer将参数注册的时候拷贝到堆上，执行时再（将参数和返回值）拷贝回栈上

go会分配不同规格的_defer pool，执行时从空闲_defer中取一个出来用，没有合适的再进行堆分配。用完以后再放回空闲_defer pool。以避免频繁的堆分配和回收

优化

go1.12中defer存在的问题：

• defer信息主要存储在堆上，要在堆和栈上来回拷贝返回值和参数很慢

• defer结构体通过链表链起来，而链表的操作也很慢

go1.13中defer的优化：

• 减少了defer信息的堆分配。再通过deferprocStack将整个defer注册到defer链表中

• 将一般情况的defer信息存储在函数栈帧的局部变量区域

• 显示循环或者是隐式循环的defer还是需要用到go1.12中defer信息的堆分配

• 官方给出的性能提升是30％

go1.14中defer的优化：

• 在编译阶段插入代码，把defer函数的执行逻辑展开在所属函数内，避免创建defer结构体，而且不需要注册到defer链表。称为 open coded defer

• 与1.13一样不适用于循环中的defer

• 性能几乎提升了一个数量级

• open coded defer 中发生panic 或 调用runtime.Goexit()，后面未注册到的defer函数无法执行到，需要栈扫描。defer结构体中就多添加了一些字段，借助这些字段可以找到未注册到链表中的defer函数

结果就是defer变快了，但是panic变慢了

defer添加了局部变量去判断是否需要执行，需要执行的话就将标识df对应的位上或一下，如果是有条件的defer，需要根据具体条件去或df

deferprocStack

// deferprocStack queues a new deferred function with a defer record on the stack.
// The defer record must have its siz and fn fields initialized.
// All other fields can contain junk.
// The defer record must be immediately followed in memory by
// the arguments of the defer.
// Nosplit because the arguments on the stack won't be scanned
// until the defer record is spliced into the gp._defer list.
//go:nosplit
func deferprocStack(d *_defer) {
   // 获得当前 goroutine
gp := getg()
if gp.m.curg != gp {
// go code on the system stack can't defer
throw("defer on system stack")
}
// siz and fn are already set.
// The other fields are junk on entry to deferprocStack and
// are initialized here.
   // 初始化 _defer 信息
d.started = false
d.heap = false
d.openDefer = false
d.sp = getcallersp()
d.pc = getcallerpc()
d.framepc = 0
d.varp = 0
// The lines below implement:
//   d.panic = nil
//   d.fd = nil
//   d.link = gp._defer
//   gp._defer = d
// But without write barriers. The first three are writes to
// the stack so they don't need a write barrier, and furthermore
// are to uninitialized memory, so they must not use a write barrier.
// The fourth write does not require a write barrier because we
// explicitly mark all the defer structures, so we don't need to
// keep track of pointers to them with a write barrier.
*(*uintptr)(unsafe.Pointer(&d._panic)) = 0
*(*uintptr)(unsafe.Pointer(&d.fd)) = 0
*(*uintptr)(unsafe.Pointer(&d.link)) = uintptr(unsafe.Pointer(gp._defer))
*(*uintptr)(unsafe.Pointer(&gp._defer)) = uintptr(unsafe.Pointer(d))
return0()
// No code can go here - the C return register has
// been set and must not be clobbered.
}

来源：https://juejin.cn/post/7101887123539623944