Golang的关键字defer的使用方法

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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.
}

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