libgo/go/runtime/mfinal.go - gofrontend - Git at Google

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // Garbage collector: finalizers and block profiling.

 package runtime

 import (
 	"runtime/internal/atomic"
 	"runtime/internal/sys"
 	"unsafe"
 )

 // finblock is allocated from non-GC'd memory, so any heap pointers
 // must be specially handled.
 //
 //go:notinheap
 type finblock struct {
 	alllink *finblock
 	next    *finblock
 	cnt     uint32
 	_       int32
 	fin     [(_FinBlockSize - 2*sys.PtrSize - 2*4) / unsafe.Sizeof(finalizer{})]finalizer
 }

 var finlock mutex  // protects the following variables
 var fing *g        // goroutine that runs finalizers
 var finq *finblock // list of finalizers that are to be executed
 var finc *finblock // cache of free blocks
 var finptrmask [_FinBlockSize / sys.PtrSize / 8]byte
 var fingwait bool
 var fingwake bool
 var allfin *finblock // list of all blocks

 // NOTE: Layout known to queuefinalizer.
 type finalizer struct {
 	fn  *funcval       // function to call (may be a heap pointer)
 	arg unsafe.Pointer // ptr to object (may be a heap pointer)
 	ft  *functype      // type of fn (unlikely, but may be a heap pointer)
 	ot  *ptrtype       // type of ptr to object (may be a heap pointer)
 }

 func queuefinalizer(p unsafe.Pointer, fn *funcval, ft *functype, ot *ptrtype) {
 	lock(&finlock)
 	if finq == nil || finq.cnt == uint32(len(finq.fin)) {
 		if finc == nil {
 			finc = (*finblock)(persistentalloc(_FinBlockSize, 0, &memstats.gc_sys))
 			finc.alllink = allfin
 			allfin = finc
 			if finptrmask[0] == 0 {
 				// Build pointer mask for Finalizer array in block.
 				// We allocate values of type finalizer in
 				// finblock values. Since these values are
 				// allocated by persistentalloc, they require
 				// special scanning during GC. finptrmask is a
 				// pointer mask to use while scanning.
 				// Since all the values in finalizer are
 				// pointers, just turn all bits on.
 				for i := range finptrmask {
 					finptrmask[i] = 0xff
 				}
 			}
 		}
 		block := finc
 		finc = block.next
 		block.next = finq
 		finq = block
 	}
 	f := &finq.fin[finq.cnt]
 	atomic.Xadd(&finq.cnt, +1) // Sync with markroots
 	f.fn = fn
 	f.ft = ft
 	f.ot = ot
 	f.arg = p
 	fingwake = true
 	unlock(&finlock)
 }

 //go:nowritebarrier
 func iterate_finq(callback func(*funcval, unsafe.Pointer, *functype, *ptrtype)) {
 	for fb := allfin; fb != nil; fb = fb.alllink {
 		for i := uint32(0); i < fb.cnt; i++ {
 			f := &fb.fin[i]
 			callback(f.fn, f.arg, f.ft, f.ot)
 		}
 	}
 }

 func wakefing() *g {
 	var res *g
 	lock(&finlock)
 	if fingwait && fingwake {
 		fingwait = false
 		fingwake = false
 		res = fing
 	}
 	unlock(&finlock)
 	return res
 }

 var (
 	fingCreate uint32
 )

 func createfing() {
 	// start the finalizer goroutine exactly once
 	if fingCreate == 0 && atomic.Cas(&fingCreate, 0, 1) {
 		go runfinq()
 	}
 }

 // This is the goroutine that runs all of the finalizers
 func runfinq() {
 	setSystemGoroutine()

 	var (
 		ef   eface
 		ifac iface
 	)

 	gp := getg()
 	for {
 		lock(&finlock)
 		fb := finq
 		finq = nil
 		if fb == nil {
 			fing = gp
 			fingwait = true
 			goparkunlock(&finlock, "finalizer wait", traceEvGoBlock, 1)
 			continue
 		}
 		unlock(&finlock)
 		for fb != nil {
 			for i := fb.cnt; i > 0; i-- {
 				f := &fb.fin[i-1]

 				if f.ft == nil {
 					throw("missing type in runfinq")
 				}
 				fint := f.ft.in[0]
 				var param unsafe.Pointer
 				switch fint.kind & kindMask {
 				case kindPtr:
 					// direct use of pointer
 					param = unsafe.Pointer(&f.arg)
 				case kindInterface:
 					ityp := (*interfacetype)(unsafe.Pointer(fint))
 					if len(ityp.methods) == 0 {
 						// set up with empty interface
 						ef._type = &f.ot.typ
 						ef.data = f.arg
 						param = unsafe.Pointer(&ef)
 					} else {
 						// convert to interface with methods
 						// this conversion is guaranteed to succeed - we checked in SetFinalizer
 						ifac.tab = getitab(fint, &f.ot.typ, true)
 						ifac.data = f.arg
 						param = unsafe.Pointer(&ifac)
 					}
 				default:
 					throw("bad kind in runfinq")
 				}
 				// This is not a system goroutine while
 				// running the actual finalizer.
 				// This matters because we want this
 				// goroutine to appear in a stack dump
 				// if the finalizer crashes.
 				// The gc toolchain handles this using
 				// a global variable fingRunning,
 				// but we don't need that.
 				gp.isSystemGoroutine = false
 				reflectcall(f.ft, f.fn, false, false, &param, nil)
 				gp.isSystemGoroutine = true

 				// Drop finalizer queue heap references
 				// before hiding them from markroot.
 				// This also ensures these will be
 				// clear if we reuse the finalizer.
 				f.fn = nil
 				f.arg = nil
 				f.ot = nil
 				atomic.Store(&fb.cnt, i-1)
 			}
 			next := fb.next
 			lock(&finlock)
 			fb.next = finc
 			finc = fb
 			unlock(&finlock)
 			fb = next
 		}
 	}
 }

 // SetFinalizer sets the finalizer associated with obj to the provided
 // finalizer function. When the garbage collector finds an unreachable block
 // with an associated finalizer, it clears the association and runs
 // finalizer(obj) in a separate goroutine. This makes obj reachable again,
 // but now without an associated finalizer. Assuming that SetFinalizer
 // is not called again, the next time the garbage collector sees
 // that obj is unreachable, it will free obj.
 //
 // SetFinalizer(obj, nil) clears any finalizer associated with obj.
 //
 // The argument obj must be a pointer to an object allocated by calling
 // new, by taking the address of a composite literal, or by taking the
 // address of a local variable.
 // The argument finalizer must be a function that takes a single argument
 // to which obj's type can be assigned, and can have arbitrary ignored return
 // values. If either of these is not true, SetFinalizer may abort the
 // program.
 //
 // Finalizers are run in dependency order: if A points at B, both have
 // finalizers, and they are otherwise unreachable, only the finalizer
 // for A runs; once A is freed, the finalizer for B can run.
 // If a cyclic structure includes a block with a finalizer, that
 // cycle is not guaranteed to be garbage collected and the finalizer
 // is not guaranteed to run, because there is no ordering that
 // respects the dependencies.
 //
 // The finalizer for obj is scheduled to run at some arbitrary time after
 // obj becomes unreachable.
 // There is no guarantee that finalizers will run before a program exits,
 // so typically they are useful only for releasing non-memory resources
 // associated with an object during a long-running program.
 // For example, an os.File object could use a finalizer to close the
 // associated operating system file descriptor when a program discards
 // an os.File without calling Close, but it would be a mistake
 // to depend on a finalizer to flush an in-memory I/O buffer such as a
 // bufio.Writer, because the buffer would not be flushed at program exit.
 //
 // It is not guaranteed that a finalizer will run if the size of *obj is
 // zero bytes.
 //
 // It is not guaranteed that a finalizer will run for objects allocated
 // in initializers for package-level variables. Such objects may be
 // linker-allocated, not heap-allocated.
 //
 // A finalizer may run as soon as an object becomes unreachable.
 // In order to use finalizers correctly, the program must ensure that
 // the object is reachable until it is no longer required.
 // Objects stored in global variables, or that can be found by tracing
 // pointers from a global variable, are reachable. For other objects,
 // pass the object to a call of the KeepAlive function to mark the
 // last point in the function where the object must be reachable.
 //
 // For example, if p points to a struct that contains a file descriptor d,
 // and p has a finalizer that closes that file descriptor, and if the last
 // use of p in a function is a call to syscall.Write(p.d, buf, size), then
 // p may be unreachable as soon as the program enters syscall.Write. The
 // finalizer may run at that moment, closing p.d, causing syscall.Write
 // to fail because it is writing to a closed file descriptor (or, worse,
 // to an entirely different file descriptor opened by a different goroutine).
 // To avoid this problem, call runtime.KeepAlive(p) after the call to
 // syscall.Write.
 //
 // A single goroutine runs all finalizers for a program, sequentially.
 // If a finalizer must run for a long time, it should do so by starting
 // a new goroutine.
 func SetFinalizer(obj interface{}, finalizer interface{}) {
 	if debug.sbrk != 0 {
 		// debug.sbrk never frees memory, so no finalizers run
 		// (and we don't have the data structures to record them).
 		return
 	}
 	e := efaceOf(&obj)
 	etyp := e._type
 	if etyp == nil {
 		throw("runtime.SetFinalizer: first argument is nil")
 	}
 	if etyp.kind&kindMask != kindPtr {
 		throw("runtime.SetFinalizer: first argument is " + *etyp.string + ", not pointer")
 	}
 	ot := (*ptrtype)(unsafe.Pointer(etyp))
 	if ot.elem == nil {
 		throw("nil elem type!")
 	}

 	// find the containing object
 	_, base, _ := findObject(e.data)

 	if base == nil {
 		// 0-length objects are okay.
 		if e.data == unsafe.Pointer(&zerobase) {
 			return
 		}

 		// Global initializers might be linker-allocated.
 		//	var Foo = &Object{}
 		//	func main() {
 		//		runtime.SetFinalizer(Foo, nil)
 		//	}
 		// The relevant segments are: noptrdata, data, bss, noptrbss.
 		// We cannot assume they are in any order or even contiguous,
 		// due to external linking.
 		//
 		// For gccgo we have no reliable way to detect them,
 		// so we just return.
 		return
 	}

 	if e.data != base {
 		// As an implementation detail we allow to set finalizers for an inner byte
 		// of an object if it could come from tiny alloc (see mallocgc for details).
 		if ot.elem == nil || ot.elem.kind&kindNoPointers == 0 || ot.elem.size >= maxTinySize {
 			throw("runtime.SetFinalizer: pointer not at beginning of allocated block")
 		}
 	}

 	f := efaceOf(&finalizer)
 	ftyp := f._type
 	if ftyp == nil {
 		// switch to system stack and remove finalizer
 		systemstack(func() {
 			removefinalizer(e.data)
 		})
 		return
 	}

 	if ftyp.kind&kindMask != kindFunc {
 		throw("runtime.SetFinalizer: second argument is " + *ftyp.string + ", not a function")
 	}
 	ft := (*functype)(unsafe.Pointer(ftyp))
 	if ft.dotdotdot {
 		throw("runtime.SetFinalizer: cannot pass " + *etyp.string + " to finalizer " + *ftyp.string + " because dotdotdot")
 	}
 	if len(ft.in) != 1 {
 		throw("runtime.SetFinalizer: cannot pass " + *etyp.string + " to finalizer " + *ftyp.string)
 	}
 	fint := ft.in[0]
 	switch {
 	case fint == etyp:
 		// ok - same type
 		goto okarg
 	case fint.kind&kindMask == kindPtr:
 		if (fint.uncommontype == nil || etyp.uncommontype == nil) && (*ptrtype)(unsafe.Pointer(fint)).elem == ot.elem {
 			// ok - not same type, but both pointers,
 			// one or the other is unnamed, and same element type, so assignable.
 			goto okarg
 		}
 	case fint.kind&kindMask == kindInterface:
 		ityp := (*interfacetype)(unsafe.Pointer(fint))
 		if len(ityp.methods) == 0 {
 			// ok - satisfies empty interface
 			goto okarg
 		}
 		if getitab(fint, etyp, true) == nil {
 			goto okarg
 		}
 	}
 	throw("runtime.SetFinalizer: cannot pass " + *etyp.string + " to finalizer " + *ftyp.string)
 okarg:
 	// make sure we have a finalizer goroutine
 	createfing()

 	systemstack(func() {
 		data := f.data
 		if !isDirectIface(ftyp) {
 			data = *(*unsafe.Pointer)(data)
 		}
 		if !addfinalizer(e.data, (*funcval)(data), ft, ot) {
 			throw("runtime.SetFinalizer: finalizer already set")
 		}
 	})
 }

 // Look up pointer v in heap. Return the span containing the object,
 // the start of the object, and the size of the object. If the object
 // does not exist, return nil, nil, 0.
 func findObject(v unsafe.Pointer) (s *mspan, x unsafe.Pointer, n uintptr) {
 	c := gomcache()
 	c.local_nlookup++
 	if sys.PtrSize == 4 && c.local_nlookup >= 1<<30 {
 		// purge cache stats to prevent overflow
 		lock(&mheap_.lock)
 		purgecachedstats(c)
 		unlock(&mheap_.lock)
 	}

 	// find span
 	arena_start := mheap_.arena_start
 	arena_used := mheap_.arena_used
 	if uintptr(v) < arena_start || uintptr(v) >= arena_used {
 		return
 	}
 	p := uintptr(v) >> pageShift
 	q := p - arena_start>>pageShift
 	s = mheap_.spans[q]
 	if s == nil {
 		return
 	}
 	x = unsafe.Pointer(s.base())

 	if uintptr(v) < uintptr(x) || uintptr(v) >= uintptr(unsafe.Pointer(s.limit)) || s.state != mSpanInUse {
 		s = nil
 		x = nil
 		return
 	}

 	n = s.elemsize
 	if s.sizeclass != 0 {
 		x = add(x, (uintptr(v)-uintptr(x))/n*n)
 	}
 	return
 }

 // Mark KeepAlive as noinline so that the current compiler will ensure
 // that the argument is alive at the point of the function call.
 // If it were inlined, it would disappear, and there would be nothing
 // keeping the argument alive. Perhaps a future compiler will recognize
 // runtime.KeepAlive specially and do something more efficient.
 //go:noinline

 // KeepAlive marks its argument as currently reachable.
 // This ensures that the object is not freed, and its finalizer is not run,
 // before the point in the program where KeepAlive is called.
 //
 // A very simplified example showing where KeepAlive is required:
 // 	type File struct { d int }
 // 	d, err := syscall.Open("/file/path", syscall.O_RDONLY, 0)
 // 	// ... do something if err != nil ...
 // 	p := &File{d}
 // 	runtime.SetFinalizer(p, func(p *File) { syscall.Close(p.d) })
 // 	var buf [10]byte
 // 	n, err := syscall.Read(p.d, buf[:])
 // 	// Ensure p is not finalized until Read returns.
 // 	runtime.KeepAlive(p)
 // 	// No more uses of p after this point.
 //
 // Without the KeepAlive call, the finalizer could run at the start of
 // syscall.Read, closing the file descriptor before syscall.Read makes
 // the actual system call.
 func KeepAlive(interface{}) {}
	// Copyright 2009 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// Garbage collector: finalizers and block profiling.

	package runtime

	import (
	"runtime/internal/atomic"
	"runtime/internal/sys"
	"unsafe"
	)

	// finblock is allocated from non-GC'd memory, so any heap pointers
	// must be specially handled.
	//
	//go:notinheap
	type finblock struct {
	alllink *finblock
	next *finblock
	cnt uint32
	_ int32
	fin [(_FinBlockSize - 2sys.PtrSize - 24) / unsafe.Sizeof(finalizer{})]finalizer
	}

	var finlock mutex // protects the following variables
	var fing *g // goroutine that runs finalizers
	var finq *finblock // list of finalizers that are to be executed
	var finc *finblock // cache of free blocks
	var finptrmask [_FinBlockSize / sys.PtrSize / 8]byte
	var fingwait bool
	var fingwake bool
	var allfin *finblock // list of all blocks

	// NOTE: Layout known to queuefinalizer.
	type finalizer struct {
	fn *funcval // function to call (may be a heap pointer)
	arg unsafe.Pointer // ptr to object (may be a heap pointer)
	ft *functype // type of fn (unlikely, but may be a heap pointer)
	ot *ptrtype // type of ptr to object (may be a heap pointer)
	}

	func queuefinalizer(p unsafe.Pointer, fn funcval, ft functype, ot *ptrtype) {
	lock(&finlock)
	if finq == nil \|\| finq.cnt == uint32(len(finq.fin)) {
	if finc == nil {
	finc = (*finblock)(persistentalloc(_FinBlockSize, 0, &memstats.gc_sys))
	finc.alllink = allfin
	allfin = finc
	if finptrmask[0] == 0 {
	// Build pointer mask for Finalizer array in block.
	// We allocate values of type finalizer in
	// finblock values. Since these values are
	// allocated by persistentalloc, they require
	// special scanning during GC. finptrmask is a
	// pointer mask to use while scanning.
	// Since all the values in finalizer are
	// pointers, just turn all bits on.
	for i := range finptrmask {
	finptrmask[i] = 0xff
	}
	}
	}
	block := finc
	finc = block.next
	block.next = finq
	finq = block
	}
	f := &finq.fin[finq.cnt]
	atomic.Xadd(&finq.cnt, +1) // Sync with markroots
	f.fn = fn
	f.ft = ft
	f.ot = ot
	f.arg = p
	fingwake = true
	unlock(&finlock)
	}

	//go:nowritebarrier
	func iterate_finq(callback func(funcval, unsafe.Pointer, functype, *ptrtype)) {
	for fb := allfin; fb != nil; fb = fb.alllink {
	for i := uint32(0); i < fb.cnt; i++ {
	f := &fb.fin[i]
	callback(f.fn, f.arg, f.ft, f.ot)
	}
	}
	}

	func wakefing() *g {
	var res *g
	lock(&finlock)
	if fingwait && fingwake {
	fingwait = false
	fingwake = false
	res = fing
	}
	unlock(&finlock)
	return res
	}

	var (
	fingCreate uint32
	)

	func createfing() {
	// start the finalizer goroutine exactly once
	if fingCreate == 0 && atomic.Cas(&fingCreate, 0, 1) {
	go runfinq()
	}
	}

	// This is the goroutine that runs all of the finalizers
	func runfinq() {
	setSystemGoroutine()

	var (
	ef eface
	ifac iface
	)

	gp := getg()
	for {
	lock(&finlock)
	fb := finq
	finq = nil
	if fb == nil {
	fing = gp
	fingwait = true
	goparkunlock(&finlock, "finalizer wait", traceEvGoBlock, 1)
	continue
	}
	unlock(&finlock)
	for fb != nil {
	for i := fb.cnt; i > 0; i-- {
	f := &fb.fin[i-1]

	if f.ft == nil {
	throw("missing type in runfinq")
	}
	fint := f.ft.in[0]
	var param unsafe.Pointer
	switch fint.kind & kindMask {
	case kindPtr:
	// direct use of pointer
	param = unsafe.Pointer(&f.arg)
	case kindInterface:
	ityp := (*interfacetype)(unsafe.Pointer(fint))
	if len(ityp.methods) == 0 {
	// set up with empty interface
	ef._type = &f.ot.typ
	ef.data = f.arg
	param = unsafe.Pointer(&ef)
	} else {
	// convert to interface with methods
	// this conversion is guaranteed to succeed - we checked in SetFinalizer
	ifac.tab = getitab(fint, &f.ot.typ, true)
	ifac.data = f.arg
	param = unsafe.Pointer(&ifac)
	}
	default:
	throw("bad kind in runfinq")
	}
	// This is not a system goroutine while
	// running the actual finalizer.
	// This matters because we want this
	// goroutine to appear in a stack dump
	// if the finalizer crashes.
	// The gc toolchain handles this using
	// a global variable fingRunning,
	// but we don't need that.
	gp.isSystemGoroutine = false
	reflectcall(f.ft, f.fn, false, false, &param, nil)
	gp.isSystemGoroutine = true

	// Drop finalizer queue heap references
	// before hiding them from markroot.
	// This also ensures these will be
	// clear if we reuse the finalizer.
	f.fn = nil
	f.arg = nil
	f.ot = nil
	atomic.Store(&fb.cnt, i-1)
	}
	next := fb.next
	lock(&finlock)
	fb.next = finc
	finc = fb
	unlock(&finlock)
	fb = next
	}
	}
	}

	// SetFinalizer sets the finalizer associated with obj to the provided
	// finalizer function. When the garbage collector finds an unreachable block
	// with an associated finalizer, it clears the association and runs
	// finalizer(obj) in a separate goroutine. This makes obj reachable again,
	// but now without an associated finalizer. Assuming that SetFinalizer
	// is not called again, the next time the garbage collector sees
	// that obj is unreachable, it will free obj.
	//
	// SetFinalizer(obj, nil) clears any finalizer associated with obj.
	//
	// The argument obj must be a pointer to an object allocated by calling
	// new, by taking the address of a composite literal, or by taking the
	// address of a local variable.
	// The argument finalizer must be a function that takes a single argument
	// to which obj's type can be assigned, and can have arbitrary ignored return
	// values. If either of these is not true, SetFinalizer may abort the
	// program.
	//
	// Finalizers are run in dependency order: if A points at B, both have
	// finalizers, and they are otherwise unreachable, only the finalizer
	// for A runs; once A is freed, the finalizer for B can run.
	// If a cyclic structure includes a block with a finalizer, that
	// cycle is not guaranteed to be garbage collected and the finalizer
	// is not guaranteed to run, because there is no ordering that
	// respects the dependencies.
	//
	// The finalizer for obj is scheduled to run at some arbitrary time after
	// obj becomes unreachable.
	// There is no guarantee that finalizers will run before a program exits,
	// so typically they are useful only for releasing non-memory resources
	// associated with an object during a long-running program.
	// For example, an os.File object could use a finalizer to close the
	// associated operating system file descriptor when a program discards
	// an os.File without calling Close, but it would be a mistake
	// to depend on a finalizer to flush an in-memory I/O buffer such as a
	// bufio.Writer, because the buffer would not be flushed at program exit.
	//
	// It is not guaranteed that a finalizer will run if the size of *obj is
	// zero bytes.
	//
	// It is not guaranteed that a finalizer will run for objects allocated
	// in initializers for package-level variables. Such objects may be
	// linker-allocated, not heap-allocated.
	//
	// A finalizer may run as soon as an object becomes unreachable.
	// In order to use finalizers correctly, the program must ensure that
	// the object is reachable until it is no longer required.
	// Objects stored in global variables, or that can be found by tracing
	// pointers from a global variable, are reachable. For other objects,
	// pass the object to a call of the KeepAlive function to mark the
	// last point in the function where the object must be reachable.
	//
	// For example, if p points to a struct that contains a file descriptor d,
	// and p has a finalizer that closes that file descriptor, and if the last
	// use of p in a function is a call to syscall.Write(p.d, buf, size), then
	// p may be unreachable as soon as the program enters syscall.Write. The
	// finalizer may run at that moment, closing p.d, causing syscall.Write
	// to fail because it is writing to a closed file descriptor (or, worse,
	// to an entirely different file descriptor opened by a different goroutine).
	// To avoid this problem, call runtime.KeepAlive(p) after the call to
	// syscall.Write.
	//
	// A single goroutine runs all finalizers for a program, sequentially.
	// If a finalizer must run for a long time, it should do so by starting
	// a new goroutine.
	func SetFinalizer(obj interface{}, finalizer interface{}) {
	if debug.sbrk != 0 {
	// debug.sbrk never frees memory, so no finalizers run
	// (and we don't have the data structures to record them).
	return
	}
	e := efaceOf(&obj)
	etyp := e._type
	if etyp == nil {
	throw("runtime.SetFinalizer: first argument is nil")
	}
	if etyp.kind&kindMask != kindPtr {
	throw("runtime.SetFinalizer: first argument is " + *etyp.string + ", not pointer")
	}
	ot := (*ptrtype)(unsafe.Pointer(etyp))
	if ot.elem == nil {
	throw("nil elem type!")
	}

	// find the containing object
	_, base, _ := findObject(e.data)

	if base == nil {
	// 0-length objects are okay.
	if e.data == unsafe.Pointer(&zerobase) {
	return
	}

	// Global initializers might be linker-allocated.
	// var Foo = &Object{}
	// func main() {
	// runtime.SetFinalizer(Foo, nil)
	// }
	// The relevant segments are: noptrdata, data, bss, noptrbss.
	// We cannot assume they are in any order or even contiguous,
	// due to external linking.
	//
	// For gccgo we have no reliable way to detect them,
	// so we just return.
	return
	}

	if e.data != base {
	// As an implementation detail we allow to set finalizers for an inner byte
	// of an object if it could come from tiny alloc (see mallocgc for details).
	if ot.elem == nil \|\| ot.elem.kind&kindNoPointers == 0 \|\| ot.elem.size >= maxTinySize {
	throw("runtime.SetFinalizer: pointer not at beginning of allocated block")
	}
	}

	f := efaceOf(&finalizer)
	ftyp := f._type
	if ftyp == nil {
	// switch to system stack and remove finalizer
	systemstack(func() {
	removefinalizer(e.data)
	})
	return
	}

	if ftyp.kind&kindMask != kindFunc {
	throw("runtime.SetFinalizer: second argument is " + *ftyp.string + ", not a function")
	}
	ft := (*functype)(unsafe.Pointer(ftyp))
	if ft.dotdotdot {
	throw("runtime.SetFinalizer: cannot pass " + etyp.string + " to finalizer " + ftyp.string + " because dotdotdot")
	}
	if len(ft.in) != 1 {
	throw("runtime.SetFinalizer: cannot pass " + etyp.string + " to finalizer " + ftyp.string)
	}
	fint := ft.in[0]
	switch {
	case fint == etyp:
	// ok - same type
	goto okarg
	case fint.kind&kindMask == kindPtr:
	if (fint.uncommontype == nil \|\| etyp.uncommontype == nil) && (*ptrtype)(unsafe.Pointer(fint)).elem == ot.elem {
	// ok - not same type, but both pointers,
	// one or the other is unnamed, and same element type, so assignable.
	goto okarg
	}
	case fint.kind&kindMask == kindInterface:
	ityp := (*interfacetype)(unsafe.Pointer(fint))
	if len(ityp.methods) == 0 {
	// ok - satisfies empty interface
	goto okarg
	}
	if getitab(fint, etyp, true) == nil {
	goto okarg
	}
	}
	throw("runtime.SetFinalizer: cannot pass " + etyp.string + " to finalizer " + ftyp.string)
	okarg:
	// make sure we have a finalizer goroutine
	createfing()

	systemstack(func() {
	data := f.data
	if !isDirectIface(ftyp) {
	data = (unsafe.Pointer)(data)
	}
	if !addfinalizer(e.data, (*funcval)(data), ft, ot) {
	throw("runtime.SetFinalizer: finalizer already set")
	}
	})
	}

	// Look up pointer v in heap. Return the span containing the object,
	// the start of the object, and the size of the object. If the object
	// does not exist, return nil, nil, 0.
	func findObject(v unsafe.Pointer) (s *mspan, x unsafe.Pointer, n uintptr) {
	c := gomcache()
	c.local_nlookup++
	if sys.PtrSize == 4 && c.local_nlookup >= 1<<30 {
	// purge cache stats to prevent overflow
	lock(&mheap_.lock)
	purgecachedstats(c)
	unlock(&mheap_.lock)
	}

	// find span
	arena_start := mheap_.arena_start
	arena_used := mheap_.arena_used
	if uintptr(v) < arena_start \|\| uintptr(v) >= arena_used {
	return
	}
	p := uintptr(v) >> pageShift
	q := p - arena_start>>pageShift
	s = mheap_.spans[q]
	if s == nil {
	return
	}
	x = unsafe.Pointer(s.base())

	if uintptr(v) < uintptr(x) \|\| uintptr(v) >= uintptr(unsafe.Pointer(s.limit)) \|\| s.state != mSpanInUse {
	s = nil
	x = nil
	return
	}

	n = s.elemsize
	if s.sizeclass != 0 {
	x = add(x, (uintptr(v)-uintptr(x))/n*n)
	}
	return
	}

	// Mark KeepAlive as noinline so that the current compiler will ensure
	// that the argument is alive at the point of the function call.
	// If it were inlined, it would disappear, and there would be nothing
	// keeping the argument alive. Perhaps a future compiler will recognize
	// runtime.KeepAlive specially and do something more efficient.
	//go:noinline

	// KeepAlive marks its argument as currently reachable.
	// This ensures that the object is not freed, and its finalizer is not run,
	// before the point in the program where KeepAlive is called.
	//
	// A very simplified example showing where KeepAlive is required:
	// type File struct { d int }
	// d, err := syscall.Open("/file/path", syscall.O_RDONLY, 0)
	// // ... do something if err != nil ...
	// p := &File{d}
	// runtime.SetFinalizer(p, func(p *File) { syscall.Close(p.d) })
	// var buf [10]byte
	// n, err := syscall.Read(p.d, buf[:])
	// // Ensure p is not finalized until Read returns.
	// runtime.KeepAlive(p)
	// // No more uses of p after this point.
	//
	// Without the KeepAlive call, the finalizer could run at the start of
	// syscall.Read, closing the file descriptor before syscall.Read makes
	// the actual system call.
	func KeepAlive(interface{}) {}