src/runtime/select.go - go - 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.

 package runtime

 // This file contains the implementation of Go select statements.

 import "unsafe"

 const (
 	debugSelect = false
 )

 var (
 	chansendpc = funcPC(chansend)
 	chanrecvpc = funcPC(chanrecv)
 )

 func selectsize(size uintptr) uintptr {
 	selsize := unsafe.Sizeof(_select{}) +
 		(size-1)*unsafe.Sizeof(_select{}.scase[0]) +
 		size*unsafe.Sizeof(*_select{}.lockorder) +
 		size*unsafe.Sizeof(*_select{}.pollorder)
 	return round(selsize, _Int64Align)
 }

 func newselect(sel *_select, selsize int64, size int32) {
 	if selsize != int64(selectsize(uintptr(size))) {
 		print("runtime: bad select size ", selsize, ", want ", selectsize(uintptr(size)), "\n")
 		gothrow("bad select size")
 	}
 	sel.tcase = uint16(size)
 	sel.ncase = 0
 	sel.lockorder = (**hchan)(add(unsafe.Pointer(&sel.scase), uintptr(size)*unsafe.Sizeof(_select{}.scase[0])))
 	sel.pollorder = (*uint16)(add(unsafe.Pointer(sel.lockorder), uintptr(size)*unsafe.Sizeof(*_select{}.lockorder)))

 	if debugSelect {
 		print("newselect s=", sel, " size=", size, "\n")
 	}
 }

 //go:nosplit
 func selectsend(sel *_select, c *hchan, elem unsafe.Pointer) (selected bool) {
 	// nil cases do not compete
 	if c != nil {
 		selectsendImpl(sel, c, getcallerpc(unsafe.Pointer(&sel)), elem, uintptr(unsafe.Pointer(&selected))-uintptr(unsafe.Pointer(&sel)))
 	}
 	return
 }

 // cut in half to give stack a chance to split
 func selectsendImpl(sel *_select, c *hchan, pc uintptr, elem unsafe.Pointer, so uintptr) {
 	i := sel.ncase
 	if i >= sel.tcase {
 		gothrow("selectsend: too many cases")
 	}
 	sel.ncase = i + 1
 	cas := (*scase)(add(unsafe.Pointer(&sel.scase), uintptr(i)*unsafe.Sizeof(sel.scase[0])))

 	cas.pc = pc
 	cas._chan = c
 	cas.so = uint16(so)
 	cas.kind = _CaseSend
 	cas.elem = elem

 	if debugSelect {
 		print("selectsend s=", sel, " pc=", hex(cas.pc), " chan=", cas._chan, " so=", cas.so, "\n")
 	}
 }

 //go:nosplit
 func selectrecv(sel *_select, c *hchan, elem unsafe.Pointer) (selected bool) {
 	// nil cases do not compete
 	if c != nil {
 		selectrecvImpl(sel, c, getcallerpc(unsafe.Pointer(&sel)), elem, nil, uintptr(unsafe.Pointer(&selected))-uintptr(unsafe.Pointer(&sel)))
 	}
 	return
 }

 //go:nosplit
 func selectrecv2(sel *_select, c *hchan, elem unsafe.Pointer, received *bool) (selected bool) {
 	// nil cases do not compete
 	if c != nil {
 		selectrecvImpl(sel, c, getcallerpc(unsafe.Pointer(&sel)), elem, received, uintptr(unsafe.Pointer(&selected))-uintptr(unsafe.Pointer(&sel)))
 	}
 	return
 }

 func selectrecvImpl(sel *_select, c *hchan, pc uintptr, elem unsafe.Pointer, received *bool, so uintptr) {
 	i := sel.ncase
 	if i >= sel.tcase {
 		gothrow("selectrecv: too many cases")
 	}
 	sel.ncase = i + 1
 	cas := (*scase)(add(unsafe.Pointer(&sel.scase), uintptr(i)*unsafe.Sizeof(sel.scase[0])))
 	cas.pc = pc
 	cas._chan = c
 	cas.so = uint16(so)
 	cas.kind = _CaseRecv
 	cas.elem = elem
 	cas.receivedp = received

 	if debugSelect {
 		print("selectrecv s=", sel, " pc=", hex(cas.pc), " chan=", cas._chan, " so=", cas.so, "\n")
 	}
 }

 //go:nosplit
 func selectdefault(sel *_select) (selected bool) {
 	selectdefaultImpl(sel, getcallerpc(unsafe.Pointer(&sel)), uintptr(unsafe.Pointer(&selected))-uintptr(unsafe.Pointer(&sel)))
 	return
 }

 func selectdefaultImpl(sel *_select, callerpc uintptr, so uintptr) {
 	i := sel.ncase
 	if i >= sel.tcase {
 		gothrow("selectdefault: too many cases")
 	}
 	sel.ncase = i + 1
 	cas := (*scase)(add(unsafe.Pointer(&sel.scase), uintptr(i)*unsafe.Sizeof(sel.scase[0])))
 	cas.pc = callerpc
 	cas._chan = nil
 	cas.so = uint16(so)
 	cas.kind = _CaseDefault

 	if debugSelect {
 		print("selectdefault s=", sel, " pc=", hex(cas.pc), " so=", cas.so, "\n")
 	}
 }

 func sellock(sel *_select) {
 	lockslice := sliceStruct{unsafe.Pointer(sel.lockorder), int(sel.ncase), int(sel.ncase)}
 	lockorder := *(*[]*hchan)(unsafe.Pointer(&lockslice))
 	var c *hchan
 	for _, c0 := range lockorder {
 		if c0 != nil && c0 != c {
 			c = c0
 			lock(&c.lock)
 		}
 	}
 }

 func selunlock(sel *_select) {
 	// We must be very careful here to not touch sel after we have unlocked
 	// the last lock, because sel can be freed right after the last unlock.
 	// Consider the following situation.
 	// First M calls runtime·park() in runtime·selectgo() passing the sel.
 	// Once runtime·park() has unlocked the last lock, another M makes
 	// the G that calls select runnable again and schedules it for execution.
 	// When the G runs on another M, it locks all the locks and frees sel.
 	// Now if the first M touches sel, it will access freed memory.
 	n := int(sel.ncase)
 	r := 0
 	lockslice := sliceStruct{unsafe.Pointer(sel.lockorder), n, n}
 	lockorder := *(*[]*hchan)(unsafe.Pointer(&lockslice))
 	// skip the default case
 	if n > 0 && lockorder[0] == nil {
 		r = 1
 	}
 	for i := n - 1; i >= r; i-- {
 		c := lockorder[i]
 		if i > 0 && c == lockorder[i-1] {
 			continue // will unlock it on the next iteration
 		}
 		unlock(&c.lock)
 	}
 }

 func selparkcommit(gp *g, sel *_select) bool {
 	selunlock(sel)
 	return true
 }

 func block() {
 	gopark(nil, nil, "select (no cases)") // forever
 }

 // overwrites return pc on stack to signal which case of the select
 // to run, so cannot appear at the top of a split stack.
 //go:nosplit
 func selectgo(sel *_select) {
 	pc, offset := selectgoImpl(sel)
 	*(*bool)(add(unsafe.Pointer(&sel), uintptr(offset))) = true
 	setcallerpc(unsafe.Pointer(&sel), pc)
 }

 // selectgoImpl returns scase.pc and scase.so for the select
 // case which fired.
 func selectgoImpl(sel *_select) (uintptr, uint16) {
 	if debugSelect {
 		print("select: sel=", sel, "\n")
 	}

 	scaseslice := sliceStruct{unsafe.Pointer(&sel.scase), int(sel.ncase), int(sel.ncase)}
 	scases := *(*[]scase)(unsafe.Pointer(&scaseslice))

 	var t0 int64
 	if blockprofilerate > 0 {
 		t0 = cputicks()
 		for i := 0; i < int(sel.ncase); i++ {
 			scases[i].releasetime = -1
 		}
 	}

 	// The compiler rewrites selects that statically have
 	// only 0 or 1 cases plus default into simpler constructs.
 	// The only way we can end up with such small sel.ncase
 	// values here is for a larger select in which most channels
 	// have been nilled out.  The general code handles those
 	// cases correctly, and they are rare enough not to bother
 	// optimizing (and needing to test).

 	// generate permuted order
 	pollslice := sliceStruct{unsafe.Pointer(sel.pollorder), int(sel.ncase), int(sel.ncase)}
 	pollorder := *(*[]uint16)(unsafe.Pointer(&pollslice))
 	for i := 0; i < int(sel.ncase); i++ {
 		pollorder[i] = uint16(i)
 	}
 	for i := 1; i < int(sel.ncase); i++ {
 		o := pollorder[i]
 		j := int(fastrand1()) % (i + 1)
 		pollorder[i] = pollorder[j]
 		pollorder[j] = o
 	}

 	// sort the cases by Hchan address to get the locking order.
 	// simple heap sort, to guarantee n log n time and constant stack footprint.
 	lockslice := sliceStruct{unsafe.Pointer(sel.lockorder), int(sel.ncase), int(sel.ncase)}
 	lockorder := *(*[]*hchan)(unsafe.Pointer(&lockslice))
 	for i := 0; i < int(sel.ncase); i++ {
 		j := i
 		c := scases[j]._chan
 		for j > 0 && lockorder[(j-1)/2].sortkey() < c.sortkey() {
 			k := (j - 1) / 2
 			lockorder[j] = lockorder[k]
 			j = k
 		}
 		lockorder[j] = c
 	}
 	for i := int(sel.ncase) - 1; i >= 0; i-- {
 		c := lockorder[i]
 		lockorder[i] = lockorder[0]
 		j := 0
 		for {
 			k := j*2 + 1
 			if k >= i {
 				break
 			}
 			if k+1 < i && lockorder[k].sortkey() < lockorder[k+1].sortkey() {
 				k++
 			}
 			if c.sortkey() < lockorder[k].sortkey() {
 				lockorder[j] = lockorder[k]
 				j = k
 				continue
 			}
 			break
 		}
 		lockorder[j] = c
 	}
 	/*
 		for i := 0; i+1 < int(sel.ncase); i++ {
 			if lockorder[i].sortkey() > lockorder[i+1].sortkey() {
 				print("i=", i, " x=", lockorder[i], " y=", lockorder[i+1], "\n")
 				gothrow("select: broken sort")
 			}
 		}
 	*/

 	// lock all the channels involved in the select
 	sellock(sel)

 	var (
 		gp     *g
 		done   uint32
 		sg     *sudog
 		c      *hchan
 		k      *scase
 		sglist *sudog
 		sgnext *sudog
 	)

 loop:
 	// pass 1 - look for something already waiting
 	var dfl *scase
 	var cas *scase
 	for i := 0; i < int(sel.ncase); i++ {
 		cas = &scases[pollorder[i]]
 		c = cas._chan

 		switch cas.kind {
 		case _CaseRecv:
 			if c.dataqsiz > 0 {
 				if c.qcount > 0 {
 					goto asyncrecv
 				}
 			} else {
 				sg = c.sendq.dequeue()
 				if sg != nil {
 					goto syncrecv
 				}
 			}
 			if c.closed != 0 {
 				goto rclose
 			}

 		case _CaseSend:
 			if raceenabled {
 				racereadpc(unsafe.Pointer(c), cas.pc, chansendpc)
 			}
 			if c.closed != 0 {
 				goto sclose
 			}
 			if c.dataqsiz > 0 {
 				if c.qcount < c.dataqsiz {
 					goto asyncsend
 				}
 			} else {
 				sg = c.recvq.dequeue()
 				if sg != nil {
 					goto syncsend
 				}
 			}

 		case _CaseDefault:
 			dfl = cas
 		}
 	}

 	if dfl != nil {
 		selunlock(sel)
 		cas = dfl
 		goto retc
 	}

 	// pass 2 - enqueue on all chans
 	gp = getg()
 	done = 0
 	for i := 0; i < int(sel.ncase); i++ {
 		cas = &scases[pollorder[i]]
 		c = cas._chan
 		sg := acquireSudog()
 		sg.g = gp
 		// Note: selectdone is adjusted for stack copies in stack.c:adjustsudogs
 		sg.selectdone = (*uint32)(noescape(unsafe.Pointer(&done)))
 		sg.elem = cas.elem
 		sg.releasetime = 0
 		if t0 != 0 {
 			sg.releasetime = -1
 		}
 		sg.waitlink = gp.waiting
 		gp.waiting = sg

 		switch cas.kind {
 		case _CaseRecv:
 			c.recvq.enqueue(sg)

 		case _CaseSend:
 			c.sendq.enqueue(sg)
 		}
 	}

 	// wait for someone to wake us up
 	gp.param = nil
 	gopark(unsafe.Pointer(funcPC(selparkcommit)), unsafe.Pointer(sel), "select")

 	// someone woke us up
 	sellock(sel)
 	sg = (*sudog)(gp.param)
 	gp.param = nil

 	// pass 3 - dequeue from unsuccessful chans
 	// otherwise they stack up on quiet channels
 	// record the successful case, if any.
 	// We singly-linked up the SudoGs in case order, so when
 	// iterating through the linked list they are in reverse order.
 	cas = nil
 	sglist = gp.waiting
 	// Clear all selectdone and elem before unlinking from gp.waiting.
 	// They must be cleared before being put back into the sudog cache.
 	// Clear before unlinking, because if a stack copy happens after the unlink,
 	// they will not be updated, they will be left pointing to the old stack,
 	// which creates dangling pointers, which may be detected by the
 	// garbage collector.
 	for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
 		sg1.selectdone = nil
 		sg1.elem = nil
 	}
 	gp.waiting = nil
 	for i := int(sel.ncase) - 1; i >= 0; i-- {
 		k = &scases[pollorder[i]]
 		if sglist.releasetime > 0 {
 			k.releasetime = sglist.releasetime
 		}
 		if sg == sglist {
 			cas = k
 		} else {
 			c = k._chan
 			if k.kind == _CaseSend {
 				c.sendq.dequeueSudoG(sglist)
 			} else {
 				c.recvq.dequeueSudoG(sglist)
 			}
 		}
 		sgnext = sglist.waitlink
 		sglist.waitlink = nil
 		releaseSudog(sglist)
 		sglist = sgnext
 	}

 	if cas == nil {
 		goto loop
 	}

 	c = cas._chan

 	if c.dataqsiz > 0 {
 		gothrow("selectgo: shouldn't happen")
 	}

 	if debugSelect {
 		print("wait-return: sel=", sel, " c=", c, " cas=", cas, " kind=", cas.kind, "\n")
 	}

 	if cas.kind == _CaseRecv {
 		if cas.receivedp != nil {
 			*cas.receivedp = true
 		}
 	}

 	if raceenabled {
 		if cas.kind == _CaseRecv && cas.elem != nil {
 			raceWriteObjectPC(c.elemtype, cas.elem, cas.pc, chanrecvpc)
 		} else if cas.kind == _CaseSend {
 			raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
 		}
 	}

 	selunlock(sel)
 	goto retc

 asyncrecv:
 	// can receive from buffer
 	if raceenabled {
 		if cas.elem != nil {
 			raceWriteObjectPC(c.elemtype, cas.elem, cas.pc, chanrecvpc)
 		}
 		raceacquire(chanbuf(c, c.recvx))
 		racerelease(chanbuf(c, c.recvx))
 	}
 	if cas.receivedp != nil {
 		*cas.receivedp = true
 	}
 	if cas.elem != nil {
 		memmove(cas.elem, chanbuf(c, c.recvx), uintptr(c.elemsize))
 	}
 	memclr(chanbuf(c, c.recvx), uintptr(c.elemsize))
 	c.recvx++
 	if c.recvx == c.dataqsiz {
 		c.recvx = 0
 	}
 	c.qcount--
 	sg = c.sendq.dequeue()
 	if sg != nil {
 		gp = sg.g
 		selunlock(sel)
 		if sg.releasetime != 0 {
 			sg.releasetime = cputicks()
 		}
 		goready(gp)
 	} else {
 		selunlock(sel)
 	}
 	goto retc

 asyncsend:
 	// can send to buffer
 	if raceenabled {
 		raceacquire(chanbuf(c, c.sendx))
 		racerelease(chanbuf(c, c.sendx))
 		raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
 	}
 	memmove(chanbuf(c, c.sendx), cas.elem, uintptr(c.elemsize))
 	c.sendx++
 	if c.sendx == c.dataqsiz {
 		c.sendx = 0
 	}
 	c.qcount++
 	sg = c.recvq.dequeue()
 	if sg != nil {
 		gp = sg.g
 		selunlock(sel)
 		if sg.releasetime != 0 {
 			sg.releasetime = cputicks()
 		}
 		goready(gp)
 	} else {
 		selunlock(sel)
 	}
 	goto retc

 syncrecv:
 	// can receive from sleeping sender (sg)
 	if raceenabled {
 		if cas.elem != nil {
 			raceWriteObjectPC(c.elemtype, cas.elem, cas.pc, chanrecvpc)
 		}
 		racesync(c, sg)
 	}
 	selunlock(sel)
 	if debugSelect {
 		print("syncrecv: sel=", sel, " c=", c, "\n")
 	}
 	if cas.receivedp != nil {
 		*cas.receivedp = true
 	}
 	if cas.elem != nil {
 		memmove(cas.elem, sg.elem, uintptr(c.elemsize))
 	}
 	sg.elem = nil
 	gp = sg.g
 	gp.param = unsafe.Pointer(sg)
 	if sg.releasetime != 0 {
 		sg.releasetime = cputicks()
 	}
 	goready(gp)
 	goto retc

 rclose:
 	// read at end of closed channel
 	selunlock(sel)
 	if cas.receivedp != nil {
 		*cas.receivedp = false
 	}
 	if cas.elem != nil {
 		memclr(cas.elem, uintptr(c.elemsize))
 	}
 	if raceenabled {
 		raceacquire(unsafe.Pointer(c))
 	}
 	goto retc

 syncsend:
 	// can send to sleeping receiver (sg)
 	if raceenabled {
 		raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
 		racesync(c, sg)
 	}
 	selunlock(sel)
 	if debugSelect {
 		print("syncsend: sel=", sel, " c=", c, "\n")
 	}
 	if sg.elem != nil {
 		memmove(sg.elem, cas.elem, uintptr(c.elemsize))
 	}
 	sg.elem = nil
 	gp = sg.g
 	gp.param = unsafe.Pointer(sg)
 	if sg.releasetime != 0 {
 		sg.releasetime = cputicks()
 	}
 	goready(gp)

 retc:
 	if cas.releasetime > 0 {
 		blockevent(cas.releasetime-t0, 2)
 	}
 	return cas.pc, cas.so

 sclose:
 	// send on closed channel
 	selunlock(sel)
 	panic("send on closed channel")
 }

 func (c *hchan) sortkey() uintptr {
 	// TODO(khr): if we have a moving garbage collector, we'll need to
 	// change this function.
 	return uintptr(unsafe.Pointer(c))
 }

 // A runtimeSelect is a single case passed to rselect.
 // This must match ../reflect/value.go:/runtimeSelect
 type runtimeSelect struct {
 	dir selectDir
 	typ unsafe.Pointer // channel type (not used here)
 	ch  *hchan         // channel
 	val unsafe.Pointer // ptr to data (SendDir) or ptr to receive buffer (RecvDir)
 }

 // These values must match ../reflect/value.go:/SelectDir.
 type selectDir int

 const (
 	_             selectDir = iota
 	selectSend              // case Chan <- Send
 	selectRecv              // case <-Chan:
 	selectDefault           // default
 )

 func reflect_rselect(cases []runtimeSelect) (chosen int, recvOK bool) {
 	// flagNoScan is safe here, because all objects are also referenced from cases.
 	size := selectsize(uintptr(len(cases)))
 	sel := (*_select)(mallocgc(size, nil, flagNoScan))
 	newselect(sel, int64(size), int32(len(cases)))
 	r := new(bool)
 	for i := range cases {
 		rc := &cases[i]
 		switch rc.dir {
 		case selectDefault:
 			selectdefaultImpl(sel, uintptr(i), 0)
 		case selectSend:
 			if rc.ch == nil {
 				break
 			}
 			selectsendImpl(sel, rc.ch, uintptr(i), rc.val, 0)
 		case selectRecv:
 			if rc.ch == nil {
 				break
 			}
 			selectrecvImpl(sel, rc.ch, uintptr(i), rc.val, r, 0)
 		}
 	}

 	pc, _ := selectgoImpl(sel)
 	chosen = int(pc)
 	recvOK = *r
 	return
 }

 func (q *waitq) dequeueSudoG(s *sudog) {
 	var prevsgp *sudog
 	l := &q.first
 	for {
 		sgp := *l
 		if sgp == nil {
 			return
 		}
 		if sgp == s {
 			*l = sgp.next
 			if q.last == sgp {
 				q.last = prevsgp
 			}
 			s.next = nil
 			return
 		}
 		l = &sgp.next
 		prevsgp = sgp
 	}
 }
	// 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.

	package runtime

	// This file contains the implementation of Go select statements.

	import "unsafe"

	const (
	debugSelect = false
	)

	var (
	chansendpc = funcPC(chansend)
	chanrecvpc = funcPC(chanrecv)
	)

	func selectsize(size uintptr) uintptr {
	selsize := unsafe.Sizeof(_select{}) +
	(size-1)*unsafe.Sizeof(_select{}.scase[0]) +
	sizeunsafe.Sizeof(_select{}.lockorder) +
	sizeunsafe.Sizeof(_select{}.pollorder)
	return round(selsize, _Int64Align)
	}

	func newselect(sel *_select, selsize int64, size int32) {
	if selsize != int64(selectsize(uintptr(size))) {
	print("runtime: bad select size ", selsize, ", want ", selectsize(uintptr(size)), "\n")
	gothrow("bad select size")
	}
	sel.tcase = uint16(size)
	sel.ncase = 0
	sel.lockorder = (*hchan)(add(unsafe.Pointer(&sel.scase), uintptr(size)unsafe.Sizeof(_select{}.scase[0])))
	sel.pollorder = (uint16)(add(unsafe.Pointer(sel.lockorder), uintptr(size)unsafe.Sizeof(*_select{}.lockorder)))

	if debugSelect {
	print("newselect s=", sel, " size=", size, "\n")
	}
	}

	//go:nosplit
	func selectsend(sel _select, c hchan, elem unsafe.Pointer) (selected bool) {
	// nil cases do not compete
	if c != nil {
	selectsendImpl(sel, c, getcallerpc(unsafe.Pointer(&sel)), elem, uintptr(unsafe.Pointer(&selected))-uintptr(unsafe.Pointer(&sel)))
	}
	return
	}

	// cut in half to give stack a chance to split
	func selectsendImpl(sel _select, c hchan, pc uintptr, elem unsafe.Pointer, so uintptr) {
	i := sel.ncase
	if i >= sel.tcase {
	gothrow("selectsend: too many cases")
	}
	sel.ncase = i + 1
	cas := (scase)(add(unsafe.Pointer(&sel.scase), uintptr(i)unsafe.Sizeof(sel.scase[0])))

	cas.pc = pc
	cas._chan = c
	cas.so = uint16(so)
	cas.kind = _CaseSend
	cas.elem = elem

	if debugSelect {
	print("selectsend s=", sel, " pc=", hex(cas.pc), " chan=", cas._chan, " so=", cas.so, "\n")
	}
	}

	//go:nosplit
	func selectrecv(sel _select, c hchan, elem unsafe.Pointer) (selected bool) {
	// nil cases do not compete
	if c != nil {
	selectrecvImpl(sel, c, getcallerpc(unsafe.Pointer(&sel)), elem, nil, uintptr(unsafe.Pointer(&selected))-uintptr(unsafe.Pointer(&sel)))
	}
	return
	}

	//go:nosplit
	func selectrecv2(sel _select, c hchan, elem unsafe.Pointer, received *bool) (selected bool) {
	// nil cases do not compete
	if c != nil {
	selectrecvImpl(sel, c, getcallerpc(unsafe.Pointer(&sel)), elem, received, uintptr(unsafe.Pointer(&selected))-uintptr(unsafe.Pointer(&sel)))
	}
	return
	}

	func selectrecvImpl(sel _select, c hchan, pc uintptr, elem unsafe.Pointer, received *bool, so uintptr) {
	i := sel.ncase
	if i >= sel.tcase {
	gothrow("selectrecv: too many cases")
	}
	sel.ncase = i + 1
	cas := (scase)(add(unsafe.Pointer(&sel.scase), uintptr(i)unsafe.Sizeof(sel.scase[0])))
	cas.pc = pc
	cas._chan = c
	cas.so = uint16(so)
	cas.kind = _CaseRecv
	cas.elem = elem
	cas.receivedp = received

	if debugSelect {
	print("selectrecv s=", sel, " pc=", hex(cas.pc), " chan=", cas._chan, " so=", cas.so, "\n")
	}
	}

	//go:nosplit
	func selectdefault(sel *_select) (selected bool) {
	selectdefaultImpl(sel, getcallerpc(unsafe.Pointer(&sel)), uintptr(unsafe.Pointer(&selected))-uintptr(unsafe.Pointer(&sel)))
	return
	}

	func selectdefaultImpl(sel *_select, callerpc uintptr, so uintptr) {
	i := sel.ncase
	if i >= sel.tcase {
	gothrow("selectdefault: too many cases")
	}
	sel.ncase = i + 1
	cas := (scase)(add(unsafe.Pointer(&sel.scase), uintptr(i)unsafe.Sizeof(sel.scase[0])))
	cas.pc = callerpc
	cas._chan = nil
	cas.so = uint16(so)
	cas.kind = _CaseDefault

	if debugSelect {
	print("selectdefault s=", sel, " pc=", hex(cas.pc), " so=", cas.so, "\n")
	}
	}

	func sellock(sel *_select) {
	lockslice := sliceStruct{unsafe.Pointer(sel.lockorder), int(sel.ncase), int(sel.ncase)}
	lockorder := ([]*hchan)(unsafe.Pointer(&lockslice))
	var c *hchan
	for _, c0 := range lockorder {
	if c0 != nil && c0 != c {
	c = c0
	lock(&c.lock)
	}
	}
	}

	func selunlock(sel *_select) {
	// We must be very careful here to not touch sel after we have unlocked
	// the last lock, because sel can be freed right after the last unlock.
	// Consider the following situation.
	// First M calls runtime·park() in runtime·selectgo() passing the sel.
	// Once runtime·park() has unlocked the last lock, another M makes
	// the G that calls select runnable again and schedules it for execution.
	// When the G runs on another M, it locks all the locks and frees sel.
	// Now if the first M touches sel, it will access freed memory.
	n := int(sel.ncase)
	r := 0
	lockslice := sliceStruct{unsafe.Pointer(sel.lockorder), n, n}
	lockorder := ([]*hchan)(unsafe.Pointer(&lockslice))
	// skip the default case
	if n > 0 && lockorder[0] == nil {
	r = 1
	}
	for i := n - 1; i >= r; i-- {
	c := lockorder[i]
	if i > 0 && c == lockorder[i-1] {
	continue // will unlock it on the next iteration
	}
	unlock(&c.lock)
	}
	}

	func selparkcommit(gp g, sel _select) bool {
	selunlock(sel)
	return true
	}

	func block() {
	gopark(nil, nil, "select (no cases)") // forever
	}

	// overwrites return pc on stack to signal which case of the select
	// to run, so cannot appear at the top of a split stack.
	//go:nosplit
	func selectgo(sel *_select) {
	pc, offset := selectgoImpl(sel)
	(bool)(add(unsafe.Pointer(&sel), uintptr(offset))) = true
	setcallerpc(unsafe.Pointer(&sel), pc)
	}

	// selectgoImpl returns scase.pc and scase.so for the select
	// case which fired.
	func selectgoImpl(sel *_select) (uintptr, uint16) {
	if debugSelect {
	print("select: sel=", sel, "\n")
	}

	scaseslice := sliceStruct{unsafe.Pointer(&sel.scase), int(sel.ncase), int(sel.ncase)}
	scases := ([]scase)(unsafe.Pointer(&scaseslice))

	var t0 int64
	if blockprofilerate > 0 {
	t0 = cputicks()
	for i := 0; i < int(sel.ncase); i++ {
	scases[i].releasetime = -1
	}
	}

	// The compiler rewrites selects that statically have
	// only 0 or 1 cases plus default into simpler constructs.
	// The only way we can end up with such small sel.ncase
	// values here is for a larger select in which most channels
	// have been nilled out. The general code handles those
	// cases correctly, and they are rare enough not to bother
	// optimizing (and needing to test).

	// generate permuted order
	pollslice := sliceStruct{unsafe.Pointer(sel.pollorder), int(sel.ncase), int(sel.ncase)}
	pollorder := ([]uint16)(unsafe.Pointer(&pollslice))
	for i := 0; i < int(sel.ncase); i++ {
	pollorder[i] = uint16(i)
	}
	for i := 1; i < int(sel.ncase); i++ {
	o := pollorder[i]
	j := int(fastrand1()) % (i + 1)
	pollorder[i] = pollorder[j]
	pollorder[j] = o
	}

	// sort the cases by Hchan address to get the locking order.
	// simple heap sort, to guarantee n log n time and constant stack footprint.
	lockslice := sliceStruct{unsafe.Pointer(sel.lockorder), int(sel.ncase), int(sel.ncase)}
	lockorder := ([]*hchan)(unsafe.Pointer(&lockslice))
	for i := 0; i < int(sel.ncase); i++ {
	j := i
	c := scases[j]._chan
	for j > 0 && lockorder[(j-1)/2].sortkey() < c.sortkey() {
	k := (j - 1) / 2
	lockorder[j] = lockorder[k]
	j = k
	}
	lockorder[j] = c
	}
	for i := int(sel.ncase) - 1; i >= 0; i-- {
	c := lockorder[i]
	lockorder[i] = lockorder[0]
	j := 0
	for {
	k := j*2 + 1
	if k >= i {
	break
	}
	if k+1 < i && lockorder[k].sortkey() < lockorder[k+1].sortkey() {
	k++
	}
	if c.sortkey() < lockorder[k].sortkey() {
	lockorder[j] = lockorder[k]
	j = k
	continue
	}
	break
	}
	lockorder[j] = c
	}
	/*
	for i := 0; i+1 < int(sel.ncase); i++ {
	if lockorder[i].sortkey() > lockorder[i+1].sortkey() {
	print("i=", i, " x=", lockorder[i], " y=", lockorder[i+1], "\n")
	gothrow("select: broken sort")
	}
	}
	*/

	// lock all the channels involved in the select
	sellock(sel)

	var (
	gp *g
	done uint32
	sg *sudog
	c *hchan
	k *scase
	sglist *sudog
	sgnext *sudog
	)

	loop:
	// pass 1 - look for something already waiting
	var dfl *scase
	var cas *scase
	for i := 0; i < int(sel.ncase); i++ {
	cas = &scases[pollorder[i]]
	c = cas._chan

	switch cas.kind {
	case _CaseRecv:
	if c.dataqsiz > 0 {
	if c.qcount > 0 {
	goto asyncrecv
	}
	} else {
	sg = c.sendq.dequeue()
	if sg != nil {
	goto syncrecv
	}
	}
	if c.closed != 0 {
	goto rclose
	}

	case _CaseSend:
	if raceenabled {
	racereadpc(unsafe.Pointer(c), cas.pc, chansendpc)
	}
	if c.closed != 0 {
	goto sclose
	}
	if c.dataqsiz > 0 {
	if c.qcount < c.dataqsiz {
	goto asyncsend
	}
	} else {
	sg = c.recvq.dequeue()
	if sg != nil {
	goto syncsend
	}
	}

	case _CaseDefault:
	dfl = cas
	}
	}

	if dfl != nil {
	selunlock(sel)
	cas = dfl
	goto retc
	}

	// pass 2 - enqueue on all chans
	gp = getg()
	done = 0
	for i := 0; i < int(sel.ncase); i++ {
	cas = &scases[pollorder[i]]
	c = cas._chan
	sg := acquireSudog()
	sg.g = gp
	// Note: selectdone is adjusted for stack copies in stack.c:adjustsudogs
	sg.selectdone = (*uint32)(noescape(unsafe.Pointer(&done)))
	sg.elem = cas.elem
	sg.releasetime = 0
	if t0 != 0 {
	sg.releasetime = -1
	}
	sg.waitlink = gp.waiting
	gp.waiting = sg

	switch cas.kind {
	case _CaseRecv:
	c.recvq.enqueue(sg)

	case _CaseSend:
	c.sendq.enqueue(sg)
	}
	}

	// wait for someone to wake us up
	gp.param = nil
	gopark(unsafe.Pointer(funcPC(selparkcommit)), unsafe.Pointer(sel), "select")

	// someone woke us up
	sellock(sel)
	sg = (*sudog)(gp.param)
	gp.param = nil

	// pass 3 - dequeue from unsuccessful chans
	// otherwise they stack up on quiet channels
	// record the successful case, if any.
	// We singly-linked up the SudoGs in case order, so when
	// iterating through the linked list they are in reverse order.
	cas = nil
	sglist = gp.waiting
	// Clear all selectdone and elem before unlinking from gp.waiting.
	// They must be cleared before being put back into the sudog cache.
	// Clear before unlinking, because if a stack copy happens after the unlink,
	// they will not be updated, they will be left pointing to the old stack,
	// which creates dangling pointers, which may be detected by the
	// garbage collector.
	for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
	sg1.selectdone = nil
	sg1.elem = nil
	}
	gp.waiting = nil
	for i := int(sel.ncase) - 1; i >= 0; i-- {
	k = &scases[pollorder[i]]
	if sglist.releasetime > 0 {
	k.releasetime = sglist.releasetime
	}
	if sg == sglist {
	cas = k
	} else {
	c = k._chan
	if k.kind == _CaseSend {
	c.sendq.dequeueSudoG(sglist)
	} else {
	c.recvq.dequeueSudoG(sglist)
	}
	}
	sgnext = sglist.waitlink
	sglist.waitlink = nil
	releaseSudog(sglist)
	sglist = sgnext
	}

	if cas == nil {
	goto loop
	}

	c = cas._chan

	if c.dataqsiz > 0 {
	gothrow("selectgo: shouldn't happen")
	}

	if debugSelect {
	print("wait-return: sel=", sel, " c=", c, " cas=", cas, " kind=", cas.kind, "\n")
	}

	if cas.kind == _CaseRecv {
	if cas.receivedp != nil {
	*cas.receivedp = true
	}
	}

	if raceenabled {
	if cas.kind == _CaseRecv && cas.elem != nil {
	raceWriteObjectPC(c.elemtype, cas.elem, cas.pc, chanrecvpc)
	} else if cas.kind == _CaseSend {
	raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
	}
	}

	selunlock(sel)
	goto retc

	asyncrecv:
	// can receive from buffer
	if raceenabled {
	if cas.elem != nil {
	raceWriteObjectPC(c.elemtype, cas.elem, cas.pc, chanrecvpc)
	}
	raceacquire(chanbuf(c, c.recvx))
	racerelease(chanbuf(c, c.recvx))
	}
	if cas.receivedp != nil {
	*cas.receivedp = true
	}
	if cas.elem != nil {
	memmove(cas.elem, chanbuf(c, c.recvx), uintptr(c.elemsize))
	}
	memclr(chanbuf(c, c.recvx), uintptr(c.elemsize))
	c.recvx++
	if c.recvx == c.dataqsiz {
	c.recvx = 0
	}
	c.qcount--
	sg = c.sendq.dequeue()
	if sg != nil {
	gp = sg.g
	selunlock(sel)
	if sg.releasetime != 0 {
	sg.releasetime = cputicks()
	}
	goready(gp)
	} else {
	selunlock(sel)
	}
	goto retc

	asyncsend:
	// can send to buffer
	if raceenabled {
	raceacquire(chanbuf(c, c.sendx))
	racerelease(chanbuf(c, c.sendx))
	raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
	}
	memmove(chanbuf(c, c.sendx), cas.elem, uintptr(c.elemsize))
	c.sendx++
	if c.sendx == c.dataqsiz {
	c.sendx = 0
	}
	c.qcount++
	sg = c.recvq.dequeue()
	if sg != nil {
	gp = sg.g
	selunlock(sel)
	if sg.releasetime != 0 {
	sg.releasetime = cputicks()
	}
	goready(gp)
	} else {
	selunlock(sel)
	}
	goto retc

	syncrecv:
	// can receive from sleeping sender (sg)
	if raceenabled {
	if cas.elem != nil {
	raceWriteObjectPC(c.elemtype, cas.elem, cas.pc, chanrecvpc)
	}
	racesync(c, sg)
	}
	selunlock(sel)
	if debugSelect {
	print("syncrecv: sel=", sel, " c=", c, "\n")
	}
	if cas.receivedp != nil {
	*cas.receivedp = true
	}
	if cas.elem != nil {
	memmove(cas.elem, sg.elem, uintptr(c.elemsize))
	}
	sg.elem = nil
	gp = sg.g
	gp.param = unsafe.Pointer(sg)
	if sg.releasetime != 0 {
	sg.releasetime = cputicks()
	}
	goready(gp)
	goto retc

	rclose:
	// read at end of closed channel
	selunlock(sel)
	if cas.receivedp != nil {
	*cas.receivedp = false
	}
	if cas.elem != nil {
	memclr(cas.elem, uintptr(c.elemsize))
	}
	if raceenabled {
	raceacquire(unsafe.Pointer(c))
	}
	goto retc

	syncsend:
	// can send to sleeping receiver (sg)
	if raceenabled {
	raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
	racesync(c, sg)
	}
	selunlock(sel)
	if debugSelect {
	print("syncsend: sel=", sel, " c=", c, "\n")
	}
	if sg.elem != nil {
	memmove(sg.elem, cas.elem, uintptr(c.elemsize))
	}
	sg.elem = nil
	gp = sg.g
	gp.param = unsafe.Pointer(sg)
	if sg.releasetime != 0 {
	sg.releasetime = cputicks()
	}
	goready(gp)

	retc:
	if cas.releasetime > 0 {
	blockevent(cas.releasetime-t0, 2)
	}
	return cas.pc, cas.so

	sclose:
	// send on closed channel
	selunlock(sel)
	panic("send on closed channel")
	}

	func (c *hchan) sortkey() uintptr {
	// TODO(khr): if we have a moving garbage collector, we'll need to
	// change this function.
	return uintptr(unsafe.Pointer(c))
	}

	// A runtimeSelect is a single case passed to rselect.
	// This must match ../reflect/value.go:/runtimeSelect
	type runtimeSelect struct {
	dir selectDir
	typ unsafe.Pointer // channel type (not used here)
	ch *hchan // channel
	val unsafe.Pointer // ptr to data (SendDir) or ptr to receive buffer (RecvDir)
	}

	// These values must match ../reflect/value.go:/SelectDir.
	type selectDir int

	const (
	_ selectDir = iota
	selectSend // case Chan <- Send
	selectRecv // case <-Chan:
	selectDefault // default
	)

	func reflect_rselect(cases []runtimeSelect) (chosen int, recvOK bool) {
	// flagNoScan is safe here, because all objects are also referenced from cases.
	size := selectsize(uintptr(len(cases)))
	sel := (*_select)(mallocgc(size, nil, flagNoScan))
	newselect(sel, int64(size), int32(len(cases)))
	r := new(bool)
	for i := range cases {
	rc := &cases[i]
	switch rc.dir {
	case selectDefault:
	selectdefaultImpl(sel, uintptr(i), 0)
	case selectSend:
	if rc.ch == nil {
	break
	}
	selectsendImpl(sel, rc.ch, uintptr(i), rc.val, 0)
	case selectRecv:
	if rc.ch == nil {
	break
	}
	selectrecvImpl(sel, rc.ch, uintptr(i), rc.val, r, 0)
	}
	}

	pc, _ := selectgoImpl(sel)
	chosen = int(pc)
	recvOK = *r
	return
	}

	func (q waitq) dequeueSudoG(s sudog) {
	var prevsgp *sudog
	l := &q.first
	for {
	sgp := *l
	if sgp == nil {
	return
	}
	if sgp == s {
	*l = sgp.next
	if q.last == sgp {
	q.last = prevsgp
	}
	s.next = nil
	return
	}
	l = &sgp.next
	prevsgp = sgp
	}
	}