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

 // scase.kind values.
 // Known to compiler.
 // Changes here must also be made in src/cmd/compile/internal/gc/select.go's walkselectcases.
 const (
 	caseNil = iota
 	caseRecv
 	caseSend
 	caseDefault
 )

 // Select case descriptor.
 // Known to compiler.
 // Changes here must also be made in src/cmd/internal/gc/select.go's scasetype.
 type scase struct {
 	c           *hchan         // chan
 	elem        unsafe.Pointer // data element
 	kind        uint16
 	pc          uintptr // race pc (for race detector / msan)
 	releasetime int64
 }

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

 func selectsetpc(cas *scase) {
 	cas.pc = getcallerpc()
 }

 func sellock(scases []scase, lockorder []uint16) {
 	var c *hchan
 	for _, o := range lockorder {
 		c0 := scases[o].c
 		if c0 != nil && c0 != c {
 			c = c0
 			lock(&c.lock)
 		}
 	}
 }

 func selunlock(scases []scase, lockorder []uint16) {
 	// 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.
 	for i := len(scases) - 1; i >= 0; i-- {
 		c := scases[lockorder[i]].c
 		if c == nil {
 			break
 		}
 		if i > 0 && c == scases[lockorder[i-1]].c {
 			continue // will unlock it on the next iteration
 		}
 		unlock(&c.lock)
 	}
 }

 func selparkcommit(gp *g, _ unsafe.Pointer) bool {
 	// This must not access gp's stack (see gopark). In
 	// particular, it must not access the *hselect. That's okay,
 	// because by the time this is called, gp.waiting has all
 	// channels in lock order.
 	var lastc *hchan
 	for sg := gp.waiting; sg != nil; sg = sg.waitlink {
 		if sg.c != lastc && lastc != nil {
 			// As soon as we unlock the channel, fields in
 			// any sudog with that channel may change,
 			// including c and waitlink. Since multiple
 			// sudogs may have the same channel, we unlock
 			// only after we've passed the last instance
 			// of a channel.
 			unlock(&lastc.lock)
 		}
 		lastc = sg.c
 	}
 	if lastc != nil {
 		unlock(&lastc.lock)
 	}
 	return true
 }

 func block() {
 	gopark(nil, nil, waitReasonSelectNoCases, traceEvGoStop, 1) // forever
 }

 // selectgo implements the select statement.
 //
 // cas0 points to an array of type [ncases]scase, and order0 points to
 // an array of type [2*ncases]uint16. Both reside on the goroutine's
 // stack (regardless of any escaping in selectgo).
 //
 // selectgo returns the index of the chosen scase, which matches the
 // ordinal position of its respective select{recv,send,default} call.
 // Also, if the chosen scase was a receive operation, it reports whether
 // a value was received.
 func selectgo(cas0 *scase, order0 *uint16, ncases int) (int, bool) {
 	if debugSelect {
 		print("select: cas0=", cas0, "\n")
 	}

 	cas1 := (*[1 << 16]scase)(unsafe.Pointer(cas0))
 	order1 := (*[1 << 17]uint16)(unsafe.Pointer(order0))

 	scases := cas1[:ncases:ncases]
 	pollorder := order1[:ncases:ncases]
 	lockorder := order1[ncases:][:ncases:ncases]

 	// Replace send/receive cases involving nil channels with
 	// caseNil so logic below can assume non-nil channel.
 	for i := range scases {
 		cas := &scases[i]
 		if cas.c == nil && cas.kind != caseDefault {
 			*cas = scase{}
 		}
 	}

 	var t0 int64
 	if blockprofilerate > 0 {
 		t0 = cputicks()
 		for i := 0; i < ncases; 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
 	for i := 1; i < ncases; i++ {
 		j := fastrandn(uint32(i + 1))
 		pollorder[i] = pollorder[j]
 		pollorder[j] = uint16(i)
 	}

 	// sort the cases by Hchan address to get the locking order.
 	// simple heap sort, to guarantee n log n time and constant stack footprint.
 	for i := 0; i < ncases; i++ {
 		j := i
 		// Start with the pollorder to permute cases on the same channel.
 		c := scases[pollorder[i]].c
 		for j > 0 && scases[lockorder[(j-1)/2]].c.sortkey() < c.sortkey() {
 			k := (j - 1) / 2
 			lockorder[j] = lockorder[k]
 			j = k
 		}
 		lockorder[j] = pollorder[i]
 	}
 	for i := ncases - 1; i >= 0; i-- {
 		o := lockorder[i]
 		c := scases[o].c
 		lockorder[i] = lockorder[0]
 		j := 0
 		for {
 			k := j*2 + 1
 			if k >= i {
 				break
 			}
 			if k+1 < i && scases[lockorder[k]].c.sortkey() < scases[lockorder[k+1]].c.sortkey() {
 				k++
 			}
 			if c.sortkey() < scases[lockorder[k]].c.sortkey() {
 				lockorder[j] = lockorder[k]
 				j = k
 				continue
 			}
 			break
 		}
 		lockorder[j] = o
 	}

 	if debugSelect {
 		for i := 0; i+1 < ncases; i++ {
 			if scases[lockorder[i]].c.sortkey() > scases[lockorder[i+1]].c.sortkey() {
 				print("i=", i, " x=", lockorder[i], " y=", lockorder[i+1], "\n")
 				throw("select: broken sort")
 			}
 		}
 	}

 	// lock all the channels involved in the select
 	sellock(scases, lockorder)

 	var (
 		gp     *g
 		sg     *sudog
 		c      *hchan
 		k      *scase
 		sglist *sudog
 		sgnext *sudog
 		qp     unsafe.Pointer
 		nextp  **sudog
 	)

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

 		switch cas.kind {
 		case caseNil:
 			continue

 		case caseRecv:
 			sg = c.sendq.dequeue()
 			if sg != nil {
 				goto recv
 			}
 			if c.qcount > 0 {
 				goto bufrecv
 			}
 			if c.closed != 0 {
 				goto rclose
 			}

 		case caseSend:
 			if raceenabled {
 				racereadpc(c.raceaddr(), cas.pc, chansendpc)
 			}
 			if c.closed != 0 {
 				goto sclose
 			}
 			sg = c.recvq.dequeue()
 			if sg != nil {
 				goto send
 			}
 			if c.qcount < c.dataqsiz {
 				goto bufsend
 			}

 		case caseDefault:
 			dfli = casi
 			dfl = cas
 		}
 	}

 	if dfl != nil {
 		selunlock(scases, lockorder)
 		casi = dfli
 		cas = dfl
 		goto retc
 	}

 	// pass 2 - enqueue on all chans
 	gp = getg()
 	if gp.waiting != nil {
 		throw("gp.waiting != nil")
 	}
 	nextp = &gp.waiting
 	for _, casei := range lockorder {
 		casi = int(casei)
 		cas = &scases[casi]
 		if cas.kind == caseNil {
 			continue
 		}
 		c = cas.c
 		sg := acquireSudog()
 		sg.g = gp
 		sg.isSelect = true
 		// No stack splits between assigning elem and enqueuing
 		// sg on gp.waiting where copystack can find it.
 		sg.elem = cas.elem
 		sg.releasetime = 0
 		if t0 != 0 {
 			sg.releasetime = -1
 		}
 		sg.c = c
 		// Construct waiting list in lock order.
 		*nextp = sg
 		nextp = &sg.waitlink

 		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(selparkcommit, nil, waitReasonSelect, traceEvGoBlockSelect, 1)

 	sellock(scases, lockorder)

 	gp.selectDone = 0
 	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 lock order.
 	casi = -1
 	cas = nil
 	sglist = gp.waiting
 	// Clear all elem before unlinking from gp.waiting.
 	for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
 		sg1.isSelect = false
 		sg1.elem = nil
 		sg1.c = nil
 	}
 	gp.waiting = nil

 	for _, casei := range lockorder {
 		k = &scases[casei]
 		if k.kind == caseNil {
 			continue
 		}
 		if sglist.releasetime > 0 {
 			k.releasetime = sglist.releasetime
 		}
 		if sg == sglist {
 			// sg has already been dequeued by the G that woke us up.
 			casi = int(casei)
 			cas = k
 		} else {
 			c = k.c
 			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 {
 		// We can wake up with gp.param == nil (so cas == nil)
 		// when a channel involved in the select has been closed.
 		// It is easiest to loop and re-run the operation;
 		// we'll see that it's now closed.
 		// Maybe some day we can signal the close explicitly,
 		// but we'd have to distinguish close-on-reader from close-on-writer.
 		// It's easiest not to duplicate the code and just recheck above.
 		// We know that something closed, and things never un-close,
 		// so we won't block again.
 		goto loop
 	}

 	c = cas.c

 	if debugSelect {
 		print("wait-return: cas0=", cas0, " c=", c, " cas=", cas, " kind=", cas.kind, "\n")
 	}

 	if cas.kind == caseRecv {
 		recvOK = 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)
 		}
 	}
 	if msanenabled {
 		if cas.kind == caseRecv && cas.elem != nil {
 			msanwrite(cas.elem, c.elemtype.size)
 		} else if cas.kind == caseSend {
 			msanread(cas.elem, c.elemtype.size)
 		}
 	}

 	selunlock(scases, lockorder)
 	goto retc

 bufrecv:
 	// 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 msanenabled && cas.elem != nil {
 		msanwrite(cas.elem, c.elemtype.size)
 	}
 	recvOK = true
 	qp = chanbuf(c, c.recvx)
 	if cas.elem != nil {
 		typedmemmove(c.elemtype, cas.elem, qp)
 	}
 	typedmemclr(c.elemtype, qp)
 	c.recvx++
 	if c.recvx == c.dataqsiz {
 		c.recvx = 0
 	}
 	c.qcount--
 	selunlock(scases, lockorder)
 	goto retc

 bufsend:
 	// can send to buffer
 	if raceenabled {
 		raceacquire(chanbuf(c, c.sendx))
 		racerelease(chanbuf(c, c.sendx))
 		raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
 	}
 	if msanenabled {
 		msanread(cas.elem, c.elemtype.size)
 	}
 	typedmemmove(c.elemtype, chanbuf(c, c.sendx), cas.elem)
 	c.sendx++
 	if c.sendx == c.dataqsiz {
 		c.sendx = 0
 	}
 	c.qcount++
 	selunlock(scases, lockorder)
 	goto retc

 recv:
 	// can receive from sleeping sender (sg)
 	recv(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
 	if debugSelect {
 		print("syncrecv: cas0=", cas0, " c=", c, "\n")
 	}
 	recvOK = true
 	goto retc

 rclose:
 	// read at end of closed channel
 	selunlock(scases, lockorder)
 	recvOK = false
 	if cas.elem != nil {
 		typedmemclr(c.elemtype, cas.elem)
 	}
 	if raceenabled {
 		raceacquire(c.raceaddr())
 	}
 	goto retc

 send:
 	// can send to a sleeping receiver (sg)
 	if raceenabled {
 		raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
 	}
 	if msanenabled {
 		msanread(cas.elem, c.elemtype.size)
 	}
 	send(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
 	if debugSelect {
 		print("syncsend: cas0=", cas0, " c=", c, "\n")
 	}
 	goto retc

 retc:
 	if cas.releasetime > 0 {
 		blockevent(cas.releasetime-t0, 1)
 	}
 	return casi, recvOK

 sclose:
 	// send on closed channel
 	selunlock(scases, lockorder)
 	panic(plainError("send on closed channel"))
 }

 func (c *hchan) sortkey() uintptr {
 	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
 )

 //go:linkname reflect_rselect reflect.rselect
 func reflect_rselect(cases []runtimeSelect) (int, bool) {
 	if len(cases) == 0 {
 		block()
 	}
 	sel := make([]scase, len(cases))
 	order := make([]uint16, 2*len(cases))
 	for i := range cases {
 		rc := &cases[i]
 		switch rc.dir {
 		case selectDefault:
 			sel[i] = scase{kind: caseDefault}
 		case selectSend:
 			sel[i] = scase{kind: caseSend, c: rc.ch, elem: rc.val}
 		case selectRecv:
 			sel[i] = scase{kind: caseRecv, c: rc.ch, elem: rc.val}
 		}
 		if raceenabled || msanenabled {
 			selectsetpc(&sel[i])
 		}
 	}

 	return selectgo(&sel[0], &order[0], len(cases))
 }

 func (q *waitq) dequeueSudoG(sgp *sudog) {
 	x := sgp.prev
 	y := sgp.next
 	if x != nil {
 		if y != nil {
 			// middle of queue
 			x.next = y
 			y.prev = x
 			sgp.next = nil
 			sgp.prev = nil
 			return
 		}
 		// end of queue
 		x.next = nil
 		q.last = x
 		sgp.prev = nil
 		return
 	}
 	if y != nil {
 		// start of queue
 		y.prev = nil
 		q.first = y
 		sgp.next = nil
 		return
 	}

 	// x==y==nil. Either sgp is the only element in the queue,
 	// or it has already been removed. Use q.first to disambiguate.
 	if q.first == sgp {
 		q.first = nil
 		q.last = nil
 	}
 }
	// 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

	// scase.kind values.
	// Known to compiler.
	// Changes here must also be made in src/cmd/compile/internal/gc/select.go's walkselectcases.
	const (
	caseNil = iota
	caseRecv
	caseSend
	caseDefault
	)

	// Select case descriptor.
	// Known to compiler.
	// Changes here must also be made in src/cmd/internal/gc/select.go's scasetype.
	type scase struct {
	c *hchan // chan
	elem unsafe.Pointer // data element
	kind uint16
	pc uintptr // race pc (for race detector / msan)
	releasetime int64
	}

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

	func selectsetpc(cas *scase) {
	cas.pc = getcallerpc()
	}

	func sellock(scases []scase, lockorder []uint16) {
	var c *hchan
	for _, o := range lockorder {
	c0 := scases[o].c
	if c0 != nil && c0 != c {
	c = c0
	lock(&c.lock)
	}
	}
	}

	func selunlock(scases []scase, lockorder []uint16) {
	// 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.
	for i := len(scases) - 1; i >= 0; i-- {
	c := scases[lockorder[i]].c
	if c == nil {
	break
	}
	if i > 0 && c == scases[lockorder[i-1]].c {
	continue // will unlock it on the next iteration
	}
	unlock(&c.lock)
	}
	}

	func selparkcommit(gp *g, _ unsafe.Pointer) bool {
	// This must not access gp's stack (see gopark). In
	// particular, it must not access the *hselect. That's okay,
	// because by the time this is called, gp.waiting has all
	// channels in lock order.
	var lastc *hchan
	for sg := gp.waiting; sg != nil; sg = sg.waitlink {
	if sg.c != lastc && lastc != nil {
	// As soon as we unlock the channel, fields in
	// any sudog with that channel may change,
	// including c and waitlink. Since multiple
	// sudogs may have the same channel, we unlock
	// only after we've passed the last instance
	// of a channel.
	unlock(&lastc.lock)
	}
	lastc = sg.c
	}
	if lastc != nil {
	unlock(&lastc.lock)
	}
	return true
	}

	func block() {
	gopark(nil, nil, waitReasonSelectNoCases, traceEvGoStop, 1) // forever
	}

	// selectgo implements the select statement.
	//
	// cas0 points to an array of type [ncases]scase, and order0 points to
	// an array of type [2*ncases]uint16. Both reside on the goroutine's
	// stack (regardless of any escaping in selectgo).
	//
	// selectgo returns the index of the chosen scase, which matches the
	// ordinal position of its respective select{recv,send,default} call.
	// Also, if the chosen scase was a receive operation, it reports whether
	// a value was received.
	func selectgo(cas0 scase, order0 uint16, ncases int) (int, bool) {
	if debugSelect {
	print("select: cas0=", cas0, "\n")
	}

	cas1 := (*[1 << 16]scase)(unsafe.Pointer(cas0))
	order1 := (*[1 << 17]uint16)(unsafe.Pointer(order0))

	scases := cas1[:ncases:ncases]
	pollorder := order1[:ncases:ncases]
	lockorder := order1[ncases:][:ncases:ncases]

	// Replace send/receive cases involving nil channels with
	// caseNil so logic below can assume non-nil channel.
	for i := range scases {
	cas := &scases[i]
	if cas.c == nil && cas.kind != caseDefault {
	*cas = scase{}
	}
	}

	var t0 int64
	if blockprofilerate > 0 {
	t0 = cputicks()
	for i := 0; i < ncases; 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
	for i := 1; i < ncases; i++ {
	j := fastrandn(uint32(i + 1))
	pollorder[i] = pollorder[j]
	pollorder[j] = uint16(i)
	}

	// sort the cases by Hchan address to get the locking order.
	// simple heap sort, to guarantee n log n time and constant stack footprint.
	for i := 0; i < ncases; i++ {
	j := i
	// Start with the pollorder to permute cases on the same channel.
	c := scases[pollorder[i]].c
	for j > 0 && scases[lockorder[(j-1)/2]].c.sortkey() < c.sortkey() {
	k := (j - 1) / 2
	lockorder[j] = lockorder[k]
	j = k
	}
	lockorder[j] = pollorder[i]
	}
	for i := ncases - 1; i >= 0; i-- {
	o := lockorder[i]
	c := scases[o].c
	lockorder[i] = lockorder[0]
	j := 0
	for {
	k := j*2 + 1
	if k >= i {
	break
	}
	if k+1 < i && scases[lockorder[k]].c.sortkey() < scases[lockorder[k+1]].c.sortkey() {
	k++
	}
	if c.sortkey() < scases[lockorder[k]].c.sortkey() {
	lockorder[j] = lockorder[k]
	j = k
	continue
	}
	break
	}
	lockorder[j] = o
	}

	if debugSelect {
	for i := 0; i+1 < ncases; i++ {
	if scases[lockorder[i]].c.sortkey() > scases[lockorder[i+1]].c.sortkey() {
	print("i=", i, " x=", lockorder[i], " y=", lockorder[i+1], "\n")
	throw("select: broken sort")
	}
	}
	}

	// lock all the channels involved in the select
	sellock(scases, lockorder)

	var (
	gp *g
	sg *sudog
	c *hchan
	k *scase
	sglist *sudog
	sgnext *sudog
	qp unsafe.Pointer
	nextp **sudog
	)

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

	switch cas.kind {
	case caseNil:
	continue

	case caseRecv:
	sg = c.sendq.dequeue()
	if sg != nil {
	goto recv
	}
	if c.qcount > 0 {
	goto bufrecv
	}
	if c.closed != 0 {
	goto rclose
	}

	case caseSend:
	if raceenabled {
	racereadpc(c.raceaddr(), cas.pc, chansendpc)
	}
	if c.closed != 0 {
	goto sclose
	}
	sg = c.recvq.dequeue()
	if sg != nil {
	goto send
	}
	if c.qcount < c.dataqsiz {
	goto bufsend
	}

	case caseDefault:
	dfli = casi
	dfl = cas
	}
	}

	if dfl != nil {
	selunlock(scases, lockorder)
	casi = dfli
	cas = dfl
	goto retc
	}

	// pass 2 - enqueue on all chans
	gp = getg()
	if gp.waiting != nil {
	throw("gp.waiting != nil")
	}
	nextp = &gp.waiting
	for _, casei := range lockorder {
	casi = int(casei)
	cas = &scases[casi]
	if cas.kind == caseNil {
	continue
	}
	c = cas.c
	sg := acquireSudog()
	sg.g = gp
	sg.isSelect = true
	// No stack splits between assigning elem and enqueuing
	// sg on gp.waiting where copystack can find it.
	sg.elem = cas.elem
	sg.releasetime = 0
	if t0 != 0 {
	sg.releasetime = -1
	}
	sg.c = c
	// Construct waiting list in lock order.
	*nextp = sg
	nextp = &sg.waitlink

	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(selparkcommit, nil, waitReasonSelect, traceEvGoBlockSelect, 1)

	sellock(scases, lockorder)

	gp.selectDone = 0
	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 lock order.
	casi = -1
	cas = nil
	sglist = gp.waiting
	// Clear all elem before unlinking from gp.waiting.
	for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
	sg1.isSelect = false
	sg1.elem = nil
	sg1.c = nil
	}
	gp.waiting = nil

	for _, casei := range lockorder {
	k = &scases[casei]
	if k.kind == caseNil {
	continue
	}
	if sglist.releasetime > 0 {
	k.releasetime = sglist.releasetime
	}
	if sg == sglist {
	// sg has already been dequeued by the G that woke us up.
	casi = int(casei)
	cas = k
	} else {
	c = k.c
	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 {
	// We can wake up with gp.param == nil (so cas == nil)
	// when a channel involved in the select has been closed.
	// It is easiest to loop and re-run the operation;
	// we'll see that it's now closed.
	// Maybe some day we can signal the close explicitly,
	// but we'd have to distinguish close-on-reader from close-on-writer.
	// It's easiest not to duplicate the code and just recheck above.
	// We know that something closed, and things never un-close,
	// so we won't block again.
	goto loop
	}

	c = cas.c

	if debugSelect {
	print("wait-return: cas0=", cas0, " c=", c, " cas=", cas, " kind=", cas.kind, "\n")
	}

	if cas.kind == caseRecv {
	recvOK = 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)
	}
	}
	if msanenabled {
	if cas.kind == caseRecv && cas.elem != nil {
	msanwrite(cas.elem, c.elemtype.size)
	} else if cas.kind == caseSend {
	msanread(cas.elem, c.elemtype.size)
	}
	}

	selunlock(scases, lockorder)
	goto retc

	bufrecv:
	// 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 msanenabled && cas.elem != nil {
	msanwrite(cas.elem, c.elemtype.size)
	}
	recvOK = true
	qp = chanbuf(c, c.recvx)
	if cas.elem != nil {
	typedmemmove(c.elemtype, cas.elem, qp)
	}
	typedmemclr(c.elemtype, qp)
	c.recvx++
	if c.recvx == c.dataqsiz {
	c.recvx = 0
	}
	c.qcount--
	selunlock(scases, lockorder)
	goto retc

	bufsend:
	// can send to buffer
	if raceenabled {
	raceacquire(chanbuf(c, c.sendx))
	racerelease(chanbuf(c, c.sendx))
	raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
	}
	if msanenabled {
	msanread(cas.elem, c.elemtype.size)
	}
	typedmemmove(c.elemtype, chanbuf(c, c.sendx), cas.elem)
	c.sendx++
	if c.sendx == c.dataqsiz {
	c.sendx = 0
	}
	c.qcount++
	selunlock(scases, lockorder)
	goto retc

	recv:
	// can receive from sleeping sender (sg)
	recv(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
	if debugSelect {
	print("syncrecv: cas0=", cas0, " c=", c, "\n")
	}
	recvOK = true
	goto retc

	rclose:
	// read at end of closed channel
	selunlock(scases, lockorder)
	recvOK = false
	if cas.elem != nil {
	typedmemclr(c.elemtype, cas.elem)
	}
	if raceenabled {
	raceacquire(c.raceaddr())
	}
	goto retc

	send:
	// can send to a sleeping receiver (sg)
	if raceenabled {
	raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
	}
	if msanenabled {
	msanread(cas.elem, c.elemtype.size)
	}
	send(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
	if debugSelect {
	print("syncsend: cas0=", cas0, " c=", c, "\n")
	}
	goto retc

	retc:
	if cas.releasetime > 0 {
	blockevent(cas.releasetime-t0, 1)
	}
	return casi, recvOK

	sclose:
	// send on closed channel
	selunlock(scases, lockorder)
	panic(plainError("send on closed channel"))
	}

	func (c *hchan) sortkey() uintptr {
	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
	)

	//go:linkname reflect_rselect reflect.rselect
	func reflect_rselect(cases []runtimeSelect) (int, bool) {
	if len(cases) == 0 {
	block()
	}
	sel := make([]scase, len(cases))
	order := make([]uint16, 2*len(cases))
	for i := range cases {
	rc := &cases[i]
	switch rc.dir {
	case selectDefault:
	sel[i] = scase{kind: caseDefault}
	case selectSend:
	sel[i] = scase{kind: caseSend, c: rc.ch, elem: rc.val}
	case selectRecv:
	sel[i] = scase{kind: caseRecv, c: rc.ch, elem: rc.val}
	}
	if raceenabled \|\| msanenabled {
	selectsetpc(&sel[i])
	}
	}

	return selectgo(&sel[0], &order[0], len(cases))
	}

	func (q waitq) dequeueSudoG(sgp sudog) {
	x := sgp.prev
	y := sgp.next
	if x != nil {
	if y != nil {
	// middle of queue
	x.next = y
	y.prev = x
	sgp.next = nil
	sgp.prev = nil
	return
	}
	// end of queue
	x.next = nil
	q.last = x
	sgp.prev = nil
	return
	}
	if y != nil {
	// start of queue
	y.prev = nil
	q.first = y
	sgp.next = nil
	return
	}

	// x==y==nil. Either sgp is the only element in the queue,
	// or it has already been removed. Use q.first to disambiguate.
	if q.first == sgp {
	q.first = nil
	q.last = nil
	}
	}