src/runtime/chan.go - go - Git at Google

 // Copyright 2014 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 channels.

 import "unsafe"

 const (
 	maxAlign  = 8
 	hchanSize = unsafe.Sizeof(hchan{}) + uintptr(-int(unsafe.Sizeof(hchan{}))&(maxAlign-1))
 	debugChan = false
 )

 type hchan struct {
 	qcount   uint           // total data in the queue
 	dataqsiz uint           // size of the circular queue
 	buf      unsafe.Pointer // points to an array of dataqsiz elements
 	elemsize uint16
 	closed   uint32
 	elemtype *_type // element type
 	sendx    uint   // send index
 	recvx    uint   // receive index
 	recvq    waitq  // list of recv waiters
 	sendq    waitq  // list of send waiters
 	lock     mutex
 }

 type waitq struct {
 	first *sudog
 	last  *sudog
 }

 //go:linkname reflect_makechan reflect.makechan
 func reflect_makechan(t *chantype, size int64) *hchan {
 	return makechan(t, size)
 }

 func makechan(t *chantype, size int64) *hchan {
 	elem := t.elem

 	// compiler checks this but be safe.
 	if elem.size >= 1<<16 {
 		throw("makechan: invalid channel element type")
 	}
 	if hchanSize%maxAlign != 0 || elem.align > maxAlign {
 		throw("makechan: bad alignment")
 	}
 	if size < 0 || int64(uintptr(size)) != size || (elem.size > 0 && uintptr(size) > (_MaxMem-hchanSize)/uintptr(elem.size)) {
 		panic("makechan: size out of range")
 	}

 	var c *hchan
 	if elem.kind&kindNoPointers != 0 || size == 0 {
 		// Allocate memory in one call.
 		// Hchan does not contain pointers interesting for GC in this case:
 		// buf points into the same allocation, elemtype is persistent.
 		// SudoG's are referenced from their owning thread so they can't be collected.
 		// TODO(dvyukov,rlh): Rethink when collector can move allocated objects.
 		c = (*hchan)(mallocgc(hchanSize+uintptr(size)*uintptr(elem.size), nil, flagNoScan))
 		if size > 0 && elem.size != 0 {
 			c.buf = add(unsafe.Pointer(c), hchanSize)
 		} else {
 			// race detector uses this location for synchronization
 			// Also prevents us from pointing beyond the allocation (see issue 9401).
 			c.buf = unsafe.Pointer(c)
 		}
 	} else {
 		c = new(hchan)
 		c.buf = newarray(elem, uintptr(size))
 	}
 	c.elemsize = uint16(elem.size)
 	c.elemtype = elem
 	c.dataqsiz = uint(size)

 	if debugChan {
 		print("makechan: chan=", c, "; elemsize=", elem.size, "; elemalg=", elem.alg, "; dataqsiz=", size, "\n")
 	}
 	return c
 }

 // chanbuf(c, i) is pointer to the i'th slot in the buffer.
 func chanbuf(c *hchan, i uint) unsafe.Pointer {
 	return add(c.buf, uintptr(i)*uintptr(c.elemsize))
 }

 // entry point for c <- x from compiled code
 //go:nosplit
 func chansend1(t *chantype, c *hchan, elem unsafe.Pointer) {
 	chansend(t, c, elem, true, getcallerpc(unsafe.Pointer(&t)))
 }

 /*
  * generic single channel send/recv
  * If block is not nil,
  * then the protocol will not
  * sleep but return if it could
  * not complete.
  *
  * sleep can wake up with g.param == nil
  * when a channel involved in the sleep has
  * been closed.  it is easiest to loop and re-run
  * the operation; we'll see that it's now closed.
  */
 func chansend(t *chantype, c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
 	if raceenabled {
 		raceReadObjectPC(t.elem, ep, callerpc, funcPC(chansend))
 	}

 	if c == nil {
 		if !block {
 			return false
 		}
 		gopark(nil, nil, "chan send (nil chan)", traceEvGoStop, 2)
 		throw("unreachable")
 	}

 	if debugChan {
 		print("chansend: chan=", c, "\n")
 	}

 	if raceenabled {
 		racereadpc(unsafe.Pointer(c), callerpc, funcPC(chansend))
 	}

 	// Fast path: check for failed non-blocking operation without acquiring the lock.
 	//
 	// After observing that the channel is not closed, we observe that the channel is
 	// not ready for sending. Each of these observations is a single word-sized read
 	// (first c.closed and second c.recvq.first or c.qcount depending on kind of channel).
 	// Because a closed channel cannot transition from 'ready for sending' to
 	// 'not ready for sending', even if the channel is closed between the two observations,
 	// they imply a moment between the two when the channel was both not yet closed
 	// and not ready for sending. We behave as if we observed the channel at that moment,
 	// and report that the send cannot proceed.
 	//
 	// It is okay if the reads are reordered here: if we observe that the channel is not
 	// ready for sending and then observe that it is not closed, that implies that the
 	// channel wasn't closed during the first observation.
 	if !block && c.closed == 0 && ((c.dataqsiz == 0 && c.recvq.first == nil) ||
 		(c.dataqsiz > 0 && c.qcount == c.dataqsiz)) {
 		return false
 	}

 	var t0 int64
 	if blockprofilerate > 0 {
 		t0 = cputicks()
 	}

 	lock(&c.lock)
 	if c.closed != 0 {
 		unlock(&c.lock)
 		panic("send on closed channel")
 	}

 	if c.dataqsiz == 0 { // synchronous channel
 		sg := c.recvq.dequeue()
 		if sg != nil { // found a waiting receiver
 			if raceenabled {
 				racesync(c, sg)
 			}
 			unlock(&c.lock)

 			recvg := sg.g
 			if sg.elem != nil {
 				syncsend(c, sg, ep)
 			}
 			recvg.param = unsafe.Pointer(sg)
 			if sg.releasetime != 0 {
 				sg.releasetime = cputicks()
 			}
 			goready(recvg, 3)
 			return true
 		}

 		if !block {
 			unlock(&c.lock)
 			return false
 		}

 		// no receiver available: block on this channel.
 		gp := getg()
 		mysg := acquireSudog()
 		mysg.releasetime = 0
 		if t0 != 0 {
 			mysg.releasetime = -1
 		}
 		mysg.elem = ep
 		mysg.waitlink = nil
 		gp.waiting = mysg
 		mysg.g = gp
 		mysg.selectdone = nil
 		gp.param = nil
 		c.sendq.enqueue(mysg)
 		goparkunlock(&c.lock, "chan send", traceEvGoBlockSend, 3)

 		// someone woke us up.
 		if mysg != gp.waiting {
 			throw("G waiting list is corrupted!")
 		}
 		gp.waiting = nil
 		if gp.param == nil {
 			if c.closed == 0 {
 				throw("chansend: spurious wakeup")
 			}
 			panic("send on closed channel")
 		}
 		gp.param = nil
 		if mysg.releasetime > 0 {
 			blockevent(int64(mysg.releasetime)-t0, 2)
 		}
 		releaseSudog(mysg)
 		return true
 	}

 	// asynchronous channel
 	// wait for some space to write our data
 	var t1 int64
 	for futile := byte(0); c.qcount >= c.dataqsiz; futile = traceFutileWakeup {
 		if !block {
 			unlock(&c.lock)
 			return false
 		}
 		gp := getg()
 		mysg := acquireSudog()
 		mysg.releasetime = 0
 		if t0 != 0 {
 			mysg.releasetime = -1
 		}
 		mysg.g = gp
 		mysg.elem = nil
 		mysg.selectdone = nil
 		c.sendq.enqueue(mysg)
 		goparkunlock(&c.lock, "chan send", traceEvGoBlockSend|futile, 3)

 		// someone woke us up - try again
 		if mysg.releasetime > 0 {
 			t1 = mysg.releasetime
 		}
 		releaseSudog(mysg)
 		lock(&c.lock)
 		if c.closed != 0 {
 			unlock(&c.lock)
 			panic("send on closed channel")
 		}
 	}

 	// write our data into the channel buffer
 	if raceenabled {
 		raceacquire(chanbuf(c, c.sendx))
 		racerelease(chanbuf(c, c.sendx))
 	}
 	typedmemmove(c.elemtype, chanbuf(c, c.sendx), ep)
 	c.sendx++
 	if c.sendx == c.dataqsiz {
 		c.sendx = 0
 	}
 	c.qcount++

 	// wake up a waiting receiver
 	sg := c.recvq.dequeue()
 	if sg != nil {
 		recvg := sg.g
 		unlock(&c.lock)
 		if sg.releasetime != 0 {
 			sg.releasetime = cputicks()
 		}
 		goready(recvg, 3)
 	} else {
 		unlock(&c.lock)
 	}
 	if t1 > 0 {
 		blockevent(t1-t0, 2)
 	}
 	return true
 }

 func syncsend(c *hchan, sg *sudog, elem unsafe.Pointer) {
 	// Send on unbuffered channel is the only operation
 	// in the entire runtime where one goroutine
 	// writes to the stack of another goroutine. The GC assumes that
 	// stack writes only happen when the goroutine is running and are
 	// only done by that goroutine. Using a write barrier is sufficient to
 	// make up for violating that assumption, but the write barrier has to work.
 	// typedmemmove will call heapBitsBulkBarrier, but the target bytes
 	// are not in the heap, so that will not help. We arrange to call
 	// memmove and typeBitsBulkBarrier instead.
 	memmove(sg.elem, elem, c.elemtype.size)
 	typeBitsBulkBarrier(c.elemtype, uintptr(sg.elem), c.elemtype.size)
 	sg.elem = nil
 }

 func closechan(c *hchan) {
 	if c == nil {
 		panic("close of nil channel")
 	}

 	lock(&c.lock)
 	if c.closed != 0 {
 		unlock(&c.lock)
 		panic("close of closed channel")
 	}

 	if raceenabled {
 		callerpc := getcallerpc(unsafe.Pointer(&c))
 		racewritepc(unsafe.Pointer(c), callerpc, funcPC(closechan))
 		racerelease(unsafe.Pointer(c))
 	}

 	c.closed = 1

 	// release all readers
 	for {
 		sg := c.recvq.dequeue()
 		if sg == nil {
 			break
 		}
 		gp := sg.g
 		sg.elem = nil
 		gp.param = nil
 		if sg.releasetime != 0 {
 			sg.releasetime = cputicks()
 		}
 		goready(gp, 3)
 	}

 	// release all writers
 	for {
 		sg := c.sendq.dequeue()
 		if sg == nil {
 			break
 		}
 		gp := sg.g
 		sg.elem = nil
 		gp.param = nil
 		if sg.releasetime != 0 {
 			sg.releasetime = cputicks()
 		}
 		goready(gp, 3)
 	}
 	unlock(&c.lock)
 }

 // entry points for <- c from compiled code
 //go:nosplit
 func chanrecv1(t *chantype, c *hchan, elem unsafe.Pointer) {
 	chanrecv(t, c, elem, true)
 }

 //go:nosplit
 func chanrecv2(t *chantype, c *hchan, elem unsafe.Pointer) (received bool) {
 	_, received = chanrecv(t, c, elem, true)
 	return
 }

 // chanrecv receives on channel c and writes the received data to ep.
 // ep may be nil, in which case received data is ignored.
 // If block == false and no elements are available, returns (false, false).
 // Otherwise, if c is closed, zeros *ep and returns (true, false).
 // Otherwise, fills in *ep with an element and returns (true, true).
 func chanrecv(t *chantype, c *hchan, ep unsafe.Pointer, block bool) (selected, received bool) {
 	// raceenabled: don't need to check ep, as it is always on the stack.

 	if debugChan {
 		print("chanrecv: chan=", c, "\n")
 	}

 	if c == nil {
 		if !block {
 			return
 		}
 		gopark(nil, nil, "chan receive (nil chan)", traceEvGoStop, 2)
 		throw("unreachable")
 	}

 	// Fast path: check for failed non-blocking operation without acquiring the lock.
 	//
 	// After observing that the channel is not ready for receiving, we observe that the
 	// channel is not closed. Each of these observations is a single word-sized read
 	// (first c.sendq.first or c.qcount, and second c.closed).
 	// Because a channel cannot be reopened, the later observation of the channel
 	// being not closed implies that it was also not closed at the moment of the
 	// first observation. We behave as if we observed the channel at that moment
 	// and report that the receive cannot proceed.
 	//
 	// The order of operations is important here: reversing the operations can lead to
 	// incorrect behavior when racing with a close.
 	if !block && (c.dataqsiz == 0 && c.sendq.first == nil ||
 		c.dataqsiz > 0 && atomicloaduint(&c.qcount) == 0) &&
 		atomicload(&c.closed) == 0 {
 		return
 	}

 	var t0 int64
 	if blockprofilerate > 0 {
 		t0 = cputicks()
 	}

 	lock(&c.lock)
 	if c.dataqsiz == 0 { // synchronous channel
 		if c.closed != 0 {
 			return recvclosed(c, ep)
 		}

 		sg := c.sendq.dequeue()
 		if sg != nil {
 			if raceenabled {
 				racesync(c, sg)
 			}
 			unlock(&c.lock)

 			if ep != nil {
 				typedmemmove(c.elemtype, ep, sg.elem)
 			}
 			sg.elem = nil
 			gp := sg.g
 			gp.param = unsafe.Pointer(sg)
 			if sg.releasetime != 0 {
 				sg.releasetime = cputicks()
 			}
 			goready(gp, 3)
 			selected = true
 			received = true
 			return
 		}

 		if !block {
 			unlock(&c.lock)
 			return
 		}

 		// no sender available: block on this channel.
 		gp := getg()
 		mysg := acquireSudog()
 		mysg.releasetime = 0
 		if t0 != 0 {
 			mysg.releasetime = -1
 		}
 		mysg.elem = ep
 		mysg.waitlink = nil
 		gp.waiting = mysg
 		mysg.g = gp
 		mysg.selectdone = nil
 		gp.param = nil
 		c.recvq.enqueue(mysg)
 		goparkunlock(&c.lock, "chan receive", traceEvGoBlockRecv, 3)

 		// someone woke us up
 		if mysg != gp.waiting {
 			throw("G waiting list is corrupted!")
 		}
 		gp.waiting = nil
 		if mysg.releasetime > 0 {
 			blockevent(mysg.releasetime-t0, 2)
 		}
 		haveData := gp.param != nil
 		gp.param = nil
 		releaseSudog(mysg)

 		if haveData {
 			// a sender sent us some data. It already wrote to ep.
 			selected = true
 			received = true
 			return
 		}

 		lock(&c.lock)
 		if c.closed == 0 {
 			throw("chanrecv: spurious wakeup")
 		}
 		return recvclosed(c, ep)
 	}

 	// asynchronous channel
 	// wait for some data to appear
 	var t1 int64
 	for futile := byte(0); c.qcount <= 0; futile = traceFutileWakeup {
 		if c.closed != 0 {
 			selected, received = recvclosed(c, ep)
 			if t1 > 0 {
 				blockevent(t1-t0, 2)
 			}
 			return
 		}

 		if !block {
 			unlock(&c.lock)
 			return
 		}

 		// wait for someone to send an element
 		gp := getg()
 		mysg := acquireSudog()
 		mysg.releasetime = 0
 		if t0 != 0 {
 			mysg.releasetime = -1
 		}
 		mysg.elem = nil
 		mysg.g = gp
 		mysg.selectdone = nil

 		c.recvq.enqueue(mysg)
 		goparkunlock(&c.lock, "chan receive", traceEvGoBlockRecv|futile, 3)

 		// someone woke us up - try again
 		if mysg.releasetime > 0 {
 			t1 = mysg.releasetime
 		}
 		releaseSudog(mysg)
 		lock(&c.lock)
 	}

 	if raceenabled {
 		raceacquire(chanbuf(c, c.recvx))
 		racerelease(chanbuf(c, c.recvx))
 	}
 	if ep != nil {
 		typedmemmove(c.elemtype, ep, chanbuf(c, c.recvx))
 	}
 	memclr(chanbuf(c, c.recvx), uintptr(c.elemsize))

 	c.recvx++
 	if c.recvx == c.dataqsiz {
 		c.recvx = 0
 	}
 	c.qcount--

 	// ping a sender now that there is space
 	sg := c.sendq.dequeue()
 	if sg != nil {
 		gp := sg.g
 		unlock(&c.lock)
 		if sg.releasetime != 0 {
 			sg.releasetime = cputicks()
 		}
 		goready(gp, 3)
 	} else {
 		unlock(&c.lock)
 	}

 	if t1 > 0 {
 		blockevent(t1-t0, 2)
 	}
 	selected = true
 	received = true
 	return
 }

 // recvclosed is a helper function for chanrecv.  Handles cleanup
 // when the receiver encounters a closed channel.
 // Caller must hold c.lock, recvclosed will release the lock.
 func recvclosed(c *hchan, ep unsafe.Pointer) (selected, recevied bool) {
 	if raceenabled {
 		raceacquire(unsafe.Pointer(c))
 	}
 	unlock(&c.lock)
 	if ep != nil {
 		memclr(ep, uintptr(c.elemsize))
 	}
 	return true, false
 }

 // compiler implements
 //
 //	select {
 //	case c <- v:
 //		... foo
 //	default:
 //		... bar
 //	}
 //
 // as
 //
 //	if selectnbsend(c, v) {
 //		... foo
 //	} else {
 //		... bar
 //	}
 //
 func selectnbsend(t *chantype, c *hchan, elem unsafe.Pointer) (selected bool) {
 	return chansend(t, c, elem, false, getcallerpc(unsafe.Pointer(&t)))
 }

 // compiler implements
 //
 //	select {
 //	case v = <-c:
 //		... foo
 //	default:
 //		... bar
 //	}
 //
 // as
 //
 //	if selectnbrecv(&v, c) {
 //		... foo
 //	} else {
 //		... bar
 //	}
 //
 func selectnbrecv(t *chantype, elem unsafe.Pointer, c *hchan) (selected bool) {
 	selected, _ = chanrecv(t, c, elem, false)
 	return
 }

 // compiler implements
 //
 //	select {
 //	case v, ok = <-c:
 //		... foo
 //	default:
 //		... bar
 //	}
 //
 // as
 //
 //	if c != nil && selectnbrecv2(&v, &ok, c) {
 //		... foo
 //	} else {
 //		... bar
 //	}
 //
 func selectnbrecv2(t *chantype, elem unsafe.Pointer, received *bool, c *hchan) (selected bool) {
 	// TODO(khr): just return 2 values from this function, now that it is in Go.
 	selected, *received = chanrecv(t, c, elem, false)
 	return
 }

 //go:linkname reflect_chansend reflect.chansend
 func reflect_chansend(t *chantype, c *hchan, elem unsafe.Pointer, nb bool) (selected bool) {
 	return chansend(t, c, elem, !nb, getcallerpc(unsafe.Pointer(&t)))
 }

 //go:linkname reflect_chanrecv reflect.chanrecv
 func reflect_chanrecv(t *chantype, c *hchan, nb bool, elem unsafe.Pointer) (selected bool, received bool) {
 	return chanrecv(t, c, elem, !nb)
 }

 //go:linkname reflect_chanlen reflect.chanlen
 func reflect_chanlen(c *hchan) int {
 	if c == nil {
 		return 0
 	}
 	return int(c.qcount)
 }

 //go:linkname reflect_chancap reflect.chancap
 func reflect_chancap(c *hchan) int {
 	if c == nil {
 		return 0
 	}
 	return int(c.dataqsiz)
 }

 //go:linkname reflect_chanclose reflect.chanclose
 func reflect_chanclose(c *hchan) {
 	closechan(c)
 }

 func (q *waitq) enqueue(sgp *sudog) {
 	sgp.next = nil
 	x := q.last
 	if x == nil {
 		sgp.prev = nil
 		q.first = sgp
 		q.last = sgp
 		return
 	}
 	sgp.prev = x
 	x.next = sgp
 	q.last = sgp
 }

 func (q *waitq) dequeue() *sudog {
 	for {
 		sgp := q.first
 		if sgp == nil {
 			return nil
 		}
 		y := sgp.next
 		if y == nil {
 			q.first = nil
 			q.last = nil
 		} else {
 			y.prev = nil
 			q.first = y
 			sgp.next = nil // mark as removed (see dequeueSudog)
 		}

 		// if sgp participates in a select and is already signaled, ignore it
 		if sgp.selectdone != nil {
 			// claim the right to signal
 			if *sgp.selectdone != 0 || !cas(sgp.selectdone, 0, 1) {
 				continue
 			}
 		}

 		return sgp
 	}
 }

 func racesync(c *hchan, sg *sudog) {
 	racerelease(chanbuf(c, 0))
 	raceacquireg(sg.g, chanbuf(c, 0))
 	racereleaseg(sg.g, chanbuf(c, 0))
 	raceacquire(chanbuf(c, 0))
 }
	// Copyright 2014 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 channels.

	import "unsafe"

	const (
	maxAlign = 8
	hchanSize = unsafe.Sizeof(hchan{}) + uintptr(-int(unsafe.Sizeof(hchan{}))&(maxAlign-1))
	debugChan = false
	)

	type hchan struct {
	qcount uint // total data in the queue
	dataqsiz uint // size of the circular queue
	buf unsafe.Pointer // points to an array of dataqsiz elements
	elemsize uint16
	closed uint32
	elemtype *_type // element type
	sendx uint // send index
	recvx uint // receive index
	recvq waitq // list of recv waiters
	sendq waitq // list of send waiters
	lock mutex
	}

	type waitq struct {
	first *sudog
	last *sudog
	}

	//go:linkname reflect_makechan reflect.makechan
	func reflect_makechan(t chantype, size int64) hchan {
	return makechan(t, size)
	}

	func makechan(t chantype, size int64) hchan {
	elem := t.elem

	// compiler checks this but be safe.
	if elem.size >= 1<<16 {
	throw("makechan: invalid channel element type")
	}
	if hchanSize%maxAlign != 0 \|\| elem.align > maxAlign {
	throw("makechan: bad alignment")
	}
	if size < 0 \|\| int64(uintptr(size)) != size \|\| (elem.size > 0 && uintptr(size) > (_MaxMem-hchanSize)/uintptr(elem.size)) {
	panic("makechan: size out of range")
	}

	var c *hchan
	if elem.kind&kindNoPointers != 0 \|\| size == 0 {
	// Allocate memory in one call.
	// Hchan does not contain pointers interesting for GC in this case:
	// buf points into the same allocation, elemtype is persistent.
	// SudoG's are referenced from their owning thread so they can't be collected.
	// TODO(dvyukov,rlh): Rethink when collector can move allocated objects.
	c = (hchan)(mallocgc(hchanSize+uintptr(size)uintptr(elem.size), nil, flagNoScan))
	if size > 0 && elem.size != 0 {
	c.buf = add(unsafe.Pointer(c), hchanSize)
	} else {
	// race detector uses this location for synchronization
	// Also prevents us from pointing beyond the allocation (see issue 9401).
	c.buf = unsafe.Pointer(c)
	}
	} else {
	c = new(hchan)
	c.buf = newarray(elem, uintptr(size))
	}
	c.elemsize = uint16(elem.size)
	c.elemtype = elem
	c.dataqsiz = uint(size)

	if debugChan {
	print("makechan: chan=", c, "; elemsize=", elem.size, "; elemalg=", elem.alg, "; dataqsiz=", size, "\n")
	}
	return c
	}

	// chanbuf(c, i) is pointer to the i'th slot in the buffer.
	func chanbuf(c *hchan, i uint) unsafe.Pointer {
	return add(c.buf, uintptr(i)*uintptr(c.elemsize))
	}

	// entry point for c <- x from compiled code
	//go:nosplit
	func chansend1(t chantype, c hchan, elem unsafe.Pointer) {
	chansend(t, c, elem, true, getcallerpc(unsafe.Pointer(&t)))
	}

	/*
	* generic single channel send/recv
	* If block is not nil,
	* then the protocol will not
	* sleep but return if it could
	* not complete.
	*
	* sleep can wake up with g.param == nil
	* when a channel involved in the sleep has
	* been closed. it is easiest to loop and re-run
	* the operation; we'll see that it's now closed.
	*/
	func chansend(t chantype, c hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
	if raceenabled {
	raceReadObjectPC(t.elem, ep, callerpc, funcPC(chansend))
	}

	if c == nil {
	if !block {
	return false
	}
	gopark(nil, nil, "chan send (nil chan)", traceEvGoStop, 2)
	throw("unreachable")
	}

	if debugChan {
	print("chansend: chan=", c, "\n")
	}

	if raceenabled {
	racereadpc(unsafe.Pointer(c), callerpc, funcPC(chansend))
	}

	// Fast path: check for failed non-blocking operation without acquiring the lock.
	//
	// After observing that the channel is not closed, we observe that the channel is
	// not ready for sending. Each of these observations is a single word-sized read
	// (first c.closed and second c.recvq.first or c.qcount depending on kind of channel).
	// Because a closed channel cannot transition from 'ready for sending' to
	// 'not ready for sending', even if the channel is closed between the two observations,
	// they imply a moment between the two when the channel was both not yet closed
	// and not ready for sending. We behave as if we observed the channel at that moment,
	// and report that the send cannot proceed.
	//
	// It is okay if the reads are reordered here: if we observe that the channel is not
	// ready for sending and then observe that it is not closed, that implies that the
	// channel wasn't closed during the first observation.
	if !block && c.closed == 0 && ((c.dataqsiz == 0 && c.recvq.first == nil) \|\|
	(c.dataqsiz > 0 && c.qcount == c.dataqsiz)) {
	return false
	}

	var t0 int64
	if blockprofilerate > 0 {
	t0 = cputicks()
	}

	lock(&c.lock)
	if c.closed != 0 {
	unlock(&c.lock)
	panic("send on closed channel")
	}

	if c.dataqsiz == 0 { // synchronous channel
	sg := c.recvq.dequeue()
	if sg != nil { // found a waiting receiver
	if raceenabled {
	racesync(c, sg)
	}
	unlock(&c.lock)

	recvg := sg.g
	if sg.elem != nil {
	syncsend(c, sg, ep)
	}
	recvg.param = unsafe.Pointer(sg)
	if sg.releasetime != 0 {
	sg.releasetime = cputicks()
	}
	goready(recvg, 3)
	return true
	}

	if !block {
	unlock(&c.lock)
	return false
	}

	// no receiver available: block on this channel.
	gp := getg()
	mysg := acquireSudog()
	mysg.releasetime = 0
	if t0 != 0 {
	mysg.releasetime = -1
	}
	mysg.elem = ep
	mysg.waitlink = nil
	gp.waiting = mysg
	mysg.g = gp
	mysg.selectdone = nil
	gp.param = nil
	c.sendq.enqueue(mysg)
	goparkunlock(&c.lock, "chan send", traceEvGoBlockSend, 3)

	// someone woke us up.
	if mysg != gp.waiting {
	throw("G waiting list is corrupted!")
	}
	gp.waiting = nil
	if gp.param == nil {
	if c.closed == 0 {
	throw("chansend: spurious wakeup")
	}
	panic("send on closed channel")
	}
	gp.param = nil
	if mysg.releasetime > 0 {
	blockevent(int64(mysg.releasetime)-t0, 2)
	}
	releaseSudog(mysg)
	return true
	}

	// asynchronous channel
	// wait for some space to write our data
	var t1 int64
	for futile := byte(0); c.qcount >= c.dataqsiz; futile = traceFutileWakeup {
	if !block {
	unlock(&c.lock)
	return false
	}
	gp := getg()
	mysg := acquireSudog()
	mysg.releasetime = 0
	if t0 != 0 {
	mysg.releasetime = -1
	}
	mysg.g = gp
	mysg.elem = nil
	mysg.selectdone = nil
	c.sendq.enqueue(mysg)
	goparkunlock(&c.lock, "chan send", traceEvGoBlockSend\|futile, 3)

	// someone woke us up - try again
	if mysg.releasetime > 0 {
	t1 = mysg.releasetime
	}
	releaseSudog(mysg)
	lock(&c.lock)
	if c.closed != 0 {
	unlock(&c.lock)
	panic("send on closed channel")
	}
	}

	// write our data into the channel buffer
	if raceenabled {
	raceacquire(chanbuf(c, c.sendx))
	racerelease(chanbuf(c, c.sendx))
	}
	typedmemmove(c.elemtype, chanbuf(c, c.sendx), ep)
	c.sendx++
	if c.sendx == c.dataqsiz {
	c.sendx = 0
	}
	c.qcount++

	// wake up a waiting receiver
	sg := c.recvq.dequeue()
	if sg != nil {
	recvg := sg.g
	unlock(&c.lock)
	if sg.releasetime != 0 {
	sg.releasetime = cputicks()
	}
	goready(recvg, 3)
	} else {
	unlock(&c.lock)
	}
	if t1 > 0 {
	blockevent(t1-t0, 2)
	}
	return true
	}

	func syncsend(c hchan, sg sudog, elem unsafe.Pointer) {
	// Send on unbuffered channel is the only operation
	// in the entire runtime where one goroutine
	// writes to the stack of another goroutine. The GC assumes that
	// stack writes only happen when the goroutine is running and are
	// only done by that goroutine. Using a write barrier is sufficient to
	// make up for violating that assumption, but the write barrier has to work.
	// typedmemmove will call heapBitsBulkBarrier, but the target bytes
	// are not in the heap, so that will not help. We arrange to call
	// memmove and typeBitsBulkBarrier instead.
	memmove(sg.elem, elem, c.elemtype.size)
	typeBitsBulkBarrier(c.elemtype, uintptr(sg.elem), c.elemtype.size)
	sg.elem = nil
	}

	func closechan(c *hchan) {
	if c == nil {
	panic("close of nil channel")
	}

	lock(&c.lock)
	if c.closed != 0 {
	unlock(&c.lock)
	panic("close of closed channel")
	}

	if raceenabled {
	callerpc := getcallerpc(unsafe.Pointer(&c))
	racewritepc(unsafe.Pointer(c), callerpc, funcPC(closechan))
	racerelease(unsafe.Pointer(c))
	}

	c.closed = 1

	// release all readers
	for {
	sg := c.recvq.dequeue()
	if sg == nil {
	break
	}
	gp := sg.g
	sg.elem = nil
	gp.param = nil
	if sg.releasetime != 0 {
	sg.releasetime = cputicks()
	}
	goready(gp, 3)
	}

	// release all writers
	for {
	sg := c.sendq.dequeue()
	if sg == nil {
	break
	}
	gp := sg.g
	sg.elem = nil
	gp.param = nil
	if sg.releasetime != 0 {
	sg.releasetime = cputicks()
	}
	goready(gp, 3)
	}
	unlock(&c.lock)
	}

	// entry points for <- c from compiled code
	//go:nosplit
	func chanrecv1(t chantype, c hchan, elem unsafe.Pointer) {
	chanrecv(t, c, elem, true)
	}

	//go:nosplit
	func chanrecv2(t chantype, c hchan, elem unsafe.Pointer) (received bool) {
	_, received = chanrecv(t, c, elem, true)
	return
	}

	// chanrecv receives on channel c and writes the received data to ep.
	// ep may be nil, in which case received data is ignored.
	// If block == false and no elements are available, returns (false, false).
	// Otherwise, if c is closed, zeros *ep and returns (true, false).
	// Otherwise, fills in *ep with an element and returns (true, true).
	func chanrecv(t chantype, c hchan, ep unsafe.Pointer, block bool) (selected, received bool) {
	// raceenabled: don't need to check ep, as it is always on the stack.

	if debugChan {
	print("chanrecv: chan=", c, "\n")
	}

	if c == nil {
	if !block {
	return
	}
	gopark(nil, nil, "chan receive (nil chan)", traceEvGoStop, 2)
	throw("unreachable")
	}

	// Fast path: check for failed non-blocking operation without acquiring the lock.
	//
	// After observing that the channel is not ready for receiving, we observe that the
	// channel is not closed. Each of these observations is a single word-sized read
	// (first c.sendq.first or c.qcount, and second c.closed).
	// Because a channel cannot be reopened, the later observation of the channel
	// being not closed implies that it was also not closed at the moment of the
	// first observation. We behave as if we observed the channel at that moment
	// and report that the receive cannot proceed.
	//
	// The order of operations is important here: reversing the operations can lead to
	// incorrect behavior when racing with a close.
	if !block && (c.dataqsiz == 0 && c.sendq.first == nil \|\|
	c.dataqsiz > 0 && atomicloaduint(&c.qcount) == 0) &&
	atomicload(&c.closed) == 0 {
	return
	}

	var t0 int64
	if blockprofilerate > 0 {
	t0 = cputicks()
	}

	lock(&c.lock)
	if c.dataqsiz == 0 { // synchronous channel
	if c.closed != 0 {
	return recvclosed(c, ep)
	}

	sg := c.sendq.dequeue()
	if sg != nil {
	if raceenabled {
	racesync(c, sg)
	}
	unlock(&c.lock)

	if ep != nil {
	typedmemmove(c.elemtype, ep, sg.elem)
	}
	sg.elem = nil
	gp := sg.g
	gp.param = unsafe.Pointer(sg)
	if sg.releasetime != 0 {
	sg.releasetime = cputicks()
	}
	goready(gp, 3)
	selected = true
	received = true
	return
	}

	if !block {
	unlock(&c.lock)
	return
	}

	// no sender available: block on this channel.
	gp := getg()
	mysg := acquireSudog()
	mysg.releasetime = 0
	if t0 != 0 {
	mysg.releasetime = -1
	}
	mysg.elem = ep
	mysg.waitlink = nil
	gp.waiting = mysg
	mysg.g = gp
	mysg.selectdone = nil
	gp.param = nil
	c.recvq.enqueue(mysg)
	goparkunlock(&c.lock, "chan receive", traceEvGoBlockRecv, 3)

	// someone woke us up
	if mysg != gp.waiting {
	throw("G waiting list is corrupted!")
	}
	gp.waiting = nil
	if mysg.releasetime > 0 {
	blockevent(mysg.releasetime-t0, 2)
	}
	haveData := gp.param != nil
	gp.param = nil
	releaseSudog(mysg)

	if haveData {
	// a sender sent us some data. It already wrote to ep.
	selected = true
	received = true
	return
	}

	lock(&c.lock)
	if c.closed == 0 {
	throw("chanrecv: spurious wakeup")
	}
	return recvclosed(c, ep)
	}

	// asynchronous channel
	// wait for some data to appear
	var t1 int64
	for futile := byte(0); c.qcount <= 0; futile = traceFutileWakeup {
	if c.closed != 0 {
	selected, received = recvclosed(c, ep)
	if t1 > 0 {
	blockevent(t1-t0, 2)
	}
	return
	}

	if !block {
	unlock(&c.lock)
	return
	}

	// wait for someone to send an element
	gp := getg()
	mysg := acquireSudog()
	mysg.releasetime = 0
	if t0 != 0 {
	mysg.releasetime = -1
	}
	mysg.elem = nil
	mysg.g = gp
	mysg.selectdone = nil

	c.recvq.enqueue(mysg)
	goparkunlock(&c.lock, "chan receive", traceEvGoBlockRecv\|futile, 3)

	// someone woke us up - try again
	if mysg.releasetime > 0 {
	t1 = mysg.releasetime
	}
	releaseSudog(mysg)
	lock(&c.lock)
	}

	if raceenabled {
	raceacquire(chanbuf(c, c.recvx))
	racerelease(chanbuf(c, c.recvx))
	}
	if ep != nil {
	typedmemmove(c.elemtype, ep, chanbuf(c, c.recvx))
	}
	memclr(chanbuf(c, c.recvx), uintptr(c.elemsize))

	c.recvx++
	if c.recvx == c.dataqsiz {
	c.recvx = 0
	}
	c.qcount--

	// ping a sender now that there is space
	sg := c.sendq.dequeue()
	if sg != nil {
	gp := sg.g
	unlock(&c.lock)
	if sg.releasetime != 0 {
	sg.releasetime = cputicks()
	}
	goready(gp, 3)
	} else {
	unlock(&c.lock)
	}

	if t1 > 0 {
	blockevent(t1-t0, 2)
	}
	selected = true
	received = true
	return
	}

	// recvclosed is a helper function for chanrecv. Handles cleanup
	// when the receiver encounters a closed channel.
	// Caller must hold c.lock, recvclosed will release the lock.
	func recvclosed(c *hchan, ep unsafe.Pointer) (selected, recevied bool) {
	if raceenabled {
	raceacquire(unsafe.Pointer(c))
	}
	unlock(&c.lock)
	if ep != nil {
	memclr(ep, uintptr(c.elemsize))
	}
	return true, false
	}

	// compiler implements
	//
	// select {
	// case c <- v:
	// ... foo
	// default:
	// ... bar
	// }
	//
	// as
	//
	// if selectnbsend(c, v) {
	// ... foo
	// } else {
	// ... bar
	// }
	//
	func selectnbsend(t chantype, c hchan, elem unsafe.Pointer) (selected bool) {
	return chansend(t, c, elem, false, getcallerpc(unsafe.Pointer(&t)))
	}

	// compiler implements
	//
	// select {
	// case v = <-c:
	// ... foo
	// default:
	// ... bar
	// }
	//
	// as
	//
	// if selectnbrecv(&v, c) {
	// ... foo
	// } else {
	// ... bar
	// }
	//
	func selectnbrecv(t chantype, elem unsafe.Pointer, c hchan) (selected bool) {
	selected, _ = chanrecv(t, c, elem, false)
	return
	}

	// compiler implements
	//
	// select {
	// case v, ok = <-c:
	// ... foo
	// default:
	// ... bar
	// }
	//
	// as
	//
	// if c != nil && selectnbrecv2(&v, &ok, c) {
	// ... foo
	// } else {
	// ... bar
	// }
	//
	func selectnbrecv2(t chantype, elem unsafe.Pointer, received bool, c *hchan) (selected bool) {
	// TODO(khr): just return 2 values from this function, now that it is in Go.
	selected, *received = chanrecv(t, c, elem, false)
	return
	}

	//go:linkname reflect_chansend reflect.chansend
	func reflect_chansend(t chantype, c hchan, elem unsafe.Pointer, nb bool) (selected bool) {
	return chansend(t, c, elem, !nb, getcallerpc(unsafe.Pointer(&t)))
	}

	//go:linkname reflect_chanrecv reflect.chanrecv
	func reflect_chanrecv(t chantype, c hchan, nb bool, elem unsafe.Pointer) (selected bool, received bool) {
	return chanrecv(t, c, elem, !nb)
	}

	//go:linkname reflect_chanlen reflect.chanlen
	func reflect_chanlen(c *hchan) int {
	if c == nil {
	return 0
	}
	return int(c.qcount)
	}

	//go:linkname reflect_chancap reflect.chancap
	func reflect_chancap(c *hchan) int {
	if c == nil {
	return 0
	}
	return int(c.dataqsiz)
	}

	//go:linkname reflect_chanclose reflect.chanclose
	func reflect_chanclose(c *hchan) {
	closechan(c)
	}

	func (q waitq) enqueue(sgp sudog) {
	sgp.next = nil
	x := q.last
	if x == nil {
	sgp.prev = nil
	q.first = sgp
	q.last = sgp
	return
	}
	sgp.prev = x
	x.next = sgp
	q.last = sgp
	}

	func (q waitq) dequeue() sudog {
	for {
	sgp := q.first
	if sgp == nil {
	return nil
	}
	y := sgp.next
	if y == nil {
	q.first = nil
	q.last = nil
	} else {
	y.prev = nil
	q.first = y
	sgp.next = nil // mark as removed (see dequeueSudog)
	}

	// if sgp participates in a select and is already signaled, ignore it
	if sgp.selectdone != nil {
	// claim the right to signal
	if *sgp.selectdone != 0 \|\| !cas(sgp.selectdone, 0, 1) {
	continue
	}
	}

	return sgp
	}
	}

	func racesync(c hchan, sg sudog) {
	racerelease(chanbuf(c, 0))
	raceacquireg(sg.g, chanbuf(c, 0))
	racereleaseg(sg.g, chanbuf(c, 0))
	raceacquire(chanbuf(c, 0))
	}