gcc/libgo/runtime/chan.goc

// 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
#include "runtime.h"
#include "arch.h"
#include "go-type.h"
#include "race.h"
#include "malloc.h"
#include "chan.h"

uint32 runtime_Hchansize = sizeof(Hchan);

static	void	dequeueg(WaitQ*);
static	SudoG*	dequeue(WaitQ*);
static	void	enqueue(WaitQ*, SudoG*);
static	void	racesync(Hchan*, SudoG*);

static Hchan*
makechan(ChanType *t, int64 hint)
{
	Hchan *c;
	uintptr n;
	const Type *elem;

	elem = t->__element_type;

	// compiler checks this but be safe.
	if(elem->__size >= (1<<16))
		runtime_throw("makechan: invalid channel element type");

	if(hint < 0 || (intgo)hint != hint || (elem->__size > 0 && (uintptr)hint > (MaxMem - sizeof(*c)) / elem->__size))
		runtime_panicstring("makechan: size out of range");

	n = sizeof(*c);
	n = ROUND(n, elem->__align);

	// allocate memory in one call
	c = (Hchan*)runtime_mallocgc(sizeof(*c) + hint*elem->__size, (uintptr)t | TypeInfo_Chan, 0);
	c->elemsize = elem->__size;
	c->elemtype = elem;
	c->dataqsiz = hint;

	if(debug)
		runtime_printf("makechan: chan=%p; elemsize=%D; dataqsiz=%D\n",
			c, (int64)elem->__size, (int64)c->dataqsiz);

	return c;
}

func reflect.makechan(t *ChanType, size uint64) (c *Hchan) {
	c = makechan(t, size);
}

Hchan*
__go_new_channel(ChanType *t, uintptr hint)
{
	return makechan(t, hint);
}

Hchan*
__go_new_channel_big(ChanType *t, uint64 hint)
{
	return makechan(t, hint);
}

/*
 * generic single channel send/recv
 * if the bool pointer is nil,
 * then the full exchange will
 * occur. if pres 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.
 */
static bool
chansend(ChanType *t, Hchan *c, byte *ep, bool block, void *pc)
{
	SudoG *sg;
	SudoG mysg;
	G* gp;
	int64 t0;
	G* g;

	g = runtime_g();

	if(raceenabled)
		runtime_racereadobjectpc(ep, t->__element_type, runtime_getcallerpc(&t), chansend);

	if(c == nil) {
		USED(t);
		if(!block)
			return false;
		runtime_park(nil, nil, "chan send (nil chan)");
		return false;  // not reached
	}

	if(runtime_gcwaiting())
		runtime_gosched();

	if(debug) {
		runtime_printf("chansend: chan=%p\n", c);
	}

	t0 = 0;
	mysg.releasetime = 0;
	if(runtime_blockprofilerate > 0) {
		t0 = runtime_cputicks();
		mysg.releasetime = -1;
	}

	runtime_lock(c);
	if(raceenabled)
		runtime_racereadpc(c, pc, chansend);
	if(c->closed)
		goto closed;

	if(c->dataqsiz > 0)
		goto asynch;

	sg = dequeue(&c->recvq);
	if(sg != nil) {
		if(raceenabled)
			racesync(c, sg);
		runtime_unlock(c);

		gp = sg->g;
		gp->param = sg;
		if(sg->elem != nil)
			runtime_memmove(sg->elem, ep, c->elemsize);
		if(sg->releasetime)
			sg->releasetime = runtime_cputicks();
		runtime_ready(gp);
		return true;
	}

	if(!block) {
		runtime_unlock(c);
		return false;
	}

	mysg.elem = ep;
	mysg.g = g;
	mysg.selectdone = nil;
	g->param = nil;
	enqueue(&c->sendq, &mysg);
	runtime_parkunlock(c, "chan send");

	if(g->param == nil) {
		runtime_lock(c);
		if(!c->closed)
			runtime_throw("chansend: spurious wakeup");
		goto closed;
	}

	if(mysg.releasetime > 0)
		runtime_blockevent(mysg.releasetime - t0, 2);

	return true;

asynch:
	if(c->closed)
		goto closed;

	if(c->qcount >= c->dataqsiz) {
		if(!block) {
			runtime_unlock(c);
			return false;
		}
		mysg.g = g;
		mysg.elem = nil;
		mysg.selectdone = nil;
		enqueue(&c->sendq, &mysg);
		runtime_parkunlock(c, "chan send");

		runtime_lock(c);
		goto asynch;
	}

	if(raceenabled) {
		runtime_raceacquire(chanbuf(c, c->sendx));
		runtime_racerelease(chanbuf(c, c->sendx));
	}

	runtime_memmove(chanbuf(c, c->sendx), ep, c->elemsize);
	if(++c->sendx == c->dataqsiz)
		c->sendx = 0;
	c->qcount++;

	sg = dequeue(&c->recvq);
	if(sg != nil) {
		gp = sg->g;
		runtime_unlock(c);
		if(sg->releasetime)
			sg->releasetime = runtime_cputicks();
		runtime_ready(gp);
	} else
		runtime_unlock(c);
	if(mysg.releasetime > 0)
		runtime_blockevent(mysg.releasetime - t0, 2);
	return true;

closed:
	runtime_unlock(c);
	runtime_panicstring("send on closed channel");
	return false;  // not reached
}


static bool
chanrecv(ChanType *t, Hchan* c, byte *ep, bool block, bool *received)
{
	SudoG *sg;
	SudoG mysg;
	G *gp;
	int64 t0;
	G *g;

	if(runtime_gcwaiting())
		runtime_gosched();

	// raceenabled: don't need to check ep, as it is always on the stack.

	if(debug)
		runtime_printf("chanrecv: chan=%p\n", c);

	g = runtime_g();

	if(c == nil) {
		USED(t);
		if(!block)
			return false;
		runtime_park(nil, nil, "chan receive (nil chan)");
		return false;  // not reached
	}

	t0 = 0;
	mysg.releasetime = 0;
	if(runtime_blockprofilerate > 0) {
		t0 = runtime_cputicks();
		mysg.releasetime = -1;
	}

	runtime_lock(c);
	if(c->dataqsiz > 0)
		goto asynch;

	if(c->closed)
		goto closed;

	sg = dequeue(&c->sendq);
	if(sg != nil) {
		if(raceenabled)
			racesync(c, sg);
		runtime_unlock(c);

		if(ep != nil)
			runtime_memmove(ep, sg->elem, c->elemsize);
		gp = sg->g;
		gp->param = sg;
		if(sg->releasetime)
			sg->releasetime = runtime_cputicks();
		runtime_ready(gp);

		if(received != nil)
			*received = true;
		return true;
	}

	if(!block) {
		runtime_unlock(c);
		return false;
	}

	mysg.elem = ep;
	mysg.g = g;
	mysg.selectdone = nil;
	g->param = nil;
	enqueue(&c->recvq, &mysg);
	runtime_parkunlock(c, "chan receive");

	if(g->param == nil) {
		runtime_lock(c);
		if(!c->closed)
			runtime_throw("chanrecv: spurious wakeup");
		goto closed;
	}

	if(received != nil)
		*received = true;
	if(mysg.releasetime > 0)
		runtime_blockevent(mysg.releasetime - t0, 2);
	return true;

asynch:
	if(c->qcount <= 0) {
		if(c->closed)
			goto closed;

		if(!block) {
			runtime_unlock(c);
			if(received != nil)
				*received = false;
			return false;
		}
		mysg.g = g;
		mysg.elem = nil;
		mysg.selectdone = nil;
		enqueue(&c->recvq, &mysg);
		runtime_parkunlock(c, "chan receive");

		runtime_lock(c);
		goto asynch;
	}

	if(raceenabled) {
		runtime_raceacquire(chanbuf(c, c->recvx));
		runtime_racerelease(chanbuf(c, c->recvx));
	}

	if(ep != nil)
		runtime_memmove(ep, chanbuf(c, c->recvx), c->elemsize);
	runtime_memclr(chanbuf(c, c->recvx), c->elemsize);
	if(++c->recvx == c->dataqsiz)
		c->recvx = 0;
	c->qcount--;

	sg = dequeue(&c->sendq);
	if(sg != nil) {
		gp = sg->g;
		runtime_unlock(c);
		if(sg->releasetime)
			sg->releasetime = runtime_cputicks();
		runtime_ready(gp);
	} else
		runtime_unlock(c);

	if(received != nil)
		*received = true;
	if(mysg.releasetime > 0)
		runtime_blockevent(mysg.releasetime - t0, 2);
	return true;

closed:
	if(ep != nil)
		runtime_memclr(ep, c->elemsize);
	if(received != nil)
		*received = false;
	if(raceenabled)
		runtime_raceacquire(c);
	runtime_unlock(c);
	if(mysg.releasetime > 0)
		runtime_blockevent(mysg.releasetime - t0, 2);
	return true;
}

// The compiler generates a call to __go_send_small to send a value 8
// bytes or smaller.
void
__go_send_small(ChanType *t, Hchan* c, uint64 val)
{
	union
	{
		byte b[sizeof(uint64)];
		uint64 v;
	} u;
	byte *v;

	u.v = val;
#ifndef WORDS_BIGENDIAN
	v = u.b;
#else
	v = u.b + sizeof(uint64) - t->__element_type->__size;
#endif
	chansend(t, c, v, true, runtime_getcallerpc(&t));
}

// The compiler generates a call to __go_send_big to send a value
// larger than 8 bytes or smaller.
void
__go_send_big(ChanType *t, Hchan* c, byte* v)
{
	chansend(t, c, v, true, runtime_getcallerpc(&t));
}

// The compiler generates a call to __go_receive to receive a
// value from a channel.
void
__go_receive(ChanType *t, Hchan* c, byte* v)
{
	chanrecv(t, c, v, true, nil);
}

_Bool runtime_chanrecv2(ChanType *t, Hchan* c, byte* v)
  __asm__ (GOSYM_PREFIX "runtime.chanrecv2");

_Bool
runtime_chanrecv2(ChanType *t, Hchan* c, byte* v)
{
	bool received = false;

	chanrecv(t, c, v, true, &received);
	return received;
}

// compiler implements
//
//	select {
//	case c <- v:
//		... foo
//	default:
//		... bar
//	}
//
// as
//
//	if selectnbsend(c, v) {
//		... foo
//	} else {
//		... bar
//	}
//
func selectnbsend(t *ChanType, c *Hchan, elem *byte) (selected bool) {
	selected = chansend(t, c, elem, false, runtime_getcallerpc(&t));
}

// compiler implements
//
//	select {
//	case v = <-c:
//		... foo
//	default:
//		... bar
//	}
//
// as
//
//	if selectnbrecv(&v, c) {
//		... foo
//	} else {
//		... bar
//	}
//
func selectnbrecv(t *ChanType, elem *byte, c *Hchan) (selected bool) {
	selected = chanrecv(t, c, elem, false, nil);
}

// 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 *byte, received *bool, c *Hchan) (selected bool) {
	bool r;

	selected = chanrecv(t, c, elem, false, received == nil ? nil : &r);
	if(received != nil)
		*received = r;
}

func reflect.chansend(t *ChanType, c *Hchan, elem *byte, nb bool) (selected bool) {
	selected = chansend(t, c, elem, !nb, runtime_getcallerpc(&t));
}

func reflect.chanrecv(t *ChanType, c *Hchan, nb bool, elem *byte) (selected bool, received bool) {
	received = false;
	selected = chanrecv(t, c, elem, !nb, &received);
}

static Select* newselect(int32);

func newselect(size int32) (sel *byte) {
 	sel = (byte*)newselect(size);
}

static Select*
newselect(int32 size)
{
	int32 n;
	Select *sel;

	n = 0;
	if(size > 1)
		n = size-1;

	// allocate all the memory we need in a single allocation
	// start with Select with size cases
	// then lockorder with size entries
	// then pollorder with size entries
	sel = runtime_mal(sizeof(*sel) +
		n*sizeof(sel->scase[0]) +
		size*sizeof(sel->lockorder[0]) +
		size*sizeof(sel->pollorder[0]));

	sel->tcase = size;
	sel->ncase = 0;
	sel->lockorder = (void*)(sel->scase + size);
	sel->pollorder = (void*)(sel->lockorder + size);

	if(debug)
		runtime_printf("newselect s=%p size=%d\n", sel, size);
	return sel;
}

// cut in half to give stack a chance to split
static void selectsend(Select *sel, Hchan *c, int index, void *elem);

func selectsend(sel *Select, c *Hchan, elem *byte, index int32) {
	// nil cases do not compete
	if(c != nil)
		selectsend(sel, c, index, elem);
}

static void
selectsend(Select *sel, Hchan *c, int index, void *elem)
{
	int32 i;
	Scase *cas;

	i = sel->ncase;
	if(i >= sel->tcase)
		runtime_throw("selectsend: too many cases");
	sel->ncase = i+1;
	cas = &sel->scase[i];

	cas->index = index;
	cas->chan = c;
	cas->kind = CaseSend;
	cas->sg.elem = elem;

	if(debug)
		runtime_printf("selectsend s=%p index=%d chan=%p\n",
			sel, cas->index, cas->chan);
}

// cut in half to give stack a chance to split
static void selectrecv(Select *sel, Hchan *c, int index, void *elem, bool*);

func selectrecv(sel *Select, c *Hchan, elem *byte, index int32) {
	// nil cases do not compete
	if(c != nil)
		selectrecv(sel, c, index, elem, nil);
}

func selectrecv2(sel *Select, c *Hchan, elem *byte, received *bool, index int32) {
	// nil cases do not compete
	if(c != nil)
		selectrecv(sel, c, index, elem, received);
}

static void
selectrecv(Select *sel, Hchan *c, int index, void *elem, bool *received)
{
	int32 i;
	Scase *cas;

	i = sel->ncase;
	if(i >= sel->tcase)
		runtime_throw("selectrecv: too many cases");
	sel->ncase = i+1;
	cas = &sel->scase[i];
	cas->index = index;
	cas->chan = c;

	cas->kind = CaseRecv;
	cas->sg.elem = elem;
	cas->receivedp = received;

	if(debug)
		runtime_printf("selectrecv s=%p index=%d chan=%p\n",
			sel, cas->index, cas->chan);
}

// cut in half to give stack a chance to split
static void selectdefault(Select*, int);

func selectdefault(sel *Select, index int32) {
	selectdefault(sel, index);
}

static void
selectdefault(Select *sel, int32 index)
{
	int32 i;
	Scase *cas;

	i = sel->ncase;
	if(i >= sel->tcase)
		runtime_throw("selectdefault: too many cases");
	sel->ncase = i+1;
	cas = &sel->scase[i];
	cas->index = index;
	cas->chan = nil;

	cas->kind = CaseDefault;

	if(debug)
		runtime_printf("selectdefault s=%p index=%d\n",
			sel, cas->index);
}

static void
sellock(Select *sel)
{
	uint32 i;
	Hchan *c, *c0;

	c = nil;
	for(i=0; i<sel->ncase; i++) {
		c0 = sel->lockorder[i];
		if(c0 && c0 != c) {
			c = sel->lockorder[i];
			runtime_lock(c);
		}
	}
}

static void
selunlock(Select *sel)
{
	int32 i, n, r;
	Hchan *c;

	// 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 = (int32)sel->ncase;
	r = 0;
	// skip the default case
	if(n>0 && sel->lockorder[0] == nil)
		r = 1;
	for(i = n-1; i >= r; i--) {
		c = sel->lockorder[i];
		if(i>0 && sel->lockorder[i-1] == c)
			continue;  // will unlock it on the next iteration
		runtime_unlock(c);
	}
}

static bool
selparkcommit(G *gp, void *sel)
{
	USED(gp);
	selunlock(sel);
	return true;
}

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

static int selectgo(Select**);

// selectgo(sel *byte);

func selectgo(sel *Select) (ret int32) {
	return selectgo(&sel);
}

static int
selectgo(Select **selp)
{
	Select *sel;
	uint32 o, i, j, k, done;
	int64 t0;
	Scase *cas, *dfl;
	Hchan *c;
	SudoG *sg;
	G *gp;
	int index;
	G *g;

	sel = *selp;
	if(runtime_gcwaiting())
		runtime_gosched();

	if(debug)
		runtime_printf("select: sel=%p\n", sel);

	g = runtime_g();

	t0 = 0;
	if(runtime_blockprofilerate > 0) {
		t0 = runtime_cputicks();
		for(i=0; i<sel->ncase; i++)
			sel->scase[i].sg.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=0; i<sel->ncase; i++)
		sel->pollorder[i] = i;
	for(i=1; i<sel->ncase; i++) {
		o = sel->pollorder[i];
		j = runtime_fastrand1()%(i+1);
		sel->pollorder[i] = sel->pollorder[j];
		sel->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.
	for(i=0; i<sel->ncase; i++) {
		j = i;
		c = sel->scase[j].chan;
		while(j > 0 && sel->lockorder[k=(j-1)/2] < c) {
			sel->lockorder[j] = sel->lockorder[k];
			j = k;
		}
		sel->lockorder[j] = c;
	}
	for(i=sel->ncase; i-->0; ) {
		c = sel->lockorder[i];
		sel->lockorder[i] = sel->lockorder[0];
		j = 0;
		for(;;) {
			k = j*2+1;
			if(k >= i)
				break;
			if(k+1 < i && sel->lockorder[k] < sel->lockorder[k+1])
				k++;
			if(c < sel->lockorder[k]) {
				sel->lockorder[j] = sel->lockorder[k];
				j = k;
				continue;
			}
			break;
		}
		sel->lockorder[j] = c;
	}
	/*
	for(i=0; i+1<sel->ncase; i++)
		if(sel->lockorder[i] > sel->lockorder[i+1]) {
			runtime_printf("i=%d %p %p\n", i, sel->lockorder[i], sel->lockorder[i+1]);
			runtime_throw("select: broken sort");
		}
	*/
	sellock(sel);

loop:
	// pass 1 - look for something already waiting
	dfl = nil;
	for(i=0; i<sel->ncase; i++) {
		o = sel->pollorder[i];
		cas = &sel->scase[o];
		c = cas->chan;

		switch(cas->kind) {
		case CaseRecv:
			if(c->dataqsiz > 0) {
				if(c->qcount > 0)
					goto asyncrecv;
			} else {
				sg = dequeue(&c->sendq);
				if(sg != nil)
					goto syncrecv;
			}
			if(c->closed)
				goto rclose;
			break;

		case CaseSend:
			if(raceenabled)
				runtime_racereadpc(c, runtime_selectgo, chansend);
			if(c->closed)
				goto sclose;
			if(c->dataqsiz > 0) {
				if(c->qcount < c->dataqsiz)
					goto asyncsend;
			} else {
				sg = dequeue(&c->recvq);
				if(sg != nil)
					goto syncsend;
			}
			break;

		case CaseDefault:
			dfl = cas;
			break;
		}
	}

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


	// pass 2 - enqueue on all chans
	done = 0;
	for(i=0; i<sel->ncase; i++) {
		o = sel->pollorder[i];
		cas = &sel->scase[o];
		c = cas->chan;
		sg = &cas->sg;
		sg->g = g;
		sg->selectdone = &done;

		switch(cas->kind) {
		case CaseRecv:
			enqueue(&c->recvq, sg);
			break;

		case CaseSend:
			enqueue(&c->sendq, sg);
			break;
		}
	}

	g->param = nil;
	runtime_park(selparkcommit, sel, "select");

	sellock(sel);
	sg = g->param;

	// pass 3 - dequeue from unsuccessful chans
	// otherwise they stack up on quiet channels
	for(i=0; i<sel->ncase; i++) {
		cas = &sel->scase[i];
		if(cas != (Scase*)sg) {
			c = cas->chan;
			if(cas->kind == CaseSend)
				dequeueg(&c->sendq);
			else
				dequeueg(&c->recvq);
		}
	}

	if(sg == nil)
		goto loop;

	cas = (Scase*)sg;
	c = cas->chan;

	if(c->dataqsiz > 0)
		runtime_throw("selectgo: shouldn't happen");

	if(debug)
		runtime_printf("wait-return: sel=%p c=%p cas=%p kind=%d\n",
			sel, c, cas, cas->kind);

	if(cas->kind == CaseRecv) {
		if(cas->receivedp != nil)
			*cas->receivedp = true;
	}

	if(raceenabled) {
		if(cas->kind == CaseRecv && cas->sg.elem != nil)
			runtime_racewriteobjectpc(cas->sg.elem, c->elemtype, selectgo, chanrecv);
		else if(cas->kind == CaseSend)
			runtime_racereadobjectpc(cas->sg.elem, c->elemtype, selectgo, chansend);
	}

	selunlock(sel);
	goto retc;

asyncrecv:
	// can receive from buffer
	if(raceenabled) {
		if(cas->sg.elem != nil)
			runtime_racewriteobjectpc(cas->sg.elem, c->elemtype, selectgo, chanrecv);
		runtime_raceacquire(chanbuf(c, c->recvx));
		runtime_racerelease(chanbuf(c, c->recvx));
	}
	if(cas->receivedp != nil)
		*cas->receivedp = true;
	if(cas->sg.elem != nil)
		runtime_memmove(cas->sg.elem, chanbuf(c, c->recvx), c->elemsize);
	runtime_memclr(chanbuf(c, c->recvx), c->elemsize);
	if(++c->recvx == c->dataqsiz)
		c->recvx = 0;
	c->qcount--;
	sg = dequeue(&c->sendq);
	if(sg != nil) {
		gp = sg->g;
		selunlock(sel);
		if(sg->releasetime)
			sg->releasetime = runtime_cputicks();
		runtime_ready(gp);
	} else {
		selunlock(sel);
	}
	goto retc;

asyncsend:
	// can send to buffer
	if(raceenabled) {
		runtime_raceacquire(chanbuf(c, c->sendx));
		runtime_racerelease(chanbuf(c, c->sendx));
		runtime_racereadobjectpc(cas->sg.elem, c->elemtype, selectgo, chansend);
	}
	runtime_memmove(chanbuf(c, c->sendx), cas->sg.elem, c->elemsize);
	if(++c->sendx == c->dataqsiz)
		c->sendx = 0;
	c->qcount++;
	sg = dequeue(&c->recvq);
	if(sg != nil) {
		gp = sg->g;
		selunlock(sel);
		if(sg->releasetime)
			sg->releasetime = runtime_cputicks();
		runtime_ready(gp);
	} else {
		selunlock(sel);
	}
	goto retc;

syncrecv:
	// can receive from sleeping sender (sg)
	if(raceenabled) {
		if(cas->sg.elem != nil)
			runtime_racewriteobjectpc(cas->sg.elem, c->elemtype, selectgo, chanrecv);
		racesync(c, sg);
	}
	selunlock(sel);
	if(debug)
		runtime_printf("syncrecv: sel=%p c=%p o=%d\n", sel, c, o);
	if(cas->receivedp != nil)
		*cas->receivedp = true;
	if(cas->sg.elem != nil)
		runtime_memmove(cas->sg.elem, sg->elem, c->elemsize);
	gp = sg->g;
	gp->param = sg;
	if(sg->releasetime)
		sg->releasetime = runtime_cputicks();
	runtime_ready(gp);
	goto retc;

rclose:
	// read at end of closed channel
	selunlock(sel);
	if(cas->receivedp != nil)
		*cas->receivedp = false;
	if(cas->sg.elem != nil)
		runtime_memclr(cas->sg.elem, c->elemsize);
	if(raceenabled)
		runtime_raceacquire(c);
	goto retc;

syncsend:
	// can send to sleeping receiver (sg)
	if(raceenabled) {
		runtime_racereadobjectpc(cas->sg.elem, c->elemtype, selectgo, chansend);
		racesync(c, sg);
	}
	selunlock(sel);
	if(debug)
		runtime_printf("syncsend: sel=%p c=%p o=%d\n", sel, c, o);
	if(sg->elem != nil)
		runtime_memmove(sg->elem, cas->sg.elem, c->elemsize);
	gp = sg->g;
	gp->param = sg;
	if(sg->releasetime)
		sg->releasetime = runtime_cputicks();
	runtime_ready(gp);

retc:
	// return index corresponding to chosen case
	index = cas->index;
	if(cas->sg.releasetime > 0)
		runtime_blockevent(cas->sg.releasetime - t0, 2);
	runtime_free(sel);
	return index;

sclose:
	// send on closed channel
	selunlock(sel);
	runtime_panicstring("send on closed channel");
	return 0;  // not reached
}

// This struct must match ../reflect/value.go:/runtimeSelect.
typedef struct runtimeSelect runtimeSelect;
struct runtimeSelect
{
	uintptr dir;
	ChanType *typ;
	Hchan *ch;
	byte *val;
};

// This enum must match ../reflect/value.go:/SelectDir.
enum SelectDir {
	SelectSend = 1,
	SelectRecv,
	SelectDefault,
};

func reflect.rselect(cases Slice) (chosen int, recvOK bool) {
	int32 i;
	Select *sel;
	runtimeSelect* rcase, *rc;

	chosen = -1;
	recvOK = false;

	rcase = (runtimeSelect*)cases.__values;

	sel = newselect(cases.__count);
	for(i=0; i<cases.__count; i++) {
		rc = &rcase[i];
		switch(rc->dir) {
		case SelectDefault:
			selectdefault(sel, i);
			break;
		case SelectSend:
			if(rc->ch == nil)
				break;
			selectsend(sel, rc->ch, i, rc->val);
			break;
		case SelectRecv:
			if(rc->ch == nil)
				break;
			selectrecv(sel, rc->ch, i, rc->val, &recvOK);
			break;
		}
	}

	chosen = (intgo)(uintptr)selectgo(&sel);
}

static void closechan(Hchan *c, void *pc);

func closechan(c *Hchan) {
	closechan(c, runtime_getcallerpc(&c));
}

func reflect.chanclose(c *Hchan) {
	closechan(c, runtime_getcallerpc(&c));
}

static void
closechan(Hchan *c, void *pc)
{
	SudoG *sg;
	G* gp;

	if(c == nil)
		runtime_panicstring("close of nil channel");

	if(runtime_gcwaiting())
		runtime_gosched();

	runtime_lock(c);
	if(c->closed) {
		runtime_unlock(c);
		runtime_panicstring("close of closed channel");
	}

	if(raceenabled) {
		runtime_racewritepc(c, pc, runtime_closechan);
		runtime_racerelease(c);
	}

	c->closed = true;

	// release all readers
	for(;;) {
		sg = dequeue(&c->recvq);
		if(sg == nil)
			break;
		gp = sg->g;
		gp->param = nil;
		if(sg->releasetime)
			sg->releasetime = runtime_cputicks();
		runtime_ready(gp);
	}

	// release all writers
	for(;;) {
		sg = dequeue(&c->sendq);
		if(sg == nil)
			break;
		gp = sg->g;
		gp->param = nil;
		if(sg->releasetime)
			sg->releasetime = runtime_cputicks();
		runtime_ready(gp);
	}

	runtime_unlock(c);
}

void
__go_builtin_close(Hchan *c)
{
	runtime_closechan(c);
}

func reflect.chanlen(c *Hchan) (len int) {
	if(c == nil)
		len = 0;
	else
		len = c->qcount;
}

intgo
__go_chan_len(Hchan *c)
{
	return reflect_chanlen(c);
}

func reflect.chancap(c *Hchan) (cap int) {
	if(c == nil)
		cap = 0;
	else
		cap = c->dataqsiz;
}

intgo
__go_chan_cap(Hchan *c)
{
	return reflect_chancap(c);
}

static SudoG*
dequeue(WaitQ *q)
{
	SudoG *sgp;

loop:
	sgp = q->first;
	if(sgp == nil)
		return nil;
	q->first = sgp->link;

	// 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 || !runtime_cas(sgp->selectdone, 0, 1))
			goto loop;
	}

	return sgp;
}

static void
dequeueg(WaitQ *q)
{
	SudoG **l, *sgp, *prevsgp;
	G *g;

	g = runtime_g();
	prevsgp = nil;
	for(l=&q->first; (sgp=*l) != nil; l=&sgp->link, prevsgp=sgp) {
		if(sgp->g == g) {
			*l = sgp->link;
			if(q->last == sgp)
				q->last = prevsgp;
			break;
		}
	}
}

static void
enqueue(WaitQ *q, SudoG *sgp)
{
	sgp->link = nil;
	if(q->first == nil) {
		q->first = sgp;
		q->last = sgp;
		return;
	}
	q->last->link = sgp;
	q->last = sgp;
}

static void
racesync(Hchan *c, SudoG *sg)
{
	runtime_racerelease(chanbuf(c, 0));
	runtime_raceacquireg(sg->g, chanbuf(c, 0));
	runtime_racereleaseg(sg->g, chanbuf(c, 0));
	runtime_raceacquire(chanbuf(c, 0));
}