re PR go/52358 (math FAILs on Solaris 8 and 9)

[gcc.git] / libgo / runtime / chan.c

// 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.

#include "runtime.h"
#include "go-type.h"

#define NOSELGEN        1

static  int32   debug   = 0;

typedef struct  WaitQ   WaitQ;
typedef struct  SudoG   SudoG;
typedef struct  Select  Select;
typedef struct  Scase   Scase;

typedef struct  __go_type_descriptor    Type;
typedef struct  __go_channel_type       ChanType;

struct  SudoG
{
        G*      g;              // g and selgen constitute
        uint32  selgen;         // a weak pointer to g
        SudoG*  link;
        byte*   elem;           // data element
};

struct  WaitQ
{
        SudoG*  first;
        SudoG*  last;
};

struct  Hchan
{
        uint32  qcount;                 // total data in the q
        uint32  dataqsiz;               // size of the circular q
        uint16  elemsize;
        bool    closed;
        uint8   elemalign;
        uint32  sendx;                  // send index
        uint32  recvx;                  // receive index
        WaitQ   recvq;                  // list of recv waiters
        WaitQ   sendq;                  // list of send waiters
        Lock;
};

// Buffer follows Hchan immediately in memory.
// chanbuf(c, i) is pointer to the i'th slot in the buffer.
#define chanbuf(c, i) ((byte*)((c)+1)+(uintptr)(c)->elemsize*(i))

enum
{
        // Scase.kind
        CaseRecv,
        CaseSend,
        CaseDefault,
};

struct  Scase
{
        SudoG   sg;                     // must be first member (cast to Scase)
        Hchan*  chan;                   // chan
        uint16  kind;
        uint16  index;                  // index to return
        bool*   receivedp;              // pointer to received bool (recv2)
};

struct  Select
{
        uint16  tcase;                  // total count of scase[]
        uint16  ncase;                  // currently filled scase[]
        uint16* pollorder;              // case poll order
        Hchan** lockorder;              // channel lock order
        Scase   scase[1];               // one per case (in order of appearance)
};

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

Hchan*
runtime_makechan_c(ChanType *t, int64 hint)
{
        Hchan *c;
        int32 n;
        const Type *elem;
        
        elem = t->__element_type;

        if(hint < 0 || (int32)hint != hint || (elem->__size > 0 && (uintptr)hint > ((uintptr)-1) / elem->__size))
                runtime_panicstring("makechan: size out of range");

        n = sizeof(*c);

        // allocate memory in one call
        c = (Hchan*)runtime_mal(n + hint*elem->__size);
        c->elemsize = elem->__size;
        c->elemalign = elem->__align;
        c->dataqsiz = hint;

        if(debug)
                runtime_printf("makechan: chan=%p; elemsize=%lld; elemalign=%d; dataqsiz=%d\n",
                        c, (long long)elem->__size, elem->__align, c->dataqsiz);

        return c;
}

// For reflect
//      func makechan(typ *ChanType, size uint32) (chan)
uintptr reflect_makechan(ChanType *, uint32)
  asm ("libgo_reflect.reflect.makechan");

uintptr
reflect_makechan(ChanType *t, uint32 size)
{
        void *ret;
        Hchan *c;

        c = runtime_makechan_c(t, size);
        ret = runtime_mal(sizeof(void*));
        __builtin_memcpy(ret, &c, sizeof(void*));
        return (uintptr)ret;
}

// makechan(t *ChanType, hint int64) (hchan *chan any);
Hchan*
__go_new_channel(ChanType *t, uintptr hint)
{
        return runtime_makechan_c(t, hint);
}

Hchan*
__go_new_channel_big(ChanType *t, uint64 hint)
{
        return runtime_makechan_c(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.
 */
void
runtime_chansend(ChanType *t, Hchan *c, byte *ep, bool *pres)
{
        SudoG *sg;
        SudoG mysg;
        G* gp;
        G* g;

        g = runtime_g();

        if(c == nil) {
                USED(t);
                if(pres != nil) {
                        *pres = false;
                        return;
                }
                g->status = Gwaiting;
                g->waitreason = "chan send (nil chan)";
                runtime_gosched();
                return;  // not reached
        }

        if(runtime_gcwaiting)
                runtime_gosched();

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

        runtime_lock(c);
        if(c->closed)
                goto closed;

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

        sg = dequeue(&c->recvq);
        if(sg != nil) {
                runtime_unlock(c);
                
                gp = sg->g;
                gp->param = sg;
                if(sg->elem != nil)
                        runtime_memmove(sg->elem, ep, c->elemsize);
                runtime_ready(gp);

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

        if(pres != nil) {
                runtime_unlock(c);
                *pres = false;
                return;
        }

        mysg.elem = ep;
        mysg.g = g;
        mysg.selgen = NOSELGEN;
        g->param = nil;
        g->status = Gwaiting;
        g->waitreason = "chan send";
        enqueue(&c->sendq, &mysg);
        runtime_unlock(c);
        runtime_gosched();

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

        return;

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

        if(c->qcount >= c->dataqsiz) {
                if(pres != nil) {
                        runtime_unlock(c);
                        *pres = false;
                        return;
                }
                mysg.g = g;
                mysg.elem = nil;
                mysg.selgen = NOSELGEN;
                g->status = Gwaiting;
                g->waitreason = "chan send";
                enqueue(&c->sendq, &mysg);
                runtime_unlock(c);
                runtime_gosched();

                runtime_lock(c);
                goto asynch;
        }
        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);
                runtime_ready(gp);
        } else
                runtime_unlock(c);
        if(pres != nil)
                *pres = true;
        return;

closed:
        runtime_unlock(c);
        runtime_panicstring("send on closed channel");
}


void
runtime_chanrecv(ChanType *t, Hchan* c, byte *ep, bool *selected, bool *received)
{
        SudoG *sg;
        SudoG mysg;
        G *gp;
        G *g;

        if(runtime_gcwaiting)
                runtime_gosched();

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

        g = runtime_g();

        if(c == nil) {
                USED(t);
                if(selected != nil) {
                        *selected = false;
                        return;
                }
                g->status = Gwaiting;
                g->waitreason = "chan receive (nil chan)";
                runtime_gosched();
                return;  // not reached
        }

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

        if(c->closed)
                goto closed;

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

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

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

        if(selected != nil) {
                runtime_unlock(c);
                *selected = false;
                return;
        }

        mysg.elem = ep;
        mysg.g = g;
        mysg.selgen = NOSELGEN;
        g->param = nil;
        g->status = Gwaiting;
        g->waitreason = "chan receive";
        enqueue(&c->recvq, &mysg);
        runtime_unlock(c);
        runtime_gosched();

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

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

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

                if(selected != nil) {
                        runtime_unlock(c);
                        *selected = false;
                        if(received != nil)
                                *received = false;
                        return;
                }
                mysg.g = g;
                mysg.elem = nil;
                mysg.selgen = NOSELGEN;
                g->status = Gwaiting;
                g->waitreason = "chan receive";
                enqueue(&c->recvq, &mysg);
                runtime_unlock(c);
                runtime_gosched();

                runtime_lock(c);
                goto asynch;
        }
        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);
                runtime_ready(gp);
        } else
                runtime_unlock(c);

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

closed:
        if(ep != nil)
                runtime_memclr(ep, c->elemsize);
        if(selected != nil)
                *selected = true;
        if(received != nil)
                *received = false;
        runtime_unlock(c);
}

// 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 *p;

        u.v = val;
#ifndef WORDS_BIGENDIAN
        p = u.b;
#else
        p = u.b + sizeof(uint64) - t->__element_type->__size;
#endif
        runtime_chansend(t, c, p, nil);
}

// 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* p)
{
        runtime_chansend(t, c, p, nil);
}

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

        u.v = 0;
#ifndef WORDS_BIGENDIAN
        p = u.b;
#else
        p = u.b + sizeof(uint64) - t->__element_type->__size;
#endif
        runtime_chanrecv(t, c, p, nil, nil);
        return u.v;
}

// The compiler generates a call to __go_receive_big to receive a
// value larger than 8 bytes.
void
__go_receive_big(ChanType *t, Hchan* c, byte* p)
{
        runtime_chanrecv(t, c, p, nil, nil);
}

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

_Bool
runtime_chanrecv2(ChanType *t, Hchan* c, byte* p)
{
        bool received;

        runtime_chanrecv(t, c, p, nil, &received);
        return received;
}

// func selectnbsend(c chan any, elem any) bool
//
// compiler implements
//
//      select {
//      case c <- v:
//              ... foo
//      default:
//              ... bar
//      }
//
// as
//
//      if selectnbsend(c, v) {
//              ... foo
//      } else {
//              ... bar
//      }
//
_Bool
runtime_selectnbsend(ChanType *t, Hchan *c, byte *p)
{
        bool res;

        runtime_chansend(t, c, p, &res);
        return res;
}

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

        runtime_chanrecv(t, c, v, &selected, nil);
        return selected;
}       

// func selectnbrecv2(elem *any, ok *bool, c chan any) bool
//
// compiler implements
//
//      select {
//      case v, ok = <-c:
//              ... foo
//      default:
//              ... bar
//      }
//
// as
//
//      if c != nil && selectnbrecv2(&v, &ok, c) {
//              ... foo
//      } else {
//              ... bar
//      }
//
_Bool
runtime_selectnbrecv2(ChanType *t, byte *v, _Bool *received, Hchan *c)
{
        bool selected;
        bool r;

        r = false;
        runtime_chanrecv(t, c, v, &selected, received == nil ? nil : &r);
        if(received != nil)
                *received = r;
        return selected;
}       

// For reflect:
//      func chansend(c chan, val iword, nb bool) (selected bool)
// where an iword is the same word an interface value would use:
// the actual data if it fits, or else a pointer to the data.

_Bool reflect_chansend(ChanType *, Hchan *, uintptr, _Bool)
  __asm__("libgo_reflect.reflect.chansend");

_Bool
reflect_chansend(ChanType *t, Hchan *c, uintptr val, _Bool nb)
{
        bool selected;
        bool *sp;
        byte *vp;
        
        if(nb) {
                selected = false;
                sp = (bool*)&selected;
        } else {
                selected = true;
                sp = nil;
        }
        if(__go_is_pointer_type(t->__element_type))
                vp = (byte*)&val;
        else
                vp = (byte*)val;
        runtime_chansend(t, c, vp, sp);
        return selected;
}

// For reflect:
//      func chanrecv(c chan, nb bool) (val iword, selected, received bool)
// where an iword is the same word an interface value would use:
// the actual data if it fits, or else a pointer to the data.

struct chanrecv_ret
{
        uintptr val;
        _Bool selected;
        _Bool received;
};

struct chanrecv_ret reflect_chanrecv(ChanType *, Hchan *, _Bool)
  __asm__("libgo_reflect.reflect.chanrecv");

struct chanrecv_ret
reflect_chanrecv(ChanType *t, Hchan *c, _Bool nb)
{
        struct chanrecv_ret ret;
        byte *vp;
        bool *sp;
        bool selected;
        bool received;

        if(nb) {
                selected = false;
                sp = &selected;
        } else {
                ret.selected = true;
                sp = nil;
        }
        received = false;
        if(__go_is_pointer_type(t->__element_type)) {
                vp = (byte*)&ret.val;
        } else {
                vp = runtime_mal(t->__element_type->__size);
                ret.val = (uintptr)vp;
        }
        runtime_chanrecv(t, c, vp, sp, &received);
        if(nb)
                ret.selected = selected;
        ret.received = received;
        return ret;
}

static void newselect(int32, Select**);

// newselect(size uint32) (sel *byte);

void* runtime_newselect(int) __asm__("runtime.newselect");

void*
runtime_newselect(int size)
{
        Select *sel;

        newselect(size, &sel);
        return (void*)sel;
}

static void
newselect(int32 size, Select **selp)
{
        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);
        *selp = sel;

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

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

// selectsend(sel *byte, hchan *chan any, elem *any) (selected bool);

void runtime_selectsend(Select *, Hchan *, void *, int)
  __asm__("runtime.selectsend");

void
runtime_selectsend(Select *sel, Hchan *c, void *elem, int index)
{
        // nil cases do not compete
        if(c == nil)
                return;
        
        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*);

// selectrecv(sel *byte, hchan *chan any, elem *any) (selected bool);

void runtime_selectrecv(Select *, Hchan *, void *, int)
  __asm__("runtime.selectrecv");

void
runtime_selectrecv(Select *sel, Hchan *c, void *elem, int index)
{
        // nil cases do not compete
        if(c == nil)
                return;

        selectrecv(sel, c, index, elem, nil);
}

// selectrecv2(sel *byte, hchan *chan any, elem *any, received *bool) (selected bool);

void runtime_selectrecv2(Select *, Hchan *, void *, bool *, int)
  __asm__("runtime.selectrecv2");

void
runtime_selectrecv2(Select *sel, Hchan *c, void *elem, bool *received, int index)
{
        // nil cases do not compete
        if(c == nil)
                return;

        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);

// selectdefault(sel *byte) (selected bool);

void runtime_selectdefault(Select *, int) __asm__("runtime.selectdefault");

void
runtime_selectdefault(Select *sel, int index)
{
        selectdefault(sel, index);
}

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

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

void
runtime_block(void)
{
        G *g;

        g = runtime_g();
        g->status = Gwaiting;   // forever
        g->waitreason = "select (no cases)";
        runtime_gosched();
}

static int selectgo(Select**);

// selectgo(sel *byte);

int runtime_selectgo(Select *) __asm__("runtime.selectgo");

int
runtime_selectgo(Select *sel)
{
        return selectgo(&sel);
}

static int
selectgo(Select **selp)
{
        Select *sel;
        uint32 o, i, j;
        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();

        // 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.
        for(i=0; i<sel->ncase; i++) {
                c = sel->scase[i].chan;
                for(j=i; j>0 && sel->lockorder[j-1] >= c; j--)
                        sel->lockorder[j] = sel->lockorder[j-1];
                sel->lockorder[j] = c;
        }
        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(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
        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->selgen = g->selgen;

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

        g->param = nil;
        g->status = Gwaiting;
        g->waitreason = "select";
        selunlock(sel);
        runtime_gosched();

        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: shouldnt 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;
        }

        selunlock(sel);
        goto retc;

asyncrecv:
        // can receive from buffer
        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);
                runtime_ready(gp);
        } else {
                selunlock(sel);
        }
        goto retc;

asyncsend:
        // can send to buffer
        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);
                runtime_ready(gp);
        } else {
                selunlock(sel);
        }
        goto retc;

syncrecv:
        // can receive from sleeping sender (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;
        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);
        goto retc;

syncsend:
        // can send to sleeping receiver (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;
        runtime_ready(gp);

retc:
        // return index corresponding to chosen case
        index = cas->index;
        runtime_free(sel);
        return index;

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

// closechan(sel *byte);
void
runtime_closechan(Hchan *c)
{
        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");
        }

        c->closed = true;

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

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

        runtime_unlock(c);
}

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

// For reflect
//      func chanclose(c chan)

void reflect_chanclose(uintptr) __asm__("libgo_reflect.reflect.chanclose");

void
reflect_chanclose(uintptr c)
{
        runtime_closechan((Hchan*)c);
}

// For reflect
//      func chanlen(c chan) (len int32)

int32 reflect_chanlen(uintptr) __asm__("libgo_reflect.reflect.chanlen");

int32
reflect_chanlen(uintptr ca)
{
        Hchan *c;
        int32 len;

        c = (Hchan*)ca;
        if(c == nil)
                len = 0;
        else
                len = c->qcount;
        return len;
}

int
__go_chan_len(Hchan *c)
{
        return reflect_chanlen((uintptr)c);
}

// For reflect
//      func chancap(c chan) (cap int32)

int32 reflect_chancap(uintptr) __asm__("libgo_reflect.reflect.chancap");

int32
reflect_chancap(uintptr ca)
{
        Hchan *c;
        int32 cap;

        c = (Hchan*)ca;
        if(c == nil)
                cap = 0;
        else
                cap = c->dataqsiz;
        return cap;
}

int
__go_chan_cap(Hchan *c)
{
        return reflect_chancap((uintptr)c);
}

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

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

        // if sgp is stale, ignore it
        if(sgp->selgen != NOSELGEN &&
                (sgp->selgen != sgp->g->selgen ||
                !runtime_cas(&sgp->g->selgen, sgp->selgen, sgp->selgen + 2))) {
                //prints("INVALID PSEUDOG POINTER\n");
                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;
}