text: /*
  ** $Id: lgc.c,v 2.38.1.1 2007/12/27 13:02:25 roberto Exp $
  ** Garbage Collector
  ** See Copyright Notice in lua.h
  */
  
  #include <string.h>
  
  #define lgc_c
  #define LUA_CORE
  
  #include "lua.h"
  
  #include "ldebug.h"
  #include "ldo.h"
  #include "lfunc.h"
  #include "lgc.h"
  #include "lmem.h"
  #include "lobject.h"
  #include "lstate.h"
  #include "lstring.h"
  #include "ltable.h"
  #include "ltm.h"
  
  
  #define GCSTEPSIZE	1024u
  #define GCSWEEPMAX	40
  #define GCSWEEPCOST	10
  #define GCFINALIZECOST	100
  
  
  #define maskmarks	cast_byte(~(bitmask(BLACKBIT)|WHITEBITS))
  
  #define makewhite(g,x)	\
     ((x)->gch.marked = cast_byte(((x)->gch.marked & maskmarks) | luaC_white(g)))
  
  #define white2gray(x)	reset2bits((x)->gch.marked, WHITE0BIT, WHITE1BIT)
  #define black2gray(x)	resetbit((x)->gch.marked, BLACKBIT)
  
  #define stringmark(s)	reset2bits((s)->tsv.marked, WHITE0BIT, WHITE1BIT)
  
  
  #define isfinalized(u)		testbit((u)->marked, FINALIZEDBIT)
  #define markfinalized(u)	l_setbit((u)->marked, FINALIZEDBIT)
  
  
  #define KEYWEAK         bitmask(KEYWEAKBIT)
  #define VALUEWEAK       bitmask(VALUEWEAKBIT)
  
  
  
  #define markvalue(g,o) { checkconsistency(o); \
    if (iscollectable(o) && iswhite(gcvalue(o))) reallymarkobject(g,gcvalue(o)); }
  
  #define markobject(g,t) { if (iswhite(obj2gco(t))) \
  		reallymarkobject(g, obj2gco(t)); }
  
  
  #define setthreshold(g)  (g->GCthreshold = (g->estimate/100) * g->gcpause)
  
  
  static void removeentry (Node *n) {
    lua_assert(ttisnil(gval(n)));
    if (iscollectable(gkey(n)))
      setttype(gkey(n), LUA_TDEADKEY);  /* dead key; remove it */
  }
  
  
  static void reallymarkobject (global_State *g, GCObject *o) {
    lua_assert(iswhite(o) && !isdead(g, o));
    white2gray(o);
    switch (o->gch.tt) {
      case LUA_TSTRING: {
        return;
      }
      case LUA_TUSERDATA: {
        Table *mt = gco2u(o)->metatable;
        gray2black(o);  /* udata are never gray */
        if (mt) markobject(g, mt);
        markobject(g, gco2u(o)->env);
        return;
      }
      case LUA_TUPVAL: {
        UpVal *uv = gco2uv(o);
        markvalue(g, uv->v);
        if (uv->v == &uv->u.value)  /* closed? */
          gray2black(o);  /* open upvalues are never black */
        return;
      }
      case LUA_TFUNCTION: {
        gco2cl(o)->c.gclist = g->gray;
        g->gray = o;
        break;
      }
      case LUA_TTABLE: {
        gco2h(o)->gclist = g->gray;
        g->gray = o;
        break;
      }
      case LUA_TTHREAD: {
        gco2th(o)->gclist = g->gray;
        g->gray = o;
        break;
      }
      case LUA_TPROTO: {
        gco2p(o)->gclist = g->gray;
        g->gray = o;
        break;
      }
      default: lua_assert(0);
    }
  }
  
  
  static void marktmu (global_State *g) {
    GCObject *u = g->tmudata;
    if (u) {
      do {
        u = u->gch.next;
        makewhite(g, u);  /* may be marked, if left from previous GC */
        reallymarkobject(g, u);
      } while (u != g->tmudata);
    }
  }
  
  
  /* move `dead' udata that need finalization to list `tmudata' */
  size_t luaC_separateudata (lua_State *L, int all) {
    global_State *g = G(L);
    size_t deadmem = 0;
    GCObject **p = &g->mainthread->next;
    GCObject *curr;
    while ((curr = *p) != NULL) {
      if (!(iswhite(curr) || all) || isfinalized(gco2u(curr)))
        p = &curr->gch.next;  /* don't bother with them */
      else if (fasttm(L, gco2u(curr)->metatable, TM_GC) == NULL) {
        markfinalized(gco2u(curr));  /* don't need finalization */
        p = &curr->gch.next;
      }
      else {  /* must call its gc method */
        deadmem += sizeudata(gco2u(curr));
        markfinalized(gco2u(curr));
        *p = curr->gch.next;
        /* link `curr' at the end of `tmudata' list */
        if (g->tmudata == NULL)  /* list is empty? */
          g->tmudata = curr->gch.next = curr;  /* creates a circular list */
        else {
          curr->gch.next = g->tmudata->gch.next;
          g->tmudata->gch.next = curr;
          g->tmudata = curr;
        }
      }
    }
    return deadmem;
  }
  
  
  static int traversetable (global_State *g, Table *h) {
    int i;
    int weakkey = 0;
    int weakvalue = 0;
    const TValue *mode;
    if (h->metatable)
      markobject(g, h->metatable);
    mode = gfasttm(g, h->metatable, TM_MODE);
    if (mode && ttisstring(mode)) {  /* is there a weak mode? */
      weakkey = (strchr(svalue(mode), 'k') != NULL);
      weakvalue = (strchr(svalue(mode), 'v') != NULL);
      if (weakkey || weakvalue) {  /* is really weak? */
        h->marked &= ~(KEYWEAK | VALUEWEAK);  /* clear bits */
        h->marked |= cast_byte((weakkey << KEYWEAKBIT) |
                               (weakvalue << VALUEWEAKBIT));
        h->gclist = g->weak;  /* must be cleared after GC, ... */
        g->weak = obj2gco(h);  /* ... so put in the appropriate list */
      }
    }
    if (weakkey && weakvalue) return 1;
    if (!weakvalue) {
      i = h->sizearray;
      while (i--)
        markvalue(g, &h->array[i]);
    }
    i = sizenode(h);
    while (i--) {
      Node *n = gnode(h, i);
      lua_assert(ttype(gkey(n)) != LUA_TDEADKEY || ttisnil(gval(n)));
      if (ttisnil(gval(n)))
        removeentry(n);  /* remove empty entries */
      else {
        lua_assert(!ttisnil(gkey(n)));
        if (!weakkey) markvalue(g, gkey(n));
        if (!weakvalue) markvalue(g, gval(n));
      }
    }
    return weakkey || weakvalue;
  }
  
  
  /*
  ** All marks are conditional because a GC may happen while the
  ** prototype is still being created
  */
  static void traverseproto (global_State *g, Proto *f) {
    int i;
    if (f->source) stringmark(f->source);
    for (i=0; i<f->sizek; i++)  /* mark literals */
      markvalue(g, &f->k[i]);
    for (i=0; i<f->sizeupvalues; i++) {  /* mark upvalue names */
      if (f->upvalues[i])
        stringmark(f->upvalues[i]);
    }
    for (i=0; i<f->sizep; i++) {  /* mark nested protos */
      if (f->p[i])
        markobject(g, f->p[i]);
    }
    for (i=0; i<f->sizelocvars; i++) {  /* mark local-variable names */
      if (f->locvars[i].varname)
        stringmark(f->locvars[i].varname);
    }
  }
  
  
  
  static void traverseclosure (global_State *g, Closure *cl) {
    markobject(g, cl->c.env);
    if (cl_isC(cl)) {
      int i;
      for (i=0; i<cl->c.nupvalues; i++)  /* mark its upvalues */
        markvalue(g, &cl->c.upvalue[i]);
    }
    else {
      int i;
      lua_assert(cl->l.nupvalues == cl->l.p->nups);
      markobject(g, cl->l.p);
      for (i=0; i<cl->l.nupvalues; i++) { /* mark its upvalues */
        if(cl->l.upvals[i])
          markobject(g, cl->l.upvals[i]);
      }
    }
  }
  
  
  static void checkstacksizes (lua_State *L, StkId max) {
    int ci_used = cast_int(L->ci - L->base_ci);  /* number of `ci' in use */
    int s_used = cast_int(max - L->stack);  /* part of stack in use */
    if (L->size_ci > LUAI_MAXCALLS)  /* handling overflow? */
      return;  /* do not touch the stacks */
    if (4*ci_used < L->size_ci && 2*BASIC_CI_SIZE < L->size_ci)
      luaD_reallocCI(L, L->size_ci/2);  /* still big enough... */
    condhardstacktests(luaD_reallocCI(L, ci_used + 1));
    if (4*s_used < L->stacksize &&
        2*(BASIC_STACK_SIZE+EXTRA_STACK) < L->stacksize)
      luaD_reallocstack(L, L->stacksize/2);  /* still big enough... */
    condhardstacktests(luaD_reallocstack(L, s_used));
  }
  
  
  static void traversestack (global_State *g, lua_State *l) {
    StkId o, lim;
    CallInfo *ci;
    markvalue(g, gt(l));
    lim = l->top;
    if(l->stack == NULL) return; /* no stack to traverse */
    for (ci = l->base_ci; ci <= l->ci; ci++) {
      lua_assert(ci->top <= l->stack_last);
      if (lim < ci->top) lim = ci->top;
    }
    for (o = l->stack; o < l->top; o++)
      markvalue(g, o);
    for (; o <= lim; o++)
      setnilvalue(o);
    if (!isfixedstack(l)) /* if stack size is fixed, can't resize it. */
      checkstacksizes(l, lim);
  }
  
  
  /*
  ** traverse one gray object, turning it to black.
  ** Returns `quantity' traversed.
  */
  static l_mem propagatemark (global_State *g) {
    GCObject *o = g->gray;
    lua_assert(isgray(o));
    gray2black(o);
    switch (o->gch.tt) {
      case LUA_TTABLE: {
        Table *h = gco2h(o);
        g->gray = h->gclist;
        if (traversetable(g, h))  /* table is weak? */
          black2gray(o);  /* keep it gray */
        return sizeof(Table) + sizeof(TValue) * h->sizearray +
                               sizeof(Node) * sizenode(h);
      }
      case LUA_TFUNCTION: {
        Closure *cl = gco2cl(o);
        g->gray = cl->c.gclist;
        traverseclosure(g, cl);
        return (cl_isC(cl)) ? sizeCclosure(cl->c.nupvalues) :
                             sizeLclosure(cl->l.nupvalues);
      }
      case LUA_TTHREAD: {
        lua_State *th = gco2th(o);
        g->gray = th->gclist;
        th->gclist = g->grayagain;
        g->grayagain = o;
        black2gray(o);
        traversestack(g, th);
        return sizeof(lua_State) + sizeof(TValue) * th->stacksize +
                                   sizeof(CallInfo) * th->size_ci;
      }
      case LUA_TPROTO: {
        Proto *p = gco2p(o);
        g->gray = p->gclist;
        traverseproto(g, p);
        return sizeof(Proto) + sizeof(Instruction) * p->sizecode +
                               sizeof(Proto *) * p->sizep +
                               sizeof(TValue) * p->sizek + 
                               sizeof(int) * p->sizelineinfo +
                               sizeof(LocVar) * p->sizelocvars +
                               sizeof(TString *) * p->sizeupvalues;
      }
      default: lua_assert(0); return 0;
    }
  }
  
  
  static size_t propagateall (global_State *g) {
    size_t m = 0;
    while (g->gray) m += propagatemark(g);
    return m;
  }
  
  
  /*
  ** The next function tells whether a key or value can be cleared from
  ** a weak table. Non-collectable objects are never removed from weak
  ** tables. Strings behave as `values', so are never removed too. for
  ** other objects: if really collected, cannot keep them; for userdata
  ** being finalized, keep them in keys, but not in values
  */
  static int iscleared (const TValue *o, int iskey) {
    if (!iscollectable(o)) return 0;
    if (ttisstring(o)) {
      stringmark(rawtsvalue(o));  /* strings are `values', so are never weak */
      return 0;
    }
    return iswhite(gcvalue(o)) ||
      (ttisuserdata(o) && (!iskey && isfinalized(uvalue(o))));
  }
  
  
  /*
  ** clear collected entries from weaktables
  */
  static void cleartable (GCObject *l) {
    while (l) {
      Table *h = gco2h(l);
      int i = h->sizearray;
      lua_assert(testbit(h->marked, VALUEWEAKBIT) ||
                 testbit(h->marked, KEYWEAKBIT));
      if (testbit(h->marked, VALUEWEAKBIT)) {
        while (i--) {
          TValue *o = &h->array[i];
          if (iscleared(o, 0))  /* value was collected? */
            setnilvalue(o);  /* remove value */
        }
      }
      i = sizenode(h);
      while (i--) {
        Node *n = gnode(h, i);
        if (!ttisnil(gval(n)) &&  /* non-empty entry? */
            (iscleared(key2tval(n), 1) || iscleared(gval(n), 0))) {
          setnilvalue(gval(n));  /* remove value ... */
          removeentry(n);  /* remove entry from table */
        }
      }
      l = h->gclist;
    }
  }
  
  
  static void freeobj (lua_State *L, GCObject *o) {
    switch (o->gch.tt) {
      case LUA_TPROTO: luaF_freeproto(L, gco2p(o)); break;
      case LUA_TFUNCTION: luaF_freeclosure(L, gco2cl(o)); break;
      case LUA_TUPVAL: luaF_freeupval(L, gco2uv(o)); break;
      case LUA_TTABLE: luaH_free(L, gco2h(o)); break;
      case LUA_TTHREAD: {
        lua_assert(gco2th(o) != L && gco2th(o) != G(L)->mainthread);
        luaE_freethread(L, gco2th(o));
        break;
      }
      case LUA_TSTRING: {
        G(L)->strt.nuse--;
        luaM_freemem(L, o, sizestring(gco2ts(o)));
        break;
      }
      case LUA_TUSERDATA: {
        luaM_freemem(L, o, sizeudata(gco2u(o)));
        break;
      }
      default: lua_assert(0);
    }
  }
  
  
  
  #define sweepwholelist(L,p)	sweeplist(L,p,MAX_LUMEM)
  
  
  static GCObject **sweeplist (lua_State *L, GCObject **p, lu_mem count) {
    GCObject *curr;
    global_State *g = G(L);
    int deadmask = otherwhite(g);
    while ((curr = *p) != NULL && count-- > 0) {
      if (curr->gch.tt == LUA_TTHREAD)  /* sweep open upvalues of each thread */
        sweepwholelist(L, &gco2th(curr)->openupval);
      if ((curr->gch.marked ^ WHITEBITS) & deadmask) {  /* not dead? */
        lua_assert(!isdead(g, curr) || testbit(curr->gch.marked, FIXEDBIT));
        makewhite(g, curr);  /* make it white (for next cycle) */
        p = &curr->gch.next;
      }
      else {  /* must erase `curr' */
        lua_assert(isdead(g, curr) || deadmask == bitmask(SFIXEDBIT));
        *p = curr->gch.next;
        freeobj(L, curr);
      }
    }
    return p;
  }
  
  
  static void checkSizes (lua_State *L) {
    global_State *g = G(L);
    /* check size of string hash */
    if (g->strt.nuse < cast(lu_int32, g->strt.size/4) &&
        g->strt.size > MINSTRTABSIZE*2)
      luaS_resize(L, g->strt.size/2);  /* table is too big */
    /* it is not safe to re-size the buffer if it is in use. */
    if (luaZ_bufflen(&g->buff) > 0) return;
    /* check size of buffer */
    if (luaZ_sizebuffer(&g->buff) > LUA_MINBUFFER*2) {  /* buffer too big? */
      size_t newsize = luaZ_sizebuffer(&g->buff) / 2;
      luaZ_resizebuffer(L, &g->buff, newsize);
    }
  }
  
  
  static void GCTM (lua_State *L) {
    global_State *g = G(L);
    GCObject *o = g->tmudata->gch.next;  /* get first element */
    Udata *udata = rawgco2u(o);
    const TValue *tm;
    /* remove udata from `tmudata' */
    if (o == g->tmudata)  /* last element? */
      g->tmudata = NULL;
    else
      g->tmudata->gch.next = udata->uv.next;
    udata->uv.next = g->mainthread->next;  /* return it to `root' list */
    g->mainthread->next = o;
    makewhite(g, o);
    tm = fasttm(L, udata->uv.metatable, TM_GC);
    if (tm != NULL) {
      lu_byte oldah = L->allowhook;
      lu_mem oldt = g->GCthreshold;
      L->allowhook = 0;  /* stop debug hooks during GC tag method */
      g->GCthreshold = 2*g->totalbytes;  /* avoid GC steps */
      setobj2s(L, L->top, tm);
      setuvalue(L, L->top+1, udata);
      L->top += 2;
      luaD_call(L, L->top - 2, 0);
      L->allowhook = oldah;  /* restore hooks */
      g->GCthreshold = oldt;  /* restore threshold */
    }
  }
  
  
  /*
  ** Call all GC tag methods
  */
  void luaC_callGCTM (lua_State *L) {
    while (G(L)->tmudata)
      GCTM(L);
  }
  
  
  void luaC_freeall (lua_State *L) {
    global_State *g = G(L);
    int i;
    g->currentwhite = WHITEBITS | bitmask(SFIXEDBIT);  /* mask to collect all elements */
    sweepwholelist(L, &g->rootgc);
    for (i = 0; i < g->strt.size; i++)  /* free all string lists */
      sweepwholelist(L, &g->strt.hash[i]);
  }
  
  
  static void markmt (global_State *g) {
    int i;
    for (i=0; i<NUM_TAGS; i++)
      if (g->mt[i]) markobject(g, g->mt[i]);
  }
  
  
  /* mark root set */
  static void markroot (lua_State *L) {
    global_State *g = G(L);
    g->gray = NULL;
    g->grayagain = NULL;
    g->weak = NULL;
    markobject(g, g->mainthread);
    /* make global table be traversed before main stack */
    markvalue(g, gt(g->mainthread));
    markvalue(g, registry(L));
    markmt(g);
    g->gcstate = GCSpropagate;
  }
  
  
  static void remarkupvals (global_State *g) {
    UpVal *uv;
    for (uv = g->uvhead.u.l.next; uv != &g->uvhead; uv = uv->u.l.next) {
      lua_assert(uv->u.l.next->u.l.prev == uv && uv->u.l.prev->u.l.next == uv);
      if (isgray(obj2gco(uv)))
        markvalue(g, uv->v);
    }
  }
  
  
  static void atomic (lua_State *L) {
    global_State *g = G(L);
    size_t udsize;  /* total size of userdata to be finalized */
    /* remark occasional upvalues of (maybe) dead threads */
    remarkupvals(g);
    /* traverse objects cautch by write barrier and by 'remarkupvals' */
    propagateall(g);
    /* remark weak tables */
    g->gray = g->weak;
    g->weak = NULL;
    lua_assert(!iswhite(obj2gco(g->mainthread)));
    markobject(g, L);  /* mark running thread */
    markmt(g);  /* mark basic metatables (again) */
    propagateall(g);
    /* remark gray again */
    g->gray = g->grayagain;
    g->grayagain = NULL;
    propagateall(g);
    udsize = luaC_separateudata(L, 0);  /* separate userdata to be finalized */
    marktmu(g);  /* mark `preserved' userdata */
    udsize += propagateall(g);  /* remark, to propagate `preserveness' */
    cleartable(g->weak);  /* remove collected objects from weak tables */
    /* flip current white */
    g->currentwhite = cast_byte(otherwhite(g));
    g->sweepstrgc = 0;
    g->sweepgc = &g->rootgc;
    g->gcstate = GCSsweepstring;
    g->estimate = g->totalbytes - udsize;  /* first estimate */
  }
  
  
  static l_mem singlestep (lua_State *L) {
    global_State *g = G(L);
    /*lua_checkmemory(L);*/
    switch (g->gcstate) {
      case GCSpause: {
        markroot(L);  /* start a new collection */
        return 0;
      }
      case GCSpropagate: {
        if (g->gray)
          return propagatemark(g);
        else {  /* no more `gray' objects */
          atomic(L);  /* finish mark phase */
          return 0;
        }
      }
      case GCSsweepstring: {
        lu_mem old = g->totalbytes;
        sweepwholelist(L, &g->strt.hash[g->sweepstrgc++]);
        if (g->sweepstrgc >= g->strt.size)  /* nothing more to sweep? */
          g->gcstate = GCSsweep;  /* end sweep-string phase */
        lua_assert(old >= g->totalbytes);
        g->estimate -= old - g->totalbytes;
        return GCSWEEPCOST;
      }
      case GCSsweep: {
        lu_mem old = g->totalbytes;
        g->sweepgc = sweeplist(L, g->sweepgc, GCSWEEPMAX);
        if (*g->sweepgc == NULL) {  /* nothing more to sweep? */
          checkSizes(L);
          g->gcstate = GCSfinalize;  /* end sweep phase */
        }
        lua_assert(old >= g->totalbytes);
        g->estimate -= old - g->totalbytes;
        return GCSWEEPMAX*GCSWEEPCOST;
      }
      case GCSfinalize: {
        if (g->tmudata) {
          GCTM(L);
          if (g->estimate > GCFINALIZECOST)
            g->estimate -= GCFINALIZECOST;
          return GCFINALIZECOST;
        }
        else {
          g->gcstate = GCSpause;  /* end collection */
          g->gcdept = 0;
          return 0;
        }
      }
      default: lua_assert(0); return 0;
    }
  }
  
  
  void luaC_step (lua_State *L) {
    global_State *g = G(L);
    if(is_block_gc(L)) return;
    set_block_gc(L);
    l_mem lim = (GCSTEPSIZE/100) * g->gcstepmul;
    if (lim == 0)
      lim = (MAX_LUMEM-1)/2;  /* no limit */
    g->gcdept += g->totalbytes - g->GCthreshold;
    if (g->estimate > g->totalbytes)
      g->estimate = g->totalbytes;
    do {
      lim -= singlestep(L);
      if (g->gcstate == GCSpause)
        break;
    } while (lim > 0);
    if (g->gcstate != GCSpause) {
      if (g->gcdept < GCSTEPSIZE)
        g->GCthreshold = g->totalbytes + GCSTEPSIZE;  /* - lim/g->gcstepmul;*/
      else {
        g->gcdept -= GCSTEPSIZE;
        g->GCthreshold = g->totalbytes;
      }
    }
    else {
      lua_assert(g->totalbytes >= g->estimate);
      setthreshold(g);
    }
    unset_block_gc(L);
  }
  
  
  void luaC_fullgc (lua_State *L) {
    global_State *g = G(L);
    if(is_block_gc(L)) return;
    set_block_gc(L);
    if (g->gcstate <= GCSpropagate) {
      /* reset sweep marks to sweep all elements (returning them to white) */
      g->sweepstrgc = 0;
      g->sweepgc = &g->rootgc;
      /* reset other collector lists */
      g->gray = NULL;
      g->grayagain = NULL;
      g->weak = NULL;
      g->gcstate = GCSsweepstring;
    }
    lua_assert(g->gcstate != GCSpause && g->gcstate != GCSpropagate);
    /* finish any pending sweep phase */
    while (g->gcstate != GCSfinalize) {
      lua_assert(g->gcstate == GCSsweepstring || g->gcstate == GCSsweep);
      singlestep(L);
    }
    markroot(L);
    while (g->gcstate != GCSpause) {
      singlestep(L);
    }
    setthreshold(g);
    unset_block_gc(L);
  }
  
  
  void luaC_barrierf (lua_State *L, GCObject *o, GCObject *v) {
    global_State *g = G(L);
    lua_assert(isblack(o) && iswhite(v) && !isdead(g, v) && !isdead(g, o));
    lua_assert(g->gcstate != GCSfinalize && g->gcstate != GCSpause);
    lua_assert(ttype(&o->gch) != LUA_TTABLE);
    /* must keep invariant? */
    if (g->gcstate == GCSpropagate)
      reallymarkobject(g, v);  /* restore invariant */
    else  /* don't mind */
      makewhite(g, o);  /* mark as white just to avoid other barriers */
  }
  
  
  void luaC_barrierback (lua_State *L, Table *t) {
    global_State *g = G(L);
    GCObject *o = obj2gco(t);
    lua_assert(isblack(o) && !isdead(g, o));
    lua_assert(g->gcstate != GCSfinalize && g->gcstate != GCSpause);
    black2gray(o);  /* make table gray (again) */
    t->gclist = g->grayagain;
    g->grayagain = o;
  }
  
  
  void luaC_marknew (lua_State *L, GCObject *o) {
    global_State *g = G(L);
    o->gch.marked = luaC_white(g);
    if (g->gcstate == GCSpropagate)
      reallymarkobject(g, o);  /* mark new objects as gray during propagate state. */
  }
  
  
  void luaC_link (lua_State *L, GCObject *o, lu_byte tt) {
    global_State *g = G(L);
    o->gch.next = g->rootgc;
    g->rootgc = o;
    o->gch.marked = luaC_white(g);
    o->gch.tt = tt;
  }
  
  
  void luaC_linkupval (lua_State *L, UpVal *uv) {
    global_State *g = G(L);
    GCObject *o = obj2gco(uv);
    o->gch.next = g->rootgc;  /* link upvalue into `rootgc' list */
    g->rootgc = o;
    if (isgray(o)) { 
      if (g->gcstate == GCSpropagate) {
        gray2black(o);  /* closed upvalues need barrier */
        luaC_barrier(L, uv, uv->v);
      }
      else {  /* sweep phase: sweep it (turning it into white) */
        makewhite(g, o);
        lua_assert(g->gcstate != GCSfinalize && g->gcstate != GCSpause);
      }
    }
  }
  
  