text: /*
  ** $Id: lgc.c,v 2.116 2011/12/02 13:18:41 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"
  
  
  
  /* how much to allocate before next GC step */
  #define GCSTEPSIZE	1024
  
  /* maximum number of elements to sweep in each single step */
  #define GCSWEEPMAX	40
  
  /* cost of sweeping one element */
  #define GCSWEEPCOST	1
  
  /* maximum number of finalizers to call in each GC step */
  #define GCFINALIZENUM	4
  
  /* cost of marking the root set */
  #define GCROOTCOST	10
  
  /* cost of atomic step */
  #define GCATOMICCOST	1000
  
  /* basic cost to traverse one object (to be added to the links the
     object may have) */
  #define TRAVCOST	5
  
  
  /*
  ** standard negative debt for GC; a reasonable "time" to wait before
  ** starting a new cycle
  */
  #define stddebt(g)	(-cast(l_mem, gettotalbytes(g)/100) * g->gcpause)
  
  
  /*
  ** 'makewhite' erases all color bits plus the old bit and then
  ** sets only the current white bit
  */
  #define maskcolors	(~(bit2mask(BLACKBIT, OLDBIT) | WHITEBITS))
  #define makewhite(g,x)	\
   (gch(x)->marked = cast_byte((gch(x)->marked & maskcolors) | luaC_white(g)))
  
  #define white2gray(x)	resetbits(gch(x)->marked, WHITEBITS)
  #define black2gray(x)	resetbit(gch(x)->marked, BLACKBIT)
  
  #define stringmark(s)	((void)((s) && resetbits((s)->tsv.marked, WHITEBITS)))
  
  
  #define isfinalized(x)		testbit(gch(x)->marked, FINALIZEDBIT)
  
  #define checkdeadkey(n)	lua_assert(!ttisdeadkey(gkey(n)) || ttisnil(gval(n)))
  
  
  #define checkconsistency(obj)  \
    lua_longassert(!iscollectable(obj) || righttt(obj))
  
  
  #define markvalue(g,o) { checkconsistency(o); \
    if (valiswhite(o)) reallymarkobject(g,gcvalue(o)); }
  
  #define markobject(g,t) { if ((t) && iswhite(obj2gco(t))) \
  		reallymarkobject(g, obj2gco(t)); }
  
  static void reallymarkobject (global_State *g, GCObject *o);
  
  
  /*
  ** {======================================================
  ** Generic functions
  ** =======================================================
  */
  
  
  /*
  ** one after last element in a hash array
  */
  #define gnodelast(h)	gnode(h, cast(size_t, sizenode(h)))
  
  
  /*
  ** link table 'h' into list pointed by 'p'
  */
  #define linktable(h,p)	((h)->gclist = *(p), *(p) = obj2gco(h))
  
  
  /*
  ** if key is not marked, mark its entry as dead (therefore removing it
  ** from the table)
  */
  static void removeentry (Node *n) {
    lua_assert(ttisnil(gval(n)));
    if (valiswhite(gkey(n)))
      setdeadvalue(gkey(n));  /* unused and unmarked key; remove it */
  }
  
  
  /*
  ** 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 objects
  ** being finalized, keep them in keys, but not in values
  */
  static int iscleared (const TValue *o) {
    if (!iscollectable(o)) return 0;
    else if (ttisstring(o)) {
      stringmark(rawtsvalue(o));  /* strings are `values', so are never weak */
      return 0;
    }
    else return iswhite(gcvalue(o));
  }
  
  
  /*
  ** barrier that moves collector forward, that is, mark the white object
  ** being pointed by a black object.
  */
  void luaC_barrier_ (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(isgenerational(g) || g->gcstate != GCSpause);
    lua_assert(gch(o)->tt != LUA_TTABLE);
    if (keepinvariant(g))  /* must keep invariant? */
      reallymarkobject(g, v);  /* restore invariant */
    else {  /* sweep phase */
      lua_assert(issweepphase(g));
      makewhite(g, o);  /* mark main obj. as white to avoid other barriers */
    }
  }
  
  
  /*
  ** barrier that moves collector backward, that is, mark the black object
  ** pointing to a white object as gray again. (Current implementation
  ** only works for tables; access to 'gclist' is not uniform across
  ** different types.)
  */
  void luaC_barrierback_ (lua_State *L, GCObject *o) {
    global_State *g = G(L);
    lua_assert(isblack(o) && !isdead(g, o) && gch(o)->tt == LUA_TTABLE);
    black2gray(o);  /* make object gray (again) */
    gco2t(o)->gclist = g->grayagain;
    g->grayagain = o;
  }
  
  
  /*
  ** barrier for prototypes. When creating first closure (cache is
  ** NULL), use a forward barrier; this may be the only closure of the
  ** prototype (if it is a "regular" function, with a single instance)
  ** and the prototype may be big, so it is better to avoid traversing
  ** it again. Otherwise, use a backward barrier, to avoid marking all
  ** possible instances.
  */
  LUAI_FUNC void luaC_barrierproto_ (lua_State *L, Proto *p, Closure *c) {
    global_State *g = G(L);
    lua_assert(isblack(obj2gco(p)));
    if (p->cache == NULL) {  /* first time? */
      luaC_objbarrier(L, p, c);
    }
    else {  /* use a backward barrier */
      black2gray(obj2gco(p));  /* make prototype gray (again) */
      p->gclist = g->grayagain;
      g->grayagain = obj2gco(p);
    }
  }
  
  
  /*
  ** check color (and invariants) for an upvalue that was closed,
  ** i.e., moved into the 'allgc' list
  */
  void luaC_checkupvalcolor (global_State *g, UpVal *uv) {
    GCObject *o = obj2gco(uv);
    lua_assert(!isblack(o));  /* open upvalues are never black */
    if (isgray(o)) {
      if (keepinvariant(g)) {
        resetoldbit(o);  /* see MOVE OLD rule */
        gray2black(o);  /* it is being visited now */
        markvalue(g, uv->v);
      }
      else {
        lua_assert(issweepphase(g));
        makewhite(g, o);
      }
    }
  }
  
  
  /*
  ** create a new collectable object (with given type and size) and link
  ** it to '*list'. 'offset' tells how many bytes to allocate before the
  ** object itself (used only by states).
  */
  GCObject *luaC_newobj (lua_State *L, int tt, size_t sz, GCObject **list,
                         int offset) {
    global_State *g = G(L);
    GCObject *o = obj2gco(cast(char *, luaM_newobject(L, tt, sz)) + offset);
    if (list == NULL)
      list = &g->allgc;  /* standard list for collectable objects */
    gch(o)->marked = luaC_white(g);
    gch(o)->tt = tt;
    gch(o)->next = *list;
    *list = o;
    return o;
  }
  
  /* }====================================================== */
  
  
  
  /*
  ** {======================================================
  ** Mark functions
  ** =======================================================
  */
  
  
  /*
  ** mark an object. Userdata and closed upvalues are visited and turned
  ** black here. Strings remain gray (it is the same as making them
  ** black). Other objects are marked gray and added to appropriate list
  ** to be visited (and turned black) later. (Open upvalues are already
  ** linked in 'headuv' list.)
  */
  static void reallymarkobject (global_State *g, GCObject *o) {
    lua_assert(iswhite(o) && !isdead(g, o));
    white2gray(o);
    switch (gch(o)->tt) {
      case LUA_TSTRING: {
        return;  /* for strings, gray is as good as black */
      }
      case LUA_TUSERDATA: {
        Table *mt = gco2u(o)->metatable;
        markobject(g, mt);
        markobject(g, gco2u(o)->env);
        gray2black(o);  /* all pointers marked */
        return;
      }
      case LUA_TUPVAL: {
        UpVal *uv = gco2uv(o);
        markvalue(g, uv->v);
        if (uv->v == &uv->u.value)  /* closed? (open upvalues remain gray) */
          gray2black(o);  /* make it black */
        return;
      }
      case LUA_TFUNCTION: {
        gco2cl(o)->c.gclist = g->gray;
        g->gray = o;
        break;
      }
      case LUA_TTABLE: {
        linktable(gco2t(o), &g->gray);
        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);
    }
  }
  
  
  /*
  ** mark metamethods for basic types
  */
  static void markmt (global_State *g) {
    int i;
    for (i=0; i < LUA_NUMTAGS; i++)
      markobject(g, g->mt[i]);
  }
  
  
  /*
  ** mark all objects in list of being-finalized
  */
  static void markbeingfnz (global_State *g) {
    GCObject *o;
    for (o = g->tobefnz; o != NULL; o = gch(o)->next) {
      makewhite(g, o);
      reallymarkobject(g, o);
    }
  }
  
  
  /*
  ** mark all values stored in marked open upvalues. (See comment in
  ** 'lstate.h'.)
  */
  static void remarkupvals (global_State *g) {
    UpVal *uv;
    for (uv = g->uvhead.u.l.next; uv != &g->uvhead; uv = uv->u.l.next) {
      if (isgray(obj2gco(uv)))
        markvalue(g, uv->v);
    }
  }
  
  
  /*
  ** mark root set and reset all gray lists, to start a new
  ** incremental (or full) collection
  */
  static void markroot (global_State *g) {
    g->gray = g->grayagain = NULL;
    g->weak = g->allweak = g->ephemeron = NULL;
    markobject(g, g->mainthread);
    markvalue(g, &g->l_registry);
    markmt(g);
    markbeingfnz(g);  /* mark any finalizing object left from previous cycle */
  }
  
  /* }====================================================== */
  
  
  /*
  ** {======================================================
  ** Traverse functions
  ** =======================================================
  */
  
  static void traverseweakvalue (global_State *g, Table *h) {
    Node *n, *limit = gnodelast(h);
    /* if there is array part, assume it may have white values (do not
       traverse it just to check) */
    int hasclears = (h->sizearray > 0);
    for (n = gnode(h, 0); n < limit; n++) {
      checkdeadkey(n);
      if (ttisnil(gval(n)))  /* entry is empty? */
        removeentry(n);  /* remove it */
      else {
        lua_assert(!ttisnil(gkey(n)));
        markvalue(g, gkey(n));  /* mark key */
        if (!hasclears && iscleared(gval(n)))  /* is there a white value? */
          hasclears = 1;  /* table will have to be cleared */
      }
    }
    if (hasclears)
      linktable(h, &g->weak);  /* has to be cleared later */
    else  /* no white values */
      linktable(h, &g->grayagain);  /* no need to clean */
  }
  
  
  static int traverseephemeron (global_State *g, Table *h) {
    int marked = 0;  /* true if an object is marked in this traversal */
    int hasclears = 0;  /* true if table has white keys */
    int prop = 0;  /* true if table has entry "white-key -> white-value" */
    Node *n, *limit = gnodelast(h);
    int i;
    /* traverse array part (numeric keys are 'strong') */
    for (i = 0; i < h->sizearray; i++) {
      if (valiswhite(&h->array[i])) {
        marked = 1;
        reallymarkobject(g, gcvalue(&h->array[i]));
      }
    }
    /* traverse hash part */
    for (n = gnode(h, 0); n < limit; n++) {
      checkdeadkey(n);
      if (ttisnil(gval(n)))  /* entry is empty? */
        removeentry(n);  /* remove it */
      else if (iscleared(gkey(n))) {  /* key is not marked (yet)? */
        hasclears = 1;  /* table must be cleared */
        if (valiswhite(gval(n)))  /* value not marked yet? */
          prop = 1;  /* must propagate again */
      }
      else if (valiswhite(gval(n))) {  /* value not marked yet? */
        marked = 1;
        reallymarkobject(g, gcvalue(gval(n)));  /* mark it now */
      }
    }
    if (prop)
      linktable(h, &g->ephemeron);  /* have to propagate again */
    else if (hasclears)  /* does table have white keys? */
      linktable(h, &g->allweak);  /* may have to clean white keys */
    else  /* no white keys */
      linktable(h, &g->grayagain);  /* no need to clean */
    return marked;
  }
  
  
  static void traversestrongtable (global_State *g, Table *h) {
    Node *n, *limit = gnodelast(h);
    int i;
    for (i = 0; i < h->sizearray; i++)  /* traverse array part */
      markvalue(g, &h->array[i]);
    for (n = gnode(h, 0); n < limit; n++) {  /* traverse hash part */
      checkdeadkey(n);
      if (ttisnil(gval(n)))  /* entry is empty? */
        removeentry(n);  /* remove it */
      else {
        lua_assert(!ttisnil(gkey(n)));
        markvalue(g, gkey(n));  /* mark key */
        markvalue(g, gval(n));  /* mark value */
      }
    }
  }
  
  
  static int traversetable (global_State *g, Table *h) {
    const TValue *mode = gfasttm(g, h->metatable, TM_MODE);
    markobject(g, h->metatable);
    if (mode && ttisstring(mode)) {  /* is there a weak mode? */
      int weakkey = (strchr(svalue(mode), 'k') != NULL);
      int weakvalue = (strchr(svalue(mode), 'v') != NULL);
      if (weakkey || weakvalue) {  /* is really weak? */
        black2gray(obj2gco(h));  /* keep table gray */
        if (!weakkey) {  /* strong keys? */
          traverseweakvalue(g, h);
          return TRAVCOST + sizenode(h);
        }
        else if (!weakvalue) {  /* strong values? */
          traverseephemeron(g, h);
          return TRAVCOST + h->sizearray + sizenode(h);
        }
        else {
          linktable(h, &g->allweak);  /* nothing to traverse now */
          return TRAVCOST;
        }
      }  /* else go through */
    }
    traversestrongtable(g, h);
    return TRAVCOST + h->sizearray + (2 * sizenode(h));
  }
  
  
  static int traverseproto (global_State *g, Proto *f) {
    int i;
    if (f->cache && iswhite(obj2gco(f->cache)))
      f->cache = NULL;  /* allow cache to be collected */
    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 */
      stringmark(f->upvalues[i].name);
    for (i = 0; i < f->sizep; i++)  /* mark nested protos */
      markobject(g, f->p[i]);
    for (i = 0; i < f->sizelocvars; i++)  /* mark local-variable names */
      stringmark(f->locvars[i].varname);
    return TRAVCOST + f->sizek + f->sizeupvalues + f->sizep + f->sizelocvars;
  }
  
  
  static int traverseclosure (global_State *g, Closure *cl) {
    if (cl->c.isC) {
      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->sizeupvalues);
      markobject(g, cl->l.p);  /* mark its prototype */
      for (i=0; i<cl->l.nupvalues; i++)  /* mark its upvalues */
        markobject(g, cl->l.upvals[i]);
    }
    return TRAVCOST + cl->c.nupvalues;
  }
  
  
  static int traversestack (global_State *g, lua_State *L) {
    StkId o = L->stack;
    if (o == NULL)
      return 1;  /* stack not completely built yet */
    for (; o < L->top; o++)
      markvalue(g, o);
    if (g->gcstate == GCSatomic) {  /* final traversal? */
      StkId lim = L->stack + L->stacksize;  /* real end of stack */
      for (; o < lim; o++)  /* clear not-marked stack slice */
        setnilvalue(o);
    }
    return TRAVCOST + cast_int(o - L->stack);
  }
  
  
  /*
  ** traverse one gray object, turning it to black (except for threads,
  ** which are always gray).
  ** Returns number of values traversed.
  */
  static int propagatemark (global_State *g) {
    GCObject *o = g->gray;
    lua_assert(isgray(o));
    gray2black(o);
    switch (gch(o)->tt) {
      case LUA_TTABLE: {
        Table *h = gco2t(o);
        g->gray = h->gclist;
        return traversetable(g, h);
      }
      case LUA_TFUNCTION: {
        Closure *cl = gco2cl(o);
        g->gray = cl->c.gclist;
        return traverseclosure(g, cl);
      }
      case LUA_TTHREAD: {
        lua_State *th = gco2th(o);
        g->gray = th->gclist;
        th->gclist = g->grayagain;
        g->grayagain = o;
        black2gray(o);
        return traversestack(g, th);
      }
      case LUA_TPROTO: {
        Proto *p = gco2p(o);
        g->gray = p->gclist;
        return traverseproto(g, p);
      }
      default: lua_assert(0); return 0;
    }
  }
  
  
  static void propagateall (global_State *g) {
    while (g->gray) propagatemark(g);
  }
  
  
  static void propagatelist (global_State *g, GCObject *l) {
    lua_assert(g->gray == NULL);  /* no grays left */
    g->gray = l;
    propagateall(g);  /* traverse all elements from 'l' */
  }
  
  /*
  ** retraverse all gray lists. Because tables may be reinserted in other
  ** lists when traversed, traverse the original lists to avoid traversing
  ** twice the same table (which is not wrong, but inefficient)
  */
  static void retraversegrays (global_State *g) {
    GCObject *weak = g->weak;  /* save original lists */
    GCObject *grayagain = g->grayagain;
    GCObject *ephemeron = g->ephemeron;
    g->weak = g->grayagain = g->ephemeron = NULL;
    propagateall(g);  /* traverse main gray list */
    propagatelist(g, grayagain);
    propagatelist(g, weak);
    propagatelist(g, ephemeron);
  }
  
  
  static void convergeephemerons (global_State *g) {
    int changed;
    do {
      GCObject *w;
      GCObject *next = g->ephemeron;  /* get ephemeron list */
      g->ephemeron = NULL;  /* tables will return to this list when traversed */
      changed = 0;
      while ((w = next) != NULL) {
        next = gco2t(w)->gclist;
        if (traverseephemeron(g, gco2t(w))) {  /* traverse marked some value? */
          propagateall(g);  /* propagate changes */
          changed = 1;  /* will have to revisit all ephemeron tables */
        }
      }
    } while (changed);
  }
  
  /* }====================================================== */
  
  
  /*
  ** {======================================================
  ** Sweep Functions
  ** =======================================================
  */
  
  
  /*
  ** clear entries with unmarked keys from all weaktables in list 'l' up
  ** to element 'f'
  */
  static void clearkeys (GCObject *l, GCObject *f) {
    for (; l != f; l = gco2t(l)->gclist) {
      Table *h = gco2t(l);
      Node *n, *limit = gnodelast(h);
      for (n = gnode(h, 0); n < limit; n++) {
        if (!ttisnil(gval(n)) && (iscleared(gkey(n)))) {
          setnilvalue(gval(n));  /* remove value ... */
          removeentry(n);  /* and remove entry from table */
        }
      }
    }
  }
  
  
  /*
  ** clear entries with unmarked values from all weaktables in list 'l' up
  ** to element 'f'
  */
  static void clearvalues (GCObject *l, GCObject *f) {
    for (; l != f; l = gco2t(l)->gclist) {
      Table *h = gco2t(l);
      Node *n, *limit = gnodelast(h);
      int i;
      for (i = 0; i < h->sizearray; i++) {
        TValue *o = &h->array[i];
        if (iscleared(o))  /* value was collected? */
          setnilvalue(o);  /* remove value */
      }
      for (n = gnode(h, 0); n < limit; n++) {
        if (!ttisnil(gval(n)) && iscleared(gval(n))) {
          setnilvalue(gval(n));  /* remove value ... */
          removeentry(n);  /* and remove entry from table */
        }
      }
    }
  }
  
  
  static void freeobj (lua_State *L, GCObject *o) {
    switch (gch(o)->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, gco2t(o)); break;
      case LUA_TTHREAD: luaE_freethread(L, gco2th(o)); break;
      case LUA_TUSERDATA: luaM_freemem(L, o, sizeudata(gco2u(o))); break;
      case LUA_TSTRING: {
        G(L)->strt.nuse--;
        luaM_freemem(L, o, sizestring(gco2ts(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);
  
  
  /*
  ** sweep the (open) upvalues of a thread and resize its stack and
  ** list of call-info structures.
  */
  static void sweepthread (lua_State *L, lua_State *L1) {
    if (L1->stack == NULL) return;  /* stack not completely built yet */
    sweepwholelist(L, &L1->openupval);  /* sweep open upvalues */
    luaE_freeCI(L1);  /* free extra CallInfo slots */
    /* should not change the stack during an emergency gc cycle */
    if (G(L)->gckind != KGC_EMERGENCY)
      luaD_shrinkstack(L1);
  }
  
  
  /*
  ** sweep at most 'count' elements from a list of GCObjects erasing dead
  ** objects, where a dead (not alive) object is one marked with the "old"
  ** (non current) white and not fixed.
  ** In non-generational mode, change all non-dead objects back to white,
  ** preparing for next collection cycle.
  ** In generational mode, keep black objects black, and also mark them as
  ** old; stop when hitting an old object, as all objects after that
  ** one will be old too.
  ** When object is a thread, sweep its list of open upvalues too.
  */
  static GCObject **sweeplist (lua_State *L, GCObject **p, lu_mem count) {
    global_State *g = G(L);
    int ow = otherwhite(g);
    int toclear, toset;  /* bits to clear and to set in all live objects */
    int tostop;  /* stop sweep when this is true */
    l_mem debt = g->GCdebt;  /* current debt */
    if (isgenerational(g)) {  /* generational mode? */
      toclear = ~0;  /* clear nothing */
      toset = bitmask(OLDBIT);  /* set the old bit of all surviving objects */
      tostop = bitmask(OLDBIT);  /* do not sweep old generation */
    }
    else {  /* normal mode */
      toclear = maskcolors;  /* clear all color bits + old bit */
      toset = luaC_white(g);  /* make object white */
      tostop = 0;  /* do not stop */
    }
    while (*p != NULL && count-- > 0) {
      GCObject *curr = *p;
      int marked = gch(curr)->marked;
      if (isdeadm(ow, marked)) {  /* is 'curr' dead? */
        *p = gch(curr)->next;  /* remove 'curr' from list */
        freeobj(L, curr);  /* erase 'curr' */
      }
      else {
        if (gch(curr)->tt == LUA_TTHREAD)
          sweepthread(L, gco2th(curr));  /* sweep thread's upvalues */
        if (testbits(marked, tostop)) {
          static GCObject *nullp = NULL;
          p = &nullp;  /* stop sweeping this list */
          break;
        }
        /* update marks */
        gch(curr)->marked = cast_byte((marked & toclear) | toset);
        p = &gch(curr)->next;  /* go to next element */
      }
    }
    luaE_setdebt(g, debt);  /* sweeping should not change debt */
    return p;
  }
  
  /* }====================================================== */
  
  
  /*
  ** {======================================================
  ** Finalization
  ** =======================================================
  */
  
  static void checkSizes (lua_State *L) {
    global_State *g = G(L);
    if (g->gckind != KGC_EMERGENCY) {  /* do not change sizes in emergency */
      int hs = g->strt.size / 2;  /* half the size of the string table */
      if (g->strt.nuse < cast(lu_int32, hs))  /* using less than that half? */
        luaS_resize(L, hs);  /* halve its size */
      luaZ_freebuffer(L, &g->buff);  /* free concatenation buffer */
    }
  }
  
  
  static GCObject *udata2finalize (global_State *g) {
    GCObject *o = g->tobefnz;  /* get first element */
    lua_assert(isfinalized(o));
    g->tobefnz = gch(o)->next;  /* remove it from 'tobefnz' list */
    gch(o)->next = g->allgc;  /* return it to 'allgc' list */
    g->allgc = o;
    resetbit(gch(o)->marked, SEPARATED);  /* mark that it is not in 'tobefnz' */
    lua_assert(!isold(o));  /* see MOVE OLD rule */
    if (!keepinvariant(g))  /* not keeping invariant? */
      makewhite(g, o);  /* "sweep" object */
    return o;
  }
  
  
  static void dothecall (lua_State *L, void *ud) {
    UNUSED(ud);
    luaD_call(L, L->top - 2, 0, 0);
  }
  
  
  static void GCTM (lua_State *L, int propagateerrors) {
    global_State *g = G(L);
    const TValue *tm;
    TValue v;
    setgcovalue(L, &v, udata2finalize(g));
    tm = luaT_gettmbyobj(L, &v, TM_GC);
    if (tm != NULL && ttisfunction(tm)) {  /* is there a finalizer? */
      int status;
      lu_byte oldah = L->allowhook;
      int running  = g->gcrunning;
      L->allowhook = 0;  /* stop debug hooks during GC metamethod */
      g->gcrunning = 0;  /* avoid GC steps */
      setobj2s(L, L->top, tm);  /* push finalizer... */
      setobj2s(L, L->top + 1, &v);  /* ... and its argument */
      L->top += 2;  /* and (next line) call the finalizer */
      status = luaD_pcall(L, dothecall, NULL, savestack(L, L->top - 2), 0);
      L->allowhook = oldah;  /* restore hooks */
      g->gcrunning = running;  /* restore state */
      if (status != LUA_OK && propagateerrors) {  /* error while running __gc? */
        if (status == LUA_ERRRUN) {  /* is there an error msg.? */
          luaO_pushfstring(L, "error in __gc metamethod (%s)",
                                          lua_tostring(L, -1));
          status = LUA_ERRGCMM;  /* error in __gc metamethod */
        }
        luaD_throw(L, status);  /* re-send error */
      }
    }
  }
  
  
  /*
  ** move all unreachable objects (or 'all' objects) that need
  ** finalization from list 'finobj' to list 'tobefnz' (to be finalized)
  */
  static void separatetobefnz (lua_State *L, int all) {
    global_State *g = G(L);
    GCObject **p = &g->finobj;
    GCObject *curr;
    GCObject **lastnext = &g->tobefnz;
    /* find last 'next' field in 'tobefnz' list (to add elements in its end) */
    while (*lastnext != NULL)
      lastnext = &gch(*lastnext)->next;
    while ((curr = *p) != NULL) {  /* traverse all finalizable objects */
      lua_assert(!isfinalized(curr));
      lua_assert(testbit(gch(curr)->marked, SEPARATED));
      if (!(all || iswhite(curr)))  /* not being collected? */
        p = &gch(curr)->next;  /* don't bother with it */
      else {
        l_setbit(gch(curr)->marked, FINALIZEDBIT); /* won't be finalized again */
        *p = gch(curr)->next;  /* remove 'curr' from 'finobj' list */
        gch(curr)->next = *lastnext;  /* link at the end of 'tobefnz' list */
        *lastnext = curr;
        lastnext = &gch(curr)->next;
      }
    }
  }
  
  
  /*
  ** if object 'o' has a finalizer, remove it from 'allgc' list (must
  ** search the list to find it) and link it in 'finobj' list.
  */
  void luaC_checkfinalizer (lua_State *L, GCObject *o, Table *mt) {
    global_State *g = G(L);
    if (testbit(gch(o)->marked, SEPARATED) || /* obj. is already separated... */
        isfinalized(o) ||                           /* ... or is finalized... */
        gfasttm(g, mt, TM_GC) == NULL)                /* or has no finalizer? */
      return;  /* nothing to be done */
    else {  /* move 'o' to 'finobj' list */
      GCObject **p;
      for (p = &g->allgc; *p != o; p = &gch(*p)->next) ;
      *p = gch(o)->next;  /* remove 'o' from root list */
      gch(o)->next = g->finobj;  /* link it in list 'finobj' */
      g->finobj = o;
      l_setbit(gch(o)->marked, SEPARATED);  /* mark it as such */
      resetoldbit(o);  /* see MOVE OLD rule */
    }
  }
  
  /* }====================================================== */
  
  
  /*
  ** {======================================================
  ** GC control
  ** =======================================================
  */
  
  
  #define sweepphases  \
  	(bitmask(GCSsweepstring) | bitmask(GCSsweepudata) | bitmask(GCSsweep))
  
  /*
  ** change GC mode
  */
  void luaC_changemode (lua_State *L, int mode) {
    global_State *g = G(L);
    if (mode == g->gckind) return;  /* nothing to change */
    if (mode == KGC_GEN) {  /* change to generational mode */
      /* make sure gray lists are consistent */
      luaC_runtilstate(L, bitmask(GCSpropagate));
      g->lastmajormem = gettotalbytes(g);
      g->gckind = KGC_GEN;
    }
    else {  /* change to incremental mode */
      /* sweep all objects to turn them back to white
         (as white has not changed, nothing extra will be collected) */
      g->sweepstrgc = 0;
      g->gcstate = GCSsweepstring;
      g->gckind = KGC_NORMAL;
      luaC_runtilstate(L, ~sweepphases);
    }
  }
  
  
  /*
  ** call all pending finalizers
  */
  static void callallpendingfinalizers (lua_State *L, int propagateerrors) {
    global_State *g = G(L);
    while (g->tobefnz) {
      resetoldbit(g->tobefnz);
      GCTM(L, propagateerrors);
    }
  }
  
  
  void luaC_freeallobjects (lua_State *L) {
    global_State *g = G(L);
    int i;
    separatetobefnz(L, 1);  /* separate all objects with finalizers */
    lua_assert(g->finobj == NULL);
    callallpendingfinalizers(L, 0);
    g->currentwhite = WHITEBITS; /* this "white" makes all objects look dead */
    g->gckind = KGC_NORMAL;
    sweepwholelist(L, &g->finobj);  /* finalizers can create objs. in 'finobj' */
    sweepwholelist(L, &g->allgc);
    for (i = 0; i < g->strt.size; i++)  /* free all string lists */
      sweepwholelist(L, &g->strt.hash[i]);
    lua_assert(g->strt.nuse == 0);
  }
  
  
  static void atomic (lua_State *L) {
    global_State *g = G(L);
    GCObject *origweak, *origall;
    lua_assert(!iswhite(obj2gco(g->mainthread)));
    markobject(g, L);  /* mark running thread */
    /* registry and global metatables may be changed by API */
    markvalue(g, &g->l_registry);
    markmt(g);  /* mark basic metatables */
    /* remark occasional upvalues of (maybe) dead threads */
    remarkupvals(g);
    /* traverse objects caught by write barrier and by 'remarkupvals' */
    retraversegrays(g);
    convergeephemerons(g);
    /* at this point, all strongly accessible objects are marked. */
    /* clear values from weak tables, before checking finalizers */
    clearvalues(g->weak, NULL);
    clearvalues(g->allweak, NULL);
    origweak = g->weak; origall = g->allweak;
    separatetobefnz(L, 0);  /* separate objects to be finalized */
    markbeingfnz(g);  /* mark userdata that will be finalized */
    propagateall(g);  /* remark, to propagate `preserveness' */
    convergeephemerons(g);
    /* at this point, all resurrected objects are marked. */
    /* remove dead objects from weak tables */
    clearkeys(g->ephemeron, NULL);  /* clear keys from all ephemeron tables */
    clearkeys(g->allweak, NULL);  /* clear keys from all allweak tables */
    /* clear values from resurrected weak tables */
    clearvalues(g->weak, origweak);
    clearvalues(g->allweak, origall);
    g->sweepstrgc = 0;  /* prepare to sweep strings */
    g->gcstate = GCSsweepstring;
    g->currentwhite = cast_byte(otherwhite(g));  /* flip current white */
    /*lua_checkmemory(L);*/
  }
  
  
  static l_mem singlestep (lua_State *L) {
    global_State *g = G(L);
    switch (g->gcstate) {
      case GCSpause: {
        if (!isgenerational(g))
          markroot(g);  /* start a new collection */
        /* in any case, root must be marked */
        lua_assert(!iswhite(obj2gco(g->mainthread))
                && !iswhite(gcvalue(&g->l_registry)));
        g->gcstate = GCSpropagate;
        return GCROOTCOST;
      }
      case GCSpropagate: {
        if (g->gray)
          return propagatemark(g);
        else {  /* no more `gray' objects */
          g->gcstate = GCSatomic;  /* finish mark phase */
          atomic(L);
          return GCATOMICCOST;
        }
      }
      case GCSsweepstring: {
        if (g->sweepstrgc < g->strt.size) {
          sweepwholelist(L, &g->strt.hash[g->sweepstrgc++]);
          return GCSWEEPCOST;
        }
        else {  /* no more strings to sweep */
          g->sweepgc = &g->finobj;  /* prepare to sweep finalizable objects */
          g->gcstate = GCSsweepudata;
          return 0;
        }
      }
      case GCSsweepudata: {
        if (*g->sweepgc) {
          g->sweepgc = sweeplist(L, g->sweepgc, GCSWEEPMAX);
          return GCSWEEPMAX*GCSWEEPCOST;
        }
        else {
          g->sweepgc = &g->allgc;  /* go to next phase */
          g->gcstate = GCSsweep;
          return GCSWEEPCOST;
        }
      }
      case GCSsweep: {
        if (*g->sweepgc) {
          g->sweepgc = sweeplist(L, g->sweepgc, GCSWEEPMAX);
          return GCSWEEPMAX*GCSWEEPCOST;
        }
        else {
          /* sweep main thread */
          GCObject *mt = obj2gco(g->mainthread);
          sweeplist(L, &mt, 1);
          checkSizes(L);
          g->gcstate = GCSpause;  /* finish collection */
          return GCSWEEPCOST;
        }
      }
      default: lua_assert(0); return 0;
    }
  }
  
  
  /*
  ** advances the garbage collector until it reaches a state allowed
  ** by 'statemask'
  */
  void luaC_runtilstate (lua_State *L, int statesmask) {
    global_State *g = G(L);
    while (!testbit(statesmask, g->gcstate))
      singlestep(L);
  }
  
  
  static void generationalcollection (lua_State *L) {
    global_State *g = G(L);
    if (g->lastmajormem == 0) {  /* signal for another major collection? */
      luaC_fullgc(L, 0);  /* perform a full regular collection */
      g->lastmajormem = gettotalbytes(g);  /* update control */
    }
    else {
      luaC_runtilstate(L, ~bitmask(GCSpause));  /* run complete cycle */
      luaC_runtilstate(L, bitmask(GCSpause));
      if (gettotalbytes(g) > g->lastmajormem/100 * g->gcmajorinc)
        g->lastmajormem = 0;  /* signal for a major collection */
    }
    luaE_setdebt(g, stddebt(g));
  }
  
  
  static void step (lua_State *L) {
    global_State *g = G(L);
    l_mem lim = g->gcstepmul;  /* how much to work */
    do {  /* always perform at least one single step */
      lim -= singlestep(L);
    } while (lim > 0 && g->gcstate != GCSpause);
    if (g->gcstate != GCSpause)
      luaE_setdebt(g, g->GCdebt - GCSTEPSIZE);
    else
      luaE_setdebt(g, stddebt(g));
  }
  
  
  /*
  ** performs a basic GC step even if the collector is stopped
  */
  void luaC_forcestep (lua_State *L) {
    global_State *g = G(L);
    int i;
    if (isgenerational(g)) generationalcollection(L);
    else step(L);
    for (i = 0; i < GCFINALIZENUM && g->tobefnz; i++)
      GCTM(L, 1);  /* Call a few pending finalizers */
  }
  
  
  /*
  ** performs a basic GC step only if collector is running
  */
  void luaC_step (lua_State *L) {
    if (G(L)->gcrunning) luaC_forcestep(L);
  }
  
  
  /*
  ** performs a full GC cycle; if "isemergency", does not call
  ** finalizers (which could change stack positions)
  */
  void luaC_fullgc (lua_State *L, int isemergency) {
    global_State *g = G(L);
    int origkind = g->gckind;
    lua_assert(origkind != KGC_EMERGENCY);
    if (!isemergency)   /* do not run finalizers during emergency GC */
      callallpendingfinalizers(L, 1);
    if (keepinvariant(g)) {  /* marking phase? */
      /* must sweep all objects to turn them back to white
         (as white has not changed, nothing will be collected) */
      g->sweepstrgc = 0;
      g->gcstate = GCSsweepstring;
    }
    g->gckind = isemergency ? KGC_EMERGENCY : KGC_NORMAL;
    /* finish any pending sweep phase to start a new cycle */
    luaC_runtilstate(L, bitmask(GCSpause));
    /* run entire collector */
    luaC_runtilstate(L, ~bitmask(GCSpause));
    luaC_runtilstate(L, bitmask(GCSpause));
    if (origkind == KGC_GEN) {  /* generational mode? */
      /* generational mode must always start in propagate phase */
      luaC_runtilstate(L, bitmask(GCSpropagate));
    }
    g->gckind = origkind;
    luaE_setdebt(g, stddebt(g));
    if (!isemergency)   /* do not run finalizers during emergency GC */
      callallpendingfinalizers(L, 1);
  }
  
  /* }====================================================== */
  
  
  