text: /*
  ** $Id: lgc.c,v 2.212 2016/03/31 19:02:03 roberto Exp $
  ** Garbage Collector
  ** See Copyright Notice in lua.h
  */
  
  #define lgc_c
  #define LUA_CORE
  
  #include "lprefix.h"
  
  
  #include <string.h>
  
  #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"
  
  
  /*
  ** internal state for collector while inside the atomic phase. The
  ** collector should never be in this state while running regular code.
  */
  #define GCSinsideatomic		(GCSpause + 1)
  
  /*
  ** cost of sweeping one element (the size of a small object divided
  ** by some adjust for the sweep speed)
  */
  #define GCSWEEPCOST	((sizeof(TString) + 4) / 4)
  
  /* maximum number of elements to sweep in each single step */
  #define GCSWEEPMAX	(cast_int((GCSTEPSIZE / GCSWEEPCOST) / 4))
  
  /* cost of calling one finalizer */
  #define GCFINALIZECOST	GCSWEEPCOST
  
  
  /*
  ** macro to adjust 'stepmul': 'stepmul' is actually used like
  ** 'stepmul / STEPMULADJ' (value chosen by tests)
  */
  #define STEPMULADJ		200
  
  
  /*
  ** macro to adjust 'pause': 'pause' is actually used like
  ** 'pause / PAUSEADJ' (value chosen by tests)
  */
  #define PAUSEADJ		100
  
  
  /*
  ** 'makewhite' erases all color bits then sets only the current white
  ** bit
  */
  #define maskcolors	(~(bitmask(BLACKBIT) | WHITEBITS))
  #define makewhite(g,x)	\
   (x->marked = cast_byte((x->marked & maskcolors) | luaC_white(g)))
  
  #define white2gray(x)	resetbits(x->marked, WHITEBITS)
  #define black2gray(x)	resetbit(x->marked, BLACKBIT)
  
  
  #define valiswhite(x)   (iscollectable(x) && iswhite(gcvalue(x)))
  
  #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 (iswhite(t)) reallymarkobject(g, obj2gco(t)); }
  
  /*
  ** mark an object that can be NULL (either because it is really optional,
  ** or it was stripped as debug info, or inside an uncompleted structure)
  */
  #define markobjectN(g,t)	{ if (t) markobject(g,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 collectable object 'o' into list pointed by 'p'
  */
  #define linkgclist(o,p)	((o)->gclist = (p), (p) = obj2gco(o))
  
  
  /*
  ** If key is not marked, mark its entry as dead. This allows key to be
  ** collected, but keeps its entry in the table.  A dead node is needed
  ** when Lua looks up for a key (it may be part of a chain) and when
  ** traversing a weak table (key might be removed from the table during
  ** traversal). Other places never manipulate dead keys, because its
  ** associated nil value is enough to signal that the entry is logically
  ** empty.
  */
  static void removeentry (Node *n) {
    lua_assert(ttisnil(gval(n)));
    if (valiswhite(gkey(n)))
      setdeadvalue(wgkey(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 (global_State *g, const TValue *o) {
    if (!iscollectable(o)) return 0;
    else if (ttisstring(o)) {
      markobject(g, tsvalue(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. (If in sweep phase, clear the black
  ** object to white [sweep it] to avoid other barrier calls for this
  ** same 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));
    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.
  */
  void luaC_barrierback_ (lua_State *L, Table *t) {
    global_State *g = G(L);
    lua_assert(isblack(t) && !isdead(g, t));
    black2gray(t);  /* make table gray (again) */
    linkgclist(t, g->grayagain);
  }
  
  
  /*
  ** barrier for assignments to closed upvalues. Because upvalues are
  ** shared among closures, it is impossible to know the color of all
  ** closures pointing to it. So, we assume that the object being assigned
  ** must be marked.
  */
  void luaC_upvalbarrier_ (lua_State *L, UpVal *uv) {
    global_State *g = G(L);
    GCObject *o = gcvalue(uv->v);
    lua_assert(!upisopen(uv));  /* ensured by macro luaC_upvalbarrier */
    if (keepinvariant(g))
      markobject(g, o);
  }
  
  
  void luaC_fix (lua_State *L, GCObject *o) {
    global_State *g = G(L);
    if (g->allgc != o)
  	  return;  /* if object is not 1st in 'allgc' list, it is in global short string table */
    white2gray(o);  /* they will be gray forever */
    g->allgc = o->next;  /* remove object from 'allgc' list */
    o->next = g->fixedgc;  /* link it to 'fixedgc' list */
    g->fixedgc = o;
  }
  
  
  /*
  ** create a new collectable object (with given type and size) and link
  ** it to 'allgc' list.
  */
  GCObject *luaC_newobj (lua_State *L, int tt, size_t sz) {
    global_State *g = G(L);
    GCObject *o = cast(GCObject *, luaM_newobject(L, novariant(tt), sz));
    o->marked = luaC_white(g);
    o->tt = tt;
    o->next = g->allgc;
    g->allgc = o;
    return o;
  }
  
  /* }====================================================== */
  
  
  
  /*
  ** {======================================================
  ** Mark functions
  ** =======================================================
  */
  
  
  /*
  ** mark an object. Userdata, strings, and closed upvalues are visited
  ** and turned black here. 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) {
   reentry:
    white2gray(o);
    switch (o->tt) {
      case LUA_TSHRSTR: {
        gray2black(o);
        g->GCmemtrav += sizelstring(gco2ts(o)->shrlen);
        break;
      }
      case LUA_TLNGSTR: {
        gray2black(o);
        g->GCmemtrav += sizelstring(gco2ts(o)->u.lnglen);
        break;
      }
      case LUA_TUSERDATA: {
        TValue uvalue;
        markobjectN(g, gco2u(o)->metatable);  /* mark its metatable */
        gray2black(o);
        g->GCmemtrav += sizeudata(gco2u(o));
        getuservalue(g->mainthread, gco2u(o), &uvalue);
        if (valiswhite(&uvalue)) {  /* markvalue(g, &uvalue); */
          o = gcvalue(&uvalue);
          goto reentry;
        }
        break;
      }
      case LUA_TLCL: {
        linkgclist(gco2lcl(o), g->gray);
        break;
      }
      case LUA_TCCL: {
        linkgclist(gco2ccl(o), g->gray);
        break;
      }
      case LUA_TTABLE: {
        linkgclist(gco2t(o), g->gray);
        break;
      }
      case LUA_TTHREAD: {
        linkgclist(gco2th(o), g->gray);
        break;
      }
      case LUA_TPROTO: {
        linkgclist(gco2p(o), g->gray);
        break;
      }
      default: lua_assert(0); break;
    }
  }
  
  
  /*
  ** mark metamethods for basic types
  */
  static void markmt (global_State *g) {
    int i;
    for (i=0; i < LUA_NUMTAGS; i++)
      markobjectN(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 = o->next)
      markobject(g, o);
  }
  
  
  /*
  ** Mark all values stored in marked open upvalues from non-marked threads.
  ** (Values from marked threads were already marked when traversing the
  ** thread.) Remove from the list threads that no longer have upvalues and
  ** not-marked threads.
  */
  static void remarkupvals (global_State *g) {
    lua_State *thread;
    lua_State **p = &g->twups;
    while ((thread = *p) != NULL) {
      lua_assert(!isblack(thread));  /* threads are never black */
      if (isgray(thread) && thread->openupval != NULL)
        p = &thread->twups;  /* keep marked thread with upvalues in the list */
      else {  /* thread is not marked or without upvalues */
        UpVal *uv;
        *p = thread->twups;  /* remove thread from the list */
        thread->twups = thread;  /* mark that it is out of list */
        for (uv = thread->openupval; uv != NULL; uv = uv->u.open.next) {
          if (uv->u.open.touched) {
            markvalue(g, uv->v);  /* remark upvalue's value */
            uv->u.open.touched = 0;
          }
        }
      }
    }
  }
  
  
  /*
  ** mark root set and reset all gray lists, to start a new collection
  */
  static void restartcollection (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
  ** =======================================================
  */
  
  /*
  ** Traverse a table with weak values and link it to proper list. During
  ** propagate phase, keep it in 'grayagain' list, to be revisited in the
  ** atomic phase. In the atomic phase, if table has any white value,
  ** put it in 'weak' list, to be cleared.
  */
  static void traverseweakvalue (global_State *g, Table *h) {
    Node *n, *limit = gnodelast(h);
    /* if there is array part, assume it may have white values (it is not
       worth traversing it now just to check) */
    int hasclears = (h->sizearray > 0);
    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 */
        if (!hasclears && iscleared(g, gval(n)))  /* is there a white value? */
          hasclears = 1;  /* table will have to be cleared */
      }
    }
    if (g->gcstate == GCSpropagate)
      linkgclist(h, g->grayagain);  /* must retraverse it in atomic phase */
    else if (hasclears)
      linkgclist(h, g->weak);  /* has to be cleared later */
  }
  
  
  /*
  ** Traverse an ephemeron table and link it to proper list. Returns true
  ** iff any object was marked during this traversal (which implies that
  ** convergence has to continue). During propagation phase, keep table
  ** in 'grayagain' list, to be visited again in the atomic phase. In
  ** the atomic phase, if table has any white->white entry, it has to
  ** be revisited during ephemeron convergence (as that key may turn
  ** black). Otherwise, if it has any white key, table has to be cleared
  ** (in the atomic phase).
  */
  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 hasww = 0;  /* true if table has entry "white-key -> white-value" */
    Node *n, *limit = gnodelast(h);
    unsigned int i;
    /* traverse array part */
    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(g, gkey(n))) {  /* key is not marked (yet)? */
        hasclears = 1;  /* table must be cleared */
        if (valiswhite(gval(n)))  /* value not marked yet? */
          hasww = 1;  /* white-white entry */
      }
      else if (valiswhite(gval(n))) {  /* value not marked yet? */
        marked = 1;
        reallymarkobject(g, gcvalue(gval(n)));  /* mark it now */
      }
    }
    /* link table into proper list */
    if (g->gcstate == GCSpropagate)
      linkgclist(h, g->grayagain);  /* must retraverse it in atomic phase */
    else if (hasww)  /* table has white->white entries? */
      linkgclist(h, g->ephemeron);  /* have to propagate again */
    else if (hasclears)  /* table has white keys? */
      linkgclist(h, g->allweak);  /* may have to clean white keys */
    return marked;
  }
  
  
  static void traversestrongtable (global_State *g, Table *h) {
    Node *n, *limit = gnodelast(h);
    unsigned 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 lu_mem traversetable (global_State *g, Table *h) {
    const char *weakkey, *weakvalue;
    const TValue *mode = gfasttm(g, h->metatable, TM_MODE);
    markobjectN(g, h->metatable);
    if (mode && ttisstring(mode) &&  /* is there a weak mode? */
        ((weakkey = strchr(svalue(mode), 'k')),
         (weakvalue = strchr(svalue(mode), 'v')),
         (weakkey || weakvalue))) {  /* is really weak? */
      black2gray(h);  /* keep table gray */
      if (!weakkey)  /* strong keys? */
        traverseweakvalue(g, h);
      else if (!weakvalue)  /* strong values? */
        traverseephemeron(g, h);
      else  /* all weak */
        linkgclist(h, g->allweak);  /* nothing to traverse now */
    }
    else  /* not weak */
      traversestrongtable(g, h);
    return sizeof(Table) + sizeof(TValue) * h->sizearray +
                           sizeof(Node) * cast(size_t, sizenode(h));
  }
  
  static int marksharedproto (global_State *g, SharedProto *f) {
    int i;
    if (g != f->l_G)
      return 0;
    markobjectN(g, f->source);
    for (i = 0; i < f->sizeupvalues; i++)  /* mark upvalue names */
      markobjectN(g, f->upvalues[i].name);
    for (i = 0; i < f->sizelocvars; i++)  /* mark local-variable names */
      markobjectN(g, f->locvars[i].varname);
    return sizeof(Instruction) * f->sizecode +
           sizeof(int) * f->sizelineinfo +
           sizeof(LocVar) * f->sizelocvars +
           sizeof(Upvaldesc) * f->sizeupvalues;
  }
  
  /*
  ** Traverse a prototype. (While a prototype is being build, its
  ** arrays can be larger than needed; the extra slots are filled with
  ** NULL, so the use of 'markobjectN')
  */
  static int traverseproto (global_State *g, Proto *f) {
    int i,nk,np;
    if (f->cache && iswhite(f->cache))
      f->cache = NULL;  /* allow cache to be collected */
    if (f->sp == NULL)
      return sizeof(Proto);
    nk = (f->k == NULL) ? 0 : f->sp->sizek;
    np = (f->p == NULL) ? 0 : f->sp->sizep;
    for (i = 0; i < nk; i++)  /* mark literals */
      markvalue(g, &f->k[i]);
    for (i = 0; i < np; i++)  /* mark nested protos */
      markobjectN(g, f->p[i]);
    return sizeof(Proto) + sizeof(Proto *) * np +
                           sizeof(TValue) * nk +
                           marksharedproto(g, f->sp);
  }
  
  
  static lu_mem traverseCclosure (global_State *g, CClosure *cl) {
    int i;
    for (i = 0; i < cl->nupvalues; i++)  /* mark its upvalues */
      markvalue(g, &cl->upvalue[i]);
    return sizeCclosure(cl->nupvalues);
  }
  
  /*
  ** open upvalues point to values in a thread, so those values should
  ** be marked when the thread is traversed except in the atomic phase
  ** (because then the value cannot be changed by the thread and the
  ** thread may not be traversed again)
  */
  static lu_mem traverseLclosure (global_State *g, LClosure *cl) {
    int i;
    markobjectN(g, cl->p);  /* mark its prototype */
    for (i = 0; i < cl->nupvalues; i++) {  /* mark its upvalues */
      UpVal *uv = cl->upvals[i];
      if (uv != NULL) {
        if (upisopen(uv) && g->gcstate != GCSinsideatomic)
          uv->u.open.touched = 1;  /* can be marked in 'remarkupvals' */
        else
          markvalue(g, uv->v);
      }
    }
    return sizeLclosure(cl->nupvalues);
  }
  
  
  static lu_mem traversethread (global_State *g, lua_State *th) {
    StkId o = th->stack;
    if (o == NULL)
      return 1;  /* stack not completely built yet */
    lua_assert(g->gcstate == GCSinsideatomic ||
               th->openupval == NULL || isintwups(th));
    for (; o < th->top; o++)  /* mark live elements in the stack */
      markvalue(g, o);
    if (g->gcstate == GCSinsideatomic) {  /* final traversal? */
      StkId lim = th->stack + th->stacksize;  /* real end of stack */
      for (; o < lim; o++)  /* clear not-marked stack slice */
        setnilvalue(o);
      /* 'remarkupvals' may have removed thread from 'twups' list */ 
      if (!isintwups(th) && th->openupval != NULL) {
        th->twups = g->twups;  /* link it back to the list */
        g->twups = th;
      }
    }
    else if (g->gckind != KGC_EMERGENCY)
      luaD_shrinkstack(th); /* do not change stack in emergency cycle */
    return (sizeof(lua_State) + sizeof(TValue) * th->stacksize +
            sizeof(CallInfo) * th->nci);
  }
  
  
  /*
  ** traverse one gray object, turning it to black (except for threads,
  ** which are always gray).
  */
  static void propagatemark (global_State *g) {
    lu_mem size;
    GCObject *o = g->gray;
    lua_assert(isgray(o));
    gray2black(o);
    switch (o->tt) {
      case LUA_TTABLE: {
        Table *h = gco2t(o);
        g->gray = h->gclist;  /* remove from 'gray' list */
        size = traversetable(g, h);
        break;
      }
      case LUA_TLCL: {
        LClosure *cl = gco2lcl(o);
        g->gray = cl->gclist;  /* remove from 'gray' list */
        size = traverseLclosure(g, cl);
        break;
      }
      case LUA_TCCL: {
        CClosure *cl = gco2ccl(o);
        g->gray = cl->gclist;  /* remove from 'gray' list */
        size = traverseCclosure(g, cl);
        break;
      }
      case LUA_TTHREAD: {
        lua_State *th = gco2th(o);
        g->gray = th->gclist;  /* remove from 'gray' list */
        linkgclist(th, g->grayagain);  /* insert into 'grayagain' list */
        black2gray(o);
        size = traversethread(g, th);
        break;
      }
      case LUA_TPROTO: {
        Proto *p = gco2p(o);
        g->gray = p->gclist;  /* remove from 'gray' list */
        size = traverseproto(g, p);
        break;
      }
      default: lua_assert(0); return;
    }
    g->GCmemtrav += size;
  }
  
  
  static void propagateall (global_State *g) {
    while (g->gray) propagatemark(g);
  }
  
  
  static void convergeephemerons (global_State *g) {
    int changed;
    do {
      GCObject *w;
      GCObject *next = g->ephemeron;  /* get ephemeron list */
      g->ephemeron = NULL;  /* tables may 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 (global_State *g, 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(g, 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 (global_State *g, GCObject *l, GCObject *f) {
    for (; l != f; l = gco2t(l)->gclist) {
      Table *h = gco2t(l);
      Node *n, *limit = gnodelast(h);
      unsigned int i;
      for (i = 0; i < h->sizearray; i++) {
        TValue *o = &h->array[i];
        if (iscleared(g, o))  /* value was collected? */
          setnilvalue(o);  /* remove value */
      }
      for (n = gnode(h, 0); n < limit; n++) {
        if (!ttisnil(gval(n)) && iscleared(g, gval(n))) {
          setnilvalue(gval(n));  /* remove value ... */
          removeentry(n);  /* and remove entry from table */
        }
      }
    }
  }
  
  
  void luaC_upvdeccount (lua_State *L, UpVal *uv) {
    lua_assert(uv->refcount > 0);
    uv->refcount--;
    if (uv->refcount == 0 && !upisopen(uv))
      luaM_free(L, uv);
  }
  
  
  static void freeLclosure (lua_State *L, LClosure *cl) {
    int i;
    for (i = 0; i < cl->nupvalues; i++) {
      UpVal *uv = cl->upvals[i];
      if (uv)
        luaC_upvdeccount(L, uv);
    }
    luaM_freemem(L, cl, sizeLclosure(cl->nupvalues));
  }
  
  
  static void freeobj (lua_State *L, GCObject *o) {
    switch (o->tt) {
      case LUA_TPROTO: luaF_freeproto(L, gco2p(o)); break;
      case LUA_TLCL: {
        freeLclosure(L, gco2lcl(o));
        break;
      }
      case LUA_TCCL: {
        luaM_freemem(L, o, sizeCclosure(gco2ccl(o)->nupvalues));
        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_TSHRSTR:
        luaS_remove(L, gco2ts(o));  /* remove it from hash table */
        luaM_freemem(L, o, sizelstring(gco2ts(o)->shrlen));
        break;
      case LUA_TLNGSTR: {
        luaM_freemem(L, o, sizelstring(gco2ts(o)->u.lnglen));
        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 at most 'count' elements from a list of GCObjects erasing dead
  ** objects, where a dead object is one marked with the old (non current)
  ** white; change all non-dead objects back to white, preparing for next
  ** collection cycle. Return where to continue the traversal or NULL if
  ** list is finished.
  */
  static GCObject **sweeplist (lua_State *L, GCObject **p, lu_mem count) {
    global_State *g = G(L);
    int ow = otherwhite(g);
    int white = luaC_white(g);  /* current white */
    while (*p != NULL && count-- > 0) {
      GCObject *curr = *p;
      int marked = curr->marked;
      if (isdeadm(ow, marked)) {  /* is 'curr' dead? */
        *p = curr->next;  /* remove 'curr' from list */
        freeobj(L, curr);  /* erase 'curr' */
      }
      else {  /* change mark to 'white' */
        curr->marked = cast_byte((marked & maskcolors) | white);
        p = &curr->next;  /* go to next element */
      }
    }
    return (*p == NULL) ? NULL : p;
  }
  
  
  /*
  ** sweep a list until a live object (or end of list)
  */
  static GCObject **sweeptolive (lua_State *L, GCObject **p) {
    GCObject **old = p;
    do {
      p = sweeplist(L, p, 1);
    } while (p == old);
    return p;
  }
  
  /* }====================================================== */
  
  
  /*
  ** {======================================================
  ** Finalization
  ** =======================================================
  */
  
  /*
  ** If possible, shrink string table
  */
  static void checkSizes (lua_State *L, global_State *g) {
    if (g->gckind != KGC_EMERGENCY) {
      l_mem olddebt = g->GCdebt;
      if (g->strt.nuse < g->strt.size / 4)  /* string table too big? */
        luaS_resize(L, g->strt.size / 2);  /* shrink it a little */
      g->GCestimate += g->GCdebt - olddebt;  /* update estimate */
    }
  }
  
  
  static GCObject *udata2finalize (global_State *g) {
    GCObject *o = g->tobefnz;  /* get first element */
    lua_assert(tofinalize(o));
    g->tobefnz = o->next;  /* remove it from 'tobefnz' list */
    o->next = g->allgc;  /* return it to 'allgc' list */
    g->allgc = o;
    resetbit(o->marked, FINALIZEDBIT);  /* object is "normal" again */
    if (issweepphase(g))
      makewhite(g, o);  /* "sweep" object */
    return o;
  }
  
  
  static void dothecall (lua_State *L, void *ud) {
    UNUSED(ud);
    luaD_callnoyield(L, L->top - 2, 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 object? */
          const char *msg = (ttisstring(L->top - 1))
                              ? svalue(L->top - 1)
                              : "no message";
          luaO_pushfstring(L, "error in __gc metamethod (%s)", msg);
          status = LUA_ERRGCMM;  /* error in __gc metamethod */
        }
        luaD_throw(L, status);  /* re-throw error */
      }
    }
  }
  
  
  /*
  ** call a few (up to 'g->gcfinnum') finalizers
  */
  static int runafewfinalizers (lua_State *L) {
    global_State *g = G(L);
    unsigned int i;
    lua_assert(!g->tobefnz || g->gcfinnum > 0);
    for (i = 0; g->tobefnz && i < g->gcfinnum; i++)
      GCTM(L, 1);  /* call one finalizer */
    g->gcfinnum = (!g->tobefnz) ? 0  /* nothing more to finalize? */
                      : g->gcfinnum * 2;  /* else call a few more next time */
    return i;
  }
  
  
  /*
  ** call all pending finalizers
  */
  static void callallpendingfinalizers (lua_State *L) {
    global_State *g = G(L);
    while (g->tobefnz)
      GCTM(L, 0);
  }
  
  
  /*
  ** find last 'next' field in list 'p' list (to add elements in its end)
  */
  static GCObject **findlast (GCObject **p) {
    while (*p != NULL)
      p = &(*p)->next;
    return p;
  }
  
  
  /*
  ** move all unreachable objects (or 'all' objects) that need
  ** finalization from list 'finobj' to list 'tobefnz' (to be finalized)
  */
  static void separatetobefnz (global_State *g, int all) {
    GCObject *curr;
    GCObject **p = &g->finobj;
    GCObject **lastnext = findlast(&g->tobefnz);
    while ((curr = *p) != NULL) {  /* traverse all finalizable objects */
      lua_assert(tofinalize(curr));
      if (!(iswhite(curr) || all))  /* not being collected? */
        p = &curr->next;  /* don't bother with it */
      else {
        *p = curr->next;  /* remove 'curr' from 'finobj' list */
        curr->next = *lastnext;  /* link at the end of 'tobefnz' list */
        *lastnext = curr;
        lastnext = &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 (tofinalize(o) ||                 /* obj. is already marked... */
        gfasttm(g, mt, TM_GC) == NULL)   /* or has no finalizer? */
      return;  /* nothing to be done */
    else {  /* move 'o' to 'finobj' list */
      GCObject **p;
      if (issweepphase(g)) {
        makewhite(g, o);  /* "sweep" object 'o' */
        if (g->sweepgc == &o->next)  /* should not remove 'sweepgc' object */
          g->sweepgc = sweeptolive(L, g->sweepgc);  /* change 'sweepgc' */
      }
      /* search for pointer pointing to 'o' */
      for (p = &g->allgc; *p != o; p = &(*p)->next) { /* empty */ }
      *p = o->next;  /* remove 'o' from 'allgc' list */
      o->next = g->finobj;  /* link it in 'finobj' list */
      g->finobj = o;
      l_setbit(o->marked, FINALIZEDBIT);  /* mark it as such */
    }
  }
  
  /* }====================================================== */
  
  
  
  /*
  ** {======================================================
  ** GC control
  ** =======================================================
  */
  
  
  /*
  ** Set a reasonable "time" to wait before starting a new GC cycle; cycle
  ** will start when memory use hits threshold. (Division by 'estimate'
  ** should be OK: it cannot be zero (because Lua cannot even start with
  ** less than PAUSEADJ bytes).
  */
  static void setpause (global_State *g) {
    l_mem threshold, debt;
    l_mem estimate = g->GCestimate / PAUSEADJ;  /* adjust 'estimate' */
    lua_assert(estimate > 0);
    threshold = (g->gcpause < MAX_LMEM / estimate)  /* overflow? */
              ? estimate * g->gcpause  /* no overflow */
              : MAX_LMEM;  /* overflow; truncate to maximum */
    debt = gettotalbytes(g) - threshold;
    luaE_setdebt(g, debt);
  }
  
  
  /*
  ** Enter first sweep phase.
  ** The call to 'sweeplist' tries to make pointer point to an object
  ** inside the list (instead of to the header), so that the real sweep do
  ** not need to skip objects created between "now" and the start of the
  ** real sweep.
  */
  static void entersweep (lua_State *L) {
    global_State *g = G(L);
    g->gcstate = GCSswpallgc;
    lua_assert(g->sweepgc == NULL);
    g->sweepgc = sweeplist(L, &g->allgc, 1);
  }
  
  
  void luaC_freeallobjects (lua_State *L) {
    global_State *g = G(L);
    separatetobefnz(g, 1);  /* separate all objects with finalizers */
    lua_assert(g->finobj == NULL);
    callallpendingfinalizers(L);
    lua_assert(g->tobefnz == NULL);
    g->currentwhite = WHITEBITS; /* this "white" makes all objects look dead */
    g->gckind = KGC_NORMAL;
    sweepwholelist(L, &g->finobj);
    sweepwholelist(L, &g->allgc);
    sweepwholelist(L, &g->fixedgc);  /* collect fixed objects */
    lua_assert(g->strt.nuse == 0);
  }
  
  
  static l_mem atomic (lua_State *L) {
    global_State *g = G(L);
    l_mem work;
    GCObject *origweak, *origall;
    GCObject *grayagain = g->grayagain;  /* save original list */
    lua_assert(g->ephemeron == NULL && g->weak == NULL);
    lua_assert(!iswhite(g->mainthread));
    g->gcstate = GCSinsideatomic;
    g->GCmemtrav = 0;  /* start counting work */
    markobject(g, L);  /* mark running thread */
    /* registry and global metatables may be changed by API */
    markvalue(g, &g->l_registry);
    markmt(g);  /* mark global metatables */
    /* remark occasional upvalues of (maybe) dead threads */
    remarkupvals(g);
    propagateall(g);  /* propagate changes */
    work = g->GCmemtrav;  /* stop counting (do not recount 'grayagain') */
    g->gray = grayagain;
    propagateall(g);  /* traverse 'grayagain' list */
    g->GCmemtrav = 0;  /* restart counting */
    convergeephemerons(g);
    /* at this point, all strongly accessible objects are marked. */
    /* Clear values from weak tables, before checking finalizers */
    clearvalues(g, g->weak, NULL);
    clearvalues(g, g->allweak, NULL);
    origweak = g->weak; origall = g->allweak;
    work += g->GCmemtrav;  /* stop counting (objects being finalized) */
    separatetobefnz(g, 0);  /* separate objects to be finalized */
    g->gcfinnum = 1;  /* there may be objects to be finalized */
    markbeingfnz(g);  /* mark objects that will be finalized */
    propagateall(g);  /* remark, to propagate 'resurrection' */
    g->GCmemtrav = 0;  /* restart counting */
    convergeephemerons(g);
    /* at this point, all resurrected objects are marked. */
    /* remove dead objects from weak tables */
    clearkeys(g, g->ephemeron, NULL);  /* clear keys from all ephemeron tables */
    clearkeys(g, g->allweak, NULL);  /* clear keys from all 'allweak' tables */
    /* clear values from resurrected weak tables */
    clearvalues(g, g->weak, origweak);
    clearvalues(g, g->allweak, origall);
    luaS_clearcache(g);
    g->currentwhite = cast_byte(otherwhite(g));  /* flip current white */
    work += g->GCmemtrav;  /* complete counting */
    return work;  /* estimate of memory marked by 'atomic' */
  }
  
  
  static lu_mem sweepstep (lua_State *L, global_State *g,
                           int nextstate, GCObject **nextlist) {
    if (g->sweepgc) {
      l_mem olddebt = g->GCdebt;
      g->sweepgc = sweeplist(L, g->sweepgc, GCSWEEPMAX);
      g->GCestimate += g->GCdebt - olddebt;  /* update estimate */
      if (g->sweepgc)  /* is there still something to sweep? */
        return (GCSWEEPMAX * GCSWEEPCOST);
    }
    /* else enter next state */
    g->gcstate = nextstate;
    g->sweepgc = nextlist;
    return 0;
  }
  
  
  static lu_mem singlestep (lua_State *L) {
    global_State *g = G(L);
    switch (g->gcstate) {
      case GCSpause: {
        g->GCmemtrav = g->strt.size * sizeof(GCObject*);
        restartcollection(g);
        g->gcstate = GCSpropagate;
        return g->GCmemtrav;
      }
      case GCSpropagate: {
        g->GCmemtrav = 0;
        lua_assert(g->gray);
        propagatemark(g);
         if (g->gray == NULL)  /* no more gray objects? */
          g->gcstate = GCSatomic;  /* finish propagate phase */
        return g->GCmemtrav;  /* memory traversed in this step */
      }
      case GCSatomic: {
        lu_mem work;
        propagateall(g);  /* make sure gray list is empty */
        work = atomic(L);  /* work is what was traversed by 'atomic' */
        entersweep(L);
        g->GCestimate = gettotalbytes(g);  /* first estimate */;
        return work;
      }
      case GCSswpallgc: {  /* sweep "regular" objects */
        return sweepstep(L, g, GCSswpfinobj, &g->finobj);
      }
      case GCSswpfinobj: {  /* sweep objects with finalizers */
        return sweepstep(L, g, GCSswptobefnz, &g->tobefnz);
      }
      case GCSswptobefnz: {  /* sweep objects to be finalized */
        return sweepstep(L, g, GCSswpend, NULL);
      }
      case GCSswpend: {  /* finish sweeps */
        makewhite(g, g->mainthread);  /* sweep main thread */
        checkSizes(L, g);
        g->gcstate = GCScallfin;
        return 0;
      }
      case GCScallfin: {  /* call remaining finalizers */
        if (g->tobefnz && g->gckind != KGC_EMERGENCY) {
          int n = runafewfinalizers(L);
          return (n * GCFINALIZECOST);
        }
        else {  /* emergency mode or no more finalizers */
          g->gcstate = GCSpause;  /* finish collection */
          return 0;
        }
      }
      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);
  }
  
  
  /*
  ** get GC debt and convert it from Kb to 'work units' (avoid zero debt
  ** and overflows)
  */
  static l_mem getdebt (global_State *g) {
    l_mem debt = g->GCdebt;
    int stepmul = g->gcstepmul;
    if (debt <= 0) return 0;  /* minimal debt */
    else {
      debt = (debt / STEPMULADJ) + 1;
      debt = (debt < MAX_LMEM / stepmul) ? debt * stepmul : MAX_LMEM;
      return debt;
    }
  }
  
  /*
  ** performs a basic GC step when collector is running
  */
  void luaC_step (lua_State *L) {
    global_State *g = G(L);
    l_mem debt = getdebt(g);  /* GC deficit (be paid now) */
    if (!g->gcrunning) {  /* not running? */
      luaE_setdebt(g, -GCSTEPSIZE * 10);  /* avoid being called too often */
      return;
    }
    do {  /* repeat until pause or enough "credit" (negative debt) */
      lu_mem work = singlestep(L);  /* perform one single step */
      debt -= work;
    } while (debt > -GCSTEPSIZE && g->gcstate != GCSpause);
    if (g->gcstate == GCSpause)
      setpause(g);  /* pause until next cycle */
    else {
      debt = (debt / g->gcstepmul) * STEPMULADJ;  /* convert 'work units' to Kb */
      luaE_setdebt(g, debt);
      runafewfinalizers(L);
    }
  }
  
  
  /*
  ** Performs a full GC cycle; if 'isemergency', set a flag to avoid
  ** some operations which could change the interpreter state in some
  ** unexpected ways (running finalizers and shrinking some structures).
  ** Before running the collection, check 'keepinvariant'; if it is true,
  ** there may be some objects marked as black, so the collector has
  ** to sweep all objects to turn them back to white (as white has not
  ** changed, nothing will be collected).
  */
  void luaC_fullgc (lua_State *L, int isemergency) {
    global_State *g = G(L);
    lua_assert(g->gckind == KGC_NORMAL);
    if (isemergency) g->gckind = KGC_EMERGENCY;  /* set flag */
    if (keepinvariant(g)) {  /* black objects? */
      entersweep(L); /* sweep everything to turn them back to white */
    }
    /* finish any pending sweep phase to start a new cycle */
    luaC_runtilstate(L, bitmask(GCSpause));
    luaC_runtilstate(L, ~bitmask(GCSpause));  /* start new collection */
    luaC_runtilstate(L, bitmask(GCScallfin));  /* run up to finalizers */
    /* estimate must be correct after a full GC cycle */
    lua_assert(g->GCestimate == gettotalbytes(g));
    luaC_runtilstate(L, bitmask(GCSpause));  /* finish collection */
    g->gckind = KGC_NORMAL;
    setpause(g);
  }
  
  /* }====================================================== */
  
  
  