text: /*
  ** $Id: lgc.c,v 2.251 2018/02/23 13:21:27 roberto Exp roberto $
  ** Garbage Collector
  ** See Copyright Notice in lua.h
  */
  
  #define lgc_c
  #define LUA_CORE
  
  #include "lprefix.h"
  
  #include <stdio.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"
  
  
  /*
  ** Maximum number of elements to sweep in each single step.
  ** (Large enough to dissipate fixed overheads but small enough
  ** to allow small steps for the collector.)
  */
  #define GCSWEEPMAX	100
  
  /*
  ** Maximum number of finalizers to call in each single step.
  */
  #define GCFINMAX	10
  
  
  /*
  ** Cost of calling one finalizer.
  */
  #define GCFINALIZECOST	50
  
  
  /*
  ** The equivalent, in bytes, of one unit of "work" (visiting a slot,
  ** sweeping an object, etc.)
  */
  #define WORK2MEM	sizeof(TValue)
  
  
  /*
  ** macro to adjust 'pause': 'pause' is actually used like
  ** 'pause / PAUSEADJ' (value chosen by tests)
  */
  #define PAUSEADJ		100
  
  
  /* mask to erase all color bits (plus gen. related stuff) */
  #define maskcolors	(~(bitmask(BLACKBIT) | WHITEBITS | AGEBITS))
  
  
  /* macro to erase all color bits then sets only the current white bit */
  #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 keyiswhite(n)   (keyiscollectable(n) && iswhite(gckey(n)))
  
  
  #define checkconsistency(obj)  \
    lua_longassert(!iscollectable(obj) || righttt(obj))
  
  /*
  ** Protected access to objects in values
  */
  #define gcvalueN(o)     (iscollectable(o) ? gcvalue(o) : NULL)
  
  
  #define markvalue(g,o) { checkconsistency(o); \
    if (valiswhite(o)) reallymarkobject(g,gcvalue(o)); }
  
  #define markkey(g, n)	{ if keyiswhite(n) reallymarkobject(g,gckey(n)); }
  
  #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);
  static lu_mem atomic (lua_State *L);
  
  
  /*
  ** {======================================================
  ** Generic functions
  ** =======================================================
  */
  
  
  /*
  ** one after last element in a hash array
  */
  #define gnodelast(h)	gnode(h, cast_sizet(sizenode(h)))
  
  
  static GCObject **getgclist (GCObject *o) {
    switch (o->tt) {
      case LUA_TTABLE: return &gco2t(o)->gclist;
      case LUA_TLCL: return &gco2lcl(o)->gclist;
      case LUA_TCCL: return &gco2ccl(o)->gclist;
      case LUA_TTHREAD: return &gco2th(o)->gclist;
      case LUA_TPROTO: return &gco2p(o)->gclist;
      case LUA_TUSERDATA: {
        Udata *u = gco2u(o);
        lua_assert(u->nuvalue > 0);
        return &u->gclist;
      }
      default: lua_assert(0); return 0;
    }
  }
  
  
  /*
  ** Link a collectable object 'o' with a known type into list pointed by 'p'.
  */
  #define linkgclist(o,p)	((o)->gclist = (p), (p) = obj2gco(o))
  
  
  /*
  ** Link a generic collectable object 'o' into list pointed by 'p'.
  */
  #define linkobjgclist(o,p) (*getgclist(o) = (p), (p) = obj2gco(o))
  
  
  
  /*
  ** Clear keys for empty entries in tables. If entry is empty
  ** and its key is not marked, mark its entry as dead. This allows the
  ** collection of the key, but keeps its entry in the table (its removal
  ** could break a chain). The main feature of a dead key is that it must
  ** be different from any other value, to do not disturb searches. 
  ** Other places never manipulate dead keys, because its associated empty
  ** value is enough to signal that the entry is logically empty.
  */
  static void clearkey (Node *n) {
    lua_assert(isempty(gval(n)));
    if (keyiswhite(n))
      setdeadkey(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 GCObject *o) {
    if (o == NULL) return 0;  /* non-collectable value */
    else if (novariant(o->tt) == LUA_TSTRING) {
      markobject(g, o);  /* strings are 'values', so are never weak */
      return 0;
    }
    else return iswhite(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 */
      if (isold(o)) {
        lua_assert(!isold(v));  /* white object could not be old */
        setage(v, G_OLD0);  /* restore generational 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, GCObject *o) {
    global_State *g = G(L);
    lua_assert(isblack(o) && !isdead(g, o));
    lua_assert(g->gckind != KGC_GEN || (isold(o) && getage(o) != G_TOUCHED1));
    if (getage(o) != G_TOUCHED2)  /* not already in gray list? */
      linkobjgclist(o, g->grayagain);  /* link it in 'grayagain' */
    black2gray(o);  /* make table gray (again) */
    setage(o, G_TOUCHED1);  /* touched in current cycle */
  }
  
  
  /*
  ** Barrier for prototype's cache of closures.  For an 'old1'
  ** object, making it gray stops it from being visited by 'markold',
  ** so it is linked in the 'grayagain' list to ensure it will be
  ** visited. Otherwise, it goes to 'protogray', as only its 'cache' field
  ** needs to be revisited.  (A prototype to be in this barrier must be
  ** already finished, so its other fields cannot change and do not need
  ** to be revisited.)
  */
  LUAI_FUNC void luaC_protobarrier_ (lua_State *L, Proto *p) {
    global_State *g = G(L);
    lua_assert(g->gckind != KGC_GEN || isold(p));
    if (getage(p) == G_OLD1)  /* still need to be visited? */
      linkgclist(p, g->grayagain);  /* link it in 'grayagain' */
    else
      linkgclist(p, g->protogray);  /* link it in 'protogray' */
    black2gray(p);  /* make prototype gray (to avoid other barriers) */
  }
  
  
  void luaC_fix (lua_State *L, GCObject *o) {
    global_State *g = G(L);
    lua_assert(g->allgc == o);  /* object must be 1st in 'allgc' list! */
    white2gray(o);  /* they will be gray forever */
    setage(o, G_OLD);  /* and old 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. They are kept gray
  ** to avoid barriers, as their values will be revisited by the thread.)
  */
  static void reallymarkobject (global_State *g, GCObject *o) {
    white2gray(o);
    switch (o->tt) {
      case LUA_TSHRSTR:
      case LUA_TLNGSTR: {
        gray2black(o);
        break;
      }
      case LUA_TUPVAL: {
        UpVal *uv = gco2upv(o);
        if (!upisopen(uv))  /* open upvalues are kept gray */
          gray2black(o);
        markvalue(g, uv->v);  /* mark its content */
        break;
      }
      case LUA_TUSERDATA: {
        Udata *u = gco2u(o);
        if (u->nuvalue == 0) {  /* no user values? */
          markobjectN(g, u->metatable);  /* mark its metatable */
          gray2black(o);  /* nothing else to mark */
          break;
        }
        /* else *//* FALLTHROUGH */
      }
      case LUA_TLCL: case LUA_TCCL: case LUA_TTABLE:
      case LUA_TTHREAD: case LUA_TPROTO: {
        linkobjgclist(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 lu_mem markbeingfnz (global_State *g) {
    GCObject *o;
    lu_mem count = 0;
    for (o = g->tobefnz; o != NULL; o = o->next) {
      count++;
      markobject(g, o);
    }
    return count;
  }
  
  
  /*
  ** 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 int remarkupvals (global_State *g) {
    lua_State *thread;
    lua_State **p = &g->twups;
    int work = 0;
    while ((thread = *p) != NULL) {
      work++;
      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) {
          work++;
          if (!iswhite(uv))  /* upvalue already visited? */
            markvalue(g, uv->v);  /* mark its value */
        }
      }
    }
    return work;
  }
  
  
  /*
  ** 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 = g->protogray = 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 */
      if (isempty(gval(n)))  /* entry is empty? */
        clearkey(n);  /* clear its key */
      else {
        lua_assert(!keyisnil(n));
        markkey(g, n);
        if (!hasclears && iscleared(g, gcvalueN(gval(n))))  /* a white value? */
          hasclears = 1;  /* table will have to be cleared */
      }
    }
    if (g->gcstate == GCSatomic && hasclears)
      linkgclist(h, g->weak);  /* has to be cleared later */
    else
      linkgclist(h, g->grayagain);  /* must retraverse it in atomic phase */
  }
  
  
  /*
  ** 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). In generational mode, it (like all visited
  ** tables) must be kept in some gray list for post-processing.
  */
  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++) {
      if (isempty(gval(n)))  /* entry is empty? */
        clearkey(n);  /* clear its key */
      else if (iscleared(g, gckeyN(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 */
    else if (g->gckind == KGC_GEN)
      linkgclist(h, g->grayagain);  /* keep it in some list */
    else
      gray2black(h);
    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 */
      if (isempty(gval(n)))  /* entry is empty? */
        clearkey(n);  /* clear its key */
      else {
        lua_assert(!keyisnil(n));
        markkey(g, n);
        markvalue(g, gval(n));
      }
    }
    if (g->gckind == KGC_GEN) {
      linkgclist(h, g->grayagain);  /* keep it in some gray list */
      black2gray(h);
    }
  }
  
  
  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? */
        ((void)((weakkey = strchr(svalue(mode), 'k'))),
         (void)((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 1 + h->sizearray + 2 * allocsizenode(h);
  }
  
  
  /*
  ** Check the cache of a prototype, to keep invariants. If the
  ** cache is white, clear it. (A cache should not prevent the
  ** collection of its reference.) Otherwise, if in generational
  ** mode, check the generational invariant. If the cache is old,
  ** everything is ok. If the prototype is 'old0', everything
  ** is ok too. (It will naturally be visited again.) If the
  ** prototype is older than 'old0', then its cache (which is new)
  ** must be visited again in the next collection, so the prototype
  ** goes to the 'protogray' list. (If the prototype has a cache,
  ** it is already immutable and does not need other barriers;
  ** then, it can become gray without problems for its other fields.)
  */
  static void checkprotocache (global_State *g, Proto *p) {
    if (p->cache) {
      if (iswhite(p->cache))
        p->cache = NULL;  /* allow cache to be collected */
      else if (g->gckind == KGC_GEN && !isold(p->cache) && getage(p) >= G_OLD1) {
        linkgclist(p, g->protogray);  /* link it in 'protogray' */
        black2gray(p);  /* make prototype gray */
      }
    }
    p->cachemiss = 0;  /* restart counting */
  }
  
  
  /*
  ** 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;
    checkprotocache(g, f);
    markobjectN(g, 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 */
      markobjectN(g, f->upvalues[i].name);
    for (i = 0; i < f->sizep; i++)  /* mark nested protos */
      markobjectN(g, f->p[i]);
    for (i = 0; i < f->sizelocvars; i++)  /* mark local-variable names */
      markobjectN(g, f->locvars[i].varname);
    return 1 + f->sizek + f->sizeupvalues + f->sizep + f->sizelocvars;
  }
  
  
  static int traverseCclosure (global_State *g, CClosure *cl) {
    int i;
    for (i = 0; i < cl->nupvalues; i++)  /* mark its upvalues */
      markvalue(g, &cl->upvalue[i]);
    return 1 + cl->nupvalues;
  }
  
  /*
  ** Traverse a Lua closure, marking its prototype and its upvalues.
  ** (Both can be NULL while closure is being created.)
  */
  static int traverseLclosure (global_State *g, LClosure *cl) {
    int i;
    markobjectN(g, cl->p);  /* mark its prototype */
    for (i = 0; i < cl->nupvalues; i++) {  /* visit its upvalues */
      UpVal *uv = cl->upvals[i];
      markobjectN(g, uv);  /* mark upvalue */
    }
    return 1 + cl->nupvalues;
  }
  
  
  /*
  ** Traverse a thread, marking the elements in the stack up to its top
  ** and cleaning the rest of the stack in the final traversal.
  ** That ensures that the entire stack have valid (non-dead) objects.
  */
  static int 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 == GCSatomic ||
               th->openupval == NULL || isintwups(th));
    for (; o < th->top; o++)  /* mark live elements in the stack */
      markvalue(g, s2v(o));
    if (g->gcstate == GCSatomic) {  /* final traversal? */
      StkId lim = th->stack + th->stacksize;  /* real end of stack */
      for (; o < lim; o++)  /* clear not-marked stack slice */
        setnilvalue(s2v(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->gcemergency)
      luaD_shrinkstack(th); /* do not change stack in emergency cycle */
    return 1 + th->stacksize;
  }
  
  
  static int traverseudata (global_State *g, Udata *u) {
    int i;
    markobjectN(g, u->metatable);  /* mark its metatable */
    for (i = 0; i < u->nuvalue; i++)
      markvalue(g, &u->uv[i].uv);
    return 1 + u->nuvalue;
  }
  
  
  /*
  ** traverse one gray object, turning it to black (except for threads,
  ** which are always gray).
  */
  static lu_mem propagatemark (global_State *g) {
    GCObject *o = g->gray;
    gray2black(o);
    g->gray = *getgclist(o);  /* remove from 'gray' list */
    switch (o->tt) {
      case LUA_TTABLE: return traversetable(g, gco2t(o));
      case LUA_TUSERDATA: return traverseudata(g, gco2u(o));
      case LUA_TLCL: return traverseLclosure(g, gco2lcl(o));
      case LUA_TCCL: return traverseCclosure(g, gco2ccl(o));
      case LUA_TPROTO: return traverseproto(g, gco2p(o));
      case LUA_TTHREAD: {
        lua_State *th = gco2th(o);
        linkgclist(th, g->grayagain);  /* insert into 'grayagain' list */
        black2gray(o);
        return traversethread(g, th);
      }
      default: lua_assert(0); return 0;
    }
  }
  
  
  static lu_mem propagateall (global_State *g) {
    lu_mem tot = 0;
    while (g->gray)
      tot += propagatemark(g);
    return tot;
  }
  
  
  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
  ** =======================================================
  */
  
  static void clearprotolist (global_State *g) {
    GCObject *p = g->protogray;
    g->protogray = NULL;
    while (p != NULL) {
      Proto *pp = gco2p(p);
      GCObject *next = pp->gclist;
      lua_assert(isgray(pp) && (pp->cache != NULL || pp->cachemiss >= MAXMISS));
      gray2black(pp);
      checkprotocache(g, pp);
      p = next;
    }
  }
  
  
  /*
  ** clear entries with unmarked keys from all weaktables in list 'l'
  */
  static void clearbykeys (global_State *g, GCObject *l) {
    for (; l; l = gco2t(l)->gclist) {
      Table *h = gco2t(l);
      Node *limit = gnodelast(h);
      Node *n;
      for (n = gnode(h, 0); n < limit; n++) {
        if (iscleared(g, gckeyN(n)))  /* unmarked key? */
          setempty(gval(n));  /* remove entry */
        if (isempty(gval(n)))  /* is entry empty? */
          clearkey(n);  /* clear its key */
      }
    }
  }
  
  
  /*
  ** clear entries with unmarked values from all weaktables in list 'l' up
  ** to element 'f'
  */
  static void clearbyvalues (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, gcvalueN(o)))  /* value was collected? */
          setempty(o);  /* remove entry */
      }
      for (n = gnode(h, 0); n < limit; n++) {
        if (iscleared(g, gcvalueN(gval(n))))  /* unmarked value? */
          setempty(gval(n));  /* remove entry */
        if (isempty(gval(n)))  /* is entry empty? */
          clearkey(n);  /* clear its key */
      }
    }
  }
  
  
  static void freeupval (lua_State *L, UpVal *uv) {
    if (upisopen(uv))
      luaF_unlinkupval(uv);
    luaM_free(L, uv);
  }
  
  
  static void freeobj (lua_State *L, GCObject *o) {
    switch (o->tt) {
      case LUA_TPROTO:
        luaF_freeproto(L, gco2p(o));
        break;
      case LUA_TUPVAL:
        freeupval(L, gco2upv(o));
        break;
      case LUA_TLCL:
        luaM_freemem(L, o, sizeLclosure(gco2lcl(o)->nupvalues));
        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: {
        Udata *u = gco2u(o);
        luaM_freemem(L, o, sizeudata(u->nuvalue, u->len));
        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);
    }
  }
  
  
  /*
  ** sweep at most 'countin' 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. ('*countout' gets the number of elements traversed.)
  */
  static GCObject **sweeplist (lua_State *L, GCObject **p, int countin,
                               int *countout) {
    global_State *g = G(L);
    int ow = otherwhite(g);
    int i;
    int white = luaC_white(g);  /* current white */
    for (i = 0; *p != NULL && i < countin; i++) {
      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 */
      }
    }
    if (countout)
      *countout = i;  /* number of elements traversed */
    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, NULL);
    } while (p == old);
    return p;
  }
  
  /* }====================================================== */
  
  
  /*
  ** {======================================================
  ** Finalization
  ** =======================================================
  */
  
  /*
  ** If possible, shrink string table.
  */
  static void checkSizes (lua_State *L, global_State *g) {
    if (!g->gcemergency) {
      l_mem olddebt = g->GCdebt;
      if (g->strt.nuse < g->strt.size / 4)  /* string table too big? */
        luaS_resize(L, g->strt.size / 2);
      g->GCestimate += g->GCdebt - olddebt;  /* correct estimate */
    }
  }
  
  
  /*
  ** Get the next udata to be finalized from the 'tobefnz' list, and
  ** link it back into the 'allgc' list.
  */
  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;
    lua_assert(!g->gcemergency);
    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 */
      L->ci->callstatus |= CIST_FIN;  /* will run a finalizer */
      status = luaD_pcall(L, dothecall, NULL, savestack(L, L->top - 2), 0);
      L->ci->callstatus &= ~CIST_FIN;  /* not running a finalizer anymore */
      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(s2v(L->top - 1)))
                              ? svalue(s2v(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 finalizers
  */
  static int runafewfinalizers (lua_State *L, int n) {
    global_State *g = G(L);
    int i;
    for (i = 0; i < n && g->tobefnz; i++)
      GCTM(L, 1);  /* call one finalizer */
    return i;
  }
  
  
  /*
  ** call all pending finalizers
  */
  static void callallpendingfinalizers (lua_State *L, int propagateerrors) {
    global_State *g = G(L);
    while (g->tobefnz)
      GCTM(L, propagateerrors);
  }
  
  
  /*
  ** 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).
  ** (Note that objects after 'finobjold' cannot be white, so they
  ** don't need to be traversed. In incremental mode, 'finobjold' is NULL,
  ** so the whole list is traversed.)
  */
  static void separatetobefnz (global_State *g, int all) {
    GCObject *curr;
    GCObject **p = &g->finobj;
    GCObject **lastnext = findlast(&g->tobefnz);
    while ((curr = *p) != g->finobjold) {  /* traverse all finalizable objects */
      lua_assert(tofinalize(curr));
      if (!(iswhite(curr) || all))  /* not being collected? */
        p = &curr->next;  /* don't bother with it */
      else {
        if (curr == g->finobjsur)  /* removing 'finobjsur'? */
          g->finobjsur = curr->next;  /* correct it */
        *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' */
      }
      else {  /* correct pointers into 'allgc' list, if needed */
        if (o == g->survival)
          g->survival = o->next;
        if (o == g->old)
          g->old = o->next;
        if (o == g->reallyold)
          g->reallyold = o->next;
      }
      /* 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 */
    }
  }
  
  /* }====================================================== */
  
  
  /*
  ** {======================================================
  ** Generational Collector
  ** =======================================================
  */
  
  
  /* mask to erase all color bits, not changing gen-related stuff */
  #define maskgencolors	(~(bitmask(BLACKBIT) | WHITEBITS))
  
  
  /*
  ** Sweep a list of objects, deleting dead ones and turning
  ** the non dead to old (without changing their colors).
  */
  static void sweep2old (lua_State *L, GCObject **p) {
    GCObject *curr;
    while ((curr = *p) != NULL) {
      if (iswhite(curr)) {  /* is 'curr' dead? */
        lua_assert(isdead(G(L), curr));
        *p = curr->next;  /* remove 'curr' from list */
        freeobj(L, curr);  /* erase 'curr' */
      }
      else {  /* all surviving objects become old */
        setage(curr, G_OLD);
        p = &curr->next;  /* go to next element */
      }
    }
  }
  
  
  /*
  ** Sweep for generational mode. Delete dead objects. (Because the
  ** collection is not incremental, there are no "new white" objects
  ** during the sweep. So, any white object must be dead.) For
  ** non-dead objects, advance their ages and clear the color of
  ** new objects. (Old objects keep their colors.)
  */
  static GCObject **sweepgen (lua_State *L, global_State *g, GCObject **p,
                              GCObject *limit) {
    static lu_byte nextage[] = {
      G_SURVIVAL,  /* from G_NEW */
      G_OLD1,      /* from G_SURVIVAL */
      G_OLD1,      /* from G_OLD0 */
      G_OLD,       /* from G_OLD1 */
      G_OLD,       /* from G_OLD (do not change) */
      G_TOUCHED1,  /* from G_TOUCHED1 (do not change) */
      G_TOUCHED2   /* from G_TOUCHED2 (do not change) */
    };
    int white = luaC_white(g);
    GCObject *curr;
    while ((curr = *p) != limit) {
      if (iswhite(curr)) {  /* is 'curr' dead? */
        lua_assert(!isold(curr) && isdead(g, curr));
        *p = curr->next;  /* remove 'curr' from list */
        freeobj(L, curr);  /* erase 'curr' */
      }
      else {  /* correct mark and age */
        if (getage(curr) == G_NEW)
          curr->marked = cast_byte((curr->marked & maskgencolors) | white);
        setage(curr, nextage[getage(curr)]);
        p = &curr->next;  /* go to next element */
      }
    }
    return p;
  }
  
  
  /*
  ** Traverse a list making all its elements white and clearing their
  ** age.
  */
  static void whitelist (global_State *g, GCObject *p) {
    int white = luaC_white(g);
    for (; p != NULL; p = p->next)
      p->marked = cast_byte((p->marked & maskcolors) | white);
  }
  
  
  /*
  ** Correct a list of gray objects. Because this correction is
  ** done after sweeping, young objects can be white and still
  ** be in the list. They are only removed.
  ** For tables and userdata, advance 'touched1' to 'touched2'; 'touched2'
  ** objects become regular old and are removed from the list.
  ** For threads, just remove white ones from the list.
  */
  static GCObject **correctgraylist (GCObject **p) {
    GCObject *curr;
    while ((curr = *p) != NULL) {
      switch (curr->tt) {
        case LUA_TTABLE: case LUA_TUSERDATA: {
          GCObject **next = getgclist(curr);
          if (getage(curr) == G_TOUCHED1) {  /* touched in this cycle? */
            lua_assert(isgray(curr));
            gray2black(curr);  /* make it black, for next barrier */
            changeage(curr, G_TOUCHED1, G_TOUCHED2);
            p = next;  /* go to next element */
          }
          else {
            if (!iswhite(curr)) {
              lua_assert(isold(curr));
              if (getage(curr) == G_TOUCHED2)
                changeage(curr, G_TOUCHED2, G_OLD);
              gray2black(curr);  /* make it black */
            }
            *p = *next;  /* remove 'curr' from gray list */
          }
          break;
        }
        case LUA_TTHREAD: {
          lua_State *th = gco2th(curr);
          lua_assert(!isblack(th));
          if (iswhite(th))  /* new object? */
            *p = th->gclist;  /* remove from gray list */
          else  /* old threads remain gray */
            p = &th->gclist;  /* go to next element */
          break;
        }
        default: lua_assert(0);  /* nothing more could be gray here */
      }
    }
    return p;
  }
  
  
  /*
  ** Correct all gray lists, coalescing them into 'grayagain'.
  */
  static void correctgraylists (global_State *g) {
    GCObject **list = correctgraylist(&g->grayagain);
    *list = g->weak; g->weak = NULL;
    list = correctgraylist(list);
    *list = g->allweak; g->allweak = NULL;
    list = correctgraylist(list);
    *list = g->ephemeron; g->ephemeron = NULL;
    correctgraylist(list);
  }
  
  
  /*
  ** Mark 'old1' objects when starting a new young collection.
  ** Gray objects are already in some gray list, and so will be visited
  ** in the atomic step.
  */
  static void markold (global_State *g, GCObject *from, GCObject *to) {
    GCObject *p;
    for (p = from; p != to; p = p->next) {
      if (getage(p) == G_OLD1) {
        lua_assert(!iswhite(p));
        if (isblack(p)) {
          black2gray(p);  /* should be '2white', but gray works too */
          reallymarkobject(g, p);
        }
      }
    }
  }
  
  
  /*
  ** Finish a young-generation collection.
  */
  static void finishgencycle (lua_State *L, global_State *g) {
    correctgraylists(g);
    checkSizes(L, g);
    g->gcstate = GCSpropagate;  /* skip restart */
    if (!g->gcemergency)
      callallpendingfinalizers(L, 1);
  }
  
  
  /*
  ** Does a young collection. First, mark 'old1' objects.  (Only survival
  ** and "recent old" lists can contain 'old1' objects. New lists cannot
  ** contain 'old1' objects, at most 'old0' objects that were already
  ** visited when marked old.) Then does the atomic step. Then,
  ** sweep all lists and advance pointers. Finally, finish the collection.
  */
  static void youngcollection (lua_State *L, global_State *g) {
    GCObject **psurvival;  /* to point to first non-dead survival object */
    lua_assert(g->gcstate == GCSpropagate);
    markold(g, g->survival, g->reallyold);
    markold(g, g->finobj, g->finobjrold);
    atomic(L);
  
    /* sweep nursery and get a pointer to its last live element */
    psurvival = sweepgen(L, g, &g->allgc, g->survival);
    /* sweep 'survival' and 'old' */
    sweepgen(L, g, psurvival, g->reallyold);
    g->reallyold = g->old;
    g->old = *psurvival;  /* 'survival' survivals are old now */
    g->survival = g->allgc;  /* all news are survivals */
  
    /* repeat for 'finobj' lists */
    psurvival = sweepgen(L, g, &g->finobj, g->finobjsur);
    /* sweep 'survival' and 'old' */
    sweepgen(L, g, psurvival, g->finobjrold);
    g->finobjrold = g->finobjold;
    g->finobjold = *psurvival;  /* 'survival' survivals are old now */
    g->finobjsur = g->finobj;  /* all news are survivals */
  
    sweepgen(L, g, &g->tobefnz, NULL);
  
    finishgencycle(L, g);
  }
  
  
  /*
  ** Enter generational mode. Must go until the end of an atomic cycle
  ** to ensure that all threads are in the gray list. Then, turn all
  ** objects into old and finishes the collection.
  */
  static void entergen (lua_State *L, global_State *g) {
    luaC_runtilstate(L, bitmask(GCSpause));  /* prepare to start a new cycle */
    luaC_runtilstate(L, bitmask(GCSpropagate));  /* start new cycle */
    atomic(L);
    /* sweep all elements making them old */
    sweep2old(L, &g->allgc);
    /* everything alive now is old */
    g->reallyold = g->old = g->survival = g->allgc;
  
    /* repeat for 'finobj' lists */
    sweep2old(L, &g->finobj);
    g->finobjrold = g->finobjold = g->finobjsur = g->finobj;
  
    sweep2old(L, &g->tobefnz);
  
    g->gckind = KGC_GEN;
    g->GCestimate = gettotalbytes(g);  /* base for memory control */
    finishgencycle(L, g);
  }
  
  
  /*
  ** Enter incremental mode. Turn all objects white, make all
  ** intermediate lists point to NULL (to avoid invalid pointers),
  ** and go to pause state.
  */
  static void enterinc (global_State *g) {
    whitelist(g, g->allgc);
    g->reallyold = g->old = g->survival = NULL;
    whitelist(g, g->finobj);
    whitelist(g, g->tobefnz);
    g->finobjrold = g->finobjold = g->finobjsur = NULL;
    g->gcstate = GCSpause;
    g->gckind = KGC_INC;
  }
  
  
  /*
  ** Change collector mode to 'newmode'.
  */
  void luaC_changemode (lua_State *L, int newmode) {
    global_State *g = G(L);
    if (newmode != g->gckind) {
      if (newmode == KGC_GEN)  /* entering generational mode? */
        entergen(L, g);
      else
        enterinc(g);  /* entering incremental mode */
    }
  }
  
  
  /*
  ** Does a full collection in generational mode.
  */
  static void fullgen (lua_State *L, global_State *g) {
    enterinc(g);
    entergen(L, g);
  }
  
  
  /*
  ** Does a generational "step". If memory grows 'genmajormul'% larger
  ** than last major collection (kept in 'g->GCestimate'), does a major
  ** collection. Otherwise, does a minor collection and set debt to make
  ** another collection when memory grows 'genminormul'% larger.
  ** 'GCdebt <= 0' means an explicit call to GC step with "size" zero;
  ** in that case, always do a minor collection.
  */
  static void genstep (lua_State *L, global_State *g) {
    lu_mem majorbase = g->GCestimate;
    int majormul = getgcparam(g->genmajormul);
    if (g->GCdebt > 0 &&
        gettotalbytes(g) > (majorbase / 100) * (100 + majormul)) {
      fullgen(L, g);
    }
    else {
      lu_mem mem;
      youngcollection(L, g);
      mem = gettotalbytes(g);
      luaE_setdebt(g, -(cast(l_mem, (mem / 100)) * g->genminormul));
      g->GCestimate = majorbase;  /* preserve base value */
    }
  }
  
  /* }====================================================== */
  
  
  /*
  ** {======================================================
  ** GC control
  ** =======================================================
  */
  
  
  /*
  ** Set the "time" to wait before starting a new GC cycle; cycle will
  ** start when memory use hits the threshold of ('estimate' * pause /
  ** PAUSEADJ). (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;
    int pause = getgcparam(g->gcpause);
    l_mem estimate = g->GCestimate / PAUSEADJ;  /* adjust 'estimate' */
    lua_assert(estimate > 0);
    threshold = (pause < MAX_LMEM / estimate)  /* overflow? */
              ? estimate * pause  /* no overflow */
              : MAX_LMEM;  /* overflow; truncate to maximum */
    debt = gettotalbytes(g) - threshold;
    if (debt > 0) debt = 0;
    luaE_setdebt(g, debt);
  }
  
  
  /*
  ** Enter first sweep phase.
  ** The call to 'sweeptolive' makes the 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 = sweeptolive(L, &g->allgc);
  }
  
  
  /*
  ** Delete all objects in list 'p' until (but not including) object
  ** 'limit'.
  */
  static void deletelist (lua_State *L, GCObject *p, GCObject *limit) {
    while (p != limit) {
      GCObject *next = p->next;
      freeobj(L, p);
      p = next;
    }
  }
  
  
  /*
  ** Call all finalizers of the objects in the given Lua state, and
  ** then free all objects, except for the main thread.
  */
  void luaC_freeallobjects (lua_State *L) {
    global_State *g = G(L);
    luaC_changemode(L, KGC_INC);
    separatetobefnz(g, 1);  /* separate all objects with finalizers */
    lua_assert(g->finobj == NULL);
    callallpendingfinalizers(L, 0);
    deletelist(L, g->allgc, obj2gco(g->mainthread));
    deletelist(L, g->finobj, NULL);
    deletelist(L, g->fixedgc, NULL);  /* collect fixed objects */
    lua_assert(g->strt.nuse == 0);
  }
  
  
  static lu_mem atomic (lua_State *L) {
    global_State *g = G(L);
    lu_mem work = 0;
    GCObject *origweak, *origall;
    GCObject *grayagain = g->grayagain;  /* save original list */
    g->grayagain = NULL;
    lua_assert(g->ephemeron == NULL && g->weak == NULL);
    lua_assert(!iswhite(g->mainthread));
    g->gcstate = GCSatomic;
    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 */
    work += remarkupvals(g);
    work += propagateall(g);  /* propagate changes */
    g->gray = grayagain;
    work += propagateall(g);  /* traverse 'grayagain' list */
    convergeephemerons(g);
    /* at this point, all strongly accessible objects are marked. */
    /* Clear values from weak tables, before checking finalizers */
    clearbyvalues(g, g->weak, NULL);
    clearbyvalues(g, g->allweak, NULL);
    origweak = g->weak; origall = g->allweak;
    separatetobefnz(g, 0);  /* separate objects to be finalized */
    work += markbeingfnz(g);  /* mark objects that will be finalized */
    work += propagateall(g);  /* remark, to propagate 'resurrection' */
    convergeephemerons(g);
    /* at this point, all resurrected objects are marked. */
    /* remove dead objects from weak tables */
    clearbykeys(g, g->ephemeron);  /* clear keys from all ephemeron tables */
    clearbykeys(g, g->allweak);  /* clear keys from all 'allweak' tables */
    /* clear values from resurrected weak tables */
    clearbyvalues(g, g->weak, origweak);
    clearbyvalues(g, g->allweak, origall);
    luaS_clearcache(g);
    clearprotolist(g);
    g->currentwhite = cast_byte(otherwhite(g));  /* flip current white */
    lua_assert(g->gray == NULL);
    return work;  /* estimate of slots marked by 'atomic' */
  }
  
  
  static int sweepstep (lua_State *L, global_State *g,
                        int nextstate, GCObject **nextlist) {
    if (g->sweepgc) {
      l_mem olddebt = g->GCdebt;
      int count;
      g->sweepgc = sweeplist(L, g->sweepgc, GCSWEEPMAX, &count);
      g->GCestimate += g->GCdebt - olddebt;  /* update estimate */
      return count;
    }
    else {  /* enter next state */
      g->gcstate = nextstate;
      g->sweepgc = nextlist;
      return 0;  /* no work done */
    }
  }
  
  
  static lu_mem singlestep (lua_State *L) {
    global_State *g = G(L);
    switch (g->gcstate) {
      case GCSpause: {
        restartcollection(g);
        g->gcstate = GCSpropagate;
        return 1;
      }
      case GCSpropagate: {
        if (g->gray == NULL) {  /* no more gray objects? */
          g->gcstate = GCSenteratomic;  /* finish propagate phase */
          return 0;
        }
        else
          return propagatemark(g);  /* traverse one gray object */
      }
      case GCSenteratomic: {
        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 */
        checkSizes(L, g);
        g->gcstate = GCScallfin;
        return 0;
      }
      case GCScallfin: {  /* call remaining finalizers */
        if (g->tobefnz && !g->gcemergency) {
          int n = runafewfinalizers(L, GCFINMAX);
          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);
  }
  
  
  /*
  ** Performs a basic incremental step. The debt and step size are
  ** converted from bytes to "units of work"; then the function loops
  ** running single steps until adding that many units of work or
  ** finishing a cycle (pause state). Finally, it sets the debt that
  ** controls when next step will be performed.
  */
  static void incstep (lua_State *L, global_State *g) {
    int stepmul = (getgcparam(g->gcstepmul) | 1);  /* avoid division by 0 */
    l_mem debt = (g->GCdebt / WORK2MEM) * stepmul;
    l_mem stepsize = (g->gcstepsize <= log2maxs(l_mem))
                   ? ((cast(l_mem, 1) << g->gcstepsize) / WORK2MEM) * stepmul
                   : MAX_LMEM;  /* overflow; keep maximum value */
    do {  /* repeat until pause or enough "credit" (negative debt) */
      lu_mem work = singlestep(L);  /* perform one single step */
      debt -= work;
    } while (debt > -stepsize && g->gcstate != GCSpause);
    if (g->gcstate == GCSpause)
      setpause(g);  /* pause until next cycle */
    else {
      debt = (debt / stepmul) * WORK2MEM;  /* convert 'work units' to bytes */
      luaE_setdebt(g, debt);
    }
  }
  
  /*
  ** performs a basic GC step if collector is running
  */
  void luaC_step (lua_State *L) {
    global_State *g = G(L);
    if (g->gcrunning) {  /* running? */
      if (g->gckind == KGC_INC)
        incstep(L, g);
      else
        genstep(L, g);
    }
  }
  
  
  /*
  ** Perform a full collection in incremental mode.
  ** 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).
  */
  static void fullinc (lua_State *L, global_State *g) {
    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(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 */
    setpause(g);
  }
  
  
  /*
  ** 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).
  */
  void luaC_fullgc (lua_State *L, int isemergency) {
    global_State *g = G(L);
    lua_assert(!g->gcemergency);
    g->gcemergency = isemergency;  /* set flag */
    if (g->gckind == KGC_INC)
      fullinc(L, g);
    else
      fullgen(L, g);
    g->gcemergency = 0;
  }
  
  /* }====================================================== */
  
  
  