Lua2RTT/lua-5.3.4/lgc.c

/*
** $Id: lgc.c,v 2.215 2016/12/22 13:08:50 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);
    lua_assert(g->allgc == o);  /* object must be 1st in 'allgc' list! */
    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, allocsizenode(h));
}


/*
** 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;
    if (f->cache && iswhite(f->cache))
        f->cache = NULL;  /* allow cache to be collected */
    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 sizeof(Proto) + sizeof(Instruction) * f->sizecode +
           sizeof(Proto *) * f->sizep +
           sizeof(TValue) * f->sizek +
           sizeof(int) * f->sizelineinfo +
           sizeof(LocVar) * f->sizelocvars +
           sizeof(Upvaldesc) * f->sizeupvalues;
}


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 */
        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(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);
}

/* }====================================================== */