lua-5.2.3/src/lgc.c

/*
** $Id: lgc.c,v 2.140.1.2 2013/04/26 18:22:05 roberto Exp $
** Garbage Collector
** See Copyright Notice in lua.h
*/

#include <string.h>

#define lgc_c
#define LUA_CORE

#include "lua.h"

#include "ldebug.h"
#include "ldo.h"
#include "lfunc.h"
#include "lgc.h"
#include "lmem.h"
#include "lobject.h"
#include "lstate.h"
#include "lstring.h"
#include "ltable.h"
#include "ltm.h"

/*
** 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))

/* maximum number of finalizers to call in each GC step */
#define GCFINALIZENUM 4

/*
** 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 plus the old bit and then
** sets only the current white bit
*/
#define maskcolors (~(bit2mask(BLACKBIT, OLDBIT) | WHITEBITS))
#define makewhite(g, x) \
	(gch(x)->marked = cast_byte((gch(x)->marked & maskcolors) | luaC_white(g)))

#define white2gray(x) resetbits(gch(x)->marked, WHITEBITS)
#define black2gray(x) resetbit(gch(x)->marked, BLACKBIT)

#define isfinalized(x) testbit(gch(x)->marked, FINALIZEDBIT)

#define checkdeadkey(n) lua_assert(!ttisdeadkey(gkey(n)) || ttisnil(gval(n)))

#define checkconsistency(obj) \
	lua_longassert(!iscollectable(obj) || righttt(obj))

#define markvalue(g, o)                                     \
	{                                                       \
		checkconsistency(o);                                \
		if (valiswhite(o)) reallymarkobject(g, gcvalue(o)); \
	}

#define markobject(g, t)                     \
	{                                        \
		if ((t) && iswhite(obj2gco(t)))      \
			reallymarkobject(g, obj2gco(t)); \
	}

static void reallymarkobject(global_State *g, GCObject *o);

/*
** {======================================================
** Generic functions
** =======================================================
*/

/*
** one after last element in a hash array
*/
#define gnodelast(h) gnode(h, cast(size_t, sizenode(h)))

/*
** link table 'h' into list pointed by 'p'
*/
#define linktable(h, p) ((h)->gclist = *(p), *(p) = obj2gco(h))

/*
** if key is not marked, mark its entry as dead (therefore removing it
** from the table)
*/
static void removeentry(Node *n)
{
	lua_assert(ttisnil(gval(n)));
	if (valiswhite(gkey(n)))
		setdeadvalue(gkey(n)); /* unused and unmarked key; remove it */
}

/*
** tells whether a key or value can be cleared from a weak
** table. Non-collectable objects are never removed from weak
** tables. Strings behave as `values', so are never removed too. for
** other objects: if really collected, cannot keep them; for objects
** being finalized, keep them in keys, but not in values
*/
static int iscleared(global_State *g, const TValue *o)
{
	if (!iscollectable(o))
		return 0;
	else if (ttisstring(o))
	{
		markobject(g, rawtsvalue(o)); /* strings are `values', so are never weak */
		return 0;
	}
	else
		return iswhite(gcvalue(o));
}

/*
** barrier that moves collector forward, that is, mark the white object
** being pointed by a black object.
*/
void luaC_barrier_(lua_State *L, GCObject *o, GCObject *v)
{
	global_State *g = G(L);
	lua_assert(isblack(o) && iswhite(v) && !isdead(g, v) && !isdead(g, o));
	lua_assert(g->gcstate != GCSpause);
	lua_assert(gch(o)->tt != LUA_TTABLE);
	if (keepinvariantout(g))    /* must keep invariant? */
		reallymarkobject(g, v); /* restore invariant */
	else
	{ /* sweep phase */
		lua_assert(issweepphase(g));
		makewhite(g, o); /* mark main obj. as white to avoid other barriers */
	}
}

/*
** barrier that moves collector backward, that is, mark the black object
** pointing to a white object as gray again. (Current implementation
** only works for tables; access to 'gclist' is not uniform across
** different types.)
*/
void luaC_barrierback_(lua_State *L, GCObject *o)
{
	global_State *g = G(L);
	lua_assert(isblack(o) && !isdead(g, o) && gch(o)->tt == LUA_TTABLE);
	black2gray(o); /* make object gray (again) */
	gco2t(o)->gclist = g->grayagain;
	g->grayagain = o;
}

/*
** barrier for prototypes. When creating first closure (cache is
** NULL), use a forward barrier; this may be the only closure of the
** prototype (if it is a "regular" function, with a single instance)
** and the prototype may be big, so it is better to avoid traversing
** it again. Otherwise, use a backward barrier, to avoid marking all
** possible instances.
*/
LUAI_FUNC void luaC_barrierproto_(lua_State *L, Proto *p, Closure *c)
{
	global_State *g = G(L);
	lua_assert(isblack(obj2gco(p)));
	if (p->cache == NULL)
	{ /* first time? */
		luaC_objbarrier(L, p, c);
	}
	else
	{                           /* use a backward barrier */
		black2gray(obj2gco(p)); /* make prototype gray (again) */
		p->gclist = g->grayagain;
		g->grayagain = obj2gco(p);
	}
}

/*
** check color (and invariants) for an upvalue that was closed,
** i.e., moved into the 'allgc' list
*/
void luaC_checkupvalcolor(global_State *g, UpVal *uv)
{
	GCObject *o = obj2gco(uv);
	lua_assert(!isblack(o)); /* open upvalues are never black */
	if (isgray(o))
	{
		if (keepinvariant(g))
		{
			resetoldbit(o); /* see MOVE OLD rule */
			gray2black(o);  /* it is being visited now */
			markvalue(g, uv->v);
		}
		else
		{
			lua_assert(issweepphase(g));
			makewhite(g, o);
		}
	}
}

/*
** create a new collectable object (with given type and size) and link
** it to '*list'. 'offset' tells how many bytes to allocate before the
** object itself (used only by states).
*/
GCObject *luaC_newobj(lua_State *L, int tt, size_t sz, GCObject **list,
					  int offset)
{
	global_State *g = G(L);
	char *raw = cast(char *, luaM_newobject(L, novariant(tt), sz));
	GCObject *o = obj2gco(raw + offset);
	if (list == NULL)
		list = &g->allgc; /* standard list for collectable objects */
	gch(o)->marked = luaC_white(g);
	gch(o)->tt = tt;
	gch(o)->next = *list;
	*list = o;
	return o;
}

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

/*
** {======================================================
** Mark functions
** =======================================================
*/

/*
** mark an object. Userdata, 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)
{
	lu_mem size;
	white2gray(o);
	switch (gch(o)->tt)
	{
		case LUA_TSHRSTR:
		case LUA_TLNGSTR:
		{
			size = sizestring(gco2ts(o));
			break; /* nothing else to mark; make it black */
		}
		case LUA_TUSERDATA:
		{
			Table *mt = gco2u(o)->metatable;
			markobject(g, mt);
			markobject(g, gco2u(o)->env);
			size = sizeudata(gco2u(o));
			break;
		}
		case LUA_TUPVAL:
		{
			UpVal *uv = gco2uv(o);
			markvalue(g, uv->v);
			if (uv->v != &uv->u.value) /* open? */
				return;                /* open upvalues remain gray */
			size = sizeof(UpVal);
			break;
		}
		case LUA_TLCL:
		{
			gco2lcl(o)->gclist = g->gray;
			g->gray = o;
			return;
		}
		case LUA_TCCL:
		{
			gco2ccl(o)->gclist = g->gray;
			g->gray = o;
			return;
		}
		case LUA_TTABLE:
		{
			linktable(gco2t(o), &g->gray);
			return;
		}
		case LUA_TTHREAD:
		{
			gco2th(o)->gclist = g->gray;
			g->gray = o;
			return;
		}
		case LUA_TPROTO:
		{
			gco2p(o)->gclist = g->gray;
			g->gray = o;
			return;
		}
		default:
			lua_assert(0);
			return;
	}
	gray2black(o);
	g->GCmemtrav += size;
}

/*
** mark metamethods for basic types
*/
static void markmt(global_State *g)
{
	int i;
	for (i = 0; i < LUA_NUMTAGS; i++)
		markobject(g, g->mt[i]);
}

/*
** mark all objects in list of being-finalized
*/
static void markbeingfnz(global_State *g)
{
	GCObject *o;
	for (o = g->tobefnz; o != NULL; o = gch(o)->next)
	{
		makewhite(g, o);
		reallymarkobject(g, o);
	}
}

/*
** mark all values stored in marked open upvalues. (See comment in
** 'lstate.h'.)
*/
static void remarkupvals(global_State *g)
{
	UpVal *uv;
	for (uv = g->uvhead.u.l.next; uv != &g->uvhead; uv = uv->u.l.next)
	{
		if (isgray(obj2gco(uv)))
			markvalue(g, uv->v);
	}
}

/*
** mark root set and reset all gray lists, to start a new
** incremental (or full) collection
*/
static void 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
** =======================================================
*/

static void traverseweakvalue(global_State *g, Table *h)
{
	Node *n, *limit = gnodelast(h);
	/* if there is array part, assume it may have white values (do not
     traverse it just to check) */
	int hasclears = (h->sizearray > 0);
	for (n = gnode(h, 0); n < limit; n++)
	{
		checkdeadkey(n);
		if (ttisnil(gval(n))) /* entry is empty? */
			removeentry(n);   /* remove it */
		else
		{
			lua_assert(!ttisnil(gkey(n)));
			markvalue(g, gkey(n));                   /* mark key */
			if (!hasclears && iscleared(g, gval(n))) /* is there a white value? */
				hasclears = 1;                       /* table will have to be cleared */
		}
	}
	if (hasclears)
		linktable(h, &g->weak);      /* has to be cleared later */
	else                             /* no white values */
		linktable(h, &g->grayagain); /* no need to clean */
}

static int traverseephemeron(global_State *g, Table *h)
{
	int marked = 0;    /* true if an object is marked in this traversal */
	int hasclears = 0; /* true if table has white keys */
	int prop = 0;      /* true if table has entry "white-key -> white-value" */
	Node *n, *limit = gnodelast(h);
	int i;
	/* traverse array part (numeric keys are 'strong') */
	for (i = 0; i < h->sizearray; i++)
	{
		if (valiswhite(&h->array[i]))
		{
			marked = 1;
			reallymarkobject(g, gcvalue(&h->array[i]));
		}
	}
	/* traverse hash part */
	for (n = gnode(h, 0); n < limit; n++)
	{
		checkdeadkey(n);
		if (ttisnil(gval(n))) /* entry is empty? */
			removeentry(n);   /* remove it */
		else if (iscleared(g, gkey(n)))
		{                            /* key is not marked (yet)? */
			hasclears = 1;           /* table must be cleared */
			if (valiswhite(gval(n))) /* value not marked yet? */
				prop = 1;            /* must propagate again */
		}
		else if (valiswhite(gval(n)))
		{ /* value not marked yet? */
			marked = 1;
			reallymarkobject(g, gcvalue(gval(n))); /* mark it now */
		}
	}
	if (prop)
		linktable(h, &g->ephemeron); /* have to propagate again */
	else if (hasclears)              /* does table have white keys? */
		linktable(h, &g->allweak);   /* may have to clean white keys */
	else                             /* no white keys */
		linktable(h, &g->grayagain); /* no need to clean */
	return marked;
}

static void traversestrongtable(global_State *g, Table *h)
{
	Node *n, *limit = gnodelast(h);
	int i;
	for (i = 0; i < h->sizearray; i++) /* traverse array part */
		markvalue(g, &h->array[i]);
	for (n = gnode(h, 0); n < limit; n++)
	{ /* traverse hash part */
		checkdeadkey(n);
		if (ttisnil(gval(n))) /* entry is empty? */
			removeentry(n);   /* remove it */
		else
		{
			lua_assert(!ttisnil(gkey(n)));
			markvalue(g, gkey(n)); /* mark key */
			markvalue(g, gval(n)); /* mark value */
		}
	}
}

static lu_mem traversetable(global_State *g, Table *h)
{
	const char *weakkey, *weakvalue;
	const TValue *mode = gfasttm(g, h->metatable, TM_MODE);
	markobject(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(obj2gco(h)); /* keep table gray */
		if (!weakkey)           /* strong keys? */
			traverseweakvalue(g, h);
		else if (!weakvalue) /* strong values? */
			traverseephemeron(g, h);
		else                           /* all weak */
			linktable(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 traverseproto(global_State *g, Proto *f)
{
	int i;
	if (f->cache && iswhite(obj2gco(f->cache)))
		f->cache = NULL; /* allow cache to be collected */
	markobject(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 */
		markobject(g, f->upvalues[i].name);
	for (i = 0; i < f->sizep; i++) /* mark nested protos */
		markobject(g, f->p[i]);
	for (i = 0; i < f->sizelocvars; i++) /* mark local-variable names */
		markobject(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);
}

static lu_mem traverseLclosure(global_State *g, LClosure *cl)
{
	int i;
	markobject(g, cl->p);               /* mark its prototype */
	for (i = 0; i < cl->nupvalues; i++) /* mark its upvalues */
		markobject(g, cl->upvals[i]);
	return sizeLclosure(cl->nupvalues);
}

static lu_mem traversestack(global_State *g, lua_State *th)
{
	int n = 0;
	StkId o = th->stack;
	if (o == NULL)
		return 1;            /* stack not completely built yet */
	for (; o < th->top; o++) /* mark live elements in the stack */
		markvalue(g, 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(o);
	}
	else
	{ /* count call infos to compute size */
		CallInfo *ci;
		for (ci = &th->base_ci; ci != th->ci; ci = ci->next)
			n++;
	}
	return sizeof(lua_State) + sizeof(TValue) * th->stacksize +
		   sizeof(CallInfo) * n;
}

/*
** 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 (gch(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 */
			th->gclist = g->grayagain;
			g->grayagain = o; /* insert into 'grayagain' list */
			black2gray(o);
			size = traversestack(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 propagatelist(global_State *g, GCObject *l)
{
	lua_assert(g->gray == NULL); /* no grays left */
	g->gray = l;
	propagateall(g); /* traverse all elements from 'l' */
}

/*
** retraverse all gray lists. Because tables may be reinserted in other
** lists when traversed, traverse the original lists to avoid traversing
** twice the same table (which is not wrong, but inefficient)
*/
static void retraversegrays(global_State *g)
{
	GCObject *weak = g->weak; /* save original lists */
	GCObject *grayagain = g->grayagain;
	GCObject *ephemeron = g->ephemeron;
	g->weak = g->grayagain = g->ephemeron = NULL;
	propagateall(g); /* traverse main gray list */
	propagatelist(g, grayagain);
	propagatelist(g, weak);
	propagatelist(g, ephemeron);
}

static void convergeephemerons(global_State *g)
{
	int changed;
	do
	{
		GCObject *w;
		GCObject *next = g->ephemeron; /* get ephemeron list */
		g->ephemeron = NULL;           /* tables will return to this list when traversed */
		changed = 0;
		while ((w = next) != NULL)
		{
			next = gco2t(w)->gclist;
			if (traverseephemeron(g, gco2t(w)))
			{                    /* traverse marked some value? */
				propagateall(g); /* propagate changes */
				changed = 1;     /* will have to revisit all ephemeron tables */
			}
		}
	} while (changed);
}

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

/*
** {======================================================
** Sweep Functions
** =======================================================
*/

/*
** clear entries with unmarked keys from all weaktables in list 'l' up
** to element 'f'
*/
static void clearkeys(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);
		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 */
			}
		}
	}
}

static void freeobj(lua_State *L, GCObject *o)
{
	switch (gch(o)->tt)
	{
		case LUA_TPROTO:
			luaF_freeproto(L, gco2p(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_TUPVAL:
			luaF_freeupval(L, gco2uv(o));
			break;
		case LUA_TTABLE:
			luaH_free(L, gco2t(o));
			break;
		case LUA_TTHREAD:
			luaE_freethread(L, gco2th(o));
			break;
		case LUA_TUSERDATA:
			luaM_freemem(L, o, sizeudata(gco2u(o)));
			break;
		case LUA_TSHRSTR:
			G(L)->strt.nuse--;
			/* go through */
		case LUA_TLNGSTR:
		{
			luaM_freemem(L, o, sizestring(gco2ts(o)));
			break;
		}
		default:
			lua_assert(0);
	}
}

#define sweepwholelist(L, p) sweeplist(L, p, MAX_LUMEM)
static GCObject **sweeplist(lua_State *L, GCObject **p, lu_mem count);

/*
** sweep the (open) upvalues of a thread and resize its stack and
** list of call-info structures.
*/
static void sweepthread(lua_State *L, lua_State *L1)
{
	if (L1->stack == NULL) return;     /* stack not completely built yet */
	sweepwholelist(L, &L1->openupval); /* sweep open upvalues */
	luaE_freeCI(L1);                   /* free extra CallInfo slots */
	/* should not change the stack during an emergency gc cycle */
	if (G(L)->gckind != KGC_EMERGENCY)
		luaD_shrinkstack(L1);
}

/*
** sweep at most 'count' elements from a list of GCObjects erasing dead
** objects, where a dead (not alive) object is one marked with the "old"
** (non current) white and not fixed.
** In non-generational mode, change all non-dead objects back to white,
** preparing for next collection cycle.
** In generational mode, keep black objects black, and also mark them as
** old; stop when hitting an old object, as all objects after that
** one will be old too.
** When object is a thread, sweep its list of open upvalues too.
*/
static GCObject **sweeplist(lua_State *L, GCObject **p, lu_mem count)
{
	global_State *g = G(L);
	int ow = otherwhite(g);
	int toclear, toset; /* bits to clear and to set in all live objects */
	int tostop;         /* stop sweep when this is true */
	if (isgenerational(g))
	{                             /* generational mode? */
		toclear = ~0;             /* clear nothing */
		toset = bitmask(OLDBIT);  /* set the old bit of all surviving objects */
		tostop = bitmask(OLDBIT); /* do not sweep old generation */
	}
	else
	{                          /* normal mode */
		toclear = maskcolors;  /* clear all color bits + old bit */
		toset = luaC_white(g); /* make object white */
		tostop = 0;            /* do not stop */
	}
	while (*p != NULL && count-- > 0)
	{
		GCObject *curr = *p;
		int marked = gch(curr)->marked;
		if (isdeadm(ow, marked))
		{                         /* is 'curr' dead? */
			*p = gch(curr)->next; /* remove 'curr' from list */
			freeobj(L, curr);     /* erase 'curr' */
		}
		else
		{
			if (testbits(marked, tostop))
				return NULL; /* stop sweeping this list */
			if (gch(curr)->tt == LUA_TTHREAD)
				sweepthread(L, gco2th(curr)); /* sweep thread's upvalues */
			/* update marks */
			gch(curr)->marked = cast_byte((marked & toclear) | toset);
			p = &gch(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, int *n)
{
	GCObject **old = p;
	int i = 0;
	do
	{
		i++;
		p = sweeplist(L, p, 1);
	} while (p == old);
	if (n) *n += i;
	return p;
}

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

/*
** {======================================================
** Finalization
** =======================================================
*/

static void checkSizes(lua_State *L)
{
	global_State *g = G(L);
	if (g->gckind != KGC_EMERGENCY)
	{                                          /* do not change sizes in emergency */
		int hs = g->strt.size / 2;             /* half the size of the string table */
		if (g->strt.nuse < cast(lu_int32, hs)) /* using less than that half? */
			luaS_resize(L, hs);                /* halve its size */
		luaZ_freebuffer(L, &g->buff);          /* free concatenation buffer */
	}
}

static GCObject *udata2finalize(global_State *g)
{
	GCObject *o = g->tobefnz; /* get first element */
	lua_assert(isfinalized(o));
	g->tobefnz = gch(o)->next; /* remove it from 'tobefnz' list */
	gch(o)->next = g->allgc;   /* return it to 'allgc' list */
	g->allgc = o;
	resetbit(gch(o)->marked, SEPARATED); /* mark that it is not in 'tobefnz' */
	lua_assert(!isold(o));               /* see MOVE OLD rule */
	if (!keepinvariantout(g))            /* not keeping invariant? */
		makewhite(g, o);                 /* "sweep" object */
	return o;
}

static void dothecall(lua_State *L, void *ud)
{
	UNUSED(ud);
	luaD_call(L, L->top - 2, 0, 0);
}

static void GCTM(lua_State *L, int propagateerrors)
{
	global_State *g = G(L);
	const TValue *tm;
	TValue v;
	setgcovalue(L, &v, udata2finalize(g));
	tm = luaT_gettmbyobj(L, &v, TM_GC);
	if (tm != NULL && ttisfunction(tm))
	{ /* is there a finalizer? */
		int status;
		lu_byte oldah = L->allowhook;
		int running = g->gcrunning;
		L->allowhook = 0;            /* stop debug hooks during GC metamethod */
		g->gcrunning = 0;            /* avoid GC steps */
		setobj2s(L, L->top, tm);     /* push finalizer... */
		setobj2s(L, L->top + 1, &v); /* ... and its argument */
		L->top += 2;                 /* and (next line) call the finalizer */
		status = luaD_pcall(L, dothecall, NULL, savestack(L, L->top - 2), 0);
		L->allowhook = oldah;   /* restore hooks */
		g->gcrunning = running; /* restore state */
		if (status != LUA_OK && propagateerrors)
		{ /* error while running __gc? */
			if (status == LUA_ERRRUN)
			{ /* is there an error 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 */
		}
	}
}

/*
** move all unreachable objects (or 'all' objects) that need
** finalization from list 'finobj' to list 'tobefnz' (to be finalized)
*/
static void separatetobefnz(lua_State *L, int all)
{
	global_State *g = G(L);
	GCObject **p = &g->finobj;
	GCObject *curr;
	GCObject **lastnext = &g->tobefnz;
	/* find last 'next' field in 'tobefnz' list (to add elements in its end) */
	while (*lastnext != NULL)
		lastnext = &gch(*lastnext)->next;
	while ((curr = *p) != NULL)
	{ /* traverse all finalizable objects */
		lua_assert(!isfinalized(curr));
		lua_assert(testbit(gch(curr)->marked, SEPARATED));
		if (!(iswhite(curr) || all)) /* not being collected? */
			p = &gch(curr)->next;    /* don't bother with it */
		else
		{
			l_setbit(gch(curr)->marked, FINALIZEDBIT); /* won't be finalized again */
			*p = gch(curr)->next;                      /* remove 'curr' from 'finobj' list */
			gch(curr)->next = *lastnext;               /* link at the end of 'tobefnz' list */
			*lastnext = curr;
			lastnext = &gch(curr)->next;
		}
	}
}

/*
** if object 'o' has a finalizer, remove it from 'allgc' list (must
** search the list to find it) and link it in 'finobj' list.
*/
void luaC_checkfinalizer(lua_State *L, GCObject *o, Table *mt)
{
	global_State *g = G(L);
	if (testbit(gch(o)->marked, SEPARATED) || /* obj. is already separated... */
		isfinalized(o) ||                     /* ... or is finalized... */
		gfasttm(g, mt, TM_GC) == NULL)        /* or has no finalizer? */
		return;                               /* nothing to be done */
	else
	{ /* move 'o' to 'finobj' list */
		GCObject **p;
		GCheader *ho = gch(o);
		if (g->sweepgc == &ho->next)
		{ /* avoid removing current sweep object */
			lua_assert(issweepphase(g));
			g->sweepgc = sweeptolive(L, g->sweepgc, NULL);
		}
		/* search for pointer pointing to 'o' */
		for (p = &g->allgc; *p != o; p = &gch(*p)->next)
		{ /* empty */
		}
		*p = ho->next;        /* remove 'o' from root list */
		ho->next = g->finobj; /* link it in list 'finobj' */
		g->finobj = o;
		l_setbit(ho->marked, SEPARATED); /* mark it as such */
		if (!keepinvariantout(g))        /* not keeping invariant? */
			makewhite(g, o);             /* "sweep" object */
		else
			resetoldbit(o); /* see MOVE OLD rule */
	}
}

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

/*
** {======================================================
** GC control
** =======================================================
*/

/*
** set a reasonable "time" to wait before starting a new GC cycle;
** cycle will start when memory use hits threshold
*/
static void setpause(global_State *g, l_mem estimate)
{
	l_mem debt, threshold;
	estimate = estimate / PAUSEADJ;                /* adjust 'estimate' */
	threshold = (g->gcpause < MAX_LMEM / estimate) /* overflow? */
					? estimate * g->gcpause        /* no overflow */
					: MAX_LMEM;                    /* overflow; truncate to maximum */
	debt = -cast(l_mem, threshold - gettotalbytes(g));
	luaE_setdebt(g, debt);
}

#define sweepphases \
	(bitmask(GCSsweepstring) | bitmask(GCSsweepudata) | bitmask(GCSsweep))

/*
** enter first sweep phase (strings) and prepare pointers for other
** sweep phases.  The calls to 'sweeptolive' make pointers 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.
** Returns how many objects it swept.
*/
static int entersweep(lua_State *L)
{
	global_State *g = G(L);
	int n = 0;
	g->gcstate = GCSsweepstring;
	lua_assert(g->sweepgc == NULL && g->sweepfin == NULL);
	/* prepare to sweep strings, finalizable objects, and regular objects */
	g->sweepstrgc = 0;
	g->sweepfin = sweeptolive(L, &g->finobj, &n);
	g->sweepgc = sweeptolive(L, &g->allgc, &n);
	return n;
}

/*
** change GC mode
*/
void luaC_changemode(lua_State *L, int mode)
{
	global_State *g = G(L);
	if (mode == g->gckind) return; /* nothing to change */
	if (mode == KGC_GEN)
	{ /* change to generational mode */
		/* make sure gray lists are consistent */
		luaC_runtilstate(L, bitmask(GCSpropagate));
		g->GCestimate = gettotalbytes(g);
		g->gckind = KGC_GEN;
	}
	else
	{ /* change to incremental mode */
		/* sweep all objects to turn them back to white
       (as white has not changed, nothing extra will be collected) */
		g->gckind = KGC_NORMAL;
		entersweep(L);
		luaC_runtilstate(L, ~sweepphases);
	}
}

/*
** call all pending finalizers
*/
static void callallpendingfinalizers(lua_State *L, int propagateerrors)
{
	global_State *g = G(L);
	while (g->tobefnz)
	{
		resetoldbit(g->tobefnz);
		GCTM(L, propagateerrors);
	}
}

void luaC_freeallobjects(lua_State *L)
{
	global_State *g = G(L);
	int i;
	separatetobefnz(L, 1); /* separate all objects with finalizers */
	lua_assert(g->finobj == NULL);
	callallpendingfinalizers(L, 0);
	g->currentwhite = WHITEBITS; /* this "white" makes all objects look dead */
	g->gckind = KGC_NORMAL;
	sweepwholelist(L, &g->finobj); /* finalizers can create objs. in 'finobj' */
	sweepwholelist(L, &g->allgc);
	for (i = 0; i < g->strt.size; i++) /* free all string lists */
		sweepwholelist(L, &g->strt.hash[i]);
	lua_assert(g->strt.nuse == 0);
}

static l_mem atomic(lua_State *L)
{
	global_State *g = G(L);
	l_mem work = -cast(l_mem, g->GCmemtrav); /* start counting work */
	GCObject *origweak, *origall;
	lua_assert(!iswhite(obj2gco(g->mainthread)));
	markobject(g, L); /* mark running thread */
	/* registry and global metatables may be changed by API */
	markvalue(g, &g->l_registry);
	markmt(g); /* mark basic metatables */
	/* remark occasional upvalues of (maybe) dead threads */
	remarkupvals(g);
	propagateall(g);      /* propagate changes */
	work += g->GCmemtrav; /* stop counting (do not (re)count grays) */
	/* traverse objects caught by write barrier and by 'remarkupvals' */
	retraversegrays(g);
	work -= g->GCmemtrav; /* 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(L, 0); /* separate objects to be finalized */
	markbeingfnz(g);       /* mark objects that will be finalized */
	propagateall(g);       /* remark, to propagate `preserveness' */
	work -= g->GCmemtrav;  /* 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);
	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 singlestep(lua_State *L)
{
	global_State *g = G(L);
	switch (g->gcstate)
	{
		case GCSpause:
		{
			/* start to count memory traversed */
			g->GCmemtrav = g->strt.size * sizeof(GCObject *);
			lua_assert(!isgenerational(g));
			restartcollection(g);
			g->gcstate = GCSpropagate;
			return g->GCmemtrav;
		}
		case GCSpropagate:
		{
			if (g->gray)
			{
				lu_mem oldtrav = g->GCmemtrav;
				propagatemark(g);
				return g->GCmemtrav - oldtrav; /* memory traversed in this step */
			}
			else
			{ /* no more `gray' objects */
				lu_mem work;
				int sw;
				g->gcstate = GCSatomic;       /* finish mark phase */
				g->GCestimate = g->GCmemtrav; /* save what was counted */
				;
				work = atomic(L);      /* add what was traversed by 'atomic' */
				g->GCestimate += work; /* estimate of total memory traversed */
				sw = entersweep(L);
				return work + sw * GCSWEEPCOST;
			}
		}
		case GCSsweepstring:
		{
			int i;
			for (i = 0; i < GCSWEEPMAX && g->sweepstrgc + i < g->strt.size; i++)
				sweepwholelist(L, &g->strt.hash[g->sweepstrgc + i]);
			g->sweepstrgc += i;
			if (g->sweepstrgc >= g->strt.size) /* no more strings to sweep? */
				g->gcstate = GCSsweepudata;
			return i * GCSWEEPCOST;
		}
		case GCSsweepudata:
		{
			if (g->sweepfin)
			{
				g->sweepfin = sweeplist(L, g->sweepfin, GCSWEEPMAX);
				return GCSWEEPMAX * GCSWEEPCOST;
			}
			else
			{
				g->gcstate = GCSsweep;
				return 0;
			}
		}
		case GCSsweep:
		{
			if (g->sweepgc)
			{
				g->sweepgc = sweeplist(L, g->sweepgc, GCSWEEPMAX);
				return GCSWEEPMAX * GCSWEEPCOST;
			}
			else
			{
				/* sweep main thread */
				GCObject *mt = obj2gco(g->mainthread);
				sweeplist(L, &mt, 1);
				checkSizes(L);
				g->gcstate = GCSpause; /* finish collection */
				return GCSWEEPCOST;
			}
		}
		default:
			lua_assert(0);
			return 0;
	}
}

/*
** advances the garbage collector until it reaches a state allowed
** by 'statemask'
*/
void luaC_runtilstate(lua_State *L, int statesmask)
{
	global_State *g = G(L);
	while (!testbit(statesmask, g->gcstate))
		singlestep(L);
}

static void generationalcollection(lua_State *L)
{
	global_State *g = G(L);
	lua_assert(g->gcstate == GCSpropagate);
	if (g->GCestimate == 0)
	{                                     /* signal for another major collection? */
		luaC_fullgc(L, 0);                /* perform a full regular collection */
		g->GCestimate = gettotalbytes(g); /* update control */
	}
	else
	{
		lu_mem estimate = g->GCestimate;
		luaC_runtilstate(L, bitmask(GCSpause)); /* run complete (minor) cycle */
		g->gcstate = GCSpropagate;              /* skip restart */
		if (gettotalbytes(g) > (estimate / 100) * g->gcmajorinc)
			g->GCestimate = 0; /* signal for a major collection */
		else
			g->GCestimate = estimate; /* keep estimate from last major coll. */
	}
	setpause(g, gettotalbytes(g));
	lua_assert(g->gcstate == GCSpropagate);
}

static void incstep(lua_State *L)
{
	global_State *g = G(L);
	l_mem debt = g->GCdebt;
	int stepmul = g->gcstepmul;
	if (stepmul < 40) stepmul = 40; /* avoid ridiculous low values (and 0) */
	/* convert debt from Kb to 'work units' (avoid zero debt and overflows) */
	debt = (debt / STEPMULADJ) + 1;
	debt = (debt < MAX_LMEM / stepmul) ? debt * stepmul : MAX_LMEM;
	do
	{                                /* always perform at least one single step */
		lu_mem work = singlestep(L); /* do some work */
		debt -= work;
	} while (debt > -GCSTEPSIZE && g->gcstate != GCSpause);
	if (g->gcstate == GCSpause)
		setpause(g, g->GCestimate); /* pause until next cycle */
	else
	{
		debt = (debt / stepmul) * STEPMULADJ; /* convert 'work units' to Kb */
		luaE_setdebt(g, debt);
	}
}

/*
** performs a basic GC step
*/
void luaC_forcestep(lua_State *L)
{
	global_State *g = G(L);
	int i;
	if (isgenerational(g))
		generationalcollection(L);
	else
		incstep(L);
	/* run a few finalizers (or all of them at the end of a collect cycle) */
	for (i = 0; g->tobefnz && (i < GCFINALIZENUM || g->gcstate == GCSpause); i++)
		GCTM(L, 1); /* call one finalizer */
}

/*
** performs a basic GC step only if collector is running
*/
void luaC_step(lua_State *L)
{
	global_State *g = G(L);
	if (g->gcrunning)
		luaC_forcestep(L);
	else
		luaE_setdebt(g, -GCSTEPSIZE); /* avoid being called too often */
}

/*
** performs a full GC cycle; if "isemergency", does not call
** finalizers (which could change stack positions)
*/
void luaC_fullgc(lua_State *L, int isemergency)
{
	global_State *g = G(L);
	int origkind = g->gckind;
	lua_assert(origkind != KGC_EMERGENCY);
	if (isemergency) /* do not run finalizers during emergency GC */
		g->gckind = KGC_EMERGENCY;
	else
	{
		g->gckind = KGC_NORMAL;
		callallpendingfinalizers(L, 1);
	}
	if (keepinvariant(g))
	{ /* may there be some black objects? */
		/* must sweep all objects to turn them back to white
       (as white has not changed, nothing will be collected) */
		entersweep(L);
	}
	/* 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(GCSpause));  /* run entire collection */
	if (origkind == KGC_GEN)
	{ /* generational mode? */
		/* generational mode must be kept in propagate phase */
		luaC_runtilstate(L, bitmask(GCSpropagate));
	}
	g->gckind = origkind;
	setpause(g, gettotalbytes(g));
	if (!isemergency) /* do not run finalizers during emergency GC */
		callallpendingfinalizers(L, 1);
}

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