mudgangster

lgc.cc (36672B)

---

 /*
 ** $Id: lgc.c,v 2.215.1.2 2017/08/31 16:15:27 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 ... */
       }
       if (ttisnil(gval(n)))  /* is entry empty? */
         removeentry(n);  /* 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);
 }
 
 /* }====================================================== */