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    1 /*
    2 ** $Id: lgc.c $
    3 ** Garbage Collector
    4 ** See Copyright Notice in lua.h
    5 */
    6 
    7 #define lgc_c
    8 #define LUA_CORE
    9 
   10 #include "lprefix.h"
   11 
   12 #include <stdio.h>
   13 #include <string.h>
   14 
   15 
   16 #include "lua.h"
   17 
   18 #include "ldebug.h"
   19 #include "ldo.h"
   20 #include "lfunc.h"
   21 #include "lgc.h"
   22 #include "lmem.h"
   23 #include "lobject.h"
   24 #include "lstate.h"
   25 #include "lstring.h"
   26 #include "ltable.h"
   27 #include "ltm.h"
   28 
   29 
   30 /*
   31 ** Maximum number of elements to sweep in each single step.
   32 ** (Large enough to dissipate fixed overheads but small enough
   33 ** to allow small steps for the collector.)
   34 */
   35 #define GCSWEEPMAX  100
   36 
   37 /*
   38 ** Maximum number of finalizers to call in each single step.
   39 */
   40 #define GCFINMAX    10
   41 
   42 
   43 /*
   44 ** Cost of calling one finalizer.
   45 */
   46 #define GCFINALIZECOST  50
   47 
   48 
   49 /*
   50 ** The equivalent, in bytes, of one unit of "work" (visiting a slot,
   51 ** sweeping an object, etc.)
   52 */
   53 #define WORK2MEM    sizeof(TValue)
   54 
   55 
   56 /*
   57 ** macro to adjust 'pause': 'pause' is actually used like
   58 ** 'pause / PAUSEADJ' (value chosen by tests)
   59 */
   60 #define PAUSEADJ        100
   61 
   62 
   63 /* mask with all color bits */
   64 #define maskcolors  (bitmask(BLACKBIT) | WHITEBITS)
   65 
   66 /* mask with all GC bits */
   67 #define maskgcbits      (maskcolors | AGEBITS)
   68 
   69 
   70 /* macro to erase all color bits then set only the current white bit */
   71 #define makewhite(g,x)  \
   72   (x->marked = cast_byte((x->marked & ~maskcolors) | luaC_white(g)))
   73 
   74 /* make an object gray (neither white nor black) */
   75 #define set2gray(x) resetbits(x->marked, maskcolors)
   76 
   77 
   78 /* make an object black (coming from any color) */
   79 #define set2black(x)  \
   80   (x->marked = cast_byte((x->marked & ~WHITEBITS) | bitmask(BLACKBIT)))
   81 
   82 
   83 #define valiswhite(x)   (iscollectable(x) && iswhite(gcvalue(x)))
   84 
   85 #define keyiswhite(n)   (keyiscollectable(n) && iswhite(gckey(n)))
   86 
   87 
   88 /*
   89 ** Protected access to objects in values
   90 */
   91 #define gcvalueN(o)     (iscollectable(o) ? gcvalue(o) : NULL)
   92 
   93 
   94 #define markvalue(g,o) { checkliveness(g->mainthread,o); \
   95   if (valiswhite(o)) reallymarkobject(g,gcvalue(o)); }
   96 
   97 #define markkey(g, n)   { if keyiswhite(n) reallymarkobject(g,gckey(n)); }
   98 
   99 #define markobject(g,t) { if (iswhite(t)) reallymarkobject(g, obj2gco(t)); }
  100 
  101 /*
  102 ** mark an object that can be NULL (either because it is really optional,
  103 ** or it was stripped as debug info, or inside an uncompleted structure)
  104 */
  105 #define markobjectN(g,t)    { if (t) markobject(g,t); }
  106 
  107 static void reallymarkobject (global_State *g, GCObject *o);
  108 static lu_mem atomic (lua_State *L);
  109 static void entersweep (lua_State *L);
  110 
  111 
  112 /*
  113 ** {======================================================
  114 ** Generic functions
  115 ** =======================================================
  116 */
  117 
  118 
  119 /*
  120 ** one after last element in a hash array
  121 */
  122 #define gnodelast(h)    gnode(h, cast_sizet(sizenode(h)))
  123 
  124 
  125 static GCObject **getgclist (GCObject *o) {
  126   switch (o->tt) {
  127     case LUA_VTABLE: return &gco2t(o)->gclist;
  128     case LUA_VLCL: return &gco2lcl(o)->gclist;
  129     case LUA_VCCL: return &gco2ccl(o)->gclist;
  130     case LUA_VTHREAD: return &gco2th(o)->gclist;
  131     case LUA_VPROTO: return &gco2p(o)->gclist;
  132     case LUA_VUSERDATA: {
  133       Udata *u = gco2u(o);
  134       lua_assert(u->nuvalue > 0);
  135       return &u->gclist;
  136     }
  137     default: lua_assert(0); return 0;
  138   }
  139 }
  140 
  141 
  142 /*
  143 ** Link a collectable object 'o' with a known type into the list 'p'.
  144 ** (Must be a macro to access the 'gclist' field in different types.)
  145 */
  146 #define linkgclist(o,p) linkgclist_(obj2gco(o), &(o)->gclist, &(p))
  147 
  148 static void linkgclist_ (GCObject *o, GCObject **pnext, GCObject **list) {
  149   lua_assert(!isgray(o));  /* cannot be in a gray list */
  150   *pnext = *list;
  151   *list = o;
  152   set2gray(o);  /* now it is */
  153 }
  154 
  155 
  156 /*
  157 ** Link a generic collectable object 'o' into the list 'p'.
  158 */
  159 #define linkobjgclist(o,p) linkgclist_(obj2gco(o), getgclist(o), &(p))
  160 
  161 
  162 
  163 /*
  164 ** Clear keys for empty entries in tables. If entry is empty, mark its
  165 ** entry as dead. This allows the collection of the key, but keeps its
  166 ** entry in the table: its removal could break a chain and could break
  167 ** a table traversal.  Other places never manipulate dead keys, because
  168 ** its associated empty value is enough to signal that the entry is
  169 ** logically empty.
  170 */
  171 static void clearkey (Node *n) {
  172   lua_assert(isempty(gval(n)));
  173   if (keyiscollectable(n))
  174     setdeadkey(n);  /* unused key; remove it */
  175 }
  176 
  177 
  178 /*
  179 ** tells whether a key or value can be cleared from a weak
  180 ** table. Non-collectable objects are never removed from weak
  181 ** tables. Strings behave as 'values', so are never removed too. for
  182 ** other objects: if really collected, cannot keep them; for objects
  183 ** being finalized, keep them in keys, but not in values
  184 */
  185 static int iscleared (global_State *g, const GCObject *o) {
  186   if (o == NULL) return 0;  /* non-collectable value */
  187   else if (novariant(o->tt) == LUA_TSTRING) {
  188     markobject(g, o);  /* strings are 'values', so are never weak */
  189     return 0;
  190   }
  191   else return iswhite(o);
  192 }
  193 
  194 
  195 /*
  196 ** Barrier that moves collector forward, that is, marks the white object
  197 ** 'v' being pointed by the black object 'o'.  In the generational
  198 ** mode, 'v' must also become old, if 'o' is old; however, it cannot
  199 ** be changed directly to OLD, because it may still point to non-old
  200 ** objects. So, it is marked as OLD0. In the next cycle it will become
  201 ** OLD1, and in the next it will finally become OLD (regular old). By
  202 ** then, any object it points to will also be old.  If called in the
  203 ** incremental sweep phase, it clears the black object to white (sweep
  204 ** it) to avoid other barrier calls for this same object. (That cannot
  205 ** be done is generational mode, as its sweep does not distinguish
  206 ** whites from deads.)
  207 */
  208 void luaC_barrier_ (lua_State *L, GCObject *o, GCObject *v) {
  209   global_State *g = G(L);
  210   lua_assert(isblack(o) && iswhite(v) && !isdead(g, v) && !isdead(g, o));
  211   if (keepinvariant(g)) {  /* must keep invariant? */
  212     reallymarkobject(g, v);  /* restore invariant */
  213     if (isold(o)) {
  214       lua_assert(!isold(v));  /* white object could not be old */
  215       setage(v, G_OLD0);  /* restore generational invariant */
  216     }
  217   }
  218   else {  /* sweep phase */
  219     lua_assert(issweepphase(g));
  220     if (g->gckind == KGC_INC)  /* incremental mode? */
  221       makewhite(g, o);  /* mark 'o' as white to avoid other barriers */
  222   }
  223 }
  224 
  225 
  226 /*
  227 ** barrier that moves collector backward, that is, mark the black object
  228 ** pointing to a white object as gray again.
  229 */
  230 void luaC_barrierback_ (lua_State *L, GCObject *o) {
  231   global_State *g = G(L);
  232   lua_assert(isblack(o) && !isdead(g, o));
  233   lua_assert((g->gckind == KGC_GEN) == (isold(o) && getage(o) != G_TOUCHED1));
  234   if (getage(o) == G_TOUCHED2)  /* already in gray list? */
  235     set2gray(o);  /* make it gray to become touched1 */
  236   else  /* link it in 'grayagain' and paint it gray */
  237     linkobjgclist(o, g->grayagain);
  238   if (isold(o))  /* generational mode? */
  239     setage(o, G_TOUCHED1);  /* touched in current cycle */
  240 }
  241 
  242 
  243 void luaC_fix (lua_State *L, GCObject *o) {
  244   global_State *g = G(L);
  245   lua_assert(g->allgc == o);  /* object must be 1st in 'allgc' list! */
  246   set2gray(o);  /* they will be gray forever */
  247   setage(o, G_OLD);  /* and old forever */
  248   g->allgc = o->next;  /* remove object from 'allgc' list */
  249   o->next = g->fixedgc;  /* link it to 'fixedgc' list */
  250   g->fixedgc = o;
  251 }
  252 
  253 
  254 /*
  255 ** create a new collectable object (with given type and size) and link
  256 ** it to 'allgc' list.
  257 */
  258 GCObject *luaC_newobj (lua_State *L, int tt, size_t sz) {
  259   global_State *g = G(L);
  260   GCObject *o = cast(GCObject *, luaM_newobject(L, novariant(tt), sz));
  261   o->marked = luaC_white(g);
  262   o->tt = tt;
  263   o->next = g->allgc;
  264   g->allgc = o;
  265   return o;
  266 }
  267 
  268 /* }====================================================== */
  269 
  270 
  271 
  272 /*
  273 ** {======================================================
  274 ** Mark functions
  275 ** =======================================================
  276 */
  277 
  278 
  279 /*
  280 ** Mark an object.  Userdata with no user values, strings, and closed
  281 ** upvalues are visited and turned black here.  Open upvalues are
  282 ** already indirectly linked through their respective threads in the
  283 ** 'twups' list, so they don't go to the gray list; nevertheless, they
  284 ** are kept gray to avoid barriers, as their values will be revisited
  285 ** by the thread or by 'remarkupvals'.  Other objects are added to the
  286 ** gray list to be visited (and turned black) later.  Both userdata and
  287 ** upvalues can call this function recursively, but this recursion goes
  288 ** for at most two levels: An upvalue cannot refer to another upvalue
  289 ** (only closures can), and a userdata's metatable must be a table.
  290 */
  291 static void reallymarkobject (global_State *g, GCObject *o) {
  292   switch (o->tt) {
  293     case LUA_VSHRSTR:
  294     case LUA_VLNGSTR: {
  295       set2black(o);  /* nothing to visit */
  296       break;
  297     }
  298     case LUA_VUPVAL: {
  299       UpVal *uv = gco2upv(o);
  300       if (upisopen(uv))
  301         set2gray(uv);  /* open upvalues are kept gray */
  302       else
  303         set2black(uv);  /* closed upvalues are visited here */
  304       markvalue(g, uv->v);  /* mark its content */
  305       break;
  306     }
  307     case LUA_VUSERDATA: {
  308       Udata *u = gco2u(o);
  309       if (u->nuvalue == 0) {  /* no user values? */
  310         markobjectN(g, u->metatable);  /* mark its metatable */
  311         set2black(u);  /* nothing else to mark */
  312         break;
  313       }
  314       /* else... */
  315     }  /* FALLTHROUGH */
  316     case LUA_VLCL: case LUA_VCCL: case LUA_VTABLE:
  317     case LUA_VTHREAD: case LUA_VPROTO: {
  318       linkobjgclist(o, g->gray);  /* to be visited later */
  319       break;
  320     }
  321     default: lua_assert(0); break;
  322   }
  323 }
  324 
  325 
  326 /*
  327 ** mark metamethods for basic types
  328 */
  329 static void markmt (global_State *g) {
  330   int i;
  331   for (i=0; i < LUA_NUMTAGS; i++)
  332     markobjectN(g, g->mt[i]);
  333 }
  334 
  335 
  336 /*
  337 ** mark all objects in list of being-finalized
  338 */
  339 static lu_mem markbeingfnz (global_State *g) {
  340   GCObject *o;
  341   lu_mem count = 0;
  342   for (o = g->tobefnz; o != NULL; o = o->next) {
  343     count++;
  344     markobject(g, o);
  345   }
  346   return count;
  347 }
  348 
  349 
  350 /*
  351 ** For each non-marked thread, simulates a barrier between each open
  352 ** upvalue and its value. (If the thread is collected, the value will be
  353 ** assigned to the upvalue, but then it can be too late for the barrier
  354 ** to act. The "barrier" does not need to check colors: A non-marked
  355 ** thread must be young; upvalues cannot be older than their threads; so
  356 ** any visited upvalue must be young too.) Also removes the thread from
  357 ** the list, as it was already visited. Removes also threads with no
  358 ** upvalues, as they have nothing to be checked. (If the thread gets an
  359 ** upvalue later, it will be linked in the list again.)
  360 */
  361 static int remarkupvals (global_State *g) {
  362   lua_State *thread;
  363   lua_State **p = &g->twups;
  364   int work = 0;  /* estimate of how much work was done here */
  365   while ((thread = *p) != NULL) {
  366     work++;
  367     if (!iswhite(thread) && thread->openupval != NULL)
  368       p = &thread->twups;  /* keep marked thread with upvalues in the list */
  369     else {  /* thread is not marked or without upvalues */
  370       UpVal *uv;
  371       lua_assert(!isold(thread) || thread->openupval == NULL);
  372       *p = thread->twups;  /* remove thread from the list */
  373       thread->twups = thread;  /* mark that it is out of list */
  374       for (uv = thread->openupval; uv != NULL; uv = uv->u.open.next) {
  375         lua_assert(getage(uv) <= getage(thread));
  376         work++;
  377         if (!iswhite(uv)) {  /* upvalue already visited? */
  378           lua_assert(upisopen(uv) && isgray(uv));
  379           markvalue(g, uv->v);  /* mark its value */
  380         }
  381       }
  382     }
  383   }
  384   return work;
  385 }
  386 
  387 
  388 static void cleargraylists (global_State *g) {
  389   g->gray = g->grayagain = NULL;
  390   g->weak = g->allweak = g->ephemeron = NULL;
  391 }
  392 
  393 
  394 /*
  395 ** mark root set and reset all gray lists, to start a new collection
  396 */
  397 static void restartcollection (global_State *g) {
  398   cleargraylists(g);
  399   markobject(g, g->mainthread);
  400   markvalue(g, &g->l_registry);
  401   markmt(g);
  402   markbeingfnz(g);  /* mark any finalizing object left from previous cycle */
  403 }
  404 
  405 /* }====================================================== */
  406 
  407 
  408 /*
  409 ** {======================================================
  410 ** Traverse functions
  411 ** =======================================================
  412 */
  413 
  414 
  415 /*
  416 ** Check whether object 'o' should be kept in the 'grayagain' list for
  417 ** post-processing by 'correctgraylist'. (It could put all old objects
  418 ** in the list and leave all the work to 'correctgraylist', but it is
  419 ** more efficient to avoid adding elements that will be removed.) Only
  420 ** TOUCHED1 objects need to be in the list. TOUCHED2 doesn't need to go
  421 ** back to a gray list, but then it must become OLD. (That is what
  422 ** 'correctgraylist' does when it finds a TOUCHED2 object.)
  423 */
  424 static void genlink (global_State *g, GCObject *o) {
  425   lua_assert(isblack(o));
  426   if (getage(o) == G_TOUCHED1) {  /* touched in this cycle? */
  427     linkobjgclist(o, g->grayagain);  /* link it back in 'grayagain' */
  428   }  /* everything else do not need to be linked back */
  429   else if (getage(o) == G_TOUCHED2)
  430     changeage(o, G_TOUCHED2, G_OLD);  /* advance age */
  431 }
  432 
  433 
  434 /*
  435 ** Traverse a table with weak values and link it to proper list. During
  436 ** propagate phase, keep it in 'grayagain' list, to be revisited in the
  437 ** atomic phase. In the atomic phase, if table has any white value,
  438 ** put it in 'weak' list, to be cleared.
  439 */
  440 static void traverseweakvalue (global_State *g, Table *h) {
  441   Node *n, *limit = gnodelast(h);
  442   /* if there is array part, assume it may have white values (it is not
  443      worth traversing it now just to check) */
  444   int hasclears = (h->alimit > 0);
  445   for (n = gnode(h, 0); n < limit; n++) {  /* traverse hash part */
  446     if (isempty(gval(n)))  /* entry is empty? */
  447       clearkey(n);  /* clear its key */
  448     else {
  449       lua_assert(!keyisnil(n));
  450       markkey(g, n);
  451       if (!hasclears && iscleared(g, gcvalueN(gval(n))))  /* a white value? */
  452         hasclears = 1;  /* table will have to be cleared */
  453     }
  454   }
  455   if (g->gcstate == GCSatomic && hasclears)
  456     linkgclist(h, g->weak);  /* has to be cleared later */
  457   else
  458     linkgclist(h, g->grayagain);  /* must retraverse it in atomic phase */
  459 }
  460 
  461 
  462 /*
  463 ** Traverse an ephemeron table and link it to proper list. Returns true
  464 ** iff any object was marked during this traversal (which implies that
  465 ** convergence has to continue). During propagation phase, keep table
  466 ** in 'grayagain' list, to be visited again in the atomic phase. In
  467 ** the atomic phase, if table has any white->white entry, it has to
  468 ** be revisited during ephemeron convergence (as that key may turn
  469 ** black). Otherwise, if it has any white key, table has to be cleared
  470 ** (in the atomic phase). In generational mode, some tables
  471 ** must be kept in some gray list for post-processing; this is done
  472 ** by 'genlink'.
  473 */
  474 static int traverseephemeron (global_State *g, Table *h, int inv) {
  475   int marked = 0;  /* true if an object is marked in this traversal */
  476   int hasclears = 0;  /* true if table has white keys */
  477   int hasww = 0;  /* true if table has entry "white-key -> white-value" */
  478   unsigned int i;
  479   unsigned int asize = luaH_realasize(h);
  480   unsigned int nsize = sizenode(h);
  481   /* traverse array part */
  482   for (i = 0; i < asize; i++) {
  483     if (valiswhite(&h->array[i])) {
  484       marked = 1;
  485       reallymarkobject(g, gcvalue(&h->array[i]));
  486     }
  487   }
  488   /* traverse hash part; if 'inv', traverse descending
  489      (see 'convergeephemerons') */
  490   for (i = 0; i < nsize; i++) {
  491     Node *n = inv ? gnode(h, nsize - 1 - i) : gnode(h, i);
  492     if (isempty(gval(n)))  /* entry is empty? */
  493       clearkey(n);  /* clear its key */
  494     else if (iscleared(g, gckeyN(n))) {  /* key is not marked (yet)? */
  495       hasclears = 1;  /* table must be cleared */
  496       if (valiswhite(gval(n)))  /* value not marked yet? */
  497         hasww = 1;  /* white-white entry */
  498     }
  499     else if (valiswhite(gval(n))) {  /* value not marked yet? */
  500       marked = 1;
  501       reallymarkobject(g, gcvalue(gval(n)));  /* mark it now */
  502     }
  503   }
  504   /* link table into proper list */
  505   if (g->gcstate == GCSpropagate)
  506     linkgclist(h, g->grayagain);  /* must retraverse it in atomic phase */
  507   else if (hasww)  /* table has white->white entries? */
  508     linkgclist(h, g->ephemeron);  /* have to propagate again */
  509   else if (hasclears)  /* table has white keys? */
  510     linkgclist(h, g->allweak);  /* may have to clean white keys */
  511   else
  512     genlink(g, obj2gco(h));  /* check whether collector still needs to see it */
  513   return marked;
  514 }
  515 
  516 
  517 static void traversestrongtable (global_State *g, Table *h) {
  518   Node *n, *limit = gnodelast(h);
  519   unsigned int i;
  520   unsigned int asize = luaH_realasize(h);
  521   for (i = 0; i < asize; i++)  /* traverse array part */
  522     markvalue(g, &h->array[i]);
  523   for (n = gnode(h, 0); n < limit; n++) {  /* traverse hash part */
  524     if (isempty(gval(n)))  /* entry is empty? */
  525       clearkey(n);  /* clear its key */
  526     else {
  527       lua_assert(!keyisnil(n));
  528       markkey(g, n);
  529       markvalue(g, gval(n));
  530     }
  531   }
  532   genlink(g, obj2gco(h));
  533 }
  534 
  535 
  536 static lu_mem traversetable (global_State *g, Table *h) {
  537   const char *weakkey, *weakvalue;
  538   const TValue *mode = gfasttm(g, h->metatable, TM_MODE);
  539   markobjectN(g, h->metatable);
  540   if (mode && ttisstring(mode) &&  /* is there a weak mode? */
  541       (cast_void(weakkey = strchr(svalue(mode), 'k')),
  542        cast_void(weakvalue = strchr(svalue(mode), 'v')),
  543        (weakkey || weakvalue))) {  /* is really weak? */
  544     if (!weakkey)  /* strong keys? */
  545       traverseweakvalue(g, h);
  546     else if (!weakvalue)  /* strong values? */
  547       traverseephemeron(g, h, 0);
  548     else  /* all weak */
  549       linkgclist(h, g->allweak);  /* nothing to traverse now */
  550   }
  551   else  /* not weak */
  552     traversestrongtable(g, h);
  553   return 1 + h->alimit + 2 * allocsizenode(h);
  554 }
  555 
  556 
  557 static int traverseudata (global_State *g, Udata *u) {
  558   int i;
  559   markobjectN(g, u->metatable);  /* mark its metatable */
  560   for (i = 0; i < u->nuvalue; i++)
  561     markvalue(g, &u->uv[i].uv);
  562   genlink(g, obj2gco(u));
  563   return 1 + u->nuvalue;
  564 }
  565 
  566 
  567 /*
  568 ** Traverse a prototype. (While a prototype is being build, its
  569 ** arrays can be larger than needed; the extra slots are filled with
  570 ** NULL, so the use of 'markobjectN')
  571 */
  572 static int traverseproto (global_State *g, Proto *f) {
  573   int i;
  574   markobjectN(g, f->source);
  575   for (i = 0; i < f->sizek; i++)  /* mark literals */
  576     markvalue(g, &f->k[i]);
  577   for (i = 0; i < f->sizeupvalues; i++)  /* mark upvalue names */
  578     markobjectN(g, f->upvalues[i].name);
  579   for (i = 0; i < f->sizep; i++)  /* mark nested protos */
  580     markobjectN(g, f->p[i]);
  581   for (i = 0; i < f->sizelocvars; i++)  /* mark local-variable names */
  582     markobjectN(g, f->locvars[i].varname);
  583   return 1 + f->sizek + f->sizeupvalues + f->sizep + f->sizelocvars;
  584 }
  585 
  586 
  587 static int traverseCclosure (global_State *g, CClosure *cl) {
  588   int i;
  589   for (i = 0; i < cl->nupvalues; i++)  /* mark its upvalues */
  590     markvalue(g, &cl->upvalue[i]);
  591   return 1 + cl->nupvalues;
  592 }
  593 
  594 /*
  595 ** Traverse a Lua closure, marking its prototype and its upvalues.
  596 ** (Both can be NULL while closure is being created.)
  597 */
  598 static int traverseLclosure (global_State *g, LClosure *cl) {
  599   int i;
  600   markobjectN(g, cl->p);  /* mark its prototype */
  601   for (i = 0; i < cl->nupvalues; i++) {  /* visit its upvalues */
  602     UpVal *uv = cl->upvals[i];
  603     markobjectN(g, uv);  /* mark upvalue */
  604   }
  605   return 1 + cl->nupvalues;
  606 }
  607 
  608 
  609 /*
  610 ** Traverse a thread, marking the elements in the stack up to its top
  611 ** and cleaning the rest of the stack in the final traversal. That
  612 ** ensures that the entire stack have valid (non-dead) objects.
  613 ** Threads have no barriers. In gen. mode, old threads must be visited
  614 ** at every cycle, because they might point to young objects.  In inc.
  615 ** mode, the thread can still be modified before the end of the cycle,
  616 ** and therefore it must be visited again in the atomic phase. To ensure
  617 ** these visits, threads must return to a gray list if they are not new
  618 ** (which can only happen in generational mode) or if the traverse is in
  619 ** the propagate phase (which can only happen in incremental mode).
  620 */
  621 static int traversethread (global_State *g, lua_State *th) {
  622   UpVal *uv;
  623   StkId o = th->stack;
  624   if (isold(th) || g->gcstate == GCSpropagate)
  625     linkgclist(th, g->grayagain);  /* insert into 'grayagain' list */
  626   if (o == NULL)
  627     return 1;  /* stack not completely built yet */
  628   lua_assert(g->gcstate == GCSatomic ||
  629              th->openupval == NULL || isintwups(th));
  630   for (; o < th->top; o++)  /* mark live elements in the stack */
  631     markvalue(g, s2v(o));
  632   for (uv = th->openupval; uv != NULL; uv = uv->u.open.next)
  633     markobject(g, uv);  /* open upvalues cannot be collected */
  634   if (g->gcstate == GCSatomic) {  /* final traversal? */
  635     for (; o < th->stack_last + EXTRA_STACK; o++)
  636       setnilvalue(s2v(o));  /* clear dead stack slice */
  637     /* 'remarkupvals' may have removed thread from 'twups' list */
  638     if (!isintwups(th) && th->openupval != NULL) {
  639       th->twups = g->twups;  /* link it back to the list */
  640       g->twups = th;
  641     }
  642   }
  643   else if (!g->gcemergency)
  644     luaD_shrinkstack(th); /* do not change stack in emergency cycle */
  645   return 1 + stacksize(th);
  646 }
  647 
  648 
  649 /*
  650 ** traverse one gray object, turning it to black.
  651 */
  652 static lu_mem propagatemark (global_State *g) {
  653   GCObject *o = g->gray;
  654   nw2black(o);
  655   g->gray = *getgclist(o);  /* remove from 'gray' list */
  656   switch (o->tt) {
  657     case LUA_VTABLE: return traversetable(g, gco2t(o));
  658     case LUA_VUSERDATA: return traverseudata(g, gco2u(o));
  659     case LUA_VLCL: return traverseLclosure(g, gco2lcl(o));
  660     case LUA_VCCL: return traverseCclosure(g, gco2ccl(o));
  661     case LUA_VPROTO: return traverseproto(g, gco2p(o));
  662     case LUA_VTHREAD: return traversethread(g, gco2th(o));
  663     default: lua_assert(0); return 0;
  664   }
  665 }
  666 
  667 
  668 static lu_mem propagateall (global_State *g) {
  669   lu_mem tot = 0;
  670   while (g->gray)
  671     tot += propagatemark(g);
  672   return tot;
  673 }
  674 
  675 
  676 /*
  677 ** Traverse all ephemeron tables propagating marks from keys to values.
  678 ** Repeat until it converges, that is, nothing new is marked. 'dir'
  679 ** inverts the direction of the traversals, trying to speed up
  680 ** convergence on chains in the same table.
  681 **
  682 */
  683 static void convergeephemerons (global_State *g) {
  684   int changed;
  685   int dir = 0;
  686   do {
  687     GCObject *w;
  688     GCObject *next = g->ephemeron;  /* get ephemeron list */
  689     g->ephemeron = NULL;  /* tables may return to this list when traversed */
  690     changed = 0;
  691     while ((w = next) != NULL) {  /* for each ephemeron table */
  692       Table *h = gco2t(w);
  693       next = h->gclist;  /* list is rebuilt during loop */
  694       nw2black(h);  /* out of the list (for now) */
  695       if (traverseephemeron(g, h, dir)) {  /* marked some value? */
  696         propagateall(g);  /* propagate changes */
  697         changed = 1;  /* will have to revisit all ephemeron tables */
  698       }
  699     }
  700     dir = !dir;  /* invert direction next time */
  701   } while (changed);  /* repeat until no more changes */
  702 }
  703 
  704 /* }====================================================== */
  705 
  706 
  707 /*
  708 ** {======================================================
  709 ** Sweep Functions
  710 ** =======================================================
  711 */
  712 
  713 
  714 /*
  715 ** clear entries with unmarked keys from all weaktables in list 'l'
  716 */
  717 static void clearbykeys (global_State *g, GCObject *l) {
  718   for (; l; l = gco2t(l)->gclist) {
  719     Table *h = gco2t(l);
  720     Node *limit = gnodelast(h);
  721     Node *n;
  722     for (n = gnode(h, 0); n < limit; n++) {
  723       if (iscleared(g, gckeyN(n)))  /* unmarked key? */
  724         setempty(gval(n));  /* remove entry */
  725       if (isempty(gval(n)))  /* is entry empty? */
  726         clearkey(n);  /* clear its key */
  727     }
  728   }
  729 }
  730 
  731 
  732 /*
  733 ** clear entries with unmarked values from all weaktables in list 'l' up
  734 ** to element 'f'
  735 */
  736 static void clearbyvalues (global_State *g, GCObject *l, GCObject *f) {
  737   for (; l != f; l = gco2t(l)->gclist) {
  738     Table *h = gco2t(l);
  739     Node *n, *limit = gnodelast(h);
  740     unsigned int i;
  741     unsigned int asize = luaH_realasize(h);
  742     for (i = 0; i < asize; i++) {
  743       TValue *o = &h->array[i];
  744       if (iscleared(g, gcvalueN(o)))  /* value was collected? */
  745         setempty(o);  /* remove entry */
  746     }
  747     for (n = gnode(h, 0); n < limit; n++) {
  748       if (iscleared(g, gcvalueN(gval(n))))  /* unmarked value? */
  749         setempty(gval(n));  /* remove entry */
  750       if (isempty(gval(n)))  /* is entry empty? */
  751         clearkey(n);  /* clear its key */
  752     }
  753   }
  754 }
  755 
  756 
  757 static void freeupval (lua_State *L, UpVal *uv) {
  758   if (upisopen(uv))
  759     luaF_unlinkupval(uv);
  760   luaM_free(L, uv);
  761 }
  762 
  763 
  764 static void freeobj (lua_State *L, GCObject *o) {
  765   switch (o->tt) {
  766     case LUA_VPROTO:
  767       luaF_freeproto(L, gco2p(o));
  768       break;
  769     case LUA_VUPVAL:
  770       freeupval(L, gco2upv(o));
  771       break;
  772     case LUA_VLCL: {
  773       LClosure *cl = gco2lcl(o);
  774       luaM_freemem(L, cl, sizeLclosure(cl->nupvalues));
  775       break;
  776     }
  777     case LUA_VCCL: {
  778       CClosure *cl = gco2ccl(o);
  779       luaM_freemem(L, cl, sizeCclosure(cl->nupvalues));
  780       break;
  781     }
  782     case LUA_VTABLE:
  783       luaH_free(L, gco2t(o));
  784       break;
  785     case LUA_VTHREAD:
  786       luaE_freethread(L, gco2th(o));
  787       break;
  788     case LUA_VUSERDATA: {
  789       Udata *u = gco2u(o);
  790       luaM_freemem(L, o, sizeudata(u->nuvalue, u->len));
  791       break;
  792     }
  793     case LUA_VSHRSTR: {
  794       TString *ts = gco2ts(o);
  795       luaS_remove(L, ts);  /* remove it from hash table */
  796       luaM_freemem(L, ts, sizelstring(ts->shrlen));
  797       break;
  798     }
  799     case LUA_VLNGSTR: {
  800       TString *ts = gco2ts(o);
  801       luaM_freemem(L, ts, sizelstring(ts->u.lnglen));
  802       break;
  803     }
  804     default: lua_assert(0);
  805   }
  806 }
  807 
  808 
  809 /*
  810 ** sweep at most 'countin' elements from a list of GCObjects erasing dead
  811 ** objects, where a dead object is one marked with the old (non current)
  812 ** white; change all non-dead objects back to white, preparing for next
  813 ** collection cycle. Return where to continue the traversal or NULL if
  814 ** list is finished. ('*countout' gets the number of elements traversed.)
  815 */
  816 static GCObject **sweeplist (lua_State *L, GCObject **p, int countin,
  817                              int *countout) {
  818   global_State *g = G(L);
  819   int ow = otherwhite(g);
  820   int i;
  821   int white = luaC_white(g);  /* current white */
  822   for (i = 0; *p != NULL && i < countin; i++) {
  823     GCObject *curr = *p;
  824     int marked = curr->marked;
  825     if (isdeadm(ow, marked)) {  /* is 'curr' dead? */
  826       *p = curr->next;  /* remove 'curr' from list */
  827       freeobj(L, curr);  /* erase 'curr' */
  828     }
  829     else {  /* change mark to 'white' */
  830       curr->marked = cast_byte((marked & ~maskgcbits) | white);
  831       p = &curr->next;  /* go to next element */
  832     }
  833   }
  834   if (countout)
  835     *countout = i;  /* number of elements traversed */
  836   return (*p == NULL) ? NULL : p;
  837 }
  838 
  839 
  840 /*
  841 ** sweep a list until a live object (or end of list)
  842 */
  843 static GCObject **sweeptolive (lua_State *L, GCObject **p) {
  844   GCObject **old = p;
  845   do {
  846     p = sweeplist(L, p, 1, NULL);
  847   } while (p == old);
  848   return p;
  849 }
  850 
  851 /* }====================================================== */
  852 
  853 
  854 /*
  855 ** {======================================================
  856 ** Finalization
  857 ** =======================================================
  858 */
  859 
  860 /*
  861 ** If possible, shrink string table.
  862 */
  863 static void checkSizes (lua_State *L, global_State *g) {
  864   if (!g->gcemergency) {
  865     if (g->strt.nuse < g->strt.size / 4) {  /* string table too big? */
  866       l_mem olddebt = g->GCdebt;
  867       luaS_resize(L, g->strt.size / 2);
  868       g->GCestimate += g->GCdebt - olddebt;  /* correct estimate */
  869     }
  870   }
  871 }
  872 
  873 
  874 /*
  875 ** Get the next udata to be finalized from the 'tobefnz' list, and
  876 ** link it back into the 'allgc' list.
  877 */
  878 static GCObject *udata2finalize (global_State *g) {
  879   GCObject *o = g->tobefnz;  /* get first element */
  880   lua_assert(tofinalize(o));
  881   g->tobefnz = o->next;  /* remove it from 'tobefnz' list */
  882   o->next = g->allgc;  /* return it to 'allgc' list */
  883   g->allgc = o;
  884   resetbit(o->marked, FINALIZEDBIT);  /* object is "normal" again */
  885   if (issweepphase(g))
  886     makewhite(g, o);  /* "sweep" object */
  887   else if (getage(o) == G_OLD1)
  888     g->firstold1 = o;  /* it is the first OLD1 object in the list */
  889   return o;
  890 }
  891 
  892 
  893 static void dothecall (lua_State *L, void *ud) {
  894   UNUSED(ud);
  895   luaD_callnoyield(L, L->top - 2, 0);
  896 }
  897 
  898 
  899 static void GCTM (lua_State *L) {
  900   global_State *g = G(L);
  901   const TValue *tm;
  902   TValue v;
  903   lua_assert(!g->gcemergency);
  904   setgcovalue(L, &v, udata2finalize(g));
  905   tm = luaT_gettmbyobj(L, &v, TM_GC);
  906   if (!notm(tm)) {  /* is there a finalizer? */
  907     int status;
  908     lu_byte oldah = L->allowhook;
  909     int running  = g->gcrunning;
  910     L->allowhook = 0;  /* stop debug hooks during GC metamethod */
  911     g->gcrunning = 0;  /* avoid GC steps */
  912     setobj2s(L, L->top++, tm);  /* push finalizer... */
  913     setobj2s(L, L->top++, &v);  /* ... and its argument */
  914     L->ci->callstatus |= CIST_FIN;  /* will run a finalizer */
  915     status = luaD_pcall(L, dothecall, NULL, savestack(L, L->top - 2), 0);
  916     L->ci->callstatus &= ~CIST_FIN;  /* not running a finalizer anymore */
  917     L->allowhook = oldah;  /* restore hooks */
  918     g->gcrunning = running;  /* restore state */
  919     if (l_unlikely(status != LUA_OK)) {  /* error while running __gc? */
  920       luaE_warnerror(L, "__gc metamethod");
  921       L->top--;  /* pops error object */
  922     }
  923   }
  924 }
  925 
  926 
  927 /*
  928 ** Call a few finalizers
  929 */
  930 static int runafewfinalizers (lua_State *L, int n) {
  931   global_State *g = G(L);
  932   int i;
  933   for (i = 0; i < n && g->tobefnz; i++)
  934     GCTM(L);  /* call one finalizer */
  935   return i;
  936 }
  937 
  938 
  939 /*
  940 ** call all pending finalizers
  941 */
  942 static void callallpendingfinalizers (lua_State *L) {
  943   global_State *g = G(L);
  944   while (g->tobefnz)
  945     GCTM(L);
  946 }
  947 
  948 
  949 /*
  950 ** find last 'next' field in list 'p' list (to add elements in its end)
  951 */
  952 static GCObject **findlast (GCObject **p) {
  953   while (*p != NULL)
  954     p = &(*p)->next;
  955   return p;
  956 }
  957 
  958 
  959 /*
  960 ** Move all unreachable objects (or 'all' objects) that need
  961 ** finalization from list 'finobj' to list 'tobefnz' (to be finalized).
  962 ** (Note that objects after 'finobjold1' cannot be white, so they
  963 ** don't need to be traversed. In incremental mode, 'finobjold1' is NULL,
  964 ** so the whole list is traversed.)
  965 */
  966 static void separatetobefnz (global_State *g, int all) {
  967   GCObject *curr;
  968   GCObject **p = &g->finobj;
  969   GCObject **lastnext = findlast(&g->tobefnz);
  970   while ((curr = *p) != g->finobjold1) {  /* traverse all finalizable objects */
  971     lua_assert(tofinalize(curr));
  972     if (!(iswhite(curr) || all))  /* not being collected? */
  973       p = &curr->next;  /* don't bother with it */
  974     else {
  975       if (curr == g->finobjsur)  /* removing 'finobjsur'? */
  976         g->finobjsur = curr->next;  /* correct it */
  977       *p = curr->next;  /* remove 'curr' from 'finobj' list */
  978       curr->next = *lastnext;  /* link at the end of 'tobefnz' list */
  979       *lastnext = curr;
  980       lastnext = &curr->next;
  981     }
  982   }
  983 }
  984 
  985 
  986 /*
  987 ** If pointer 'p' points to 'o', move it to the next element.
  988 */
  989 static void checkpointer (GCObject **p, GCObject *o) {
  990   if (o == *p)
  991     *p = o->next;
  992 }
  993 
  994 
  995 /*
  996 ** Correct pointers to objects inside 'allgc' list when
  997 ** object 'o' is being removed from the list.
  998 */
  999 static void correctpointers (global_State *g, GCObject *o) {
 1000   checkpointer(&g->survival, o);
 1001   checkpointer(&g->old1, o);
 1002   checkpointer(&g->reallyold, o);
 1003   checkpointer(&g->firstold1, o);
 1004 }
 1005 
 1006 
 1007 /*
 1008 ** if object 'o' has a finalizer, remove it from 'allgc' list (must
 1009 ** search the list to find it) and link it in 'finobj' list.
 1010 */
 1011 void luaC_checkfinalizer (lua_State *L, GCObject *o, Table *mt) {
 1012   global_State *g = G(L);
 1013   if (tofinalize(o) ||                 /* obj. is already marked... */
 1014       gfasttm(g, mt, TM_GC) == NULL)   /* or has no finalizer? */
 1015     return;  /* nothing to be done */
 1016   else {  /* move 'o' to 'finobj' list */
 1017     GCObject **p;
 1018     if (issweepphase(g)) {
 1019       makewhite(g, o);  /* "sweep" object 'o' */
 1020       if (g->sweepgc == &o->next)  /* should not remove 'sweepgc' object */
 1021         g->sweepgc = sweeptolive(L, g->sweepgc);  /* change 'sweepgc' */
 1022     }
 1023     else
 1024       correctpointers(g, o);
 1025     /* search for pointer pointing to 'o' */
 1026     for (p = &g->allgc; *p != o; p = &(*p)->next) { /* empty */ }
 1027     *p = o->next;  /* remove 'o' from 'allgc' list */
 1028     o->next = g->finobj;  /* link it in 'finobj' list */
 1029     g->finobj = o;
 1030     l_setbit(o->marked, FINALIZEDBIT);  /* mark it as such */
 1031   }
 1032 }
 1033 
 1034 /* }====================================================== */
 1035 
 1036 
 1037 /*
 1038 ** {======================================================
 1039 ** Generational Collector
 1040 ** =======================================================
 1041 */
 1042 
 1043 static void setpause (global_State *g);
 1044 
 1045 
 1046 /*
 1047 ** Sweep a list of objects to enter generational mode.  Deletes dead
 1048 ** objects and turns the non dead to old. All non-dead threads---which
 1049 ** are now old---must be in a gray list. Everything else is not in a
 1050 ** gray list. Open upvalues are also kept gray.
 1051 */
 1052 static void sweep2old (lua_State *L, GCObject **p) {
 1053   GCObject *curr;
 1054   global_State *g = G(L);
 1055   while ((curr = *p) != NULL) {
 1056     if (iswhite(curr)) {  /* is 'curr' dead? */
 1057       lua_assert(isdead(g, curr));
 1058       *p = curr->next;  /* remove 'curr' from list */
 1059       freeobj(L, curr);  /* erase 'curr' */
 1060     }
 1061     else {  /* all surviving objects become old */
 1062       setage(curr, G_OLD);
 1063       if (curr->tt == LUA_VTHREAD) {  /* threads must be watched */
 1064         lua_State *th = gco2th(curr);
 1065         linkgclist(th, g->grayagain);  /* insert into 'grayagain' list */
 1066       }
 1067       else if (curr->tt == LUA_VUPVAL && upisopen(gco2upv(curr)))
 1068         set2gray(curr);  /* open upvalues are always gray */
 1069       else  /* everything else is black */
 1070         nw2black(curr);
 1071       p = &curr->next;  /* go to next element */
 1072     }
 1073   }
 1074 }
 1075 
 1076 
 1077 /*
 1078 ** Sweep for generational mode. Delete dead objects. (Because the
 1079 ** collection is not incremental, there are no "new white" objects
 1080 ** during the sweep. So, any white object must be dead.) For
 1081 ** non-dead objects, advance their ages and clear the color of
 1082 ** new objects. (Old objects keep their colors.)
 1083 ** The ages of G_TOUCHED1 and G_TOUCHED2 objects cannot be advanced
 1084 ** here, because these old-generation objects are usually not swept
 1085 ** here.  They will all be advanced in 'correctgraylist'. That function
 1086 ** will also remove objects turned white here from any gray list.
 1087 */
 1088 static GCObject **sweepgen (lua_State *L, global_State *g, GCObject **p,
 1089                             GCObject *limit, GCObject **pfirstold1) {
 1090   static const lu_byte nextage[] = {
 1091     G_SURVIVAL,  /* from G_NEW */
 1092     G_OLD1,      /* from G_SURVIVAL */
 1093     G_OLD1,      /* from G_OLD0 */
 1094     G_OLD,       /* from G_OLD1 */
 1095     G_OLD,       /* from G_OLD (do not change) */
 1096     G_TOUCHED1,  /* from G_TOUCHED1 (do not change) */
 1097     G_TOUCHED2   /* from G_TOUCHED2 (do not change) */
 1098   };
 1099   int white = luaC_white(g);
 1100   GCObject *curr;
 1101   while ((curr = *p) != limit) {
 1102     if (iswhite(curr)) {  /* is 'curr' dead? */
 1103       lua_assert(!isold(curr) && isdead(g, curr));
 1104       *p = curr->next;  /* remove 'curr' from list */
 1105       freeobj(L, curr);  /* erase 'curr' */
 1106     }
 1107     else {  /* correct mark and age */
 1108       if (getage(curr) == G_NEW) {  /* new objects go back to white */
 1109         int marked = curr->marked & ~maskgcbits;  /* erase GC bits */
 1110         curr->marked = cast_byte(marked | G_SURVIVAL | white);
 1111       }
 1112       else {  /* all other objects will be old, and so keep their color */
 1113         setage(curr, nextage[getage(curr)]);
 1114         if (getage(curr) == G_OLD1 && *pfirstold1 == NULL)
 1115           *pfirstold1 = curr;  /* first OLD1 object in the list */
 1116       }
 1117       p = &curr->next;  /* go to next element */
 1118     }
 1119   }
 1120   return p;
 1121 }
 1122 
 1123 
 1124 /*
 1125 ** Traverse a list making all its elements white and clearing their
 1126 ** age. In incremental mode, all objects are 'new' all the time,
 1127 ** except for fixed strings (which are always old).
 1128 */
 1129 static void whitelist (global_State *g, GCObject *p) {
 1130   int white = luaC_white(g);
 1131   for (; p != NULL; p = p->next)
 1132     p->marked = cast_byte((p->marked & ~maskgcbits) | white);
 1133 }
 1134 
 1135 
 1136 /*
 1137 ** Correct a list of gray objects. Return pointer to where rest of the
 1138 ** list should be linked.
 1139 ** Because this correction is done after sweeping, young objects might
 1140 ** be turned white and still be in the list. They are only removed.
 1141 ** 'TOUCHED1' objects are advanced to 'TOUCHED2' and remain on the list;
 1142 ** Non-white threads also remain on the list; 'TOUCHED2' objects become
 1143 ** regular old; they and anything else are removed from the list.
 1144 */
 1145 static GCObject **correctgraylist (GCObject **p) {
 1146   GCObject *curr;
 1147   while ((curr = *p) != NULL) {
 1148     GCObject **next = getgclist(curr);
 1149     if (iswhite(curr))
 1150       goto remove;  /* remove all white objects */
 1151     else if (getage(curr) == G_TOUCHED1) {  /* touched in this cycle? */
 1152       lua_assert(isgray(curr));
 1153       nw2black(curr);  /* make it black, for next barrier */
 1154       changeage(curr, G_TOUCHED1, G_TOUCHED2);
 1155       goto remain;  /* keep it in the list and go to next element */
 1156     }
 1157     else if (curr->tt == LUA_VTHREAD) {
 1158       lua_assert(isgray(curr));
 1159       goto remain;  /* keep non-white threads on the list */
 1160     }
 1161     else {  /* everything else is removed */
 1162       lua_assert(isold(curr));  /* young objects should be white here */
 1163       if (getage(curr) == G_TOUCHED2)  /* advance from TOUCHED2... */
 1164         changeage(curr, G_TOUCHED2, G_OLD);  /* ... to OLD */
 1165       nw2black(curr);  /* make object black (to be removed) */
 1166       goto remove;
 1167     }
 1168     remove: *p = *next; continue;
 1169     remain: p = next; continue;
 1170   }
 1171   return p;
 1172 }
 1173 
 1174 
 1175 /*
 1176 ** Correct all gray lists, coalescing them into 'grayagain'.
 1177 */
 1178 static void correctgraylists (global_State *g) {
 1179   GCObject **list = correctgraylist(&g->grayagain);
 1180   *list = g->weak; g->weak = NULL;
 1181   list = correctgraylist(list);
 1182   *list = g->allweak; g->allweak = NULL;
 1183   list = correctgraylist(list);
 1184   *list = g->ephemeron; g->ephemeron = NULL;
 1185   correctgraylist(list);
 1186 }
 1187 
 1188 
 1189 /*
 1190 ** Mark black 'OLD1' objects when starting a new young collection.
 1191 ** Gray objects are already in some gray list, and so will be visited
 1192 ** in the atomic step.
 1193 */
 1194 static void markold (global_State *g, GCObject *from, GCObject *to) {
 1195   GCObject *p;
 1196   for (p = from; p != to; p = p->next) {
 1197     if (getage(p) == G_OLD1) {
 1198       lua_assert(!iswhite(p));
 1199       changeage(p, G_OLD1, G_OLD);  /* now they are old */
 1200       if (isblack(p))
 1201         reallymarkobject(g, p);
 1202     }
 1203   }
 1204 }
 1205 
 1206 
 1207 /*
 1208 ** Finish a young-generation collection.
 1209 */
 1210 static void finishgencycle (lua_State *L, global_State *g) {
 1211   correctgraylists(g);
 1212   checkSizes(L, g);
 1213   g->gcstate = GCSpropagate;  /* skip restart */
 1214   if (!g->gcemergency)
 1215     callallpendingfinalizers(L);
 1216 }
 1217 
 1218 
 1219 /*
 1220 ** Does a young collection. First, mark 'OLD1' objects. Then does the
 1221 ** atomic step. Then, sweep all lists and advance pointers. Finally,
 1222 ** finish the collection.
 1223 */
 1224 static void youngcollection (lua_State *L, global_State *g) {
 1225   GCObject **psurvival;  /* to point to first non-dead survival object */
 1226   GCObject *dummy;  /* dummy out parameter to 'sweepgen' */
 1227   lua_assert(g->gcstate == GCSpropagate);
 1228   if (g->firstold1) {  /* are there regular OLD1 objects? */
 1229     markold(g, g->firstold1, g->reallyold);  /* mark them */
 1230     g->firstold1 = NULL;  /* no more OLD1 objects (for now) */
 1231   }
 1232   markold(g, g->finobj, g->finobjrold);
 1233   markold(g, g->tobefnz, NULL);
 1234   atomic(L);
 1235 
 1236   /* sweep nursery and get a pointer to its last live element */
 1237   g->gcstate = GCSswpallgc;
 1238   psurvival = sweepgen(L, g, &g->allgc, g->survival, &g->firstold1);
 1239   /* sweep 'survival' */
 1240   sweepgen(L, g, psurvival, g->old1, &g->firstold1);
 1241   g->reallyold = g->old1;
 1242   g->old1 = *psurvival;  /* 'survival' survivals are old now */
 1243   g->survival = g->allgc;  /* all news are survivals */
 1244 
 1245   /* repeat for 'finobj' lists */
 1246   dummy = NULL;  /* no 'firstold1' optimization for 'finobj' lists */
 1247   psurvival = sweepgen(L, g, &g->finobj, g->finobjsur, &dummy);
 1248   /* sweep 'survival' */
 1249   sweepgen(L, g, psurvival, g->finobjold1, &dummy);
 1250   g->finobjrold = g->finobjold1;
 1251   g->finobjold1 = *psurvival;  /* 'survival' survivals are old now */
 1252   g->finobjsur = g->finobj;  /* all news are survivals */
 1253 
 1254   sweepgen(L, g, &g->tobefnz, NULL, &dummy);
 1255   finishgencycle(L, g);
 1256 }
 1257 
 1258 
 1259 /*
 1260 ** Clears all gray lists, sweeps objects, and prepare sublists to enter
 1261 ** generational mode. The sweeps remove dead objects and turn all
 1262 ** surviving objects to old. Threads go back to 'grayagain'; everything
 1263 ** else is turned black (not in any gray list).
 1264 */
 1265 static void atomic2gen (lua_State *L, global_State *g) {
 1266   cleargraylists(g);
 1267   /* sweep all elements making them old */
 1268   g->gcstate = GCSswpallgc;
 1269   sweep2old(L, &g->allgc);
 1270   /* everything alive now is old */
 1271   g->reallyold = g->old1 = g->survival = g->allgc;
 1272   g->firstold1 = NULL;  /* there are no OLD1 objects anywhere */
 1273 
 1274   /* repeat for 'finobj' lists */
 1275   sweep2old(L, &g->finobj);
 1276   g->finobjrold = g->finobjold1 = g->finobjsur = g->finobj;
 1277 
 1278   sweep2old(L, &g->tobefnz);
 1279 
 1280   g->gckind = KGC_GEN;
 1281   g->lastatomic = 0;
 1282   g->GCestimate = gettotalbytes(g);  /* base for memory control */
 1283   finishgencycle(L, g);
 1284 }
 1285 
 1286 
 1287 /*
 1288 ** Enter generational mode. Must go until the end of an atomic cycle
 1289 ** to ensure that all objects are correctly marked and weak tables
 1290 ** are cleared. Then, turn all objects into old and finishes the
 1291 ** collection.
 1292 */
 1293 static lu_mem entergen (lua_State *L, global_State *g) {
 1294   lu_mem numobjs;
 1295   luaC_runtilstate(L, bitmask(GCSpause));  /* prepare to start a new cycle */
 1296   luaC_runtilstate(L, bitmask(GCSpropagate));  /* start new cycle */
 1297   numobjs = atomic(L);  /* propagates all and then do the atomic stuff */
 1298   atomic2gen(L, g);
 1299   return numobjs;
 1300 }
 1301 
 1302 
 1303 /*
 1304 ** Enter incremental mode. Turn all objects white, make all
 1305 ** intermediate lists point to NULL (to avoid invalid pointers),
 1306 ** and go to the pause state.
 1307 */
 1308 static void enterinc (global_State *g) {
 1309   whitelist(g, g->allgc);
 1310   g->reallyold = g->old1 = g->survival = NULL;
 1311   whitelist(g, g->finobj);
 1312   whitelist(g, g->tobefnz);
 1313   g->finobjrold = g->finobjold1 = g->finobjsur = NULL;
 1314   g->gcstate = GCSpause;
 1315   g->gckind = KGC_INC;
 1316   g->lastatomic = 0;
 1317 }
 1318 
 1319 
 1320 /*
 1321 ** Change collector mode to 'newmode'.
 1322 */
 1323 void luaC_changemode (lua_State *L, int newmode) {
 1324   global_State *g = G(L);
 1325   if (newmode != g->gckind) {
 1326     if (newmode == KGC_GEN)  /* entering generational mode? */
 1327       entergen(L, g);
 1328     else
 1329       enterinc(g);  /* entering incremental mode */
 1330   }
 1331   g->lastatomic = 0;
 1332 }
 1333 
 1334 
 1335 /*
 1336 ** Does a full collection in generational mode.
 1337 */
 1338 static lu_mem fullgen (lua_State *L, global_State *g) {
 1339   enterinc(g);
 1340   return entergen(L, g);
 1341 }
 1342 
 1343 
 1344 /*
 1345 ** Set debt for the next minor collection, which will happen when
 1346 ** memory grows 'genminormul'%.
 1347 */
 1348 static void setminordebt (global_State *g) {
 1349   luaE_setdebt(g, -(cast(l_mem, (gettotalbytes(g) / 100)) * g->genminormul));
 1350 }
 1351 
 1352 
 1353 /*
 1354 ** Does a major collection after last collection was a "bad collection".
 1355 **
 1356 ** When the program is building a big structure, it allocates lots of
 1357 ** memory but generates very little garbage. In those scenarios,
 1358 ** the generational mode just wastes time doing small collections, and
 1359 ** major collections are frequently what we call a "bad collection", a
 1360 ** collection that frees too few objects. To avoid the cost of switching
 1361 ** between generational mode and the incremental mode needed for full
 1362 ** (major) collections, the collector tries to stay in incremental mode
 1363 ** after a bad collection, and to switch back to generational mode only
 1364 ** after a "good" collection (one that traverses less than 9/8 objects
 1365 ** of the previous one).
 1366 ** The collector must choose whether to stay in incremental mode or to
 1367 ** switch back to generational mode before sweeping. At this point, it
 1368 ** does not know the real memory in use, so it cannot use memory to
 1369 ** decide whether to return to generational mode. Instead, it uses the
 1370 ** number of objects traversed (returned by 'atomic') as a proxy. The
 1371 ** field 'g->lastatomic' keeps this count from the last collection.
 1372 ** ('g->lastatomic != 0' also means that the last collection was bad.)
 1373 */
 1374 static void stepgenfull (lua_State *L, global_State *g) {
 1375   lu_mem newatomic;  /* count of traversed objects */
 1376   lu_mem lastatomic = g->lastatomic;  /* count from last collection */
 1377   if (g->gckind == KGC_GEN)  /* still in generational mode? */
 1378     enterinc(g);  /* enter incremental mode */
 1379   luaC_runtilstate(L, bitmask(GCSpropagate));  /* start new cycle */
 1380   newatomic = atomic(L);  /* mark everybody */
 1381   if (newatomic < lastatomic + (lastatomic >> 3)) {  /* good collection? */
 1382     atomic2gen(L, g);  /* return to generational mode */
 1383     setminordebt(g);
 1384   }
 1385   else {  /* another bad collection; stay in incremental mode */
 1386     g->GCestimate = gettotalbytes(g);  /* first estimate */;
 1387     entersweep(L);
 1388     luaC_runtilstate(L, bitmask(GCSpause));  /* finish collection */
 1389     setpause(g);
 1390     g->lastatomic = newatomic;
 1391   }
 1392 }
 1393 
 1394 
 1395 /*
 1396 ** Does a generational "step".
 1397 ** Usually, this means doing a minor collection and setting the debt to
 1398 ** make another collection when memory grows 'genminormul'% larger.
 1399 **
 1400 ** However, there are exceptions.  If memory grows 'genmajormul'%
 1401 ** larger than it was at the end of the last major collection (kept
 1402 ** in 'g->GCestimate'), the function does a major collection. At the
 1403 ** end, it checks whether the major collection was able to free a
 1404 ** decent amount of memory (at least half the growth in memory since
 1405 ** previous major collection). If so, the collector keeps its state,
 1406 ** and the next collection will probably be minor again. Otherwise,
 1407 ** we have what we call a "bad collection". In that case, set the field
 1408 ** 'g->lastatomic' to signal that fact, so that the next collection will
 1409 ** go to 'stepgenfull'.
 1410 **
 1411 ** 'GCdebt <= 0' means an explicit call to GC step with "size" zero;
 1412 ** in that case, do a minor collection.
 1413 */
 1414 static void genstep (lua_State *L, global_State *g) {
 1415   if (g->lastatomic != 0)  /* last collection was a bad one? */
 1416     stepgenfull(L, g);  /* do a full step */
 1417   else {
 1418     lu_mem majorbase = g->GCestimate;  /* memory after last major collection */
 1419     lu_mem majorinc = (majorbase / 100) * getgcparam(g->genmajormul);
 1420     if (g->GCdebt > 0 && gettotalbytes(g) > majorbase + majorinc) {
 1421       lu_mem numobjs = fullgen(L, g);  /* do a major collection */
 1422       if (gettotalbytes(g) < majorbase + (majorinc / 2)) {
 1423         /* collected at least half of memory growth since last major
 1424            collection; keep doing minor collections */
 1425         setminordebt(g);
 1426       }
 1427       else {  /* bad collection */
 1428         g->lastatomic = numobjs;  /* signal that last collection was bad */
 1429         setpause(g);  /* do a long wait for next (major) collection */
 1430       }
 1431     }
 1432     else {  /* regular case; do a minor collection */
 1433       youngcollection(L, g);
 1434       setminordebt(g);
 1435       g->GCestimate = majorbase;  /* preserve base value */
 1436     }
 1437   }
 1438   lua_assert(isdecGCmodegen(g));
 1439 }
 1440 
 1441 /* }====================================================== */
 1442 
 1443 
 1444 /*
 1445 ** {======================================================
 1446 ** GC control
 1447 ** =======================================================
 1448 */
 1449 
 1450 
 1451 /*
 1452 ** Set the "time" to wait before starting a new GC cycle; cycle will
 1453 ** start when memory use hits the threshold of ('estimate' * pause /
 1454 ** PAUSEADJ). (Division by 'estimate' should be OK: it cannot be zero,
 1455 ** because Lua cannot even start with less than PAUSEADJ bytes).
 1456 */
 1457 static void setpause (global_State *g) {
 1458   l_mem threshold, debt;
 1459   int pause = getgcparam(g->gcpause);
 1460   l_mem estimate = g->GCestimate / PAUSEADJ;  /* adjust 'estimate' */
 1461   lua_assert(estimate > 0);
 1462   threshold = (pause < MAX_LMEM / estimate)  /* overflow? */
 1463             ? estimate * pause  /* no overflow */
 1464             : MAX_LMEM;  /* overflow; truncate to maximum */
 1465   debt = gettotalbytes(g) - threshold;
 1466   if (debt > 0) debt = 0;
 1467   luaE_setdebt(g, debt);
 1468 }
 1469 
 1470 
 1471 /*
 1472 ** Enter first sweep phase.
 1473 ** The call to 'sweeptolive' makes the pointer point to an object
 1474 ** inside the list (instead of to the header), so that the real sweep do
 1475 ** not need to skip objects created between "now" and the start of the
 1476 ** real sweep.
 1477 */
 1478 static void entersweep (lua_State *L) {
 1479   global_State *g = G(L);
 1480   g->gcstate = GCSswpallgc;
 1481   lua_assert(g->sweepgc == NULL);
 1482   g->sweepgc = sweeptolive(L, &g->allgc);
 1483 }
 1484 
 1485 
 1486 /*
 1487 ** Delete all objects in list 'p' until (but not including) object
 1488 ** 'limit'.
 1489 */
 1490 static void deletelist (lua_State *L, GCObject *p, GCObject *limit) {
 1491   while (p != limit) {
 1492     GCObject *next = p->next;
 1493     freeobj(L, p);
 1494     p = next;
 1495   }
 1496 }
 1497 
 1498 
 1499 /*
 1500 ** Call all finalizers of the objects in the given Lua state, and
 1501 ** then free all objects, except for the main thread.
 1502 */
 1503 void luaC_freeallobjects (lua_State *L) {
 1504   global_State *g = G(L);
 1505   luaC_changemode(L, KGC_INC);
 1506   separatetobefnz(g, 1);  /* separate all objects with finalizers */
 1507   lua_assert(g->finobj == NULL);
 1508   callallpendingfinalizers(L);
 1509   deletelist(L, g->allgc, obj2gco(g->mainthread));
 1510   deletelist(L, g->finobj, NULL);
 1511   deletelist(L, g->fixedgc, NULL);  /* collect fixed objects */
 1512   lua_assert(g->strt.nuse == 0);
 1513 }
 1514 
 1515 
 1516 static lu_mem atomic (lua_State *L) {
 1517   global_State *g = G(L);
 1518   lu_mem work = 0;
 1519   GCObject *origweak, *origall;
 1520   GCObject *grayagain = g->grayagain;  /* save original list */
 1521   g->grayagain = NULL;
 1522   lua_assert(g->ephemeron == NULL && g->weak == NULL);
 1523   lua_assert(!iswhite(g->mainthread));
 1524   g->gcstate = GCSatomic;
 1525   markobject(g, L);  /* mark running thread */
 1526   /* registry and global metatables may be changed by API */
 1527   markvalue(g, &g->l_registry);
 1528   markmt(g);  /* mark global metatables */
 1529   work += propagateall(g);  /* empties 'gray' list */
 1530   /* remark occasional upvalues of (maybe) dead threads */
 1531   work += remarkupvals(g);
 1532   work += propagateall(g);  /* propagate changes */
 1533   g->gray = grayagain;
 1534   work += propagateall(g);  /* traverse 'grayagain' list */
 1535   convergeephemerons(g);
 1536   /* at this point, all strongly accessible objects are marked. */
 1537   /* Clear values from weak tables, before checking finalizers */
 1538   clearbyvalues(g, g->weak, NULL);
 1539   clearbyvalues(g, g->allweak, NULL);
 1540   origweak = g->weak; origall = g->allweak;
 1541   separatetobefnz(g, 0);  /* separate objects to be finalized */
 1542   work += markbeingfnz(g);  /* mark objects that will be finalized */
 1543   work += propagateall(g);  /* remark, to propagate 'resurrection' */
 1544   convergeephemerons(g);
 1545   /* at this point, all resurrected objects are marked. */
 1546   /* remove dead objects from weak tables */
 1547   clearbykeys(g, g->ephemeron);  /* clear keys from all ephemeron tables */
 1548   clearbykeys(g, g->allweak);  /* clear keys from all 'allweak' tables */
 1549   /* clear values from resurrected weak tables */
 1550   clearbyvalues(g, g->weak, origweak);
 1551   clearbyvalues(g, g->allweak, origall);
 1552   luaS_clearcache(g);
 1553   g->currentwhite = cast_byte(otherwhite(g));  /* flip current white */
 1554   lua_assert(g->gray == NULL);
 1555   return work;  /* estimate of slots marked by 'atomic' */
 1556 }
 1557 
 1558 
 1559 static int sweepstep (lua_State *L, global_State *g,
 1560                       int nextstate, GCObject **nextlist) {
 1561   if (g->sweepgc) {
 1562     l_mem olddebt = g->GCdebt;
 1563     int count;
 1564     g->sweepgc = sweeplist(L, g->sweepgc, GCSWEEPMAX, &count);
 1565     g->GCestimate += g->GCdebt - olddebt;  /* update estimate */
 1566     return count;
 1567   }
 1568   else {  /* enter next state */
 1569     g->gcstate = nextstate;
 1570     g->sweepgc = nextlist;
 1571     return 0;  /* no work done */
 1572   }
 1573 }
 1574 
 1575 
 1576 static lu_mem singlestep (lua_State *L) {
 1577   global_State *g = G(L);
 1578   lu_mem work;
 1579   lua_assert(!g->gcstopem);  /* collector is not reentrant */
 1580   g->gcstopem = 1;  /* no emergency collections while collecting */
 1581   switch (g->gcstate) {
 1582     case GCSpause: {
 1583       restartcollection(g);
 1584       g->gcstate = GCSpropagate;
 1585       work = 1;
 1586       break;
 1587     }
 1588     case GCSpropagate: {
 1589       if (g->gray == NULL) {  /* no more gray objects? */
 1590         g->gcstate = GCSenteratomic;  /* finish propagate phase */
 1591         work = 0;
 1592       }
 1593       else
 1594         work = propagatemark(g);  /* traverse one gray object */
 1595       break;
 1596     }
 1597     case GCSenteratomic: {
 1598       work = atomic(L);  /* work is what was traversed by 'atomic' */
 1599       entersweep(L);
 1600       g->GCestimate = gettotalbytes(g);  /* first estimate */;
 1601       break;
 1602     }
 1603     case GCSswpallgc: {  /* sweep "regular" objects */
 1604       work = sweepstep(L, g, GCSswpfinobj, &g->finobj);
 1605       break;
 1606     }
 1607     case GCSswpfinobj: {  /* sweep objects with finalizers */
 1608       work = sweepstep(L, g, GCSswptobefnz, &g->tobefnz);
 1609       break;
 1610     }
 1611     case GCSswptobefnz: {  /* sweep objects to be finalized */
 1612       work = sweepstep(L, g, GCSswpend, NULL);
 1613       break;
 1614     }
 1615     case GCSswpend: {  /* finish sweeps */
 1616       checkSizes(L, g);
 1617       g->gcstate = GCScallfin;
 1618       work = 0;
 1619       break;
 1620     }
 1621     case GCScallfin: {  /* call remaining finalizers */
 1622       if (g->tobefnz && !g->gcemergency) {
 1623         g->gcstopem = 0;  /* ok collections during finalizers */
 1624         work = runafewfinalizers(L, GCFINMAX) * GCFINALIZECOST;
 1625       }
 1626       else {  /* emergency mode or no more finalizers */
 1627         g->gcstate = GCSpause;  /* finish collection */
 1628         work = 0;
 1629       }
 1630       break;
 1631     }
 1632     default: lua_assert(0); return 0;
 1633   }
 1634   g->gcstopem = 0;
 1635   return work;
 1636 }
 1637 
 1638 
 1639 /*
 1640 ** advances the garbage collector until it reaches a state allowed
 1641 ** by 'statemask'
 1642 */
 1643 void luaC_runtilstate (lua_State *L, int statesmask) {
 1644   global_State *g = G(L);
 1645   while (!testbit(statesmask, g->gcstate))
 1646     singlestep(L);
 1647 }
 1648 
 1649 
 1650 /*
 1651 ** Performs a basic incremental step. The debt and step size are
 1652 ** converted from bytes to "units of work"; then the function loops
 1653 ** running single steps until adding that many units of work or
 1654 ** finishing a cycle (pause state). Finally, it sets the debt that
 1655 ** controls when next step will be performed.
 1656 */
 1657 static void incstep (lua_State *L, global_State *g) {
 1658   int stepmul = (getgcparam(g->gcstepmul) | 1);  /* avoid division by 0 */
 1659   l_mem debt = (g->GCdebt / WORK2MEM) * stepmul;
 1660   l_mem stepsize = (g->gcstepsize <= log2maxs(l_mem))
 1661                  ? ((cast(l_mem, 1) << g->gcstepsize) / WORK2MEM) * stepmul
 1662                  : MAX_LMEM;  /* overflow; keep maximum value */
 1663   do {  /* repeat until pause or enough "credit" (negative debt) */
 1664     lu_mem work = singlestep(L);  /* perform one single step */
 1665     debt -= work;
 1666   } while (debt > -stepsize && g->gcstate != GCSpause);
 1667   if (g->gcstate == GCSpause)
 1668     setpause(g);  /* pause until next cycle */
 1669   else {
 1670     debt = (debt / stepmul) * WORK2MEM;  /* convert 'work units' to bytes */
 1671     luaE_setdebt(g, debt);
 1672   }
 1673 }
 1674 
 1675 /*
 1676 ** performs a basic GC step if collector is running
 1677 */
 1678 void luaC_step (lua_State *L) {
 1679   global_State *g = G(L);
 1680   lua_assert(!g->gcemergency);
 1681   if (g->gcrunning) {  /* running? */
 1682     if(isdecGCmodegen(g))
 1683       genstep(L, g);
 1684     else
 1685       incstep(L, g);
 1686   }
 1687 }
 1688 
 1689 
 1690 /*
 1691 ** Perform a full collection in incremental mode.
 1692 ** Before running the collection, check 'keepinvariant'; if it is true,
 1693 ** there may be some objects marked as black, so the collector has
 1694 ** to sweep all objects to turn them back to white (as white has not
 1695 ** changed, nothing will be collected).
 1696 */
 1697 static void fullinc (lua_State *L, global_State *g) {
 1698   if (keepinvariant(g))  /* black objects? */
 1699     entersweep(L); /* sweep everything to turn them back to white */
 1700   /* finish any pending sweep phase to start a new cycle */
 1701   luaC_runtilstate(L, bitmask(GCSpause));
 1702   luaC_runtilstate(L, bitmask(GCScallfin));  /* run up to finalizers */
 1703   /* estimate must be correct after a full GC cycle */
 1704   lua_assert(g->GCestimate == gettotalbytes(g));
 1705   luaC_runtilstate(L, bitmask(GCSpause));  /* finish collection */
 1706   setpause(g);
 1707 }
 1708 
 1709 
 1710 /*
 1711 ** Performs a full GC cycle; if 'isemergency', set a flag to avoid
 1712 ** some operations which could change the interpreter state in some
 1713 ** unexpected ways (running finalizers and shrinking some structures).
 1714 */
 1715 void luaC_fullgc (lua_State *L, int isemergency) {
 1716   global_State *g = G(L);
 1717   lua_assert(!g->gcemergency);
 1718   g->gcemergency = isemergency;  /* set flag */
 1719   if (g->gckind == KGC_INC)
 1720     fullinc(L, g);
 1721   else
 1722     fullgen(L, g);
 1723   g->gcemergency = 0;
 1724 }
 1725 
 1726 /* }====================================================== */
 1727 
 1728