lua/lgc.c
changeset 0 d7ee4e2ed49d
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/lua/lgc.c	Tue Dec 17 22:16:27 2013 -0500
@@ -0,0 +1,1220 @@
+/*
+** $Id: lgc.c,v 2.140.1.2 2013/04/26 18:22:05 roberto Exp $
+** Garbage Collector
+** See Copyright Notice in lua.h
+*/
+
+#include <string.h>
+
+#define lgc_c
+#define LUA_CORE
+
+#include "lua.h"
+
+#include "ldebug.h"
+#include "ldo.h"
+#include "lfunc.h"
+#include "lgc.h"
+#include "lmem.h"
+#include "lobject.h"
+#include "lstate.h"
+#include "lstring.h"
+#include "ltable.h"
+#include "ltm.h"
+
+
+
+/*
+** cost of sweeping one element (the size of a small object divided
+** by some adjust for the sweep speed)
+*/
+#define GCSWEEPCOST	((sizeof(TString) + 4) / 4)
+
+/* maximum number of elements to sweep in each single step */
+#define GCSWEEPMAX	(cast_int((GCSTEPSIZE / GCSWEEPCOST) / 4))
+
+/* maximum number of finalizers to call in each GC step */
+#define GCFINALIZENUM	4
+
+
+/*
+** macro to adjust 'stepmul': 'stepmul' is actually used like
+** 'stepmul / STEPMULADJ' (value chosen by tests)
+*/
+#define STEPMULADJ		200
+
+
+/*
+** macro to adjust 'pause': 'pause' is actually used like
+** 'pause / PAUSEADJ' (value chosen by tests)
+*/
+#define PAUSEADJ		100
+
+
+/*
+** 'makewhite' erases all color bits plus the old bit and then
+** sets only the current white bit
+*/
+#define maskcolors	(~(bit2mask(BLACKBIT, OLDBIT) | WHITEBITS))
+#define makewhite(g,x)	\
+ (gch(x)->marked = cast_byte((gch(x)->marked & maskcolors) | luaC_white(g)))
+
+#define white2gray(x)	resetbits(gch(x)->marked, WHITEBITS)
+#define black2gray(x)	resetbit(gch(x)->marked, BLACKBIT)
+
+
+#define isfinalized(x)		testbit(gch(x)->marked, FINALIZEDBIT)
+
+#define checkdeadkey(n)	lua_assert(!ttisdeadkey(gkey(n)) || ttisnil(gval(n)))
+
+
+#define checkconsistency(obj)  \
+  lua_longassert(!iscollectable(obj) || righttt(obj))
+
+
+#define markvalue(g,o) { checkconsistency(o); \
+  if (valiswhite(o)) reallymarkobject(g,gcvalue(o)); }
+
+#define markobject(g,t) { if ((t) && iswhite(obj2gco(t))) \
+		reallymarkobject(g, obj2gco(t)); }
+
+static void reallymarkobject (global_State *g, GCObject *o);
+
+
+/*
+** {======================================================
+** Generic functions
+** =======================================================
+*/
+
+
+/*
+** one after last element in a hash array
+*/
+#define gnodelast(h)	gnode(h, cast(size_t, sizenode(h)))
+
+
+/*
+** link table 'h' into list pointed by 'p'
+*/
+#define linktable(h,p)	((h)->gclist = *(p), *(p) = obj2gco(h))
+
+
+/*
+** if key is not marked, mark its entry as dead (therefore removing it
+** from the table)
+*/
+static void removeentry (Node *n) {
+  lua_assert(ttisnil(gval(n)));
+  if (valiswhite(gkey(n)))
+    setdeadvalue(gkey(n));  /* unused and unmarked key; remove it */
+}
+
+
+/*
+** tells whether a key or value can be cleared from a weak
+** table. Non-collectable objects are never removed from weak
+** tables. Strings behave as `values', so are never removed too. for
+** other objects: if really collected, cannot keep them; for objects
+** being finalized, keep them in keys, but not in values
+*/
+static int iscleared (global_State *g, const TValue *o) {
+  if (!iscollectable(o)) return 0;
+  else if (ttisstring(o)) {
+    markobject(g, rawtsvalue(o));  /* strings are `values', so are never weak */
+    return 0;
+  }
+  else return iswhite(gcvalue(o));
+}
+
+
+/*
+** barrier that moves collector forward, that is, mark the white object
+** being pointed by a black object.
+*/
+void luaC_barrier_ (lua_State *L, GCObject *o, GCObject *v) {
+  global_State *g = G(L);
+  lua_assert(isblack(o) && iswhite(v) && !isdead(g, v) && !isdead(g, o));
+  lua_assert(g->gcstate != GCSpause);
+  lua_assert(gch(o)->tt != LUA_TTABLE);
+  if (keepinvariantout(g))  /* must keep invariant? */
+    reallymarkobject(g, v);  /* restore invariant */
+  else {  /* sweep phase */
+    lua_assert(issweepphase(g));
+    makewhite(g, o);  /* mark main obj. as white to avoid other barriers */
+  }
+}
+
+
+/*
+** barrier that moves collector backward, that is, mark the black object
+** pointing to a white object as gray again. (Current implementation
+** only works for tables; access to 'gclist' is not uniform across
+** different types.)
+*/
+void luaC_barrierback_ (lua_State *L, GCObject *o) {
+  global_State *g = G(L);
+  lua_assert(isblack(o) && !isdead(g, o) && gch(o)->tt == LUA_TTABLE);
+  black2gray(o);  /* make object gray (again) */
+  gco2t(o)->gclist = g->grayagain;
+  g->grayagain = o;
+}
+
+
+/*
+** barrier for prototypes. When creating first closure (cache is
+** NULL), use a forward barrier; this may be the only closure of the
+** prototype (if it is a "regular" function, with a single instance)
+** and the prototype may be big, so it is better to avoid traversing
+** it again. Otherwise, use a backward barrier, to avoid marking all
+** possible instances.
+*/
+LUAI_FUNC void luaC_barrierproto_ (lua_State *L, Proto *p, Closure *c) {
+  global_State *g = G(L);
+  lua_assert(isblack(obj2gco(p)));
+  if (p->cache == NULL) {  /* first time? */
+    luaC_objbarrier(L, p, c);
+  }
+  else {  /* use a backward barrier */
+    black2gray(obj2gco(p));  /* make prototype gray (again) */
+    p->gclist = g->grayagain;
+    g->grayagain = obj2gco(p);
+  }
+}
+
+
+/*
+** check color (and invariants) for an upvalue that was closed,
+** i.e., moved into the 'allgc' list
+*/
+void luaC_checkupvalcolor (global_State *g, UpVal *uv) {
+  GCObject *o = obj2gco(uv);
+  lua_assert(!isblack(o));  /* open upvalues are never black */
+  if (isgray(o)) {
+    if (keepinvariant(g)) {
+      resetoldbit(o);  /* see MOVE OLD rule */
+      gray2black(o);  /* it is being visited now */
+      markvalue(g, uv->v);
+    }
+    else {
+      lua_assert(issweepphase(g));
+      makewhite(g, o);
+    }
+  }
+}
+
+
+/*
+** create a new collectable object (with given type and size) and link
+** it to '*list'. 'offset' tells how many bytes to allocate before the
+** object itself (used only by states).
+*/
+GCObject *luaC_newobj (lua_State *L, int tt, size_t sz, GCObject **list,
+                       int offset) {
+  global_State *g = G(L);
+  char *raw = cast(char *, luaM_newobject(L, novariant(tt), sz));
+  GCObject *o = obj2gco(raw + offset);
+  if (list == NULL)
+    list = &g->allgc;  /* standard list for collectable objects */
+  gch(o)->marked = luaC_white(g);
+  gch(o)->tt = tt;
+  gch(o)->next = *list;
+  *list = o;
+  return o;
+}
+
+/* }====================================================== */
+
+
+
+/*
+** {======================================================
+** Mark functions
+** =======================================================
+*/
+
+
+/*
+** mark an object. Userdata, strings, and closed upvalues are visited
+** and turned black here. Other objects are marked gray and added
+** to appropriate list to be visited (and turned black) later. (Open
+** upvalues are already linked in 'headuv' list.)
+*/
+static void reallymarkobject (global_State *g, GCObject *o) {
+  lu_mem size;
+  white2gray(o);
+  switch (gch(o)->tt) {
+    case LUA_TSHRSTR:
+    case LUA_TLNGSTR: {
+      size = sizestring(gco2ts(o));
+      break;  /* nothing else to mark; make it black */
+    }
+    case LUA_TUSERDATA: {
+      Table *mt = gco2u(o)->metatable;
+      markobject(g, mt);
+      markobject(g, gco2u(o)->env);
+      size = sizeudata(gco2u(o));
+      break;
+    }
+    case LUA_TUPVAL: {
+      UpVal *uv = gco2uv(o);
+      markvalue(g, uv->v);
+      if (uv->v != &uv->u.value)  /* open? */
+        return;  /* open upvalues remain gray */
+      size = sizeof(UpVal);
+      break;
+    }
+    case LUA_TLCL: {
+      gco2lcl(o)->gclist = g->gray;
+      g->gray = o;
+      return;
+    }
+    case LUA_TCCL: {
+      gco2ccl(o)->gclist = g->gray;
+      g->gray = o;
+      return;
+    }
+    case LUA_TTABLE: {
+      linktable(gco2t(o), &g->gray);
+      return;
+    }
+    case LUA_TTHREAD: {
+      gco2th(o)->gclist = g->gray;
+      g->gray = o;
+      return;
+    }
+    case LUA_TPROTO: {
+      gco2p(o)->gclist = g->gray;
+      g->gray = o;
+      return;
+    }
+    default: lua_assert(0); return;
+  }
+  gray2black(o);
+  g->GCmemtrav += size;
+}
+
+
+/*
+** mark metamethods for basic types
+*/
+static void markmt (global_State *g) {
+  int i;
+  for (i=0; i < LUA_NUMTAGS; i++)
+    markobject(g, g->mt[i]);
+}
+
+
+/*
+** mark all objects in list of being-finalized
+*/
+static void markbeingfnz (global_State *g) {
+  GCObject *o;
+  for (o = g->tobefnz; o != NULL; o = gch(o)->next) {
+    makewhite(g, o);
+    reallymarkobject(g, o);
+  }
+}
+
+
+/*
+** mark all values stored in marked open upvalues. (See comment in
+** 'lstate.h'.)
+*/
+static void remarkupvals (global_State *g) {
+  UpVal *uv;
+  for (uv = g->uvhead.u.l.next; uv != &g->uvhead; uv = uv->u.l.next) {
+    if (isgray(obj2gco(uv)))
+      markvalue(g, uv->v);
+  }
+}
+
+
+/*
+** mark root set and reset all gray lists, to start a new
+** incremental (or full) collection
+*/
+static void restartcollection (global_State *g) {
+  g->gray = g->grayagain = NULL;
+  g->weak = g->allweak = g->ephemeron = NULL;
+  markobject(g, g->mainthread);
+  markvalue(g, &g->l_registry);
+  markmt(g);
+  markbeingfnz(g);  /* mark any finalizing object left from previous cycle */
+}
+
+/* }====================================================== */
+
+
+/*
+** {======================================================
+** Traverse functions
+** =======================================================
+*/
+
+static void traverseweakvalue (global_State *g, Table *h) {
+  Node *n, *limit = gnodelast(h);
+  /* if there is array part, assume it may have white values (do not
+     traverse it just to check) */
+  int hasclears = (h->sizearray > 0);
+  for (n = gnode(h, 0); n < limit; n++) {
+    checkdeadkey(n);
+    if (ttisnil(gval(n)))  /* entry is empty? */
+      removeentry(n);  /* remove it */
+    else {
+      lua_assert(!ttisnil(gkey(n)));
+      markvalue(g, gkey(n));  /* mark key */
+      if (!hasclears && iscleared(g, gval(n)))  /* is there a white value? */
+        hasclears = 1;  /* table will have to be cleared */
+    }
+  }
+  if (hasclears)
+    linktable(h, &g->weak);  /* has to be cleared later */
+  else  /* no white values */
+    linktable(h, &g->grayagain);  /* no need to clean */
+}
+
+
+static int traverseephemeron (global_State *g, Table *h) {
+  int marked = 0;  /* true if an object is marked in this traversal */
+  int hasclears = 0;  /* true if table has white keys */
+  int prop = 0;  /* true if table has entry "white-key -> white-value" */
+  Node *n, *limit = gnodelast(h);
+  int i;
+  /* traverse array part (numeric keys are 'strong') */
+  for (i = 0; i < h->sizearray; i++) {
+    if (valiswhite(&h->array[i])) {
+      marked = 1;
+      reallymarkobject(g, gcvalue(&h->array[i]));
+    }
+  }
+  /* traverse hash part */
+  for (n = gnode(h, 0); n < limit; n++) {
+    checkdeadkey(n);
+    if (ttisnil(gval(n)))  /* entry is empty? */
+      removeentry(n);  /* remove it */
+    else if (iscleared(g, gkey(n))) {  /* key is not marked (yet)? */
+      hasclears = 1;  /* table must be cleared */
+      if (valiswhite(gval(n)))  /* value not marked yet? */
+        prop = 1;  /* must propagate again */
+    }
+    else if (valiswhite(gval(n))) {  /* value not marked yet? */
+      marked = 1;
+      reallymarkobject(g, gcvalue(gval(n)));  /* mark it now */
+    }
+  }
+  if (prop)
+    linktable(h, &g->ephemeron);  /* have to propagate again */
+  else if (hasclears)  /* does table have white keys? */
+    linktable(h, &g->allweak);  /* may have to clean white keys */
+  else  /* no white keys */
+    linktable(h, &g->grayagain);  /* no need to clean */
+  return marked;
+}
+
+
+static void traversestrongtable (global_State *g, Table *h) {
+  Node *n, *limit = gnodelast(h);
+  int i;
+  for (i = 0; i < h->sizearray; i++)  /* traverse array part */
+    markvalue(g, &h->array[i]);
+  for (n = gnode(h, 0); n < limit; n++) {  /* traverse hash part */
+    checkdeadkey(n);
+    if (ttisnil(gval(n)))  /* entry is empty? */
+      removeentry(n);  /* remove it */
+    else {
+      lua_assert(!ttisnil(gkey(n)));
+      markvalue(g, gkey(n));  /* mark key */
+      markvalue(g, gval(n));  /* mark value */
+    }
+  }
+}
+
+
+static lu_mem traversetable (global_State *g, Table *h) {
+  const char *weakkey, *weakvalue;
+  const TValue *mode = gfasttm(g, h->metatable, TM_MODE);
+  markobject(g, h->metatable);
+  if (mode && ttisstring(mode) &&  /* is there a weak mode? */
+      ((weakkey = strchr(svalue(mode), 'k')),
+       (weakvalue = strchr(svalue(mode), 'v')),
+       (weakkey || weakvalue))) {  /* is really weak? */
+    black2gray(obj2gco(h));  /* keep table gray */
+    if (!weakkey)  /* strong keys? */
+      traverseweakvalue(g, h);
+    else if (!weakvalue)  /* strong values? */
+      traverseephemeron(g, h);
+    else  /* all weak */
+      linktable(h, &g->allweak);  /* nothing to traverse now */
+  }
+  else  /* not weak */
+    traversestrongtable(g, h);
+  return sizeof(Table) + sizeof(TValue) * h->sizearray +
+                         sizeof(Node) * cast(size_t, sizenode(h));
+}
+
+
+static int traverseproto (global_State *g, Proto *f) {
+  int i;
+  if (f->cache && iswhite(obj2gco(f->cache)))
+    f->cache = NULL;  /* allow cache to be collected */
+  markobject(g, f->source);
+  for (i = 0; i < f->sizek; i++)  /* mark literals */
+    markvalue(g, &f->k[i]);
+  for (i = 0; i < f->sizeupvalues; i++)  /* mark upvalue names */
+    markobject(g, f->upvalues[i].name);
+  for (i = 0; i < f->sizep; i++)  /* mark nested protos */
+    markobject(g, f->p[i]);
+  for (i = 0; i < f->sizelocvars; i++)  /* mark local-variable names */
+    markobject(g, f->locvars[i].varname);
+  return sizeof(Proto) + sizeof(Instruction) * f->sizecode +
+                         sizeof(Proto *) * f->sizep +
+                         sizeof(TValue) * f->sizek +
+                         sizeof(int) * f->sizelineinfo +
+                         sizeof(LocVar) * f->sizelocvars +
+                         sizeof(Upvaldesc) * f->sizeupvalues;
+}
+
+
+static lu_mem traverseCclosure (global_State *g, CClosure *cl) {
+  int i;
+  for (i = 0; i < cl->nupvalues; i++)  /* mark its upvalues */
+    markvalue(g, &cl->upvalue[i]);
+  return sizeCclosure(cl->nupvalues);
+}
+
+static lu_mem traverseLclosure (global_State *g, LClosure *cl) {
+  int i;
+  markobject(g, cl->p);  /* mark its prototype */
+  for (i = 0; i < cl->nupvalues; i++)  /* mark its upvalues */
+    markobject(g, cl->upvals[i]);
+  return sizeLclosure(cl->nupvalues);
+}
+
+
+static lu_mem traversestack (global_State *g, lua_State *th) {
+  int n = 0;
+  StkId o = th->stack;
+  if (o == NULL)
+    return 1;  /* stack not completely built yet */
+  for (; o < th->top; o++)  /* mark live elements in the stack */
+    markvalue(g, o);
+  if (g->gcstate == GCSatomic) {  /* final traversal? */
+    StkId lim = th->stack + th->stacksize;  /* real end of stack */
+    for (; o < lim; o++)  /* clear not-marked stack slice */
+      setnilvalue(o);
+  }
+  else {  /* count call infos to compute size */
+    CallInfo *ci;
+    for (ci = &th->base_ci; ci != th->ci; ci = ci->next)
+      n++;
+  }
+  return sizeof(lua_State) + sizeof(TValue) * th->stacksize +
+         sizeof(CallInfo) * n;
+}
+
+
+/*
+** traverse one gray object, turning it to black (except for threads,
+** which are always gray).
+*/
+static void propagatemark (global_State *g) {
+  lu_mem size;
+  GCObject *o = g->gray;
+  lua_assert(isgray(o));
+  gray2black(o);
+  switch (gch(o)->tt) {
+    case LUA_TTABLE: {
+      Table *h = gco2t(o);
+      g->gray = h->gclist;  /* remove from 'gray' list */
+      size = traversetable(g, h);
+      break;
+    }
+    case LUA_TLCL: {
+      LClosure *cl = gco2lcl(o);
+      g->gray = cl->gclist;  /* remove from 'gray' list */
+      size = traverseLclosure(g, cl);
+      break;
+    }
+    case LUA_TCCL: {
+      CClosure *cl = gco2ccl(o);
+      g->gray = cl->gclist;  /* remove from 'gray' list */
+      size = traverseCclosure(g, cl);
+      break;
+    }
+    case LUA_TTHREAD: {
+      lua_State *th = gco2th(o);
+      g->gray = th->gclist;  /* remove from 'gray' list */
+      th->gclist = g->grayagain;
+      g->grayagain = o;  /* insert into 'grayagain' list */
+      black2gray(o);
+      size = traversestack(g, th);
+      break;
+    }
+    case LUA_TPROTO: {
+      Proto *p = gco2p(o);
+      g->gray = p->gclist;  /* remove from 'gray' list */
+      size = traverseproto(g, p);
+      break;
+    }
+    default: lua_assert(0); return;
+  }
+  g->GCmemtrav += size;
+}
+
+
+static void propagateall (global_State *g) {
+  while (g->gray) propagatemark(g);
+}
+
+
+static void propagatelist (global_State *g, GCObject *l) {
+  lua_assert(g->gray == NULL);  /* no grays left */
+  g->gray = l;
+  propagateall(g);  /* traverse all elements from 'l' */
+}
+
+/*
+** retraverse all gray lists. Because tables may be reinserted in other
+** lists when traversed, traverse the original lists to avoid traversing
+** twice the same table (which is not wrong, but inefficient)
+*/
+static void retraversegrays (global_State *g) {
+  GCObject *weak = g->weak;  /* save original lists */
+  GCObject *grayagain = g->grayagain;
+  GCObject *ephemeron = g->ephemeron;
+  g->weak = g->grayagain = g->ephemeron = NULL;
+  propagateall(g);  /* traverse main gray list */
+  propagatelist(g, grayagain);
+  propagatelist(g, weak);
+  propagatelist(g, ephemeron);
+}
+
+
+static void convergeephemerons (global_State *g) {
+  int changed;
+  do {
+    GCObject *w;
+    GCObject *next = g->ephemeron;  /* get ephemeron list */
+    g->ephemeron = NULL;  /* tables will return to this list when traversed */
+    changed = 0;
+    while ((w = next) != NULL) {
+      next = gco2t(w)->gclist;
+      if (traverseephemeron(g, gco2t(w))) {  /* traverse marked some value? */
+        propagateall(g);  /* propagate changes */
+        changed = 1;  /* will have to revisit all ephemeron tables */
+      }
+    }
+  } while (changed);
+}
+
+/* }====================================================== */
+
+
+/*
+** {======================================================
+** Sweep Functions
+** =======================================================
+*/
+
+
+/*
+** clear entries with unmarked keys from all weaktables in list 'l' up
+** to element 'f'
+*/
+static void clearkeys (global_State *g, GCObject *l, GCObject *f) {
+  for (; l != f; l = gco2t(l)->gclist) {
+    Table *h = gco2t(l);
+    Node *n, *limit = gnodelast(h);
+    for (n = gnode(h, 0); n < limit; n++) {
+      if (!ttisnil(gval(n)) && (iscleared(g, gkey(n)))) {
+        setnilvalue(gval(n));  /* remove value ... */
+        removeentry(n);  /* and remove entry from table */
+      }
+    }
+  }
+}
+
+
+/*
+** clear entries with unmarked values from all weaktables in list 'l' up
+** to element 'f'
+*/
+static void clearvalues (global_State *g, GCObject *l, GCObject *f) {
+  for (; l != f; l = gco2t(l)->gclist) {
+    Table *h = gco2t(l);
+    Node *n, *limit = gnodelast(h);
+    int i;
+    for (i = 0; i < h->sizearray; i++) {
+      TValue *o = &h->array[i];
+      if (iscleared(g, o))  /* value was collected? */
+        setnilvalue(o);  /* remove value */
+    }
+    for (n = gnode(h, 0); n < limit; n++) {
+      if (!ttisnil(gval(n)) && iscleared(g, gval(n))) {
+        setnilvalue(gval(n));  /* remove value ... */
+        removeentry(n);  /* and remove entry from table */
+      }
+    }
+  }
+}
+
+
+static void freeobj (lua_State *L, GCObject *o) {
+  switch (gch(o)->tt) {
+    case LUA_TPROTO: luaF_freeproto(L, gco2p(o)); break;
+    case LUA_TLCL: {
+      luaM_freemem(L, o, sizeLclosure(gco2lcl(o)->nupvalues));
+      break;
+    }
+    case LUA_TCCL: {
+      luaM_freemem(L, o, sizeCclosure(gco2ccl(o)->nupvalues));
+      break;
+    }
+    case LUA_TUPVAL: luaF_freeupval(L, gco2uv(o)); break;
+    case LUA_TTABLE: luaH_free(L, gco2t(o)); break;
+    case LUA_TTHREAD: luaE_freethread(L, gco2th(o)); break;
+    case LUA_TUSERDATA: luaM_freemem(L, o, sizeudata(gco2u(o))); break;
+    case LUA_TSHRSTR:
+      G(L)->strt.nuse--;
+      /* go through */
+    case LUA_TLNGSTR: {
+      luaM_freemem(L, o, sizestring(gco2ts(o)));
+      break;
+    }
+    default: lua_assert(0);
+  }
+}
+
+
+#define sweepwholelist(L,p)	sweeplist(L,p,MAX_LUMEM)
+static GCObject **sweeplist (lua_State *L, GCObject **p, lu_mem count);
+
+
+/*
+** sweep the (open) upvalues of a thread and resize its stack and
+** list of call-info structures.
+*/
+static void sweepthread (lua_State *L, lua_State *L1) {
+  if (L1->stack == NULL) return;  /* stack not completely built yet */
+  sweepwholelist(L, &L1->openupval);  /* sweep open upvalues */
+  luaE_freeCI(L1);  /* free extra CallInfo slots */
+  /* should not change the stack during an emergency gc cycle */
+  if (G(L)->gckind != KGC_EMERGENCY)
+    luaD_shrinkstack(L1);
+}
+
+
+/*
+** sweep at most 'count' elements from a list of GCObjects erasing dead
+** objects, where a dead (not alive) object is one marked with the "old"
+** (non current) white and not fixed.
+** In non-generational mode, change all non-dead objects back to white,
+** preparing for next collection cycle.
+** In generational mode, keep black objects black, and also mark them as
+** old; stop when hitting an old object, as all objects after that
+** one will be old too.
+** When object is a thread, sweep its list of open upvalues too.
+*/
+static GCObject **sweeplist (lua_State *L, GCObject **p, lu_mem count) {
+  global_State *g = G(L);
+  int ow = otherwhite(g);
+  int toclear, toset;  /* bits to clear and to set in all live objects */
+  int tostop;  /* stop sweep when this is true */
+  if (isgenerational(g)) {  /* generational mode? */
+    toclear = ~0;  /* clear nothing */
+    toset = bitmask(OLDBIT);  /* set the old bit of all surviving objects */
+    tostop = bitmask(OLDBIT);  /* do not sweep old generation */
+  }
+  else {  /* normal mode */
+    toclear = maskcolors;  /* clear all color bits + old bit */
+    toset = luaC_white(g);  /* make object white */
+    tostop = 0;  /* do not stop */
+  }
+  while (*p != NULL && count-- > 0) {
+    GCObject *curr = *p;
+    int marked = gch(curr)->marked;
+    if (isdeadm(ow, marked)) {  /* is 'curr' dead? */
+      *p = gch(curr)->next;  /* remove 'curr' from list */
+      freeobj(L, curr);  /* erase 'curr' */
+    }
+    else {
+      if (testbits(marked, tostop))
+        return NULL;  /* stop sweeping this list */
+      if (gch(curr)->tt == LUA_TTHREAD)
+        sweepthread(L, gco2th(curr));  /* sweep thread's upvalues */
+      /* update marks */
+      gch(curr)->marked = cast_byte((marked & toclear) | toset);
+      p = &gch(curr)->next;  /* go to next element */
+    }
+  }
+  return (*p == NULL) ? NULL : p;
+}
+
+
+/*
+** sweep a list until a live object (or end of list)
+*/
+static GCObject **sweeptolive (lua_State *L, GCObject **p, int *n) {
+  GCObject ** old = p;
+  int i = 0;
+  do {
+    i++;
+    p = sweeplist(L, p, 1);
+  } while (p == old);
+  if (n) *n += i;
+  return p;
+}
+
+/* }====================================================== */
+
+
+/*
+** {======================================================
+** Finalization
+** =======================================================
+*/
+
+static void checkSizes (lua_State *L) {
+  global_State *g = G(L);
+  if (g->gckind != KGC_EMERGENCY) {  /* do not change sizes in emergency */
+    int hs = g->strt.size / 2;  /* half the size of the string table */
+    if (g->strt.nuse < cast(lu_int32, hs))  /* using less than that half? */
+      luaS_resize(L, hs);  /* halve its size */
+    luaZ_freebuffer(L, &g->buff);  /* free concatenation buffer */
+  }
+}
+
+
+static GCObject *udata2finalize (global_State *g) {
+  GCObject *o = g->tobefnz;  /* get first element */
+  lua_assert(isfinalized(o));
+  g->tobefnz = gch(o)->next;  /* remove it from 'tobefnz' list */
+  gch(o)->next = g->allgc;  /* return it to 'allgc' list */
+  g->allgc = o;
+  resetbit(gch(o)->marked, SEPARATED);  /* mark that it is not in 'tobefnz' */
+  lua_assert(!isold(o));  /* see MOVE OLD rule */
+  if (!keepinvariantout(g))  /* not keeping invariant? */
+    makewhite(g, o);  /* "sweep" object */
+  return o;
+}
+
+
+static void dothecall (lua_State *L, void *ud) {
+  UNUSED(ud);
+  luaD_call(L, L->top - 2, 0, 0);
+}
+
+
+static void GCTM (lua_State *L, int propagateerrors) {
+  global_State *g = G(L);
+  const TValue *tm;
+  TValue v;
+  setgcovalue(L, &v, udata2finalize(g));
+  tm = luaT_gettmbyobj(L, &v, TM_GC);
+  if (tm != NULL && ttisfunction(tm)) {  /* is there a finalizer? */
+    int status;
+    lu_byte oldah = L->allowhook;
+    int running  = g->gcrunning;
+    L->allowhook = 0;  /* stop debug hooks during GC metamethod */
+    g->gcrunning = 0;  /* avoid GC steps */
+    setobj2s(L, L->top, tm);  /* push finalizer... */
+    setobj2s(L, L->top + 1, &v);  /* ... and its argument */
+    L->top += 2;  /* and (next line) call the finalizer */
+    status = luaD_pcall(L, dothecall, NULL, savestack(L, L->top - 2), 0);
+    L->allowhook = oldah;  /* restore hooks */
+    g->gcrunning = running;  /* restore state */
+    if (status != LUA_OK && propagateerrors) {  /* error while running __gc? */
+      if (status == LUA_ERRRUN) {  /* is there an error object? */
+        const char *msg = (ttisstring(L->top - 1))
+                            ? svalue(L->top - 1)
+                            : "no message";
+        luaO_pushfstring(L, "error in __gc metamethod (%s)", msg);
+        status = LUA_ERRGCMM;  /* error in __gc metamethod */
+      }
+      luaD_throw(L, status);  /* re-throw error */
+    }
+  }
+}
+
+
+/*
+** move all unreachable objects (or 'all' objects) that need
+** finalization from list 'finobj' to list 'tobefnz' (to be finalized)
+*/
+static void separatetobefnz (lua_State *L, int all) {
+  global_State *g = G(L);
+  GCObject **p = &g->finobj;
+  GCObject *curr;
+  GCObject **lastnext = &g->tobefnz;
+  /* find last 'next' field in 'tobefnz' list (to add elements in its end) */
+  while (*lastnext != NULL)
+    lastnext = &gch(*lastnext)->next;
+  while ((curr = *p) != NULL) {  /* traverse all finalizable objects */
+    lua_assert(!isfinalized(curr));
+    lua_assert(testbit(gch(curr)->marked, SEPARATED));
+    if (!(iswhite(curr) || all))  /* not being collected? */
+      p = &gch(curr)->next;  /* don't bother with it */
+    else {
+      l_setbit(gch(curr)->marked, FINALIZEDBIT); /* won't be finalized again */
+      *p = gch(curr)->next;  /* remove 'curr' from 'finobj' list */
+      gch(curr)->next = *lastnext;  /* link at the end of 'tobefnz' list */
+      *lastnext = curr;
+      lastnext = &gch(curr)->next;
+    }
+  }
+}
+
+
+/*
+** if object 'o' has a finalizer, remove it from 'allgc' list (must
+** search the list to find it) and link it in 'finobj' list.
+*/
+void luaC_checkfinalizer (lua_State *L, GCObject *o, Table *mt) {
+  global_State *g = G(L);
+  if (testbit(gch(o)->marked, SEPARATED) || /* obj. is already separated... */
+      isfinalized(o) ||                           /* ... or is finalized... */
+      gfasttm(g, mt, TM_GC) == NULL)                /* or has no finalizer? */
+    return;  /* nothing to be done */
+  else {  /* move 'o' to 'finobj' list */
+    GCObject **p;
+    GCheader *ho = gch(o);
+    if (g->sweepgc == &ho->next) {  /* avoid removing current sweep object */
+      lua_assert(issweepphase(g));
+      g->sweepgc = sweeptolive(L, g->sweepgc, NULL);
+    }
+    /* search for pointer pointing to 'o' */
+    for (p = &g->allgc; *p != o; p = &gch(*p)->next) { /* empty */ }
+    *p = ho->next;  /* remove 'o' from root list */
+    ho->next = g->finobj;  /* link it in list 'finobj' */
+    g->finobj = o;
+    l_setbit(ho->marked, SEPARATED);  /* mark it as such */
+    if (!keepinvariantout(g))  /* not keeping invariant? */
+      makewhite(g, o);  /* "sweep" object */
+    else
+      resetoldbit(o);  /* see MOVE OLD rule */
+  }
+}
+
+/* }====================================================== */
+
+
+/*
+** {======================================================
+** GC control
+** =======================================================
+*/
+
+
+/*
+** set a reasonable "time" to wait before starting a new GC cycle;
+** cycle will start when memory use hits threshold
+*/
+static void setpause (global_State *g, l_mem estimate) {
+  l_mem debt, threshold;
+  estimate = estimate / PAUSEADJ;  /* adjust 'estimate' */
+  threshold = (g->gcpause < MAX_LMEM / estimate)  /* overflow? */
+            ? estimate * g->gcpause  /* no overflow */
+            : MAX_LMEM;  /* overflow; truncate to maximum */
+  debt = -cast(l_mem, threshold - gettotalbytes(g));
+  luaE_setdebt(g, debt);
+}
+
+
+#define sweepphases  \
+	(bitmask(GCSsweepstring) | bitmask(GCSsweepudata) | bitmask(GCSsweep))
+
+
+/*
+** enter first sweep phase (strings) and prepare pointers for other
+** sweep phases.  The calls to 'sweeptolive' make pointers point to an
+** object inside the list (instead of to the header), so that the real
+** sweep do not need to skip objects created between "now" and the start
+** of the real sweep.
+** Returns how many objects it swept.
+*/
+static int entersweep (lua_State *L) {
+  global_State *g = G(L);
+  int n = 0;
+  g->gcstate = GCSsweepstring;
+  lua_assert(g->sweepgc == NULL && g->sweepfin == NULL);
+  /* prepare to sweep strings, finalizable objects, and regular objects */
+  g->sweepstrgc = 0;
+  g->sweepfin = sweeptolive(L, &g->finobj, &n);
+  g->sweepgc = sweeptolive(L, &g->allgc, &n);
+  return n;
+}
+
+
+/*
+** change GC mode
+*/
+void luaC_changemode (lua_State *L, int mode) {
+  global_State *g = G(L);
+  if (mode == g->gckind) return;  /* nothing to change */
+  if (mode == KGC_GEN) {  /* change to generational mode */
+    /* make sure gray lists are consistent */
+    luaC_runtilstate(L, bitmask(GCSpropagate));
+    g->GCestimate = gettotalbytes(g);
+    g->gckind = KGC_GEN;
+  }
+  else {  /* change to incremental mode */
+    /* sweep all objects to turn them back to white
+       (as white has not changed, nothing extra will be collected) */
+    g->gckind = KGC_NORMAL;
+    entersweep(L);
+    luaC_runtilstate(L, ~sweepphases);
+  }
+}
+
+
+/*
+** call all pending finalizers
+*/
+static void callallpendingfinalizers (lua_State *L, int propagateerrors) {
+  global_State *g = G(L);
+  while (g->tobefnz) {
+    resetoldbit(g->tobefnz);
+    GCTM(L, propagateerrors);
+  }
+}
+
+
+void luaC_freeallobjects (lua_State *L) {
+  global_State *g = G(L);
+  int i;
+  separatetobefnz(L, 1);  /* separate all objects with finalizers */
+  lua_assert(g->finobj == NULL);
+  callallpendingfinalizers(L, 0);
+  g->currentwhite = WHITEBITS; /* this "white" makes all objects look dead */
+  g->gckind = KGC_NORMAL;
+  sweepwholelist(L, &g->finobj);  /* finalizers can create objs. in 'finobj' */
+  sweepwholelist(L, &g->allgc);
+  for (i = 0; i < g->strt.size; i++)  /* free all string lists */
+    sweepwholelist(L, &g->strt.hash[i]);
+  lua_assert(g->strt.nuse == 0);
+}
+
+
+static l_mem atomic (lua_State *L) {
+  global_State *g = G(L);
+  l_mem work = -cast(l_mem, g->GCmemtrav);  /* start counting work */
+  GCObject *origweak, *origall;
+  lua_assert(!iswhite(obj2gco(g->mainthread)));
+  markobject(g, L);  /* mark running thread */
+  /* registry and global metatables may be changed by API */
+  markvalue(g, &g->l_registry);
+  markmt(g);  /* mark basic metatables */
+  /* remark occasional upvalues of (maybe) dead threads */
+  remarkupvals(g);
+  propagateall(g);  /* propagate changes */
+  work += g->GCmemtrav;  /* stop counting (do not (re)count grays) */
+  /* traverse objects caught by write barrier and by 'remarkupvals' */
+  retraversegrays(g);
+  work -= g->GCmemtrav;  /* restart counting */
+  convergeephemerons(g);
+  /* at this point, all strongly accessible objects are marked. */
+  /* clear values from weak tables, before checking finalizers */
+  clearvalues(g, g->weak, NULL);
+  clearvalues(g, g->allweak, NULL);
+  origweak = g->weak; origall = g->allweak;
+  work += g->GCmemtrav;  /* stop counting (objects being finalized) */
+  separatetobefnz(L, 0);  /* separate objects to be finalized */
+  markbeingfnz(g);  /* mark objects that will be finalized */
+  propagateall(g);  /* remark, to propagate `preserveness' */
+  work -= g->GCmemtrav;  /* restart counting */
+  convergeephemerons(g);
+  /* at this point, all resurrected objects are marked. */
+  /* remove dead objects from weak tables */
+  clearkeys(g, g->ephemeron, NULL);  /* clear keys from all ephemeron tables */
+  clearkeys(g, g->allweak, NULL);  /* clear keys from all allweak tables */
+  /* clear values from resurrected weak tables */
+  clearvalues(g, g->weak, origweak);
+  clearvalues(g, g->allweak, origall);
+  g->currentwhite = cast_byte(otherwhite(g));  /* flip current white */
+  work += g->GCmemtrav;  /* complete counting */
+  return work;  /* estimate of memory marked by 'atomic' */
+}
+
+
+static lu_mem singlestep (lua_State *L) {
+  global_State *g = G(L);
+  switch (g->gcstate) {
+    case GCSpause: {
+      /* start to count memory traversed */
+      g->GCmemtrav = g->strt.size * sizeof(GCObject*);
+      lua_assert(!isgenerational(g));
+      restartcollection(g);
+      g->gcstate = GCSpropagate;
+      return g->GCmemtrav;
+    }
+    case GCSpropagate: {
+      if (g->gray) {
+        lu_mem oldtrav = g->GCmemtrav;
+        propagatemark(g);
+        return g->GCmemtrav - oldtrav;  /* memory traversed in this step */
+      }
+      else {  /* no more `gray' objects */
+        lu_mem work;
+        int sw;
+        g->gcstate = GCSatomic;  /* finish mark phase */
+        g->GCestimate = g->GCmemtrav;  /* save what was counted */;
+        work = atomic(L);  /* add what was traversed by 'atomic' */
+        g->GCestimate += work;  /* estimate of total memory traversed */ 
+        sw = entersweep(L);
+        return work + sw * GCSWEEPCOST;
+      }
+    }
+    case GCSsweepstring: {
+      int i;
+      for (i = 0; i < GCSWEEPMAX && g->sweepstrgc + i < g->strt.size; i++)
+        sweepwholelist(L, &g->strt.hash[g->sweepstrgc + i]);
+      g->sweepstrgc += i;
+      if (g->sweepstrgc >= g->strt.size)  /* no more strings to sweep? */
+        g->gcstate = GCSsweepudata;
+      return i * GCSWEEPCOST;
+    }
+    case GCSsweepudata: {
+      if (g->sweepfin) {
+        g->sweepfin = sweeplist(L, g->sweepfin, GCSWEEPMAX);
+        return GCSWEEPMAX*GCSWEEPCOST;
+      }
+      else {
+        g->gcstate = GCSsweep;
+        return 0;
+      }
+    }
+    case GCSsweep: {
+      if (g->sweepgc) {
+        g->sweepgc = sweeplist(L, g->sweepgc, GCSWEEPMAX);
+        return GCSWEEPMAX*GCSWEEPCOST;
+      }
+      else {
+        /* sweep main thread */
+        GCObject *mt = obj2gco(g->mainthread);
+        sweeplist(L, &mt, 1);
+        checkSizes(L);
+        g->gcstate = GCSpause;  /* finish collection */
+        return GCSWEEPCOST;
+      }
+    }
+    default: lua_assert(0); return 0;
+  }
+}
+
+
+/*
+** advances the garbage collector until it reaches a state allowed
+** by 'statemask'
+*/
+void luaC_runtilstate (lua_State *L, int statesmask) {
+  global_State *g = G(L);
+  while (!testbit(statesmask, g->gcstate))
+    singlestep(L);
+}
+
+
+static void generationalcollection (lua_State *L) {
+  global_State *g = G(L);
+  lua_assert(g->gcstate == GCSpropagate);
+  if (g->GCestimate == 0) {  /* signal for another major collection? */
+    luaC_fullgc(L, 0);  /* perform a full regular collection */
+    g->GCestimate = gettotalbytes(g);  /* update control */
+  }
+  else {
+    lu_mem estimate = g->GCestimate;
+    luaC_runtilstate(L, bitmask(GCSpause));  /* run complete (minor) cycle */
+    g->gcstate = GCSpropagate;  /* skip restart */
+    if (gettotalbytes(g) > (estimate / 100) * g->gcmajorinc)
+      g->GCestimate = 0;  /* signal for a major collection */
+    else
+      g->GCestimate = estimate;  /* keep estimate from last major coll. */
+
+  }
+  setpause(g, gettotalbytes(g));
+  lua_assert(g->gcstate == GCSpropagate);
+}
+
+
+static void incstep (lua_State *L) {
+  global_State *g = G(L);
+  l_mem debt = g->GCdebt;
+  int stepmul = g->gcstepmul;
+  if (stepmul < 40) stepmul = 40;  /* avoid ridiculous low values (and 0) */
+  /* convert debt from Kb to 'work units' (avoid zero debt and overflows) */
+  debt = (debt / STEPMULADJ) + 1;
+  debt = (debt < MAX_LMEM / stepmul) ? debt * stepmul : MAX_LMEM;
+  do {  /* always perform at least one single step */
+    lu_mem work = singlestep(L);  /* do some work */
+    debt -= work;
+  } while (debt > -GCSTEPSIZE && g->gcstate != GCSpause);
+  if (g->gcstate == GCSpause)
+    setpause(g, g->GCestimate);  /* pause until next cycle */
+  else {
+    debt = (debt / stepmul) * STEPMULADJ;  /* convert 'work units' to Kb */
+    luaE_setdebt(g, debt);
+  }
+}
+
+
+/*
+** performs a basic GC step
+*/
+void luaC_forcestep (lua_State *L) {
+  global_State *g = G(L);
+  int i;
+  if (isgenerational(g)) generationalcollection(L);
+  else incstep(L);
+  /* run a few finalizers (or all of them at the end of a collect cycle) */
+  for (i = 0; g->tobefnz && (i < GCFINALIZENUM || g->gcstate == GCSpause); i++)
+    GCTM(L, 1);  /* call one finalizer */
+}
+
+
+/*
+** performs a basic GC step only if collector is running
+*/
+void luaC_step (lua_State *L) {
+  global_State *g = G(L);
+  if (g->gcrunning) luaC_forcestep(L);
+  else luaE_setdebt(g, -GCSTEPSIZE);  /* avoid being called too often */
+}
+
+
+
+/*
+** performs a full GC cycle; if "isemergency", does not call
+** finalizers (which could change stack positions)
+*/
+void luaC_fullgc (lua_State *L, int isemergency) {
+  global_State *g = G(L);
+  int origkind = g->gckind;
+  lua_assert(origkind != KGC_EMERGENCY);
+  if (isemergency)  /* do not run finalizers during emergency GC */
+    g->gckind = KGC_EMERGENCY;
+  else {
+    g->gckind = KGC_NORMAL;
+    callallpendingfinalizers(L, 1);
+  }
+  if (keepinvariant(g)) {  /* may there be some black objects? */
+    /* must sweep all objects to turn them back to white
+       (as white has not changed, nothing will be collected) */
+    entersweep(L);
+  }
+  /* finish any pending sweep phase to start a new cycle */
+  luaC_runtilstate(L, bitmask(GCSpause));
+  luaC_runtilstate(L, ~bitmask(GCSpause));  /* start new collection */
+  luaC_runtilstate(L, bitmask(GCSpause));  /* run entire collection */
+  if (origkind == KGC_GEN) {  /* generational mode? */
+    /* generational mode must be kept in propagate phase */
+    luaC_runtilstate(L, bitmask(GCSpropagate));
+  }
+  g->gckind = origkind;
+  setpause(g, gettotalbytes(g));
+  if (!isemergency)   /* do not run finalizers during emergency GC */
+    callallpendingfinalizers(L, 1);
+}
+
+/* }====================================================== */
+
+