ravi/src/lgc.c

/*
** $Id: lgc.c,v 2.215.1.2 2017/08/31 16:15:27 roberto Exp $
** Garbage Collector
** See Copyright Notice in lua.h
*/

#define lgc_c
#define LUA_CORE

#include "lprefix.h"


#include <string.h>

#include "lua.h"

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


/*
** internal state for collector while inside the atomic phase. The
** collector should never be in this state while running regular code.
*/
#define GCSinsideatomic		(GCSpause + 1)

/*
** cost of sweeping one element (the size of a small object divided
** by some adjust for the sweep speed)
*/
#define GCSWEEPCOST	((sizeof(TString) + 4) / 4)

/* maximum number of elements to sweep in each single step */
#define GCSWEEPMAX	(cast_int((GCSTEPSIZE / GCSWEEPCOST) / 4))

/* cost of calling one finalizer */
#define GCFINALIZECOST	GCSWEEPCOST


/*
** macro to adjust 'stepmul': 'stepmul' is actually used like
** 'stepmul / STEPMULADJ' (value chosen by tests)
*/
#define STEPMULADJ		200


/*
** macro to adjust 'pause': 'pause' is actually used like
** 'pause / PAUSEADJ' (value chosen by tests)
*/
#define PAUSEADJ		100


/*
** 'makewhite' erases all color bits then sets only the current white
** bit
*/
#define maskcolors	(~(bitmask(BLACKBIT) | WHITEBITS))
#define makewhite(g,x)	\
 (x->marked = cast_byte((x->marked & maskcolors) | luaC_white(g)))

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


#define valiswhite(x)   (iscollectable(x) && iswhite(gcvalue(x)))

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


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


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

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

/*
** mark an object that can be NULL (either because it is really optional,
** or it was stripped as debug info, or inside an uncompleted structure)
*/
#define markobjectN(g,t)	{ if (t) markobject(g,t); }

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


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


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


/*
** link collectable object 'o' into list pointed by 'p'
*/
#define linkgclist(o,p)	((o)->gclist = (p), (p) = obj2gco(o))


/*
** If key is not marked, mark its entry as dead. This allows key to be
** collected, but keeps its entry in the table.  A dead node is needed
** when Lua looks up for a key (it may be part of a chain) and when
** traversing a weak table (key might be removed from the table during
** traversal). Other places never manipulate dead keys, because its
** associated nil value is enough to signal that the entry is logically
** empty.
*/
static void removeentry (Node *n) {
  lua_assert(ttisnil(gval(n)));
  if (valiswhite(gkey(n)))
    setdeadvalue(wgkey(n));  /* unused and unmarked key; remove it */
}


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

/*
Following description is taken from:
http://wiki.luajit.org/New-Garbage-Collector#gc-algorithms_tri-color-incremental-mark-sweep

Newly allocated objects are white. The mark phase starts at the GC roots.
Marking a reachable object means flipping the color of it from white to
gray and pushing it onto a gray stack (or re-chaining it onto a gray list).
The gray stack is iteratively processed, removing one gray object at a time.
A gray object is traversed and all objects reachable from it are marked,
like above. After an object has been traversed, it's turned from gray to
black. The sweep phase works just like the two-color algorithm above.

This algorithm is incremental: the collector can operate in small steps,
processing only a couple of objects from the gray stack and then let the
mutator run again for a while. This spreads out the GC pauses into
many short intervals, which is important for highly interactive
workloads (e.g. games or internet servers).

But there's one catch: the mutator might get in the way of the collector
and store a reference to a white (unprocessed) object at a black
(processed) object. This object would never be marked and will be
freed by the sweep, even though it's clearly still referenced from a
reachable object, i.e. it should be kept alive.

To avoid this scenario, one has to preserve the tri-color invariant:
a black object may never hold a reference to a white object. This is
done with a write barrier, which has to be checked after every write.
If the invariant has been violated, a fixup step is needed.
There are two alternatives:

1. Either turn the black object gray and push it back onto the gray stack.
This is moving the barrier "back", because the object has to be reprocessed
later on. This is beneficial for container objects, because they usually
receive several stores in succession. This avoids a barrier for the next
objects that are stored into it (which are likely white, too).

2. Or immediately mark the white object, turning it gray and push it onto
the gray stack. This moves the barrier "forward", because it implicitly
drives the GC forward. This works best for objects that only receive
isolated stores.

There are many optimizations to turn this into a practical algorithm.
Here are the most important:

* Stacks should always be kept gray and re-traversed just before the
final sweep phase. This avoids a write barrier for stores to stack slots,
which are the most common kind of stores.

* Objects which have no references to child objects can immediately be
turned from white to black and don't need to go through the gray stack.

* The sweep phase can be made incremental by using two whites and
flipping between them just before entering the sweep phase. Objects with
the 'current' white need to be kept. Only objects with the
'other' white should be freed.

In Lua, Tables use backward barriers, all other traversable objects
use forward barriers.
*/

/*
** barrier that moves collector forward, that is, mark the white object
** being pointed by a black object. (If in sweep phase, clear the black
** object to white [sweep it] to avoid other barrier calls for this
** same object.)
**
** Here we have a black object pointing / referencing a white object
** So to preserve tri-color invariant the white object must
** be marked and turned gray or black. Userdata, strings and upvalues
** are turned black, whereas functions, threads, tables and protos are turned
** gray.
**
** Example: userdata o references user value v, or
**          function proto o references newly added constant v
*/
void luaC_barrier_(lua_State *L, GCObject *o, GCObject *v) {
  global_State *g = G(L);
  lua_assert(isblack(o) && iswhite(v) && !isdead(g, v) && !isdead(g, o));
  if (keepinvariant(g))     /* must keep invariant? (not in sweep phase) */
    reallymarkobject(g, v); /* restore invariant - turn white object to gray or black */
  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.
**
** Here we have a container (table) being assigned a value, so the
** table if black must be turned to gray as black objects cannot point
** to white objects.
*/
void luaC_barrierback_ (lua_State *L, Table *t) {
  global_State *g = G(L);
  lua_assert(isblack(t) && !isdead(g, t));
  black2gray(t);  /* make table gray (again) */
  linkgclist(t, g->grayagain);
}

/*
** barrier for assignments to closed upvalues. Because upvalues are
** shared among closures, it is impossible to know the color of all
** closures pointing to it. So, we assume that the object being assigned
** must be marked.
*/
void luaC_upvalbarrier_ (lua_State *L, UpVal *uv) {
  global_State *g = G(L);
  GCObject *o = gcvalue(uv->v);
  lua_assert(!upisopen(uv));  /* ensured by macro luaC_upvalbarrier */
  if (keepinvariant(g))
    markobject(g, o);
}


void luaC_fix (lua_State *L, GCObject *o) {
  global_State *g = G(L);
  lua_assert(g->allgc == o);  /* object must be 1st in 'allgc' list! */
  white2gray(o);  /* they will be gray forever */
  g->allgc = o->next;  /* remove object from 'allgc' list */
  o->next = g->fixedgc;  /* link it to 'fixedgc' list */
  g->fixedgc = o;
}


/*
** create a new collectable object (with given type and size) and link
** it to 'allgc' list.
*/
GCObject *luaC_newobj (lua_State *L, int tt, size_t sz) {
  global_State *g = G(L);
  GCObject *o = cast(GCObject *, luaM_newobject(L, novariant(tt), sz));
  o->marked = luaC_white(g);
  lua_assert(tt <= 127); /* RAVI: Must fit in a byte */
  o->tt = tt;
  o->next = g->allgc;
  g->allgc = o;
  return o;
}

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



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


/*
** mark an object. Userdata, strings, and closed upvalues are visited
** and turned black here. Other objects (functions, tables, threads, protos)
** are marked gray and added
** to appropriate list to be visited (and turned black) later. (Open
** upvalues are already linked in 'headuv' list.)
*/
static void reallymarkobject (global_State *g, GCObject *o) {
 reentry:
  white2gray(o);
  switch (o->tt) {
    case LUA_TSHRSTR: {
      gray2black(o);
      g->GCmemtrav += sizelstring(gco2ts(o)->shrlen);
      break;
    }
    case LUA_TLNGSTR: {
      gray2black(o);
      g->GCmemtrav += sizelstring(gco2ts(o)->u.lnglen);
      break;
    }
    case LUA_TUSERDATA: {
      TValue uvalue;
      markobjectN(g, gco2u(o)->metatable);  /* mark its metatable */
      gray2black(o);
      g->GCmemtrav += sizeudata(gco2u(o));
      getuservalue(g->mainthread, gco2u(o), &uvalue);
      if (valiswhite(&uvalue)) {  /* markvalue(g, &uvalue); */
        o = gcvalue(&uvalue);
        goto reentry;
      }
      break;
    }
    case LUA_TLCL: {
      linkgclist(gco2lcl(o), g->gray);
      break;
    }
    case LUA_TCCL: {
      linkgclist(gco2ccl(o), g->gray);
      break;
    }
    /* RAVI changes */
    case RAVI_TIARRAY:
    case RAVI_TFARRAY:
    case LUA_TTABLE: {
      linkgclist(gco2t(o), g->gray);
      break;
    }
    case LUA_TTHREAD: {
      linkgclist(gco2th(o), g->gray);
      break;
    }
    case LUA_TPROTO: {
      linkgclist(gco2p(o), g->gray);
      break;
    }
    default: lua_assert(0); break;
  }
}


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


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


/*
** Mark all values stored in marked open upvalues from non-marked threads.
** (Values from marked threads were already marked when traversing the
** thread.) Remove from the list threads that no longer have upvalues and
** not-marked threads.
*/
static void remarkupvals (global_State *g) {
  lua_State *thread;
  lua_State **p = &g->twups;
  while ((thread = *p) != NULL) {
    lua_assert(!isblack(thread));  /* threads are never black */
    if (isgray(thread) && thread->openupval != NULL)
      p = &thread->twups;  /* keep marked thread with upvalues in the list */
    else {  /* thread is not marked or without upvalues */
      UpVal *uv;
      *p = thread->twups;  /* remove thread from the list */
      thread->twups = thread;  /* mark that it is out of list */
      for (uv = thread->openupval; uv != NULL; uv = uv->u.open.next) {
        if (uv->u.open.touched) {
          markvalue(g, uv->v);  /* remark upvalue's value */
          uv->u.open.touched = 0;
        }
      }
    }
  }
}


/*
** mark root set and reset all gray lists, to start a new collection
*/
static void restartcollection (global_State *g) {
  g->gray = g->grayagain = NULL;
  g->weak = g->allweak = g->ephemeron = NULL;
  markobject(g, g->mainthread);
  markvalue(g, &g->l_registry);
  markmt(g);
  markbeingfnz(g);  /* mark any finalizing object left from previous cycle */
}

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


/*
** {======================================================
** Traverse functions
** =======================================================
*/

/*
** Traverse a table with weak values and link it to proper list. During
** propagate phase, keep it in 'grayagain' list, to be revisited in the
** atomic phase. In the atomic phase, if table has any white value,
** put it in 'weak' list, to be cleared.
*/
static void traverseweakvalue (global_State *g, Table *h) {
  Node *n, *limit = gnodelast(h);
  /* if there is array part, assume it may have white values (it is not
     worth traversing it now just to check) */
  int hasclears = (h->sizearray > 0);
  for (n = gnode(h, 0); n < limit; n++) {  /* traverse hash part */
    checkdeadkey(n);
    if (ttisnil(gval(n)))  /* entry is empty? */
      removeentry(n);  /* remove it */
    else {
      lua_assert(!ttisnil(gkey(n)));
      markvalue(g, gkey(n));  /* mark key */
      if (!hasclears && iscleared(g, gval(n)))  /* is there a white value? */
        hasclears = 1;  /* table will have to be cleared */
    }
  }
  if (g->gcstate == GCSpropagate)
    linkgclist(h, g->grayagain);  /* must retraverse it in atomic phase */
  else if (hasclears)
    linkgclist(h, g->weak);  /* has to be cleared later */
}


/*
** Traverse an ephemeron table and link it to proper list. Returns true
** iff any object was marked during this traversal (which implies that
** convergence has to continue). During propagation phase, keep table
** in 'grayagain' list, to be visited again in the atomic phase. In
** the atomic phase, if table has any white->white entry, it has to
** be revisited during ephemeron convergence (as that key may turn
** black). Otherwise, if it has any white key, table has to be cleared
** (in the atomic phase).
*/
static int traverseephemeron (global_State *g, Table *h) {
  int marked = 0;  /* true if an object is marked in this traversal */
  int hasclears = 0;  /* true if table has white keys */
  int hasww = 0;  /* true if table has entry "white-key -> white-value" */
  Node *n, *limit = gnodelast(h);
  unsigned int i;
  /* traverse array part */
  for (i = 0; i < h->sizearray; i++) {
    if (valiswhite(&h->array[i])) {
      marked = 1;
      reallymarkobject(g, gcvalue(&h->array[i]));
    }
  }
  /* traverse hash part */
  for (n = gnode(h, 0); n < limit; n++) {
    checkdeadkey(n);
    if (ttisnil(gval(n)))  /* entry is empty? */
      removeentry(n);  /* remove it */
    else if (iscleared(g, gkey(n))) {  /* key is not marked (yet)? */
      hasclears = 1;  /* table must be cleared */
      if (valiswhite(gval(n)))  /* value not marked yet? */
        hasww = 1;  /* white-white entry */
    }
    else if (valiswhite(gval(n))) {  /* value not marked yet? */
      marked = 1;
      reallymarkobject(g, gcvalue(gval(n)));  /* mark it now */
    }
  }
  /* link table into proper list */
  if (g->gcstate == GCSpropagate)
    linkgclist(h, g->grayagain);  /* must retraverse it in atomic phase */
  else if (hasww)  /* table has white->white entries? */
    linkgclist(h, g->ephemeron);  /* have to propagate again */
  else if (hasclears)  /* table has white keys? */
    linkgclist(h, g->allweak);  /* may have to clean white keys */
  return marked;
}


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


static lu_mem traversetable (global_State *g, Table *h) {
  const char *weakkey, *weakvalue;
  const TValue *mode = gfasttm(g, h->metatable, TM_MODE);
  markobjectN(g, h->metatable);
  if (mode && ttisstring(mode) &&  /* is there a weak mode? */
      ((weakkey = strchr(svalue(mode), 'k')),
       (weakvalue = strchr(svalue(mode), 'v')),
       (weakkey || weakvalue))) {  /* is really weak? */
    black2gray(h);  /* keep table gray */
    if (!weakkey)  /* strong keys? */
      traverseweakvalue(g, h);
    else if (!weakvalue)  /* strong values? */
      traverseephemeron(g, h);
    else  /* all weak */
      linkgclist(h, g->allweak);  /* nothing to traverse now */
  }
  else  /* not weak */
    traversestrongtable(g, h);
  return sizeof(Table) + sizeof(TValue) * h->sizearray +
                         sizeof(Node) * cast(size_t, allocsizenode(h));
}


/*
** Traverse a prototype. (While a prototype is being build, its
** arrays can be larger than needed; the extra slots are filled with
** NULL, so the use of 'markobjectN')
*/
static int traverseproto (global_State *g, Proto *f) {
  int i;
  if (f->cache && iswhite(f->cache))
    f->cache = NULL;  /* allow cache to be collected */
  markobjectN(g, f->source);
  for (i = 0; i < f->sizek; i++)  /* mark literals */
    markvalue(g, &f->k[i]);
  for (i = 0; i < f->sizeupvalues; i++) { /* mark upvalue names */
    markobjectN(g, f->upvalues[i].name);
    markobjectN(g, f->upvalues[i].usertype); /* RAVI change */
  }
  for (i = 0; i < f->sizep; i++) /* mark nested protos */
    markobjectN(g, f->p[i]);
  for (i = 0; i < f->sizelocvars; i++) { /* mark local-variable names */
    markobjectN(g, f->locvars[i].varname);
    markobjectN(g, f->locvars[i].usertype); /* RAVI change */
  }
  return sizeof(Proto) + sizeof(Instruction) * f->sizecode +
                         sizeof(Proto *) * f->sizep +
                         sizeof(TValue) * f->sizek +
                         sizeof(int) * f->sizelineinfo +
                         sizeof(LocVar) * f->sizelocvars +
                         sizeof(Upvaldesc) * f->sizeupvalues;
}


static lu_mem traverseCclosure (global_State *g, CClosure *cl) {
  int i;
  for (i = 0; i < cl->nupvalues; i++)  /* mark its upvalues */
    markvalue(g, &cl->upvalue[i]);
  return sizeCclosure(cl->nupvalues);
}

/*
** open upvalues point to values in a thread, so those values should
** be marked when the thread is traversed except in the atomic phase
** (because then the value cannot be changed by the thread and the
** thread may not be traversed again)
*/
static lu_mem traverseLclosure (global_State *g, LClosure *cl) {
  int i;
  markobjectN(g, cl->p);  /* mark its prototype */
  for (i = 0; i < cl->nupvalues; i++) {  /* mark its upvalues */
    UpVal *uv = cl->upvals[i];
    if (uv != NULL) {
      if (upisopen(uv) && g->gcstate != GCSinsideatomic)
        uv->u.open.touched = 1;  /* can be marked in 'remarkupvals' */
      else
        markvalue(g, uv->v);
    }
  }
  return sizeLclosure(cl->nupvalues);
}


static lu_mem traversethread (global_State *g, lua_State *th) {
  StkId o = th->stack;
  if (o == NULL)
    return 1;  /* stack not completely built yet */
  lua_assert(g->gcstate == GCSinsideatomic ||
             th->openupval == NULL || isintwups(th));
  for (; o < th->top; o++)  /* mark live elements in the stack */
    markvalue(g, o);
  if (g->gcstate == GCSinsideatomic) {  /* final traversal? */
    StkId lim = th->stack + th->stacksize;  /* real end of stack */
    for (; o < lim; o++)  /* clear not-marked stack slice */
      setnilvalue(o);
    /* 'remarkupvals' may have removed thread from 'twups' list */
    if (!isintwups(th) && th->openupval != NULL) {
      th->twups = g->twups;  /* link it back to the list */
      g->twups = th;
    }
  }
  else if (g->gckind != KGC_EMERGENCY)
    luaD_shrinkstack(th); /* do not change stack in emergency cycle */
  return (sizeof(lua_State) + sizeof(TValue) * th->stacksize +
          sizeof(CallInfo) * th->nci);
}


/*
** traverse one gray object, turning it to black (except for threads,
** which are always gray).
*/
static void propagatemark (global_State *g) {
  lu_mem size;
  GCObject *o = g->gray;
  lua_assert(isgray(o));
  gray2black(o);
  switch (o->tt) {
    /* RAVI changes */
    case RAVI_TIARRAY:
    case RAVI_TFARRAY:
    case LUA_TTABLE: {
      Table *h = gco2t(o);
      g->gray = h->gclist;  /* remove from 'gray' list */
      size = traversetable(g, h);
      break;
    }
    case LUA_TLCL: {
      LClosure *cl = gco2lcl(o);
      g->gray = cl->gclist;  /* remove from 'gray' list */
      size = traverseLclosure(g, cl);
      break;
    }
    case LUA_TCCL: {
      CClosure *cl = gco2ccl(o);
      g->gray = cl->gclist;  /* remove from 'gray' list */
      size = traverseCclosure(g, cl);
      break;
    }
    case LUA_TTHREAD: {
      lua_State *th = gco2th(o);
      g->gray = th->gclist;  /* remove from 'gray' list */
      linkgclist(th, g->grayagain);  /* insert into 'grayagain' list */
      black2gray(o);
      size = traversethread(g, th);
      break;
    }
    case LUA_TPROTO: {
      Proto *p = gco2p(o);
      g->gray = p->gclist;  /* remove from 'gray' list */
      size = traverseproto(g, p);
      break;
    }
    default: lua_assert(0); return;
  }
  g->GCmemtrav += size;
}


static void propagateall (global_State *g) {
  while (g->gray) propagatemark(g);
}


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

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


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


/*
** clear entries with unmarked keys from all weaktables in list 'l' up
** to element 'f'
*/
static void clearkeys (global_State *g, GCObject *l, GCObject *f) {
  for (; l != f; l = gco2t(l)->gclist) {
    Table *h = gco2t(l);
    Node *n, *limit = gnodelast(h);
    for (n = gnode(h, 0); n < limit; n++) {
      if (!ttisnil(gval(n)) && (iscleared(g, gkey(n)))) {
        setnilvalue(gval(n));  /* remove value ... */
      }
      if (ttisnil(gval(n)))  /* is entry empty? */
        removeentry(n);  /* remove entry from table */
    }
  }
}


/*
** clear entries with unmarked values from all weaktables in list 'l' up
** to element 'f'
*/
static void clearvalues (global_State *g, GCObject *l, GCObject *f) {
  for (; l != f; l = gco2t(l)->gclist) {
    Table *h = gco2t(l);
    Node *n, *limit = gnodelast(h);
    unsigned int i;
    for (i = 0; i < h->sizearray; i++) {
      TValue *o = &h->array[i];
      if (iscleared(g, o))  /* value was collected? */
        setnilvalue(o);  /* remove value */
    }
    for (n = gnode(h, 0); n < limit; n++) {
      if (!ttisnil(gval(n)) && iscleared(g, gval(n))) {
        setnilvalue(gval(n));  /* remove value ... */
        removeentry(n);  /* and remove entry from table */
      }
    }
  }
}


void luaC_upvdeccount (lua_State *L, UpVal *uv) {
  lua_assert(uv->refcount > 0);
  uv->refcount--;
  if (uv->refcount == 0 && !upisopen(uv))
    luaM_free(L, uv);
}


static void freeLclosure (lua_State *L, LClosure *cl) {
  int i;
  for (i = 0; i < cl->nupvalues; i++) {
    UpVal *uv = cl->upvals[i];
    if (uv)
      luaC_upvdeccount(L, uv);
  }
  luaM_freemem(L, cl, sizeLclosure(cl->nupvalues));
}


static void freeobj (lua_State *L, GCObject *o) {
  switch (o->tt) {
    case LUA_TPROTO: luaF_freeproto(L, gco2p(o)); break;
    case LUA_TLCL: {
      freeLclosure(L, gco2lcl(o));
      break;
    }
    case LUA_TCCL: {
      luaM_freemem(L, o, sizeCclosure(gco2ccl(o)->nupvalues));
      break;
    }
    /* RAVI changes */
    case RAVI_TFARRAY:
    case RAVI_TIARRAY:
    case LUA_TTABLE: luaH_free(L, gco2t(o)); break;
    case LUA_TTHREAD: luaE_freethread(L, gco2th(o)); break;
    case LUA_TUSERDATA: luaM_freemem(L, o, sizeudata(gco2u(o))); break;
    case LUA_TSHRSTR:
      luaS_remove(L, gco2ts(o));  /* remove it from hash table */
      luaM_freemem(L, o, sizelstring(gco2ts(o)->shrlen));
      break;
    case LUA_TLNGSTR: {
      luaM_freemem(L, o, sizelstring(gco2ts(o)->u.lnglen));
      break;
    }
    default: lua_assert(0);
  }
}


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


/*
** sweep at most 'count' elements from a list of GCObjects erasing dead
** objects, where a dead object is one marked with the old (non current)
** white; change all non-dead objects back to white, preparing for next
** collection cycle. Return where to continue the traversal or NULL if
** list is finished.
*/
static GCObject **sweeplist (lua_State *L, GCObject **p, lu_mem count) {
  global_State *g = G(L);
  int ow = otherwhite(g);
  int white = luaC_white(g);  /* current white */
  while (*p != NULL && count-- > 0) {
    GCObject *curr = *p;
    int marked = curr->marked;
    if (isdeadm(ow, marked)) {  /* is 'curr' dead? */
      *p = curr->next;  /* remove 'curr' from list */
      freeobj(L, curr);  /* erase 'curr' */
    }
    else {  /* change mark to 'white' */
      curr->marked = cast_byte((marked & maskcolors) | white);
      p = &curr->next;  /* go to next element */
    }
  }
  return (*p == NULL) ? NULL : p;
}


/*
** sweep a list until a live object (or end of list)
*/
static GCObject **sweeptolive (lua_State *L, GCObject **p) {
  GCObject **old = p;
  do {
    p = sweeplist(L, p, 1);
  } while (p == old);
  return p;
}

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


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

/*
** If possible, shrink string table
*/
static void checkSizes (lua_State *L, global_State *g) {
  if (g->gckind != KGC_EMERGENCY) {
    l_mem olddebt = g->GCdebt;
    if (g->strt.nuse < g->strt.size / 4)  /* string table too big? */
      luaS_resize(L, g->strt.size / 2);  /* shrink it a little */
    g->GCestimate += g->GCdebt - olddebt;  /* update estimate */
  }
}


static GCObject *udata2finalize (global_State *g) {
  GCObject *o = g->tobefnz;  /* get first element */
  lua_assert(tofinalize(o));
  g->tobefnz = o->next;  /* remove it from 'tobefnz' list */
  o->next = g->allgc;  /* return it to 'allgc' list */
  g->allgc = o;
  resetbit(o->marked, FINALIZEDBIT);  /* object is "normal" again */
  if (issweepphase(g))
    makewhite(g, o);  /* "sweep" object */
  return o;
}


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


static void GCTM (lua_State *L, int propagateerrors) {
  global_State *g = G(L);
  const TValue *tm;
  TValue v;
  setgcovalue(L, &v, udata2finalize(g));
  tm = luaT_gettmbyobj(L, &v, TM_GC);
  if (tm != NULL && ttisfunction(tm)) {  /* is there a finalizer? */
    int status;
    lu_byte oldah = L->allowhook;
    int running  = g->gcrunning;
    L->allowhook = 0;  /* stop debug hooks during GC metamethod */
    g->gcrunning = 0;  /* avoid GC steps */
    setobj2s(L, L->top, tm);  /* push finalizer... */
    setobj2s(L, L->top + 1, &v);  /* ... and its argument */
    L->top += 2;  /* and (next line) call the finalizer */
    L->ci->callstatus |= CIST_FIN;  /* will run a finalizer */
    status = luaD_pcall(L, dothecall, NULL, savestack(L, L->top - 2), 0);
    L->ci->callstatus &= ~CIST_FIN;  /* not running a finalizer anymore */
    L->allowhook = oldah;  /* restore hooks */
    g->gcrunning = running;  /* restore state */
    if (status != LUA_OK && propagateerrors) {  /* error while running __gc? */
      if (status == LUA_ERRRUN) {  /* is there an error object? */
        const char *msg = (ttisstring(L->top - 1))
                            ? svalue(L->top - 1)
                            : "no message";
        luaO_pushfstring(L, "error in __gc metamethod (%s)", msg);
        status = LUA_ERRGCMM;  /* error in __gc metamethod */
      }
      luaD_throw(L, status);  /* re-throw error */
    }
  }
}


/*
** call a few (up to 'g->gcfinnum') finalizers
*/
static int runafewfinalizers (lua_State *L) {
  global_State *g = G(L);
  unsigned int i;
  lua_assert(!g->tobefnz || g->gcfinnum > 0);
  for (i = 0; g->tobefnz && i < g->gcfinnum; i++)
    GCTM(L, 1);  /* call one finalizer */
  g->gcfinnum = (!g->tobefnz) ? 0  /* nothing more to finalize? */
                    : g->gcfinnum * 2;  /* else call a few more next time */
  return i;
}


/*
** call all pending finalizers
*/
static void callallpendingfinalizers (lua_State *L) {
  global_State *g = G(L);
  while (g->tobefnz)
    GCTM(L, 0);
}


/*
** find last 'next' field in list 'p' list (to add elements in its end)
*/
static GCObject **findlast (GCObject **p) {
  while (*p != NULL)
    p = &(*p)->next;
  return p;
}


/*
** move all unreachable objects (or 'all' objects) that need
** finalization from list 'finobj' to list 'tobefnz' (to be finalized)
*/
static void separatetobefnz (global_State *g, int all) {
  GCObject *curr;
  GCObject **p = &g->finobj;
  GCObject **lastnext = findlast(&g->tobefnz);
  while ((curr = *p) != NULL) {  /* traverse all finalizable objects */
    lua_assert(tofinalize(curr));
    if (!(iswhite(curr) || all))  /* not being collected? */
      p = &curr->next;  /* don't bother with it */
    else {
      *p = curr->next;  /* remove 'curr' from 'finobj' list */
      curr->next = *lastnext;  /* link at the end of 'tobefnz' list */
      *lastnext = curr;
      lastnext = &curr->next;
    }
  }
}


/*
** if object 'o' has a finalizer, remove it from 'allgc' list (must
** search the list to find it) and link it in 'finobj' list.
*/
void luaC_checkfinalizer (lua_State *L, GCObject *o, Table *mt) {
  global_State *g = G(L);
  if (tofinalize(o) ||                 /* obj. is already marked... */
      gfasttm(g, mt, TM_GC) == NULL)   /* or has no finalizer? */
    return;  /* nothing to be done */
  else {  /* move 'o' to 'finobj' list */
    GCObject **p;
    if (issweepphase(g)) {
      makewhite(g, o);  /* "sweep" object 'o' */
      if (g->sweepgc == &o->next)  /* should not remove 'sweepgc' object */
        g->sweepgc = sweeptolive(L, g->sweepgc);  /* change 'sweepgc' */
    }
    /* search for pointer pointing to 'o' */
    for (p = &g->allgc; *p != o; p = &(*p)->next) { /* empty */ }
    *p = o->next;  /* remove 'o' from 'allgc' list */
    o->next = g->finobj;  /* link it in 'finobj' list */
    g->finobj = o;
    l_setbit(o->marked, FINALIZEDBIT);  /* mark it as such */
  }
}

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



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


/*
** Set a reasonable "time" to wait before starting a new GC cycle; cycle
** will start when memory use hits threshold. (Division by 'estimate'
** should be OK: it cannot be zero (because Lua cannot even start with
** less than PAUSEADJ bytes).
*/
static void setpause (global_State *g) {
  l_mem threshold, debt;
  l_mem estimate = g->GCestimate / PAUSEADJ;  /* adjust 'estimate' */
  lua_assert(estimate > 0);
  threshold = (g->gcpause < MAX_LMEM / estimate)  /* overflow? */
            ? estimate * g->gcpause  /* no overflow */
            : MAX_LMEM;  /* overflow; truncate to maximum */
  debt = gettotalbytes(g) - threshold;
  luaE_setdebt(g, debt);
}


/*
** Enter first sweep phase.
** The call to 'sweeplist' tries to make pointer point to an object
** inside the list (instead of to the header), so that the real sweep do
** not need to skip objects created between "now" and the start of the
** real sweep.
*/
static void entersweep (lua_State *L) {
  global_State *g = G(L);
  g->gcstate = GCSswpallgc;
  lua_assert(g->sweepgc == NULL);
  g->sweepgc = sweeplist(L, &g->allgc, 1);
}


void luaC_freeallobjects (lua_State *L) {
  global_State *g = G(L);
  separatetobefnz(g, 1);  /* separate all objects with finalizers */
  lua_assert(g->finobj == NULL);
  callallpendingfinalizers(L);
  lua_assert(g->tobefnz == NULL);
  g->currentwhite = WHITEBITS; /* this "white" makes all objects look dead */
  g->gckind = KGC_NORMAL;
  sweepwholelist(L, &g->finobj);
  sweepwholelist(L, &g->allgc);
  sweepwholelist(L, &g->fixedgc);  /* collect fixed objects */
  lua_assert(g->strt.nuse == 0);
}


static l_mem atomic (lua_State *L) {
  global_State *g = G(L);
  l_mem work;
  GCObject *origweak, *origall;
  GCObject *grayagain = g->grayagain;  /* save original list */
  lua_assert(g->ephemeron == NULL && g->weak == NULL);
  lua_assert(!iswhite(g->mainthread));
  g->gcstate = GCSinsideatomic;
  g->GCmemtrav = 0;  /* start counting work */
  markobject(g, L);  /* mark running thread */
  /* registry and global metatables may be changed by API */
  markvalue(g, &g->l_registry);
  markmt(g);  /* mark global metatables */
  /* remark occasional upvalues of (maybe) dead threads */
  remarkupvals(g);
  propagateall(g);  /* propagate changes */
  work = g->GCmemtrav;  /* stop counting (do not recount 'grayagain') */
  g->gray = grayagain;
  propagateall(g);  /* traverse 'grayagain' list */
  g->GCmemtrav = 0;  /* restart counting */
  convergeephemerons(g);
  /* at this point, all strongly accessible objects are marked. */
  /* Clear values from weak tables, before checking finalizers */
  clearvalues(g, g->weak, NULL);
  clearvalues(g, g->allweak, NULL);
  origweak = g->weak; origall = g->allweak;
  work += g->GCmemtrav;  /* stop counting (objects being finalized) */
  separatetobefnz(g, 0);  /* separate objects to be finalized */
  g->gcfinnum = 1;  /* there may be objects to be finalized */
  markbeingfnz(g);  /* mark objects that will be finalized */
  propagateall(g);  /* remark, to propagate 'resurrection' */
  g->GCmemtrav = 0;  /* restart counting */
  convergeephemerons(g);
  /* at this point, all resurrected objects are marked. */
  /* remove dead objects from weak tables */
  clearkeys(g, g->ephemeron, NULL);  /* clear keys from all ephemeron tables */
  clearkeys(g, g->allweak, NULL);  /* clear keys from all 'allweak' tables */
  /* clear values from resurrected weak tables */
  clearvalues(g, g->weak, origweak);
  clearvalues(g, g->allweak, origall);
  luaS_clearcache(g);
  g->currentwhite = cast_byte(otherwhite(g));  /* flip current white */
  work += g->GCmemtrav;  /* complete counting */
  return work;  /* estimate of memory marked by 'atomic' */
}


static lu_mem sweepstep (lua_State *L, global_State *g,
                         int nextstate, GCObject **nextlist) {
  if (g->sweepgc) {
    l_mem olddebt = g->GCdebt;
    g->sweepgc = sweeplist(L, g->sweepgc, GCSWEEPMAX);
    g->GCestimate += g->GCdebt - olddebt;  /* update estimate */
    if (g->sweepgc)  /* is there still something to sweep? */
      return (GCSWEEPMAX * GCSWEEPCOST);
  }
  /* else enter next state */
  g->gcstate = nextstate;
  g->sweepgc = nextlist;
  return 0;
}


static lu_mem singlestep (lua_State *L) {
  global_State *g = G(L);
  switch (g->gcstate) {
    case GCSpause: {
      g->GCmemtrav = g->strt.size * sizeof(GCObject*);
      restartcollection(g);
      g->gcstate = GCSpropagate;
      return g->GCmemtrav;
    }
    case GCSpropagate: {
      g->GCmemtrav = 0;
      lua_assert(g->gray);
      propagatemark(g);
       if (g->gray == NULL)  /* no more gray objects? */
        g->gcstate = GCSatomic;  /* finish propagate phase */
      return g->GCmemtrav;  /* memory traversed in this step */
    }
    case GCSatomic: {
      lu_mem work;
      propagateall(g);  /* make sure gray list is empty */
      work = atomic(L);  /* work is what was traversed by 'atomic' */
      entersweep(L);
      g->GCestimate = gettotalbytes(g);  /* first estimate */;
      return work;
    }
    case GCSswpallgc: {  /* sweep "regular" objects */
      return sweepstep(L, g, GCSswpfinobj, &g->finobj);
    }
    case GCSswpfinobj: {  /* sweep objects with finalizers */
      return sweepstep(L, g, GCSswptobefnz, &g->tobefnz);
    }
    case GCSswptobefnz: {  /* sweep objects to be finalized */
      return sweepstep(L, g, GCSswpend, NULL);
    }
    case GCSswpend: {  /* finish sweeps */
      makewhite(g, g->mainthread);  /* sweep main thread */
      checkSizes(L, g);
      g->gcstate = GCScallfin;
      return 0;
    }
    case GCScallfin: {  /* call remaining finalizers */
      if (g->tobefnz && g->gckind != KGC_EMERGENCY) {
        int n = runafewfinalizers(L);
        return (n * GCFINALIZECOST);
      }
      else {  /* emergency mode or no more finalizers */
        g->gcstate = GCSpause;  /* finish collection */
        return 0;
      }
    }
    default: lua_assert(0); return 0;
  }
}


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


/*
** get GC debt and convert it from Kb to 'work units' (avoid zero debt
** and overflows)
*/
static l_mem getdebt (global_State *g) {
  l_mem debt = g->GCdebt;
  int stepmul = g->gcstepmul;
  if (debt <= 0) return 0;  /* minimal debt */
  else {
    debt = (debt / STEPMULADJ) + 1;
    debt = (debt < MAX_LMEM / stepmul) ? debt * stepmul : MAX_LMEM;
    return debt;
  }
}

/*
** performs a basic GC step when collector is running
*/
void luaC_step (lua_State *L) {
  global_State *g = G(L);
  l_mem debt = getdebt(g);  /* GC deficit (be paid now) */
  if (!g->gcrunning) {  /* not running? */
    luaE_setdebt(g, -GCSTEPSIZE * 10);  /* avoid being called too often */
    return;
  }
  do {  /* repeat until pause or enough "credit" (negative debt) */
    lu_mem work = singlestep(L);  /* perform one single step */
    debt -= work;
  } while (debt > -GCSTEPSIZE && g->gcstate != GCSpause);
  if (g->gcstate == GCSpause)
    setpause(g);  /* pause until next cycle */
  else {
    debt = (debt / g->gcstepmul) * STEPMULADJ;  /* convert 'work units' to Kb */
    luaE_setdebt(g, debt);
    runafewfinalizers(L);
  }
}


/*
** Performs a full GC cycle; if 'isemergency', set a flag to avoid
** some operations which could change the interpreter state in some
** unexpected ways (running finalizers and shrinking some structures).
** Before running the collection, check 'keepinvariant'; if it is true,
** there may be some objects marked as black, so the collector has
** to sweep all objects to turn them back to white (as white has not
** changed, nothing will be collected).
*/
void luaC_fullgc (lua_State *L, int isemergency) {
  global_State *g = G(L);
  lua_assert(g->gckind == KGC_NORMAL);
  if (isemergency) g->gckind = KGC_EMERGENCY;  /* set flag */
  if (keepinvariant(g)) {  /* black objects? */
    entersweep(L); /* sweep everything to turn them back to white */
  }
  /* finish any pending sweep phase to start a new cycle */
  luaC_runtilstate(L, bitmask(GCSpause));
  luaC_runtilstate(L, ~bitmask(GCSpause));  /* start new collection */
  luaC_runtilstate(L, bitmask(GCScallfin));  /* run up to finalizers */
  /* estimate must be correct after a full GC cycle */
  lua_assert(g->GCestimate == gettotalbytes(g));
  luaC_runtilstate(L, bitmask(GCSpause));  /* finish collection */
  g->gckind = KGC_NORMAL;
  setpause(g);
}

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