#define checkGC(L,c)  \
  Protect( luaC_condGC(L,{L->top = (c);  /* limit of live values */ \
                          luaC_step(L); \
                          L->top = ci->top;})  /* restore top */ \
           luai_threadyield(L); )

/*只有当debt大于0时才正是开始执行GC,在实际运行时,此值通常小于负的一个步长, 随着新对象的创建,达到一个步长时,则启动GC */
#define luaC_condGC(L,c) \
	{if (G(L)->GCdebt > 0) {c;}; condchangemem(L);}   
#define luaC_checkGC(L)		luaC_condGC(L, luaC_step(L);)

/*
** 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) */  /*所有新创对象都会增加debt的值*/
  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 */ /*work代表单次step(mark或者sweep)的成本*/
    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);
  }
}

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); /*启动GC,并mark根部(全局变量表和内部的thread(又称协程, 每个协程有自己的独立逻辑栈))*/
      g->gcstate = GCSpropagate;
      return g->GCmemtrav;  /*返回work成本,用于增量GC控制*/
    }
    case GCSpropagate: {
      g->GCmemtrav = 0;
      lua_assert(g->gray);
      propagatemark(g);     /*Traverse灰色链表中的一个对象*/
       if (g->gray == NULL)  /* no more gray objects? */
        g->gcstate = GCSatomic;  /* finish propagate phase */ /*当灰色链接中没有对象后,迁移到下一个状态*/
      return g->GCmemtrav;  /* memory traversed in this step */  /*返回work成本,用于增量GC控制*/
    }
    case GCSatomic: {
      lu_mem work;
      int sw;
      propagateall(g);  /* make sure gray list is empty */  /*再次检查灰色链表是否还有对象*/
      work = atomic(L);  /* work is what was traversed by 'atomic' */ /*这块操作是不可中断的,主要是将一些写屏障导致的灰色对象继续traverse*/
      sw = entersweep(L);
      g->GCestimate = gettotalbytes(g);  /* first estimate */;
      return work + sw * GCSWEEPCOST;
    }
    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;
  }
}

      vmcase(OP_SETLIST) {
        int n = GETARG_B(i);
        int c = GETARG_C(i);
        unsigned int last;
        Table *h;
        if (n == 0) n = cast_int(L->top - ra) - 1;
        if (c == 0) {
          lua_assert(GET_OPCODE(*ci->u.l.savedpc) == OP_EXTRAARG);
          c = GETARG_Ax(*ci->u.l.savedpc++);
        }
        luai_runtimecheck(L, ttistable(ra));
        h = hvalue(ra);
        last = ((c-1)*LFIELDS_PER_FLUSH) + n;
        if (last > h->sizearray)  /* needs more space? */
          luaH_resizearray(L, h, last);  /* pre-allocate it at once */
        for (; n > 0; n--) {
          TValue *val = ra+n;
          luaH_setint(L, h, last--, val);
          luaC_barrierback(L, h, val);
        }
        L->top = ci->top;  /* correct top (in case of previous open call) */
        vmbreak;
      }

#define luaC_barrierback(L,p,v) {  \
	if (iscollectable(v) && isblack(p) && iswhite(gcvalue(v)))  \
	luaC_barrierback_(L,p); }

/*
** barrier that moves collector backward, that is, mark the black object
** pointing to a white object as gray again.
*/
void luaC_barrierback_ (lua_State *L, Table *t) {
  global_State *g = G(L);
  lua_assert(isblack(t) && !isdead(g, t));
  black2gray(t);  /* make table gray (again) */
  linkgclist(t, g->grayagain);
}