Why set useless stack slice(above L->top) to nil during gc atomic phase ?

[张伟智 <robotk@...>]

Lua 5.2.4
In lgc.c propagatemark -> traversestack, line 502-506,
clear the slice in the range of [th->top + 1, th->stack + th->stacksize),
What if don't clear not-marked stack slice ?
Thanks.

 495 static lu_mem traversestack (global_State *g, lua_State *th) {
 496   int n = 0;
 497   StkId o = th->stack;
 498   if (o == NULL)
 499     return 1;  /* stack not completely built yet */
 500   for (; o < th->top; o++)  /* mark live elements in the stack */
 501     markvalue(g, o); 
 502   if (g->gcstate == GCSatomic) {  /* final traversal? */
 503     StkId lim = th->stack + th->stacksize;  /* real end of stack */
 504     for (; o < lim; o++)  /* clear not-marked stack slice */
 505       setnilvalue(o);
 506   } 
 507   else {  /* count call infos to compute size */
 508     CallInfo *ci;
 509     for (ci = &th->base_ci; ci != th->ci; ci = ci->next)
 510       n++;
 511   }   
 512   return sizeof(lua_State) + sizeof(TValue) * th->stacksize +
 513          sizeof(CallInfo) * n;
 514 }