do not try to ensure that 'sweepgc' points to a live object

when entering sweep phase ('entersweep'); that may be too
expensive to be done still inside the atomic step. Walking
one single object more often than not will work.

lgc.c

@@ -1,5 +1,5 @@
 /*
-** $Id: lgc.c,v 2.210 2015/11/03 18:10:44 roberto Exp roberto $
+** $Id: lgc.c,v 2.211 2015/12/10 18:12:30 roberto Exp roberto $
 ** Garbage Collector
 ** See Copyright Notice in lua.h
 */
@@ -754,14 +754,11 @@ static GCObject **sweeplist (lua_State *L, GCObject **p, lu_mem count) {
 /*
 ** sweep a list until a live object (or end of list)
 */
-static GCObject **sweeptolive (lua_State *L, GCObject **p, int *n) {
+static GCObject **sweeptolive (lua_State *L, GCObject **p) {
   GCObject **old = p;
-  int i = 0;
   do {
-    i++;
     p = sweeplist(L, p, 1);
   } while (p == old);
-  if (n) *n += i;
   return p;
 }
 
@@ -909,7 +906,7 @@ void luaC_checkfinalizer (lua_State *L, GCObject *o, Table *mt) {
     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, NULL);  /* change 'sweepgc' */
+        g->sweepgc = sweeptolive(L, g->sweepgc);  /* change 'sweepgc' */
     }
     /* search for pointer pointing to 'o' */
     for (p = &g->allgc; *p != o; p = &(*p)->next) { /* empty */ }
@@ -951,19 +948,16 @@ static void setpause (global_State *g) {
 
 /*
 ** Enter first sweep phase.
-** The call to 'sweeptolive' makes 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.
-** Returns how many objects it swept.
+** 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 int entersweep (lua_State *L) {
+static void entersweep (lua_State *L) {
   global_State *g = G(L);
-  int n = 0;
   g->gcstate = GCSswpallgc;
   lua_assert(g->sweepgc == NULL);
-  g->sweepgc = sweeptolive(L, &g->allgc, &n);
-  return n;
+  g->sweepgc = sweeplist(L, &g->allgc, 1);
 }
 
 
@@ -1064,12 +1058,11 @@ static lu_mem singlestep (lua_State *L) {
     }
     case GCSatomic: {
       lu_mem work;
-      int sw;
       propagateall(g);  /* make sure gray list is empty */
       work = atomic(L);  /* work is what was traversed by 'atomic' */
-      sw = entersweep(L);
+      entersweep(L);
       g->GCestimate = gettotalbytes(g);  /* first estimate */;
-      return work + sw * GCSWEEPCOST;
+      return work;
     }
     case GCSswpallgc: {  /* sweep "regular" objects */
       return sweepstep(L, g, GCSswpfinobj, &g->finobj);