corrected some places where an old object could end up in front

of a new one + minimal documentation about this problem

lgc.c

@@ -1,5 +1,5 @@
 /*
-** $Id: lgc.c,v 2.92 2010/05/07 18:43:24 roberto Exp roberto $
+** $Id: lgc.c,v 2.93 2010/05/10 16:46:49 roberto Exp roberto $
 ** Garbage Collector
 ** See Copyright Notice in lua.h
 */
@@ -164,7 +164,7 @@ void luaC_checkupvalcolor (global_State *g, UpVal *uv) {
   lua_assert(!isblack(o));  /* open upvalues are never black */
   if (isgray(o)) {
     if (keepinvariant(g)) {
-      resetoldbit(o);
+      resetoldbit(o);  /* see MOVE OLD rule */
       gray2black(o);  /* it is being visited now */
       markvalue(g, uv->v);
     }
@@ -670,6 +670,7 @@ static Udata *udata2finalize (global_State *g) {
   u->uv.next = g->allgc;  /* return it to 'allgc' list */
   g->allgc = o;
   resetbit(u->uv.marked, SEPARATED);  /* mark that it is not in 'tobefnz' */
+  resetoldbit(o); /* see MOVE OLD rule */
   if (!keepinvariant(g))  /* not keeping invariant? */
     makewhite(g, o);  /* "sweep" object */
   return u;
@@ -729,7 +730,6 @@ void luaC_separateudata (lua_State *L, int all) {
       p = &gch(curr)->next;  /* don't bother with it */
     else {
       l_setbit(gch(curr)->marked, FINALIZEDBIT); /* won't be finalized again */
-      resetoldbit(curr); /* may be old when 'all' is true */
       *p = gch(curr)->next;  /* remove 'curr' from 'udgc' list */
       gch(curr)->next = *lastnext;  /* link at the end of 'tobefnz' list */
       *lastnext = curr;
@@ -756,7 +756,7 @@ void luaC_checkfinalizer (lua_State *L, Udata *u) {
     u->uv.next = g->udgc;  /* link it in list 'udgc' */
     g->udgc = obj2gco(u);
     l_setbit(u->uv.marked, SEPARATED);  /* mark it as such */
-    resetoldbit(obj2gco(u));
+    resetoldbit(obj2gco(u));  /* see MOVE OLD rule */
   }
 }
 
@@ -854,7 +854,6 @@ static void atomic (lua_State *L) {
 
 static l_mem singlestep (lua_State *L) {
   global_State *g = G(L);
-  /*lua_checkmemory(L);*/
   switch (g->gcstate) {
     case GCSpause: {
       if (!isgenerational(g))