New way to keep hints for table length

Instead of using 'alimit' for keeping the size of the array and at
the same time being a hint for '#t', a table now keeps these two
values separate. The Table structure has a field 'asize' with the
size of the array, while the length hint is kept in the array itself.
That way, tables with no array part waste no space with that field.
Moreover, the space for the hint may have zero cost for small arrays,
if the array of tags plus the hint still fits in a single word.

lgc.c

@@ -486,7 +486,7 @@ 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->alimit > 0);
+  int hasclears = (h->asize > 0);
   for (n = gnode(h, 0); n < limit; n++) {  /* traverse hash part */
     if (isempty(gval(n)))  /* entry is empty? */
       clearkey(n);  /* clear its key */
@@ -508,7 +508,7 @@ static void traverseweakvalue (global_State *g, Table *h) {
 ** Traverse the array part of a table.
 */
 static int traversearray (global_State *g, Table *h) {
-  unsigned asize = luaH_realasize(h);
+  unsigned asize = h->asize;
   int marked = 0;  /* true if some object is marked in this traversal */
   unsigned i;
   for (i = 0; i < asize; i++) {
@@ -604,7 +604,7 @@ static l_mem traversetable (global_State *g, Table *h) {
   }
   else  /* not weak */
     traversestrongtable(g, h);
-  return 1 + 2*sizenode(h) + h->alimit;
+  return 1 + 2*sizenode(h) + h->asize;
 }
 
 
@@ -790,7 +790,7 @@ static void clearbyvalues (global_State *g, GCObject *l, GCObject *f) {
     Table *h = gco2t(l);
     Node *n, *limit = gnodelast(h);
     unsigned int i;
-    unsigned int asize = luaH_realasize(h);
+    unsigned int asize = h->asize;
     for (i = 0; i < asize; i++) {
       GCObject *o = gcvalarr(h, i);
       if (iscleared(g, o))  /* value was collected? */

lobject.h

@@ -763,24 +763,12 @@ typedef union Node {
 	  checkliveness(L,io_); }
 
 
-/*
-** About 'alimit': if 'isrealasize(t)' is true, then 'alimit' is the
-** real size of 'array'. Otherwise, the real size of 'array' is the
-** smallest power of two not smaller than 'alimit' (or zero iff 'alimit'
-** is zero); 'alimit' is then used as a hint for #t.
-*/
-
-#define BITRAS		(1 << 7)
-#define isrealasize(t)		(!((t)->flags & BITRAS))
-#define setrealasize(t)		((t)->flags &= cast_byte(~BITRAS))
-#define setnorealasize(t)	((t)->flags |= BITRAS)
-
 
 typedef struct Table {
   CommonHeader;
   lu_byte flags;  /* 1<<p means tagmethod(p) is not present */
-  lu_byte lsizenode;  /* log2 of size of 'node' array */
-  unsigned int alimit;  /* "limit" of 'array' array */
+  lu_byte lsizenode;  /* log2 of number of slots of 'node' array */
+  unsigned int asize;  /* number of slots in 'array' array */
   Value *array;  /* array part */
   Node *node;
   struct Table *metatable;