small optimization in 'lua_arith' (avoids overhead in the common case

of both arguments being numbers)

lapi.c

@@ -1,5 +1,5 @@
 /*
-** $Id: lapi.c,v 2.80 2009/06/17 17:52:57 roberto Exp roberto $
+** $Id: lapi.c,v 2.81 2009/06/17 18:38:54 roberto Exp roberto $
 ** Lua API
 ** See Copyright Notice in lua.h
 */
@@ -278,7 +278,11 @@ LUA_API int lua_rawequal (lua_State *L, int index1, int index2) {
 LUA_API void  lua_arith (lua_State *L, int op) {
   lua_lock(L);
   api_checknelems(L, 2);
-  luaV_arith(L, L->top - 2, L->top - 2, L->top - 1, op - LUA_OPADD + TM_ADD);
+  if (ttisnumber(L->top - 2) && ttisnumber(L->top - 1))
+    changenvalue(L->top - 2,
+                 luaO_arith(op, nvalue(L->top - 2), nvalue(L->top - 1)));
+  else
+    luaV_arith(L, L->top - 2, L->top - 2, L->top - 1, op - LUA_OPADD + TM_ADD);
   L->top--;
   lua_unlock(L);
 }