no need to ensure any stack space for panic function + some changes

in 'tryfuncTM' (small simplification)

ldo.c

@@ -1,5 +1,5 @@
 /*
-** $Id: ldo.c,v 2.133 2014/11/02 19:33:33 roberto Exp roberto $
+** $Id: ldo.c,v 2.134 2014/11/10 17:42:04 roberto Exp roberto $
 ** Stack and Call structure of Lua
 ** See Copyright Notice in lua.h
 */
@@ -123,9 +123,9 @@ l_noret luaD_throw (lua_State *L, int errcode) {
       if (g->panic) {  /* panic function? */
         seterrorobj(L, errcode, L->top);  /* assume EXTRA_STACK */
         if (L->ci->top < L->top)
-          L->ci->top = L->top + 2;
+          L->ci->top = L->top;  /* pushing msg. can break this invariant */
         lua_unlock(L);
-        g->panic(L);  /* call it (last chance to jump out) */
+        g->panic(L);  /* call panic function (last chance to jump out) */
       }
       abort();
     }
@@ -289,24 +289,18 @@ static StkId adjust_varargs (lua_State *L, Proto *p, int actual) {
 /*
 ** Check whether __call metafield of 'func' is a function. If so, put
 ** it in stack below original 'func' so that 'luaD_precall' can call
-** it. Raise an error if __call metafield is not a function. (The
-** check 'luaD_checkstack' avoids problems of errors in tag methods,
-** because both tag methods and error messages may need EXTRA_STACK.)
+** it. Raise an error if __call metafield is not a function.
 */
-static StkId tryfuncTM (lua_State *L, StkId func) {
+static void tryfuncTM (lua_State *L, StkId func) {
   const TValue *tm = luaT_gettmbyobj(L, func, TM_CALL);
   StkId p;
-  ptrdiff_t funcr = savestack(L, func);
-  if (!ttisfunction(tm)) {
-    luaD_checkstack(L, 1);
+  if (!ttisfunction(tm))
     luaG_typeerror(L, func, "call");
-  }
   /* Open a hole inside the stack at 'func' */
-  for (p = L->top; p > func; p--) setobjs2s(L, p, p-1);
-  incr_top(L);
-  func = restorestack(L, funcr);  /* previous call may change stack */
+  for (p = L->top; p > func; p--)
+    setobjs2s(L, p, p-1);
+  L->top++;  /* slot ensured by caller */
   setobj2s(L, func, tm);  /* tag method is the new function to be called */
-  return func;
 }
 
 
@@ -376,7 +370,9 @@ int luaD_precall (lua_State *L, StkId func, int nresults) {
       return 0;
     }
     default: {  /* not a function */
-      func = tryfuncTM(L, func);  /* retry with 'function' tag method */
+      luaD_checkstack(L, 1);  /* ensure space for metamethod */
+      func = restorestack(L, funcr);  /* previous call may change stack */
+      tryfuncTM(L, func);  /* try to get '__call' metamethod */
       return luaD_precall(L, func, nresults);  /* now it must be a function */
     }
   }