How and when to use GCPRO?

[Leo <sdl.web@gmail.com>]

Hello,

I wonder if anyone can point me to a more detailed explanation of how
and when to use GCPRO other than what's in the elisp manual.

Is there any symptom in a gdb session that one should suspect a GCPRO is
missing?

For example, Could someone comment on the following function I just
wrote, which tries to add support for feature expressions like in common
lisp?

static Lisp_Object
eval_feature_expression (form)
     Lisp_Object form;
{
  Lisp_Object val, exp;
  struct gcpro gcpro1;

  if (! NILP(Fatom(form)))
    return NILP(Fmemq(form, Vfeatures)) ? Qnil:Qt;

  if (EQ (Fcar(form), Qnot))
    {
      if (XINT(Flength(form)) != 2)
        error ("Invalid feature expression");
      val = eval_feature_expression (Fcar(Fcdr(form)));
      return NILP(val) ? Qt:Qnil;
    }

  if (EQ (Fcar(form), Qand))
    {
      if (NILP (Fcdr (form)))
        error ("Invalid feature expression");
      val = eval_feature_expression (Fcar (Fcdr (form)));
      if (NILP (Fcdr (Fcdr (form))))
        return val;
      else if (NILP(val))
        return Qnil;
      else
        {
          GCPRO1(exp);
          exp = Fcons (Qand, (Fcdr(Fcdr(form))));
          val = eval_feature_expression (exp);
          UNGCPRO;
          return val;
        }
    }

  if (EQ (Fcar(form), Qor))
    {
      if (NILP (Fcdr (form)))
        error ("Invalid feature expression");
      val = eval_feature_expression (Fcar (Fcdr (form)));
      if (NILP (Fcdr (Fcdr (form))))
        return val;
      else if (NILP (val))
        {
          GCPRO1(exp);
          exp = Fcons (Qor, (Fcdr(Fcdr(form))));
          UNGCPRO;
          return eval_feature_expression (exp);
        }
      else
        return Qt;
    }
  error ("Invalid feature expression");
}

I am also reading the XEmacs internals manual on GCPROing but I have
little idea what is still true for GNU Emacs.

Thanks in advance and happy holidays.

Leo