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From: Richard Stallman <rms@gnu.org>
Newsgroups: gmane.emacs.devel
Subject: Re: Preventing stack overflows with alloca.
Date: Fri, 18 Jun 2004 23:19:15 -0400
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    Since old_value is not a lisp object here,
    we would need to fix (hack) GC like this:

      for (bind = specpdl; bind != specpdl_ptr; bind++)
	{
	  mark_object (bind->symbol);
	  if (bind->func != safe_alloca_unwind)
	    mark_object (bind->old_value);
	}

Or else use make_save_value to convert it to a clean Lisp object.
Then GC won't need changing.