bug#75322: SAFE_ALLOCA assumed to root Lisp_Objects/SSDATA(string)

[Eli Zaretskii <eliz@gnu.org>]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: pipcet@protonmail.com,  75322@debbugs.gnu.org
> Date: Sun, 05 Jan 2025 12:37:42 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> If you mean the two patches I sent with "these two", then no. I prefer
> >> using SAFE_ALLOCA_LISP because that introduces an exact root.
> >
> > I guess I'm confused, then.  The first patch replaces calls to
> > SAFE_NALLOCA by SAFE_ALLOCA_LISP, the second patch modifies
> > SAFE_NALLOCA to call igc_xnmalloc_ambig.  That's why I thought they
> > were alternatives.
> >
> > If they are not alternatives, then why did you replace SAFE_NALLOCA in
> > the first patch?
> 
> I checked other uses of SAFE_NALLOCA that were not yet mentioned, and
> found another problematic case. (Something with struct itree_node *,
> don't remember the function name, it's in some other mail). There were
> too many grep hits for SAFE_NALLOCA for me, so I shot with a canon :-).

OK.

So can we talk about the relative merits and demerits of using
SAFE_ALLOCA_LISP vs SAFE_NALLOCA?  First, why is it better to have an
exact root than an ambiguous root?  And second, SAFE_ALLOCA_LISP
conses a Lisp vector, which will increase GC pressure, so isn't
SAFE_NALLOCA preferable at least in some cases?