GNU bug report logs - #55481
mostly nonrecursive printing [PATCH]

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Mattias Engdegård <mattiase <at> acm.org>
To: bug-gnu-emacs <at> gnu.org
Subject: mostly nonrecursive printing [PATCH]
Date: Tue, 17 May 2022 15:59:16 +0200

[Message part 1 (text/plain, inline)]

Printing deeply nested values typically crashes Emacs by running out of C stack since the printer uses C recursion to traverse nested data structures. This is a long-standing problem that keeps turning up; see bug#52753 for one example.

The attached patch eliminates most of the recursion: conses, vectors, records, hash tables and char tables are now traversed without consuming C stack. This should cover the vast majority of objects that form deep structures.

Of note:

- Performance seems to be slightly improved (about 2.5 % faster for printing a long list of symbols) but that's not really the point of the patch.

- The patch does not attempt to fix the bogus #N notation for circular lists (bug#55395) but tries to stay bug-compatible for easier comparison.

- Some special syntax is context-conditional: (\, X) is only printed as ,X if surrounded by a positive number of backquote forms. It's not clear what we gain from this; using the special syntax for the backquote, comma and comma-at forms unconditionally would simplify matter without any apparent inconvenience to the user. Right now, the patch does not remove recursion for printing these forms.

- This patch does not address reading nested values, where a similar problem exists.

[print-nonrec.diff (application/octet-stream, attachment)]

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