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bug#33887: 26.1; Emacs hangs for several seconds when going to the end o

From: Stefan Monnier
Subject: bug#33887: 26.1; Emacs hangs for several seconds when going to the end of an XML file in nXML mode
Date: Thu, 17 Jan 2019 17:57:07 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

> The profile (see below) blames syntax-ppss called by
> sgml-syntax-propertize, so I suspect commit 0055190, which added
> sgml-syntax-propertize-inside to sgml-syntax-propertize.

Hmm... actually, the syntax-ppss calls that take time are directly made
from within sgml-syntax-propertize rather than from within
sgml-syntax-propertize-inside (which doesn't even appear in your profile
(in my profile I get 8099 units of time in sgml-syntax-propertize, of
which 7611 in syntax-ppss and only 77 in sgml-syntax-propertize-inside).
The problem seems to come from the following syntax propertize rule:

     ;; Double quotes outside of tags should not introduce strings.
     ;; Be careful to call `syntax-ppss' on a position before the one we're
     ;; going to change, so as not to need to flush the data we just computed.
     ("\"" (0 (if (prog1 (zerop (car (syntax-ppss (match-beginning 0))))
                    (goto-char (match-end 0)))
                  (string-to-syntax "."))))

If I comment it out, the delay is *much* smaller.

The problem being that " are quite common characters in XML files, so
the regexp matches often and we call syntax-ppss each time, so we end up
calling syntax-ppss very often.

I'm trying to figure out how to avoid calling syntax-ppss for every
" character.  I'm thinking of looking at pairs of " chars and only do
extra work if there's a < or > between the two.


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