GNU bug report logs -
#74281
30.0.91; font-lock mode hangs on scrolling large Scheme file
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Reported by: Divya Ranjan <divya <at> subvertising.org>
Date: Sat, 9 Nov 2024 16:06:01 UTC
Severity: normal
Found in version 30.0.91
Done: Stefan Monnier <monnier <at> iro.umontreal.ca>
Bug is archived. No further changes may be made.
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> Ping! Divya, could you please try Stefan's suggestions and report
> back?
FWIW, I think the change is an improvement in any case, so we could
install it. Since it's in a "core" file, it might be too risky for
`emacs-30`, so I'm thinking of installing it in `master`, WDYT?
Stefan
>> Cc: Divya Ranjan <divya <at> subvertising.org>, 74281 <at> debbugs.gnu.org
>> From: Stefan Monnier <monnier <at> iro.umontreal.ca>
>> Date: Thu, 14 Nov 2024 11:56:37 -0500
>>
>> > Stefan, what tools do we have to investigate slowness related to
>> > parse-partial-sexp? Or maybe you have suggestions for how to speed up
>> > font-lock in this case?
>>
>> Hmm... `parse-partial-sexp` is normally expected to be fast, unless it
>> has to scan a lot of text.
>>
>> > Here's the profile I get while moving with C-p through the above file:
>>
>> A stab in the dark, but maybe the relevant call is the one in:
>>
>> (state (if (or syntax-ppss-table
>> (not font-lock--syntax-table-affects-ppss))
>> (syntax-ppss start)
>> ;; If `syntax-ppss' doesn't have its own syntax-table and
>> ;; we have installed our own syntax-table which
>> ;; differs from the standard one in ways which affects PPSS,
>> ;; then we can't use `syntax-ppss' since that would pollute
>> ;; and be polluted by its cache.
>> (parse-partial-sexp (point-min) start)))
>>
>> so the origin of the slowdown would be the (?#. "w 14") in the setting
>> below in `scheme.el`:
>>
>> (setq font-lock-defaults
>> '((scheme-font-lock-keywords
>> scheme-font-lock-keywords-1 scheme-font-lock-keywords-2)
>> nil t (("+-*/.<>=!?$%_&~^:" . "w") (?#. "w 14"))
>> beginning-of-defun
>> (font-lock-mark-block-function . mark-defun)))
>>
>> in which case, setting a `syntax-ppss-table` should fix the problem, tho
>> we could also fix it by being more careful: AFAICT the purpose of this
>> (?#. "w 14") is only to change the syntax of `#` from "prefix" to "word"
>> without changing the comment-related flags, so it shouldn't cause
>> `font-lock--syntax-table-affects-ppss` to be set.
>> So, we could solve it by improving the code that sets
>> `font-lock--syntax-table-affects-ppss`, as in the patch below.
>>
>>
>> Stefan
>>
>>
>> diff --git a/lisp/font-lock.el b/lisp/font-lock.el
>> index 203131bfd5a..f6299920c0a 100644
>> --- a/lisp/font-lock.el
>> +++ b/lisp/font-lock.el
>> @@ -1955,14 +1955,15 @@ font-lock-set-defaults
>> (dolist (char (if (numberp (car selem))
>> (list (car selem))
>> (mapcar #'identity (car selem))))
>> - (unless (memq (car (aref font-lock-syntax-table char))
>> - '(1 2 3)) ;"." "w" "_"
>> - (setq font-lock--syntax-table-affects-ppss t))
>> - (modify-syntax-entry char syntax font-lock-syntax-table)
>> - (unless (memq (car (aref font-lock-syntax-table char))
>> - '(1 2 3)) ;"." "w" "_"
>> - (setq font-lock--syntax-table-affects-ppss t))
>> - ))))
>> + (let ((old-syntax (aref font-lock-syntax-table char)))
>> + (modify-syntax-entry char syntax font-lock-syntax-table)
>> + (let ((new-syntax (aref font-lock-syntax-table char)))
>> + (unless (and (equal (cdr old-syntax) (cdr new-syntax))
>> + (memq (logand (car old-syntax) 255) '(1 2 3 6))
>> + (memq (logand (car new-syntax) 255) '(1 2 3 6))
>> + (equal (ash (car old-syntax) -8)
>> + (ash (car new-syntax) -8)))
>> + (setq font-lock--syntax-table-affects-ppss t))))))))
>> ;; (nth 4 defaults) used to hold `font-lock-beginning-of-syntax-function',
>> ;; but that was removed in 25.1, so if it's a cons cell, we assume that
>> ;; it's part of the variable alist.
>>
>>
>>
>>
>>
This bug report was last modified 140 days ago.
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