Re: GNU Emacs 24 Performance Issues in 'add-text-properties

[Stefan Monnier]

> Actually, I wrote the commenting because font-lock-mode is too slow on large
> C/C++ files.

Performance of c-mode's highlighting on large files can indeed be
problematic.  This said, Emacs's own src/xdisp.c (1MB, 30K lines) is
a test case which Emacs maintainers use on a regular basis
(intentionally or not), so we try to fix those problems, and we
generally consider them as bugs.

If you have a file where Emacs's major mode suffers from performance
problems, please M-x report-emacs-bug.  It's not always easy to fix, but
we do try to find some solution.

> Thanks for your response. This does work well with no interference from
> font-lock-mode because I turn and keep font-lock-mode off when I edit C/C++.

When I try your code on my machine (on Emacs's src/xdisp.c, with
font-lock disabled), it indeed takes a few seconds to run in c-mode, but
is "instantaneous" in fundamental-mode.  Running it twice in succession in
c-mode, only the first run is slow.

Running it through M-x profiler-start, I get the following profile:

- command-execute                                                5900  98%
 - call-interactively                                            5900  98%
  - execute-extended-command                                     5773  96%
   - command-execute                                             5760  96%
    - call-interactively                                         5760  96%
     - mark-comments                                             5744  95%
      - while                                                    5740  95%
       - add-text-properties                                     5704  95%
        - c-after-change                                         5012  83%
         - let*                                                  4996  83%
          - unwind-protect                                       4996  83%
           - progn                                               4996  83%
            - save-restriction                                   4996  83%
             - let                                               4996  83%
              - unwind-protect                                   4992  83%
               - progn                                           4984  83%
                - save-excursion                                 4808  80%
                 - mapc                                          4808  80%
                  - #<lambda 0xe067877>                          4808  80%
                   - funcall                                     4808  80%
                    - c-neutralize-syntax-in-and-mark-CPP        4800  80%
                    ...

so as you can see, even though I did disable font-lock-mode, c-mode
still has an after-change-function installed (probably used because
syntactic info is also needed for indentation and is cached to avoid
terrible slowdowns in large files) and it takes the lion's share of
the time.


        Stefan