Re: cc-mode fontification feels random

[Alan Mackenzie]

Hello, Eli.

On Thu, Jun 10, 2021 at 20:22:50 +0300, Eli Zaretskii wrote:
> > From: Daniel Colascione <dancol@dancol.org>
> > Date: Thu, 10 Jun 2021 10:07:52 -0700
> > Cc: rudalics@gmx.at, monnier@iro.umontreal.ca, rms@gnu.org, 
> > emacs-devel@gnu.org

> > > Then I suggest to set it to 2 by default.

> > Performance is reasonable most of the time.

> Not IME.

I have measured CC Mode's scrolling performance using:

(defmacro time-it (&rest forms)
  "Time the running of a sequence of forms using `float-time'.
Call like this: \"M-: (time-it (foo ...) (bar ...) ...)\"."
  `(let ((start (float-time)))
    ,@forms
    (- (float-time) start)))

together with

M-: (time-it (scroll-up-window) (sit-for 0))

on regions of text which are not yet fontified.  My window has 65 lines
of buffer text.  Starting at the middle of xdisp.c, I see the following
timings for the first few scrolls:

   0.026s, 0.025s, 0.026s, 0.078s, 0.026s, 0.027s.

That is, with the exception of the fourth timing, the scroll operation
takes a little over 1/40 second.

This is in an Emacs-28 compiled with default optimisation, on a 4
year-old first generation Ryzen machine.

For me personally, this scrolling speed, in conjunction with
fast-but-imprecise-scrolling, is acceptable.  I also accept there are
people with slower machines.

-- 
Alan Mackenzie (Nuremberg, Germany).