Re: cc-mode fontification feels random

[Daniel Colascione]

On 6/11/21 11:22 AM, Eli Zaretskii wrote:

> > Cc: rudalics@gmx.at, monnier@iro.umontreal.ca, rms@gnu.org,
> >   emacs-devel@gnu.org
> > From: Daniel Colascione <dancol@dancol.org>
> > Date: Fri, 11 Jun 2021 11:02:34 -0700
> >
> > > >      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.
> > > I suggest to compare these times with Emacs 23 to see how we
> > > regressed.
> > Regression is acceptable in exchange for correctness so long as absolute
> > performance is adequate. We're not using 80486s anymore.
> Here are my times using an optimized build of Emacs 27.2 on a 3.4GHz
> Core i7 box:
>
>    0.015625
>    0.03125
>    0.015625
>    0.046875
>    0.09375
>    0.0625
>    0.015625
>    0.03125
>    0.015625
>    0.03125
>    0.015625
>    0.03125
>
> You consider this to be adequate performance for a single
> window-scroll?  (I don't have an optimized build of Emacs 28, but
> there's no reason to believe it is faster; quite the opposite.)

native-comp?

> And here's the top part of the profile while running the above
> benchmark:
>
>    - redisplay_internal (C function)                                 159  65%
>     - jit-lock-function                                              158  65%
>      - jit-lock-fontify-now                                          158  65%
>       - jit-lock--run-functions                                      158  65%
>        - run-hook-wrapped                                            158  65%
>         - #<compiled -0x1ffffffff8a67860>                            158  65%
>         - font-lock-fontify-region                                  157  65%
>          - c-font-lock-fontify-region                               157  65%
>           - font-lock-default-fontify-region                        146  60%
>            - font-lock-fontify-keywords-region                      143  59%
>             - c-font-lock-declarations                               97  40%
>              - c-find-decl-spots                                     97  40%
>               - #<compiled -0x1ffffffff94b65d0>                      73  30%
>                - c-forward-decl-or-cast-1                            38  15%
>                 - c-forward-type                                     22   9%
>                  - c-check-qualified-type                             7   2%
>
> We can stick our heads in the sand as much as we want, but facts are
> stubborn things.

Hrm. That doesn't seem consistent with Alan's report that we spend a ton 
of time doing work like deciding whether a brace occurs at top-level. My 
question stands: what core facilities can we add to accelerate cc-mode's 
parsing here? There's got to be some efficiency we can gain here.