Re: The Emacs master is much slower than the emacs-27 branch.

[Gregory Heytings]

[Apparently this email, which I sent a few hours ago, did not reach the 
mailing list.  I apologize if you receive it twice.]

> I've got pretty much up to date copies of both the master and the 
> emacs-27 branches.
>
> When I run my customary benchmark (see below) on master's src/xdisp.c in 
> master, it's taking around 34s.  On emacs-27, it's taking 22s.
>
> That's a massive slowdown.  Does anybody know why this is happening? 
> Should I raise a bug report?
>
> The benchmark I run is this:
>
> (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)))
>
> (defun time-scroll (&optional arg)
>  (interactive "P")
>  (message "%s"
>           (time-it
>            (condition-case nil
>                (while t
>                  (if arg (scroll-down) (scroll-up))
>                  (sit-for 0))
>              (error nil)))))
>
> On a freshly loaded buffer, do M-: (time-scroll).

Did you use emacs -Q?  What is your platform?

Out of curiosity, I tried your benchmark on my computer (Debian GNU/Linux) 
for the following Emacs versions.  I ran the benchmark four times for each 
version, and the results were stable for each version:

Emacs 23.4.1: 5.2s (that's not a typo: five seconds)
Emacs 24.5.1: 28.2s
Emacs 25.2.2: 27.6s
Emacs 26.3: 16.9s
Emacs 27.1: 18.5s
Emacs 28.0.50: 19.5s

I see a slowdown between 27 and 28, but not a massive one.

FWIW, here are the results of that benchmark after disabling 
global-font-lock-mode:

Emacs 23.4.1: 1.1s
Emacs 24.5.1: 1.8s
Emacs 25.2.2: 1.8s
Emacs 26.3: 3.0s
Emacs 27.1: 3.3s
Emacs 28.0.50: 3.3s