Re: Redisplay slower in Emacs 28 than Emacs 27

[Jean Louis]

* Gregory Heytings via "Emacs development discussions. <emacs-devel@gnu.org> 
[2020-12-07 17:55]:
> 
> Following the "The Emacs master is much slower than the emacs-27 branch."
> thread, I tested Alan's benchmark on the master branch.  The platform I used
> is a 64-bit GNU/Linux computer, with GCC 10.2.
> 
> It turns out that the 10-15% slowdown of redisplay in Emacs 28, compared to
> Emacs 27, is due to a single character change, namely the "!" character
> removed in commit 165fd028.
> 
> I attach three graphs of the timings calculated on various revisions of
> master.  The benchmark ran ten times on each revision; Emacs was compiled
> with CFLAGS='-O2'.  "timing-1.png" shows that the slowdown happens between
> revision 68ff32a5 and 7445560d, "timing-2.png" narrows this to revisions
> 4c5043c5 to d3c73fbf, and "timing-3.png" identifies revision 165fd028 as the
> cause of the problem.

Great work! You may also document how you did that and maybe other
people can help in debugging this way "what went wrong".