bug#21028: Performance regression in revision af1a69f4d17a482c359d98c00ef86fac835b5fac (Apr 2014).

[Clément Pit--Claudel]

On 2016-12-22 11:25, Eli Zaretskii wrote:
> I indeed see slow redisplay with your test files (but only after
> changing 'append' to 'prepend', because otherwise Emacs selects a
> different font for the arrows).

I see, thanks; prepend is clearly not what I want, since I trying to set these 
fonts as a fallback.

> But when I looked at the display
> closely, I found that the arrows are not displayed using Segoe UI
> Emoji font, they are displayed by some other font.  In fact, the only
> characters in that test which use the Segoe UI Emoji font are blanks!
> Did you try using "C-u C-x =" to see which characters are displayed
> using the Emoji font in your case?

I did; append works fine here. Everything is displayed with XITS Math, as 
expected.  prepend uses Segoe or XITS for all text, which I don't want.  It's 
good for testing, though, since I guess that otherwise your Emacs picks other 
fonts that are available on your machine.

> I think that's because Segoe UI Emoji doesn't have glyphs for the
> arrows, at least on my system.  So I modified your 21028.fixed.el
> recipe as follows:

Thanks, but I'm not sure I see why you're suggesting this :/ If you see the 
slowdown, then it seems that managed to reproduce the problem, right?  Is there 
anything preventing us from applying the fix that you described last month?

The example only shows arrows because I tried to make it minimal; my bug report 
is about the fact that, when I set up Emacs to display characters the way I 
want them, it gets slow.  I readily admit that changing the font set-up may 
make the problem go away, but it's not a satisfactory solution (I still get 
support requests from users of my packages about degraded performance due to 
prettify-symbols-mode, and it's hard to tell them to use different fonts).

Recall that the original bug report didn't even use symbols: I first found out 
about this when trying to edit text written in Chinese.  I ran into the problem 
regularly in various settings since then, until I reverted the faulty commit 
(and later reapplied the safe part of it that you isolated).  Since then I've 
never run into this issue.  I've also confirmed on other users' laptops that 
reverting the faulty commit fixes the issue.

The fixes that you suggested to my minimal example do work around the issue in 
that case (another work around for me is to just remove the Segoe UI line), but 
these fixes are not applicable to my full font configuration.  I can post my 
full configuration if you want to suggest improvements that would work around 
the issue in my specific case (alternatively, it's available here: 
https://github.com/cpitclaudel/.emacs.d/blob/master/init/fonts.el), but this 
seems like a waste of your time: there's no doubt that others will run into the 
same issues (indeed, they already do), and fixing their configurations one by 
one isn't practical (assuming we even hear about them).

Cheers,
Clément.

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