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

[Eli Zaretskii]

> Cc: 21028@debbugs.gnu.org
> From: Clément Pit--Claudel <clement.pitclaudel@live.com>
> Date: Thu, 16 Mar 2017 17:23:13 -0400
> 
> >> In these fast example I'm still adding Symbola with 'unicode — but
> >> adding Ubuntu Mono with 'latin.
> > 
> > Ubuntu Mono _is_ the problem.  Symbola has very good coverage of
> > almost all Unicode blocks, so it is not the problem.
> 
> Ok, I think I understand better now.  But I'm still confused: why would the 
> following be slow, then?
> 
>     25.1/src/emacs -Q --eval "(set-fontset-font \"fontset-default\" 'unicode 
> (font-spec :name \"Symbola\") nil 'prepend)"
> 
> To clarify: it isn't slow with the example of plenty of ⇒ characters, but 
> it's still slow if I use a more diverse set of symbols or letters, like e.g.
> 
>     ⍺⍶ᷧΑΆἉἍᾍἏᾏᾉἋᾋᾹΆᾼἈἌᾌἎᾎᾈἊᾊΆᾺᾸαάἁἅᾅἇᾇἃᾃᾁᾱάᾴᾶᾷἀἄᾄἆᾆἂᾂᾀάὰᾲᾰᾳⱭⱰɑᶐꬰꭤɒ𝚨𝜜𝜶𝛂𝛢𝛼𝝖𝞐𝞪𝝰ᵅᶛ

If you show a concrete test case, I could look into it when I have
time.

> > What happens if you don't do this?  Which font is used for math
> > symbols?  Or are the problems with symbols other than math symbols?
> > IOW, I don't understand fully why you need to set up XITS Math in the
> > fontset, what is missing/incorrect/ugly if you don't?
> 
> I think this is due to my not explaining it properly, or trying to hard to 
> make minimal examples.  In can answer separately for myself and for the other 
> user configurations I've looked at.
> 
> For myself, it's a matter of monospace fonts: that is, I want all characters 
> to occupy the same width, so I don't use Symbola: instead, I use a version of 
> XITS Math edited with FontForge to have equal widths for all characters.

So you could instead edit Symbola to have a fixed width, and then you
wouldn't need to touch the default fontset, is that right?

> I also prefer as many characters to be displayed by the same font.

That already works in Emacs 25, unless I misunderstand what you mean.
The default font is used for all the characters it supports.  For
fallback fonts, those from fontset-default, you need to set up their
supported scripts for your liking, based on what they support and on
whether or not you like the glyphs they produce for each script they
support.  Using 'unicode' is only recommended for a few fonts whose
coverage is really almost universal.

> For other users, we had cases were Emacs displayed an empty rectangle until 
> we added an explicit set-fontset-font with Symbola.

This should be fixed in Emacs 25 out of the box, AFAIU.