bug#70036: 30.0.50; Move file-truename to the C level

[Theodor Thornhill via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Theodor Thornhill <theo@thornhill.no> writes:

> Eli Zaretskii <eliz@gnu.org> writes:
>
>>> Cc: 70036@debbugs.gnu.org
>>> Date: Wed, 27 Mar 2024 22:43:25 +0100
>>> From:  Theodor Thornhill via "Bug reports for GNU Emacs,
>>>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
>>> 
>>> Felician Nemeth <felician.nemeth@gmail.com> writes:
>>> 
>>> >> I've been studying Eglots performance and have been noticing a couple
>>> >> of things that I find very interesting. It seems like `file-truename`
>>> >> is in the hot path
>>> >
>>> > I think Eglot repeatedly calls file-truename with the same argument (or
>>> > with an argument from a small set of filenames.)
>>> 
>>> This is true, to some extent.
>>
>> Can someone explain why Eglot needs to call file-truename in the first
>> place?
>

[...]

> handler. We could store the last received diagnostics in the server
> object, and do a quick lookup from known buffers there.
>
> Theo


It seems to be there from the very early days, like in 2018. My guess is
that we simply never noticed the performance issues here due to other
stuff like json parsing etc.

CC'd Joao to get some perspective whether or not it is intentional or
needed at all.

Theo