bug#50777: Dropping EIEIO from xref (for performance)

[Arthur Miller <arthur.miller@live.com>]

Dmitry Gutov <dgutov@yandex.ru> writes:

> Resending with the intended recipients (except for the maintainers who are
> automatically subscribed anyway, to spare the inboxes).
>
> With the recent discussion about our Lisp overhead when the search returns many
> matches, I went a couple more rounds to try to identify the hotspots.
>
> My main benchmark for that is
>
>   (benchmark 1 '(project-find-regexp "list"))
>
> in the Emacs repo. The 3 recent commits have reduced that time from 8s to 6.5s
> on my machine (under best conditions: warm disk cache and fresh Emacs
> session). The machine is pretty fast, so I figure we can multiply that timing by
> 2x, to imagine the average user experience.
>
> Having exhausted the obvious steps, I have looked at the difference between
> defclass and cl-defstruct in this respect. Earlier quick tests indicated there
> would be little to no difference, but now that I've wrote the full patch, it's
> significant.
>
> The attached patch drops the timing of the above benchmark further down to 4.1s
> here.
>
> Both allocation ('make-instance' vs auto-generated 'xref-make-*' struct
> constructors) and accessing the fields show increased performance, with the
> former showing the most improvement: I guess EIEIO's type checking added some
> extra overhead.
>
> That creates some incompatibility (third-party packages can't inherit from
> 'xref-location' anymore, or use 'make-instance', 'oref' or 'with-slots' with our
> values), but OTOH a quick survey of existing packages that integrate with Xref
> shows they wisely don't do that already. Not the largest, popular ones I have
> reviewed, at least.
>
> The ones that do can be updated to rely on the recommended public interface: the
> xref-make-* constructors and the generic functions to access the data. This will
> keep the code compatible with both new and previous versions of Xref.
>
> I'd like to push it soon, so users of Emacs 28 can enjoy the speedup.
>
> What do people think?

Yes please.