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Re: Native compilation: the bird-eye view

From: Stefan Monnier
Subject: Re: Native compilation: the bird-eye view
Date: Sun, 23 Aug 2020 14:20:06 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux)

>> I don't have a clear understanding of the problem this is trying to solve.
>> Could you summarize it?
> Sure,
> we need a rule to go from the source file to the eln filename.
> /xxx/foo.el -> foo-something1.eln
> /yyy/foo.el -> foo-something2.eln
> Given we store all eln "flatted" in a single directory indeed we need to
> have "something1" different from "something2".
> I then came-up with the current idea of using the source content as
> input of the hash, this should be robust.

Yes, the use of the content-hash should solve this problem "once and
forall".  Maybe the performance cost will force us to reevaluate this
decision, but for now I'm happy with it, so consider it a "solved problem".

> At the moment we are using 2 separate hashes to make the cleaning (easy)
> possible, this is a separate problem that's correct.  The idea is that
> with the two hashes when recompiling is easy to just remove the old file
> as you can identify using the first hash the old one.

I must be missing something: in which way does the filename-hash help here?
Oh, you mean we could look for <same-name-hash>-*.eln and remove them?

What I was thinking of doing was: when we generate a new .elc file, just
before saving the file over the old .elc file, we compute the hash of the
old .elc file and erase any matching .eln file.

> Additionally to that, at any point in time, if you have two file
> sharing the first hash you know that all but the most recent can
> be removed.

Then we could move the content-hash from the .eln filename to within the
.eln file itself.

Note that for the "system-wide" .eln files it can be trickier because we
may want to generate them once and then use them with Emacs installed at
difference locations, so we'd then want to try and avoid using the
absolute file name (and instead use file names that are relative to the
root of the Emacs install or something).

> I think this may be problematic as we could have two different Emacsen
> with two different load-path that are sharing the eln-cache folders.
> This may be a bit of patological case tho.

No, it's definitely not pathological.  It's pretty common for packages to
modify the `load-path` upon first use, so your `load-path` can change
long after startup.

Not to mention situations where you have several different configs
(e.g. I always have 2 Emacs sessions, one dedicated to Gnus and
a "general" one and they share *some* of the config but not all, so
they don't have the same `load-path`).


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