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Re: Updating *.el files and native compilation

From: Andrea Corallo
Subject: Re: Updating *.el files and native compilation
Date: Mon, 10 May 2021 15:44:16 +0000
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux)

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Andrea Corallo <akrl@sdf.org>
>> Cc: emacs-devel@gnu.org
>> Date: Mon, 10 May 2021 07:35:53 +0000
>> >   . if, for some reason, Emacs loads an incompatible .eln file, then
>> >     some Lisp programs could crash the Emacs session, is that correct?
>> >     If so, how do we make sure such incompatible changes always cause
>> >     a new native compilation that yields a different file name for the
>> >     .eln file?
>> Yes but this should not happen, every change that can introduce an
>> incompatibility has to be accounted in the `comp-abi-hash' computation
>> and AFAIK ATM it is.
> Some changes don't require updating comp-abi-hash, but still create
> *.eln files with different hashes in its name.  AFAIU, that happens
> when the primitives don't change, but the .el file itself changes,
> isn't that so?


> In any case, are you saying that the only situations where loading and
> using a .eln file could crash Emacs are those which are handled by
> changing comp-abi-hash?


> If so, how can we make sure we never fail to update comp-abi-hash when
> that is needed?

The vast majority of the cases is when some subr is added or a signature
is changed and we account for this automatically.  The rest should be
only about changes specific to the eln load mechanism in comp.c.  I hope
who does that is very well aware of consequences but I don't know if
there's a way we can automatically guard against these changes breaking
the system (I guess there's not).



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