bug#46256: [feature/native-comp] AOT eln files ignored if run from build tree

[Andy Moreton <andrewjmoreton@gmail.com>]

On Sat 06 Mar 2021, Eli Zaretskii wrote:

>> From: Andy Moreton <andrewjmoreton@gmail.com>
>> Date: Sat, 06 Mar 2021 00:33:10 +0000
>> 
>> > This is a conservative choice, we may want to look only at
>> > '--with-wide-int' but I'm wondering if that's really the only sensitive
>> > input therefore having `system-configuration-options' in the equation
>> > looked safer to me at least for now.
>> 
>> I agree that it is a conservative choice, but that still misses features
>> that are enabled/disabled by default in the configury.
>> 
>> Thus the ABI hash should also include `system-configuration-features'.
>
> Which of the features you think might affect the ABI?  I reviewed them
> a couple of days ago and didn't find any I could convince myself
> should affect that, but maybe I missed something.

As a reasonably recent example, adding bignum support by default added
"GMP" to `system-configuration-features' when rebuilding after pulling
upstream changes, without the user changing the configure command line
saved in `system-configuration-options'.

That kind of change may not happen often, but for developers building
commits near the switchover point (e.g. git bisect), it adds a silent
incompatibility that may be hard to diagnose.

> Or maybe we should ask Andrea to try to provide a more detailed
> definition of what exactly constitutes the "ABI" in this case.

Adding some design notes notes to the repo on this and other aspects of
the design would indeed be useful.

    AndyM