[libc-dev] [musl] Powerpc Linux 'scv' system call ABI proposal take 2

[Florian Weimer via libc-dev]

* Nicholas Piggin via Libc-alpha:

> We may or may not be getting a new ABI that will use instructions not 
> supported by old processors.
>
> https://sourceware.org/legacy-ml/binutils/2019-05/msg00331.html
>
> Current ABI continues to work of course and be the default for some 
> time, but building for new one would give some opportunity to drop
> such support for old procs, at least for glibc.

If I recall correctly, during last year's GNU Tools Cauldron, I think
it was pretty clear that this was only to be used for intra-DSO ABIs,
not cross-DSO optimization.  Relocatable object files have an ABI,
too, of course, so that's why there's a ABI documentation needed.

For cross-DSO optimization, the link editor would look at the DSO
being linked in, check if it uses the -mfuture ABI, and apply some
shortcuts.  But at that point, if the DSO is swapped back to a version
built without -mfuture, it no longer works with those newly linked
binaries against the -mfuture version.  Such a thing is a clear ABI
bump, and based what I remember from Cauldron, that is not the plan
here.

(I don't have any insider knowledge—I just don't want people to read
this think: gosh, yet another POWER ABI bump.  But the PCREL stuff
*is* exciting!)