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Message-ID: <87wo6fchh1.fsf@mid.deneb.enyo.de>
Date: Thu, 16 Apr 2020 22:18:18 +0200
From: Florian Weimer <fw@...eb.enyo.de>
To: Nicholas Piggin via Libc-alpha <libc-alpha@...rceware.org>
Cc: Rich Felker <dalias@...c.org>, Nicholas Piggin <npiggin@...il.com>, libc-dev@...ts.llvm.org, linuxppc-dev@...ts.ozlabs.org, musl@...ts.openwall.com
Subject: Re: Powerpc Linux 'scv' system call ABI proposal take 2
* 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!)
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