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From: Jeffrey Walton <noloader@gmail.com>
Date: Thu, 16 Apr 2020 11:21:56 -0400
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Subject: Re: [musl] Powerpc Linux 'scv' system call ABI proposal take 2
To: musl@lists.openwall.com
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On Wed, Apr 15, 2020 at 8:17 PM Nicholas Piggin <npiggin@gmail.com> wrote:
>
> Excerpts from Rich Felker's message of April 16, 2020 8:55 am:
> > On Thu, Apr 16, 2020 at 07:45:09AM +1000, Nicholas Piggin wrote:
> >> I would like to enable Linux support for the powerpc 'scv' instruction,
> >> as a faster system call instruction.
> >>
> >> This requires two things to be defined: Firstly a way to advertise to
> >> userspace that kernel supports scv, and a way to allocate and advertise
> >> support for individual scv vectors. Secondly, a calling convention ABI
> >> for this new instruction.
> >> ...
> > Note that any libc that actually makes use of the new functionality is
> > not going to be able to make clobbers conditional on support for it;
> > branching around different clobbers is going to defeat any gains vs
> > always just treating anything clobbered by either method as clobbered.
>
> Well it would have to test HWCAP and patch in or branch to two
> completely different sequences including register save/restores yes.
> You could have the same asm and matching clobbers to put the sequence
> inline and then you could patch the one sc/scv instruction I suppose.

Could GCC function multiversioning work here?
https://gcc.gnu.org/wiki/FunctionMultiVersioning

It seems like selecting a runtime version of a function is the sort of
thing you are trying to do.

Jeff