Re: [PATCH] aarch64: allow adding/removing just feature flags via .arch

[Marcus Shawcroft <marcus dot shawcroft at gmail dot com>]

On 21 October 2014 09:12, Jan Beulich <JBeulich@suse.com> wrote:

> I understand the desire for consistency with ARM, but before going
> that route I'd like to point out that there are shortcomings with that
> directive there, and a patch to eliminate those
> (https://sourceware.org/ml/binutils/2013-04/msg00070.html) didn't
> get accepted (in fact there never was any kind of response).
> Admittedly right now there are no features setting multiple bits in
> AArch64 yet, but I kind of suspect this will change over time. Hence
> what I'd minimally like to know is whether at least that level of
> divergence (i.e. using distinct values to enable/merge and to disable
> a feature) would be acceptable, or whether to go the dual model
> ("no" and "no-" prefixes having different meaning) also proposed as
> an alternative there.
>
> Jan
>

Hi Jan,  My (vague) recollection of the intent when .arch_extension
was added back in 2010 was that it should have incremental behaviour
and that seems to be consistent with what the documentation in
c-arm.texi says.  I've not studied the details of your patch
https://sourceware.org/ml/binutils/2013-04/msg00070.htm but I think
its intent is sane.   I think you should ping your original patch.
Richard is out until week after next so don;t expect an immediate
response.

For aarch64 I think we should add .arch_extension to allow incremental
changes to feature bits.  nofoo should have the effect of disabling
only foo and hence no need to add no-foo variants.

/Marcus