Re: [PATCH] x86: fix AVX* dependencies of ".arch .nosse*"

["H.J. Lu" <hjl.tools@gmail.com>]

On Thu, Feb 13, 2020 at 5:44 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 13.02.2020 12:50, H.J. Lu wrote:
> > On Thu, Feb 13, 2020 at 1:23 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> Since ".arch .avx*" enables SSE*, disabling SSE* should also disable
> >> AVX*. Together with 7deea9aad866 ('x86: fix SSE4a dependencies of
> >> ".arch .nosse*"') I think this makes clear that the whole .arch logic
> >> needs an overhaul, such that the mechanism to enable features implies
> >> the reverse operation when disabling any, without having to modify two
> >> places. Arm64's approach may be worthwhile to consider cloning.
> >>
> >> Note that while commit 1848e567343e ("Update x86 CPU_XXX_FLAGS
> >> handling") introducing the testcase which needs fixing here explicitly
> >> says "Don't disable AVX nor AVX512 when disabling SSE", I don't see why
> >> this would be. Furthermore it also says "Don't disable AVX512 when
> >> disabling AVX", which too has been undone meanwhile (commit 89199bb5a027
> >> ["ix86: Disable AVX512F when disabling AVX2"], PR gas/24359). The only
> >> sensible (consistent) alternative therefore would be to avoid enabling
> >> SSE* with ".arch .avx*", like is done for SSE* wrt MMX (in turn leading
> >> to inconsistencies with SSE insns accessing MMX registers).
> >>
> >
> > The intended usages are to build an object:
> >
> > 1. Without MMX, but with SSE, AVX and AVX512.
> > 2. Without SEE nor MMX, but with AVX and AVX512.
> > 3. Without SSE, MMX, AVX, but with AVX512.
> >
> > enforced by assembler.
>
> For one - this isn't spelled out anywhere in the docs. And then it's
> also counter-intuitive. Plus your point 3 isn't true afaict, as per
> commit 89199bb5a027 mentioned above.

We should revisit it.

> >  Your patch doesn't help it.
>
> My patch gets the SSE/AVX interaction into the same state as your
> 89199bb5a027 did for the AVX/AVX512F, so improves consistency. MMX
> is different, as it acts on a different register file.


-- 
H.J.