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

[H.J. Lu]

On Thu, Feb 13, 2020 at 6:52 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 13.02.2020 15:17, H.J. Lu wrote:
> > 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.
>
> What is the point of a mode like what you outline in 3 above

We can use it to build a pure AVX512 object.

> anyway? Such a mode is even contrary to -O converting EVEX to
> VEX where possible.

Well, we should check if AVX is enabled when doing it.

-- 
H.J.