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

[Jan Beulich <jbeulich at suse dot com>]

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.

>  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.

Jan