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[PATCH] x86: accept SSE* insns accessing MMX registers with ".arch .nommx"

Jan Beulich jbeulich@suse.com
Thu Feb 13 16:01:00 GMT 2020 
On 13.02.2020 16:54, H.J. Lu wrote:
> On Thu, Feb 13, 2020 at 6:50 AM Jan Beulich <jbeulich@suse.com> wrote:
>>
>> On 13.02.2020 15:15, H.J. Lu wrote:
>>> On Thu, Feb 13, 2020 at 5:39 AM Jan Beulich <jbeulich@suse.com> wrote:
>>>>
>>>> On 13.02.2020 12:43, H.J. Lu wrote:
>>>>> On Thu, Feb 13, 2020 at 1:24 AM Jan Beulich <jbeulich@suse.com> wrote:
>>>>>>
>>>>>> These are regular SSE/SSE2 insns, and hence should be accepted
>>>>>> irrespective of MMX as a feature being enabled. Since the MMX CPU
>>>>>> feature is intentionally not implicitly enabled by ".arch .sse", MMX
>>>>>
>>>>> This begs the question if ".arch .sse" should enable MMX.
>>>>
>>>> Well, I understood not doing so was intentional, for allowing
>>>> people to write SSE code which is free of MMX insns.
>>>
>>> True.
>>>
>>>>>> registers may be left unrecognized only when both MMX and SSE are
>>>>>> disabled. The errors previously generated on most of these insns in this
>>>>>> mode perhaps wasn't actually the biggest problem - in "noprefix" mode,
>>>>>> mm<N> source operands would silently have been taken as memory operands
>>>>>> of those names, i.e. wrong code was silently generated.
>>>>>
>>>>> This is the real problem.  What happens if YMM/ZMM are used with
>>>>> AVX disabled?
>>>>
>>>> I don't understand. When AVX is disabled, ymm<N> are expected to
>>>> be ordinary identifiers. Same for AVX512F and zmm<N> as well as
>>>> k<N>. When MMX is disabled the situation is different, due to
>>>> those SSE insns which access MMX registers.
>>>
>>> I think we should disallow naked MMX register in SSE instructions
>>> if cpummx is 0.
>>
>> This is not going to be acceptable in Intel syntax mode. Having to
>> use % prefixes there makes no sense at all, unless explicitly
>> asking for it by ".intel_syntax prefix".
> 
> I was referring to "The errors previously generated on most of these
> insns in this
> mode perhaps wasn't actually the biggest problem - in "noprefix" mode,
> mm<N> source operands would silently have been taken as memory operands
> of those names, i.e. wrong code was silently generated."
> 
> We shouldn't allow MMX registers with ".arch .sse" if MMX isn't enabled.

I disagree, and hence this patch: With ".arch .sse" _all_ SSE insns
should be legitimate to use.

Jan
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