GNU_PROPERTY_X86_* questions

[Jan Beulich]

On 28.10.2019 15:50,  H.J. Lu  wrote:
> On Fri, Oct 25, 2019 at 8:05 AM Jan Beulich <jbeulich@suse.com> wrote:
>>
>> On 10.10.2019 17:16,  H.J. Lu  wrote:
>>> On Thu, Oct 10, 2019 at 6:33 AM Jan Beulich <jbeulich@suse.com> wrote:
>>>>
>>>> H.J.,
>>>>
>>>> I think I'm seeing a number of issues with the handling of these
>>>> properties in gas, but without the original commit referencing a
>>>> specification I can't really determine if some or all of these
>>>> are actually "working as designed".
>>>>
>>>> Some of the points are:
>>>> - What is the actual meaning of a set bit? I.e. should e.g. -O
>>>
>>> https://github.com/hjl-tools/x86-psABI/wiki/X86-psABI
>>>
>>>>   converting between ISAs actually be reflected in the produced
>>>>   note?
>>>
>>> It records what are in the object file.
>>>
>>>> - What is the rule of ISAs to get (or not get) a flag defined?
>>>>   E.g. why are there AVX, AVX2, and FMA flags, but no XOP nor
>>>>   FMA4 ones?
>>>
>>> One can propose them.
>>>
>>>> - What is the rule of register sets getting feature flags
>>>>   defined? E.g. why are there XMM etc flags but there's no mask
>>>>   register one?
>>>
>>> One can propose them.
>>
>> The document above does not explain in any way what is supposed to
>> force on the individual GNU_PROPERTY_X86_FEATURE_2_* flags. While
>> one can reasonably deduce this for GNU_PROPERTY_X86_ISA_1_* (albeit
>> there's at least one bug in gas), but not for these. IOW it in
>> particular does not answer ...
>>
>>>> - What are the rules for the feature flags actually getting set?
>>>>   I find it puzzling that there are straight use
>>>>   i.has_reg{x,y,z}mm flags (which could be easily done away
>>>>   with), but for MMX quite a bit of extra logic is involved.
>>
>> ... this question.
>>
>>> MMX is set for pure MMX related instructions.
>>
>> Which only shift the question to "what is a pure MMX related
>> instruction". I'd assume one touching any MMX register. But I'd
>> also assume the same for e.g. GNU_PROPERTY_X86_FEATURE_2_XMM wrt
>> XMM registers, yet that's not spelled out anywhere. In turn only
>> the _exact_ spelling of this determines if there are further gas
>> bugs here.
> 
> GNU_PROPERTY_X86_FEATURE_2 describes processor capabilities
> in terms of processor states.

I.e. processor registers? If so, how is e.g. FXSR different from
X87+XMM, and how come there's a separate MMX when its state is a
subset of X87? Further, if so, how come the MMX handling is more
complicated than the XMM one? It should all be tied to what
registers an insn uses then, shouldn't it?

If I'm understanding your reply correctly, then at least for this
item I think I can see a fairly clear picture of the conditions
under which adding a new flag would make sense to be proposed.
But the same continues to not be true for the ISA side of things.

Jan