GNU_PROPERTY_X86_* questions

[H.J. Lu]

On Mon, Oct 28, 2019 at 8:09 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> 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?

There is no requirement to use MMX when X87 or FXSR are used.
A program can use X87+XMM with XSAVE without using MMX nor
FXSR.

> 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.
>

ISA is different.   One can use AVX ISA with XMM, but without YMM.

-- 
H.J.