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

["H.J. Lu" <hjl dot tools at gmail dot com>]

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.

-- 
H.J.