[PATCH 3/5] x86/APX: VMOVDQ{A,U} can also be expressed

[Cui, Lili]

> Subject: Re: [PATCH 3/5] x86/APX: VMOVDQ{A,U} can also be expressed
> 
> On 11.09.2024 09:50, Cui, Lili wrote:
> >> Interestingly unlike VROUND{P,S}{S,D} and VPERM{F,I}128, but like
> >> V{BROADCAST,EXTRACT,INSERT}{F,I}128 they weren't even present in the
> >> x86- 64-apx-egpr-inval testcase, hence why I overlooked that these,
> >> too, can be encoded, (once again) using suitable AVX512F counterparts.
> >>
> >
> > I found some of our previous discussions. It would be better if these
> optimizations could be put under the option instead of by default.
> >
> > 1. The compiler will do this during the backend instruction selection phase.
> Binutils should only do instruction translation, not instruction selection.
> 
> I'm afraid this again takes the compiler-only perspective. Please always keep in
> mind that hand-written assembly is where any of this matters. If we want ...
> 
> > 2. We can only convert some instructions, not all instructions. When users use
> eGPR-s illegally, some will report an error, while others will not, which is very
> confusing.
> > 3. Binutils needs to report errors for illegal instructions to ensure the
> correctness of the compiler.
> 
> ... a mode like this, we need an extra flag the compiler passes us.
> That could be the existing -f, requiring that to be made work first.
> This would have the advantage of asm() constructs then being treatable like
> hand-written assembly (once suitably made work), while other code could be
> checked as you say.
> 
> Right now the assumption is for the compiler to emit sane code.
> 
> > 4. I don't know if there are special cases where gcc doesn't want EVEX to be
> generated.
> 
> If such a need existed, insns would need marking accordingly by e.g.
> pseudo prefixes ({vex} / {vex3} for the particular insns this patch is about.
> 

Normally, binutils always downgrade EVEX code to VEX or shorter code by default, but upgrading the wrong VEX to another EVEX instruction by default will give people an unexpected first impression, since the apx spec indicates that this is illegal.
I am worried that this promotion will become more and more common, laying hidden dangers, and eventually it will get out of control.

Lili.

> Jan