Re: [PATCH] x86: Add pmovzx/pmovsx patterns with SI/DI operands

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

On Sun, Sep 30, 2018 at 1:53 AM Marc Glisse <marc.glisse@inria.fr> wrote:
>
> On Sat, 29 Sep 2018, H.J. Lu wrote:
>
> > Add pmovzx/pmovsx patterns with SI and DI operands for pmovzx/pmovsx
> > instructions which only read the low 4 or 8 bytes from the source.
>
> Hello,
>
> I am wondering a few things (these are questions, I am not asking for
> changes):
>
> Should we change the builtin and make it take a shorter argument, so it is
> visible to gimple optimizers that the high part is unused? But then would
> that shorter type be v8qi (we don't really have that type) or di (risks
> trying to use general regs?)?

I think this may lead to other issues you pointed out.

> > +(define_insn "*sse4_1_<code>v8qiv8hi2<mask_name>"
> > +  [(set (match_operand:V8HI 0 "register_operand" "=Yr,*x,v")
> > +     (any_extend:V8HI
> > +       (vec_select:V8QI
> > +         (subreg:V16QI
> > +           (vec_concat:V2DI
> > +             (match_operand:DI 1 "nonimmediate_operand" "Yrm,*xm,vm")
> > +             (const_int 0)) 0)
> > +         (parallel [(const_int 0) (const_int 1)
> > +                    (const_int 2) (const_int 3)
> > +                    (const_int 4) (const_int 5)
> > +                    (const_int 6) (const_int 7)]))))]
>
> There is code in simplify-rtx.c that handles (vec_select (vec_concat x
> y) z) when vec_select only picks from x. We could extend it to handle an
> intermediate subreg/cast, which would yield something like:
> (any_extend:V8HI (subreg:V8QI (match_operand:DI)))
> or maybe even
> (any_extend:V8HI (match_operand:V8QI))
> Would this be likely to work? Is it desirable?
>

We need vector instructions with source memory or XMM operand in SI/DImode
only for these patterns.  Exposed it to simplify-rtx.c may lead to unexpected
results.  But we don't know for sure unless we try.

-- 
H.J.