Re: [PATCH 27/37] target/i386: Use tcg gvec ops for pmovmskb

[Richard Henderson]

On 9/14/22 23:59, Paolo Bonzini wrote:
> On Tue, Sep 13, 2022 at 10:17 AM Richard Henderson
> <richard.henderson@linaro.org> wrote:
> > On 9/12/22 00:04, Paolo Bonzini wrote:
> > > +    while (vec_len > 8) {
> > > +        vec_len -= 8;
> > > +        tcg_gen_shli_tl(s->T0, s->T0, 8);
> > > +        tcg_gen_ld8u_tl(t, cpu_env, offsetof(CPUX86State, xmm_t0.ZMM_B(vec_len 
> > > - 1)));
> > > +        tcg_gen_or_tl(s->T0, s->T0, t);
> > >        }
> >
> > The shl + or is deposit, for those hosts that have it,
> > and will be re-expanded to shl + or for those that don't:
> >
> >       tcg_gen_ld8u_tl(t, ...);
> >       tcg_gen_deposit_tl(s->T0, t, s->T0, 8, TARGET_LONG_BITS - 8);
>
> What you get from that is an shl(t, 56) followed by extract2 (i.e.
> SHRD). Yeah there are targets with a native deposit (x86 itself could
> add PDEP/PEXT support I guess) but I find it hard to believe that it
> outperforms a simple shl + or.

Perhaps the shl+shrd (or shrd+rol if the deposit is slightly different) is over-cleverness 
on my part in the expansion, and pdep requires a constant mask.

But for other hosts, deposit is the same cost as shift.


r~