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

[Paolo Bonzini]

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.

If we want to get clever, I should instead load ZMM_B(vec_len - 1)
directly into the *high* byte of t, using ZMM_L or ZMM_Q, and then
issue the extract2 myself.

Paolo