Re: [PATCH v2 2/3] target/ppc: Implement Vector Extract Mask

[Richard Henderson]

On 12/3/21 5:00 AM, Richard Henderson wrote:
> On 11/12/21 6:14 AM, matheus.ferst@eldorado.org.br wrote:
> > From: Matheus Ferst <matheus.ferst@eldorado.org.br>
> >
> > Implement the following PowerISA v3.1 instructions:
> > vextractbm: Vector Extract Byte Mask
> > vextracthm: Vector Extract Halfword Mask
> > vextractwm: Vector Extract Word Mask
> > vextractdm: Vector Extract Doubleword Mask
> > vextractqm: Vector Extract Quadword Mask
> >
> > Suggested-by: Richard Henderson <richard.henderson@linaro.org>
> > Signed-off-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
> > ---
> > v2:
> > - Applied rth suggestion to do_vextractm
> > ---
> >   target/ppc/insn32.decode            |  6 +++
> >   target/ppc/translate/vmx-impl.c.inc | 60 +++++++++++++++++++++++++++++
> >   2 files changed, 66 insertions(+)
> >
> > diff --git a/target/ppc/insn32.decode b/target/ppc/insn32.decode
> > index 9a28f1d266..639ac22bf0 100644
> > --- a/target/ppc/insn32.decode
> > +++ b/target/ppc/insn32.decode
> > @@ -419,6 +419,12 @@ VEXPANDWM       000100 ..... 00010 ..... 11001000010    
> > @VX_tb
> >   VEXPANDDM       000100 ..... 00011 ..... 11001000010    @VX_tb
> >   VEXPANDQM       000100 ..... 00100 ..... 11001000010    @VX_tb
> > +VEXTRACTBM      000100 ..... 01000 ..... 11001000010    @VX_tb
> > +VEXTRACTHM      000100 ..... 01001 ..... 11001000010    @VX_tb
> > +VEXTRACTWM      000100 ..... 01010 ..... 11001000010    @VX_tb
> > +VEXTRACTDM      000100 ..... 01011 ..... 11001000010    @VX_tb
> > +VEXTRACTQM      000100 ..... 01100 ..... 11001000010    @VX_tb
> > +
> >   # VSX Load/Store Instructions
> >   LXV             111101 ..... ..... ............ . 001   @DQ_TSX
> > diff --git a/target/ppc/translate/vmx-impl.c.inc 
> > b/target/ppc/translate/vmx-impl.c.inc
> > index 58aca58f0f..dd7337c2f2 100644
> > --- a/target/ppc/translate/vmx-impl.c.inc
> > +++ b/target/ppc/translate/vmx-impl.c.inc
> > @@ -1539,6 +1539,66 @@ static bool trans_VEXPANDQM(DisasContext *ctx, arg_VX_tb 
> > *a)
> >       return true;
> >   }
> > +static bool do_vextractm(DisasContext *ctx, arg_VX_tb *a, unsigned vece)
> > +{
> > +    const uint64_t elem_width = 8 << vece, elem_count_half = 8 >> vece;
> > +    TCGv_i64 t, b, tmp;
> > +
> > +    REQUIRE_INSNS_FLAGS2(ctx, ISA310);
> > +    REQUIRE_VECTOR(ctx);
> > +
> > +    t = tcg_const_i64(0);
> > +    b = tcg_temp_new_i64();
> > +    tmp = tcg_temp_new_i64();
> > +
> > +    for (int w = 0; w < 2; w++) {
> > +        get_avr64(b, a->vrb, w);
> > +
> > +        for (int i = 0; i < elem_count_half; i++) {
> > +            int in_bit = (i + 1) * elem_width - 1;
> > +            int out_bit = w * elem_count_half + i;
> > +
> > +            if (in_bit > out_bit) {
> > +                tcg_gen_shri_i64(tmp, b, in_bit - out_bit);
> > +            } else {
> > +                tcg_gen_shli_i64(tmp, b, out_bit - in_bit);
> > +            }
> > +            tcg_gen_andi_i64(tmp, tmp, 1 << out_bit);
> > +            tcg_gen_or_i64(t, t, tmp);
> > +        }
> > +    }
> > +    tcg_gen_trunc_i64_tl(cpu_gpr[a->vrt], t);
>
> Pardon me.  I realized after the fact that we can run the same algorithm as for mtvsrm (in 
> the next patch) in reverse.
>
>    & dup(1)
> .......a.......b.......c.......d.......e.......f.......g.......h
>    >> 32 - 4
> ...................................a.......b.......c.......d....
>    |
> .......a.......b.......c.......d...a...e...b...f...c...g...d...h
>    >> 16 - 2
> .....................a.......b.......c.......d...a...e...b...f..
>    |
> .......a.......b.....a.c.....b.d...a.c.e...b.d.f.a.c.e.g.b.d.f.h
>    >> 8 - 1
> ..............a.......b.....a.c.....b.d...a.c.e...b.d.f.a.c.e.g.
>    |
> .......a......ab.....abc....abcd...abcde..abcdef.abcdefgabcdefgh
>    & 0xff
> ........................................................abcdefgh
>
> where one of the two final masks can be done via deposit:
>
>      tcg_gen_andi_i64(hi, hi, 0xff);
>      tcg_gen_deposit_i64(lo, lo, hi, 8, 56);
>
> Which will reduce the instruction count of this implementation by half.

Oops, ENOCOFFEE.  Of course the input bit comes from the msb of the element, not the lsb. 
 Three different options:

(1) Begin with a shift of elem_count_half - 1, then do the above,

(2) Change the initial mask to the msb, then extract from elem_count_half - 1.

(3) Do left shifts so that we collect the bits at the msb of
    the word.  This probably results in the easiest concatenation
    in the end:

    tcg_gen_shri_i64(hi, hi, 64 - elem_count_half);
    tcg_gen_extract2_i64(lo, lo, hi, 64 - 2 * elem_count_half);


r~