Re: [PATCH 07/33] target/ppc: Implement cntlzdm

[Matheus K. Ferst]

On 22/10/2021 20:16, Richard Henderson wrote:
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> On 10/21/21 12:45 PM, matheus.ferst@eldorado.org.br wrote:
> > +uint64_t helper_CNTLZDM(uint64_t src, uint64_t mask)
> > +{
> > +    uint64_t sel_bit, count = 0;
> > +
> > +    while (mask != 0) {
> > +        sel_bit = 0x8000000000000000ULL >> clz64(mask);
> > +
> > +        if (src & sel_bit) {
> > +            break;
> > +        }
>
> We need to count how many mask are set left of mask & src.
> How about
>
>      sh = clz64(src & mask);
>      if (sh == 0) {
>          return 0;
>      }
>      return ctpop64(mask >> (64 - sh));
>
> which could probably be implemented inline relatively easy.

Thanks for this suggestion Richard, we'll try to inline it.

> > +static bool trans_CNTLZDM(DisasContext *ctx, arg_X *a)
> > +{
> > +    REQUIRE_64BIT(ctx);
> > +    REQUIRE_INSNS_FLAGS2(ctx, ISA310);
> > +#if defined(TARGET_PPC64)
> > +    gen_helper_CNTLZDM(cpu_gpr[a->ra], cpu_gpr[a->rt], cpu_gpr[a->rb]);
> > +#else
> > +    qemu_build_not_reached();
> > +#endif
> > +    return true;
> > +}
>
> Why the ifdef here?  Oh, I see. You could just use target_long in the 
> helper to avoid
> that.  And if not, you should move the helper into an ifdef too.

That's the same case of cfuged. There is a vector version of this 
instruction (vclzdm) that is not 64-bits only (at least on paper), so it 
should receive i64 and cannot be inside an ifdef(TARGET_PPC64). I'll add 
this info to the commit message.

If we dismiss the possibility of a future 32-bits implementation of 
PowerISA v3.1, we can move the helper inside the ifdef and add 
REQUIRE_64BITS in vclzdm/vctzdm (and vcfuged, vpdepd, vpextd, etc.)

--
Matheus K. Ferst
Instituto de Pesquisas ELDORADO <http://www.eldorado.org.br/>
Analista de Software
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