Re: [Qemu-devel] [PATCH v2 5/6] target-ppc: add vprtyb[w/d/q] instructions

[David Gibson]

On Wed, Oct 26, 2016 at 10:22:10PM -0700, Richard Henderson wrote:
> On 10/26/2016 08:47 PM, David Gibson wrote:
> > > > +void helper_vprtybq(ppc_avr_t *r, ppc_avr_t *b)
> > > > +{
> > > > +    int i;
> > > > +    uint8_t s = 0;
> > > > +    for (i = 0; i < 16; i++) {
> > > > +        s ^= (b->u8[i] & 1);
> > > > +    }
> > > > +    r->u64[LO_IDX] = (!s) ? 0 : 1;
> > > > +    r->u64[HI_IDX] = 0;
> > > > +}
> > > > +
> > I think you can implement these better.  First mask with 0x01010101
> > (of the appropriate length) to extract the LSB bits of each byte.
> > Then XOR the two halves together, then quarters and so forth,
> > ln2(size) times to arrive at the parity.  This is similar to the usual
> > Hamming weight implementation.
> > 
> 
> You don't even have to mask with 0x01010101 to start.  Just fold halves til
> you get to the byte level and then mask with 1.

Good point.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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