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

[Richard Henderson]

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.


r~