Hi,
Yep. At present to multiply for 2^-1 I'm using in C:
static inline uint64_t d2_64(uint64_t v)
{
uint64_t mask = v & 0x0101010101010101U;
mask = (mask << 8) - mask;
v = (v >> 1) & 0x7f7f7f7f7f7f7f7fU;
v ^= mask & 0x8e8e8e8e8e8e8e8eU;
return v;
}
and for SSE2:
asm volatile("movdqa %xmm2,%xmm4");
asm volatile("pxor %xmm5,%xmm5");
asm volatile("psllw $7,%xmm4");
asm volatile("psrlw $1,%xmm2");
asm volatile("pcmpgtb %xmm4,%xmm5");
asm volatile("pand %xmm6,%xmm2"); with xmm6 == 7f7f7f7f7f7f...
asm volatile("pand %xmm3,%xmm5"); with xmm3 == 8e8e8e8e8e...
asm volatile("pxor %xmm5,%xmm2");
where xmm2 is the intput/output
Ciao,
Andrea
On Wed, Nov 20, 2013 at 7:43 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> It is also possible to quickly multiply by 2^-1 which makes for an interesting R parity.
>
> Andrea Mazzoleni <amadvance@xxxxxxxxx> wrote:
>>Hi David,
>>
>>>> The choice of ZFS to use powers of 4 was likely not optimal,
>>>> because to multiply by 4, it has to do two multiplications by 2.
>>> I can agree with that. I didn't copy ZFS's choice here
>>David, it was not my intention to suggest that you copied from ZFS.
>>Sorry to have expressed myself badly. I just mentioned ZFS because it's
>>an implementation that I know uses powers of 4 to generate triple
>>parity, and I saw in the code that it's implemented with two
>>multiplication
>>by 2.
>>
>>Ciao,
>>Andrea
>
> --
> Sent from my mobile phone. Please pardon brevity and lack of formatting.
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