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Date: Wed, 20 Nov 2013 10:59:32 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Andrea Mazzoleni <amadvance@gmail.com>
CC: David Brown <david.brown@hesbynett.no>,
        Linux RAID Mailing List <linux-raid@vger.kernel.org>,
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        David Smith <creamyfish@gmail.com>
Subject: Re: Triple parity and beyond
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On 11/20/2013 10:56 AM, Andrea Mazzoleni wrote:
> 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
> 

Now, that doesn't sound like something that can get neatly meshed into
the Cauchy matrix scheme, I assume.  It is somewhat nice to have a
scheme which is arbitrarily expandable without having to fall back to
dual parity during the restripe operation.  It probably also reduces the
amount of code necessary.

	-hpa