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Date: Wed, 20 Nov 2013 13:06:51 -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>,
        Btrfs BTRFS <linux-btrfs@vger.kernel.org>,
        David Smith <creamyfish@gmail.com>
Subject: Re: Triple parity and beyond
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On 11/20/2013 01:04 PM, Andrea Mazzoleni wrote:
> Hi Peter,
> 
>>> static inline uint64_t d2_64(uint64_t v)
>>> {
>>>         uint64_t mask = v & 0x0101010101010101U;
>>>         mask = (mask << 8) - mask;
>>
>> (mask << 7) I assume...
> No. It's "(mask << 8) - mask". We want to expand the bit at position 0
> (in each byte) to the full byte, resulting in 0xFF if the bit is at 1,
> and 0x00 if the bit is 0.
> 
> (0 << 8) - 0 = 0x00
> (1 << 8) - 1 = 0x100 - 1 = 0xFF
> 

Oh, right... it is the same as (v << 1) - (v >> 7) except everything is
shifted over one.

	-hpa