From: "H. Peter Anvin" <hpa@zytor.com>
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
Date: Wed, 20 Nov 2013 13:06:51 -0800 [thread overview]
Message-ID: <528D246B.8050605@zytor.com> (raw)
In-Reply-To: <CAGKKSfVQhRX4=e+UGOATq5L02X0_muhsfgBzVc4KqT=dfTYSyg@mail.gmail.com>
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
next prev parent reply other threads:[~2013-11-20 21:07 UTC|newest]
Thread overview: 96+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-11-18 22:08 Triple parity and beyond Andrea Mazzoleni
2013-11-18 22:12 ` H. Peter Anvin
2013-11-18 22:35 ` Andrea Mazzoleni
2013-11-18 23:25 ` H. Peter Anvin
2013-11-19 10:16 ` David Brown
2013-11-19 17:36 ` Andrea Mazzoleni
2013-11-19 22:51 ` Drew
2013-11-20 0:54 ` Chris Murphy
2013-11-20 1:23 ` John Williams
2013-11-20 10:35 ` David Brown
2013-11-20 10:31 ` David Brown
2013-11-20 18:09 ` John Williams
2013-11-20 18:44 ` Andrea Mazzoleni
2013-11-21 6:15 ` Stan Hoeppner
2013-11-21 8:32 ` David Brown
2013-11-20 18:34 ` Andrea Mazzoleni
2013-11-20 18:43 ` H. Peter Anvin
2013-11-20 18:56 ` Andrea Mazzoleni
2013-11-20 18:59 ` H. Peter Anvin
2013-11-20 21:21 ` Andrea Mazzoleni
2013-11-20 19:00 ` H. Peter Anvin
2013-11-20 21:04 ` Andrea Mazzoleni
2013-11-20 21:06 ` H. Peter Anvin [this message]
2013-11-21 8:36 ` David Brown
2013-11-19 17:28 ` Andrea Mazzoleni
2013-11-19 20:29 ` Ric Wheeler
2013-11-20 16:16 ` James Plank
2013-11-20 19:05 ` Andrea Mazzoleni
2013-11-20 19:10 ` H. Peter Anvin
2013-11-20 20:30 ` James Plank
2013-11-20 21:23 ` Andrea Mazzoleni
2013-11-27 2:50 ` ronnie sahlberg
2013-11-20 21:28 ` H. Peter Anvin
2013-11-21 1:28 ` Stan Hoeppner
2013-11-21 2:46 ` John Williams
2013-11-21 6:52 ` Stan Hoeppner
2013-11-21 7:05 ` John Williams
2013-11-21 22:57 ` Stan Hoeppner
2013-11-21 23:38 ` John Williams
2013-11-22 9:35 ` Stan Hoeppner
2013-11-22 15:01 ` John Williams
2013-11-22 22:28 ` Stan Hoeppner
2013-11-22 23:07 ` NeilBrown
2013-11-23 3:46 ` Stan Hoeppner
2013-11-23 5:04 ` NeilBrown
2013-11-23 5:34 ` John Williams
2013-11-23 7:12 ` NeilBrown
2013-11-24 4:03 ` Stan Hoeppner
2013-11-24 5:14 ` John Williams
2013-11-24 21:13 ` Stan Hoeppner
2013-11-24 23:28 ` Rudy Zijlstra
[not found] ` <l6u3h9$l72$2@ger.gmane.org>
2013-11-25 2:04 ` Stan Hoeppner
2013-11-25 9:15 ` David Brown
2013-11-24 5:19 ` Russell Coker
2013-11-24 21:44 ` Stan Hoeppner
2013-11-24 22:31 ` Mark Knecht
2013-11-25 2:14 ` Russell Coker
2013-11-25 9:20 ` David Brown
2013-11-21 8:08 ` joystick
2013-11-22 0:30 ` Stan Hoeppner
2013-11-22 0:33 ` H. Peter Anvin
2013-11-22 0:45 ` David Brown
2013-11-21 9:07 ` David Brown
2013-11-21 9:54 ` Adam Goryachev
2013-11-21 10:32 ` David Brown
2013-11-22 8:12 ` Russell Coker
2013-11-25 18:23 ` Pasi Kärkkäinen
2013-11-22 8:13 ` Stan Hoeppner
2013-11-22 13:15 ` David Brown
2013-11-22 16:07 ` Stan Hoeppner
2013-11-22 22:59 ` NeilBrown
2013-11-23 17:39 ` David Brown
2013-11-22 16:50 ` Mark Knecht
2013-11-22 19:51 ` Duncan
2013-11-22 8:38 ` Stan Hoeppner
2013-11-22 13:24 ` David Brown
2013-11-28 7:16 ` Stan Hoeppner
2013-11-28 7:36 ` Russell Coker
2013-11-28 9:56 ` David Brown
2013-11-21 19:56 ` Piergiorgio Sartor
2013-11-19 18:12 ` Piergiorgio Sartor
2013-11-20 10:44 ` David Brown
2013-11-20 21:59 ` Piergiorgio Sartor
2013-11-21 10:13 ` David Brown
2013-11-21 17:37 ` Goffredo Baroncelli
2013-11-21 20:05 ` Piergiorgio Sartor
2013-11-21 20:31 ` David Brown
2013-11-21 20:52 ` Piergiorgio Sartor
2013-11-22 0:32 ` David Brown
2013-11-22 20:32 ` Piergiorgio Sartor
2013-11-26 18:10 ` joystick
2013-11-20 21:38 ` Andrea Mazzoleni
2013-11-20 22:29 ` Piergiorgio Sartor
2013-11-23 7:55 ` Andrea Mazzoleni
2013-11-23 22:10 ` Piergiorgio Sartor
2013-11-24 9:39 ` Andrea Mazzoleni
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