From: "Maciej Żenczykowski" <maze@google.com>
To: David Laight <David.Laight@aculab.com>
Cc: netdev@vger.kernel.org
Subject: Re: [PATCH] net: pfifo_fast - use ffs(x)-1 instead of array lookup
Date: Tue, 13 Mar 2012 13:37:25 -0700 [thread overview]
Message-ID: <CANP3RGe6zQzsuvZ=qpcWFou6EY99HhWq0ekf6UkfqKCvj+eiRA@mail.gmail.com> (raw)
In-Reply-To: <AE90C24D6B3A694183C094C60CF0A2F6026B6EA1@saturn3.aculab.com>
but a 16-bit constant will be better than a 32-bit one on 16-bit platforms
On Tue, Mar 13, 2012 at 9:55 AM, David Laight <David.Laight@aculab.com> wrote:
>
>> > + /* For i in 0..7 returns {-1, 0, 1, 0, 2, 0, 1, 0}[i] */
>> > + return ((26468 >> (i+i)) & 3) - 1;
>>
>> That expression doesn't seem quite right to me ...
>> return (int)(0x12131210 >> (i * 4)) & 3) - 1;
>> probably does what is wanted.
>
> Hmmm... I'm going blind - I read that as 'i+1' not 'i+i'.
> But using 'i * 4' and putting the constant in base 16
> make the code rather less obscure.
>
> David
>
>
--
Maciej A. Żenczykowski
Kernel Networking Developer @ Google
1600 Amphitheatre Parkway, Mountain View, CA 94043
tel: +1 (650) 253-0062
prev parent reply other threads:[~2012-03-13 20:37 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-03-13 5:12 [PATCH] net: pfifo_fast - use ffs(x)-1 instead of array lookup Maciej Żenczykowski
2012-03-13 5:13 ` Maciej Żenczykowski
2012-03-13 5:54 ` David Miller
2012-03-13 7:21 ` Maciej Żenczykowski
2012-03-13 7:23 ` Maciej Żenczykowski
2012-03-13 7:25 ` David Miller
2012-03-13 7:25 ` David Miller
2012-03-13 11:04 ` David Laight
2012-03-13 16:55 ` David Laight
2012-03-13 17:34 ` Eric Dumazet
2012-03-13 20:37 ` Maciej Żenczykowski [this message]
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