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Date: Thu, 04 Dec 2014 08:21:04 +0100
From: Hans Petter Selasky <hps@selasky.org>
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To: John-Mark Gurney <jmg@funkthat.com>
Subject: Re: svn commit: r275468 - head/sys/dev/usb/controller
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On 12/04/14 01:54, John-Mark Gurney wrote:
> Hans Petter Selasky wrote this message on Wed, Dec 03, 2014 at 21:55 +0000:
>> Author: hselasky
>> Date: Wed Dec  3 21:55:44 2014
>> New Revision: 275468
>> URL: https://svnweb.freebsd.org/changeset/base/275468
>>
>> Log:
>>    Optimise the bit searching loops, by quickly skipping the 16 first set
>>    bits if all the 16 first bits are set. This way the worst case
>>    searching time is reduced from 32 to 16 cycles.
>
> You could use ffs instead:
> 	x = ffs(~map);
> 	if (x) {
> 		x--;
> 		/* normal code */
> 	}
>
> This has the benefit of using a single instruction on platforms that
> support it (bsfl on i386), though apparently, we haven't optimized this
> for all platforms...  arm has a version for int, but their ffsl does
> the same linear search instead of just calling ffs, or at least
> detecting if sizeof(long) == sizeof(int) and calling ffs...
>

Yes, I'm aware about that, but like you say it is not optimised for all 
platforms yet. So I'm not sure if it will give any benefit for the 
platform the driver is running on ....

I'll see if I can change the logic inside the C-version of ffs() and the 
do the swap like you suggest.

--HPS