linux-kernel - Re: [patch/rfc 2/4] pcf875x I2C GPIO expander driver

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Date:	Fri, 30 Nov 2007 08:09:56 -0600
From:	Bill Gatliff <bgat@...lgatliff.com>
To:	Jean Delvare <khali@...ux-fr.org>
CC:	David Brownell <david-b@...bell.net>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Felipe Balbi <felipebalbi@...rs.sourceforge.net>,
	Haavard Skinnemoen <hskinnemoen@...el.com>,
	Andrew Victor <andrew@...people.com>,
	Tony Lindgren <tony@...mide.com>,
	eric miao <eric.y.miao@...il.com>,
	Kevin Hilman <khilman@...sta.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Ben Dooks <ben@...nity.fluff.org>
Subject: Re: [patch/rfc 2/4] pcf875x I2C GPIO expander driver

Jean Delvare wrote:
> Hi Bill,
>
> On Fri, 30 Nov 2007 07:04:10 -0600, Bill Gatliff wrote:
>   
>> Jean Delvare wrote:
>>     
>>> !!(value & (1 << offset))
>>> is more efficiently written
>>> (value >> offset) & 1
>>>       
>> ... but not more efficiently implemented.
>>
>> Your version requires code to do the shift on live data at runtime.  
>> David's version lets the compiler create the mask once, at compile-time.
>>     
>
> I don't understand. How could the compiler create the mask at
> compile-time when "offset" is only known at runtime?
>   

Oops.  :)  I was thinking of some different code.  Disregard.

/me got up too early this morning, after working too late last night!


b.g.

-- 
Bill Gatliff
bgat@...lgatliff.com

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