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

From: Bill Gatliff <bgat@billgatliff.com>
To: Jean Delvare <khali@linux-fr.org>
Cc: David Brownell <david-b@pacbell.net>,
	Linux Kernel list <linux-kernel@vger.kernel.org>,
	Felipe Balbi <felipebalbi@users.sourceforge.net>,
	Haavard Skinnemoen <hskinnemoen@atmel.com>,
	Andrew Victor <andrew@sanpeople.com>,
	Tony Lindgren <tony@atomide.com>,
	eric miao <eric.y.miao@gmail.com>,
	Kevin Hilman <khilman@mvista.com>,
	Paul Mundt <lethal@linux-sh.org>,
	Ben Dooks <ben@trinity.fluff.org>
Subject: Re: [patch/rfc 2/4] pcf875x I2C GPIO expander driver
Date: Fri, 30 Nov 2007 08:09:56 -0600	[thread overview]
Message-ID: <475019B4.1090603@billgatliff.com> (raw)
In-Reply-To: <20071130143654.3fd39ffe@hyperion.delvare>

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@billgatliff.com