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

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Message-ID: <20071130143654.3fd39ffe@hyperion.delvare>
Date:	Fri, 30 Nov 2007 14:36:54 +0100
From:	Jean Delvare <khali@...ux-fr.org>
To:	Bill Gatliff <bgat@...lgatliff.com>
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

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?

Anyway, I just checked and gcc (4.1.2 on x86_86) generates the same
code for both expressions, so there's not much to argue about. David,
feel free to ignore my comment if you prefer your implementation :)

-- 
Jean Delvare
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