linux-kernel - Re: [PATCH] common implementation of iterative div/mod

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Date:	Tue, 13 May 2008 23:46:27 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Segher Boessenkool <segher@...nel.crashing.org>,
	Robert Hancock <hancockr@...w.ca>,
	Christian Kujau <lists@...dbynature.de>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	john stultz <johnstul@...ibm.com>
Subject: Re: [PATCH] common implementation of iterative div/mod

On Thu, 08 May 2008 16:16:41 +0100 Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> We have a few instances of the open-coded iterative div/mod loop, used
> when we don't expcet the dividend to be much bigger than the divisor.
> Unfortunately modern gcc's have the tendency to strength "reduce" this
> into a full mod operation, which isn't necessarily any faster, and
> even if it were, doesn't exist if gcc implements it in libgcc.
> 
> The workaround is to put a dummy asm statement in the loop to prevent
> gcc from performing the transformation.
> 
> This patch creates a single implementation of this loop, and uses it
> to replace the open-coded versions I know about.

Believe it or not, this patch causes one of my test machines to fail to
find its disk.

good dmesg: http://userweb.kernel.org/~akpm/dmesg-t61p.txt
bad dmesg: http://userweb.kernel.org/~akpm/dmesg-t61p-dead.txt
config: http://userweb.kernel.org/~akpm/config-t61p.txt
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