linux-kernel - Re: [PATCH] fork_init: fix division by zero

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Date:	Wed, 10 Dec 2008 11:25:12 -0600
From:	Scott Wood <scottwood@...escale.com>
To:	Yuri Tikhonov <yur@...raft.com>
Cc:	Al Viro <viro@...IV.linux.org.uk>, Wolfgang Denk <wd@...x.de>,
	Detlev Zundel <dzu@...x.de>, linux-kernel@...r.kernel.org,
	Milton Miller <miltonm@....com>, linuxppc-dev@...abs.org,
	Geert Uytterhoeven <Geert.Uytterhoeven@...ycom.com>,
	Ilya Yanok <yanok@...raft.com>
Subject: Re: [PATCH] fork_init: fix division by zero

On Wed, Dec 10, 2008 at 01:29:12PM +0300, Yuri Tikhonov wrote:
> On Wednesday, December 10, 2008 you wrote:
> > x * y / z is parsed as (x * y) / z, not x * (y / z).
> 
>  Here we believe in preprocessor: since all PAGE_SIZE, 8, and 
> THREAD_SIZE are the constants we expect it will calculate this.
> 
>  E.g. here is the result from this line as produced by cross-gcc 
> 4.2.2:
> 
>         lis     r9,0
>         rlwinm  r29,r29,2,16,29
>         stw     r29,0(r9)
> 
>  As you see - only rotate-left, i.e. multiplication to the constant.

Yes, and also note that it is masking the result by 0xfffc, to preserve
the effect of any overflow in (x * y).

-Scott
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