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Message-ID: <1795455197.20081210132912@emcraft.com>
Date: Wed, 10 Dec 2008 13:29:12 +0300
From: Yuri Tikhonov <yur@...raft.com>
To: Al Viro <viro@...IV.linux.org.uk>
CC: Geert Uytterhoeven <Geert.Uytterhoeven@...ycom.com>,
<linux-kernel@...r.kernel.org>, <linuxppc-dev@...abs.org>,
Detlev Zundel <dzu@...x.de>, Wolfgang Denk <wd@...x.de>,
Milton Miller <miltonm@....com>, Ilya Yanok <yanok@...raft.com>
Subject: Re[2]: [PATCH] fork_init: fix division by zero
Hello Al,
On Wednesday, December 10, 2008 you wrote:
> On Wed, Dec 10, 2008 at 01:01:13PM +0300, Yuri Tikhonov wrote:
>> >> + max_threads = mempages * PAGE_SIZE / (8 * THREAD_SIZE);
>> > ^^^^^^^^^^^^^^^^^^^^
>> >> +#endif
>>
>> > Can't this overflow, e.g. on 32-bit machines with HIGHMEM?
>>
>> The multiplier here is not PAGE_SIZE, but [PAGE_SIZE / (8 *
>> THREAD_SIZE)], and this value is expected to be rather small (2, 4, or
>> so).
> 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.
In any case, adding braces as follows probably would be better:
+ max_threads = mempages * (PAGE_SIZE / (8 * THREAD_SIZE));
Right ?
> Only assignment operators (and ?:, in a sense that a ? b : c ? d : e is
> parsed as a ? b : (c ? d : e)) are right-to-left. The rest is left-to-right.
Regards, Yuri
--
Yuri Tikhonov, Senior Software Engineer
Emcraft Systems, www.emcraft.com
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