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To: gentoo-amd64@lists.gentoo.org
From: Nikos Chantziaras <realnc@arcor.de>
Subject: [gentoo-amd64] Re: GCC-4.5.2 Has Serious Problems
Date: Fri, 01 Jul 2011 03:11:09 +0300
Organization: Lucas Barks
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References: <20110630174530.9bcbcd47.frank.peters@comcast.net>	<iuj17v$o6s$1@dough.gmane.org>	<iuj1o7$o6s$2@dough.gmane.org> <20110630200424.0d6d49ef.frank.peters@comcast.net>
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On 07/01/2011 03:04 AM, Frank Peters wrote:
> On Fri, 01 Jul 2011 02:44:36 +0300
> Nikos Chantziaras<realnc@arcor.de>  wrote:
>
>>>
>>> Your code is buggy, because you're breaking C's aliasing rules. You are
>>> not allowed to use a different pointer type to dereference a variable of
>>> a different type. Doing so results in undefined behavior.
>>
>
> Well, the error occurs only within the loop structure.  If
> I manually "unroll" the loop using the same pointers there
> is no error when compiled with "O2."
>
> IOW, the problem is how the loop is optimized under -O2.

When there's "undefined behavior", it really means just that: you cannot 
make any assumptions about when the result is going to be correct and 
when not.  It might as well depend on whether it was raining yesterday.