Gentoo Archives: gentoo-amd64

From: Nikos Chantziaras <realnc@×××××.de>
To: gentoo-amd64@l.g.o
Subject: [gentoo-amd64] Re: GCC-4.5.2 Has Serious Problems
Date: Fri, 01 Jul 2011 00:12:29
Message-Id: iuj3a1$1rd$
In Reply to: Re: [gentoo-amd64] Re: GCC-4.5.2 Has Serious Problems by Frank Peters
On 07/01/2011 03:04 AM, Frank Peters wrote:
> On Fri, 01 Jul 2011 02:44:36 +0300 > Nikos Chantziaras<realnc@×××××.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.


Subject Author
[gentoo-amd64] Re: GCC-4.5.2 Has Serious Problems Duncan <1i5t5.duncan@×××.net>