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$1@dough.gmane.org
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.

Replies

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