To: linux-kernel@vger.kernel.org
From: torvalds@transmeta.com (Linus Torvalds)
Subject: Re: Invalid compilation without -fno-strict-aliasing
Date: Wed, 26 Feb 2003 17:26:37 +0000 (UTC)
Organization: Transmeta Corporation
Message-ID: <b3itcd$2bi$1@penguin.transmeta.com>
References: <20030225234646.GB30611@bougret.hpl.hp.com>
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In article <20030225234646.GB30611@bougret.hpl.hp.com>,
Jean Tourrilhes  <jt@bougret.hpl.hp.com> wrote:
>
>	It looks like a compiler bug to me...

Why do you think the kernel uses "-fno-strict-aliasing"?

The gcc people are more interested in trying to find out what can be
allowed by the c99 specs than about making things actually _work_. The
aliasing code in particular is not even worth enabling, it's just not
possible to sanely tell gcc when some things can alias.

>	Some users have complained that when the following code is
>compiled without the -fno-strict-aliasing, the order of the write and
>memcpy is inverted (which mean a bogus len is mem-copied into the
>stream).

The "problem" is that we inline the memcpy(), at which point gcc won't
care about the fact that it can alias, so they'll just re-order
everything and claim it's out own fault.  Even though there is no sane
way for us to even tell gcc about it. 

I tried to get a sane way a few years ago, and the gcc developers really
didn't care about the real world in this area. I'd be surprised if that
had changed, judging by the replies I have already seen.

I'm not going to bother to fight it. 

			Linus
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