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Message-Id: <200302261538.h1QFcAiF004085@eeyore.valparaiso.cl>
To: jt@hpl.hp.com
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Invalid compilation without -fno-strict-aliasing 
In-Reply-To: Your message of "Tue, 25 Feb 2003 15:46:46 PST."
             <20030225234646.GB30611@bougret.hpl.hp.com> 
Date: Wed, 26 Feb 2003 16:38:10 +0100
From: Horst von Brand <vonbrand@inf.utfsm.cl>
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Jean Tourrilhes <jt@bougret.hpl.hp.com> said:
> 	It looks like a compiler bug to me...
> 	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).
> 	Code (from linux/include/net/iw_handler.h) :
> --------------------------------------------
> static inline char *
> iwe_stream_add_event(char *	stream,		/* Stream of events */
> 		     char *	ends,		/* End of stream */
> 		     struct iw_event *iwe,	/* Payload */
> 		     int	event_len)	/* Real size of payload */
> {
> 	/* Check if it's possible */
> 	if((stream + event_len) < ends) {
> 		iwe->len = event_len;
> 		memcpy(stream, (char *) iwe, event_len);
> 		stream += event_len;
> 	}
> 	return stream;
> }
> --------------------------------------------
> 	IMHO, the compiler should have enough context to know that the
> reordering is dangerous. Any suggestion to make this simple code more
> bullet proof is welcomed.

The compiler is free to assume char *stream and struct iw_event *iwe point
to separate areas of memory, due to strict aliasing.
-- 
Dr. Horst H. von Brand                   User #22616 counter.li.org
Departamento de Informatica                     Fono: +56 32 654431
Universidad Tecnica Federico Santa Maria              +56 32 654239
Casilla 110-V, Valparaiso, Chile                Fax:  +56 32 797513
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