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Date: Wed, 9 Jan 2002 23:37:26 -0500
From: Tim Hollebeek <tim@hollebeek.com>
To: dewar@gnat.com
Cc: groudier@free.fr, jamagallon@able.es, Dautrevaux@microprocess.com,
        gcc@gcc.gnu.org, jtv@xs4all.nl, linux-kernel@vger.kernel.org,
        paulus@samba.org, tim@hollebeek.com, trini@kernel.crashing.org,
        velco@fadata.bg
Subject: Re: [PATCH] C undefined behavior fix
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> Nope, that's not enough, it's not that simple. Yes your
> example of a-a is of course straightforward but what about
> 
>     b = (a & 1) | (b & 1);
> 
> if a is volatile and b is known to be odd, can the read of a be eliminated?
> The answer should be no (and I think the standard guarantees this), but the
> reasoning is completely different from thinking about the fact that a may
> change unexpectedly, since obviously no matter what value comes from
> reading a, b will be set to 1 if b is known to be odd.

But presumably the transformation to:

a; b = 1;

is ok.  where 'a;' represents the operation "read the value a into a
register and then let that register die".
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