RE: [PATCH] C undefined behavior fix

Bernard Dautrevaux
2002-01-09T10:51:30Z

> -----Original Message-----
> From: dewar@gnat.com [mailto:dewar@gnat.com]
> Sent: Wednesday, January 09, 2002 11:42 AM
> To: bernd@gams.at; gcc@gcc.gnu.org; linux-kernel@vger.kernel.org
> Subject: Re: [PATCH] C undefined behavior fix
>
>
> <<Especially if there are cases were this optimization yields
> a slower =
>
> access (or even worse indirect bugs).
> E.g. if the referenced "volatile short" is a hardware register and the
> access is multiplexed over a slow 8 bit bus. There are
> embedded systems
> around where this is the case and the (cross-)compiler has no way to
> know this (except it can be told by the programmer).
> >>
>
> Well that of course is a situation where the compiler is
> being deliberately
> misinformed as to the relative costs of various machine
> instructions, and
> that is definitely a problem. One can even imagine hardware
> (not such a hard
> feat, one of our customers had such hardware) where a word
> access works, but
> a byte access fails due to hardware shortcuts,

Tht's quite often the case with MMIO, and the only portable way to give a
hint to the compiler that it should refrain from optimizing is "volatile";
that's why I think the compiler should not do this optimization on volatile
objects at all.

Bernard

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Bernard Dautrevaux
Microprocess Ingenierie
97 bis, rue de Colombes
92400 COURBEVOIE
FRANCE
Tel: +33 (0) 1 47 68 80 80
Fax: +33 (0) 1 47 88 97 85
e-mail: dautrevaux@microprocess.com
b.dautrevaux@usa.net
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