RE: [PATCH] C undefined behavior fix

[Bernard Dautrevaux]

> -----Original Message-----
> From: [e-mail] [mailto:[e-mail]]
> Sent: Wednesday, January 09, 2002 11:42 AM
> To: [e-mail]; [e-mail]; [e-mail]
> 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

--------------------------------------------
Bernard Dautrevaux
Microprocess Ingenierie
97 bis, rue de Colombes
92400 COURBEVOIE
FRANCE
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e-mail:	[e-mail]
		[e-mail]
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