Messages in this thread | | | Date | Wed, 9 Jan 2002 22:59:35 +0100 (CET) | From | Gérard Roudier <> | Subject | RE: [PATCH] C undefined behavior fix |
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On Wed, 9 Jan 2002, Bernard Dautrevaux wrote:
> > -----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.
The C programming language and MMIO are two different things that must not be mixed.
'volatile' is not enough a portable a concept to deal with MMIO. You also need to tell about what is atomic and what is not atomic regarding accesses through the involved BUS, for example. As a result, you may use 'volatile' to avoid assembly for MMIO, but you want to put the code in architecture dependant code.
The atomicity issue also applies to memory-like access obviously, but the compiler is expected to know about the architecture capabilities. If this may differ for a given architecture family, then some compiler options should be made available.
Gérard.
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