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Re: [PATCH][4.8] C++ memory model bitfield handling rewrite


On Tue, 7 Feb 2012, Jakub Jelinek wrote:

> On Tue, Feb 07, 2012 at 04:01:31PM +0000, Jonathan Wakely wrote:
> > > What would it have said for -fabi-version=1 where for
> > > we place s.i and s.d into the same byte?
> > 
> > I think it says they shouldn't be in the same byte :-)
> 
> They don't, except for compatibility with the old ABI.
> I think easiest would be either to error out on -fabi-version=1
> mixed with an explicit option to request C++11 memory model,
> or just ignore it (perhaps with a warning), people who care about the strict
> memory model requirements just shouldn't use -fabi-version=1.
> Using atomic modifications of the byte that has parts of the tail
> padding bits used by another class would be IMHO an overkill.

I agree.  The question is of course whether the C++ memory model
folks thought about this issue in case there is an ABI that requires
this tail-padding reuse (they could have, for example, declared
that s.i and s.d are in the same bitfield - that would have made
things easy).

Richard.


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