[PATCH] Fix PRs 52080, 52097 and 48124, rewrite bitfield expansion, enable the C++ memory model wrt bitfields everywhere

[Richard Guenther]

On Mon, 26 Mar 2012, Eric Botcazou wrote:

> > I think we indeed can't really in stor-layout, so the only place is very
> > likely get_bit_range.
> 
> Something like that for example.  I think the expmed.c hunk should be applied 
> to the 4.7 branch as well, because the new code in store_bit_field is quite 
> dangerous without it.
> 
> 
> 	* expmed.c (store_bit_field): Assert that BITREGION_START is a multiple
> 	of a unit before computing the offset in units.
> 	* expr.c (get_bit_range): Return the null range if the enclosing record
> 	is part of a larger bit field.

The patch looks reasonable - can we compute this backward from the
result of the outer get_inner_reference call and the outermost
field-decl though?  Or make get_inner_reference compute that while
analyzing the full reference and return a flag?  OTOH it shouldn't
be too expensive.

I agree the assert should go to the banch as well, though the code
only ever triggers there with --param allow-store-data-races=0.

Thanks,
Richard.