Re: [C++0x] contiguous bitfields race implementation

[Jason Merrill <jason at redhat dot com>]

On 09/02/2011 04:53 AM, Richard Guenther wrote:
> On Thu, Sep 1, 2011 at 5:19 PM, Jason Merrill<jason@redhat.com>  wrote:
> > I think it would make sense to expose this information to the back end
> > somehow.  A hook would do the trick: call it type_data_size or type_min_size
> > or some such, which in the C++ front end would return TYPE_SIZE
> > (CLASSTYPE_AS_BASE (t)) for classes or just TYPE_SIZE for other types.
>
> That's too late to work with LTO, you'd need to store that information
> permanently somewhere.

OK.

> Maybe move this whole C++ specific bitfield handling where it belongs,
> namely to the C++ frontend?

I don't think that is the way to go; C is adopting the same memory 
model, and this is the only sane thing to do with bit-fields.

> I suggest to always not re-use tail padding for now (I believe if your
> parent object is a COMPONENT_REF, thus, x.parent.bitfield,
> you can use the TYPE_SIZE vs. field-decl DECL_SIZE discrepancy
> to decide about whether the tail-padding was reused, but please
> double-check that ;)))

But you don't always have a COMPONENT_REF; you still need to avoid 
touching the tail padding when you just have a pointer to the type 
because it might be a base sub-object.

I wonder what would break if C++ just set TYPE_SIZE to the as-base size?

Jason