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

[Richard Guenther <richard.guenther@gmail.com>]

On Thu, Sep 1, 2011 at 5:19 PM, Jason Merrill <jason@redhat.com> wrote:
> On 09/01/2011 11:10 AM, Aldy Hernandez wrote:
>>>
>>> Basically you can only touch the size of the CLASSTYPE_AS_BASE variant.
>>> For many classes this will be the same as the size of the class itself.
>>
>> All this code is in the middle end, so we're language agnostic.
>>
>> What do we need here, a hook to query the front-end, or is it too late?
>> Or will we have to play it conservative and never touch the padding
>> (regardless of language)?
>
> 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.

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

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 discrepance
to decide about whether the tail-padding was reused, but please
double-check that ;)))

Richard.

> Jason
>
>