Share ebo helper throughout lib

[François Dumont]

Ok, thanks for those very interesting feedbacks.

On 25/07/2018 22:04, Jonathan Wakely wrote:
> On 25/07/18 21:53 +0200, Marc Glisse wrote:
>> On Wed, 25 Jul 2018, François Dumont wrote:
>>
>>> Â Â Â  It has already been noticed that there are 2 ebo helpers in the 
>>> lib. Here is a patch to use 1.
>>>
>>>
>>> Â Â Â  * include/bits/ebo_helper.h: New.
>>> Â Â Â  * include/Makefile.am: Add latter.
>>> Â Â Â  * include/Makefile.in: Regenerate.
>>> Â Â Â  * include/bits/hashtable_policy.h: Adapt.
>>> Â Â Â  * include/bits/shared_ptr_base.h: Adapt.
>>>
>>> Tested under linux x86_64.
>>>
>>> Ok to commit ?
>>
>> I don't think we support [[no_unique_address]] yet, but assuming we 
>> soon will and we enable it also for C++03 (at least with the 
>> __attribute__ syntax and/or in system headers), do you know if some 
>
> Yes, I hope we'll have that soon.
>
>> similar helper will still be necessary, with a simpler 
>> implementation, or if the attribute will magically get rid of it?
>
> We'll be able to replace some uses of EBO with the attribute
> (specifically, in std::tuple). In some places we'll want to only apply
> the attribute under the same conditions as we currently use the EBO,
> because otherwise we'd change the layout ("compressing" the member
> using the attribute where we previously didn't compress it).
>
> In some cases that will be OK because it's an internal implementation
> detail, or because we can replace e.g. _Sp_counted_deleter with
> _Sp_counted_deleter_v2. In other cases we must avoid any layout change
> (e.g. std::tuple).
>
> Concretely, we probably don't want to change the layout of the
> hashtable types. We could change the layout for shared_ptr
> _Sp_counted_xxx types (gaining some additional space savings for final
> types that currently can't be EBO'd) as long as we rename them to
> avoid the linker trying to combine incompatible definitions. So on
> that basis, maybe we don't want to bother changing the _Sp_ebo_helper
> for now.
>
>
>