Re: atomic operations for shared_ptr ?

[Florian Weimer <fweimer at redhat dot com>]

On 08/26/2013 01:04 PM, Marc Glisse wrote:
> On Mon, 26 Aug 2013, Florian Weimer wrote:
>
> > On 08/25/2013 01:52 PM, Oleg Endo wrote:
> >
> > > Just for my understanding, on targets that use 32 bit pointers and have
> > > a 64 bit atomic (compare and) exchange, that would be a no-brainer,
> > > wouldn't it?
> >
> > Unfortunately, no.  At least i686 requires 8-byte alignment for the
> > pair, while we currently only provide 4-byte alignment.  Similarly for
> > x86_64, where it's 16-byte alignment vs 8-byte.
>
> If changing the alignment in the library would help, please make sure
> there is a PR open with the ABI keyword. There may be some form of ABI
> change in 4.10, though I don't know if it will be large enough to allow
> this...

I'm not sure if we can actually reliably allocate objects with 16 byte 
alignment on the heap (for x86_64, that is).  If not, achieving this 
alignment would be rather invasive.

> (I hate binary compatibility taken too far)

This seems to require pretty far-reaching layout changes, and shared_ptr 
has been around as std::tr1::shared_ptr for a long time.

--
Florian Weimer / Red Hat Product Security Team