[libstdc++/65033] Give alignment info to libatomic

[Hans-Peter Nilsson]

On Tue, 7 Apr 2015, Jonathan Wakely wrote:
> On 05/04/15 21:07 -0400, Hans-Peter Nilsson wrote:
> > On Fri, 3 Apr 2015, Jonathan Wakely wrote:
> >
> > > On 03/04/15 05:24 -0400, Hans-Peter Nilsson wrote:
> > > > On Thu, 2 Apr 2015, Hans-Peter Nilsson wrote:
> > > > > Why then use __alignof(_M_i) (the object-alignment)
> > > > > instead of _S_alignment (the deduced alas insufficiently
> > > > > increased type-alignment)?
> > >
> > > Isn't the object aligned to _S_alignment?
> >
> > We did specify that with the alignas.  Is the alignof always
> > exactly the same as an alignas, if one is specified?  (And will
> > that not change in a future amendment, standard and/or
> > implementation?)  Either way, is there a test-case to guard all
> > this?
>
> The language guarantees that's what alignas() does, if the argument is
> a valid alignment (which it must be if we derive it from some other
> type's alignment).

I'm more worried about alignof reporting a higher value for a
specific object than alignas to be wrong.

Your question quoted just below seems to indicate a similar
worry.

> > > Or is it different if a std::atomic<T> is included in some other
> > > struct and the user forces a different alignment on it? I don't think
> > > we really need to support that, users shouldn't be doing that.
> >
> > Why do we even need to ask those questions, when the patch takes
> > care of the per-type business without doubt?
>
> Well if we know the object is guaranteed to be correctly aligned we
> might not even need a fake, minimally aligned pointer. We could go
> back to passing &_M_i or just a null pointer to __atomic_is_lock_free.

The target has a say in __atomic_is_lock_free and could impose
some crazy pointer-value-specific condition like "only in the
first half of a page" (anything can happen to work around chip
errata) so I suggest staying with an alignment-generated
pointer.  No, I only dreamt that up, shoot it down if we only
care for current targets.

> The whole point of alignas() is to fix the alignment to a known value.

And alignof and __alignof to report *exactly* that, I hope?

brgds, H-P