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?
Those questions wouldn't even be asked if we used _S_alignment
for the fake-pointer too, just as a matter of defensive
programming.
> Instead of changing every case in the condition to include sizeof why
> not just do it afterwards using sizeof(_Tp), in the _S_alignment
> calculation?
Doh.
> We know sizeof(_Tp) == sizeof(corresponding integer type) because
> that's the whole point of the conditionals! See attachment.
>
> > @@ -216,7 +216,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > is_lock_free() const noexcept
> > {
> > // Produce a fake, minimally aligned pointer.
> > - void *__a = reinterpret_cast<void *>(-__alignof(_M_i));
> > + void *__a = reinterpret_cast<void *>(-_S_alignment);
> > return __atomic_is_lock_free(sizeof(_M_i), __a);
> > }
>
> If _M_i is aligned to _S_alignment then what difference does the
> change above make?
>
> It doesn't matter if the value is per-object if we've forced all such
> objects to have the same alignment, does it?
>
> 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?
> The attached patch against trunk should have the same result with much
> less effort.
>
> It doesn't include the changes to the reinterpret_cast<void *>
> expressions to produce a minimally aligned pointer, but I think this
> is progress, thanks :-)
Progress is good. :)
brgds, H-P
