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
