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

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

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).

Those questions wouldn't even be asked if we used _S_alignment
for the fake-pointer too, just as a matter of defensive

Instead of changing every case in the condition to include sizeof why
not just do it afterwards using sizeof(_Tp), in the _S_alignment


We know sizeof(_Tp) == sizeof(corresponding integer type) because
that's the whole point of the conditionals! See attachment.

>       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?

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

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