[cpp-threads] Review of N2995 and resulting email threads

[Peter Dimov]

Paul E. McKenney wrote:
> Hello!
>
> It was suggested that I review N2995 and the related email threads and
> comment appropriately.

...

> o 1.10p4: Add note:
>
> [Note: Atomic and locking objects are not trivially
> copyable [3.9], so executions that access atomic
> or locking objects as non-atomic objects, for example
> through an array of character type, will have undefined
> behaviour - end note]
>
> --- I believe that this is straightforward enough to qualify
> --- as editorial.

I still think that this note is incorrect, as there are ways to read an 
atomic object without UB. In fact, you acknowledge this objection of mine 
in:

> o 1.10p14: Use of casts or unions can result in atomic and
> non-atomic accesses to the same memory location, so the "at least
> one of which is not atomic" must remain.  Peter Dimov pointed out
> that memcpy() can have the same effect.  Therefore, I am -not-
> OK with the suggested modifications to the first sentence --
> if any of the accesses are non-atomic, there can be a data race.
> It is not necessary for -all- of them to be non-atomic.