[cpp-threads] Yet another visibility question

[Peter Dimov]

Boehm, Hans wrote:
> Partially correcting myself:
>
> I think the analysis below applies if x is an atomic, even if it is
> accessed with unordered operations.  If x is an ordinary variable,
> there is a race between the assert and the assignment to x.  And I
> think that's what we want here.

x is an ordinary variable in the example.

It's not clear what we want to happen in this case. What I want is for 
reference counting to be as efficient as possible (within the confines of 
the model, of course.)

If the causality requirements of the model effectively imply that in the 
real world the store to x can never race with the assert, then I wouldn't 
want to be forced to put a redundant ordered fetchadd(&y, 0) between the two 
just to satisfy the spec.


>>>> // x y initially zero
>>>>
>>>> // threads 1 to N:
>>>>
>>>> assert( x == 0 );
>>>>
>>>> if( fetchadd_release( &y, 1 ) == N - 1 ) {
>>>>    x = 1;
>>>> }