[dmd-concurrency] draft 7

[Fawzi Mohamed]

On 2-feb-10, at 04:59, Andrei Alexandrescu wrote:

> Fawzi Mohamed wrote:
>> On 2-feb-10, at 01:34, Andrei Alexandrescu wrote:
>>> Fawzi Mohamed wrote:
>>>> you don't need a barrier, you need a volatile statement to avoid  
>>>> compiler optimizations
>>>
>>> That depends on the platform. On contemporary Intel machines I  
>>> agree there's no need for a barrier.
>> as I explained in another post (that I wrote several hours ago, but  
>> unfortunately was just sent (I closed the computer too fast...) I  
>> think that you don't really understand what barriers do.
>
> It's possible I am not expressing myself clearly. My understanding  
> is described at length in this article:
>
> http://www.aristeia.com/Papers/DDJ_Jul_Aug_2004_revised.pdf

yes in that case you need a barrier because you expect that some other  
memory (the one that holds the singleton) is initialized *if* you are  
able to read the value of the pointer pointing to it.
Thus you need to introduce a partial ordering that guarantees that you  
will never see the pointer pointing to the memory before seeing the  
initialized memory.
You need a partial ordering in the memory write/reads.
This is not connected (in general, maybe on some hardware it is) to  
reading the updated value read from memory.
What breaks an access to a single atomic value is not the absence of a  
barrier, but putting it in a register and not bothering to load it  
each time (something that a compiler might to in some occasions as  
optimization).
I hope now the issue is clearer.

Fawzi