No memory model

[qznc]

On Tuesday, 22 October 2013 at 12:46:16 UTC, Moritz Maxeiner 
wrote:
> On Tuesday, 22 October 2013 at 10:45:29 UTC, qznc wrote:
>> On Tuesday, 22 October 2013 at 10:44:41 UTC, qznc wrote:
>>> You are a little bit off. Sequential consistency (SC) does 
>>> require the compiler to insert locking.
>>
>> Arg. Does NOT require the compiler to insert locking. ;)
>
> Alright, I more or less paraphrased the definition of SC 
> (allegedly) made by Lamport, which is given on the Wikipedia 
> page (http://en.wikipedia.org/wiki/Sequential_consistency), but 
> in this case you answered your own question, I think. If SC 
> does only guarantee this, then you'd need the atomic operations 
> for correct synchronisation, as SC doesn't guarantee it?

The usual phrase is "sequential consistency for data-race-free 
programs". The advice for unaware programmers is to avoid race 
conditions and properly synchronize everything with locks. Only 
then you get SC.

My example contains a data-race, hence no SC (according to Java 
and C++ memory models).

If you really want to write such code, then you must understand 
the whole complexity of the memory model. Then you can figure 
out, if the behavior of your program is defined by language 
semantics. In my example, the memory models says "undefined 
behavior". If you make the necessary changes (atomic operations, 
memory fences, etc), then the memory model says "defined 
behavior: you might get pass the assert successfully or not at 
all".