Something needs to happen with shared, and soon.

[Alex Rønne Petersen]

On 14-11-2012 15:14, Andrei Alexandrescu wrote:
> On 11/14/12 1:19 AM, Walter Bright wrote:
>> On 11/13/2012 11:56 PM, Jonathan M Davis wrote:
>>> Being able to have double-checked locking work would be valuable, and
>>> having
>>> memory barriers would reduce race condition weirdness when locks
>>> aren't used
>>> properly, so I think that it would be desirable to have memory barriers.
>>
>> I'm not saying "memory barriers are bad". I'm saying that having the
>> compiler blindly insert them for shared reads/writes is far from the
>> right way to do it.
>
> Let's not hasten. That works for Java and C#, and is allowed in C++.
>
> Andrei
>
>

I need some clarification here: By memory barrier, do you mean x86's 
mfence, sfence, and lfence? Because as Walter said, inserting those 
blindly when unnecessary can lead to terrible performance because it 
practically murders pipelining.

(And note that you can't optimize this either; since the dependencies 
memory barriers are supposed to express are subtle and not detectable by 
a compiler, the compiler would always have to insert them because it 
can't know when it would be safe not to.)

-- 
Alex Rønne Petersen
alex at lycus.org
http://lycus.org