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
a...@lycus.org
http://lycus.org
