On 04/05/17 15:05, Jonathan Wakely wrote: > On 4 May 2017 at 13:58, Toebs Douglass wrote: >> On 04/05/17 14:44, Jonathan Wakely wrote: >>> Although there are also relaxed atomic operations (on atomic types) >>> which are not synchronization operations. Not all atomic operations >>> provide sequential consistency. >> >> If you had multiple cores issuing CAS with consume on a single variable, >> you'd get total order, I think? and you wouldn't with relaxed because >> with relaxed you wouldn't get a compiler barrier. > > Forget about consume. It's underspecified, poorly understood and > poorly implemented. GCC says it promotes consume to acquire. However, I *think* I understand consume (I almost certainly don't, of course :-) and it is something I do use in my code - I hope GCC gets it right one day. I may be completely wrong, but I might go out on a limb and say it's the difference between emitting no barriers (relaxed), a compiler barrier (consume), or a compiler and memory barrier (acquire).
