> So on x86, both read and write barriers are complete no-ops, because > all reads are ordered, and all writes are ordered. So those only need > compiler barriers to guarantee that the compiler itself doesn't > re-order them. > > (Side note: earlier reads are also guaranteed to happen before later > writes, so it's really only writes that can be delayed past reads, but > we don't haev a barrier for that situation anyway. Also note that all > of this is not "real" ordering, but only a guarantee that the > user-visible semantics are AS IF they were actually ordered - if > things are local in cache, ordering doesn't matter because no external > CPU can *see* what the ordering was). However in the local case *FENCE still orders, so it's actually not a nop. Just normally you can't tell the difference in ordering semantics, but it's visible in side effects like RDTSC. -Andi
