I'm have been checking out the new fence API's in Java (Unsafe/VarHandle).
I understand how the higher level API are translated to the logical fences. E.g. release fence -> LoadStore+StoreStore. There are some great post including https://shipilev.net/blog/2014/on-the-fence-with-dependencies/ Great explanation how a release fence needs to be combined with a StoreLoad to preserve sequential consistency Also this post is great on the topic: https://preshing.com/20120913/acquire-and-release-semantics/ When I zoom into hardware things are a bit more blurry. X86 provides the following guarantees: Loads won't be reordered with older loads [LoadLoad] Stores won't be reordered with older stores (TSO) [StoreStore] Stores won't be reordered with older loads [LoadStore] One fundamental fence is the MFENCE because it will provide StoreLoad semantics. And on X86 the Unsafe.fullFence can be compiled to a MFENCE (in practice it uses the lock addl ... but that isn't relevant for this discussion). This will prevent stores to be reordered with older stores and will make sure the memory is visible to other CPU's (by waiting for the store buffer to be drained). The SFENCE was a bit more obscure to be because X86 proves TSO; so what is the point of adding a [StoreStore] fence is the platform provides it out of the box (so prevents stores to be reordered with older stores). Apparently there are certain instructions like those of SSE that are weakly ordered and these need to have this SFENCE. Ok. I can live with that. But the LFENCE I can't place. Initially I thought it would provide a similar fix as the SFENCE; so prevent load load reordering for weakly ordered instructions like those of SSE. But apparently the LFENCE is a very different beast. Could someone shed some light on the purpose of the LFENCE? -- You received this message because you are subscribed to the Google Groups "mechanical-sympathy" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/mechanical-sympathy/52527501-bffd-4a82-96fa-3fa618bec111%40googlegroups.com.
