On Sun, Mar 18, 2018 at 9:47 PM, shivaram via golang-nuts <golang-nuts@googlegroups.com> wrote: > > I noticed that internally, the language implementation seems to rely on the > atomicity of reads to single-word values: > > https://github.com/golang/go/blob/bd859439e72a0c48c64259f7de9f175aae3b9c37/src/runtime/chan.go#L160
In the machine level, words like "atomicity" are overloaded with different meanings. I think what you are saying is that the runtime package is assuming that a load of a machine word will never read an interleaving of two different store of a machine word. It will always read the value written by a single store, though exactly which store it sees is unknown. This is true on all the processors that Go supports. > As I understand it, this atomicity is provided by the cache coherence > algorithms of modern architectures. Accordingly, the implementations in > sync.atomic of word-sized loads (e.g., LoadUint32 on 386 and LoadUint64 on > amd64) use ordinary MOV instructions: > > https://github.com/golang/go/blob/bd859439e72a0c48c64259f7de9f175aae3b9c37/src/sync/atomic/asm_386.s#L146 > > https://github.com/golang/go/blob/bd859439e72a0c48c64259f7de9f175aae3b9c37/src/sync/atomic/asm_amd64.s#L103 > > However, word-sized stores on these architectures use special instructions: > > https://github.com/golang/go/blob/bd859439e72a0c48c64259f7de9f175aae3b9c37/src/sync/atomic/asm_amd64.s#L133 > > Given that the APIs being implemented don't provide any global ordering > guarantees, what's the reason they can't be implemented solely with MOV? You are not giving the correct reason for why atomic.LoadUint32 and LoadUint64 can use ordinary MOV instructions on x86 processors. The LoadUint32, etc., functions guarantee much more than that they read a value that is not an interleaving a multiple writes. They are also load-acquire operations, meaning that when the function completes, the caller will see not only the value that was loaded but also all other values that some other processor core wrote before writing to the address being loaded (assuming the write was done using StoreUint32, etc.). It happens that on x86 you can implement load-acquire using a simple MOV instruction. Most other multicore processors use a more complex memory model, and their sync/atomic implementations are accordingly more complex. Ian -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
