On Mon, Apr 19, 2021 at 10:26:57AM +0200, Peter Zijlstra wrote: > https://godbolt.org/z/85xoPxeE5 That wants _Atomic on the seq definition for clang. > void writer(void) > { > atomic_store_explicit(&seq, seq+1, memory_order_relaxed); > atomic_thread_fence(memory_order_acquire); > > X = 1; > Y = 2; > > atomic_store_explicit(&seq, seq+1, memory_order_release); > } > > gives: > > writer: > adrp x1, .LANCHOR0 > add x0, x1, :lo12:.LANCHOR0 > ldr w2, [x1, #:lo12:.LANCHOR0] > add w2, w2, 1 > str w2, [x0] > dmb ishld > ldr w1, [x1, #:lo12:.LANCHOR0] > mov w3, 1 > mov w2, 2 > stp w3, w2, [x0, 4] > add w1, w1, w3 > stlr w1, [x0] > ret > > Which, afaict, is completely buggered. What it seems to be doing is > turning the seq load into a load-acquire, but what we really need is to > make sure the seq store (increment) is ordered before the other stores. Put differently, what you seem to want is store-acquire, but there ain't no such thing.
