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.
