Re: [PATCH 00/13] [RFC] Rust support

[Peter Zijlstra]

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.