On Thu, 2017-10-19 at 13:58 +0200, Mattias Rönnblom wrote:
> Hi.
>
> I have this code:
>
> #include <stdatomic.h>
>
> int ready;
> int message;
>
> void send_x4711(int m) {
> message = m*4711;
> atomic_thread_fence(memory_order_release);
> ready = 1;
> }
>
> When I compile it with GCC 7.2 -O3 -std=c11 on x86_64 it produces the
> following code:
>
> send_x4711:
> .LFB0:
> .LVL0:
> imul edi, edi, 4711
> .LVL1:
> mov DWORD PTR ready[rip], 1
> mov DWORD PTR message[rip], edi
> ret
>
> I expected the store to 'message' and 'ready' to be in program order.
>
> Did I misunderstand the semantics of
> atomic_thread_fence+memory_order_release?
Yes. You must make your program data-race-free. This is required by
C11. No other thread can observe "ready" without a data race or other
synchronization, so the fence is a noop in this program snippet.
