This is the mail archive of the
gcc@gcc.gnu.org
mailing list for the GCC project.
Re: atomic_thread_fence() semantics
- From: Torvald Riegel <triegel at redhat dot com>
- To: Mattias Rönnblom <hofors at lysator dot liu dot se>
- Cc: gcc at gcc dot gnu dot org
- Date: Fri, 20 Oct 2017 12:47:38 +0200
- Subject: Re: atomic_thread_fence() semantics
- Authentication-results: sourceware.org; auth=none
- Authentication-results: ext-mx07.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com
- Authentication-results: ext-mx07.extmail.prod.ext.phx2.redhat.com; spf=fail smtp.mailfrom=triegel at redhat dot com
- Dmarc-filter: OpenDMARC Filter v1.3.2 mx1.redhat.com 4218EC047B6D
- References: <351d54cc-4da1-dd20-aace-17c5b6244e1a@lysator.liu.se>
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.