[PATCH 3/5] x86: Define atomic_full_barrier using __sync_synchronize

[Uros Bizjak]

On Mon, Sep 8, 2025 at 2:38 PM Uros Bizjak <ubizjak@gmail.com> wrote:
>
> For x86_64 targets, __sync_synchronize emits a full 64-bit
> 'LOCK ORQ $0x0,(%rsp)' instead of 'LOCK ORL $0x0,(%rsp)'.
>
> No functional changes.
>
> Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
> Cc: Florian Weimer <fweimer@redhat.com>
> Cc: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
> Cc: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
> Cc: Collin Funk <collin.funk1@gmail.com>
> Cc: H.J.Lu <hjl.tools@gmail.com>
> Cc: Carlos O'Donell <carlos@redhat.com>
> ---
>  sysdeps/x86/atomic-machine.h | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/sysdeps/x86/atomic-machine.h b/sysdeps/x86/atomic-machine.h
> index d5b2d49031..c0c2c3437a 100644
> --- a/sysdeps/x86/atomic-machine.h
> +++ b/sysdeps/x86/atomic-machine.h
> @@ -26,15 +26,14 @@
>
>  #ifdef __x86_64__
>  # define __HAVE_64B_ATOMICS            1
> -# define SP_REG                                "rsp"
>  #else
>  /* Since the Pentium, i386 CPUs have supported 64-bit atomics, but the
>     i386 psABI supplement provides only 4-byte alignment for uint64_t
>     inside structs, so it is currently not possible to use 64-bit
>     atomics on this platform.  */
>  # define __HAVE_64B_ATOMICS            0
> -# define SP_REG                                "esp"
>  #endif
> +
>  #define ATOMIC_EXCHANGE_USES_CAS       0
>
>  #define atomic_compare_and_exchange_val_acq(mem, newval, oldval) \
> @@ -74,10 +73,7 @@
>  #define catomic_exchange_and_add(mem, value) \
>    __atomic_fetch_add (mem, value, __ATOMIC_ACQUIRE)
>
> -/* We don't use mfence because it is supposedly slower due to having to
> -   provide stronger guarantees (e.g., regarding self-modifying code).  */
> -#define atomic_full_barrier() \
> -    __asm __volatile (LOCK_PREFIX "orl $0, (%%" SP_REG ")" ::: "memory")
> +#define atomic_full_barrier() __sync_synchronize ()

Oh... this could be:

#define atomic_full_barrier() __atomic_thread_fence (__ATOMIC_SEQ_CST);

to keep primitives consistent with c11. This one is functionally
equivalent to __sync_synchronize ().

Uros.