Re: [Qemu-devel] [PATCH] atomic.h: Work around gcc spurious "unused value" warning

[Philippe Mathieu-Daudé]

On 05/10/2018 11:36 AM, Peter Maydell wrote:
> Some versions of gcc produce a spurious warning if the result of
> __atomic_compare_echange_n() is not used and the type involved
> is a signed 8 bit value:
>   error: value computed is not used [-Werror=unused-value]
> This has been seen on at least
>  gcc (Ubuntu 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609
> 
> Work around this by using an explicit cast to void to indicate
> that we don't care about the return value.
> 
> We don't currently use our atomic_cmpxchg() macro on any signed
> 8 bit types, but the upcoming support for the Arm v8.1-Atomics
> will require it.
> 
> Signed-off-by: Peter Maydell <address@hidden>

Reviewed-by: Philippe Mathieu-Daudé <address@hidden>

> ---
> Sending out my workaround as an actual patch. If this is OK I'll
> put in via target-arm.next as it's a dependency for the v8.1-atomics
> series.
> 
>  include/qemu/atomic.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
> index d73c9e14d7..9ed39effd3 100644
> --- a/include/qemu/atomic.h
> +++ b/include/qemu/atomic.h
> @@ -187,7 +187,7 @@
>  /* Returns the eventual value, failed or not */
>  #define atomic_cmpxchg__nocheck(ptr, old, new)    ({                    \
>      typeof_strip_qual(*ptr) _old = (old);                               \
> -    __atomic_compare_exchange_n(ptr, &_old, new, false,                 \
> +    (void)__atomic_compare_exchange_n(ptr, &_old, new, false,           \
>                                __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);      \
>      _old;                                                               \
>  })
>