atomic.h: Work around gcc spurious "unused value" warning

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

Posted by Peter Maydell 7 years, 5 months ago

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 <peter.maydell@linaro.org>
---
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;                                                               \
 })
-- 
2.17.0

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

Posted by Richard Henderson 7 years, 5 months ago

On 05/10/2018 07:36 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 <peter.maydell@linaro.org>
> ---
> 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(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


r~

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

Posted by Peter Maydell 7 years, 5 months ago

On 10 May 2018 at 15:37, Richard Henderson <rth@twiddle.net> wrote:
> On 05/10/2018 07:36 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 <peter.maydell@linaro.org>
>> ---
>> 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(-)
>
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

Thanks. Do we care about trying to follow up on the gcc side
to find out if this is a definitely-fixed bug?

-- PMM

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

Posted by Richard Henderson 7 years, 5 months ago

On 05/10/2018 07:38 AM, Peter Maydell wrote:
> Thanks. Do we care about trying to follow up on the gcc side
> to find out if this is a definitely-fixed bug?

Given I did not see the problem with gcc 6 as shipped with debian 9, I'm not
overly inclined to dig further than that.


r~

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

Posted by Philippe Mathieu-Daudé 7 years, 5 months ago

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 <peter.maydell@linaro.org>

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

> ---
> 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;                                                               \
>  })
>

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

Posted by Paolo Bonzini 7 years, 5 months ago

On 10/05/2018 16:36, 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 <peter.maydell@linaro.org>
> ---
> 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.

Yes, please go ahead!

Paolo

>  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;                                                               \
>  })
>