* [Qemu-devel] [PATCH] atomic.h: Work around gcc spurious "unused value" warning
@ 2018-05-10 14:36 Peter Maydell
2018-05-10 14:37 ` Richard Henderson
` (2 more replies)
0 siblings, 3 replies; 6+ messages in thread
From: Peter Maydell @ 2018-05-10 14:36 UTC (permalink / raw)
To: qemu-devel; +Cc: patches, Richard Henderson, Paolo Bonzini
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
^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [Qemu-devel] [PATCH] atomic.h: Work around gcc spurious "unused value" warning
2018-05-10 14:36 [Qemu-devel] [PATCH] atomic.h: Work around gcc spurious "unused value" warning Peter Maydell
@ 2018-05-10 14:37 ` Richard Henderson
2018-05-10 14:38 ` Peter Maydell
2018-05-10 14:58 ` Philippe Mathieu-Daudé
2018-05-10 17:09 ` Paolo Bonzini
2 siblings, 1 reply; 6+ messages in thread
From: Richard Henderson @ 2018-05-10 14:37 UTC (permalink / raw)
To: Peter Maydell, qemu-devel; +Cc: patches, Paolo Bonzini
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~
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Qemu-devel] [PATCH] atomic.h: Work around gcc spurious "unused value" warning
2018-05-10 14:37 ` Richard Henderson
@ 2018-05-10 14:38 ` Peter Maydell
2018-05-10 14:43 ` Richard Henderson
0 siblings, 1 reply; 6+ messages in thread
From: Peter Maydell @ 2018-05-10 14:38 UTC (permalink / raw)
To: Richard Henderson; +Cc: QEMU Developers, patches, Paolo Bonzini
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
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Qemu-devel] [PATCH] atomic.h: Work around gcc spurious "unused value" warning
2018-05-10 14:38 ` Peter Maydell
@ 2018-05-10 14:43 ` Richard Henderson
0 siblings, 0 replies; 6+ messages in thread
From: Richard Henderson @ 2018-05-10 14:43 UTC (permalink / raw)
To: Peter Maydell; +Cc: QEMU Developers, patches, Paolo Bonzini
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~
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Qemu-devel] [PATCH] atomic.h: Work around gcc spurious "unused value" warning
2018-05-10 14:36 [Qemu-devel] [PATCH] atomic.h: Work around gcc spurious "unused value" warning Peter Maydell
2018-05-10 14:37 ` Richard Henderson
@ 2018-05-10 14:58 ` Philippe Mathieu-Daudé
2018-05-10 17:09 ` Paolo Bonzini
2 siblings, 0 replies; 6+ messages in thread
From: Philippe Mathieu-Daudé @ 2018-05-10 14:58 UTC (permalink / raw)
To: Peter Maydell, qemu-devel; +Cc: Paolo Bonzini, Richard Henderson, patches
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; \
> })
>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Qemu-devel] [PATCH] atomic.h: Work around gcc spurious "unused value" warning
2018-05-10 14:36 [Qemu-devel] [PATCH] atomic.h: Work around gcc spurious "unused value" warning Peter Maydell
2018-05-10 14:37 ` Richard Henderson
2018-05-10 14:58 ` Philippe Mathieu-Daudé
@ 2018-05-10 17:09 ` Paolo Bonzini
2 siblings, 0 replies; 6+ messages in thread
From: Paolo Bonzini @ 2018-05-10 17:09 UTC (permalink / raw)
To: Peter Maydell, qemu-devel; +Cc: patches, Richard Henderson
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; \
> })
>
^ permalink raw reply [flat|nested] 6+ messages in thread
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2018-05-10 14:36 [Qemu-devel] [PATCH] atomic.h: Work around gcc spurious "unused value" warning Peter Maydell
2018-05-10 14:37 ` Richard Henderson
2018-05-10 14:38 ` Peter Maydell
2018-05-10 14:43 ` Richard Henderson
2018-05-10 14:58 ` Philippe Mathieu-Daudé
2018-05-10 17:09 ` Paolo Bonzini
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