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

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.

Backports commit cd95fc28fb6d8afced0d70ce52c294d0761a9daa from qemu
This commit is contained in:
Peter Maydell 2018-05-14 07:26:37 -04:00 committed by Lioncash
parent 150125de3f
commit b22f822858
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@ -194,7 +194,7 @@ void _ReadWriteBarrier(void);
/* 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; \
})