Re: [Qemu-devel] [RFC v1 05/12] atomic: introduce cmpxchg_bool

[Eric Blake <eblake@redhat.com>]

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On 06/03/2016 01:12 PM, Alex Bennée wrote:

>>> +#define atomic_bool_cmpxchg(ptr, old, new)                              \
>>> +    ({                                                                  \
>>> +    typeof(*ptr) _old = (old), _new = (new);                            \
>>> +    bool r;                                                             \
>>> +    r = __atomic_compare_exchange(ptr, &_old, &_new, false,             \
>>> +                                  __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);  \
>>> +    r;                                                                  \
>>> +    })
>>> +
>>> +
>>
>> Could be more simple:
>>
>> #define atomic_bool_cmpxchg(ptr, old, new)                              \
>>     ({                                                                  \
>>     typeof(*ptr) _old = (old), _new = (new);                            \
>>     __atomic_compare_exchange(ptr, &_old, &_new, false,                 \
>>                               __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);      \
>>     })
> 
> OK that makes sense. I'll have to ask my toolchain colleague what the
> rules are for results from {} blocks.

It's a gcc extension, and the rule is that the value of the overall ({})
is the value of the last statement in the block.  No need for a
temporary variable 'r' if you can just use __atomic_compare_exchange()
as the last statement.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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