[PATCH v5 2/3] server: Add USD support with timestamp updates.

[Alexandre Julliard]

Rémi Bernon <rbernon at codeweavers.com> writes:

> On 5/7/20 10:45 PM, Alexandre Julliard wrote:
>> Rémi Bernon <rbernon at codeweavers.com> writes:
>>
>>> +#if defined(__GNUC__) && ((__GNUC__ > 4) || ((__GNUC__ == 4) && (__GNUC_MINOR__ >= 7)))
>>> +    __atomic_store_n(&ptr->SystemTime.High2Time, system_time_high, __ATOMIC_SEQ_CST);
>>> +    __atomic_store_n(&ptr->SystemTime.LowPart, system_time_low, __ATOMIC_SEQ_CST);
>>> +    __atomic_store_n(&ptr->SystemTime.High1Time, system_time_high, __ATOMIC_SEQ_CST);
>>> +
>>> +    __atomic_store_n(&ptr->InterruptTime.High2Time, interrupt_time_high, __ATOMIC_SEQ_CST);
>>> +    __atomic_store_n(&ptr->InterruptTime.LowPart, interrupt_time_low, __ATOMIC_SEQ_CST);
>>> +    __atomic_store_n(&ptr->InterruptTime.High1Time, interrupt_time_high, __ATOMIC_SEQ_CST);
>>> +
>>> +    __atomic_store_n(&ptr->TickCount.High2Time, tick_count_high, __ATOMIC_SEQ_CST);
>>> +    __atomic_store_n(&ptr->TickCount.LowPart, tick_count_low, __ATOMIC_SEQ_CST);
>>> +    __atomic_store_n(&ptr->TickCount.High1Time, tick_count_high, __ATOMIC_SEQ_CST);
>>> +    __atomic_store_n(&ptr->TickCountLowDeprecated, tick_count_low, __ATOMIC_SEQ_CST);
>>
>> Is this gcc-specific code really necessary?
>>
>
> I'm not sure it is strictly necessary but then it depends what
> guarantees we want to have. The volatile qualifier only acts as a
> compiler barrier and will make sure that the stores aren't optimized
> or reordered by the compiler. However it doesn't enforce anything on
> the CPU. I believe that X86 has global store ordering guarantees, but
> a quick search tells me that ARM or POWER don't, so the stores to the
> various members may be seen out of order from another CPU depending on
> the architecture.

My concern is that if the code is necessary, it means that the fallback
path for older gccs would yield a broken implementation. But if the
fallback works correctly on x86 that's probably good enough.

-- 
Alexandre Julliard
julliard at winehq.org