Fixing core.atomic

[rm]

On 02/06/2021 20:33, Max Haughton wrote:
> On Wednesday, 2 June 2021 at 15:30:46 UTC, Ola Fosheim Grøstad wrote:
>> On Wednesday, 2 June 2021 at 15:19:59 UTC, Ola Fosheim Grøstad wrote:
>>> On Wednesday, 2 June 2021 at 15:09:54 UTC, rm wrote:
>>>> inc/dec are implemented in terms of fetch_add.
>>>
>>> IIRC some architectures provide more efficient inc/dec atomics 
>>> without fetch? I haven't looked at that in years, so I have no idea 
>>> what the contemporary situation is.
>>
>> No, I think that was wrong, I think they usually return the original 
>> value (or set a flag or whatever). But it doesn't matter. We should 
>> just look at what the common contemporary processors provide and look 
>> at instructions per clock cycles throughput. I guess last generation 
>> ARM/Intel/AMD is sufficient?
> 
> Are they always fixed latency? No dependence on the load store queue 
> state (etc.)  for example?

At least on x86-TSO, an atomic operation forces the cache to be flushed 
to memory.