On 9/16/19 12:58 PM, Wilco Dijkstra wrote:
Hi Richard,
>> So what is the behaviour when you explicitly select a specific CPU?
>
> Selecting a specific cpu selects the specific architecture that the cpu
> supports, does it not? Thus the architecture example above still
applies.
>
> Unless I don't understand what distinction that you're making?
When you select a CPU the goal is that we optimize and schedule for that
specific microarchitecture. That implies using atomics that work best for
that core rather than outlining them.
I think we want to go ahead with this framework to enable the portable
deployment of LSE atomics.
More CPU-specific fine-tuning can come later separately.
Thanks,
Kyrill
>> I'd say that by the time GCC10 is released and used in distros,
systems without
>> LSE atomics would be practically non-existent. So we should favour
LSE atomics
>> by default.
>
> I suppose. Does it not continue to be true that an a53 is more
impacted by the
> branch prediction than an a76?
That's hard to say for sure - the cost of taken branches (3 in just a
few instructions for
the outlined atomics) might well affect big/wide cores more. Also note
Cortex-A55
(successor of Cortex-A53) has LSE atomics.
Wilco
