Re: [PATCH, AArch64, v3 0/6] LSE atomics out-of-line

[Richard Henderson <rth at twiddle dot net>]

On 9/17/19 6:55 AM, Wilco Dijkstra wrote:
> Hi Kyrill,
> 
>>> 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.
> 
> I'm not talking about CPU-specific fine-tuning, but ensuring we don't penalize
> performance when a user selects the specific CPU their application will run on.
> And in that case outlining is unnecessary.

>From aarch64_override_options:

Given both -march=foo -mcpu=bar, then the architecture will be foo and -mcpu
will be treated as -mtune=bar, but will not use any insn not in foo.

Given only -mcpu=foo, then the architecture will be the one supported by foo.

So if foo supports LSE, then we will not outline the functions, no matter how
we arrive at foo.


r~