Add FEAT_CMH support

[ricbal02]

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Hi all,

This is a PR to propose the addition of new intrinsics required as part of FEAT_CMH.

Quality Level: ALP

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[Wilco1]

floating point??? Let's stick to scalar integer only. I don't believe there is any use of FP variants anyway.

[Lukacma]

I think the idea was to be consistent with the store intrinsic. Or is there a reason to have different type support between the 2 ?

[Wilco1]

Since it can't be done on compare&exchange loops, you'd have to check that for all architectures where CMH is allowed that FP atomics are a must-have and not optional. It's complex.

[ricbal02]

Happy to remove from the fetch and leave on the store, the lack of consistency may be confusing but I doubt it will be used much tbh

[Lukacma]

I am not sure I follow @Wilco1. We have a fetch instructions for floating point add. Do we not to have hintable builtins for these ?

[Wilco1]

We don't have a good way to emit the instruction yet without the hint... I think it's best done separately since it only applies when FEAT_LSFE is available. Note LSFE also defines lots LDFMIN/MAX variants.

[Lukacma]

Sure, but with the hint it will have to be emitted by a builtin in any case. So unless you want to have 2 separate hint builtins 1 for integer and 1 for floating point type it will need to be added here. Regarding the feature constraint integer variant is also guarded by FEAT_LSE.

But if you have strong preference for separating codegen fo FEAT_LSFE instructions to separate work, we can leave it be for now and extend later.

[Wilco1]

.