Add FMV feature names priority[ABCDE]

[andrewcarlotti]

These feature names allow developers to specify a priority order for their
function versions that differs from the default priority order for the
specified architectural features.

This builds upon #370.

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[labrinea]

LGTM. Needs rebase and separation from #370. Perhaps it's not the most intuitive syntax but does the job in my opinion. @jroelofs @DanielKristofKiss what are your thoughts?

[jroelofs]

LGTM. Needs rebase and separation from #370. Perhaps it's not the most intuitive syntax but does the job in my opinion. @jroelofs @DanielKristofKiss what are your thoughts?

bikeshed: maybe priority1/priority2/etc. would be more clear naming-wise? seems fine either way, to me.

[labrinea]

bikeshed: maybe priority1/priority2/etc. would be more clear naming-wise? seems fine either way, to me.

I had the exact same thought. I don't mind either way.

[andrewcarlotti]

Re. the bikeshedding - this may well be a cultural difference, but to me it's unclear whether priority1 would mean the first (highest) priority, or the lowest one, whereas it feels unambiguous to me lettered priorities.

Some mostly non-technical examples:
Smaller numbers are ranked higher - e.g. 1st place in a competition.
Larger numbers are ranked higher - e.g. English GCSE exam grades, or the priority numbers in the table of features in this document.

Earlier letters are ranked higher - e.g. Many UK exam grade systems, UK road classification (A, B, C roads, ignoring motorways)
Later letters are ranked higher - I can't think of any examples for this.

I recognise that my vague associations with letter/number orderings might be specific to my own life experience, and other people may see this differently, so I'm happy to change this if you disagree with my thinking.

[jroelofs]

All ambiguity would be removed if they were named e.g. priority9998/priority9999/etc. That said, I don't have a preference here. Letters seem fine, since if you're having to adjust priorities manually, you're probably going to have to look at the table anyway.

[DanielKristofKiss]

my 2 cents,

can we specify priorityA+priorityB+crc' ? or even priorityA+priorityA+crc' ?

If we go with priorityN then I'd prefer to specify it with a formula instead of typing in all numbers.
This could be a pro besides priority\d+ format.

Original intention was developers rarely need to know about the actual priorities as allocated values should mostly fine for the usual cases. But if they need to deal with the ordering of the selection then maybe exact values are better.

Bit different idea: more complex case could be specify order\d+ and the selection algorithm goes over the ordered item(0toN)first then priority (M to default).

[labrinea]

@DanielKristofKiss can you please explain a little more as I am not sure I understood. Are you suggesting we use an explicit priority syntax like the one for example in RISC-V: https://github.com/riscv-non-isa/riscv-c-api-doc/pull/85/files ? That would be harder to implement, for the current proposal the machinery is in place, only these pseudo fmv features are needed to be implemented.

can we specify priorityA+priorityB+crc ? or even priorityA+priorityA+crc ?

Yes, like with every other feature, these special features can be used in conjuction with others. Repeating the same feature in a version does not get diagnosed, but has no effect (at least in LLVM).