[clang] [clang] Add support for `__ptrauth` being applied to integer types (PR #137580)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > Thank you for your patience!
> > In general, I think adding a new qualifier to Clang needs to come with significant justification for the implementation and maintenance costs. In this specific case, those costs are somewhat amortized because we already have the `__ptrauth` qualifier and this piggybacks off that work. But I don't think the benefits justify the costs: if the user writes `__ptrauth` on an integer type, we can determine whether that type is "good enough" or not and issue a diagnostic for invalid or suspicious uses, same as if the user spelled it `__ptrauth_restricted_intptr`.
> 
> It was a very 50/50 decision to have a different spelling at the time, so I'm fine with dropping this, I just need to do some work to determine how to do the feature checks nicely.

Excellent, thank you!

> > The implementation already has to update typechecking in various places regardless of which way we spell the qualifiers, so there should not be too much extra implementation burden from only using one qualifier.
> 
> The implementation actually becomes simpler - it removes a bunch of work for debugging, and the type checking itself is just "__ptrauth only applies to pointers and __ptrauth_restricted_intptr only applies to integers".
> 
> > So on balance, I think we should only upstream the single qualifier and use a header file + feature tests for compatibility with older compilers. Is that something you can get behind? (You're still the expert in this space, so if I'm trivializing concerns, please speak up!)
> 
> @AaronBallman we might need a feature check -- maybe (please help with what it should be called, I'm terrible at naming) `__ptrauth_can_haz_intptrs`? It's a little weird as I think support for the ptrauth qualifier has not yet been in any official clang release, so from an end user's point of view `__ptrauth` will always have worked on pointer sized integers. It's possible that we might actually be able to manage by assuming that the lack of the `__ptrauth_restricted_intptr` qualifier means `__ptrauth` works on pointers and pointer sized integers but it will require some checking first.
> 
> In the mean time I'm going to update this PR to remove the new spelling and related shenanigans, and then update tests.

Would it make sense to add the feature macro to your downstream instead? e.g., downstream supports `__has_feature(ptrauth_restricted_intptr)` and upstream would return `false` for it.


https://github.com/llvm/llvm-project/pull/137580