Code and data live in the same address space, and the address-space needs of the system are the main input to the basic signature width, so the basic signatures widths are currently the same, and the only difference is TBI.
You could imagine a system where code was always loaded into a restricted subset of the address space and so code pointers could use wider signatures.
> Can I write an application that uses the top bits of data pointers?
Apple's ABIs actually consider the top 8 bits of data pointers to be outside the addressable range on all its 64-bit targets, including x86_64. ARM64 TBI just means that you don't need to explicitly mask off those bits before doing loads and stores. But there are caveats:
- ARMv8.5 memory tagging uses bits 56-59, so you should probably stick to just the top four bits in case Apple ever uses memory tagging.
- IIRC the first ARM64 iOS release didn't enable TBI, so if your deployment target goes really far back, you do still need to mask.
- The ABI for pointers expects those bits to be clear on normal ABI boundaries. This means you need to mask before handing pointers off to other code; on the upside, however, you don't need to worry about those bits being set when you receive a pointer.