[llvm-branch-commits] [DataLayout][LangRef] Split non-integral and unstable pointer properties (PR #105735)

[Alexander Richardson via llvm-branch-commits]

https://github.com/arichardson created https://github.com/llvm/llvm-project/pull/105735

Theses are finer-grained versions of isNonIntegralAddressSpace() and
isNonIntegralPointerType() where the current semantics prohibit
introduction of both ptrtoint and inttoptr instructions. These semantics
are too strict for some targets (e.g. AMDGPU/CHERI) where ptrtoint has
a stable value, but the pointer cannot be recreated just from the
address since there is additional metadata.
Currently, marking a pointer address space as non-integral also marks it
as having an unstable bitwise representation (e.g. when pointers can be
changed by a copying GC). This property inhibits a lot of
optimizations that are perfectly legal for other non-integral pointers
such as fat pointers or CHERI capabilities that have a well-defined
bitwise representation but can't be created with only an address.

This change splits the two properties and allows for address spaces to
be marked as unstable or non-integral (or both) independently using
the 'p' part of the DataLayout string a 'u' following the p marks the
address space as unstable and a 'n' marks it as non-integral.

This does not change the checks in any of the passes yet - we
currently keep the existing non-integral behaviour. In the future I plan
to audit calls to DL.isNonIntegral[PointerType]() and replace them with
the DL.shouldAvoid{IntToPtr,PtrToInt}() checks that allow for more
optimizations.