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Tracking Issue for pointer::mask #98290
Description
Feature gate: #![feature(ptr_mask)]
This is a tracking issue for <*const T>::mask and <*mut T>::mask methods that allow masking pointers.
Public API
impl<T: ?Sized> *const T {
pub fn mask(self, mask: usize) -> *const T;
}
impl<T: ?Sized> *mut T {
pub fn mask(self, mask: usize) -> *mut T;
}Steps / History
- Implementation: Add pointer masking convenience functions #96946
- Final comment period (FCP)
- Stabilization PR
Unresolved Questions
- None yet.
Activity
hsanzg commentedon Jul 21, 2024
Are there plans to make the
ptr_mask(and thereforeptr::mask) usable fromconstcontexts?This currently depends on making
ptr::map_addrandptr::with_addrconst, which would need to be discussed in the context of the strict provenance project.RalfJung commentedon Jul 21, 2024
These functions cannot in general be made const-compatible, since their behavior depends on the exact bits of a pointer value which are not known at compile-time.
with_addrwill never beconst. (More specifically, subtracting two pointers will never be possible at const-time, andwith_addrdepends on that ability.)Certain tricks could be played for the specific case where the original allocation was sufficiently aligned to make the lowest bits of the pointers statically predictable. That would need new specialized intrinsics though, it cannot be implemented via
with_addr.RalfJung commentedon Sep 14, 2024
Don't we also need an operation that corresponds to
ptr.map_addr(|a| a | mask)to make this sufficient for tagged pointers? But somehow LLVM seems to only support&, not|, on pointers?(Maybe LLVM should just support what we call
addrandwith_addr, then it wouldn't need a bunch of new pointer versions for each integer operation...)Cc @nikic
nikic commentedon Sep 14, 2024
LLVM currently doesn't have something like
addrbecause the necessary analysis infrastructure to capitalize on it isn't in place (yet). LLVM currently combines address capture and provenance escape into one concept, and the addr vs ptrtoint distinction only really becomes relevant once you separate them. This is something I have on my long term TODO list, but never get around to...RalfJung commentedon Sep 14, 2024