> A block of memory cannot be read as an array of simd types sometimes and scalar types otherwise.
As far as I can tell, it is currently the case that, using raw pointers, this is not actually undefined behavior (but I never entirely trust my conclusions on these matters).
"&mut T and &T follow LLVM’s scoped noalias model" [1][referring to 2 and 3] but I am fairly sure this does not currently apply to raw pointers, and "provenance is implicitly shared with all pointers transitively derived from the original pointer through operations like offset, borrowing, and pointer casts." [4]
LLVM can represent various aliasing relationships, modulo some risk of C++ inspired bugs in some passes. They might all be stamped out now. I remember a bug report about one that was open for many years.
I'm happy to hear rust can (probably) represent the same relationships LLVM can. C++ cannot, at least as of about two years ago when I last looked through the corresponding papers. All it can do is different types do not alias, where atomic_int and int are different types.
No, LLVM definitely still has big problems. https://github.com/llvm/llvm-project/issues/45725 is an example, the symptom in Rust is that you can write what is in effect a pointer comparison in which LLVM ends up claiming that two things are different, although they are also identical...
As far as I can tell, it is currently the case that, using raw pointers, this is not actually undefined behavior (but I never entirely trust my conclusions on these matters).
"&mut T and &T follow LLVM’s scoped noalias model" [1][referring to 2 and 3] but I am fairly sure this does not currently apply to raw pointers, and "provenance is implicitly shared with all pointers transitively derived from the original pointer through operations like offset, borrowing, and pointer casts." [4]
[1] https://doc.rust-lang.org/reference/behavior-considered-unde...
[2] https://llvm.org/docs/LangRef.html#pointeraliasing
[3] "noalias" under https://llvm.org/docs/LangRef.html#parameter-attributes
[4] https://doc.rust-lang.org/core/ptr/index.html
Also excellent are
https://faultlore.com/blah/fix-rust-pointers
https://www.ralfj.de/blog/2018/07/24/pointers-and-bytes.html
https://www.ralfj.de/blog/2020/12/14/provenance.html
https://www.ralfj.de/blog/2022/04/11/provenance-exposed.html
It seems likely you'd already be familiar with these; I'm just putting them out there for anyone interested.