[llvm] [LangRef] Document accessing memory outside of object is UB. (PR #128429)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

UB from out of bounds accesses is a consequence of provenance, and as such specified in the pointer aliasing section. It says:

> Any memory access must be done through a pointer value associated with an address range of the memory access, otherwise the behavior is undefined.

Notably, this applies even if the out of bounds memory is part of  *different* allocated object (than the one the pointer is *based on*).

(I think the pointer aliasing section could use a rewrite though, because the way it is documented matches neither contemporary terminology, nor our current understanding of the semantics.)

https://github.com/llvm/llvm-project/pull/128429