[llvm] [RFC][LangRef] Specify that the accessed bytes of concurrent atomics must be either disjoint or the same (PR #204329)

[Fabian Ritter via llvm-commits]

================
@@ -4057,12 +4057,17 @@ Given that definition, R{sub}`byte` is defined as follows:
 -  Otherwise, if R{sub}`byte` may see exactly one write,
    R{sub}`byte` returns the value written by that write.
 -  Otherwise, if R is atomic, and all the writes R{sub}`byte` may
-   see are atomic, it chooses one of the values written. See the {ref}`Atomic
+   see are atomic, and R and the writes all access the exact same set of
+   bytes, it chooses one of the values written. See the {ref}`Atomic
    Memory Ordering Constraints <ordering>` section for additional
    constraints on how the choice is made. Targets may impose additional
    requirements on R and the writes it may see based on their `syncscope`.
 -  Otherwise R{sub}`byte` returns `undef`.
 
+Defined atomic accesses cannot tear: If a byte subaccess R{sub}`byte1`
----------------
ritter-x2a wrote:

Done.

https://github.com/llvm/llvm-project/pull/204329