[llvm] [RFC] IR: Define noalias.addrspace metadata (PR #102461)

[Rahul Joshi via llvm-commits]

================
@@ -8021,6 +8021,43 @@ it will contain a list of ids, including the ids of the callsites in the
 full inline sequence, in order from the leaf-most call's id to the outermost
 inlined call.
 
+
+'``noalias.addrspace``' Metadata
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The ``noalias.addrspace`` metadata is used to identify memory
+operations which cannot access a range of address spaces. It is
+attached to memory instructions, including :ref:`atomicrmw
+<i_atomicrmw>`, :ref:`cmpxchg <i_cmpxchg>`, and :ref:`call <i_call>`
+instructions.
+
+This follows the same form as :ref:`range metadata <range-metadata>`,
+except the field entries must be of type `i32`. The interpretation is
+the same numeric address spaces as applied to IR values.
+
+Example:
+
+.. code-block:: llvm
+
+    ; %ptr cannot point to an object allocated in addrspace(5)
+    %rmw.valid = atomicrmw and ptr %ptr, i64 %value seq_cst, !noalias.addrspace !0
+
+    ; Undefined behavior. The underlying object is allocated in one of the listed
+    ; address spaces.
+    %alloca = alloca i64, addrspace(5)
+    %alloca.cast = addrspacecast ptr addrspace(5) %alloca to ptr
+    %rmw.ub = atomicrmw and ptr %alloca.cast, i64 %value seq_cst, !noalias.addrspace !0
+
+    !0 = !{i32 5, i32 6}
----------------
jurahul wrote:

I can think of use cases where the set of address spaces is non-contiguous and the range base representation can be clunky as each range will be of size 1 and any coalescing is coincidental. Would it be possible to support both list and ranges, for example:

```
!0 = !{!"range" i32 5, i32 6} // a list or ranges follows
!0 = !{!"list" i32 5} or a list of address spaces follows
```


https://github.com/llvm/llvm-project/pull/102461