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

[Harald van Dijk via llvm-commits]

================
@@ -8020,6 +8020,42 @@ 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}
+
+
+This is intended for use on targets with a notion of generic address
+spaces, which at runtime resolve to different physical memory
+spaces. The interpretation of the address space values is target
+specific. The behavior is undefined if the runtime memory address does
+resolve to an object defined in one of the indicated address spaces.
----------------
hvdijk wrote:

It is not clear to me from this what the intended behaviour is when this metadata is used to specify an object is not in the generic address space. Does that effectively mean this code must be unreachable, or does that say nothing because the concrete address space any object is in will not be the generic address space?

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