[llvm] AMDGPU: Preliminary documentation for named barriers (PR #165502)

[Nicolai Hähnle via llvm-commits]

================
@@ -1179,6 +1179,53 @@ is conservatively correct for OpenCL.
                              other operations within the same address space.
      ======================= ===================================================
 
+Target Types
+------------
+
+The AMDGPU backend implements some target extension types.
+
+.. _amdgpu-types-named-barriers:
+
+Named Barriers
+~~~~~~~~~~~~~~
+
+Named barriers are represented as memory objects of type
+``target("amdgcn.named.barrier", 0)``. They are allocated as global variables
----------------
nhaehnle wrote:

I also find the framing as "lowering hints" weird. If we did that, then logically speaking we'd have to say that global objects are lowering hints for addresses, which... sure, that's true in a sense, but why would you phrase it that way?

https://github.com/llvm/llvm-project/pull/165502