[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:

We still have a philosophical disagreement. There *is* addressable backing memory in hardware, it's just not the kind of memory that consists of the usual bits and bytes. But it doesn't really matter for what's written in this PR.

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