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

[Pierre van Houtryve 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
----------------
Pierre-vh wrote:

I'd call a spade a spade and say these are token objects with no actual storage in LDS, and that they're only used as lowering hints for named barrier IDs.

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