[llvm] [NVPTX] Add support for "blocksareclusters" kernel attr (PR #152265)

[Alex MacLean via llvm-commits]

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@@ -92,6 +92,12 @@ Function Attributes
     dimension. Specifying a different cluster dimension at launch will result in
     a runtime error or kernel launch failure. Only supported for Hopper+.
 
+``"nvvm.blocksareclusters"``
+    This attribute implies that the grid launch configuration for the corresponding
+    kernel function is specifying the number of clusters instead of the number of thread
+    blocks. This attribute is only allowed for kernel functions and requires
+    ``nvvm.reqntid`` and ``nvvm.cluster_dim`` attributes.
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AlexMaclean wrote:

I'm still having some trouble wrapping my head around the semantics here. Does this attribute solely have the impact of changing the meaning of the other two attributes? Maybe it would help to contrast what ``nvvm.reqntid`` and ``nvvm.cluster_dim`` normally mean vs. when this attribute is present.

https://github.com/llvm/llvm-project/pull/152265