[llvm] [AMDGPU] Add waterfall intrinsics (PR #192409)

[Sebastian Neubauer via llvm-commits]

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@@ -1776,6 +1776,59 @@ The AMDGPU backend implements the following LLVM IR intrinsics.
                                                    * :ref:`Synchronization Scope<amdgpu-intrinsics-syncscope-metadata-operand>`.
                                                      Note that the scope used must ensure that the L2 cache will be hit.
 
+  llvm.amdgcn.waterfall.begin                      Marks the beginning of a waterfall region of code.
+
+                                                   The compiler generates a waterfall loop around the region.
+                                                   A waterfall loop handles the case where an operation that requires
+                                                   a uniform operand (e.g., held in an SGPR) is applied to a divergent operand
+                                                   (held in a VGPR, with values varying per lane).
+                                                   Each iteration of the waterfall loop activates a subset of lanes that
+                                                   share the same value of the VGPR (the value in the first active lane).
+                                                   The operation is then executed using that value as the uniform operand.
+                                                   If the VGPR is already uniform, the waterfall loop executes only once.
+                                                   The worst case for a waterfall loop is one iteration per lane (all lanes
+                                                   have different values of the VGPR), but it is not common in practice.
+
+                                                   The intrinsic takes a previous token
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Flakebi wrote:

The previous here sounds wrong (not saying it _is_ wrong, just reads weirdly for someone not familiar with the implementation details). Maybe add a second sentence that explains the intentional use of the incoming token?
Edit: I see that the last paragraph explains it. Maybe add this paragraph behind the current last paragraph?
I.e. explain the intention and then explain the implementation, not the other way around.

https://github.com/llvm/llvm-project/pull/192409