[llvm] [AMDGPU][Attributor] Rework update of `AAAMDWavesPerEU` (PR #123995)

[Johannes Doerfert via llvm-commits]

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@@ -1323,6 +1298,60 @@ struct AAAMDGPUNoAGPR
 
 const char AAAMDGPUNoAGPR::ID = 0;
 
+/// The final check and update of the attribute 'amdgpu-waves-per-eu' based on
+/// the determined 'amdgpu-flat-work-group-size' attribute. We can't do this
+/// during attributor run because the two attributes grow in opposite direction,
+/// we should not use any intermediate value to calculate waves per eu until we
+/// have a determined flat workgroup size.
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jdoerfert wrote:

I still believe the wording here is misleading. In our last (long) discussion, I tried to propose a different view on the same issue, as "grows in the opposite direction" is not a conceptual problem. I would recommend looking back at that conversation, maybe you can expand on the above a bit to help people understand the conceptual issue.

https://github.com/llvm/llvm-project/pull/123995