[llvm] [RFC][AMDGPU][Doc] Add documentation about ABI occupancy (PR #224103)

[Siu Chi Chan via llvm-commits]

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@@ -21963,6 +22006,7 @@ The following sub-directives may appear inside the block:
      ``.amdgpu_num_vgpr`` *value*           Architectural VGPRs used (u32)
      ``.amdgpu_num_agpr`` *value*           Accumulator VGPRs used (u32)
      ``.amdgpu_private_segment_size`` *n*   Private segment size in bytes (u32)
+     ``.amdgpu_occupancy`` *value*          Occupancy used to compile the function (u32)
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scchan wrote:

Can we call it something like resource-bounded occupancy?  It might be more intuitive to think about it like: we need to guarantee this amount of resources and therefore, we have to enforce this occupancy...

https://github.com/llvm/llvm-project/pull/224103