[llvm] [AMDGPU] Add amdgpu-accum-offset attribute to bound available arch VGPR (PR #218745)

[via llvm-commits]

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JoshuaGrindstaff wrote:

What do you mean by "offset" VGPRs? I want to cap VGPRs with `amdgpu-accum-offset` so that the function can run with the offset of any reachable kernel. 

Do you mean why I don't approach the limit from above and remove VGPR from the top based on an inherited usage of AGPRs (different from `amdgpu-agpr-alloc`), I don't do that since different callers can have different total sizes. If kernel A has 256 total registers while kernel B can only have 128 registers, the usage of 30 AGPRs would mean something different for kernel A then it would be for kernel B. 
A    B
  \  /
   C
A - 216 accum-offset, 40 agpr-alloc
B - 98 accum-offset, 30 agpr-alloc

Even though A has a higher agpr count here, we should only consider B's agpr-alloc/accum-offset since it's the one that will affect C.

https://github.com/llvm/llvm-project/pull/218745