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

[Shilei Tian via llvm-commits]

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shiltian wrote:

But in this case, C will also need to get `agpr-alloc=50` after propagation right?

Essentially I think this attribute is to be propagated from caller to callee, while the other existing attribute is to be propagated from callee to caller. We can't propagate any single one in both direction. This new attribute is essentially to tell the RA how many VGPRs need to be "removed" from the total number of VGPRs the corresponding occupancy can have.

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