[llvm] AMDGPU: Add amdgpu-num-agpr attribute to control AGPR allocation (PR #128034)

[Shilei Tian via llvm-commits]

shiltian wrote:

I found these ranges are very confusing. If I require minimum 0 AGPRs, that means I can require 1, 2, 3, etc. AGPRs, because they are all larger than the minimum. Why does it mean no AGPR?

https://github.com/llvm/llvm-project/pull/128034