[llvm] Fix SIFixSGPRCopies To Handle Physical Registers (PR #149859)

[Patrick Simmons via llvm-commits]

linuxrocks123 wrote:

> You can't simply insert a readfirstlane to make this correct. In general you need to emit a waterfall loop, and you need more context than a single physical register copy. @Shoreshen is working on introducing waterfall loops

I have a question, if you don't mind my asking, since I'm new to this architecture and this is my first issue: why do VGPR to SGPR copies differ in what is needed to make them correct if the registers are virtual versus physical?  SIFixSGPRCopies can apparently handle virtual VGPR to SGPR copies with existing code, so why is something different needed for physical VGPR to SGPR copies.

Also, if @Shoreshen is working on waterfall loops, does that mean he is likely to create a PR that would duplicate the work of fixing this fuzzer testcase?  If so, I should probably drop this and take another issue instead.

https://github.com/llvm/llvm-project/pull/149859