[llvm] [TableGen][GISel] Learn to import patterns with physreg defs (PR #120343)

[Sergei Barannikov via llvm-commits]

s-barannikov wrote:

I'd like some help with "VReg has no regclass after selection" issue, I'm not sure how to fix it.

* Insert `ConstrainOperandToRegClass` to an arbitrary register class? Probably wrong.
* Insert `ConstrainSelectedInstOperands` for the virtreg <-- physreg COPY in the hope that backends will constrain the virtual register? They currently don't.
* Insert some new `ConstrainXXX` designed specifically for virtreg <-- physreg copies?
* Fix the linked FIXME somehow? Probably needs a target hook one way or another, and I don't see a way of how to plug it in.
* Let it be for now? Hopefully :)


https://github.com/llvm/llvm-project/pull/120343