[llvm] [TRI][RISCV] Add methods to get common register class of two registers (PR #118435)

[Paul Kirth via llvm-commits]

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@@ -222,6 +222,25 @@ TargetRegisterInfo::getMinimalPhysRegClass(MCRegister reg, MVT VT) const {
   return BestRC;
 }
 
+const TargetRegisterClass *TargetRegisterInfo::getCommonMinimalPhysRegClass(
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ilovepi wrote:

The two functions are almost identical, can we maybe use a template to contain all the inner logic? The only difference I see that's hard to factor out is `Ty.isValid()` vs `VT == MVT::Other`. Since they're both `bool`s, maybe that could be passed in as a param, and the templating could take care of the rest?

Its very little code, but these kind of need to be kept in sync otherwise.

https://github.com/llvm/llvm-project/pull/118435