[llvm] [LoongArch] lower SCALAR_TO_VECTOR to INSERT_VECTOR_ELT (PR #122863)

[Yingchi Long via llvm-commits]

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@@ -255,6 +255,7 @@ LoongArchTargetLowering::LoongArchTargetLowering(const TargetMachine &TM,
       setOperationAction(ISD::SETCC, VT, Legal);
       setOperationAction(ISD::VSELECT, VT, Legal);
       setOperationAction(ISD::VECTOR_SHUFFLE, VT, Custom);
+      setOperationAction(ISD::SCALAR_TO_VECTOR, VT, Custom);
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inclyc wrote:

> Why not make it `Legal` and define patterns in .td files.

Hi @SixWeining, I thought that in `.td` files, we can only lower operations to machine instructions. However, if this is done during ISel lowering, it gets canonicalized into a different standard SDNode. Could there be potential benefits if we incorporate some DAGCombine patterns in this approach?

https://github.com/llvm/llvm-project/pull/122863