[llvm] [RISCV] Reduce LMUL when index is known when lowering insert_vector_elt (PR #66087)

[Luke Lau via llvm-commits]

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@@ -7510,6 +7531,10 @@ SDValue RISCVTargetLowering::lowerINSERT_VECTOR_ELT(SDValue Op,
     Policy = RISCVII::TAIL_AGNOSTIC;
   SDValue Slideup = getVSlideup(DAG, Subtarget, DL, ContainerVT, Vec, ValInVec,
                                 Idx, Mask, InsertVL, Policy);
+
+  if (ContainerVT != OrigContainerVT)
+    Slideup = DAG.getNode(ISD::INSERT_SUBVECTOR, DL, OrigContainerVT, OrigVec,
+                          Slideup, DAG.getVectorIdxConstant(0, DL));
   if (!VecVT.isFixedLengthVector())
     return Slideup;
   return convertFromScalableVector(VecVT, Slideup, DAG, Subtarget);
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lukel97 wrote:

Making a mental note here re: #65997, all the return paths here should be covered by a reinsert into the correct original vector type after shrinking

https://github.com/llvm/llvm-project/pull/66087