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Author: zhaoqi5

llvm/lib/Target/LoongArch/LoongArchISelLowering.cpp

@@ -2627,7 +2627,12 @@ LoongArchTargetLowering::lowerEXTRACT_VECTOR_ELT(SDValue Op,
       llvm_unreachable("Unexpected type");
     case MVT::v32i8:
     case MVT::v16i16: {
+      // Consider the source vector as v8i32 type.
       SDValue NewVec = DAG.getBitcast(MVT::v8i32, Vec);
+
+      // Compute the adjusted index and use it to broadcast the vector.
+      // The original desired i8/i16 element is now replicated in each
+      // i32 lane of the splatted vector.
       SDValue NewIdx = DAG.getNode(
           LoongArchISD::BSTRPICK, DL, GRLenVT, Idx,
           DAG.getConstant(31, DL, GRLenVT),
@@ -2637,6 +2642,9 @@ LoongArchTargetLowering::lowerEXTRACT_VECTOR_ELT(SDValue Op,
           DAG.getNode(LoongArchISD::XVPERM, DL, MVT::v8i32, NewVec, SplatIdx);
       SDValue SplatVec = DAG.getBitcast(VecTy, SplatValue);
 
+      // Compute the local index of the original i8/i16 element within the
+      // i32 element and then use it to broadcast the vector. Each elements
+      // of the vector will be the desired element.
       SDValue LocalIdx = DAG.getNode(
           ISD::AND, DL, GRLenVT, Idx,
           DAG.getConstant(((VecTy == MVT::v32i8) ? 3 : 1), DL, GRLenVT));
@@ -2657,7 +2665,11 @@ LoongArchTargetLowering::lowerEXTRACT_VECTOR_ELT(SDValue Op,
     }
     case MVT::v4i64:
     case MVT::v4f64: {
+      // Consider the source vector as v8i32 type.
       SDValue NewVec = DAG.getBitcast(MVT::v8i32, Vec);
+
+      // Split the original element index into low and high parts:
+      // Lo = Idx * 2, Hi = Idx * 2 + 1.
       SDValue SplatIdx = DAG.getSplatBuildVector(MVT::v8i32, DL, Idx);
       SDValue SplatIdxLo =
           DAG.getNode(LoongArchISD::VSLLI, DL, MVT::v8i32, SplatIdx,
@@ -2667,10 +2679,15 @@ LoongArchTargetLowering::lowerEXTRACT_VECTOR_ELT(SDValue Op,
                       DAG.getSplatBuildVector(MVT::v8i32, DL,
                                               DAG.getConstant(1, DL, GRLenVT)));
 
+      // Use the broadcasted index to broadcast the low and high parts of the
+      // vector separately.
       SDValue SplatVecLo =
           DAG.getNode(LoongArchISD::XVPERM, DL, MVT::v8i32, NewVec, SplatIdxLo);
       SDValue SplatVecHi =
           DAG.getNode(LoongArchISD::XVPERM, DL, MVT::v8i32, NewVec, SplatIdxHi);
+
+      // Combine the low and high i32 parts to reconstruct the original i64/f64
+      // element.
       SDValue SplatValue = DAG.getNode(LoongArchISD::VILVL, DL, MVT::v8i32,
                                        SplatVecHi, SplatVecLo);
       SDValue ExtractVec = DAG.getBitcast(VecTy, SplatValue);