[LoongArch] Override cost hooks to expose more DAG combine opportunities

Author: zhaoqi5

llvm/lib/Target/LoongArch/LoongArchISelLowering.cpp

@@ -9094,3 +9094,20 @@ bool LoongArchTargetLowering::SimplifyDemandedBitsForTargetNode(
   return TargetLowering::SimplifyDemandedBitsForTargetNode(
       Op, OriginalDemandedBits, OriginalDemandedElts, Known, TLO, Depth);
 }
+
+bool LoongArchTargetLowering::isExtractSubvectorCheap(EVT ResVT, EVT SrcVT,
+                                                      unsigned Index) const {
+  if (!isOperationLegalOrCustom(ISD::EXTRACT_SUBVECTOR, ResVT))
+    return false;
+
+  // Extract a 128-bit subvector from index 0 of a 256-bit vector is free.
+  return Index == 0;
+}
+
+bool LoongArchTargetLowering::isExtractVecEltCheap(EVT VT,
+                                                   unsigned Index) const {
+  EVT EltVT = VT.getScalarType();
+
+  // Extract a scalar FP value from index 0 of a vector is free.
+  return (EltVT == MVT::f32 || EltVT == MVT::f64) && Index == 0;
+}

llvm/lib/Target/LoongArch/LoongArchISelLowering.h

@@ -332,6 +332,10 @@ class LoongArchTargetLowering : public TargetLowering {
                                          TargetLoweringOpt &TLO,
                                          unsigned Depth) const override;
 
+  bool isExtractSubvectorCheap(EVT ResVT, EVT SrcVT,
+                               unsigned Index) const override;
+  bool isExtractVecEltCheap(EVT VT, unsigned Index) const override;
+
 private:
   /// Target-specific function used to lower LoongArch calling conventions.
   typedef bool LoongArchCCAssignFn(const DataLayout &DL, LoongArchABI::ABI ABI,