[RISCV][TTI] Scale the cost of trunc/fptrunc/fpext with LMUL (#87101)

Use the destination data type to measure the LMUL size for
latency/throughput cost

Author: arcbbb

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -927,6 +927,7 @@ InstructionCost RISCVTTIImpl::getCastInstrCost(unsigned Opcode, Type *Dst,
   if (!IsTypeLegal)
     return BaseT::getCastInstrCost(Opcode, Dst, Src, CCH, CostKind, I);
 
+  std::pair<InstructionCost, MVT> SrcLT = getTypeLegalizationCost(Src);
   std::pair<InstructionCost, MVT> DstLT = getTypeLegalizationCost(Dst);
 
   int ISD = TLI->InstructionOpcodeToISD(Opcode);
@@ -961,13 +962,31 @@ InstructionCost RISCVTTIImpl::getCastInstrCost(unsigned Opcode, Type *Dst,
       // Instead we use the following instructions to truncate to mask vector:
       // vand.vi v8, v8, 1
       // vmsne.vi v0, v8, 0
-      return 2;
+      return getRISCVInstructionCost({RISCV::VAND_VI, RISCV::VMSNE_VI},
+                                     SrcLT.second, CostKind);
     }
     [[fallthrough]];
   case ISD::FP_EXTEND:
-  case ISD::FP_ROUND:
+  case ISD::FP_ROUND: {
     // Counts of narrow/widen instructions.
-    return std::abs(PowDiff);
+    unsigned SrcEltSize = Src->getScalarSizeInBits();
+    unsigned DstEltSize = Dst->getScalarSizeInBits();
+
+    unsigned Op = (ISD == ISD::TRUNCATE)    ? RISCV::VNSRL_WI
+                  : (ISD == ISD::FP_EXTEND) ? RISCV::VFWCVT_F_F_V
+                                            : RISCV::VFNCVT_F_F_W;
+    InstructionCost Cost = 0;
+    for (; SrcEltSize != DstEltSize;) {
+      MVT ElementMVT = (ISD == ISD::TRUNCATE)
+                           ? MVT::getIntegerVT(DstEltSize)
+                           : MVT::getFloatingPointVT(DstEltSize);
+      MVT DstMVT = DstLT.second.changeVectorElementType(ElementMVT);
+      DstEltSize =
+          (DstEltSize > SrcEltSize) ? DstEltSize >> 1 : DstEltSize << 1;
+      Cost += getRISCVInstructionCost(Op, DstMVT, CostKind);
+    }
+    return Cost;
+  }
   case ISD::FP_TO_SINT:
   case ISD::FP_TO_UINT:
   case ISD::SINT_TO_FP: