[RISCV] Use vnclip(u) to handle fp_to_(s/u)int_sat that needs additional narrowing. (#100071)

If vncvt doesn't produce the destination type directly, use vnclip to do
additional narrowing with saturation.

Author: topperc

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -2955,10 +2955,6 @@ static SDValue lowerFP_TO_INT_SAT(SDValue Op, SelectionDAG &DAG,
   if (SatVT != DstEltVT)
     return SDValue();
 
-  // FIXME: Don't support narrowing by more than 1 steps for now.
-  if (SrcEltSize > (2 * DstEltSize))
-    return SDValue();
-
   MVT DstContainerVT = DstVT;
   MVT SrcContainerVT = SrcVT;
   if (DstVT.isFixedLengthVector()) {
@@ -2986,9 +2982,29 @@ static SDValue lowerFP_TO_INT_SAT(SDValue Op, SelectionDAG &DAG,
     Src = DAG.getNode(RISCVISD::FP_EXTEND_VL, DL, InterVT, Src, Mask, VL);
   }
 
+  MVT CvtContainerVT = DstContainerVT;
+  MVT CvtEltVT = DstEltVT;
+  if (SrcEltSize > (2 * DstEltSize)) {
+    CvtEltVT = MVT::getIntegerVT(SrcEltVT.getSizeInBits() / 2);
+    CvtContainerVT = CvtContainerVT.changeVectorElementType(CvtEltVT);
+  }
+
   unsigned RVVOpc =
       IsSigned ? RISCVISD::VFCVT_RTZ_X_F_VL : RISCVISD::VFCVT_RTZ_XU_F_VL;
-  SDValue Res = DAG.getNode(RVVOpc, DL, DstContainerVT, Src, Mask, VL);
+  SDValue Res = DAG.getNode(RVVOpc, DL, CvtContainerVT, Src, Mask, VL);
+
+  while (CvtContainerVT != DstContainerVT) {
+    CvtEltVT = MVT::getIntegerVT(CvtEltVT.getSizeInBits() / 2);
+    CvtContainerVT = CvtContainerVT.changeVectorElementType(CvtEltVT);
+    // Rounding mode here is arbitrary since we aren't shifting out any bits.
+    unsigned ClipOpc = IsSigned ? RISCVISD::VNCLIP_VL : RISCVISD::VNCLIPU_VL;
+    Res = DAG.getNode(
+        ClipOpc, DL, CvtContainerVT,
+        {Res, DAG.getConstant(0, DL, CvtContainerVT),
+         DAG.getUNDEF(CvtContainerVT), Mask,
+         DAG.getTargetConstant(RISCVVXRndMode::RNU, DL, Subtarget.getXLenVT()),
+         VL});
+  }
 
   SDValue SplatZero = DAG.getNode(
       RISCVISD::VMV_V_X_VL, DL, DstContainerVT, DAG.getUNDEF(DstContainerVT),