[llvm] [AArch64][SVE] Use SVE for scalar FP converts in streaming[-compatible] functions (1/n) (PR #118505)

[Paul Walker via llvm-commits]

================
@@ -19083,13 +19083,70 @@ static SDValue performVectorCompareAndMaskUnaryOpCombine(SDNode *N,
   return SDValue();
 }
 
+/// Tries to replace scalar FP <-> INT conversions with SVE in streaming
+/// functions, this can help to reduce the number of fmovs to/from GPRs.
+static SDValue
+tryToReplaceScalarFPConversionWithSVE(SDNode *N, SelectionDAG &DAG,
+                                      TargetLowering::DAGCombinerInfo &DCI,
+                                      const AArch64Subtarget *Subtarget) {
+  if (N->isStrictFPOpcode())
+    return SDValue();
+
+  if (DCI.isBeforeLegalizeOps())
+    return SDValue();
+
+  if (!Subtarget->isSVEorStreamingSVEAvailable() ||
+      (!Subtarget->isStreaming() && !Subtarget->isStreamingCompatible()))
+    return SDValue();
+
+  auto isSupportedType = [](EVT VT) {
+    return VT != MVT::bf16 && VT != MVT::f128;
+  };
+
+  if (!isSupportedType(N->getValueType(0)) ||
+      !isSupportedType(N->getOperand(0).getValueType()))
+    return SDValue();
+
+  SDValue SrcVal = N->getOperand(0);
+  EVT SrcTy = SrcVal.getValueType();
+  EVT DestTy = N->getValueType(0);
+
+  EVT SrcVecTy;
+  EVT DestVecTy;
+  if (DestTy.bitsGT(SrcTy)) {
+    DestVecTy = getPackedSVEVectorVT(DestTy);
+    SrcVecTy = DestVecTy.changeVectorElementType(SrcTy);
+  } else {
+    SrcVecTy = getPackedSVEVectorVT(SrcTy);
+    DestVecTy = SrcVecTy.changeVectorElementType(DestTy);
+  }
+
+  // Ensure the resulting src/dest vector type is legal.
+  if (SrcVecTy == MVT::nxv2i32 || DestVecTy == MVT::nxv2i32)
+    return SDValue();
+
+  SDLoc DL(N);
+  SDValue ZeroIdx = DAG.getVectorIdxConstant(0, DL);
+  SDValue Vec = DAG.getNode(ISD::INSERT_VECTOR_ELT, DL, SrcVecTy,
+                            DAG.getUNDEF(SrcVecTy), SrcVal, ZeroIdx);
+  SDValue Convert = DAG.getNode(N->getOpcode(), DL, DestVecTy, Vec);
+  SDValue Scalar =
+      DAG.getNode(ISD::EXTRACT_VECTOR_ELT, DL, DestTy, Convert, ZeroIdx);
+  return Scalar;
----------------
paulwalker-arm wrote:

```suggestion
  return DAG.getNode(ISD::EXTRACT_VECTOR_ELT, DL, DestTy, Convert, ZeroIdx);
```

https://github.com/llvm/llvm-project/pull/118505