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

[Paul Walker via llvm-commits]

================
@@ -18989,13 +18989,75 @@ 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,
+                                      const AArch64Subtarget *Subtarget) {
+  if (N->isStrictFPOpcode())
+    return SDValue();
+
+  if (!Subtarget->isSVEorStreamingSVEAvailable() ||
+      (!Subtarget->isStreaming() && !Subtarget->isStreamingCompatible()))
+    return SDValue();
+
+  auto isSupportedType = [](EVT VT) {
+    if (!VT.isSimple())
+      return false;
+    // There are SVE instructions that can convert to/from all pairs of these
+    // int and float types. Note: We don't bother with i8 or i16 as those are
+    // illegal types for scalars.
+    return is_contained({MVT::i32, MVT::i64, MVT::f16, MVT::f32, MVT::f64},
+                        VT.getSimpleVT().SimpleTy);
+  };
+
+  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);
+
+  bool IsI32ToF64 = SrcTy == MVT::i32 && DestTy == MVT::f64;
+  bool isF64ToI32 = SrcTy == MVT::f64 && DestTy == MVT::i32;
+
+  // Conversions between f64 and i32 are a special case as nxv2i32 is an illegal
+  // type (unlike the equivalent nxv2f32 for floating-point types).
+  // TODO: Support these conversations.
+  if (IsI32ToF64 || isF64ToI32)
+    return SDValue();
+
+  EVT SrcVecTy;
+  EVT DestVecTy;
+  if (DestTy.bitsGT(SrcTy)) {
+    DestVecTy = getPackedSVEVectorVT(DestTy);
+    SrcVecTy = SrcTy == MVT::i32 ? getPackedSVEVectorVT(SrcTy)
+                                 : DestVecTy.changeVectorElementType(SrcTy);
----------------
paulwalker-arm wrote:

Can this be just `SrcVecTy = DestVecTy.changeVectorElementType(SrcTy)` now that you're bailing for the special case above?

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