[llvm] [AArch64][SVE] Use SVE for scalar FP converts in streaming[-compatible] functions (PR #112564)

[Benjamin Maxwell via llvm-commits]

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@@ -18929,13 +18929,67 @@ static SDValue performVectorCompareAndMaskUnaryOpCombine(SDNode *N,
   return SDValue();
 }
 
+static bool
+shouldUseSVEForScalarFPConversion(SDNode *N,
+                                  const AArch64Subtarget *Subtarget) {
+  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 we are in a streaming[-compatible] function, use SVE for scalar FP <->
+  // INT conversions as this can help avoid movs between GPRs and FPRs, which
+  // could be quite expensive.
+  return !N->isStrictFPOpcode() && Subtarget->isSVEorStreamingSVEAvailable() &&
+         (Subtarget->isStreaming() || Subtarget->isStreamingCompatible()) &&
+         isSupportedType(N->getValueType(0)) &&
+         isSupportedType(N->getOperand(0).getValueType());
+}
+
+/// Replaces a scalar FP <-> INT conversion with an SVE (scalable) one, wrapped
+/// with an insert and extract.
+static SDValue replaceScalarFPConversionWithSVE(SDNode *N, SelectionDAG &DAG) {
+  assert(!N->isStrictFPOpcode() && "strict fp ops not supported");
+  SDValue SrcVal = N->getOperand(0);
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MacDue wrote:

Waiting till after legalization has the side-effect of promoting  `f16` to `f32`, which leads to extra unnecessary `fcvt`s being emitted. 

https://github.com/llvm/llvm-project/pull/112564