[llvm] add narrowExtractedVectorUnaryOp to simplify cast nodes (PR #87977)

[Simon Pilgrim via llvm-commits]

================
@@ -24083,6 +24083,55 @@ static SDValue narrowInsertExtractVectorBinOp(SDNode *Extract,
                      BinOp->getFlags());
 }
 
+/// If we are extracting a subvector produced by a wide unary operator try
+/// to use a narrow unary operator and/or avoid extraction.
+static SDValue narrowExtractedVectorUnaryOp(SDNode *Extract, SelectionDAG &DAG,
+                                          bool LegalOperations) {
+  const TargetLowering &TLI = DAG.getTargetLoweringInfo();
+  SDValue UnaryOp = Extract->getOperand(0);
+  unsigned UnaryOpcode = UnaryOp.getOpcode();
+  
+  if (UnaryOpcode != ISD::FP_TO_SINT || UnaryOp->getNumValues() != 1)
+    return SDValue();
+
+  // The extract index must be a constant, so we can map it to a concat operand.
+  auto *ExtractIndexC = dyn_cast<ConstantSDNode>(Extract->getOperand(1));
+  if (!ExtractIndexC)
+    return SDValue();
+
+  EVT WideUVT = UnaryOp.getValueType();
+  if (!WideUVT.isFixedLengthVector())
+    return SDValue();
+  
+  EVT VT = Extract->getValueType(0);
+  unsigned ExtractIndex = ExtractIndexC->getZExtValue();
+  assert(ExtractIndex % VT.getVectorNumElements() == 0 &&
+         "Extract index is not a multiple of the vector length.");
+
+  // Bail out if this is not a proper multiple width extraction.
+  unsigned WideWidth = WideUVT.getSizeInBits();
+  unsigned NarrowWidth = VT.getSizeInBits();
+  if (WideWidth % NarrowWidth != 0)
+    return SDValue();
+
+  unsigned NarrowingRatio = WideWidth / NarrowWidth;
+  unsigned WideNumElts = WideUVT.getVectorNumElements();
+
+  // Bail out if the target does not support a narrower version of the unaryop.
+  EVT NarrowUVT = EVT::getVectorVT(*DAG.getContext(), WideUVT.getScalarType(),
+                                   WideNumElts / NarrowingRatio);
+  if (!TLI.isOperationLegalOrCustomOrPromote(UnaryOpcode, NarrowUVT,
+                                             LegalOperations))
+    return SDValue();
+  
----------------
RKSimon wrote:

You need to add similar code to extract_subvector stage here: https://github.com/llvm/llvm-project/blob/170c525d79a4ab3659041b0655ac9697768fc915/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp#L24157-L24169

https://github.com/llvm/llvm-project/pull/87977