[llvm] [AArch64] Transform add(x, abs(y)) -> saba(x, y, 0) (PR #156615)

[David Sherwood via llvm-commits]

================
@@ -21913,6 +21914,56 @@ static SDValue performExtBinopLoadFold(SDNode *N, SelectionDAG &DAG) {
   return DAG.getNode(N->getOpcode(), DL, VT, Ext0, NShift);
 }
 
+// Transform the following:
+// - add(x, abs(y)) -> saba(x, y, 0)
+// - add(x, zext(abs(y))) -> sabal(x, y, 0)
+static SDValue performAddSABACombine(SDNode *N,
+                                     TargetLowering::DAGCombinerInfo &DCI) {
+  if (N->getOpcode() != ISD::ADD)
+    return SDValue();
+
+  EVT VT = N->getValueType(0);
+  if (!VT.isFixedLengthVector())
+    return SDValue();
+
+  SDValue N0 = N->getOperand(0);
+  SDValue N1 = N->getOperand(1);
+
+  auto MatchAbsOrZExtAbs = [](SDValue V0, SDValue V1, SDValue &AbsOp,
+                              SDValue &Other, bool &IsZExt) {
+    Other = V1;
----------------
david-arm wrote:

I realise with your code below it's not a problem right now, but I think it' still worth only setting `Other` if we found a match.

https://github.com/llvm/llvm-project/pull/156615