[llvm] [PowerPC] Add custom lowering for ssubo (PR #111748)

[Amy Kwan via llvm-commits]

================
@@ -11967,6 +11971,36 @@ SDValue PPCTargetLowering::LowerFP_EXTEND(SDValue Op, SelectionDAG &DAG) const {
   llvm_unreachable("ERROR:Should return for all cases within swtich.");
 }
 
+SDValue PPCTargetLowering::LowerSSUBO(SDValue Op, SelectionDAG &DAG) const {
+
+  SDLoc dl(Op);
+
+  SDValue LHS64 = DAG.getNode(ISD::ANY_EXTEND, dl, MVT::i64, Op.getOperand(0));
+  SDValue RHS64 = DAG.getNode(ISD::ANY_EXTEND, dl, MVT::i64, Op.getOperand(1));
+
+  SDValue Sub = DAG.getNode(ISD::SUB, dl, MVT::i64, LHS64, RHS64);
+
+  SDValue Extsw = DAG.getNode(ISD::SIGN_EXTEND_INREG, dl, MVT::i64, Sub,
+                              DAG.getValueType(MVT::i32));
+
+  SDValue Xor = DAG.getNode(ISD::XOR, dl, MVT::i64, Extsw, Sub);
+
+  SDValue Addic = DAG.getNode(ISD::ADDC, dl, DAG.getVTList(MVT::i64, MVT::Glue),
+                              Xor, DAG.getConstant(-1, dl, MVT::i64));
+
+  SDValue Overflow =
+      DAG.getNode(ISD::SUBE, dl, DAG.getVTList(MVT::i64, MVT::Glue), Xor, Addic,
+                  Addic.getValue(1));
+
+  SDValue OverflowTrunc =
+      DAG.getNode(ISD::TRUNCATE, dl, Op.getNode()->getValueType(1), Overflow);
+  SDValue SubTrunc =
+      (Sub->getValueType(0) != Op.getNode()->getValueType(0))
----------------
amy-kwan wrote:

Nit: Can probably pull out `Op.getNode()->getValueType(0)`.

https://github.com/llvm/llvm-project/pull/111748