[llvm] [PowerPC] Lower ucmp using subtractions (PR #146446)

[via llvm-commits]

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@@ -12521,6 +12525,33 @@ SDValue PPCTargetLowering::LowerSSUBO(SDValue Op, SelectionDAG &DAG) const {
   return DAG.getMergeValues({Sub, OverflowTrunc}, dl);
 }
 
+// Lower unsigned 3-way compare producing -1/0/1.
+SDValue PPCTargetLowering::LowerUCMP(SDValue Op, SelectionDAG &DAG) const {
+  SDLoc DL(Op);
+  SDValue A = DAG.getFreeze(Op.getOperand(0));
+  SDValue B = DAG.getFreeze(Op.getOperand(1));
+  EVT OpVT = A.getValueType();   // operand type
+  EVT ResVT = Op.getValueType(); // result type
+
+  // First compute diff = A - B (will become subf).
+  SDValue Diff = DAG.getNode(ISD::SUB, DL, OpVT, A, B);
+
+  // Generate B - A using SUBC to capture carry.
+  SDVTList VTs = DAG.getVTList(OpVT, MVT::i32);
+  SDValue SubC = DAG.getNode(PPCISD::SUBC, DL, VTs, B, A);
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AZero13 wrote:

> Are PPC SUBC/SUBE just the same as the normal sub-with-overflow, just with glue? Can this be a generic expansion with those instead, and transform those into the target glue nodes? 

NO. 

Sube on PowerPC has a unique characteristic. It's not a-b-carry, it's a - b + carry. 

Because of this, only PowerPC gets to have a lowering like this. 

https://github.com/llvm/llvm-project/pull/146446