[llvm] [X86][CodeGen] Support lowering for CCMP/CTEST (PR #91747)

[Shengchen Kan via llvm-commits]

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@@ -49217,6 +49228,147 @@ static SDValue combineBMILogicOp(SDNode *N, SelectionDAG &DAG,
   return SDValue();
 }
 
+static SDValue combineX86SubCmpForFlags(SDNode *N, SDValue Flag,
+                                        SelectionDAG &DAG,
+                                        TargetLowering::DAGCombinerInfo &DCI,
+                                        const X86Subtarget &ST) {
+  // cmp(setcc(cc, X), 0)
+  // brcond ne
+  //  ->
+  //    X
+  //    brcond cc
+
+  // sub(setcc(cc, X), 1)
+  // brcond ne
+  //  ->
+  //    X
+  //    brcond ~cc
+  //
+  // if only flag has users
+
+  SDValue SetCC = N->getOperand(0);
+
+  // TODO: Remove the check hasCCMP() and update the non-APX tests.
+  if (!ST.hasCCMP() || SetCC.getOpcode() != X86ISD::SETCC || !Flag.hasOneUse())
+    return SDValue();
+
+  // Check the only user of flag is `brcond ne`.
+  SDNode *BrCond = *Flag->uses().begin();
+  if (BrCond->getOpcode() != X86ISD::BRCOND)
+    return SDValue();
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KanRobert wrote:

Is there such a limitation? Starting from the SUB/CMP seems more intuitive to me.

https://github.com/llvm/llvm-project/pull/91747