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

[Shengchen Kan via llvm-commits]

================
@@ -54508,7 +54513,97 @@ static bool onlyZeroFlagUsed(SDValue Flags) {
   return true;
 }
 
+static SDValue
+combineX86SubCmpToCcmpHelper(SDNode *N, SDValue Flag, SelectionDAG &DAG,
+                             TargetLowering::DAGCombinerInfo &DCI,
+                             const X86Subtarget &ST) {
+  // sub(and(setcc(cc0, flag0), setcc(cc1, sub (X, Y))), 1)
+  // brcond ne
+  //
+  // OR
+  //
+  // sub(and(setcc(cc0, flag0), setcc(cc1, sub (X, Y))), 1)
+  // brcond ne
----------------
KanRobert wrote:

We already considered the commutative law in the function.
```
  // and is commutable. Try to commute the operands and then test again.
  if (SetCC1.getOperand(1).getOpcode() != X86ISD::SUB) {
    std::swap(SetCC0, SetCC1);
    if (SetCC1.getOperand(1).getOpcode() != X86ISD::SUB)
      return SDValue();
  }
```

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