[llvm] [RISCV] Prefer (select (x < 0), y, z) -> x >> (XLEN - 1) & (y - z) + z even with Zicond. (PR #125772)

[Yingwei Zheng via llvm-commits]

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@@ -8378,6 +8378,33 @@ SDValue RISCVTargetLowering::lowerSELECT(SDValue Op, SelectionDAG &DAG) const {
     if (isa<ConstantSDNode>(TrueV) && isa<ConstantSDNode>(FalseV)) {
       const APInt &TrueVal = TrueV->getAsAPIntVal();
       const APInt &FalseVal = FalseV->getAsAPIntVal();
+
+      // Prefer these over Zicond to avoid materializing an immediate:
+      //   (select (x < 0), y, z)  -> x >> (XLEN - 1) & (y - z) + z
+      //   (select (x > -1), z, y) -> x >> (XLEN - 1) & (y - z) + z
+      if (CondV.getOpcode() == ISD::SETCC &&
+          CondV.getOperand(0).getValueType() == VT && CondV.hasOneUse()) {
+        ISD::CondCode CCVal = cast<CondCodeSDNode>(CondV.getOperand(2))->get();
+        if ((CCVal == ISD::SETLT && isNullConstant(CondV.getOperand(1))) ||
+            (CCVal == ISD::SETGT && isAllOnesConstant(CondV.getOperand(1)))) {
+          int64_t TrueImm = TrueVal.getSExtValue();
+          int64_t FalseImm = FalseVal.getSExtValue();
+          if (CCVal == ISD::SETGT)
+            std::swap(TrueImm, FalseImm);
+          if (isInt<12>(TrueImm) && isInt<12>(FalseImm) &&
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dtcxzyw wrote:

Do we really need `isInt<12>(FalseImm)`?


https://github.com/llvm/llvm-project/pull/125772