[llvm] [RISCV][ISelLowering] Use Zicond for FP selects on Zfinx/Zdinx (PR #169299)

[Craig Topper via llvm-commits]

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@@ -9555,6 +9555,54 @@ SDValue RISCVTargetLowering::lowerSELECT(SDValue Op, SelectionDAG &DAG) const {
   if (SDValue V = lowerSelectToBinOp(Op.getNode(), DAG, Subtarget))
     return V;
 
+  // When there is no cost for GPR <-> FGPR, we can use zicond select for
+  // floating value when CondV is int type
+  bool FPinGPR = Subtarget.hasStdExtZfinx();
+
+  // We can handle FGPR without spliting into hi/lo parts
+  bool FitsInGPR = TypeSize::isKnownLE(VT.getSizeInBits(),
+                                       Subtarget.getXLenVT().getSizeInBits());
+
+  bool UseZicondForFPSel = Subtarget.hasStdExtZicond() && FPinGPR &&
+                           VT.isFloatingPoint() && FitsInGPR;
+
+  if (UseZicondForFPSel) {
+    MVT XLenIntVT = Subtarget.getXLenVT();
+
+    auto CastToInt = [&](SDValue V) -> SDValue {
+      // Treat +0.0 as integer 0 to enable single 'czero' instruction
+      // generation.
+      if (auto *CFP = dyn_cast<ConstantFPSDNode>(V)) {
+        if (CFP->isZero() && !CFP->isNegative())
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topperc wrote:

Oh that only exists in APFloat.

https://github.com/llvm/llvm-project/pull/169299