[llvm] [RISCV] Optimize lowering of VECREDUCE_FMINIMUM/VECREDUCE_FMAXIMUM. (PR #85165)

[Philip Reames via llvm-commits]

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@@ -9571,9 +9576,30 @@ SDValue RISCVTargetLowering::lowerFPVECREDUCE(SDValue Op,
     VectorVal = convertToScalableVector(ContainerVT, VectorVal, DAG, Subtarget);
   }
 
+  MVT ResVT = Op.getSimpleValueType();
   auto [Mask, VL] = getDefaultVLOps(VecVT, ContainerVT, DL, DAG, Subtarget);
-  return lowerReductionSeq(RVVOpcode, Op.getSimpleValueType(), ScalarVal,
-                           VectorVal, Mask, VL, DL, DAG, Subtarget);
+  SDValue Res = lowerReductionSeq(RVVOpcode, ResVT, ScalarVal, VectorVal, Mask,
+                                  VL, DL, DAG, Subtarget);
+  if (Op.getOpcode() != ISD::VECREDUCE_FMINIMUM &&
+      Op.getOpcode() != ISD::VECREDUCE_FMAXIMUM)
+    return Res;
+
+  if (Op->getFlags().hasNoNaNs())
+    return Res;
+
+  // Force output to NaN if any element is Nan.
+  SDValue IsNan =
+      DAG.getNode(RISCVISD::SETCC_VL, DL, Mask.getValueType(),
+                  {VectorVal, VectorVal, DAG.getCondCode(ISD::SETNE),
+                   DAG.getUNDEF(Mask.getValueType()), Mask, VL});
+  MVT XLenVT = Subtarget.getXLenVT();
+  SDValue CPop = DAG.getNode(RISCVISD::VCPOP_VL, DL, XLenVT, IsNan, Mask, VL);
+  SDValue NoNaNs = DAG.getSetCC(DL, XLenVT, CPop,
+                                DAG.getConstant(0, DL, XLenVT), ISD::SETEQ);
+  return DAG.getSelect(
+      DL, ResVT, NoNaNs, Res,
+      DAG.getConstantFP(APFloat::getNaN(DAG.EVTToAPFloatSemantics(ResVT)), DL,
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preames wrote:

I think this is okay, but let me call out a possible concern here to confirm.

This is not propagating the nan payload (if any) from the argument.  Instead, it is returning a canonical nan if any of the inputs were nan (of any payload).  I don't believe we're required to propagate nan payloads per relevant spec documents, but would be good to confirm you agree.  

https://github.com/llvm/llvm-project/pull/85165