expandFMINIMUMNUM_FMAXIMUMNUM: Improve compare between zeros

1. On GPR32 platform, expandIS_FPCLASS may fail due to ISD::BITCAST
double to int64 may fail. Let's FP_ROUND double to float first.
Since we use it if MinMax is zero only, so the flushing won't
break anything.

2. Only one IS_FPCLASS is needed. MinMax will always be RHS if equal.
So we can select between LHS and MinMax.
It will even safe if FP_ROUND flush a small LHS, as if LHS is not zero
then, MinMax won't be Zero, so we will always use MinMax.

Author: wzssyqa

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -8690,6 +8690,7 @@ SDValue TargetLowering::expandFMINIMUMNUM_FMAXIMUMNUM(SDNode *Node,
     RHS = DAG.getSelectCC(DL, RHS, RHS, LHS, RHS, ISD::SETUO);
   }
 
+  // Please always prefer RHS if equal.
   SDValue MinMax =
       DAG.getSelectCC(DL, LHS, RHS, LHS, RHS, IsMax ? ISD::SETGT : ISD::SETLT);
 
@@ -8704,13 +8705,27 @@ SDValue TargetLowering::expandFMINIMUMNUM_FMAXIMUMNUM(SDNode *Node,
       DAG.getTargetConstant(IsMax ? fcPosZero : fcNegZero, DL, MVT::i32);
   SDValue IsZero = DAG.getSetCC(DL, CCVT, MinMax,
                                 DAG.getConstantFP(0.0, DL, VT), ISD::SETEQ);
-  SDValue LCmp = DAG.getSelect(
-      DL, VT, DAG.getNode(ISD::IS_FPCLASS, DL, CCVT, LHS, TestZero), LHS,
+  unsigned BitSize = VT.getScalarSizeInBits();
+  EVT IntVT = EVT::getIntegerVT(*DAG.getContext(), BitSize);
+  EVT FloatVT = EVT::getFloatingPointVT(32);
+  if (VT.isVector()) {
+    IntVT =
+        EVT::getVectorVT(*DAG.getContext(), IntVT, VT.getVectorElementCount());
+    FloatVT = EVT::getVectorVT(*DAG.getContext(), FloatVT,
+                               VT.getVectorElementCount());
+  }
+  SDValue LHSTrunc = LHS;
+  if (!isOperationLegal(ISD::BITCAST, IntVT) &&
+      !isOperationLegal(ISD::IS_FPCLASS, VT)) {
+    LHSTrunc = DAG.getNode(ISD::FP_ROUND, DL, FloatVT, LHS,
+                           DAG.getIntPtrConstant(0, DL, /*isTarget=*/true));
+  }
+  // It's OK to select from LHS and MinMax, with only one ISD::IS_FPCLASS, as
+  // we preferred RHS when generate MinMax, if the operands are equal.
+  SDValue RetZero = DAG.getSelect(
+      DL, VT, DAG.getNode(ISD::IS_FPCLASS, DL, CCVT, LHSTrunc, TestZero), LHS,
       MinMax, Flags);
-  SDValue RCmp = DAG.getSelect(
-      DL, VT, DAG.getNode(ISD::IS_FPCLASS, DL, CCVT, RHS, TestZero), RHS, LCmp,
-      Flags);
-  return DAG.getSelect(DL, VT, IsZero, RCmp, MinMax, Flags);
+  return DAG.getSelect(DL, VT, IsZero, RetZero, MinMax, Flags);
 }
 
 /// Returns a true value if if this FPClassTest can be performed with an ordered