optimize is_finite on floating points

Author: folkertdev

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -9122,8 +9122,15 @@ SDValue TargetLowering::expandIS_FPCLASS(EVT ResultVT, SDValue Op,
     ; // Detect finite numbers of f80 by checking individual classes because
       // they have different settings of the explicit integer bit.
   else if ((Test & fcFinite) == fcFinite) {
-    // finite(V) ==> abs(V) < exp_mask
-    PartialRes = DAG.getSetCC(DL, ResultVT, AbsV, ExpMaskV, ISD::SETLT);
+    // finite(V) ==> (a << 1) < (inf << 1)
+    //
+    // See https://github.com/llvm/llvm-project/issues/169270, this is slightly
+    // shorter than the `finite(V) ==> abs(V) < exp_mask` formula used before.
+    SDValue One = DAG.getShiftAmountConstant(1, IntVT, DL);
+    SDValue TwiceOp = DAG.getNode(ISD::SHL, DL, IntVT, OpAsInt, One);
+    SDValue TwiceInf = DAG.getNode(ISD::SHL, DL, IntVT, ExpMaskV, One);
+
+    PartialRes = DAG.getSetCC(DL, ResultVT, TwiceOp, TwiceInf, ISD::SETULT);
     Test &= ~fcFinite;
   } else if ((Test & fcFinite) == fcPosFinite) {
     // finite(V) && V > 0 ==> V < exp_mask