Add note about a subtle bug in this code. Does not effect the main

architectures that LLVM targets, because they don't use this code.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@90564 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp

@@ -1180,6 +1180,8 @@ ExpandShiftWithUnknownAmountBit(SDNode *N, SDValue &Lo, SDValue &Hi) {
     LoS = DAG.getNode(ISD::SHL, dl, NVT, InL, Amt);
     HiS = DAG.getNode(ISD::OR, dl, NVT,
                       DAG.getNode(ISD::SHL, dl, NVT, InH, Amt),
+    // FIXME: If Amt is zero, the following shift generates an undefined result
+    // on some architectures.
                       DAG.getNode(ISD::SRL, dl, NVT, InL, AmtLack));
 
     // Long: ShAmt >= NVTBits
@@ -1194,6 +1196,8 @@ ExpandShiftWithUnknownAmountBit(SDNode *N, SDValue &Lo, SDValue &Hi) {
     HiS = DAG.getNode(ISD::SRL, dl, NVT, InH, Amt);
     LoS = DAG.getNode(ISD::OR, dl, NVT,
                       DAG.getNode(ISD::SRL, dl, NVT, InL, Amt),
+    // FIXME: If Amt is zero, the following shift generates an undefined result
+    // on some architectures.
                       DAG.getNode(ISD::SHL, dl, NVT, InH, AmtLack));
 
     // Long: ShAmt >= NVTBits
@@ -1208,6 +1212,8 @@ ExpandShiftWithUnknownAmountBit(SDNode *N, SDValue &Lo, SDValue &Hi) {
     HiS = DAG.getNode(ISD::SRA, dl, NVT, InH, Amt);
     LoS = DAG.getNode(ISD::OR, dl, NVT,
                       DAG.getNode(ISD::SRL, dl, NVT, InL, Amt),
+    // FIXME: If Amt is zero, the following shift generates an undefined result
+    // on some architectures.
                       DAG.getNode(ISD::SHL, dl, NVT, InH, AmtLack));
 
     // Long: ShAmt >= NVTBits