DAG: Avoid asserting on libcall action if function is unavailable

Eventually the set of available functions will be a program
dependent property, which could diverge from the static table of
functions for the subtarget. In that case, fall back to the usual
expansion.

Author: arsenm

llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp

@@ -4102,14 +4102,20 @@ void DAGTypeLegalizer::ExpandIntRes_CTPOP(SDNode *N, SDValue &Lo, SDValue &Hi) {
 
   if (TLI.getOperationAction(ISD::CTPOP, VT) == TargetLoweringBase::LibCall) {
     RTLIB::Libcall LC = RTLIB::getCTPOP(VT);
-    assert(LC != RTLIB::UNKNOWN_LIBCALL && TLI.getLibcallName(LC) &&
+    assert(LC != RTLIB::UNKNOWN_LIBCALL &&
            "LibCall explicitly requested, but not available");
-    TargetLowering::MakeLibCallOptions CallOptions;
-    EVT IntVT =
-        EVT::getIntegerVT(*DAG.getContext(), DAG.getLibInfo().getIntSize());
-    SDValue Res = TLI.makeLibCall(DAG, LC, IntVT, Op, CallOptions, DL).first;
-    SplitInteger(DAG.getSExtOrTrunc(Res, DL, VT), Lo, Hi);
-    return;
+
+    if (RTLIB::LibcallImpl LCImpl = TLI.getLibcallImpl(LC)) {
+      TargetLowering::MakeLibCallOptions CallOptions;
+      EVT IntVT =
+          EVT::getIntegerVT(*DAG.getContext(), DAG.getLibInfo().getIntSize());
+      SDValue Res =
+          TLI.makeLibCall(DAG, LCImpl, IntVT, Op, CallOptions, DL).first;
+      SplitInteger(DAG.getSExtOrTrunc(Res, DL, VT), Lo, Hi);
+      return;
+    }
+
+    // If the function is not available, fall back on the expansion.
   }
 
   // ctpop(HiLo) -> ctpop(Hi)+ctpop(Lo)