DAG: Avoid using getLibcallName when looking for a divrem call (llvm#170413)

Also introduce an error if it's not available, which is not yet
testable.

Author: arsenm

llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

@@ -2381,8 +2381,19 @@ SelectionDAGLegalize::ExpandDivRemLibCall(SDNode *Node,
   Entry.IsZExt = !isSigned;
   Args.push_back(Entry);
 
-  SDValue Callee = DAG.getExternalSymbol(TLI.getLibcallName(LC),
-                                         TLI.getPointerTy(DAG.getDataLayout()));
+  RTLIB::LibcallImpl LibcallImpl = TLI.getLibcallImpl(LC);
+  if (LibcallImpl == RTLIB::Unsupported) {
+    DAG.getContext()->emitError(Twine("no libcall available for ") +
+                                Node->getOperationName(&DAG));
+    SDValue Poison = DAG.getPOISON(RetVT);
+    Results.push_back(Poison);
+    Results.push_back(Poison);
+    return;
+  }
+
+  SDValue Callee =
+      DAG.getExternalSymbol(TLI.getLibcallImplName(LibcallImpl).data(),
+                            TLI.getPointerTy(DAG.getDataLayout()));
 
   SDLoc dl(Node);
   TargetLowering::CallLoweringInfo CLI(DAG);