Index: llvm/include/llvm/CodeGen/GlobalISel/CallLowering.h
===================================================================
--- llvm/include/llvm/CodeGen/GlobalISel/CallLowering.h
+++ llvm/include/llvm/CodeGen/GlobalISel/CallLowering.h
@@ -144,9 +144,17 @@
   /// class abstracts the differences.
   struct ValueHandler {
     ValueHandler(bool IsIncoming, MachineIRBuilder &MIRBuilder,
-                 MachineRegisterInfo &MRI, CCAssignFn *AssignFn)
-        : MIRBuilder(MIRBuilder), MRI(MRI), AssignFn(AssignFn),
-          IsIncomingArgumentHandler(IsIncoming) {}
+                 MachineRegisterInfo &MRI, CCAssignFn *AssignFn_,
+                 CCAssignFn *AssignFnVarArg_ = nullptr)
+        : MIRBuilder(MIRBuilder), MRI(MRI), AssignFn(AssignFn_),
+          AssignFnVarArg(AssignFnVarArg_),
+          IsIncomingArgumentHandler(IsIncoming) {
+
+      // Some targets change the handler depending on whether the call is
+      // varargs or not. If
+      if (!AssignFnVarArg)
+        AssignFnVarArg = AssignFn;
+    }
 
     virtual ~ValueHandler() = default;
 
@@ -226,13 +234,26 @@
     virtual bool assignArg(unsigned ValNo, EVT OrigVT, MVT ValVT, MVT LocVT,
                            CCValAssign::LocInfo LocInfo, const ArgInfo &Info,
                            ISD::ArgFlagsTy Flags, CCState &State) {
-      return AssignFn(ValNo, ValVT, LocVT, LocInfo, Flags, State);
+      return getAssignFn(State.isVarArg())(ValNo, ValVT, LocVT, LocInfo, Flags,
+                                           State);
     }
 
     MachineIRBuilder &MIRBuilder;
     MachineRegisterInfo &MRI;
+
+    /// Assignment function to use for a general call.
     CCAssignFn *AssignFn;
 
+    /// Assignment function to use for a variadic call. This is usually the same
+    /// as AssignFn.
+    CCAssignFn *AssignFnVarArg;
+
+    /// Select the appropriate assignment function depending on whether this is
+    /// a variadic call.
+    CCAssignFn *getAssignFn(bool IsVarArg) const {
+      return IsVarArg ? AssignFnVarArg : AssignFn;
+    }
+
   private:
     bool IsIncomingArgumentHandler;
     virtual void anchor();
@@ -240,8 +261,9 @@
 
   struct IncomingValueHandler : public ValueHandler {
     IncomingValueHandler(MachineIRBuilder &MIRBuilder, MachineRegisterInfo &MRI,
-                         CCAssignFn *AssignFn)
-        : ValueHandler(true, MIRBuilder, MRI, AssignFn) {}
+                         CCAssignFn *AssignFn_,
+                         CCAssignFn *AssignFnVarArg_ = nullptr)
+        : ValueHandler(true, MIRBuilder, MRI, AssignFn_, AssignFnVarArg_) {}
 
     /// Insert G_ASSERT_ZEXT/G_ASSERT_SEXT or other hint instruction based on \p
     /// VA, returning the new register if a hint was inserted.
@@ -254,8 +276,9 @@
 
   struct OutgoingValueHandler : public ValueHandler {
     OutgoingValueHandler(MachineIRBuilder &MIRBuilder, MachineRegisterInfo &MRI,
-                         CCAssignFn *AssignFn)
-        : ValueHandler(false, MIRBuilder, MRI, AssignFn) {}
+                         CCAssignFn *AssignFn,
+                         CCAssignFn *AssignFnVarArg = nullptr)
+        : ValueHandler(false, MIRBuilder, MRI, AssignFn, AssignFnVarArg) {}
   };
 
 protected:
Index: llvm/lib/Target/AArch64/GISel/AArch64CallLowering.cpp
===================================================================
--- llvm/lib/Target/AArch64/GISel/AArch64CallLowering.cpp
+++ llvm/lib/Target/AArch64/GISel/AArch64CallLowering.cpp
@@ -138,7 +138,8 @@
                  const CallLowering::ArgInfo &Info, ISD::ArgFlagsTy Flags,
                  CCState &State) override {
     applyStackPassedSmallTypeDAGHack(OrigVT, ValVT, LocVT);
-    return AssignFn(ValNo, ValVT, LocVT, LocInfo, Flags, State);
+    return getAssignFn(State.isVarArg())(ValNo, ValVT, LocVT, LocInfo, Flags,
+                                         State);
   }
 
   /// How the physical register gets marked varies between formal
@@ -187,9 +188,9 @@
                      MachineInstrBuilder MIB, CCAssignFn *AssignFn,
                      CCAssignFn *AssignFnVarArg, bool IsReturn,
                      bool IsTailCall = false, int FPDiff = 0)
-      : OutgoingValueHandler(MIRBuilder, MRI, AssignFn), MIB(MIB),
-        AssignFnVarArg(AssignFnVarArg), IsReturn(IsReturn),
-        IsTailCall(IsTailCall), FPDiff(FPDiff), StackSize(0), SPReg(0),
+      : OutgoingValueHandler(MIRBuilder, MRI, AssignFn, AssignFnVarArg),
+        MIB(MIB), IsReturn(IsReturn), IsTailCall(IsTailCall), FPDiff(FPDiff),
+        StackSize(0), SPReg(0),
         Subtarget(MIRBuilder.getMF().getSubtarget<AArch64Subtarget>()) {}
 
   Register getStackAddress(uint64_t Size, int64_t Offset,
@@ -296,7 +297,6 @@
   }
 
   MachineInstrBuilder MIB;
-  CCAssignFn *AssignFnVarArg;
 
   /// Track if this is used for a return instead of function argument
   /// passing. We apply a hack to i1/i8/i16 stack passed values, but do not use