Index: include/llvm/CodeGen/GlobalISel/CallLowering.h
===================================================================
--- include/llvm/CodeGen/GlobalISel/CallLowering.h
+++ include/llvm/CodeGen/GlobalISel/CallLowering.h
@@ -17,6 +17,7 @@
 
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/CodeGen/CallingConvLower.h"
+#include "llvm/CodeGen/GlobalISel/MachineIRBuilder.h"
 #include "llvm/CodeGen/MachineValueType.h"
 #include "llvm/CodeGen/TargetCallingConv.h"
 #include "llvm/IR/CallSite.h"
@@ -29,7 +30,6 @@
 
 class DataLayout;
 class Function;
-class MachineIRBuilder;
 class MachineOperand;
 struct MachinePointerInfo;
 class MachineRegisterInfo;
@@ -102,7 +102,8 @@
     virtual bool assignArg(unsigned ValNo, MVT ValVT, MVT LocVT,
                            CCValAssign::LocInfo LocInfo, const ArgInfo &Info,
                            CCState &State) {
-      return AssignFn(ValNo, ValVT, LocVT, LocInfo, Info.Flags, State);
+      return AssignFn ?
+          AssignFn(ValNo, ValVT, LocVT, LocInfo, Info.Flags, State) : false;
     }
 
     MachineIRBuilder &MIRBuilder;
@@ -126,13 +127,6 @@
   void setArgFlags(ArgInfo &Arg, unsigned OpNum, const DataLayout &DL,
                    const FuncInfoTy &FuncInfo) const;
 
-  /// Invoke Handler::assignArg on each of the given \p Args and then use
-  /// \p Callback to move them to the assigned locations.
-  ///
-  /// \return True if everything has succeeded, false otherwise.
-  bool handleAssignments(MachineIRBuilder &MIRBuilder, ArrayRef<ArgInfo> Args,
-                         ValueHandler &Callback) const;
-
 public:
   CallLowering(const TargetLowering *TLI) : TLI(TLI) {}
   virtual ~CallLowering() = default;
@@ -188,6 +182,61 @@
     return false;
   }
 
+  /// Invoke Handler::assignArg on each of the given \p Args and then use
+  /// \p Callback to move them to the assigned locations.
+  ///
+  /// \return True if everything has succeeded, false otherwise.
+  virtual bool handleAssignments(MachineIRBuilder &MIRBuilder,
+                                 ArrayRef<ArgInfo> Args,
+                                 ValueHandler &Handler) const {
+    MachineFunction &MF = MIRBuilder.getMF();
+    const Function &F = MF.getFunction();
+    const DataLayout &DL = F.getParent()->getDataLayout();
+
+    SmallVector<CCValAssign, 16> ArgLocs;
+    // TODO: Override CCState with custom <Target>CCState.
+    CCState CCInfo(F.getCallingConv(), F.isVarArg(), MF, ArgLocs,
+            F.getContext());
+
+    unsigned NumArgs = Args.size();
+    for (unsigned i = 0; i != NumArgs; ++i) {
+      MVT CurVT = MVT::getVT(Args[i].Ty);
+      // TODO: Override assignArg, cast from CCState to custom <Target>CCState.
+      if (Handler.assignArg(i, CurVT, CurVT, CCValAssign::Full, Args[i],
+                  CCInfo)) {
+        return false;
+      }
+    }
+
+    for (unsigned i = 0, e = Args.size(), j = 0; i != e; ++i, ++j) {
+      assert(j < ArgLocs.size() && "Skipped too many arg locs");
+
+      CCValAssign &VA = ArgLocs[j];
+      assert(VA.getValNo() == i && "Location doesn't correspond to current arg");
+
+      if (VA.needsCustom()) {
+        j += Handler.assignCustomValue(Args[i], makeArrayRef(ArgLocs).slice(j));
+        continue;
+      }
+
+      if (VA.isRegLoc())
+        Handler.assignValueToReg(Args[i].Reg, VA.getLocReg(), VA);
+      else if (VA.isMemLoc()) {
+        unsigned Size = VA.getValVT() == MVT::iPTR
+                          ? DL.getPointerSize()
+                          : alignTo(VA.getValVT().getSizeInBits(), 8) / 8;
+        unsigned Offset = VA.getLocMemOffset();
+        MachinePointerInfo MPO;
+        unsigned StackAddr = Handler.getStackAddress(Size, Offset, MPO);
+        Handler.assignValueToAddress(Args[i].Reg, StackAddr, Size, MPO, VA);
+      } else {
+        // FIXME: Support byvals and other weirdness
+        return false;
+      }
+    }
+    return true;
+  }
+
   /// Lower the given call instruction, including argument and return value
   /// marshalling.
   ///
Index: lib/CodeGen/GlobalISel/CallLowering.cpp
===================================================================
--- lib/CodeGen/GlobalISel/CallLowering.cpp
+++ lib/CodeGen/GlobalISel/CallLowering.cpp
@@ -13,7 +13,6 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/CodeGen/GlobalISel/CallLowering.h"
-#include "llvm/CodeGen/GlobalISel/MachineIRBuilder.h"
 #include "llvm/CodeGen/MachineOperand.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/CodeGen/TargetLowering.h"
@@ -104,52 +103,6 @@
                                     const DataLayout &DL,
                                     const CallInst &FuncInfo) const;
 
-bool CallLowering::handleAssignments(MachineIRBuilder &MIRBuilder,
-                                     ArrayRef<ArgInfo> Args,
-                                     ValueHandler &Handler) const {
-  MachineFunction &MF = MIRBuilder.getMF();
-  const Function &F = MF.getFunction();
-  const DataLayout &DL = F.getParent()->getDataLayout();
-
-  SmallVector<CCValAssign, 16> ArgLocs;
-  CCState CCInfo(F.getCallingConv(), F.isVarArg(), MF, ArgLocs, F.getContext());
-
-  unsigned NumArgs = Args.size();
-  for (unsigned i = 0; i != NumArgs; ++i) {
-    MVT CurVT = MVT::getVT(Args[i].Ty);
-    if (Handler.assignArg(i, CurVT, CurVT, CCValAssign::Full, Args[i], CCInfo))
-      return false;
-  }
-
-  for (unsigned i = 0, e = Args.size(), j = 0; i != e; ++i, ++j) {
-    assert(j < ArgLocs.size() && "Skipped too many arg locs");
-
-    CCValAssign &VA = ArgLocs[j];
-    assert(VA.getValNo() == i && "Location doesn't correspond to current arg");
-
-    if (VA.needsCustom()) {
-      j += Handler.assignCustomValue(Args[i], makeArrayRef(ArgLocs).slice(j));
-      continue;
-    }
-
-    if (VA.isRegLoc())
-      Handler.assignValueToReg(Args[i].Reg, VA.getLocReg(), VA);
-    else if (VA.isMemLoc()) {
-      unsigned Size = VA.getValVT() == MVT::iPTR
-                          ? DL.getPointerSize()
-                          : alignTo(VA.getValVT().getSizeInBits(), 8) / 8;
-      unsigned Offset = VA.getLocMemOffset();
-      MachinePointerInfo MPO;
-      unsigned StackAddr = Handler.getStackAddress(Size, Offset, MPO);
-      Handler.assignValueToAddress(Args[i].Reg, StackAddr, Size, MPO, VA);
-    } else {
-      // FIXME: Support byvals and other weirdness
-      return false;
-    }
-  }
-  return true;
-}
-
 unsigned CallLowering::ValueHandler::extendRegister(unsigned ValReg,
                                                     CCValAssign &VA) {
   LLT LocTy{VA.getLocVT()};