Index: lib/Target/AArch64/AArch64ISelLowering.cpp
===================================================================
--- lib/Target/AArch64/AArch64ISelLowering.cpp
+++ lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -3148,6 +3148,17 @@
     // We currently pass all varargs at 8-byte alignment.
     StackOffset = ((StackOffset + 7) & ~7);
     FuncInfo->setVarArgsStackIndex(MFI.CreateFixedObject(4, StackOffset, true));
+
+    if (MFI.hasMustTailInVarArgFunc()) {
+      SmallVector<MVT, 2> RegParmTypes;
+      RegParmTypes.push_back(MVT::i64);
+      RegParmTypes.push_back(MVT::f128);
+      // Compute the set of forwarded registers. The rest are scratch.
+      SmallVectorImpl<ForwardedRegister> &Forwards =
+                                       FuncInfo->getForwardedMustTailRegParms();
+      CCInfo.analyzeMustTailForwardedRegisters(Forwards, RegParmTypes,
+                                               CC_AArch64_AAPCS);
+    }
   }
 
   unsigned StackArgSize = CCInfo.getNextStackOffset();
@@ -3608,6 +3619,14 @@
   SmallVector<SDValue, 8> MemOpChains;
   auto PtrVT = getPointerTy(DAG.getDataLayout());
 
+  if (IsVarArg && CLI.CS && CLI.CS.isMustTailCall()) {
+    const auto &Forwards = FuncInfo->getForwardedMustTailRegParms();
+    for (const auto &F : Forwards) {
+      SDValue Val = DAG.getCopyFromReg(Chain, DL, F.VReg, F.VT);
+      RegsToPass.push_back(std::make_pair(unsigned(F.PReg), Val));
+    }
+  }
+
   // Walk the register/memloc assignments, inserting copies/loads.
   for (unsigned i = 0, realArgIdx = 0, e = ArgLocs.size(); i != e;
        ++i, ++realArgIdx) {
Index: lib/Target/AArch64/AArch64MachineFunctionInfo.h
===================================================================
--- lib/Target/AArch64/AArch64MachineFunctionInfo.h
+++ lib/Target/AArch64/AArch64MachineFunctionInfo.h
@@ -18,6 +18,7 @@
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallVector.h"
+#include "llvm/CodeGen/CallingConvLower.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/MC/MCLinkerOptimizationHint.h"
 #include <cassert>
@@ -97,6 +98,9 @@
   /// attribute, in which case it is set to false at construction.
   Optional<bool> HasRedZone;
 
+  /// ForwardedMustTailRegParms - A list of virtual and physical registers
+  /// that must be forwarded to every musttail call.
+  SmallVector<ForwardedRegister, 1> ForwardedMustTailRegParms;
 public:
   AArch64FunctionInfo() = default;
 
@@ -209,6 +213,10 @@
     LOHRelated.insert(Args.begin(), Args.end());
   }
 
+  SmallVectorImpl<ForwardedRegister> &getForwardedMustTailRegParms() {
+    return ForwardedMustTailRegParms;
+  }
+
 private:
   // Hold the lists of LOHs.
   MILOHContainer LOHContainerSet;
Index: test/CodeGen/AArch64/vararg-tallcall.ll
===================================================================
--- /dev/null
+++ test/CodeGen/AArch64/vararg-tallcall.ll
@@ -0,0 +1,31 @@
+; RUN: llc -mtriple=aarch64-windows-msvc %s -o - | FileCheck %s
+; RUN: llc -mtriple=aarch64-linux-gnu %s -o - | FileCheck %s
+
+target datalayout = "e-m:w-p:64:64-i32:32-i64:64-i128:128-n32:64-S128"
+
+%class.X = type { i8 }
+%struct.B = type { i32 (...)** }
+
+$"??_9B@@$BA@AA" = comdat any
+
+; Function Attrs: noinline optnone
+define linkonce_odr void @"??_9B@@$BA@AA"(%struct.B* %this, ...) #1 comdat align 2  {
+entry:
+  %this.addr = alloca %struct.B*, align 8
+  store %struct.B* %this, %struct.B** %this.addr, align 8
+  %this1 = load %struct.B*, %struct.B** %this.addr, align 8
+  %0 = bitcast %struct.B* %this1 to void (%struct.B*, ...)***
+  %vtable = load void (%struct.B*, ...)**, void (%struct.B*, ...)*** %0, align 8
+  %vfn = getelementptr inbounds void (%struct.B*, ...)*, void (%struct.B*, ...)** %vtable, i64 0
+  %1 = load void (%struct.B*, ...)*, void (%struct.B*, ...)** %vfn, align 8
+  musttail call void (%struct.B*, ...) %1(%struct.B* %this1, ...)
+  ret void
+                                                  ; No predecessors!
+  ret void
+}
+
+attributes #1 = { noinline optnone "thunk" }
+
+; CHECK: ldr     x8, [x0]
+; CHECK: ldr     x8, [x8]
+; CHECK: br      x8