diff --git a/clang/lib/CodeGen/CGVTables.cpp b/clang/lib/CodeGen/CGVTables.cpp
--- a/clang/lib/CodeGen/CGVTables.cpp
+++ b/clang/lib/CodeGen/CGVTables.cpp
@@ -165,6 +165,7 @@
   llvm::Type *Ty = CGM.getTypes().GetFunctionType(FnInfo);
   llvm::Value *Callee = CGM.GetAddrOfFunction(GD, Ty, /*ForVTable=*/true);
   llvm::Function *BaseFn = cast<llvm::Function>(Callee);
+  assert(!BaseFn->isDeclaration() && "cannot clone undefined variadic method");
 
   // Clone to thunk.
   llvm::ValueToValueMapTy VMap;
@@ -201,6 +202,8 @@
   Builder.SetInsertPoint(&*ThisStore);
   llvm::Value *AdjustedThisPtr =
       CGM.getCXXABI().performThisAdjustment(*this, ThisPtr, Thunk.This);
+  AdjustedThisPtr = Builder.CreateBitCast(AdjustedThisPtr,
+                                          ThisStore->getOperand(0)->getType());
   ThisStore->setOperand(0, AdjustedThisPtr);
 
   if (!Thunk.Return.isEmpty()) {
@@ -291,14 +294,17 @@
                           *this, LoadCXXThisAddress(), Thunk->This)
           : LoadCXXThis();
 
-  if (CurFnInfo->usesInAlloca() || IsUnprototyped) {
-    // We don't handle return adjusting thunks, because they require us to call
-    // the copy constructor.  For now, fall through and pretend the return
-    // adjustment was empty so we don't crash.
+  // If perfect forwarding is required a variadic method, a method using
+  // inalloca, or an unprototyped thunk, use musttail. Emit an error if this
+  // thunk requires a return adjustment, since that is impossible with musttail.
+  if (CurFnInfo->usesInAlloca() || CurFnInfo->isVariadic() || IsUnprototyped) {
     if (Thunk && !Thunk->Return.isEmpty()) {
       if (IsUnprototyped)
         CGM.ErrorUnsupported(
             MD, "return-adjusting thunk with incomplete parameter type");
+      else if (CurFnInfo->isVariadic())
+        llvm_unreachable("shouldn't try to emit musttail return-adjusting "
+                         "thunks for variadic functions");
       else
         CGM.ErrorUnsupported(
             MD, "non-trivial argument copy for return-adjusting thunk");
@@ -549,16 +555,32 @@
 
   CGM.SetLLVMFunctionAttributesForDefinition(GD.getDecl(), ThunkFn);
 
+  // Thunks for variadic methods are special because in general variadic
+  // arguments cannot be perferctly forwarded. In the general case, clang
+  // implements such thunks by cloning the original function body. However, for
+  // thunks with no return adjustment on targets that support musttail, we can
+  // use musttail to perfectly forward the variadic arguments.
+  bool ShouldCloneVarArgs = false;
   if (!IsUnprototyped && ThunkFn->isVarArg()) {
-    // Varargs thunks are special; we can't just generate a call because
-    // we can't copy the varargs.  Our implementation is rather
-    // expensive/sucky at the moment, so don't generate the thunk unless
-    // we have to.
-    // FIXME: Do something better here; GenerateVarArgsThunk is extremely ugly.
+    ShouldCloneVarArgs = true;
+    if (TI.Return.isEmpty()) {
+      switch (CGM.getTriple().getArch()) {
+      case llvm::Triple::x86_64:
+      case llvm::Triple::x86:
+      case llvm::Triple::aarch64:
+        ShouldCloneVarArgs = false;
+        break;
+      default:
+        break;
+      }
+    }
+  }
+
+  if (ShouldCloneVarArgs) {
     if (UseAvailableExternallyLinkage)
       return ThunkFn;
-    ThunkFn = CodeGenFunction(CGM).GenerateVarArgsThunk(ThunkFn, FnInfo, GD,
-                                                        TI);
+    ThunkFn =
+        CodeGenFunction(CGM).GenerateVarArgsThunk(ThunkFn, FnInfo, GD, TI);
   } else {
     // Normal thunk body generation.
     CodeGenFunction(CGM).generateThunk(ThunkFn, FnInfo, GD, TI, IsUnprototyped);
diff --git a/clang/test/CodeGenCXX/linetable-virtual-variadic.cpp b/clang/test/CodeGenCXX/linetable-virtual-variadic.cpp
--- a/clang/test/CodeGenCXX/linetable-virtual-variadic.cpp
+++ b/clang/test/CodeGenCXX/linetable-virtual-variadic.cpp
@@ -1,5 +1,6 @@
-// RUN: %clang_cc1 -triple x86_64-apple-darwin -emit-llvm -debug-info-kind=line-tables-only %s -o - | FileCheck %s
-// RUN: %clang_cc1 -triple x86_64-apple-darwin -emit-llvm -debug-info-kind=line-directives-only %s -o - | FileCheck %s
+// Sparc64 is used because AArch64 and X86_64 would both use musttail.
+// RUN: %clang_cc1 -triple sparc64-linux-gnu -emit-llvm -debug-info-kind=line-tables-only %s -o - | FileCheck %s
+// RUN: %clang_cc1 -triple sparc64-linux-gnu -emit-llvm -debug-info-kind=line-directives-only %s -o - | FileCheck %s
 // Crasher for PR22929.
 class Base {
   virtual void VariadicFunction(...);
diff --git a/clang/test/CodeGenCXX/thunks-variadic-covariant.cpp b/clang/test/CodeGenCXX/thunks-variadic-covariant.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/thunks-variadic-covariant.cpp
@@ -0,0 +1,41 @@
+// RUN: %clang_cc1 %s -triple=x86_64-windows-msvc -munwind-tables -emit-llvm -o - -O1 -disable-llvm-passes | FileCheck --check-prefix=MSVC %s
+// RUN: %clang_cc1 %s -triple=x86_64-windows-gnu -munwind-tables -emit-llvm -o - -O1 -disable-llvm-passes | FileCheck --check-prefix=ITANIUM %s
+
+struct A {
+  virtual A *f(int x, ...);
+};
+struct B {
+  virtual B *f(int x, ...);
+};
+struct C : A, B {
+  virtual void c();
+  virtual C *f(int x, ...);
+};
+C *makeC();
+C *C::f(int x, ...) { return makeC(); }
+C c;
+
+// MSVC-LABEL: define dso_local %struct.C* @"?f@C@@UEAAPEAU1@HZZ"
+// MSVC-SAME: (%struct.C* %this, i32 %x, ...)
+// MSVC: call %struct.C* @"?makeC@@YAPEAUC@@XZ"()
+
+// MSVC-LABEL: define weak_odr dso_local %struct.C* @"?f@C@@W7EAAPEAUB@@HZZ"
+// MSVC-SAME: (%struct.C* %this, i32 %x, ...)
+// MSVC: call %struct.C* @"?makeC@@YAPEAUC@@XZ"()
+// MSVC: getelementptr inbounds i8, i8* {{.*}}, i32 8
+// MSVC: ret %struct.C* %7
+
+// MSVC-LABEL: define linkonce_odr dso_local %struct.C* @"?f@C@@W7EAAPEAU1@HZZ"
+// MSVC-SAME: (%struct.C* %this, i32 %x, ...)
+// MSVC: getelementptr i8, i8* %{{[^,]*}}, i32 -8
+// MSVC: musttail call %struct.C* (%struct.C*, i32, ...) @"?f@C@@UEAAPEAU1@HZZ"(%struct.C* %{{[^,]*}}, i32 %{{[^,]*}}, ...)
+
+
+// ITANIUM-LABEL: define dso_local %struct.C* @_ZN1C1fEiz
+// ITANIUM-SAME: (%struct.C* %this, i32 %x, ...)
+// ITANIUM: call %struct.C* @_Z5makeCv()
+
+// ITANIUM-LABEL: define dso_local %struct.C* @_ZTchn8_h8_N1C1fEiz
+// ITANIUM-SAME: (%struct.C* %this, i32 %x, ...)
+// ITANIUM: call %struct.C* @_Z5makeCv()
+// ITANIUM: getelementptr inbounds i8, i8* %{{[^,]*}}, i64 8
diff --git a/clang/test/CodeGenCXX/thunks-variadic.cpp b/clang/test/CodeGenCXX/thunks-variadic.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/thunks-variadic.cpp
@@ -0,0 +1,24 @@
+// RUN: %clang_cc1 %s -triple=x86_64-windows-msvc -munwind-tables -emit-llvm -o - -O1 -disable-llvm-passes | FileCheck --check-prefix=MSVC %s
+// RUN: %clang_cc1 %s -triple=x86_64-windows-gnu -munwind-tables -emit-llvm -o - -O1 -disable-llvm-passes | FileCheck --check-prefix=ITANIUM %s
+
+namespace test1 {
+class a {
+  virtual void b(const char *, ...);
+};
+class c {
+  virtual void b(const char *, ...);
+};
+class e : c, a {
+  void b(const char *, ...) override;
+} d;
+
+// MSVC-LABEL: define linkonce_odr dso_local void @"?b@e@test1@@G7EAAXPEBDZZ"
+// MSVC-SAME: (%"class.test1::e"* %this, i8* %[[ARG:[^,]+]], ...)
+// MSVC: getelementptr i8, i8* %{{.*}}, i32 -8
+// MSVC: musttail call void (%"class.test1::e"*, i8*, ...) @"?b@e@test1@@EEAAXPEBDZZ"(%"class.test1::e"* %{{.*}}, i8* %[[ARG]], ...)
+
+// ITANIUM-LABEL: define available_externally dso_local void @_ZThn8_N5test11e1bEPKcz
+// ITANIUM-SAME: (%"class.test1::e"* %this, i8* %{{.*}}, ...)
+// ITANIUM: getelementptr inbounds i8, i8* %{{.*}}, i64 -8
+// ITANIUM: musttail call void (%"class.test1::e"*, i8*, ...) @_ZN5test11e1bEPKcz(%"class.test1::e"* %{{.*}}, i8* %{{.*}}, ...)
+}
diff --git a/clang/test/CodeGenCXX/thunks.cpp b/clang/test/CodeGenCXX/thunks.cpp
--- a/clang/test/CodeGenCXX/thunks.cpp
+++ b/clang/test/CodeGenCXX/thunks.cpp
@@ -1,6 +1,7 @@
-// RUN: %clang_cc1 %s -triple=x86_64-pc-linux-gnu -munwind-tables -emit-llvm -o - | FileCheck --check-prefix=CHECK --check-prefix=CHECK-NONOPT %s
-// RUN: %clang_cc1 %s -triple=x86_64-pc-linux-gnu -debug-info-kind=standalone -dwarf-version=5 -munwind-tables -emit-llvm -o - | FileCheck --check-prefix=CHECK --check-prefix=CHECK-NONOPT --check-prefix=CHECK-DBG %s
-// RUN: %clang_cc1 %s -triple=x86_64-pc-linux-gnu -munwind-tables -emit-llvm -o - -O1 -disable-llvm-passes | FileCheck --check-prefix=CHECK --check-prefix=CHECK-OPT %s
+// Sparc64 is used because AArch64 and X86_64 would both use musttail.
+// RUN: %clang_cc1 %s -triple=sparc64-pc-linux-gnu -munwind-tables -emit-llvm -o - | FileCheck --check-prefix=CHECK --check-prefix=CHECK-NONOPT %s
+// RUN: %clang_cc1 %s -triple=sparc64-pc-linux-gnu -debug-info-kind=standalone -dwarf-version=5 -munwind-tables -emit-llvm -o - | FileCheck --check-prefix=CHECK --check-prefix=CHECK-NONOPT --check-prefix=CHECK-DBG %s
+// RUN: %clang_cc1 %s -triple=sparc64-pc-linux-gnu -munwind-tables -emit-llvm -o - -O1 -disable-llvm-passes | FileCheck --check-prefix=CHECK --check-prefix=CHECK-OPT %s
 
 namespace Test1 {