Index: lib/Target/AArch64/AArch64CallingConvention.td
===================================================================
--- lib/Target/AArch64/AArch64CallingConvention.td
+++ lib/Target/AArch64/AArch64CallingConvention.td
@@ -288,6 +288,14 @@
                            (sub (sequence "X%u", 1, 28), X15, X16, X17, X18),
                            (sequence "D%u", 0, 31))>;
 
+// CSRs that are handled by prologue, epilogue.
+def CSR_AArch64_CXX_TLS_Darwin_PE
+    : CalleeSavedRegs<(add LR, FP)>;
+
+// CSRs that are handled explicitly via copies.
+def CSR_AArch64_CXX_TLS_Darwin_ViaCopy
+    : CalleeSavedRegs<(sub CSR_AArch64_CXX_TLS_Darwin, LR, FP)>;
+
 // The ELF stub used for TLS-descriptor access saves every feasible
 // register. Only X0 and LR are clobbered.
 def CSR_AArch64_TLS_ELF
Index: lib/Target/AArch64/AArch64FastISel.cpp
===================================================================
--- lib/Target/AArch64/AArch64FastISel.cpp
+++ lib/Target/AArch64/AArch64FastISel.cpp
@@ -3646,6 +3646,9 @@
   if (F.isVarArg())
     return false;
 
+  if (TLI.supportSplitCSR(FuncInfo.MF))
+    return false;
+
   // Build a list of return value registers.
   SmallVector<unsigned, 4> RetRegs;
 
Index: lib/Target/AArch64/AArch64ISelLowering.h
===================================================================
--- lib/Target/AArch64/AArch64ISelLowering.h
+++ lib/Target/AArch64/AArch64ISelLowering.h
@@ -385,6 +385,14 @@
   bool isCheapToSpeculateCtlz() const override {
     return true;
   }
+  bool supportSplitCSR(MachineFunction *MF) const override {
+    return MF->getFunction()->getCallingConv() == CallingConv::CXX_FAST_TLS &&
+           MF->getFunction()->hasFnAttribute(Attribute::NoUnwind);
+  }
+  void initializeSplitCSR(MachineBasicBlock *Entry) const override;
+  void insertCopiesSplitCSR(
+      MachineBasicBlock *Entry,
+      const SmallVectorImpl<MachineBasicBlock *> &Exits) const override;
 
 private:
   bool isExtFreeImpl(const Instruction *Ext) const override;
Index: lib/Target/AArch64/AArch64ISelLowering.cpp
===================================================================
--- lib/Target/AArch64/AArch64ISelLowering.cpp
+++ lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -3271,6 +3271,19 @@
     Flag = Chain.getValue(1);
     RetOps.push_back(DAG.getRegister(VA.getLocReg(), VA.getLocVT()));
   }
+  const AArch64RegisterInfo *TRI = Subtarget->getRegisterInfo();
+  const MCPhysReg *I =
+      TRI->getCalleeSavedRegsViaCopy(&DAG.getMachineFunction());
+  if (I) {
+    for (; *I; ++I) {
+      if (AArch64::GPR64RegClass.contains(*I))
+        RetOps.push_back(DAG.getRegister(*I, MVT::i64));
+      else if (AArch64::FPR64RegClass.contains(*I))
+        RetOps.push_back(DAG.getRegister(*I, MVT::getFloatingPointVT(64)));
+      else
+        llvm_unreachable("Unexpected register class in CSRsViaCopy!");
+    }
+  }
 
   RetOps[0] = Chain; // Update chain.
 
@@ -10003,3 +10016,49 @@
       IRB.CreateConstGEP1_32(IRB.CreateCall(ThreadPointerFunc), TlsOffset),
       Type::getInt8PtrTy(IRB.getContext())->getPointerTo(0));
 }
+
+void AArch64TargetLowering::initializeSplitCSR(MachineBasicBlock *Entry) const {
+  // Update IsSplitCSR in AArch64unctionInfo.
+  AArch64FunctionInfo *AFI = Entry->getParent()->getInfo<AArch64FunctionInfo>();
+  AFI->setIsSplitCSR(true);
+}
+
+void AArch64TargetLowering::insertCopiesSplitCSR(
+    MachineBasicBlock *Entry,
+    const SmallVectorImpl<MachineBasicBlock *> &Exits) const {
+  const AArch64RegisterInfo *TRI = Subtarget->getRegisterInfo();
+  const MCPhysReg *IStart = TRI->getCalleeSavedRegsViaCopy(Entry->getParent());
+  if (!IStart)
+    return;
+
+  const TargetInstrInfo *TII = Subtarget->getInstrInfo();
+  MachineRegisterInfo *MRI = &Entry->getParent()->getRegInfo();
+  for (const MCPhysReg *I = IStart; *I; ++I) {
+    const TargetRegisterClass *RC = nullptr;
+    if (AArch64::GPR64RegClass.contains(*I))
+      RC = &AArch64::GPR64RegClass;
+    else if (AArch64::FPR64RegClass.contains(*I))
+      RC = &AArch64::FPR64RegClass;
+    else
+      llvm_unreachable("Unexpected register class in CSRsViaCopy!");
+
+    unsigned NewVR = MRI->createVirtualRegister(RC);
+    // Create copy from CSR to a virtual register.
+    // FIXME: this currently does not emit CFI pseudo-instructions, it works
+    // fine for CXX_FAST_TLS since the C++-style TLS access functions should be
+    // nounwind. If we want to generalize this later, we may need to emit
+    // CFI pseudo-instructions.
+    assert(Entry->getParent()->getFunction()->hasFnAttribute(
+               Attribute::NoUnwind) &&
+           "Function should be nounwind in insertCopiesSplitCSR!");
+    Entry->addLiveIn(*I);
+    BuildMI(*Entry, Entry->begin(), DebugLoc(), TII->get(TargetOpcode::COPY),
+            NewVR)
+        .addReg(*I);
+
+    for (auto *Exit : Exits)
+      BuildMI(*Exit, Exit->begin(), DebugLoc(), TII->get(TargetOpcode::COPY),
+              *I)
+          .addReg(NewVR);
+  }
+}
Index: lib/Target/AArch64/AArch64MachineFunctionInfo.h
===================================================================
--- lib/Target/AArch64/AArch64MachineFunctionInfo.h
+++ lib/Target/AArch64/AArch64MachineFunctionInfo.h
@@ -72,16 +72,22 @@
   /// registers.
   unsigned VarArgsFPRSize;
 
+  /// True if this function has a subset of CSRs that is handled explicitly via
+  /// copies.
+  bool IsSplitCSR;
+
 public:
   AArch64FunctionInfo()
       : BytesInStackArgArea(0), ArgumentStackToRestore(0), HasStackFrame(false),
         NumLocalDynamicTLSAccesses(0), VarArgsStackIndex(0), VarArgsGPRIndex(0),
-        VarArgsGPRSize(0), VarArgsFPRIndex(0), VarArgsFPRSize(0) {}
+        VarArgsGPRSize(0), VarArgsFPRIndex(0), VarArgsFPRSize(0),
+        IsSplitCSR(false) {}
 
   explicit AArch64FunctionInfo(MachineFunction &MF)
       : BytesInStackArgArea(0), ArgumentStackToRestore(0), HasStackFrame(false),
         NumLocalDynamicTLSAccesses(0), VarArgsStackIndex(0), VarArgsGPRIndex(0),
-        VarArgsGPRSize(0), VarArgsFPRIndex(0), VarArgsFPRSize(0) {
+        VarArgsGPRSize(0), VarArgsFPRIndex(0), VarArgsFPRSize(0),
+        IsSplitCSR(false) {
     (void)MF;
   }
 
@@ -96,6 +102,9 @@
   bool hasStackFrame() const { return HasStackFrame; }
   void setHasStackFrame(bool s) { HasStackFrame = s; }
 
+  bool isSplitCSR() const { return IsSplitCSR; }
+  void setIsSplitCSR(bool s) { IsSplitCSR = s; }
+
   void setLocalStackSize(unsigned Size) { LocalStackSize = Size; }
   unsigned getLocalStackSize() const { return LocalStackSize; }
 
Index: lib/Target/AArch64/AArch64RegisterInfo.h
===================================================================
--- lib/Target/AArch64/AArch64RegisterInfo.h
+++ lib/Target/AArch64/AArch64RegisterInfo.h
@@ -35,6 +35,7 @@
 
   /// Code Generation virtual methods...
   const MCPhysReg *getCalleeSavedRegs(const MachineFunction *MF) const override;
+  const MCPhysReg *getCalleeSavedRegsViaCopy(const MachineFunction *MF) const override;
   const uint32_t *getCallPreservedMask(const MachineFunction &MF,
                                        CallingConv::ID) const override;
 
Index: lib/Target/AArch64/AArch64RegisterInfo.cpp
===================================================================
--- lib/Target/AArch64/AArch64RegisterInfo.cpp
+++ lib/Target/AArch64/AArch64RegisterInfo.cpp
@@ -15,6 +15,7 @@
 #include "AArch64RegisterInfo.h"
 #include "AArch64FrameLowering.h"
 #include "AArch64InstrInfo.h"
+#include "AArch64MachineFunctionInfo.h"
 #include "AArch64Subtarget.h"
 #include "MCTargetDesc/AArch64AddressingModes.h"
 #include "llvm/ADT/BitVector.h"
@@ -47,11 +48,22 @@
   if (MF->getFunction()->getCallingConv() == CallingConv::AnyReg)
     return CSR_AArch64_AllRegs_SaveList;
   if (MF->getFunction()->getCallingConv() == CallingConv::CXX_FAST_TLS)
-    return CSR_AArch64_CXX_TLS_Darwin_SaveList;
+    return MF->getInfo<AArch64FunctionInfo>()->isSplitCSR() ?
+           CSR_AArch64_CXX_TLS_Darwin_PE_SaveList :
+           CSR_AArch64_CXX_TLS_Darwin_SaveList;
   else
     return CSR_AArch64_AAPCS_SaveList;
 }
 
+const MCPhysReg *
+AArch64RegisterInfo::getCalleeSavedRegsViaCopy(const MachineFunction *MF) const {
+  assert(MF && "Invalid MachineFunction pointer.");
+  if (MF->getFunction()->getCallingConv() == CallingConv::CXX_FAST_TLS &&
+      MF->getInfo<AArch64FunctionInfo>()->isSplitCSR())
+    return CSR_AArch64_CXX_TLS_Darwin_ViaCopy_SaveList;
+  return nullptr;
+}
+
 const uint32_t *
 AArch64RegisterInfo::getCallPreservedMask(const MachineFunction &MF,
                                           CallingConv::ID CC) const {
Index: test/CodeGen/AArch64/cxx-tlscc.ll
===================================================================
--- test/CodeGen/AArch64/cxx-tlscc.ll
+++ test/CodeGen/AArch64/cxx-tlscc.ll
@@ -13,7 +13,7 @@
 declare %struct.S* @_ZN1SD1Ev(%struct.S* returned)
 declare i32 @_tlv_atexit(void (i8*)*, i8*, i8*)
 
-define cxx_fast_tlscc nonnull %struct.S* @_ZTW2sg() {
+define cxx_fast_tlscc nonnull %struct.S* @_ZTW2sg() nounwind {
   %.b.i = load i1, i1* @__tls_guard, align 1
   br i1 %.b.i, label %__tls_init.exit, label %init.i
 
@@ -28,50 +28,49 @@
 }
 
 ; CHECK-LABEL: _ZTW2sg
-; CHECK-DAG: stp d31, d30
-; CHECK-DAG: stp d29, d28
-; CHECK-DAG: stp d27, d26
-; CHECK-DAG: stp d25, d24
-; CHECK-DAG: stp d23, d22
-; CHECK-DAG: stp d21, d20
-; CHECK-DAG: stp d19, d18
-; CHECK-DAG: stp d17, d16
-; CHECK-DAG: stp d7, d6
-; CHECK-DAG: stp d5, d4
-; CHECK-DAG: stp d3, d2
-; CHECK-DAG: stp d1, d0
-; CHECK-DAG: stp x20, x19
-; CHECK-DAG: stp x14, x13
-; CHECK-DAG: stp x12, x11
-; CHECK-DAG: stp x10, x9
-; CHECK-DAG: stp x8, x7
-; CHECK-DAG: stp x6, x5
-; CHECK-DAG: stp x4, x3
-; CHECK-DAG: stp x2, x1
-; CHECK-DAG: stp x29, x30
+; CHECK-NOT: stp d31, d30
+; CHECK-NOT: stp d29, d28
+; CHECK-NOT: stp d27, d26
+; CHECK-NOT: stp d25, d24
+; CHECK-NOT: stp d23, d22
+; CHECK-NOT: stp d21, d20
+; CHECK-NOT: stp d19, d18
+; CHECK-NOT: stp d17, d16
+; CHECK-NOT: stp d7, d6
+; CHECK-NOT: stp d5, d4
+; CHECK-NOT: stp d3, d2
+; CHECK-NOT: stp d1, d0
+; CHECK-NOT: stp x20, x19
+; CHECK-NOT: stp x14, x13
+; CHECK-NOT: stp x12, x11
+; CHECK-NOT: stp x10, x9
+; CHECK-NOT: stp x8, x7
+; CHECK-NOT: stp x6, x5
+; CHECK-NOT: stp x4, x3
+; CHECK-NOT: stp x2, x1
 ; CHECK: blr
 ; CHECK: tbnz w{{.*}}, #0, [[BB_end:.?LBB0_[0-9]+]]
 ; CHECK: blr
 ; CHECK: tlv_atexit
 ; CHECK: [[BB_end]]:
 ; CHECK: blr
-; CHECK-DAG: ldp x2, x1
-; CHECK-DAG: ldp x4, x3
-; CHECK-DAG: ldp x6, x5
-; CHECK-DAG: ldp x8, x7
-; CHECK-DAG: ldp x10, x9
-; CHECK-DAG: ldp x12, x11
-; CHECK-DAG: ldp x14, x13
-; CHECK-DAG: ldp x20, x19
-; CHECK-DAG: ldp d1, d0
-; CHECK-DAG: ldp d3, d2
-; CHECK-DAG: ldp d5, d4
-; CHECK-DAG: ldp d7, d6
-; CHECK-DAG: ldp d17, d16
-; CHECK-DAG: ldp d19, d18
-; CHECK-DAG: ldp d21, d20
-; CHECK-DAG: ldp d23, d22
-; CHECK-DAG: ldp d25, d24
-; CHECK-DAG: ldp d27, d26
-; CHECK-DAG: ldp d29, d28
-; CHECK-DAG: ldp d31, d30
+; CHECK-NOT: ldp x2, x1
+; CHECK-NOT: ldp x4, x3
+; CHECK-NOT: ldp x6, x5
+; CHECK-NOT: ldp x8, x7
+; CHECK-NOT: ldp x10, x9
+; CHECK-NOT: ldp x12, x11
+; CHECK-NOT: ldp x14, x13
+; CHECK-NOT: ldp x20, x19
+; CHECK-NOT: ldp d1, d0
+; CHECK-NOT: ldp d3, d2
+; CHECK-NOT: ldp d5, d4
+; CHECK-NOT: ldp d7, d6
+; CHECK-NOT: ldp d17, d16
+; CHECK-NOT: ldp d19, d18
+; CHECK-NOT: ldp d21, d20
+; CHECK-NOT: ldp d23, d22
+; CHECK-NOT: ldp d25, d24
+; CHECK-NOT: ldp d27, d26
+; CHECK-NOT: ldp d29, d28
+; CHECK-NOT: ldp d31, d30