Index: lib/Target/X86/CMakeLists.txt
===================================================================
--- lib/Target/X86/CMakeLists.txt
+++ lib/Target/X86/CMakeLists.txt
@@ -21,6 +21,7 @@
 add_public_tablegen_target(X86CommonTableGen)
 
 set(sources
+  ShadowCallStack.cpp
   X86AsmPrinter.cpp
   X86CallFrameOptimization.cpp
   X86CallingConv.cpp
Index: lib/Target/X86/ShadowCallStack.cpp
===================================================================
--- /dev/null
+++ lib/Target/X86/ShadowCallStack.cpp
@@ -0,0 +1,325 @@
+//===------- ShadowCallStack.cpp - Shadow Call Stack pass -----------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// The ShadowCallStack pass instruments function prologs/epilogs to check that
+// the return address has not been corrupted during the execution of the
+// function. The return address is stored in a 'shadow call stack' addressed
+// using the %gs segment register.
+//
+//===----------------------------------------------------------------------===//
+
+#include "X86.h"
+#include "X86InstrBuilder.h"
+#include "X86InstrInfo.h"
+#include "X86Subtarget.h"
+
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineModuleInfo.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Target/TargetInstrInfo.h"
+
+using namespace llvm;
+
+namespace llvm {
+void initializeShadowCallStackPass(PassRegistry &);
+}
+
+namespace {
+
+class ShadowCallStack : public MachineFunctionPass {
+public:
+  static char ID;
+
+  ShadowCallStack() : MachineFunctionPass(ID) {
+    initializeShadowCallStackPass(*PassRegistry::getPassRegistry());
+  }
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    MachineFunctionPass::getAnalysisUsage(AU);
+  }
+
+  bool runOnMachineFunction(MachineFunction &Fn) override;
+
+private:
+  // Do not instrument leaf functions with this many or fewer instructions. The
+  // shadow call stack instrumented prolog/epilog are slightly race-y reading
+  // and checking the saved return address, so it is better to not instrument
+  // functions that have fewer instructions than the instrumented prolog/epilog
+  // race.
+  static const size_t SkipLeafInstructions = 3;
+};
+
+char ShadowCallStack::ID = 0;
+} // end anonymous namespace.
+
+static void addProlog(MachineFunction &Fn, const TargetInstrInfo *TII,
+                      MachineBasicBlock &MBB, const DebugLoc &DL);
+static void addPrologLeaf(MachineFunction &Fn, const TargetInstrInfo *TII,
+                          MachineBasicBlock &MBB, const DebugLoc &DL,
+                          MCPhysReg FreeRegister);
+
+static void addEpilog(const TargetInstrInfo *TII, MachineBasicBlock &MBB,
+                      MachineInstr &MI, MachineBasicBlock &TrapBB);
+static void addEpilogLeaf(const TargetInstrInfo *TII, MachineBasicBlock &MBB,
+                          MachineInstr &MI, MachineBasicBlock &TrapBB,
+                          MCPhysReg FreeRegister);
+// Generate a longer epilog that only uses r10 when a tailcall branches to r11.
+static void addEpilogOnlyR10(const TargetInstrInfo *TII, MachineBasicBlock &MBB,
+                             MachineInstr &MI, MachineBasicBlock &TrapBB);
+
+// Helper function to add ModR/M references for [Seg: Reg + Offset] memory
+// accesses
+static inline const MachineInstrBuilder &
+addSegmentedMem(const MachineInstrBuilder &MIB, MCPhysReg Seg, MCPhysReg Reg,
+                int Offset = 0) {
+  return MIB.addReg(Reg).addImm(1).addReg(0).addImm(Offset).addReg(Seg);
+}
+
+static void addProlog(MachineFunction &Fn, const TargetInstrInfo *TII,
+                      MachineBasicBlock &MBB, const DebugLoc &DL) {
+  const MCPhysReg ReturnReg = X86::R10;
+  const MCPhysReg OffsetReg = X86::R11;
+
+  auto MBBI = MBB.begin();
+  // mov r10, [rsp]
+  addDirectMem(BuildMI(MBB, MBBI, DL, TII->get(X86::MOV64rm)).addDef(ReturnReg),
+               X86::RSP);
+  // xor r11, r11
+  BuildMI(MBB, MBBI, DL, TII->get(X86::XOR64rr))
+      .addDef(OffsetReg)
+      .addReg(OffsetReg, RegState::Undef)
+      .addReg(OffsetReg, RegState::Undef);
+  // add QWORD [gs:r11], 8
+  addSegmentedMem(BuildMI(MBB, MBBI, DL, TII->get(X86::ADD64mi8)), X86::GS,
+                  OffsetReg)
+      .addImm(8);
+  // mov r11, [gs:r11]
+  addSegmentedMem(
+      BuildMI(MBB, MBBI, DL, TII->get(X86::MOV64rm)).addDef(OffsetReg), X86::GS,
+      OffsetReg);
+  // mov [gs:r11], r10
+  addSegmentedMem(BuildMI(MBB, MBBI, DL, TII->get(X86::MOV64mr)), X86::GS,
+                  OffsetReg)
+      .addReg(ReturnReg);
+}
+
+static void addPrologLeaf(MachineFunction &Fn, const TargetInstrInfo *TII,
+                          MachineBasicBlock &MBB, const DebugLoc &DL,
+                          MCPhysReg FreeRegister) {
+  // mov REG, [rsp]
+  addDirectMem(BuildMI(MBB, MBB.begin(), DL, TII->get(X86::MOV64rm))
+                   .addDef(FreeRegister),
+               X86::RSP);
+}
+
+static void addEpilog(const TargetInstrInfo *TII, MachineBasicBlock &MBB,
+                      MachineInstr &MI, MachineBasicBlock &TrapBB) {
+  const DebugLoc &DL = MI.getDebugLoc();
+
+  // xor r11, r11
+  BuildMI(MBB, MI, DL, TII->get(X86::XOR64rr))
+      .addDef(X86::R11)
+      .addReg(X86::R11, RegState::Undef)
+      .addReg(X86::R11, RegState::Undef);
+  // mov r10, [gs:r11]
+  addSegmentedMem(BuildMI(MBB, MI, DL, TII->get(X86::MOV64rm)).addDef(X86::R10),
+                  X86::GS, X86::R11);
+  // mov r10, [gs:r10]
+  addSegmentedMem(BuildMI(MBB, MI, DL, TII->get(X86::MOV64rm)).addDef(X86::R10),
+                  X86::GS, X86::R10);
+  // sub QWORD [gs:r11], 8
+  // This instruction should not be moved up to avoid a signal race.
+  addSegmentedMem(BuildMI(MBB, MI, DL, TII->get(X86::SUB64mi8)),
+                  X86::GS, X86::R11)
+      .addImm(8);
+  // cmp [rsp], r10
+  addDirectMem(BuildMI(MBB, MI, DL, TII->get(X86::CMP64mr)), X86::RSP)
+      .addReg(X86::R10);
+  // jne trap
+  BuildMI(MBB, MI, DL, TII->get(X86::JNE_1)).addMBB(&TrapBB);
+  MBB.addSuccessor(&TrapBB);
+}
+
+static void addEpilogLeaf(const TargetInstrInfo *TII, MachineBasicBlock &MBB,
+                          MachineInstr &MI, MachineBasicBlock &TrapBB,
+                          MCPhysReg FreeRegister) {
+  const DebugLoc &DL = MI.getDebugLoc();
+
+  // cmp [rsp], REG
+  addDirectMem(BuildMI(MBB, MI, DL, TII->get(X86::CMP64mr)), X86::RSP)
+      .addReg(FreeRegister);
+  // jne trap
+  BuildMI(MBB, MI, DL, TII->get(X86::JNE_1)).addMBB(&TrapBB);
+  MBB.addSuccessor(&TrapBB);
+}
+
+static void addEpilogOnlyR10(const TargetInstrInfo *TII, MachineBasicBlock &MBB,
+                             MachineInstr &MI, MachineBasicBlock &TrapBB) {
+  const DebugLoc &DL = MI.getDebugLoc();
+
+  // xor r10, r10
+  BuildMI(MBB, MI, DL, TII->get(X86::XOR64rr))
+      .addDef(X86::R10)
+      .addReg(X86::R10, RegState::Undef)
+      .addReg(X86::R10, RegState::Undef);
+  // mov r10, [gs:r10]
+  addSegmentedMem(BuildMI(MBB, MI, DL, TII->get(X86::MOV64rm)).addDef(X86::R10),
+                  X86::GS, X86::R10);
+  // mov r10, [gs:r10]
+  addSegmentedMem(BuildMI(MBB, MI, DL, TII->get(X86::MOV64rm)).addDef(X86::R10),
+                  X86::GS, X86::R10);
+  // sub QWORD [gs:0], 8
+  // This instruction should not be moved up to avoid a signal race.
+  addSegmentedMem(BuildMI(MBB, MI, DL, TII->get(X86::SUB64mi8)), X86::GS, 0)
+      .addImm(8);
+  // cmp [rsp], r10
+  addDirectMem(BuildMI(MBB, MI, DL, TII->get(X86::CMP64mr)), X86::RSP)
+      .addReg(X86::R10);
+  // jne trap
+  BuildMI(MBB, MI, DL, TII->get(X86::JNE_1)).addMBB(&TrapBB);
+  MBB.addSuccessor(&TrapBB);
+}
+
+bool ShadowCallStack::runOnMachineFunction(MachineFunction &Fn) {
+  if (!Fn.getFunction().hasFnAttribute(Attribute::ShadowCallStack) ||
+      Fn.getFunction().hasFnAttribute(Attribute::Naked))
+    return false;
+
+  if (Fn.empty() || !Fn.getRegInfo().tracksLiveness())
+    return false;
+
+  // FIXME: Skip functions that have r10 or r11 live on entry (r10 can be live
+  // on entry for parameters with the nest attribute.)
+  if (Fn.front().isLiveIn(X86::R10) || Fn.front().isLiveIn(X86::R11))
+    return false;
+
+  // FIXME: Skip functions with conditional and r10 tail calls for now.
+  bool HasReturn = false;
+  for (auto &MBB : Fn) {
+    if (MBB.empty())
+      continue;
+
+    const MachineInstr &MI = MBB.instr_back();
+    if (MI.isReturn())
+      HasReturn = true;
+
+    if (MI.isReturn() && MI.isCall()) {
+      if (MI.findRegisterUseOperand(X86::EFLAGS))
+        return false;
+      // This should only be possible on Windows 64 (see GR64_TC versus
+      // GR64_TCW64.)
+      if (MI.findRegisterUseOperand(X86::R10) ||
+          MI.hasRegisterImplicitUseOperand(X86::R10))
+        return false;
+    }
+  }
+
+  if (!HasReturn)
+    return false;
+
+  // For leaf functions:
+  // 1. Do not instrument very short functions where it would not improve that
+  //    function's security.
+  // 2. Detect if there is an unused caller-saved register we can reserve to
+  //    hold the return address instead of writing/reading it from the shadow
+  //    call stack.
+  MCPhysReg LeafFuncRegister = X86::NoRegister;
+  if (!Fn.getFrameInfo().adjustsStack()) {
+    size_t InstructionCount = 0;
+    std::bitset<X86::NUM_TARGET_REGS> UsedRegs;
+    for (auto &MBB : Fn) {
+      for (auto &LiveIn : MBB.liveins())
+        UsedRegs.set(LiveIn.PhysReg);
+      for (auto &MI : MBB) {
+        InstructionCount++;
+        for (auto &Op : MI.operands())
+          if (Op.isReg() && Op.isDef())
+            UsedRegs.set(Op.getReg());
+      }
+    }
+
+    if (InstructionCount <= SkipLeafInstructions)
+      return false;
+
+    std::bitset<X86::NUM_TARGET_REGS> CalleeSavedRegs;
+    const MCPhysReg *CSRegs = Fn.getRegInfo().getCalleeSavedRegs();
+    for (size_t i = 0; CSRegs[i]; i++)
+      CalleeSavedRegs.set(CSRegs[i]);
+
+    const TargetRegisterInfo *TRI = Fn.getSubtarget().getRegisterInfo();
+    for (auto &Reg : X86::GR64_NOSPRegClass.getRegisters()) {
+      // FIXME: Optimization opportunity: spill/restore a callee-saved register
+      // if a caller-saved register is unavailable.
+      if (CalleeSavedRegs.test(Reg))
+        continue;
+
+      bool Used = false;
+      for (MCSubRegIterator SR(Reg, TRI, true); SR.isValid(); ++SR)
+        if ((Used = UsedRegs.test(*SR)))
+          break;
+
+      if (!Used) {
+        LeafFuncRegister = Reg;
+        break;
+      }
+    }
+  }
+
+  const bool LeafFuncOptimization = LeafFuncRegister != X86::NoRegister;
+  if (LeafFuncOptimization)
+    // Mark the leaf function register live-in for all MBBs except the entry MBB
+    for (auto I = ++Fn.begin(), E = Fn.end(); I != E; ++I)
+      I->addLiveIn(LeafFuncRegister);
+
+  MachineBasicBlock &MBB = Fn.front();
+  const MachineBasicBlock *NonEmpty = MBB.empty() ? MBB.getFallThrough() : &MBB;
+  const DebugLoc &DL = NonEmpty->front().getDebugLoc();
+
+  const TargetInstrInfo *TII = Fn.getSubtarget().getInstrInfo();
+  if (LeafFuncOptimization)
+    addPrologLeaf(Fn, TII, MBB, DL, LeafFuncRegister);
+  else
+    addProlog(Fn, TII, MBB, DL);
+
+  MachineBasicBlock *Trap = nullptr;
+  for (auto &MBB : Fn) {
+    if (MBB.empty())
+      continue;
+
+    MachineInstr &MI = MBB.instr_back();
+    if (MI.isReturn()) {
+      if (!Trap) {
+        Trap = Fn.CreateMachineBasicBlock();
+        BuildMI(Trap, MI.getDebugLoc(), TII->get(X86::TRAP));
+        Fn.push_back(Trap);
+      }
+
+      if (LeafFuncOptimization)
+        addEpilogLeaf(TII, MBB, MI, *Trap, LeafFuncRegister);
+      else if (MI.findRegisterUseOperand(X86::R11))
+        addEpilogOnlyR10(TII, MBB, MI, *Trap);
+      else
+        addEpilog(TII, MBB, MI, *Trap);
+    }
+  }
+
+  return true;
+}
+
+INITIALIZE_PASS(ShadowCallStack, "shadow-call-stack", "Shadow Call Stack",
+                false, false)
+
+FunctionPass *llvm::createShadowCallStackPass() {
+  return new ShadowCallStack();
+}
Index: lib/Target/X86/X86.h
===================================================================
--- lib/Target/X86/X86.h
+++ lib/Target/X86/X86.h
@@ -50,6 +50,11 @@
 /// transition penalty between functions encoded with AVX and SSE.
 FunctionPass *createX86IssueVZeroUpperPass();
 
+/// This pass instruments the function prolog to save the return address to a
+/// 'shadow call stack' and the function epilog to check that the return address
+/// did not changed during function execution.
+FunctionPass *createShadowCallStackPass();
+
 /// This pass inserts ENDBR instructions before indirect jump/call
 /// destinations as part of CET IBT mechanism.
 FunctionPass *createX86IndirectBranchTrackingPass();
Index: lib/Target/X86/X86TargetMachine.cpp
===================================================================
--- lib/Target/X86/X86TargetMachine.cpp
+++ lib/Target/X86/X86TargetMachine.cpp
@@ -58,6 +58,7 @@
 
 void initializeWinEHStatePassPass(PassRegistry &);
 void initializeFixupLEAPassPass(PassRegistry &);
+void initializeShadowCallStackPass(PassRegistry &);
 void initializeX86CallFrameOptimizationPass(PassRegistry &);
 void initializeX86CmovConverterPassPass(PassRegistry &);
 void initializeX86ExecutionDomainFixPass(PassRegistry &);
@@ -76,6 +77,7 @@
   initializeFixupBWInstPassPass(PR);
   initializeEvexToVexInstPassPass(PR);
   initializeFixupLEAPassPass(PR);
+  initializeShadowCallStackPass(PR);
   initializeX86CallFrameOptimizationPass(PR);
   initializeX86CmovConverterPassPass(PR);
   initializeX86ExecutionDomainFixPass(PR);
@@ -470,6 +472,7 @@
     addPass(createBreakFalseDeps());
   }
 
+  addPass(createShadowCallStackPass());
   addPass(createX86IndirectBranchTrackingPass());
 
   if (UseVZeroUpper)
Index: test/CodeGen/X86/O0-pipeline.ll
===================================================================
--- test/CodeGen/X86/O0-pipeline.ll
+++ test/CodeGen/X86/O0-pipeline.ll
@@ -49,6 +49,7 @@
 ; CHECK-NEXT:       Post-RA pseudo instruction expansion pass
 ; CHECK-NEXT:       X86 pseudo instruction expansion pass
 ; CHECK-NEXT:       Analyze Machine Code For Garbage Collection
+; CHECK-NEXT:       Shadow Call Stack
 ; CHECK-NEXT:       X86 Indirect Branch Tracking
 ; CHECK-NEXT:       X86 vzeroupper inserter
 ; CHECK-NEXT:       Contiguously Lay Out Funclets
Index: test/CodeGen/X86/O3-pipeline.ll
===================================================================
--- test/CodeGen/X86/O3-pipeline.ll
+++ test/CodeGen/X86/O3-pipeline.ll
@@ -141,6 +141,7 @@
 ; CHECK-NEXT:       ReachingDefAnalysis
 ; CHECK-NEXT:       X86 Execution Dependency Fix
 ; CHECK-NEXT:       BreakFalseDeps
+; CHECK-NEXT:       Shadow Call Stack
 ; CHECK-NEXT:       X86 Indirect Branch Tracking
 ; CHECK-NEXT:       X86 vzeroupper inserter
 ; CHECK-NEXT:       MachineDominator Tree Construction
Index: test/CodeGen/X86/shadow-call-stack.mir
===================================================================
--- /dev/null
+++ test/CodeGen/X86/shadow-call-stack.mir
@@ -0,0 +1,204 @@
+# RUN: llc -mtriple=x86_64-unknown-linux-gnu -run-pass shadow-call-stack -verify-machineinstrs -o - %s | FileCheck %s
+--- |
+
+  define void @no_return() #0 { ret void }
+  define void @normal_return() #0 { ret void }
+  define void @normal_return_leaf_func() #0 { ret void }
+  define void @short_leaf_func() #0 { ret void }
+  define void @normal_tail_call() #0 { ret void }
+  define void @r11_tail_call() #0 { ret void }
+  define void @conditional_tail_call() #0 { ret void }
+  define void @r10_live_in() #0 { ret void }
+
+  attributes #0 = { shadowcallstack }
+
+...
+---
+# CHECK-LABEL: name: no_return
+name: no_return
+tracksRegLiveness: true
+frameInfo:
+  adjustsStack: true # not a leaf function
+body: |
+  ; CHECK: bb.0:
+  bb.0:
+    ; CHECK: $eax = MOV32ri 13
+    $eax = MOV32ri 13
+...
+---
+# CHECK-LABEL: name: normal_return
+name: normal_return
+tracksRegLiveness: true
+frameInfo:
+  adjustsStack: true # not a leaf function
+body: |
+  ; CHECK: bb.0:
+  bb.0:
+    ; CHECK: $r10 = MOV64rm $rsp, 1, $noreg, 0, $noreg
+    ; CHECK-NEXT: $r11 = XOR64rr undef $r11, undef $r11, implicit-def $eflags
+    ; CHECK-NEXT: ADD64mi8 $r11, 1, $noreg, 0, $gs, 8, implicit-def $eflags
+    ; CHECK-NEXT: $r11 = MOV64rm $r11, 1, $noreg, 0, $gs
+    ; CHECK-NEXT: MOV64mr $r11, 1, $noreg, 0, $gs, $r10
+    ; CHECK-NEXT: $eax = MOV32ri 13
+    $eax = MOV32ri 13
+
+    ; CHECK-NEXT: $r11 = XOR64rr undef $r11, undef $r11, implicit-def $eflags
+    ; CHECK-NEXT: $r10 = MOV64rm $r11, 1, $noreg, 0, $gs
+    ; CHECK-NEXT: $r10 = MOV64rm $r10, 1, $noreg, 0, $gs
+    ; CHECK-NEXT: SUB64mi8 $r11, 1, $noreg, 0, $gs, 8, implicit-def $eflags
+    ; CHECK-NEXT: CMP64mr $rsp, 1, $noreg, 0, $noreg, $r10, implicit-def $eflags
+    ; CHECK-NEXT: JNE_1 %bb.1, implicit $eflags
+    ; CHECK-NEXT: RETQ $eax
+    RETQ $eax
+
+    ; CHECK: bb.1:
+    ; CHECK-NEXT; TRAP
+...
+---
+# CHECK-LABEL: name: normal_return_leaf_func
+name: normal_return_leaf_func
+tracksRegLiveness: true
+frameInfo:
+  adjustsStack: false # leaf function
+body: |
+  ; CHECK: bb.0:
+  ; CHECK: liveins: $rcx
+  bb.0:
+    liveins: $rcx
+
+    ; CHECK: $rdx = MOV64rm $rsp, 1, $noreg, 0, $noreg
+    ; CHECK-NEXT: $eax = MOV32ri 0
+    $eax = MOV32ri 0
+    ; CHECK-NEXT: CMP64ri8 $rcx, 5, implicit-def $eflags
+    CMP64ri8 $rcx, 5, implicit-def $eflags
+    ; CHECK-NEXT: JA_1 %bb.1, implicit $eflags
+    JA_1 %bb.1, implicit $eflags
+    ; CHECK-NEXT: JMP_1 %bb.2
+    JMP_1 %bb.2
+
+  ; CHECK: bb.1
+  ; CHECK: liveins: $eax, $rdx
+  bb.1:
+    liveins: $eax
+
+    ; CHECKT: $eax = MOV32ri 1
+    $eax = MOV32ri 1
+
+  ; CHECK: bb.2
+  ; CHECK: liveins: $eax, $rdx
+  bb.2:
+    liveins: $eax
+
+    ; CHECK: CMP64mr $rsp, 1, $noreg, 0, $noreg, $rdx, implicit-def $eflags
+    ; CHECK-NEXT: JNE_1 %bb.3, implicit $eflags
+    ; CHECK-NEXT: RETQ $eax
+    RETQ $eax
+
+    ; CHECK: bb.3:
+    ; CHECK-NEXT; TRAP
+...
+---
+# CHECK-LABEL: name: short_leaf_func
+name: short_leaf_func
+tracksRegLiveness: true
+frameInfo:
+  adjustsStack: false # leaf function
+body: |
+  ; CHECK: bb.0:
+  bb.0:
+    ; CHECK: $eax = MOV32ri 13
+    $eax = MOV32ri 13
+
+    ; CHECK-NEXT: RETQ $eax
+    RETQ $eax
+...
+---
+# CHECK-LABEL: name: normal_tail_call
+name: normal_tail_call
+tracksRegLiveness: true
+frameInfo:
+  adjustsStack: true # not a leaf function
+body: |
+  ; CHECK: bb.0:
+  bb.0:
+    ; CHECK: $r10 = MOV64rm $rsp, 1, $noreg, 0, $noreg
+    ; CHECK-NEXT: $r11 = XOR64rr undef $r11, undef $r11, implicit-def $eflags
+    ; CHECK-NEXT: ADD64mi8 $r11, 1, $noreg, 0, $gs, 8, implicit-def $eflags
+    ; CHECK-NEXT: $r11 = MOV64rm $r11, 1, $noreg, 0, $gs
+    ; CHECK-NEXT: MOV64mr $r11, 1, $noreg, 0, $gs, $r10
+    ; CHECK-NEXT: $eax = MOV32ri 13
+    $eax = MOV32ri 13
+
+    ; CHECK-NEXT: $r11 = XOR64rr undef $r11, undef $r11, implicit-def $eflags
+    ; CHECK-NEXT: $r10 = MOV64rm $r11, 1, $noreg, 0, $gs
+    ; CHECK-NEXT: $r10 = MOV64rm $r10, 1, $noreg, 0, $gs
+    ; CHECK-NEXT: SUB64mi8 $r11, 1, $noreg, 0, $gs, 8, implicit-def $eflags
+    ; CHECK-NEXT: CMP64mr $rsp, 1, $noreg, 0, $noreg, $r10, implicit-def $eflags
+    ; CHECK-NEXT: JNE_1 %bb.1, implicit $eflags
+    ; CHECK-NEXT: TAILJMPr64 $rax
+    TAILJMPr64 $rax
+
+    ; CHECK: bb.1:
+    ; CHECK-NEXT; TRAP
+...
+---
+# CHECK-LABEL: name: r11_tail_call
+name: r11_tail_call
+tracksRegLiveness: true
+frameInfo:
+  adjustsStack: true # not a leaf function
+body: |
+  ; CHECK: bb.0:
+  bb.0:
+    ; CHECK: $r10 = MOV64rm $rsp, 1, $noreg, 0, $noreg
+    ; CHECK-NEXT: $r11 = XOR64rr undef $r11, undef $r11, implicit-def $eflags
+    ; CHECK-NEXT: ADD64mi8 $r11, 1, $noreg, 0, $gs, 8, implicit-def $eflags
+    ; CHECK-NEXT: $r11 = MOV64rm $r11, 1, $noreg, 0, $gs
+    ; CHECK-NEXT: MOV64mr $r11, 1, $noreg, 0, $gs, $r10
+    ; CHECK-NEXT: $eax = MOV32ri 13
+    $eax = MOV32ri 13
+
+    ; CHECK-NEXT: $r10 = XOR64rr undef $r10, undef $r10, implicit-def $eflags
+    ; CHECK-NEXT: $r10 = MOV64rm $r10, 1, $noreg, 0, $gs
+    ; CHECK-NEXT: $r10 = MOV64rm $r10, 1, $noreg, 0, $gs
+    ; CHECK-NEXT: SUB64mi8 $noreg, 1, $noreg, 0, $gs, 8, implicit-def $eflags
+    ; CHECK-NEXT: CMP64mr $rsp, 1, $noreg, 0, $noreg, $r10, implicit-def $eflags
+    ; CHECK-NEXT: JNE_1 %bb.1, implicit $eflags
+    ; CHECK-NEXT: TAILJMPr64 undef $r11
+    TAILJMPr64 undef $r11
+
+    ; CHECK: bb.1:
+    ; CHECK-NEXT; TRAP
+...
+---
+# CHECK-LABEL: name: conditional_tail_call
+name: conditional_tail_call
+tracksRegLiveness: true
+frameInfo:
+  adjustsStack: true # not a leaf function
+body: |
+  ; CHECK: bb.0:
+  bb.0:
+    ; CHECK: $eax = MOV32ri 13
+    $eax = MOV32ri 13
+
+    ; CHECK-NEXT: TAILJMPd64_CC @conditional_tail_call, undef $eflags
+    TAILJMPd64_CC @conditional_tail_call, undef $eflags
+...
+---
+# CHECK-LABEL: name: r10_live_in
+name: r10_live_in
+tracksRegLiveness: true
+frameInfo:
+  adjustsStack: true # not a leaf function
+body: |
+  ; CHECK: bb.0:
+  ; CHECK: liveins: $r10
+  bb.0:
+    liveins: $r10
+
+    ; CHECK: $eax = MOV32ri 13
+    $eax = MOV32ri 13
+    ; CHECK-NEXT: RETQ $eax
+    RETQ $eax
+...