Delete MacOSXFrameBackchain unwind logic (NFC)

[vsk]

Summary

This code looks like a good reference for building a new unwinder, but
is currently unused, so there's no need to keep it.

[aprantl]

Deleting dead code is always good; I'll let Jason sign this off though.

[vsk]

Ping?

[jasonmolenda]

Sorry for not replying to this. Yes the code is unused - it was the initial unwinder implementation when lldb is very young, before we had our current one. The main reason to keep it around is (as you say) an example of the simplest unwinder someone might use when porting lldb to a new platform. I don't feel strongly about it one way or the other, I'm fine with removing it if that's what you want to do.

lldb/lldb.xcodeproj/project.pbxproj

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lldb/source/Plugins/Process/Utility/CMakeLists.txt

add_lldb_library(lldbPluginProcessUtility PLUGIN
   RegisterContextFreeBSD_x86_64.cpp
   RegisterContextHistory.cpp
   RegisterContextLinux_i386.cpp
   RegisterContextLinux_x86_64.cpp
   RegisterContextLinux_mips64.cpp
   RegisterContextLinux_mips.cpp
   RegisterContextLinux_s390x.cpp
   RegisterContextLLDB.cpp
-  RegisterContextMacOSXFrameBackchain.cpp
   RegisterContextMach_arm.cpp
   RegisterContextMach_i386.cpp
   RegisterContextMach_x86_64.cpp
   RegisterContextMemory.cpp
   RegisterContextNetBSD_x86_64.cpp
   RegisterContextOpenBSD_i386.cpp
   RegisterContextOpenBSD_x86_64.cpp
   RegisterContextPOSIX_arm.cpp
   RegisterContextPOSIX_arm64.cpp
   RegisterContextPOSIX_mips64.cpp
   RegisterContextPOSIX_powerpc.cpp
   RegisterContextPOSIX_ppc64le.cpp
   RegisterContextPOSIX_s390x.cpp
   RegisterContextPOSIX_x86.cpp
   RegisterContextThreadMemory.cpp
   RegisterInfoPOSIX_arm.cpp
   RegisterInfoPOSIX_arm64.cpp
   RegisterInfoPOSIX_ppc64le.cpp
   StopInfoMachException.cpp
   ThreadMemory.cpp
   UnwindLLDB.cpp
-  UnwindMacOSXFrameBackchain.cpp
 
   LINK_LIBS
     lldbBreakpoint
     lldbCore
     lldbDataFormatters
     lldbExpression
     lldbHost
     lldbSymbol
     lldbTarget
     lldbUtility
   LINK_COMPONENTS
     Support
   )

lldb/source/Plugins/Process/Utility/RegisterContextMacOSXFrameBackchain.h

-//===-- RegisterContextMacOSXFrameBackchain.h -------------------*- C++ -*-===//
-//
-//                     The LLVM Compiler Infrastructure
-//
-// This file is distributed under the University of Illinois Open Source
-// License. See LICENSE.TXT for details.
-//
-//===----------------------------------------------------------------------===//
-
-#ifndef lldb_RegisterContextMacOSXFrameBackchain_h_
-#define lldb_RegisterContextMacOSXFrameBackchain_h_
-
-// C Includes
-// C++ Includes
-// Other libraries and framework includes
-// Project includes
-#include "lldb/Target/RegisterContext.h"
-#include "lldb/lldb-private.h"
-
-#include "UnwindMacOSXFrameBackchain.h"
-
-class RegisterContextMacOSXFrameBackchain
-    : public lldb_private::RegisterContext {
-public:
-  RegisterContextMacOSXFrameBackchain(
-      lldb_private::Thread &thread, uint32_t concrete_frame_idx,
-      const UnwindMacOSXFrameBackchain::Cursor &cursor);
-
-  ~RegisterContextMacOSXFrameBackchain() override;
-
-  void InvalidateAllRegisters() override;
-
-  size_t GetRegisterCount() override;
-
-  const lldb_private::RegisterInfo *GetRegisterInfoAtIndex(size_t reg) override;
-
-  size_t GetRegisterSetCount() override;
-
-  const lldb_private::RegisterSet *GetRegisterSet(size_t reg_set) override;
-
-  bool ReadRegister(const lldb_private::RegisterInfo *reg_info,
-                    lldb_private::RegisterValue &value) override;
-
-  bool WriteRegister(const lldb_private::RegisterInfo *reg_info,
-                     const lldb_private::RegisterValue &value) override;
-
-  bool ReadAllRegisterValues(lldb::DataBufferSP &data_sp) override;
-
-  bool WriteAllRegisterValues(const lldb::DataBufferSP &data_sp) override;
-
-  uint32_t ConvertRegisterKindToRegisterNumber(lldb::RegisterKind kind,
-                                               uint32_t num) override;
-
-private:
-  UnwindMacOSXFrameBackchain::Cursor m_cursor;
-  bool m_cursor_is_valid;
-
-  DISALLOW_COPY_AND_ASSIGN(RegisterContextMacOSXFrameBackchain);
-};
-
-#endif // lldb_RegisterContextMacOSXFrameBackchain_h_

lldb/source/Plugins/Process/Utility/RegisterContextMacOSXFrameBackchain.cpp

-//===-- RegisterContextMacOSXFrameBackchain.cpp -----------------*- C++ -*-===//
-//
-//                     The LLVM Compiler Infrastructure
-//
-// This file is distributed under the University of Illinois Open Source
-// License. See LICENSE.TXT for details.
-//
-//===----------------------------------------------------------------------===//
-
-#include "RegisterContextMacOSXFrameBackchain.h"
-
-// C Includes
-// C++ Includes
-// Other libraries and framework includes
-#include "lldb/Core/RegisterValue.h"
-#include "lldb/Core/Scalar.h"
-#include "lldb/Target/Thread.h"
-#include "lldb/Utility/DataBufferHeap.h"
-#include "lldb/Utility/DataExtractor.h"
-#include "lldb/Utility/StreamString.h"
-// Project includes
-#include "lldb/Utility/StringExtractorGDBRemote.h"
-
-using namespace lldb;
-using namespace lldb_private;
-
-//----------------------------------------------------------------------
-// RegisterContextMacOSXFrameBackchain constructor
-//----------------------------------------------------------------------
-RegisterContextMacOSXFrameBackchain::RegisterContextMacOSXFrameBackchain(
-    Thread &thread, uint32_t concrete_frame_idx,
-    const UnwindMacOSXFrameBackchain::Cursor &cursor)
-    : RegisterContext(thread, concrete_frame_idx), m_cursor(cursor),
-      m_cursor_is_valid(true) {}
-
-//----------------------------------------------------------------------
-// Destructor
-//----------------------------------------------------------------------
-RegisterContextMacOSXFrameBackchain::~RegisterContextMacOSXFrameBackchain() {}
-
-void RegisterContextMacOSXFrameBackchain::InvalidateAllRegisters() {
-  m_cursor_is_valid = false;
-}
-
-size_t RegisterContextMacOSXFrameBackchain::GetRegisterCount() {
-  return m_thread.GetRegisterContext()->GetRegisterCount();
-}
-
-const RegisterInfo *
-RegisterContextMacOSXFrameBackchain::GetRegisterInfoAtIndex(size_t reg) {
-  return m_thread.GetRegisterContext()->GetRegisterInfoAtIndex(reg);
-}
-
-size_t RegisterContextMacOSXFrameBackchain::GetRegisterSetCount() {
-  return m_thread.GetRegisterContext()->GetRegisterSetCount();
-}
-
-const RegisterSet *
-RegisterContextMacOSXFrameBackchain::GetRegisterSet(size_t reg_set) {
-  return m_thread.GetRegisterContext()->GetRegisterSet(reg_set);
-}
-
-bool RegisterContextMacOSXFrameBackchain::ReadRegister(
-    const RegisterInfo *reg_info, RegisterValue &value) {
-  if (!m_cursor_is_valid)
-    return false;
-
-  uint64_t reg_value = LLDB_INVALID_ADDRESS;
-
-  switch (reg_info->kinds[eRegisterKindGeneric]) {
-  case LLDB_REGNUM_GENERIC_PC:
-    if (m_cursor.pc == LLDB_INVALID_ADDRESS)
-      return false;
-    reg_value = m_cursor.pc;
-    break;
-
-  case LLDB_REGNUM_GENERIC_FP:
-    if (m_cursor.fp == LLDB_INVALID_ADDRESS)
-      return false;
-    reg_value = m_cursor.fp;
-    break;
-
-  default:
-    return false;
-  }
-
-  switch (reg_info->encoding) {
-  case eEncodingInvalid:
-  case eEncodingVector:
-    break;
-
-  case eEncodingUint:
-  case eEncodingSint:
-    value.SetUInt(reg_value, reg_info->byte_size);
-    return true;
-
-  case eEncodingIEEE754:
-    switch (reg_info->byte_size) {
-    case sizeof(float):
-      if (sizeof(float) == sizeof(uint32_t)) {
-        value.SetUInt32(reg_value, RegisterValue::eTypeFloat);
-        return true;
-      } else if (sizeof(float) == sizeof(uint64_t)) {
-        value.SetUInt64(reg_value, RegisterValue::eTypeFloat);
-        return true;
-      }
-      break;
-
-    case sizeof(double):
-      if (sizeof(double) == sizeof(uint32_t)) {
-        value.SetUInt32(reg_value, RegisterValue::eTypeDouble);
-        return true;
-      } else if (sizeof(double) == sizeof(uint64_t)) {
-        value.SetUInt64(reg_value, RegisterValue::eTypeDouble);
-        return true;
-      }
-      break;
-
-// TOOD: need a better way to detect when "long double" types are
-// the same bytes size as "double"
-#if !defined(__arm__) && !defined(__arm64__) && !defined(__aarch64__) &&       \
-    !defined(_MSC_VER) && !defined(__mips__) && !defined(__powerpc__) &&       \
-    !defined(__ANDROID__)
-    case sizeof(long double):
-      if (sizeof(long double) == sizeof(uint32_t)) {
-        value.SetUInt32(reg_value, RegisterValue::eTypeLongDouble);
-        return true;
-      } else if (sizeof(long double) == sizeof(uint64_t)) {
-        value.SetUInt64(reg_value, RegisterValue::eTypeLongDouble);
-        return true;
-      }
-      break;
-#endif
-    }
-    break;
-  }
-  return false;
-}
-
-bool RegisterContextMacOSXFrameBackchain::WriteRegister(
-    const RegisterInfo *reg_info, const RegisterValue &value) {
-  // Not supported yet. We could easily add support for this by remembering the
-  // address of each entry (it would need to be part of the cursor)
-  return false;
-}
-
-bool RegisterContextMacOSXFrameBackchain::ReadAllRegisterValues(
-    lldb::DataBufferSP &data_sp) {
-  // libunwind frames can't handle this it doesn't always have all register
-  // values. This call should only be called on frame zero anyway so there
-  // shouldn't be any problem
-  return false;
-}
-
-bool RegisterContextMacOSXFrameBackchain::WriteAllRegisterValues(
-    const lldb::DataBufferSP &data_sp) {
-  // Since this class doesn't respond to "ReadAllRegisterValues()", it must not
-  // have been the one that saved all the register values. So we just let the
-  // thread's register context (the register context for frame zero) do the
-  // writing.
-  return m_thread.GetRegisterContext()->WriteAllRegisterValues(data_sp);
-}
-
-uint32_t
-RegisterContextMacOSXFrameBackchain::ConvertRegisterKindToRegisterNumber(
-    lldb::RegisterKind kind, uint32_t num) {
-  return m_thread.GetRegisterContext()->ConvertRegisterKindToRegisterNumber(
-      kind, num);
-}

lldb/source/Plugins/Process/Utility/UnwindMacOSXFrameBackchain.h

-//===-- UnwindMacOSXFrameBackchain.h ----------------------------*- C++ -*-===//
-//
-//                     The LLVM Compiler Infrastructure
-//
-// This file is distributed under the University of Illinois Open Source
-// License. See LICENSE.TXT for details.
-//
-//===----------------------------------------------------------------------===//
-
-#ifndef lldb_UnwindMacOSXFrameBackchain_h_
-#define lldb_UnwindMacOSXFrameBackchain_h_
-
-// C Includes
-// C++ Includes
-#include <vector>
-
-// Other libraries and framework includes
-// Project includes
-#include "lldb/Target/Unwind.h"
-#include "lldb/lldb-private.h"
-
-class UnwindMacOSXFrameBackchain : public lldb_private::Unwind {
-public:
-  UnwindMacOSXFrameBackchain(lldb_private::Thread &thread);
-
-  ~UnwindMacOSXFrameBackchain() override = default;
-
-protected:
-  void DoClear() override { m_cursors.clear(); }
-
-  uint32_t DoGetFrameCount() override;
-
-  bool DoGetFrameInfoAtIndex(uint32_t frame_idx, lldb::addr_t &cfa,
-                             lldb::addr_t &pc) override;
-
-  lldb::RegisterContextSP
-  DoCreateRegisterContextForFrame(lldb_private::StackFrame *frame) override;
-
-  friend class RegisterContextMacOSXFrameBackchain;
-
-  struct Cursor {
-    lldb::addr_t pc; // Program counter
-    lldb::addr_t fp; // Frame pointer for us with backchain
-  };
-
-private:
-  std::vector<Cursor> m_cursors;
-
-  size_t GetStackFrameData_i386(const lldb_private::ExecutionContext &exe_ctx);
-
-  size_t
-  GetStackFrameData_x86_64(const lldb_private::ExecutionContext &exe_ctx);
-
-  //------------------------------------------------------------------
-  // For UnwindMacOSXFrameBackchain only
-  //------------------------------------------------------------------
-  DISALLOW_COPY_AND_ASSIGN(UnwindMacOSXFrameBackchain);
-};
-
-#endif // lldb_UnwindMacOSXFrameBackchain_h_

lldb/source/Plugins/Process/Utility/UnwindMacOSXFrameBackchain.cpp

-//===-- UnwindMacOSXFrameBackchain.cpp --------------------------*- C++ -*-===//
-//
-//                     The LLVM Compiler Infrastructure
-//
-// This file is distributed under the University of Illinois Open Source
-// License. See LICENSE.TXT for details.
-//
-//===----------------------------------------------------------------------===//
-
-#include "lldb/Symbol/Function.h"
-#include "lldb/Symbol/ObjectFile.h"
-#include "lldb/Symbol/Symbol.h"
-#include "lldb/Target/ExecutionContext.h"
-#include "lldb/Target/Process.h"
-#include "lldb/Target/Target.h"
-#include "lldb/Target/Thread.h"
-#include "lldb/Utility/ArchSpec.h"
-
-#include "RegisterContextMacOSXFrameBackchain.h"
-
-using namespace lldb;
-using namespace lldb_private;
-
-UnwindMacOSXFrameBackchain::UnwindMacOSXFrameBackchain(Thread &thread)
-    : Unwind(thread), m_cursors() {}
-
-uint32_t UnwindMacOSXFrameBackchain::DoGetFrameCount() {
-  if (m_cursors.empty()) {
-    ExecutionContext exe_ctx(m_thread.shared_from_this());
-    Target *target = exe_ctx.GetTargetPtr();
-    if (target) {
-      const ArchSpec &target_arch = target->GetArchitecture();
-      // Frame zero should always be supplied by the thread...
-      exe_ctx.SetFrameSP(m_thread.GetStackFrameAtIndex(0));
-
-      if (target_arch.GetAddressByteSize() == 8)
-        GetStackFrameData_x86_64(exe_ctx);
-      else
-        GetStackFrameData_i386(exe_ctx);
-    }
-  }
-  return m_cursors.size();
-}
-
-bool UnwindMacOSXFrameBackchain::DoGetFrameInfoAtIndex(uint32_t idx,
-                                                       addr_t &cfa,
-                                                       addr_t &pc) {
-  const uint32_t frame_count = GetFrameCount();
-  if (idx < frame_count) {
-    if (m_cursors[idx].pc == LLDB_INVALID_ADDRESS)
-      return false;
-    if (m_cursors[idx].fp == LLDB_INVALID_ADDRESS)
-      return false;
-
-    pc = m_cursors[idx].pc;
-    cfa = m_cursors[idx].fp;
-
-    return true;
-  }
-  return false;
-}
-
-lldb::RegisterContextSP
-UnwindMacOSXFrameBackchain::DoCreateRegisterContextForFrame(StackFrame *frame) {
-  lldb::RegisterContextSP reg_ctx_sp;
-  uint32_t concrete_idx = frame->GetConcreteFrameIndex();
-  const uint32_t frame_count = GetFrameCount();
-  if (concrete_idx < frame_count)
-    reg_ctx_sp.reset(new RegisterContextMacOSXFrameBackchain(
-        m_thread, concrete_idx, m_cursors[concrete_idx]));
-  return reg_ctx_sp;
-}
-
-size_t UnwindMacOSXFrameBackchain::GetStackFrameData_i386(
-    const ExecutionContext &exe_ctx) {
-  m_cursors.clear();
-
-  StackFrame *first_frame = exe_ctx.GetFramePtr();
-
-  Process *process = exe_ctx.GetProcessPtr();
-  if (process == NULL)
-    return 0;
-
-  struct Frame_i386 {
-    uint32_t fp;
-    uint32_t pc;
-  };
-
-  RegisterContext *reg_ctx = m_thread.GetRegisterContext().get();
-  assert(reg_ctx);
-
-  Cursor cursor;
-  cursor.pc = reg_ctx->GetPC(LLDB_INVALID_ADDRESS);
-  cursor.fp = reg_ctx->GetFP(0);
-
-  Frame_i386 frame = {static_cast<uint32_t>(cursor.fp),
-                      static_cast<uint32_t>(cursor.pc)};
-
-  m_cursors.push_back(cursor);
-
-  const size_t k_frame_size = sizeof(frame);
-  Status error;
-  while (frame.fp != 0 && frame.pc != 0 && ((frame.fp & 7) == 0)) {
-    // Read both the FP and PC (8 bytes)
-    if (process->ReadMemory(frame.fp, &frame.fp, k_frame_size, error) !=
-        k_frame_size)
-      break;
-    if (frame.pc >= 0x1000) {
-      cursor.pc = frame.pc;
-      cursor.fp = frame.fp;
-      m_cursors.push_back(cursor);
-    }
-  }
-  if (!m_cursors.empty()) {
-    lldb::addr_t first_frame_pc = m_cursors.front().pc;
-    if (first_frame_pc != LLDB_INVALID_ADDRESS) {
-      const uint32_t resolve_scope =
-          eSymbolContextModule | eSymbolContextCompUnit |
-          eSymbolContextFunction | eSymbolContextSymbol;
-
-      SymbolContext first_frame_sc(
-          first_frame->GetSymbolContext(resolve_scope));
-      const AddressRange *addr_range_ptr = NULL;
-      AddressRange range;
-      if (first_frame_sc.function)
-        addr_range_ptr = &first_frame_sc.function->GetAddressRange();
-      else if (first_frame_sc.symbol) {
-        range.GetBaseAddress() = first_frame_sc.symbol->GetAddress();
-        range.SetByteSize(first_frame_sc.symbol->GetByteSize());
-        addr_range_ptr = &range;
-      }
-
-      if (addr_range_ptr) {
-        if (first_frame->GetFrameCodeAddress() ==
-            addr_range_ptr->GetBaseAddress()) {
-          // We are at the first instruction, so we can recover the previous PC
-          // by dereferencing the SP
-          lldb::addr_t first_frame_sp = reg_ctx->GetSP(0);
-          // Read the real second frame return address into frame.pc
-          if (first_frame_sp &&
-              process->ReadMemory(first_frame_sp, &frame.pc, sizeof(frame.pc),
-                                  error) == sizeof(frame.pc)) {
-            cursor.fp = m_cursors.front().fp;
-            cursor.pc = frame.pc; // Set the new second frame PC
-
-            // Insert the second frame
-            m_cursors.insert(m_cursors.begin() + 1, cursor);
-
-            m_cursors.front().fp = first_frame_sp;
-          }
-        }
-      }
-    }
-  }
-  //    uint32_t i=0;
-  //    printf("      PC                 FP\n");
-  //    printf("      ------------------ ------------------ \n");
-  //    for (i=0; i<m_cursors.size(); ++i)
-  //    {
-  //        printf("[%3u] 0x%16.16" PRIx64 " 0x%16.16" PRIx64 "\n", i,
-  //        m_cursors[i].pc, m_cursors[i].fp);
-  //    }
-  return m_cursors.size();
-}
-
-size_t UnwindMacOSXFrameBackchain::GetStackFrameData_x86_64(
-    const ExecutionContext &exe_ctx) {
-  m_cursors.clear();
-
-  Process *process = exe_ctx.GetProcessPtr();
-  if (process == NULL)
-    return 0;
-
-  StackFrame *first_frame = exe_ctx.GetFramePtr();
-
-  struct Frame_x86_64 {
-    uint64_t fp;
-    uint64_t pc;
-  };
-
-  RegisterContext *reg_ctx = m_thread.GetRegisterContext().get();
-  assert(reg_ctx);
-
-  Cursor cursor;
-  cursor.pc = reg_ctx->GetPC(LLDB_INVALID_ADDRESS);
-  cursor.fp = reg_ctx->GetFP(0);
-
-  Frame_x86_64 frame = {cursor.fp, cursor.pc};
-
-  m_cursors.push_back(cursor);
-  Status error;
-  const size_t k_frame_size = sizeof(frame);
-  while (frame.fp != 0 && frame.pc != 0 && ((frame.fp & 7) == 0)) {
-    // Read both the FP and PC (16 bytes)
-    if (process->ReadMemory(frame.fp, &frame.fp, k_frame_size, error) !=
-        k_frame_size)
-      break;
-
-    if (frame.pc >= 0x1000) {
-      cursor.pc = frame.pc;
-      cursor.fp = frame.fp;
-      m_cursors.push_back(cursor);
-    }
-  }
-  if (!m_cursors.empty()) {
-    lldb::addr_t first_frame_pc = m_cursors.front().pc;
-    if (first_frame_pc != LLDB_INVALID_ADDRESS) {
-      const uint32_t resolve_scope =
-          eSymbolContextModule | eSymbolContextCompUnit |
-          eSymbolContextFunction | eSymbolContextSymbol;
-
-      SymbolContext first_frame_sc(
-          first_frame->GetSymbolContext(resolve_scope));
-      const AddressRange *addr_range_ptr = NULL;
-      AddressRange range;
-      if (first_frame_sc.function)
-        addr_range_ptr = &first_frame_sc.function->GetAddressRange();
-      else if (first_frame_sc.symbol) {
-        range.GetBaseAddress() = first_frame_sc.symbol->GetAddress();
-        range.SetByteSize(first_frame_sc.symbol->GetByteSize());
-        addr_range_ptr = &range;
-      }
-
-      if (addr_range_ptr) {
-        if (first_frame->GetFrameCodeAddress() ==
-            addr_range_ptr->GetBaseAddress()) {
-          // We are at the first instruction, so we can recover the previous PC
-          // by dereferencing the SP
-          lldb::addr_t first_frame_sp = reg_ctx->GetSP(0);
-          // Read the real second frame return address into frame.pc
-          if (process->ReadMemory(first_frame_sp, &frame.pc, sizeof(frame.pc),
-                                  error) == sizeof(frame.pc)) {
-            cursor.fp = m_cursors.front().fp;
-            cursor.pc = frame.pc; // Set the new second frame PC
-
-            // Insert the second frame
-            m_cursors.insert(m_cursors.begin() + 1, cursor);
-
-            m_cursors.front().fp = first_frame_sp;
-          }
-        }
-      }
-    }
-  }
-  return m_cursors.size();
-}

lldb/source/Target/Thread.cpp

 //===-- Thread.cpp ----------------------------------------------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 // C Includes
 // C++ Includes
 // Other libraries and framework includes
 // Project includes
 #include "lldb/Target/Thread.h"
 #include "Plugins/Process/Utility/UnwindLLDB.h"
-#include "Plugins/Process/Utility/UnwindMacOSXFrameBackchain.h"
 #include "lldb/Breakpoint/BreakpointLocation.h"
 #include "lldb/Core/Debugger.h"
 #include "lldb/Core/FormatEntity.h"
 #include "lldb/Core/Module.h"
 #include "lldb/Core/State.h"
 #include "lldb/Core/ValueObject.h"
 #include "lldb/Host/Host.h"
 #include "lldb/Interpreter/OptionValueFileSpecList.h"
if (!m_unwinder_ap) {
     case llvm::Triple::mips64el:
     case llvm::Triple::ppc:
     case llvm::Triple::ppc64:
     case llvm::Triple::ppc64le:
     case llvm::Triple::systemz:
     case llvm::Triple::hexagon:
       m_unwinder_ap.reset(new UnwindLLDB(*this));
       break;
-
     default:
-      if (target_arch.GetTriple().getVendor() == llvm::Triple::Apple)
-        m_unwinder_ap.reset(new UnwindMacOSXFrameBackchain(*this));
       break;
     }
   }
   return m_unwinder_ap.get();
 }
 
 void Thread::Flush() {
   ClearStackFrames();