diff --git a/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
new file mode 100644
index 0000000..d2a8bfe
--- /dev/null
+++ b/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
@@ -0,0 +1,416 @@
+//===-- llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp --*- C++ -*--===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file contains support for writing Microsoft CodeView debug info.
+//
+//===----------------------------------------------------------------------===//
+
+#include "CodeViewDebug.h"
+#include "llvm/DebugInfo/CodeView/CodeView.h"
+#include "llvm/DebugInfo/CodeView/SymbolRecord.h"
+#include "llvm/MC/MCExpr.h"
+#include "llvm/MC/MCSymbol.h"
+#include "llvm/Support/COFF.h"
+
+using namespace llvm::codeview;
+
+namespace llvm {
+
+StringRef CodeViewDebug::getFullFilepath(const MDNode *S) {
+  assert(S);
+  assert((isa<DICompileUnit>(S) || isa<DIFile>(S) || isa<DISubprogram>(S) ||
+          isa<DILexicalBlockBase>(S)) &&
+         "Unexpected scope info");
+
+  auto *Scope = cast<DIScope>(S);
+  StringRef Dir = Scope->getDirectory(),
+            Filename = Scope->getFilename();
+  std::string &Filepath =
+      DirAndFilenameToFilepathMap[std::make_pair(Dir, Filename)];
+  if (!Filepath.empty())
+    return Filepath;
+
+  // Clang emits directory and relative filename info into the IR, but CodeView
+  // operates on full paths.  We could change Clang to emit full paths too, but
+  // that would increase the IR size and probably not needed for other users.
+  // For now, just concatenate and canonicalize the path here.
+  if (Filename.find(':') == 1)
+    Filepath = Filename;
+  else
+    Filepath = (Dir + "\\" + Filename).str();
+
+  // Canonicalize the path.  We have to do it textually because we may no longer
+  // have access the file in the filesystem.
+  // First, replace all slashes with backslashes.
+  std::replace(Filepath.begin(), Filepath.end(), '/', '\\');
+
+  // Remove all "\.\" with "\".
+  size_t Cursor = 0;
+  while ((Cursor = Filepath.find("\\.\\", Cursor)) != std::string::npos)
+    Filepath.erase(Cursor, 2);
+
+  // Replace all "\XXX\..\" with "\".  Don't try too hard though as the original
+  // path should be well-formatted, e.g. start with a drive letter, etc.
+  Cursor = 0;
+  while ((Cursor = Filepath.find("\\..\\", Cursor)) != std::string::npos) {
+    // Something's wrong if the path starts with "\..\", abort.
+    if (Cursor == 0)
+      break;
+
+    size_t PrevSlash = Filepath.rfind('\\', Cursor - 1);
+    if (PrevSlash == std::string::npos)
+      // Something's wrong, abort.
+      break;
+
+    Filepath.erase(PrevSlash, Cursor + 3 - PrevSlash);
+    // The next ".." might be following the one we've just erased.
+    Cursor = PrevSlash;
+  }
+
+  // Remove all duplicate backslashes.
+  Cursor = 0;
+  while ((Cursor = Filepath.find("\\\\", Cursor)) != std::string::npos)
+    Filepath.erase(Cursor, 1);
+
+  return Filepath;
+}
+
+void CodeViewDebug::maybeRecordLocation(DebugLoc DL,
+                                                const MachineFunction *MF) {
+  const MDNode *Scope = DL.getScope();
+  if (!Scope)
+    return;
+  unsigned LineNumber = DL.getLine();
+  // Skip this line if it is longer than the maximum we can record.
+  if (LineNumber > COFF::CVL_MaxLineNumber)
+    return;
+
+  unsigned ColumnNumber = DL.getCol();
+  // Truncate the column number if it is longer than the maximum we can record.
+  if (ColumnNumber > COFF::CVL_MaxColumnNumber)
+    ColumnNumber = 0;
+
+  StringRef Filename = getFullFilepath(Scope);
+
+  // Skip this instruction if it has the same file:line as the previous one.
+  assert(CurFn);
+  if (!CurFn->Instrs.empty()) {
+    const InstrInfoTy &LastInstr = InstrInfo[CurFn->Instrs.back()];
+    if (LastInstr.Filename == Filename && LastInstr.LineNumber == LineNumber &&
+        LastInstr.ColumnNumber == ColumnNumber)
+      return;
+  }
+  FileNameRegistry.add(Filename);
+
+  MCSymbol *MCL = Asm->MMI->getContext().createTempSymbol();
+  Asm->OutStreamer->EmitLabel(MCL);
+  CurFn->Instrs.push_back(MCL);
+  InstrInfo[MCL] = InstrInfoTy(Filename, LineNumber, ColumnNumber);
+}
+
+CodeViewDebug::CodeViewDebug(AsmPrinter *AP)
+    : Asm(nullptr), CurFn(nullptr) {
+  MachineModuleInfo *MMI = AP->MMI;
+
+  // If module doesn't have named metadata anchors or COFF debug section
+  // is not available, skip any debug info related stuff.
+  if (!MMI->getModule()->getNamedMetadata("llvm.dbg.cu") ||
+      !AP->getObjFileLowering().getCOFFDebugSymbolsSection())
+    return;
+
+  // Tell MMI that we have debug info.
+  MMI->setDebugInfoAvailability(true);
+  Asm = AP;
+}
+
+void CodeViewDebug::endModule() {
+  if (FnDebugInfo.empty())
+    return;
+
+  // FIXME: For functions that are comdat, we should emit separate .debug$S
+  // sections that are comdat associative with the main function instead of
+  // having one big .debug$S section.
+  assert(Asm != nullptr);
+  Asm->OutStreamer->SwitchSection(
+      Asm->getObjFileLowering().getCOFFDebugSymbolsSection());
+  Asm->EmitInt32(COFF::DEBUG_SECTION_MAGIC);
+
+  // The COFF .debug$S section consists of several subsections, each starting
+  // with a 4-byte control code (e.g. 0xF1, 0xF2, etc) and then a 4-byte length
+  // of the payload followed by the payload itself.  The subsections are 4-byte
+  // aligned.
+
+  // Emit per-function debug information.  This code is extracted into a
+  // separate function for readability.
+  for (size_t I = 0, E = VisitedFunctions.size(); I != E; ++I)
+    emitDebugInfoForFunction(VisitedFunctions[I]);
+
+  // This subsection holds a file index to offset in string table table.
+  Asm->OutStreamer->AddComment("File index to string table offset subsection");
+  Asm->EmitInt32(unsigned(ModuleSubstreamKind::FileChecksums));
+  size_t NumFilenames = FileNameRegistry.Infos.size();
+  Asm->EmitInt32(8 * NumFilenames);
+  for (size_t I = 0, E = FileNameRegistry.Filenames.size(); I != E; ++I) {
+    StringRef Filename = FileNameRegistry.Filenames[I];
+    // For each unique filename, just write its offset in the string table.
+    Asm->EmitInt32(FileNameRegistry.Infos[Filename].StartOffset);
+    // The function name offset is not followed by any additional data.
+    Asm->EmitInt32(0);
+  }
+
+  // This subsection holds the string table.
+  Asm->OutStreamer->AddComment("String table");
+  Asm->EmitInt32(unsigned(ModuleSubstreamKind::StringTable));
+  Asm->EmitInt32(FileNameRegistry.LastOffset);
+  // The payload starts with a null character.
+  Asm->EmitInt8(0);
+
+  for (size_t I = 0, E = FileNameRegistry.Filenames.size(); I != E; ++I) {
+    // Just emit unique filenames one by one, separated by a null character.
+    Asm->OutStreamer->EmitBytes(FileNameRegistry.Filenames[I]);
+    Asm->EmitInt8(0);
+  }
+
+  // No more subsections. Fill with zeros to align the end of the section by 4.
+  Asm->OutStreamer->EmitFill((-FileNameRegistry.LastOffset) % 4, 0);
+
+  clear();
+}
+
+static void EmitLabelDiff(MCStreamer &Streamer,
+                          const MCSymbol *From, const MCSymbol *To,
+                          unsigned int Size = 4) {
+  MCSymbolRefExpr::VariantKind Variant = MCSymbolRefExpr::VK_None;
+  MCContext &Context = Streamer.getContext();
+  const MCExpr *FromRef = MCSymbolRefExpr::create(From, Variant, Context),
+               *ToRef   = MCSymbolRefExpr::create(To, Variant, Context);
+  const MCExpr *AddrDelta =
+      MCBinaryExpr::create(MCBinaryExpr::Sub, ToRef, FromRef, Context);
+  Streamer.EmitValue(AddrDelta, Size);
+}
+
+void CodeViewDebug::emitDebugInfoForFunction(const Function *GV) {
+  // For each function there is a separate subsection
+  // which holds the PC to file:line table.
+  const MCSymbol *Fn = Asm->getSymbol(GV);
+  assert(Fn);
+
+  const FunctionInfo &FI = FnDebugInfo[GV];
+  if (FI.Instrs.empty())
+    return;
+  assert(FI.End && "Don't know where the function ends?");
+
+  StringRef FuncName;
+  if (auto *SP = getDISubprogram(GV))
+    FuncName = SP->getDisplayName();
+
+  // If our DISubprogram name is empty, use the mangled name.
+  if (FuncName.empty())
+    FuncName = GlobalValue::getRealLinkageName(GV->getName());
+
+  // Emit a symbol subsection, required by VS2012+ to find function boundaries.
+  MCSymbol *SymbolsBegin = Asm->MMI->getContext().createTempSymbol(),
+           *SymbolsEnd = Asm->MMI->getContext().createTempSymbol();
+  Asm->OutStreamer->AddComment("Symbol subsection for " + Twine(FuncName));
+  Asm->EmitInt32(unsigned(ModuleSubstreamKind::Symbols));
+  EmitLabelDiff(*Asm->OutStreamer, SymbolsBegin, SymbolsEnd);
+  Asm->OutStreamer->EmitLabel(SymbolsBegin);
+  {
+    MCSymbol *ProcSegmentBegin = Asm->MMI->getContext().createTempSymbol(),
+             *ProcSegmentEnd = Asm->MMI->getContext().createTempSymbol();
+    EmitLabelDiff(*Asm->OutStreamer, ProcSegmentBegin, ProcSegmentEnd, 2);
+    Asm->OutStreamer->EmitLabel(ProcSegmentBegin);
+
+    Asm->EmitInt16(unsigned(SymbolRecordKind::S_GPROC32_ID));
+
+    // Some bytes of this segment don't seem to be required for basic debugging,
+    // so just fill them with zeroes.
+    Asm->OutStreamer->EmitFill(12, 0);
+    // This is the important bit that tells the debugger where the function
+    // code is located and what's its size:
+    EmitLabelDiff(*Asm->OutStreamer, Fn, FI.End);
+    Asm->OutStreamer->EmitFill(12, 0);
+    Asm->OutStreamer->EmitCOFFSecRel32(Fn);
+    Asm->OutStreamer->EmitCOFFSectionIndex(Fn);
+    Asm->EmitInt8(0);
+    // Emit the function display name as a null-terminated string.
+    Asm->OutStreamer->EmitBytes(FuncName);
+    Asm->EmitInt8(0);
+    Asm->OutStreamer->EmitLabel(ProcSegmentEnd);
+
+    // We're done with this function.
+    Asm->EmitInt16(0x0002);
+    Asm->EmitInt16(unsigned(SymbolRecordKind::S_PROC_ID_END));
+  }
+  Asm->OutStreamer->EmitLabel(SymbolsEnd);
+  // Every subsection must be aligned to a 4-byte boundary.
+  Asm->OutStreamer->EmitFill((-FuncName.size()) % 4, 0);
+
+  // PCs/Instructions are grouped into segments sharing the same filename.
+  // Pre-calculate the lengths (in instructions) of these segments and store
+  // them in a map for convenience.  Each index in the map is the sequential
+  // number of the respective instruction that starts a new segment.
+  DenseMap<size_t, size_t> FilenameSegmentLengths;
+  size_t LastSegmentEnd = 0;
+  StringRef PrevFilename = InstrInfo[FI.Instrs[0]].Filename;
+  for (size_t J = 1, F = FI.Instrs.size(); J != F; ++J) {
+    if (PrevFilename == InstrInfo[FI.Instrs[J]].Filename)
+      continue;
+    FilenameSegmentLengths[LastSegmentEnd] = J - LastSegmentEnd;
+    LastSegmentEnd = J;
+    PrevFilename = InstrInfo[FI.Instrs[J]].Filename;
+  }
+  FilenameSegmentLengths[LastSegmentEnd] = FI.Instrs.size() - LastSegmentEnd;
+
+  // Emit a line table subsection, required to do PC-to-file:line lookup.
+  Asm->OutStreamer->AddComment("Line table subsection for " + Twine(FuncName));
+  Asm->EmitInt32(unsigned(ModuleSubstreamKind::Lines));
+  MCSymbol *LineTableBegin = Asm->MMI->getContext().createTempSymbol(),
+           *LineTableEnd = Asm->MMI->getContext().createTempSymbol();
+  EmitLabelDiff(*Asm->OutStreamer, LineTableBegin, LineTableEnd);
+  Asm->OutStreamer->EmitLabel(LineTableBegin);
+
+  // Identify the function this subsection is for.
+  Asm->OutStreamer->EmitCOFFSecRel32(Fn);
+  Asm->OutStreamer->EmitCOFFSectionIndex(Fn);
+  // Insert flags after a 16-bit section index.
+  Asm->EmitInt16(COFF::DEBUG_LINE_TABLES_HAVE_COLUMN_RECORDS);
+
+  // Length of the function's code, in bytes.
+  EmitLabelDiff(*Asm->OutStreamer, Fn, FI.End);
+
+  // PC-to-linenumber lookup table:
+  MCSymbol *FileSegmentEnd = nullptr;
+
+  // The start of the last segment:
+  size_t LastSegmentStart = 0;
+
+  auto FinishPreviousChunk = [&] {
+    if (!FileSegmentEnd)
+      return;
+    for (size_t ColSegI = LastSegmentStart,
+                ColSegEnd = ColSegI + FilenameSegmentLengths[LastSegmentStart];
+         ColSegI != ColSegEnd; ++ColSegI) {
+      unsigned ColumnNumber = InstrInfo[FI.Instrs[ColSegI]].ColumnNumber;
+      assert(ColumnNumber <= COFF::CVL_MaxColumnNumber);
+      Asm->EmitInt16(ColumnNumber); // Start column
+      Asm->EmitInt16(0);            // End column
+    }
+    Asm->OutStreamer->EmitLabel(FileSegmentEnd);
+  };
+
+  for (size_t J = 0, F = FI.Instrs.size(); J != F; ++J) {
+    MCSymbol *Instr = FI.Instrs[J];
+    assert(InstrInfo.count(Instr));
+
+    if (FilenameSegmentLengths.count(J)) {
+      // We came to a beginning of a new filename segment.
+      FinishPreviousChunk();
+      StringRef CurFilename = InstrInfo[FI.Instrs[J]].Filename;
+      assert(FileNameRegistry.Infos.count(CurFilename));
+      size_t IndexInStringTable =
+          FileNameRegistry.Infos[CurFilename].FilenameID;
+      // Each segment starts with the offset of the filename
+      // in the string table.
+      Asm->OutStreamer->AddComment(
+          "Segment for file '" + Twine(CurFilename) + "' begins");
+      MCSymbol *FileSegmentBegin = Asm->MMI->getContext().createTempSymbol();
+      Asm->OutStreamer->EmitLabel(FileSegmentBegin);
+      Asm->EmitInt32(8 * IndexInStringTable);
+
+      // Number of PC records in the lookup table.
+      size_t SegmentLength = FilenameSegmentLengths[J];
+      Asm->EmitInt32(SegmentLength);
+
+      // Full size of the segment for this filename, including the prev two
+      // records.
+      FileSegmentEnd = Asm->MMI->getContext().createTempSymbol();
+      EmitLabelDiff(*Asm->OutStreamer, FileSegmentBegin, FileSegmentEnd);
+      LastSegmentStart = J;
+    }
+
+    // The first PC with the given linenumber and the linenumber itself.
+    EmitLabelDiff(*Asm->OutStreamer, Fn, Instr);
+    uint32_t LineNumber = InstrInfo[Instr].LineNumber;
+    assert(LineNumber <= COFF::CVL_MaxLineNumber);
+    uint32_t LineData = LineNumber | COFF::CVL_IsStatement;
+    Asm->EmitInt32(LineData);
+  }
+
+  FinishPreviousChunk();
+  Asm->OutStreamer->EmitLabel(LineTableEnd);
+}
+
+void CodeViewDebug::beginFunction(const MachineFunction *MF) {
+  assert(!CurFn && "Can't process two functions at once!");
+
+  if (!Asm || !Asm->MMI->hasDebugInfo())
+    return;
+
+  const Function *GV = MF->getFunction();
+  assert(FnDebugInfo.count(GV) == false);
+  VisitedFunctions.push_back(GV);
+  CurFn = &FnDebugInfo[GV];
+
+  // Find the end of the function prolog.
+  // FIXME: is there a simpler a way to do this? Can we just search
+  // for the first instruction of the function, not the last of the prolog?
+  DebugLoc PrologEndLoc;
+  bool EmptyPrologue = true;
+  for (const auto &MBB : *MF) {
+    if (PrologEndLoc)
+      break;
+    for (const auto &MI : MBB) {
+      if (MI.isDebugValue())
+        continue;
+
+      // First known non-DBG_VALUE and non-frame setup location marks
+      // the beginning of the function body.
+      // FIXME: do we need the first subcondition?
+      if (!MI.getFlag(MachineInstr::FrameSetup) && MI.getDebugLoc()) {
+        PrologEndLoc = MI.getDebugLoc();
+        break;
+      }
+      EmptyPrologue = false;
+    }
+  }
+  // Record beginning of function if we have a non-empty prologue.
+  if (PrologEndLoc && !EmptyPrologue) {
+    DebugLoc FnStartDL = PrologEndLoc.getFnDebugLoc();
+    maybeRecordLocation(FnStartDL, MF);
+  }
+}
+
+void CodeViewDebug::endFunction(const MachineFunction *MF) {
+  if (!Asm || !CurFn)  // We haven't created any debug info for this function.
+    return;
+
+  const Function *GV = MF->getFunction();
+  assert(FnDebugInfo.count(GV));
+  assert(CurFn == &FnDebugInfo[GV]);
+
+  if (CurFn->Instrs.empty()) {
+    FnDebugInfo.erase(GV);
+    VisitedFunctions.pop_back();
+  } else {
+    CurFn->End = Asm->getFunctionEnd();
+  }
+  CurFn = nullptr;
+}
+
+void CodeViewDebug::beginInstruction(const MachineInstr *MI) {
+  // Ignore DBG_VALUE locations and function prologue.
+  if (!Asm || MI->isDebugValue() || MI->getFlag(MachineInstr::FrameSetup))
+    return;
+  DebugLoc DL = MI->getDebugLoc();
+  if (DL == PrevInstLoc || !DL)
+    return;
+  maybeRecordLocation(DL, Asm->MF);
+}
+}