Index: clang/lib/Basic/Targets/X86.h
===================================================================
--- clang/lib/Basic/Targets/X86.h
+++ clang/lib/Basic/Targets/X86.h
@@ -893,7 +893,6 @@
   CygwinX86_64TargetInfo(const llvm::Triple &Triple, const TargetOptions &Opts)
       : X86_64TargetInfo(Triple, Opts) {
     this->WCharType = TargetInfo::UnsignedShort;
-    TLSSupported = false;
   }
 
   void getTargetDefines(const LangOptions &Opts,
@@ -907,6 +906,10 @@
     if (Opts.CPlusPlus)
       Builder.defineMacro("_GNU_SOURCE");
   }
+
+  BuiltinVaListKind getBuiltinVaListKind() const override {
+    return TargetInfo::CharPtrBuiltinVaList;
+  }
 };
 
 class LLVM_LIBRARY_VISIBILITY DarwinX86_64TargetInfo
Index: clang/lib/Driver/CMakeLists.txt
===================================================================
--- clang/lib/Driver/CMakeLists.txt
+++ clang/lib/Driver/CMakeLists.txt
@@ -54,6 +54,7 @@
   ToolChains/CrossWindows.cpp
   ToolChains/CSKYToolChain.cpp
   ToolChains/Cuda.cpp
+  ToolChains/Cygwin.cpp
   ToolChains/Darwin.cpp
   ToolChains/DragonFly.cpp
   ToolChains/Flang.cpp
Index: clang/lib/Driver/Driver.cpp
===================================================================
--- clang/lib/Driver/Driver.cpp
+++ clang/lib/Driver/Driver.cpp
@@ -20,6 +20,7 @@
 #include "ToolChains/Contiki.h"
 #include "ToolChains/CrossWindows.h"
 #include "ToolChains/Cuda.h"
+#include "ToolChains/Cygwin.h"
 #include "ToolChains/Darwin.h"
 #include "ToolChains/DragonFly.h"
 #include "ToolChains/FreeBSD.h"
@@ -6161,6 +6162,9 @@
         else
           TC = std::make_unique<toolchains::Generic_GCC>(*this, Target, Args);
         break;
+      case llvm::Triple::Cygnus:
+        TC = std::make_unique<toolchains::Cygwin>(*this, Target, Args);
+        break;
       case llvm::Triple::GNU:
         TC = std::make_unique<toolchains::MinGW>(*this, Target, Args);
         break;
Index: clang/lib/Driver/ToolChains/Clang.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Clang.cpp
+++ clang/lib/Driver/ToolChains/Clang.cpp
@@ -6149,6 +6149,12 @@
                    options::OPT_foperator_names, false))
     CmdArgs.push_back("-fno-operator-names");
 
+  if (Triple.isOSCygMing() && Args.hasArg(options::OPT_static)) {
+    CmdArgs.push_back("-flto-visibility-public-std");
+    if (getToolChain().GetCXXStdlibType(Args) == ToolChain::CST_Libcxx)
+      CmdArgs.push_back("-D_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS");
+  }
+
   // Forward -f (flag) options which we can pass directly.
   Args.AddLastArg(CmdArgs, options::OPT_femit_all_decls);
   Args.AddLastArg(CmdArgs, options::OPT_fheinous_gnu_extensions);
@@ -6574,7 +6580,7 @@
   // -fuse-cxa-atexit is default.
   if (!Args.hasFlag(
           options::OPT_fuse_cxa_atexit, options::OPT_fno_use_cxa_atexit,
-          !RawTriple.isOSAIX() && !RawTriple.isOSWindows() &&
+          !RawTriple.isOSAIX() && !(RawTriple.isOSWindows() && !RawTriple.isWindowsCygwinEnvironment()) &&
               ((RawTriple.getVendor() != llvm::Triple::MipsTechnologies) ||
                RawTriple.hasEnvironment())) ||
       KernelOrKext)
Index: clang/lib/Driver/ToolChains/Cygwin.h
===================================================================
--- /dev/null
+++ clang/lib/Driver/ToolChains/Cygwin.h
@@ -0,0 +1,110 @@
+//===--- Cygwin.h - Cygwin ToolChain Implementations --------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_LIB_DRIVER_TOOLCHAINS_CYGWIN_H
+#define LLVM_CLANG_LIB_DRIVER_TOOLCHAINS_CYGWIN_H
+
+#include "Cuda.h"
+#include "Gnu.h"
+#include "ROCm.h"
+#include "clang/Driver/Tool.h"
+#include "clang/Driver/ToolChain.h"
+#include "llvm/Support/ErrorOr.h"
+
+namespace clang {
+namespace driver {
+namespace tools {
+
+/// Cygwin -- Directly call GNU Binutils assembler and linker
+namespace Cygwin {
+class LLVM_LIBRARY_VISIBILITY Assembler : public Tool {
+public:
+  Assembler(const ToolChain &TC) : Tool("Cygwin::Assemble", "assembler", TC) {}
+
+  bool hasIntegratedCPP() const override { return false; }
+
+  void ConstructJob(Compilation &C, const JobAction &JA,
+                    const InputInfo &Output, const InputInfoList &Inputs,
+                    const llvm::opt::ArgList &TCArgs,
+                    const char *LinkingOutput) const override;
+};
+
+class LLVM_LIBRARY_VISIBILITY Linker : public Tool {
+public:
+  Linker(const ToolChain &TC) : Tool("Cygwin::Linker", "linker", TC) {}
+
+  bool hasIntegratedCPP() const override { return false; }
+  bool isLinkJob() const override { return true; }
+
+  void ConstructJob(Compilation &C, const JobAction &JA,
+                    const InputInfo &Output, const InputInfoList &Inputs,
+                    const llvm::opt::ArgList &TCArgs,
+                    const char *LinkingOutput) const override;
+
+private:
+  void AddLibGCC(const llvm::opt::ArgList &Args,
+                 llvm::opt::ArgStringList &CmdArgs) const;
+};
+} // end namespace Cygwin
+} // end namespace tools
+
+namespace toolchains {
+
+class LLVM_LIBRARY_VISIBILITY Cygwin : public ToolChain {
+public:
+  Cygwin(const Driver &D, const llvm::Triple &Triple,
+        const llvm::opt::ArgList &Args);
+
+  bool IsIntegratedAssemblerDefault() const override;
+  // bool IsUnwindTablesDefault(const llvm::opt::ArgList &Args) const override;
+  bool isPICDefault() const override;
+  bool isPIEDefault(const llvm::opt::ArgList &Args) const override;
+  bool isPICDefaultForced() const override;
+
+  llvm::ExceptionHandling GetExceptionModel(
+      const llvm::opt::ArgList &Args) const override;
+
+  void
+  AddClangSystemIncludeArgs(const llvm::opt::ArgList &DriverArgs,
+                            llvm::opt::ArgStringList &CC1Args) const override;
+  void AddClangCXXStdlibIncludeArgs(
+      const llvm::opt::ArgList &DriverArgs,
+      llvm::opt::ArgStringList &CC1Args) const override;
+  void AddCXXStdlibLibArgs(const llvm::opt::ArgList &DriverArgs,
+                           llvm::opt::ArgStringList &CC1Args) const override;
+
+  void AddCudaIncludeArgs(const llvm::opt::ArgList &DriverArgs,
+                          llvm::opt::ArgStringList &CC1Args) const override;
+
+  void printVerboseInfo(raw_ostream &OS) const override;
+
+protected:
+  Tool *getTool(Action::ActionClass AC) const override;
+  Tool *buildLinker() const override;
+  Tool *buildAssembler() const override;
+
+private:
+  CudaInstallationDetector CudaInstallation;
+
+  std::string Base;
+  std::string GccLibDir;
+  std::string Ver;
+  std::string Arch;
+  mutable std::unique_ptr<tools::gcc::Preprocessor> Preprocessor;
+  mutable std::unique_ptr<tools::gcc::Compiler> Compiler;
+  void findGccLibDir();
+  llvm::ErrorOr<std::string> findGcc();
+  llvm::ErrorOr<std::string> findClangRelativeSysroot();
+};
+
+} // end namespace toolchains
+} // end namespace driver
+} // end namespace clang
+
+#endif // LLVM_CLANG_LIB_DRIVER_TOOLCHAINS_CYGWIN_H
Index: clang/lib/Driver/ToolChains/Cygwin.cpp
===================================================================
--- /dev/null
+++ clang/lib/Driver/ToolChains/Cygwin.cpp
@@ -0,0 +1,496 @@
+//===--- Cygwin.cpp - CygwinToolChain Implementation ------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "Cygwin.h"
+#include "CommonArgs.h"
+#include "clang/Config/config.h"
+#include "clang/Driver/Compilation.h"
+#include "clang/Driver/Driver.h"
+#include "clang/Driver/DriverDiagnostic.h"
+#include "clang/Driver/InputInfo.h"
+#include "clang/Driver/Options.h"
+#include "clang/Driver/SanitizerArgs.h"
+#include "llvm/Option/ArgList.h"
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/Path.h"
+#include "llvm/Support/VirtualFileSystem.h"
+#include <system_error>
+
+using namespace clang::diag;
+using namespace clang::driver;
+using namespace clang;
+using namespace llvm::opt;
+
+/// Cygwin Tools
+void tools::Cygwin::Assembler::ConstructJob(Compilation &C, const JobAction &JA,
+                                           const InputInfo &Output,
+                                           const InputInfoList &Inputs,
+                                           const ArgList &Args,
+                                           const char *LinkingOutput) const {
+  claimNoWarnArgs(Args);
+  ArgStringList CmdArgs;
+
+  if (getToolChain().getArch() == llvm::Triple::x86) {
+    CmdArgs.push_back("--32");
+  } else if (getToolChain().getArch() == llvm::Triple::x86_64) {
+    CmdArgs.push_back("--64");
+  }
+
+  Args.AddAllArgValues(CmdArgs, options::OPT_Wa_COMMA, options::OPT_Xassembler);
+
+  CmdArgs.push_back("-o");
+  CmdArgs.push_back(Output.getFilename());
+
+  for (const auto &II : Inputs)
+    CmdArgs.push_back(II.getFilename());
+
+  const char *Exec = Args.MakeArgString(getToolChain().GetProgramPath("as"));
+  C.addCommand(std::make_unique<Command>(JA, *this, ResponseFileSupport::None(),
+                                         Exec, CmdArgs, Inputs, Output));
+
+  if (Args.hasArg(options::OPT_gsplit_dwarf))
+    SplitDebugInfo(getToolChain(), C, *this, JA, Args, Output,
+                   SplitDebugName(JA, Args, Inputs[0], Output));
+}
+
+void tools::Cygwin::Linker::AddLibGCC(const ArgList &Args,
+                                     ArgStringList &CmdArgs) const {
+  // Make use of compiler-rt if --rtlib option is used
+  ToolChain::RuntimeLibType RLT = getToolChain().GetRuntimeLibType(Args);
+  bool Static = Args.hasArg(options::OPT_static_libgcc) ||
+                Args.hasArg(options::OPT_static);
+  // bool Shared = Args.hasArg(options::OPT_shared);
+  // bool CXX = getToolChain().getDriver().CCCIsCXX();
+  if (RLT == ToolChain::RLT_Libgcc) {
+
+    if (Static) {
+      CmdArgs.push_back("-lgcc");
+      CmdArgs.push_back("-lgcc_eh");
+    } else {
+      CmdArgs.push_back("-lgcc_s");
+      CmdArgs.push_back("-lgcc");
+    }
+  } else {
+    AddRunTimeLibs(getToolChain(), getToolChain().getDriver(), CmdArgs, Args);
+    CmdArgs.push_back("-lunwind");
+  }
+
+  CmdArgs.push_back("-lcygwin");
+}
+
+void tools::Cygwin::Linker::ConstructJob(Compilation &C, const JobAction &JA,
+                                        const InputInfo &Output,
+                                        const InputInfoList &Inputs,
+                                        const ArgList &Args,
+                                        const char *LinkingOutput) const {
+  const ToolChain &TC = getToolChain();
+  const Driver &D = TC.getDriver();
+  // const SanitizerArgs &Sanitize = TC.getSanitizerArgs();
+
+  ArgStringList CmdArgs;
+
+  // Silence warning for "clang -g foo.o -o foo"
+  Args.ClaimAllArgs(options::OPT_g_Group);
+  // and "clang -emit-llvm foo.o -o foo"
+  Args.ClaimAllArgs(options::OPT_emit_llvm);
+  // and for "clang -w foo.o -o foo". Other warning options are already
+  // handled somewhere else.
+  Args.ClaimAllArgs(options::OPT_w);
+
+  if (!D.SysRoot.empty())
+    CmdArgs.push_back(Args.MakeArgString("--sysroot=" + D.SysRoot));
+
+  if (Args.hasArg(options::OPT_s))
+    CmdArgs.push_back("-s");
+
+  CmdArgs.push_back("-m");
+  switch (TC.getArch()) {
+  case llvm::Triple::x86:
+    CmdArgs.push_back("i386pe");
+    break;
+  case llvm::Triple::x86_64:
+    CmdArgs.push_back("i386pep");
+    break;
+  default:
+    llvm_unreachable("Unsupported target architecture.");
+  }
+
+  if (!Args.getLastArgValue(options::OPT_fuse_ld_EQ, "link").equals_insensitive("lld")) {
+    if (TC.getArch() == llvm::Triple::x86) {
+      CmdArgs.push_back("--wrap");
+      CmdArgs.push_back("_Znwj");
+      CmdArgs.push_back("--wrap");
+      CmdArgs.push_back("_Znaj");
+      CmdArgs.push_back("--wrap");
+      CmdArgs.push_back("_ZnwjRKSt9nothrow_t");
+      CmdArgs.push_back("--wrap");
+      CmdArgs.push_back("_ZnajRKSt9nothrow_t");
+    } else {
+      CmdArgs.push_back("--wrap");
+      CmdArgs.push_back("_Znwm");
+      CmdArgs.push_back("--wrap");
+      CmdArgs.push_back("_Znam");
+      CmdArgs.push_back("--wrap");
+      CmdArgs.push_back("_ZnwmRKSt9nothrow_t");
+      CmdArgs.push_back("--wrap");
+      CmdArgs.push_back("_ZnamRKSt9nothrow_t");
+    }
+    CmdArgs.push_back("--wrap");
+    CmdArgs.push_back("_ZdlPv");
+    CmdArgs.push_back("--wrap");
+    CmdArgs.push_back("_ZdaPv");
+    CmdArgs.push_back("--wrap");
+    CmdArgs.push_back("_ZdlPvRKSt9nothrow_t");
+    CmdArgs.push_back("--wrap");
+    CmdArgs.push_back("_ZdaPvKSt9nothrow_t");
+  }
+
+  if (Args.hasArg(options::OPT_mwindows)) {
+    CmdArgs.push_back("--subsystem");
+    CmdArgs.push_back("windows");
+  } else if (Args.hasArg(options::OPT_mconsole)) {
+    CmdArgs.push_back("--subsystem");
+    CmdArgs.push_back("console");
+  }
+
+  if (Args.hasArg(options::OPT_static))
+    CmdArgs.push_back("-Bstatic");
+  else {
+    if (Args.hasArg(options::OPT_mdll))
+      CmdArgs.push_back("--dll");
+    else if (Args.hasArg(options::OPT_shared))
+      CmdArgs.push_back("--shared");
+    else if (Args.hasArg(options::OPT_rdynamic))
+      CmdArgs.push_back("--export-all-symbols");
+    CmdArgs.push_back("-Bdynamic");
+    if (Args.hasArg(options::OPT_mdll) || Args.hasArg(options::OPT_shared)) {
+      CmdArgs.push_back("-e");
+      if (TC.getArch() == llvm::Triple::x86)
+        CmdArgs.push_back("__cygwin_dll_entry@12");
+      else
+        CmdArgs.push_back("_cygwin_dll_entry");
+      CmdArgs.push_back("--enable-auto-image-base");
+    } else {
+      if (TC.getArch() == llvm::Triple::x86)
+        CmdArgs.push_back("--large-address-aware");
+      CmdArgs.push_back("--tsaware");
+    }
+  }
+
+  CmdArgs.push_back("-o");
+  CmdArgs.push_back(Output.getFilename());
+
+  Args.AddAllArgs(CmdArgs, options::OPT_e);
+  // FIXME: add -N, -n flags
+  Args.AddLastArg(CmdArgs, options::OPT_r);
+  Args.AddLastArg(CmdArgs, options::OPT_s);
+  Args.AddLastArg(CmdArgs, options::OPT_t);
+  Args.AddAllArgs(CmdArgs, options::OPT_u_Group);
+  Args.AddLastArg(CmdArgs, options::OPT_Z_Flag);
+
+  if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles)) {
+    if (Args.hasArg(options::OPT_shared) || Args.hasArg(options::OPT_mdll)) {
+      CmdArgs.push_back(Args.MakeArgString(TC.GetFilePath("crtbeginS.o")));
+    } else {
+      CmdArgs.push_back(Args.MakeArgString(TC.GetFilePath("crt0.o")));
+      if (Args.hasArg(options::OPT_pg))
+        CmdArgs.push_back(Args.MakeArgString(TC.GetFilePath("gcrt0.o")));
+      CmdArgs.push_back(Args.MakeArgString(TC.GetFilePath("crtbegin.o")));
+    }
+  }
+
+  Args.AddAllArgs(CmdArgs, options::OPT_L);
+  TC.AddFilePathLibArgs(Args, CmdArgs);
+  AddLinkerInputs(TC, Inputs, Args, CmdArgs, JA);
+
+  // TODO: Add ASan stuff here
+
+  // TODO: Add profile stuff here
+
+  if (TC.ShouldLinkCXXStdlib(Args)) {
+    bool OnlyLibstdcxxStatic = Args.hasArg(options::OPT_static_libstdcxx) &&
+                               !Args.hasArg(options::OPT_static);
+    if (OnlyLibstdcxxStatic)
+      CmdArgs.push_back("-Bstatic");
+    TC.AddCXXStdlibLibArgs(Args, CmdArgs);
+    if (OnlyLibstdcxxStatic)
+      CmdArgs.push_back("-Bdynamic");
+  }
+
+  if (!Args.hasArg(options::OPT_nostdlib)) {
+    if (!Args.hasArg(options::OPT_nodefaultlibs)) {
+      if (Args.hasArg(options::OPT_static))
+        CmdArgs.push_back("--start-group");
+
+      if (Args.hasArg(options::OPT_fopenmp))
+        CmdArgs.push_back("-lgomp");
+
+      AddLibGCC(Args, CmdArgs);
+
+      if (Args.hasArg(options::OPT_pg))
+        CmdArgs.push_back("-lgmon");
+
+      // Add system libraries.
+      if (Args.hasArg(options::OPT_mwindows)) {
+        CmdArgs.push_back("-lgdi32");
+        CmdArgs.push_back("-lcomdlg32");
+      }
+      CmdArgs.push_back("-ladvapi32");
+      CmdArgs.push_back("-lshell32");
+      CmdArgs.push_back("-luser32");
+      CmdArgs.push_back("-lkernel32");
+
+      if (Args.hasArg(options::OPT_static))
+        CmdArgs.push_back("--end-group");
+    }
+
+    if (!Args.hasArg(options::OPT_nostartfiles)) {
+      // Add crtfastmath.o if available and fast math is enabled.
+      TC.addFastMathRuntimeIfAvailable(Args, CmdArgs);
+
+      if (!Args.hasArg(options::OPT_shared))
+        CmdArgs.push_back(Args.MakeArgString(TC.GetFilePath("default-manifest.o")));
+      CmdArgs.push_back(Args.MakeArgString(TC.GetFilePath("crtend.o")));
+    }
+  }
+  const char *Exec = Args.MakeArgString(TC.GetLinkerPath());
+  C.addCommand(std::make_unique<Command>(JA, *this,
+                                         ResponseFileSupport::AtFileUTF8(),
+                                         Exec, CmdArgs, Inputs, Output));
+}
+
+// Simplified from Generic_GCC::GCCInstallationDetector::ScanLibDirForGCCTriple.
+static bool findGccVersion(StringRef LibDir, std::string &GccLibDir,
+                           std::string &Ver) {
+  auto Version = toolchains::Generic_GCC::GCCVersion::Parse("0.0.0");
+  std::error_code EC;
+  for (llvm::sys::fs::directory_iterator LI(LibDir, EC), LE; !EC && LI != LE;
+       LI = LI.increment(EC)) {
+    StringRef VersionText = llvm::sys::path::filename(LI->path());
+    auto CandidateVersion =
+        toolchains::Generic_GCC::GCCVersion::Parse(VersionText);
+    if (CandidateVersion.Major == -1)
+      continue;
+    if (CandidateVersion <= Version)
+      continue;
+    Ver = VersionText;
+    GccLibDir = LI->path();
+  }
+  return Ver.size();
+}
+
+void toolchains::Cygwin::findGccLibDir() {
+  llvm::SmallVector<llvm::SmallString<32>, 2> Archs;
+  Archs.emplace_back(getTriple().getArchName());
+  Archs[0] += "-pc-cygwin";
+  Archs.emplace_back("cygwin");
+  if (Arch.empty())
+    Arch = Archs[0].str();
+  // lib: Arch Linux, Ubuntu, Windows
+  // lib64: openSUSE Linux
+  for (StringRef CandidateLib : {"lib", "lib64"}) {
+    for (StringRef CandidateArch : Archs) {
+      llvm::SmallString<1024> LibDir(Base);
+      llvm::sys::path::append(LibDir, CandidateLib, "gcc", CandidateArch);
+      if (findGccVersion(LibDir, GccLibDir, Ver)) {
+        Arch = CandidateArch;
+        return;
+      }
+    }
+  }
+}
+
+llvm::ErrorOr<std::string> toolchains::Cygwin::findGcc() {
+  llvm::SmallVector<llvm::SmallString<32>, 2> Gccs;
+  Gccs.emplace_back(getTriple().getArchName());
+  Gccs[0] += "-pc-cygwin-gcc";
+  Gccs.emplace_back("cygwin-gcc");
+  // Please do not add "gcc" here
+  for (StringRef CandidateGcc : Gccs)
+    if (llvm::ErrorOr<std::string> GPPName = llvm::sys::findProgramByName(CandidateGcc))
+      return GPPName;
+  return make_error_code(std::errc::no_such_file_or_directory);
+}
+
+llvm::ErrorOr<std::string> toolchains::Cygwin::findClangRelativeSysroot() {
+  llvm::SmallVector<llvm::SmallString<32>, 2> Subdirs;
+  Subdirs.emplace_back(getTriple().str());
+  Subdirs.emplace_back(getTriple().getArchName());
+  Subdirs[1] += "-pc-cygwin";
+  StringRef ClangRoot =
+      llvm::sys::path::parent_path(getDriver().getInstalledDir());
+  StringRef Sep = llvm::sys::path::get_separator();
+  for (StringRef CandidateSubdir : Subdirs) {
+    if (llvm::sys::fs::is_directory(ClangRoot + Sep + CandidateSubdir)) {
+      Arch = CandidateSubdir;
+      return (ClangRoot + Sep + CandidateSubdir).str();
+    }
+  }
+  return make_error_code(std::errc::no_such_file_or_directory);
+}
+
+toolchains::Cygwin::Cygwin(const Driver &D, const llvm::Triple &Triple,
+                         const ArgList &Args)
+    : ToolChain(D, Triple, Args), CudaInstallation(D, Triple, Args) {
+  getProgramPaths().push_back(getDriver().getInstalledDir());
+
+  // Look for <clang-bin>/../<triplet>; if found, use <clang-bin>/.. as the
+  // base as it could still be a base for a gcc setup with libgcc.
+  if (llvm::ErrorOr<std::string> TargetSubdir = findClangRelativeSysroot())
+    Base = llvm::sys::path::parent_path(TargetSubdir.get());
+  else if (llvm::ErrorOr<std::string> GPPName = findGcc())
+    Base = llvm::sys::path::parent_path(
+        llvm::sys::path::parent_path(GPPName.get()));
+  else
+    Base = llvm::sys::path::parent_path(getDriver().getInstalledDir());
+
+  Base += llvm::sys::path::get_separator();
+  findGccLibDir();
+  // GccLibDir must precede Base/lib so that the
+  // correct crtbegin.o ,cetend.o would be found.
+  getFilePaths().push_back(GccLibDir);
+  getFilePaths().push_back(
+      (Base + Arch + llvm::sys::path::get_separator() + "lib").str());
+  getFilePaths().push_back(getDriver().SysRoot + "/usr/lib");
+  getFilePaths().push_back(getDriver().SysRoot + "/usr/lib/w32api");
+}
+
+bool toolchains::Cygwin::IsIntegratedAssemblerDefault() const { return true; }
+
+Tool *toolchains::Cygwin::getTool(Action::ActionClass AC) const {
+  switch (AC) {
+  case Action::PreprocessJobClass:
+    if (!Preprocessor)
+      Preprocessor.reset(new tools::gcc::Preprocessor(*this));
+    return Preprocessor.get();
+  case Action::CompileJobClass:
+    if (!Compiler)
+      Compiler.reset(new tools::gcc::Compiler(*this));
+    return Compiler.get();
+  default:
+    return ToolChain::getTool(AC);
+  }
+}
+
+Tool *toolchains::Cygwin::buildAssembler() const {
+  return new tools::Cygwin::Assembler(*this);
+}
+
+Tool *toolchains::Cygwin::buildLinker() const {
+  return new tools::Cygwin::Linker(*this);
+}
+
+#if 0
+bool toolchains::Cygwin::IsUnwindTablesDefault(const ArgList &Args) const {
+  return getArch() == llvm::Triple::x86_64;
+}
+#endif
+
+bool toolchains::Cygwin::isPICDefault() const {
+  return getArch() == llvm::Triple::x86_64;
+}
+
+bool toolchains::Cygwin::isPIEDefault(const llvm::opt::ArgList &Args) const { return false; }
+
+bool toolchains::Cygwin::isPICDefaultForced() const {
+  return getArch() == llvm::Triple::x86_64;
+}
+
+llvm::ExceptionHandling
+toolchains::Cygwin::GetExceptionModel(const ArgList &Args) const {
+  if (getArch() == llvm::Triple::x86_64)
+    return llvm::ExceptionHandling::WinEH;
+  return llvm::ExceptionHandling::DwarfCFI;
+}
+
+void toolchains::Cygwin::AddCudaIncludeArgs(const ArgList &DriverArgs,
+                                           ArgStringList &CC1Args) const {
+  CudaInstallation.AddCudaIncludeArgs(DriverArgs, CC1Args);
+}
+
+void toolchains::Cygwin::printVerboseInfo(raw_ostream &OS) const {
+  CudaInstallation.print(OS);
+}
+
+void toolchains::Cygwin::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
+                                                  ArgStringList &CC1Args) const {
+  if (DriverArgs.hasArg(options::OPT_nostdinc))
+    return;
+
+  if (!DriverArgs.hasArg(options::OPT_nobuiltininc)) {
+    SmallString<1024> P(getDriver().ResourceDir);
+    llvm::sys::path::append(P, "include");
+    addSystemInclude(DriverArgs, CC1Args, P.str());
+  }
+
+  if (!DriverArgs.hasArg(options::OPT_nostdlibinc))
+    addSystemInclude(DriverArgs, CC1Args, getDriver().SysRoot + "/usr/local/include");
+
+  if (!DriverArgs.hasArg(options::OPT_nobuiltininc)) {
+    SmallString<1024> P(getDriver().ResourceDir);
+    llvm::sys::path::append(P, "include");
+    addSystemInclude(DriverArgs, CC1Args, P.str());
+  }
+
+  if (DriverArgs.hasArg(options::OPT_nostdlibinc))
+    return;
+
+  addSystemInclude(DriverArgs, CC1Args,
+                   Base + Arch + llvm::sys::path::get_separator() + "include");
+  addSystemInclude(DriverArgs, CC1Args, getDriver().SysRoot + "/usr/include");
+  addSystemInclude(DriverArgs, CC1Args, getDriver().SysRoot + "/usr/include/w32api");
+}
+
+void toolchains::Cygwin::AddClangCXXStdlibIncludeArgs(
+    const ArgList &DriverArgs, ArgStringList &CC1Args) const {
+  if (DriverArgs.hasArg(options::OPT_nostdlibinc) ||
+      DriverArgs.hasArg(options::OPT_nostdincxx))
+    return;
+
+  StringRef Slash = llvm::sys::path::get_separator();
+
+  switch (GetCXXStdlibType(DriverArgs)) {
+  case ToolChain::CST_Libcxx:
+    addSystemInclude(DriverArgs, CC1Args,
+                     Base + "include" + Slash + "c++" + Slash + "v1");
+    break;
+
+  case ToolChain::CST_Libstdcxx:
+    llvm::SmallVector<llvm::SmallString<1024>, 4> CppIncludeBases;
+    CppIncludeBases.emplace_back(Base);
+    llvm::sys::path::append(CppIncludeBases[0], Arch, "include", "c++");
+    CppIncludeBases.emplace_back(Base);
+    llvm::sys::path::append(CppIncludeBases[1], Arch, "include", "c++", Ver);
+    CppIncludeBases.emplace_back(Base);
+    llvm::sys::path::append(CppIncludeBases[2], "include", "c++", Ver);
+    CppIncludeBases.emplace_back(GccLibDir);
+    llvm::sys::path::append(CppIncludeBases[3], "include", "c++");
+    for (auto &CppIncludeBase : CppIncludeBases) {
+      addSystemInclude(DriverArgs, CC1Args, CppIncludeBase);
+      CppIncludeBase += Slash;
+      addSystemInclude(DriverArgs, CC1Args, CppIncludeBase + Arch);
+      addSystemInclude(DriverArgs, CC1Args, CppIncludeBase + "backward");
+    }
+    break;
+  }
+}
+
+void toolchains::Cygwin::AddCXXStdlibLibArgs(const ArgList &Args,
+                                    ArgStringList &CmdArgs) const {
+  switch (GetCXXStdlibType(Args)) {
+  case ToolChain::CST_Libcxx:
+    CmdArgs.push_back("-lc++");
+    CmdArgs.push_back("-lc++abi");
+    break;
+  case ToolChain::CST_Libstdcxx:
+    CmdArgs.push_back("-lstdc++");
+    CmdArgs.push_back("-liconv");
+    break;
+  }
+}
Index: clang/lib/Lex/InitHeaderSearch.cpp
===================================================================
--- clang/lib/Lex/InitHeaderSearch.cpp
+++ clang/lib/Lex/InitHeaderSearch.cpp
@@ -318,14 +318,7 @@
   case llvm::Triple::RTEMS:
     break;
   case llvm::Triple::Win32:
-    switch (triple.getEnvironment()) {
-    default: llvm_unreachable("Include management is handled in the driver.");
-    case llvm::Triple::Cygnus:
-      AddPath("/usr/include/w32api", System, false);
-      break;
-    case llvm::Triple::GNU:
-      break;
-    }
+    llvm_unreachable("Include management is handled in the driver.");
     break;
   default:
     break;
@@ -353,17 +346,7 @@
   llvm::Triple::OSType os = triple.getOS();
   switch (os) {
   case llvm::Triple::Win32:
-    switch (triple.getEnvironment()) {
-    default: llvm_unreachable("Include management is handled in the driver.");
-    case llvm::Triple::Cygnus:
-      // Cygwin-1.7
-      AddMinGWCPlusPlusIncludePaths("/usr/lib/gcc", "i686-pc-cygwin", "4.7.3");
-      AddMinGWCPlusPlusIncludePaths("/usr/lib/gcc", "i686-pc-cygwin", "4.5.3");
-      AddMinGWCPlusPlusIncludePaths("/usr/lib/gcc", "i686-pc-cygwin", "4.3.4");
-      // g++-4 / Cygwin-1.5
-      AddMinGWCPlusPlusIncludePaths("/usr/lib/gcc", "i686-pc-cygwin", "4.3.2");
-      break;
-    }
+    llvm_unreachable("Include management is handled in the driver.");
     break;
   case llvm::Triple::DragonFly:
     AddPath("/usr/include/c++/5.0", CXXSystem, false);
Index: llvm/lib/Target/X86/X86TargetMachine.cpp
===================================================================
--- llvm/lib/Target/X86/X86TargetMachine.cpp
+++ llvm/lib/Target/X86/X86TargetMachine.cpp
@@ -207,8 +207,8 @@
 }
 
 static CodeModel::Model
-getEffectiveX86CodeModel(std::optional<CodeModel::Model> CM, bool JIT,
-                         bool Is64Bit) {
+getEffectiveX86CodeModel(const Triple &TT, std::optional<CodeModel::Model> CM, bool JIT) {
+  bool Is64Bit = TT.getArch() == Triple::x86_64;
   if (CM) {
     if (*CM == CodeModel::Tiny)
       report_fatal_error("Target does not support the tiny CodeModel", false);
@@ -216,6 +216,8 @@
   }
   if (JIT)
     return Is64Bit ? CodeModel::Large : CodeModel::Small;
+  if (TT.isWindowsCygwinEnvironment())
+    return Is64Bit ? CodeModel::Medium : CodeModel::Small;
   return CodeModel::Small;
 }
 
@@ -230,7 +232,7 @@
     : LLVMTargetMachine(
           T, computeDataLayout(TT), TT, CPU, FS, Options,
           getEffectiveRelocModel(TT, JIT, RM),
-          getEffectiveX86CodeModel(CM, JIT, TT.getArch() == Triple::x86_64),
+          getEffectiveX86CodeModel(TT, CM, JIT),
           OL),
       TLOF(createTLOF(getTargetTriple())), IsJIT(JIT) {
   // On PS4/PS5, the "return address" of a 'noreturn' call must still be within