DWARF parsing and writing support using LLVM (#2520)

This imports LLVM code for DWARF handling. That code has the
Apache 2 license like us. It's also the same code used to
emit DWARF in the common toolchain, so it seems like a safe choice.

This adds two passes: --dwarfdump which runs the same code LLVM
runs for llvm-dwarfdump. This shows we can parse it ok, and will
be useful for debugging. And --dwarfupdate writes out the DWARF
sections (unchanged from what we read, so it just roundtrips - for
updating we need #2515).

This puts LLVM in thirdparty which is added here.

All the LLVM code is behind USE_LLVM_DWARF, which is on
by default, but off in JS for now, as it increases code size by 20%.

This current approach imports the LLVM files directly. This is not
how they are intended to be used, so it required a bunch of
local changes - more than I expected actually, for the platform-specific
stuff. For now this seems to work, so it may be good enough, but
in the long term we may want to switch to linking against libllvm.
A downside to doing that is that binaryen users would need to
have an LLVM build, and even in the waterfall builds we'd have a
problem - while we ship LLVM there anyhow, we constantly update
it, which means that binaryen would need to be on latest llvm all
the time too (which otherwise, given DWARF is quite stable, we
might not need to constantly update).

An even larger issue is that as I did this work I learned about how
DWARF works in LLVM, and while the reading code is easy to
reuse, the writing code is trickier. The main code path is heavily
integrated with the MC layer, which we don't have - we might want
to create a "fake MC layer" for that, but it sounds hard. Instead,
there is the YAML path which is used mostly for testing, and which
can convert DWARF to and from YAML and from binary. Using
the non-YAML parts there, we can convert binary DWARF to
the YAML layer's nice Info data, then convert that to binary. This
works, however, this is not the path LLVM uses normally, and it
supports only some basic DWARF sections - I had to add ranges
support, in fact. So if we need more complex things, we may end
up needing to use the MC layer approach, or consider some other
DWARF library. However, hopefully that should not affect the core
binaryen code which just calls a library for DWARF stuff.

Helps #2400

1 files changed, 1444 insertions, 0 deletions

diff --git a/third_party/llvm-project/include/llvm/Support/FileSystem.h b/third_party/llvm-project/include/llvm/Support/FileSystem.h
new file mode 100644
index 000000000..a29a9d787
--- /dev/null
+++ b/third_party/llvm-project/include/llvm/Support/FileSystem.h
@@ -0,0 +1,1444 @@
+//===- llvm/Support/FileSystem.h - File System OS Concept -------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file declares the llvm::sys::fs namespace. It is designed after
+// TR2/boost filesystem (v3), but modified to remove exception handling and the
+// path class.
+//
+// All functions return an error_code and their actual work via the last out
+// argument. The out argument is defined if and only if errc::success is
+// returned. A function may return any error code in the generic or system
+// category. However, they shall be equivalent to any error conditions listed
+// in each functions respective documentation if the condition applies. [ note:
+// this does not guarantee that error_code will be in the set of explicitly
+// listed codes, but it does guarantee that if any of the explicitly listed
+// errors occur, the correct error_code will be used ]. All functions may
+// return errc::not_enough_memory if there is not enough memory to complete the
+// operation.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_SUPPORT_FILESYSTEM_H
+#define LLVM_SUPPORT_FILESYSTEM_H
+
+#include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/Twine.h"
+#include "llvm/Config/llvm-config.h"
+#include "llvm/Support/Chrono.h"
+#include "llvm/Support/Error.h"
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/ErrorOr.h"
+#include "llvm/Support/MD5.h"
+#include <cassert>
+#include <cstdint>
+#include <ctime>
+#include <memory>
+#include <stack>
+#include <string>
+#include <system_error>
+#include <tuple>
+#include <vector>
+
+#ifdef HAVE_SYS_STAT_H
+#include <sys/stat.h>
+#endif
+
+namespace llvm {
+namespace sys {
+namespace fs {
+
+#if defined(_WIN32)
+// A Win32 HANDLE is a typedef of void*
+using file_t = void *;
+#else
+using file_t = int;
+#endif
+
+extern const file_t kInvalidFile;
+
+/// An enumeration for the file system's view of the type.
+enum class file_type {
+  status_error,
+  file_not_found,
+  regular_file,
+  directory_file,
+  symlink_file,
+  block_file,
+  character_file,
+  fifo_file,
+  socket_file,
+  type_unknown
+};
+
+/// space_info - Self explanatory.
+struct space_info {
+  uint64_t capacity;
+  uint64_t free;
+  uint64_t available;
+};
+
+enum perms {
+  no_perms = 0,
+  owner_read = 0400,
+  owner_write = 0200,
+  owner_exe = 0100,
+  owner_all = owner_read | owner_write | owner_exe,
+  group_read = 040,
+  group_write = 020,
+  group_exe = 010,
+  group_all = group_read | group_write | group_exe,
+  others_read = 04,
+  others_write = 02,
+  others_exe = 01,
+  others_all = others_read | others_write | others_exe,
+  all_read = owner_read | group_read | others_read,
+  all_write = owner_write | group_write | others_write,
+  all_exe = owner_exe | group_exe | others_exe,
+  all_all = owner_all | group_all | others_all,
+  set_uid_on_exe = 04000,
+  set_gid_on_exe = 02000,
+  sticky_bit = 01000,
+  all_perms = all_all | set_uid_on_exe | set_gid_on_exe | sticky_bit,
+  perms_not_known = 0xFFFF
+};
+
+// Helper functions so that you can use & and | to manipulate perms bits:
+inline perms operator|(perms l, perms r) {
+  return static_cast<perms>(static_cast<unsigned short>(l) |
+                            static_cast<unsigned short>(r));
+}
+inline perms operator&(perms l, perms r) {
+  return static_cast<perms>(static_cast<unsigned short>(l) &
+                            static_cast<unsigned short>(r));
+}
+inline perms &operator|=(perms &l, perms r) {
+  l = l | r;
+  return l;
+}
+inline perms &operator&=(perms &l, perms r) {
+  l = l & r;
+  return l;
+}
+inline perms operator~(perms x) {
+  // Avoid UB by explicitly truncating the (unsigned) ~ result.
+  return static_cast<perms>(
+      static_cast<unsigned short>(~static_cast<unsigned short>(x)));
+}
+
+class UniqueID {
+  uint64_t Device;
+  uint64_t File;
+
+public:
+  UniqueID() = default;
+  UniqueID(uint64_t Device, uint64_t File) : Device(Device), File(File) {}
+
+  bool operator==(const UniqueID &Other) const {
+    return Device == Other.Device && File == Other.File;
+  }
+  bool operator!=(const UniqueID &Other) const { return !(*this == Other); }
+  bool operator<(const UniqueID &Other) const {
+    return std::tie(Device, File) < std::tie(Other.Device, Other.File);
+  }
+
+  uint64_t getDevice() const { return Device; }
+  uint64_t getFile() const { return File; }
+};
+
+/// Represents the result of a call to directory_iterator::status(). This is a
+/// subset of the information returned by a regular sys::fs::status() call, and
+/// represents the information provided by Windows FileFirstFile/FindNextFile.
+class basic_file_status {
+protected:
+  #if defined(LLVM_ON_UNIX)
+  time_t fs_st_atime = 0;
+  time_t fs_st_mtime = 0;
+  uint32_t fs_st_atime_nsec = 0;
+  uint32_t fs_st_mtime_nsec = 0;
+  uid_t fs_st_uid = 0;
+  gid_t fs_st_gid = 0;
+  off_t fs_st_size = 0;
+  #elif defined (_WIN32)
+  uint32_t LastAccessedTimeHigh = 0;
+  uint32_t LastAccessedTimeLow = 0;
+  uint32_t LastWriteTimeHigh = 0;
+  uint32_t LastWriteTimeLow = 0;
+  uint32_t FileSizeHigh = 0;
+  uint32_t FileSizeLow = 0;
+  #endif
+  file_type Type = file_type::status_error;
+  perms Perms = perms_not_known;
+
+public:
+  basic_file_status() = default;
+
+  explicit basic_file_status(file_type Type) : Type(Type) {}
+
+  #if defined(LLVM_ON_UNIX)
+  basic_file_status(file_type Type, perms Perms, time_t ATime,
+                    uint32_t ATimeNSec, time_t MTime, uint32_t MTimeNSec,
+                    uid_t UID, gid_t GID, off_t Size)
+      : fs_st_atime(ATime), fs_st_mtime(MTime),
+        fs_st_atime_nsec(ATimeNSec), fs_st_mtime_nsec(MTimeNSec),
+        fs_st_uid(UID), fs_st_gid(GID),
+        fs_st_size(Size), Type(Type), Perms(Perms) {}
+#elif defined(_WIN32)
+  basic_file_status(file_type Type, perms Perms, uint32_t LastAccessTimeHigh,
+                    uint32_t LastAccessTimeLow, uint32_t LastWriteTimeHigh,
+                    uint32_t LastWriteTimeLow, uint32_t FileSizeHigh,
+                    uint32_t FileSizeLow)
+      : LastAccessedTimeHigh(LastAccessTimeHigh),
+        LastAccessedTimeLow(LastAccessTimeLow),
+        LastWriteTimeHigh(LastWriteTimeHigh),
+        LastWriteTimeLow(LastWriteTimeLow), FileSizeHigh(FileSizeHigh),
+        FileSizeLow(FileSizeLow), Type(Type), Perms(Perms) {}
+  #endif
+
+  // getters
+  file_type type() const { return Type; }
+  perms permissions() const { return Perms; }
+
+  /// The file access time as reported from the underlying file system.
+  ///
+  /// Also see comments on \c getLastModificationTime() related to the precision
+  /// of the returned value.
+  TimePoint<> getLastAccessedTime() const;
+
+  /// The file modification time as reported from the underlying file system.
+  ///
+  /// The returned value allows for nanosecond precision but the actual
+  /// resolution is an implementation detail of the underlying file system.
+  /// There is no guarantee for what kind of resolution you can expect, the
+  /// resolution can differ across platforms and even across mountpoints on the
+  /// same machine.
+  TimePoint<> getLastModificationTime() const;
+
+  #if defined(LLVM_ON_UNIX)
+  uint32_t getUser() const { return fs_st_uid; }
+  uint32_t getGroup() const { return fs_st_gid; }
+  uint64_t getSize() const { return fs_st_size; }
+  #elif defined (_WIN32)
+  uint32_t getUser() const {
+    return 9999; // Not applicable to Windows, so...
+  }
+
+  uint32_t getGroup() const {
+    return 9999; // Not applicable to Windows, so...
+  }
+
+  uint64_t getSize() const {
+    return (uint64_t(FileSizeHigh) << 32) + FileSizeLow;
+  }
+  #endif
+
+  // setters
+  void type(file_type v) { Type = v; }
+  void permissions(perms p) { Perms = p; }
+};
+
+/// Represents the result of a call to sys::fs::status().
+class file_status : public basic_file_status {
+  friend bool equivalent(file_status A, file_status B);
+
+  #if defined(LLVM_ON_UNIX)
+  dev_t fs_st_dev = 0;
+  nlink_t fs_st_nlinks = 0;
+  ino_t fs_st_ino = 0;
+  #elif defined (_WIN32)
+  uint32_t NumLinks = 0;
+  uint32_t VolumeSerialNumber = 0;
+  uint32_t FileIndexHigh = 0;
+  uint32_t FileIndexLow = 0;
+  #endif
+
+public:
+  file_status() = default;
+
+  explicit file_status(file_type Type) : basic_file_status(Type) {}
+
+  #if defined(LLVM_ON_UNIX)
+  file_status(file_type Type, perms Perms, dev_t Dev, nlink_t Links, ino_t Ino,
+              time_t ATime, uint32_t ATimeNSec,
+              time_t MTime, uint32_t MTimeNSec,
+              uid_t UID, gid_t GID, off_t Size)
+      : basic_file_status(Type, Perms, ATime, ATimeNSec, MTime, MTimeNSec,
+                          UID, GID, Size),
+        fs_st_dev(Dev), fs_st_nlinks(Links), fs_st_ino(Ino) {}
+  #elif defined(_WIN32)
+  file_status(file_type Type, perms Perms, uint32_t LinkCount,
+              uint32_t LastAccessTimeHigh, uint32_t LastAccessTimeLow,
+              uint32_t LastWriteTimeHigh, uint32_t LastWriteTimeLow,
+              uint32_t VolumeSerialNumber, uint32_t FileSizeHigh,
+              uint32_t FileSizeLow, uint32_t FileIndexHigh,
+              uint32_t FileIndexLow)
+      : basic_file_status(Type, Perms, LastAccessTimeHigh, LastAccessTimeLow,
+                          LastWriteTimeHigh, LastWriteTimeLow, FileSizeHigh,
+                          FileSizeLow),
+        NumLinks(LinkCount), VolumeSerialNumber(VolumeSerialNumber),
+        FileIndexHigh(FileIndexHigh), FileIndexLow(FileIndexLow) {}
+  #endif
+
+  UniqueID getUniqueID() const;
+  uint32_t getLinkCount() const;
+};
+
+/// @}
+/// @name Physical Operators
+/// @{
+
+/// Make \a path an absolute path.
+///
+/// Makes \a path absolute using the \a current_directory if it is not already.
+/// An empty \a path will result in the \a current_directory.
+///
+/// /absolute/path   => /absolute/path
+/// relative/../path => <current-directory>/relative/../path
+///
+/// @param path A path that is modified to be an absolute path.
+void make_absolute(const Twine &current_directory, SmallVectorImpl<char> &path);
+
+/// Make \a path an absolute path.
+///
+/// Makes \a path absolute using the current directory if it is not already. An
+/// empty \a path will result in the current directory.
+///
+/// /absolute/path   => /absolute/path
+/// relative/../path => <current-directory>/relative/../path
+///
+/// @param path A path that is modified to be an absolute path.
+/// @returns errc::success if \a path has been made absolute, otherwise a
+///          platform-specific error_code.
+std::error_code make_absolute(SmallVectorImpl<char> &path);
+
+/// Create all the non-existent directories in path.
+///
+/// @param path Directories to create.
+/// @returns errc::success if is_directory(path), otherwise a platform
+///          specific error_code. If IgnoreExisting is false, also returns
+///          error if the directory already existed.
+std::error_code create_directories(const Twine &path,
+                                   bool IgnoreExisting = true,
+                                   perms Perms = owner_all | group_all);
+
+/// Create the directory in path.
+///
+/// @param path Directory to create.
+/// @returns errc::success if is_directory(path), otherwise a platform
+///          specific error_code. If IgnoreExisting is false, also returns
+///          error if the directory already existed.
+std::error_code create_directory(const Twine &path, bool IgnoreExisting = true,
+                                 perms Perms = owner_all | group_all);
+
+/// Create a link from \a from to \a to.
+///
+/// The link may be a soft or a hard link, depending on the platform. The caller
+/// may not assume which one. Currently on windows it creates a hard link since
+/// soft links require extra privileges. On unix, it creates a soft link since
+/// hard links don't work on SMB file systems.
+///
+/// @param to The path to hard link to.
+/// @param from The path to hard link from. This is created.
+/// @returns errc::success if the link was created, otherwise a platform
+/// specific error_code.
+std::error_code create_link(const Twine &to, const Twine &from);
+
+/// Create a hard link from \a from to \a to, or return an error.
+///
+/// @param to The path to hard link to.
+/// @param from The path to hard link from. This is created.
+/// @returns errc::success if the link was created, otherwise a platform
+/// specific error_code.
+std::error_code create_hard_link(const Twine &to, const Twine &from);
+
+/// Collapse all . and .. patterns, resolve all symlinks, and optionally
+///        expand ~ expressions to the user's home directory.
+///
+/// @param path The path to resolve.
+/// @param output The location to store the resolved path.
+/// @param expand_tilde If true, resolves ~ expressions to the user's home
+///                     directory.
+std::error_code real_path(const Twine &path, SmallVectorImpl<char> &output,
+                          bool expand_tilde = false);
+
+/// Expands ~ expressions to the user's home directory. On Unix ~user
+/// directories are resolved as well.
+///
+/// @param path The path to resolve.
+void expand_tilde(const Twine &path, SmallVectorImpl<char> &output);
+
+/// Get the current path.
+///
+/// @param result Holds the current path on return.
+/// @returns errc::success if the current path has been stored in result,
+///          otherwise a platform-specific error_code.
+std::error_code current_path(SmallVectorImpl<char> &result);
+
+/// Set the current path.
+///
+/// @param path The path to set.
+/// @returns errc::success if the current path was successfully set,
+///          otherwise a platform-specific error_code.
+std::error_code set_current_path(const Twine &path);
+
+/// Remove path. Equivalent to POSIX remove().
+///
+/// @param path Input path.
+/// @returns errc::success if path has been removed or didn't exist, otherwise a
+///          platform-specific error code. If IgnoreNonExisting is false, also
+///          returns error if the file didn't exist.
+std::error_code remove(const Twine &path, bool IgnoreNonExisting = true);
+
+/// Recursively delete a directory.
+///
+/// @param path Input path.
+/// @returns errc::success if path has been removed or didn't exist, otherwise a
+///          platform-specific error code.
+std::error_code remove_directories(const Twine &path, bool IgnoreErrors = true);
+
+/// Rename \a from to \a to.
+///
+/// Files are renamed as if by POSIX rename(), except that on Windows there may
+/// be a short interval of time during which the destination file does not
+/// exist.
+///
+/// @param from The path to rename from.
+/// @param to The path to rename to. This is created.
+std::error_code rename(const Twine &from, const Twine &to);
+
+/// Copy the contents of \a From to \a To.
+///
+/// @param From The path to copy from.
+/// @param To The path to copy to. This is created.
+std::error_code copy_file(const Twine &From, const Twine &To);
+
+/// Copy the contents of \a From to \a To.
+///
+/// @param From The path to copy from.
+/// @param ToFD The open file descriptor of the destination file.
+std::error_code copy_file(const Twine &From, int ToFD);
+
+/// Resize path to size. File is resized as if by POSIX truncate().
+///
+/// @param FD Input file descriptor.
+/// @param Size Size to resize to.
+/// @returns errc::success if \a path has been resized to \a size, otherwise a
+///          platform-specific error_code.
+std::error_code resize_file(int FD, uint64_t Size);
+
+/// Compute an MD5 hash of a file's contents.
+///
+/// @param FD Input file descriptor.
+/// @returns An MD5Result with the hash computed, if successful, otherwise a
+///          std::error_code.
+ErrorOr<MD5::MD5Result> md5_contents(int FD);
+
+/// Version of compute_md5 that doesn't require an open file descriptor.
+ErrorOr<MD5::MD5Result> md5_contents(const Twine &Path);
+
+/// @}
+/// @name Physical Observers
+/// @{
+
+/// Does file exist?
+///
+/// @param status A basic_file_status previously returned from stat.
+/// @returns True if the file represented by status exists, false if it does
+///          not.
+bool exists(const basic_file_status &status);
+
+enum class AccessMode { Exist, Write, Execute };
+
+/// Can the file be accessed?
+///
+/// @param Path Input path.
+/// @returns errc::success if the path can be accessed, otherwise a
+///          platform-specific error_code.
+std::error_code access(const Twine &Path, AccessMode Mode);
+
+/// Does file exist?
+///
+/// @param Path Input path.
+/// @returns True if it exists, false otherwise.
+inline bool exists(const Twine &Path) {
+  return !access(Path, AccessMode::Exist);
+}
+
+/// Can we execute this file?
+///
+/// @param Path Input path.
+/// @returns True if we can execute it, false otherwise.
+bool can_execute(const Twine &Path);
+
+/// Can we write this file?
+///
+/// @param Path Input path.
+/// @returns True if we can write to it, false otherwise.
+inline bool can_write(const Twine &Path) {
+  return !access(Path, AccessMode::Write);
+}
+
+/// Do file_status's represent the same thing?
+///
+/// @param A Input file_status.
+/// @param B Input file_status.
+///
+/// assert(status_known(A) || status_known(B));
+///
+/// @returns True if A and B both represent the same file system entity, false
+///          otherwise.
+bool equivalent(file_status A, file_status B);
+
+/// Do paths represent the same thing?
+///
+/// assert(status_known(A) || status_known(B));
+///
+/// @param A Input path A.
+/// @param B Input path B.
+/// @param result Set to true if stat(A) and stat(B) have the same device and
+///               inode (or equivalent).
+/// @returns errc::success if result has been successfully set, otherwise a
+///          platform-specific error_code.
+std::error_code equivalent(const Twine &A, const Twine &B, bool &result);
+
+/// Simpler version of equivalent for clients that don't need to
+///        differentiate between an error and false.
+inline bool equivalent(const Twine &A, const Twine &B) {
+  bool result;
+  return !equivalent(A, B, result) && result;
+}
+
+/// Is the file mounted on a local filesystem?
+///
+/// @param path Input path.
+/// @param result Set to true if \a path is on fixed media such as a hard disk,
+///               false if it is not.
+/// @returns errc::success if result has been successfully set, otherwise a
+///          platform specific error_code.
+std::error_code is_local(const Twine &path, bool &result);
+
+/// Version of is_local accepting an open file descriptor.
+std::error_code is_local(int FD, bool &result);
+
+/// Simpler version of is_local for clients that don't need to
+///        differentiate between an error and false.
+inline bool is_local(const Twine &Path) {
+  bool Result;
+  return !is_local(Path, Result) && Result;
+}
+
+/// Simpler version of is_local accepting an open file descriptor for
+///        clients that don't need to differentiate between an error and false.
+inline bool is_local(int FD) {
+  bool Result;
+  return !is_local(FD, Result) && Result;
+}
+
+/// Does status represent a directory?
+///
+/// @param Path The path to get the type of.
+/// @param Follow For symbolic links, indicates whether to return the file type
+///               of the link itself, or of the target.
+/// @returns A value from the file_type enumeration indicating the type of file.
+file_type get_file_type(const Twine &Path, bool Follow = true);
+
+/// Does status represent a directory?
+///
+/// @param status A basic_file_status previously returned from status.
+/// @returns status.type() == file_type::directory_file.
+bool is_directory(const basic_file_status &status);
+
+/// Is path a directory?
+///
+/// @param path Input path.
+/// @param result Set to true if \a path is a directory (after following
+///               symlinks, false if it is not. Undefined otherwise.
+/// @returns errc::success if result has been successfully set, otherwise a
+///          platform-specific error_code.
+std::error_code is_directory(const Twine &path, bool &result);
+
+/// Simpler version of is_directory for clients that don't need to
+///        differentiate between an error and false.
+inline bool is_directory(const Twine &Path) {
+  bool Result;
+  return !is_directory(Path, Result) && Result;
+}
+
+/// Does status represent a regular file?
+///
+/// @param status A basic_file_status previously returned from status.
+/// @returns status_known(status) && status.type() == file_type::regular_file.
+bool is_regular_file(const basic_file_status &status);
+
+/// Is path a regular file?
+///
+/// @param path Input path.
+/// @param result Set to true if \a path is a regular file (after following
+///               symlinks), false if it is not. Undefined otherwise.
+/// @returns errc::success if result has been successfully set, otherwise a
+///          platform-specific error_code.
+std::error_code is_regular_file(const Twine &path, bool &result);
+
+/// Simpler version of is_regular_file for clients that don't need to
+///        differentiate between an error and false.
+inline bool is_regular_file(const Twine &Path) {
+  bool Result;
+  if (is_regular_file(Path, Result))
+    return false;
+  return Result;
+}
+
+/// Does status represent a symlink file?
+///
+/// @param status A basic_file_status previously returned from status.
+/// @returns status_known(status) && status.type() == file_type::symlink_file.
+bool is_symlink_file(const basic_file_status &status);
+
+/// Is path a symlink file?
+///
+/// @param path Input path.
+/// @param result Set to true if \a path is a symlink file, false if it is not.
+///               Undefined otherwise.
+/// @returns errc::success if result has been successfully set, otherwise a
+///          platform-specific error_code.
+std::error_code is_symlink_file(const Twine &path, bool &result);
+
+/// Simpler version of is_symlink_file for clients that don't need to
+///        differentiate between an error and false.
+inline bool is_symlink_file(const Twine &Path) {
+  bool Result;
+  if (is_symlink_file(Path, Result))
+    return false;
+  return Result;
+}
+
+/// Does this status represent something that exists but is not a
+///        directory or regular file?
+///
+/// @param status A basic_file_status previously returned from status.
+/// @returns exists(s) && !is_regular_file(s) && !is_directory(s)
+bool is_other(const basic_file_status &status);
+
+/// Is path something that exists but is not a directory,
+///        regular file, or symlink?
+///
+/// @param path Input path.
+/// @param result Set to true if \a path exists, but is not a directory, regular
+///               file, or a symlink, false if it does not. Undefined otherwise.
+/// @returns errc::success if result has been successfully set, otherwise a
+///          platform-specific error_code.
+std::error_code is_other(const Twine &path, bool &result);
+
+/// Get file status as if by POSIX stat().
+///
+/// @param path Input path.
+/// @param result Set to the file status.
+/// @param follow When true, follows symlinks.  Otherwise, the symlink itself is
+///               statted.
+/// @returns errc::success if result has been successfully set, otherwise a
+///          platform-specific error_code.
+std::error_code status(const Twine &path, file_status &result,
+                       bool follow = true);
+
+/// A version for when a file descriptor is already available.
+std::error_code status(int FD, file_status &Result);
+
+#ifdef _WIN32
+/// A version for when a file descriptor is already available.
+std::error_code status(file_t FD, file_status &Result);
+#endif
+
+/// Get file creation mode mask of the process.
+///
+/// @returns Mask reported by umask(2)
+/// @note There is no umask on Windows. This function returns 0 always
+///       on Windows. This function does not return an error_code because
+///       umask(2) never fails. It is not thread safe.
+unsigned getUmask();
+
+/// Set file permissions.
+///
+/// @param Path File to set permissions on.
+/// @param Permissions New file permissions.
+/// @returns errc::success if the permissions were successfully set, otherwise
+///          a platform-specific error_code.
+/// @note On Windows, all permissions except *_write are ignored. Using any of
+///       owner_write, group_write, or all_write will make the file writable.
+///       Otherwise, the file will be marked as read-only.
+std::error_code setPermissions(const Twine &Path, perms Permissions);
+
+/// Vesion of setPermissions accepting a file descriptor.
+/// TODO Delete the path based overload once we implement the FD based overload
+/// on Windows.
+std::error_code setPermissions(int FD, perms Permissions);
+
+/// Get file permissions.
+///
+/// @param Path File to get permissions from.
+/// @returns the permissions if they were successfully retrieved, otherwise a
+///          platform-specific error_code.
+/// @note On Windows, if the file does not have the FILE_ATTRIBUTE_READONLY
+///       attribute, all_all will be returned. Otherwise, all_read | all_exe
+///       will be returned.
+ErrorOr<perms> getPermissions(const Twine &Path);
+
+/// Get file size.
+///
+/// @param Path Input path.
+/// @param Result Set to the size of the file in \a Path.
+/// @returns errc::success if result has been successfully set, otherwise a
+///          platform-specific error_code.
+inline std::error_code file_size(const Twine &Path, uint64_t &Result) {
+  file_status Status;
+  std::error_code EC = status(Path, Status);
+  if (EC)
+    return EC;
+  Result = Status.getSize();
+  return std::error_code();
+}
+
+/// Set the file modification and access time.
+///
+/// @returns errc::success if the file times were successfully set, otherwise a
+///          platform-specific error_code or errc::function_not_supported on
+///          platforms where the functionality isn't available.
+std::error_code setLastAccessAndModificationTime(int FD, TimePoint<> AccessTime,
+                                                 TimePoint<> ModificationTime);
+
+/// Simpler version that sets both file modification and access time to the same
+/// time.
+inline std::error_code setLastAccessAndModificationTime(int FD,
+                                                        TimePoint<> Time) {
+  return setLastAccessAndModificationTime(FD, Time, Time);
+}
+
+/// Is status available?
+///
+/// @param s Input file status.
+/// @returns True if status() != status_error.
+bool status_known(const basic_file_status &s);
+
+/// Is status available?
+///
+/// @param path Input path.
+/// @param result Set to true if status() != status_error.
+/// @returns errc::success if result has been successfully set, otherwise a
+///          platform-specific error_code.
+std::error_code status_known(const Twine &path, bool &result);
+
+enum CreationDisposition : unsigned {
+  /// CD_CreateAlways - When opening a file:
+  ///   * If it already exists, truncate it.
+  ///   * If it does not already exist, create a new file.
+  CD_CreateAlways = 0,
+
+  /// CD_CreateNew - When opening a file:
+  ///   * If it already exists, fail.
+  ///   * If it does not already exist, create a new file.
+  CD_CreateNew = 1,
+
+  /// CD_OpenExisting - When opening a file:
+  ///   * If it already exists, open the file with the offset set to 0.
+  ///   * If it does not already exist, fail.
+  CD_OpenExisting = 2,
+
+  /// CD_OpenAlways - When opening a file:
+  ///   * If it already exists, open the file with the offset set to 0.
+  ///   * If it does not already exist, create a new file.
+  CD_OpenAlways = 3,
+};
+
+enum FileAccess : unsigned {
+  FA_Read = 1,
+  FA_Write = 2,
+};
+
+enum OpenFlags : unsigned {
+  OF_None = 0,
+  F_None = 0, // For compatibility
+
+  /// The file should be opened in text mode on platforms that make this
+  /// distinction.
+  OF_Text = 1,
+  F_Text = 1, // For compatibility
+
+  /// The file should be opened in append mode.
+  OF_Append = 2,
+  F_Append = 2, // For compatibility
+
+  /// Delete the file on close. Only makes a difference on windows.
+  OF_Delete = 4,
+
+  /// When a child process is launched, this file should remain open in the
+  /// child process.
+  OF_ChildInherit = 8,
+
+  /// Force files Atime to be updated on access. Only makes a difference on windows.
+  OF_UpdateAtime = 16,
+};
+
+/// Create a potentially unique file name but does not create it.
+///
+/// Generates a unique path suitable for a temporary file but does not
+/// open or create the file. The name is based on \a Model with '%'
+/// replaced by a random char in [0-9a-f]. If \a MakeAbsolute is true
+/// then the system's temp directory is prepended first. If \a MakeAbsolute
+/// is false the current directory will be used instead.
+///
+/// This function does not check if the file exists. If you want to be sure
+/// that the file does not yet exist, you should use use enough '%' characters
+/// in your model to ensure this. Each '%' gives 4-bits of entropy so you can
+/// use 32 of them to get 128 bits of entropy.
+///
+/// Example: clang-%%-%%-%%-%%-%%.s => clang-a0-b1-c2-d3-e4.s
+///
+/// @param Model Name to base unique path off of.
+/// @param ResultPath Set to the file's path.
+/// @param MakeAbsolute Whether to use the system temp directory.
+void createUniquePath(const Twine &Model, SmallVectorImpl<char> &ResultPath,
+                      bool MakeAbsolute);
+
+/// Create a uniquely named file.
+///
+/// Generates a unique path suitable for a temporary file and then opens it as a
+/// file. The name is based on \a Model with '%' replaced by a random char in
+/// [0-9a-f]. If \a Model is not an absolute path, the temporary file will be
+/// created in the current directory.
+///
+/// Example: clang-%%-%%-%%-%%-%%.s => clang-a0-b1-c2-d3-e4.s
+///
+/// This is an atomic operation. Either the file is created and opened, or the
+/// file system is left untouched.
+///
+/// The intended use is for files that are to be kept, possibly after
+/// renaming them. For example, when running 'clang -c foo.o', the file can
+/// be first created as foo-abc123.o and then renamed.
+///
+/// @param Model Name to base unique path off of.
+/// @param ResultFD Set to the opened file's file descriptor.
+/// @param ResultPath Set to the opened file's absolute path.
+/// @returns errc::success if Result{FD,Path} have been successfully set,
+///          otherwise a platform-specific error_code.
+std::error_code createUniqueFile(const Twine &Model, int &ResultFD,
+                                 SmallVectorImpl<char> &ResultPath,
+                                 unsigned Mode = all_read | all_write);
+
+/// Simpler version for clients that don't want an open file. An empty
+/// file will still be created.
+std::error_code createUniqueFile(const Twine &Model,
+                                 SmallVectorImpl<char> &ResultPath,
+                                 unsigned Mode = all_read | all_write);
+
+/// Represents a temporary file.
+///
+/// The temporary file must be eventually discarded or given a final name and
+/// kept.
+///
+/// The destructor doesn't implicitly discard because there is no way to
+/// properly handle errors in a destructor.
+class TempFile {
+  bool Done = false;
+  TempFile(StringRef Name, int FD);
+
+public:
+  /// This creates a temporary file with createUniqueFile and schedules it for
+  /// deletion with sys::RemoveFileOnSignal.
+  static Expected<TempFile> create(const Twine &Model,
+                                   unsigned Mode = all_read | all_write);
+  TempFile(TempFile &&Other);
+  TempFile &operator=(TempFile &&Other);
+
+  // Name of the temporary file.
+  std::string TmpName;
+
+  // The open file descriptor.
+  int FD = -1;
+
+  // Keep this with the given name.
+  Error keep(const Twine &Name);
+
+  // Keep this with the temporary name.
+  Error keep();
+
+  // Delete the file.
+  Error discard();
+
+  // This checks that keep or delete was called.
+  ~TempFile();
+};
+
+/// Create a file in the system temporary directory.
+///
+/// The filename is of the form prefix-random_chars.suffix. Since the directory
+/// is not know to the caller, Prefix and Suffix cannot have path separators.
+/// The files are created with mode 0600.
+///
+/// This should be used for things like a temporary .s that is removed after
+/// running the assembler.
+std::error_code createTemporaryFile(const Twine &Prefix, StringRef Suffix,
+                                    int &ResultFD,
+                                    SmallVectorImpl<char> &ResultPath);
+
+/// Simpler version for clients that don't want an open file. An empty
+/// file will still be created.
+std::error_code createTemporaryFile(const Twine &Prefix, StringRef Suffix,
+                                    SmallVectorImpl<char> &ResultPath);
+
+std::error_code createUniqueDirectory(const Twine &Prefix,
+                                      SmallVectorImpl<char> &ResultPath);
+
+/// Get a unique name, not currently exisiting in the filesystem. Subject
+/// to race conditions, prefer to use createUniqueFile instead.
+///
+/// Similar to createUniqueFile, but instead of creating a file only
+/// checks if it exists. This function is subject to race conditions, if you
+/// want to use the returned name to actually create a file, use
+/// createUniqueFile instead.
+std::error_code getPotentiallyUniqueFileName(const Twine &Model,
+                                             SmallVectorImpl<char> &ResultPath);
+
+/// Get a unique temporary file name, not currently exisiting in the
+/// filesystem. Subject to race conditions, prefer to use createTemporaryFile
+/// instead.
+///
+/// Similar to createTemporaryFile, but instead of creating a file only
+/// checks if it exists. This function is subject to race conditions, if you
+/// want to use the returned name to actually create a file, use
+/// createTemporaryFile instead.
+std::error_code
+getPotentiallyUniqueTempFileName(const Twine &Prefix, StringRef Suffix,
+                                 SmallVectorImpl<char> &ResultPath);
+
+inline OpenFlags operator|(OpenFlags A, OpenFlags B) {
+  return OpenFlags(unsigned(A) | unsigned(B));
+}
+
+inline OpenFlags &operator|=(OpenFlags &A, OpenFlags B) {
+  A = A | B;
+  return A;
+}
+
+inline FileAccess operator|(FileAccess A, FileAccess B) {
+  return FileAccess(unsigned(A) | unsigned(B));
+}
+
+inline FileAccess &operator|=(FileAccess &A, FileAccess B) {
+  A = A | B;
+  return A;
+}
+
+/// @brief Opens a file with the specified creation disposition, access mode,
+/// and flags and returns a file descriptor.
+///
+/// The caller is responsible for closing the file descriptor once they are
+/// finished with it.
+///
+/// @param Name The path of the file to open, relative or absolute.
+/// @param ResultFD If the file could be opened successfully, its descriptor
+///                 is stored in this location. Otherwise, this is set to -1.
+/// @param Disp Value specifying the existing-file behavior.
+/// @param Access Value specifying whether to open the file in read, write, or
+///               read-write mode.
+/// @param Flags Additional flags.
+/// @param Mode The access permissions of the file, represented in octal.
+/// @returns errc::success if \a Name has been opened, otherwise a
+///          platform-specific error_code.
+std::error_code openFile(const Twine &Name, int &ResultFD,
+                         CreationDisposition Disp, FileAccess Access,
+                         OpenFlags Flags, unsigned Mode = 0666);
+
+/// @brief Opens a file with the specified creation disposition, access mode,
+/// and flags and returns a platform-specific file object.
+///
+/// The caller is responsible for closing the file object once they are
+/// finished with it.
+///
+/// @param Name The path of the file to open, relative or absolute.
+/// @param Disp Value specifying the existing-file behavior.
+/// @param Access Value specifying whether to open the file in read, write, or
+///               read-write mode.
+/// @param Flags Additional flags.
+/// @param Mode The access permissions of the file, represented in octal.
+/// @returns errc::success if \a Name has been opened, otherwise a
+///          platform-specific error_code.
+Expected<file_t> openNativeFile(const Twine &Name, CreationDisposition Disp,
+                                FileAccess Access, OpenFlags Flags,
+                                unsigned Mode = 0666);
+
+/// Converts from a Posix file descriptor number to a native file handle.
+/// On Windows, this retreives the underlying handle. On non-Windows, this is a
+/// no-op.
+file_t convertFDToNativeFile(int FD);
+
+#ifndef _WIN32
+inline file_t convertFDToNativeFile(int FD) { return FD; }
+#endif
+
+/// Return an open handle to standard in. On Unix, this is typically FD 0.
+/// Returns kInvalidFile when the stream is closed.
+file_t getStdinHandle();
+
+/// Return an open handle to standard out. On Unix, this is typically FD 1.
+/// Returns kInvalidFile when the stream is closed.
+file_t getStdoutHandle();
+
+/// Return an open handle to standard error. On Unix, this is typically FD 2.
+/// Returns kInvalidFile when the stream is closed.
+file_t getStderrHandle();
+
+/// Reads \p Buf.size() bytes from \p FileHandle into \p Buf. Returns the number
+/// of bytes actually read. On Unix, this is equivalent to `return ::read(FD,
+/// Buf.data(), Buf.size())`, with error reporting. Returns 0 when reaching EOF.
+///
+/// @param FileHandle File to read from.
+/// @param Buf Buffer to read into.
+/// @returns The number of bytes read, or error.
+Expected<size_t> readNativeFile(file_t FileHandle, MutableArrayRef<char> Buf);
+
+/// Reads \p Buf.size() bytes from \p FileHandle at offset \p Offset into \p
+/// Buf. If 'pread' is available, this will use that, otherwise it will use
+/// 'lseek'. Returns the number of bytes actually read. Returns 0 when reaching
+/// EOF.
+///
+/// @param FileHandle File to read from.
+/// @param Buf Buffer to read into.
+/// @param Offset Offset into the file at which the read should occur.
+/// @returns The number of bytes read, or error.
+Expected<size_t> readNativeFileSlice(file_t FileHandle,
+                                     MutableArrayRef<char> Buf,
+                                     uint64_t Offset);
+
+/// @brief Opens the file with the given name in a write-only or read-write
+/// mode, returning its open file descriptor. If the file does not exist, it
+/// is created.
+///
+/// The caller is responsible for closing the file descriptor once they are
+/// finished with it.
+///
+/// @param Name The path of the file to open, relative or absolute.
+/// @param ResultFD If the file could be opened successfully, its descriptor
+///                 is stored in this location. Otherwise, this is set to -1.
+/// @param Flags Additional flags used to determine whether the file should be
+///              opened in, for example, read-write or in write-only mode.
+/// @param Mode The access permissions of the file, represented in octal.
+/// @returns errc::success if \a Name has been opened, otherwise a
+///          platform-specific error_code.
+inline std::error_code
+openFileForWrite(const Twine &Name, int &ResultFD,
+                 CreationDisposition Disp = CD_CreateAlways,
+                 OpenFlags Flags = OF_None, unsigned Mode = 0666) {
+  return openFile(Name, ResultFD, Disp, FA_Write, Flags, Mode);
+}
+
+/// @brief Opens the file with the given name in a write-only or read-write
+/// mode, returning its open file descriptor. If the file does not exist, it
+/// is created.
+///
+/// The caller is responsible for closing the freeing the file once they are
+/// finished with it.
+///
+/// @param Name The path of the file to open, relative or absolute.
+/// @param Flags Additional flags used to determine whether the file should be
+///              opened in, for example, read-write or in write-only mode.
+/// @param Mode The access permissions of the file, represented in octal.
+/// @returns a platform-specific file descriptor if \a Name has been opened,
+///          otherwise an error object.
+inline Expected<file_t> openNativeFileForWrite(const Twine &Name,
+                                               CreationDisposition Disp,
+                                               OpenFlags Flags,
+                                               unsigned Mode = 0666) {
+  return openNativeFile(Name, Disp, FA_Write, Flags, Mode);
+}
+
+/// @brief Opens the file with the given name in a write-only or read-write
+/// mode, returning its open file descriptor. If the file does not exist, it
+/// is created.
+///
+/// The caller is responsible for closing the file descriptor once they are
+/// finished with it.
+///
+/// @param Name The path of the file to open, relative or absolute.
+/// @param ResultFD If the file could be opened successfully, its descriptor
+///                 is stored in this location. Otherwise, this is set to -1.
+/// @param Flags Additional flags used to determine whether the file should be
+///              opened in, for example, read-write or in write-only mode.
+/// @param Mode The access permissions of the file, represented in octal.
+/// @returns errc::success if \a Name has been opened, otherwise a
+///          platform-specific error_code.
+inline std::error_code openFileForReadWrite(const Twine &Name, int &ResultFD,
+                                            CreationDisposition Disp,
+                                            OpenFlags Flags,
+                                            unsigned Mode = 0666) {
+  return openFile(Name, ResultFD, Disp, FA_Write | FA_Read, Flags, Mode);
+}
+
+/// @brief Opens the file with the given name in a write-only or read-write
+/// mode, returning its open file descriptor. If the file does not exist, it
+/// is created.
+///
+/// The caller is responsible for closing the freeing the file once they are
+/// finished with it.
+///
+/// @param Name The path of the file to open, relative or absolute.
+/// @param Flags Additional flags used to determine whether the file should be
+///              opened in, for example, read-write or in write-only mode.
+/// @param Mode The access permissions of the file, represented in octal.
+/// @returns a platform-specific file descriptor if \a Name has been opened,
+///          otherwise an error object.
+inline Expected<file_t> openNativeFileForReadWrite(const Twine &Name,
+                                                   CreationDisposition Disp,
+                                                   OpenFlags Flags,
+                                                   unsigned Mode = 0666) {
+  return openNativeFile(Name, Disp, FA_Write | FA_Read, Flags, Mode);
+}
+
+/// @brief Opens the file with the given name in a read-only mode, returning
+/// its open file descriptor.
+///
+/// The caller is responsible for closing the file descriptor once they are
+/// finished with it.
+///
+/// @param Name The path of the file to open, relative or absolute.
+/// @param ResultFD If the file could be opened successfully, its descriptor
+///                 is stored in this location. Otherwise, this is set to -1.
+/// @param RealPath If nonnull, extra work is done to determine the real path
+///                 of the opened file, and that path is stored in this
+///                 location.
+/// @returns errc::success if \a Name has been opened, otherwise a
+///          platform-specific error_code.
+std::error_code openFileForRead(const Twine &Name, int &ResultFD,
+                                OpenFlags Flags = OF_None,
+                                SmallVectorImpl<char> *RealPath = nullptr);
+
+/// @brief Opens the file with the given name in a read-only mode, returning
+/// its open file descriptor.
+///
+/// The caller is responsible for closing the freeing the file once they are
+/// finished with it.
+///
+/// @param Name The path of the file to open, relative or absolute.
+/// @param RealPath If nonnull, extra work is done to determine the real path
+///                 of the opened file, and that path is stored in this
+///                 location.
+/// @returns a platform-specific file descriptor if \a Name has been opened,
+///          otherwise an error object.
+Expected<file_t>
+openNativeFileForRead(const Twine &Name, OpenFlags Flags = OF_None,
+                      SmallVectorImpl<char> *RealPath = nullptr);
+
+/// @brief Close the file object.  This should be used instead of ::close for
+/// portability. On error, the caller should assume the file is closed, as is
+/// the case for Process::SafelyCloseFileDescriptor
+///
+/// @param F On input, this is the file to close.  On output, the file is
+/// set to kInvalidFile.
+///
+/// @returns An error code if closing the file failed. Typically, an error here
+/// means that the filesystem may have failed to perform some buffered writes.
+std::error_code closeFile(file_t &F);
+
+std::error_code getUniqueID(const Twine Path, UniqueID &Result);
+
+/// Get disk space usage information.
+///
+/// Note: Users must be careful about "Time Of Check, Time Of Use" kind of bug.
+/// Note: Windows reports results according to the quota allocated to the user.
+///
+/// @param Path Input path.
+/// @returns a space_info structure filled with the capacity, free, and
+/// available space on the device \a Path is on. A platform specific error_code
+/// is returned on error.
+ErrorOr<space_info> disk_space(const Twine &Path);
+
+/// This class represents a memory mapped file. It is based on
+/// boost::iostreams::mapped_file.
+class mapped_file_region {
+public:
+  enum mapmode {
+    readonly, ///< May only access map via const_data as read only.
+    readwrite, ///< May access map via data and modify it. Written to path.
+    priv ///< May modify via data, but changes are lost on destruction.
+  };
+
+private:
+  /// Platform-specific mapping state.
+  size_t Size;
+  void *Mapping;
+#ifdef _WIN32
+  sys::fs::file_t FileHandle;
+#endif
+  mapmode Mode;
+
+  std::error_code init(sys::fs::file_t FD, uint64_t Offset, mapmode Mode);
+
+public:
+  mapped_file_region() = delete;
+  mapped_file_region(mapped_file_region&) = delete;
+  mapped_file_region &operator =(mapped_file_region&) = delete;
+
+  /// \param fd An open file descriptor to map. Does not take ownership of fd.
+  mapped_file_region(sys::fs::file_t fd, mapmode mode, size_t length, uint64_t offset,
+                     std::error_code &ec);
+
+  ~mapped_file_region();
+
+  size_t size() const;
+  char *data() const;
+
+  /// Get a const view of the data. Modifying this memory has undefined
+  /// behavior.
+  const char *const_data() const;
+
+  /// \returns The minimum alignment offset must be.
+  static int alignment();
+};
+
+/// Return the path to the main executable, given the value of argv[0] from
+/// program startup and the address of main itself. In extremis, this function
+/// may fail and return an empty path.
+std::string getMainExecutable(const char *argv0, void *MainExecAddr);
+
+/// @}
+/// @name Iterators
+/// @{
+
+/// directory_entry - A single entry in a directory.
+class directory_entry {
+  // FIXME: different platforms make different information available "for free"
+  // when traversing a directory. The design of this class wraps most of the
+  // information in basic_file_status, so on platforms where we can't populate
+  // that whole structure, callers end up paying for a stat().
+  // std::filesystem::directory_entry may be a better model.
+  std::string Path;
+  file_type Type = file_type::type_unknown; // Most platforms can provide this.
+  bool FollowSymlinks = true;               // Affects the behavior of status().
+  basic_file_status Status;                 // If available.
+
+public:
+  explicit directory_entry(const Twine &Path, bool FollowSymlinks = true,
+                           file_type Type = file_type::type_unknown,
+                           basic_file_status Status = basic_file_status())
+      : Path(Path.str()), Type(Type), FollowSymlinks(FollowSymlinks),
+        Status(Status) {}
+
+  directory_entry() = default;
+
+  void replace_filename(const Twine &Filename, file_type Type,
+                        basic_file_status Status = basic_file_status());
+
+  const std::string &path() const { return Path; }
+  // Get basic information about entry file (a subset of fs::status()).
+  // On most platforms this is a stat() call.
+  // On windows the information was already retrieved from the directory.
+  ErrorOr<basic_file_status> status() const;
+  // Get the type of this file.
+  // On most platforms (Linux/Mac/Windows/BSD), this was already retrieved.
+  // On some platforms (e.g. Solaris) this is a stat() call.
+  file_type type() const {
+    if (Type != file_type::type_unknown)
+      return Type;
+    auto S = status();
+    return S ? S->type() : file_type::type_unknown;
+  }
+
+  bool operator==(const directory_entry& RHS) const { return Path == RHS.Path; }
+  bool operator!=(const directory_entry& RHS) const { return !(*this == RHS); }
+  bool operator< (const directory_entry& RHS) const;
+  bool operator<=(const directory_entry& RHS) const;
+  bool operator> (const directory_entry& RHS) const;
+  bool operator>=(const directory_entry& RHS) const;
+};
+
+namespace detail {
+
+  struct DirIterState;
+
+  std::error_code directory_iterator_construct(DirIterState &, StringRef, bool);
+  std::error_code directory_iterator_increment(DirIterState &);
+  std::error_code directory_iterator_destruct(DirIterState &);
+
+  /// Keeps state for the directory_iterator.
+  struct DirIterState {
+    ~DirIterState() {
+      directory_iterator_destruct(*this);
+    }
+
+    intptr_t IterationHandle = 0;
+    directory_entry CurrentEntry;
+  };
+
+} // end namespace detail
+
+/// directory_iterator - Iterates through the entries in path. There is no
+/// operator++ because we need an error_code. If it's really needed we can make
+/// it call report_fatal_error on error.
+class directory_iterator {
+  std::shared_ptr<detail::DirIterState> State;
+  bool FollowSymlinks = true;
+
+public:
+  explicit directory_iterator(const Twine &path, std::error_code &ec,
+                              bool follow_symlinks = true)
+      : FollowSymlinks(follow_symlinks) {
+    State = std::make_shared<detail::DirIterState>();
+    SmallString<128> path_storage;
+    ec = detail::directory_iterator_construct(
+        *State, path.toStringRef(path_storage), FollowSymlinks);
+  }
+
+  explicit directory_iterator(const directory_entry &de, std::error_code &ec,
+                              bool follow_symlinks = true)
+      : FollowSymlinks(follow_symlinks) {
+    State = std::make_shared<detail::DirIterState>();
+    ec = detail::directory_iterator_construct(
+        *State, de.path(), FollowSymlinks);
+  }
+
+  /// Construct end iterator.
+  directory_iterator() = default;
+
+  // No operator++ because we need error_code.
+  directory_iterator &increment(std::error_code &ec) {
+    ec = directory_iterator_increment(*State);
+    return *this;
+  }
+
+  const directory_entry &operator*() const { return State->CurrentEntry; }
+  const directory_entry *operator->() const { return &State->CurrentEntry; }
+
+  bool operator==(const directory_iterator &RHS) const {
+    if (State == RHS.State)
+      return true;
+    if (!RHS.State)
+      return State->CurrentEntry == directory_entry();
+    if (!State)
+      return RHS.State->CurrentEntry == directory_entry();
+    return State->CurrentEntry == RHS.State->CurrentEntry;
+  }
+
+  bool operator!=(const directory_iterator &RHS) const {
+    return !(*this == RHS);
+  }
+};
+
+namespace detail {
+
+  /// Keeps state for the recursive_directory_iterator.
+  struct RecDirIterState {
+    std::stack<directory_iterator, std::vector<directory_iterator>> Stack;
+    uint16_t Level = 0;
+    bool HasNoPushRequest = false;
+  };
+
+} // end namespace detail
+
+/// recursive_directory_iterator - Same as directory_iterator except for it
+/// recurses down into child directories.
+class recursive_directory_iterator {
+  std::shared_ptr<detail::RecDirIterState> State;
+  bool Follow;
+
+public:
+  recursive_directory_iterator() = default;
+  explicit recursive_directory_iterator(const Twine &path, std::error_code &ec,
+                                        bool follow_symlinks = true)
+      : State(std::make_shared<detail::RecDirIterState>()),
+        Follow(follow_symlinks) {
+    State->Stack.push(directory_iterator(path, ec, Follow));
+    if (State->Stack.top() == directory_iterator())
+      State.reset();
+  }
+
+  // No operator++ because we need error_code.
+  recursive_directory_iterator &increment(std::error_code &ec) {
+    const directory_iterator end_itr = {};
+
+    if (State->HasNoPushRequest)
+      State->HasNoPushRequest = false;
+    else {
+      file_type type = State->Stack.top()->type();
+      if (type == file_type::symlink_file && Follow) {
+        // Resolve the symlink: is it a directory to recurse into?
+        ErrorOr<basic_file_status> status = State->Stack.top()->status();
+        if (status)
+          type = status->type();
+        // Otherwise broken symlink, and we'll continue.
+      }
+      if (type == file_type::directory_file) {
+        State->Stack.push(directory_iterator(*State->Stack.top(), ec, Follow));
+        if (State->Stack.top() != end_itr) {
+          ++State->Level;
+          return *this;
+        }
+        State->Stack.pop();
+      }
+    }
+
+    while (!State->Stack.empty()
+           && State->Stack.top().increment(ec) == end_itr) {
+      State->Stack.pop();
+      --State->Level;
+    }
+
+    // Check if we are done. If so, create an end iterator.
+    if (State->Stack.empty())
+      State.reset();
+
+    return *this;
+  }
+
+  const directory_entry &operator*() const { return *State->Stack.top(); }
+  const directory_entry *operator->() const { return &*State->Stack.top(); }
+
+  // observers
+  /// Gets the current level. Starting path is at level 0.
+  int level() const { return State->Level; }
+
+  /// Returns true if no_push has been called for this directory_entry.
+  bool no_push_request() const { return State->HasNoPushRequest; }
+
+  // modifiers
+  /// Goes up one level if Level > 0.
+  void pop() {
+    assert(State && "Cannot pop an end iterator!");
+    assert(State->Level > 0 && "Cannot pop an iterator with level < 1");
+
+    const directory_iterator end_itr = {};
+    std::error_code ec;
+    do {
+      if (ec)
+        report_fatal_error("Error incrementing directory iterator.");
+      State->Stack.pop();
+      --State->Level;
+    } while (!State->Stack.empty()
+             && State->Stack.top().increment(ec) == end_itr);
+
+    // Check if we are done. If so, create an end iterator.
+    if (State->Stack.empty())
+      State.reset();
+  }
+
+  /// Does not go down into the current directory_entry.
+  void no_push() { State->HasNoPushRequest = true; }
+
+  bool operator==(const recursive_directory_iterator &RHS) const {
+    return State == RHS.State;
+  }
+
+  bool operator!=(const recursive_directory_iterator &RHS) const {
+    return !(*this == RHS);
+  }
+};
+
+/// @}
+
+} // end namespace fs
+} // end namespace sys
+} // end namespace llvm
+
+#endif // LLVM_SUPPORT_FILESYSTEM_H