[libc++] Use __int128_t to represent file_time_type.

Summary:
The ``file_time_type`` time point is used to represent the write times for files.
Its job is to act as part of a C++ wrapper for less ideal system interfaces. The
underlying filesystem uses the ``timespec`` struct for the same purpose.

However, the initial implementation of ``file_time_type`` could not represent
either the range or resolution of ``timespec``, making it unsuitable. Fixing
this requires an implementation which uses more than 64 bits to store the
time point.

I primarily considered two solutions: Using ``__int128_t`` and using a
arithmetic emulation of ``timespec``. Each has its pros and cons, and both
come with more than one complication.

However, after a lot of consideration, I decided on using `__int128_t`. This patch implements that change.

Please see the [FileTimeType Design Document](http://libcxx.llvm.org/docs/DesignDocs/FileTimeType.html) for more information.

Reviewers: mclow.lists, ldionne, joerg, arthur.j.odwyer, EricWF

Reviewed By: EricWF

Subscribers: christof, K-ballo, cfe-commits, BillyONeal

Differential Revision: https://reviews.llvm.org/D49774

llvm-svn: 337960

1 files changed, 31 insertions, 1 deletions

diff --git a/libcxx/include/experimental/filesystem b/libcxx/include/experimental/filesystem
index 060c7e0335b..dbc03595d42 100644
--- a/libcxx/include/experimental/filesystem
+++ b/libcxx/include/experimental/filesystem
@@ -260,7 +260,37 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL_FILESYSTEM
 
-typedef chrono::time_point<std::chrono::system_clock>  file_time_type;
+struct _FilesystemClock {
+#if !defined(_LIBCPP_HAS_NO_INT128)
+  typedef __int128_t rep;
+  typedef nano period;
+#else
+  typedef long long rep;
+  typedef nano period;
+#endif
+
+  typedef chrono::duration<rep, period> duration;
+  typedef chrono::time_point<_FilesystemClock> time_point;
+
+  static _LIBCPP_CONSTEXPR_AFTER_CXX11 const bool is_steady = false;
+
+  _LIBCPP_FUNC_VIS static time_point now() noexcept;
+
+  _LIBCPP_INLINE_VISIBILITY
+  static time_t to_time_t(const time_point& __t) noexcept {
+    typedef chrono::duration<rep> __secs;
+    return time_t(
+        chrono::duration_cast<__secs>(__t.time_since_epoch()).count());
+  }
+
+  _LIBCPP_INLINE_VISIBILITY
+  static time_point from_time_t(time_t __t) noexcept {
+    typedef chrono::duration<rep> __secs;
+    return time_point(__secs(__t));
+  }
+};
+
+typedef chrono::time_point<_FilesystemClock> file_time_type;
 
 struct _LIBCPP_TYPE_VIS space_info
 {