[libcxx] r290806 - chrono: address post commit comments from Howard

[Saleem Abdulrasool via cfe-commits]

Author: compnerd
Date: Sun Jan  1 16:04:38 2017
New Revision: 290806

URL: http://llvm.org/viewvc/llvm-project?rev=290806&view=rev
Log:
chrono: address post commit comments from Howard

Drawing some inspiration from code from Bill O'Neal as pointed out by
Howard, rework the code to avoid an overflow in the duration.  Adjust
the style to match libc++ style as well.

Create a local typedef for the FILETIME duration (100-ns units).  Use
this to define the difference between the NT and the UNIX epochs (which
previously overflowed due to the representation limits due to the
bouncing to ns).  Return the FILETIME duration biased by the NT-to-UNIX
epoch conversion.

Use of the custom duration makes it easier to read and reason about the
code.

Modified:
    libcxx/trunk/src/chrono.cpp

Modified: libcxx/trunk/src/chrono.cpp
URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/src/chrono.cpp?rev=290806&r1=290805&r2=290806&view=diff
==============================================================================
--- libcxx/trunk/src/chrono.cpp (original)
+++ libcxx/trunk/src/chrono.cpp Sun Jan  1 16:04:38 2017
@@ -46,26 +46,30 @@ system_clock::time_point
 system_clock::now() _NOEXCEPT
 {
 #if defined(_WIN32)
-  // The Windows epoch is Jan 1 1601, the Unix epoch Jan 1 1970.  The difference
-  // in nanoseconds is the windows epoch offset.
-  static const constexpr __int64 kWindowsEpochOffset = 0x19db1ded53e8000;
+  // FILETIME is in 100ns units
+  using filetime_duration =
+      _VSTD::chrono::duration<__int64,
+                              _VSTD::ratio_multiply<_VSTD::ratio<100, 1>,
+                                                    nanoseconds::period>>;
 
-  FILETIME ftSystemTime;
+  // The Windows epoch is Jan 1 1601, the Unix epoch Jan 1 1970.
+  static _LIBCPP_CONSTEXPR const filetime_duration
+      nt_to_unix_epoch{11644473600};
+
+  FILETIME ft;
 #if _WIN32_WINNT >= _WIN32_WINNT_WIN8
 #if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP)
-  GetSystemTimePreciseAsFileTime(&ftSystemTime);
+  GetSystemTimePreciseAsFileTime(&ft);
 #else
-  GetSystemTimeAsFileTime(&ftSystemTime);
+  GetSystemTimeAsFileTime(&ft);
 #endif
 #else
-  GetSystemTimeAsFileTime(&ftSystemTime);
+  GetSystemTimeAsFileTime(&ft);
 #endif
-  __int64 llWinTimeNS =
-      ((static_cast<__int64>(ftSystemTime.dwHighDateTime) << 32) |
-       static_cast<__int64>(ftSystemTime.dwLowDateTime)) *
-      100;
-  return time_point(duration_cast<duration>(
-      (nanoseconds(llWinTimeNS - kWindowsEpochOffset))));
+
+  filetime_duration d{(static_cast<__int64>(ft.dwHighDateTime) << 32) |
+                       static_cast<__int64>(ft.dwLowDateTime)};
+  return time_point(duration_cast<duration>(d - nt_to_unix_epoch));
 #else
 #ifdef CLOCK_REALTIME
     struct timespec tp;