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chrono: address post commit comments from Howard

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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.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@290806 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Saleem Abdulrasool
2017-01-01 22:04:38 +00:00
parent 4ed4bca5f3
commit 864ad6d129
1 changed files with 17 additions and 13 deletions
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30
src/chrono.cpp
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@@ -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;