[libc++] Bring back mach_absolute_time implementation of steady_clock

[ldionne]

Summary

This is meant to unblock Chrome, as discussed in https://llvm.org/D74489.

[ldionne]

Please try this out and let me know if that solves your problems.

[rnk]

I patched this in and confirmed that it fixes our build issue, thanks!

I see that you didn't change the documentation back to claim support for Mac 10.9. That's fine with me, I consider this to be a temporary state of affairs.

[ldionne]

In D95177#2516251, @rnk wrote:

I patched this in and confirmed that it fixes our build issue, thanks!

I see that you didn't change the documentation back to claim support for Mac 10.9. That's fine with me, I consider this to be a temporary state of affairs.

We're on the same page. :-)

libcxx/src/chrono.cpp

 #  define WIN32_LEAN_AND_MEAN
 #  define VC_EXTRA_LEAN
 #  include <windows.h>
 #  if _WIN32_WINNT >= _WIN32_WINNT_WIN8
 #    include <winapifamily.h>
 #  endif
 #endif // defined(_LIBCPP_WIN32API)
 
+#if __has_include(<mach/mach_time.h>)
+# include <mach/mach_time.h>
+#endif
+
 #if defined(__ELF__) && defined(_LIBCPP_LINK_RT_LIB)
 #  pragma comment(lib, "rt")
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 namespace chrono
 {

 //  better for it to not exist and have the rest of libc++ use system_clock
 //  instead.
 //
 
 #ifndef _LIBCPP_HAS_NO_MONOTONIC_CLOCK
 
 #if defined(__APPLE__)
 
+// TODO(ldionne):
+// This old implementation of steady_clock is retained until Chrome drops supports
+// for macOS < 10.12. The issue is that they link libc++ statically into their
+// application, which means that libc++ must support being built for such deployment
+// targets. See https://llvm.org/D74489 for details.
+#if (defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ < 101200) || \
+    (defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ < 100000) || \
+    (defined(__ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__ < 100000) || \
+    (defined(__ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__ < 30000)
+# define _LIBCPP_USE_OLD_MACH_ABSOLUTE_TIME
+#endif
+
+#if defined(_LIBCPP_USE_OLD_MACH_ABSOLUTE_TIME)
+
+//   mach_absolute_time() * MachInfo.numer / MachInfo.denom is the number of
+//   nanoseconds since the computer booted up.  MachInfo.numer and MachInfo.denom
+//   are run time constants supplied by the OS.  This clock has no relationship
+//   to the Gregorian calendar.  It's main use is as a high resolution timer.
+
+// MachInfo.numer / MachInfo.denom is often 1 on the latest equipment.  Specialize
+//   for that case as an optimization.
+
+static steady_clock::rep steady_simplified() {
+    return static_cast<steady_clock::rep>(mach_absolute_time());
+}
+static double compute_steady_factor() {
+    mach_timebase_info_data_t MachInfo;
+    mach_timebase_info(&MachInfo);
+    return static_cast<double>(MachInfo.numer) / MachInfo.denom;
+}
+
+static steady_clock::rep steady_full() {
+    static const double factor = compute_steady_factor();
+    return static_cast<steady_clock::rep>(mach_absolute_time() * factor);
+}
+
+typedef steady_clock::rep (*FP)();
+
+static FP init_steady_clock() {
+    mach_timebase_info_data_t MachInfo;
+    mach_timebase_info(&MachInfo);
+    if (MachInfo.numer == MachInfo.denom)
+        return &steady_simplified;
+    return &steady_full;
+}
+
+static steady_clock::time_point __libcpp_steady_clock_now() {
+    static FP fp = init_steady_clock();
+    return steady_clock::time_point(steady_clock::duration(fp()));
+}
+
+#else // vvvvv default behavior for Apple platforms  vvvvv
+
 // On Apple platforms, only CLOCK_UPTIME_RAW, CLOCK_MONOTONIC_RAW or
 // mach_absolute_time are able to time functions in the nanosecond range.
 // Furthermore, only CLOCK_MONOTONIC_RAW is truly monotonic, because it
 // also counts cycles when the system is asleep. Thus, it is the only
 // acceptable implementation of steady_clock.
 static steady_clock::time_point __libcpp_steady_clock_now() {
     struct timespec tp;
     if (0 != clock_gettime(CLOCK_MONOTONIC_RAW, &tp))
         __throw_system_error(errno, "clock_gettime(CLOCK_MONOTONIC_RAW) failed");
     return steady_clock::time_point(seconds(tp.tv_sec) + nanoseconds(tp.tv_nsec));
 }
 
+#endif
+
 #elif defined(_LIBCPP_WIN32API)
 
 // https://msdn.microsoft.com/en-us/library/windows/desktop/ms644905(v=vs.85).aspx says:
 //    If the function fails, the return value is zero. <snip>
 //    On systems that run Windows XP or later, the function will always succeed
 //      and will thus never return zero.
 
 static LARGE_INTEGER