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[libc++] Bring back mach_absolute_time implementation of steady_clock 

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

Differential Revision: https://reviews.llvm.org/D95177
This commit is contained in:
Louis Dionne 2021-01-21 17:53:29 -05:00
parent 73de3df1d2
commit faa440786c
1 changed files with 59 additions and 0 deletions
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59
libcxx/src/chrono.cpp
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@ -33,6 +33,10 @@
# 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
@ -121,6 +125,59 @@ system_clock::from_time_t(time_t t) _NOEXCEPT
#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
@ -133,6 +190,8 @@ static steady_clock::time_point __libcpp_steady_clock_now() {
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: