[libc++] Remove workarounds for the lack of clock_gettime on older macOS platforms

This increases the Mac OS requirement for building libc++ to 10.12.
Note that it doesn't change whether the *headers* still support older
platforms -- it's only that macOS >= 10.12 is required to build the
dylib from sources.

Differential Revision: https://reviews.llvm.org/D74489

Author: ldionne

libcxx/docs/index.rst

@@ -85,25 +85,46 @@ reasons, but some of the major ones are:
 Platform and Compiler Support
 -----------------------------
 
-libc++ is known to work on the following platforms, using gcc and
-clang.
-Note that functionality provided by ``<atomic>`` is only functional with clang
-and GCC.
+For using the libc++ headers
+############################
+The libc++ headers are known to work on the following platforms, using GCC and
+Clang. Note that functionality provided by ``<atomic>`` is only functional with
+Clang and GCC.
+
+============ ==================== ============
+OS           Arch                 Compilers
+============ ==================== ============
+macOS 10.9+  i386, x86_64         Clang, GCC
+FreeBSD 10+  i386, x86_64, ARM    Clang, GCC
+Linux        i386, x86_64         Clang, GCC
+============ ==================== ============
+
+The following minimum compiler versions are required:
+
+* Clang 4.0 and above
+* GCC 5.0 and above.
+
+The C++03 dialect is only supported with Clang.
+
+For building the libc++ library
+###############################
+Building the libc++ library (static or shared) requires some features from
+the operating system. As such, it has its own set of (slightly different)
+system requirements.
 
 ============ ==================== ============ ========================
 OS           Arch                 Compilers    ABI Library
 ============ ==================== ============ ========================
-macOS        i386, x86_64         Clang, GCC   libc++abi
+macOS 10.12+ i386, x86_64         Clang, GCC   libc++abi
 FreeBSD 10+  i386, x86_64, ARM    Clang, GCC   libcxxrt, libc++abi
 Linux        i386, x86_64         Clang, GCC   libc++abi
 ============ ==================== ============ ========================
 
-The following minimum compiler versions are strongly recommended.
+The following minimum compiler versions are required:
 
 * Clang 4.0 and above
 * GCC 5.0 and above.
 
-The C++03 dialect is only supported for Clang compilers.
 
 C++ Dialect Support
 ---------------------

libcxx/src/chrono.cpp

@@ -9,7 +9,7 @@
 #include "chrono"
 #include "cerrno"        // errno
 #include "system_error"  // __throw_system_error
-#include <time.h>        // clock_gettime, CLOCK_MONOTONIC and CLOCK_REALTIME
+#include <time.h>        // clock_gettime and CLOCK_{MONOTONIC,REALTIME,MONOTONIC_RAW}
 #include "include/apple_availability.h"
 
 #if __has_include(<unistd.h>)
@@ -21,28 +21,20 @@
 #endif
 
 #if defined(_LIBCPP_WIN32API)
-#define WIN32_LEAN_AND_MEAN
-#define VC_EXTRA_LEAN
-#include <windows.h>
-#if _WIN32_WINNT >= _WIN32_WINNT_WIN8
-#include <winapifamily.h>
-#endif
+#  define WIN32_LEAN_AND_MEAN
+#  define VC_EXTRA_LEAN
+#  include <windows.h>
+#  if _WIN32_WINNT >= _WIN32_WINNT_WIN8
+#    include <winapifamily.h>
+#  endif
 #else
-#if !defined(CLOCK_REALTIME) || !defined(_LIBCPP_USE_CLOCK_GETTIME)
-#include <sys/time.h> // for gettimeofday and timeval
-#endif
+#  if !defined(CLOCK_REALTIME)
+#    include <sys/time.h>        // for gettimeofday and timeval
+#  endif // !defined(CLOCK_REALTIME)
 #endif // defined(_LIBCPP_WIN32API)
 
-#if !defined(_LIBCPP_HAS_NO_MONOTONIC_CLOCK)
-#if __APPLE__
-#include <mach/mach_time.h>  // mach_absolute_time, mach_timebase_info_data_t
-#elif !defined(_LIBCPP_WIN32API) && !defined(CLOCK_MONOTONIC)
-#error "Monotonic clock not implemented"
-#endif
-#endif
-
 #if defined(__ELF__) && defined(_LIBCPP_LINK_RT_LIB)
-#pragma comment(lib, "rt")
+#  pragma comment(lib, "rt")
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
@@ -82,7 +74,7 @@ system_clock::now() _NOEXCEPT
                        static_cast<__int64>(ft.dwLowDateTime)};
   return time_point(duration_cast<duration>(d - nt_to_unix_epoch));
 #else
-#if defined(_LIBCPP_USE_CLOCK_GETTIME) && defined(CLOCK_REALTIME)
+#if defined(CLOCK_REALTIME)
   struct timespec tp;
   if (0 != clock_gettime(CLOCK_REALTIME, &tp))
     __throw_system_error(errno, "clock_gettime(CLOCK_REALTIME) failed");
@@ -91,7 +83,7 @@ system_clock::now() _NOEXCEPT
     timeval tv;
     gettimeofday(&tv, 0);
     return time_point(seconds(tv.tv_sec) + microseconds(tv.tv_usec));
-#endif // _LIBCPP_USE_CLOCK_GETTIME && CLOCK_REALTIME
+#endif // CLOCK_REALTIME
 #endif
 }
 
@@ -118,8 +110,15 @@ const bool steady_clock::is_steady;
 
 #if defined(__APPLE__)
 
-// Darwin libc versions >= 1133 provide ns precision via CLOCK_MONOTONIC_RAW
-#if defined(_LIBCPP_USE_CLOCK_GETTIME) && defined(CLOCK_MONOTONIC_RAW)
+#if !defined(CLOCK_MONOTONIC_RAW)
+#  error "Building libc++ on Apple platforms requires CLOCK_MONOTONIC_RAW"
+#endif
+
+// 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.
 steady_clock::time_point
 steady_clock::now() _NOEXCEPT
 {
@@ -129,60 +128,6 @@ steady_clock::now() _NOEXCEPT
     return time_point(seconds(tp.tv_sec) + nanoseconds(tp.tv_nsec));
 }
 
-#else
-//   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;
-}
-
-steady_clock::time_point
-steady_clock::now() _NOEXCEPT
-{
-    static FP fp = init_steady_clock();
-    return time_point(duration(fp()));
-}
-#endif // defined(_LIBCPP_USE_CLOCK_GETTIME) && defined(CLOCK_MONOTONIC_RAW)
-
 #elif defined(_LIBCPP_WIN32API)
 
 // https://msdn.microsoft.com/en-us/library/windows/desktop/ms644905(v=vs.85).aspx says:
@@ -210,13 +155,6 @@ steady_clock::now() _NOEXCEPT
 
 #elif defined(CLOCK_MONOTONIC)
 
-// On Apple platforms only CLOCK_UPTIME_RAW, CLOCK_MONOTONIC_RAW or
-// mach_absolute_time are able to time functions in the nanosecond range.
-// Thus, they are the only acceptable implementations of steady_clock.
-#ifdef __APPLE__
-#error "Never use CLOCK_MONOTONIC for steady_clock::now on Apple platforms"
-#endif
-
 steady_clock::time_point
 steady_clock::now() _NOEXCEPT
 {
@@ -227,7 +165,7 @@ steady_clock::now() _NOEXCEPT
 }
 
 #else
-#error "Monotonic clock not implemented"
+#  error "Monotonic clock not implemented"
 #endif
 
 #endif // !_LIBCPP_HAS_NO_MONOTONIC_CLOCK

libcxx/src/filesystem/operations.cpp

@@ -36,13 +36,9 @@
 #define _LIBCPP_USE_COPYFILE
 #endif
 
-#if !defined(__APPLE__) && _POSIX_TIMERS > 0
-#define _LIBCPP_USE_CLOCK_GETTIME
-#endif
-
-#if !defined(CLOCK_REALTIME) || !defined(_LIBCPP_USE_CLOCK_GETTIME)
+#if !defined(CLOCK_REALTIME)
 #include <sys/time.h> // for gettimeofday and timeval
-#endif                // !defined(CLOCK_REALTIME)
+#endif // !defined(CLOCK_REALTIME)
 
 #if defined(__ELF__) && defined(_LIBCPP_LINK_RT_LIB)
 #pragma comment(lib, "rt")
@@ -490,7 +486,7 @@ const bool _FilesystemClock::is_steady;
 
 _FilesystemClock::time_point _FilesystemClock::now() noexcept {
   typedef chrono::duration<rep> __secs;
-#if defined(_LIBCPP_USE_CLOCK_GETTIME) && defined(CLOCK_REALTIME)
+#if defined(CLOCK_REALTIME)
   typedef chrono::duration<rep, nano> __nsecs;
   struct timespec tp;
   if (0 != clock_gettime(CLOCK_REALTIME, &tp))
@@ -502,7 +498,7 @@ _FilesystemClock::time_point _FilesystemClock::now() noexcept {
   timeval tv;
   gettimeofday(&tv, 0);
   return time_point(__secs(tv.tv_sec) + __microsecs(tv.tv_usec));
-#endif // _LIBCPP_USE_CLOCK_GETTIME && CLOCK_REALTIME
+#endif // CLOCK_REALTIME
 }
 
 filesystem_error::~filesystem_error() {}

libcxx/src/include/apple_availability.h

@@ -28,24 +28,6 @@
 #endif
 #endif // __ENVIRONMENT_.*_VERSION_MIN_REQUIRED__
 
-#if defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__)
-#if __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ >= 101200
-#define _LIBCPP_USE_CLOCK_GETTIME
-#endif
-#elif defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__)
-#if __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ >= 100000
-#define _LIBCPP_USE_CLOCK_GETTIME
-#endif
-#elif defined(__ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__)
-#if __ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__ >= 100000
-#define _LIBCPP_USE_CLOCK_GETTIME
-#endif
-#elif defined(__ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__)
-#if __ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__ >= 30000
-#define _LIBCPP_USE_CLOCK_GETTIME
-#endif
-#endif // __ENVIRONMENT_.*_VERSION_MIN_REQUIRED__
-
 #if defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__)
 #if __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ >= 101500
 #define _LIBCPP_USE_ULOCK