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

[ldionne]

Summary

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.

[ldionne]

Note: this was suggested in https://reviews.llvm.org/D74341

[ldionne]

Feedback on this change was requested in http://lists.llvm.org/pipermail/libcxx-dev/2020-February/000688.html and nobody objected, so I think we're good to move forward.

@EricWF @mclow.lists Good?

[ldionne]

Rebase onto master and update documentation

[thakis]

We're looking into bumping libc++ in chromium, and this is a problem for us. We statically link libc++, and we still support 10.11. (We _just_ dropped support for 10.10 in our last release iirc.) What do you recommend as path forward?

[ldionne]

In D74489#2497044, @thakis wrote:

We're looking into bumping libc++ in chromium, and this is a problem for us. We statically link libc++, and we still support 10.11. (We _just_ dropped support for 10.10 in our last release iirc.) What do you recommend as path forward?

Ugh, that's unfortunate. I see two options:

1. We revert the patch here, or
2. You carry a local patch until you don't support 10.11 anymore

The underlying question I'm really curious to hear the answer to is why don't you use the system libc++.dylib? How do you make sure that you don't have ODR issues when linking against macOS system frameworks that might be linking against libc++.dylib?

[rnk]

In D74489#2505156, @ldionne wrote:

In D74489#2497044, @thakis wrote:

We're looking into bumping libc++ in chromium, and this is a problem for us. We statically link libc++, and we still support 10.11. (We _just_ dropped support for 10.10 in our last release iirc.) What do you recommend as path forward?

Ugh, that's unfortunate. I see two options:

1. We revert the patch here, or
2. You carry a local patch until you don't support 10.11 anymore

I would prefer a revert if possible, since we are currently using a mirror of libc++ and don't have a good way to carry a local patch. We could create one if necessary, but it's work that will hopefully become obsolete in another year.

The underlying question I'm really curious to hear the answer to is why don't you use the system libc++.dylib? How do you make sure that you don't have ODR issues when linking against macOS system frameworks that might be linking against libc++.dylib?

I believe Chrome statically links its own libc++, uses the unstable ABI, and disables the libc++ visibility annotations. So, all our libc++ symbols should be hidden, and should not conflict with the dylib symbols. I see some evidence in the build files that people have run into ODR issues in the past and have worked around them by doing things like -D_LIBCPP_ABI_VERSION=Cr in certain configurations.

[ldionne]

See tentative fix in https://reviews.llvm.org/D95177

libcxx/docs/index.rst

* STLport and the Apache libstdcxx library are two other popular
   Faced with a rewrite, we decided to start from scratch and evaluate every
   design decision from first principles based on experience.
   Further, both projects are apparently abandoned: STLport 5.2.1 was
   released in Oct'08, and STDCXX 4.2.1 in May'08.
 
 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
 ---------------------
 
 * C++11 - Complete
 * `C++14 - Complete <http://libcxx.llvm.org/cxx1y_status.html>`__
 * `C++17 - In Progress <http://libcxx.llvm.org/cxx1z_status.html>`__
 * `Post C++14 Technical Specifications - In Progress <http://libcxx.llvm.org/ts1z_status.html>`__

libcxx/src/chrono.cpp

 //===------------------------- chrono.cpp ---------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #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>)
 #include <unistd.h>
 #endif
 
 #if !defined(__APPLE__) && _POSIX_TIMERS > 0
 #define _LIBCPP_USE_CLOCK_GETTIME
 #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
 
 namespace chrono
 {
 
 // system_clock

 #else
   GetSystemTimeAsFileTime(&ft);
 #endif
 
   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
-#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");
   return time_point(seconds(tp.tv_sec) + microseconds(tp.tv_nsec / 1000));
 #else
     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
 }
 
 time_t
 system_clock::to_time_t(const time_point& t) _NOEXCEPT
 {
     return time_t(duration_cast<seconds>(t.time_since_epoch()).count());
 }

 // Warning:  If this is not truly steady, then it is non-conforming.  It is
 //  better for it to not exist and have the rest of libc++ use system_clock
 //  instead.
 
 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
 {
     struct timespec tp;
     if (0 != clock_gettime(CLOCK_MONOTONIC_RAW, &tp))
         __throw_system_error(errno, "clock_gettime(CLOCK_MONOTONIC_RAW) failed");
     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:
 //    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
steady_clock::now() _NOEXCEPT
 
   LARGE_INTEGER counter;
   (void) QueryPerformanceCounter(&counter);
   return time_point(duration(counter.QuadPart * nano::den / freq.QuadPart));
 }
 
 #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
 {
     struct timespec tp;
     if (0 != clock_gettime(CLOCK_MONOTONIC, &tp))
         __throw_system_error(errno, "clock_gettime(CLOCK_MONOTONIC) failed");
     return time_point(seconds(tp.tv_sec) + nanoseconds(tp.tv_nsec));
 }
 
 #else
-#error "Monotonic clock not implemented"
+#  error "Monotonic clock not implemented"
 #endif
 
 #endif // !_LIBCPP_HAS_NO_MONOTONIC_CLOCK
 
 }
 
 _LIBCPP_END_NAMESPACE_STD

libcxx/src/filesystem/operations.cpp

 #include <sys/sendfile.h>
 #define _LIBCPP_USE_SENDFILE
 #endif
 #elif defined(__APPLE__) || __has_include(<copyfile.h>)
 #include <copyfile.h>
 #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")
 #endif
 
 #if defined(_LIBCPP_COMPILER_GCC)
 #if _GNUC_VER < 500
 #pragma GCC diagnostic ignored "-Wmissing-field-initializers"

 using parser::createView;
 using parser::PathParser;
 using parser::string_view_t;
 
 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))
     __throw_system_error(errno, "clock_gettime(CLOCK_REALTIME) failed");
   return time_point(__secs(tp.tv_sec) +
                     chrono::duration_cast<duration>(__nsecs(tp.tv_nsec)));
 #else
   typedef chrono::duration<rep, micro> __microsecs;
   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() {}
 
 void filesystem_error::__create_what(int __num_paths) {
   const char* derived_what = system_error::what();
   __storage_->__what_ = [&]() -> string {
     const char* p1 = path1().native().empty() ? "\"\"" : path1().c_str();

libcxx/src/include/apple_availability.h

 #endif
 #elif defined(__ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__)
 #if __ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__ >= 40000
 #define _LIBCPP_USE_UTIMENSAT
 #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
 #endif
 #elif defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__)
 #if __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ >= 130000
 #define _LIBCPP_USE_ULOCK
 #endif
 #elif defined(__ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__)