[libcxx] Partially address a FIXME in steady_clock::now()

[jroelofs]

Ping

[mclow.lists]

On a system w/o _POSIX_MONOTONIC_CLOCK, this will change from "libc++ will not build" to "it will build a non-conforming library"

That doesn't seem like an improvement to me.

[jroelofs]

Is it possible to build a monotonic clock out of a non-monotonic clock in
software only?

I could force it to be monotonic with some static storage and a max=, but would
that be correct?

Jon

[jroelofs]

mclow and I discussed this offline, and max= is not going to work:

20.12.7.2/1: Objects of class steady_clock represent clocks for which values of time_point never decrease as physical time advances and for which values of time_point advance at a steady rate relative to real time.

I'll look into what breaks if steady_clock isn't provided on systems without _POSIX_MONOTONIC_CLOCK, and go from there.

[jroelofs]

On systems without a monotonic clock, it's probably better to not have steady_clock than to have a non-conforming one.

Since steady_clock is used for timed mutexes and such, something like: http://reviews.llvm.org/D3969 is probably necessary to actually build libc++ without a a monotonic clock. I'll be picking that patch up for upstreaming again here shortly.

[jroelofs]

Vaguely depends on http://reviews.llvm.org/D3969

Also... ping?

[jroelofs]

Add XFAILs

[jroelofs]

ping x2

libcxx/test/lit.site.cfg

11 ↗

(On Diff #12840)

FYI, I don't intend for this to go upstream... this is just here as an example usage.

[danalbert]

include/chrono

943

Should there be a warning that the system won't use steady_clock?

libcxx/test/utilities/time/time.clock/time.clock.steady/consistency.pass.cpp

14 ↗

(On Diff #12840)

I'd use UNSUPPORTED instead.

[danalbert]

include/__config

644

As mentioned on http://reviews.llvm.org/D3969, _LIBCPP_HAS_NO_MONOTONIC_CLOCK will more closely match the other configs.

[jroelofs]

include/chrono

943

Implementing high_resolution_clock via a typedef to system_clock is explicitly allowed by the standard. I don't see a reason for a warning here.

libcxx/test/utilities/time/time.clock/time.clock.steady/consistency.pass.cpp

14 ↗

(On Diff #12840)

Ok

[mclow.lists]

include/chrono

944

I concur - no need for a warning.

[danalbert]

include/chrono

943

Ah, alright. Fine by me.

[jroelofs]

Rename the flag to _LIBCPP_HAS_NO_MONOTONIC_CLOCK, and change the XFAILs to UNSUPPORTEDs.

[danalbert]

LGTM.

[jroelofs]

r216949

include/__config

 #endif
 
 #if defined(ELAST)
 #define _LIBCPP_ELAST ELAST
 #elif defined(__linux__)
 #define _LIBCPP_ELAST 4095
 #elif defined(_NEWLIB_VERSION)
 #define _LIBCPP_ELAST __ELASTERROR
 #else
 // Warn here so that the person doing the libcxx port has an easier time:
 #warning This platform's ELAST hasn't been ported yet
 #endif
 
 #ifndef _LIBCPP_TRIVIAL_PAIR_COPY_CTOR
 #  define _LIBCPP_TRIVIAL_PAIR_COPY_CTOR 1
 #endif
 
+#if (!defined(_POSIX_TIMERS) || _POSIX_TIMERS <= 0) || \
+    (!defined(_POSIX_MONOTONIC_CLOCK) || _POSIX_MONOTONIC_CLOCK <= 0)
+#define _LIBCPP_HAS_NO_MONOTONIC_CLOCK
+#endif
+
 #ifndef _LIBCPP_STD_VER
 #  if  __cplusplus <= 201103L
 #    define _LIBCPP_STD_VER 11
 #  elif __cplusplus <= 201402L
 #    define _LIBCPP_STD_VER 14
 #  else
 #    define _LIBCPP_STD_VER 15  // current year, or date of c++17 ratification
 #  endif

include/chrono

public:
     typedef chrono::time_point<system_clock> time_point;
     static _LIBCPP_CONSTEXPR_AFTER_CXX11 const bool is_steady = false;
 
     static time_point now() _NOEXCEPT;
     static time_t     to_time_t  (const time_point& __t) _NOEXCEPT;
     static time_point from_time_t(time_t __t) _NOEXCEPT;
 };
 
+#ifndef _LIBCPP_HAS_NO_MONOTONIC_CLOCK
 class _LIBCPP_TYPE_VIS steady_clock
 {
 public:
     typedef nanoseconds                                   duration;
     typedef duration::rep                                 rep;
     typedef duration::period                              period;
     typedef chrono::time_point<steady_clock, duration>    time_point;
     static _LIBCPP_CONSTEXPR_AFTER_CXX11 const bool is_steady = true;
 
     static time_point now() _NOEXCEPT;
 };
 
 typedef steady_clock high_resolution_clock;
+#else
+typedef system_clock high_resolution_clock;
+#endif
 
 } // chrono
 
 #if _LIBCPP_STD_VER > 11
 // Suffixes for duration literals [time.duration.literals]
 inline namespace literals
 { 
   inline namespace chrono_literals

src/chrono.cpp

 }
 
 system_clock::time_point
 system_clock::from_time_t(time_t t) _NOEXCEPT
 {
     return system_clock::time_point(seconds(t));
 }
 
+#ifndef _LIBCPP_HAS_NO_MONOTONIC_CLOCK
 // steady_clock
 
 const bool steady_clock::is_steady;
 
 #ifdef __APPLE__
 //   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

 {
     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));
 }
 #endif  // __APPLE__
 
+#endif // !_LIBCPP_HAS_NO_MONOTONIC_CLOCK
+
 }
 
 _LIBCPP_END_NAMESPACE_STD

test/utilities/time/time.clock/time.clock.steady/consistency.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is dual licensed under the MIT and the University of Illinois Open
 // Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This test uses new symbols that were not defined in the libc++ shipped on
 // darwin11 and darwin12:
 // XFAIL: with_system_lib=x86_64-apple-darwin11
 // XFAIL: with_system_lib=x86_64-apple-darwin12
+// UNSUPPORTED: no-monotonic-clock
 
 // <chrono>
 
 // steady_clock
 
 // check clock invariants
 
 #include <chrono>

test/utilities/time/time.clock/time.clock.steady/now.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is dual licensed under the MIT and the University of Illinois Open
 // Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
+//
+// UNSUPPORTED: no-monotonic-clock
 
 // <chrono>
 
 // steady_clock
 
 // static time_point now();
 
 #include <chrono>