--enable-libstdcxx-time changes ABI?

[Kenny Simpson <theonetruekenny at yahoo dot com>]

Hello,
  http://gcc.gnu.org/onlinedocs/libstdc++/manual/configure.html documents the --enable-libstdcxx-time options, but, unlike some of the other options, does not mention that it affects ABI.

or maybe I'm not understanding ABI-breakage.

My understanding of the affect of this is that  std::chrono::system_clock::time_point would become a different type.

std::chrono::system_clock::time_point is defined as:
      typedef chrono::time_point<system_clock, duration>        time_point;

and 'duration' depends on a configuration-time setting based the enable-libstdcxx-time:

    struct system_clock
    {
#ifdef _GLIBCXX_USE_CLOCK_REALTIME
      typedef chrono::nanoseconds                               duration;
#elif defined(_GLIBCXX_USE_GETTIMEOFDAY)
      typedef chrono::microseconds                              duration;
#else
      typedef chrono::seconds                                   duration;
#endif

What makes this affect the ABI is system_clock::now:
      static time_point
      now() noexcept;

Since only the return type is affected, and since the return type is not part of the mangled name, the exported symbol is not affected.

However, if a program at runtime were to run against a libstdc++.so built with a differing option, the results would be times/durations that are off by large factors - silently.  The program could have compiled assuming nanoseconds, only to run against a libstdc++ that returns microseconds.

As such, it seems it would never be safe to mix code that was built with differing flavors of libstdcxx-time.  If this is true, a note in the documentation would be nice - even if this isn't considered ABI-breakage since the exported symbols aren't affected.

thanks,
-Kenny