[patch] Default to --enable-libstdcxx-time=auto

[Benjamin De Kosnik]

> > It scraps the renaming/aliasing approach, and just creates a
> > compatibility-chrono.cc that mimics the default configuration in
> > 4.8.0.
> 
> Yeah, I think that is reasonable, with one nit, see below.

Cool, incorporated.
 
> > Users who specially-configured a build with --enable-libstdcxx-time
> > configure options in 4.8.0 are breaking an experimental
> > C++ ABI anyway, so I'm not going to solve for anything but the
> > default case.
> > 
> > For chrono.cc, there is an inline namespace that mangles
> > system_clock and steady_clock in a new way to prevent ambiguous
> > uses. In addition, I make a controversial decision and just
> > forward-declare clocks that libstdc++ cannot do correctly, instead
> > of using typedefs that clearly are only going to get us into
> > trouble as we change things down the road. That's probably only
> > appropriate for trunk.
> 
> > +    // To support the (forward) evolving definition of the
> > library's
> > +    // defined clocks, wrap inside inline namespace so that these
> > +    // types are uniquely mangled. This way, new code can use the
> > +    // current clocks, while the library can contain old
> > definitions.
> > +    // At some point, when these clocks settle down, the inlined
> > +    // namespaces can be removed.
> > +    // XXX GLIBCXX_ABI Deprecated
> > +    inline namespace _V2 {
> > +
> >      /// system_clock
> >      struct system_clock
> >      {
> 
> In this _V2 inline namespace, I think we should change that:
>     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
> into:
>     struct system_clock
>     {
>       typedef chrono::nanoseconds
> duration;
> 
> Won't the templates then DTRT in:
>   return time_point(duration(chrono::seconds(tv.tv_sec)
> 		    + chrono::microseconds(tv.tv_usec)));
> (multiply microseconds by 1000 and seconds by 1000000000)
>       return system_clock::from_time_t(__sec);
> (multiply seconds by 1000000000)?

Yes.
 
> While this change isn't really necessary for Linux, where usually
> _GLIBCXX_USE_CLOCK_REALTIME will be in 4.8.1+ defined and thus the
> nanoseconds resolution will be used anyway, on other OSes it might
> be already a problem, say on Solaris or FreeBSD GCC 4.8.1 will have
> by default likely microseconds resolution, while on the trunk
> nanoseconds. By making it always count in nanoseconds, even if on
> some OSes the low 3 or 9 decimal digits will be always zero, we'd
> prepared to change the system_clock::now() implementation any time to
> provide better resolution, as a libstdc++ internal implementation
> detail.
> 
> Or is this undesirable for users for some reason?

I think your rationale is sound here. I've added some of these comments
to the code.

> 
> > @@ -742,10 +751,18 @@ _GLIBCXX_END_NAMESPACE_VERSION
> >        now() noexcept;
> >      };
> >  #else
> > -    typedef system_clock steady_clock;
> > +    // Forward declare only.
> > +    struct steady_clock;
> >  #endif
> >  
> > +#ifdef _GLIBCXX_USE_CLOCK_REALTIME
> >      typedef system_clock high_resolution_clock;
> > +#else
> > +    // Forward declare only.
> > +    struct high_resolution_clock;
> > +#endif
> > +
> > +  } // end inline namespace _V2
> 
> Yeah, I bet this hunk should be left out from 4.8.1 version of the
> patch.

I did something different, anyway.

Un-guardedly declare the clocks, and then just use the definitions in
the .cc file to define QoI. That way there is a symbol exported in all
configs, but one that can be improve in the future w/o pain. 

> Perhaps we also want some testcase that will roughly test it, like
> grab time(NULL), std::chrono::system_clock::now() and
> std::chrono::steady_clock::now(), then sleep(3), grab both clocks
> again and time(NULL) last, then if the difference between time() is
> at least 2 seconds and less than say 10, verify the difference
> between the clocks is say in a 1 to 20 seconds interval (yeah, it
> could fail badly if one changes system time in between once or
> several times)?  But maybe it would be too flakey.

Agreed, certainly room for improvement. Let's separate out this part
though.

I'm tempted to check this patch into trunk. Jakub?

tested x86_64/linux

-benjamin
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