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


On Fri, May 24, 2013 at 08:26:32PM -0700, Benjamin De Kosnik wrote:
> 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.

> 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)?

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?

> @@ -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.

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.

	Jakub


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