microseconds ought to be enough for anybody

[Ryan Lortie <desrt desrt ca>]

Hello

After an extensive process of dithering and soul-searching, I've come to
the following conclusions about time in GLib:

  - we have too many time types

      no explanation required.

  - microseconds ought to be enough for anybody

     A nanosecond is on the scale of individual clock cycles and
     processors are getting slower, not faster.  I have an extremely
     hard time imagining this sort of precision being meaningful (much
     less, actually useful) in our platform.  Heck; our mainloop is
     still based on milliseconds.

  - double isn't as good as int64

     Using doubles for time intervals is extremely wasteful.  Doubles
     were designed to represent extremely small and extremely large
     numbers -- orders of magnitude that we never meaningfully encounter
     while dealing with time.  Those extra bits used to express order of
     magnitude are lost for expressing actual information.  If we used
     fixed point, we get those 12 extra bits back.

     I also don't like the strange feeling that I get when I consider
     that the precision of double-based time expression slowly decreases
     with each passing year.  Not that it's really a practical
     consideration -- just a weird feeling.

  - use of int64-as-microseconds is already extremely common

     This is another reason to avoid doubles.

  - who cares about stupid platforms?

     I spent a significant amount of time hand-wringing over
     hypothetical "stupid" platforms that have the epoch of their
     monotonic clock in biblical times.  Although that's a theoretical
     possibility, I don't know of any such platforms and if they do
     exist, we can cross that bridge when we get there.  Our primary
     concern should be Linux -- and Linux is quite sane here.

The conclusion of all of this is one point: barring substantial
complaints, the be-all and end-all of time in glib is going to be
microseconds stored in a gint64.

GDateTime already calls this a "GTimeSpan".  That's OK.


I will do the following:

  - rip out the newly introduced GTimeSpec and all APIs using it

  - deprecate GTimeVal and all APIs using it

  - replace both with APIs using int64 of microseconds

  - deprecate GTimer

    with int64-based time, it's trivially easy to do it yourself


Somewhat related (but not directly): I'd like to deprecate GDate (and
the half-dozen or so related types) in favour of eventual replacement
with a GDateTime-style immutable opaque structure.  I guess we could
call that GDay (pronounced "g'day") for lack of a better available name
or we could wait until glib4 and replace GDate with GDate at that point.
Probably we should consider support for non-Gregorian calendars before
that point anyway (the fabled GCalendar API).

I'm going to start pushing this work into a branch today and I'll merge
it to master quite soon unless there are objections.

Cheers