bug#12430: Glitches caused by addition of psec to timers

[Eli Zaretskii]

> Date: Thu, 13 Sep 2012 11:48:48 -0700
> From: Paul Eggert <eggert@cs.ucla.edu>
> CC: Stefan Monnier <monnier@iro.umontreal.ca>, 12430@debbugs.gnu.org
> 
> On 09/13/2012 09:57 AM, Eli Zaretskii wrote:
> > Wouldn't it be better to leave timers at their previous size of 8, and
> > instead allow the usecs element be a floating-point number?
> 
> That could introduce off-by-one errors due to rounding
> issues.

Not with correct programs that _want_ the sub-microsecond resolution.

> We could have the usecs element be a list, though.
> That is, (USECS PSECS) would stand for the sum of
> USECS microseconds and PSECS picoseconds, whereas
> a plain USECS would be treated as a microseconds count,
> as before.

Yes, that's also a possibility, although it is less compatible (e.g.,
fails the 'numberp' test).  Any backward-compatible structure is
better than an incompatible one.  If 'timerp' just checks the number
of elements in the array, so could some code out there.