bug#39944: 27.0.90; JIT Stealth timer errors

[Paul Eggert <eggert@cs.ucla.edu>]

>> Although comments in that area no doubt could use improvement, I'd rather not 
>> see comments like this:
>> 
>>        /* Add TM_YEAR_BASE to mpz[0].  */
>>        mpz_add_ui (mpz[0], mpz[0], TM_YEAR_BASE);
> 
> There are quite a few of similar comments there,

I don't see any similar comments for simple calls from Emacs to mpz_add_ui. And 
I doubt whether there should be, any more than image.c should have comments like 
this:

   /* Use components from GC to draw a line on DPY's PIXMAP from (X, Y)
      to (X+WIDTH-1, Y+HEIGHT-1).  */
   XDrawLine (dpy, pixmap, gc, x, y, x + width - 1, y + height - 1);

Emacs does not do comments like that, because they would clutter the code if you 
already know what XDrawLine does; and if you don't know, it's easy enough to 
look it up - which you should do anyway, and you'll need to do it anyway if you 
want to understand commentary like "components from GC" and "DPY's PIXMAP".

Similarly for nearly every library function Emacs calls. GMP should be no 
different from other libraries in this respect.

> But I really meant that stuff like this lacks a comment:
> 
>       hz = make_integer_mpz ();
>       mpz_swap (mpz[0], *iticks);
>       ticks = make_integer_mpz ();
> 
> And also functions like timespec_ticks, lisp_time_hz_ticks, and
> lisp_time_seconds, which don't have a single comment describing how
> they do their thing.

I added a few comments here and there. But I didn't go down to the level of 
commenting every call.

>> But it might be better in master to remove the "Invalid time format"
>> check entirely.
> 
> I'm fine with removing that test, if we are sure that invoking
> run-at-time with something utterly un-timely, like a symbol or a a
> string that cannot be a valid time description, will trigger an error
> (presumably from timer-set-time).

Yes, that's what happens. I removed the test in master.