Re: Edebug corrupting point in buffers; we need buffer-point and set-buffer-point, perhaps.

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 31 Oct 2022 15:46:07 +0000
> Cc: emacs-devel@gnu.org
> From: Alan Mackenzie <acm@muc.de>
> 
> > I'm not sure performance in a debugger is a reason good enough to add
> > 2 more primitives.  The fact that we didn't need them until now should
> > tell us something, no?
> 
> Well, I timed it.  With 207 buffers, creating an alist of (buffer .
> buffere-point) with my new function was 17 times as fast as using
> with-current-buffer and point.

17 times faster doesn't yet tell how important is the speedup, because
you give no absolute numbers, and they are what's important here.

> But on the other hand, these two functions feel like they ought to exist.
> They could save a lot of clumsy programming with swapping the buffer,
> just to get or set point.

There's nothing clumsy with what we did, and the fact that we did
manage without them speaks volumes.

> > > +DEFUN ("buffer-point", Fbuffer_point, Sbuffer_point, 1, 1, 0,
> > > +       doc: /* Return the buffer point of BUFFER-OR-NAME.
> > > +The argument may be a buffer or the name of an existing buffer.  */)
> > > +  (Lisp_Object buffer_or_name)
> 
> > Why not an optional argument to 'point'?  And why in buffer.c and not
> > in editfns.c?
> 
> I'm not sure what you mean by an optional argument, here.

I mean (point &optional buffer), of course, what else could I mean?

> > > +  return (make_fixnum (b->pt));
> 
> > Please never-ever use b->pt etc. directly.  We have BUF_PT and other
> > macros for that, and for a good reason.
> 
> BUF_PT and friends work specifically on current_buffer.

No, they don't:

  /* Position of point in buffer.  */
  INLINE ptrdiff_t
  BUF_PT (struct buffer *buf)
  {
    return (buf == current_buffer ? PT
	    : NILP (BVAR (buf, pt_marker)) ? buf->pt
	    : marker_position (BVAR (buf, pt_marker)));
  }

> The whole idea of the new functions is to avoid having to switch
> buffers.

We do this from C in a gazillion of places.